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\DOI{10.5802/crchim.242}
\datereceived{2023-03-26}
\daterevised{2023-07-09}
\dateaccepted{2023-07-10}
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\begin{document}

\begin{noXML}

\RubricEF{Review article}{Article de synthèse}

\title{Electrogeneration of non-electroactive and non-conducting
materials: a counterintuitive concept for the functionalization and
nanostructuration of electrode surfaces}

\alttitle{G\'{e}n\'{e}ration de mat\'{e}riaux non \'{e}lectroactifs et
non conducteurs par voie \'{e}lectrochimique : un concept
contre-intuitif de fonctionnalisation  et de nanostructuration
de surface d'\'{e}lectrodes}

\author{\firstname{Alain} \lastname{Walcarius}\CDRorcid{0000-0003-3633-200X}}
\address{Universit\'{e} de Lorraine, CNRS, LCPME, 54000 Nancy, France}
\email{alain.walcarius@univ-lorraine.fr}

\def\thanksname{Note}
\thanks{Alain Walcarius is the 2019 recipient of the Langevin Prize.}

%\thanks{\textbf{Note.}
%Alain Walcarius est le laur\'{e}at 2019 du Prix Langevin.}

\begin{abstract}
Electrodeposition is a long-lasting and efficient process to generate
thin or thick films on conductive supports. It is mainly based on
electrochemically induced redox reactions involving electroactive
precursors, which are intended to form solid deposits onto the
electrode surface. More recently, a rather counterintuitive approach
has emerged by exploiting electrochemistry to generate
non-electroactive and non-conductive thin films (e.g., sol--gel
derived materials), based on the electrogeneration of a catalyst that
is likely to induce indirectly the formation of a thin film
(i.e., without direct electron transfer with the precursors).
This account summarizes the major advances made by our group in this
field, focusing primarily on electro-induced sol--gel bioencapsulation
and electro-assisted self-assembly of oriented and functionalized
mesoporous silica films.
\end{abstract}

\begin{altabstract}
L'\'{e}lectrod\'{e}p\^{o}t est un moyen employ\'{e} de longue date
pour produire efficacement des films minces ou \'{e}pais sur
diff\'{e}rents supports conducteurs. Il implique principalement des
r\'{e}actions redox induites par voie \'{e}lectrochimique au niveau de
pr\'{e}curseurs \'{e}lectroactifs, qui sous-tendent la formation de
d\'{e}p\^{o}ts solides \`{a} la surface d'une \'{e}lectrode. Plus
r\'{e}cemment, une autre approche assez contre-intuitive est n\'{e}e de
l'exploitation de l'\'{e}lectrochimie pour g\'{e}n\'{e}rer des
films minces non \'{e}lectroactifs et non conducteurs (comme des
mat\'{e}riaux d\'{e}riv\'{e}s de la chimie sol--gel), bas\'{e}e sur la
g\'{e}n\'{e}ration \'{e}lectrochimique d'un catalyseur susceptible
d'induire indirectement la formation d'un film mince
(c'est-\`{a}-dire sans transfert \'{e}lectronique direct avec les
pr\'{e}curseurs). Cette mise au point r\'{e}sume les principales
avanc\'{e}es effectu\'{e}es par notre groupe dans ce domaine, en se
focalisant principalement sur la bioencapsulation sol--gel
\'{e}lectro-induite et l'auto-assemblage \'{e}lectrochimiquement
assist\'{e} de films de silice m\'{e}soporeuse orient\'{e}s et
fonctionnalis\'{e}s.
\end{altabstract}

\keywords{\kwd{Electrodeposition}
\kwd{Thin films}
\kwd{Sol--gel materials}
\kwd{Mesoporous silica}
\kwd{Modified electrodes}
\kwd{(Bio)electrocatalysis}
\kwd{Electroanalysis}}

\altkeywords{\kwd{\'{E}lectrod\'{e}p\^{o}t}
\kwd{Films minces}
\kwd{Mat\'{e}riaux sol--gel}
\kwd{Silices m\'{e}soporeuses}
\kwd{\'{E}lectrodes modifi\'{e}es}
\kwd{(Bio)\'{e}lectrocatalyse}
\kwd{\'{E}lectroanalyse}}

\maketitle

\vspace*{14pt}

\twocolumngrid

\end{noXML}

\section{Introduction}

Electrochemistry has a long history of surface modification ability,
either by coating conductive surfaces with thin of thick films, or by
covalent\unskip\break grafting of molecular moieties or
supramolecular assemblies
onto electrode materials. The concept of electrodeposition is known for
more than a century when metal layers were deposited onto an electrode
surface by electrolysis~\cite{1}. It was then extended to the
deposition of other electroactive coatings on conductive surfaces and
the field of electrodeposition expanded considerably in the
80's~\cite{2}, being still growing nowadays~\cite{3}. It is mainly
based on electrochemically induced redox reactions involving
electroactive precursors, which are intended to form solid deposits
onto the electrode surface by electroplating (for metals~\cite{4,5,6},
alloys~\cite{6,7} or semiconductors~\cite{8}),
electropolymerization (for, e.g., conducting
polymers~\cite{9}) or other electrodeposition methods (for
oxides~\cite{10,11} or composite materials~\cite{3,12}). More   
diverse and/or sophisticated\unskip\break materials can be also generated by
electrodeposition (e.g., metal--organic frameworks~\cite{13},
nanostructured catalysts~\cite{14}, nanocomposites~\cite{15} or
other functional (nano)materials~\cite{16,17}). The resulting
coatings are most often electroactive and/or conductive. In parallel,
both oxidative and reductive electrografting methods have been
developed to attach organic layers to solid conducting
substrates~\cite{18} and such electrochemically induced surface
modification can be followed further by post-functionalization
processes to design advanced electrochemical interfaces with more
complex molecular structures useful for targeted
applications~\cite{19,20}. Again, most of them were electroactive
and/or conductive.

On the other hand, notwithstanding their insulating properties and
non-electroactive nature, several inorganic materials have been used in
connection to electrochemistry, such as clays~\cite{21,22,23,24,25,26},
zeolites~\cite{27,28,29,30,31,32,33}, zeolitic imidazolate
frameworks~\cite{34,35,36} or large band-gap metal oxides (most often
silica gels or silica-based organic--inorganic
hybrids~\cite{37,38,39,40}, sol--gel
materials~\cite{40,41,42,43,44,45,46,47}, mesoporous
silica~\cite{31,47,48,49,50,51}, or other nanostructured/nano-sized
metal oxides~\cite{52,53,54}). They are attractive for the additional
properties they can bring to an electrochemical interface
(e.g., preconcentration and/or permselective features, host
for biomolecules and/or catalysts). They can also be chemically
modified in order to become electroactive (e.g., by
impregnation/adsorption/ion exchange of redox-active species, or via
the formation of hybrid materials made of organic moieties covalently
attached to the inorganic structure). At the beginning, the
intersection between electrochemistry and such materials was focusing
mainly onto two parallel and complementary approaches~\cite{55}: 
\begin{enumerate}[{(i)}]
\item the use of electrochemical characterization methods to get
information on the materials properties (i.e., guest
transport~\cite{56,57,58,59,60,61,62,63}, size/charge
selectivity~\cite{63,64,65,66,67}, charge transfer mechanisms in the
insulating porous media~\cite{68,69,70,71,72}, recognition and/or
accumulation properties~\cite{73,74,75,76,77,78,79,80,81});
\item the exploitation of the remarkable properties of these materials
used as electrode modifiers for selected applications, mainly in the
field of electrochemical sensors and biosensors (see selected review
papers~\cite{24,25,28,30,31,37,40,48,49,50,51,52,53,54,82,83,84,85,86}),
but also in\unskip\break
electrocatalysis~\cite{87,88,89,90}, electrochemical energy storage and
conversion~\cite{34,90,91,92}, corrosion protection~\cite{93},
electrochromism~\cite{94}, and more recently molecular
electronics~\cite{95,96}.
\end{enumerate}
Because they are insulating materials, their use in connection to
electrochemistry requires a close contact to a conductive electrode
surface. This can be basically achieved by dispersing particles in a
conductive composite matrix (e.g., carbon
paste~\cite{97,98,99,100,101,102,103}, carbon
ink~\cite{103,104,105,106}) or via the deposition of the materials as
thin/thick films onto a solid electrode support (see examples of
electrodes covered with clay platelets
films~\cite{107,108,109,110,111}, zeolite particulate or continuous
films~\cite{31,64,112,113,114,115,116,117,118}, silica-based or other
sol--gel derived films~\cite{46,47,119,120,121}, composite deposits
containing mesoporous silica particles~\cite{31,122,123,124,125} or
continuous mesoporous silica films~\cite{31,126,127,128}). For film
preparation, the approaches frequently used are solvent casting,
spin-coating, dip-coating, layer-by-layer assembly, or via the
Langmuir--Blodgett methodology.

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Schematic
illustration of the electro-assisted deposition of a silica film onto
an electrode surface from a hydrolysed sol at pH 3 (yellow part) by
applying a cathodic potential likely to increase pH at the
electrode/solution interface (pink part) to induce gelation of the sol
and formation of the silica network by condensation of
Si(OH)$_{4}$ precursors catalysed
by the electrogenerated OH$^{-}$\unskip\break species.}
\vspace*{-6pt}
\end{figure*}

In recent decades, significant efforts have been made to prepare thin
films of non-conductive and non-electroactive materials by indirect
electrochemically induced deposition. This method is based on the
electrochemical generation of species that are likely to react with
precursors in solution in the vicinity of the electrode, leading to the
deposition or growth of solid phases onto the electrode surface mainly
driven by local pH
changes~\cite{51,86,89,121,128,129,130,131,132,133,134,135,136,137}.
This is conceptually distinct from electrophoretic deposition,
i.e., a technique involving the application of an electric
field to a solution containing suspended matter, leading to the
deposition of charged particles onto a conductive substrate~\cite{138},
which was mainly applied to the formation of metal oxide and ceramic
coatings~\cite{139,140,141,142} or the deposition of other advanced
(bio)materials~\cite{143,144,145}. In the present case, a reagent is
generated via an electron transfer reaction at the electrode surface
and so-generated species are involved in subsequent reactions inducing
the transformation of solution-phase precursors (via precipitation or
polycondensation) into a desired material coating the electrode
support. Examples are available for metal oxides~\cite{129,130},
polymers~\cite{131,132}, sol--gel derived materials~\cite{133,134},
layered double hydroxides~\cite{89,135}, ordered mesoporous thin
films~\cite{51,86,128,136,137}. They can be formed according to
different driving forces. Precipitation of metal oxides and hydroxides
or layered double hydroxides can be performed by electrogenerated
$\mathrm{OH}^{-}$ ions~\cite{89,129,130,135}, pH tuning in the
electrode/solution interfacial region (as a result of water
electrolysis~\cite{146}) to induce changes in solubility of polymeric
materials in suspension due to variation in their
hydrophilic/hydrophobic ratio~\cite{131,132}, or the electrogeneration
of a catalyst that is likely to induce the growth of sol--gel films by
accelerating the polycondensation reactions~\cite{133,134}.
Mesostructures are obtained if operating in the presence of surfactant
templates~\cite{51,86,128,136,137}. In all cases, the electrogeneration
of the catalyst leads to the indirect formation of a thin film,
i.e., without direct electron transfer with the precursors,
and such synthesis concept can be considered as counterintuitive as it
enables the preparation of non-conductive and non-electroactive
materials by an electrochemical way. This account intends to summarize
the major advances primarily made by our group in this field, focusing
mainly on electrochemically induced sol--gel bioencapsulation and
electrochemically assisted self-assembly of oriented and functionalized
mesoporous silica films, also considering recent applications developed
by other groups.

\section{The concept of electrochemically assisted deposition of sol--gel films by local pH tuning}

The concept of sol--gel electrodeposition was born~in 1999 when
attempting to coat metal surfaces with hydrophobic organosilane for 
corrosion protection \cite{147}, and  then extended to the formation of
non-functionalized silica layers~\cite{133,148} or organically modified
silicate films~\cite{112,149} on electrodes. As shown in
Figure~\ref{fig1}, it is based on the use of a silica sol containing
tetraalkoxysilane precursors that are hydrolysed in moderate acidic
medium (pH 3, Equation~(\ref{eqn1})) and an electrode biased at a suitable
cathodic potential likely to generate $\mathrm{OH}^{-}$ species by water
reduction (imposing pH 9--10 at the electrode/solution interface) acting
as a catalyst for polycondensation of the hydrolysed precursors 
(Equation~(\ref{eqn2})), leading to concomitant silica film growth onto the
electrode surface.
{\begin{eqnarray}
&\mathrm{Si}(\mathrm{OR})_{4}+4\mathrm{H}_{2}\mathrm{O}\overset{\mathrm{H}^{+}}{\Rightarrow}
\mathrm{Si}(\mathrm{OH})_{4}+4\mathrm{ROH} 
\label{eqn1}\Seqnsplit
& 2\mathrm{Si}(\mathrm{OH})_{4}\overset{\mathrm{OH}^{-}}{\Rightarrow }
(\mathrm{OH})_{3}\mathrm{Si}-\mathrm{O}-\mathrm{Si}
(\mathrm{OH})_{3}+\mathrm{H}_{2}\mathrm{O}
\label{eqn2}
\end{eqnarray}}\unskip

The presence of nitrate ions in the electrosynthesis medium can
contribute to promote further the cathodic alkalization at the origin
of electro-assisted deposition of sol--gel films~\cite{150}. Although
mainly applied to the generation of silica-based thin films, the method
can also be implemented to obtain other metal oxide layers
(zirconia~\cite{151}, titania~\cite{152}) or for the
co-electrodeposition of binary silica-metal oxide
coatings~\cite{153,154,155}. 

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Electro-assisted deposition of
amino-functionalized silica films onto a gold electrode at an applied
potential of ${-}1.2$~V from a sol solution containing tetraethoxysilane
(TEOS) and aminopropyltriethoxysilane (APTES) in 90:10 molar ratio. (A)
Frequency--time plot obtained by quartz crystal microbalance. (B)
Variation of the film thickness as a function of electrodeposition
time. (C) Atomic force microscopy imaging of two films obtained
respectively after 10 and 60~s electrodeposition. {Reprinted and
adapted from Sibottier \etal\ \cite{149}, Copyright
(2006) American Chemical Society}.}
\end{figure*}


\begin{figure*}
\includegraphics{fig03}
\vspace*{-2pt}
\caption{\label{fig3}Illustration of the various possibilities
offered by the electro-assisted deposition of silica films.}
\vspace*{-4pt}
\end{figure*}


The parameters affecting the growth process are mainly (1) the nature
of the precursor and its concentration, (2) the value of the applied
cathodic potential, (3) the deposition time, and to lesser extent the
temperature and supporting
electrolyte~\cite{133,147,149,156,157,158,159,160}. In particular, the
film thickness (i.e., the amount of deposited material) has a
strong dependence on the applied potential, being much larger when
operating at more cathodic potentials because they induce higher
interfacial pH values (due to more $\mathrm{OH}^{-}$ species generated). The
time of electrolysis is also an important parameter governing the film
growth as one could expect thicker deposits at longer electrodeposition
times. However, if the film thickness is indeed increasing linearly in
the sub-micrometer range with the time of applying the cathodic
potential in a first stage, it rises up exponentially to form much
thicker  (${>}1~\rmmu\mathrm{m}$) and rougher porous deposits in a second
stage. As illustrated in Figure~\ref{fig2}, these two steps and their
consequences on film thickness and roughness can be evidenced from
quartz crystal microbalance measurements (Figure~\ref{fig2}A), from
thickness variation as a function of deposition time
(Figure~\ref{fig2}B), and by atomic force microscopy imaging
(Figure~\ref{fig2}C)~\cite{149}. The second faster gelation step
leading to the presence of thicker deposits in the form of particulate
aggregates is due to the fact that the polycondensation catalysts
($\mathrm{OH}^{-}$ species) are generated at the electrode/solution interface in
a thickness of hundreds of micrometers (corresponding to the diffusion
layer originating from the application of the cathodic potential). A
quantitative description of the kinetics associated with the growth of
sol--gel films generated by electrochemically assisted deposition can be
derived from numerical simulations~\cite{161}. Finally, varying the
concentration of precursors\unskip\break enables to tune the film growth
rate~\cite{159,160} and the use of very dilute sol solutions leads to
the formation of ultra-thin (${<}$10~nm) silica films~\cite{156}. The
replacement of some tetraalkoxysilane precursors by
organo-trialkoxysilane reagents also leads to thinner films due to
slower hydrolysis/condensation reactions~\cite{157}. For more details
on the fundamentals of sol--gel electrochemical deposition, the reader
is directed to an excellent book chapter on the topic~\cite{160}.

The concept of electrochemically assisted sol--gel deposition opens new
ways for electrode surface modification, as illustrated in Figure~\ref{fig3}. 
\begin{itemize}
\item Firstly, it offers the advantage of depositing layers of
uniform thickness over the whole surface of non-flat supports (such as
metal or carbon nanofibers~\cite{162,163,164}, streaked metal
surfaces~\cite{133,165}, ordered macroporous metals~\cite{156,166})
or micro- and nanoscale objects (carbon nanotubes~\cite{167}), the
electrode geometry and size having some impact on film formation and
properties~\cite{168}. It can cover selectively specific portions of
heterogeneous supports, either via electrodeposition only onto the
conducting portions of the support (printed circuit, gold mesh,~silver
grid printed on polyethylene\unskip\break terephthalate)~\cite{169,170} or by
taking advantage of  different overpotentials for the generation of
$\mathrm{OH}^{-}$ catalyst (Pt nanoparticles on carbon or Pt--Au bimetallic
macroporous electrodes), enabling selective sol--gel electro-assisted
deposition onto Pt~\cite{166,171}, whereas the films obtained by
dip-coating covered the whole surface of the Pt/C or Pt/Au supports.
Of related interest is the formation of free-standing porous silica
microstructures by exploiting adhesion differences of the deposit on
hydrophilic and hydrophobic surfaces, via the selective removal of
silica from the more hydrophobic areas of the substrate (thiol-modified
Au) while keeping those firmly attached to the hydrophilic ones
(indium-tin oxide)~\cite{172}. 

\item Secondly, patterning is achievable by local scale
electro-assisted deposition of sol--gel dots using a micrometric size
electrode and a scanning electrochemical microscope in order to
approach the microelectrode closely above the surface of a solid
support enabling to generate locally the interfacial pH change to
induce gelation and film formation under the
microelectrode~\cite{173,174,175}. Getting patterns of locally deposited
sol--gel material can be also performed by electrochemistry at arrays of
liquid--liquid micro- or nanointerfaces, yet via a slightly different
mechanism involving the electro-assisted transfer of
cetyltrimethylammonium surfactant from an organic phase to an aqueous
phase containing the tetraalkoxysilane precursor, leading to film
formation at such soft interfaces~\cite{176,177,178}. 

\item Thirdly, composite films can be obtained by co-deposition of
metal and sol--gel\unskip\break interpenetrated layers by adding a metal precursor in
the starting sol, which is reduced simultaneously to silica gel
formation (examples are available for Cu~\cite{179}, 
Zn~\cite{180,181}, Ni~\cite{182} and Au~\cite{183}).
Co-electrodeposition of silica and other metal oxides is also possible
(e.g., $\mathrm{TiO}_{2}$~\cite{153} or $\mathrm{MnO}_{2}$~\cite{184}).
{Sometimes, the silica part of the composite is dissolved to get porous
metals or metal oxides~\cite{182,183,184}. Conductive polymer--silica
hybrid thin films} can be obtained {by combining electropolymerization
and sol--gel electrodeposition from a medium containing together the
organic polymer precursor (e.g., pyrrole) and a
tetraalkoxysilane. It involves performing the anodic polymerization of
the conductive polymer and the cathodic sol--gel electrodeposition
successively (separating both processes enables a better control of
deposition parameters and thus an easy manipulation of film composition
and thickness)  \cite{185,186}. Other kind of organic--inorganic hybrid
films based on polymer--silica composites can be indirectly
electrodeposited via interfacial pH change, as illustrated for
organotrialkoxysilane-chitosan hybrid gels~\cite{187,188}. 

\item Fourthly, nano-object can be physically entrapped within
sol--gel films in the course of their electrochemically induced growth,
offering a way to immobilize durably some particles that are somewhat
difficult to accommodate onto electrode surfaces in a mechanically
stable form. Examples are available for clays~\cite{189,190}, metal or
metal oxide nanoparticles~\cite{191,192,193}, carbon
nanomaterials~\cite{167,170,194,195}, giving rise to composite films
applicable in various fields (sensing, electrocatalysis, protective
coatings). An interesting approach is the controlled growth of sol--gel
matrices with different thickness and functional groups as nanoparticle
imprinted thin films on electrodes that are likely to recognize and
detect selectively gold nanoparticles based on their dimensions
\cite{196}. The electrochemical generation of sol--gel films can be also
performed in the presence of polymeric nanobeads, leading to their
entrapment in the inorganic coatings, and their subsequent dissolution
leads to pore formation and enhanced mass transport of the resulting
porous films~\cite{197}.

\item Fifthly, adding molecules or biomolecules and even
microorganisms in the starting sol enables in principle their
entrapment in the film during the formation of the silica framework.
This is indeed successful for redox proteins, as pioneeringly
demonstrated for haemoglobin and glucose oxidase in electrodeposited
silica films~\cite{198,199,200}, taking advantage of} the room temperature
sol--gel processing (which appears as a robust and versatile way for the
effective immobilization of biologicals~\cite{201,202,203}). The use of
additives in the starting sol may help at improving the biocomposite
film properties, such as polymers (i.e., polyelectrolytes
to favour bioencapsulation/biocompatibility)~\cite{204}, noble metal
nanoparticles (to enhance conductivity)~\cite{200,205} or
biodegradable polymeric nanoparticles~\cite{206}. The immobilization of
larger biological materials (e.g., bacteria) in
electrogenerated sol--gel films is also possible, leading to the
production of electroactive artificial biofilms~\cite{207,208}. On the
other hand, if examples are available for entrapping smaller molecular
species in electrodeposited sol--gel films (e.g., methyl
parathion or dopamine via molecular imprinting during electro-assisted
deposition~\cite{209,210}), they are usually not durably immobilized
and are quickly lost in solution, requiring suitable strategies to
attach them more firmly (as pointed out for NADH, for
example)~\cite{211}. More detail is given in Section~\ref{sec3}.

\item Sixthly, to the electrosynthesis medium can be doped with
organosilane reagents that are likely to co-condense with the
tetraalkoxysilane precursors to form an organic--inorganic hybrid film
with the organic components being covalently bonded to the silica
framework~\cite{147,149,165}. This enables to avoid leaching of the
organo-functional groups, as common for other types of sol--gel
materials~\cite{212,213,214,215}, thanks to the non-hydrolysable
metal--carbon bond in metal alkoxides~\cite{216}. The organo-functional
groups are selected for their particular properties, promoting tailor
made features to the hybrid films, such as preconcentration
ability~\cite{80,165,217}, molecular recognition~\cite{209,218}, ion
exchange~\cite{219,220}, electrocatalysis~\cite{221,222}, selective
permeation~\cite{61,157,223,224},
superhydrophobicity~\cite{225,226,227} or protection against
corrosion~\cite{227,228,229}. 

\item Seventhly, combining the electro-assisted sol--gel formation
with the inclusion of a surfactant template enables to get highly
ordered and nanostructured deposits (see, e.g., pioneering
works for vertically aligned mesoporous silica films~\cite{230} or
lamellar structured zinc oxide films~\cite{136}). In this case, the
applied potential does not only contribute to enforce the pH change at
the electrode--solution interface but it also contributes to some
organization of the surfactant species onto the electrode surface
(adsorbed micelles or hemi-micelles formed as a result of
potential-controlled surface aggregation of surfactants at electrode
surfaces \cite{231}). This combination of electrochemical modulation of
the sol--gel process and electrochemical interfacial surfactant
templating leads to the growth of nanostructured oxides with
preferential pore orientation~\cite{135,136,230,232}. The development
of this rapidly expanding field is presented in detail in Section~\ref{sec4}.
Note that mesoporous metals can be formed by direct electrodeposition
through organized surfactant phases on electrodes~\cite{233,234}, or
mesoporous metal oxides films from electrochemically induced
precipitation in the presence of ionic
surfactants~\cite{136,235,236,237,238}, but this is conceptually
distinct from the electro-induced self-assembly of sol--gel films by
generating the polycondensation catalyst~\cite{230}. Of related
interest is the electrogeneration of porous silica films by inclusion
of $\rmbeta$-cyclodextrin~\cite{239}, taking advantage of the templating
ability of cyclodextrin assemblies~\cite{240} and the
potential-controlled adsorption of such stacks onto an electrode
surface~\cite{241}. Dendrimer pillars forming at solid--liquid
interfaces~\cite{242,243} can be also exploited as template for the
electrogeneration of nanoporous silica~\cite{244}. 

\item Finally, one can combine nanostructuration and
functionalization. Because post-synthesis grafting may lead to pore
blocking, the co-condensation approach is often preferred for the
electro-assisted self-assembly of functionalized mesoporous silica
films~\cite{137}. It is however restricted to rather simple
organo-functional groups (e.g., aminopropyl) and quite low
organic group contents (i.e., 10\%)~\cite{245}. An elegant
way to overcome this limitation is to combine the electro-assisted
self-assembly of azide- and/or thiol-functionalized films with click
chemistry, a method likely to generate nanostructured silica films
bearing a richness of organo-functional groups~\cite{137,246} and even
multifunctional nanostructures by exploiting two different click
reactions~\cite{247}. This opens the door to a series of advanced
applications and most recent developments are presented and discussed
in Section~\ref{sec4.3}. 
\end{itemize}

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}Illustration of sol--gel bioencapsulation.
(A) Comparison of cyclic voltammograms (CV) recorded using GCE with
electrodeposited silica gel entrapping Hb (1,3) or not (2), as obtained
in anaerobic (1) or aerobic (2,3) conditions. Electrodeposition was
performed from a sol solution containing 227~mM tetraethoxysilane
(TEOS) and 20~${\rmmu}$M Hb, by applying a potential of ${-}1.2$~V
for 10~s. (A$_{1}$~$=$ data; A$_{2}$~$=$ illustrative
scheme). (B) Variation of the cathodic peak currents measured at GCE
with electrodeposited silica gel entrapping Hb and GOx, in an
air-saturated phosphate buffer solution (pH 6.5) to which increasing
amounts of glucose have been added, as a function of glucose
concentration. Inset represents CV curves recorded in the absence (1)
and in the presence of glucose at 0.02~mM (2), 0.04~mM (3), and 0.06~mM
(4). The biocomposite film was prepared as in (A) but from a TEOS-based
sol containing 20~${\rmmu}$M Hb and 50~${\rmmu}$M GOx in
mixture. (B$_{1}$~$=$ data; B$_{2}$~$=$ illustrative
scheme). {Reprinted from Nadzhafova \etal~\cite{198}, 
Copyright (2007), with permission from Elsevier Science}.}
\vspace*{-4pt}
\end{figure*}



\section{Electrochemically induced sol--gel bioencapsulation and
extension to the entrapment of other materials}\label{sec3}

Thanks to their synthesis at room temperature and under mild
conditions, sol--gel oxides are good hosts for the encapsulation and
stabilization of biochemicals such as enzymes, antibodies, living cells
and bacteria~\cite{248}. Sol--gel bioencapsulation, by keeping the
biological activity of the immobilized biorecognition
elements~\cite{201,202,203}, leads to bioceramic materials that are
attractive for the development of biosensors~\cite{249,250,251}. For
bioelectrochemical applications, the bioceramic materials needs to be
in contact with a conductive surface, which is usually achieved by
dispersing biocomposite particles into conductive matrices or by
coating bioceramic films formed by evaporation on electrodes 
\cite{40,43,252}, which is however not appropriate for uniform
deposition on surfaces with complex morphologies. As briefly
aforementioned, sol--gel bioencapsulation can be electrochemically
induced to form biocomposite thin films, even on non-flat
surfaces~\cite{156,204,205,253}, leading to a novel one-step
electrochemical approach of biomolecule immobilization for
bioelectrochemical applications (beside electrodeposited polymers, for
instance)~\cite{254}. Historical aspects and major\unskip\break advances made in
this field are summarized hereafter, ending with few words on the
electro-assisted sol--gel encapsulation of non-biological materials.



\subsection{Feasibility of electrochemically induced sol--gel bioencapsulation}

The first examples of electrochemically induced sol--gel
bioencapsulation were reported in 2007 by two independent groups
(Nadzhafova \etal~\cite{198} and Jia  \etal~\cite{199}), describing the
one-step immobilization of glucose oxidase (GOx) and/or haemoglobin
(Hb) in an electrogenerated silica film. Once immobilized, the
biomolecules kept their biological activity, as pointed out for GOx via
the detection of glucose~\cite{198,199}, and the redox proteins were
electrochemically accessible in spite of the insulating nature of the
silica host, as demonstrated for Hb~\cite{198,200}. This is attributed
to the biocompatibility of sol--gel matrices~\cite{255,256}, which can
even lead to enhanced stability when an enzyme is encapsulated in
silica~\cite{257}. Figure~\ref{fig4} illustrates the proof-of-concept. In part A
of the figure, one can see the typical reversible voltammetric signal
of a glassy carbon electrode (GCE) covered with an electrogenerated
silica film with encapsulated haemoglobin in a deaerated solution
(curve 1) and its electrocatalytic response in the presence of oxygen
(curve 3), with control experiment showing no response to oxygen when
using a haemoglobin-free silica film on GCE (curve 2). In case of
co-immobilized Hb and GOx, the biocomposite film becomes sensitive to
the addition of increasing concentrations of glucose in the medium,
resulting in the progressive decrease of the electrocatalytic signal
(Figure~\ref{fig4}B) due to oxygen consumption during the enzymatic
bioconversion of glucose~\cite{198}. This demonstrates both the
electrochemical and biochemical activities of the encapsulated
biomolecules. For very open silica films (i.e., as those
formed at very cathodic potentials for which the generation of hydrogen
bubbles induces the formation of a highly porous silica matrix), the
bioelectrochemical response of encapsulated glucose oxidase may drop.
This is due to some enzyme loss from the silica layer but it remains
better than the poor stability of enzymes physically adsorbed onto
electrode surfaces~\cite{199}. On the other hand, GOx nanoconfinement
in well-structured silica material such as inverse opal matrix 
revealed\unskip\break
enhanced 
enzyme activity relative to that of GOx in aqueous
solution~\cite{258}. Note that bioceramic thin films can be also
prepared by spin-coating or dip-coating on electrodes and used for
electrochemical biosensing~\cite{252}. This approach was restricted to
flat surfaces only and usually required multi-step preparation
procedures to get multilayer configurations to ensure stability,
contrary to the one-step electro-assisted sol--gel deposition of
biocomposite films, which is applicable to uniform film formation on
microstructured electrodes as well~\cite{156,204,253}.




\subsection{Generalization, importance of additives}

The first attempts to extend the above electro-induced sol--gel
bioencapsulation method to other systems, simply by replacing Hb and
GOx by other enzymes in the sol solution, failed as illustrated in
Figure~\ref{fig5}A for \textsc{d}-sorbitol dehydrogenase (DSDH) for
which no response to \textsc{d}-sorbitol is observed~\cite{259}. The
reason is the lack of favorable interactions between the negatively
charged silica network and the anionic groups on the DSDH surface. An
effective way to circumvent this problem is to add a cationic
polyelectrolyte in the synthesis medium (i.e.,
{poly(dimethyldiallylammonium chloride), PDMDAAC, or protonated
polyethyleneimine, PEI)}, acting as stabilizer by favoring
electrostatic interactions during the biocomposite
formation~\cite{259}. In this case, the encapsulated DSDH recovered its
biological activity and the biocomposite film is operating in both
anodic and cathodic modes in the presence of \textsc{d}-sorbitol and
\textsc{d}-fructose, respectively (Figure~\ref{fig5}B). The method is
compatible with the co-immobilization of several biomolecules (e.g.,
DSDH and diaphorase~\cite{259} or glucose dehydrogenase and
diaphorase~\cite{260}) and has been used so far for the entrapment of
various other enzymes such as lipase~\cite{253}, laccase~\cite{261},
polyol dehydrogenases~\cite{262}, or membrane-bounded (S)-mandelate
dehydrogenase~\cite{163}. The biopolymer chitosan has been also used as
stabilizer~\cite{261,263} and the co-encapsulation of gold
nanoparticles contributed to further enhancement of the bioelectrode
response~\cite{260,261,264}, thanks to the preservation of encapsulated
enzymes in silicate composite films with additives (polymers, smart
nanomaterials) as well as their contribution to ensure porosity for
reactants~\cite{251,265}. Finally, one has to mention\unskip\break alternative
electrodeposition methods, such as the generation of free standing
silica-based biocomposite films by electrochemically induced ion
transfer at liquid/liquid interfaces~\cite{266} or with the help of an
electrode/organic-phase/aqueous-electrolyte three-phase
junction~\cite{267}.

\begin{figure*}
\includegraphics{fig05}
\vspace*{-2pt}
\caption{\label{fig5}(A, B) Electrochemical responses 
to \textsc{d}-sorbitol (A, B$_{1}$) and to fructose (B$_{2}$)
measured at GCE modified by DSDH with using sol--gel electro-assisted
deposition done at ${-}1.3$~V for 60 s from a sol containing 0.17~M
TEOS, 3.3~mg${\cdot}$cm$^{-3}$ DSDH, respectively without (A)
and with 6.7 wt\% PDMDAAC (B). (A) Responses in the absence of
\textsc{d}-sorbitol (dashed line) and in the presence of 12 mM
\textsc{d}-sorbitol (solid lines). (B) Responses in the absence of
enzymatic substrate (dashed lines) and in the presence of
\textsc{d}-sorbitol (B$_{1}$) or \textsc{d}-fructose
(B$_{2}$) from 1 to 8~mM (solid lines). CVs were recorded
either in tris-HCl buffer (pH 9) containing 1~mM NAD$^{+}$ (B$_{1}$) 
or in 0.1~M phosphate buffer (pH 6.5) containing 1
mM NADH (B$_{2}$). (C) CV responses to \textsc{d}-sorbitol of flat
and macroporous gold electrodes modified by silica thin films with
co-encapsulated DSDH and diaphorase electrodeposited at ${-}1.1$~V for
30~s from a sol containing 6.7 wt\% PDMDAAC, 0.17~M TEOS, 
3.3~mg${\cdot}$cm$^{-3}$ DSDH and 0.83~mg${\cdot}$cm$^{-3}$
diaphorase. (C$_{1}$) Responses of a flat gold electrode in
the absence (dashed line) and in the presence of increasing
concentrations of \textsc{d}-sorbitol from 1 to 7 mM (solid lines).
(C$_{2}$) Responses of a gold macroporous electrode (three
half-layers, see scheme in right bottom) in the absence (dashed line)
and in the presence of increasing concentrations of \textsc{d}-sorbitol
from 0.5 to 2.5~mM (solid lines). All CVs were performed in tris-HCl
buffer at pH~9 in the presence of 1~mM NAD$^{+}$ and 0.1~mM 
ferrocenedimethanol in solution. {Reprinted from Wang  \etal
\cite{259}, Copyright (2011), with permission from Elsevier Science}.}
\vspace*{-4pt}
\end{figure*}


Interestingly, the biocomposite films electrogenerated onto large area
electrode supports (such as electrospun platinum nanofibers~\cite{162},
ordered gold macroporous inverse opal structures~\cite{156}, dealloyed
nanoporous gold~\cite{253}, or carbon nanotubes layers~\cite{268}) led
to dramatic improvement of the catalytic efficiency of the
bioelectrode. This is illustrated in Figure~\ref{fig5}C, showing
clearly a much larger bioelectrocatalytic response (by {ca.}\  two
orders of magnitude) when passing from a silica film with
co-encapsulated DSDH and diaphorase electrodeposited on flat gold
electrode (Figure~\ref{fig5}C$_{1}$) to the same biocomposite layer
formed onto the internal surfaces of a macroporous gold electrode 
(Figure~\ref{fig5}C$_{2}$). Such advantage is attributed to the ability
of the electrochemically assisted deposition process to form thin films
of uniform thickness over the entire large area internal surface of the
macroporous gold electrode without any pore clogging effect~\cite{156}.
This is basically promising for the development of bioreactors directed
to the quantitative bioconversion of the enzymatic substrates, on the
condition of being able to immobilize all the necessary components
(i.e., the biomolecule and its cofactor, the redox mediator)
to get reagent-free bioelectrocatalytic devices (the data depicted in
Figure~\ref{fig5}C were obtained from biocomposite films entrapping only the
enzymes, while the $\mathrm{NAD}^{+}$ cofactor and ferrocenedimethanol mediator
were added to the solution~\cite{259}).

\subsection{Co-immobilization of mediators and cofactors}

In addition to biocompatibility, another interest of silica-based
sol--gel materials for bioelectrochemistry purposes is the ease of
chemical modification, either by physical entrapment of various
electroactive and/or catalytic reagents and additives or via covalent
grafting thanks to a rich organosilane chemistry, which has been
largely exploited in the field of electrochemical biosensors 
\cite{251,269}. In view of developing bioelectrocatalytic reactors,
this has to be extended to large area porous electrodes and the
electrochemically assisted deposition may help on that matter. In
particular, we have investigated the field of NAD(H)-dependent
dehydrogenases operating in both oxidation and reduction modes. One
already knows from above that enzyme immobilization can be achieved by
physical entrapment (Figure~\ref{fig5}). For the confinement of the
necessary mediators (useful to regenerate the $\mathrm{NAD}^{+}$ {and NADH
cofactors~\cite{270} because their direct electrochemical reduction or
oxidation may lead to enzymatically inactive or adventitious side
products~\cite{271}), several strategies are available. When wishing to
work in the anodic side, numerous redox polymers can be
(electro)deposited---or molecular redox mediators 
(electro)grafted---onto electrode surfaces for electrocatalytic oxidation of NADH and thus
$\mathrm{NAD}^{+}$ regeneration~\cite{272,273}. In our group, we have explored
different systems compatible with electro-induced sol--gel
bioencapsulation. They encompass the use of a ferrocene-functionalized
PEI polymer acting both as a polyelectrolyte stabilizing the enzyme in
the silica matrix and as a mediator for NADH oxidation~\cite{211}, the
one-step co-entrapment of the enzyme together with carbon nanotubes
wrapped with an osmium-complex modified polymer
electrocatalyst~\cite{274} or modified with adsorbed vitamin
$\mathrm{K}_{3}$~\cite{260}, and the electropolymerization of
poly(methylene green) mediator onto the electrode surface prior to the
electrodeposition of the biocomposite gel~\cite{275}. When wishing to
work in the cathodic side, fewer redox mediators are available for the
efficient non-enzymatic regeneration of NADH from $\mathrm{NAD}^{+}$
reduction~\cite{276,277}. The most effective ones are rhodium
complexes, such as (2,2$'$-bipyridyl)
(pentamethylcyclopentadienyl)-Rhodium ([Cp*Rh(bpy)H$_{2}$O]$^{2+}$
or [Cp*Rh(bpy)Cl]$^{+}$)~\cite{278,279}. Their covalent
immobilization in an active form onto~an electrode surface is however
challenging as it is subject to inactivation in the presence of
enzymes~\cite{280} or organic ligands~\cite{281} (i.e., likely
to affect the coordination chemistry of Rh centers). We found a way to
solve this problem by combining diazonium electrografting, click
chemistry and metal complexation, to have [Cp*Rh(bpy)Cl]$^{+}$ species
covalently bonded to carbon electrodes (Figure~\ref{fig6}A), giving
rise to effective electrocatalytic reduction of $\mathrm{NAD}^{+}$
(Figure~\ref{fig6}B)~\cite{282}. Then, the DSDH enzyme can be
co-immobilized, by either covalent attachment to the electrode
surface~\cite{283} or entrapment in a silica layer overcoated onto the
porous carbon electrode bearing the Rh mediator~\cite{284}. In both
cases the enzyme and [Cp*Rh(bpy)Cl]$^{+}$ species were physically
separated from each other, leading to effective bioelectrocatalytic
responses to \textsc{d}-fructose when DSDH was co-immobilized with the
Rh mediator while no response was observed in the control experiment
without enzyme (see respectively curves (a) and (b) in Figure~\ref{fig6}C).


\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}(A) Scheme illustrating the synthetic route
followed for the functionalization of a carbon electrode surface with
{[}Cp*Rh$^{\mathrm{III}}$(bpy)Cl{]}$^{+}$. (B) NADH
regeneration mediated by
{[}Cp*Rh$^{\mathrm{III}}$(bpy)Cl{]}$^{+}$ covalently
bonded to a carbon felt--carbon nanotubes (CF-CNT-Rh)
electrode. CV curves recorded at a potential scan 
rate of 5~mV${\cdot}$s$^{-1}$ 
using the CF-CNT-Rh electrode in 50 mM PBS buffer
(pH~6.5), under nitrogen, to gradually added NAD$^{+}$:
(a) 0~mM, (b) 0.5~mM, (c) 1~mM, (d) 2~mM, (e) 3~mM, and (f) 4~mM.
{Reprinted with permission from Zhang \etal \cite{282}, 
Copyright (2017) American Chemical Society.} 
(C)~Configuration and electroenzymatic process involved in the
bioelectrochemical system with DSDH immobilized in a silica layer
overcoated onto a bucky paper functionalized with the rhodium complex.
Amperometric response to additions of 0.5~mM \textsc{d}-fructose (each
addition is marked with an arrow) recorded using a 1~cm$^{2}$ 
electrode (a)~in the presence of DSDH in the
silica layer and (b)~in the absence of enzyme in the composite
electrode. The measurements were performed under nitrogen at ${-}0.72$~V
vs. Ag/AgCl in 50~mM PBS (pH~6.5) with 1~mM NADH in solution.
{From Zhang \etal \cite{284}, reproduced
with permission of WILEY-VCH Verlag GmbH}.}
\end{figure*}


The remaining challenge is the co-immobilization of the
$\mathrm{NAD}^{+}$/NADH
cofactor. Indeed, its simple entrapment in the sol--gel matrix led to
rapid leaching in solution and thus poor operational stability of the
bioelectrode in the absence of additional protective silica
overlayer~\cite{285}. Covalent bonding would lead to more durable
immobilization, as shown for biosensors based on NADH bonded to
water-soluble polymers~\cite{286,287}, but special care should be taken
to the regioselectivity of the coupling reaction in order to maintain
the enzymatic activity of the cofactor~\cite{288}. We have been able to
reach the goal of durable} cofactor immobilization in sol--gel
bio-composite thin films by using a suitable organosilane,
3-glycidopropyltrimethoxysilane (GPS), which is likely to bind
$\mathrm{NAD}^{+}$ (through the C-6 amino group rather than the N-1 ring
nitrogen via Dimroth rearrangement~\cite{289}) in the course of the
sol--gel process~\cite{211}. In such a way, when co-immobilized with
DSDH and diaphorase (DI) enzymes and ferrocene-functionalized PEI
(PEI-Fc) as mediator and polyelectrolyte, the resulting biocomposite
film on GCE (Figure~\ref{fig7}A) can be used as a reagent-less
bioelectrode for the conversion of \textsc{d}-sorbitol (as pointed out
by CV, see Figure~\ref{fig7}B). In this case, long-term operational
stability is achieved (several hours, see blue line in 
Figure~\ref{fig7}C), whereas the simple entrapment of the cofactor in
the film led to very fast vanishing of the bioelectrochemical response
(see red line in Figure~\ref{fig7}C) due to its fast leaching in
solution~\cite{211}. This approach can be extended to other electrode
supports and other immobilized mediators, such as poly(methylene green)
or Os-complex modified polymer on carbon nanotubes, which were covered
with an electrodeposited sol--gel silica-GPS-NAD-DSDH and applied to
the reagent-less biotransformation of \textsc{d}-sorbitol, with good
stability upon continuous use in hydrodynamic
conditions\unskip\break \cite{274,290}.


\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}(A) Illustration of the electrochemical
pathway used with the immobilized cofactor NAD$^{+}$ for
the detection of the dehydrogenase enzymatic substrate. (B)
Cyclic voltammograms recorded with GCE/(TEOS
$+$ GPS)/(DSDH $+$ DI)/NAD-GPS/PEI-Fc containing all elements (cofactor,
mediator and proteins) in the absence of \textsc{d}-sorbitol and in the presence
of 2.8~mM  {\textsc{d}}-sorbitol. Inset plot shows the variation of peak 
currents sampled at 0.4~V by successive additions of 0.2~mM
{\textsc{d}}-sorbitol. (C) Amperometric responses obtained at an
applied potential of ${+}$0.4~V using either GCE/(TEOS $+$ GPS)/(DSDH $+$
DI)/NAD-GPS/PEI-Fc (i.e., in the presence of GPS, blue curve) or
GCE/(TEOS)/(DSDH $+$ DI)/NAD${+}$/PEIFc (i.e., in the absence of GPS, red
curve). Measurements have been performed for 14~h oxidation under
convective conditions in 0.1~M Tris--HCl buffer (pH 9)
containing 2~mM \textsc{d}-sorbitol. {Reprinted from  Wang
\etal \cite{211}, Copyright (2012), with
permission from Elsevier Science}.} 
\end{figure*}


\subsection{Development of bioelectrocatalytic reactors}


Taking advantage of all the above features of electrochemically
assisted sol--gel bioencapsulation in terms of uniformly coating large
area porous electrodes with biocomposite layers and effective
immobilization of all the necessary ingredients\unskip\break (enzymes, cofactors,
mediators), we were able to design bioelectrocatalytic flow reactors.
For example, a lab-scale flow bioreactor comprising a porous carbon
felt electrode ($16~\mathrm{cm}^{2}$ geometric area) with CNT wrapped
by poly(methylene green) (mediator) and covered with sol--gel layer
comprising DSDH\unskip\break enzyme resulted in the quantitative transformation of
\textsc{d}-sorbitol into \textsc{d}-fructose in the mM concentration
range at the rather low overpotential of ${+}$0.1~V (vs. Ag/AgCl pseudo
reference)~\cite{275}. If combining such bioanode to an oxygen-gas
diffusion cathode, the flow bioreactor was likely to operate in an
energy-producing mode as illustrated in Figure~\ref{fig8}. It led to a
lower biotransformation yield (60\%), but reached an energy power
output up to $14.6~\rmmu\mathrm{W}{\cdot}\mathrm{cm}^{-3}$ and a conversion
rate of $1.18~\mathrm{mg}{\cdot}\mathrm{day}^{-1}$ in conditions of
bioconversion with continuous substrate feeding~\cite{275}. Such
bioreactors with enzymes encapsulated in electrodeposited silica-based
materials were found to operate continuously in a stable way for a
week, positioning the electro-assisted sol--gel bioencapsulation at a
good place among the one-step electrochemical approaches of enzyme
immobilization for bioelectrochemical applications~\cite{254}. Note
that even better performance can be reached for enzymes immobilized in
conducting polymers (as exemplified for the production of
$\mathrm{H}_{2}\mathrm{O}_{2}$ in a GOx-based biocatalytic flow
reactor~\cite{291}). Recent efforts are directed to the development of
hydrogenase-based hybrid flow bioreactors operating in the cathodic
mode with good Faradic efficiency and high turnover
numbers~\cite{292,293}.


\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}(A) Illustration of the flow bioreactor
composed of a gas-flow cathode and a bioanode made of carbon felt 
(16~cm$^{2}$) modified with carbon nanotubes, wrapped with
poly(methylene green) mediator, and subsequently subjected to
electrodeposition of a biocomposite layer from a sol solution
containing TEOS, GPS, PEI and DSDH (the enzyme concentration in the
bioanode was 0.375~mg${\cdot}$cm$^{-3}$). (B) Conversion of
\textsc{d}-sorbitol (1 mM initial concentration) to \textsc{d}-Fructose
in the bioreactor with simultaneous energy production. The 100~mL
flowing electrolyte was composed of 0.1~M Tris-HCl buffer (pH 9) and
0.1 mM NAD$^{+}$. (C) Polarization curve of the bioreactor
when flowing the of \textsc{d}-sorbitol solution. {Reprinted
from Mazurenko \etal \cite{275}, Copyright
(2016), with permission from Elsevier\unskip\break 
Science}.}
\end{figure*}


\begin{figure*}
\includegraphics{fig09}
\caption{\label{fig9}(A) Illustration of an electroactive
artificial biofilm made of \textit{Pseudomonas fluorescens} bacteria
and cytochrome c co-immobilized in a silica sol--gel film formed by
electrochemically assisted deposition on glassy carbon electrode (GCE).
(B) Cyclic voltammograms for (a) bare GCE, (b) cytochrome c in a
sol--gel film on GCE, (c) cytochrome c and
\textit{Pseudomonas fluorescens} in a sol--gel film on GCE.
Inset: Cyclic voltammetric response of \textit{P. fluorescens} alone in
the sol--gel film. (C) Amperometric current responses
measured upon the addition of glucose using GCE modified with the
artificial biofilm depicted in (A). The measurements were performed
under stirring in phosphate buffer (pH 7) at room temperature at an
applied potential of ${+}$0.15~V vs.\ Ag/AgCl reference electrode.
{Reprinted from Ghach \etal \cite{208}, 
Copyright (2014), with permission from Elsevier Science}.}
\end{figure*}


\subsection{Encapsulation of bacteria: towards\newline electroactive artificial biofilms}

The efficient electrochemical communication between living cells and
electrode surfaces is a challenge of fundamental interdisciplinary
research and the immobilization of such biological/living material in
an (electro)active form onto a suitable conductive support is a key
prerequisite to their effective application in
(bio)electrochemistry~\cite{294}. The electrochemically assisted
sol--gel deposition may contribute to the field, by providing a way to
encapsulate bacteria in thin silica films in order to make them more
practical for use in bioelectronics devices. This was first
demonstrated by forming bacteria-silica composite films, either by
sol--gel electrodeposition over bacteria immobilised on ITO electrode or
in one-step where the sol and the bacteria were deposited at the same
time~\cite{207}. In both cases, rehalose, poly(ethylene glycol) and
chitosan were added to the sol to improve cell viability. With optimum
conditions, 95\% cell viability was observed after one month. The
proof-of-concept of sol--gel based ``artificial'' biofilms on electrodes
was then developed using cytochrome c as a biological redox shuttle
between \textit{Pseudomonas fluorescens} bacteria (both entrapped in a
sol--gel film, Figure~\ref{fig9}A) and a glassy\unskip\break carbon electrode
surface~\cite{208}. The electrochemical addressability of the bacteria
in the film was evidenced from the increase in current response of
cytochrome c in the presence of \textit{P. fluorescens}
(Figure~\ref{fig9}B). The feasibility of bacteria/cytochrome c electron
transfer in the sol--gel layer was also proven by detecting the
microbial communication with the electrode upon successive additions of
glucose as electron donor for respiration (Figure~\ref{fig9}C). As
shown for \textit{G. sulfurreducens} cells immobilized in composite
materials associating silica gel and carbon felt fibers, such
ready-to-use artificial bioelectrodes are promising for the versatile
time- and cost-saving development of microbial electrochemical
systems~\cite{295}.




\subsection{Extension to the entrapment of other\newline (non-biological) materials}

Using similar approaches as for bacteria entrapment by electro-assisted
sol--gel deposition, hard inorganic particles have been encapsulated
into electrogenerated silica films, such as clays~\cite{189} or
organoclays~\cite{190}, metal oxides~\cite{193}, gold
nanoparticles~\cite{191,192,196,296}, graphene oxide 
\cite{194,195}, or carbon nanotubes~\cite{170,274,290,297,298}.
Again,~two strategies were employed, either the electrodeposition of
the silica layer onto preformed  assemblies\unskip\break 
of these nano-objects onto
the electrode surface 
\cite{189,190,193,194,196,274,290,297,298}, or the one-step
co-electrodeposition of the composite film from a sol solution
containing the nano-objects in suspension 
\cite{170,191,192,195,296}. In the particular case of mixed
polypyrrole-silica molecular imprinted polymer (PPy@sol--gel MIP)
films~\cite{297,298}, one does not know what was the driving force of
the composite electrodeposition process (PPy electropolymerization or
sol--gel electro-assisted precipitation), even if one knows that
molecularly imprinted silica films can be generated by electro-assisted
sol--gel deposition~\cite{210}. Several applications have been developed
by exploiting the particular properties of the nanomaterials entrapped
in the silica layer and their durable immobilization on the electrode
surface. They encompass electrochemical sensors (for metal
ions~\cite{189}, biologically relevant molecules~\cite{191}, dugs or
pharmaceuticals~\cite{190,297,298}, or even the nanoparticles
themselves~\cite{196}), electrocatalysis~\cite{191,193},
electrochemiluminescence~\cite{192}, corrosion protection~\cite{195},
or antireflection and non-linear optics~\cite{170}.


\begin{figure*}
\includegraphics{fig10}
\vspace*{4pt}
\caption{\label{fig10}Schematic illustration of the
electrochemically assisted self-assembly of a vertically aligned
mesoporous silica film onto an electrode surface from a sol solution
containing Si(OEt)$_{4}$ precursors, a cationic surfactant
and a supporting electrolyte. {Reprinted with permission from 
Mousty and Walcarius \cite{135}, Copyright (2015) Springer}.}
\vspace*{5pt}
\end{figure*}


\section{Nanostructuration of electrode surfaces by electrochemically
assisted deposition in the presence of surfactant templates}\label{sec4}

An important feature regarding the application of isolating thin films
on electrodes is their propensity to allow efficient accessibility to
the underlying conductive surface and fast transport processes through
the film. For silica-based materials, this can be achieved with the aid
of widely open inorganic frameworks (such as silica aerogels 
\cite{299,300}) or three-dimensional nanostructured architectures (as
ordered mesoporous silica films generated by evaporation-induced
self-assembly on electrodes~\cite{59,301}. The porosity can be also
created in electrogenerated silica films. The straightforward way is to
work at very cathodic potentials for sol--gel electrodeposition in order
to exploit the hydrogen bubbles produced during water reduction as a
dynamic template in forming the silica matrix~\cite{302,303} (as
otherwise applied to the production of porous metallic foams 
\cite{304,305}). Three-dimensional macroporous and mesoporous silica
films can be also obtained from the electro-assisted deposition through
assemblies of polymeric\unskip\break particles (mainly polystyrene
(nano)beads~\cite{197,224,306}) or dendrimers~\cite{224,244}), among
some other related approaches~\cite{47}. An elegant way to control both
the film structure at the nanoscale and mesopore orientation is via the
combination of electrochemical interfacial surfactant templating and
electrochemical modulation of the sol--gel process, as show hereafter
through the concept of electrochemically assisted self-assembly (EASA).

\subsection{The electrochemically assisted self-assembly (EASA)
concept: an efficient route to ordered and vertically oriented
mesoporous silica films}

When performing the electrochemically assisted deposition of sol--gel
films using tetraethoxysilane (TEOS) in the presence of a cationic
surfactant (cetyltrimethyl-ammonium bromide, CTAB) in the starting sol,
the applied potential is not only responsible for the supply of the
polycondensation catalysts ($\mathrm{OH}^{-}$ species), but it also contributes
to arrange the surfactant molecules in the form of transient
hemi-micelles, thus contributing to the vertical growth of silica walls
around this template~\cite{135,137,232}, as schematically depicted in
Figure~\ref{fig10}. The EASA concept was discovered by our group in
2007~\cite{230}, and then developed and optimized by thoroughly
investigating the various parameters (TEOS and CTAB concentrations, sol
aging, electrolysis mode, deposition time) likely to affect the film
structure, morphology and thickness~\cite{307}. It indeed enables to
get quickly a continuous and uniform film covering the whole electrode
surface (Figure~\ref{fig11}A), which is made of regularly organized
mesopore channels in a hexagonal geometry (Figure~\ref{fig11}B$_{1}$),
all perfectly oriented perpendicular to the plane of the membrane
(Figure~\ref{fig11}B$_{2}$). The hexagonal ordering and the vertically
oriented mesostructure are evidenced by both electron diffraction (see
inset in Figure~\ref{fig11}B$_{1}$) and grazing-incidence X-ray
diffraction (GIXD) showing the presence of diffraction spots in the
equatorial plane (Figure~\ref{fig11}C). After surfactant removal (by
either calcination or solvent extraction~\cite{308}), the ordered and
oriented mesostructure is maintained and the porosity released, without
altering the pore diameter or lattice parameter~\cite{307}. The
electrodeposition can be performed potentiostatically (by applying a
suitable cathodic potential) or in galvanostatic conditions (by
applying a negative current). Increasing the TEOS concentration and/or
lengthening the electrolysis time and/or applying a more cathodic
potential (or a higher current density in the galvanostatic mode),
contributed to accelerate the film growth, and thus increasing its
thickness, but also resulted in the formation of a side-product
(particulate aggregates) over the uniform mesoporous film (see 
Figure~\ref{fig11}D$_{1}$). This is also visible on the GIXD pattern
(Figure~\ref{fig11}D$_{2}$) by the presence of out-of-plane rings (in
addition to the in-plane spots), indicating additional materials with
wormlike mesostructures. By carefully controlling the electrodeposition
conditions (i.e., deposition time ${<}$ 15~s; [TEOS]
${<}$ 125~mM; CTAB/TEOS ratio ${<}$ 0.32), however, one can avoid the
precipitation of such particles. So, one does obtain uniform and
aggregate-free films on various solid supports (gold, platinum, glassy
carbon, ITO and copper \cite{230,307}), including on supports of
various size and shape (microfibers~\cite{168}, exfoliated
graphite~\cite{309} or macroporous 3D graphene \cite{310}), and even at
the local scale (with an ultramicroelectrode generating locally the
hydroxyl catalyst~\cite{174}). Applying EASA through polystyrene bead
assemblies led to hierarchical macro-mesoporous films with the vertical
orientation maintained throughout the whole membrane~\cite{311}. Other
methods were then described to form vertically aligned hexagonal
mesostructures (St\"{o}ber-solution growth~\cite{312} or oil-induced
co-assembly~\cite{313}), but comparative studies by independent
groups~\cite{314,315} suggested faster and easier access to highly
ordered films on electrodes using the EASA process, positioning it
among the most versatile ways to ordered mesoporous films with
preferential orientation control~\cite{316}.




\begin{figure*}
\includegraphics{fig11}
\vspace*{3pt}
\caption{\label{fig11}(A) FE-SEM micrograph obtained for
electrodeposited surfactant-templated mesoporous silica film on ITO
(cross-section view $+$ enlargement), as generated for short time (10 s);
(B) High-magnification TEM micrographs obtained for a
surfactant-templated mesoporous silica film electrogenerated on glassy
carbon: top view (B$_{1}$) and cross-sectional view
(B$_{2}$); the inset in B$_{1}$ shows the
electron diffraction pattern of the mesostructured film; (C)
Grazing-Incidence X-Ray Diffraction (GIXD) pattern of the film obtained
as in (A); (D) FE-SEM micrograph (D$_{1}$, top view $+$
magnified cross-section in inset) and GIXD pattern (D$_{2}$)
observed for a film prepared similarly as in (A) but at longer
electrodeposition time (30~s). {Reproduced with permission from
Walcarius \etal \cite{230}, Copyright (2007) Nature Publishing Group,
and from Goux \etal \cite{307}, Copyright (2009) American
Chemical Society.}}
\vspace*{4pt}
\end{figure*}


The film thickness can be basically tuned by adjusting the sol
composition and the deposition parameters that are likely to control
the film growth rate. However, too low TEOS or CTAB concentrations and
too short deposition times prevented film formation~\cite{307}, thus
limiting the access to ultra-thin films, while too high TEOS or CTAB
concentrations and too long electrodeposition led to the precipitation
of particulate aggregates on top of the ordered and oriented mesoporous
coatings\unskip\break (Figure~\ref{fig11}D$_{1}$). Some strategies were 
recently\unskip\break
proposed to  circumvent these limitations, such as the soft wet etching
to decrease film thickness~\cite{317}, and the alternative
current-assisted deposition or hydrodynamic electrochemistry approaches
to get aggregate-free films beyond microns in thickness~\cite{318,319}.
Applying EASA in sequential deposition mode enabled to form
multilayered mesoporous silica films up to 400~nm in thickness without
huge aggregates on top of them~\cite{320}. Most investigations made so
far with EASA films were based on samples prepared using either CTAB or
OTAB (octadecyltrimethylammonium bromide) as surfactant, leading to
materials with pore sizes in the 2--3~nm range~\cite{128,137,321}.
Actually, the range of accessible mesopore diameters can be extended
from 1.3 to 4.4~nm by adjusting the alkyl chain length of the
alkylammonium template (from $\mathrm{C}_{12}$ to $\mathrm{C}_{22}$)~\cite{128,322}.
Other cationic compounds such as alkyl-imidazolium-based ionic liquids
have been used to generate mesoporous silica films with 2.5--3.5~nm
wide oriented nanochannels~\cite{323,324}, the driving force of the
synthesis being always the cooperative self-assembly induced by
favourable electrostatic interactions between the positively charged
surfactant and both the negatively charged silica precursors and the
negatively charged support~\cite{325}. Such electrostatic attraction
was otherwise exploited for the EASA growth of oriented 
mesoporous\unskip\break
metal--organic framework thin films upon electro-assisted pH
increase~\cite{326}. Finally, an important point for further practical
applications is the good adhesion of the mesoporous layer onto the
underlying electrode surface. If it is straightforward for silica films
on hydroxide-containing electrode materials (such as ITO or FTO) thanks
to their covalent bonding~\cite{307,327}, or on reduced graphene oxide
thanks to strong interactions with surface oxygen-containing groups and
hydrophobic $\rmpi$-conjugated structure \cite{325,328}, their durable
attachment to other electrodes may require suitable pre-treatments.
They encompass notably the electrochemical activation of glassy carbon
surfaces~\cite{329,330}, the electrografting of
3-aminopropyl-triethoxysilane (APTES) on carbon electrodes 
\cite{331,332}, and the chemisorption of
3-mercaptopropyl-trimethoxysilane (MPTMS) on gold electrodes 
\cite{333,334}, APTES and MPTMS acting as a ``molecular glue'' between
the electrode surface and the electrodeposited silica  layer.

\subsection{Size and charge selectivity at the molecular level}

Figure~\ref{fig12} illustrates the typical voltammetric responses
obtained for redox probes of different charge (the cationic
$\mathrm{Ru}(\mathrm{NH}_{3})_{6}^{3+}$, the neutral
ferrocenedimethanol $\mathrm{Fc}(\mathrm{MeOH})_{2}$, and the anionic
$\mathrm{Fe}(\mathrm{CN})_{6}^{3-}$) using ITO electrodes covered with
an oriented mesoporous silica film, respectively before and after
removal of the surfactant template. As shown, both
$\mathrm{Ru}(\mathrm{NH}_{3})_{6}^{3+}$ and
$\mathrm{Fe}(\mathrm{CN})_{6}^{3-}$ species did not give any noticeable
signal for as-synthesized films (see red curves in parts A and C in
Figure~\ref{fig12}) because they are blocked by the surfactant present
in the channels. This also confirmed the good quality of the composite
membrane over  the whole electrode surface area (in case of
defects/cracks, faradic currents would have been observed~\cite{168}).
On the contrary, the neutral probe $\mathrm{Fc}(\mathrm{MeOH})_{2}$ was
likely to dissolve in the surfactant liquid crystalline phase and to
reach the underlying electrode surface to be detected
(Figure~\ref{fig12}B, red curve), yet at more anodic potentials than on
bare ITO due to stabilization effects~\cite{335}. After template
removal, the films should be in principle permeable to all the probes.
This is indeed the case for $\mathrm{Ru}(\mathrm{NH}_{3})_{6}^{3+}$ and
$\mathrm{Fc}(\mathrm{MeOH})_{2}$, which exhibited peak currents of the
same order of magnitude (even slightly larger) as on bare ITO (compare
blue and dashed black curves in parts A and B in Figure~\ref{fig12}),
but not for $\mathrm{Fe}(\mathrm{CN})_{6}^{3-}$ for which lower
currents were observed suggesting that the film acted somehow as a
barrier to reach the electrode surface (Figure~\ref{fig12}C, blue
curve). These results can be rationalized by taking into accounts the
negatively charged walls of the silica nanochannels (due to the
presence of silanolate groups), acting therefore as an electrostatic
barrier to anionic species while facilitating the ingress of cations,
conferring to these films a real charge selectivity at the molecular
level~\cite{128,335}. The permselective properties of such vertically
aligned mesoporous silica membrane on electrode are schematically
summarized in Figure~\ref{fig12}D.

\begin{figure*}
\includegraphics{fig12}
\caption{\label{fig12}(A--C) CV curves
recorded at a bare ITO electrode (curves in black) and at ITO covered
with an oriented mesoporous silica film (pore $\xvarnothing = 2.0$~nm)
electrode, respectively before (red curves) and after (blue curves)
surfactant removal using (A) $\mathrm{Ru}(\mathrm{NH}_{3})_{6}^{3+}$ 
cations, (B) neutral Fc(MeOH)$_{2}$, and (C) $\mathrm{Fe}(\mathrm{CN})
_{6}^{3-}$ anions in solution.
{Adapted from Karman \etal \cite{335}, with permission of WILEY-VCH
Verlag GmbH}. (D)~Schematic illustration of the permselective
properties of the films corresponding to the experiments~\mbox{(A--C).}}
\vspace*{-5.2pt}
\end{figure*}

The rejection of anions is even more effective, almost totally
blocking, if operating in a much diluted electrolyte medium (see black
curve in Figure~\ref{fig13}A). Actually, varying the ionic strength of
the solution is a convenient way to tune the permselective properties
of the film towards anions by playing with the thickness of the
electrical double layer (EDL)~\cite{335}. The EDL overlaps over the
whole mesopore channels in diluted electrolyte media, whereas high
ionic strengths induce EDL thickness becoming smaller than the pore
radius, thus making possible the free diffusion of anions through the
channels (see illustrative schemes in Figure~\ref{fig13}A) resulting
correspondingly in large voltammetric signals (see blue curve in
Figure~\ref{fig13}A). The charge selectivity can be reversed by shifting
pH towards lower values~\cite{336}, and such effect of pH-modulated ion
transport and amplified redox response is illustrated in Figure~\ref{fig13}B 
for a bulky polyanionic probe (polyoxometalate, POM). As shown, a more
acidic medium will induce the protonation of surface silanol groups and
contribute to make the nanochannels positively charged, thus enhancing
the ingress of the negatively charged POM species through the membrane
and their electrochemical response on the underlying electrode (compare
blue and black curves in Figure~\ref{fig13}B). Again, these processes
are influenced by the ionic strength of the medium, but also by the
membrane pore size and POM probe charge~\cite{337}. Besides
electrostatic effects, permselectivity can be also determined by
lipophilic effects or external forces (convection, migration), and
detailed discussion on the permselective behavior of silica nanochannel
membranes (including those prepared by the St\"{o}ber growth
method~\cite{312}) is available\unskip\break elsewhere \cite{51}.


\begin{figure*}
\includegraphics{fig13}
\caption{\label{fig13}Effect ionic
strength and pH on charge selectivity at ITO electrode covered with a
surfactant-extracted mesoporous silica film (pore $\xvarnothing = 2.0$~nm). 
(A) CV curves recorded at the film electrode using
$\mathrm{Fe}(\mathrm{CN})_{6}^{4-}$ anions in solutions of low and high ionic
strengths (respectively 1 and 300~mM NaNO$_{3}$).
{Adapted from Karman \etal \cite{335}, with permission of WILEY-VCH Verlag
GmbH}. (B) CV curves recorded at the film
electrode using a Keggin-type heteropolyanion (POM) in solutions
respectively adjusted at pH 0.3 and 4. 
{Adapted from Vil\`{a} \etal \cite{336}, 
with permission from Elsevier
Science}.}
\vspace*{4pt}
\end{figure*}


\begin{figure*}
\includegraphics{fig14}
\caption{\label{fig14}CV curves recorded in 0.12~mM
9-ferrocenyl-terminated dendrimer (in THF $+$~0.2~M
TBABF$_{4}$) using (a) bare ITO and (b) ITO electrodes
modified with (A) CTAB-based and (B) OTAB-based mesoporous silica films
after surfactant removal. On the right part, both high resolution STEM
of the films (the sides of the red hexagon represent the \textsc{d}-spacing and
the yellow circle one mesopore aperture) and simplified schemes of the
systems have been added. {Reprinted with permission from 
Vil\`{a} \etal \cite{321}, Copyright (2016)
 American Chemical Society.}}
\end{figure*}





Another important characteristic of electrodes coated with vertically
oriented mesoporous silica films with monodisperse pore diameters in
the nanometer range is their ability to differentiate redox probes
based on their size. Indeed, if small probes (as above) are likely to
cross the membrane and give well-defined voltammetric signals
(Figure~\ref{fig12}), bigger molecules or biomolecules (such as
haemoglobin, for instance) are not electrochemically accessible as they
cannot reach the underlying electrode surface~\cite{338}. This imparts
size selectivity to the films, at the molecular level, as better
demonstrated by using an electroactive nanoferrocenyl dendrimer (with
dimensions of $1.9 \times 2.4 \times 2.7~\mathrm{nm}^{3}$) and two
oriented mesoporous films with distinct but close pore diameters
(i.e., 2.0 and 2.9~nm)~\cite{321}. As shown in
Figure~\ref{fig14}, the dendrimer gives rise to well-defined reversible
voltammetric signals at a bare ITO electrode (see dashed black curves
in Figure~\ref{fig14}) but they are totally suppressed when the ITO
surface is covered with a mesoporous film of 2.0~nm aperture (see red
curve in Figure~\ref{fig14}A) and partly restored for nanochannels with
2.9~nm diameter (see red curve in Figure~\ref{fig14}B). In this latter
case, the lower peak current intensities compared to bare ITO are due
to some diffusional limitations for such large probe in the silica
nanochannels~\cite{321}. This demonstrates unambiguously the molecular
sieving properties of these oriented honeycomb structured membranes
with tunable pore size. When removed from their support, this kind of
thin membranes with adjustable-size vertical mesochannels are expected
to be good candidates for, e.g., nanofiltration or
nanosize-based separation~\cite{339,340}, or nanovalves that enable
unidirectional molecular\unskip\break transport \cite{341}.




\subsection{Functionalization of the vertically aligned nanochannels}
\label{sec4.3}

\subsubsection{Growth of nanofilaments through the mesopore channels}
\label{sec4.3.1}

In the same way that we commonly use track-etched polycarbonate
membranes or anodic aluminum oxide templates for designing 1D
nanostructures arrays~\cite{342,343,344,345}, one could think about
using our oriented mesoporous silica films as a mold for the
electrochemical growth of nanofilaments, with the wish to reach smaller
dimensions due to the nanometer-scale diameters of the vertically
aligned nanochannels. Even if metal ions can be electrochemically
reduced on electrodes coated with mesoporous silica films (as pointed
out for copper or silver detection by anodic stripping voltammetry, for
instance~\cite{245,346}), the electrodeposition of ultra-small size
metallic nanowires with macroscopic continuity remains
challenging~\cite{347,348}. A recent interesting study reports the
interest of electrodeposition from a supercritical fluid to produce tin
nanowires through nanoporous membranes, but too small pore diameters
(${<}$7~nm) do not allow the formation of stable nanowires in this
template~\cite{349}. Actually, when attempting to electrodeposit
metallic nanowires through EASA-generated films, they were usually of
larger diameter than the silica nanochannels (e.g., 50~nm for
palladium wires~\cite{350}), suggesting a growth out of the mesoporous
membrane. An exception is the growth of individual copper nanowires
from the top of the mesoporous film to the underlying electrode support
(using a scanning electrochemical microscopy configuration), leading to
filaments behaving as atomic contacts with improved stability when
protected by the silica nanochannels than those formed on a bare
electrode~\cite{351}. Nevertheless, 2--3~nm size metallic 1D
nanostructures arrays that would have been electrodeposited in a stable
form through oriented mesoporous silica films are not yet described. In
the following paragraph, we will discuss if it would be possible for
``synthetic\unskip\break 
metals''.

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig15}
{\vspace*{1pt}}
\caption{\label{fig15}(A$_{1}$) Potentiostatic
electropolymerization of PANI at an ITO electrode modified with a
vertically aligned mesoporous silica membrane as the hard template (up
to Q $=$ 3~mC), with the electropolymerization at a bare ITO as inset. (I)
Induction and nucleation; (II) PANI nanowire formation within the
mesochannels; (III) 3D PANI growth. Solution was made of 0.1 M aniline
in 0.5~M H$_{2}$SO$_{4}$, and applied potential
was ${+}$0.85~V. (A$_{2}$) Schematic view of the three
electropolymerization stages. 
(A$_{3}$) CVs recorded at 50~mV${\cdot}$s$^{-1}$ in 0.5~M
H$_{2}$SO$_{4}$ after PANI electropolymerization
for Q $=$ 1.5~mC~(a) and 2.7~mC~(b). {Reprinted with permission
from Gamero-Quijano \etal \cite{359},
Copyright (2017) American Chemical Society.}  (B) Schematic illustration
of the generation of PANI nanofilaments by combining electrografting of
the monodiazonium salt of p-phenylenediamine and electropolymerization
of aniline, with subsequent removal of silica to get the template-free
nanofilaments (photos of the deposits are also shown).
{Reprinted with permission from Ullah \etal \cite{366},
Copyright (2022) The Royal Society of
Chemistry}.}
{\vspace*{1pt}}
\end{figure*}


Several reports are dealing with the synthesis of conducting polymers
by electropolymerization using vertically oriented silica nanochannels
as a template, notably for polypyrrole (PPy)~\cite{352,353},
polythiophene~\cite{354,355} or  
poly(3,4-ethylenedioxythiophene)
(PEDOT)~\cite{356,357},~polyaniline (PANI)~\cite{358,359} and
polyquinone 
\cite{360,361}. Sometimes the silica membrane is deposited
onto a preformed layer of bulk polymer prior to nanowires growth in
order to ensure a good adhesion of these latter onto the electrode
support~\cite{353,357,361}. However, the observed diameters are mostly
much larger (10--30~nm~\cite{353,354,362}) than the aperture of the
silica nanochannels (2--3~nm), suggesting a growth outside the
membrane, even if thinner deposits ({ca.}\  3--5~nm size) are
reported in some cases~\cite{355,358,359}. Of related interest is the
electrogeneration of Prussian Blue nanowires, exhibiting an
electrochemical behavior suggesting very small size but without direct
evidence of the nanofilaments in the nanochannels~\cite{363}. A way to
control carefully the growth of nanofilaments through oriented silica
membranes is demonstrated for PANI by applying electropolymerization in
the potentiostatic mode~\cite{359}. As shown in
Figure~\ref{fig15}A$_{1}$, one can clearly identify three stages in the
process: (I) a first nucleation step with low currents, (II) a second
step corresponding to a rate-limited growth of 1D PANI nanofilaments in
the nanochannels (plateau current), and (III) an exponential current
increase corresponding to the 3D growth of the polymer on top of the
silica membrane (see a schematic illustration of the three situations
in Figure~\ref{fig15}A$_{2}$). On bare ITO, one can only observe the
nucleation and 3D growth steps (Figure~\ref{fig15}A$_{1}$, inset). Once
formed, PANI deposits are electroactive (see typical CV curves in
Figure~\ref{fig15}A$_{3}$, corresponding to the
leucoemeraldine-emeraldine redox switching of PANI nanofilaments alone
(a) and with bulky deposits on top (b)), with higher peak currents in
the latter case as expected from larger amounts of produced polymer.
Interestingly, the anodic-to-cathodic peak-to-peak separation for the
individual nanofilaments separated from each other by the silica walls
was close to 60 mV, a much lower value than that observed for bulk PANI
films ({ca.}\  120--130~mV), suggesting a more reversible
electrochemical behavior in the former case~\cite{359} and higher
specific capacitance~\cite{364}. Unfortunately, such PANI nanofilaments
are not durably attached to the underlying electrode support (ITO in
this case) and are lost in solution when dissolving the silica
membrane~\cite{365}. This problem can be solved by preforming a polymer
layer prior to electro-assisted self-assembly of the silica membrane
and \mbox{electropolymerization,} as \mbox{aforementioned}~\cite{353,357,361}. If
wishing to have only the polymer nanofilaments covalently attached to
an untreated electrode surface, however, it is necessary to develop
another strategy. This is achieved by the electrografting of the
monomer at the bottom of nanochannels prior to electropolymerization,
as illustrated for PANI (Figure~\ref{fig15}B). This approach is indeed
successful and PANI nanofilaments stay on the electrode support after
silica dissolution~\cite{365}. Same as above (Figure~\ref{fig15}A$_{1}$),
one can control the nanofilaments length by tuning the
electropolymerization time, as well as the amount of deposited PANI by
adjusting the pore diameters of the silica membrane~\cite{366}. These
silica-free PANI nanofilaments are all electrochemically accessible
(ideal thin layer behavior) contrarily to those confined in the silica
membrane for which some diffusional limitations do exist due to
resistance to mass transport during doping-dedoping
\mbox{processes~\cite{366}.}\looseness=1


\subsubsection{Covalent attachment of organo-functional\newline groups}

In order to extend the scope of applications of the oriented mesoporous
silica films on electrodes (see Section~\ref{sec4.4}), one should be
able to get them in the form of organic--inorganic hybrids with
organo-functional groups firmly attached to the silica walls. If some
reagents (e.g.,
$\mathrm{Ru}(\mathrm{bpy})_{3}^{2+}$~\cite{367} or
microperoxydase-11~\cite{368}) can be introduced in the nanochannels
via favorable electrostatic interactions or adsorption, their long-term
immobilization might be an issue, and strategies to bind the
organo-functional groups covalently to the silica network are needed.
As common for silica-based materials in general and mesoporous silica
in particular~\cite{212,213}, this can be basically achieved by using
{organo-trialkoxysilane reagents that are either grafted onto
as-synthesized silica materials or involved in co-condensation
reactions with the tetraalkoxysilane precursors to produce the}
organic--inorganic hybrids in one step. In spite of some reports on the
post-synthesis grafting of oriented mesoporous silica films 
\cite{369,370}, this approach usually leads to pore blocking as a
result of anarchical polymerization of the grafting reagents in the
nanochannels due to the presence of residual water~\cite{246}. The
co-condensation route is an alternative~\cite{157,168,245,369}, but
often limited to quite small and uncharged moieties (e.g.,
methyl, amino- or mercapto-propyl groups) in quite low amounts
(i.e., 10\% for aminated films) so as not to disturb the
surfactant template organization in the EASA process and the resulting
film ordering and\break orientation~\cite{245}.


\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig16}
{\vspace*{2pt}}
\caption{\label{fig16}(A) Illustration of the EASA co-condensation process of
(A$_{1}$) AzPTMS and TEOS or (A$_{2}$) MPTMS and
TEOS in the presence of CTAB to get vertically aligned mesoporous
silica films respectively functionalized with (A$_{1}$)
azide or (A$_{2}$) thiol groups, which are further
derivatized following (A$_{1}$) the Huisgen azide--alkyne
click coupling or (A$_{2}$) the thiol--ene reaction. (B)
Cyclic voltammograms (4 successive cycles) recorded in DMF (0.1 M TBAPF
6) using an ITO electrode modified with mesoporous silica thin films
incorporating azide and thiol groups, as prepared from a sol solution
containing AzPTMS/MPTMS/TEOS at the 20/20/60 molar ratio, which were
subjected to Huisgen reaction using ethynylcobaltocenium and thiol--ene
coupling using vinylferrocene. {From Vil\`{a} \etal \cite{247},
reproduced with permission of
WILEY-VCH Verlag GmbH.} (C) Cyclic voltammograms recorded in
acetonitrile (${+}$0.1~M TBAClO$_{4}$) at 20~mV${\cdot}$s$^{-1}$ 
using ITO electrodes covered with
ferrocene-functionalized silica films (schematically represented on the
left part) as prepared from various AzPTMS/TEOS ratios (100 mM total
concentration). {From Vil\`{a} \etal \cite{246}, 
reproduced with permission of WILEY-VCH Verlag GmbH.}
(D) Simplified scheme illustrating the electron transfer processes in
vertically-oriented ferrocene-functionalized mesoporous silica film in
the absence or presence of a ferrocenyl dendrimer in solution (left)
and CV curves recorded in THF (${+}$0.2~M TBABF$_{4}$) without
(a) or with 0.12 mM 9-ferrocenyl-terminated dendrimer (b), using an ITO
electrode modified with a CTAB-based mesoporous silica film
incorporating azide groups (as obtained from a sol prepared with 20\%
AzPTMS and 80\% TEOS) subjected to CuAAC coupling with
propargyl-ferrocene, as recorded at a potential scan 
rate of 5~mV${\cdot}$s$^{-1}$ 
(right). {Reprinted with permission
from Vil\`{a} \etal \cite{321}, Copyright (2016) 
American Chemical Society.}}
{\vspace*{2pt}}
\end{figure*}



A more versatile and universal approach is the combination of EASA with
click chemistry, as illustrated in Figure~\ref{fig16}A. The basic idea is to
generate first either azide- or thiol-functionalized mesoporous silica
films by co-condensation of TEOS with 3-azidopropyltrimethoxysilane
(AzPTMS) or MPTMS, respectively, and then coupling them by either the
azide--alkyne Huisgen cycloaddition (Figure~\ref{fig16}A$_{1}$) or the
thiol--ene reaction (Figure~\ref{fig16}A$_{2}$) to get basically any
type of organic group covalently attached to the silica
walls~\cite{246,247}. One can indeed obtain highly ordered and oriented
mesoporous silica films from sol solutions containing up to 40\%
AzPTMS~\cite{246} or 30\% MPTMS \cite{247} relative to the total silane
precursors content without altering the mesostructure. The unexpectedly
high azide content compatible with keeping a high level of film
structuration/orientation is probably due to the zwitterionic nature of
azide groups, acting as stabilizer between the positively charged
surfactant molecules and the negatively charged silanolate
moieties~\cite{246}. In spite of the confined space of the
nanochannels, the click coupling is effective in both cases (Huisgen
cycloaddition and thiol--ene reaction). Moreover, bi-functionalized
films containing both azide and thiol functions can be produced by EASA
and derivatized according to the two click reactions, successively in
any order thanks to the different catalysts used for each coupling
\cite{247}. The organo-functional groups are covalently bonded to the
silica nanochannels, randomly and uniformly distributed over the entire
internal surface area of the mesopores (contrary to more heterogeneous
grafting~\cite{371}), at a coverage level that can be adjusted by
controlling the amount of organosilanes in the synthesis
medium~\cite{246,247}. It is also possible to attach very
reactive/unstable species (such as tetrazine~\cite{372}) in a smooth
way. One can even selectively functionalize one or two silica layers in
multi-layered mesoporous films~\cite{320}. In principle, one could also
think about other reactions, such as the amino-carboxyl or
amino-aldehyde bonding's, which are possible from amine-functionalized
mesoporous silica films~\cite{373} but one would be limited by the
level of functionalization (max 10\% APTES to get ordered and oriented
films by EASA~\cite{245}). Covalent functionalization of such oriented
mesoporous films can be performed only at the pore bottom, either by
grafting an hydroxylated electrode surface (as ITO) with an
organo-alkoxysilane reagent prior to the electrogeneration of the
silica film (as pointed out for propyl-phosphonate and cyanopropyl
groups~\cite{374}) or by electrografting selectively the underlying
electrode support after film formation (i.e., via
electrochemical reduction of a diazonium salt, as shown for binding
ferrocene units~\cite{375}). Finally, the use of organosilane
precursors bearing more than one alkoxy group (such as tetra-silylated
cyclam, for instance) enables the electro-assisted generation of
oriented films containing the organo-functional groups inside the
silica walls~\cite{315}.




If redox-active moieties such as ferrocene or cobaltocenium derivatives
are used to functionalize the silica walls, they are electrochemically
accessible in spite of the insulating nature of the membrane, giving
rise to well-defined voltammetric signals (Figure~\ref{fig16}B). The
charge transfer mechanism is based on electron hopping between adjacent
redox species eventually combined to diffusion processes 
\cite{246,247,371,376}, somewhat similar to what happens with redox
polymers (polymer backbones with pendant redox centers)~\cite{377}. In
the present case, the redox-active moieties are firmly attached to a
rigid inorganic structure, without the possibility of motion (contrary
to the chains mobility in non-reticulated redox polymers for which
interplay of electron hopping and bounded diffusion is
possible~\cite{378}), except perhaps via some flexibility of the
binding arm. As expected, the intensity of the voltammetric peak
currents is rising up with increasing the content of electroactive
units immobilized in the nanochannels (see Figure~\ref{fig16}C for the
particular case of ferrocene groups clicked onto oriented mesoporous
silica films prepared from increased amounts of AzPTMS). At too low
concentrations of electroactive centers (i.e., for films
prepared from less than 2.5\% AzPTMS), the voltammetric response
vanishes rapidly because the distance separating the redox units
becomes too long to enable effective electron hopping~\cite{246}, as
otherwise reported for other assemblies of redox centers~\cite{379}.
When investigating the effect of potential scan rates in cyclic
voltammetry, one realizes that the charge transfer processes are mainly
governed by diffusion (linear dependence of peak currents on the square
root of scan rates), except at very low scan rates
(${<}10~\mathrm{mV}{\cdot}\mathrm{s}^{-1}$) where a thin-layer behavior
becomes dominating~\cite{376}. In the diffusion regime, the
rate-determining step is dominated by the percolation of electrons
passing from one ferrocene group to another one, especially at low
ferrocene contents. At higher ferrocene contents, the free space in the
nanochannels becomes smaller and the overall charge transfer process
(involving also charge neutralization with counter-ions) can be limited
by the transport of the electrolyte anions in a so-confined environment
(as an anion is necessary to compensate the positive charge generated
during the oxidation of ferrocene into ferricinium). This is notably
evidenced with using a large electrolyte anion (i.e.,
$p$-toluene sulfonate), contributing to much lower currents due to
restricted diffusion compared to a smaller anion (i.e.,
perchlorate), in neutralizing the ferricinium species generated by the
electrochemical oxidation of ferrocene~\cite{376}. It is therefore
necessary to find the best compromise between a sufficiently high level
of functionalization to fasten the charge transfer by electron hopping
but not too high to enable easy mass transport during charge
compensation by the electrolyte ions.




\subsubsection{Long-range charge transfer through insulating hybrid films}

As stated above, charge transfer reactions can occur in the insulating
silica membrane, either via the presence of conducting polymer
nanofilaments (Figure~\ref{fig15}A$_{3}$) or by electron hopping
between redox moieties covalently bonded to the silica walls
(Figure~\ref{fig16}B,C). The question arises now: can we perform
long-range charge transfer reactions through the entire thickness
(${\sim}$100~nm) of the insulating silica film by taking advantage of
the vertical orientation of the nanochannels filled with electroactive
materials? The answer is fully yes for conducting polymer nanofilaments
as they can be quantitatively and reversibly oxidized/reduced,
including when the polymer is grown beyond the silica
membrane~\cite{353,366}. An additional demonstration is given by
Conductive Atomic Force Microscopy (C-AFM) experiments, resulting in
high conductance when approaching a Pt-coated C-AFM tip, due to the
formation of molecular junctions with the vertical PANI wires in the
silica nanochannels~\cite{380}. For mesoporous silica films bearing
redox units covalently attached to the nanochannels, long-range charge
transfer is also possible through the whole membrane, as pointed out
using a size-excluded redox probe that can only be electrochemically
transformed onto the top of the functionalized film~\cite{321}. For
example, a vertically aligned ferrocene-functionalized film is likely
to oxidize the film-excluded nonaferrocenyl dendrimer (see the increase
in peak current in the presence of the dendrimer in
Figure~\ref{fig16}D). This requires the application of rather low
potential scan rates (${<}10~\mathrm{mV}{\cdot}\mathrm{s}^{-1}$) to be
effective, and {ca.}\  one ferrocene out of the nine in the
dendrimer is electrochemically accessible, as a result of the quite
restricted charge/mass transport processes in such films (as discussed
in the previous section)~\cite{321}. Anyway, it works and it can even
be extended to multilayered films containing the redox units in all the
superimposed layers, confirming good communication between
them~\cite{320}. Much faster and reversible charge transfer can be
achieved with spin crossover complexes (such as
$\mathrm{Fe}(\mathrm{Htrz})_{3}$ (Htrz $=$ 1,2,4,${-}$1H-triazole))
immobilized as a coordination polymer in a mesoporous silica 
film\unskip\break
generated by EASA~\cite{381}. In that case, the film growth is based on
the ability of $\mathrm{Fe}(\mathrm{Htrz})_{3}$ to act itself as a real
template (instead of the surfactant) during the self-assembly of the
oriented mesostructure. Finally, the possibility of redox cycling for
freely diffusing redox probes in a nanochannel membrane sandwiched
between two ITO electrodes, thanks to the facilitated transport offered
by the vertical orientation~\cite{382}, is also of related interest but
this is no more a reagent-free system.

\subsection{Applications of the vertically oriented\newline mesoporous
(organo)silica films} \label{sec4.4}

Due to their regular, widely open and vertically oriented pore
structure, the mesoporous silica thin films generated by EASA are
ensuring fast transport processes from the external solution to the
underlying electrode surface, making them very attractive for various
applications, as illustrated in Figures~\ref{fig17}, \ref{fig18}
and~\ref{fig19}. Films with both filled pores (Figure~\ref{fig17}),
template-free open pores (Figure~\ref{fig18}) and functionalized/grafted
channels (Figure~\ref{fig19}) have been exploited, notably for
electroanalytical purposes (Table~\ref{tab1}) but not only. The main
recent developments are briefly presented and discussed\break hereafter.



\begin{figure*}
\includegraphics{fig17}
{\vspace*{8pt}}
\caption{\label{fig17}Illustration of some electrochemical
applications involving EASA-derived vertically aligned silica
nanochannel membranes with filled pores. (A) Hydrophobic-based
accumulation/detection and antifouling (possibility to preconcentrate
small lipophilic species while rejecting large molecules/interferences
and charged ions. One the right, one can see DPV curves of
{vertically-oriented mesoporous silica film on
plasma-treated 3D graphene} electrode in response to different added
concentrations of uric acid in human serum and the corresponding
calibration curve compared to that on bare electrode. {Reprinted
from Zhu \etal \cite{387}, Copyright (2022), with
permission from Elsevier Science.} (B) Superhydrophobic surfaces and
corrosion protection (superhydrophobic film containing a corrosion
inhibitor). On the right is a photograph of common liquids beading up
on the surfaces of bare aluminum alloy and superhydrophobic
benzotriazole-loaded CTAB-filled mesoporous silica film
electrodeposited on aluminum alloy substrate and further grafted with
dodecyltrimethoxysilane (liquids from left to right: top row, green
tea, milk and orange juice; bottom row, coffee, Gatorade and tomato
juice). {Reprinted from Zhao \etal \cite{441}, 
Copyright (2020), with permission from Elsevier
Science.} (C)~Electrocatalysis with coordination polymer template:
Fe(Htrz)$_{3}$ (Htrz $=$ 1,2,4,${-}$1H-triazole) confined into a
mesoporous silica matrix during its formation by EASA on ITO electrode.
In the center, FIA responses to three successive injections of 100
${\rmmu}$L aliquots of H$_{2}$O$_{2}$ (in an
aqueous carrier solution of 0.1~M NaNO$_{3}$ flowing at 
1~mL${\cdot}$min$^{-1}$) at 30, 150 and 300~${\rmmu}$M; on the
right, CV curves recorded at various potential scan rates (in the
0.2--1.2~V${\cdot}$s$^{-1}$ range) in 0.1 M
NaNO$_{3}$ aqueous solution, as depicted on a TEM view of
the mesoporous film. {{Adapted from}}
{Ahoulou \etal \cite{410}, with
permission of WILEY-VCH Verlag GmbH, and \cite{381}, 
Copyright (2019)} {American Chemical Society.} (D)
Electropolymerized nanowires of polyaniline (PANI) that are formed in
the vertical nanochannels and stood free by dissolution of the silica
membrane: electrochromic response and photographs at switching
potentials (20~s each) of ${+}$0.3~V (colouration) and ${-}$0.2~V (bleaching),
and cyclic voltammograms recorded at various potential scan rates from
10 to 200~mV${\cdot}$s$^{-1}$ (every 10~mV${\cdot}$s$^{-1}$). 
{Adapted with permission from 
Ullah \etal \cite{366}, Copyright (2022)
 The Royal Society of Chemistry}.}
\end{figure*}


\begin{table*}[p!]
\caption{Electrochemical sensors designed from EASA-based mesoporous silica thin films on electrode\label{tab1}}
\footnotesize
\tabcolsep=4pt
\begin{tabular}{llccccc}
\thead
\xmorerows{1}{Analyte$^{\mathrm{a}}$}  & 
\xmorerows{1}{Electrode configuration$^{\mathrm{b}}$} & 
\morerows{1}{\parbox[t]{1.5cm}{Detection procedure$^{\mathrm{c}}$}} &
\xmorerows{1}{Technique$^{\mathrm{d}}$}  & 
\multicolumn{2}{c}{Analytical performance} & \xmorerows{1}{Ref.} \\
\cline{5-6}
 &  &  &  & Concentration range & Det. limit &  \\ 
\endthead
Acetaminophen  & \morerows{2}{GCE/rGO/VMSF} & \morerows{2}{Acc.--Det.} & \morerows{2}{DPV} & 2 ${\times}$ 10$^{-7}$--1.0 ${\times}$ 10$^{-4}$ M & 9.7 ${\times}$ 10$^{-9}$ M & \morerows{2}{\cite{395}} \\
Norepinephrine &&&&                                1 ${\times}$ 10$^{-7}$--3.0 ${\times}$ 10$^{-5}$ M & 1.4 ${\times}$ 10$^{-8}$ M & \\
Tryptophan     &&&&                                8 ${\times}$ 10$^{-7}$--2.0 ${\times}$ 10$^{-4}$ M & 3.6 ${\times}$ 10$^{-8}$ M\vspace*{5pt} &\\

Ag$^{+}$ & CF/(VMSF-SH) & Acc.--Det. & DPV & 2.5 ${\times}$ 10$^{-6}$--2.0 ${\times}$ 10$^{-6}$ M & - & \cite{168}\vspace*{5pt} \\ 

4-Aminophenol & ITO/VMSF & Acc.--Det. & DPV & 5 ${\times}$ 10$^{-7}$--4.0 ${\times}$ 10$^{-4}$ M & 3.2 ${\times}$ 10$^{-7}$ M & \cite{395}\vspace*{5pt} \\ 

Ascorbic acid & \morerows{1}{\parbox[t]{2.5cm}{AuE/(VMSF-GPS-\newline DNAzyme)}}  & \morerows{1}{Indirect det.} & \morerows{1}{SWV} & 1.0 ${\times}$ 10$^{-6}$--1.2 ${\times}$ 10$^{-4}$ M  & 7.0 ${\times}$ 10$^{-7}$ M  & \morerows{1}{\cite{413}} \\ 
Cu$^{2+}$     &&&&                                              310 ppb--4.5 ppm & 250 ppb\vspace*{5pt}  &\\

\morerows{1}{\parbox[t]{1.5cm}{\textit{tert}-Butyl-\newline hydroquinone}} & \morerows{1}{GCE/ErGO/VMSF} & \morerows{1}{Acc.--Det.} & \morerows{1}{DPV} & 1 ${\times}$ 10$^{-9}$--5 ${\times}$ 10$^{-7}$  M \&  & \morerows{1}{2.3 ${\times}$ 10$^{-10}$ M} & \morerows{1}{\cite{396}} \\ 
                                 &&&&                                 5 ${\times}$ 10$^{-7}$--1.2 ${\times}$ 10$^{-4}$ M\vspace*{5pt} & \\

Carbendazim & GCE/BN-rGO/VMSF & Acc.--Det. & DPV & 5 ${\times}$ 10$^{-9}$--7 ${\times}$ 10$^{-6}$ M & 2.3 ${\times}$ 10$^{-9}$ M & \cite{400}\vspace*{5pt} \\ 

\morerows{1}{Catechol}     & \morerows{1}{\parbox[t]{2cm}{AuE/ErGO/\newline(VMSF-CTAB)}}  & \morerows{2}{Acc.--Det.} & \morerows{2}{DPV} & 1 ${\times}$ 10$^{-6}$--2.0 ${\times}$ 10$^{-5}$  M \&  & \morerows{1}{1.7 ${\times}$ 10$^{-8}$ M}  & \morerows{2}{\cite{384}} \\ 
             &&&&                                         2.0 ${\times}$ 10$^{-5}$--6.0 ${\times}$ 10$^{-5}$  M  & &\\
Hydroquinone & AuE/ErGO/VMSF &&&                          1 ${\times}$ 10$^{-6}$--6.0 ${\times}$ 10$^{-5}$ M  & 1.5 ${\times}$ 10$^{-8}$ M\vspace*{5pt}  &\\

\morerows{1}{CA15-3} & \morerows{1}{\parbox[t]{2.5cm}{ITO/(VMSF-GPS-Ab,\newline Ru(bpy)$_{3}^{2+}$)}} & \morerows{1}{Genosensing} & ECL  & 0.0001--100~U${\cdot}$mL$^{-1}$  & 9~${\rmmu}$U${\cdot}$mL$^{-1}$  & \morerows{1}{\cite{414}} \\ 
       &&&                                                     DPV  & 0.01--100~U${\cdot}$mL$^{-1}$    & 5.4~mU${\cdot}$mL$^{-1}$\vspace*{5pt} &\\

CEA & \morerows{1}{\parbox[t]{3cm}{ITO/(VMSF-Ru(bpy)$_{3}^{2+}$)/\newline(VMSF-NH$_{2}$)/GluA-Ab}} & \morerows{1}{Genosensing} & ECL   & 0.0001--50~ng${\cdot}$mL$^{-1}$  & 0.06~pg${\cdot}$mL$^{-1}$  & \morerows{1}{\cite{415}} \\ 
CA15-3 &&&                                                                  DPV   & 0.005--500~U${\cdot}$mL$^{-1}$   & 2.3~mU${\cdot}$mL$^{-1}$\vspace*{5pt} &\\

C-reactive protein & \parbox[t]{3.2cm}{SPCE/ErGO/\newline (VMSF-NH$_{2}$)/GluA-Ab} & Genosensing & DPV & 0.01--100~ng${\cdot}$mL$^{-1}$ & 8~pg${\cdot}$mL$^{-1}$ & \cite{419}\vspace*{5pt} \\ 

\morerows{1}{Chlorpromazine} & \morerows{1}{SPCE/ErGO/VMSF} & \morerows{1}{Acc.--Det.} & \morerows{1}{DPV} & 3 ${\times}$ 10$^{-7}$--2.3 ${\times}$ 10$^{-5}$  M \& & 6.1 ${\times}$ 10$^{-9}$ M  & \morerows{1}{\cite{390}} \\ 
&&&&                                                 10$^{-6}$--10$^{-5}$ M (blood)  & 1.6 ${\times}$ 10$^{-8}$ M\vspace*{5pt} & \\

\morerows{1}{Clindamycin} & \morerows{1}{\parbox[t]{2.5cm}{PET-ITO/\newline(VMSF-Ru(bpy)$_{3}^{2+}$)}} & \morerows{1}{Direct det.} & \morerows{1}{ECL} & 10 nM--25 ${\rmmu}$M \&  & \morerows{1}{4 nM} & \morerows{1}{\cite{422}} \\ 
&&&&                                                                      25--70 ${\rmmu}$M \& \vspace*{5pt} & & \\

Clozapine & ep-SPCE/VMSF & Acc.--Det. & DPV & 50 nM--20 ${\rmmu}$M  & 2.8 ${\times}$ 10$^{-8}$ M & \cite{391}\vspace*{5pt} \\ 

Cu$^{2+}$ & GCE/(clay + VMSF)  & Acc.--Det. & SWV & 1 ${\times}$ 10$^{-7}$--1.0 ${\times}$ 10$^{-6}$ M & - & \cite{189}\vspace*{5pt} \\ 

Cu$^{2+}$ & GCE/(VMSF-NH$_{2}$)  & Acc.--Det. & ASDPV & 1 ${\times}$ 10$^{-8}$--1.0 ${\times}$ 10$^{-6}$ M & 3 ${\times}$ 10$^{-9}$ M & \cite{245}\vspace*{5pt} \\ 

Cysteine & ITO/(VMSF-Fc)  & Mediated EC & A, FIA & 3 ${\times}$ 10$^{-6}$--1.0 ${\times}$ 10$^{-4}$ M & - & \cite{222}\vspace*{5pt} \\ 

Diclofenac & \parbox[t]{2.5cm}{GCE/APTES/\newline(Amino-AT $+$ VMSF)} & Acc.--Det. & SWV & 3 ${\times}$ 10$^{-7}$--2.0 ${\times}$ 10$^{-5}$ M & 5.3 ${\times}$ 10$^{-8}$ M & \cite{190}\vspace*{5pt} \\ 

DNA & \parbox[t]{2.5cm}{AuE/(VMSF-GPS-\newline DNA Ab)}  & Genosensing & CV & 5 ${\times}$ 10$^{-9}$--7.0 ${\times}$ 10$^{-7}$ M & 2.5 ${\times}$ 10$^{-9}$ M & \cite{422}\vspace*{5pt} \\ 

\morerows{1}{Dopamine} & \morerows{1}{GCE/VMSF} & \morerows{1}{Acc.--Det.} & \morerows{1}{DPV} & 10 nM--1.0 ${\rmmu}$M \&  & \morerows{1}{0.88 nM}   & \morerows{1}{\cite{329}} \\ 
&&&&                                     0.05--1.0 ${\rmmu}$M (serum)\vspace*{5pt}   & & \\

\morerows{1}{Dopamine} & \morerows{1}{AuE/(VMSF-GQDs)} & \morerows{1}{Acc.--EC det.} & \morerows{1}{DPV} & 200 nM--20 ${\rmmu}$M \&  & \morerows{1}{120 nM}  & \morerows{1}{\cite{407}} \\ 
&&&&                                             20--100 ${\rmmu}$M\vspace*{5pt} &&\\
\tabularBreak{0}

Dopamine  & \morerows{1}{GCE/(VMSF-NMe$_{3}^+$)}  & \morerows{1}{Acc.--Det.} & \morerows{1}{DPV} & 2.0 ${\times}$ 10$^{-5}$--2.26 ${\times}$ 10$^{-4}$  M  & 9 ${\times}$ 10$^{-6}$ M  & \morerows{1}{\cite{404}} \\ 
Ascorbic acid &&&&                                       4.9 ${\times}$ 10$^{-5}$--2.65 ${\times}$ 10$^{-3}$ M   & 1.1 ${\times}$ 10$^{-5}$ M\vspace*{5pt} & \\

Doxorubicin & GCE/VMSF & Acc.--Det. & DPV & 5 ${\times}$ 10$^{-10}$--2.3 ${\times}$ 10$^{-5}$ M & 2 ${\times}$ 10$^{-10}$ M & \cite{392}\vspace*{5pt} \\ 

Doxorubicin & GCE/ErGO/VMSF & Acc.--Det. & DPV & 1 ${\times}$ 10$^{-9}$--2.0 ${\times}$ 10$^{-5}$ M & 7.7 ${\times}$ 10$^{-10}$ M & \cite{393}\vspace*{5pt} \\ 

Guanine & ITO/(VMSF-Ru(bpy)$_{3}^{2+}$) & EC & CV & 2 ${\times}$ 10$^{-7}$--5.0 ${\times}$ 10$^{-4}$ M & 5.8 ${\times}$ 10$^{-8}$ M & \cite{409}\vspace*{5pt} \\ 

Hydrogen peroxide & ITO/(VMSF-PB wires)  & Mediated EC & CV & 1 ${\times}$ 10$^{-4}$--1 ${\times}$ 10$^{-2}$ M &  & \cite{363}\vspace*{5pt} \\ 

Hydrogen peroxide & ITO/(VMSF-Fe(Htrz)$_{3}$)  & Mediated EC & A, FIA & 5 ${\times}$ 10$^{-6}$--3.0 ${\times}$ 10$^{-4}$ M & 2 ${\times}$ 10$^{-6}$ M & \cite{410}\vspace*{5pt} \\ 

Hydrogen peroxide & CFME/(VMSF-PtNS) & EC & A & 5 ${\times}$ 10$^{-4}$--2.0 ${\times}$ 10$^{-3}$ M & - & \cite{408}\vspace*{5pt} \\ 

\morerows{1}{Imidacloprid} & \morerows{1}{GCE/ErGO/VMSF} & \morerows{1}{Acc.--Det.} & \morerows{1}{CV} & 1.0--50~${\rmmu}$g${\cdot}$mL$^{-1}$ \&  & \morerows{1}{0.3~${\rmmu}$g${\cdot}$mL$^{-1}$} & \morerows{1}{\cite{401}} \\ 
&&&&                                             50--400~${\rmmu}$g${\cdot}$mL$^{-1}$\vspace*{5pt} && \\

Insulin & \parbox[t]{3cm}{GCE/APTES/\newline(VMSF-NH$_{2}$)/\newline GluA-cDNA-Apt.} & Aptasensing & DPV & 1.0 ${\times}$ 10$^{-8}$--3.5 ${\times}$ 10$^{-7}$ M & 3.0 ${\times}$ 10$^{-9}$ M & \cite{421}\vspace*{5pt} \\ 

Isoproturon & GCE/APTES/VMSF & EO amplif. & SWV & 1.0 ${\times}$ 10$^{-5}$--1.0 ${\times}$ 10$^{-4}$ M & - & \cite{405}\vspace*{5pt} \\ 

Lidocaine & FTO/(VMSF-Ru(bpy)$_{3}^{2+}$) & Direct det. & ECL & 1.0 ${\times}$ 10$^{-8}$--5.0 ${\times}$ 10$^{-5}$ M & 8 ${\times}$ 10$^{-9}$ M & \cite{327}\vspace*{5pt} \\ 

Methidazine & 3DG/VMSF & Acc.--EC det. & DPV & 5.0 ${\times}$ 10$^{-8}$--1 ${\times}$ 10$^{-5}$ M & 3.0 ${\times}$ 10$^{-8}$ M & \cite{394}\vspace*{5pt} \\ 

Nitrate  & \morerows{5}{ITO/(VMSF-Fc)} & \morerows{5}{Indirect det.} & \morerows{5}{IC-A} & 2.0 ${\times}$ 10$^{-5}$--1.0 ${\times}$ 10$^{-4}$  M     & 1.6 ${\times}$ 10$^{-5}$ M   & \morerows{5}{\cite{412}} \\ 
Fluoride &&&&                                     5 ${\times}$ 10$^{-6}$--5.0 ${\times}$ 10$^{-5}$  M       & 3 ${\times}$ 10$^{-6}$ M     &\\
Chloride &&&&                                     5 ${\times}$ 10$^{-6}$--5.0 ${\times}$ 10$^{-5}$  M       & 4 ${\times}$ 10$^{-6}$ M     &\\
Bromide  &&&&                                     1.0 ${\times}$ 10$^{-5}$--7.5 ${\times}$ 10$^{-5}$  M     & 1.0 ${\times}$ 10$^{-5}$ M   &\\
Phosphate&&&&                                     2.0 ${\times}$ 10$^{-5}$--1.0 ${\times}$ 10$^{-4}$  M     & 1.2 ${\times}$ 10$^{-5}$ M   &\\
Sulfate  &&&&                                     1.5 ${\times}$ 10$^{-5}$--1.0 ${\times}$ 10$^{-4}$ M      & 1.0 ${\times}$ 10$^{-5}$ M\vspace*{5pt}   &\\

Paraquat & GCE/APTES/VMSF & Acc.--Det. & SWV & 1.0 ${\times}$ 10$^{-8}$--1.0 ${\times}$ 10$^{-5}$ M & 4 ${\times}$ 10$^{-9}$ M & \cite{332}\vspace*{5pt} \\ 

\morerows{1}{Paraquat} & \morerows{1}{GCE/3DG-CNT/VMSF} & \morerows{1}{Acc.--Det.} & \morerows{1}{DPV} & 2 ${\times}$ 10$^{-9}$--1.0 ${\times}$ 10$^{-8}$  M \&  & \morerows{1}{1.17 ${\times}$ 10$^{-9}$ M} & \morerows{1}{\cite{389}} \\ 
&&&&                                             1.0 ${\times}$ 10$^{-8}$--1.0 ${\times}$ 10$^{-5}$ M\vspace*{5pt} &  & \\

PSA & \parbox[t]{2.5cm}{ITO/(VMSF-NH$_{2}$)/\newline GluA-Ab)} & Genosensing & DPV & 10~pg${\cdot}$mL$^{-1}$--1~${\rmmu}$g${\cdot}$mL$^{-1}$ & 8.1~pg${\cdot}$mL$^{-1}$ & \cite{416}\vspace*{5pt} \\ 

PSA & ITO/VMSF/GPS-Ab & Genosensing & ECL & 1~pg${\cdot}$mL$^{-1}$--100~ng${\cdot}$mL$^{-1}$ & 0.1~pg${\cdot}$mL$^{-1}$ & \cite{417}\vspace*{5pt} \\ 

PSA & \parbox[t]{2.2cm}{AuE/(VMSF-GPS-PSA Apt.)}  & Aptasensing & DPV & 1--300~ng${\cdot}$mL$^{-1}$ & 0.28~ng${\cdot}$mL$^{-1}$ & \cite{418}\vspace*{5pt} \\ 

\morerows{1}{Rutin} & \morerows{1}{GCE/ErGO/VMSF} & \morerows{1}{Acc.--Det.} & \morerows{1}{DPV} & 3 ${\times}$ 10$^{-7}$--2 ${\times}$ 10$^{-6}$  M \&  & \morerows{1}{2.3 ${\times}$ 10$^{-9}$ M}& \morerows{1}{\cite{399}} \\ 
&&&&                                       2 ${\times}$ 10$^{-6}$--4.0 ${\times}$ 10$^{-5}$ M\vspace*{5pt} && \\

\morerows{1}{Serotonin} & \morerows{3}{ITO/ErGO-CNT/VMSF} & \morerows{3}{Acc.--Det.} & \morerows{3}{DPV} & 1 ${\times}$ 10$^{-7}$--1.0 ${\times}$ 10$^{-5}$  M    & \morerows{1}{5.4 ${\times}$ 10$^{-9}$ M}  & \morerows{3}{\cite{397}} \\ 
&&&&                                               1.0 ${\times}$ 10$^{-5}$--3.0 ${\times}$ 10$^{-5}$ M   & &\\
\morerows{1}{Melatonin} &&&&                                     1 ${\times}$ 10$^{-6}$--2.0 ${\times}$ 10$^{-5}$  M    & \morerows{1}{1.41 ${\times}$ 10$^{-8}$ M} & \\
&&&&                                               2.0 ${\times}$ 10$^{-5}$--1.0 ${\times}$ 10$^{-4}$ M\vspace*{5pt}   & & \\
\tabularBreak{0}

\morerows{1}{\parbox[t]{2cm}{2,4,6-trichloro-\newline phenol}} & \morerows{1}{3DG/VMSF} & \morerows{1}{Acc.--EC det.} & \morerows{1}{DPV} & 1.0 ${\times}$ 10$^{-8}$--1 ${\times}$ 10$^{-7}$  M \&  & \morerows{1}{2.4 ${\times}$ 10$^{-9}$ M} & \morerows{1}{\cite{406}} \\ 
&&&&                                                      1 ${\times}$ 10$^{-7}$--1.5 ${\times}$ 10$^{-5}$ M\vspace*{5pt} && \\

\morerows{1}{Uric acid} & \morerows{1}{pl-3DG/(VMSF-CTAB)} & \morerows{1}{Acc.--Det.} & \morerows{1}{DPV} & 100 nM--30 ${\rmmu}$M \&  & \morerows{1}{2.3 ${\times}$ 10$^{-8}$ M} & \morerows{1}{\cite{387}} \\ 
&&&&                                                30--150 ${\rmmu}$M\vspace*{5pt}  &&\\

Uric acid & SPCE/ErGO/(VMSF-NH$_{2}$) & Acc.--Det. & DPV & 5 ${\times}$ 10$^{-7}$--1.80 ${\times}$ 10$^{-4}$ M & 1.29 ${\times}$ 10$^{-7}$ M & \cite{403}
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$Abbreviations: CA15-3, cancer antigen 15-3;
CEA, carcino-embryonic antigen; DNA, deoxyribonucleic acid; PSA,
prostate specific antigen.
$^{\mathrm{b}}$Abbreviations: GCE, glassy carbon electrode; rGO,
reduced graphene oxide; VMSF, vertically-oriented mesoporous silica
film; CF, carbon fiber; VMSF-SH, thiol-functionalized VMSF; ITO,
indium-tin oxide; AuE, gold electrode; GPS, (3-glycidopropyl)
trimethoxysilane; DNAzyme, deoxyribozyme; ErGO, electrochemically
reduced graphene oxide; BN-rGO, boron nitride-reduced graphene oxide;
CTAB, cetyltrimethylammonium bromide; Ab, antibody; Ru(bpy)$_{3}^{2+}$,
tris(2,2'-bipyridyl)ruthenium(II); VMSF-NH$_{2}$, amine-functionalized
VMSF; GluA, glutaraldehyde; SPCE, screen-printed carbon electrode;
PET-ITO, polyethyleneterephthalate coated with indium-tin oxide;
ep-SPCE, electrochemically pretreated screen-printed electrode; Fc,
ferrocene; APTES, aminopropyl-triethoxysilane; Amino-AT,
amino-attapulgite; GQDs, graphene quantum dots; VMSF-NMe$_{3}^+$,
trimethylammonium-functionalized VMSF; PB, Prussian Blue;
Fe(Htrz)$_{3}$, iron-triazole coordination polymer (Htrz $=$
1,2,4,${-}$1H-triazole); CFME, carbon fiber microelectrode; PtNS, platinum
nanostructure; cDNA, complementary DNA oligonucleotide; Apt., aptamer;
FTO, fluorine-doped tin oxide; 3DG, 3D graphene; 3DG-CNT, 3D
graphene-carbon nanotube assemblies; PSA Apt., prostate specific
antigen aptamer; pl-3DG, plasma-treated 3D graphene.
$^{\mathrm{c}}$Abbreviations: Acc., accumulation; Det., detection; EC,
electrocatalysis; EO amplif., electro-oligomerisation signal
amplification.
$^{\mathrm{d}}$Abbreviations: DPV, differential pulse voltammetry; SWV, square
wave voltammetry; ECL, electrochemiluminescence; ASDPV, anodic
stripping differential pulse voltammetry; A, amperometry; FIA, Flow
injection analysis; CV, cyclic voltammetry; IC-A, ion chromatography
with amperometric detection.}
{\vspace*{-3pt}}
\end{table*}

\begin{figure*}
{\vspace*{3pt}}
\includegraphics{fig18}
{\vspace*{3pt}}
\caption{\label{fig18}Illustration of some electrochemical
applications involving EASA-derived vertically aligned silica
nanochannel membranes with open pores. (A) Charge selective
preconcentration and detection (possibility to accumulate cations while
rejecting anions): paraquat sensing by square wave voltammetry recorded
in Meuse River water samples at bare (blue) and modified (red)
electrodes, respectively, after spiking with of 0, 10, 20, 30, and 40
nM paraquat (Inset: peak current for paraquat added concentrations of
10, 20, 30, and 40 nM at the modified electrode). {Reprinted
from Nasir \etal \cite{332}, Copyright
(2018)} {American Chemical Society.} (B) Antifouling 
barrier---size selectivity (possibility to detect small species
while rejecting the big ones): cyclic voltammetry
detection of a beta-blocker (propranolol, at
100~${\rmmu}$M) in the presence of Haemoglobin (5
${\rmmu}$M) at an ITO electrode with (plain green curve) or
without SiO$_{2}$ {film (dashed
blue curve)}. {Reprinted from Serrano \etal \cite{338}, 
Copyright (2015), with permission from Elsevier
Science.} (C) Electrochemiluminescence of transparent FTO electrode
coated with a mesoporous silica film bearing
$\mathrm{Ru}(\mathrm{bpy})_{3}^{2+}$ species in the
nanochannels: FIA responses to three successive injections of lidocaine
at increasing concentrations in the 0--50~${\rmmu}$M range and
corresponding calibration curve (inset). {Reprinted with
permission from Liang \etal \cite{327}, 
Copyright (2021) The Royal Society of Chemistry.} (D)~Immuno-,
apta- and genosensors: from a mesoporous silica film grafted with DNA
strands, for instance, and a redox probe in solution (here
$\mathrm{Fe}(\mathrm{CN})_{6}^{4-}$), the voltammetric response
decreases after hybridization, proportionally to the concentration of
complementary DNA. {Adapted with permission from Saadaoui 
\etal \cite{420}, Copyright (2016) Springer}.}
{\vspace*{3pt}}
\end{figure*}


\begin{figure*}
\vspace*{3pt}
\includegraphics{fig19}
\vspace*{4pt}
\caption{\label{fig19}Illustration of some electrochemical applications involving
EASA-derived vertically aligned silica nanochannel membranes with
grafted pores. (A) Indirect amperometric detection of non-electroactive
anions (the oxidation of ferrocene needs an anion to neutralize the
generated ferricinium cation). On the top right part, one can see the
square wave amperometric detection of 0.1 mM
$\mathrm{NO}_{3}^{-}$ (5~successive injections each 2
min in an electrolyte-free water flow at a rate of 
1~mL${\cdot}$min$^{-1}$, by applying ${\Delta}E = 0.5$~V
at a frequency of 1.25 Hz), respectively at unmodified ITO (black
curve) and the ferrocene-functionalized mesoporous silica film
electrode (red curve); at the bottom is the calibration curves obtained
for F$^{-}$, Cl$^{-}$, Br$^{-}$ and NO$_{3}^{-}$ after separation by ion
chromatography. {Reprinted from Karman \mbox{\etal} \cite{412}, 
with permission from Elsevier Science.} (B)
Electrocatalytic sensors (possibility to detect thiols via the
ferrocene mediator): flow injection analysis of 50~${\rmmu}$M
of glutathione (GSH, red arrows) and cysteine (Cys, black arrows) at a
ferrocene-functionalized mesoporous silica film electrode (applied
potential $= {+}$0.65 V; flow rate $=1$~mL${\cdot}$min$^{-1}$).
{Reprinted from Maheshwari \etal \cite{222}, 
with permission of WILEY-VCH Verlag GmbH}. (C) Pseudocapacitive
materials: a ferrocene-functionalized mesoporous silica layer generated
on graphene electrode is likely to provide a cycling performance at
constant current density of 13.3~mA${\cdot}$cm$^{-2}$
without noticeable capacity decrease after 10,000 cycles.
{Reprinted from Wang \etal \cite{309}, 
with permission from Elsevier Science.} (D) Electrochromic
coatings: an ITO electrode coated with a mesoporous silica film
decorated with {bis(terpyridine) iron(II)} complexes, for instance, is
likely to undergo fast redox switching (see CV curve in the right part)
and exhibit colour change (see the transmittance variation at 565 nm
for different potential pulses width of 20 and 10 s applied between
${+}$0.5 and ${+}$1.2~V). {Reprinted from Vil\`{a} and Walcarius \cite{431},
with permission of MDPI}.}
\vspace*{4pt}
\end{figure*}


\subsubsection{Preconcentration electroanalysis and\newline permselective coatings}

The charge and size selectivity (Figures~\ref{fig12}--\ref{fig14})
imparting permselectivity properties to these films can be exploited
for the accumulation/rejection of target species/interference. Neutral
analytes such as biological molecules, drugs or
pharmaceuticals~\cite{383,384}, on one hand, and pollutants or
hazardous species such as pesticides or explosives~\cite{385,386} on
the other hand, are likely to preconcentrate into the hydrophobic
surfactant phase of the films. This process is quite selective (ionic
species and large interferent molecules being excluded), enabling their
determination in real media (whole blood, serum, urine, environmental
samples, foodstuffs, etc.). An illustrative example is the
determination of uric acid in in serum, showing clearly the
accumulation effect of the film compared to the bare electrode
(Figure~\ref{fig17}A), with an anti-interference and antifouling
behavior in such a complex matrix~\cite{387}. The hydrophobic-based
accumulation/detection is usually characterized by calibration curves
with two linear dynamic concentration ranges of different sensitivities
due to more effective preconcentration from diluted samples~\cite{384}.
This is also the case for surfactant-extracted films that are likely to
accumulate cationic species by favorable electrostatic interactions, as
pointed out for paraquat sensing in river water
(Figure~\ref{fig18}A)~\cite{332}. At low \mbox{concentration} (nM to
${\rmmu}$M range) the voltammetric signal is directly proportional to
the square root of potential scan rate (pure adsorption behavior)
whereas it becomes more and more diffusion-controlled at higher
paraquat concentrations (mM range)~\cite{388}, explaining the existence
of the two dynamic ranges in the calibration~\cite{389}. Various
organic molecules have been determined by accumulation/detection at
EASA-derived mesoporous films, mainly by differential pulse voltammetry
(Table~\ref{tab1}), including drugs or
pharmaceuticals~\cite{390,391,392,393,394,395}, biologically relevant
molecules~\cite{329,395,396,397}, organic dyes~\cite{398,399} and
pesticides~\cite{400,401}. Other examples are available for similar
films prepared by the St\"{o}ber growth method~\cite{51}, notably based
on the modulation of their permeability properties (by the presence or
absence of surfactant in the nanochannels) for the direct analysis of
both hydrophilic and lipophilic species (as pointed out for
antioxidants detection in fruit juice~\cite{402}). The question of
long-term operational stability was not often considered, which should
be rather low for such thin walls made of amorphous silica gel, being
evidently not stable at all at high pH~\cite{390}. Sometimes, additives
like clay particles were added to the film to improve somehow the
accumulation efficiency~\cite{189,190}. The covalent bonding of
organo-functional groups (e.g., thiol~\cite{168}, amine~\cite{245,403}
or alkylammonium~\cite{404}) to the mesoporous silica films is a
possibility to enhance further the performance in terms of better
sensitivity and selectivity. Another way of improving the
preconcentration efficiency is via electro-oligomerization stripping
voltammetry, a concept in which the analyte (here isoproturon) is
oxidized in the form of oligomeric deposits that are then detected by
cathodic stripping square wave voltammetry~\cite{405}. An important
feature is the protecting role played by the nanochannels of
monodisperse diameter to reject interference and (bio)fouling agents.
As an example, one can cite the electrochemical detection of the
betablocker propranolol in the presence of haemoglobin, which is not
possible on bare electrode due to adsorption of the protein. It becomes
feasible with the mesoporous silica film acting as a physical barrier
preventing the biomolecule from reaching the electrode surface whereas
small propranolol species are likely to pass through the film and be
oxidized on the underlying ITO electrode\break
(Figure~\ref{fig18}B)~\cite{338}.



\subsubsection{Electrocatalytic detection, indirect amperometric
detection, and sensors based on hybridization reactions and/or
electrochemiluminescence}

When the mesoporous silica films are deposited onto nanomaterials-based
electrodes (such as reduced graphene oxide or carbon nanotubes, for
instance), the analyte preconcentration might be accompanied by
electrocatalytic effects. It is notably the case when three-dimensional
graphene is used as a support for the films~\cite{394,406}.
Nanocatalysts (such as graphene quantum dots~\cite{407} or platinum
nanoparticles~\cite{408}) can be used to modify the silica layer and
the resulting nanocomposite be applied to the electrocatalytic
detection of various analytes (Table~\ref{tab1}). Beside direct
electrocatalysis, mediated electrocatalysis is otherwise exploited by
incorporating a redox mediator in the nanochannels. Examples are
available for silica films simply impregnated with metal complexes
(such as $\mathrm{Ru}(\mathrm{bpy})_{3}^{2+}$ for guanine
detection~\cite{409}), or filled with redox polymers (such as Prussian
Blue~\cite{363} or poly-$\mathrm{Fe}(\mathrm{Htrz})_{3}$~\cite{410} for
$\mathrm{H}_{2}\mathrm{O}_{2}$ sensing thanks to fast electron transfer
in the insulating membrane, see Figure~\ref{fig17}B). Other examples are
films grafted with molecular redox mediators (such as ferrocene for
preferential determination of cysteine over glutathione
(Figure~\ref{fig19}B)~\cite{222}, or catechol for NADH
detection~\cite{373}). Finally, conducting polymers electropolymerized
in the channels are also likely to act as electrocatalysts, alone or in
the presence of additional metal nanoparticles~\cite{358,411}.\looseness=1

The indirect amperometric detection of non-electroactive anions can be
achieved at ferrocene-functionalized mesoporous silica films on the
conceptual basis of charge compensation (the oxidation of ferrocene
into ferricinium requires the ingress of an anion from the solution to
neutralize the so-generated cation), leading to peak currents each time
an aliquot of solution containing an anion is injected at the electrode
surface (Figure~\ref{fig19}A). The flow injection analysis is performed
using a pure water mobile phase (to avoid high background currents) and
the amperometric signal consists of a first capacitive contribution
(due to conductivity change upon electrolyte injection) followed by a
faradic one (arising from ferrocene oxidation and charge neutralization
by the injected anion). At a bare electrode, one
observes only the
capacitive contribution. The~\mbox{amperometric} response is directly
proportional to the anion concentration (Figure~\ref{fig19}A, bottom),
which tends to level off at larger values because of saturation of the
ferrocene sites. The modified electrode can be used as a detector in
ion chromatography, enabling the detection of several anions at the
same time~\cite{412}. Of related interest is the indirect detection of
copper and ascorbic acid by exploiting a DNAzyme-based molecular
gate~\cite{413}. The strategy is the activation of the DNAzyme attached
on top of the mesoporous film by the analytes, resulting in a molecular
switching able to control the diffusion of a redox probe
($\mathrm{Fe}(\mathrm{CN})_{6}^{3-/4-}$) through the nanochannels and
their modulated detection on the underlying electrode surface (the
current drop being related to the analyte concentration).

A similar ``modulation'' principle is applied for the development of
geno-, apta- or \mbox{immunosensors,} \mbox{implying} the grafting of an aptamer or
an antibody onto the external surface of the mesoporous silica film and
using a redox probe in solution. Upon hybridization by the
complementary aptamer or antigen, the probe response is decreasing
proportionally to the analyte concentration (Figure~\ref{fig18}D). This
was applied to the detection of cancer~\cite{414,415} and prostate
specific~\cite{416,417,418} antigens, C-reactive protein~\cite{419},
DNA~\cite{420} or insulin~\cite{421} (Table~\ref{tab1}). The used
electrochemical technique was most often differential pulse voltammetry
but it could be also electrogenerated \mbox{chemiluminescence.} 
\mbox{Actually,} \mbox{mesoporous} \mbox{silica} films doped 
with $\mathrm{Ru}(\mathrm{bpy})_{3}^{2+}$
species are attractive platforms for amplified electrochemiluminescence
(ECL) sensing~\cite{310,422,423,424}, as illustrated for the flow
injection analysis of lidocaine in serum (Figure~\ref{fig18}C) for which
the small-diameter nanochannels also protected the ECL sensor from
biofouling~\cite{327}. The enhanced ECL signal is clearly due to the
$\mathrm{Ru}(\mathrm{bpy})_{3}^{2+}$ enrichment in the film attributed
to electrostatic attraction effects~\cite{423}. Improvement of
long-term operational stability can be achieved by operating in a
bilayer configuration, with a positively-charged mesoporous film
deposited on top of the negatively-charged one bearing
$\mathrm{Ru}(\mathrm{bpy})_{3}^{2+}$ species, in order to avoid their
leaching in the surrounding solution~\cite{415}.


\subsubsection{Pseudocapacitive hybrid films} 

As aforementioned, ferrocene-functionalized and oriented mesoporous
silica films are characterized by well-defined voltammetric response
(Figure~\ref{fig16}C) thanks to fast redox kinetics associated to the
electron-hopping mechanism~\cite{376}. As deposited on ITO electrode,
such functionalized films behave as transparent pseudocapacitive
materials. They give an almost linear potential response during the
galvanostatic charge--discharge processes at various current densities
($0.4\ndash 20~\mathrm{A}{\cdot}\mathrm{cm}^{-3}$), with an appreciable
delivered capacity of $1.10~\mathrm{mC}{\cdot}\mathrm{cm}^{-2}$ (at a
current density of $0.4~\mathrm{A}{\cdot}\mathrm{cm}^{-3}$) keeping 48\% of
its value at higher current density
($20~\mathrm{A}{\cdot}\mathrm{cm}^{-3}$)~\cite{425}. In order to increase the
loading amount of ferrocene groups per surface area, for the sake of
increasing capacity, similar films were generated by EASA on a
free-standing graphene foam prepared by electro-exfoliation of a
graphite electrode~\cite{309}, exhibiting also well-defined and
reproducible charge/discharge curves (Figure~\ref{fig19}C). Based on
these curves, the capacity can reach $196~\mathrm{mC}{\cdot}\mathrm{cm}^{-2}$
(or capacitance of $326~\mathrm{mF}{\cdot}\mathrm{cm}^{-2}$) at a current
density of $2~\mathrm{mA}{\cdot}\mathrm{cm}^{-2}$~\cite{309}, with a good
operational stability (no noticeable capacity loss after 10,000 cycles,
see Figure~\ref{fig19}C). They can be also manufactured in the form of
graphene-based flexible electrodes~\cite{426}.

Well-aligned 1D-polyaniline (PANI) nanofilaments as nanoarrays
generated by electropolymerization through silica nanochannels membrane
(Figure~\ref{fig17}D) exhibit improved electrochemical performance
compared to bulk PANI deposits~\cite{364,366}. They are promising for
application as \mbox{electrochemical} supercapacitors, reaching high specific
capacitance ($3.00~\mathrm{mF}{\cdot}\mathrm{cm}^{-2}$ at
$20~\rmmu\mathrm{A}{\cdot}\mathrm{cm}^{-2}$, i.e., an increase by
152\% in comparison to PANI directly deposited on ITO
substrate)~\cite{364}. Similar observations can be made with
polypyrrole (PPy) nanowire electrodeposits, for which specific
capacitance values 60 times higher than bulk PPy was
reported~\cite{362}, but the nanowires diameter was around 30~nm
suggesting that they probably grew also out of the mesoporous membrane.
Such nanocomposites based on conducting polymer 1D nanostructures can
be considered as prospective electrode materials for the development of
supercapacitors~\cite{427,428}.\looseness=1

\subsubsection{Electrochromic and multi-stimuli smart coatings}

As discussed in Section~\ref{sec4.3.1}, nanofilaments of conducting polymers
(i.e., PANI) can be generated by electropolymerization through
the mesoporous films and the resulting vertically aligned nanoarrays
can be freed from silicic matrix by dissolution while kept covalently
bonded to the transparent ITO electrode surface
(Figure~\ref{fig15})~\cite{365}. When free, such PANI nanowires exhibit
very efficient charge transfer reactions and very fast color change
upon reversible switching between the leucoemeraldine and emeraldine
forms (Figure~\ref{fig17}D), stable for hundreds of cycles, with
response time of {ca.}\  0.1~s~\cite{366}, which is among the
fastest one for electrochromic materials based on PANI
nanostructures~\cite{429}. Similar electrochromic devices based on
PEDOT nanowires grown through nanochannels on FTO electrode are
reported~\cite{430}.

Electrochromism can be also induced in the oriented mesoporous films by
covalent bonding of metal polypyridyl complexes onto the silica walls,
as illustrated for bis(terpyridine) iron(II) species
(Figure~\ref{fig19}D). Again, one can exploit the rather fast charge
transfer reactions via electron hopping to ensure color change by redox
switching~\cite{431}. The grafting of tris(bipyridine) iron(II)
complexes imparts to the hybrid film both electro- and photochemical
activity through the manipulation of the oxidation state and spin state
of the confined species, being promising for the development of
optoelectronic devices~\cite{432}. Even rather sensitive/reactive
moieties such as tetrazine derivatives can be successfully immobilized
in a stable form, while \mbox{keeping}
their \mbox{electrochemical} and fluorescence
properties, the intensity of which being proportional to the
functionalization level~\cite{372}.

\subsubsection{Miscellaneous}

In addition to the above uses, the permselective properties and
separation ability of mesoporous silica membranes with vertically
aligned nanochannels are basically attractive for other applications
such as gated molecular transport~\cite{62,375}, current
rectification~\cite{375,433}, ionic and molecular
separations~\cite{51}, electro-osmotic pumping~\cite{434}, drug
release~\cite{435}, thermoelectric measurements~\cite{436}, or
integration with microfluidics~\cite{437}. An extreme situation
concerns layers that completely block access to the underlying
substrate, such as that encountered for the electrodeposited
superhydrophobic coatings~\cite{438}. To achieve this goal, the
mesoporous silica film containing CTAB in the oriented channels is
grafted with long alkyl-chains (using dodecyltrimethoxysilane) on top
of the membrane~\cite{439}, sometimes with an additional corrosion
inhibitor (i.e., 1H-benzotriazole) dissolved in the surfactant
phase~\cite{440,441}. The resulting films offer good resistance in
various electrolyte solutions and their superhydrophobic properties are
kept when contacting common liquid products, as demonstrated by contact
angle observations (Figure~\ref{fig17}B). They also exhibit active
corrosion protection of metals and alloys~\cite{440,441}.\unskip\break
Of related
interest are the self-defensive antibacterial coatings bearing
antimicrobial agents in the mesopores and a molecularly engineered
functional assembly grafted on top of the film with pH/enzyme dual
stimuli-responsiveness and multiple release modes~\cite{442}.

Unlike blocking/protecting the electrode surface, the exploitation of
charge transfer through independent nanochannels in such insulating
membrane might be interesting in molecular electronics, optoelectronics
or super-dense magnetic memory storage devices~\cite{95}. This domain
is still in its infancy, but one can cite some pioneering works as
protected atomic switches based on single copper nanowires~\cite{351},
PANI nanofilaments-based molecular junctions in silica
nanochannels~\cite{380}, oriented films with anchored copper
phosphonate functional groups with nonlinear optical
properties~\cite{443}, or a dielectric silica layer acting as
artificial memristive\unskip\break 
synapse~\cite{444}.


\subsection{Non-silicate films generated by electro-assisted self-assembly}
\subsubsection{Oriented mesoporous oxides} 

The generation of lamellar mesostructures via electrochemical
interfacial surfactant templating has been largely developed for
ZnO~\cite{136,235,236,445,446,447,448} and extended to other metal
oxides or hydroxides such as
$\mathrm{Ni}(\mathrm{OH})_{2}$~\cite{238,449},
$\mathrm{Co}(\mathrm{OH})_{2}$~\cite{450,451},
$\mathrm{SnO}_{2}$~\cite{452}, $\mathrm{TiO}_{2}$~\cite{453} or
$\mathrm{MnO}_{2}$~\cite{237}. In those cases, anionic surfactants were
mainly used, often with the addition of co-solvents, co-surfactants and
cationic ions, the electrodeposition being achievable in both
potentiostatic and galvanostatic modes, as well as in cathodic and
anodic conditions~\cite{445}.

The electrodeposition mechanisms have been largely discussed for the
production of lamellar structured ZnO films 
\cite{235,236,445,446,447,448}. It was first demonstrated that
reduction of nitrate species in a slightly acidic zinc nitrate solution
containing dilute (${<}$CMC) sodium dodecyl sulfate (SDS) led to a pH
increase at the electrode surface, inducing thereby the precipitation
of ZnO in a lamellar structure due to the bilayer arrangement of the
SDS surfactant molecules~\cite{235,445}. The driving force of the
ordering is the electrostatic interactions between the
$\mathrm{Zn}^{2+}$ (or hydrated
$[\mathrm{Zn}(\mathrm{H}_{2}\mathrm{O})_{4}]^{2+}$) cations and the
negatively charged SDS self-assembled onto the electrode surface.
Contrary to the silica-based films, the applied potential needed to be
less negative (i.e., ${-}0.5$~V vs. Ag/AgCl) to get the ordered
mesoporosity and avoid metallic zinc deposition. The morphology of the
films was different (Figure~\ref{fig20}A$_{1}$), the mesostructure was
lamellar (Figure~\ref{fig20}A$_{2}$) instead of hexagonal, and the
repeat distance was function of the surfactant concentration or type
(Figure~\ref{fig20}A$_{3}$) as a result of different packing during the
self-assembly growth~\cite{235}. No ordered film can be produced when
using CTAB (instead of SDS) because the positively charged head group
cannot coordinate zinc cations~\cite{445}. Depending on the surfactant
concentration and applied potential, the formation of a nanoparticulate
ZnO phase can be observed~\cite{446,448}. The electrodeposition can be
also performed in the anodic direction. In this case, the starting zinc
solution is prepared at high pH (i.e., 13.3) where
$\mathrm{Zn}^{\mathrm{II}}$ species are in the form of anions
($[\mathrm{Zn}(\mathrm{OH})_{3}]^{-}$ or
$[\mathrm{Zn}(\mathrm{OH})_{4}]^{2-})$ and pH is decreased at the
electrode surface by oxidizing ascorbate species added to the medium,
leading to the precipitation of ZnO. Surprisingly, the lamellar
structures were obtained only with anionic surfactants (like SDS or
lauric acid, this time at concentrations above CMC), and not with the
positively charged ones (as CTAB)~\cite{236}. This indicates that the
direct electrostatic interactions between the surfactant and the
inorganic precursors are not the main factor affecting the
self-assembly process (it is thought that the electrolyte cations play
a role in the process)~\cite{236}. The morphology of the deposits is
still in the form of ZnO plates (Figure~\ref{fig20}B$_{1}$) with some
oriented mesopores (Figure~\ref{fig20}B$_{2}$) and a lamellar organization
depending on the type of anionic surfactant (with more vertical order
with SDS than with lauric acid, compare parts $\mathrm{B}_{3}$ and
$\mathrm{B}_{4}$ in Figure~\ref{fig20}). Adding alkylammonium to the
electrodeposition medium further improved the quality of the mesoporous
ordering (Figure~\ref{fig20}B$_{5}$--B$_{8}$), probably due to the
reduction of electrostatic repulsions affecting the ordering and domain
size of the bilayer\unskip\break 
assemblies~\cite{445}.

\begin{figure*}
\includegraphics{fig20}
\caption{\label{fig20}(A) Cathodically electrodeposited ZnO
films. (A$_{1}$) SEM image (scale bar: 1~${\rmmu}$m)
and (A$_{2}$) TEM view of lamellar-structured ZnO films
deposited (at ${-}$0.5~V for 10 min at 70~\textdegree C) from 0.02 M
Zn(NO$_{3}$)$_{2}$ solutions containing either
0.25 wt\% of SDS (A$_{1}$) or 0.25 wt\% of AOT
(A$_{2}$) anionic surfactants. (A$_{3}$)
Small-angle XRD patterns of ZnO films deposited from 0.02 M
Zn(NO$_{3}$)$_{2}$ solutions containing (a) 0.02
wt\% of SHDS, (b) 0.02 wt\% of SDBS, (c) 0.25 wt\% of DP, (d) 0.25
wt\% of AOT, and (e) 0.25 wt\% of SDS, with possible variation of SDS
lamellar assemblies that can lead to different repeat distances on the
right. {Reprinted from Tan \etal \cite{235}, 
Copyright (2005)} {American Chemical Society.} (B)
Anodically electrodeposited ZnO films. (B$_{1}$) SEM image
and (B$_{2}$) TEM view of lamellar-structured ZnO films
deposited (at ${+}$0.15~V for 30 min at 65~\textdegree C) from solutions
containing 0.02 M ascorbic acid and 0.01 M
Zn(NO$_{3}$)$_{2}$ (at a pH of 13.3) and 5 wt\%
of lauric acid surfactant. (B$_{3}$, B$_{4}$) 2D
gi-SAXS of ZnO films deposited (B$_{3}$) with 10~wt\% of
SDS and (B$_{4}$) with 5 wt\% of lauric acid.
(B$_{5}$--B$_{8}$) SEM (B$_{5}$,
B$_{6}$) and TEM (B$_{7}$, B$_{8}$)
images of ZnO films templated by (B$_{5}$,
B$_{7}$) 5 wt\% of SDS and 1 wt\% of CTAB or
(B$_{6}$, B$_{8}$) 5 wt\% of SDS and 1 wt\% of
TAB. {Reprinted from} {{Steinmiller and Choi}} \cite{236},
Copyright (2007) {American Chemical Society.} 
(C)~Lamellar hybrid nanostructures electrodeposited with the aid of
conjugated surfactant molecules. (C$_{1}$)~Morphology: 
(a,b)~SEM images (top (a) and cross-sectional (b) views) of deposited ZnO
platelets using PyBA (magnified in the inset for image (a)); (c) TEM
image of lamellar sheets within platelets, magnified in the inset; 
(d)~schematic diagram of lamellar ordering composed of inorganic Zn-rich
regions and bilayers of PyBA surfactant. {Reproduced with
permission from Sofos \etal \cite{454}, Copyright
(2009) Nature Publishing Group.} (C$_{2}$) Structures of the
terthiophene-based surfactants (with systematic modifications of their
geometry, valency, and flexibility) used for the cathodic
electrodeposition of lamellar ZnO films on ITO and PEDOT:PSS-coated
electrodes: TAA(T)$_{2}$ and TBA(T)$_{2}$ do not
lead to ZnO film growth, 3TCA grows isotropic lamellae on both
substrates, lamellar coverage from 3TDCA deposition are too weak for a
2D gi-SAXS signal on ITO and isotropic on PEDOT:PSS, 3TBA directs
parallel growth on ITO and perpendicular growth on PEDOT:PSS, and 3TDBA
deposits lamellae that are preferentially parallel on ITO and isotropic
on PEDOT:PSS. {Reprinted from Bruns \etal \cite{456}, 
Copyright (2013) American Chemical Society}.}
\end{figure*}

Other examples of combining self-assembly and electrodeposition in a
single step process are available, notably based on families of
surfactants constituted of one or two carboxylate head(s) and
conjugated segments/linkers such as pyrene or terthiophene
derivatives~\cite{454,455,456}. For instance, lamellar hybrid films
alternating sheets of ZnO and 1-pyrenebutyric acid (PyBA) can be
obtained by electro-assisted precipitation of self-assembled
$\mathrm{Zn}(\mathrm{OH})_{2}/\mathrm{PyBA}$ structures on ITO
electrode thanks to favorable zinc-carboxylate interactions and
$\rmpi$--$\rmpi$ binding of pyrene moieties, and further annealing at
150~\textdegree C to transform $\mathrm{Zn}(\mathrm{OH})_{2}$ into
ZnO~\cite{454}. The morphology is very similar to that obtained for
SDS/ZnO films, with near-vertical randomly oriented sheets revealing a
lamellar mesostructure having a periodicity of 3.2~nm, with 0.7--1.0~nm
thick inorganic walls separated by organic layers of 2.4~nm in
thickness (corresponding to PyBA bilayer), as illustrated in part
$\mathrm{C}_{1}$ of Figure~\ref{fig20}. Adjusting the PyBA
concentration in the electrosynthesis medium enables to control between
bloomlike and flakelike growth~\cite{455}. The vertical orientation is
better achieved when covering the ITO surface with
poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS)
defining both hydrophobic and hydrophilic regions. In addition to the
substrate, the orientation and long-range alignment of hybrid lamellae
are also affected by the molecular structure of the conjugated
template, as illustrated for terthiophene-based surfactants
(Figure~\ref{fig20}C$_{2}$). Monovalent and/or flexible surfactants
gave rise to more anisotropic nanostructures with higher densities,
longer persistence lengths, and lower polydispersity, compared to the
divalent and/or rigid ones, whereas no ordering at all can be observed
with T-shape surfactants~\cite{456}. Such hybrid films combining
macroscopically oriented $n$-type semiconductor (ZnO) and $p$-type
semiconducting supramolecular phases (conjugated surfactants) are
promising for both photoconductivity and photovoltaic
efficiency~\cite{454,456,457}.

Contrary to the oriented mesoporous silica films for which the porosity
can be revealed by template extraction, no surfactant removal was
reported for the lamellar mesoporous oxides and hydroxides, most
probably because their structure cannot be maintained and it would
collapse upon removing the organic phase. Nevertheless, besides the
above photoconductors alternating $n$-type and $p$-type domains, other
lamellar organic--inorganic hybrids started to find some applications.
The ZnO/SDS films exhibit good optical properties, better than bulk ZnO
electrodeposited in the absence of surfactant~\cite{447}. They can be
loaded with dyes and used as thin films in dye-sensitized solar cell
approaching a power conversion efficiency of 2\%  \cite{458,459}. Their
photocatalytic properties were also exploited for methylene blue
photodegradation~\cite{460}. Finally, such electrogenerated mesoporous
hybrid films are promising for pseudo- or supercapacitor electrodes, as
pointed out with Ni-doped $\mathrm{Co}(\mathrm{OH})_{2}/\mathrm{ITO}$
nanowires or mesoporous $\mathrm{Mn}_{2}\mathrm{O}_{3}$
film~\cite{461,462}.

\subsubsection{Other mesoporous films electrogenerated in the presence of surfactants}

If we leave aside the ordered mesoporous metals or alloys, yet at the
origin of the elaboration of oriented mesoporous films by
electrochemical interfacial surfactant templating \cite{233,234}, but
implying direct metal electrodeposition (different from the concept of
indirect electrogeneration of thin films as discussed in this account)
and being the subject of existing well-documented
reviews~\cite{463,464,465}, there are almost no examples of other
mesoporous films electrogenerated in the presence of
structure-directing agents. The closest approach is \mbox{probably} the
formation of chitosan-based composite films by combining the
precipitation of chitosan by electro-induced pH change (easily
achievable from an acidic solution at reducing potentials~\cite{466})
with the layer-by-layer assembly technique (as shown for
alginate--chitosan multilayers~\cite{467}) or with other co-deposition
or assembly methods~\cite{468}. On the other hand, it is surprising
that the electropolymerization of conducting polymers has still not
been combined with the electrochemical interfacial surfactant
templating, especially regarding the need for conducting polymer
nanostructures for various applications~\cite{469,470,471} and the
existing soft template-assisted (yet non-electrochemical)
polymerization methods~\cite{472}. An exception is the electrochemical
polymerization performed around the walls of ``soap bubble'' templates
(but the resulting porous conducting polymers were not
ordered)~\cite{473}. Perhaps one can cite the electrodeposition of
enzymes-integrated mesoporous composite films by interfacial
templating~\cite{474}, but they are actually made of amphiphilic
phospholipid-templated mesoporous ZnO layers covalently inlaid with the
proteins (thus being somehow related to the family of mesoporous oxides
presented above). Finally, vertically aligned mesoporous carbon thin
films have
been also\unskip\break developed from surfactant template assemblies on
silver electrodes~\cite{475}, but they were not electrochemically
produced.

\section{Conclusions}

The electrochemically assisted sol--gel deposition of
non-electroactive and non-conductive thin films is a counterintuitive
approach overcoming some limitations of the conventional
electrodeposition techniques. Uniform silica sol--gel thin films can
be produced in a controlled way by electrochemically generated
interfacial pH change onto various electrode surfaces, including of
irregular topography and heterogeneous nature, as well as at the local
scale, or in the form of composites by encapsulation of nano-objects.
Using additional organosilanes in the electrosynthesis medium enables
to get organic--inorganic hybrid films. The method is compatible with
the entrapment of biomolecules without preventing their biological
activity, with promising applications in the field of
bioelectrocatalytic reactors. Sol--gel electrogeneration in the presence
of \mbox{surfactant} template led to the growth of vertically-aligned
mesoporous silica nanochannel films via electro-assisted self-assembly.
These films exhibit size and charge selectivity at the molecular level,
which can be exploited notably for electrochemical sensing
(preconcentration electroanalysis, permselective coatings). The
oriented nanochannel membranes can used to electropolymerize conducting
polymer nanofilaments, durably attached to the electrode surface thanks
to electrografting, and exhibiting attractive properties for
applications (electrochromism, pseudocapacitors). Combining the
electro-assisted self-assembly to click chemistry offers a versatile
way to oriented mesoporous films bearing a variety of organo-functional
groups that are accessible to external reagents thanks to the vertical
alignment of nanochannels of monodisperse diameter. The incorporated
redox moieties remain electrochemically accessible, enabling long-range
electron transfer, with applications in electroanalysis,
electrocatalysis, or electrochemical energy storage. Now that the
concepts have been established and considering the numerous
possibilities of functionalization, no doubt that further innovative
applications will emerge, opening new challenges and prospects in such
burgeoning field.

\section*{Conflicts of interest}
The author has no conflict of interest to declare.

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