Vertically aligned mesoporous silica films bearing covalently bound tris(2,2-bipyridyl)ruthenium(II) redox photocatalysts

Mahmoud Rahal, Cheryl Karman, Neus Vilà, Christelle Despas, Mariela

Alicia Brites Helu, Jaâfar Ghanbaja, Malik Sebbat, Akshay Silswal,

Gediminas Jonusauskas, Nathan Mcclenaghan, et al.

<figure><img src="image_1.png" /></figure>

To cite this version:

Mahmoud Rahal, Cheryl Karman, Neus Vilà, Christelle Despas, Mariela Alicia Brites Helu, et al. Vertically aligned mesoporous silica films bearing covalently bound tris(2,2-bipyridyl)ruthenium(II) redox photocatalysts. Microporous and Mesoporous Materials, 2025, 387, pp.113524. 10.1016/j.micromeso.2025.113524. hal-04914351

HAL Id: hal-04914351

https://uha.hal.science/hal-04914351v1

Submitted on 27 Jan 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Vertically aligned mesoporous silica films bearing covalently bound tris(2,2'-bipyridyl)ruthenium(II) redox photocatalysts

Mahmoud Rahal,ª Cheryl Karman,ª Neus Vilà,ª Christelle Despas,ª Mariela Alicia

Brites Helu,ª Jaafar Ghanbaja,ᵇ Malik Sebbat,ᶜ Akshay Silswal,ᵈ Gediminas

Jonusauskas,ᵉ Nathan McClenaghan,ᵈ Emmanuel Oheix,ᵉ,ᵉ Bénédicte Lebeau,ᵉ,ᵉ

Morgan Cormier,ᵉ Jean-Philippe Goddard,ᵉ,\* Alain Walcariusª,\*

<footnote>ª Université de Lorraine, CNRS, Laboratoire de Chimie Physique et Microbiologie pour les Matériaux et l'Environnement (LCPME), Nancy F-54000, France</footnote>

<footnote>ᵇ Université de Lorraine, CNRS, Institut Jean Lamour (IJL), Nancy F-54000, France</footnote>

<footnote>ᶜ Laboratoire d'Innovation Moléculaire et Applications (LIMA), UMR 7042, Université de Haute-Alsace (UHA), Université de Strasbourg, CNRS, 68100 Mulhouse (France)</footnote>

<footnote>ᵈ Université de Bordeaux, CNRS, Institut des Sciences des Moléculaires (ISM) UMR 5255, 33405 Talence, France</footnote>

<footnote>ᵉ Université de Bordeaux, CNRS, Laboratoire Ondes et Matière d'Aquitaine (LOMA) UMR 5798, 33405 Talence, France</footnote>

<footnote>f Université de Haute-Alsace (UHA), CNRS, IS2M UMR 7361, 68100 Mulhouse, France</footnote>

<footnote>g Université de Strasbourg, 67000 Strasbourg, France</footnote>

<footnote>* Corresponding authors: Alain Walcarius, alain.walcarius@univ-lorraine.fr, LCPME, 405 rue de Vandoeuvre, 54600 Villers-les-Nancy, France; Jean-Philippe Goddard, jean-philippe.goddard@uha.fr, LIMA, 2 rue des Frères Lumière, 68093 Mulhouse, France.</footnote>
**Abstract**

This work reports a strategy to prepare mesoporous silica films with preferential vertical alignment of pore channels with covalently attached tris(2,2'-bipyridyl)ruthenium(II) redox photocatalysts, i.e. [Ru(bpy)₂(bpy')]²⁺ (prepared from a precursor grafting agent containing 2,2'-bipyridine (bpy) and derivatized 4,4'-dimethyl-2,2'-bipyridine (bpy') units). This was achieved by the electrochemically assisted self-assembly of vertically-aligned mesoporous silica films containing varying amounts of azide groups and their subsequent derivatization by click coupling with an alkyne-bearing [Ru(bpy)₂(bpy')]²⁺ reagent. The effectiveness of the functionalization processes was monitored by infrared spectroscopy, indicating a best compromise reached for an azide content large enough to immobilize a high content of [Ru(bpy)₂(bpy')]²⁺ moieties, but not too large to avoid steric constraints and leading to lower yields of the click reaction. The structural integrity (hexagonal order and mesopore orientation) after functionalization was checked by transmission electron microscopy and grazing-incidence small-angle X-ray scattering. The films were generated on transparent indium-tin oxide electrodes and cyclic voltammetry experiments demonstrated a good electrochemical response via electron hopping between adjacent [Ru(bpy)₂(bpy')]²⁺ moieties immobilized in the nanochannels, which was dependent on the functionalization level and proved stable upon repetitive potential cycling as a result of their covalent attachment to the silica walls. These films also exhibit luminescence, which can be exploited for photoelectrochemical applications, e.g., detection by electrochemiluminescence or electrochemical regeneration of photocatalyst, as illustrated here from preliminary experiments.

**Keywords**

Electrochemically assisted self-assembly (EASA); mesoporous silica thin films; vertical orientation; ruthenium tris(bipyridine); photocatalyst; electrochemical regeneration.
# 1. Introduction

Ordered mesoporous silica films exhibiting a widely open structure are attractive for applications in several fields [1]. They are typically prepared by soft-templating approaches using surfactant assemblies or block copolymers as structure-directing agents [2]. So far, the most versatile method to deposit lyotropic liquid crystalline mesophases onto solid support surfaces is via the so-called evaporation induced self-assembly (EISA) process, involving the controlled evaporation of a sol solution containing the silica precursors and soft templates by dip-coating [2,3]. Although operational, the EISA method suffers from some limitations, especially when attempting to orient the mesopore channels vertically relative to the support and align them parallel to each other (i.e., an ideal configuration to ensure fast transport properties), so that alternative methodologies have appeared recently [4]. Apart from rather sporadically reported approaches such as field-induced alignment controls [5], epitaxial growth [6], air flow control [7], substrate pre-modification [8] or oil-induced co-assembly [9], the most widely used methods to generate vertically-oriented mesoporous silica films (VMSF) are: (1) the electrochemically assisted self-assembly (EASA) [10,11] and (2) the Stöber-solution growth [12]. The so-obtained VMSF exhibit permselective properties at the molecular level [13,14], which are attractive for applications particularly in selective detection and separation [14-18].

EASA offers access to highly ordered and oriented films on electrodes, in a way reported to be faster and easier compared to other methods [19,20], via the *in situ* combination of electrochemically-triggered pH-induced polycondensation of the silica precursors and electrochemical interfacial surfactant templating [13]. The method is compatible with functionalization with organic groups covalently attached to the silica walls, basically by post-synthesis grafting [21,22] or co-condensation [22-25] using organosilane reagents. These approaches are however restricted to rather ‘simple’ functions and in limited amounts to keep the quality of the oriented nanostructure [23] or to avoid pore blocking [26]. This is why an alternative functionalization route has been proposed, based on the EASA synthesis of azide functionalized VMSF followed by click coupling via the copper catalyzed Azide-Alkyne Cycloaddition (CuAAC) [26]. Two different organo-functional groups can be also selectively anchored to silica walls by using bi-functionalized VMSF (e.g., containing thiol and azide groups) and combining two click reactions (e.g., thiol-ene and azide-alkyne) [27]. For instance, redox-active functions such as metallocenes [26-28], tetrazine derivatives [29] or polypyridyl metal complexes [30,31], have been successfully immobilized into the oriented nanochannels
by covalent binding to the silica walls. Once attached to VMSF, these moieties kept their electrochemical activity [26-31], their spectroscopic or photochemical features [29,31] and their electrochromic properties [30]. In spite of the insulating nature of the silica matrix, long-range (~100 nm) charge transfer phenomena have been demonstrated [32], the charge transfer mechanism being identified as an electron hopping between adjacent redox functions [33], as common for static assemblies of redox centers or in redox polymers [34-36].

Ruthenium(II) polypyridine complexes serve in one-electron transfer reactions (photoredox), in energy transfer reactions (¹O₂ generation, for instance), or even as two-photon absorbers and sensitizers [37-40]. In particular, tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)₃]²⁺) is attractive for preparative organic chemistry by exploiting its ability to absorb light in the visible domain, generating a high energy excited state ([Ru(bpy)₃]²⁺*) likely to be involved in a wide range of photooxidation or photoreduction processes [37,41,42]. [Ru(bpy)₃]²⁺ is electroactive and the [Ru(bpy)₃]³⁺/²⁺ redox couple is known to be a reversible electrochemical system with a standard potential equal to +1.26 V (in 1 M H₂SO₄) [43,44], this value being likely to be affected to some degree by the medium [45,46]. So far, [Ru(bpy)₃]²⁺ has been investigated at VMSF electrode only as a weakly bound species (i.e., by soaking the film electrode into a [Ru(bpy)₃]²⁺ solution) [47-51], by exploiting the ability of positively-charged species to accumulate into the film thanks to favourable electrostatic interactions with the negatively-charged silica surface [52]. Although they found interest for applications in electrochemiluminescence sensing [47-49], immunosensing [50] or electrocatalysis [51], the long-term operational stability might be restricted by the desorption of such weakly-bound species. For instance, [Ru(bpy)₃]²⁺-loaded mesoporous silica retained the probe inside the mesopore channels when appropriately capped with an organic network blocking the outlet of the pores, but [Ru(bpy)₃]²⁺ species were released out of the material when unlocking the gate by opening/breaking the organic network [53-55]. In view of what has been done for the functionalization of other substrates by Ru(II) polypyridine complexes [56,57], it is thus necessary to find a way to immobilize such photoredox catalysts more durably to the VMSF, namely via a covalent bond, if we wish to avoid its leaching from the film during its use.

In this work, we report the preparation of VMSF bearing covalently bound tris(2,2'-bipyridyl)ruthenium(II) redox photocatalysts (actually in the form of [Ru(bpy)₂(bpy')]²⁺, with bpy = 2,2'-bipyridine and bpy' = 4-[(2-propynyloxy)methyl]-4'-methyl-2,2'-bipyridine), by sequential EASA generation of azide-functionalized VMSF and subsequent click coupling by CuAAC with an alkyne-bearing [Ru(bpy)₂(bpy')]²⁺ reagent. The obtained films are
characterized by spectroscopic and electrochemical techniques, notably with respect to the
functionalization level, and preliminary data are given on their potential luminescence and
photocatalytic properties.

**2. Material and methods**

**2.1. Reagents**

The precursor 3-azidopropyltriethoxysilane (AzPTES) was prepared according to published procedures [26,58] using 3-chloropropyltriethoxysilane (CIPTES, 97%, Sigma Aldrich) and sodium azide (NaN₃, >99%, Labosi) as reagents in dry acetonitrile (ACN, 99.9%, Sigma-Aldrich) medium containing tetrabutylammonium bromide (NBu₄Br, 99%, Sigma-Aldrich). Detail of AzPTES synthesis is given in Supplementary Material (section 1.1 and Fig. S1). The following chemicals were used to synthesize the azide-functionalized VMSF materials: tetraethoxysilane (TEOS, 98%, Thermo Scientific) and the above AzPTES molecule as precursors, cetyltrimethylammonium bromide (CTAB, 99%, Sigma Aldrich) as surfactant, ethanol (95-96%, Merck) and high-purity water (18.2 MΩ cm, obtained from a Purelab Option-Q from ELGA) as solvents, sodium nitrate (NaNO₃, 99%, Fluka) as electrolyte, and HCl (37%, Riedel de Haen) to adjust pH of the sol solution. All these reagents were used as received. The two reagents used to prepare the alkyne-functionalized [Ru(bpy)₂(bpy')]Cl₂ complex (intended to derivatize the azide-functionalized VMSF) were respectively 4-[(2-propynyloxy)methyl]-4'-methyl-2,2'-bipyridine (synthesized similarly as previously reported procedures [59-61]) and *cis*-dichlorobis(2,2'-bipyridine)ruthenium(II) hydrate (*cis*-[Ru(bpy)₂]Cl₂·xH₂O, 97%, Sigma Aldrich). The click coupling also involved the use of additional chemicals, such as copper(II) acetate (Cu(CH₃COO)₂·H₂O, 98%, Sigma Aldrich), ascorbic acid (100%, Prolabo), dimethylformamide (DMF, 99.8%, Sigma Aldrich) and sodium diethylthiocarbamate trihydrate ((C₂H₅)₂NCSSNa·3H₂O, 99%, Sigma Aldrich). The redox probes used for the film permeability measurements were ferrocene dimethanol (Fc(MeOH)₂, 97%, Sigma Aldrich) and hexaammineruthenium(III) chloride (Ru(NH₃)₆Cl₃, 98%, Sigma Aldrich). The solutions used for other electrochemical experiments (characterization of [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF) were composed of acetonitrile solvent with tetrabutylammonium perchlorate (TBAClO₄, 99.9%, Merck) as supporting electrolyte.
## 2.2. Preparation of the films onto indium-tin oxide (ITO) electrode

Figure 1 illustrates the overall film preparation pathway. It first involved the generation of azide-functionalized VMSF by EASA (Fig. 1A) using a procedure slightly adapted from [26,31]. This was achieved by applying a suitable cathodic potential (i.e., -1.3 V for 20 s) to an ITO electrode immersed in a hydrolyzed sol solution (200 mM [TEOS+AzPTES] and 64 mM CTAB) in order to induce the condensation of the precursors by electrochemically-induced pH change at the working electrode surface around CTAB assemblies via electrochemical interfacial surfactant templating (Fig. 1A₁). The sol solution was prepared in hydroalcoholic medium (1:1, water:ethanol, v/v) containing the precursors at various molar ratios (from 2.5/97.5 to 40/60 AzPTES/TEOS) and 0.1 M $NaNO_3$ as supporting electrolyte, which was adjusted to pH 3 by adding HCl and aged for 2.5 h prior to use for film growth by EASA (Fig. 1A₂). After film formation, the electrode was rinsed with water and kept overnight in an oven at 130°C. The CTAB template was removed from the mesoporous silica films by chemical extraction using an ethanol solution containing 0.1 M HCl for 15 min, followed by rinsing with distilled water and ethanol. The obtained azide-functionalized films were then derivatized according to a subsequent click coupling by CuAAC with an alkyne-bearing $[Ru(bpy)_2(bpy')]^2+$, in a quite similar way as applied for mesoporous silica particles [61]. Briefly, the alkyne-bearing $[Ru(bpy)_2(bpy')]^2+$ compound was synthesized by allowing to react equimolar amounts (typically 0.2 mmol in 20 mL solution) of 4-[(2-propynyloxy)methyl]-4'-methyl-2,2'-bipyridine (bpy', see detail of its synthesis in section 1.2 and Figs. S2-S5 in Supplementary Material) and $Ru(bpy)_2Cl_2$ in hydroalcoholic medium (water:ethanol, 3:7, v/v) under reflux for 16 h (Fig. 1B), following the procedure described in [59]. The resulting product was recovered as a solid after precipitation using 1 mL of a solution of ammonium hexafluorophosphate. In a final step (Fig. 1C), $[Ru(bpy)_2(bpy')]^2+$ was attached to azide-functionalized VMSF by dipping the electrode in a 20 mL solution of DMF/H₂O (7:3, v/v) containing the alkyne-bearing $[Ru(bpy)_2(bpy')]^2+$ compound (20 mg), along with $Cu(CO_2CH_3)_2·H_2O$ (14 mg) and ascorbic acid (70 mg), which was allowed to react for 36 h in the dark. The films were then successively rinsed with DMF, 0.1 M of sodium diethyldithiocarbamate in ethanol, and acetonitrile (for 10 min each) in order to remove the possible unreacted species and remaining traces of the catalyst.

## 2.3. Apparatus and characterization procedures

VMSF preparation by EASA and electrochemical characterization of the obtained films were carried out using the model PGSTAT 101 A AutoLab potentiostat monitored by the
NOVA software (Metrohm), with electrochemical cells in a three-electrode configuration (bare and film-modified ITO working electrodes, Ag/AgCl reference electrode (Metrohm) and platinum rod counter electrode). For EASA, they were arranged by placing the ITO electrode at the bottom of a home-made one-compartment cell (see supplementary material in Ref. [32]), whereas a standard three-electrode configuration was used for the electrochemical characterization. Permeability measurements were performed by cyclic voltammetry (CV) in aqueous solutions containing either 0.5 mM Fc(MeOH)₂ or 0.5m M Ru(NH₃)₆³⁺ as redox probes (+0.1 M NaNO₃ as supporting electrolyte). [Ru(bpy)₂(bpy')²]²⁺-functionalized VMSFs were also characterized by CV, in acetonitrile (+0.1 M TBACIO₄) medium.

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>
**Figure 1.** (A) Scheme illustrating the EASA process to generate VMSF from a sol solution containing silica precursors and surfactant molecules by applying an electric field between a working electrode (W.E.) and a counter electrode (C.E.): (A₁) general case; (A₂) particular case of azide-functionalized VMSF. (B) Reaction performed to get the alkyne-bearing [Ru(bpy)₂(bpy')]<sup>2+</sup> compound used here, and (C) its click coupling with the azide-functionalized VMSF material.

Other physico-chemical techniques were used to characterize the functionalized films. The mesostructure of the film after functionalization was checked by transmission electron microscopy (TEM) using a Philips CM20 microscope at an accelerated voltage of 200 kV. It was also analysed by grazing-incidence small-angle X-ray scattering (GISAXS), by collecting data on a “SAXSess mc²” instrument (Anton Paar) equipped with Cu-Kα source (λ = 0.1542 nm) operating at 40 kV and 50 mA and a CCD detector (Princeton Instruments). A Nicolet 8700 apparatus equipped with a diffuse reflectance accessory was used to carry out Fourier transform infrared spectroscopy (FTIR) measurements. The optical absorption spectra were obtained using a Cary 60 UV-Vis spectrophotometer with the aid or the Cary WinUV-Scan software in a range of wavelengths between 300 and 800 nm. Blank measurements were used to compensate the background absorption and were recorded using bare ITO substrates. Fluorescence and electrochemiluminescence measurements were carried out using JASCO FP-8200 and SAFAS Xenius XM instruments, respectively. For photoelectrochemical experiments, the photocurrents were measured at constant potential (typically -0.1 V vs. Ag/AgCl) while illuminating the [Ru(bpy)₂(bpy')]<sup>2+</sup>-functionalized VMSF at 400 nm using the Lightning Cure LC8 Spot Light-source apparatus (Hamamatsu) at a power of 4.5 W. Measurements were carried out using a transparent electrochemical cell with the film-modified ITO working electrode positioned orthogonally to the light source, at a distance of 5 cm, in an aqueous solutions containing the supporting electrolyte (0.1 M NaNO₃) and the photosensitizer, (methylviologen, MV²⁺, at various concentrations, from 1 to 20 mM). Finally, ¹H NMR spectra of the various precursor intermediates (described in section 1 in Supplementary Material) were recorded in CDCl₃ in order to check their chemical structure and purity, using the Bruker Avance III apparatus operating at 400 MHz.
# 3. Results and discussion

## 3.1. Preparation of azide-functionalized films onto ITO electrode

Figure 2 exhibits typical FTIR data obtained for the azide-functionalized VMSF materials. Compared to a non-functionalized film (curve (a) in Fig. 2A), the azide-containing film (curve (b) in Fig. 2A) is characterized by an additional vibration at 2105 cm⁻¹ corresponding to the stretching of azide groups [62,63]. All other vibrations are located at the same wavelengths for both films, corresponding to either the silica framework (i.e., 801 cm⁻¹ and 1075 cm⁻¹ with a shoulder at 1220 cm⁻¹ for the Si-O-Si stretching modes, and 964 cm⁻¹ for Si-OH silica [64]) or the CTA⁺ template (i.e., 2921 cm⁻¹ and 2850 cm⁻¹ for the asymmetric and symmetric C-H stretching modes [65,66] and 1463 cm⁻¹ for C-H scissoring vibration [67]). After washing the azide-functionalized VMSF in ethanol/HCl, successful removal of the CTA⁺ template is confirmed by the disappearance of its characteristic bands at 2921 cm⁻¹ and 2850 cm⁻¹ (consistent with previous observation [68]) while keeping intact the signal of azide groups (curve (c) in Fig. 2A). The presence of propyl groups acting as arms between the azide groups and the silica surface is even visible by weak absorptions at 2921 cm⁻¹ and 2850 cm⁻¹ corresponding to C-H stretching of CH₂ groups [65] (see insert in Fig. 2A). The small absorption band at 1625 cm⁻¹ is due to adsorbed water molecules on the silica surface [67]. As expected, increasing the AzPTES/TEOS ratio in the synthesis medium led to the formation of VMSF containing more and more azide groups (as it can be seen via the evolution of the intensity of the signal at 2105 cm⁻¹, see Fig. 2B) and it is now well-established that larger azide content does not affect the structural integrity of the oriented mesoporous film. Indeed, it is known that azide-functionalized mesoporous silica films prepared by EASA, as used here, are made of vertically-aligned hexagonally-packed mesopore channels that are highly ordered over the whole underlying electrode support up to 40% AzPTES [26], so no additional structural characterization is needed at this stage (we will provide such investigation for the [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF to confirm the integrity of the oriented mesostructure, see section 3.2).
<figure><img src="image_7.png" /><figcaption>**Figure 2.** (A) Normalized (ν = 1075 cm⁻¹) FTIR spectra of (a) an as-synthesized non-functionalized mesoporous silica film, and (b,c) an azide-functionalized mesoporous silica film respectively (b) before and (c) after surfactant removal. (B) Magnification of the azide stretching vibration region for azide-functionalized VMSF materials prepared from various AzPTES/TEOS ratios: (1) 2.5/97.5, (2) 5/95, (3) 10/90, (4) 15/85, (5) 20/80, and (6) 25/75.</figcaption></figure>

Figure 3 depicts cyclic voltammograms obtained for the azide-functionalized films on an ITO electrode using two redox probes, i.e., neutral $Fc(MeOH)_2$ (see Fig. 3A,C) and positively charged $Ru(NH_3)_6^{3+}$ (see Fig. 3B,D), which are useful to characterize the film permeability [69]. Before surfactant extraction, no noticeable response of $Ru(NH_3)_6^{3+}$ was observed (Fig. 3B) because this probe is not able to cross the hydrophobic CTA⁺-filled mesochannels, confirming a defect-free film over the whole electrode surface. In contrast, $Fc(MeOH)_2$ is likely to dissolve into the surfactant phase and reach the electrode surface, giving a signal typically shifted to the anodic potentials (Fig. 3A) [10]. After template removal, both redox probes gave rise to well-defined and intense peak currents (see green curves on Fig. 3A,B) indicating fast diffusion through the surfactant-free vertical nanochannels. These signals were almost identical for $Fc(MeOH)_2$ independently on the functionalization level (only a slight decrease was observed for the sample prepared from 25/75 AzPTES/TEOS ratio, see Fig. 3C), demonstrating an easy access through the mesopores even at quite high azidopropyl group contents. For the larger and positively charged $Ru(NH_3)_6^{3+}$ probe, the decrease in peak intensity was a bit more pronounced, which is indicative of some diffusional limitations, but still peak currents decreased by less than 50% in the worst case (25/75 AzPTES/TEOS ratio, see Fig. 3D). The above results thus suggest that azide groups in all these films should be accessible for click coupling and further derivatization with $[Ru(bpy)_2(bpy')]^2+$ moieties as schematically illustrated in Figure 1.
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

**Figure 3.** Cyclic voltammograms recorded in 0.5 mM solutions of (A) $Fc(MeOH)_2$ and (B) $Ru(NH_3)_6^{3+}$, using ITO electrodes modified with azide-functionalized VMSF (prepared from 2.5/97.5 AzPTES/TEOS ratio) respectively (1) before and (2) after surfactant extraction. (C,D) Cyclic voltammograms recorded in 0.5 mM solutions of (C) $Fc(MeOH)_2$ and (D) $Ru(NH_3)_6^{3+}$, using surfactant-extracted azide-functionalized VMSF materials prepared from various AzPTES/TEOS ratios: (1) 2.5/97.5, (2) 5/95, (3) 10/90, (4) 15/85, (5) 20/80, and (6) 25/75. Other conditions: 0.1 M $NaNO_3$ supporting electrolyte; 100 mV s$^{-1}$ potential scan rate.

## 3.2. Functionalization of the films with $[Ru(bpy)_2(bpy')']^{2+}$ and characterization

The azide-functionalized VMSF materials (Fig. 1A₂) was allowed to react with 4-[(2-propynyloxy)methyl]-4'-methyl-2,2'-bipyridine ruthenium(II) bis(2,2'-bipyridine) (Fig. 1B) to give the final $[Ru(bpy)_2(bpy')]^{2+}$-functionalized film (Fig. 1C) according to the Cu-catalyzed
alkyne-azide 1,3-dipolar cycloaddition [60,61]. The effectiveness of this reaction can be monitored by FTIR via the disappearance (or decrease) of the absorption band at 2105 cm⁻¹ typical of the azide groups [27,62,63]. As shown in Figure 4, this is indeed the case with yields close to 100% for the materials prepared with AzPTES/TEOS ratios up to 5/95, about 70% for 7.5/92.5, 45% for 10/90, and less than 20% for 12.5/87.5 and 15/85 ratios. These films are permeable to small redox probes (Fig. 3), but it seems here that the big size of the derivatization agent (i.e., alkyne-bearing [Ru(bpy)₂(bpy')]²⁺ compound, Fig. 1B) prevents its effective access to all the azide moieties in the mesopore channels for the materials prepared with AzPTES/TEOS ratios greater than 7.5/92.5, contrary to what was reported for smaller moieties such as ferrocene derivatives for which CuAAC was effective up to 40/60 AzPTES/TEOS ratio [26]. In principle one could also evidence the characteristic vibration bands of the triazole rings formed during CuAAC (i.e., stretching vibration of C=C and N=N at 1461 cm⁻¹ and 1655 cm⁻¹ [70]) but precise assignment of all bands in that region is impossible due to overlap with that of adsorbed water (at 1625 cm⁻¹ [67]) and especially those characteristics of the 2,2'-bipyridine units of the [Ru(bpy)₂(bpy')]²⁺ complex (occurring in the 1400-1650 cm⁻¹ region [71,72]).

<figure><img src="image_12.png" /></figure>

**Figure 4.** (A) Normalized (v = 1075 cm⁻¹) FTIR spectra of azide-functionalized mesoporous silica films prepared from various AzPTES/TEOS ratios: (a) 2.5/97.5, (b) 5/95, (c) 7.5/92.5, (d) 10/90, (e) 12.5/87.5, and (f) 15/85, respectively before (black curves) and after click coupling with [Ru(bpy)₂(bpy')]²⁺ (red curves). (B) Variation of the click coupling efficiency (% determined from the decrease in intensity of the azide stretching vibration band at 2105 cm⁻¹) as a function of the AzPTES content in the film synthesis medium.
Figure 5 shows that after functionalization with [Ru(bpy)₂(bpy')']²⁺, VMSF materials kept their structural integrity, as expected from previous reports on EASA-generated mesoporous silica films functionalized by click chemistry [26-31]. As pointed out by TEM, the [Ru(bpy)₂(bpy')']²⁺-functionalized films are characterized by a hexagonal arrangement of mesopore channels oriented vertically relative to the plane of the membrane, with a film thickness of about 150 nm and pore-to-pore distance (d-spacing) close to 4 nm (see the top view and cross section micrographs on Fig. 5A for a film prepared from 5% AzPTES). A more accurate d-spacing value of 3.7 nm can be evaluated from STEM imaging (see Fig. S6 in Supplementary Material). The ordered and oriented mesostructuration is maintained over the whole film, regardless of the level of functionalization, as revealed by GISAXS experiments (see Fig. 5B for films prepared from 2.5% and 10% AzPTES). This is indicated by the presence of diffraction spots in the equatorial plane (z = 0) with scattering maxima located at (+/-) 1.7 nm⁻¹ from which one can calculate a d-spacing of 3.7 nm, in agreement with the values reported for related mesoporous silica films prepared by EASA using CTAB as surfactant template [19,73,74]. The weak intensity of the peaks (and associated noise) is probably due to the presence of the [Ru(bpy)₂(bpy')']²⁺ complex in the pores, contributing to reduce the X-ray scattering contrast in the material.

<figure><img src="image_13.png" /><figcaption>**Figure 5.** (A) Bright field TEM top views (top) and cross sections (bottom) at two magnifications of a [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF (prepared from AzPTES/TEOS ratio of 5/95). (B) GISAXS patterns at the incidence angle of 0.25° (top) and corresponding intensity profiles plotted against q_y (bottom) for ITO electrodes covered with</figcaption></figure>
[Ru(bpy)₂(bpy')]²⁺-functionalized VMSF materials prepared from AzPTES/TEOS ratios of (B₁) 2.5/97.5 and (B₂) 10/90.

Figure 6 shows typical cyclic voltammograms recorded with ITO electrodes covered by [Ru(bpy)₂(bpy')]²⁺-functionalized films prepared from VMSF materials containing various amounts of azide groups. They first indicate that [Ru(bpy)₂(bpy')]²⁺ complexes immobilized in such an insulating silica matrix are electrochemically active, in agreement with previous observations for other redox species [26-33]. The anodic signal corresponds to the oxidation of [Ru(bpy)₂(bpy')]²⁺ to [Ru(bpy)₂(bpy')]³⁺ and is located at +1.30 V, a value very close to that of [Ru(bpy)₂(bpy')]²⁺ in solution (see Fig. S7 in Supplementary Material), perhaps suggesting little effect of the confinement on the electrochemical properties. However, the immobilized redox couple is less reversible than in solution, with an anodic-to-cathodic peak-to-peak separation of 110–140 mV depending on the functionalization level (at 20 mV s⁻¹, compared to 80 mV for [Ru(bpy)₂(bpy')]²⁺ in solution at the same potential scan rate), and a cathodic peak intensity, *i*<sub>pc</sub>, observed on scan reversal being less than the anodic one, *i*<sub>pa</sub> (i.e., *i*<sub>pc</sub>/*i*<sub>pa</sub> ~ 0.75–0.9 depending on the potential scan rate and functionalization level). The intensity of peak currents is also very dependent on the AzPTES/TEOS ratio used for film synthesis (Fig. 6A), growing almost proportionally between 2.5/97.5 and 5/95, then levelling off at 7.5/92.5 and starting to decrease for 10/90, and falling down rapidly for greater AzPTES/TEOS ratios (Fig. 6B), with voltammetric signals losing their reversibility (Fig. 6A). One can hypothesize that this variation reflects the content of [Ru(bpy)₂(bpy')]²⁺ in the film (as pointed out for ferrocene-functionalized VMSF [26,33]), even if one cannot be sure that all the immobilized species are electrochemically accessible. These results are consistent with the trend observed from FTIR characterization (Fig. 4B).
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 6.** (A) Cyclic voltammograms recorded in blank electrolyte solution (0.1 M TBACIO₄ in acetonitrile) at a potential scan rate of 100 mV s⁻¹, using ITO electrodes modified with [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF (prepared from AzPTES/TEOS ratios ranging between 2.5/97.5 and 15/85). (B) Corresponding variation of anodic peak currents, Iₚₐ, as a function of the AzPTES content in the film synthesis medium. (C,D) Cyclic voltammograms recorded in the same solution as in (A) at various potential scan rates (from 10 to 100 mV s⁻¹), using azide-functionalized VMSF materials prepared from AzPTES/TEOS ratios of (C) 2.5/97.5 and (D) 10/90.</figcaption></figure>

The anodic-to-cathodic peak-to-peak separation increases as the potential scan rate increases (Fig. 6C&D), suggesting quite slow charge transfer kinetics. The charge transfer mechanism for redox species covalently immobilized in the insulating silica material involves electron
hopping between adjacent redox centers, which is expected to be facilitated in more concentrated films [33]. Nevertheless, the oxidation of $[Ru(bpy)_2(bpy')]^{2+}$ to $[Ru(bpy)_2(bpy')]^{3+}$ also involves the penetration of electrolyte anions inside the VMSF to maintain charge balance, and this process can be rate-determining in the case of slow mass transport in the porous material [33]. As a consequence, one does need to have a density of redox moieties large enough for $[Ru(bpy)_2(bpy')]^{2+}$ centers being close to each other and favour electron hopping but not too high to keep the nanochannels porous enough to enable easy mass transport of the neutralizing anions coming from the electrolyte solution. The most intense peak currents are observed for VMSF prepared from AzPTES/TEOS ratios ranging between 5/95 and 10/90 (Fig. 6A&B). In all cases, peak currents vary linearly with the square root of the potential scan rate (Fig. 6C&D), indicating charge transfer processes limited by the diffusion (i.e., the pseudo-diffusion of the electron in the hopping process and/or the diffusion of charge-compensating anions through the film [33]).

### 3.3. Main properties of the $[Ru(bpy)_2(bpy')]^{2+}$-functionalized films

The aforementioned results show that $[Ru(bpy)_2(bpy')]^{2+}$ moieties covalently bonded to VMSF remain electrochemically accessible. An interesting point is related to the covalent bonding of such redox probes to the silica film, leading to a durable immobilization and consequently to a quite stable electrochemical response upon multisweep cyclic voltammetry. This is illustrated in Figure 7 where multiple cyclic voltammograms recorded using an ITO electrode modified with azide-functionalized VMSF (prepared from an AzPTES/TEOS ratio of 5/95) reveal a small and continuous decrease in peak current intensity after successive potential scans (i.e., only 17% lost after 150 cycles, Fig. 7A), which is connected to the incomplete reduction of the oxidized $[Ru(bpy)_2(bpy')]^{3+}$ species on scan reversal (in agreement with $i_{pc}/i_{pa}$ ratios lower than unity observed above, Fig. 6). After investigating the electrode, the solutions do not show any absorption bands for $[Ru(bpy)_2(bpy')]^{2+}$ species, confirming the absence of any noticeable leaching under these conditions. By contrast, a bare mesoporous silica film prepared by the same EASA method (but without azide groups and no click coupling), which was impregnated with $[Ru(bpy)_3]^{2+}$ ions (which are known to accumulate in the nanochannels via electrostatic interactions with the silica surface [47-52]), results in a very fast decrease in peak current intensity (i.e., 80% lost after only 15 cycles, Fig. 7B). This can be explained by the progressive leaching of the loosely bound $[Ru(bpy)_3]^{2+}$ species in solution [52, 75]. The multisweep voltammetric data depicted in Figure 7 have been obtained in organic medium
(acetonitrile), but similar observations can be made in aqueous medium for which much better operational stability can be reached with [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF (i.e., 14 % decrease in peak currents after 150 cycles, see Fig. S8A in Supplementary Material) compared to the [Ru(bpy)₃]²⁺-impregnated mesoporous silica film (i.e., 50 % decrease in peak currents after 15 cycles and 80 % lost after 150 cycles, see Fig. S8B in Supplementary Material). All these results confirm the interest of the covalent route to immobilize durably the redox probe in an electroactive form in VMSF.

<figure><img src="image_18.png" /><figcaption>**Figure 7.** Consecutive cyclic voltammograms (150 scans) recorded in blank electrolyte solution (0.1 M TBACIO₄ in acetonitrile) at a potential scan rate of 100 mV s⁻¹, using an ITO electrode modified with (A) [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF (prepared from AzPTES/TEOS ratio of 5/95) or (B) a bare mesoporous silica film previously impregnated with [Ru(bpy)₃]²⁺ species (1 h soaking in 1 mM [Ru(bpy)₃]Cl₂).</figcaption></figure>

The material is also photoactive, as illustrated by Figure S9 in Supplementary Material. The typical UV-visible spectrum of [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF (Fig. S9A) exhibits a first broad absorption band centered at ~305 nm (assigned to the π-π* transition of bipyridyl ligand [38]) and another one located around 445 nm (attributed to metal-to-ligand charge transfer, MLCT [38]), consistent with what was observed for [Ru(bpy)₃]²⁺ physically adsorbed into mesoporous silica particles [76,77]. Note that the position of the MLCT is the same as for the complex in solution (see insert in Fig. S9A), confirming its structural integrity when immobilized in the mesoporous silica film. Exciting the film immersed in either
acetonitrile or water, at 450 nm, enables to observe emission spectra in both media (Fig. S9B), with maxima at around 595 nm (Fig. S9B₁) and 580 nm (Fig. S9B₂), respectively. These values are slightly blue-shifted with respect to the free complex in solution due to possible interactions between [Ru(bpy)₂(bpy')]²⁺ and the silica surface [72, 77, 78]. The luminescence is clearly visible only after removal of oxygen by bubbling nitrogen in the media (Fig. S9B) because of the well-established quenching of [Ru(bpy)₂(bpy')]²⁺* in the presence of molecular oxygen [79], an effect that can be notably exploited for oxygen sensing [76].

**3.4. Possible applications of the [Ru(bpy)₂(bpy')]²⁺-functionalized films**

This last section intends to give a flavour of possible applications of [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF without any optimization but just highlighting the proofs of concept.

Ruthenium tris(bipyridine) derivatives immobilized onto an electrode surface are well-known for their ability to electrochemically generate chemiluminescence (i.e., electrochemiluminescence, ECL) in the presence of tri-*n*-propylamine (TPA), via a mechanism involving the oxidation of both [Ru(bpy)₃]²⁺ and TPA, forming excited TPA* species reacting with the electrogenerated [Ru(bpy)₃]³⁺ to produce the luminescent [Ru(bpy)₃]²⁺* state [80]. So far, in the field of VMSF, only ECL sensing applications based on weakly bound [Ru(bpy)₃]²⁺ species interacting electrostatically with the mesoporous silica walls have been reported [47-49]. We demonstrate here that [Ru(bpy)₂(bpy')]²⁺ covalently immobilized in VMSF also exhibits ECL properties (see Fig. S10 in Supplementary Material). It is indeed likely to electrocatalyze the oxidation of TPA, exhibiting a catalytic current for [Ru(bpy)₂(bpy')]²⁺ 15 times larger than in the absence of TPA (Fig. S10A), which is much higher than that observed with [Ru(bpy)₃]²⁺ entrapped in silica nanoparticles [81]. The ECL response is also well-defined (Fig. S10B), similar to those reported for [Ru(bpy)₃]²⁺-doped silica nanoparticles [81, 82]. Upon successive detection (10 consecutive scans, Fig. S10C), the ECL intensity decreased to stabilize at about 2×10⁵ (arbitrary units). This is due to the consumption of TPA molecules near the electrode surface, a problem that can be circumvented by stopping the experiment and waiting for ca. 5 min (in order to allow the diffusion of fresh TPA molecules into the silica nanochannels) to get back the initial response (ECL intensity of about 7×10⁵).

More original is the possible interest of such system in redox photocatalysis, as it could be a platform for the reagent-free electrochemical regeneration of the photocatalyst (i.e., a concept just demonstrated for [Ru(bpy)₃]²⁺ in solution [83]). Figure 8 illustrates the feasibility
of such an approach using [Ru(bpy)₂(bpy')]²⁺ moieties attached to VMSF as photosensitizer, methylviologen (MV²⁺) as quencher and oxygen as reagent. Upon irradiation at 400 nm, one can clearly see cathodic photocurrents corresponding to the electrochemical reduction of [Ru(bpy)₂(bpy')]³⁺ species generated during the reaction between the photoexcited *[Ru(bpy)₂(bpy')]²⁺ state and MV²⁺ (Eq. 1), according to a reaction pathway known for the Ru(bpy)₃²⁺/MV²⁺ system [84,85], while the formed MV·⁺ oxidizes into MV²⁺ by reaction with molecular oxygen in aerated medium [86]. According to the literature, the reaction product of this last reaction is probably H₂O₂ [86], but one cannot confirm its presence in solution as it would be formed at the electrode/solution interface only.

$$*[Ru(bpy)_{2}(bpy')]^{2+} + MV^{2+} \rightarrow [Ru(bpy)_{2}(bpy')]^{3+} + MV^{\bullet+} \quad (1)$$

The photocurrents increase when increasing the concentration of the quencher (MV²⁺), proving the efficacy of the electrochemical regeneration of the [Ru(bpy)₂(bpy')]²⁺ photocatalyst immobilized in the mesoporous silica film. Note that in the absence of MV²⁺, some signals can be also observed (see Fig. S11 in Supplementary Material) because oxygen can also play the role of quencher [87], but the thus-generated photocurrents were of lower intensity compared to those recorded in presence of MV²⁺ (Fig. 8). The photoelectrochemical response is stable upon repeated on/off cycles, without any noticeable decrease in intensity under illumination, suggesting its possible exploitation for the reagentless electrochemical regeneration of photocatalysts without the need for any additional sacrificial reagent (contrary to what is usually necessary in redox photocatalysis [88,89]). The good photostability of the film is consistent with previous observations indicating the interest of [Ru(bpy)₃]²⁺ embedment in silica for improving the photostability of the photosensitizer [90].
<figure><img src="image_19.png" /><figcaption>**Figure 8.** Hydrodynamic amperograms recorded in a stirred aqueous solution containing MV²⁺ at various concentrations (from 1 to 20 mM) (+0.1 M NaNO₃), using an ITO electrode modified with [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF (prepared from AzPTES/TEOS ratio of 5/95), by applying a potential of -0.1 V and irradiating at 400 nm for 10 s every 20 s periods. A scheme illustrating the expected reactions likely to occur in such an aerated medium and the origin of photocurrents is shown on the right part of the figure.</figcaption></figure>

**4. Conclusions**

Vertically-oriented mesoporous silica films, VMSF, bearing covalently bound tris(2,2'-bipyridyl)ruthenium(II) redox photocatalysts, i.e. [Ru(bpy)₂(bpy')]²⁺, can be prepared on ITO electrode by sequential electro-assisted self-assembly of azide-functionalized VMSF and subsequent click coupling by CuAAC with an alkyne-bearing [Ru(bpy)₂(bpy')]²⁺ reagent. Tuning the azide content in the material enables to control the amount of covalently bound [Ru(bpy)₂(bpy')]²⁺ moieties in the final film but the effectiveness/yield of the derivatization reaction decreased when increasing the azide content due to steric constraints. The best compromise leading the highest functionalization levels is achieved with VMSF materials prepared from AzPTES/TEOS ratios ranging between 5/95 and 10/90. Although being immobilized in an insulating silica matrix, [Ru(bpy)₂(bpy')]²⁺ moieties were electrochemically accessible thanks to an effective electron hopping mechanism, and their electrochemical response was much more stable upon multiple successive potential scans in comparison to
similar complexes incorporated into such films via electrostatic interactions. Moreover,
[Ru(bpy)₂(bpy')]²⁺-functionalized VMSF are photoactive and, as being deposited onto
transparent ITO supports, they are thus promising for photoelectrochemical applications.
Preliminary experiments point out their possible interest for electrochemiluminescence sensing
or, in photocatalysis, as a mean to regenerate electrochemically the photocatalyst without using
a sacrificial reagent.

Acknowledgements

The authors acknowledge financial supports from the French “Agence Nationale de la
Recherche” (public grant N° ANR-21-CE07-044, PHOTOMATON2) and from the “Région
Grand-Est” (“Jeune Chercheur” project ELECTROCAT). They also thank Nadia Canilho and
Lionel Richaudeau from L2CM in Nancy for GISAXS experiments, and the spectroscopy and
microscopy Service Facility (SMI) of LCPME for providing access to the FTIR apparatus.

**Competing interest statement**

The authors have no competing interests to declare.
## 5. References

[1] P. Innocenzi, L. Malfatti, Mesoporous thin films: properties and applications, Chem. Soc. Rev. 42 (2013) 4198–4216. https://doi.org/10.1039/C3CS35377J.

[2] D. Feng, J. Wei, M. Wang, Q. Yue, Y. Deng, A.M. Asiri, D. Zhao, Advances in mesoporous thin films via self-assembly process, Adv. Porous Mater. 1 (2013) 164–186. https://doi.org/10.1166/apm.2013.1013.

[3] C. Sanchez, C. Boissière, D. Grosso, C; Laberty, L. Nicole, Design, Synthesis, and properties of inorganic and hybrid thin films having periodically organized nanoporosity, Chem. Mater. 20 (2008) 682–737. https://doi.org/10.1021/cm702100t.

[4] P. Innocenzi, Mesoporous ordered films via self-assembly: trends and perspectives, Chem. Sci. 13 (2022) 13264–13279. https://doi.org/10.1039/D2SC04828K.

[5] Y. Yamauchi, Field-induced alignment controls of one-dimensional mesochannels in mesoporous materials, J. Ceram. Soc. Jpn. 121 (2013) 831–840. https://doi.org/10.2109/jcersj2.121.831.

[6] E. Richman, T. Brezesinski, S. Tolbert, Vertically oriented hexagonal mesoporous films formed through nanometre-scale epitaxy, Nat. Mater. 7 (2008) 712–717. https://doi.org/10.1038/nmat2257.

[7] B. Su, X. Lu, Q. Lu, A facile method to prepare macroscopically oriented mesostructured silica film: controlling the orientation of mesochannels in multilayer films by air flow, J. Am. Chem. Soc. 130 (2008) 14356–14357. https://doi.org/10.1021/ja8003254.

[8] J.T. Paci, H.B. Man, B. Saha, D. Ho, G.C. Schatz, Synthesis of surfactant-templated silica films with orthogonally aligned hexagonal mesophase, J. Phys. Chem. C 117 (2013) 17256–17267. https://doi.org/10.1021/jp045037a.

[9] K.-C. Kao, C.-H. Lin, T.-Y. Chen, Y.-H. Liu, C.-Y. Mou, A general method for growing large area mesoporous silica thin films on flat substrates with perpendicular nanochannels, J. Am. Chem. Soc. 137 (2015) 3779–3782. https://doi.org/10.1021/jacs.5b01180.

[10] A. Walcarius, E. Sibottier, M. Etienne, J. Ghanbaja, Electrochemically assisted self-assembly of mesoporous silica thin films, Nat. Mater. 6 (2007) 602–608. https://doi.org/10.1038/nmat1951.
[11] A. Goux, M. Etienne, E. Aubert, C. Lecomte, J. Ghanbaja, A. Walcarius, Oriented mesoporous silica films obtained by electro-assisted self-assembly (EASA), Chem. Mater. 21 (2009) 731–741. https://doi.org/10.1021/cm8029664.

[12] Z. Teng, G. Zheng, Y. Dou, W. Li, C.-Y. Mou, X. Zhang, A.M. Asiri, D. Zhao, Highly ordered mesoporous silica films with perpendicular mesochannels by a simple Stöber-solution growth approach, Angew. Chem. Int. Ed. 51 (2012) 2173–2177.
https://doi.org/10.1002/anie.201108748.

[13] A. Walcarius, Electro-induced surfactant self-assembly driven to vertical growth of oriented mesoporous films, Acc. Chem. Res. 54 (2021) 3563–3575.
https://doi.org/10.1021/acs.accounts.1c00233.

[14] P. Zhou, L. Yao, K. Chen, B. Su, Silica nanochannel membranes for electrochemical analysis and molecular sieving: a comprehensive review, Crit. Rev. Anal. Chem. 50 (2020) 424–444. https://doi.org/10.1080/10408347.2019.1642735.

[15] M.L. Scala-Benuzzi, S.N. Fernández, G. Giménez, G. Ybarra, G.J.A.A. Soler-Illia, Ordered mesoporous electrodes for sensing applications, ACS Omega 8 (2023) 24128–24152.
https://doi.org/10.1021/acsomega.3c02013.

[16] A. Walcarius, Silica-based electrochemical sensors and biosensors: Recent trends, Curr. Opin. Electrochem. 10 (2018) 88–97. https://doi.org/10.1016/j.coelec.2018.03.017.

[17] X. Fan, J. Wu, T. Zhang, J. Liu, Electrochemical/electrochemiluminescence sensors based on vertically-ordered mesoporous silica films for biomedical analytical applications, ChemBioChem 25 (2024) e202400320. https://doi.org/10.1002/cbic.202400320.

[18] O. Tananaiko, A. Walcarius, Composite silica-based films as platforms for electrochemical sensors, Chem. Rec. 24 (2024) e202300194.
https://doi.org/10.1002/tcr.202300194.

[19] C. Robertson, R. Beanland, S. A. Boden, A. L. Hector, R. J. Kashtiban, J. Sloan, D. C. Smith, A. Walcarius, Ordered mesoporous silica films with pores oriented perpendicular to a titanium nitride substrate, Phys. Chem. Chem. Phys. 17 (2015) 4763–4770.
https://doi.org/10.1039/C4CP05730A.

[20] M. Laskowska, M. Dulski, M. Marszałek, M. Zubko, L. Laskowski, Vertically aligned porous silica thin films functionalized by nickel chloride incorporated in walls, Microporous Mesoporous Mater. 276 (2019) 201–206. https://doi.org/10.1016/j.micromeso.2018.10.002.
[21] W. Li, L. Ding, Q. Wang, B. Su, Differential pulse voltammetry detection of dopamine and ascorbic acid by permselective silica mesochannels vertically attached to the electrode surface, Analyst 139 (2014) 3926-3931. https://doi.org/10.1039/C4AN00605D.

[22] F. Asadpour, M. Mazloum-Ardakani, F. Hoseynidokht, S.M. Moshtaghioun, In situ monitoring of gating approach on mesoporous silica nanoparticles thin-film generated by the EASA method for electrochemical detection of insulin, Biosens. Bioelectron. 180 (2021) 113124. https://doi.org/10.1016/j.bios.2021.113124.

[23] M. Etienne, A. Goux, E. Sibottier, A. Walcarius, Oriented mesoporous organosilica films on electrode: a new class of nanomaterials for sensing, J. Nanosci. Nanotechnol. 9 (2009) 2398-2406. https://doi.org/10.1166/jnn.2009.SE39.

[24] G. Herzog, E. Sibottier, M. Etienne, A. Walcarius, Electrochemically-assisted self-assembly of ordered and functionalized mesoporous silica films: impact of the electrode geometry and size on film formation and properties, Faraday Discus. 164 (2013) 259-273. https://doi.org/10.1039/C3FD00021D.

[25] K. Ma, L. Yang, J. Liu, J. Liu, Electrochemical sensor nanoarchitectonics for sensitive detection of uric acid in human whole blood based on screen-printed carbon electrode equipped with vertically-ordered mesoporous silica-nanochannel film, Nanomater. 12 (2022) 1157. https://doi.org/10.3390/nano12071157.

[26] N. Vilà, J. Ghanbaja, E. Aubert, A. Walcarius, Electrochemically assisted generation of highly ordered azide-functionalized mesoporous silica for oriented hybrid films, Angew. Chem. Int. Ed. 53 (2014) 2945-2950. https://doi.org/10.1002/anie.201309447.

[27] N. Vilà, J. Ghanbaja, A. Walcarius, Clickable bifunctional and vertically aligned mesoporous silica films, Adv. Mater. Interfaces 3 (2016) 1500440. https://doi.org/10.1002/admi.201500440.

[28] J. Wang, N. Vilà, A. Walcarius, Redox-active vertically-aligned mesoporous silica thin films as transparent surfaces for energy storage applications, ACS Appl. Mater. Interfaces 12 (2020) 24262-24270. https://doi.org/10.1021/acsami.0c03650.

[29] N. Vilà, C. Allain, P. Audebert, A. Walcarius, Tetrazine-functionalized and vertically-aligned mesoporous silica films with electrochemical activity and fluorescence properties, Electrochem. Commun. 59 (2015) 9-12. https://doi.org/10.1016/j.elecom.2015.06.012.
[30] N. Vilà, A. Walcarius, Bis(terpyridine) Iron(II) functionalized vertically-oriented nanostructured silica films: towards electrochromic materials, Front. Chem. 8 (2020), 830. https://doi.org/10.3389/fchem.2020.00830.

[31] S. Ahoulou, N. Vilà, S. Pillet, C. Carteret, D. Schaniel, A. Walcarius, Multi-stimuli photo and redox-active nanostructured mesoporous silica films on transparent electrodes, ChemPhysChem 22 (2021) 2464–2477. https://doi.org/10.1002/cphc.202100608.

[32] N. Vilà, E. André, R. Ciganda, J. Ruiz, D. Astruc, A. Walcarius, Molecular sieving with vertically-aligned mesoporous silica films and electronic wiring through isolating nanochannels, Chem. Mater., 28 (2016), 2511–2514. https://doi.org/10.1021/acs.chemmater.6b00716.

[33] N. Vilà, A. Walcarius, Electrochemical response of vertically-aligned, ferrocene-functionalized mesoporous silica films: effect of supporting electrolyte, Electrochim. Acta, 179 (2015), 304–314. https://doi.org/10.1016/j.electacta.2015.02.169.

[34] D.N. Blauch, J.-M. Savéant, Effects of long-range electron transfer on charge transport in static assemblies of redox centers, J. Phys. Chem. 97 (1993) 6444–6448. https://doi.org/10.1021/j100126a019.

[35] M. Kaneko, Charge transport in solid polymer matrixes with redox centers, Prog. Polym. Sci. 26 (2001) 1101–1137. https://doi.org/10.1016/S0079-6700(01)00015-6.

[36] A. Akhoury, L. Bromberg, T. A. Hatton, Interplay of Electron Hopping and Bounded Diffusion during Charge Transport in Redox Polymer Electrodes, J. Phys. Chem. B 117 (2013) 333–342. https://doi.org/10.1021/jp302157g.

[37] C.K. Prier, D.A. Rankic, D.W.C. MacMillan, Visible light photoredox catalysis with transition metal complexes: applications in organic synthesis, Chem. Rev. 113 (2013) 5322–5363. https://doi.org/10.1021/cr300503r.

[38] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von Zelewsky, Ru(II) polypyridine complexes: photophysics, photochemistry, electrochemistry, and chemiluminescence, Coord. Chem. Rev. 84 (1988) 85–277. https://doi.org/10.1016/0010-8545(88)80032-8.

[39] E.R. dos Santos, J. Pina, T. Venâncio, C. Serpa, J.M.G. Martinho, R.M. Carlos, Photoinduced energy and electron-transfer reactions by polypyridine Ruthenium(II)
complexes containing a derivatized perylene diimide, J. Phys. Chem. C 120 (2016) 22831–22843. https://doi.org/10.1021/acs.jpcc.6b06693.

[40] J.-L. Fillaut, Ruthenium (II) polypyridyl complexes as two-photon absorbers and sensitizers: Design, structure-proPERTIES relationships and applications, Coord. Chem. Rev. 518 (2024) 216050. https://doi.org/10.1016/j.ccr.2024.216050.

[41] F. Teplý, Visible-light photoredox catalysis with [Ru(bpy)₃]²⁺: general principles and the twentieth-century roots, Phys. Sci. Rev. 5 (2020) 20170171. https://doi.org/10.1515/psr-2017-0171.

[42] S.S. Rozenel, C.R. Azpilcueta, M.M. Flores-Leonar, J.P.F. Rebolledo-Chávez, L. Ortiz-Frade, C. Amador-Bedolla, E. Martin, Ruthenium tris bipyridine derivatives and their photocatalytic activity in [4 + 2] cycloadditions. An experimental and DFT study, Catal. Today 310 (2018) 2–10. https://doi.org/10.1016/j.cattod.2017.05.021.

[43] T. Saji, S. Aoyagui, Electron-transfer kinetics of transition-metal complexes in lower oxidation states: IV. Electrochemical electron-transfer rates of tris(2,2'-bipyridine) complexes of iron, ruthenium, osmium, chromium, titanium, vanadium and molybdenum, J. Electroanal. Chem. Interf. Electrochem. 63 (1975) 31–37. https://doi.org/10.1016/S0022-0728(75)80124-0.

[44] C.-T. Lin, W. Bóttcher, M. Chou, C. Creutz, N. Sutin, Mechanism of the quenching of the emission of substituted polypyridineruthenium(II) complexes by iron(III), chromium(III), and europium(III) ions, J. Am. Chem. Soc. 98 (1976) 6536–6544. https://doi.org/10.1021/ja00437a020.

[45] M.R. McDevitt, A.W. Addison, Medium effects on the redox properties tris(2,2'-bipyridyl)ruthenium complexes, Inorg. Chim. Acta 204 (1993) 141–146. https://doi.org/10.1016/S0020-1693(00)82917-5.

[46] Y. Katayama, Y. Toshimitsu, T. Miura, Electrode kinetics of tris(2,2'-bipyridine)ruthenium complexes in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid, J. Electrochem. Soc. 160 (2013) H224–H228. https://doi.org/10.1149/2.071304jes.

[47] R. Liang, J. Jiang, Y. Zheng, A. Sailjoi, J. Chen, J. Liu, H. Li, Vertically oriented mesoporous silica film modified fluorine-doped tin oxide electrode for enhanced electrochemiluminescence detection of lidocaine in serum, RSC Adv. 11 (2021) 34669. https://doi.org/10.1039/D1RA06375H.
[48] P. Zhou, B. Su, Enhanced electrochemiluminescence at silica nanochannel membrane studied by scanning electrochemical microscopy, J. Electroanal. Chem. 904 (2022) 115943.
https://doi.org/10.1016/j.jelechem.2021.115943.

[49] R. Yu, Y. Zhao, J. Liu, Solid electrochemiluminescence sensor by immobilization of emitter ruthenium(II)tris(bipyridine) in bipolar silica nanochannel film for sensitive detection of oxalate in serum and urine, Nanomater. 14 (2024) 390.
https://doi.org/10.3390/nano14050390.

[50] J. Gong, T. Zhang, P. Chen, F. Yan, J. Liu, Bipolar silica nanochannel array for dual-mode electrochemiluminescence and electrochemical immunosensing platform, Sens. Actuat. B 368 (2022) 132086. https://doi.org/10.1016/j.snb.2022.132086.

[51] L. Yang, T. Zhang, H. Zhou, F. Yan, Y. Liu, Silica nanochannels boosting Ru(bpy)₃²⁺-mediated electrochemical sensor for the detection of guanine in beer and pharmaceutical samples, Front. Nutr. 9 (2022) 987442. https://doi.org/10.3389/fnut.2022.987442.

[52] D. Basnig, N. Vilà, G. Herzog, A. Walcarius, Voltammetric behaviour of cationic redox probes at mesoporous silica film electrodes, J. Electroanal. Chem. 872 (2020) 113993.
https://doi.org/10.1016/j.jelechem.2020.113993.

[53] I. Candel, A. Bernardos, E. Climent, M.D. Marcos, R. Martínez-Máñez, F. Sancenón, J. Soto, A.M. Costero, S. Gil, M. Parra, Selective opening of nanoscopic capped mesoporous inorganic materials with nerve agent simulants; an application to design chromo-fluorogenic probes, Chem. Commun. 47 (2011) 8313-8315. https://doi.org/10.1039/C1CC12727F.

[54] Y. Salinas, A. Agostini, E. Pérez-Esteve, R. Martínez-Máñez, F. Sancenón, M. D. Marcos, J. Soto, A.M. Costero, S. Gil, M. Parra, P. Amorós, Fluorogenic detection of Tetryl and TNT explosives using nanoscopic-capped mesoporous hybrid materials, J. Mater. Chem. A 1 (2013) 3561-3564. https://doi.org/10.1039/C3TA01438J.

[55] X. He, Y. Zhao, D. He, K. Wang, F. Xu, J. Tang, Correction to ATP-responsive controlled release system using aptamer-functionalized mesoporous silica nanoparticles, Langmuir 28 (2012) 12909-12915. https://doi.org/10.1021/la302767b.

[56] A. Busemann, C. Araman, I. Flaspohler, A. Pratesi, X.-Q. Zhou, V.H.S. van Rixel, M.A. Siegler, L. Messori, S.I. van Kasteren, S. Bonnet, Alkyne functionalization of a photoactivated Ruthenium polypyridyl complex for click-enabled serum albumin interaction studies, Inorg. Chem. 59 (2020) 7710-7720. https://doi.org/10.1021/acs.inorgchem.0c00742.
[57] S. Hennessey, R. González-Gómez, K. McCarthy, C. Burke, C. Le Houérou, N. Kumar Sarangi, P. McArdle, T.E. Keyes, F. Cucinotta, P. Farràs, Enhanced photostability and photoactivity of Ruthenium polypyridyl-based photocatalysts by covalently anchoring onto reduced graphene oxide, ACS Omega 9 (2024) 13872–13882.
https://doi.org/10.1021/acsomega.3c08800.

[58] H. Hettegger, I. Sumerskii, S. Sortino, A. Potthast, T. Rosenau, Silane meets click chemistry: Towards the functionalization of wet bacterial cellulose sheets, ChemSusChem 8 (2015) 680–687. https://doi.org/10.1002/cssc.201402991.

[59] D. Ma, S.E. Bettis, K. Hanson, M. Minakova, L. Alibabaei, W. Fondrie, D.M. Ryan, G.A. Papoian, T.J. Meyer, M.L. Waters, J.M. Papanikolas, Interfacial energy conversion in RuII polypyridyl-derivatized oligoproline assemblies on TiO₂, J. Am. Chem. Soc. 135 (2013) 5250–5253. https://doi.org/10.1021/ja312143h.

[60] L. Zhang, N. Vilà, A. Walcarius, M. Etienne, Molecular and biological catalysts co-immobilization on electrode by combining diazonium electrografting and sequential click chemistry, ChemElectroChem 5 (2018) 2208–2217. https://doi.org/10.1002/celc.201800258.

[61] M. Sebbat, A. Lazar, C. Laurent, C. Despas, N. Vilà, A. Walcarius, N. McClenaghan, E. Oheix, B. Lebeau, M. Cormier, J.-P. Goddard, Heterogenization of [Ru(bpy)₃]Cl₂ on ordered mesoporous silica materials for photocatalytic applications, Synlett (2024) in press.
https://doi.org/10.1055/a-2317-3672.

[62] B. Malvi, B.R. Sarkar, D. Pati, R. Mathew, T.G. Ajithkumar, S. Sen Gupta, “ Clickable ” SBA-15 mesoporous materials: synthesis, characterization and their reaction with alkynes, J. Mater. Chem. 19 (2009) 1409–1416. https://doi.org/10.1039/B815350G.

[63] N. Moitra, P. Trens, L. Raehm, J.-O. Durand, X. Cattoën, Facile route to functionalized mesoporous silica nanoparticles by click chemistry, J. Mater. Chem. 21 (2011) 13476–13482.
https://doi.org/10.1039/C1JM12066B.

[64] L.B. Capeletti, J.H. Zimnoch, Fourier transform infrared and Raman characterization of silica-based materials, in: M.T. Stauffer (Ed.), Applications of molecular spectroscopy to current research in the chemical and biological sciences, IntechOpen, 2016, Chapter 1.
https://doi.org/10.5772/64477.
[65] S. Karaca, A. Gürses, M.E. Korucu, Investigation of the orientation of CTA+ ions in the interlayer of CTAB pillared montmorillonite, J. Chem. 2013 (2013) 274838.
https://doi.org/10.1155/2013/274838.

[66] F. Ai, G. Zhao, W. Lv, J. Lin, Facile synthesis of cetyltrimethylammonium bromide-loaded mesoporous silica nanoparticles for efficient inhibition of hepatocellular carcinoma cell proliferation, Mater. Res. Express 7 (2020) 085008. https://doi.org/10.1088/2053-1591/abad15.

[67] C. Huo, J. Ouyang, H. Yang, CuO nanoparticles encapsulated inside Al-MCM-41 mesoporous materials via direct synthetic route, Sci. Rep. 4 (2014) 3682.
https://doi.org/10.1038/srep03682.

[68] Z.-L. Hua, J.-L. Shi, L. Wang, W.-H. Zhang, Preparation of mesoporous silica films on a glass slide: surfactant template removal by solvent extraction, J. Non-Cryst. Solids 292 (2001) 177-183. https://doi.org/10.1016/S0022-3093(01)00806-7.

[69] C. Karman, N. Vilà, A. Walcarius, Amplified charge transfer for anionic redox probes through oriented mesoporous silica thin films, ChemElectroChem 3 (2016) 2130-2137.
https://doi.org/10.1002/celc.201600303.

[70] S.B. Burujeny, H. Yeganeh, M. Atai, H. Gholami, M. Sorayya, Bactericidal dental nanocomposites containing 1,2,3-triazolium-functionalized POSS additive prepared through thio-ene click polymerization, Dental Mater. 33 (2017) 119-131.
https://doi.org/10.1016/j.dental.2016.11.001.

[71] Y. Li, X. Yang, F. Yang, Y. Wang, P. Zheng, X. Liu, Effective immobilization of Ru(bpy)₃²⁺ by functional composite phosphomolybdic acid anion on an electrode surface for solid-state electrochemiluminescence to sensitive determination of NADH, Electrochim. Acta, 66 (2012), 188-192. https://doi.org/10.1016/j.electacta.2012.01.087.

[72] R.M. Sábio, M. Gressier, J.M.A. Caiut, M.-J. Menu, S.J.L. Ribeiro, Luminescent multifunctional hybrids obtained by grafting of ruthenium complexes on mesoporous silica, Mater. Lett. 174 (2016) 1-5. https://doi.org/10.1016/j.matlet.2016.03.058.

[73] W. Ullah, G. Herzog, N. Vilà, A. Walcarius, Polyaniline nanowire arrays generated through oriented mesoporous silica films: effect of pore size and spectroelectrochemical response, Faraday Discuss. 233 (2022) 77-99. https://doi.org/10.1039/d1fd00034a.
[74] N. Vilà, C. Richart, D. Nieto, M. A. Brites Helú, I. M. Mbomekallé, A. Walcarius, In situ' formation and voltammetric characterization of polymeric linear [(Mn(H₂O)₃)₂(H₂W₁₂O₄₂)]ₙ 6n- polyoxometalates in vertically-oriented mesoporous silica thin films, Electrochim. Acta 499 (2024) 144702. https://doi.org/10.1016/j.electacta.2024.144702.

[75] J. Liu, D. He, Q. Liu, X. He, K. Wang, X. Yang, J. Shangguan, J. Tang, Y. Mao, Anal. Chem. 88 (2016) 11707-11713. https://doi.org/10.1021/acs.analchem.6b03317.

[76] P. Zhang, J. Guo, Y. Wang, W. Pang, Incorporation of luminescent tris(bipyridine)ruthenium(II) complex in mesoporous silica spheres and their spectroscopic and oxygen-sensing properties, Mater. Lett. 53 (2002) 400-405.
https://doi.org/10.1016/S0167-577X(01)00514-6.

[77] E. Bottinelli, I. Miletto, G. Caputo, S. Coluccia, E. Gianotti, Photoactive Ru complex embedded in mesostructured MCM-41 nanoparticles, J. Fluoresc. 21 (2011) 901-909. DOI 10.1007/s10895-010-0605-5.

[78] M. Ogawa, T. Nakamura, J. Mori, K. Kuroda, Luminescence of tris(2,2'-bipyridine)ruthenium(II) cations ([Ru(bpy)₃]²⁺) adsorbed in mesoporous silica, J. Phys. Chem. B 104 (2000) 8554-8556. https://doi.org/10.1021/jp0015123.

[79] J. N. Demas, E. W. Harris, R. P. McBride, Energy transfer from luminescent transition metal complexes to oxygen, J. Am. Chem. Soc. 99 (1977) 3547-3551.
https://doi.org/10.1021/ja00453a001.

[80] H. Wei, E. Wang, Solid-state electrochemiluminescence of tris (2,2'-bipyridyl) ruthenium, Trends Anal. Chem. 27 (2008) 447-459. https://doi.org/10.1016/j.trac.2008.02.009.

[81] L. Qian, X. Yang, One-step synthesis of Ru(2,2'-Bipyridine)₃Cl₂-immobilized silica nanoparticles for use in electrogenerated chemiluminescence detection, Adv. Funct. Mater. 17 (2007) 1353-1358. https://doi.org/10.1002/adfm.200600253.

[82] L. Zhang, S. Dong, Electrogenerated chemiluminescence sensors using Ru(bpy)₃²⁺ doped in silica nanoparticles, Anal. Chem. 78 (2006) 5119-5123. https://doi.org/10.1021/ac060451n.

[83] M. Rahal, N. Vilà, C. Despas, M. Sebbat, N. McClenaghan, E. Oheix, B. Lebeau, M. Cormier, J.-P. Goddard, A. Walcarius, Towards electrochemical regeneration of redox photocatalysts? The example of the tris(2,2'-bipyridine)ruthenium(II)/methylviologen system, J. Electroanal. Chem. 972 (2024) 118618. https://doi.org/10.1016/j.jelechem.2024.118618.
[84] G. Jones II, V. Malba, Pyridinium quenchers of Ru(bpy)₃²⁺. Charge effects on the yield of electron transfer, J. Org. Chem. 50 (1985) 5776–5782.
https://doi.org/10.1021/jo00350a068.

[85] H. Sun, A. Yoshimura, M.Z. Hoffman, Oxidative quenching of the excited state of tris(2,2'-bipyridine)ruthenium(2+) ion by methylviologen. Variation of solution medium and temperature, J. Phys. Chem. 98 (1994) 5058–5064. https://doi.org/10.1021/j100070a019.

[86] C.L. Bird, A.T. Kuhn, Electrochemistry of the Viologens, Chem. Soc. Rev. 10 (1981) 49–82. https://doi.org/10.1039/CS9811000049.

[87] H.N. McMurray, P. Douglas, C. Busa, M.S. Garley, Oxygen quenching of tris(2,2'-bipyridine) ruthenium(II) complexes in thin organic films, J. Photochem. Photobiol. A 80 (1994) 283–288. https://doi.org/10.1016/1010-6030(94)01055-2.

[88] N.C. Castillo, L. Ding, A. Heel, T. Graule, C. Pulgarin, On the photocatalytic degradation of phenol and dichloroacetate by BiVO₄: the need of a sacrificial electron acceptor, J. Photochem. Photobiol. A 216 (2010) 221–227.
https://doi.org/10.1016/j.jphotochem.2010.08.021.

[89] Y. Pellegrin, F. Odobel, Sacrificial electron donor reagents for solar fuel production, C. R. Chim. 20 (2017) 283–295. https://doi.org/10.1016/j.crci.2015.11.036.

[90] J. Wang, S. Zhou, B. Li, X. Liu, H. Chen, H. Wang, Improving the photostability of [Ru(bpy)₃]²⁺ by embedment in silica, ChemPhotoChem 6 (2022) e202200124.
https://doi.org/10.1002/cptc.202200124.
## Research Highlights

* Electrogenerated azide-functionalized vertically-aligned mesoporous silica films

* Covalent binding of [Ru(bpy)₂(bpy')']²⁺ photocatalysts by click coupling

* [Ru(bpy)₂(bpy')']²⁺ species are electrochemically accessible in the insulating film

* These films exhibit luminescence and photo(electro)catalytic properties

## Graphical abstract

<figure><img src="image_20.png" /></figure>