Amplified Charge Transfer for Anionic Redox Probes through Oriented Mesoporous Silica Thin Films

Abstract

Diffusion of anionic species through ordered mesoporous silica films is considerably restricted due to electrostatic repulsion by the negatively charged silica surface and this effect is even worse in low ionic strength media (owing to Donnan exclusion effects). This is at the origin of the poor electrochemical response of anionic redox probes at electrodes modified with such mesoporous thin films. Here, by using indium–tin oxide (ITO) electrodes covered with vertically aligned mesoporous silica films generated by electrochemically assisted self‐assembly (EASA), we show that charge transfer for anionic redox probes [i.e. Fe(CN)63−/4−] can be dramatically amplified by using a neutral [ferrocene dimethanol, Fc(MeOH)2] or cationic [(Ru(NH3)63+] redox mediator. This has been observed by cyclic voltammetry, showing current enhancement in the presence of the mediator, which acts as a mobile electron shuttle between the charge‐excluded Fe(CN)63−/4− species from the mesoporous silica film and the underlying ITO surface owing to the ability of the mediator species to diffuse freely through the oriented mesopore channels. This behavior was significantly affected by the relative contents of Fe(CN)63−/4− probe to Fc(MeOH)2 mediator, the potential scan rate, and the supporting electrolyte concentration. The most effective improvements were reached at low Fe(CN)63−/4− concentration, high mediator content, low potential scan rate, and low ionic strength.

Nanoconfinement Effects on Redox Probe Transport in Lipid Assemblies on and in Mesoporous Silica Thin Films

“…Due to the negative charge of the silica surface, electrostatic repulsion drives the transport of anionic redox species in mesoporous silica thin films, making the influence of film structure more difficult to interpret. [ 28 ] To eliminate the electrostatic repulsion, the positively charged/neutral redox species FDM + /FDM (at a solution pH of 7.4) was chosen as the probe molecule. [ 14b ] After applying a small amplitude sinusoidal potential to the system, the complex impedance (Z(ω) = Z′+ j Z′′) can be extracted from the resulting AC current and presented in the complex plane, giving a Nyquist plot.…”

Mesoporous Silica-Based Materials for Electronics-Oriented Applications

“…These films can be generated by a versatile method called “Electrochemically Assisted Self-Assembly” (EASA) involving the vertical growth of silica walls around a cationic surfactant template under potentiostatic or galvanostatic control [40,160]. One already knows that such vertical orientation leads to fast mass transport of solution-phase redox probes through the film, resulting in highly sensitive electrochemical responses that can be exploited in the sensors field (e.g., [161,162,163,164]). We show next that these oriented films can also be used as template for conducting polymers and as support for the covalent binding of redox molecules, both giving rise to effective charge transfer with possible applications in sensing, electrocatalysis, energy or molecular electronics.…”

Critical Effect of Film Thickness on Preconcentration Electroanalysis with Oriented Mesoporous Silica Modified Electrodes

“…Here we discuss the critical effect of film thickness of the sensor's sensitivity, on the basis of deposits obtained after distinct synthesis durations. This cationic redox probe has been selected as this is the only one likely to distinguish the coating uniformity (neutral probes can dissolve in the surfactant phase and negative probes suffer from restricted diffusion through the film [35]). This cationic redox probe has been selected as this is the only one likely to distinguish the coating uniformity (neutral probes can dissolve in the surfactant phase and negative probes suffer from restricted diffusion through the film [35]).…”