Electrochemically Assisted Self-assembly Research Articles

Electrochemical response of vertically-aligned, ferrocene-functionalized mesoporous silica films: effect of the supporting electrolyte

The influence of the supporting electrolyte on the electrochemical response of ordered and oriented, ferrocene-functionalized, mesoporous silica films has been investigated. The ferrocene-functionalized films were prepared by electrochemically-assisted self-assembly, leading to azide-functionalized silica materials in a first step, which were then allowed to react with ethynylferrocene according to a Huisgen azide–alkyne cycloaddition to get the final films bearing various amounts of ferrocene groups covalently attached to the mesopore channels. In such organic–inorganic hybrid redox polymer, the charge transfer mechanism involves electron hopping between adjacent redox sites and transport of electrolyte ions through the film to maintain charge balance (i.e., ingress of anions to compensate the positive charges generated by the oxidation of ferrocene moieties). The electrochemical behavior of the ferrocene-functionalized films is thus likely to be affected by the density of redox sites in the material as well as the supporting electrolyte nature and concentration. This has been analyzed here, showing that electron transfer processes are favored in mesopore channels containing great amounts of ferrocene groups, in an electrolyte-rich medium, and for counter-anions of small size and big charge. Electron transfer kinetics are primarily governed by the density of redox sites in the material while the intensity of voltammetric signals is mainly controlled by the transport of counter-anions in the confined space of mesopore channels.

Platinum Ultramicroelectrodes Modified with Electrogenerated Surfactant‐Templated Mesoporous Organosilica Films: Effect of Film Formation Conditions on Its Performance in Preconcentration Electroanalysis

AbstractPt µdisc electrodes have been modified by mesoporous organosilica thin films by electrochemically assisted self‐assembly (EASA) of mercaptopropyltrimethoxysilane (MPTMS), tetraethoxysilane (TEOS), and the surfactant cetyltrimethylammonium bromide (CTAB). The EASA process involves the generation of hydroxide ions at the electrode/solution interface, upon the application of a cathodic current density, leading to TEOS and MPTMS polycondensation around the CTAB template and concomitant growing of a thiol‐functionalized mesoporous film onto the electrode surface. The experimental conditions (current density, deposition time, silane concentration and molar ratio between surfactant template and silane) were optimised to form a thin and permeable film likely to be used in preconcentration electroanalysis. The morphology of the film electrodes were characterised by scanning electron microscopy. The permeability properties of the modified Pt µdisc electrodes have been evidenced by cyclic voltammetry using Ru(NH3)63+ as a redox probe. The best parameters identified for the film preparation are a current density of − 8 mA cm−2 applied for 15 s in a solution containing 110 mM of hydrolysed silane precursors and 70.4 mM of CTAB. Pt µdisc electrodes modified in these conditions were used for the open‐circuit preconcentration of Hg(II) species prior to their detection by anodic stripping voltammetry in a mercury‐free solution. In the optimized conditions, a sensitivity of 14.3 mA cm−2 µM−1 was obtained for the 0.02–0.08 µM concentration range. The analytical performance of such organosilica films could decay by up to two orders of magnitude for the materials prepared in conditions other than the optimized ones, highlighting the need for a fine control of the deposition parameters to elaborate sensors based on such modified ultramicroelectrodes.

Hydroquinone functionalized oriented MCM-41 mesochannels at the electrode surface

The ordered mesoporous silica (MCM-41) thin film has been electrodeposited at the electrode surface by the method known as electrochemically assisted self assembly (EASA). The constructed structures with unique morphology have been functionalized with hydroquinone derivatives as an electron mediator. Three functionalization routes, one co-condensation and two grafting methods, have been examined for the attachment of hydroquinone derivative to the electrode surface. For all electrodes the electroactive group immobilized successfully on deposited MCM-41 thin film and all of the constructed modified electrodes show the adequate and relatively stable currents. The electrochemical behaviors of these electroactive porous electrodes and their performances in electrocatalytic oxidation of benzyl alcohol and hydrazine have been studied by electrochemical methods.

Electrochemically assisted self-assembly of ordered and functionalized mesoporous silica films: impact of the electrode geometry and size on film formation and properties

Surfactant-templated mesoporous silica thin films can be deposited onto solid electrode surfaces by electrochemically assisted self-assembly (EASA). The method involves a cathodically triggered self-assembly of cationic surfactants (cetyltrimethyl ammonium bromide, CTAB) and local pH increase leading to the polycondensation of silica precursors (i.e., tetraethoxysilane, alone or in the presence of (3-mercaptopropyl) trimethoxysilane (MPTMS)) and concomitant growth of the ordered mesoporous silica or organosilica film. The present work shows that the EASA method can be applied to film deposition on electrode supports of various morphologies, geometries and sizes (large and flat discs or non-flat streaked supports, i.e., gold CD-trodes, as well as several kinds of ultramicroelectrodes, including carbon fibers, platinum wires, and platinum microdiscs). Galvanostatic conditions were mainly preferred to potentiostatic conditions to avoid problems related to various overpotentials and surface areas experienced with the various working electrodes used here. The results indicate that film deposition was possible on each electrode support but also that both the film formation and properties were dependent on the experimental conditions for EASA. For example, passing from large electrodes to ultramicroelectrodes required the application of larger current densities to ensure film deposition, which can be due to faster loss of the hydroxyl species in solution in the case of radial or spherical diffusion, in comparison to the linear. Highly porous deposits were obtained after template removal, as ascertained by cyclic voltammetry using Ru(NH3)6(3+) as a redox probe. The advantage of better signal-to-background current ratios for ultramicroelectrodes relative to the macroscopic ones was maintained after film deposition, also resulting in higher sensitivity when used in conditions of preconcentration electroanalysis (using silver(I) or mercury(II) as a probe being accumulated by complexation to MPTMS-based films).

Electrochemical approaches for the fabrication and/or characterization of pure and hybrid templated mesoporous oxide thin films: a review

Electrochemistry can be used for fabrication and characterization of mesoporous oxide films. First, this review provides insight into the methods used to prepare templated mesoporous thin films on an electrode surface, i.e., evaporation-induced self-assembly (EISA) and electrochemically assisted self-assembly (EASA). Electrochemical characterization of mass transport processes in pure and organically functionalized mesoporous oxide films is then discussed. The electrochemical response can be basically restricted by the electron/mass transfer reaction at the electrode-film interface and diffusion through mesopore channels. The contributions of cyclic voltammetry, hydrodynamic voltammetry, electrochemical impedance spectroscopy, and scanning electrochemical microscopy to the characterization of films with distinct mesostructures are finally described, with special emphasis on identification of conditions that can affect the electrochemical response recorded with such modified electrodes.