Scanning Gel Electrochemical Microscopy for Topography and Electrochemical Imaging

Scanning electrochemical probe techniques have been widely applied for analyzing the local electrochemical activity of surfaces and interfaces. In this work, we develop a new concept of carrying out local electrochemical measurements by localizing both the electrode and the electrolyte. This is achieved through a gel probe, which is prepared by electrodepositing chitosan-gelatin gel on a microdisk electrode. It is positioned in contact with the sample surface by shear force feedback. The preliminary results indicate that the topography of the sample can be mapped by tapping the probe and recording the coordinates at a given normalized shear force signal, while the local electrochemical activity can be retrieved from local measurements with the probe touching the sample surface. The technique is denoted as scanning gel electrochemical microscopy. As compared with existing techniques, it has a major advantage of operating in air with the electrolyte immobilized in gel. This would prevent the spreading and leakage of solution on the sample surface and may lead to field applications

Electrochemically Assisted Generation of Silica Deposits Using a Surfactant Template at Liquid/Liquid Microinterfaces

The electrochemically assisted generation of mesoporous silica deposits at arrays of microscopic liquid/liquid interfaces was investigated. Ion transfer voltammetry was used in order to initiate the formation of silica material by electrochemical transfer of template species (cetyltrimethylammonium, CTA⁺), initially present in the organic phase, to the aqueous phase containing the hydrolyzed silica precursors (tetraethoxysilane, TEOS). The deposition mechanism was investigated using cyclic voltammetry, based on the analysis of diffusion layer profiles of CTA⁺ species from the organic side of the interface. The morphology of the deposits varied from hemispherical to almost flat with the potential scan rate, the spacing factor of the microinterfaces array supporting the liquid/liquid interfaces, or the initial CTA⁺ and TEOS concentrations, as evidenced by scanning electron microscopy and profilometry analyses. The amount of deposited material can be related to the amount of CTA⁺ species passing across the liquid/liquid interfaces. Confocal Raman spectroscopy was used to confirm the presence of surfactant-templated silica deposits and to analyze the effectiveness of calcination in removing the organic molecules filling the interior of the pores. After template removal, the mesoporous network became accessible to external reagents, as checked by interfacial alkylammonium cation transfer, suggesting a possible analytical interest of such modified micro-liquid/liquid interfaces

Electro-Assisted Self-Assembly of Cetyltrimethylammonium-Templated Silica Films in Aqueous Media: Critical Effect of Counteranions on the Morphology and Mesostructure Type

The electro-assisted self-assembly (EASA) of tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTABr) in hydro-alcoholic medium is now recognized to be a versatile method to generate highly ordered mesoporous silica films with unique orientation of mesopore channels normal to the underlying surface. In this work, we have evaluated the possibility to extend the method to aqueous media (i.e., without adding a cosolvent) and to determine the parameters affecting the EASA process and the resulting organization/orientation of the mesoporous framework by using electron microscopies and diffraction techniques. Contrary to water/cosolvent-based sols, the nature of the surfactant and supporting electrolyte counteranions (X^–) was found to induce drastic variations on both the morphology and the mesostructural order of the deposits formed by electrochemically induced gelification (by pH increase) of CTAX/NaX-based silica sols. These changes are triggered by different surfactant assemblies arising from lower critical micellar concentration when passing from hydro-alcoholic to aqueous medium, and they are affected by the chaotropic–cosmotropic character of the counteranions. To be brief, cosmotropic anions (such as SO₄^2–) promote the formation of thin films but suffering from poor or no ordering, whereas weakly bonded anions (such as Cl^–) favor the mesostructuration but mainly in the form of particles or aggregates, while chaotropic anions (such as Br^–) lead to rather thick deposits made of poorly organized aggregates. Mixing these anions, to get mixed micelles, enables compromises to be reached between these “extreme” behaviors and mesostructured thin films can be indeed obtained with the CTACl/Na₂SO₄ and CTABr/Na₂SO₄ media, exhibiting, respectively, some vertical or horizontal orientation of mesopore channels. This can be rationalized by taking into account the CTA⁺, X^– binding strength variations (Cl^– < SO₄^2– < Br^–), thus affecting competitive binding of negatively charged silicate species, and sphere-to-rod transition abilities (SO₄^2– ≈ Cl^– < Br^–) of the CTA⁺-based templates. Cyclic voltammetry was also used to characterize mass transport processes through the films. Finally, a preliminary work aiming at getting swelled pores of such electrogenerated films with mesitylene was carried out to evaluate the potential interest of the water-based EASA process for the entrapment of hydrophobic molecules inside the surfactant–silica phases

Vertically Aligned and Ordered One-Dimensional Mesoscale Polyaniline

The growth of vertically aligned and ordered polyaniline nanofilaments is controlled by potentiostatic polymerization through hexagonally packed and oriented mesoporous silica films. In such small pore template (2 nm in diameter), quasi-single PANI chains are likely to be produced. From chronoamperometric experiments and using films of various thicknesses (100–200 nm) it is possible to evidence the electropolymerization transients, wherein each stage of polymerization (induction period, growth, and overgrowth of polyaniline on mesoporous silica films) is clearly identified. The advantageous effect of mesostructured silica thin films as hard templates for the generation of isolated polyaniline nanofilaments is demonstrated from enhancement of the reversibility between the conductive and the nonconductive states of polyaniline and the higher electroactive surface areas displayed for all mesoporous silica/PANI composites. The possibility to control and tailor the growth of conducting polymer nanofilaments offers numerous opportunities for applications in various fields including energy, sensors and biosensors, photovoltaics, nanophotonics, or nanoelectronics

Electrogeneration of a free-standing cytochrome c-silica matrix at a soft electrified interface

Interactions of a protein with a solid−liquid or a liquid− liquid interface may destabilize its conformation and hence result in a loss of biological activity. We propose here a method for the immobilization of proteins at an electrified liquid−liquid interface. Cytochrome c (Cyt c) is encapsulated in a silica matrix through an electrochemical process at an electrified liquid−liquid interface. Silica condensation is triggered by the interfacial transfer of cationic surfactant, cetyltrimethylammonium, at the lower end of the interfacial potential window. Cyt c is then adsorbed on the previously electrodeposited silica layer, when the interfacial potential, Δo wϕ, is at the positive end of the potential window. By cycling of the potential window back and forth, silica electrodeposition and Cyt c adsorption occur sequentially as demonstrated by in situ UV−vis absorbance spectroscopy. After collection from the liquid−liquid interface, the Cyt c−silica matrix is characterized ex situ by UV−vis diffuse reflectance spectroscopy, confocal Raman microscopy, and fluorescence microscopy, showing that the protein maintained its tertiary structure during the encapsulation process. The absence of denaturation is further confirmed in situ by the absence of electrocatalytic activity toward O2 (observed in the case of Cyt c denaturation). This method of protein encapsulation may be used for other proteins (e.g., Fe−S cluster oxidoreductases, copper-containing reductases, pyrroloquinoline quinonecontaining enzymes, or flavoproteins) in the development of biphasic bioelectrosynthesis or bioelectrocatalysis applications

Covalent Immobilization of (2,2′-Bipyridyl) (Pentamethylcyclopentadienyl)-Rhodium Complex on a Porous Carbon Electrode for Efficient Electrocatalytic NADH Regeneration

Covalent bonding of (2,2′-bipyridyl) (pentamethylcyclopentadienyl)-rhodium complex at the surface of a carbon-based porous electrode was achieved by combining diazonium electrografting, Huisgen cycloaddition, and metal complexation. The immobilized catalyst was applied to electrochemical regeneration of the reduced form of nicotinamide adenine dinucleotide (NADH). The different steps of surface functionalization were characterized by X-ray photoelectron spectroscopy and electrochemistry. The Faradaic efficiency of NADH regeneration was 87%. The chemical bonding provided good stability in solution under convection over 14 days, which is much better than the simple adsorption of the Rh complex on the electrode surface. Finally, the system was tested in the presence of NAD-dependent dehydrogenases that were immobilized in a sol–gel film on the top of the functionalized porous carbon electrode. A total turnover of 3790 and turnover frequency of 164 h^–1 was observed. Two enzymatic reactions were considered: d-fructose reduction to d-sorbitol catalyzed by d-sorbitol dehydrogenase and hydroxyacetone reduction to 1,2-propanediol with galactitol dehydrogenase. The electrocatalytic regeneration of 1 mM NADH by the immobilized rhodium species was kept after 90 h electrolysis in the conditions of electroenzymatic synthesis

Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat

An electrochemical method was developed for rapid and sensitive detection of the herbicide paraquat in aqueous samples using mesoporous silica thin film modified glassy carbon electrodes (GCE). Vertically aligned mesoporous silica thin films were deposited onto GCE by electrochemically assisted self-assembly (EASA). Cyclic voltammetry revealed effective response to the cationic analyte (while rejecting anions) thanks to the charge selectivity exhibited by the negatively charged mesoporous channels. Square wave voltametry (SWV) was then used to detect paraquat via its one electron reduction process. Influence of various experimental parameters (i.e., pH, electrolyte concentration, and nature of electrolyte anions) on sensitivity was investigated and discussed with respect to the mesopore characteristics and accumulation efficiency, pointing out the key role of charge distribution in such confined spaces on these processes. Calibration plots for paraquat concentration ranging from 10 nM to 10 μM were constructed at mesoporous silica modified GCE which were linear with increasing paraquat concentration, showing dramatically enhanced sensitivity (almost 30 times) as compared to nonmodified electrodes. Finally, real samples from Meuse River (France) spiked with paraquat, without any pretreatment (except filtration), were analyzed by SWV, revealing the possible detection of paraquat at very low concentration (10–50 nM). Limit of detection (LOD) calculated from real sample analysis was found to be 12 nM, which is well below the permissible limits of paraquat in drinking water (40–400 nM) in various countries