Grégoire Herzog

Voltammetric characterisation of silicon-based microelectrode arrays and their application to mercury-free stripping voltammetry of copper ions.

This paper describes the electrochemical characterisation of a range of gold and platinum microelectrode arrays (MEAs) fabricated by standard photolithographic methods. The inter-electrode spacing, geometry, numbers and dimensions of the electrodes in the arrays were found to influence the voltammetric behaviours obtained. Excellent correlation was found between experimental data and theoretical predictions employing published models of microelectrode behaviour. Gold MEAs were evaluated for their applicability to copper determination in a soil extract sample, where agreement was found between the standard analytical method and a method based on underpotential deposition-anodic stripping voltammetry (UPD-ASV) at the MEAs, offering a mercury-free alternative for copper sensing.

Electrochemical detection of oligopeptides at silicon-fabricated micro-liquid/liquid interfaces.

The detection of peptides is an important bioanalytical challenge, as they are a generic class of potent molecules of biomedical and biopharmaceutical significance. In this work, the electrochemistry of seven oligopeptides at microscaled interfaces between two immiscible electrolyte solutions (microITIES) was investigated. Their transfer across the polarized interface was assisted by dibenzo-18-crown-6 (DB18C6). The ion transfer potentials of these oligopeptides were dependent on their hydrophobicities and their interaction with DB18C6. Micropore arrays, which were fabricated in silicon by a combination of wet and dry etch techniques, were used to enhance mass transfer and thus analytical sensitivities. The use of a gellified organic phase allowed the implementation of voltammetric stripping techniques at the liquid-organogel interface. The combination of interface miniaturization and stripping voltammetry provided limits of detection at submicromolar concentration levels. The sensitivities (calibration graph slopes) were -3205 nA microM(-1) cm(-2) for Phe-Phe, -1791 nA microM(-1) cm(-2) for Leu-Leu, -6014 nA microM(-1) cm(-2) for Lys-Lys, and -9611 nA microM(-1) cm(-2) for Lys-Lys-Lys. Mixtures of peptides were also investigated with this technique, illustrating the possibility to detect certain mixture combinations.

Single Nanoskived Nanowires for Electrochemical Applications

In this work, we fabricate gold nanowires with well controlled critical dimensions using a recently demonstrated facile approach termed nanoskiving. Nanowires are fabricated with lengths of several hundreds of micrometers and are easily electrically contacted using overlay electrodes. Following fabrication, nanowire device performance is assessed using both electrical and electrochemical characterization techniques. We observe low electrical resistances with typical linear Ohmic responses from fully packaged nanowire devices. Steady-state cyclic voltammograms in ferrocenemonocarboxylic acid demonstrate scan rate independence up to 1000 mV s(-1). Electrochemical responses are excellently described by classical Butler-Volmer kinetics, displaying a fast, heterogeneous electron transfer kinetics, k(0) = 2.27 ± 0.02 cm s(-1), α = 0.4 ± 0.01. Direct reduction of hydrogen peroxide is observed at nanowires across the 110 pM to 1 mM concentration range, without the need for chemical modification, demonstrating the potential of these devices for electrochemical applications.

Detection of food additives by voltammetry at the liquid-liquid interface.

Electrochemistry at the liquid-liquid interface enables the detection of nonredoxactive species with electroanalytical techniques. In this work, the electrochemical behavior of two food additives, aspartame and acesulfame K, was investigated. Both ions were found to undergo ion-transfer voltammetry at the liquid-liquid interface. Differential pulse voltammetry was used for the preparation of calibration curves over the concentration range of 30-350 microM with a detection limit of 30 microM. The standard addition method was applied to the determination of their concentrations in food and beverage samples such as sweeteners and sugar-free beverages. Selective electrochemically modulated liquid-liquid extraction of these species in both laboratory solutions and in beverage samples was also demonstrated. These results indicate the suitability of liquid-liquid electrochemistry as an analytical approach in food analysis.

Electropolishing of medical-grade stainless steel in preparation for surface nano-texturing

The purpose of this work is to investigate the electropolishing of medical-grade 316xa0L stainless steel to obtain a clean, smooth, and defect-free surface in preparation for surface nano-texturing. Electropolishing of steel was conducted under stationary conditions in four electrolyte mixtures: (A) 4.5xa0M H2SO4 + 11xa0M H3PO4, (B) 7.2xa0M H2SO4 + 6.5xa0M H3PO4, (C) 6.4xa0M glycerol + 6.1xa0M H3PO4, and (D) 6.1xa0M H3PO4. The influence of electrolyte composition and concentration, temperature, and electropolishing time, in conjunction with linear sweep voltammetry and chronoamperometry, on the stainless steel surface was studied. The resulting surfaces of unpolished and optimally polished stainless steel were characterized in terms of contamination, defects, topography, roughness, hydrophilicity, and chemical composition by optical and atomic force microscopies, contact angle goniometry, and X-ray photoelectron spectroscopy. It was found that the optimally polished surfaces were obtained with the following parameters: electrolyte mixture A at 2.1xa0V of applied potential at 80xa0°C for 10xa0min. This corresponded to the diffusion-limited dissolution of the surface. The root mean square surface roughness of the electropolished surface achieved was 0.4xa0nm over 2u2009×u20092xa0μm2. Surface analysis showed that electropolishing led to ultraclean surfaces with reduced roughness and contamination thickness and with Cr, P, S, Mo, Ni, and O enrichment compared to untreated surfaces.

Ion-Transfer Voltammetric Behavior of Protein Digests at Liquid|Liquid Interfaces

The development of new methods for the detection of proteins and peptides is of widespread importance. In this work, the electrochemical behavior of peptide mixtures resulting from proteolytic digestion of proteins was investigated at the polarized liquid|liquid interface (or the interface between two immiscible electrolyte solutions, ITIES). The influence of pepsin digestion on three proteins (hemoglobin, lysozyme, and cytochrome c) was studied, and it was revealed that resulting cyclic voltammograms of the three protein digests were different due to the unique peptide mixtures for a given protein. Differential pulse stripping voltammetry of protein digests enabled the detection of digested proteins at concentrations ranging between 0.55 and 4.22 microM. A limit of detection of 0.55 microM of the initial concentration of protein was achieved, demonstrating the analytical possibilities of such an electrochemical method. These results show that ion-transfer voltammetry offers the opportunity to study and develop label-free detection of peptides resulting from enzymatic digestions of proteins and may thus have a role in development of new proteomic technologies.

Electrochemical activity of phenolic calixarenes

Abstract A preliminary evaluation of the electrochemical behaviour of phenolic calix[ n ]arenes ( n =4,6) possessing either H- or tert-butyl functionality at the para -phenol position has been undertaken by cyclic voltammetry. Electrochemical activity of these calixarenes is an inherent property, due to oxidation of the phenolic moiety in both types of calixarene. Oxidation of the p -H-substituted calixarenes leads to an electrode passivation process whereas oxidation of the p -t-butyl-substituted calixarenes does not. The former is attributed to electropolymerisation via intermolecular reaction of calixarene phenoxy radical species at the para -position to produce a non-conducting deposit on the electrode surface. This process is not possible with the p -t-butyl-substituted calixarenes.

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.

Combined Raman microspectrometer and shearforce regulated SECM for corrosion and self-healing analysis.

Shearforce regulated scanning electrochemical microscopy (SECM) has been associated with Raman microspectrometry in order to perform combined electrochemical and spectrochemical analysis on reactive interfaces. The interest of the method was evaluated by analyzing local corrosion phenomena in damaged Zn(Mg, Al) self-healing coatings deposited on steel. Despite the high aspect ratio of the analyzed sample displaying here more than a 50 μm depth profile, the optimized setup allowed (1) precise electrode positioning with the help of shearforce detection, (2) electrochemical measurement at a constant distance from the sample surface, and (3) local chemical analysis of the solid surface by confocal Raman microspectroscopy performed at a constant focal distance from the sample. All in all, this new setup allows one to approach the detailed reactivity involved in defective metal samples.

Microelectrochemical Systems on Silicon Chips for the Detection of Pollutants in Seawater

The ever-growing demand for simple, fast and reliable techniques for the detection of pollutants and contaminants in the environment has sparked the development of remote detection and monitoring systems which include application specific sensors, instrumentation and signal processing. We report here the design, fabrication and characterisation of four designs of microelectrochemical systems on silicon chip for the detection of pollutants in artificial seawater. These systems were fabricated by photolithography and incorporate a Pt working microelectrode array (squares or bands), a Pt counter electrode and a Ag|AgCl reference electrode. They have been characterised by cyclic voltammetry of ferricyanide and behaved in good agreement with the theory. These systems were evaluated over 72 hours and showed good stability. Underpotential Deposition – Stripping Voltammetry experiments of Cu2+ in artificial seawater have been carried out at an array of 35 microsquares of 20u2005µmu2009×u200920u2005µm. The sensitivity achieved was (2.93±0.14)u2005µA cm−2 µM−1, with 1u2005µM being the lowest Cu2+ concentration measured. These devices provide the basis for the development into sensor systems for remote analysis applications.