Supported mesostructured thin films are of major importance for applications in optical, electrochemical and sensing devices. However, good performance is restricted to mesostructured phases ensuring good accessibility from the film surface, which would be straightforward with cylindrical pores oriented normal to the underlying support, but this remains challenging. Here, we demonstrate that electrochemistry is likely to induce self-assembly of surfactant-templated (organo)silica thin films on various conducting supports, homogeneously over wide areas. The method involves the application of a suitable cathodic potential to an electrode immersed in a surfactant-containing hydrolysed sol solution to generate the hydroxyl ions that are necessary to catalyse polycondensation of the precursors and self-assembly of hexagonally packed one-dimensional channels that grow perpendicularly to the electrode surface. The method is compatible with controlled and localized deposition on heterogeneous supports, opening the way to electrochemically driven nanolithography for designing complex patterns of widely accessible mesostructured materials.

Electrochemically assisted self-assembly of mesoporous silica thin films.

Biosensors, combining a selective biological recognition element and a sensitive transducer, are versatile analytical tools applied more and more in different fields, such as medicine, food quality and safety control, and environment pollution monitoring. They are expected to play an increasingly important role in the improvement of life quality. In this context, the present work covers recent approaches in design and development of biosensors applied for analysis of real samples of medical, environmental or industrial relevance. The described sensors meet the sensitivity, selectivity, and response time required by their applications. Moreover, they are designed to avoid contamination of the monitored systems with undesirable components and to minimise the damage of living organisms (when it comes to in vivo monitoring). (C) 2004 Elsevier B.V. All rights reserved. (Less)

Biosensors for life quality. Design, development and applications

The sol-gel immobilization of enzymes within metal oxides results in thin composite layers which are useful for biosensors, e.g., in combination with optical or electrochemical transducers. The activity of glucose oxidase (GOD) immobilized within SiO2 gels is mainly determined by the chemical and physical parameters of the sol-gel process. So, the type of the sol, the conditions of the SiO2 formation, the GOD concentration and the addition of penetration agents are studied to optimize the enzyme activity and the response of the complete amperometric biosensors. Both optical and electrochemical methods are used to measure activities of immobilized GOD. The applications of such GOD-SiO2 composites (i) directly on the electrode surface of a flat sensor, (ii) indirectly within Cuprophan® membranes of an amperometric hydrogen peroxide sensor or (iii) on zeolite beads in a cartridge followed by an H2O2 sensor are compared with regard to sensitivity and selectivity of the respective sensor systems. The best membrane sensor properties are obtained for silica hydrogel composites containing large amounts of GOD produced with sols from acidic hydrolysis of a 25% 3-aminopropyltrimethoxysilane-75% tetraethoxysilane precursor. Xerogels from acidic hydrolysed tetraethoxysilane sols within aprotic solvents like dioxane are favoured for direct application on Pt working electrodes of flat sensor structures.

Biosensor properties of glucose oxidase immobilized within SiO2 gels

In this work, we describe a versatile approach to generate highly methylated mesoporous silica films exhibiting hexagonally-packed one-dimensional mesochannels oriented uniquely perpendicular to the underlying support. It involves the electro-assisted self-assembly (EASA) of tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) with cetyltrimethylammonium bromide (CTAB) under potential control. Transmission electron microscopy and grazing-incidence X-ray diffraction indicate that high level of ordering and vertical alignment of mesopores can be maintained at rather high MTES contents (up to 50–60 mol% with respect to total precursors in the starting sol). The rate of film growth decreased concomitantly to increasing the MTES/TEOS ratio, resulting in thinner films for higher functionalization degrees. This also led to more hydrophobic films (after template extraction), which tended to become less permeable to aqueous phase solutes, as pointed out qualitatively and quantitatively by several electrochemical methods and various redox probes. Effective rate constants for electron transfer were found to be several orders of magnitude higher than for other thin silica films on electrodes, explained by preferential porosity orientation.

Electrogeneration of highly methylated mesoporous silica thin films with vertically-aligned mesochannels and electrochemical monitoring of mass transport issues

Cholesterol oxidase (ChOx) has been immobilized onto electrochemically prepared poly- N -methylpyrrole P(NMPY)- p -toluene sulphonate (PTS)/indium–tin-oxide (ITO) films by physical adsorption and entrapment techniques, respectively. The ChOx(entrapped)-P(NMPY)-PTS/ITO, ChOx(physisorbed)-P(NMPY)-PTS/ITO along with ferrocene carboxylic acid (FeMC) mediated ChOx(physisorbed)-P(NMPY)-PTS/ITO films have been characterized using cyclic voltammetry (CV), Fourier-transform-infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) techniques, respectively. Compared to ChOx(entrapped)-P(NMPY)-PTS/ITO (0.205 mA (mM cm 2 ) −1 ) and ChOx(physisorbed)-P(NMPY)-PTS/ITO (0.24 mA (mM cm 2 ) −1 ) electrodes, the ChOx(physisorbed)–FeMC(physisorbed)-P(NMPY)-PTS/ITO bio-electrode exhibits increased sensitivity (0.45 mA (mM cm 2 ) −1 ) and decreased response to interferent (ascorbic acid, uric acid, urea, glucose). A tentative mechanism for the immobilization of cholesterol oxidase onto P(NMPY)- p -toluene sulphonate (PTS) films has been proposed.

Application of electrochemically prepared poly-N-methylpyrrole-p-toluene sulphonate films to cholesterol biosensor

Mechanism of a "jumping off" ferricenium in glucose oxidase-D-glucose-ferrocene micellar electrochemical systems

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Mechanism of a "jumping off" ferricenium in glucose oxidase-D-glucose-ferrocene micellar electrochemical systems