Electrochemistry in solids prepared by sol-gel processes

Stainless steel catalyst for air pollution control: structure, properties, and activity

With the awakening of environmental awareness, the importance of air quality to human health and the proper functioning of social mechanisms is becoming increasingly prominent. The low cost and high efficiency of catalytic technique makes it a natural choice for achieving deep air purification. Stainless steel alloys have demonstrated their full potential for application in a variety of catalytic fields. The diversity of 3D networks or fibrous structures increases the turbulence within the heterogeneous catalysis, balance the temperature distribution in the reaction bed and, in combination with a highly thermally conductive skeleton, avoid agglomeration and deactivation of the active components; corrosion resistance and thermal stability are adapted to highly endothermic/exothermic or corrosive reaction environments; oxide layers formed by bulk transition metals activated by thermal treatment or etching can significantly alter the physico-chemical properties between the substrate and active species, further improving the stability of stainless steel catalysts; suitable electronic conductivity can be applied to the electrothermal catalysis, which is expected to provide guidance for the reduction of intermittent emission exhausts and the storage of renewable energy. The current applications of stainless steel as catalyst or support in the air purification have covered soot particle capture and combustion, catalytic oxidation of VOCs, SCR, and air sterilization. This paper summarizes several preparation methods and presents the relationships between the preparation process and the activity, and reviews its application and the current status of research in atmospheric environmental management, proposing the advantages and challenges of the stainless steel-based catalysts.

Electroanalysis based on stand-alone matrices and electrode-modifying films with silica sol-gel frameworks: a review

Silica sol-gel matrices and its organically modified analogues that contain aqueous electrolytes, ionic liquids, or other ionic conductors constitute stand-alone solid-state electrochemical cells when hosting electrodes or serve as modifying films on working electrodes in conventional cells. These materials facilitate a wide variety of analytical applications and are employed in various designs of power sources. In this review, analytical applications are the focus. Solid-state cells that serve as gas sensors, including in chromatographic detectors of gas-phase analytes, are described. Sol-gel films that modify working electrodes to perform functions such as hosting electrochemical catalysts and acting as size-exclusion moieties that protect the electrode from passivation by adsorption of macromolecules are discussed with emphasis on pore size, structure, and orientation. Silica sol-gel chemistry has been studied extensively; thus, factors that control its general properties as frameworks for solid-state cells and for thin films on the working electrode are well characterized. Here, recent advances such as the use of dendrimers and of nanoscale beads in conjunction with electrochemically assisted deposition of silica to template pore size and distribution are emphasized. Related topics include replacing aqueous solutions as the internal electrolyte with room-temperature ionic liquids, using the sol-gel as an anchor for functional groups and modifying electrodes with silica-based composites.

Surface renewable sol–gel composite electrode derived from 3-aminopropyl trimethoxy silane with covalently immobilized thionin

Sol-gel technique has been used for the covalent immobilization of the water-soluble mediator, thionin to construct a bulk modified, leak free composite electrode. This renewable composite electrode provides stable immobilization matrix for thionin via glutaraldehyde crosslinking. In the electrode composition the sol-gel precursor 3-aminopropyltrimethoxy silane serves as the host for immobilization of thionin, thereby preventing its leakage. An additional precursor methyl trimethoxy silane endows hydrophobicity and limits the wetting section of the modified electrode. Cyclic voltammetric characterization of the modified electrode in the potential range of 0.2 to -0.6 V exhibited stable redox peaks with a formal potential of -0.273 V, corresponding to immobilized thionin. This chemically modified electrode exhibits good electrocatalytic activity for the reduction of H(2)O(2) at a lower potential of -0.35 V. The reduction current of the modified electrode increases linearly in the range of 3.44 x 10(-6)M to 3.07 x 10(-3)M H(2)O(2) with a detection limit of 1.38 x 10(-6)M. The stable and quick response (5s) during chronoamperometry shows the potential application of the modified electrode for flow system analysis. The low potential operation (-0.35 V) favoured selective determination of H(2)O(2). The composite electrode exhibits distinct advantages of polishing in the event of surface fouling as well as simple preparation, good chemical and mechanical stability, economical and remarkable long-term stability (more than 1 year). The applicability of the present sensor for H(2)O(2) determination proposes a method for the detection of other biologically significant analytes.

Inorganic sensing using organofunctional sol-gel materials

This Account describes recent work in the development and applications of sol-gel sensors for concentrated strong acids and bases and metal ions. The use of sol-gel films doped with organic indicators for the optical sensing of concentrated strong acids (HCl, 1-10 M) and bases (NaOH, 1-10 M) has been explored, and the development of dual optical sensor approaches for ternary systems (HCl-salt-H 2O and NaOH-alcohol-H 2O) to give acid and salt, as well as base and alcohol, concentrations is discussed. The preparation of transparent, ligand-grafted sol-gel monoliths is also described, and their use in the analysis of both metal cations (Cu (2+)) and metal anions [Cr(VI)] is presented. A new model using both metal ion diffusion and immobilization by the ligands in such monoliths has been developed to give metal concentrations using the optical monolith sensors. In addition to optical sensing, a method utilizing ligand-grafted sol-gel films for analyte preconcentration in the electrochemical determination of Cr(VI) has been explored and is discussed.

Amperometric hydrogen peroxide sensor based on a sol-gel-derived ceramic carbon composite electrode with toluidine blue covalently immobilized using 3-aminopropyltrimethoxysilane

A carbon composite amperometric hydrogen peroxide sensor has been developed using a sol-gel technique. Toluidine blue (TB), which acts as the redox mediator, was covalently immobilized via glutaraldehyde crosslinking with an organically modified silane, namely 3-aminopropyltrimethoxysilane (APTMOS). Methyltrimethoxysilane (MTMOS) was used as the additional monomer; this controls the hydrophobicity of the electrode surface, thus limiting the wettability. The immobilization of TB within the sol-gel matrix was confirmed with FTIR studies. The sol-gel mixture containing TB immobilized in APTMOS and MTMOS was mixed with graphite powder in order to prepare the carbon composite electrode. The electrode was characterized using voltammetric techniques and its electrocatalytic activity for the reduction of hydrogen peroxide was also studied. The carbon composite electrode has the advantage of sensing H(2)O(2) at a lower potential and with a higher sensitivity, and interferences due to ascorbic acid, uric acid and acetaminophen were greatly minimized. The linear range for the determination of H(2)O(2) extends from 5.37 x 10(-6) to 6.15 x 10(-3) M, with a correlation coefficient of 0.9981. The detection limit was found to be 2.15 x 10(-6) M. The covalent immobilization of TB effectively prevents the leakage of the water-soluble mediator during measurements. The modified electrode, aside from electrocatalyzing the reduction of H(2)O(2), exhibits distinct advantages in terms of surface renewal in the event of surface fouling, as well as simple preparation, good chemical and mechanical stability, and good reproducibility.

Hydrophobic silica sol–gel films for biphasic electrodes and porotrodes

Hydrophobic sol-gel films from methyltrimethoxysilane (MTMOS) are deposited onto glass and tin-doped indium oxide (ITO) coated glass substrates. Uniform and microporous films of ca. 200 nm thickness are obtained and investigated by scanning electron microscopy and by electrochemical techniques. From cyclic voltammograms for the oxidation of ferrocenedimethanol in aqueous 0.1 M KNO3 apparent diffusion coefficients and free volume data for processes within the film are derived and it is demonstrated that the film morphology can be controlled by the deposition timing. Two novel types of biphasic electrodes for observing liquid/liquid ion transfer reactions are introduced: (i) an ITO electrode coated with a hydrophobic sol-gel film and (ii) a hydrophobic sol-gel film on glass sputter-coated with 20 nm porous gold (porotrode). For the t-butylferrocene redox system deposited in the form of an organic liquid, very low and morphology dependent current responses are observed on modified ITO electrodes. However, the porotrode system allows biphasic electrode reactions to be driven with high efficiency and with no significant morphology effect of the hydrophobic sol-gel film. This type of nanofilm-modified electrode system will be of interest for biphasic sensor developments.

Studies of sol-gel ceramic film incorporating methylene blue on glassy carbon: an electrocatalytic system for the simultaneous determination of ascorbic and uric acids

In this work, we investigated the immobilization of methylene blue in a methyltrimethoxysilane sol-gel ceramic film on a glassy carbon electrode. Up to a certain saturation level, under our conditions, it was found that the methylene blue was tightly held and did not leach out into aqueous solutions, even with continuous immersion for up to 1 month. The electrochemical behavior of the immobilized methylene blue was then studied. pH variation revealed that there were two distinct redox couples whose existences were pH-dependent. The methylene blue/sol-gel film was also examined as an electrocatalytic system for ascorbic and uric acid oxidations. It was revealed that this system was highly sensitive for ascorbic and uric acid sensing (practical determination limits of 5.00 nM and 1.00 nM for ascorbic acid and uric acid, respectively) and also allowed simultaneous determination of these biomolecules. The simultaneous determination of these two analytes in a human urine sample was demonstrated. The stability of the methylene blue/sol-gel film/glassy carbon electrode sensing system was good, with up to at least a month of continual operation.