Preparation and Electrochemical Properties of the Pd-modified Cu Electrode for Nitrate Reduction in Water

The reduction of nitrate has gained renewed attention due to environmental problems like overfertilization and the increasing costs of purification of drinking water. The usual techniques (e.g. ion-exchange and biofiltration) have some disadvantages1. So …     

Deposition of silica thin films formed by sol–gel method

Deposition of silica thin films on silicon wafer was investigated by in situ mass measurements with a microbalance configured for dip coating. Mass change was recorded with respect to deposition time when the substrate was fully immersed in the silica sol. Mass gain during deposition was higher than predicted from monolayer coverage of silica nano particles. This implied that deposition was facilitated by gelling of the nanoparticles on the substrate. The rate of deposition was enhanced by increasing the particle concentration in the sol and by decreasing the particle size from 12 to 5 nm. Increasing the salt concentration of the silica sol at constant pH enhanced the deposition of the silica particles. Reducing the pH of the sol from 10 to 6 decreased the deposition rate due to aggregation of the primary silica particles.     

Zeta potential orientation dependence of sapphire substrates

The zeta potential of planar sapphire substrates for three different crystallographic orientations was measured by a streaming potential technique in the presence of KCl and (CH3)4NCl electrolytes. The streaming potential was measured for large single crystalline C-plane (0001), A-plane (1120), and R-plane (1102) wafers over a full pH range at three or more ionic strengths ranging from 1 to 100 mM. The roughness of the epi-polished wafers was verified using atomic force microscopy to be on the order of atomic scale, and X-ray photoelectron spectroscopy (XPS) was used to ensure that the samples were free of silica and other contaminants. The results reveal a shift in the isoelectric point (iep) of the three samples by as much as two pH units, with the R-plane surface exhibiting the most acidic behavior and the C-plane samples having the highest iep. The iep at all ionic strengths was tightly centered around a single pH for each wafer. These values of iep are substantially different from the range of pH 8-10 consistently reported in the literature for alpha-Al2O3 particles. Particle zeta potential measurements were performed on a model powder using phase analysis light scattering, and the iep was confirmed to occur at pH 8. Modified Auger parameters (MAP) were calculated from XPS spectra of a monolayer of iridium metal deposited on the sapphire by electron beam deposition. A shift in MAP consistent with the observed differences in iep of the surfaces confirms the effect of surface structure on the transfer of charge between the Ir and sapphire, hence accounting for the changes in acidity as a function of crystallographic orientation.     

A Micro Planar Ampehometric Glucose Sensor Using an Isfet as a Reference Electrode

Abstract

A very small glucose sensor has been realized, which consists of a gold working electrode with a glucose oxidase immobilized membrane on it, and a gold counter electrode, all made on a sapphire substrate. By using the pH sensitive ISFET as a reference electrode, the potential for a solution, whose pH is constant, can be measured and irreversible metal electrodes, such as gold or platinum, can be used as working electrode and counter electrode. The sensor is very suitable for miniaturizing and mass production, because the Integrated Circuit (IC) fabrication process can be applied. The glucose oxidase immobilized membrane was also deposited by a lift off method, one of the IC processes. A glucose concentration, from 1 to 100 mg/dl, was measured with good linear current output.     

Electrochemically‐Induced Deposition of Amine‐Functionalized Silica Films on Gold Electrodes and Application to Cu(II) Detection in (Hydro)Alcoholic Medium

Well‐adherent amine‐functionalized porous silica films have been deposited on gold electrodes by combining the self‐assembly technology, the sol–gel process, and the electrochemical modulation of pH at the electrode/solution interface. A partial self‐assembled monolayer of mercaptopropyl‐trimethoxysilane (MPTMS) was first formed on disposable gold electrodes from recordable CDs (Au‐CDtrodes). The so pretreated MPTMS‐Au‐CDtrodes were immersed in a stable sol solution (pH 3) containing (3‐aminopropyl)‐triethoxysilane (APTES) and tetraethoxysilane (TEOS). Polycondensation of the APTES and TEOS precursors was then achieved by applying a negative potential for a given period of time to generate a local pH increase at the electrode/solution interface and promote the deposition of the amine functionalized silica film adhering well to the electrode surface owing to the MPTMS monolayer acting somewhat as a “molecular glue”. Various parameters affecting the electrodeposition process have been studied and the film permeability to redox probes in solution was characterized by cyclic voltammetry. The amine‐functionalized silica film electrodes were then applied to the preconcentration of copper(II) species prior to their electrochemical detection by anodic stripping differential pulse voltammetry. Getting high sensitivity has however required the application of an electrochemical pre‐activation step as the majority of the organo‐functional groups were in the form of ammonium moieties (because the film was prepared from an acidic sol). This was achieved by applying a sufficiently negative potential to the electrode surface to reduce protons and increase consequently the amine‐to‐ammonium ratio within the film and, thus, the efficiency of the precocentration process. The resulting device was then optimized for copper(II) determination in hydroalcoholic medium, giving rise to a linear response in the 0.1–10 μM concentration range.     

Direct electron transfer kinetics in horseradish peroxidase electrocatalysis

The study of direct electron transfer between enzymes and electrodes is frequently hampered by the small fraction of adsorbed proteins that remains electrochemically active. Here, we outline a strategy to overcome this limitation, which is based on a hierarchical analysis of steady-state electrocatalytic currents and the adoption of the "binary activity" hypothesis. The procedure is illustrated by studying the electrocatalytic response of horseradish peroxidase (HRP) adsorbed on graphite electrodes as a function of substrate (hydrogen peroxide) concentration, electrode potential, and solution pH. Individual contributions of the rates of substrate/enzyme reaction and of the electrode/enzyme electron exchange to the observed catalytic currents were disentangled by taking advantage of their distinct dependence on substrate concentration and electrode potential. In the absence of nonturnover currents, adoption of the "binary activity" hypothesis provided values of the standard electron-transfer rate constant for reduction of HRP Compound II that are similar to those reported previously for reduction of cytochrome c peroxidase Compound II. The variation of the catalytic currents with applied potential was analyzed in terms of the non-adiabatic Marcus-DOS electron transfer theory. The availability of a broad potential window, where catalytic currents could be recorded, facilitates an accurate determination of both the reorganization energy and the maximum electron-transfer rate for HRP Compound II reduction. The variation of these two kinetic parameters with solution pH provides some indication of the nature and location of the acid/base groups that control the electronic exchange between enzyme and electrode.     

Silica‐modified Electrodes for Electrochemical Detection of Malachite Green

New silica‐modified glassy carbon electrodes prepared with three different sorts of ordered mesoporous silica (OMS) were characterized and tested for the electrochemical detection of Malachite Green (MG). The electrodes were prepared by drop casting using silica suspensions and, for stability sake, a Nafion coating was deposited on the electrode top by the same technique. Square wave anodic stripping voltammetry was used to investigate the effect of various experimental parameters (deposition time, solution pH, silica type and concentration) on the performance of the modified electrodes. The best electrode (GC/MCM‐41‐NH₂/Nafion) with detection limit 0.36 μM, sensitivity 0.164±0.003 A/M; linear domain 1–6 μM was applied to detect MG in a commercial product commonly used as biocide in aquaria for ornamental fish.     

Deposition of nanosized latex particles onto silica and cellulose surfaces studied by optical reflectometry

Deposition of positively charged nanosized latex particles onto planar silica and cellulose substrates was studied in monovalent electrolyte solutions at pH 9.5. The deposition was probed in situ with optical reflectometry in a stagnation point flow cell. The surface coverage can be estimated reliably with island film theory as well as with a homogeneous film model, as confirmed with atomic force microscopy (AFM). The deposition kinetics on the bare surface was of first order with respect to the particle concentration, whereby the deposition rate was close to the value expected for a perfect collector. The efficiency coefficient, which was defined as the ratio of the experimental and theoretical deposition rate constants, was in the range from 0.3 to 0.7. Subsequently, the surface saturated and a limiting maximum coverage was attained (i.e., blocking). These trends were in qualitative agreement with predictions of the random sequential absorption (RSA) model, where electrostatic interactions between the particles were included. It was observed, however, that the substrate strongly influenced the maximum coverage, which was substantially higher for silica than for cellulose. The major conclusion of this work was that the nature of the substrate played an important role in a saturated layer of deposited colloidal particles.     

Electrocatalytic reduction of dioxygen at platinum particles deposited on Nafion- and clay-coated electrodes

Nafion- and clay-coated electrodes are prepared by casting a known amount of Nafion and clay solutions on the glassy carbon electrode (GC) surface. Subsequently platinum (Pt) particles are deposited on the GC electrodes and on the Nafion- and clay-coated GC electrodes. The formation of Pt particles on the modified electrode surface is analysed by scanning electron microscope, while cyclic voltammetry provides information on the anodisation of Pt particles deposited on the GC electrodes at >0.5 V(SCE) leading to the formation of platinum oxide (PtO). The involvement of PtO on the catalysed dioxygen reduction to hydrogen peroxide is reported. Macrocyclic cobalt(III) complex is also used as the electrocatalyst. The effect of pH on the reduction of PtO and dioxygen in deaerated and oxygenated solutions is studied to understand the dioxygen reduction processes. Received: 24 January 1997 / Accepted: 14 April 1997     

Factors affecting the preparation and properties of electrodeposited silica thin films functionalized with amine or thiol groups

Well-adherent sol-gel-derived silica films functionalized with amine or thiol groups have been electrogenerated on gold electrodes and both the deposition process and the film properties have been studied by various physicochemical techniques. Electrodeposition was achieved by combining the formation of a self-assembled "nanoglue" on the electrode surface, the sol-gel process, and the electrochemical manipulation of pH to catalyze polycondensation of the precursors. Gold electrodes pretreated with mercaptopropyltrimethoxysilane (MPTMS) were immersed in sol solutions containing the selected precursors (tetraethoxysilane, TEOS, in mixture with (3-aminopropyl)triethoxysilane, APTES, or MPTMS) where they underwent a cathodic electrolysis to generate the hydroxyl ions that are necessary to catalyze the formation of the organosilica films on the electrode surface. Special attention was given to analyze the effects of deposition time and applied potential and to compare APTES and MPTMS films. Characterization was made using quartz crystal microbalance, scanning electron microscopy, cyclic voltammetry, and atomic force microscopy (including in situ monitoring). The electrodeposition process was found to occur at two growing rates: a first slow stage giving rise to rather homogeneous, yet rough, films with thickness in the sub-mum range (increasing continuously when increasing the deposition time), which was followed by a faster gelification step resulting in much thicker (>1 microm) and rougher macroporous deposits. These two successive situations were observed independently on the applied potential except that more cathodic values led to narrower sub-microm ranges (as expected from the larger amounts of the electrogenerated hydroxyl catalyst). Thiol-functionalized silica films were deposited more rapidly than the amine ones and, for both of them, permeability to redox probe was found to decrease when increasing the film thickness because of higher resistance to mass transport.     

扫描电化学显微镜直接电沉积法构建葡萄糖氧化酶微米点

利用自制的凹形电极在铂基底电极上直接构建了葡萄糖氧化酶微米点. 首先, 将电聚合和电化学刻蚀法相结合制备了凹形铂微米电极. 然后将此种电极作为参比及辅助电极, 基底铂电极作为工作电极, 利用葡萄糖氧化酶在合适的条件下(浓度、一定量Triton X-100存在、电极电位等)由于电极表面pH的降低可以在铂电极上电沉积这一特性, 将酶固定在铂基底电极上, 微修饰得到了具有活性的葡萄糖氧化酶微米点. 最终用扫描电子显微镜和扫描电化学显微镜对所得微米点进行了表征. 所得微米点直径约20 μm, 且具有催化活性. 该方法简便, 干扰因素较少.     

电泳沉积制备氧化石墨烯-银电极及其氧还原催化性能

通过在1-甲基-2-吡咯烷酮（NMP）中超声剥离氧化石墨制备出稳定的氧化石墨烯（GO）分散液,添加AgNO₃使氧化石墨烯吸附Ag+而带正电荷。采用电泳沉积法使GO沉积到阴极的玻璃碳电极上,Ag+被电化学还原为单质银,均匀的分散在GO片层当中。通过AFM、SEM、Raman、XRD及元素面扫分析对制备电极的形貌、结构进行表征。在碱性环境中进行氧还原测试,结果表明GO+Ag电极的氧还原起始电位较玻璃碳电极最大正移228mV,还原电流密度最大为7.564mA·cm^-2,是玻璃碳电极的3.4倍。通过不同转速下的线性扫描曲线绘制Koutechy-Levich图,计算氧还原反应的电子转移数为3.3。     

Myoglobin/sol-gel film modified electrode: direct electrochemistry and electrochemical catalysis

Direct electrochemical and electrocatalytic behavior of myoglobin (Mb) immobilized on carbon paste electrode (CPE) by a silica sol-gel film derived from tetraethyl orthosilicate was investigated for the first time. Mb/sol-gel film modified electrodes show a pair of well-defined and nearly reversible cyclic voltammetric peaks for the Mb Fe(III)/Fe(II) redox couple at about -0.298 V (vs Ag/AgCl) in a pH 7.0 phosphate buffer solution. The formal potential of the Mb heme Fe(III)/Fe(II) couple shifted linearly with pH with a slope of 52.4 mV/pH, denoting that an electron transfer accompanies single-proton transportation. An FTIR and UV-vis spectroscopy study confirms that the secondary structure of Mb immobilized on an electrode by a sol-gel film still maintains the original arrangement. The immobilized Mb displays the features of a peroxidase and acts in an electrocatalytic manner in the reduction of oxygen, trichloroacetic acid (TCA), and nitrite. In comparison to other electrodes, the chemically modified electrodes used in this study for direct electrochemistry and electrocatalysis of Mb are easy to fabricate and fairly inexpensive. Consequently, the Mb/sol-gel film modified electrode provides a convenient way to perform electrochemical research on this kind of protein. It also has potential use in the fabrication of bioreactors and third-generation biosensors.     

Highly Efficient and Stable Palladium Nanoparticles Supported on an Ionic Liquid Silica Sol?Gel Modified Electrode

A carbon ionic liquid electrode modified with a thin layer of silica sol? gel containing phosphinite ionic liquid was prepared for deposition of palladium nanoparticles. Palladium nanoparticles were formed easily by simple contact of the modified electrode with palladium chloride solution. The novel material overcomes the shortcomings of conventionally modified electrodes with a thin layer of silica sol? gel, due to the existence of ionic liquid in silica matrix. A crack‐free sol? gel matrix was obtained and also, the uniform porous structure of the ionic liquid‐sol? gel matrix resulted in a fast mass transport and increased ionic conductivity. The electrode exhibits high electrocatalytic effects towards hydrazine and ascorbic acid. It is very stable against repetitive cycling in the applied potential window.     

Deposition of colloid particles on protein layers: fibrinogen on mica

Colloid particle deposition was applied to characterize fibrinogen (Fb) monolayers on mica, which were produced by controlled adsorption under diffusion transport. By adjusting the time of adsorption and the bulk Fb concentration, monolayers of desired surface concentration were obtained. The surface concentration of Fb was determined directly by AFM enumeration of single molecules adsorbed over the substrate surface. It was proven that Fb adsorbed irreversibly on mica both at pH 3.5 and at pH 7.4 with the rate governed by bulk transport. The electrokinetic properties of Fb monolayers produced in this way were studied using the streaming potential method. The dependence of the apparent zeta potential of Fb monolayers was determined as a function of the coverage. It was shown that for pH 3.5 the initial negative zeta potential of the mica substrate was converted to positive for Fb coverage exceeding 0.16. On the other hand, for pH 7.4, the zeta potential of a Fb-covered mica remained negative for the entire coverage range. The charge distribution in Fb monolayers was additionally studied using the colloid deposition method, in which negatively and positively charged polystyrene latex particles (ca. 800 nm in diameter) were used. An anomalous deposition of negative latex particles on substrates exhibiting a negative zeta potential was observed. Results of these experiments were quantitatively interpreted in terms of the fluctuation theory assuming that adsorption sites consisted of two and three Fb molecules, for pH 3.5 and 7.4, respectively. These results suggested that for pH 7.4, the distribution of charge on Fb molecules was heterogeneous, characterized by the presence of positive patches, whereas the average zeta potential was negative, equal to -19 mV. The utility of the colloid deposition method for studying Fb monolayers was further demonstrated in deposition experiments involving positive latex particles. It was shown that for a rather broad range of fibrinogen coverage, both the positive and the negative latex particles can adsorb on surfaces covered by Fb, which behaved, therefore, as superadsorbing surfaces. It was also concluded that the colloid deposition method can be used to determine the Fb bulk concentration for the range inaccessible for other methods.     

STUDIES OF ADHESION OF METAL PARTICLES TO SILICA PARTICLES BASED ON ZETA POTENTIAL MEASUREMENTS

The zeta-potentials of silica, copper, platinum and gold particles have been measured as a function of pH. The isoelectric points were found to be at pH 3.0, 5.8, 3.0 and 3.5, respectively. In the pH range 3.0 to 5.8 copper and silica particles are oppositely charged and accordingly the coating of silica with copper particles could be demonstrated. In the case of gold and platinum the sign of the charge is such that direct adhesion to silica particles cannot be expected and this was also demonstrated in the case of platinum.     

Adsorption of polyelectrolyte modified graphene to silica surfaces: monolayers and multilayers

Exfoliated graphene particles stabilised by the cationic polyelectrolyte polyethyleneimine (PEI) were used in conjunction with an anionic polyelectrolyte, poly(acrylic acid), to construct multilayers using the layer-by-layer technique on a silica substrate. In the first adsorption step, the surface excess of the cationic graphene was dependent on the overall charge on the nanoparticle which in turn can be tuned through modifying solution pH as PEI has weakly ionisable charged amine groups. The adsorbed amount onto the silica surface increased as the solution pH increased. Subsequently, a layer of PAA was adsorbed on top of the cationic graphene through electrostatic interaction. The multilayer could be assembled through this alternate deposition, with the influence of solution conditions investigated. The pH of the adsorbing solutions was the chief determinant of the overall adsorbed amounts, with more mass added at the elevated pH of 9 in comparison with pH 4. Atomic force microscopy confirmed that the graphene particles were adsorbed to the silica interface and that the surface coverage of the disc-like nanoparticles was complete after the deposition of five graphene-polyelectrolyte bi-layers. Furthermore, the graphene nanoparticles themselves could be modified through the consecutive addition of the oppositely charged polymers. A multilayered assembly of negatively charged graphene sheets modified with a bi-layer of PEI and PAA was also deposited on a silica surface with adsorbed PEI.     

电泳沉积制备氧化石墨烯-银电极及其氧还原催化性能

通过在1-甲基-2-吡咯烷酮(NMP)中超声剥离氧化石墨制备出稳定的氧化石墨烯(GO)分散液,添加AgNO3使氧化石墨烯吸附Ag+而带正电荷。采用电泳沉积法使GO沉积到阴极的玻璃碳电极上,Ag+被电化学还原为单质银,均匀的分散在GO片层当中。通过AFM、SEM、Raman、XRD及元素面扫分析对制备电极的形貌、结构进行表征。在碱性环境中进行氧还原测试,结果表明GO+Ag电极的氧还原起始电位较玻璃碳电极最大正移228 mV,还原电流密度最大为7.564 mA·cm-2,是玻璃碳电极的3.4倍。通过不同转速下的线性扫描曲线绘制Koutechy-Levich图,计算氧还原反应的电子转移数为3.3。     

Electrical Control of Alkanethiols Self-Assembly on a Gold Surface as an Approach for Preparation of Microelectrode Arrays

 It is shown by capacitive monitoring that the self-assembly of alkanethiols on gold electrodes and desorption of these self-assembled monolayers from the electrodes are controlled by the electrode potential. At neutral pH, chemical adsorption of alkanethiols was observed at an electrode potential of +300 mV vs SCE, but only physical adsorption was detected when the electrode potential was −1400 mV vs SCE. At electrode potentials between these values (−300 mV, −600 mV), chemical adsorption of alkanethiols occurred, but the alkanethiol monolayers were not stable in the absence of the alkanethiol in the bulk solution and were desorbed from the gold electrode. The desorption rate was higher at more negative electrode potentials. These results can be used in designing methods for electrically addressable immobilization of different receptors on (micro)electrode arrays. This has been demonstrated by deposition of two different types of alkanethiols onto a two-electrode array. Received June 24, 1998. Revision October 19, 1998.     

Polyaniline films containing palladium microparticles for electrocatalytic purposes

The deposition of palladium as microparticles on electrogenerated polyaniline (PAni) films accomplished by metal electroless precipitation or by electrodeposition has been used to prepare stable polymer-modified electrode systems. Its general morphology, metal particles size and distribution were obtained by AFM analysis. The PAni/Pd microparticles electrodes have been evaluated for their catalytic activity towards hydrogenation reactions upon organic compounds reduction using 4-nitro-benzaldehyde. Studies on rotating ring-disk electrodes have shown that relatively low Pd loadings are required to obtain effective electrocatalytic activity, provided a good metal particles dispersion of about 100 nm diameter is observed. Dedicated to Professor M. A. Vorotyntsev on the occasion of his 60th birthday