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This paper describes the construction of a mixed monolayer of ferrocenylalkanethiol and encapsulated horseradish peroxidase (HRP) at a gold electrode for amperometric detection of H(2)O(2) at trace levels. By tuning the alkanethiol chain lengths that tether the HRP enzyme and the ferrocenylalkanethiol (FcC(11)SH) mediator, facile electron transfer between FcC(11)SH and HRP can be achieved. Unlike most HRP-based electrochemical sensors, which rely on HRP-facilitated H(2)O(2) reduction (to H(2)O), the electrocatalytic current is resulted from an HRP-catalyzed oxidation reaction of H(2)O(2) (to O(2)). Upon optimizing other experimental conditions (surface coverage ratio, pH, and flow rate), the electrocatalytic reaction proceeding at the electrode was used to attain a low amperometric detection level (0.64 nM) and a dynamic range spanning over 3 orders of magnitude. Not only does the thin hydrophilic porous HRP capsule allow facile electron transfer, it also enables H(2)O(2) to permeate. More significantly, the enzymatic activity of the encapsulated HRP is retained for a considerably longer period (3 weeks) than naked HRP molecules attached to an electrode or those wired to a redox polymer thin film. By comparing to electrodes modified with denatured HRP that are subsequently encapsulated or embedded in a poly-L-lysine matrix, it is concluded that the encapsulation has significantly preserved the native structure of HRP and therefore its enzymatic activity. The electrode covered with FcC(11)SH and encapsulated HRP is shown to be capable of rapidly and reproducibly detecting H(2)O(2) present in complex sample media.

A simple, rapid, and effective photoelectrochemical method is proposed to quantitatively characterise photocatalytic degradation behaviour of organic compounds at nanoporous TiO 2 film electrodes in a thin-layer photoelectrochemical cell. This method uses the charge obtained by integrating the photocurrent originating from the photocatalytic oxidation of organic compounds to quantify the extent of degradation. Complete mineralisation was observed for all organic compounds investigated. A double-exponential kinetic rate expression was acquired using a computer simulation method, indicating two simultaneous kinetic processes. Photocurrent profiles of different organic compounds complied well with the proposed theoretical model. Both pre-exponential and exponential constants were obtained. The rate of the fast kinetic component is 10–25 times faster than that of the slow kinetic component. It was found that the identities of organic compounds have no significant effect on the photocatalytic oxidation kinetics, whereas the availability of the organic compounds to capture photoholes plays a decisive role.

A novel rapid methodology for the determination of chemical oxygen demand (COD) based on photoelectrochemical oxidative degradation principle (PECOD) was proposed and experimentally validated. With this new method, the extent of degradation of dissolved organic matter in a water sample is measured simply by directly quantifying the extent of electron transfer at a TiO2 nanoporous film electrode during an exhaustive photoelectrocatalytic degradation of organic matter in a thin layer photoelectrochemical cell. The PECOD method demonstrated in this work is a direct and absolute method. It does not require the use of standard for calibration. The method, in principle, measures the theoretical COD value due to the extraordinary high oxidation efficiency and accuracy of charge measurement. This new approach overcomes many of the current problems associated with existing oxygen demand techniques (e.g., the matrix effect, one of the serious practical problems that most rapid COD methods suffered because of the in...

The effect of potential bias, light intensity, and the concentration of the photohole scavenger (glucose) on the photocurrent responses of the nanoporous TiO2 film electrodes were investigated with the focus on the overall photoelectrocatalytic oxidation process. The electron transport in TiO2 film was the rate-limiting step at low potential bias, while the interfacial reactions became rate-limiting step at high potential bias. A linear photocurrent/potential characteristic can be obtained within a wide range of applied potential bias. Within this linear range, the electrodes behave as a constant resistance rather than a variable resistance, which is unlike a photoelectrochemical process at a bulk semiconductor electrode. The resistance consists of two components: a variant component and an invariant component. The former is inversely proportional to maximum photocurrent, due to the free electron concentration change as a result of the consumption of photoholes through interfacial reaction. The hypothesi...

A study of photoelectrochemical oxidation of methanol at nanoporous TiO2 film electrodes was carried out. The effect of variables such as applied potential, light intensity, methanol concentration and pH on the photoelectrochemical behaviour was investigated. It was found that the photoresponse was greatly influenced by these factors and a unique photocurrent/potential characteristic of the electrode in solution was observed which was similar to that of the single crystal semiconductor electrode. It was also found that the rate-limiting step for the overall photooxidation process varied depending on potential, light intensity and methanol surface coverage. In low potential region at a given light intensity and methanol concentration the migration of electron across the TiO2 film is the rate-limiting step. While in the relatively high potential region the interfacial charge transfer was found to be the rate-limiting step. A deviation of onset potential dependence on pH from the Nernstian relationship was also observed, and based on the energy band theory and the existence of deep electron traps an explanation was given. An unusual abrupt increase of photocurrent in strong basic methanol solution was observed, which was attributed to the existence of acidic hydroxyl group on TiO2 and its stronger adsorption to methanol molecule after its reaction with OH−, and the higher reactivity of the methanol thus adsorbed.

The formation process of sodium lauryl sulfate monolayers on the surface of octadecanethiol self-assembled monolayers was investigated by the method of cyclic voltammetry and ac impedance spectroscopy. The pinhole in an octadecanethiol-coated gold electrode can be patched when adding sodium lauryl sulfate to solution. The charge transfer resistance increased but the membrane capacitance decreased with the addition of sodium lauryl sulfate or other amphiphilic substance, indicating that the adsorption of sodium lauryl sulfate on the electrode surface from solution.

Au electrodes are alkylated by self-assembled organic monolayers of octadecanethiol from alcohol solution. The electron tunnelling resistance of a monolayer-coated gold electrode has been investigated by ac impedance. The relation between the fractional coverage of different defects and the corresponding film thickness at these ‘collapsed’ sites has been deduced from electron tunnelling theory. By using the concepts of average film thickness at defect (da) and average fractional coverage of defect (θa), we have obtained the θa∼da plot. The influence of the apparent standard rate constant on the shape of the θa∼da plot has been discussed. In our experiments, Fe(CN)63−/4− is used as a redox probe to study the θa∼da plot of an octadecanethiol monolayer. The θa versus da plot indicates that the defects with da

Octadecanethiol self-assembled monolayers derivatized gold electrode in different concentrations of Fe(CN)6 3−/4− was studied by cyclic voltammetry. The cyclic voltammetric responses display a series of inverse sigmoidal curves for the SAMs in the solution of various concentration of Fe(CN)6 3−/4−. A new concept of apparent effective thickness of self-assembled monolayers is established on the base of the measurements of cyclic voltammetry. It is determined that the apparent effective thickness of the SAMs is lower than the length of chain of octadecanethiol. Defect sites may be present in the alkanethiol monolayers. The apparent effective thickness can be used to describe the quality of SAMs quantitatively.

The electrochemical behavior of the gold electrode modified by hybrid bilayer membranes in different concentrations of Fe(CN)63−/Fe(CN)64− was investigated by electrochemical impedance spectroscopy and cyclic voltammetry technology. The electron transfer between gold and the redox species separated by the hybrid bilayers assembly was completed by an electron tunneling process. A detailed equivalent circuit for electron transport across the HBMs is proposed. It was found that the apparent effective thickness of the hybrid bilayer membranes was lower than that of the sum of the chain length of octadecanethiol and phospholipid so some possible collapsed sites might exist at the hybrid bilayer membranes.

The concept of an apparent effective thickness of a self-assembled monolayer is established and assessed with the measurements of ac impedance spectroscopy in different concentrations of Fe(CN) 6 3− /Fe(CN) 6 4− by alkanethiol self-assembled monolayers on a gold electrode. The complex-plane impedance plots for SAMs in solution of various concentrations of Fe(CN) 6 3− /Fe(CN) 6 4− exhibit arc shapes. The apparent resistance ( R 2 ), obtained from the simulation, decreased with increasing concentration of the redox couples. It is determined that the apparent effective thickness of the SAMs is lower than the length of chain of the alkanethiol. Defect sites may exist at the alkanethiol monolayers.

Self-assembly of octadecyl mercaptan on gold has been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV experiments show that well-assembled thiol monolayers on gold are essentially free of pinhole defects. Support for the existence of ‘collapsed’ sites in thiol monolayers is obtained by comparing the apparent electron transfer rate constant, which is obtained from CV experiments at a film covered Au electrode, with the theoretical one, which is calculated by assuming electrons tunnel across the collapsed-site-free monolayer. The empirical relationship Z Rs 1:[s(jv) a ] is used to describe the film coated Au in inert electrolyte and the degree of disorder of the monolayer structure is estimated from the a value. It is demonstrated that the admittance plane plot of an ideally polarizable electrode with constant phase element behavior is a circular arc which can be used to determine a. Results of EIS experiments show that a increases sharply after the initial adsorption time and approaches its final value slowly. This is attributed to the rapid adsorption step followed by a slow crystallization process in the kinetics of alkanethiol adsorption onto the Au electrode. For a well-assembled thiol monolayer on gold, a can approach 0.99, which means that the number of collapsed sites is not large and the surface of the monolayer Au electrode is rather smooth. © 1999 Elsevier Science S.A. All rights reserved.

The effect of Ca2+ on the electron transfer of Fe(CN)63−/4− couple at Pt supported lecithin (PC)/cholesterol (CH) bilayer lipid membrane (BLM) electrode has been studied by voltammetry and ac impedance spectroscopy. Experimental results suggest that the interaction of Ca2+ with the BLM can open some kind of channel for Fe(CN)63−/4− ions, allowing increased access of the redox couple to the electrode surface. This phenomenon may be related to the Ca2+ regulation or activation effect of ion channel in some biomembranes.

Effective blockage of recombination electron transfer of a fast electron transfer redox couple (ferrocenium/ferrocene or Fc(+)/Fc) at TiO2 nanowire array electrodes is achieved by silanization of the dye loaded TiO2 nanowire array. FT-IR clearly shows the formation of polysiloxane network at fluorine doped tin electrodes covered with TiO2 nanowire arrays and the dye molecules. Compared to the commonly used TiO2 nanoparticle film electrodes, the TiO2 nanowire array has a more spatially accessible structure, facilitating the formation of uniform polysiloxane films. Energy-dispersive X-ray spectroscopy (EDS) also reveals the presence of Si over multiple spots at the cross sections of the silanized TiO2 nanowire array electrodes. As a result, a rather high open-cell voltage Voc (0.69 V) and an enhanced efficiency (0.749 %) for DSSC with the Fc(+)/Fc couple were obtained. Contrary to the passivated TiO2 nanoparticle film electrodes at which a complex, biphasic dependence of electron lifetime on Voc was observed, we recorded a logarithm linear dependence of the lifetime on Voc after the silanization treatment. The nanowire arrays with optimal salinization treatments afford a useful surface for the study of electron recombination and photovoltaic generation in DSSCs.

The photoelectrochemical behaviour and semiconductive properties of Zn-doped sintered polycrystalline iron oxide were investigated in boric acid/borax buffer solution (pH = 8.4). Strange characteristic photoresponses of n-type semiconductive electrodes at a more positive potential, p-type semiconductive electrodes at a more negative potential and both n-type and p-type semiconductive electrodes at an intermediate potential have been measured. Through a series of experiments, a possible explanation of this phenomenon has been proposed, ie the existence of heterogeneity both on the surface and in the bulk of the electrode leads to different behaviour of the electrode at different external potentials.

Photovoltage waveforms of Mg-doped or Zn-doped iron oxide electrodes that exhibit n-type and p-type photoresponses simultaneously at certain potential interval were reported for the first time. Tranformation of n-type photoresponse at more positive potential to p-type photoresponse at more negative potential was showed. The photovoltage waveforms at different potentials were explicitly measured. It was found that the waveforms at intermediate potentials are linear combination of typical n-type and p-type waveforms. An equivalent circuit based on the existence of multiphase both on the surface and in the bulk of the electrode was advanced.

The cathodic reduction current of passivated iron electrode in sulfuric acid solution was detected for the first time when potentiodynamic sweeps were applied. The influence of different factors such as scan rate, cycling numbers, etc on it were examined. The cathodic current might be related to a certain reduction process of γ-Fe2O3.