W. John Albery

Amperometric enzyme electrodes: Part II. Conducting salts as electrode materials for the oxidation of glucose oxidase

Abstract Results are reported for the direct oxidation of the enzyme glucose oxidase on 7 different one-dimensional conducting donor acceptor salts. Experiments conducted with the enzyme in bulk solution are shown to be in good agreement with theory. Three salts, made of the cations tetrathiafulvalinium (TTF+), N-methylphenazinium or quinolinium with the anion tetracyanoquinodimethanide (TCNQ−) had the lowest background currents and were used to make membrane sensors for glucose. Analysis of the variation of current with glucose concentration identified the rate limiting processes as transport of gluycose through the membrane and electrochemical kinetics under unsaturated and saturated conditions respectively. The electrochemical rate constants for these three materials were all greater than 10−2 cm s−1. TTF+TCNQ− is the material of choice and linear calibration plots were obtained for glucose concentrations between 50 μmol dm−3 and 10 mmol dm−3.

A TRANSMISSION-LINE MODEL FOR MODIFIED ELECTRODES AND THIN-LAYER CELLS

Abstract A novel transmission line in which a distributed capacitance is placed between resistive elements R1 and R2 is proposed as a model for a polymer coated electrode or for a thin layer cell containing a binary electrolyte. For both cases the response of the transmission line to a small potential step is derived using Laplace transforms. The ac impedance can then be found for both cases. The results are discussed and are shown to provide a better model than the classical transmission line in which either R1 or R2 is zero.

Percutaneous absorption: in vivo experiments

The percutaneous absorption of esters of nicotinic acid has been studied in vivo in man. The time for erythema to be produced has been measured both when the ester is applied continuously and in ‘pulse’ experiments when the ester is removed before the erythema develops. The results show that the erythema is produced long before steady state diffusion across the epidermis is established and the penetration of methyl nicotinate is characterized by D/l2 = 2.3 × 10−4 s−1 where D is the diffusion coefficient and l the thickness of the barrier. Results using glycerol water mixtures in the external phase show that the route of penetration for methyl nicotinate is through the interstitial channels and not through the keratinized cells. Data for absorption from various creams and ointments (Barrett et al 1964) show that the route is independent of the nature of the external phase. Steady state data for the absorption of salicylic acid and carbinoxamine through the abdominal skin of guinea‐pigs (Arita et al 1970) show that the route of penetration does not change as the experiment proceeds. Data for the absorption of other substances (Michaels et al 1975) also fit the interstitial route.

Spectroscopic and electrochemical studies of charge transfer in modified electrodes

Experimental results from electrochemical e.s.r. on electrodes modified with conducting polymers such as polyaniline, polypyrrole and polythiophene are presented. The results show that the polymer exists in two forms, one stable when it is insulating (α) and the other (β) when it is conducting. In the conducting region the kinetics of the transformation of the metastable α form into the β form can be followed by e.s.r. It is further shown that in the lightly oxidised films there is a one-to-one correlation between charge injected and e.s.r. spins. These results are explained by a chemical redox model as opposed to a delocalised band.Results from a.c. impedance studies are presented. These results are explained by an equivalent circuit which includes a novel transmission line with differential resistances. Theoretical impedance plots are derived. Experimental results show that the polymer resistance and the resistance of the aqueous pores are equal. Reasons for this finding are discussed.

Percutaneous absorption: theoretical description

Equations are derived to describe the percutaneous absorption of a substance through the epidermal barrier. The treatment includes interfacial barriers and allows for the depletion of the substance in the external phase. The equations are derived both for the continuous application and for pulse experiments where the drug is applied for a time, then removed, and the response occurs some time after the removal of the drug. Competition between the drug diffusing through the keratinized cells (transcellular route) and diffusing in the interstitial channels around the cells (intercellular route) is also considered.

The kinetics of electron transfer in the thionine-coated electrode

Abstract Transient studies using potential jump and optical absorbance have been carried out on thionine-coated electrodes. Both types of transient are shown to fit a simple diffusion model. At pH 1.3 the characteristic diffusion time varies with the size of the anion present in the electrolyte, showing that diffusion of the counter ions is the rate-limiting process. At pH 5 faster, more uniform, rates are found which are attributed to rate-limiting electron hopping. The rate constant for this process is in rough agreement with the rate of the conproportionation reaction of thionine and leucothionine. The electron-hopping process also limits the maximum current that can be passed through the modified electrode in the steady state.

Interfacial transfer studied with a rotating diffusion cell

The construction of a diffusion cell is described in which species diffuse through a rotating disc of filter paper. Correct rotating disc hydrodynamics are established on both sides of the filter. Liquid liquid interfaces can be established on either side of the filter and the rate at which species are transferred from one phase to the other can be measured. By extrapolation to infinite rotation speed the rate constant for the transfer reaction at the interface can be measured. Results for 11 systems are reported. The rate constant for the interfacial transfer in the downhill direction is found for each system to be approximately 10–5 m s–1. The activation parameters for two systems are also reported.

The wall-jet ring-disc electrode: Part I. Theory

Abstract A theoretical treatment is presented for the wall-jet ring-disc electrode. Velocity profiles for the radial and axial flow in the wall-jet system are derived. A general treatment for transforming the convective diffusion equation into the same form as that for a rotating ring-disc electrode is presented. This transformation means that the results of the rotating ring-disc electrode for the limiting currents, the collection efficiency, and diffusion layer titration curves can be applied directly to the wall-jet ring-disc electrode.

The Transport and Kinetics of Minority Carriers in Illuminated Semiconductor Electrodes

The differential equation describing the transport and kinetics of photogenerated minority carriers in a semiconductor electrode is solved analytically to obtain a solution in terms of confluent hypergeometric functions. Much simpler approximate solutions are also obtained. Comparison of these solutions with results from the full expression show that the approximate solutions hold to within 5%. A simple physical significance can be attached to the different terms in the approximate solutions. The form of the photocurrent voltage curves that arise from the different solutions is discussed.

A novel mechanism of electrochemical deposition of conducting polymers: two-dimensional layer-by-layer nucleation and growth observed for poly(thiophene-3-acetid acid)

Abstract Results for a conducting polymer poly(thiophene-3-acetic acid) are presented which demonstrate that electrochemical deposition proceeds favourably through two-dimensional (2-D) layer-by-layer nucleation and growth. This is a striking contrast to previous investigations of conducting polymers which have consistently reported growth via three-dimensional (3-D) nucleation. The new findings indicate the possibility to prepare ultrathin, compact conducting polymer films which are potentially useful in molecular electronics. The 2-D mechanism is seen from, among other evidences, separated individual events of nucleation and monolayer deposition with a typical coverage of ca 1 × 10 −9 mol cm −2 monomer units and a lateral spreading rate between 0.04 and 0.08 cm s −1 . Double as well as single potential step experiments revealed that the overall process of polymer deposition is controlled by the growth rate, k , representing an electrochemical charge transfer generating intermediate cations from the neutral monomer. The influence of monomer concentration on nucleation and growth rate has also been investigated and the results discussed.