Electrode response kinetics

The response of the immobilized enzyme electrode can be made independent of the enzyme concentration by using a large excess of enzyme at the electrode surface. The electrode response is limited by the mass transport of the substrate. Using an excess of enzyme often results in longer electrode lifetimes, increased linear range, reduced susceptibiUty to pH, temperature, and interfering species (58,59). At low enzyme concentrations the electrode response is governed by the kinetics of the enzyme reaction. 

The kinetics of H2 oxidation has been investigated on a Ni/YSZ cermet nsing impedance spectroscopy at zero dc polarization. The hydrogen reaction appears to be very complex. The electrode response appears as two semicircles. The one in the high-freqnency range is assumed to arise partly from the transfer of ions across the TPB and partly from the resistance inside the electrode particles. The semicircle observed at low freqnencies is attributed to a chemical reaction resistance. The following reaction mechanism is suggested ... 

The theory of ion-selective electrode response is well developed, due to the works of Eisenman, Buck and others [23], Three models used for the description of the ISE response through the years, namely kinetic, membrane surface (or space charge) and phase boundary potential (PBP) models, although being seemingly contradictory, give similar results in most cases [7], The first two sophisticated models are out of the scope of the present chapter, as the PBP model, despite its simplicity, satisfactorily explains most of the experimental results and thus has become widely applicable. The... 

Fig. 12.7. Interaction of mercury vapour with thin gold films coated by self-assembled monolayer of 1-hexadecanethiol (a) comparison of the kinetics of resistive response for bare (open symbols) and coated (filled symbols) gold films on exposure to 10 ng/1 mercury vapour (b) influence of different mercury vapour concentrations on the resistance of coated electrodes (c) kinetics of changes of the resonance frequency of a 1-hexadecanethiol-coated piezo-quartz due to exposure to 8.3 ng/1 of mercury vapour [25].

The amount of enzyme, either in a pure or crude form, also affects the speed of response of the electrode. Yet, compromises must be made between the increase in enzyme activity and the concomitant increase in membrane thickness, which affects the rate of electrode response. As the amount of enzyme is increased, a shorter response time is observed, until an optimum level is reached. Further increase in the amount of enzyme tends to diminish the response time due to a thickening of the membrane layer and an increase in the time required for the substrate to diffuse through the membrane. Generally, using enzyme with the highest specific activity gives the thinnest membranes and the most rapid kinetics. 

Kinetic measurements of H2O2 formation result in fast electrode responses (less than 12 s) using chemically bound L-amino acid oxidase (LAAO) covering a platinum electrode (170) to assay for cysteine, leucine, tyrosine, phenylalanine, tryptophane, and methionine. Potentiometric-selective electrodes for amino acids are Constructed by immobilizing LAAO on chemically modified graphite... 

Since practically all organic reactive intermediates readily undergo electron transfer reactions, electrochemical methods play an important role in the study of their chemistry. The measurement of the electrode potential for the formation of the intermediate can lead directly to the standard free energy of the process. The kinetics of the reactions of intermediates, formed in exceedingly low concentrations can be deduced from the electrode response of the substrate from which the intermediate is derived by an electron transfer. 

The manner in which kinetic data are treated in arriving at an electrode mechanism depends primarily upon whether the technique gives a direct measure of the response of the intermediate or an indirect measure, usually the effect of the chemical reaction on the electrode response of the substrate. In the former case, the conventional way of handling the data is to compare the experimental response with theoretical data in the form of a working curve and determine the mechanism from the best fit with theoretical data. The latter case usually involves the calculation of the electrode response to a particular mechanism and then comparing some measurable quantity, for example the sweep rate dependence of the peak potential, with the theoretical value. Which type of analysis is appropriate, direct or indirect, depends upon the... 

Reference electrode performance also includes the response kinetics. For example,... 

In most cases, electrode measurements are reasonably rapid—equilibrium being reached in less than a minute but in some cases, usually in very dilute solutions, slow electrode response may require fifteen minutes to an hour for equilibrium. The normally rapid response of ion-selective electrodes make them suitable in kinetic studies and for monitoring changes in flowing process streams. The equipment used is simple, quite inexpensive, and can be made portable for field operations. The method is virtually nondestructive of the sample (once it is in the liquid state), and can be used with very small samples (< 1 ml). 

SWV seeks to address the disadvantages of both NPV and DPV by being differential and also by being fast to minimize adsorption. It cannot always reach this goal. Indeed, there are situations in which SWV is too fast, given the slow kinetics of some reactions, to be able to register any electrode response. In these cases, DPV must be employed. 

Aoyagi W, Omiya M (2013) Mechanical and electrochemical properties of an IPMC actuator with palladium electrodes in acid and alkaline solutions. Smart Mater Struct 22 055028 (10 pp) Asaka K, Oguro K (2000) Bending of Polyelectrolyte Membrane-platinum composites by electric stimuli. Part II. Response kinetics. I Electroanal Chem 480 186-198 Asaka K, Oguro K (2009a) IPMC actuators fundamentals. In Carpi F, Smela E (eds) Biomedical applications of electroactive polymer actuators. Wiley, Chichester, pp 103-119 Asaka K, Oguro K (2009b) Active microcatheter and biomedical soft devices based on IPMC actuators. In Carpi F, Smela E (eds) Biomedical applications of electroactive polymer actuators. Wiley, Chichester, pp 103-119... 

The physical model and coordinates system are shown in Fig. P6.10. The local analyte concentration at the enzyme surface is low so that the reaction kinetics are adequately described by a first-order law. This latter assumption ensures that the electrode response is proportional to the analyte concentration. 

In conventional EIS experiments the electrode response to a perturbation signal corresponds to a measurement averaged across the whole electrode surface area. However, electrochemical systems show nonuniform current and potential distributions, resulting in CPE behavior. Such distributions can be studied bythe local EIS method, which employs in situ probing of local current density distribution in the vicinity of the working electrode surface. Local EIS (LEIS) relies on the fact that AC current density in the solution very near the working electrode is proportional to the local impedance properties of the electrode [22]. The AC current spreads in the solution as a funchon of the distance from the electrode surface, and as a consequence the LEIS results depend on the distance between the probe and the surface. That allows for spatially resolved LEIS measurements of the surface topography and kinetics at the electrochemical interface. 

Bradley C.R. and Rechnitz G.A. (1984) Kinetic analysis of enzyme electrode response. Anal. Chem., 56,664-667. 

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

The Solution. The responses of working and reference electrodes to appHed voltages are important only because this information can be indicative of what goes on in the solution, or at the solution/electrode interface. The distinction between bulk (solution) and interfacial events is basically the distinction between chemical kinetics and charge transfer. 

---