Microelectrode potential

Cassidy J F and Foley M B 1993 Microelectrodes—potential Invaders Chem. Br. 29 764... 

For clusters of higher nuclearity too, the kinetic method for determining the redox potential °(M]] /M ) is based on electron transfer, for example, from mild reductants of known potential which are used as reference systems, towards charged clusters M](. [31] Note that the redox potential differs from the microelectrode potential M /M ) by the... 

Some calculations [70] were made to derive the microelectrode potential M i/M ) for silver and copper from the data in the gas phase [nuclearity-dependent M-M bond energy and IPg(M )j. The potential presents odd-even... 

Fig. 2.10 Amounts of 02 consumed as a result of photocatalytic reactions in ethanol-containing aqueous solution, with and without 0.1 M phenol present, under UV irradiation for 1 min. The amount of 02 consumed was estimated using a finite difference method from the oxygen concentration vs. time curves (light intensity, 2.5 mW/cm2 microelectrode potential, -1 V vs. SCE).

For clusters of higher nuclearity too, the kinetics method for determining the redox potential E°(M /M ) is based on the electron transfer, for example, from mild reductants of known potential which are used as reference systems, towards charged clusters M/. Note that the redox potential differs from the microelectrode potential E° (M. M /M ) by the adsorption energy of M onM (except for n = 1). The principle (Figure 5) is to observe at which step n of the cascade of coalescence reactions (14), a reaction of electron transfer, occurring between a donor S and the cluster M/ could compete with (14). Indeed n is known from the time elapsed from the end of the pulse and the start of coalescence. The donor S is produced by the same pulse as the atoms M", the radiolytic radicals being shared between M (reactions 1,7,8) and S (reactions 25, 26). 

Some calculations have been made also to derive the microelectrode potential E°(M, M .,/M ) for silver and copper from the data in the gas phase (nuclearity-dependent M-M bond energy and IPg(M )). The potential E°(M, M .,/M ) presents odd-even oscillations with n, (more stable for n even) as for IPg, but again the general trends are opposite, and an increase is found in solution due to the solvation energy. 

The selectivity coefficients of the microelectrodes were evaluated in equimolar pure solutions. The ionic strength of the standards was selected so as to approximate the value of intracellular fluid of Necturus proximal tubules. The intracellular K" and Cl concentrations were determined from the calibration curve of the respective microelectrode in standard solutions that flanked intracellular readings. The chloride microelectrode potential is insensitive to the cationic substitution of Na for K" " in the different solutions at constant ionic strength. It is also insensitive to changes in H concentration in the pH 4-8 range. Similarly, the potassium microelectrode potential is insensitive to the anionic substitution of H2P0 for Cl". 

The scan rate, u = EIAt, plays a very important role in sweep voltannnetry as it defines the time scale of the experiment and is typically in the range 5 mV s to 100 V s for nonnal macroelectrodes, although sweep rates of 10 V s are possible with microelectrodes (see later). The short time scales in which the experiments are carried out are the cause for the prevalence of non-steady-state diflfiision and the peak-shaped response. Wlien the scan rate is slow enough to maintain steady-state diflfiision, the concentration profiles with time are linear within the Nemst diflfiision layer which is fixed by natural convection, and the current-potential response reaches a plateau steady-state current. On reducing the time scale, the diflfiision layer caimot relax to its equilibrium state, the diffusion layer is thiimer and hence the currents in the non-steady-state will be higher. 

This expression is the sum of a transient tenu and a steady-state tenu, where r is the radius of the sphere. At short times after the application of the potential step, the transient tenu dominates over the steady-state tenu, and the electrode is analogous to a plane, as the depletion layer is thin compared with the disc radius, and the current varies widi time according to the Cottrell equation. At long times, the transient cunent will decrease to a negligible value, the depletion layer is comparable to the electrode radius, spherical difhision controls the transport of reactant, and the cunent density reaches a steady-state value. At times intenuediate to the limiting conditions of Cottrell behaviour or diffusion control, both transient and steady-state tenus need to be considered and thus the fiill expression must be used. Flowever, many experiments involving microelectrodes are designed such that one of the simpler cunent expressions is valid. 

The platinum microelectrode appears to act as a potentiostat and maintains the potential of the Pb-solution interface at a crack at a value that favours the re-formation of PbOj, rather than the continuous formation of PbClj which would otherwise result in excessive corrosion. 

Fig. 6. Molecular transistor based on a microelectrode array. P is a polymer layer that can be switched conductive or nonconductive by the potential of the gate electrode (from ref.

Recently, there has been a growth of interest in the development of in vitro methods for measuring toxic effects of chemicals on the central nervous system. One approach has been to conduct electrophysiological measurements on slices of the hippocampus and other brain tissues (Noraberg 2004, Kohling et al. 2005). An example of this approach is the extracellular recording of evoked potentials from neocortical slices of rodents and humans (Kohling et al. 2005). This method, which employs a three-dimensional microelectrode array, can demonstrate a loss of evoked potential after treatment of brain tissue with the neurotoxin trimethyltin. Apart from the potential of in vitro methods such as this as biomarkers, there is considerable interest in the use of them as alternative methods in the risk assessment of chemicals, a point that will be returned to in Section 16.8. 

Such clear postsynaptic potentials can be recorded intracellularly with microelectrodes in large quiescent neurons after appropriate activation but may be somewhat artificial. In practice a neuron receives a large number of excitatory and inhibitory inputs and its bombardment by mixed inputs means that its potential is continuously changing and may only move towards the threshold for depolarisation if inhibition fails or is overcome by a sudden increase in excitatory input. 

To deposit Au structures, a Au probe is approached to the surface until a positive feedback is observed. This is due to the regeneration of Cl species on the substrate while Au is deposited from AUCI4 according to the reverse reaction, leading to an increase in the local concentration of Cl. The microelectrode is then left at this position above the substrate for a certain time, after which it is withdrawn from the surface. The potential of the substrate, the electrolyte, and the pH were found to be the most significant parameters determining in determining the rate of Au electrodeposition and its structure (Amman and Mandler, 2001). 

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