Electrode system, response time

Continuous-flow methods allow use of a response time of the recording system longer than the life of the reaction to be followed. As an example, most electrodes have response times of the order of seconds. If they are built into a flow system the time determined by its dimensions and rate of flow will be substituted for the response of the electrode. The analogy with the use of the Hartridge spectroscope is obvious. There have been relatively few applications of the principle. 

The high specificity required for the analysis of physiological fluids often necessitates the incorporation of permselective membranes between the sample and the sensor. A typical configuration is presented in Fig. 7, where the membrane system comprises three distinct layers. The outer membrane. A, which encounters the sample solution is indicated by the dashed lines. It most commonly serves to eliminate high molecular weight interferences, such as other enzymes and proteins. The substrate, S, and other small molecules are allowed to enter the enzyme layer, B, which typically consist of a gelatinous material or a porous solid support. The immobilized enzyme catalyzes the conversion of substrate, S, to product, P. The substrate, product or a cofactor may be the species detected electrochemically. In many cases the electrochemical sensor may be prone to interferences and a permselective membrane, C, is required. The response time and sensitivity of the enzyme electrode will depend on the rate of permeation through layers A, B and C the kinetics of enzymatic conversion as well as the charac-... 

Such electrodes should be sufficient as a reference electrode for short-term usage or as a disposable electrode. However, the requirement of a pre-hydration time may limit its applications for fast measurements, such as POCT (the point-of-care testing), due to its slow response time. In fact, the lack of long-term stable microreference electrodes will continue to hamper the development of integrated pH sensing systems. 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

Fig. 5. Two-dimensional parametric diagram of system response at different initial concentrations of reagents in batch A, monotonic growth of Pt potential [Fig. 1(a)] V, monotonic decrease of Pt potential [Fig. 1(b)] O, Pt electrode potential first decreases and then increases in time [Fig. 1(c)] , various nonmonotonic transient regimes [Fig. l(d—f)]. Strizhak, P. E. Basylchuk, A. B. Demjanchyk, I. Fecher, F. Shcneider, F. W. Munster, A. F. Phys. Chem. Chem. Phys. 2000, 2, 4721. Reproduced by permission of The Royal Society of Chemistry on behalf of the PCCP Owner Societies.

The problem with the Clark electrode is that some of these requirements have solutions that are opposing. For instance, flow dependence may be reduced by employing a thicker membrane but this would occur at the cost of increased response time. As a result, most commercially available systems are design compromises that sacrifice a part of some desirable feature. It should he noted that an optical measurement technique where oxygen and/or electrolyte is not consumed will be free of the drawbacks mentioned above. 

Alternatively, an assembly of microelectrodes can alleviate some of the problems associated with the individual microelectrodes. Such a random array of microelectrodes (RAM) comprises about 1000 carbon fibres (each of diameter 5-7 pm) which are embedded randomly within an inert adhesive such as an epoxy resin. (The ends of the fibres need to be widely spaced.) The net result is to generate an electrode system with a superior response time and a current which is IfKK) times that of a single microelectrode. By increasing the current in this way, the sensitivity of measurement is further increased. 

A working GCE coated with an electrodeposited film of CuPtCle, together with a reference SCSE and a stainless steel wire as counterelectrode, serve for sensitive amperometric determination of H2O2 in phosphate buffer at pH 7.4 in a FIA system. Working at 4-200 mV on the oxidation of the analyte avoids interference of dissolved oxygen. The response time of the coated electrode is very fast (about 5 s) the LOD is 10 nM, with linearity in the 50 nM to 5 mM range. ... 

Current as a function of time is the system response as well as the monitored response in chronoamperometry. A typical double-potential-step chronoamperogram is shown by the solid line in Figure 3.3B. (The dashed line shows the background response to the excitation signal for a solution containing supporting electrolyte only. This current decays rapidly when the electrode has been charged to the applied potential.) The potential step initiates an instantaneous current as a result of the reduction of O to R. The current then drops as the electrolysis proceeds. 

From the theoretical point of view, the analysis of the faradaic electrochemical behavior of these systems is simpler than that corresponding to solution soluble species since the mass transport is not present, a fact which greatly simplifies the modellization of these processes (note that, in general, for a redox species in solution, a two- or three-variable problem (coordinates and time) needs to be considered, whereas at the electrode surface only time variable is needed). Thus, it is possible to deduce the electrochemical response of these molecules in a more direct way. 

At the present time, the theory of electrochemical impedance of electrodes with distributed potentials is not yet completed, and algorithms of parametrical and structural identification procedures are not available. In addition, the interpretation of the results is very complicated. For this reason, in this work we analyzed only the frequency characteristics of impedance s components in the modified electrode system. As a result, we obtained a set of response peculiarities in the frequency range under investigation. Rather low frequency dispersion was observed in a solution containing a ferri-ferrocyanide system for both active (Fig.3, curve 2) and reactive (Fig.4, curve 3) components. In our opinion, this fact confirms that the independent on frequency resistance of charge transfer determines the main contribution to the impedance. 

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