Sensor, defined amperometric

Note that this equation describes the relationship between concentration C of the component and the sensor response X. It is purposely written backwards by comparison with the usual notation used with linear sensors (e.g., optical, amperometric, etc.) discussed earlier. This convention helps to define P as the matrix of regression coefficients. 

It should be noted that GC mode experiments with amperometric tips may contain a feedback component to the current if the electrochemical process at the tip is reversible and the tip-to-specimen distance is less than about 5a. However, at greater distances or when employing a potentiometric tip, the tip acts approximately as a passive sensor, i.e., one that does not perturb the local concentration. This situation is quite distinct from feedback mode, where the product of the electrolysis at the tip is an essential reactant in the process at the specimen surface. This interdependence of tip and specimen reactions in feedback mode ensures that the biochemical process is confined to an area under the tip defined by the tip radius and diffusional spreading of the various reagents (20). In contrast, the biochemical process in GC mode is independent of the presence of the tip and may therefore occur simultaneously across the whole surface. In addition, the tip signal often does not directly provide information on the height of the tip above the surface methods to overcome this limitation are described in Sec. I.D. Finally, since the tip process and the biochemical reaction at the specimen are independent, a wide range of microsensors may be employed as the tip, e.g., ion-selective microelectrodes, which are not applicable in feedback experiments. 

Electrodes are used as sensors in either a potentiometric mode or an amperometric mode. As the names imply, potentiometric electrodes measure electrochemical activity by relating it to a potential (voltage). Amperometric electrodes measure electrochemical activity by relating it to a quantity of current (amperes). Both modes have found wide application. Ion-selective electrodes generally operate in the potentiometric mode. Amperometric sensors, conversely, generally use nonselective electrodes which can be made selective by electrochemical and nonelectrochemical modification. Potentiometric electrodes operate via a number of presently ill-defined mechanisms. However, regardless of the mechanism, the measured potential is due to an interfacial chemical equilibrium that does not involve a bulk transfer of material. Amperometric electrodes, on the other hand, do involve the bulk transfer of material. 

Next we have to define the boundary and the initial conditions. For so called zero flux sensors there is no transport of any of the participating species across the sensor/enzyme layer boundary. Such condition would apply to, e.g., optical, thermal or potentiometric enzyme sensors. In that case the first space derivatives of all variables at point x are zero. On the other hand amperometric sensors would fall into the category of non-zero-flux sensors by this definition and the flux of at least one of the species (product or substrate) would be given by the current through the electrode. 

Because EPs can be formed and modified electrochemically their application to electrochemical sensors is the natural choice. It is again the remarkable flexibility of their design which makes them universally applicable if necessary they can be prepared with the high conductivity needed for amperometric sensors. They also form well defined ohmic contacts with the metal electrodes required for conductimetric sensors. Their thickness, which is important in potentiometric sensors, can be controlled. We now take a more detail look at the three principal transduction electrochemical modes. A good source of information on electrochemical aspects of EPs is the review chapter on chemically modified electrodes by Murray [3]. 

In the preceding three sections reaction mechanisms in which the homogeneous chemical reaction was coupled with the electrode process were discussed. This coupling enables exceptionally fast chemical reactions to be investigated and their rate constants determined. Nevertheless, voltammetric methods can also be exploited for kinetic studies on chemical reactions occurring independently of the electrode process in the bulk of the solution. For this purpose all voltammetric techniques can be used for which the dependence of voltammetric response on the concentration of one or more reactants is defined in a simple way. Various amperometric sensors are mostly applied, working at the potentials of limiting current. The response need not be a diffusion-controlled current. Kinetic currents within the diffusion-controlled zone can also be taken into account. 

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