Sensor, defined potentiometric

Another problem that is common for all membrane-based solid-state sensors is the ill-defined membrane-metal interface. A large exchange current density is required to produce a reversible interface for a stable potentiometric sensor response. One approach to improving this interface is to use conducting polymers. Conducting polymers are electroactive n-conjugated polymers with mixed ionic and electronic conductivity. They... 

There are three types of reference electrodes discussed reference electrodes of the first kind, reference electrodes of the second kind, and redox reference electrodes. The first two are used with potentiometric chemical sensors, whereas the last one helps us to get around the difficult problem of comparing potentials in different solvents. There is also a pseudo-reference electrode that does not have a stable, defined, reproducible potential. It serves only as the signal return to satisfy the condition of closing the electrical circuit (see Section 5.2). Because the liquid junction always causes some leakage of the internal solution, electrodes of the first kind are particularly affected. 

The obvious advantage of the symmetrical arrangement is that the processes at all internal interfaces can be well defined and that most nonidealities at the mem-brane/solution interface tend to cancel out. Because the volume of the internal reference compartment is typically a few milliliters, the electrode does not suffer from exposure to electrically neutral compounds that would penetrate the membrane and change the composition of this solution. This type of potentiometric ion sensor has been used in the majority of basic studies of ion-selective electrodes. Most commercial ion-selective electrodes are also of this type. The drawbacks of this arrangement are also related to the presence of the internal solution and to its volume. Mainly for this reason, it is not conveniently possible to miniaturize it and to integrate it into a multisensor package. 

Symmetrical placement of the ion-selective membrane is typical for the conventional ISE. It helped us to define the operating principles of these sensors and most important, to highlight the importance of the interfaces. Although such electrodes are fundamentally sound and proven to be useful in practice, the future belongs to the miniaturized ion sensors. The reason for this is basic there is neither surface area nor size restriction implied in the Nernst or in the Nikolskij-Eisenman equations. Moreover, multivariate analysis (Chapter 10) enhances the information content in chemical sensing. It is predicated by the miniaturization of individual sensors. The miniaturization has led to the development of potentiometric sensors with solid internal contact. They include Coated Wire Electrodes (CWE), hybrid ion sensors, and ion-sensitive field-effect transistors. The internal contact can be a conductor, semiconductor, or even an insulator. The price to be paid for the convenience of these sensors is in the more restrictive design parameters. These must be followed in order to obtain sensors with performance comparable to the conventional symmetrical ion-selective electrodes. 

Instrumental control over the sensitivity of potentiometric sensors will allow controlled ion uptake by the membrane, thereby generating strong super-Nernstian responses. Advances in this direction were recently realized with double- and triple-pulse experiments, where well-defined current and potential pulses were used for accurate control of the otherwise highly transient transport and extraction process [88]. 

The stability of enzyme electrodes is difficult to define because an enzyme can lose some of its activity. Deterioration of immobilized enzyme in the potentiometric electrodes can be seen by three changes in the response characteristics (a) with age the upper limit will decrease (e.g., from 10-2 to 10 3 moll-1), (b) the slope of the analytical (calibration) curve of potential vs. log [analyte] decrease from 59.2 mV per decade (Nernstian response) to lower value, and (c) the response time of the biosensor will become longer as the enzyme ages [59]. The overall lifetime of the biosensor depends on the frequency with which the biosensor is used and the stability depends on the type of entrapment used, the concentration of enzyme in the tissue or crude extract, the optimum conditions of enzyme, the leaching out of loosely bound cofactor from the active site, a cofactor that is needed for the enzymatic activity and the stability of the base sensor. 

Thus, ion-selective membrane electrodes can be defined as electrochemical sensors that allow potentiometric measurements of the activity of particular species in aqueous and mixed solvents or partial pressures of dissolved gases in water. However, these sensors may respond to certain other ions in the sample in addition to the selected i ion interferences by such j ions are usually expressed by the Nikolskii-Eisenman Eq. (17) ... 

In contrast to direct potentiometry, the potentiometric titration technique offers the advantage of high accuracy and precision, although at the cost of increased time and increased consumption of titrants. Another advantage is that the potential break at the titration endpoint must be well defined, but the slope of the sensing electrode response need be neither reproducible nor Nernstian, and the actual potential values at the endpoint are of secondary interest. In many cases, this allows for the use of simplified sensors. 

Solid-state reference electrodes for potentiometric sensors are currently under research. The main problem to be faced in developing this type of electrode lies in connecting the ionic conducting (usually aqueous) solution with an electronic conductor. Since the reference electrode has to maintain a defined potential, the electrochemical reaction with components of the electrolyte has to be avoided. Oxides, mixed oxides, and polyoxometalate salts of transition elements can be proposed for preparing solid-state reference electrodes. Tested compounds include tungsten and molybdenum oxides (Guth et al., 2009). 

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. 

Numerous applications have been developed in the field of chemical analysis using potentiometric measurements as indicators, including the production of potentiometric sensors and titration devices. In this chapter, we will focus on the defining principles of these potentiometric methods at zero current when these systems are in thermodynamic equilibrium, which is not necessarily true for all potentiometric measurements. In particular, the following description is confined to electrochemical cells with no ionic junction. In practice, these results will also be applied to many experimental cases in which ionic junction voltages can be neglected . 

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]. 

Since the potential of selective potentiometric sensors is in a well-defined relationship with the activity of the detected ion, it is obvious that potentiometric electrodes can be used for pH measurements. The Nemst equation for the hydrogen ion is... 

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