Potentiometric sensors advantages

In comparison to voltammetry, with heated potentiometric sensors, advantages are not obvious at first glance. Anyhow, there are chances to tune such sensors in order to achieve higher sensitivity, faster response and better selectivity. Of course, experiments are restricted to all-solid ISEs which can be heated indirectly. An interesting example was presented [19], where a heated copper disc had been modified by an ionophore layer. The slope of the sensor was strongly increased by heating, and the detection Unfit was decreased by half an order of magnitude. 

Enzyme electrodes with amperometric indication have certain advantages over potentiometric sensors, chiefly because the product of the enzymic reaction is consumed at the electrode and thus the response time is decreased. For this reason, the potentiometric glucose enzyme electrode, based on reaction (8.1) followed by the reaction of HjO, with iodide ions sensed by an iodide ISE [39], has not found practical use. 

The symmetrical arrangement cannot be easily scaled down, but it has one major advantage most nonidealities cancel out. It is a good starting point for discussion of all ion potentiometric sensors. 

The sign in front of the reaction term is positive only for hydrogen peroxide. Also, the function 3Ipn can be made constant by operating the sensor in a medium of high buffer capacity. This is clearly a distinct advantage compared to the potentiometric sensors in which the buffer capacity represented a major interference. 

Potentiometric redox measurements are often performed in nonaqueous or mixed-solvent media. For such solvents various potentiometric sensors have been developed, which, under rigorously controlled conditions, give a Nemstian response over a wide ranges of activities, particularly in buffered solutions. There are some experimental limitations, such as with solvent purification and handling or use of a reference electrode without salt bridges, but there also ate important advantages. Solutes may be more soluble in such media, and redox... 

Measurement of the cell potential of a potentiometric sensor should be carried out under zero-current or quasiequilibrium conditions. A high input impedance electrometer is commonly used. The advantage of potentiometric sensor is that the sensor output and the cell potential do not depend on the electrode surface area. This can make the manufacturing of practical sensors simpler. A major disadvantage of... 

One of the advantages of mixed potential sensors is that it is possible for both electrodes to be exposed to the same gas. The elimination of a need to separate the two electrodes simplifies the sensor design, which in turn reduces fabrication costs. Although this simpler planar design is often used, the electrodes are sometimes separated to provide a more stable reference potential. As with equilibrium potentiometric sensors, the minimum operating temperature is often limited by electrolyte conductivity. However, the maximum operation temperatures for nonequilibrium sensors are typically lower than those of equilibrium sensors, because the electrode reactions tend towards equilibrium as the temperature increases. This operating temperature window depends on the electrode materials, as will be discussed later in the chapter. 

The mixed-potential type of potentiometric sensors, described in the previous sections, appears to be advantageous for combustion applications and for on-board NOx, CO, and HC sensors owing to their high sensitivities, especially in the measurement of a lower concentration (less than 100 ppm). 

Several of the procedures described in the previous sections can be advantageously carried out with double barrel tips. Such a probe consists of two capillaries (see Sec. V.B), one of which acts as the potentiometric sensor, while the other is used to determine the tip-substrate distance. For example (79), a gallium microdisk was combined with an ion-selective (K+) potentiometric probe to image K+ activity near the aperture of a capillary (see Fig. 7). Similarly (77), a double barrel tip with one channel as an open Ag/ AgCl micropipette for solution resistance measurement and the other channel as an ion-selective neutral carrier-based microelectrode for potentiometric measurements was successfully used to image concentration distributions for NH4 (Fig. 8) and Zn2+ (Fig. 9). While dual-channel tips facilitate the approach of the substrate and permit a direct determination of the absolute tip-substrate distance, their difficult fabrication severely limits their use. Reference 80 compares the above methods. 

The concept of electrode potentials, described here, has great advantages over considerations based on thermodynamic data calculated with measured potential differences of cells for application of solid-electrolyte potentiometric sensors it is simple to understand, results follow immediately and thus it is very helpful in practical cases. 

Ion-selective electrodes (ISEs) constitute an example of potentiometric sensors that offer several advantages over other analytical techniques for the analysis of environmentally important ions. Specifically, the sensing platform of a membrane-based ISE consists of an ion carrier (ionophore) entrapped within a liquid polymeric membrane. The membrane does offer some interaction with numerous species, but the main interaction governing the selectivity of the sensor is between the analyte/interferences and the ionophore. Once an ionophore that offers the preferred selectivity has been developed and the polymer components that are ionophore-compatible have been optimized, the production of a functional ISE is rather facile and rapid. Presently, ISEs have been reported for several species including metal ions, anions, surfactants, and gases (5). 

Experimental studies have also shown that selective detection is an important advantage of an electrochemical sensor, but sometimes their responses are affected by the presence of other gases (Park et al. 2003). As established in numerous experiments, cross-sensitivity is often observed when lype II or Type III potentiometric sensors are exposed to gas species that form thermodynamically more stable compounds or kinetically more favorable compounds in spite of lower thermodynamic stability (Weppner 1992). 

Potentiometric sensors responsive to iodide ion that are based on iodine-doped poly(3-methylthiophene) have been demonstrated [214,215]. Potentiometric sensors, because of the wide range of possible potential-influencing interactions [216], offer no major advantages over conventional functionalized nonconducting polymer membranes. 

The miniaturization of traditional solvent polymeric membrane-based potentiometric sensors has obvious limitations in terms of reduced lifetimes and extremely high electrical resistances. Therefore, a completely different construction is needed to overcome these limitations. One approach that takes advantage of the unique opportunities provided by nanostructures and shown to be of perspective is based on the use of nanoporous membranes. The concept behind nanopore potentiometry is based on shrinking the nanopore restriction to approach molecular dimensions so that the physical-chemical properties of the inner wall of the nanopores determine the ion transport through the nanopore. Thus, in this approach, there is no solvent polymeric membrane, but solely the surface functionality of the nanopores is generating the ion-selective behavior. 

---