Activity potentiometric devices

If the potential difference across a membrane varies with activity in the manner depicted by equation (9.7), then the potentiometric device is said to behave in a Nernstian manner. For many membranes, this sort of behaviour, or an approximation to it, is observed. 

Membrane potentiometric devices respond in a logarithmic fashion to analyte activities. The majority of chemists are more used to working with concentrations than with activities, and unless the ionic strength of a solution is known, the concentration cannot be calculated. Addition of a total ionic-strength adjusting buffer can overcome this problem, but is not always... 

Electrometer preamplifiers are used where very low current drain from a signal source is important. Input impedances of such circuits lie in the range from 10 to 10 " Q. They are used as preamplifiers for microelectrode recording, and null sensing circuits in potentiometric devices such as pH meters. The active elements in these circuits are either electrometer vacuum-tubes or insulated-gate FET s. Ordinary transistors are inapplicable because of their low input impedance. 

Potentiometric devices have a relatively high sensitivity and selectivity and abroad response range. They are also simple, rehable, and inexpensive and use commonly available instrumentation. Potentiometric enzyme-based biosensor systems have been recently reviewed. The most general approach for the determination of OP compounds (OPCs) is based on their inhibition of the activity of choline esterase enzymes. The presence of low concentrations of inhibitors strongly and specifically affects the enzyme activity. Therefore, by measuring the enzyme activity, the concentration of the OPCs can be assayed. Different enzymes, such as acetylcholine (ACh), BuCh, and, most often, pH sensors, were used as transducers in potentiometric biosensors. 

The equipment required for direct potentiometric measurements includes an ion-selective electrode (ISE), a reference electrode, and a potential-measuring device (a pH/millivolt meter that can read 0.2mV or better) (Figure 5-1). Conventional voltmeters cannot be used because only very small currents are allowed to be drawn. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species. Such electrodes exhibit a fast response and a wide linear range, are not affected by color or turbidity, are not... 

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

The PEDOT PSS-based electrochemical transistor balance at the boundary between amperometric and potentiometric sensor functionality. Its device architecture is simple and robust and these PEDOT PSS sensors can easily be manufactured using standard printing technologies. In addition, its proper ion-to-electron transduction characteristics and the possibility to integrate biologically active substances promise for simple and sensitive sensors in the... 

The type of molecular recognition reaction determines the form of the transducer used (Table 5.3). Enzymatic reactions often involve an electron transfer. This electrical activity can be measured with amperometric, potentiometric or conductometric sensors. If the bioreaction includes the generation of H+ or OH ions, then a pH sensitive dye in combination with an optical device can be used. For antibody-antigen binding, the mass change on the surface of the transducer can be detected with a piezoelectric device. Exothermic or endothermic reactions can be followed with a temperature sensor. 

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