Potentiometric transducers

The essential component of a potentiometric measurement is an indicator electrode, the potential of which is a function of the activity of the target analyte. Many types of electrodes exist (see Table 9.1), but those based on membranes are by far the most useful analytical devices. The broader field of potentiometry has been reviewed recently (1). The potential of the indicator electrode cannot be determined in isolation, and another electrode (a reference electrode) is required to complete the electrochemical cell. Undoubtedly the best known of the potentiometric indicator electrodes is the glass pH electrode, the operation and use of which has been adequately discussed (2). Ion-selective electrodes (ISEs) are also commonplace, and have been the subject of several books (3-5) there is even a review journal for ISEs (6). Unfortunately, the simplicity of fabrication and use of ISEs has given rise to the idea that ISEs are chemical sensors. At the best this is a half-truth certainly, they can behave like chemical sensors under well-controlled laboratory conditions, but in the real world their performance leaves much to be desired. Moreover, from a manufacturing point of view important features of a sensor are that it can be fabricated in relatively large numbers, and that each device is identical to all the others. Although some ISEs can be mass-produced , many cannot, and even those that do lend themselves to this form of production invariably require calibration before use. Nonetheless, in spite of the limitations of ISEs, transducers based on potentiometric membrane electrodes have much to contribute to the field of chemical sensing. 

The electrochemical cell that is formed when a membrane electrode is used in conjunction with a reference electrode is shown below. 

It is clear from this diagram that the function of the membrane is to provide a 

Pt or other inert electrode for the determination of redox potentials 

Metal electrode in reversible equilibrium with a solution of its own ions (Ag/AgN03) 

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Martinez-Eabregas, E. Alegret, S. A Practical Approach to Ghemical Sensors through Potentiometric Transducers Determination of Urea in Serum by Means of a Biosensor, /. Chem. Educ. 1994, 71, A67-A70. 

Potentiometric transducers now belong to the most mature transducers with numerous commercial products. For potentiometric transducers, a local equilibrium is established at the transducer interface at near-zero current flow, where the change... 

The use of additional membranes, which selectively convert nonionic analytes into ionic species that can be determined via ISEs is another common approach. An abundance of ingenious designs make use of biocatalysts for the development of potentiometric biosensors. Much of the earlier designs have made use of enzymes as the molecular recognition element. The products that are associated with such enzyme-catalyzed reactions can be readily monitored with the potentiometric transducer by coating the traditional electrodes with the enzyme. 

Field effect transistors are miniature, solid-state, potentiometric transducers (Figure 4.22) which can be readily mass produced. This makes them ideal for use as components in inexpensive, disposable biosensors and various types are being developed. The function of these semiconductor devices is based on the fact that when an ion is absorbed at the surface of the gate insulator (oxide) a corresponding charge will add at the semiconductor... 

Karube et al. have constructed several gas-sensing devices by using immobilized microorganisms and a potentiometric transducer. Two of them... 

The resolution of potentiometric transducers is dependent upon the construction of the resistance element. In the case of a wire-wound resistance, in order to obtain a high resistance in a small space, the resistance wire is wound on to a mandrel or card which is straight or formed into a circle or helix depending upon the motion of the contact. This limits the resolution of the transducer as the wiper moves from one wire to the next on the mandrel. The best resolution that can be obtained is about 0.01 per cent (see Section 6.10.1). Typical wire-wound potentiometers have strokes of between 0.0025 m and 0.5 m and rotational versions from about 10° of arc to 50 turns. An alternative often employed is the conductive plastic film element. This provides a continuous resistance element and thus, a zero resolution, but such elements suffer from a higher temperature coefficient of resistance. A more recent development is a combination of earlier types in which a conductive plastic coating is sprayed on to a wire-wound resistor. 

Potentiometric transducers measure the potential between the sensing element and a reference element. Thus, in contrast to amperometric transducers, practically no mass transport occurs the response depends on the development of the thermodynamic equilibrium. pH changes often correlate with the measured substance because many enzymatic reactions consume or produce protons. 

Potentiometric transducers measure the potential under conditions of constant current. This device can be used to determine the analytical quantity of interest, generally the concentration of a certain analyte. The potential that develops in the electrochemical cell is the result of the free-energy change that would occur if the chemical phenomena were to proceed until the equilibrium condition is satisfied. For electrochemical cells containing an anode and a cathode, the potential difference between the cathode electrode potential and the anode electrode potential is the potential of the electrochemical cell. If the reaction is conducted under standard-state conditions, then this equation allows the calculation of the standard cell potential. When the reaction conditions are not standard state, however, one must use the Nernst equation to determine the cell potential. Physical phenomena that do not involve explicit redox reactions, but whose initial conditions have a non-zero free energy, also will generate a potential. An example of this would be ion-concentration gradients across a semi-permeable membrane this can also be a potentiometric phenomenon and is the basis of measurements that use ion-selective electrodes (ISEs). 

For optical transducers, the measured signals are directly proportional to [P], so that, once again, reaction layer thickness and mass-transport kinetics determine the sensitivity of the biosensor, and signals are directly proportional to analyte concentration. For potentiometric transducers, signals are proportional to log[P], and therefore to log[S]. ... 

Electrochemical biosensors have some advantages over other analytical transducing systems, such as the possibility to operate in turbid media, comparable instrumental sensitivity, and possibility of miniaturization. As a consequence of miniaturization, small sample volume can be required. Modern electroanalytical techniques (i.e., square wave voltammetry, chronopotentiometry, chronoamperometry, differential pulse voltammetry) have very low detection limit (1(T7-10 9 M). In-situ or on-line measurements are both allowed. Furthermore, the equipments required for electrochemical analysis are simple and cheap when compared with most other analytical techniques (2). Basically electrochemical biosensor can be based on amperometric and potentiometric transducers, even if some examples of conductimetric as well as impedimetric biosensor are reported in literature (3-5). 

The principle for biosensors and immunosensors is based on the biochemical reaction produced at the surface of the electrode followed by the detection of one of the products formed in the biochemical reaction by using a certain transducer. In this case, the selectivity of the biochemical reaction must be correlated with the sensitivity of the transducer.264 The best correlation is accomplished when amperometric transducers are utilized. The potentiometric transducers in most cases are not suitable for biosensor design. For the design of immunosensors, the potentiometric transducer is not always able to measure the product obtained in the immunological reaction because it is known that the reaction between the antigen and the antibody is very sensitive. In certain cases the selectivity of transducer vs. different products of the biochemical reaction must also be considered. Because of the high sensitivity of the piezoelectric transducers, their selectivity is limited, and they cannot assure the best results when used in biosensor or immunosensor design. 

Binding protein/antibody-based assay on a light addressable potentiometric transducer pH changes in whole cells... 

Basically, every kind of potentiometric ion-sensitive electrode can be miniaturized using a FET as potentiometric transducer especially, ion-selective polymer membranes have been used. In addition, utilizing eri2yme systems, enzyme-modified FETs (ENFETs) can be reahzed, opening up possibilities for miniaturized biosensors (see Sect. 2.11.3, Miniaturization). 

Interesting features of potentiometric transducers are simplicity of instrumentation (only a potentiometer is needed), low cost, selectivity, and the nondestructive nature of the analytical procedure. 

Urea is a diagnostic indicator for kidney function. Urea can be potentiometrically determined following the enzymatic reaction of the enzyme urease (reaction [I]) coupled with a variety of potentiometric transducers such as the pC02 electrode, the PNH3 electrode, the pH electrode, and the PNH4 electrode ... 

Immimodiagnostics and enzyme biosensors are two of the leading technologies that have a greatest impact on the food industry. The use of these two systems has reduced the time for detection of pathogens such as Salmonella to 24 hr and has provided detection of biological compounds such as cholesterol or chymotrypsin [15]. Biosensors analyses Beta lactams in milk and presence of urea in milk that lead to production of synthetic milk, the biocomponent part of the urea biosensor is an immobilized urease yielding bacterial cell biomass isolated from soil and is coupled to the ammonium ions selective electrodes of a potentiometric transducer. The membrane potential of all types of potentiometric cell based probes is related to the activity of electrochemically-detected product [16]. 

POTENTIOMETRIC TRANSDUCERS Table 9.1 Potentiometric indicator electrodes... 

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