Potentiometric probes

University of Southampton, Southampton, England Geza Nagy 

Janus Pannonius University, Pecs, Hungary Klara Toth 

Most scanning electrochemical microscopy (SECM) experiments are conducted in the amperometric mode, yet microelectrodes have for many years been used as potentiometric devices. Not surprisingly, several SECM articles have described how the tip operated in the potentiometric mode. In this chapter we aim to present the background necessary to understand the differences between amperometric and potentiometric SECM applications. Since many aspects of SECM are covered elsewhere in this monograph, we have focused on the progress made in the held of potentiometric microelectrodes and presented it in the context of SECM experiments. Starting with an historical perspective, the key discoveries that facilitated the development and applications of micro potentiometric probes are highlighted. Fabrication techniques and recipes are reviewed. Basic theoretical principles are covered as well as properties and technical operational details. In the second half of the chapter, SECM potentiometric applications are discussed. There the differences between the conventional amperometric mode are developed and emphasized. 

Potentiometric probes are the oldest forms of electrochemical sensors. They can conveniently be used for studying many interesting chemical systems not accessible to voltammetric techniques. In particular, alkali and alkaline earth metal ion concentrations, of importance in biological systems, 

Ion-selective electrodes (ISE) are membrane-based devices with internal filling solution and internal reference electrode or with internal solid contact. Ion-selective electrodes may be classified according to the nature of the ion-selective membrane or the shape and size of the electrode arrangement. The main types of membrane electrodes are  

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Potentiometry (discussed in Chapter 5), which is of great practical importance, is a static (zero current) technique in which the information about the sample composition is obtained from measurement of the potential established across a membrane. Different types of membrane materials, possessing different ion-recognition processes, have been developed to impart high selectivity. The resulting potentiometric probes have thus been widely used for several decades for direct monitoring of ionic species such as protons or calcium, fluoride, and potassium ions in complex samples. 

The sensor is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme mease (Figme 6-11) (22). The generated ammonium ions are detected after 30-60 s to reach a steady-state potential. Alternately, the changes in the proton concentration can be probed with glass pH or other pH-sensitive electrodes. As expected for potentiometric probes, the potential is a linear function of the logarithm of the urea concentration in the sample solution. 

The main classes of dyes nsed as potentiometric probes are cationic or zwitterionic styryl dyes, cationic carbocyanines and rhodamines, anionic oxonols and hybrid oxonols and merocyanines. The particular class of dye determines factors snch as accnmnlation in cells, response mechanism and toxicity. 

The GC mode can also be used to image some enzymes that are not oxidoreductases. For the important enzymes alkaline phosphatase (ALP) and galactosidase, this has been achieved by using an enzyme substrate that is not redox active at the UME potential, whereas one of the products (p-aminophenol (PAP)) can be oxidized. The experiment is detailed in the Protocol P2 H37. The use of potentiometric probes is also possible. Table 37.2 provides an overview about the investigated enzymes. 

G. Denuault, G. Nagy and K. Toth, Potentiometric probes. In A.J. Bard and M.V. Mirkin (Eds.), Scanning Electrochemical Microscopy, Marcel Dekker, New York, Basel, 2001, pp. 397-444. 

The integration of chemically sensitive membranes with solid-state electronics has led to the evolution of miniaturized, mass-produced potentiometric probes known as ion-selective field effect transistors (ISFETs). The development of ISFETs is considered as a logical extension of coated-wire electrodes (described in Section 5.2.4). The construction of ISFETs is based on the tech-... 

The requirements for accurate recording of these two types of signals are quite different and lead to different equipment requirements. A sensitive potentiostat is required for a voltammetric probe and an electrometer is needed for a potentiometric probe. 

The high resistance of the potentiometric probe is an effective noise generator. The Johnson noise produced by a resistor at room temperature is 1.3 X 10 10 ft i 2V/Hz1/2 (26,27). The 1010 ft Rta of a micropotentiometric probe will produce 41 /xV rms of noise in a 10 Hz measurement bandwidth,... 

Although somewhat dependent on the method of electrode construction, the Rs of a potentiometric probe depends on the active tip area (24,25). One limiting factor for the ultimate size of the probe used in potentiometric measurements is the electrical characteristics. As Rs approaches 1013 fl, the noise level of the measurement will not allow greater than 1% accuracy, assuming very naively that the only noise source is the resistor Johnson noise. The noise level of small tips suggests that potentiometric tips of 0.05 /xm diameter are close to the smallest useful size. 

The response time is one of the most critical characteristics of the potentiometric probe in continuous analyzers and in in situ monitoring. Most often the activity step method is used for its determination (Fig. 4). The response time is affected by several factors such as the properties of the ion-selective... 

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. 

In the following sections we consider several potentiometric applications. Many articles do not refer directly to scanning electrochemical microscopy, but all are closely related to the SECM principles and were therefore included in the present chapter. We have not included a section on applications related to potentiometric probing of biological substrates since they are covered in Chapter 11 of this volume. 

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