Micro-reference Electrodes

Abstract

Rapid and accurate determination of ions in solutions is an increasing demand in medicine, research, routine work in laboratories as well as in process control. The development of portable chemical analysers is dependent on the ability to miniaturise the sensors. With voltammetric or amperometric techniques, the tolerated uncertainty in the potential is still relatively large since the analyte concentration is related primarily to the measured current, which is quite constant in a certain potential window. So, pseudo-reference electrodes or quasi-reference electrodes have been successfully implemented in voltammetric and amperometric microsystems. With potentiometric sensors, however, the zero-current potential is directly related to the activity of the analyte ion. The potential change at the reference electrode as a function of the sample composition must therefore be kept reliably small. Considerable effort has been made to improve the performance of ion selective electrodes. As a result, miniaturised ISEs having analytical performance comparable to macro-ISEs are now available for many ions. However, true miniaturisation of ion sensor is often hampered by the lack of suitable micro-reference electrodes. The understanding of the dimensions of a microsystem differs in the literature. The dimension of a true microelectrode should be ca. 1 μm; however, this dimension refers to the active area. The size of the electrode housing may be orders of magnitude larger. Very often, the term micro-reference electrode refers to electrodes with outer dimensions in the lower millimetre range. (cf. Janata [1]). As always, there are three basic requirements that a reference electrode has to satisfy: (1) it should have a stable potential (i.e. the potential should not depend on the composition of the sample solution), (2) the potential should establish reversibly (i.e. the potential will return rapidly to its equilibrium value after a small transient perturbation), (3) the potential should be reproducible (i.e. the same electrode potential should be obtained when the reference electrode is constructed from the same electrode/solution combination; esp. also in series production). When miniaturising a reference half-cell, the main question is to which extent these basic requirements have to be fulfilled by the reference electrode. On one hand side, when developing low-cost disposable electrodes, a simple design with cheap component parts is required; a special long-term stability must not be fulfilled. On the other hand side, sophisticated technologies are required to develop sensor systems with increased lifetime without curtailment of the performance of a macroscopic electrode. The development of new technological approaches such as the thick film and thin film techniques supported arrangements with planar reference electrodes. Although thick and thin film technologies have been optimised to a great extent, special problems with miniaturised reference electrodes have to be considered. Because the adhesion of silver to substrates like glass or SiO2 is poor, an adhesive layer of chromium or titanium is used to provide adequate adhesion. Exposed edges of these adhesive layers could be responsible for the appearance of mixed potentials, or in extreme cases chromium and titanium can be oxidised resulting in a positive potential shift. The contamination of the silver layer by interdiffusion of atoms from the underlying layer can also fatally affect the electrode performance, especially during the conversion of silver to silver chloride [2]. In contrast to macro-reference electrodes, the lifetime of miniaturised reference electrodes is limited because of the non-negligible solubility of AgCl in solutions of high Cl− concentrations (formation of chloro complexes of Ag(I)). This problem is severe in case of electrodes with AgCl layers of only a few hundred nanometres thickness. The incorporation of a liquid reservoir in miniaturised devices is technically not easy to achieve. In addition, because of the miniaturisation, the electrolyte volume is substantially reduced and rapid contamination as well as exhaustion and drying out of the reference electrolyte, even at low leakage rates, become limiting factors, i.e. attempts to miniaturise macro-reference electrodes of the second kind resulted in a serious reduction of the lifetime. Moreover, a leakage of the inner filling electrolyte of the reference compartment can disturb the response of the working electrode tremendously in small sample volumes. To avoid a dry out of the electrolyte containing gel, one can think about the miniaturisation of the liquid junction. But, the miniaturisation of the contact area between the reference electrolyte and the sample is also limited. By using a dense diaphragm, instable diffusion potentials can occur. Small pores can easily be blocked, and the contact between the reference electrolyte and the sample is interrupted. Additionally, interfering ions (e.g. redox systems from the sample, complexing agents for silver ions, or ions forming sparingly soluble compounds with silver ions) reach the reference electrode surface much faster and in higher concentrations than in the macroscopic arrangement.