Reference electrode microreference

Such electrodes should be sufficient as a reference electrode for short-term usage or as a disposable electrode. However, the requirement of a pre-hydration time may limit its applications for fast measurements, such as POCT (the point-of-care testing), due to its slow response time. In fact, the lack of long-term stable microreference electrodes will continue to hamper the development of integrated pH sensing systems. 

Volume has been the most significant limitation on the size and construction of microreference electrodes, a limitation that complements the small size of the microfabricated ion sensors (Section 6.23.2). There have been many attempts to prepare a liquid junction free microreference electrode that would be comparable in size with the integrated ion sensors, such as ion-sensitive field-effect transistors (Section 6.23.2). These attempts have followed broadly three tines of reasoning scaling down of a macroscopic reference electrode (Comte and Janata, 1978 Smith and Scott, 1986), elimination of the reference solution compartment while preserving the internal element structure (e.g., Ag/AgCl), and utilization of inert materials such as polyfluorinated hydrocarbons and the tike, particularly in the so-called reference FET configuration. 

In case of potentiometric measurements with microelectrode arrays, complete micro-reference electrodes have been suggested. The term complete microreference electrode is used here for such reference electrodes that consist of the same parts as conventional macroelectrodes. Their operating principle is essentially identical to that of the latter. Major difference is the size. Needle-type reference electrodes are frequently used for miniaturisation. The Ag/AgCl system (or a similar one) is hosted in a capillary with dimensions in the millimetre range. The capillary is thinned at one end, and the orifice houses the liquid junction. Commercially available complete reference microelectrodes have capillaries with an outer diameter of 1-2 mm at the end. The liquid junction is realised by a ceramic frit or a fibre. A real micro-reference electrode has been described by Kitade and coworkers [26]. They used a commercially available Femtotip capillary tube as the electrode body. Such tips have a capillary with an outer diameter of 1 pm and are usually used for microinjection into adherent and suspension cells. The tip is filled with a KCl agar gel to realise the liquid junction. The bare part of the silver wire has to be covered with a polystyrene membrane to avoid any additional potential between the inner electrolyte solution and the bare silver wire. 

Considerable efforts have been made to develop reliable micro-reference systems. As a result, different strategies have been proposed to constmct micro-reference compartments mostly for short-time measurements in research laboratories or for disposable devices. However, the main problems with long-term stability of microreference electrodes could not be solved satisfactorily up to now. The development of all-solid state reference electrodes is still in an early stage, and basic research is required to add the knowledge in the field of the electrochemical processes at solid-state electrodes. The main question is if it is really possible from a theoretical point of view to create a reliable reference system in the micrometre scale. 

Microreference elements (reference electrodes or reference field-effect transistors— REFETs) micromachined in silicon were precursors of screen-printed reference electrodes (e.g., [4-7]). The major motivation to look for optimal layered reference system was usually to buUd a potentiometric chip containing disposable sensing and reference electrode, preferably made in the same or similar layer application technology, e.g., thin-film technology, which can be regarded as direct ancestor of nowadays screen-printed reference elements. 

---