Miniaturised reference electrodes for field-effect sensors compatible to silicon chip technology

Ag/AgCl reference electrodes without KCl-containing membrane (“quasi-reference” electrodes) have been fabricated by means of thick-film technique (screen-printing technique) and thin-film technique (electron-beam evaporation/chlorination or electron-beam evaporation/pulsed laser deposition). The different types of reference electrodes were characterised by means of scanning electron microscopy as a function of their preparation parameters. The potentials of the “quasi-reference” electrodes were measured versus a commercial Ag/AgCl reference electrode in 3 M KCl solution. The potential stability was up to 10 h (thin-film technique) and more than 1200 h (thick-film technique). In a next step, an additional KCl-containing agar membrane was deposited and different coatings have been used to protect the reference electrode from a fast leaching out of KCl. The leaching out could be improved from originally 3 h (by using no protective coating on top of the agar membrane) up to 50 days by using polyvinylchloride/nafion (PVC/N) or polyvinylchloride (PVC) as protective coating. Additionally, poly-(2-hydroxyethyl methacrylate) (pHEMA) has been studied as an alternative material for agar.