Recent advances in all-solid-state reference electrodes: A review

Electrochemical sensors have been of interest to researchers for a long time, and despite the passage of time, they have not lost their popularity. They found particularly extensive application in the biomedical analysis of various physiological fluids. Potentiometric sensors are particularly popular because of their selectivity, simplicity and cost. Much research has been done on the miniaturization of indicator electrodes, but somehow the other essential component of a sensor - the reference electrode - has been forgotten. Its miniaturization is more problematic than the indicator electrode. In this review article, we have considered the latest advances in the field of all-solid-state reference electrodes, with a special emphasis on potentiometric sensors for biomedical analysis. We have classified all-solid-state reference electrodes into 4 different subgroups: pseudo-reference electrodes, electrodes with heterogenous membranes, electrodes with homogenous membranes and electrodes with ionic liquids. According to the literature we described the mechanism of operation of each group of sensors. In addition, we presented each mechanism in a clear figure. Cited articles were compared in terms of the reference electrode used and the proposed biomedical application. The most interesting ideas were shown in the figures. We have also identified the biggest challenges and addressed fields for further development and improvement of all-solid-state reference electrodes. We believe that a summary of recent work in the field of all-solid-state potentiometric sensors for biomedical analysis will help to organize the current state of the art and assist in further development in this field.