Thirty years of ion-selective microelectrodes: disappointments and successes.

The need to know the intracellular activity of an ion and how it changes under controlled conditions is as important today as it was 30 years ago. In 1956, one could fabricate only a H+-selective microelectrode and with a tip size not much smaller than 100 micron. Today, one can fabricate microelectrodes selective to H+, Na+, K+, Cl-, HCO3-, Ca2+, or Mg2+ (plus others) and with active tips less than 1 micron. The reduction of active tip size can be attributed mainly to the introduction of liquid ion exchanger (LIX) and neutral carrier ligands. Unfortunately, the LIX microelectrodes, as currently fabricated, do not yet function optimally as reliable and stable electrochemical measuring devices. A durable bond between the active membrane and its insulated container continues to remain the major design problem even after 30 years of development.