How to make calcium-sensitive minielectrodes

In this protocol we describe how to make and use minielectrodes for measuring [Ca(2+)] in small volumes of solution. The minielectrodes are ~2 mm in diameter and have sufficiently low resistances to be used with a standard pH meter. They are made by dipping polyethylene or borosilicate glass tubes (~5 cm long) in a membrane solution. Although the chemicals used to make these Ca(2+)-sensitive minielectrodes are expensive, they can be used to make hundreds of electrodes, each with a useful life of several months.