How to make calcium-sensitive microelectrodes

Ca(2+)-sensitive microelectrodes (CaSMs) directly measure the pCa at their tip, which can be in a small extracellular space or inside a large and robust cell. They do not add to buffering and do not require expensive equipment. But they are time-consuming to make, require a reference electrode in the same location, and tend to create a leak around the point of insertion. In addition, CaSMs only work well with a tip diameter of >1 μm. In this protocol, we describe how to make and use the electrodes and briefly consider possible problems.