Ion Selective electrodes come in four states/types. They are solid state sensors, gas sensors, liquid membrane sensors, and glass sensors.
Solid State Sensors
Solid state sensors have a pellet-type membrane that is made of compressed material that is sensitive to a specific ion. When a solid state sensor is submerged in solution, it will start to slowly dissolved (this is influenced by the solubility in the sample). Generally, the more concentrated a solution the ISE is submerged in, the more the membrane will dissolve. This dissolution is VERY SLOW. You will not submerge the electrode and then pull it out and see a marked difference.
Gas sensors measure a dissolved gas in a solution (not the gas in the air). The membrane consists of a layer of hydrophobic material to isolate the internal electrode from the sample. Dissolved gas pass through the membrane and into the electrolyte surrounding the internal electrode. The dissolved gas changes the pH of the internal electrolyte which is then read by the internal sensor. The change in pH is proportional to the concentration of the dissolved gas in your sample.
Liquid Membrane Sensors
Liquid membrane sensors are unique in that the membrane works via organic ion exchangers or neutral carries embedded in a plastic matrix. This membrane works kind of like an antibiotic with how it moves ions in and out of the sample/electrode. Liquid membranes are more fragile and less shelf-stable than solid state sensors. This is due to the organic component in the membrane. Liquid state membranes should be stored in the refrigerator when not in use.
Glass sensors have a glass bulb at their tip that is made up of a hydrated glass. An ion exchange between this specially formulated glass and your sample create a change in mV, and the reference part of the electrode coupled with the sensing portion are what allows the electrode to read your sample.