Ion-selective electrodes (ISEs) are a class of electroanalytical sensors designed to measure the concentration of specific ions in a solution. They work based on the principle of selectively responding to a particular ion while being relatively insensitive to other ions present in the sample. Ion-selective electrodes have found a wide range of applications in various fields, including environmental monitoring, clinical diagnostics, industrial processes, and research. 

The key components of an ion-selective electrode include a selective ion-sensitive membrane, a reference electrode, and an electrolyte solution. The ion-sensitive membrane is the crucial part of the electrode, as it determines the selectivity of the sensor for a particular ion. This membrane is typically made of materials like glass, crystalline solids, or polymeric materials, which contain specific ion-exchange sites or binding agents. These sites or agents interact preferentially with the target ion.

The working principle of ion-selective electrodes involves generating a potential difference across the ion-sensitive membrane in response to the activity or concentration of the target ion. This potential difference, also known as the electromotive force (EMF), is proportional to the logarithm of the ion's concentration in the solution. The Nernst