Ion-selective optical sensors (ISO sensors, or simply optodes) are members of an extensive family of nano/microsensors commonly referred to as PEBBLEs (Photonic Explorers for Biomedical use with Biologically Localized Embedding). PEBBLEs is a general term that encompasses a variety of fabrication techniques for miniaturization of well-known sensors. Optodes leverage the same response mechanisms as ion-selective electrodes, contain identical components and became popular as intracellular sensors in early 1990s. They have found broad use in medicine and biology.