Potassium cations play many important roles in living organisms, especially in electro-physiology, since they are involved in neurotransmission and muscle contractions. We report the synthesis of a ratiometric fluorescent microsensor for potassium (K+) detection, based on the fluorescent probe ION potassium green 4. Potassium-sensitive fluorescent microparticles were obtained by using silica as the core material. We obtained silica-based microsensors with sizes in the micrometer range, spherical shapes, good monodispersity, optimal selectivity and a sensitivity range of 0 to 40 mM. The microsensors also proved to be non-toxic in cell cultures and suitable for fluorescence imaging, offering new possibilities for non-invasive optical detection, quantification and in situ monitoring of K+ variations in cell culture systems.