Optical sensing materials for the selective measurement of potassium ions (K+) in water are presented. The indicator dyes are based on an aza-crown ether as a receptor and borondipyrromethenes (BODIPY) dyes as fluorophores. Fluorescence enhancement is caused by the reduction of photoinduced electron transfer (PET) upon complexation with K+ ions. The family of new indicators possesses tuneable optical properties (green to red excitation, red to NIR emission) and PET efficiencies. They exhibit high brightness with quantum yields between 0.20 and 0.47 in the "on" state and a molar absorption coefficient between 30 000 and 290 000 M-1 cm(-1). The new indicator dyes are immobilized in biocompatible hydrogel matrices to obtain stable nonleaching and fast responding (t(90) approximate to 10 s) sensing materials for continuous measurements of potassium. They are realized in various formats such as planar optodes, fiber-optic sensors, and water-dispersible polymer-based nanoparticles. Apart from fluorescence intensity measurements, self-referenced read-out of fluorescence decay time is demonstrated. All sensor materials display a high K+/Na+ selectivity and are not influenced by pH within the physiologically relevant range. Practical applicability of the materials is emphasized by application of a fiber-optic sensor to quantification of K+ in serum, which shows excellent correlation with the reference measurements.