Selective sensing Ca²⁺ with a spiropyran-based fluorometric probe

Spiropyran (SP) and its derivatives operate between their ring opening and closing forms as a versatile molecular platform for the fluorescence detection of cations and anions, using a colour change for signalling. A functionalized SP fluorescence probe, L, was prepared and characterized. Probe L can detect Ca²⁺ with a fluorescence 'turn-on' response in ethanol solution. It selectively binds Ca²⁺ to form a 1:1 ligand/metal complex, which produced a new emission band centred at 604 nm. The sensing result was clearly observed by the solution colour change from colourless to pink under visible light, and from blue to red under ultraviolet light. The detection limit was calculated to be 4.53 × 10^-8  M for Ca²⁺ . The probe provides another possibility that SP-based derivatives could be used for the development and detection of metal ions in environmental and physiological systems.