Figure 9.

Benzoxadizole-functionalised calix[4]arene probe 16 responds selectively to Ag⁺, which triggers quenching of the free ligand emission at 527 nm and emergence of a new emission band at 576 nm; the resulting metal complex is then an effective probe for HCHO, which reduces the Ag⁺ and regenerates the free probe 16 [89]. The anthraquinone-functionalised calix[4]arene 17 responds selectively to Ca²⁺ with enhanced fluorescence emission at 510 nm, and the resulting 17·Ca²⁺ complex then functions as a turn-off probe for fluoride [90]. Triazolyl-anthracene derivative 18 is a turn-off probe for Co²⁺ [91], while the structurally related benzimidazole probe 19 binds Cu²⁺ and reports this with a ratiometric fluorescence response (via changes in emissions at 311 and 380 nm) [92].