Highly selective fluorescent probe for switch-on Al^sup 3+^ detection and switch-off F^sup -^ detection

A new 2-(2′-hydroxyphenyl)thiazole-4-carboxaldehyde based fluorescent probe (1) was designed and synthesized for sequential detection of Al3+ and F− ions in methanol. Probe 1 selectively detected Al3+ through a “switch on” response driven by the selective complexation of Al3+ ions with hemiacetal 2 formed from 1 in methanol. This process was mediated by combined excited-state intramolecular proton transfer (ESIPT) and chelation-induced enhanced fluorescence (CHEF) effects. Probe 2 bound Al3+ ions in a 1:1 stoichiometry with a good association constant of Ka = 9.1 × 104 M−1. The in situ generated 2·Al3+ (1:1) complex detected F− ions sequentially through a “switch off” response, with a 1:3 stoichiometry observed between 2·Al3+ (1:1) and F−. The lowest limits of detection (LODs) for Al3+ and F− ions in methanol were determined to be 9.24 × 10−7 and 1.4 × 10−5 M, respectively. The probe was also used to detect Al3+ ions with a slightly lower LOD and binding constant in aqueous methanol (1:19).