Highly selective fluorescent probe for switch-on Al³⁺ detection and switch-off F⁻ 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 K_a = 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).