A Phosphorescent Trinuclear Gold(I) Pyrazolate Chemosensor for Silver Ion Detection and Remediation in Aqueous Media

We report a phosphorescent chemosensor based on a trinuclear Au(I) pyrazolate complex or [Au(3-CH₃,5-COOH)Pz]₃ (aka Au₃Pz₃) stabilized in aqueous chitosan (CS) polymer media. Au₃Pz₃ is synthesized in situ within aqueous CS media at pH ∼ 6.5 and room temperature (RT). Au₃Pz₃ exhibits strong red emission (λ_max ∼ 690 nm) in such solutions. On addition of silver salt to Au₃Pz₃/CS aqueous media, a bright-green emissive adduct (Au₃Pz₃/Ag⁺) with a peak maximum within 475-515 nm is developed. The silver adduct exhibits a 4-fold increase in quantum yield (0.19 ± 0.02) compared to Au₃Pz₃ alone (0.05 ± 0.01), along with a corresponding increase in phosphorescence lifetime. With almost zero interference from 15 other metal ions tested, Au₃Pz₃ exhibits extreme selectivity for Ag⁺ with nM/ppb detection limits (6.4-72 ppb, depending on %CS and on the sensitivity basis being a signal-to-noise ratio (S/N) = 3 or a baseline-corrected signal change = 10%). Au₃Pz₃ exhibits sensitivity to higher concentrations (>1 mM) of other metal ions (Tl⁺/Pb²⁺/Gd³⁺). The sensing methodology is simple, fast, convenient, and can even be detected by the naked eye. On addition of ethylenediaminetetraacetic acid (EDTA), the red Au₃Pz₃ emission can be restored. Au₃Pz₃ and its silver adduct retain their characteristic photophysical properties in thin film forms. Remarkable photostability with <7% photobleaching after 4 h of UV irradiation is attained for Au₃Pz₃ solutions or thin films.