A Rhodamine-based Fluorescent Chemodosimeter for Au³⁺ in Aqueous Solution and Living Cells

A highly selective rhodamine hydrazide-based fluorescent chemosensor for Au³⁺ detection was developed. The aqueous solution of rhodamine N-hydroxysemicarbazide (RHS), in the presence of Au³⁺, exhibited a significant 55-fold turn-on fluorescence response at 591 nm and a colorimetric change from colorless to pink. Other interested ions including Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Ba²⁺, Pb²⁺, Mn²⁺, Co²⁺, Ni²⁺, Ag⁺, Cd²⁺, Cu²⁺, Hg²⁺, Zn²⁺, Sn²⁺, Fe²⁺, Fe³⁺, Cr³⁺, Ce³⁺ did not induce any distinct color/spectral changes. The irreversible detection mechanism occurred via Au³⁺-promoted 5-exo-trig ring closure to yield 1,3,4-oxadiazole-2-one product. The RHS probe is non-responsive to other biologically relevant metal ions and the limit of detection for Au³⁺ was calculated to be 0.5 µM with a linear range of 0 to 90 µM. Fluorescence bioimaging of Au³⁺ in HepG2 cells was also successfully demonstrated.