An efficient rhodamine thiospirolactam-based fluorescent probe for detection of Hg2+ in aqueous samples

This paper described the optimized design, synthesis and application of a novel rhodamine thiospirolactam derivative as an ‘off-on’ fluorescent probe for the detection of Hg2+ in aqueous samples. The ‘off-on’ fluorescence and color signal change of the probe is based on an Hg2+-triggered domino reaction which brings on the opened-ring form of the rhodamine spirolactam to regain the conjugated system of the rhodamine skeleton. In the well designed probe, the thiospirolactam serves as both Hg2+ binding unit and electron-defect carbon centre, a phenolic hydroxyl with very strong nucleophilicity after deprotonation is chosen as the attacking unit, and a benzene ring is introduced on the linker to afford steric effects, which benefits an efficient nucleophilic reaction, with a high sensitivity towards Hg2+. It exhibits a stable response for Hg2+ from 1.0 × 10−8 to 1.0 × 10−6 M, with a detection limit of 3.0 × 10−9 M. The response of the probe to Hg2+ is highly selective and pH-insensitive, with a fast response time. All these unique features make it particularly favorable for cellular Hg2+ imaging applications. It has been preliminarily used for highly sensitive monitoring of Hg2+ levels in living cells with satisfying resolution.