A novel "turn-on" fluorescent probe based on thiocarbamoyl-DHP for Hg²⁺ detection in water samples and living cells

In this study, two fluorescent sensing probes, dihydropyridine (DHP) derivatives (DHP-CT1 and DHP-CT2) bearing phenoxy thiocarbonyl group, have been developed for Hg²⁺ detection. The tandem trimerization-cyclization of methylpropiolate with ammonium acetate gave 1.4-DHP and 1,2-DHP derivatives, which were reacted with O-phenylcarbonochloridothioate to produce DHP-CT1 and DHP-CT2, respectively. DHP-CT1 exhibits superior sensitivity and selectivity of fluorescence enhancement towards Hg²⁺ in aqueous media. The fluorescence intensity shows a good linear relationship with the concentration of Hg²⁺ in the range of 0-10 µM providing the extremely low LOD of 346 nM (69.4 ppb). The fluorescence enhancement is caused by the Hg²⁺ promoted hydrolysis of the thioamide bond releasing the fluorescent 1,4-DHP that was confirmed by NMR and HRMS. The quantitative analysis of Hg²⁺ in water samples using DHP-CT1 probe was demonstrated in aqueous solution and paper-based sensing strips. Furthermore, DHP-CT1 was also applied for monitoring intracellular Hg²⁺ in living RAW264.7 macrophages through fluorescence cell imaging.