Spirolactone and spirothiolactone rhodamine-pyrene probes for detection of Hg²⁺ with different sensing properties and its application in living cells

Two new rhodamine B-based fluorescent probes PyRbS and PyRbO containing pyrene moiety were designed and synthesized. Both of the probes showed colorimetric and fluorometric sensing abilities for Hg²⁺ with high selectivity over other metal ions. The binding analysis using Job's plot suggested 1:1 stoichiometry for the complexes formed for Hg²⁺. Compared with PyRbO, the PyRbS showed higher selectivity and sensitivity due to the thiophilic property of Hg²⁺ ion. The PyRbS exhibited the linear fluorescence quenching to Hg²⁺ in the range of 0.3 to 4.8 μM (λ_ex = 365 nm) and 0.3 to 5.4 μM (λ_ex = 515 nm), and the detection limit was 0.72 μM. Moreover, ratiometric changes of PyRbS with Hg²⁺ in absorption spectrum were observed, which could not be obtained in the combination of PyRbO with Hg²⁺. In addition, the methyl thiazolyl tetrazolium (MTT) assay demonstrated that RbPyS had low cytotoxicity and was successfully used to monitor intracellular Hg²⁺ levels in living cells.