A turn-on fluorescent probe with high selectivity for Hg²⁺ and its applications in living cells

A novel fluorescent probe L with a rhodamine B lactam structure modified with 3-methyl-2-thiophenecarboxaldehyde has been prepared based on the thiophilicity of Hg²⁺. The probe L exhibits a unique response with an "off-on-type" mode towards Hg²⁺ among other biologically relevant metal cations. The limit of detection (LOD) for probe L is 1.5 ppb. In addition, in the presence of Hg²⁺, the probe L shows a colorimetric response from colorless to pink. The recognition behavior of probe L towards Hg²⁺ has been investigated by ¹H NMR titration experiments, Job's plot, and MS and IR analyses. As a result, the ligation between the probe and Hg²⁺ leads to the scission of the spirolactam bond of free L and the restoration of its conjugated structure, which can give rise to the fluorescence enhancement of the probe L. Besides, it also can be used as a sensitivity probe in living cells for Hg²⁺ sensing, which can meet various needs in genetic and environmental samples.