A Mitochondria-Targeted Ratiometric and Colorimetric Fluorescent Probe for Hg²⁺ Based on Deselenation-Hydrolysis-Elimination Strategy

In this work, we developed the first mitochondria-targeted ratiometric and colorimetric fluorescent probe 1 for Hg2+ detection, utilizing coumarin as the fluorophore, diphenylphosphinoselenoate as the recognition group, and p-hydroxybenzyl moiety as the self-immolative spacer. Upon reaction with Hg2+, 1 exhibited a significant blue shift in absorption spectra from 492 to 426 nm (Δ66 nm), resulting in color change from orange to pale yellow under natural light. Simultaneously, the fluorescence spectra of 1 were blue-shifted from 570 to 498 nm (Δ72 nm), accompanied by a substantial increase in the emission ratio of F498nm/F570nm (R/R0 up to 892-fold) and fluorescence color change from salmon pink to blue under 365 nm ultraviolet (UV) light. These spectral changes can be attributed to the Hg2+-triggered deselenation-hydrolysis of the diphenylphosphinoselenoate moiety in 1, leading to cleavage of the P-O bond and formation of diphenylphosphinic acid and the intermediate A containing a phenolic hydroxyl group. Subsequently, fluorophore 2 was generated by the self-immolative reaction of the p-hydroxybenzyl moiety of intermediate A through a 1,6-elimination pathway. 1 demonstrated a large Stokes shift of 78 nm, high sensitivity with a detection limit of 16.7 nM, and excellent selectivity for Hg2+ even in the presence of other metal ions. Furthermore, 1 exhibited mitochondria-targeting capabilities and successfully imaged the fluctuation of intracellular Hg2+ through dual emission channels. Importantly, the novel design strategy of modifying diphenylphosphinoselenoate to the nitrogen atom of pyridine is unprecedented to the best of our knowledge.