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 <b>1</b> for Hg²⁺ detection, utilizing coumarin as the fluorophore, diphenylphosphinoselenoate as the recognition group, and <i>p</i>-hydroxybenzyl moiety as the self-immolative spacer. Upon reaction with Hg²⁺, <b>1</b> 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 <b>1</b> were blue-shifted from 570 to 498 nm (Δ72 nm), accompanied by a substantial increase in the emission ratio of F_498nm/F_570nm (<i>R</i>/<i>R</i>₀ 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 <b>1</b>, leading to cleavage of the P-O bond and formation of diphenylphosphinic acid and the intermediate <b>A</b> containing a phenolic hydroxyl group. Subsequently, fluorophore <b>2</b> was generated by the self-immolative reaction of the <i>p</i>-hydroxybenzyl moiety of intermediate <b>A</b> through a 1,6-elimination pathway. <b>1</b> 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, <b>1</b> 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.