Carbazole-Thioether Conjugates for Highly Selective and Sensitive Fluorescent Sensing of Hg 2+ Ions

Abstract

The sensitive detection and monitoring of heavy metal ions, particularly highly toxic mercury (Hg2+) even at trace levels, are vital for environmental and human health protection. In response to this challenge, we have developed and synthesized two novel thiol-based fluorescent chemodosimeter, termed thiol-1 and thiol-2, specifically designed for the selective detection of Hg2+ ions. The structural characterization of these chemodosimeter was comprehensively performed using multinuclear NMR and high-resolution mass spectrometry. Sensing studies conducted in a 1:1 DMF–PBS buffer system revealed that thiol-1 exhibits a significant fluorescence turn-on response in the presence of Hg2+ ions, even amidst competitive metal ions. In contrast, thiol-2 showed no response to Hg2+ and other tested metal ions. The detection limit of Thiol-1 toward Hg²⁺ ions was determined to be 31 nM, demonstrating its high sensitivity for trace-level detection. The sensing mechanism was further confirmed by 1H NMR and ESI-MS analyses, which clearly indicate Hg²⁺-induced deprotection of the thiol (thioacetal) unit to generate the corresponding aldehyde. In addition, density functional theory (DFT) studies provided detailed insights into the electronic structure and photophysical properties. Finally, cellular imaging experiments validated the practical applicability of Thiol-1 in biological systems, demonstrating its capability to selectively detect and visualize Hg²⁺ ions within cellular environments.