Highly sensitive and selective strategy for imaging Hg2+ using near-infrared squaraine dye in live cells and zebrafish

Abstract

This article describes the rational design and synthesis of a monothiosquaraine (MTSQ) fluorescent Hg2+ probe that exhibits near-infrared fluorescence, ultrahigh brightness, fast response, high sensitivity and excellent selectivity. Utilizing the strategy of the incorporation of the heavy atom effect and self-assembly caused quenching, the background fluorescence of MTSQ was greatly lowered and the sensitivity in PBS buffer solution was highly enhanced with a very low limit of detection (LOD = 1.2 nM). Systematic studies revealed excellent selectivity and specificity for Hg2+ over other thiophilic metal ions such as Ag+, Pb2+, and Cd2+. The mechanistic investigation indicates that self-assembly and desulfurization of MTSQ are responsible for Hg2+-triggered significantly higher fluorescence enhancement. With high sensitivity, optimal pH range of 3–7, weak background, good water-solubility and low cytotoxicity, MTSQ therefore can be applied for imaging Hg2+ in live cells and zebrafish. The very simple but effective strategy reported here can provide a more convenient approach for designing NIR probes.