A Sensitive Water-Soluble Reversible Optical Probe for Hg²⁺ Detection

We report the serendipitous discovery of an optical mercury sensor while trying to develop a water-soluble manganese probe. The sensor is based on a pentaaza macrocycle conjugated to a hemicyanine dye. The pentaaza macrocycle earlier designed in our group was used to develop photoinduced electron transfer (PET)-based "turn-on" fluorescent sensors for manganese. (1) In an attempt to increase the water-solubility of the manganese sensors we changed the dye from BODIPY to hemicyanine. The resultant molecule qHCM afforded a distinct reversible change in the absorption features and a concomitant visible color change upon binding to Hg²⁺ ions, leading to a highly water-soluble mercury sensor with a 10 ppb detection limit. The molecule acts as a reversible "ON-OFF" fluorescent sensor for Hg²⁺ with a 35 times decrease in the emission intensity in the presence of 1 equiv of Hg²⁺ ions. We have demonstrated the applicability of the probe for detecting Hg²⁺ ions in living cells and in live zebrafish larvae using confocal fluorescence microscopy with visible excitation. High selectivity and sensitivity toward Hg²⁺ detection make qHCM an attractive probe for detecting Hg²⁺ in contaminated water sources, which is a major environmental toxicity concern. We have scrutinized the altered metal-ion selectivity of the probe using density functional theory (DFT) and time-dependent DFT calculations, which show that a PET-based metal-sensing scheme is not operational in qHCM. ¹H NMR studies and DFT calculations indicate that Hg²⁺ ions coordinate to oxygen-donor atoms from both the chromophore and macrocycle, leading to sensitive mercury detection.