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

Abstract

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 Hg 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 Hg with a 35 times decrease in the emission intensity in the presence of 1 equiv of Hg ions. We have demonstrated the applicability of the probe for detecting Hg ions in living cells and in live zebrafish larvae using confocal fluorescence microscopy with visible excitation. High selectivity and sensitivity toward Hg detection make qHCM an attractive probe for detecting Hg 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. H NMR studies and DFT calculations indicate that Hg ions coordinate to oxygen-donor atoms from both the chromophore and macrocycle, leading to sensitive mercury detection.

Fluorescent Sensors for Hg2+ and Cu2+ Based on Condensation Products of 4H-1,2,4 Triazole-4-Amine and Carboxylated Benzoic Acids

“…Examples can be cited are Schiff base obtained from -α-naphthaldehyde with naphthylamine [18], N-phenylthiosemicarbazide and 2-naphthaldehyde [19] etc. Amide armed calix [4]-aza-crown based sensor containing S is reported but Hg 2+ binds to N atoms [20], sensor based on a pentaaza macrocycle conjugated to a hemicyanine dye where Hg 2+ binds to carboxylate is known [21]. Hence development of uorescence sensor for Hg 2+ without having S must be of interest.…”

Development of a Next-Generation Fluorescent Turn-On Sensor to Simultaneously Detect and Detoxify Mercury in Living Samples

“…[5][6][7] These include (a) analytical techniques such as inductively coupled plasma optical emission spectrometry (ICP-OES) with detection limit of 0.06 µg 8 , cold vapor atomic absorption spectrometry (CV AAS) which reduces Hg 2+ to elemental mercury Hg(0) and detects the atomic absorption signature of Hg with a detection limit of 0.7ng 9 and inductively coupled plasma mass spectrometry (ICP-MS) with a detection limit of 0.001 ppb 10,11 (b) fluorescent and colorimetric sensors, 5 (c) surfaceenhanced Raman spectroscopy (SERS) based sensors 12 (d) ratiometric sensors, 13 (e) photoelectrochemical sensors, 14 (f) label-free sensors, 15 (g) micro-electromechanical sensors based on surface acoustic wave and quartz crystal microbalance, 16 (h) naked eye sensors 17 (i) reusable DNA-functionalized hydrogels 18 and surfaces 19 and others. [20][21][22][23][24] We have earlier developed a rhodamine−rhodanine based 'turn-on' fluorescent sensor (RR1) for real-time monitoring of inorganic mercury (Hg 2+ ) uptake in cells and zebrafish larvae. 25 We were inspired to take the next step of developing simple and elegant strategies to simultaneously detect and detoxify Hg 2+ from biological samples with high sensitivity and efficacy.…”

“…These adverse effects include enhanced risk for severe brain damage, kidney problems, immune dysfunction, and motion disorders in humans. − The World Health Organization (WHO) and the United States Environmental Protection Agency (U.S. EPA) have prescribed stringent limits regarding the maximum contamination level for mercury in drinking water. There have been concerted efforts to develop sensitive methods for detecting mercury at low levels in environmental and biological samples. − These include (a) analytical techniques such as inductively coupled plasma optical emission spectrometry (ICP-OES) with a detection limit of 0.06 μg, cold vapor atomic absorption spectrometry (CV AAS) which reduces Hg 2+ to elemental mercury Hg(0) and detects the atomic absorption signature of Hg with a detection limit of 0.7 ng, and inductively coupled plasma mass spectrometry (ICP-MS) with a detection limit of 0.001 ppb; , (b) fluorescent and colorimetric sensors; (c) surface-enhanced Raman spectroscopy (SERS)-based sensors; (d) ratiometric sensors; (e) photoelectrochemical sensors; (f) label-free sensors; (g) microelectromechanical sensors based on surface acoustic wave and quartz crystal microbalance; (h) naked eye sensors; (i) reusable DNA-functionalized hydrogels and surfaces, and others. − We have earlier developed a rhodamine–rhodanine-based “turn-on” fluorescent sensor (RR1) for real-time monitoring of inorganic mercury (Hg 2+ ) uptake in cells and zebrafish larvae . We were inspired to take the next step of developing simple and elegant strategies to simultaneously detect and detoxify Hg 2+ from biological samples with high sensitivity and efficacy.…”

A Lysosome-Targetable Fluorescence Sensor for Ultrasensitive Detection of Hg2+in Living Cells and Real Samples

“…Various traditional approaches, e.g., inductively coupled plasma mass spectrometry, atomic absorption-emission spectrometry, electrochemical sensors, etc., are well-known to determine mercury. − However, these techniques are not suitable for in vitro analysis and on-site determination of Hg 2+ ions because they involve highly concentrated sample size and also sophisticated, costly instrumentation. In contrast, detection by fluorescence techniques has received a lot of interest due to its simplicity, convenience, high sensitivity, and real-time nondestructive detection approach. − A variety of sensor systems have been developed for detection and monitoring of Hg 2+ based on chromophores and fluorophores, − DNAzymes, − nanosystems, − oligonucleotides, etc. Also some functionalized mesoporous materials have been developed for selective detection of Hg 2+ in an aqueous medium. , Owing to having a high surface area and pore volume, they can act as a good platform for sensing.…”