Hemicyanine-based fluorescent probes with ratiometric and turn-on responses for selective and sensitive detection of Hg2+: A combined experimental and DFT study

Mercury and its compounds are among the most well-known toxic heavy metal pollutants. Even trace amounts of mercury are known cause severe health effects by irreversibly targeting the central nervous system in humans. In particular, Hg 2+ has been extensively researched to improve its detection in a variety of matrixes. Due to their high sensitivity, small-molecule fluorescent probes that can rapidly and accurately detect Hg 2+ ions are of particular interest. In the present study, two new fluorescent probes (PN-PTC and PN-DMTC) were designed and synthesized for selective Hg 2+ detection. PN-PTC features a pyridinium-naphthalene-phenyl thiocarbonate structure, while PN-DMTC incorporates a pyridinium-naphthalene-dimethyl thiocarbamate motif. Upon Hg 2+ binding, PN-PTC exhibited a ratiometric fluorescence response accompanied by a visible color transition from blue to yellow, whereas PN-DMTC showed a significant fluorescence turn-on effect. The Hg 2+ recognition mechanism, involving a Hg 2+ -promoted desulfurization-hydrolysis reaction, was confirmed through 1 H NMR, HRMS and FT-IR analysis of the hydrolysis products. Both probes showed excellent fast response times, large Stokes shifts, high sensitivity (nM limits of detection), and excellent selectivity for Hg 2+ . Additionally, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were conducted to elucidate the electronic basis of the Hg 2+ sensing. Frontier molecular orbital (FMO) analysis, molecular electrostatic potential (MEP) mapping, and simulated absorption/emission spectra corroborated the experimental results. The combined data confirmed that both intramolecular charge transfer (ICT) and photoinduced electron transfer (PET) are involved in the sensing mechanism. Finally, both probes demonstrated reliable performances in detecting Hg 2+ in real water samples, highlighting their practical utility for environmental monitoring. • Two hemicyanine probes enable Hg 2+ detection via desulfurization-hydrolysis. • Different recognition groups lead to distinct Hg 2+ sensing performances. • High selectivity, excellent sensitivity and rapid responses, with low LODs. • Density functional theory calculations verified the sensing behavior.