Rhodamine-assisted fluorescent strategy for the sensitive and selective in-field mapping of environmental pollutant Hg(II) with potential bioimaging

Mercury is a global pollutant that can damage human brain, central nervous system and endocrine moreover, other biological systems. Its real-time monitoring is a key challenge in the control of mercury emissions or pollution. In this study, a simple and efficient fluorescent sensor (PST) comprising of rhodamine B and 2-amino-5-bromopyridine was developed for the detection of environmental toxicant Hg²⁺. The newly synthesized sensor displayed a highly selective and sensitive “turn-on” response by the addition of Hg²⁺ among miscellaneous metal cations (Ag⁺, Al³⁺, Ba²⁺, Ca²⁺, Cd²⁺, Fe³⁺, Fe²⁺, K⁺, Li⁺, Mg²⁺, Mn²⁺, Cu²⁺, Pb²⁺, Ni²⁺, Na⁺ and Zn²⁺) in CH₃CN: H₂O (8:2 v/v) system. The 1:1 binding stoichiometry, limit of detection (LOD) 0.63 μM along with binding constant of 1.04 × 10⁵ M⁻¹ was calculated for the complex PST-Hg²⁺. The reversible nature of the complex was confirmed in the presence of ethylenediaminetetraacetate (EDTA). The significant enhancement of fluorometric and absorption response was attributed to the spirolactam ring opening via photo-induced electron transfer (PET) of xanthene moiety. This spirolactam ring opening was further verified by density functional theory, ¹H NMR and Fourier transform infrared spectroscopy (FT-IR). The real-time applicability of the sensor in environmental and biological fields was proved as detection kit on TLC strips and with the help of Laser confocal microscopy for live cell imaging by using HeLa cells. Conclusively, PST can be regarded as accurate ‘two in one’ kit for colorimetric recognition of Hg²⁺ in the environment and fluorescent cell imaging in live HeLa cells.