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A new phenanthridine-based multifunctional chemosensor (L), was synthesised via a green synthetic route and characterised using FT-IR, NMR and HRMS analysis. The sensing application of L towards metal ions in both solution and solid-state was studied using UV-vis and fluorescence spectroscopy, which exhibits dual-sensing behaviour for Th 4+ and Hg 2+ ions with good recyclability. In aqueous acetonitrile, L showed rapid response for the detection of environmental toxic metal ions and has a very low analytical detection limit of 125.5 pM and 1.94 nM for Th 4+ and Hg 2+ ions respectively, which is remarkably lower than the World Health Organization standard. The cation binding property of the L with Th 4+ and Hg 2+ ions was investigated by Job plot, 1 H NMR titration, HR-MS and DFT calculation. The in-situ formed ensemble L-Hg 2+ was further applied in the naked-eye detection of Cys (Cystine) and His (Histidine) over other common amino acids. The utility of L for real-time detection of Hg 2+ and Th 4+ ions was explored in various sources of environmental water samples, test paper strips, fingerprint imaging, fluorescent ink and smartphone-assisted sensing techniques, demonstrating the promising on-site visualization of the probe in controlling the toxicity levels in wastewater sources without resorting to expensive instruments., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Heavy metals emanate from natural and man-made sources, such as agricultural chemicals including fertilisers and pesticides, medical waste, and chemicals released from industries. Detection and monitoring toxic metal ions is one of the challenges confronting scientists in biological, environmental, and chemical systems. This study describes the design and synthesis of a new imidazole-based fluorescent and colourimetric chemosensor (DPICDT) for highly selective sensing of Hg 2+ and Cu 2+ ions in aqueous acetonitrile medium. The probe was synthesised by coupling benzil and substituted aldehyde using ethanolic ammonium acetate. The structure of DPICDT was confirmed via IR spectra, NMR, and HR-MS spectra. The DPICDT probe displayed a rapid naked-eye response towards Cu 2+ ions from colourless to red-purple and significant fluorescence quenching response towards Hg 2+ over other competitive metal ions in both solution and solid support. The binding modes of DPICDT with Cu 2+ and Hg 2+ ions were found to be at a 1 : 1 ratio as determined using Job plot, ESI HR-MS, and the sensing mechanism was evolved by 1 H NMR titrations, HR-MS spectra, and DFT calculations. The lower detection limit was 15.1 nM for Cu 2+ , eventually far less than the World Health Organization guideline for drinking water (Cu 2+ - 31.5 μM) and 1.17 μM for Hg 2+ (permissible concentration 2 ppb). Promisingly, the tissue paper-based DPICDT test strips and silica-supported DPICDT were developed and demonstrated for on-site application without resorting to expensive instruments., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)