Development of tissue paper-based chemosensor and demonstration for the selective detection of Cu²⁺ and Hg²⁺ ions

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 Hg²⁺ and Cu²⁺ 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 Cu²⁺ ions from colourless to red-purple and significant fluorescence quenching response towards Hg²⁺ over other competitive metal ions in both solution and solid support. The binding modes of DPICDT with Cu²⁺ and Hg²⁺ 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 ¹H NMR titrations, HR-MS spectra, and DFT calculations. The lower detection limit was 15.1 nM for Cu²⁺, eventually far less than the World Health Organization guideline for drinking water (Cu²⁺ - 31.5 μM) and 1.17 μM for Hg²⁺ (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.