Squaramide-based ultrasensitive and reversible colourimetric dual sensor for the detection of copper and metal-ion-assisted acetate anion

A tripodal squaramide–quinoline– tris(2-aminoethyl)amine (TREN) chemosensor, TSQAQ, was synthesised in good yield using diethylsquarate, 8-aminoquinoline and tris(2aminoethyl)amine as starting materials. The chemosensor was evaluated as a dual-mode colorimetric platform for the selective naked-eye detection of Cu2+ in DMSO and Cd2+ (precursor was cadmium acetate, CA) in acetonitrile. TSQAQ exhibited sensitivity towards Cu2+ via the ligand-to-metal charge transfer (LMCT) mechanism, producing an absorption peak at 450 nm and a distinctive colour change from colourless to bright yellow, irrespective of the Cu2+ counteranion screened. In contrast, the addition of CA yields an absorption peak at 420 nm and a colour transformation from colourless to pale yellow, with confirms the cooperative binding of Cd2+ by the squaramide moiety through hydrogen bonding. The limits of detection (LODs) were determined to be 97 nM (Cu2+) and 3 nM (Cd2+). Binding constants of 2.51 × 104 M-1 and 1.97 × 104 M-1 were determined using the Benesi-Hildebrand equation. The binding stoichiometry of TSQAQ with Cd2+ was established as 1:1 using the Job plot method. FTIR and 1H NMR titration studies indicated the involvement of TREN nitrogen, quinoline imine nitrogen, and squaramide carbonyl oxygen atoms in metal-ion coordination. Cu2+ sensing was reversible upon the addition of EDTA, with 94% signal recovery maintained over five cycles, enabling the implementation of an INHIBIT molecular logic gate. For practical applications, TSQAQ-coated paper strips were employed for the naked-eye detection of Cu2+ ions in real environmental water samples, including river water, seawater, and tap water.