Aminothiophenol and 7-diethylamino-4-hydroxycoumarin derived probe for reversible turn off–on–off detection of Cu2+ ions and cysteine

Here, we present a simple disulfide linked probe HTP for rapid detection of Cu²⁺ ions, which was prepared by a condensation reaction between 7-diethylamino-4-hydroxycoumarin aldehyde and 2-aminothiophenol. The disulfide linked probe HTP was characterized using ¹H NMR, ¹³C NMR, and HRMS spectroscopic analysis and confirmed by single crystal X-ray diffraction analysis. The photophysical behavior of HTP in various solvents (non-polar to polar) was studied and HTP displayed aggregation induced emission (AIE) characteristics in CH₃CN-water mixtures (0–99 %). Upon binding with Cu²⁺ ions, emission enhancement occurs along with color changes from weak green to intense yellow emission in CH₃CN/Tris-HCl buffer (20 μM, 9:1, 10 mM Tris HCl Buffer, pH = 7.4). Detection limit for Cu²⁺ ions was found to be 0.97 nM which is lower than the recommended tolerance limit by the WHO and the association constant 0.42 × 10⁸ M⁻¹ was obtained using B–H plot. Furthermore, the stoichiometric ratio 1:1 was confirmed by Job’s plot, ¹H NMR, mass spectral analysis and DFT calculations were supported the formation of HTP-Cu²⁺ complex. The reversibility of HTP with Cu²⁺ ions was achieved by cysteine with detection limit and association constant value of 1.64 µM and 0.15 × 10⁷ M⁻¹ respectively. The reversible sensing nature of HTP with Cu²⁺/cysteine was further applied for constructing a molecular logic gate (INHIBIT) and practical applications such as paper strips, cotton swabs and real water analysis.