Smartphone-assisted colorimetric detection of nickel( <scp>ii</scp> ) ions using a novel benzothiazole–quinoline dyad in semi-aqueous media

In this study, we present three different approaches for the colorimetric detection of Ni²⁺ ions using a specifically designed benzothiazole-quinoline dyad (L) synthesized <i>via</i> the Knoevenagel condensation reaction in high yield. The unique properties of L enable a rapid and selective response to Ni²⁺ ions, making it an ideal probe for practical applications. The probe L shows a pale yellow color under normal conditions. Upon interaction with Ni²⁺ ions, L undergoes a significant color change from pale yellow to bright orange, allowing for visual detection in semi-aqueous media. This rapid colorimetric response enables real-time monitoring of Ni²⁺ concentrations. The absorption maximum of L undergoes a bathochromic shift in the presence of Ni²⁺ ions due to ligand-to-metal charge transfer (LMCT). The probe L could form a 2 : 1 [L : Ni²⁺] stoichiometric complex, confirmed through Job's plot and ESI mass analysis with an estimated association constant of 2.61 × 10⁶ M^-2. The probe L could detect Ni²⁺ concentration down to 61 nM, 106 nM, and 129 nM <i>via</i> a UV-Vis spectrophotometer, smartphone-assisted RGB method, and test paper strip analysis. The binding mechanism of probe L with metal ions was studied using ¹H NMR, ESI mass spectrometry, and DFT calculations. The zeta potential analysis showed a potential of -28.38 mV for the free ligand and +12.09 mV upon complexation with Ni²⁺. More importantly, the potential application of probe L includes the quantification of Ni²⁺ ions in various water samples through all three sensing approaches.