Smartphone-assisted Colorimetric Detection of Nickel(II) Ions using Novel Benzothiazole-Quinoline Dyad in Semi-aqueous Media

Abstract

In this study, we present three different approaches for the colorimetric detection of Ni2+ ions using a specifically designed benzothiazole-quinoline dyad (L) synthesized via the Knoevenagel condensation reaction of (Benzo[d]thiazol-2-yl)acetonitrile and Quinoline-2-carbaldehyde 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 provides for real-time monitoring of Ni2+ concentrations. The absorption maxima 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 𝗑 106 M-2. The probe L could detect Ni2+ concentration down to 61 nM, 106 nM, and 129 nM via UV-Vis spectrophotometer and smartphone-assisted RGB method and test paper strips analysis. The binding mechanism of probe L with metal ions was studied using 1H NMR, ESI mass, 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.