New 8-hydroxyquinoline-based azo dyes as highly sensitive colorimetric chemosensors for nickel cation

A new series of sensitive 8-hydroxyquinoline azo-compounds (S1-S8) as sensors was synthesized by diazotization and diazo-coupling, and these compounds were characterized through UV, FTIR, ¹H-NMR, ¹³C-NMR and ESI-MS investigations. The UV-visible spectral data revealed that Ni²⁺ was sensed by compounds S2, S3, S4, and S6, producing colour changes ranging from violet to orange, and exhibited bathochromism of +8, +67, +63, and +85 nm, respectively. Optimized conditions included a mixture of ethanol/water (4 : 1) solvent system; pH 4 for compounds S3, S4, and S6, and pH 8 for compound S2; reaction time of 2 min; and a stoichiometric ratio of 3 : 1. The limits of detection for the Ni²⁺ cation demonstrated by the dyes were in the range of 0.012-0.038 μM using UV-visible spectroscopy, which is lower than the permissible value of Ni²⁺ (1.2 μM) in drinking water specified by the United States Environmental Protection Agency. Intra-day and inter-day accuracies of the Ni²⁺ cation spiked in tap and underground water (in terms of relative errors) were less than 5%, while precisions were less than 4.0% (RSD). The accuracy and precision of chemosensor-based analyses were comparable to those of the inductively coupled plasma mass spectrometry (ICP-MS) method. Colour changes of paper strips with sensors S2, S3, S4, and S6 ranged from orange to violet in the presence of nickel. Infrared analysis confirmed that sensors interacted with Ni²⁺ through nitrogen and hydroxyl moieties of quinoline. Thus, an easy-to-use, highly sensitive, and low-cost method for analyzing nickel in water samples was established.