A Convenient and Practical Colorimetric Chemosensor for Detection of Cu²⁺ Operating in Near-Perfect Water: Test Strip and Smartphone Application to Environmental Water

A convenient and practical colorimetric chemosensor, HQPC ((E)-N'-((8-hydroxy-2,3,6,7-tetra-hydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)methylene)pyrazine-2-carbohydrazide), was synthesized and evaluated for sensing Cu²⁺. HQPC exhibited a color variation from pale yellow to light apricot upon interaction with Cu²⁺ in near-perfect water, showing a low detection limit of 0.30 µM and excellent selectivity against various coexisting ions. The applicability and reusability of HQPC were validated through environmental water sample analysis and reversibility test using ethylenediaminetetraacetic acid (EDTA). The test strip and smartphone-based quantification further demonstrated the practicality and convenience of HQPC for Cu²⁺ detection. The probing mechanism of HQPC for Cu²⁺ was proposed based on ESI-MS and FT-IR analysis, ¹H NMR titration, and DFT calculations.