A reversible indole-based colorimetric chemosensor for Cu²⁺: implications for smartphone-integrated sensing, logic...

The development of portable, cost-effective, and practical analytical platforms for rapid on-site detection of Cu²⁺ remains challenging. Herein, we have reported the novel one-pot synthesis of an indole-based sensor for Cu²⁺, namely (E)-N'-((1H-indol-3-yl)methylene)picolinohydrazide (MPH), which was characterized by FT-IR, ¹H-NMR, ¹³C-NMR, elemental analysis, HRMS, and single-crystal X-ray diffraction. The sensing performance of MPH toward various metal ions was evaluated using colorimetric analysis, UV-vis spectroscopy, cyclic voltammetry, and a smartphone-assisted RGB platform in MeOH/H₂O. MPH exhibited high selectivity for Cu²⁺ ions, producing a distinct color change from colorless to intense yellow (≤1 second). Job's plot analysis revealed the formation of a 2 : 1 (MPH : Cu²⁺) complex. The titration studies revealed a distinct isosbestic point at 308 nm, indicating a clean and well-defined interconversion between the free and complexed species. The binding mechanism was investigated using FT-IR, ¹H NMR, ESI-MS, FESEM, and was supported by DFT calculations. The UV-vis method achieved a Cu²⁺ detection limit of 4.48 × 10^-6 M, surpassing EPA drinking water standards. The sensor also demonstrated excellent reversibility and recyclability with EDTA, while the RGB platform enabled real-time Cu²⁺ quantification with a detection limit of 5.5 × 10^-6 M. Practical applicability was confirmed through logic-gate operations, paper strip method, antimicrobial studies, and Cu²⁺ determination in real environmental samples. These ultrafast responses, high sensitivity, multifunctional capabilities, reusability, and real-world applicability suggest MPH as a highly effective, economical, and field-ready sensor for practical Cu²⁺ monitoring in environmental and analytical conditions.