Design and development of an azo dye-based ultra-sensitive chemosensor for the ubiquitous organic anion in aqueous solutions

Oxalate plays a crucial role in various biological and industrial applications. Its involvement in diseases, drug- and toxin-induced disorders, and disruption of industrial processes makes it a high-priority chemical for the development of more precise, selective, and straightforward analytical methods. Herein, an innovative and efficient strategy for sequentially measuring Cu²⁺ and oxalate ions is reported. During an indicator displacement process, where Cu²⁺ and azo dye, ArsenazoIII (AAIII), are used as receiver and indicator, respectively, oxalate (C₂O₄²⁻) is measured by a colorimetric method. Proof of concept results from UV–visible spectroscopy, naked eye, and signal reset of the indicator revealed a more robust interaction of the Cu²⁺ receptor with C₂O₄²⁻ than the AAIII indicator with high selectivity, good sensitivity, and lower detection limit(4.01 × 10⁻⁸ and 3.04 × 10⁻⁹mol L⁻¹, for Cu²⁺ and C₂O₄²⁻, respectively). The results of measuring C₂O₄²⁻ in actual samples (urine, mushrooms, and spinach) showed the applicability of the existing method. Most importantly, the colorimetric system based on molecular exchange in the indicator dislocation proceeding can serve as a colorimetric INHIBIT logic-gate. It works like a molecular keypad lock system that can be controlled by two users with two different sets of chemical passwords (inputs) that can be identified through optical paths.