Development of multi-analyte responsive sensors: optical discrimination of arsenite and arsenate ions, ratiometric detection of arsenite, and application in food and water samples

Abstract

5-Nitro-2-furaldehyde based Schiff base chemosensors, N4R1–N4R3 with varying strengths of electron-withdrawing groups were developed for the discriminatory sensing of arsenite and arsenate ions in semi-aqueous media. Receptor N4R1 distinguished arsenite (AsO₂⁻) and arsenate (AsO₄³⁻) anions colorimetrically in 30% aq. acetonitrile (MeCN) with a detection limit of 18 ppb for arsenite. N4R1–N4R3 also detected phosphate, arsenite and arsenate in 30% aq. dimethyl sulfoxide (Me₂SO) solution and the selectivity for arsenite over arsenate was achieved at 50% aq. Me₂SO solution with a noticeable bathochromic shift near 200 nm. All three receptors exhibited ratiometric detection for arsenite and were identified as stable over the 6–12 pH range. The computed high binding constants of the receptors N4R1–N4R3 for inorganic arsenic anions and phosphate in the range of 10⁶ M⁻¹ exposed the receptor's higher potential in the sensing process. Hydrogen bonding interaction followed by deprotonation and the ICT mechanism on binding with arsenite/arsenate/phosphate was confirmed by UV-vis and ¹H-NMR titration, electrochemical studies, density functional theory studies, and mass spectral analysis. Fabrication of disposable paper strips and solid state sensing using silica gel were employed to conveniently detect arsenite and phosphate anions in real-life samples. The potential application of the receptors for detection in environmental and real-life samples was evaluated by analyzing food and water samples spiked with arsenite and phosphate anions.