A water-soluble colorimetric chemosensor for sequential probing of Cu²⁺ and S²⁻ and its practical applications to test strips, reversible test, and water samples

A water-soluble colorimetric chemosensor NHOP ((E)-1-(2-(2-(2-hydroxy-5-nitrobenzylidene)hydrazineyl)-2-oxoethyl)pyridin-1-ium) chloride) was developed for the sequential probing of Cu²⁺ and S²⁻. NHOP underwent a color change from pale yellow to colorless in the presence of Cu²⁺ in pure water. The binding ratio between NHOP and Cu²⁺ was confirmed to be 1:1 by the Job plot and ESI-MS (electrospray ionization mass spectrometry). The detection limit of NHOP for Cu²⁺ was calculated as 0.15 μM, which was far below the EPA (Environmental Protection Agency) standard (20 μM). The NHOP-coated test strip was able to easily monitor Cu²⁺ in real-time. Meanwhile, the NHOP-Cu²⁺ complex reverted from colorless to pale yellow in the presence of S²⁻ through the demetallation. The stoichiometric ratio between NHOP-Cu²⁺ and S²⁻ was determined to be 1:1 by analyzing the Job plot and ESI-MS. The detection limit of NHOP-Cu²⁺ for S²⁻ was calculated as 0.29 μM, which was very below the WHO (World Health Organization) guideline (14.7 μM). NHOP successfully achieved the quantification for Cu²⁺ and S²⁻ in water samples. NHOP could work as a sequential probe for Cu²⁺ and S²⁻ at the biological pH range (7.0–8.4). Moreover, NHOP could successively probe Cu²⁺ and S²⁻ at least three cycles because of its reversible property. The detection mechanisms of NHOP for Cu²⁺ and NHOP-Cu²⁺ for S²⁻ were demonstrated with Job plot, ESI-MS, and DFT (density functional theory) calculations. Therefore, NHOP could work as an efficient sequential probe for Cu²⁺ and S²⁻ in environmental systems.