A Colorimetric Distinct Color Change Cu(II) 4-{[1-(2,5-dihydroxyphenyl)ethylidene]amino}-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one Chemosensor and its Application as a Paper Test Kit

A “4-{[1-(2,5-dihydroxyphenyl)ethylidene]amino}-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one” chemosensor (C1) synthesized by Condensation reaction using “4-amino-1,2-dihydro-1,5-dimethyl-2-phenylpyrazol-3-one” and “2,5-dihydroxy Actophenone”. It shows absorption peak at 326 nm due to the C=C bond (π-π* transition), while the absorption peak at 364 nm is caused by the C=O bond (n-π* transition). In the presence of copper C1 only demonstrated a change in absorption peak to higher wavelength (redshift) from 364 to 425 nm. The hue of both the C1+Cu(II) complex changed from bright yellow to dark brown colour with hypochromic shift appearance of a new peak at 425 nm. Even in the presence of other competing metal ions, the hypsochromic shift of the absorption band and the quenching of the fluorescence emission intensity were different for detecting Cu2+, in CH3OH-H2O (v/v = 6:4). Job's plot was used to compute the binding stoichiometry between C1 and Cu2+, and MASS and FTIR spectrophotometry were used to investigate the complexation process. The capacity of the C1 to bind Cu2+ was further proven using DFT simulations. The complex S1+Cu2+ has a HOMO-LUMO energy gap of 2.8002 eV, which is lesser than S1 (2.9991 eV). From these results, it can be seen that the specific binding of S1 to Cu2+ improves the stability of the S1+Cu2+ complex and effectively reduces the HOMO-LUMO energy gap. The C1 was successfully used in a real-world water sample analysis. Using the Benesi-Hildebrand and Scatchard plots, the Kb was calculated and found to be 47340M-1 and 48369M-1 respectively, showing the creation of stable complexation between Cu2+ and S1 with 1:1 Stoichiometry. The LOD of S1 for Cu2+ ion analysis was determined to be 649 nM. Strip sheets were also built and tested to detect varying amounts of Cu2+ aqueous solution, and their color change suggested that they might be used for on-site Cu2+ detection in polluted water.