Paper based field deployable sensor for naked eye monitoring of copper (II) ions; elucidation of binding mechanism by DFT studies

Abstract The study demonstrates the fabrication of test strips made from newly synthesized ortho-Vanillin based colorimetric chemosensor (probe P) that could be employed as field deployable tool for rapid and naked eye detection of Cu2+. Upon addition of Cu2+ to the chemosensor, it exhibits rapid pink color from colorless and can be easily seen through the naked eye. This probe exhibits a remarkable colorimetric “ON” response and the absorbance intensity of the probe enhances significantly in presence of Cu2+. The sensing mechanism has been deduced using FTIR, XPS, LCMS and DFT studies. The binding mechanism of the probe to Cu2+ was substantiated by DFT studies. HOMO of the probe suggests that a high electronic density resides on O, N atoms and thus these are the favorable binding site for the metal ions. Study revealed that the P + Cu2+ complex is −35.64 eV more stable than individual reactants. The Cu2+ binds to the probe in 1:1 stoichiometry with a binding constant of 2.6 × 104 M−1 as calculated by Job's plot and Benesi-Hildebrand plot. The chemosensor shows 1.8 × 10−8 M detection limit, which is considerably lesser than that of the WHO admissible limit of [Cu2+] in drinking water. Possible interfering ions namely Ca2+, Mg2+, Fe2+, Co2+, Ni2+, Cd2+, Hg2+, Mn2+, Al3+ and Cr3+ do not show any appreciable interference in the colorimetric response of the probe towards Cu2+. Particularly, the colorimetric “ON-OFF-ON” responses are proved to be repeated over 5 times by the sequential inclusion of Cu2+ and S2−. Sensitivity of the probe in real-time water and blood samples is found at par with results with AAS and ICP-OES techniques. Further, the reversibility of the probe and the easy fabrication of deployable strips for real-field naked eye detection of Cu2+ suggest importance of synthesized probe.