Development of Colorimetric Chemosensors for Selective Sensing of Inorganic Arsenic Anions and their Applications

Abstract

Arsenic, a carcinogenic metalloid, is widely present in natural water, soils, and the atmosphere, posing a significant health risk to millions worldwide. Inorganic arsenic anions such as arsenite (AsO2⁻) and arsenate (AsO43⁻) are particularly toxic compared to organic arsenic compounds. High exposure to these inorganic arsenic anions leads to severe health effects, including carcinogenic impacts on the skin, liver, kidneys, and other organs, often resulting in fatalities. The World Health Organization (WHO) has set the permissible level of inorganic arsenic anions in drinking water at 10 parts per billion (ppb), highlighting the crucial need for detecting these ions in water samples. Addressing these concerns, the present thesis aims to design, synthesize, and characterize seven series of colorimetric organic receptors for the selective detection of arsenite and arsenate ions in semi-aqueous media. The anion binding characteristics of the receptors have been evaluated in organo-aqueous solvent systems, and the binding mechanisms have been elucidated using UV-Vis titration experiments, 1H NMR titrations, cyclic voltammetric studies, and Density Functional Theory (DFT) studies. The research findings concluded that the acidic binding site of the receptors initially connected to the anions through hydrogen bonding interactions, and the excess addition of anions caused the deprotonation, constituting an intramolecular charge transfer mechanism. The stabilities of the receptors and receptor-arsenite complexes have been assessed through pH studies, revealing that the detection process is feasible within the pH range of 6–12. The binding constants and binding ratios of the receptors have been calculated using Benesi-Hildebrand (B-H) equation and B-H plots. The lower detection limits (LOD) of the receptors, achieved in parts per billion (ppb) for arsenite and arsenate, demonstrate the efficiency of the receptors. Receptors N4R1 and N7R1 could differentiate arsenite and arsenate with different colors and spectral shifts, and the receptor N7R3 containing two binding sites reported the lowest LOD of 1.79 ppb and highest binding constant of 1.6296 × 1013 M⁻2. Real-life applications of the receptors have been demonstrated through the fabrication of test strips, analysis of various environmental water and food samples, and the construction of logic gates.