Design and synthesis of new receptors for molecular recognition of environmental polluting cations

Abstract

The colorimetric chemosensors for the recognition of heavy metal ions is an exciting area in the field of supramolecular chemistry as it has various applications in chemistry, biology and environment. In view of the stereochemical and rich redox reactivity, the transition metal ions such as Cu2+, Zn2+, Mn2+ and Fe3+ play an essential role in many biological processes and involves in active participation in small molecule binding and transport. Further, the heavy metal ions such as Hg2+, Cd2+, Pb2+, arsenic are highly toxic due to their clinical and environmental reasons. Considering the facts from the literature, four new series of Schiff base molecules based on different structural backbone Viz., benzothiazole, thiocarbohydrazide, thiophene-2-carboxyaldehyde and quinoline has been designed, synthesized and characterized as chemosensors for the colorimetric detection of heavy metal ions (Cu2+, Hg2+ and Cd2+). The chemical structure of the all synthesized chemosensors is confirmed by different standard spectroscopic techniques like, FTIR, 1H-NMR, 13C-NMR, HR-MS and LC-mass (ESI-MS). The 3D structure of selected chemosensors was elucidated using Single Crystal X-Ray diffraction (SCXRD) studies. The qualitative and quantitative analysis of heavy metal ions with the designed chemosensors has been carried out using UV-Vis spectroscopic studies and the binding constant (k) for the chemosensor-metal ion complex has been calculated using Benesi–Hildebrand equation and Bindfit method. Further, the detection and quantification limits have been estimated using signal to noise ration formula given by the ICH guidelines (Q2R1) from the UV-Vis titration data. Furthermore, the binding mechanism has been proposed based on UV-Vis titration and the same has been confirmed by FT-IR, 1H-NMR and LC-mass. Finally, the experimental results were compared with the theoretical density functional theory (DFT) studies for some series of molecules. From the experimental and theoretical DFT results, it can be concluded that, the simple organic molecules could act as very good colorimetric chemosensor for the detection of heavy metal ions such as Hg2+ and Cd2+ ions.