Selective detection of an organophosphorus pesticide using a pH-sensitive metal–organic compound containing a cobalt(ii) ion

Abstract

We have introduced a metal–organic compound (MOC) containing a Co(II) ion that could have the potential to serve as a sensor for dichlorvos, an organophosphorus pesticide. The metal–organic compound [Co(3-CCA)₂(H₂O)₂], RT-2, was created by employing coumarin-3-carboxylic acid (3-CCA) as the organic ligand through a gradual layer diffusion method at ambient temperature. Single-crystal X-ray diffraction analysis of [Co(3-CCA)₂(H₂O)₂] (RT-2) revealed a slightly distorted octahedral CoO₆ coordination environment with non-coplanar trans coumarin rings exhibiting a chair-like conformation. The supramolecular architecture is stabilized by strong tetrameric hydrogen bonding, nonconventional C–H⋯O interactions, and π–π stacking, resulting in a robust 2D network. When excited at 325 nm, RT-2 displayed an emission peak at 405 nm, demonstrating an 11-fold enhancement in intensity along with a 10 nm red shift in the presence of 800 μM dichlorvos. It was observed to be highly selective for dichlorvos compared to other organophosphorus pesticides, with a detection limit (LOD) of 0.117 μM or 25.85 ppb. The investigation of luminescence lifetime also indicates that the luminescence turn-on represents the enhanced structural rigidity obtained through the establishment of a hydrogen bonding network involving dichlorvos and RT-2. The pH dependence of the luminescence properties of RT-2 was also examined in detail. It showed a large enhancement of luminescence intensity as well as the luminescence lifetime below pH = 5, with a red shift of the luminescence spectra by 10 nm, which was very similar to the results of dichlorvos detection. It provided a way to identify the binding position of dichlorvos with RT-2 and supported the proposed mechanism of sensing.