Covalent functionalization of MoS nanosheets synthesized by liquid phase exfoliation to construct electrochemical sensors for Cd (II) detection.

Surface functionalization is an effective strategy in the precise control of electronic surface states of two-dimensional materials for promoting their applications. In this study, based on the strong coordination interaction between the transition-metal centers and N atoms, the surface functionalization of few-layer MoS nanosheets was successfully prepared by liquid phase exfoliation method in N, N-dimethylformamide (DMF), 1-methyl-2-pyrrolidinone, and formamide. The cytotoxicity of surface-functionalized MoS nanosheets was for the first time evaluated by the methylthiazolyldiphenyl-tetrazoliumbromide assays. An electrochemical sensor was constructed based on glass carbon electrode (GCE) modified by MoS nanosheets obtained in DMF, which exhibits relatively higher sensitivity to Cd detection and lower cytotoxicity against MCF-7 cells. The mechanisms of surface functionalization and selectively detecting Cd were investigated by density functional theory calculations together with various spectroscopic measurements. It was found that surface-functionalized MoS nanosheets could be generated through Mo-N covalent bonds due to the orbital hybridization between the 5 s orbitals of Mo atoms and the 2p orbitals of N atoms of the solvent molecules. The high selectivity of the sensor is attributed to the coordination reaction between Cd and O donor atoms of DMF adsorbed on MoS nanosheets. The robust anti-interference is ascribed to the strong binding energy of Cd and O atoms of DMF. Under the optimum conditions, the electrochemical sensor exhibits highly sensitive and selective assaying of Cd with a measured detection limit of 0.2 nM and a linear range from 2 nM to 20 μM.