Covalent functionalization of MoS₂ 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 MoS₂ 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 MoS₂ 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 MoS₂ nanosheets obtained in DMF, which exhibits relatively higher sensitivity to Cd²⁺ detection and lower cytotoxicity against MCF-7 cells. The mechanisms of surface functionalization and selectively detecting Cd²⁺ were investigated by density functional theory calculations together with various spectroscopic measurements. It was found that surface-functionalized MoS₂ 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 Cd²⁺ and O donor atoms of DMF adsorbed on MoS₂ nanosheets. The robust anti-interference is ascribed to the strong binding energy of Cd²⁺ and O atoms of DMF. Under the optimum conditions, the electrochemical sensor exhibits highly sensitive and selective assaying of Cd²⁺ with a measured detection limit of 0.2 nM and a linear range from 2 nM to 20 μM.