Hydrothermal Synthesis of Fluorescent Functionalized MoS2 Quantum Dots for Heavy Metal Detection

Surface functional groups of molybdenum disulfide (MoS2) quantum dots (QDs) can play a fundamental role in their optical properties. Nonetheless, the use of fluorescent MoS2 QDs as optical probes still requires further investigation regarding their surface properties and functional groups. Because such features depend on the synthesis procedures employed, which influence the nanomaterials’ morphologies and structure, it becomes essential to understand the influences of the synthesis parameters on the material properties. Here, we demonstrate how to tune the surface properties of MoS2 QDs by alterations in the synthesis parameters and how they influence the copper ions (Cu2+) detection. Different stoichiometric ratio (Mo:S) between sodium molybdate:l-cysteine and reaction times of 7 or 17 h were used on the syntheses of MoS2 QDs, resulting in changes in the amount of partially oxidized functional groups (SO3/SO3H/SO42–/S2O22–) and oxygen functionalities (Mo–O/Mo═O/–COOH), according to Fourier transformed infrared and X-ray photoelectron spectroscopies analyses. The results show that the partially oxidized functionalization and sulfur-based groups of MoS2 QDs were obtained and led to remarkable changes in the photoluminescence (PL) properties, with quantum yields ranging from 7.0 to 13.1%. At the optimal synthesis condition, the MoS2 QDs could detect Cu2+ in water with a limit of detection (LoD) as low as 0.07 μM. Additionally, the MoS2 QDs obtained demonstrated the ability to discriminate Cu2+ from other common metal ions by using Principal Component Analysis (PCA). The sensing performance was attributed to the surface functional group tuning, which can be tailored by the synthesis parameters to enhance the LoD, quantum yield, or affinities toward different heavy metals.