Water-Soluble Monolayer Molybdenum Disulfide Quantum Dots with Upconversion Fluorescence

Uniform water-soluble monolayer MoS2 quantum dots (MQDs) with lateral sizes of approximate to 2.1 nm, a clearly zigzag-terminated edge, and a hexagonal lattice structure are achieved using ammonium molybdate, thiourea, and N-acetyl-L-cysteine (NAC) as precursors and the capping reagent in a facile one-pot hydrothermal approach. MQDs have good dispersity and high stability in aqueous suspension and exhibit a significantly larger direct bandgap (3.96 eV) compared to monolayer MoS2 nanosheets (1.89 eV). Pronounced blue-shifts in the wavelengths of both the excitonic absorption and intrinsic state emission with activated strong luminescence at room temperature beyond monolayer MoS2 nanosheets is demonstrated. Unusual upconversion photoluminescence is also observed and is caused by two successive transfers of energy from the near-infrared (NIR) absorption generated by the NAC capping reagent to the hexagonal structure of MQDs. Additional optical properties of MQDs may provide numerous exciting technological applications. Here, MQDs are demonstrated as a highly selective fluorescent reagent for detecting tetracycline hydrochloride under UV and NIR irradiation.