2H-WS₂ Quantum Dots Produced by Modulating the Dimension and Phase of 1T-Nanosheets for Antibody-Free Optical Sensing of Neurotransmitters

Modulating the dimensions and phases of transition metal dichalcogenides is of great interest to enhance their intrinsic properties or to create new physicochemical properties. Herein, we report an effective approach to synthesize 2H-WS₂ quantum dots (QDs) via the dimension and phase engineering of 1T-WS₂ nanosheets. The solvothermal reaction of chemically exfoliated 1T-WS₂ nanosheets in N-methyl-2-pyrrolidone (NMP) under an N₂ atmosphere induced their chopping and phase transition at lower temperature to produce 2H-WS₂ QDs with a high quantum yield (5.5 ± 0.3%). Interestingly, this chopping and phase transition process showed strong dependency on solvent; WS₂ QDs were not produced in other solvents such as 1,4-dioxane and dimethyl sulfoxide. Mechanistic investigations suggested that NMP radicals played a crucial role in the effective production of 2H-WS₂ QDs from 1T-WS₂ nanosheets. WS₂ QDs were successfully applied for the selective, sensitive, and rapid detection of dopamine in human serum (4 min, as low as 23.8 nM). The intense fluorescence of WS₂ QDs was selectively quenched upon the addition of dopamine and Au³⁺ ions due to fluorescence resonance energy transfer between WS₂ QDs and the quickly formed Au nanoparticles. This new sensing principle enabled us to discriminate dopamine from dopamine-derivative neurotransmitters including epinephrine and norepinephrine, as well as other interference compounds.