In this work, a bottom-up strategy is developed to synthesize water-soluble molybdenum disulfide quantum dots (MoS2 QDs) through a simple, one-step hydrothermal method using ammonium tetrathiomolybdate [(NH4)2MoS4] as the precursor and hydrazine hydrate as the reducing agent. The as-synthesized MoS2 QDs are few-layered with a narrow size distribution, and the average diameter is about 2.8 nm. The resultant QDs show excitation-dependent blue fluorescence due to the polydispersity of the QDs. Moreover, the fluorescence can be quenched by hyaluronic acid (HA)-functionalized gold nanoparticles through a photoinduced electron-transfer mechanism. Hyaluronidase (HAase), an endoglucosidase, can cleave HA into proangiogenic fragments and lead to the aggregation of gold nanoparticles. As a result, the electron transfer is blocked and fluorescence is recovered. On the basis of this principle, a novel fluorescence sensor for HAase is developed with a linear range from 1 to 50 U/mL and a detection limit of 0.7 U/mL.

A new fluorescent polyimide covalent organic framework (PI-COF) has been successfully synthesized through solvothermal route using tetra(4-aminophenyl) porphyrin and perylenetracarboxylic dianhydride, which possesses porous crystalline and excellent thermal stability (>500 °C). Furthermore, few-layered PI covalent organic nanosheets (PI-CONs) can be easily obtained from the fluorescent PI-COF through a facile liquid phase exfoliation approach, which were confirmed by atomic force microscopy and transmission electron microscopy analysis. It is interesting that the fluorescent intensity of PI-CONs is obviously enhanced relative to that of PI-COF. The PI-CONs have been successfully utilized as an efficient fluorescent probe for the highly sensitive and selective detection of 2,4,6-trinitrophenol (TNP). The mechanism might be attributed to the combination of electron transfer and inner filter effect based on DFT calculations and spectral overlap data. The system exhibits a good linear response toward TNP over the range from 0.5 to 10 μM with a detection limit of 0.25 μM.

A facile and eco-friendly approach for the synthesis of water-soluble WS2 quantum dots (QDs) was developed via ultrasonication and a hydrothermal process from bulk WS2. In this strategy, the dispersity of bulk WS2 in aqueous phase was improved with the aid of a surfactant (hexadecyltrimethylammonium bromide, CTAB), which could shorten the exfoliation time and improve the exfoliation efficiency to form layered WS2 nanosheets. Through hydrothermal treatment, the nanosheets were further scissored into QDs with high quality. The QDs show excellent features with narrow size distribution, good water solubility, and stable fluorescence. We find that the fluorescence of WS2 QDs can be quenched by Fe3+ through photoinduced electron transfer, and a wide detection linear range for Fe3+ is acquired. It indicates that WS2 QD can be used as a “turn-off” probe for Fe3+. In the presence of lipoic acid (LA), the fluorescence was recovered due to the stronger interaction between LA and Fe3+ than WS2 QDs. A “turn-on” sensor for LA was developed with a linear range from 1 to 10 μM and a detection limit of 0.59 μM. The strategy might be suitable for the facile synthesis of other water-soluble transition metal dichalcogenide QDs. It is expected that the water-soluble QDs have great potential applications in biological system.

In this work, a novel ratiometric fluorescence sensor has been constructed for the selective and sensitive detection of Hg, which is based on the inner filter effect (IFE) of tetraphenylporphyrin tetrasulfonic acid (TPPS) toward black phosphorus quantum dots (BP QDs). Highly fluorescent BP QDs were successfully synthesized from bulk BP by sonication-assisted solvothermal method via a top-down route. In the presence of Hg, the IFE originating from spectral overlap between the excitation of BP QDs and the absorption of TPPS is inhibited and the fluorescence of BP QDs is restored. At the same time, the red fluorescence of TPPS is quenched due to its coordination with Mn. These phenomena result from the rapid coordination between Mn and TPPS in the presence of Hg, which leads to the dramatic decrease of the absorption of TPPS. On the basis of these findings, we design a ratiometric fluorescence sensor for the detection of Hg. The as-constructed sensor reveals a good linear response to Hg ranging from 1 to 60 nM with a detection limit of 0.39 nM. Furthermore, the sensing assay is applicable to detecting Hg in real samples.

MoS2 quantum dots with two-photon fluorescence features are synthesized through a one-step solvothermal approach and successfully used for cellular bioimaging.

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