Synthesis Of Graphene-like Fluorescent Quantum Dots And Their Analytical Application

Owning to its advantanges, such as rapid-response, high-sensitivity and cost-effectivity, fluorescence sensor has been widely used in the fields of environment, biology medicine and so on. Nowadays, new kinds of inorganic two-dimensional layered material, such as molybdenum disulfide ?MoS₂? and black phosphorous ?BP?, which possess similar structure with graphene, have been applied in the design of fluorescence sensor, for example, large specific surface area, excellent catalytic performance and outstanding electron transfer efficiency. While the size of MoS₂ and BP are reduced to be less than 10 nm, zero-dimensional MoS₂ quantum dots ?MoS₂ QDs? and BP QDs can be obstained. Due to the quantum confinement and size effect, MoS₂ QDs and BP QDs possess unique optical, electronic properties and are potentially applicable in bio-imaging, photothermal therapy, catalysis, etc. However, the synthesis of MoS₂ QDs and BP QDs and their application in the construction of fluorescence sensors are in the preliminary stages. For the synthesis of quantum dots, there are several problems, for example, poor synthetic strategy, low efficiency and uncontrollable quanlity. For the analytical application, low quantum yield and few applications limit the further development of the quantum dots. Therefore, the systematical research on the synthesis and analytical applications of graphene-like quantum dots are of great importance. In this papar, we focus on the controllable synthesis, property improvement and analytical applications of MoS₂ QDs and BP QDs. Through "Botom-Up" and "Top-Down" strategies, MoS₂ QDs and BP QDs with improved fluorescence property and solubility are successfully synthesized effectively. And the dependency of fluorescence property on the phase conpositions of MoS₂ QDs is studied. At the same time, we apply MoS₂ QDs and BP QDs in bio-image and the design of fluorescence sensor. The main contents are as follows:Chapter 1. IntroductionThis chapter gives the outlines and reviews of the synthesis, doping, modification composition of MoS₂ and BP QDs, as well as the applications of the QDs on different fields. Finally, the purpose and significance of our work are presented.Chapter 2. A facile and one-step ethanol-thermal synthesis of MoS₂ quantum dots for two-photon fluorescence imagingTwo-photon fluorescence ?TPF? is an optical phenomenon, in which fluorophore absorb two infrared photons simultaneously and then relax a single photon with higher energy. Comparing to one-photon fluorescence, TPF possesses great advantanges, such as low tissue autofluorescence, large penetration depth, and reduced photobleaching. In this chapter, MoS₂ QDs with TPF property are synthesized through a one step ethanol-thermal method via a "Top-Down" strategy. This blue fluorescent MoS₂ QDs have good biocompatibility. They can be applied in TPF cell imaging.Chapter 3. One-step synthesis of water-soluble MoS₂ quantum dots via a hydro thermal method as a fluorescent probe for hyaluronidase detectionIn this chapter, via a "Botom-Up" strategy, water-soluble molybdenum disulfide quantum dots ?MoS₂ QDs? are synthesized through a one-step hydrothermal method using ammonium tetrathiomolylybdate [?NH₄?₂MoS₄] as the precursor. The fluorescence of the MoS₂ QDs can be quenched by hysluronic acid ?HA? stablilized gold nanoparticles ?AuNPs? through a photoinduced electron transfer mechanism. Hyalutonidase ?HAase? can cleave HA into fragments and lead to the aggregation of AuNPs. As a result, the electron transfer is blocked and the 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 unit mL-1 and a detection limit of 0.7 unit mL^-1.Chapter 4. Fluorescence enhancement of MoS₂ quantum dots and its application on ratiometric fluorescent detection of hydrogen peroxideDepending on the arrangement of the S atoms, several prototype of MoS₂ have been reported, such as 2H and 1T phases. Their physical, electrical, optical properties are quite different from each other, due to their different electronic structures. For example, 1T-MoS₂ nanosheets possess no photoluminescence property, while 2T-MoS₂ nanosheets are strongly photoluminescent. In this chapter, MoS₂ QDs are synthesized through hydrothermal method using sodium mulybdate and sulfer powder as precursor and further thermally annealed under higher temperature to tune their phase compositions. The main phase conpositions of the QDs transferred from 1T to 2H, and the fluorescence property are improved. This method can be used in the fluorescence enhancement of other transition-metal dichalcogenide quantum dots. Furthermore, the excitation and emission spectra of MoS₂ QDs exist large spectral overlaps with the absorption of Fe???meso-Tetra?N-methyl-4-pyridyl?porphine pentachloride ?Fe-TMPyP?, while for gold nanoclusters ?AuNCs?, there is a spectral overlap between their excitation spectrum and the absorption of TPPS. As a result, the fluorescence of the MoS₂ QDs/AuNCs solution can be quenched by Fe-TMPyP through inner filter effect ?IFE?. Hydrogen peroxide ?H₂O₂? can oxidize and desolve Fe-TMPyP, which leads to the fluorescence recovery of MoS₂ QDs, while AuNCs would not be affected. On the basis of these phenomena, H₂O₂ can be detected through a new ratiometric fluorescence sensing system. The linear response range of the sensing system is 1 to 100 ?M with a detection limit of 0.62 ?M.Chaper 5. Solvothermal synthesis of highly fluorescent black phosphorus quantum dots for ratiometric fluorescent detection of Hg²⁺Black phosphorus, as one of three main allotropes of phosphorus, possesses silmar structure with graphene. In the chapter, green fluorescent BP quantums dots are synthesized through by a sonication-assisted solvothermal method. The as-prepared QDs possess ultrasmall size, good photostability and relative pH resistance. In addition, due to the large spectral overlap between the excitation of BP QDs and the absorption of tetraphenylporphyrin tetrasulfonic acid ?TPPS?, the fluorescence of BP QDs can be quenched due to IFE. In the prescence of Hg²⁺, Mn²⁺ can coordinate with TPPS rapidly, which leads to the decrease of the absorption of TPPS and the fluorescence recovery of BP QDs. At the same time, the red fluorescence of TPPS is quenched after its coordination with Mn²⁺. Basing on these phenomena, a radiometrix fluorescence sensor for Hg²⁺ is developed. This sensor shows a good linear response to Hg²⁺ in the range of 1 to 60 nM, with a detection limit of 0.39 nM.