Synthesis, Photoluminescence Mechanism And Aqueous Applications Of Fluorescent Carbon Materials

There are many forms of carbon existing in nature, including graphite, diamond,carbon nanotube and so on. Carbon material is a kind of environmentally friendlymaterials, which has draw incresing attention in recent years. Macroscopic carbonmaterials often lack the appropriate band gap, which are difficult to be developed asideal fluorescent materials. The fluorescent materials often contain inorganicfluorescent material, such as alkaline earth metals and rare earth elements; organicfluorescent materials, such as small organic molecules, polymer as well ascoordination compounds. Besides, the semiconductor nanocrystals or nano clustersare rising kind of fluorescent materials. By regulating the size and surface chemistry,the carbon can be developed to be a new type of fluorescent material. In this paper,we are focusing on the carbon dots (graphene quantum dots (GQDs), carbon nanodots(CNDs), polymer dots (PDs)), the most typical fluorescent carbon materials. Thesynthesis methods, physical and chemical properties, fluorescence mechanism andnovel applications on carbon dots were investigated in detail. There were severalcritical points of innovation in this paper: taked the lead in nano-solution phasesynthesis to GQDs as well as cell imaging applications, used ultrafast spectroscopy tostudy photoluminescent (PL) mechanism of GQDs for the first time; prepared the PLCNDs with the highest quantum yield and developed the molecule state PLmechanism; proposed the concept of non-conjugated polymer dots, and investigatedthe enhanced PL in crosslinking PDs. Specifically, there were three aspectsintroducing in detail in the following paragraphes.Firstly, we take the GQDs with relatively simple chemical structure as a modelsystem to understand the PL mechanism. The solvothermally GQDs from grapheneoxide possessed edge grafted nitrogen and water solubility. In the following work, wefocused on regulating surface chemistry and figuring out the PL center of the GQDs.Using column chromatography separation, the GQDs with tunable oxidation degreeand PL emission (blue to green) were prepared. By alkyl amines grafting or reducing,the surface chemistry of the GQDs were tuned, and the defect state (molecular state)PL changed to intrinsic state PL. By ultrafast transient absorption spectroscopystudying, we attributed the green PL of the GQDs to two relatively independentmolecule states and an intrinsic state with short fluorescent life. Due to the excellentstability, good biocompatibility, outstanding PL properties of the GQDs, we exploitedtheir applications in single-photon, two-photon biological imaging, biosensors andhybrid solar cells.Secondly, when the PL mechanism of the fluorescent carbon materials wasunderstanded in depth using the GQDs system, the bottom-up approach with small molecules or polymers was adopted to prepare carbon materials in large scale. Thesmall molecules with abundant carboxyl/hydroxyl and amino groups were used toprepare CNDs by hydrothermal routes. By regulating the types of molecules,temperature and time of carbonization, we can understand the impact of chemicalcomposition on the formation of CNDs. By optimizing the synthesis conditions, thehighly PL CNDs with quantum yield as high as80%were obtained. Furthermore,three typical CDs were chose to investigate the origin of the green PL by studing thephotophysical processes: the C=O groups (carbonyl, carboxyl or amide) connected onthe carbon core were proved to be the PL centers. In addition, due to the highquantum yield, low toxicity of CNDs, the fluorescent ink on macroscopic andmicrocosmic surface, nano-composites, ion detection and cell imaging were exploited.Further, using four representative CNDs, the size, quantum yield and the surfacepotential of CNDs were investigated how to affect the cell imaging applications.Finally, we further developed novel kinds of CDs, named PDs. The PDspossessed both the fluorescent properties of carbon materials, and performance ofpolymers, which is significant for practical applications. The nonconjugated linearpolymers with easy dehydration groups were selected to form the PDs by moderatelycarbonizing, such as polyvinyl alcohol, polysaccharides, polyethylene imine. Theprepared PDs have single PL emission center, low cytotoxicity, which were applied incell imaging fields. Moreover, by precipitation assembling organic synthesized GQDs,the other kind of PDs were obtained. The PL mechanism was also exploited. Further,we investigated the PL behavior of crosslinked PDs system: polymer crosslinking caneffectively reduce the vibration and rotation of nonradiative relaxation consumption,thereby enhancing the fluorescence properties. At the end, the functionalnanocomposites were prepared using the fluorescent PDs: benefited from the PDs, thenanocomposites contained not only the PL emission center, but also the connectedpolymer chains.In conclusion, from understanding PL mechanism of GQDs to producing CNDsand PDs in large scale, we followed the connection between theory and practicalapplication in this paper. The expanding the types, diverse synthesis, clear PLmechanism and novel aqueous applications of fluorescent carbon materials wereachieved in the present paper.