Synthesis And Properties Of Water-Soluble Carbon Quantum Dot Fluorescence Probes

Carbon quantum dots are a kind of novel ultrasmall carbon nanoparticles with excellent optical properties, low toxicity, good biocompatibility and low cost. Applying carbon quantum dots as a highly sensitive, multi-channel fluorescence probe in early detection of serious diseases such as malignant tumors is an important research project. In this dissertation, we focus on synthesis of carbon dots (C-dots) and graphene quantum dots (GQDs) and study of their properties such as photoluminescent (PL), magnetic properties, self-assembly properties, e-induced fractal growth, biological fluorescence labeling. Besides, we discuss the fluorescence mechanism of C-dots and GQDs by systemic experimental data and rational deduction. The main contents are as follows:a) The quantum yields (QYs) of most C-dots synthesized thus far are less than 15%; more work was concerned about the liquid-state PL properties of C-dots, however, little work was done on their other properties such as solid-state PL, magnetic and self-assembly properties. Herein, we synthesized water soluble blue fluorescent C-dots with high QY (40.6%), narrow full width half maximum (FWHM), good photostability and up-conversion fluorescence by thermal decomposition of EDTA sodium salts at low temperatures (300~400°C). The fluorescence of C-dots is strongly affected by pyrolysis temperature, precursors, solvents, pH, annealing conditions, and photocatalysis etc. Besides, the hydrophilic C-dots can be easily transferred from water to toluene by surface functionalization. We further fabricated carbon thin films by spin-coating hydrophobic C-dots on different substrates. We also studied the magnetic properties of pyrolysis products and first reported carbon nano-materials with both fluorescence and magnetism, which provides novel materials for applications in carbon-based magneto-optical integrated nanodevices. Finally, we reported interesting self-assembly properties of C-dots, including "oriented attachment", evaporation-induced and substrate-related self-assembly.b) GQDs show potential applications in future nano-electronic devices, spin devices, and optoelectronic devices. But the current synthesis methods such as etching method and the bottom-up chemical route are expensive and inefficient. Herein, we reported a top-down route—hydrothermally cutting of 200°C reduced graphene nanosheets into water soluble GQDs with an average diameter of ~ 9.6 nm and 1 to 3 layers. Interestingly, the as-prepared GQDs can emit bright blue fluorescence (430 nm) with QY of 6.9% and good photostability when excited at 320 nm. Besides, the GQDs can be easily transferred from water to toluene by surface functionalization. We studied the optical properties of hydrophobic GQDs. Note that it's the initiate work reporting a chemical route to synthesize water soluble GQDs and finding the fluorescence properties of GQDs, which makes GQDs have potential applications in future nanoelectronic devices, spin devices, optoelectronic devices and biological fluorescence probes.c) Graphene shows promising applications in energy, environmental and biological fields due to its excellent electrical, thermal, mechanical properties and chemical stability. But little attention has been paid to its PL properties, which limits its applications in optical devices, optoelectronic devices and biological imaging, etc. Herein, we first fabricated ultrasmall (average diameter ~ 3.1 nm), well-crystallized GQDs by hydrothermally cutting 600°C thermal reduced graphene nanosheets under strong alkalic condition. The as-synthesized GQDs can emit strong green fluorescence with the maximum emission peak centered at 500 nm and QY of 7.5% when excited at 420 nm. The GQDs show good photostability, pH-dependent PL and self-absorption properties. Interestingly, we observed in-situ fractal growth process of GQDs under electron beam irradiation in TEM. We further discussed the mechanism of fractal growth. More importantly, we first applied the water soluble green fluorescent GQD as biological fluorescence probes to image HeLa cells, which further extends applications of graphene materials. d) C-dots and GQDs show promising applications in biological, environmental, and electronic devices. However, the PL mechanism remains unclear. Herein, we analyzed the typical edge structure of sp~2 clusters in C-dots and GQDs such as zigzag sites and armchair sites. Besides, we calculated the energy gap ofÏ€â†'Ï€* transitions based on time-dependent (TD) density functional theory (DFT) as a function of the number of aromatic rings. Then, combined with optical properties and electron paramagnetic resonance (EPR) characterization of C-dots and GQDs, we put forward that the fluorescence may be from the the zigzag structure with triplet carbine (σ~1Ï€~1) in sp~2 clusters. We first use systemic spectroscopic characterizations to deduce the relationship of microstructures of C-dots and GQDs and PL, which provides theoretical basis for the controlled synthesis of highly efficient carbon-based luminescent materials.