Synthesis, Characterization, Formation Mechanism And Sensor Applications Of Photoluminescent Carbon Dots

Carbon dots (C-dots), an emerging star in carbonaceous nanomaterials since their discovery by purifying single-walled carbon nanotubes in 2004, have attracted tremendous attention due to their unique optical properties, low environmental hazard, excellent biocompatibility, and robust chemical inertness. The growing interest in applications of C-dots in bioimaging, cancer therapy, sensors, optoelectronics, and catalysis comes a need for low-cost and scale-scale production of this unique material. Despite a variety of synthetic methods have been developed for synthesis of C-dots, they usually suffer from expensive precursors, special equipment, harsh synthetic conditions, and complex processes, and get trapped in small scale and low production yield. How the C-dots generate from molecular precursors is another important issue. However, little attention has been paid to the formation process of C-dots due to the difficulty of monitoring their complicated reaction path, which leads to the unclear formation mechanism. A deeper insight into the formation mechanism of C-dots is of great significance not only for understanding the construction chemistry of carbon nanomaterials but also for guiding the large-scale synthesis. On the other hand, if C-dots with special properties are obtained, it is necessary to further explore their applications. In this dissertation, we have developed several novel methods for synthesis of C-dots, and systematically investigated their formation process, and further explored their applications in analytical chemistry. This dissertation includes the following five parts:1. Oxygen-driven, high-efficiency production of nitrogen-doped carbon dots from alkanolamines and their application for two-photon cellular imagingA novel oxygen-driven method has been developed for low-cost, large-scale, and high-efficiency production of nitrogen-doped carbon dots (N-C-dots) by bubbling pure oxygen into monoethanolamine (MEA) under heating condition. We find that the addition of pure oxygen significantly increases the reaction rate, and makes it feasible for one-pot gram scale fabrication (3.36 g) of highly photoluminescent N-C-dots in a couple of hours (2.0 h). With an instantaneous nucleation and gradual growth mechanism, precise control over the particle size of the N-C-dots from 2.0 to 16.1 nm is achieved by simply prolong the heating time from 0.5 to 4.0 h. The as-prepared N-C-dots contain aromatic CN heterocycles in the core and have plentiful hydrophilic groups on the surface. Practically, the oxygen-driven method can be used to synthesize fluorescent N-C-dots from other alkanolamines such as diethanolamine (DEA) and triethanolamine (TEA), which shows general universality. Due to the strong up-conversion photoluminescence, good aqueous dispersibility, high photostability, excellent biocompatibility, and low cytotoxicity, the N-C-dots are demonstrated to be promising two-photon probes for high contrast bioimaging applications.2. How do nitrogen-doped carbon dots generate from molecular precursors? An investigation of formation mechanism and a solution-based large-scale synthesisA bottom-up method, using monoethanolamine (MEA) as both a passivation agent and a solvent, has been developed for rapid and massive synthesis of nitrogen-doped carbon dots (N-C-dots) from citric acid under heating conditions. This method requires relative mild temperature (170℃) without special equipment, and affords one-pot large-scale production (39.96 g) of high-quality N-C-dots (quantum yield of 40.3%) in a few minutes (10 minutes). Significantly, an interesting formation process of N-C-dots, for the first time, has been monitored by transmission electron microscopy, ultraviolet-visible absorbance spectroscopy, photoluminescence spectroscopy, Fourier transformed infrared spectroscopy, and thermogravimetric analysis, and a corresponding formation mechanism including polymerization, aromatization, nucleation, and growth, is proposed. It is important that the MEA-based synthesis of N-C-dots can be extended to various precursors, such as glucose, ascorbic acid, cysteine, and glutathione, which show general university. Furthermore, the N-C-dots with strong fluorescence, excellent optical stability, and low cytotoxicity, are successfully applied as fluorescent probes for bioimaging.3. Waste frying oil as a precursor for one-step synthesis of sulfur-doped carbon dots with pH-sensitive photoluminescenceProper disposal of waste frying oil (WFO) is an important waste-management concern. In this paper, we develop a facile method to reuse WFO for one-step synthesis of sulfur-doped carbon dots (S-C-dots) with the assistance of concentrated sulfuric acid. The as-synthesized S-C-dots are uniform in size and show partial disordered graphite-like structure. Different from the doping-free or nitrogen-doped carbon dots, the S-C-dots perform a strong ultraviolet emission at 378 nm due to successful sulfur-doping. Noticeably, the S-C-dots exhibit a distinct pH-sensitive feature and the intensity of photoluminescence increases linearly in the pH range from 3 to 9. Furthermore, possessing fascinating optical properties, high photostability, and low cytotoxicity, the S-C-dots have served as fluorescent probes for cell imaging. The simple method developed here presents a new way for effective reuse of WFO and realizes the encouraging "waste-to-treasure" conversion.4. Green and size-controllable synthesis of photoluminescent carbon nanoparticles from waste plastic bagsWe have developed a facile approach for green and size-controllable synthesis of photoluminescent carbon nanoparticles (CNPs) by hydrothermal treatment of various waste plastic bags (WPBs) in low-concentration H2O2 solutions (≤5.0 wt%). This approach requires none of toxic regents, severe synthetic conditions, or complicated procedures. Fine control over the particle size of the CNPs is achieved by simply changing the H2O2 concentration, and the higher H2O2 concentration leads to the smaller particle size of the CNPs. An interesting formation mechanism of the CNPs derived from WPBs has been proposed including thermo-oxidative degradation, polymerization, carbonization, and passivation. It is found that the CNPs can selectively quantify the concentration of Fe3+ from 10 to 400 μM with a detection limit as low as 2.8 μM. Moreover, the strong photoluminescence, excellent optical stability, low cytotoxicity, and good water-dispersibility of the CNPs make them suitable candidates for cellular imaging.5. Ethanol in aqueous hydrogen peroxide solution:hydrothermal synthesis of highly photoluminescent carbon dots as multifunctional nanosensorsA novel synthetic strategy has been developed for facile, green and low-cost fabrication of highly photoluminescent C-dots by hydrothermal treatment of ethanol in aqueous hydrogen peroxide (H2O2) solution. Noticeably, the synthesized C-dots present an unexpectedly large quantum yield of 38.7% without any post-treatments. In contrast to the most amorphous C-dots, the ethanol-derived C-dots possess an essentially crystalline nature as evidenced by the high-resolution transmission electron microscopy and selected-area electron diffraction. It is found that the C-dots can serve as multifunctional fluorescence nanosensors to detect pH, temperature, and the concentration of hypochlorite ion (CIO-). The PL intensity of C-dots decreases dramatically as pH increases from 3 to 11. Based on this feature, a C-dots coated fluorescent paper for visual detection of pH by naked eyes has been successfully prepared. The C-dots reveals a linear and reversible PL response toward the temperature in the range of 10-80℃, suggesting the great potential for design of temperature-sensitive devices. The selective quantification of CIO- concentration from 0.1 to 10 μM with a detection limit as low as 0.08 μM is achieved by ClO--induced PL quenching of C-dots. Moreover, the C-dots applied for ClO- assay in real water samples with satisfactory recovery is demonstrated.