Aconitic Acid Derived Fluorescent Carbon Nanomaterials:synthesis And Analytical Application

In recent years, fluorescent carbon nanomaterials have attracted increasing attention and subsequent rapid development because of their environmentally friendly characteristics. As the new member of fluorescent carbon materials, carbon dots（CDs） have attracted much attention since they were first reported in 2004. Compared to traditional organic dyes, CDs have shown excellent optical stability, good biocompatibility and low cytotoxicity, and accordingly, they are expected to have potential application prospects in the field of life analysis. Throughout the development status of CDs, there are still two problems to be solved. On the one hand, a large amount of CDs have a relatively low quantum yield that is not beneficial for their practical applications. On the other hand, most CDs have a single fluorescence emission with blue that limits their applications in biological imaging. Therefore, it is a challenging and hot topic for finding new precursors, expanding and improving the preparation methods to obtain CDs with high quantum yield and multicolor fluorescence emission. The dissertation focuses on improving the quantum yield and developing novel fluorescent CDs with tunable emission wavelength. On the basis of the foregoing, a series of aconitic acid（AA） derived fluorescent carbon nanomaterials were prepared with urea, ethylenediamine（EDA） or 1,2,4-triaminebenzene dihydrochloride（TBD） as the co-doping reagents respectively via scientific and reasonable design. Subsequently, potential applications in analytical field based on their high quantum yield and tunable emission properties were investigated. There are four chapters in this dissertation:Chapter one: The discovery and research status of fluorescent carbon nanomaterials were briefly introduced, focusing on generalizing and summarizing the preparation methods of CDs with the aims to improve their quantum yield and control their fluorescence emission wavelength.Chapter two: A facile one-step microwave-assisted synthesis approach was developed for the fabrication of new fluorescent CDs with high quantum yield and blue fluorescence emission,by using AA and EDA as the carbon precursor and water as the medium. The effects of molar ratio of reactants, microwave power and microwave time on the fluorescence properties weresystematically investigated. The experimental results indicated that when the molar ratio of AA to EDA was 1:3, the microwave power was 700 W and the microwave time was 6 min, the resulting CDs exhibited the highest quantum yield. The as-prepared CDs had an average diameter of ca. 2.9 nm and could be well dispersed. The AA derived CDs（i.e. AA-CDs） were water-soluble and the aqueous solution exhibited bright blue fluorescence with an absolute quantum yield of 45% under the optimized excitation wavelength of 360 nm and emission wavelength of 450 nm. Subsequently, an “on-off-on” fluorescent sensor was established and then successfully applied for turn-off detection of Hg²⁺ ions and turn-on detection of cysteine（Cys） in aqueous solution, with the detection limit of 5.5 and 30 n M, respectively. Furthermore, this system showed excellent stability and reversibility, and was successfully applied as fluorescent nanoprobes for determination of Hg²⁺ ions in various water environment, as well as label-free imaging of Hg²⁺ and Cys in living cells.Chapter three: As a continuing study described in Chapter two, we obtained four types of water-soluble CDs（AA&EDA-MW-CDs, AA&EDA-HT-CDs, AA&Urea-MW-CDs,AA&Urea-HT-CDs） with excellent optical properties through hydrothermal or microwave-assisted approach, and introducing AA as the carbon precursor, EDA or urea as the co-doping reagents. Among these, AA&Urea-MW-CDs, fabricated by microwave irradiation using AA and urea as reagents, emitted bright yellow fluorescence. The effects of molar ratio of reactants, microwave power and microwave time on the fluorescent properties were systematically investigated. The experimental results indicated that the optimized conditions were: the amounts of AA and urea were 0.0871 g and 0.3604 g, the microwave power was 700 W and the microwave time was 5 min. Then, the products were isolated by reverse phase silica gel using 10%（v/v） of methanol as the eluant, by which their quantum yield could reach as high as44%. Further characterization demonstrated that the resulting product possessed good dispersity and optical stability, and excellent biocompatibility. Compared with CDs emitting blue fluorescence, the as-prepared CDs emitting yellow fluorescence allowed to minimize the fluorescence interference arisen from biological tissues, and accordingly, the as-prepared CDs were successfully applied for cellular bioimaging of adherent SMMC-7221 hepatoma cells.Chapter four: To further investigate the influences of precursors on fluorescence properties,an aromatic amine 1,2,4-triaminebenzene dihydrochloride with conjugated structure was chosen as the co-doping reagent to obtain water-soluble, AA derived red fluorescent CDs via a facile one-step solid phase synthesis under argon protection. Subsequently, a preliminary study was performed to characterise its fluorescence properties. In addition, the changing colour and the ever-changing fluorescence intensity of AA&TBD aqueous solution were observed inexperiments, which took place when the pH of aqueous solution of the red fluorescent carbon dots changed from 2 to 12. Hence, it is found that the as-prepared red fluorescent carbon dots have potential applications in both fluorescence and colorimetric pH probes, which can be also applied in cellular bioimaging.