Surface Modification With Small Organic Molecules And Nitrogen Doping Of Carbon Quantum Dots And Their Applications In Bioimaging And Sensing

Carbon quantum dots as new promising carbon materials have great prospective applications in photovoltaic devices and photocatalysis, in particular in bioimaging, due to their excellent properties, which include unusual fluorescence, low toxicity, good biocompatibility, and strong resistance to photodegradation and bleaching. Especially, the luminescence properties have been receiving growing research interest.There are still some obstacles which hinder their practical applications in bioimaging, such as relatively low luminescence quantum yields, shifting fluorescence emissions and unclear fluorescence mechanism. Research shows that chemical modification of carbon quantum dots can not only alter their dispersibility, but also improve their quantum yields and effectively tune their photoluminescence. Due to the unique electronic and structural properties of carbon quantum dots, surface modification and elemental doping can be used to enhance the solubility and optical performance, the resulting fluorescence are more stable and durable compared to ordinary fluorescent molecules. Therefore, it is an urgent need for systematic study in this area. The functionalization of carbon quantum dots is, essential to expand the applications of carbon materials in bioimaging and sensors, since it can modulate and enhanced luminescence, and afford a series of useful intermediates or act as precursors for many subsequent organic chemical reaction and applications.In this dissertation, we systematically investigated surface chemistry of graphene quantum dots functionalized with a series of small organic molecules combining experimental and theoretical approaches. We report a one-pot solvothermal synthesis of N-doped carbon quantum dots, the as-prepared NCQDs were compared with carbon quantum dots, it was found that NCQDs can be used to detect pH values, metal ions and H2O2. The main contents are as follows:1. We synthesized a series of carbon quantum dots functionalized by different small organic molecules. Experimental results indicated that surface functionalization with functional groups such as alcohol, amine and thiol can effectively tune the fluorescence of carbon quantum dots, the emission efficiency of1,2-ethylenediamine functionalized carbon quantum dots reached up to17.6%. We employed theoretical calculations to explore the fluorescence enhancement mechanism. Cellular toxicity tests and bioimaging experiment demonstrated that functionalized carbon quantum dots showed low toxicity, comparable to pristine carbon quantum dots, but revealed a much better bioimaging performance than pristine carbon quantum dots.2. We have reported the one-pot hydrothermal synthesis of NCQDs from different nitrogen sources and synthesized a series of NCQDs with different nitrogen contents. The introduction of nitrogen atoms largely increased the quantum yield of NCQDs and highest emission efficiency is up to36.3%. The fluorescence enhancement may originate from more polyaromatic structures induced by incorporated nitrogen atoms and protonation of nitrogen atoms on dots. It was found that NCQDs can act as a multifunctional fluorescence sensing platform because they can be used to detect pH values, Ag(Ⅰ), Fe(Ⅲ) and H2O2in aqueous solution. Cellular toxicity test showed NCQDs still retain low toxicity to cells despite the introduction of a great deal of nitrogen atoms. Moreover, bioimaging experiments demonstrated that NCQDs have stronger resistance to photobleaching than CQDs and more excellent fluorescence labeling performance.