Preparation,optical Properties And Applications Of Novel Nitrogen Doped Carbon Dots Derived From Nitrogen-containing Heterocyclic Organic Small Molecules

Due to their unique optical properties and application prospects in various fields,carbon dots?CDs?have attracted widespread interest of researchers since they were found in single-walled carbon nanotubes in 2004.CDs have the advantages of simple synthesis and a wide range of raw materials.At the same time,they have excellent fluorescence and phosphorescence properties,photobleaching resistance,low toxicity,biocompatibility,etc.,making them useful in optoelectronic devices,information encryption and decryption,detection,biological imaging and other aspects.Nowadays,due to the needs of production and life,many harmful substances,such as explosives and metal ions,remain in the wastewater,which seriously threat the need for green and sustainable growth of living organisms.In order to achieve long-term monitoring and prevention,chemists have tried using various luminescent substances to detect these harmful substances,but it is difficult to achieve detection in water solution.In general,only a single substance can be detected,and the detection limit is difficult to achieve the ideal situation.Therefore,it is still a great challenge to use a single luminescent material in aqueous solution to achieve high sensitivity detection of a variety of substances at the same time.Metal-free room-temperature phosphorescent?RTP?materials have a wide range of applications in the fields of detection,information encryption,and biological imaging due to their long lifetime,light stability,and biocompatibility.In order to obtain non-toxic,non-metallic RTP materials,the following two problems are generally faced: one is that it is difficult for small organic molecules to achieve inter-system crossing?ISC?from singlet to triplet state;the other is the triplet exciton is easily quenched,and it is also easy to return to the ground state in the form of non-radiative transitions.These two problems severely limit the development of phosphorescent materials.In this thesis,two new nitrogen-doped CDs were synthesized using the nitrogen-containing heterocyclic organic compounds 5-chloromethyl-8-hydroxyquinoline?LQ?and 5-amino-1,10-phenanthroline?Aphen?as the main carbon sources.The water-soluble CDs prepared by LQ and citric acid?CA?have a good coordination effect,and can be used to detect the Al³⁺and trinitrophenol?TNP?in aqueous solution.The CDs synthesized from Aphen and polyvinyl alcohol?PVA?have the fluorescence properties in both solution and solid state and RTP.The prepared CDs can be used in explosive sensing,pH detection,white LED construction and the security of banknote.The specific content includes:?1?One-step solvothermal route was developed to synthesize nitrogen-doped CDs using CA and LQ as raw materials.The prepared CDs have strong blue-white fluorescence emission under the excitation of 405 nm with a fluorescent quantum yield?QY?of 25% and high ion concentration stability.The water-soluble CDs rich in 8-hydroxyquinoline?HQ?groups on the surface can be used to detect Al³⁺by fluorescence enhancement and TNP by fluorescence quenching,with detection limits of 229 nM and 44.4 nM,respectively.The detection mechanism of Al³⁺is that the LQ groups on the surface of CDs can enhance the fluorescence by forming coordination complexes,generating fluorescence synergy and limiting non-radiative transitions of CDs.The detection mechanism of TNP is attributed to the internal filtering effect and static quenching.?2?Nitrogen-doped CDs-1 showing both fluorescence and RTP was synthesized from PVA and Aphen by a one-step hydrothermal method.The CDs-1 emits blue-green fluorescence in aqueous solution,while emits white fluorescence and yellow-green RTP in the solid state with a RTP life up to 103 ms,showing a long afterglow that can be recognized by the naked eye for 3 s.The RTP performance of CDs-1 is derived from C-N/C=N in Aphen and C=O produced during carbonization,which helps to achieve ISC.At the same time,the hydrogen bonds formed between the C=N/CN or C=O bonds and-OH of the not completely carbonized PVA chains on the surface of CDs-1 have the advantages of moisture resistance,oxygen resistance,and increasing the rigidity of CDs-1.Based on the fluorescence characteristics of CDs-1,their aqueous solution can be used to selectively detect TNP with a detection limit of 153 nM,and at the same time the fluorescence is sensitive to pH value with a good linear response between acidic and alkaline,respectively.In addition,solid-state CDs-1 can be used to build white LED and applied to banknote anti-counterfeiting material under long excitation wavelength.In summary,this work provides a new idea for one-step synthesis of multifunctional CDs.