Study On The Preparation And Application Of Carbon Dots(CDs) And Their Carbon-nitrogen Complexes

Carbon dot（CDs）is an emerging fluorescent nanomaterial.It has many advantages such as abundant raw materials,non-toxic and environmentally friendly,simple preparation,adjustable fluorescent color,and good biocompatibility.It has received more and more attention in the fields of fluorescence detection,photocatalysis,bioimaging,and anti-counterfeiting.Because of its ultra-small size and good biocompatibility,pure CDs have unique advantages in the fields of bioimaging and biotherapy.What’s more,the abundant functional groups on the surface of CDs determine that it is extremely easy to compound with a variety of materials.The application of materials loaded with CDs in the fields of fluorescence detection,photocatalysis and anti-counterfeiting will be more diverse.Due to the complex and diverse luminescence mechanisms of CDs,there is no unified conclusion yet,and there is still much room for exploration in the preparation and application of CDs and their composites.Therefore,we focuses on selecting different raw materials,selecting synthesis methods,focusing on the preparation process of CDs and their compound with g-C₃N₄,developing diversified applications of pure CDs,and realizing the control of the morphology of g-C₃N₄ by CDs,exploring the relationship between the composition,structure and luminescence properties of CDs and their complexes.The results show that CDs and their complexes have broad application prospects in the fields of fluorescence detection,photocatalysis,anti-counterfeiting and biological imaging.The main works content of our paper have the following aspects:1.Full-spectrum nitrogen-doped carbon dots（NCDs）are simply prepared by a one-step sintering method.The NCDs show typical excitation wavelength dependence,are sensitive to temperature and p H,have low cytotoxicity and excellent water solubility.Using these excellent properties,we have developed the application of NCDs in inkjet printing for multiple anti-counterfeiting and cell imaging.2.CDs prepared with sodium alginate and urea as precursors.The CDs retain the main functional groups of sodium alginate and are highly sensitive to heavy metal ions.They can not only be used for the fluorescence detection of heavy metal ions in water,but also can also form hydrogels with a large amount of heavy metal ions in the water to realize the removal of heavy metal ions.This is the first report that pure CDs can be used to remove heavy metal ions from water.The removal form of hydrogel is novel and easy to extract.Therefore,we comprehensively developed CDs to detect and remove heavy metal ions.3.The red NCDs with independent excitation wavelength are synthesized by hydrothermal method,g-C₃N₄ is prepared by classical thermal sintering method,and NCDs@g-C₃N₄ composite is synthesized by secondary hydrothermal method.Compared with pure g-C₃N₄ which has a complete two-dimensional lamella structure,the NCDs@g-C₃N₄ composite has rich pore structure.This is the first report that NCDs have a similar pore-forming effect.In addition to broadening the spectral response range of g-C₃N₄ and reducing the electron-hole recombination rate of g-C₃N₄,NCDs can also make the NCDs@g-C₃N₄ complex have the pore structure.We have studied the pore-forming mechanism of NCDs,and developed the application of NCDs,g-C₃N₄ and NCDs@g-C₃N₄ in the field of photocatalytic degradation of pollutants and fluorescent anti-counterfeiting.4.A two-step method was used to synthesize positively charged CDs with abundant amino groups and compound with negatively charged g-C₃N₄ to prepare ultra-light NCDs/g-C₃N₄ aerogels with abundant pores and large specific surface area.Due to the effects of electrostatic attraction and hydrogen bonding,the addition of positively charged CDs triggers the self-assembly of negatively charged g-C₃N₄.NCDs/g-C₃N₄ with excellent hydrogen production properties are formed in a pure aqueous solution without a binder.The photocatalytic hydrogen production performance of NCDs/g-C₃N₄ aerogel is as high as 53993μmol g^-1,and the hydrogen production rate is 13499μmol h^-1 g^-1.Compared with pure g-C₃N₄,the hydrogen production performance has been greatly improved.Among the currently reported hydrogen production materials,the properties are also extremely high.In short,this article uses CDs as the research object.For pure CDs,the CDs are rich in surface states and sensitive to the environment.The application of CDs in the field of multiple anti-counterfeiting and multiple bioimaging is developed.For CDs and heavy metal ions,the properties that can be coordinated to form bonds have developed the application of CDs for the fluorescence detection and removal of heavy metal ions.For carbon-nitrogen complexes of CDs,the spectrum of CDs can be flexibly adjusted to make up for the shortcomings of the narrow spectrum utilization range of g-C₃N₄.The photo-induced electron transfer properties of CDs are used to effectively reduce the hole recombination rate of photogenerated electron-vacancy of g-C₃N₄.Use the abundant functional groups on the surface of the CDs to control the morphology of g-C₃N₄.This is also the highlight of our work on the complex of CDs and g-C₃N₄ in this paper.Morphology control,prepared porous NCDs@g-C₃N₄ composite and NCDs/g-C₃N₄ aerogel,two carbon-nitrogen composites with unique morphologies,providing new ideas for morphology optimization of two-dimensional materials and development Its application in the photodegradation of pollutants and photocatalytic hydrogen production fills the gap of CDs morphology control in the field of photocatalysis.