Microwave-assisted Synthesis Of Two Kinds Of Carbon Dots And Their Properties

In recent years,as a new kind of carbon nanomaterial,carbon dots（CDs）have attracted attention from various fields because of their excellent optical properties,water solubility,biocompatibility,chemical and physical stability,and ease of modification.And due to its great potential in many aspects,such as biomedicine,catalysis,optoelectronics,etc.,it is always interesting to explore and improve the synthesis of carbon dots.However,structures of most CDs cannot be obtained because of their raw materials and synthetic methods.Thus,it is difficult to know the exact photoluminescence（PL）mechanism of the carbon dots,which makes it hard to improve the quantum yield of carbon dots.Meanwhile,although various methods for synthesizing multi-color fluorescent carbon dots have been proposed,most of them are either time-consuming or involved in hard conditions,which makes it hard to apply on large scale.In this paper,through microwave-assisted method,a new kind of blue emitting carbon dots（A-CDs）was fast fabricated,using L-asparagine as raw material.With the combination of the simple and known structure of starting material and a series of characterizations,the possible structure of A-CDs was proposed.Then,combining with the possible structure and lifetime result of A-CDs,the PL origin of A-CDs was from the the graphite-like structure and surface defects.Finally,we found that A-CDs with excellent biocompatibility had potential applications in long-term cell imaging.Further,in order to obtain carbon dots with a longer emission wavelength by microwave method,another carbon dots（F-CDs）was synthesized with D-fructose and NH₄HCO₃.F-CDs had concentration-dependent multicolor emission,which meant with the concentration increasing of F-CDs,the maximum fluorescence emission wavelength of F-CDs was red-shifted.And when the concentration of F-CDs increased from 20μg/mL to 2 mg/mL,the maximum emission wavelength was red-shifted from 404nm to 583nm,covering the blue-violet region to the orange-red region.Characterizations were carried to find that the F-CDs consist of a graphite-like core and abundant surface functional groups.What’s more,in combination with TEM,the aggregation process of F-CDs in aqueous solution was discussed.Then,taking the lifetime study into account,it could be concluded that the arising of multi levels and the increasing of the sp² domains mainly caused the red shift of the optimal emission wavelength of F-CDs with the increasing of concentration.At last,we successfully applied F-CDs to multi-color fluorescent inks and multi-channel,multi-color cell imaging.This thesis is divided into the following four chapters.Chapter 1:This chapter mainly introduces the physical and chemical properties,PL mechanism,synthesis methods of carbon dots,and their applications,especially in biomedical field.Chapter 2:Nitrogen-doped blue emitting carbon dots（A-CDs）were rapidly synthesized by microwave method within 15 minutes using L-asparagine as a precursor.The A-CDs were monodispersed,quasi-spherical,and had an average size of 2.17±0.70nm.Also,A-CDs showed a distinct lattice structure,indicating that small molecules without sp² carbon could form carbon dots with a graphite-like core,through microwave method.With a series of characterizations,the possible structure of A-CDs was put forward.By analyzing the proposed structure and the lifetime,the PL origin of A-CDs was from the graphite-like core and the surface defects.Moreover,A-CDs had good fluorescence stability,and especially exhibited high PL stability in high ionic strengths.Besides,A-CDs had low cytotoxicity and good biocompatibility.With the incubation of 800μg/mL A-CDs for 24 h,there was no significant effect on cell morphology of He La cells,And A-CDs still emitted bright blue light in cytoplasm,which meant the A-CDs’promising for long-time bioimaging.Chapter 3:In order to obtain a new carbon dots with a longer emission wavelength,in this chapter,we synthesized a multicolor emission carbon dots（F-CDs）using D-fructose and NH₄HCO₃ as raw materials.Varying the concentration,the optimal emission wavelength of F-CDs could be adjusted from 404nm to 583nm,covering nearly the entire visible region.In addition,the sp² carbon core of F-CDs and abundant surface functional groups provided a basis for concentration-dependent multicolor emission.TEM results showed concentration-induced F-CDs aggregation process.Combined with lifetime results,F-CDs concentration-dependent multicolor emission was associated with the appearance of new multi levels and increased sp²domains.As for the alleviation of the fluorescence quenching caused by self-absorption,it might be attributed to the reason similar to the crosslinked enhanced emission（CEE）effect.In the meantime,F-CDs exhibited excellent fluorescence stability and good biocompatibility,and has advantages in multi-color fluorescent inks and multi-channel,multi-color fine imaging.Chapter 4:Conclusions of this work and perspective for future works.