Mechanistic Study Of Improving The Photothermal Performance Of Carbon Nanodots Based On Sulfur Doping And Assembly Effect

Carbon nanodots,as a new type of carbon-based nanomaterial,have advantages such as strong water solubility,good dispersibility,good biocompatibility,and easy functionalization modification.Therefore,they have been widely used in various biomedical fields.However,as a photothermal conversion material,carbon nanodots exhibit low photothermal conversion performance.Currently,the two main methods for effectively improving the photothermal conversion performance of carbon nanodots include:(1)Element doping during the synthesis of carbon nanodots.Element doping is a method of introducing heteroatoms by generating n-type or p-type carriers to change the electronic structure.Since various metal elements cannot be metabolized normally in the body,non-metal elements are usually selected for doping carbon nanodots.In element doping experiments,the solvent thermal method is the simplest,most effective,and direct doping method,which has been widely used.However,the mechanism of solvent in changing the photothermal conversion performance of carbon nanodots has not been reported yet and is also a scientific problem that needs to be urgently solved.(2)Carbon nanodots can also enhance the photothermal conversion efficiency through assembly effects.Studies have shown that the energy dissipation mode before and after the assembly of carbon nanodots changes,which significantly enhances the photothermal conversion performance of the assembled body compared to the free carbon nanodots.Among the assembly methods,non-covalent bonds are widely used due to their high efficiency and variety of types.However,how to develop an effective set of experimental and analytical methods to elucidate the assembly mechanism of noncovalent bonded assemblies is another scientific problem that we urgently need to solve.In order to solve the above two scientific problems,we conducted corresponding studies on sulfur-doped carbon nanodots and hydrogen-bonded assemblies of carbon nanodots,respectively.In Chapter 2,we took sulfur-doped carbon nanodots as an example and provided a set of experimental and analytical methods to elucidate the specific role of solvents in element doping by regulating the solvent ratio.In this study,we synthesized sulfurdoped carbon nanodots using a one-step solvent thermal method and analyzed and discussed the regulatory role of different solvent ratios on the optical properties of carbon nanodots.We used transmission electron microscopy,zeta potential,dynamic light scattering,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,UV-visible spectroscopy,and fluorescence spectroscopy to discuss the differences between sulfur-doped and non-doped carbon nanodots in particle size,surface charge,composition elements,UV-visible spectrum,and fluorescence spectrum.The research results showed that when the ratio of formamide to dimethyl sulfoxide reached 5:6,the near-infrared absorption performance and photothermal conversion efficiency of carbon nanodots were the best(51.36%).Further in vitro and in vivo antibacterial experiments also proved that the sulfur-doped carbon nanodots made by the above ratio had good photothermal sterilization performance.The above analytical methods can be extended to other element doping systems and also lay a solid foundation for the extensive application of element doping systems.In Chapter 3,we used the hydrogen-bonded assembly of carbon nanodots as an example.By changing the molecular structure,p H value,ion strength,concentration,and environmental temperature of five different buffer solutions,we observed changes in the color of the assembly in solution,UV-Vis spectra,and the absorbance at A680,and analyzed the disassembly process and assembly mechanism.Studies have shown that for the hydrochloric acid-urea(HCl-urea)system,the assembly structure is stabilized by two types of hydrogen bonds.The first is the hydrogen bond between the carboxyl groups of citric acid carbon nanodots,which stabilizes the internal structure of the assembly.The second is the hydrogen bond between the amino group of urea and the carboxyl group of citric acid carbon nanodots,which stabilizes the external structure of the assembly.This work not only elucidates the hydrogen-bonded assembly mechanism of carbon nanodots but also provides a new set of experimental and analytical methods for deconstructing assemblies,which can be applied to other assembly systems and lays a solid theoretical foundation for their widespread application.In summary,we provide solutions to two problems related to improving the photothermal conversion efficiency of carbon nanodots in this study,using carbon nanodots doped with sulfur and the hydrogen-bonded assembly of carbon nanodots as examples.Firstly,by adjusting the solvent ratio,we provide a set of experimental and analytical methods to elucidate the specific role of the solvent in element doping.Secondly,by changing the buffer solution parameters and analyzing the changes in the optical properties of the assembly,we provide a new set of experimental and analytical methods to systematically elucidate the disassembly process and assembly mechanism of the assembly.The above two works lay a good theoretical foundation for improving the photo-thermal conversion performance of carbon nanodots.