Structural Performance Of Functional Carbon Dots Prepared Via Biomass Hydrothermal/Solvothermal Carbonization

Compared with metal quantum dots,CDs as a new type of carbon nanomaterial,have the advantages of excellent water solubility,high photostability,low toxicity,easy functionalization and great biocompatibility.In this study,a series of functionalized modified CDs with tunable optical properties were prepared by a hydrothermal/solvothermal carbonization method using biomass molecules as raw materials.The optical properties of CDs were optimized for achieving high fluorescence quantum yield,multicolor luminescence,solid-state and phosphorescence emission,and the corresponding luminescence mechanism was also explored in detail.A series of new optical composite materials were prepared with CDs as functional components,revealing the mechanism of interaction between the various components of the composites and their influence on the properties of the materials.The applications of functionalized modified CDs in analytical detection,cellular imaging and white light emitting diodes were optimized,and the influence mechanism of the composition,structure and properties of modified CDs on its applications was explored.The current work can be divided into the following six aspects:NCDs were prepared by hydrothermal carbonization using citric acid as carbon source and p-phenylenediamine as nitrogen source.The surface morphology,crystal structure,elemental composition and optical properties of NCDs were characterized by means of TEM,AFM,FT-IR,XPS,fluorescence and UV-Vis absorption spectroscopy.NCDs possessed graphitized carbon core structure,the lattice spacing was 0.21 nm,the average particle size and height were 3.67 nm and 1.13 nm,respectively.The N content was as high as 17.09%,and the surface C=N,C-N-C,N-（C）₃ and H-N-（C）₂ exist in the form of N-containing functional groups.The fluorescence had the characteristics of excitation wavelength-and p H-dependence.Based on the fluorescence inner filter effect and static quenching mechanism,the fluorescence of NCDs was selectively quenched by NO₂^-,with a linear detection range of 0.02～40μM and a LOD of 21.2 n M.NCDs showed low cytotoxicity and were used for multicolour fluorescence imaging of Hep G2 cells.Borax was added into the reaction system for the preparation of NCDs to obtain NB-CQDs.NB-CQDs are quasi-spherical nanoparticles with graphitized carbon core structure,the average particle size and lattice spacing were 3.53 nm and 0.21 nm,respectively.The surface possessed N-and B-containing groups such as C=N,C-N-C,N-（C）₃,H-N-（C）₂,B-O and B-C.The fluorescence emission was dependent on excitation wavelength and p H,and the emission intensity and maximum wavelength were affected by the types and structures of the surface groups.Based on the fluorescence inner filter effect and the acetone solvent effect,the fluorescence quenching and fluorescence enhancement detection methods for dopamine and acetone were constructed used NB-CQDs,respectively.NB-CQD/polyvinyl alcohol composite films were prepared for fluorescent visual monitoring of acetone gas.The linear detection range of NB-CQDs for dopamine was 0.1～70μM,and the LOD is 11 n M.The linear detection range for acetone was 1～200μM,and the LOD is 0.54μM.Cu-NCDs were prepared by solvothermal carbonization using folic acid as carbon and nitrogen sources.The obtained Cu-NCDs were spherical particles with an average particle size of 3.57 nm,a graphitic carbon core structure and a lattice spacing of 0.21 nm.N-and Cu-containing groups such as C=N,C-N-C,N-（C）₃,H-N-（C）₂ and N-Cu-N had successfully carried out element doping and surface passivation of CDs.The Cu doping effectively improved the photoluminescence properties of Cu-NCDs.Under the excitation wavelength of340 nm,there were dual emission peaks at 410 nm and 470 nm,and the fluorescence emission center at 410 nm might originate from the doping and coordination of Cu,which changed the surface functional groups and carbon core structure network of CDs.Based on the non-redox process and static quenching mechanism,Cu-NCDs were used for the detection of trace ascorbic acid,with a linear detection range of 0.02～40μM and a LOD of 17.8 n M.Using glucosamine as carbon source and nitrogen source,3-[2-（2-aminoethylamino）ethylamino]propyl-trimethoxysilane as organosilane-functionalized reagent,and acetone as reaction solvent,Si-CQDs with solid-state fluorescence emission were prepared by solvothermal carbonization method.The Si-CQDs had an average particle size of 3.66 nm,a graphite-like carbon core structure and a lattice spacing of 0.21 nm.The Si element was modified on the surface of CDs by means of C=N,C-N,Si-O-C and Si-O-Si groups.The optical properties showed that the Si-CQD acetone solution emitted blue fluorescence at 460nm under the wavelength excitation of 380 nm,and had a concentration-dependent fluorescence emission phenomenon,that was,with the increasing of Si-CQDs concentration,the fluorescence emission peak red-shifted.Solid-state Si-CQDs emitted yellow fluorescence at595 nm under the excitation wavelength of 470 nm,and the emission also possessed excitation wavelength-and p H-dependent characteristics.Based on the fluorescence inner filter effect,the fluorescence of Si-CQDs was effectively quenched by Cr⁶⁺with a linear detection range of0.4～160μM and a LOD of 34 n M.Through the redox reaction between ascorbic acid and Cr⁶⁺,the quenched fluorescence of the Si-CQD/Cr⁶⁺hybrid system was recovered by ascorbic acid.A detection method for ascorbic acid was established,the linear detection range was 1～80μM,and the LOD was 84.6 n M.Si-CQDs showed good thermal stability and self-crosslinking ability,and were directly used as solid encapsulation films on blue-emitting In Ga N chips for constructing white light emitting diodes.One-step hydrothermal carbonization of glucose and HPW yielded CQD/HPW.The CQD/HPW was a monodisperse quasi-spherical structure with an average particle size of 1.7nm,graphitic carbon core structure and a lattice spacing of 0.21 nm.The effects of reaction temperature and glucose/HPW ratio on the physicochemical properties,especially the optical properties of CQD/HPW were investigated.High temperature promotes the fluorescence emission of CQD/HPW,but too high reaction temperature（>210°C）reduced the fluorescence intensity.With the increasing of HPW content,the fluorescence emission intensity of CQD/HPW decreased gradually.The fluorescence quantum yield for CQD/HPW was calculated to be 6.8%.Fluorescence emission was excitation wavelength-and p H-responsive.The binding mode and possible formation mechanism of CDs and HPW in CQD/HPW were analyzed:HPW was bonded to the surface of CDs through physical interaction（π-πconjugation）and the formation of stable chemical bonds（W-O-C）through hydrothermal processes.HPW promoted the dehydration and carbonization processes in the hydrothermal reaction,and act as an“electron acceptor”,effectively blocking the radiative recombination of electron/hole pairs on the surface of CDs,resulting in a weaker fluorescence intensity of CQD/HPW than that of uncomplexed CDs.Based on the fluorescence inner filter effect,the fluorescence of CQD/HPW was selectively quenched by Cr⁶⁺.Compared with uncomplexed CDs,the good electron-gathering ability of HPW effectively hindered the transfer of electrons on the surface of CQD/HPW to other metal ions,which greatly improved the detection sensitivity and selectivity of Cr⁶⁺.The sterilization efficiency was 98%.The linear detection range of Cr⁶⁺was 2～80μM,and the LOD was 0.16μM.The quenched fluorescence of this system was recovered by ascorbic acid,showing good cycling performance.Using lignin as the carbon source,o-phenylenediamine as the nitrogen source,formamide and ethanol as reaction solvent,blue-emitting CDs and yellow-emitting CDs were prepared by solvothermal carbonization under the same conditions.The differences in morphology,structure and optical properties of the two kinds of CDs were compared and analyzed,and the fluorescence source was inferred according to the change of the optical properties of CDs with the group composition,and the fluorescence regulation scheme was found.The results showed that both CDs were monodisperse quasi-spherical structures with graphitized carbon cores and contain-COOH/C=O,C-O-C,C=N,C-N-C,N-（C）₃ and H-N-（C）₂ groups on the surface,the lattice spacing is 0.21 nm.The particle size of yellow-emitting YCDs（11.59 nm）was larger than that of blue-emitting CDs（8.09 nm）and the N content was lower than that of blue-emitting CDs,indicating that the increase in the size of the sp² hybrid conjugated domain and the decreasing in the N content resulted in a red-shift of fluorescence.Encapsulating blue/yellow-emitting CDs and polyvinyl alcohol into delignified wood to construct LTW with green/red RTP emission.The RTP of LTW originates from the C=O/N groups in CDs,and the increasing of graphitization degree and the decreasing of N content led to the red shift of RTP.The LTW was used for real-time and visual monitoring of indoor formaldehyde pollution,achieving color-responsive ratiometric fluorescence and delayed RTP detection of formaldehyde in the range of 20～1500μM（LOD=1.08 n M）and 20～2000 m M（LOD=45.8n M）,respectively.