Preparation And Optical Properties Of Tunable Luminescent Carbon Dots And Their Composites

Carbon dots（CDs）are a new type of carbon-based luminescent nanomaterials with adjustable size（usually less than 20 nm）,which are mainly composed of carbon cores and connected surface groups.Compared with other luminescent nanoparticles,CDs have the characteristics of low preparation cost,high photostability,easy surface functionalization,good biocompatibility and tunable photoluminescence,etc.,which make them have great application value in biomedicine,fluorescence sensing,catalysis,optoelectronic equipment and anti-counterfeiting.Since the discovery of CDs in 2004,the research on CDs with monochromatic fluorescence（especially blue and green）has become more and more mature.In order to broaden the application of CDs,the recent research attentions have gradually shifted to the full-spectrum luminescence and chiroptical properties of CDs-based materials.In addition,aggregation quenching between CDs in solid state is easy to occur,and the intrinsic chemical bond vibration of CDs will cause energy consumption.To avoid these problems,the“matrix-assisted method”can be used to construct CDs-based composites,in which the suitable matrix can effectively immobilize CDs,inhibit their intramolecular vibration and rotation,and prevent oxygen quenching,thereby improving luminescence properties of CDs.Fluorescence is one of the main property of CDs.Most CDs have high-efficiency blue luminescence but low-efficiency in the long wavelength emission.Multicolor luminescent CDs have attracted much attention due to their excellent photoluminescence,and unique electronic and physicochemical properties.The luminescent properties of CDs are mainly related to the types of carbon sources and synthesis methods.At present,most of the precursors for the preparation of CDs are commercially available organic small molecules.In recent years,the use of biomass materials and other carbon-containing wastes instead of organic small molecules to prepare CDs has attracted researchers’attention,but there are few reports on fluorescence regulation based on these carbon sources.On the other hand,the hydrothermal and solvothermal methods commonly used in the preparation of CDs are suffered from cumbersome preparation process,difficult purification,and large amount of waste liquid.Therefore,the development of simple and convenient method is of great significance for the preparation of tunable luminescent CDs.Chiral CDs not only retain the luminescence properties of ordinary CDs,but also have unique chirality.Generally,the chirality of CDs is derived from the chiral carbon sources and the asymmetry induced by the chiral environment.However,due to the limitations of chiral carbon sources and the easy racemization of chiral structure at high temperature,the synthesis of chiral CDs is difficult.Moreover,most chiral CDs exhibit a single blue or green fluorescence emission,and present only ground-state chirality（circular dichroism）while not excited state chirality,thus cannot produce circularly polarized luminescence（CPL）.Therefore,researchers usually assemble CDs or chiral CDs with strong chiral matrices（i.e.chiral gels,chiral pore materials,chiral supramolecular materials,etc.）to prepare CDs-based composites with circular polarized luminescence.However,there are still few reports on CDs and their composites with multicolor circular polarized luminescence.This thesis mainly focuses on the design and preparation of CDs with tunable luminescence and optical activity.On the basis of“matrix-assisted”strategy,the tunable fluorescence and chirality in the excited state of CDs have been achieved by modulating the carbon source,reaction conditions,solvent composition and heteroatom doping.The relationship between the luminescent properties of CDs and their composition and structure has been established.The main research contents of this thesis are as follows:1.A facile one-step pyrolysis method is used to prepare CDs@zeolite composites with tunable fluorescence and room temperature phosphorescence（RTP）from the catalytically deactivated SAPO-34 zeolite as a single carbon source.Design idea:starting from the SAPO-34 zeolite deactivated from methanol to olefin reaction,the carbon species in the deactivated catalyst are carbonized to form CDs by calcination,which are confined in the SAPO-34 zeolite matrix.The effects of different calcination conditions（temperature and time）on the luminescence properties of the composites were studied,and the influences of CDs content,hetero-oxygen functional groups and surface oxidation state on the fluorescence emission wavelength of CDs were also studied.As a result,CDs@zeolite composites with adjustable fluorescence（574-500nm）and stable green RTP were prepared,which could be applied to the template of information encryption to achieve time-resolved pattern and digital encryption.This work provides a new direction for the reuse of industrially deactivated zeolite catalysts,which also provides a simple and green method for the preparation of multicolor CDs-based composites.2.A mild hydrothermal method is used to prepare novel chiral CDs from abundant amino acid enantiomers.Design idea:the amino acid enantiomers were used as both a nitrogen-containing carbon sources and an abundant chiral precursor.Under mild hydrothermal conditions,the reactions of dehydration,polymerization and carbonization occur between chiral amino acids and citric acid to produce blue-emitting luminescent CDs,meanwhile,the precursor chirality is retained on the surface structure of CDs.The effect of hydrothermal reaction time and acid-base environment on the luminescent properties of the chiral CDs was investigated.The local structure of chiral CDs was optimized and their electronic circular dichroism spectrum was simulated by theoretical calculation to elucidate the chiral origin of CDs.The relationship between the concentration of chiral CDs and circular dichroism signal was further explored.In addition,the prepared LGln-CDs and DGln-CDs exhibited high sensitivity and selectivity fluorescence detection for Fe³⁺metal ions.This work provides a universal and simple synthesis method for the preparation of CDs with chiral optical properties.3.A“matrix-assisted”strategy is used to prepare CDs@cellulose composite films with high luminescence asymmetric factor and multicolor CPL.Design idea:multicolor chiral CDs are prepared by selection of reaction raw materials（o-phenylenediamine with benzene ring structure and L-proline with five-membered ring structure）,modulation of the solvent composition,p H environment,reaction temperature and element doping.Based on the doping of N and P heteroatoms,the fluorescence quantum yields of multicolor chiral CDs have been improved.By combining these chiral CDs with cellulose nanocrystals（CNC）with different forbidden band positions,and assisting the various interactions between the host and guest,the multicolor CDs@cellulose composite films with high luminescence asymmetric factor are obtained.The composite films with stable left-handed chiral nematic structure and multicolor luminescence are expected to have potential for visualization of circular polarization detection,information encryption and anti-counterfeitings.