Photoluminescence Tuning And Mechanisms Research Of Carbon Dots

The development of nanomaterial science has greatly influenced human life styles.In recent years,carbon-based nanomaterials have attracted the attention of many scientists because of their numerous precursor sources and excellent mechanical,optical and electrical properties.Among many carbon nanomaterials,carbon dots(CDs)are regarded as another epochal carbon-based nanomaterials after fullerenes,carbon nanotubes and graphene.CDs have a wide range of applications in drug delivery,bioimaging,fluorescence sensing,optoelectronic devices,photocatalysis,electrocatalysis and information encryption due to attractive physical and chemical properties,including ultra-small size,abundant functional groups,chemical stability,biocompatibility and low toxicity.After more than a decade of joint efforts by “CDs’ researchers”,a lot of progress has been achieved in CDs-relevant fields.However,researchers are still facing many challenges:(1)Compared with the development trend of CDs,the luminescence mechanism of CDs is still unclear and controversial due to their diverse and complex structure;(2)To solve the problem of aggregation-caused quenching(ACQ),the usual way is to disperse CDs in a suitable solid matrix.It is generally think that solid matrix acts as solvent,but the interactions between CDs and matrix are often neglected;(3)The research on dynamic regulation of CDs fluorescence is very limited,which limits the exploration of CDs luminescence mechanism and its application;(4)Up to now,the optical research of CDs focuses on fluorescence regulation,but there are few researches on the realization and regulation of the optical properties of long afterglow,which limits the the application of CDs.In view of the above problems,on the basis of predecessors’ research,this thesis focuses on the preparation process and microstructure of CDs for adjusting their excitationwavelength dependence/independence properties and the preparation of dual-emission CDs.The interaction between CDs and composite matrix,the dynamic fluorescence regulation of CDs and the optical long afterglow regulation of CDs were explored,and the luminescence mechanism was also explored.The main contents of this thesis are concluded as follows:(1)The classification,preparation,luminescence mechanism and application of CDs were summarized.The luminescence property control and mechanism of CDs are summarized in detail.(2)Excitation-wavelength independent/dependent fluorescence control by polymer structures contained in carbonized polymer dots(CPDs).Three kinds of carbonized polymer dots(CPDs-1-3)with excitation-wavelength independence and two kinds of carbonized polymer dots(CPDs-4-5)with excitation-wavelength dependence were prepared by a simple solvothermal method.The structural characterization and optical properties showed that the homogeneous polymer structure contained in the CPDs is the main source of excitation-wavelength independent fluorescence,and the random copolymer was the main source of excitation-wavelength dependent fluorescence.In addition,the choice of precursor and reaction temperature has an important influence on the synthesis of the excitation-wavelength independent/dependent CPDs.This study demonstrates the luminescence mechanism of CPDs with excitation-wavelength independence or dependence,and provides a new perspective for the design of new type of CPDs.(3)Preparation and luminescence regulation of dual-emission CDs with solvent-dependence.On the basis of the above research,using p-phenylenediamine and folic acid as precursors,a kind of nitrogen-doped carbon dots(N-CDs)with dual-emission and solvent dependence was designed and synthesized by simple hydrothermal method,and it was used to detect the polarity of mixed solvent.The structural characterization and optical properties show that the random copolymer formed during the reaction process as fluorophore is responsible for the excitation-wavelength dependence of the first emission band of N-CDs;The homogenous polymer chain formed during the reaction as fluorophore is responsible for the excitation-wavelength independence of the second emission band of N-CDs.In addition,the second emission band of N-CDs has a significant response to the polarity of the solvent.Based on this unique optical property,the fluorescence detection of the mixed solvent is realized.This study provides a good basis for studying the fluorescence origin of dualemission CDs.(4)Tuning fluorescence emission behaviors by the interactions between CDs and matrix.Using citric acid and melamine as precursors,the carbon dots hydrogen bond self-assembled aggregates(HB-CDs)composite fluorescent materials were directly constructed by a simple solvothermal method.HB-CDs has the characteristic of “dual-mode fluorescence”,and the molecular state constructed by hydrogen bond network is the main source of yellow fluorescence of HB-CDs aggregates;On the contrary,when HB-CDs are in a dispersed state due to hydrogen bond fracture,various core states generated by high temperature carbonization are the main source of fluorescence of HB-CDs.This study provides a new perspective for studying the interaction between carbon dots and solid matrix.(5)Preparation and luminescence regulation of smart CDs.Smart carbon dots(M-CDs)with room temperature phosphorescence,time-dependent fluorescence and near-infrared emission was prepared by a simple solvent-free catalytic assisted strategy using o-phenylenediamine as precursor and aluminum chloride as catalyst.In the solid-state,the polymer-liked structures with amino,alongside the enhanced emission effect of hydrogen bonds crosslink were confirmed to both contribute to the generation of fluorescence and room temperature phosphorescence of M-CDs.In the solution-state,hydrogen bonding interaction can act as a tool to manipulate nanostructure of CDs,the unexpected full-color evolution resulting from adaptive expression of M-CDs as time goes on.This study has stimulated research on the dynamic fluorescence of carbon dots.(6)Preparation and regulation of organic solvent CDs-based long afterglow materials.Using ethylenediamine(EDA)and ethanolamine(DEA)as carbon sources and cyanic acid(CA)as matrix,the carbon dots@crystal composites(EDA-CDs@CA with green afterglow and DEA-CDs@CA with blue afterglow)were prepared by simple hydrothermal reaction.This study provides a new perspective for the preparation of carbon dots materials with tunable afterglow.