Preparation Of Multicolor Emissive Carbon Dots And Its Applications In Cell-imaging And LEDs

Since carbon dots（CDs）were obtained for the first time by purifying the mixed debris of fluorescent single-walled carbon nanotube in 2004,various CDs with diverse sizes,morphologies,microstructures,and properties have been developed.In comparison with traditional fluorescent dyes or semiconductor quantum dots,CDs own more fascinating merits such as low toxicity,excellent biocompatibility and chemical stability,strong photobleaching resistance,easy to surface modify and outstanding photoelectric properties.So they can be applied in the field of bioimaging,biosensor,chemical sensor,drug/gene delivery,and optoelectronic devices.In recent years,CDs with multicolor fluorescence have attracted more research interest,because they can overcome the limitations of short-wavelength emissive CDs in cell imaging,fluorescent display and LEDs.However,most of reported CDs emitted single blue-green fluorescence and the lack of multicolor obstructed further application of CDs.In addition,unlike semiconductor quantum dots,the fluorescent mechanism of carbon dots is still unclear.And it is still an enormous challenge for tuning CDs’fluorescence.In view of these problems,some researches have been carried out.The concrete contents of our work can be described as follows:1.Three kinds of carbon dots（CDs）with different photoluminescence（PL）（blue,yellow or orange）were synthesized by microwave method using three phenylenediamine isomers as raw material,respectively.They display wavelength-independent excitation wavelengths（in the range from 444 to 574 nm）,similar average particle size（from 3.7 to4.2 nm）,and fluorescent lifetimes（from 2.7 to 3.1 ns）.Color and quantum yields are related to the oxidation degree and the number of N-functional groups on their surface.The CDs are shown to be aviable nanoprobes for multicolor imaging of cancer cells with outstanding imaging effect,low toxicity and the capacity of labelling cell nucleus.Three composite phosphors were obtained by coating the various CDs on starch particle.The resulting nanomaterials emit solid-state fluorescence with a quantum yield≥16%.They were used to the fabricate luminescent blocks with blue,yellow,orange or white fluorescence and light-emitting diodes（LEDs）with controllable color temperature,color rendering index（CRI）>80 and lighting efficiency（LE）>17 lm·W^-1.2.Full-color emissive carbon dots（CDs）were prepared through a solvothermal treatment of indole in EtOH with addition of acid and oxidant（H₂O₂）.Then,the prepared mixture can be efficiently separated through silica gel column chromatography to obtain ten CDs which can emit full-color fluorescence from blue-violet to orange-red with wavelength-independent excitation wavelengths from 333 to 608 nm.The formation of these CDs was ascribed to the synchronous reaction which involves acid cation polymerization and oxidation.Thanks to the single precursor,one-pot reaction condition,these CDs’properties display regular variations,which is beneficial to reveal the emissive essence of full-color fluorescent CDs.Specifically speaking,along with the red-shift,these CDs show the increasing trend in polarity and the oxidation degree,the reducing trend in the optical energy band gap（Eg）,photoluminescence quantum yield and lifetime,clarifing the oxidation causes red-shift in fluorescence.In addition,the increase in the content of graphitic N could cause the degree of graphitization decrease.Based on the all data,it can be speculated that the fluorescent properties depend entirely on the surface state,and the N-doping（i.e.,graphitic N）or graphitization in core state just cause the cores’absorption red-shift.In other words,regardless of which energy levels an excited electron is,it ultimately relaxes into the LUMO band of surface state energy levels and then jump to the HOMO band（ground state）.And the gap of LUMO-HOMO band（Eg）depends on the oxidation degree on CDs surface.Finally,they were used to fabricate LEDs with controllable color temperature and showed the enormous prospect of illumination application.