Confined Synthesis And Meechanism Study Of Multicolor Fluorescent Carbon Dots

As an emerging nano-material of the “carbon family”,carbon dots(CDs)have been generally used in biomedicine,analytical detection,anticounterfeiting,catalysis and light-emitting devices and other fields because of their excellent optical properties,biocompatibility,stable chemical properties and easy to be synthetic.After more than ten years of study,the research on luminescence of CDs has changed from single blue-green to multi-color fluorescence and even full-spectrum fluorescence of visible light.However,most of the multicolor carbon dots are obtained by changing different precursors or by chromatographic separation,which are cumbersome and the fluorescence regulation mechanism is not perfect.Therefore,it is meaningful and challenging to explore a new synthesis method with universal and reference significance to tune the photoluminescence color of CDs,and at the same time to clarify the relationship between synthesis method and CDs structure and establish the mechanism of multicolor fluorescence emission.In this paper,CDs with different fluorescence emission were prepared by LDHs confined synthesis and traditional hydrothermal method.The influences of synthesis methods on the structure and fluorescence properties of CDs were systematically studied by structural characterization and optical test.Based on the effects of domain confined synthesis on the structure and fluorescence properties of CDs,blue,green,yellow and red CDs were prepared.The photoluminescence mechanism of four-color CDs was studied in combining with morphology,size,elemental composition,chemical structure and energy level structure.Based on the low toxicity of carbon dots prepared experimentally,it is applied in cell imaging.The fluorescent composite film prepared by four-color fluorescent CDs@PVA was applied in white light-emitting diodes(WLEDs).Using 2,5-diaminobenzene sulfonic acid and sodium dodecyl sulfate as raw materials,fluorescent CDs were prepared by hydrotalcite confined synthesis method and traditional hydrothermal method.By changing the Mg-Al ratio of the hydrotalcite laminates,regulating the charge density of the laminates,and then by high-temperature hydrothermal reaction and acid etching,localized synthetic CDs with different emission wavelengths were prepared.Combined with elemental composition and structural characterization,the effects of confined synthesis on the reaction process and CDs structure were studied.XRD and Raman indicating that CDs synthesized in confined interlayer of hydrotalciteare have a higher degree of graphitization.It can be seen that confined synthesis promotes the formation of amorphous structure of CDs,thereby effecting their fluorescence properties.O1 s high-resolution XPS spectrum shows that the oxidation state of CDs prepared by hydrotalcite confinement synthesis method is weaker than that synthesized by traditional hydrothermal method.In order to further verify the influence of confined synthesis on oxidation degree,intercalated hydrotalcite precursors with different charge density laminates were designed.By changing the Mg-Al ratio of the hydrotalcite layer,the spatial arrangement of the reactants and the oxygen content in the reaction microenvironment were controlled,and the confined synthetic CDs with different fluorescence emission wavelengths were obtained.The oxidation degree of CDs synthesized by traditional hydrothermal method and confined synthesis with different Mg-Al ratios shows that the confinement synthesis limits the oxidation degree of CDs,and the oxidation degree of CDs is correspondingly weakened when the reactants are arranged more closely.The oxidation degree of CDs synthesized by traditional hydrothermal method,hydrotalcite confined zone synthesis and hydrotalcite confined zone synthesis with different Mg/Al ratio shows that limited zone synthesis limits the oxidation degree of CDs,especially when the charge density increases,the reactants between layers are arranged more closely,the oxidation degree of CDs decreases correspondingly.Therefore,the 2D interlayer confinement synthesis will lead to an increase in the degree of graphitization,a decrease in the surface oxidation state,and a decrease in the nitrogen content of the CDs,thereby regulating the blue-shift of its emission wavelengths.Blue,green,yellow and red CDs were prepared from 2,5-DBS,2,4-DBS and DS by traditional hydrothermal method and hydrotalcite limited domain synthesis method respectively.The corresponding fluorescence emission wavelengths measured by fluorescence spectrum were 460,500,550 and 605 nm respectively,and display wavelength-independent excitation.The TEM and AFM revealed that the average particle size of four-color fluorescent CDs were about 2 nm,indicating that quantum size effect is not the predominant mechanism of multicolor fluorescence emission.Uv-vis,FT-IR and XPS characterization finally found that the fluorescence color of CDs was related to its surface state,mainly involving the content of pyrrolic nitrogen and C=O on the surface of carbon dots.Specifically,the increase of the content of pyrrole nitrogen,the increase of C=O lead to the increase of surface oxidation degree,and the red shift of the luminous wavelength of the fluorescent CDs.CCK8 cell proliferation assay showed that the CDs obtained in the experiment had good biological compatibility and low toxicity.Subsequently,the four-color CDs were used for cell multicolor imaging,showing good multicolor labeling effect.WLEDs was fabricated by assembling the three primary colors CDs@PVA films obtained by the uniform mixing of CDs and PVA with a 365 nm UV chip.