Research On Preparation Technology Of Multicolor Fluorescent Carbon Quantum Dots And Application Of Metal Ion Detection And Photocatalysis

Carbon quantum dots（CQDs）are fluorescent nanomaterials with carbon as the main element.Their applications in biomarkers and imaging,sensors,and photocatalysis have shown great potential.The appearance of multicolor fluorescent CQDs has broadened the application field of CQDs.In this paper,amino acids or tartaric acid were used as carbon source,and the solvent and hydrothermal conditions were adjusted.The multicolor fluorescent CQDs were prepared by hydrothermal method from the perspective of changing the composition of the CQDs.The morphology,structure and fluorescent properties of the prepared multicolor fluorescent CQDs were analyzed and studied,and the fluorescent emission mechanism of the multicolor fluorescent of CQDs was discussed.Based on the principle that metal ions can quench the fluorescence of CQDs,the metal ions were detected.It was found that the selectivity of CQDs with different fluorescence colors to metal ions was different.At the same time,the advantage that CQDs can improve the catalytic performance of TiO₂ powder was used to prepare CQDs/TiO₂ composite,and the photocatalytic performance of the composite was analyzed.The main achievements are as follows:（1）The blue-emitting carbon quantum dots（B-CQDs）and red-emitting carbon quantum dots（R-CQDs）were prepared using L-Methionine as carbon source and urea as nitrogen source by one-step hydrothermal treatment doping with N and S.By changing the reaction solvents（DI-water and DMF）,the maximum emission of the resulting CQDs shifted from blue to red light.Specifically,the emission wavelength of CQDs was turned from 433 nm to625 nm by embedding a new functional group（-CONH-）on the surface of CQDs.The photoluminescence quantum yields of the B-CQDs and R-CQDs reach up to 64%and 61%,respectively.In terms of application,the R-CQDs were used to detect metal ions,and it is demonstrated that there was a linear relationship between ln（F/F₀）and the Fe³⁺ions concentration in the range 0-0.5 mmol/L with a detection limit of 0.067μM,thus the R-CQDs have great potential as a fluorescent probe for Fe³⁺ions detection.Furthermore,the B-CQDs and R-CQDs were composited with TiO₂ to prepare the CQDs/TiO₂ composite material.It was found that both of them can improve the catalytic performance of TiO₂.（2）The green-emitting carbon quantum dots（G-CQDs）were prepared using tartaric acid and bran as double carbon sources by solvothermal treatment with the photoluminescence quantum yields as high as 46%.The morphology of G-CQDs was characterized by TEM,which showed the average diameter of G-CQDs is approximately 4.85 nm.The FT-IR spectra displayed the presence of-OH,C-N,N-H and-COOH on the surface of G-CQDs.The emission wavelength of G-CQDs was 539 nm in the case of 450 nm excitation wavelength,which corresponds to the green fluorescence.Furthermore,the G-CQDs were used as a fluorescent probe for detection of Cu²⁺ions,and demonstrated the linear distribution between ln（F/F₀）and the Cu²⁺ions concentration(the range of the Cu²⁺ions concentration was0-0.5mM),and the detection limit was 0.0507μM.Meanwhile,the catalytic properties of the G-CQDs/TiO₂ composite were analyzed.It was found that G-CQDs could improve the catalytic performance of TiO₂.（3）With tartaric acid and citric acid as the double carbon source,thiourea as the nitrogen source and sulfur source,providing N and S elements,N,S doped yellow fluorescent carbon quantum dots（Y-CQDs）were prepared by hydrothermal method.A series of analytical studies on Y-CQDs were carried out by means of TEM,FT-IR,XPS and fluorescence properties.The results showed that Y-CQDs have abundant hydroxyl groups on its surface,indicating that Y-CQDs have good hydrophilicity,and N and S were successfully doped into Y-CQDs.The maximum excitation wavelength of Y-CQDs was 500 nm,and the corresponding maximum emission wavelength was 589 nm,which belongs to the yellow light range.Different from G-CQDs,Y-CQDs had a good selectivity and sensitivity to detect Cr³⁺ions.The ratio of Cr³⁺ion concentration to fluorescence intensity showed a good linear relationship,and the detection limit was 0.06μM.