| Fluorescence carbon dots(CDs),as a new zero dimension carbon-based nanomaterial,have many advantages,such as unique photoluminescence(PL)property,good water solubility,high photostability,ease preparation and functionalization,low toxicity,and excellent biocompatibility.Benefiting from these merits,CDs have demonstrated their great application potential in the fields of optoelectronic devices,fluorescence sensing,photocatalysis,bioimaging,drug delivery,and information anti-counterfeiting.Thus far,developing renewable,low-cost and green biomass as carbon source not only meets the urgent need for large-scale and green synthesis of CDs,but also promotes the development of value-added utilization of the waste biomass.However,the synthesized biomass-based CDs in previous research always suffer from complex synthesis procedure,inferior luminescence efficiency,monotonous fluorescence color,and the lack of efficient strategy for tailoring PL behaviors,which are still big obstacles to their practical largescale application.In the present research,xylan as well as its hydrolysis product xylose were used as carbon sources to synthesize CDs.Simultaneously,heteroatom doping strategy was also adopted to tailor the excitation-related emission behaviors and optimal emission wavelengths of the xylan-and xylose-derived CDs via a facile microwave-assisted hydrothermal method.Furthermore,the intrinsic PL emission mechanism was elucidated by systematically investigating relationship between chemical structure and optical properties.In terms of the potential applications,the synthesized hemicellulose-based CDs demonstrated their valuable utilization in the field of fluorescence probe,information anti-counterfeiting,and ratiometric fluorescent pH detection,which offered a new way for the preparation of tunable PL emissive biomass-based CDs and the efficient value-added utilization of hemicellulose.The major research contents in this paper are summarized as follows:(1)A simple and direct microwave-assisted hydrothermal method was used to prepare nitrogen-doped carbon dots(N-CDs)in a single step using an ammonia solution of xylan as the precursor.The synthesized N-CDs displayed typical excitation-dependent emission(EDE),and the maximum emission was centered at 442 nm under excitation of 360 nm.Comparing with the undoped xylan-derived CDs,the nitrogen doping facilitates the increase of PL quantum yield(PLQY)from 2.0% to 4.0%,demonstrating the significantly improved PL efficiency that was contributed from nitrogen doping.In addition,the further investigation illustrated that the reaction temperature,treatment time and the addition amount of ammonia had little effect on the PL properties of N-CDs.Moreover,the synthesized N-CDs showed sensitive and selective fluorescence quenching toward the tetracycline antibiotics,and based on this,novel fluorescent probe was established for the detection of tetracycline antibiotics.(2)Based on the strategy of homogenizing the surface functional groups of CDs,the excitation-related PL emissions of xylan-derived CDs was tailored,for this purpose,the branched polyethyleneimine(PEI)with high reaction activity and plentiful amino groups was used as nitrogen dopant in the starting materials containing xylan.The xylan-derived CDs synthesized via a microwave-assisted hydrothermal route showed typical excitation-independent emission(EIE)behavior,and the maximum emission was centered at 463 nm with a PLQY of 8.0%.Further characterizations for the morphological structure,chemical composition,and optical properties demonstrated that the EIE behavior was heavily dependent on the uniform surface state rather than the particle size of CDs.Additionally,the PEI functionalized CDs showed excellent fluorescence detection performance toward tannic acid,and the established detection system was successfully applied to detect tannic acid in real lake water and white wine samples.(3)The study focused on the excitation-related PL emission behaviors of xylan-derived CDs was carried out by investigating their morphological structure,size distribution,chemical composition,and optical properties,and the obtained results demonstrate that both the EDE and EIE behaviors were highly dependent on the change of surface defect states rather than the size effect.Further analysis on the surface chemical states revealed that the multiple energy levels induced by C-OH,C-O-C and C-O functionalities were responsible for the EDE.Attributed to the high nitrogen doping efficiency,the generated uniform surface state that was induced by dramatically decreased O-containing moieties and massive C-N or C=N like groups within CDs contributed to the EIE behavior.(4)A novel green emissive xylose-derived CDs(G-CDs)with primary EIE behavior were synthesized from a starting material comprising of xylose and m-phenylenediamine in phosphoric acid solution via a simple microwave-assisted hydrothermal method.The maximum emission of the synthesized G-CDs was centered at about 518 nm under 440 nm light excitation,and the PLQY can reach up to a high value of 73.6%.Systematic investigation demonstrates that the GCDs with the same emission behavior can be synthesized from the starting material in different acid solutions and ultrapure water,while the alkaline solution was conducive to the synthesis of blue emissive CDs(B-CDs)from the starting material.Based on the analysis of morphological structure,sized distribution and chemical composition,it can be found that the different reactive solvents had large effect on the PL properties by changing the nitrogen doping efficiency.According to the significant difference in chemical states between G-CDs and B-CDs,a fluorescence mechanism that was explained by the surface defect levels was proposed,and also verified by further examining the pH-dependent PL behaviors.Specifically,the distinct surface states induced by multiple O-containing groups and protonated amino groups were highly responsible for the green emission and blue emission,respectively.Furthermore,benefiting from the switched fluorescence colors induced by pH changing,the xylose-derived G-CDs solution was demonstrated to be a promising fluorescent ink for anti-counterfeit printing.(5)Engineering surface chemical state of CDs was carried out on the basis of sulfur,nitrogen co-doping strategy by using 2,4-diaminobenzenesulfonicacid as dopant.The synergistic effect of microwave-assisted hydrothermal treatment facilitated the co-doping of sulfur and nitrogen,and thus resulted in synthesis of green-yellow emissive CDs(GY-CDs),the optical emission was centered at 533 nm under 460 nm excitation,and the absolute PLQY was as high as 42.4%.Also,by using m-phenylenediamine and ethanediamine as dopants,respectively,other two types of nitrogen-doped xylose-derived CDs with green emission and blue emission were also synthesized under the same synthetic condition,and their absolute PLQYs reached up to 86.1% and 62.7%,respectively.The investigation on the morphological structure,sized distribution and surface chemical state of these synthesized CDs were carried out systematically,and the results indicated that the tunable PL emissions were induced by the different surface chemical states.Specifically,introducing protonated amino groups and sulfur-related moieties was confirmed to be an efficiency way that can narrow the energy gap of surface defect state and thus resulted in the green-yellow PL emission.Moreover,investigation focused on the pH-dependent PL behaviors was performed,the analysis results demonstrated that the different defect state-controlled emissions of GY-CDs induced by short-and long-wavelength excitations can realize pH determination via ratiometric fluorescence assay,and importantly,this developed pH nanosensors exhibited high anti-interference performance and excellent reliability. |
PDF Full Text Request