Functional Preparation And Applications Of Biomass Cellulose-Based Carbon Quantum Dots

Due to their unique nanostructure,the carbon quantum dots?CDs?exhibite good optical properties,superior biocompatibility,and show good application potential in the fields of biochemical sensing,imaging analysis,and cells targeting.CDs derived from green and inexpensive natural precursors,also known as biomass-based CDs,have recently attracted extensive attention for their extraordinary properties,including favorable photoluminescence properties,low-cost,superior biocompatibility,especially the abundant and inexpensive nature.Nevertheless,to the best of our knowledge,biomass-based CDs mostly?1?exhibit low fluorescence quantum yield,?2?unsuitable for large-scale preparation,and?3?show emission in the limited blue range.In view of the above problems,we attempt to improve the fluorescence characteristics of biomass-based CDs and expand their application range by searching for high-efficiency preparation methods and functional regulation of microstructure of CDs.In this work,nitrogen and sulfur co-doped CDs?N/S-CDs?were synthesized by a facile approach based on one-step combustion treatment of cellulose-based biowaste.On the basis of the above,a functionalized CDs fluorescent smart hydrogel that can be used for metal ions detection was designed and fabricated.Finally,by adjusting the carbonization doping process and nano-transformation process of biomass cellulose materials,the photoluminescence?PL?-tunable CDs were successfully obtained,and their application in the field of multi-color bioimaging was further explored.The main contents of this paper include the following three parts:?1?A�catalytic combustion method�for the preparation of biomass cellulose-based N/S-CDs was proposed.Firstly,CDs were obtained by direct combustion using biomass cellulose materials?willow catkin,weed,straw,leaves,walnut shell and cotton?as the initial carbon source.However,these CDs did not show clear morphology and crystalline structure.The PL quantum yield and generated yield of these CDs were only?0.3⁰.8?%and?0.05⁰.92?%,respectively.Based on the mechanism of flame retardant and doping coordination,the N/S system was used to pretreat the biomass cellulose,then N/S-CDs were efficiently synthesized by a facile approach based on catalytic combustion treatment of biomass cellulose.Specifically,the willow catkins were taken as an example.Willow catkins were immersed in a N/S mixed solution,directly burned after drying,and then subjected to a simple purification step to obtain an aqueous solution of N/S-CDs.TEM results showed that,unlike CDs,the as-prepared N/S-CDs were roughly spherical particles with an average diameter of 7.3?nm,and mono-dispersing in aqueous media.The lattice spacing distance of resulting N/S-CDs corresponded to the in-plane lattice spacing of graphene.The XPS and FT-IR results clearly indicated that N and S atoms had been successfully doped into the prepared N/S-CDs.Even better,After the introduction of the N/S pretreatment system,the generated yield of CDs increased from 0.92%to14.3%,and the quantum yield increased from 0.7%to 13.3%,which far exceeded the average of CDs prepared using traditional methods.A possible proposal was put forward to elucidate the main regularities and mechanism in the N/S-Cellulose combustion process.Sulfuric acid can promote dehydration,while the flame-retardant effect of urea can prevent the consumption of carbon.The combination of the two components can effectively improve the yield.The nitrogen element in the N/S pretreatment system mainly entered the aromatic carbon ring by nucleophilic addition,and existed in the form of pyrrole nitrogen and pyridinium nitrogen;The sulfur element is mainly attached to N/S-CDs by electrophilic addition.The introduction of N/S pretreatment system provides a new path for the preparation of high PL quantum yield CDs.Significantly,the resulting N/S-CDs were found to possess excellent photo-stability under different conditions.The PL intensity of N/S-CDs aqueous solution was strong and even stable in a wide range of pH values from 3 to 11.In addition,the PL intensity of N/S-CDs showed almost no obvious variation with the varying concentrations of NaCl?0 to 3?M?,and long-time UV irradiation?150 W,6 h?and storage?8?months?as well.In particular,Fe³⁺ion can strongly chelate with a large number of oxygen-containing functional groups and sulfur elements on the surface of N/S-CDs.Therefore,the N/S-CDs could be used for selective and sensitive detection of Fe³⁺ion within the range of 40�700??M,and the detection limit was estimated to be0.03??M.The cytotoxicity test results revealed that the N/S-CDs have not imposed any significant toxicity to HeLa cells,highlighting their excellent biocompatibility.N/S-CDs can easily enter the cell membrane and cytoplasm through phagocytosis of cells,but not into the nucleus.Finally,we further evaluated the capability of N/S-CDs in intracellular Fe³⁺ions detection.The fluorescence intensity decreased distinctly upon the addition of Fe³⁺ions,and there was no obvious blue emission detected after the cells were treated with 40??M Fe³⁺ions,suggesting that the fluorescence intensity of N/S-CDs was related to the concentration of Fe³⁺ions and capable of labeling the Fe³⁺ions in living cells.The proposed method provides a facile avenue for the production of cellulose-based CDs for various applications in biological detection and clinical diagnosis.?2?A novel fluorescent smart hydrogel based on functional CDs for metal ions detection was designed and prepared.Most existing fluorometric CDs was trend to agglomerate as the testing time rises,which will cause erroneous measurements.In order to prevent fluorescence quenching,a novel fluorescent smart hydrogel based on PEI-CDs/hydrogel nanocomposite material for stable Fe³⁺ion detection was prepared.Firstly,the N/S-CDs were modified by polyethyleneimine?PEI?.The effects of reaction conditions during the surface passivation of CDs,such as PEI concentrations,hydrothermal temperature and reaction time on the PL properties of PEI-CDs were investigated and an optimized reaction condition was identified.FT-IR combined with XPS analysis indicated that the PEI-CDs were successfully prepared by hydrothermal method.The average diameter of PEI-CDs was around 7.8?nm.Under excitation at 360nm,the aqueous solutions of PEI-CDs?pH=3⁸?exhibited strong and stable blue emission at 445 nm.The PEI functionalization increased the quantum yield of N/S-CDs from 13.3%to 28%.It indicated that PEI functionalization can effectively improve the PL properties of CDs,and also enhance the interaction between PEI-CDs and cellulose hydrogels.The PEI-CDs were added to the cellulose solution in advance,and the fluorescent smart hydrogel was prepared by using ethanol as a crosslinking agent.Unlike cellulose hydrogels,which have no PL emission,the smart hydrogel displayed a strong blue emission band centered at 445?nm.The PEI-CDs can be well dispersed in the smart hydrogel.The SEM,FT-IR and TGA were used to investigate the interaction between PEI-CDs and microcrystalline cellulose?MCC?hydrogel matrix.These results demonstrated that there were strong hydrogen bonding interactions between hydrophilic hydroxide radicals on the surface of the MCC and the OH groups of the PEI-CDs.The highly 3D porous structure of smart hydrogel prevented the aggregation of PEI-CDs during the detection process and improved their photo-stability to some extent.The emission intensity signal of smart hydrogel displayed a good linear relationship with Fe³⁺ion concentration in the range of 10¹00??M,and the limit of detection was 65?nM.In addition,the smart hydrogel can be used as fluorescent probe with good accuracy and low RSD%?1.64%to 1.92%?for Fe³⁺ion detection in real water samples?tap water,lake water and sewage?,suggesting the applicability of the proposed method to real samples.?3?Preparation of CDs with long-wavelength and photoluminescence?tunable emission to achieve multicolor imaging in cells.Most CDs derived from natural biowastes show emission in the limited blue range and the preparation of PL-tunable biomass-based CDs remains a very challenging task.By controlling the structure and nano-transformation process of biomass cellulose carbides,the size,structure and surface state of the prepared CDs can be well controlled,and PL-tunable CDs can be successfully obtained.Firstly,a large-sized carbon target with a certain graphitization structure was prepared by carbonizing biomass cellulose materials at 900�C.The carbon target was then added into concentrated HNO₃ and H₂SO₄(V _HNO3:V _H2SO4=1:3)chemical cutting solution.As the chemical cutting temperature increases,under 360 nm excitation,the prepared CDs exhibited bright yellow?100^oC?,green?120^oC?and blue?140^oC?fluorescence,respectively.The reason why the CDs showed different emission colors was further discussed.The morphology and structure of three CDs were analyzed by TEM,FTIR,XPS and Raman.It was found that the increased chemical cutting temperature had resulted in more ordered graphitic structures,decreased size distribution?from 2.86 to 2.40 nm?and C-O bands contains,which may change the optical bandgaps of CDs,thus enabling the adjustment of emission colors.The PL spectroscopy results showed that the three CDs exhibited high PL emission intensity and good stability in a wide physiological pH range?4¹0?.After being cultured with these three CDs,the cell membrane and cytoplasmic regions of HeLa cells showed bright blue,green and yellow fluorescence signals,respectively.These PL-tunable CDs were successfully applied to multicolor bioimaging in Hela cells,indicating their potential toward multimodal sensing applications in biological environment.In summary,the most abundant biomass cellulose materials in the natural world were used as a carbon source.A�catalytic combustion method�for the preparation of biomass cellulose-based N/S-CDs was proposed.The preparation of functionalized CDs fluorescent smart hydrogel can improve the fluorescence emission stability and detection accuracy of PEI-CDs in a complex detection environment.Finally,by controlling the sizes,structures and surface states of CDs,the preparation of CDs with blue,green and yellow fluorescent emission and multi-color fluorescence imaging in cells were realized.The research of this work provides some new ideas for the preparation and application of biomass-based CDs.