Preparation Of Chitosan-based Carbon Dots And Its Application In Ascorbic Acid Analysis Of Food Samples

Carbon Dots?CDs?,an emerging fluorescent nanomaterial,has been widely applied in biomedical and fluorescent sensor fields due to its high water solubility,fluorescence stability,good biocompatibility and low toxicity,as well as abundant amino,hydroxyl and carboxyl groups on surface,which can be chemical modified to give different functional CDs.In this thesis,new N-doped carbon dots?CDs?are prepared from renewable precursors,chitosan and tartaric acid,and used as a biocompatible fluorescent sensor for the sequential detection of Fe³⁺and Ascorbic acid?AA?.Additionally,the potential application in food packaging has also been explored.The research contents and results are as follows:?1?The new N-doped carbon dots?CDs?with quantum yield of 5.22%was obtained and were systematically characterized.The particle size about 20 nm is comfirmed by transmission electron microscopy?TEM?and dynamic light scattering?DLS?.As demonstrated by Fourier transform infrared spectroscopy?FT-IR?,nuclear magnetic resonance?NMR?,X-ray diffraction?XRD?,and X-ray photoelectron spectroscopy?XPS?,the prepared CDs have rich surface functional groups,containing nitrogen and graphene structures.Furthermore,the obtained CDs possess excellent fluorescence stability in acidic and good tolerance with high ionic strength.?2?The potential application of the CDs in fluorescent sensing for metal ions was investigated by monitoring the changes in the fluorescence intensity with the addition of various metal ions.Notably,Fe³⁺?2 mM?can apparently quench the fluorescence of CDs,while the addition of other metal ions?2 mM?has no obvious quenching effect.The F/F₀displays a good linear relationship with the Fe³⁺concentration in the range of 0-145?M with a correlation coefficient?R²?of 0.9964.The detection limit for Fe³⁺was calculated to be 1.15?M.Then,the CDs-based fluorescence system as“on-off-on”fluorescent switch sensor for the detection of AA was developed because the fluorescence of CDs quenched by Fe³⁺could be remarkably restored upon the presence of AA,which was further utilized for the detection of AA.A good linear relationship in the range of 0-150?M of AA concentration and the detection limit as low as 0.02?M were found.This fluorescence method has been successfully applied in the practical determination of AA in several commercial fruit juices and fresh fruits and vegetables.Satisfactory recoveries ranging from 86 to 104%with low relative standard deviation values of less than 3.2%?n=3?were resulted.The presented CDs-based fluorescence sensor possesses appealing advantages,including easy operation,high environmental safety,visual detection,wide linear range,low detection limit,good selectivity,excellent sensitivity and rapid responsiveness,which enable it great potential for the applications in detection of AA in food field.?3?The prepared chitosan/polyvinyl alcohol?PVA?/carbon dots blend film was characterized by SEM and FT-IR.The results showed that CDs are well dispersed onto the film surface,while the addition of CDs significantly improves the incompatibility of chitosan and polyvinyl alcohol.Compared to the CH/PVA film,the resulting complex film exhibited better UV-blocking performance,fluorescent properties and mechanical properties based on the result of optical and mechanical assessments,suggesting it is a potential food packaging material.Interestingly,the study on the CDs fluorescent ink?20 mg/mL?showed good fluorescence and can be further applied in food packaging system.