The Preparation And Applications Research Of Novel Doped-fluorescent Carbon Quantum Dots

Carbon quantum dots(or carbon dots,CDs)are developed a kind of novel fluorescent nanomaterials in recent 10 years.CDs has lots of advantages such as high fluorescence intensity,good light stability,non-photobleaching,resistant to photobleaching,tunable excitation and emission wavelengths comparing with the traditional organic fluorescent dyes.At the same time,CDs also have some properties such as low toxicity,good biocompatibility,low molecular weight and small size comparing with the traditional metal fluorescent quantum dots that they have been widely used.Which made them have good potential applications in biochemical sensors,biological imaging,light catalysis and other areas.Though the synthesis and application of CDs have rapidly developed,there are still many problems demanding prompt solution in low fluorescent quantum yield(FLQY)and lack of in-depth utilization.In this work,we adopted heteroatoms doping strategies to improve FLQY of CDs.Novel doped CDs with high photoluminescence were prepared by one step pyrolysis or hydrothermal treatment of various carbon sources such as biomass peanut shells,lychee seed,litchi shell,starch and simple organic compounds citric acid(CA),thiourea,ethylenediamine and so on).Then we studied the applications of as-prepared doped-fluorescent CDs in biochemistry detection,biochemical imaging,photocatalysis,drug delivery,photodynamic therapy(PDT)and other areas.The main contents of this work are briefly summarized as follows:Part one:The synthesis of fluorescent carbon dots doped with nitrogen(N-CDs)for an average size of 1.6 nm has been developed using peanut shell as precursor.The N-CDs were prepared by the simple pyrolysis of biomass peanut shell.The morphology,structure and fluorescent properties of N-CDs were characterized by transmission electron microscopy(TEM),the X-ray photoelectron spectroscopy(XPS),the X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FTIR),UV-Vis and fluorescent spectra.At the same time,a series of experiments proved N-CDs with an excellent water solubility,good tolerance for pH,resistance to photobleaching,satisfactory stability for ionic strength,good biocompatibility and low cytotoxicity.Based on the above merits,we also applied the prepared N-CDs as an effective fluorescent probe for living cell of multicolor fluorescence imaging.Part two:A green approach was developed for the preparation of fluorescent carbon dots doped with nitrogen(N-CDs)for an average size of 1.1 nm by using biomass lychee seed as precursor.The preparation of N-CDs was performed by simply pyrolysis.The morphology,structure and fluorescent properties of N-CDs were characterized by TEM,XPS,Raman spectra,FTIR,UV-Vis and fluorescent spectra.The as-prepared N-CDs are uniform in size,and the FLQY was 10.6%by using quinine sulfate as the reference.The N-CDs exhibit various advantages including excellent water solubility,good biocompatibility and low cytotoxicity,and has satisfactory stability for ionic strength,resistance to photobleaching.Additionally,the N-CDs were employed as a fluorescence probe for the detection of methylene blue(MB),which is based on MB induced fluorescence quenching of N-CDs.This sensing system exhibits a linear relationship with MB concentrations from 2.0×10-7 mol/L to 1.0×10-5 mol/L and a detection limit as low as 5.0×10-8 mol/L.It was then successfully applied for the fluorescence imaging of HepG2 cells.Part three:A novel sulfur and nitrogen binary doped carbon dots(S,N-CDs)for an average size of 7.0 nm was synthetized by one-step manner through the hydrothermal treatment of CA and ammonium thiocyanate.We investigated the synthetic conditions,including the reaction time,the reaction temperature and the ratio of CA and thiocyanate for obtaining a very high FLQY(74.2%).Then the procedures for biomedical applications,including probing doxycycline in living cells and multicolor cell imaging were developed.The fluorescence of S,N-CDs can be specifically quenched by doxycycline,providing a convenient turn-off assay of doxycycline.This assay shows a wide linear detection range from 0.08 to 60 μmol/L(0.037～28 p.g/mL)with a low detection limit of 20 nmol/L(～0.009 μg/mL).The proposed S,N-CDs probe here exhibits an approving selectivity and can distinguish between doxycycline and tetracycline.The high sensitivity and selectivity may ensure a more accurate analysis of doxycycline in complex samples.Part four:Nitrogen and sulfur co-doped carbon dots(S,N-CDs)have been demonstrated to be environmentally friendly photoluminescence probes.Their fluorescence was quenched by free chlorine,which was utilized to develop a new technology for the detection of free chlorine.The conditions of this new technology were optimized including the concentration of S,N-CDs,response time and the pH of the solution.Then we put forward the possible reaction mechanism:the free chlorine can destroy the surface passivation layer of the S,N-CDs,forming ground state complexes without fluorescence leading to significant fluorescence quenching.Under optimal conditions,a linear relationship between the decreased fluorescence intensity of S,N-CDs and the concentration of free chlorine in the range from 0.01 to 100μmol/L with a detection limit of 5 nmol/L was obtained.The proposed method was successfully applied for the determination of free residual chlorine in tap water and swimming pool.The method is simple and results are accurate with recovery rate(91.3～106.2%).Part five:A facile hydrothermal route was developed to prepare ternary-doped carbon dots with nitrogen,boron and sulfur elements(N,B,S-CDs)for an average size of 5.3 nm using CA,thiourea,boric acid and ethylenediamine as precursor.The prepared ternary-doped CDs exhibit ultrabright luminescence with an ultrahigh FLQY(92.6%).The living cell multicolor imaging was demonstrated because the N,B,S-CDs possess good stability,water-solubility and excellent biocompatibility.Significantly,the ternary doping of nitrogen,boron and sulfur makes N,B,S-CDs display a prominent visible light photocatalytic activity for the degradation of rhodamine B(RhB).Results show that N,B,S-CDs can degrade RhB under the irradiation of visible light for 90 min and the degradation rate can reach 98%.Then we put forward the catalytic degradation mechanism based on the reaction conditions:the electrons transferred from active N,B,S-CDs to H2O2 and produced active oxygen species·OH radicals and O2-under the irradiation of visible light.The reactive radical specie of·OH and O2-were with high oxidizability,which can degrade RhB into smaller organic matters under the catalysis of N,B,S-CDs.Part six:A low temperature hydrothermal treatment method was developed for the preparation of nitrogen and phosphorus co-doped fluorescent carbon dots(N,P-CDs)by using starch in the presence of phosphoric acid and nitric acid.The FLQY was 27.0%by using quinine sulfate as the reference.The surface of the N,P-CDs was functionalized by using bovine serum albumin(BSA)as the linking agent,folic acid(FA)and the anticancer drug doxorubicin(DOX)was successfully attached.The N,P-CDs-FA-DOX functional composites act as fluorescent probes as well as drug carriers for imaging and selective target delivery.The drug loading efficiency of N,P-CDs-FA-DOX was as high as 90.9%and the percentage drug release in pH=5.5 solution was 63.9%after 48 h.At the same time,cell imaging prove that the N,P-CDs-FA and N,P-CDs-FA-DOX functional composites show specificity for HepG2 cells,almost no response to normal cells and FA plays a very important role as a navigation molecule.These features make the functional composites as a drug carrier in anticancer drug delivery with the function of the targeted cancer cells.Part seven:The synthesis of fluorescent carbon dots doped with nitrogen(CDs)has been developed using litchi shell as precursor by the simple pyrolysis.The surface of the CDs was modified by using H2N-PEG-NH2(PEG).Furthermore,Chlorin e6(Ce6)and transferrin(Tf)was conjugated on the CDs-PEG via an EDC/NHS reaction,and Tf can enhance the selectivity to cancer cells.The functionalization of carbon quantum dots doped with nitrogen for near-infrared fluorescence(CDs-PEG-Ce6-Tf)was synthesized.We developed a simple,specific identification of tumor cells and a new method of PDT by using the special optical properties of CDs-PEG-Ce6-Tf and specific recognition ability of Tf.This method use the active targeting function of Tf by the transferrin receptor mediated endocytosis into tumor cells.Photosensitizer fluorescence detection(PFD)and PDT were used successfully for diagnosis and therapy of malignant tumor.The therapeutic CDs-PEG-Ce6-Tf nanoagent is facile and safe for delivery and cancer cell targeting to simultaneously minimize side effects and enhance the efficiency of PDT.