Functionalization Of Carbon Dots And Their Applications In Bio-imaging And Sensing

Carbon dots(CDs),as a new class of fluorescent nanomaterial with size of<10 nm,possess a series of advantages,including:excellent photoluminescence property,tunable excitation and emission,good fluorescence stability,outstanding water solubility,low cell toxicity and favorable biocompatibility.The unique fluorescence characteristics of CDs facilitate their great potential applications in the field of biological chemistry and analytical chemistry.Thus,CDs are promising as a competitive alternative for organic dyes and heavy metal based quantum dots.In this dissertation,we will focus on developing rational strategies for the preparation of functionalized CDs(including element doping,surface passivation and complex formation)with high quantum yields and tunable photoluminescence behaviors,which makes it possible for them to be applied in targeting-modulated biological and chemical sensing with high sensitivity and selectivity.Chapter 1 gives an overall introduction for the classification of preparation methods,the functionalization strategies,their physical and chemical features including fluorescent mechanisms of CDs.In addition,their applications in bioimaging,drug delivery,chemical sensing,photocatalysis and optoelectronic devices are also discussed.In charpter 2,carbon dots are prepared using a green hydrothermal approach with dehydrated shiitake mushroom as the sole carbon source without any additives,shortly termed as MCDs.Carbonization,surface functionalization and nitrogen doping are involved in the hydrothermal treatment and no further modification or surface passivation is necessary.The derived MCDs are nitrogen-doped,oxygen-rich with hydroxyl,carboxyl and amine groups,with a diameter of ca.4.2 nm.They also exhibit resistance to photobleaching,excellent photoluminescence stability and favorable biocompatibility.MCDs exhibit cell permeable properties and a distinct pH-sensitive/excitation-dependent photoluminescence emission feature within pH 4.0-8.0 owing to their abundant reactive groups and excellent photoluminescence property,providing an optical probe for intracellular pH sensing and multicolor imaging of live HeLa cells.MCDs also show strong fluorescence response to hemin,which facilitates sensitive fluorescent sensing of hemin in serum and capsule samples within a linear range of 0.4-8.0 ?mol L-1 and a detection limit of 120 nmol L-1.In chapter 3,surface passivation is adopted to improve the quantum yield and photoluminescence behavior of CDs.Octa-aminopropyl polyhedral oligomeric silsesquioxane hydrochloride salt(OA-POSS)functionalized carbon dots(CDs/POSS)are prepared by a one-pot approach with glycerol as carbon source and solvent medium.OA-POSS serves as a passivation agent,and it is obtained via hydrolytic condensation of 3-aminopropyltriethoxysilane(APTES).During the functionalization process,the amino groups on OA-POSS combine with carboxylic groups on the bare CDs via the formation of amide bond to construct organic-inorganic hybrid carbon dots.The obtained CDs/POSS are well dispersed in aqueous medium with a diameter of ca.3.6 nm.It is demonstrated that CDs/POSS provide favorable photoluminescent property with a quantum yield of 24.0%,which is 9-fold higher than the bare CDs.They also exhibit resistance to photobleaching and excellent photoluminescence stability in the presence of biological sample matrix(characterized by salts,heavy metals and organic molecules),which facilitate cell imaging in biological systems.Both the photoluminescent emission wavelength and the fluorescence intensity depend closely on the excitation wavelength,and thus,it provides a potential for multicolor imaging as demonstrated by using HeLa cells and MCF-7 cells.In chapter 4,the nucleophilic substitution reaction between 4-chloro-7-nitro-2,1,3-benzoxadiazole(NBD-Cl)and-NH2 group on the amino-functionalized carbon dots(CDs)produces a novel carbon dots based hybrid,shortly as CDs@NBD.In addition to the characteristic fluorescent emission of carbon dots at?ex/?em=360/443 nm,the CDs@NBD hybrid gives rise to a new photoluminescence at?ex/?em=460/544 nm,which is significantly quenched by p-phenylenediamine(PPD)via static quenching.This provides a novel sensing approach for fluorometric detection of PPD with CDs@NBD as a probe.A linear calibration graph is obtained within two concentration ranges of 0.1-1.0 ?mol L-1 and 1.0-10.0 ?nol L-1,along with a detection limit of 56 nmol L-1 and a RSD of<3%(at 1.0 ?mol L-1).The common coexisting species in environmental and biological sample matrixes cause no obvious interferences on the detection of PPD.The colour change of CDs@NBD hybrid solution with the variation of PPD concentration also facilitates visual detection of PPD,which further demonstrates promising applications in environmental and biological sample analysis.