Carbon Nanodots:from Structure Controlling,Photoluminescence Property To Mechanism

Generally speaking,C-dots,with carbon as the main body and diameter of less than 10 nm,consist of abundant oxygen-containing groups or polymers on their surface.They have attracted immense attention due to their unique optical properties,excellent biocompatibility,facile preparation and low cost.With these merits,C-dots have been used in a wide range of applications including bioimaging,photocatalysis,light-emitting devices,etc.However,the properties and mechanism of PL need to be fully understood.The main research work in this doctoral thesis has been summarized as follows:(1)The C-dots prepared via wet oxidation contain many functional groups.Abundant carboxyl groups on the surface of C-dots were proved by the FT-IR measurement in different condition.And then,the surface of C-dots were modified by alcohols or amines based on the reactions of carboxyl groups.Of interest here is that the PL of C-dots before and after modified based on the reaction of carboxyl groups had no obvious difference,laying a solid foundation of the surface modification in the future.Based on this,C-dots were modified by(3-aminopropyl)triethoxysilane(APTES).Through the co-hydrolyzation tetraethyl orthosilicate(TEOS),the ultrabright carbon nanodots-hybridized silica nanospheres(CSNs)were obtained by Stober method,implementing the fluorescence-converging of C-dots.Because C-dots were gathered inside of silica matrix,their self-quenching and external quenching have been inhibited.The PL intensity of each CSN can reach as three to four thousand times as that of a single C-dot.In addition,the CSNs have been modified with wheat germ agglutinin(WGA),and applied in the cell lablling,improving the the quality of C-dots in bioimaging.(2)Taking the carbon nanodots(C-dots)prepared by wet oxidation as a model,the delocalization extent of ?-electron system and the functional groups of the C-dots are carefully controlled by reduction of carbonyl groups and elimination of hydroxyl groups.Combining structural characterizations and the changes of fluorescence properties,we revealed that the PL of C-dots is related to the surface state composed of ?-electron system and carbonyl groups,and the energy level is determined by the strong coupling of the?-electron system with carbonyl groups.Theoretical calculations further support the PL mechanism proposed above.Moreover,fluorescence lifetime and ultrafast dynamics suggest that the carbonyl group will result in a large ratio of non-radiation to radiation decay which should be responsible for the low QY of C-dots.Thus,high QY of C-dots can be expected if the carbonyl-group content is decreased,and the emission wavelength can be tuned by adjusting the extent of the ?-electron system and the carbonyl-group content,rather than the size.This work provides an insight into the PL of C-dots,which will facilitate to engineer fluorescent carbon-based nanomaterials with desirable emissions and QYs.(3)Combining the comparison of sensitivity of the three kinds of C-dots with different functional groups on Cu2+ and the ultrafast dynamics,the PL qunching mechanism of carbon nanodots(C-dots)by Cu2+ was investigated deeply.The coordination between Cu2+and carboxyl group on the surface of C-dots narrows the distance between them,thus resulting in efficient quenching of the PL through photoinduced electron transfe from excited C-dots to the the empty d orbits of Cu2+ within picoseconds.This will facilitate the designing C-dots-based fluorescent probe,as well as surface modification to turn the variety and quantity of functional groups on the surface of C-dots.