Preparation Of Carbon Dots And Their Optical Properties In Visible Spectrum

Carbon dots or carbon quantum dots are a new and important form of zero-dimensional carbonaceous nanomaterials generally defined as small carbon nanoparticles with various surface passivation.Ever since their original finding,there have been broad and intense interests in carbon dots for their unique and/or advantageous properties.A wide variety of potential applications of carbon dots have been pursued,including bioimaging and sensing,photocatalytic energy conversions and optoelectronics.In fact,carbon dots now represent a rapidly advancing and expanding research field,as reflected by the large increasing number of publications in the recent literature.Numerous precursors have been employed to synthesize carbon dots,including carbon-rich organic species such as aromatic compounds which significantly limit the biological applications of the carbon dots.Synthesis procedures of carbon dots can be classified into two categories:deliberate chemical functionalization method and carbonization of organic molecules or other carbon-rich precursors method（“one-pot”processing）.For deliberate chemical functionalization carbon nanoparticles,the carbon nanoparticles were obtained by treating carbon particle precursors with nitrate acid via centrifugation in multiple cycles.After these tedious procedures,the prepared carbon nanoparticles were functionalized by oligomeric and polymeric molecules under critical reaction condition.But the structure and optical properties are stable and well-illustrated.Compared with deliberate chemical functionalization method,“one-pot”processing is more convenient but less controllable,yields carbon dots of widely varying optical behaviors and controversial synthesis mechanism.Alternatively,numerous carbon dots by“one-pot”processing have also raised many debates and controversies due to their versatile optical properties and the messy UV spectral region,where the potential absorption or strong absorption of many functional groups associated with the altered,derivatized and/or unreacted organic species in the precursor mixture used in the carbonization processing,which make optical study of carbon dots very confusing.Meanwhile,the optical properties in the visible light region are more meaningful for the bio-application of carbon dots.In addition,the fluorescence properties of carbon dots,which mainly in blue and green region,limit their applications in bioimaging.Recently,many red/near IR emission carbon dots were reported by using specific precursors under mild reaction conditions.But there are still lots of confusions and controversies whether the red fluorescence properties are from the real carbon dots and the mechanism of the procedure.Therefore,it is necessary to use biomass as precursors for preparation carbon dots to improve efficiency and explore applications.In addition,there are also some drawbacks of these two methods.Thus,it is important to design a new process which combines all advantageous features of these two methods.Meanwhile,study of optical properties of carbon dots in the visible light region is important for understanding of the“one-pot”processing,constructing connection between“one-pot”processing and deliberate chemical functionalization method and defining a clear and unified understanding of the structure and optical properties of carbon dots in general.At last,red/near IR emission is essential for biological applications of carbon dots.Thus,understanding the preparing mechanism in the literatures and designing new red/near IR emission carbon dots are meaningful for broadening pratical applications of carbon dots.In this dissertation,there are four parts as following:（1）Disperse water-soluble fluorescent carbon dots were prepared by using coffee grounds as raw material by hydrothermal treatment.The as-prepared carbon dots exhibit an excitation-dependent,high photostable and pH dependent photoluminescence behavior.They are nearly not cytotoxic to Hela cells and can be applied to Hela cell imaging.Notably,the fluorescence of the carbon dots can be quenched by Fe³⁺ions,which can serve as a useful fluorescent probe for detecting Fe³⁺ions.（2）A hybrid approach combining the advantageous characteristics of deliberate functionalization of carbon nanoparticles and“one-pot”processing was applied to prepare carbon dots.The spherical and hydrophilic carbon dots were prepared by high-energy ball-milling of active carbon and potassium carbonate.They exhibit an excitation-independent and high photostable blue photoluminescence behavior and excellent photostability and a stable PL behavior based on pH.They are nearly not cytotoxic to Hela cells and can be applied to Hela cell imaging.Our hybrid approach is a potential way which can be scaled up for large-scale preparation of carbon dots.（3）A systematic comparison of carbon dots from different synthetic approaches based on carbon nanoparticles,the use of pre-existing small carbon nanoparticles sourced from pure carbon soot and the formation of similar nanoparticles in situ via one-pot thermal carbonization of organic molecular precursors“one-pot”processing was studied emphasizing spectroscopic characterizations over the visible spectrum.The results show that the carbon dots prepared by the“one-pot”processing under sufficiently robust conditions are generally no different from those from the deliberate chemical functionalization in terms of the observed optical properties and associated photoinduced redox characteristics in the application-wise more meaningful visible spectral region,suggesting a high stability or general applicability of the definition on carbon dots as surface-passivated small-carbon nanoparticles,which solves the controversories in the UV region.Implications of the reported findings to the further understanding and mechanistic explorations of carbon dots,including the necessity to focus on the core carbon nanoparticles in carbon dots in such explorations,are highlighted and discussed.（4）Citric acid and formamide were used as precursors for synthesizing red emission entities under mild processing conditions.The optical properties in the visible spectrum were investigated systematically by using carbon nanoparticles as reference.The absorption above 500 nm,which is related to red emission,is more likely from the dye,which comes from specific reactions between citric acid and formamide during the reflux.Also,the absorption and fluorescence behavior of the entities are not stable under room temperature.Based on the“host-guest”carbon dots,red emission carbon dots were successfully prepared by microwave treatment rhodamine B and polyethylene glycol.The resulting carbon dots exhibit similar absorption and fluorescence properties after vigorous dialysis compared with free rhodamine B.