Chemical Modification And Functionalization Of Carbon Quantum Dots

Carbon quantum dots （CQDs） are quasispherical nanocarbons with sizes below10nm that possess stable photoluminescence （PL）. CQDs exhibit excellent fluorescentproperties, such as photoluminescence of size-and excitation wavelength-dependentproperties, good stability and no photobleaching. CQDs also have low-toxicity andexcellent biocompatibility. These excellent properties make them have wideapplications in bioimaging, fluorescent sensing, photocatalysis and organic photovoltaicdevices, etc. In this dissertation, we focus on the research of some key questions relatedto surface chemical modification, functional composites, PL modulation and surfaceenhanced Raman scattering （SERS） effects of CQDs. The main results are summarizedas follows:A versatile diazonium chemistry method was used to graft aryl groups includingphenyl （P）,4-carboxyphenyl （CP）,4-sulfophenyl （SP） or5-sulfonaphthyl （SN） to CQDs.The number of CP, SP and SN groups grafted onto each thermally treated CQD（TT-CQD） was21,21and12, respectively. The aryl-modified CQDs are graphiticnanocrystals with lateral dimensions in the range of24nm and an average thicknesslower than1nm. Upon chemical modification with aryl groups, the PL bands of CQDswere tuned in the range of418and447nm, and their fluorescent quantum yields （QYs）were increased greatly with a maximum improvement about6times. Furthermore, thearyl-modified CQDs exhibited excellent pH tolerance. PL intensity and peak positionwere almost unchanged in a wide pH window of111. Meanwhile, we also discussedthe PL mechanism of CQDs with aryl-modification. After aryl-modification, PL ofCQDs was originated from defect states, which was different from triplet-ground-stateof carbene structure for TT-CQDs.Gold@carbon quantum dots （Au@CQDs） nanoparticles with ultrathin CQDs shellsof2nm were prepared by reducing HAuCl₄with CQDs. By adjusting the reactiontemperature and the feeding mass ratio of HAuCl₄to CQDs, the average diameter ofAu@CQDs can be modulated from8to44nm. Au@CQDs nanoparticles with anaverage diameter of24nm were applied as a substrate for SERS and it exhibited a higher SERS effect for rhodamine6G than pure gold nanoparticles with nearly the samesize. The excellent SERS effect of Au@CQDs is mainly attributed to their improvedcapability of adsorbing the aromatic probe molecules.Hydrothermal method was used to regulate the surface states of various CQDsobtained from different carbonaceous materials such as carbon fiber, graphitic powder,graphene oxide and reduced graphene oxide. All the fluorescence peaks of CQDs wereblue-shifted to440nm after hydrothermal treatment. Furthermore, the full widths at halfmaxima （FWHM） of fluorescent peaks were narrowed, and fluorescent QYs wereincreased by about2times. The improved PL properties are mainly attributed to the factthat hydrothermal treatment kept only the carbonyl related groups （carboxyl, carbonyl）among various oxygenated groups （carboxyl, carbonyl, hydroxyl and epoxy group） onoriginal CQDs. After hydrothermal treatment, mass carbonyl related groups （carboxyl,carbonyl） of CQDs benefit for their further chemical modification and functionalization.