1、This journal is The Royal Society of Chemistry 2014 Chem.Soc.Rev.Citethis:DOI:10.1039/c4cs00269eCarbon quantum dots and their applicationsShi Ying Lim, Wei Shen and Zhiqiang Gao*Fluorescent carbon nanoparticles or carbon quantum dots (CQDs) are a new class of carbonnanomaterials that have emerged re
2、cently and have garnered much interest as potential competitors toconventional semiconductor quantum dots. In addition to their comparable optical properties, CQDshave the desired advantages of low toxicity, environmental friendliness low cost and simple syntheticroutes. Moreover, surface passivatio
3、n and functionalization of CQDs allow for the control of theirphysicochemical properties. Since their discovery, CQDs have found many applications in the fields ofchemical sensing, biosensing, bioimaging, nanomedicine, photocatalysis and electrocatalysis. This articlereviews the progress in the rese
4、arch and development of CQDs with an emphasis on their synthesis,functionalization and technical applications along with some discussion on challenges and perspectivesin this exciting and promising field.1. IntroductionThe unique properties of carbonic nanomaterials such as nano-diamonds, fullerenes
5、, carbon nanotubes, graphene sheets andfluorescent carbon nanoparticles or carbon quantum dots (CQDs)have inspired extensive studies on them due to their greatpotential for a wide variety of technical applications. Among theelectronic and physicochemical characteristics of CQDs, theiroptical propert
6、ies and their fluorescence emissions in parti-cular have attracted increasing interest in recent years. For manyyears, semiconductor quantum dots have been extensively inves-tigated for their strong and tunable fluorescence emissionproperties, which enable their applications in biosensing andbioimag
7、ing. However, semiconductor quantum dots possesscertain limitations such as high toxicity due to the use of heavymetals in their production.13It is known that heavy metals arehighly toxic even at relatively low levels, which may proveprohibitive to any clinical studies.4,5This prompted the creationo
8、f CQDs to replace semiconductor quantum dots due to theirlow toxicity, biocompatibility, low cost and chemical inertness inaddition to having similar fluorescence properties.The accidental discovery of CQDs during the separation andpurification of single-walled carbon nanotubes (SWCNTs) byXu et al.
9、in 2004 triggered subsequent studies to exploit thefluorescence properties of CQDs and create a new class of viablefluorescent nanomaterials.6Fluorescent carbon nanoparticlesreceived their name carbon quantum dots in 2006 fromSun et al.7who proposed a synthetic route to produce CQDs withmuch enhance
10、d fluorescence emissions via surface passivation.CQDs are synthesised by two routes, namely the top-down route79and the bottom-up route.3,1012CQDs are typically quasi-sphericalDepartment of Chemistry, National University of Singapore, Singapore 117543.E-mail: chmgaoznus.edu.sg; Fax: +65 6779-1691; T
11、el: +65 6516-3887Shi Ying LimMs Shi Ying Lim was born inSingapore. She received her BScin Chemistry from the NationalUniversity of Singapore in 2014and is currently a graduate studentin the Department of Chemistry atthe same institution. Her researchfocuses on camera-based fluores-cence correlation
12、spectroscopy.Wei ShenMs Wei Shen received her BSc inChemistry from Zhejiang Univer-sity in 2011. She is currently agraduate student in the Depart-ment of Chemistry NationalUniversity of Singapore. Herresearch interests include bio-sensors and optical assays fornucleic acids and proteins.Received 4th
13、 August 2014DOI: 10.1039/c4cs00269ewww.rsc.org/csrChem Soc RevREVIEW ARTICLEPublished on 15 October 2014. Downloaded by Beijing University of Chemical Technology on 15/10/2014 11:12:54. View Article OnlineView JournalChem.Soc.Rev. This journal is The Royal Society of Chemistry 2014nanoparticles comp
14、rising amorphous to nanocrystalline coreswith predominantly graphitic or turbostratic carbon (sp2carbon)or graphene and graphene oxide sheets fused by diamond-likesp3hybridised carbon insertions (Fig. 1).1316Oxidised CQDscontain considerable amounts of carboxyl moieties at theirsurface. Depending on
15、 the synthetic route, the oxygen contentin the oxidised CQDs ranges from 5 to 50% (weight).14As shownin Fig. 1, there are many carboxyl moieties on the CQD surface,which impart excellent water solubility and suitable chemicallyreactive groups for further functionalization and surface passiva-tion wi
16、th various organic, polymeric, inorganic or biologicalmaterials to CQDs. Upon surface passivation, the fluorescenceproperties of CQDs are enhanced. Surface functionalizationalso modifies their physical properties, like their solubility inaqueous and non-aqueous solvents.As a group of newly emerged f
17、luorescent nanomaterials, CQDshave shown tremendous potential as versatile nanomaterialsfor a wide range of applications, including chemical sensing,biosensing, bioimaging, drug delivery, photodynamic therapy,photocatalysis and electrocatalysis. Compared to conventionalsemiconductorquantumdots, theu
18、nique attributesofCQDs,forexample their benign chemical composition, tunable fluores-cence emissions, facile functionalization and excellent physico-chemical and photochemical stability (non-photobleaching ornon-photoblinking), render them very attractive for technicalapplications. Together with oth
19、er advantages such as low costand ease of synthesis,17CQDs are in a favourable position forachieving unprecedented performance. On the other hand,complex procedures for their separation, purification andfunctionalization, their generally low quantum yields, andambiguity in their geometry, compositio
20、n and structure aresome of the issues that need to be tackled before they can trulyoutperform their semiconductor quantum dot counterparts inareas like bioimaging, biosensing and nanomedicine. Thisarticle reviews the progress in the research and developmentof CQDs and their technical applications. W
21、e first examine thefluorescence properties of CQDs and their tunable emissions.Then, we discuss the various synthetic approaches for theirproduction and possible surface passivation and functionaliza-tion routes to impart desired properties to CQDs. Finally, wediscuss in great detail the application
22、s of CQDs in chemicalsensing, biosensing, bioimaging, nanomedicine, photocatalysisand electrocatalysis, especially the advantages they could bring tothese fields. In view of several excellent review articles focusing ondierent aspects of CQDs, such as their synthesis and physico-chemical properties,
23、14,18surface functionalization,19bioimagingand biosensing19,20and photocatalysis and optoelectronics,18itis hoped that this article will provide a comprehensive overviewof the current status of CQD research and open new perspectivestoward the research and development of CQDs with muchimproved physic
24、ochemical properties.2. Fluorescence properties of CQDsCQDs are a new class of nanomaterials that have attractedsignificant attention in the past decade. Two classes of fluores-cence emission mechanisms have been proposed for CQDseven though the exact origins of their fluorescence emissionsremain de
25、batable and more research is needed in order to painta clearer picture of the mechanisms of their fluorescence emissions.The first class of fluorescence emission mechanism is that ofbandgap transitions caused by conjugated p-domains, whilethe second class involves more intricate origins associated w
26、ithsurface defects in CQDs.2.1 Fluorescence emissions from bandgap transitions ofconjugated p-domainsFor the first class of fluorescence mechanism, bandgap transi-tions arise from conjugated p-domains. These p-domains areisolated by creating sp2hybridised islands rich in p-electronsthrough the reduc
27、tion of graphene oxides obtained by usingHummers method of oxidising and exfoliating graphite flakes.21They are created in a way that there are no p-connectionsbetween the sp2islands, because any p-connections betweenthe sp2islands would lead to interisland quenching of desiredfluorescence emissions
28、.22,23In this type of bandgap transitions, single-layer graphenesheets have to be used to prevent interlayer quenching.24The single-layer graphene sheets are used as precursors forelectronically slicing into isolated p-conjugated domains, whichresemble large aromatic molecules with extended p-conjug
29、ationFig. 1 Chemical structure of CQDs. (Reproduced with permission fromref. 13.)Zhiqiang GaoDr Zhiqiang Gao is an associateprofessor at the Department ofChemistry National University ofSingapore. He received his BScand PhD in Chemistry fromWuhan University. The followingyears he worked as a postdoc
30、toralfellow at bo Akademi Universityand The Weizmann Institute ofScience. After spending three yearsin the United States and eight yearsat the Institute of Bioengineeringand Nanotechnology, he joinedNUS in April 2011. Research in hislaboratory currently includes electrochemistry, analytical chemistr
31、yand materials science.Review Article Chem Soc RevPublished on 15 October 2014. Downloaded by Beijing University of Chemical Technology on 15/10/2014 11:12:54. View Article OnlineThis journal is The Royal Society of Chemistry 2014 Chem.Soc.Rev.of specific electronic energy bandgap for optical absorp
32、tion andfluorescence emissions.25Such electronic transitions displaystrong absorption in the ultraviolet (UV) region, but weak or nofluorescence emissions (Fig. 2A). The strong absorption is likelydue to light absorption by a large amount of high density p-electrons in the sp2hybridised islands, whi
33、ch form excitonicstates while the weak emissions are possibly a result of quench-ing via radiationless relaxations to the ground state duringexciton migration to energy traps.132.2 Fluorescence emissions of surface defect-derived originsThe second class of the fluorescence mechanism arises fromsurfa
34、ce-related defective sites generally any sites that have non-perfect sp2domains will result in surface energy traps. Both sp2and sp3hybridised carbons and other functionalised surfacedefects,26,27such as carbonyl-related localised electronic states,24,28present in CQDs contribute to their multicolou
35、r emissions that areconcentrated in the blue and green regions of the visible lightspectrum. These surface defects behave like aromatic moleculesthat are individually incorporated into solid hosts, exhibiting multi-colour emissions due to the existence of multiple surface defectswith different excit
36、ation and emission properties.13,25Robertson and OReilly suggested that the optical propertiesof carbon nanomaterials which contain both sp2and sp3bondsare determined by the p-states of the sp2sites.29Thus, thebright surface defect-derived fluorescence of CQDs is due to therecombination of electronh
37、ole pairs in the strongly localised pand p* electronic levels of the sp2sites. These sites lie betweenthe bandgap of the s and s* states of the sp3matrix,30,31leading to strong visible emissions. Such electronic transitionsexhibit weak absorption in the near ultraviolet-visible (UV-vis)region but st
38、rong emissions in the visible region as shown inFig. 2B. In addition, upon surface passivation or functionaliza-tion, the surface defects become more stable to facilitate moreeffective radiative recombination of surface-confined electronsand holes, thus achieving brighter fluorescence emissions.252.
39、3 Tunable fluorescence emissions of CQDsOne unique property of CQDs is their tunable fluorescenceemissions. Generally, CQDs possess tunable emissions evenwithout any surface passivation, but they usually have very lowquantum yields due to unstable surface defects leading toreduced radiative recombin
40、ation.Upon surface passivation with organic or polymeric materials,such as poly(propionyl ethyleneimine-co-ethyleneimine) (PPEI-EI)attached to the CQD surface, surface defects are stabilised andstrong fluorescence emissions both in solution-like suspensionand in solid state were detected. The emissi
41、ons of such passivatedCQDs covered a broad range of the visible region and extendedinto the near-infrared (NIR) region as shown in Fig. 3.7It should be noted that the surface passivation agents used werenot emissive in the visible and NIR regions, thus any fluorescenceemissions observed must have or
42、iginated from the surface-passivated CQDs. The tunable emission property of CQDs isclearly demonstrated in Fig. 3. From the fluorescence spectra ofPPEI-EI-passivated CQDs, it is evident that the emissions arebroad and excitation wavelength-dependent.7,26The tunableemissions of the surface-passivated
43、 CQDs could be a result ofvaried fluorescence characteristics of particles of dierent sizesof the CQDs and the distribution of dierent emissive siteson the surface of the CQDs. However, the exact mechanismaccounting for the excitation wavelength-dependent emissionFig. 2 (A) CQDs with strong absorpti
44、on in the UV region and weakemissions and (B) CQDs with weak absorption in the near UV-vis regionbut strong multicolour emissions in the visible region. (Reproduced withpermission from ref. 13.)Fig. 3 (A) Aqueous solutions of PEG1500N-passivated CQDs (a) excited at 400 nm and (b) excited at the indi
45、cated wavelengths; (B) absorption andemission spectra of PPEI-EI passivated CQDs in water with increasing lexfrom 400 nm on the left with 20 nm increments. Inset: emission intensitiesnormalized to quantum yields. (Reproduced with permission from ref. 7.)Chem Soc Rev Review ArticlePublished on 15 Oct
46、ober 2014. Downloaded by Beijing University of Chemical Technology on 15/10/2014 11:12:54. View Article OnlineChem.Soc.Rev. This journal is The Royal Society of Chemistry 2014remains to be established and the requirement for surfacepassivation in order to produce fluorescence emissions ispoorly unde
47、rstood. Experimental observations are not helpfulsince controversial results are often observed. Moreover, theoptical properties of CQDs are closely associated with thesynthetic routes used in their preparation. For example, Sunand co-workers attributed the fluorescence emissions to theradiative rec
48、ombination of excitons of surface energy traps ofCQDs. Upon surface passivation, these energy traps are stabilisedand therefore become emissive a phenomenon that has beenobserved in semiconductor quantum dots.32They postulated thatthere must be a quantum confinement effect of emissive energytraps on
49、 the surface of the CQDs. Nonetheless, such tunablefluorescence emission property of CQDs provides an addedadvantage in the selection of different emission wavelengths withdifferent excitation wavelengths that can be applied to opticallabelling and fluorescence imaging.In addition to the excitation wavelength-dependent emission,several reports have indicated that the fluorescence emissions ofCQDs are pH-dependent.10,33,34Liu and colleagues noticed thatthe fluorescence intensity of their CQDs decreases when the pHof the solution is shifted from the optimal value of 7.0 re
