Luminescent Carbon Dots: Characteristics and Applications

Submitted bySaurabh Sonia

Project SupervisorProf. Dr. Maria A. Loib

Top Master Program in NanoscienceZernike Institute of Advanced Materials

University of Groningen(14th February, 2016)

as2848481, Top Master Nanoscience studentUniversity of Groningen

email: s.soni.1@student.rug.nlbProfessor of Photophysics and OptoElectronics

FMNS, Zernike Research Institute of Advanced MaterialsUniversity of Groningen

email: m.a.loi@student.rug.nl

Carbon Quantum Dots, or just Carbon Dots (C-Dots) are a recently discoverednew class of fluorescent materials from the nano-carbon family. These C-Dots havecome forth as potential competitors for inorganic quantum dots (QDs) and othertoxic, heavy-metal based materials, because of their characteristics like low toxicity,elemental abundance and biocompatibilty. In the last decade, numerous C-Dots syn-thesis routes have been developed, and it has been observed that different synthesismethods lead to different carbogenic core and surface structure, along with differentcharacteristic properties related to their composition, luminescence, functionalization,bio-compatibility, surface passivation, etc. In this review, it is discussed how thesedifferent synthesis routes and properties of C-Dots can be advantageous in their use inapplications in the fields of bioimaging and medicine, optoelectronic devices, chemicaland bio-sensing, etc. The report mainly focuses on the luminescence behaviour of theseC-dots, discussing their tunable optical behaviour and excitation dependent emission,good electron donor and acceptor tendencies, appreciable quantum yield (QY), elec-tron and charge transfer property, etc., and how can these be used efficiently over theexisting materials and techniques for the aforementioned applications.

CONTENTS

I. Introduction 1

II. Synthesis Methods 2A. Physical Methods 2

1. Laser Ablation Method 22. Arc Discharge Method 33. Plasma Treatment 3

B. Chemical Methods 31. Electrochemical Synthesis 32. Combustion / Hydrothermal /

Solvothermal Synthesis 43. Microwave / Induction-heater assisted

Synthesis 64. Support assisted Synthesis 6

III. Optical Properties 6A. Absorbance 6B. Photo luminescence 7C. Up-Conversion Photo luminescence 9D. Electrochemical Luminescence 10

IV. Applications 10A. Optoelectronic Devices 10B. Bioimaging 13

1. Fluorescence Imaging 132. Multiphoton Bioimaging 143. Cytotoxicity Analysis 14

C. Chemical and Bio-Sensing 15

V. Outlook 15

VI. Conclusion 17

VII. Acknowledgments 18

References 18

I. INTRODUCTION

Several class of materials have shown to exhibit lumi-nescence in different forms, and this has been studiedextensively since long by researchers for various appli-cations in optoelectronics, fluorescence imaging, sensing,photocatalysis, etc.[1–5] However, there has always been aconstant need to look for materials which lead to low costfor synthesis/fabrication, and which are non-toxic, eco-friendly, as well as biocompatible. Carbon dots (C-Dots)are one such highly fluorescent carbogenic nanoparticlesor nano-dots, with average size of roughly 1 - 10 nm,which have shown some of these properties to an appre-ciable extent[6–8].

Since their early accidental discovery by Xu, X. et al.using arc discharge-electrophoresis separation method[9]

in 20004, and synthesis by Sun, Y. P. et al. using laserablation method[10] in 2006, C-Dots have gained atten-tion as new class of highly luminescent quantum dotswith their characteristic advantages like non toxicity, bio-compatibility, elemental abundance, low cost, etc. Theseproperties of C-Dots give them an added advantage overinorganic QDs, which have been used extensively for op-toelectronic and imaging applications[1], though the prac-tical usage of QDs have been limited due to their toxicnature and use of costly heavy metals. Apart from theirstrong and tunable Photoluminescence, C-Dots are alsosuperior in terms of their solubility, functionalizability,photostability and chemical inertness.

Here, we discuss some of the various synthesis meth-ods that have been developed for C-Dots. C-Dots syn-thesis methods can be classified broadly into two routes:

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a) physical synthesis methods, like laser ablation[10–12],arc discharge[9], and plasma induced[13], and b)the chemical synthesis methods, like electrochemicalsynthesis[14–17], hydrothermal[18–20], solvothermal[21, 22],pyrolysis[23–25], microwave-assisted[26–28], and support-assisted[29, 30]. Different synthesis methods using dif-ferent precursors result in different percentage of Car-bon, Oxygen and Nitrogen, with different functionalgroups, and therefore different surface and other func-tional properties. Synthesized C-Dots using differentmethods are found to comprise of amorphous, to nano-crystalline graphitic or turbostatic carbon (sp2 Car-bon), to diamond like core structure (sp3 Carbon).Some synthesis routes also involve surface passivation,via passivating agents like poly(ethylene glycol) (PEG),poly(propionylethylenimine-co-ethylenimine) (PPEI-EI),etc., which holds importance in the fluorescence enhance-ment, functionalization, solubility, etc.

Due to their high oxygen content, many times C-Dotsare also referred to as Carbogenic quantum dots[18, 23, 29],and sometimes C-Dots with graphitic sp2 core struc-ture or 1-10 stacked graphene layers in the core,are also referred distinctively as graphene quantumdots[13, 17, 31, 32]. However, there still is no clear consentamongst researchers regarding this distinction, and theseterminologies have sometimes been used interchangeably.Therefore, for the sake of simplicity, in this review we willaddress them all as C-Dots.

Further, we discuss the interesting luminescence be-haviour of C-Dots and the different explanations thathave been put forward in different publications for theobserved luminescence. Briefly, the luminescence be-haviour of C-Dots is usually believed to be originatingdue to radiative recombination of excitons in core andsurface electronic states, surface functional groups andsurface energy traps, the quantum size confinement ef-fect, etc.[14, 17–19, 23, 24, 26, 30, 32–35] We also discuss thevarious other optical phenomenon that have been ob-served in different C-Dots, like Up Conversion Photo Lu-minescence and Near Infra Red Emission[11, 22, 36], sizeand excitation wavelength dependent emission, Electro-chemical Luminescence[16, 26], etc., and some other ob-served properties of C-Dots like their electron acceptorand donor behaviour[37] and photo-induced charge trans-fer.

These interesting properties of C-Dots have at-tracted researchers to study this new material andlook for the applications where they can be uti-lized as non-toxic and inexpensive alternatives to ex-isting materials. We, therefore, in the next sec-tion discuss the potential use of C-Dots as materialsin optoelectronic devices like solar cells[38] and LEDsfor multicolor and white light emission[24, 27, 39–41]; innanomedicine for single and multiphoton bioimaging, andbiosensing[22, 34, 36, 42, 43]; and in chemical sensing of sen-sitive metal ions and other molecules by fluorescencequenching mechanism[19, 25, 28]. The reported quantumyields of various C-Dots have also been tabulated in theoutlook section.

C-Dots have by now been reviewed many times by re-searchers now[6–8, 31, 44, 45], and these reviews with thisreport aims at encouraging further studies on this newclass of interesting material. However, C-Dots are a rela-tively new class of materials, and some of their character-istics like reason behind strong fluorescence, multiphotonemission, etc. are yet to be understood completely. Un-ambiguity in synthesis methods also result in lower con-trol on size spread and structural non-uniformity, whichin turn affects the luminescence of C-Dots. Thus furtherresearch is needed for studying the mechanisms respon-sible behind different luminescent features of C-Dots,which will also then help in better understanding andutilization of this material for applications in optoelec-tronics, bioimaging, chemical and bio-sensors, etc.

II. SYNTHESIS METHODS

A. Physical Methods

1. Laser Ablation Method

Laser Ablation synthesis of C-Dots was one of the ear-liest synthesis methods used. Sun, Y.-P. et al. used aQ-switched Nd:YAG laser on a Carbon target, preparedusing graphite powder and cement, in the presence ofwater vapor and argon gas[10]. The obtained Carbonnanoparticles exhibited a size spread with no detectablePhotoluminescence (PL). However, when refluxed withHNO3 for 12h, and passivated using PEG1500N - whichare diamine terminated oligomeric poly-(ethylene glycol)

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FIG. 1. Schematic of the one-step laser ablation synthesis ofC-Dots using graphite powder in PEG200N. (Ref [12])

molecules - the obtained C-Dots exhibited bright PL inboth solution-like suspension state and solid state. Otherorganic molecules like poly(propionylethyleneimine-co-ethyleneimine) (PPEI-EI) could also be used as pas-sivating agents. They also demonstrated synthesis ofZnS- and ZnO-doped C-Dots (CZnS-Dots and CZnS-Dots)[11], using the carbon nanoparticles obtained fromlaser ablation[10]. After refluxing with HNO3, dia-lyzing and centrifugating, Zn(CH3COO)2 was addedthrough hydrolysis with Na2S and NaOH for CZnS-Dotsand CZnO-Dots, respectively. The later was annealedto convert Zn(OH)2 to ZnO. Both C-Dots were mixedwith sodium dodecyl sulfate via sonication, were fil-tered, washed and dried, and then mixed thoroughly withPEG1500N. The mixture was heated, stirred and thencooled down followed by centrifugation to obtain CZnS-Dots and CZnO-Dots.

A single step route to create fluorescent C-Dots bylaser irradiation of graphite powder in organic solventswas proposed by Hu, S.-L. et al.[12]. Graphite pow-der dispersed in different solvents like diamine hydrate,diethanolamine, and PEG200N was irradiated with aNd:YAG pulsed laser, with ultrasonication of the mix-ture. The resulting homogeneous black suspension wascentrifuged, and a colorful supernatant containing C-Dots was separated for further analysis. The authorssuggested that the generation of high temperature andpressure by the pulsed laser transferred C-Dots into aplasma state; the cooling down of which resulted in for-mation of C-Dots, and the residual Oxygen formed thesurface functional groups. (figure 1)

2. Arc Discharge Method

Xu et al.[9] demonstrated the separation of C-Dotsfrom Single Walled Carbon Nanotubes (CNTs), preparedfrom arc discharge soot, via a preparative electrophoretic

method. Arc discharged soot was oxidized with HNO3

and extracted with basic water. Agarose gel and glassbead electrophoresis was used to separate C-Dots, whichfluoresced different colors under UV light excitation.

FIG. 2. Schematic describing plasma induced synthesis ofblue fluorescent C-Dots and their application toward whiteLEDs. (Ref [13])

3. Plasma Treatment

Li, C.-X. et al.[13] reported a plasma induced synthesismethod for blue-emitting C-Dots. Acrylamide was placedin a glass dish and was treated with air plasma (CTP-2000K) for 10 min at radio frequency power of 150 W.The resultant dark brown powder was ultrasonicated inethanol, centrifuged and filtered to separate out a trans-parent brown supernatant containing fluorescent C-Dots.

B. Chemical Methods

1. Electrochemical Synthesis

Electrochemical Oxidation process involves the use ofan oxidative agent which breakdowns the carbon precur-sor in various smaller nanoparticles. The Carbon pre-cursor gets oxidized and various functional groups areformed, resulting in enhanced solubility and PL of theC-Dots.

Liu H. et al. first showed the synthesis of C-Dotsby oxidative treatment of soot, collected from a burn-ing candle, using HNO3 as an oxidative agent[14]. Thecollected soot was hydrophobic, and composed of 91.69%Carbon. The collected soot was refluxed with HNO3,and after centrifugation of the collected suspension, theresulting light-brown supernatant exhibited yellow flu-orescence under UV-light excitation (312 nm). Theyalso used other oxidative agents, like H2O2 and AcOH,which exhibited blue fluorescence. The fluorescent C-Dots were purified using polyacrylamide gel electrophore-sis (PAGE), which resulted in violet-blue, green-yellow

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and orange-red C-Dots with different mobilities (whichcould be either due to charge or diameter). The re-sulted C-Dots were found to contain 36.79% C, 5.91% H,9.59% H and 44.66% O, and the FTIR spectroscopy and13C NMR measurements showed the presence of terminaland internal C=C bonds, C=O bonds, but no saturatedsp3 Carbon atoms. The same method was also used byRay, S. C. et al.[46] for synthesizing similar C-Dots, withgraphitic sp2 carbon core.

Tian, L. et al.[15] used the same electrochemical ox-idation method of carbon soot to synthesize graphiticC-Dots. They deposited metal nanoparticles on C-Dotsby adding AgNO3 or Cu(NO3)2 or PdCl2 solutions tothe C-Dots solution, with drop-wise addition of ascorbicacid under magnetic stirring. Change of color from light-brown to dark-red for Au, dark-yellow for Cu, and blackfor Pd indicated the formation of these C-Dots.

Zheng, L. et al.[16] performed electrochemical synthesisof C-Dots in an electrochemical cell consisting of graphiterod working electrode, an Ag/AgCl reference electrode,a Pt mesh counter electrode and a pH 7 phosphate buffersolution. With the increase in number of scan cycles ofpotential sweep from -3V to +3V, the solution turneddark-brown from yellow, indicating the formation of C-Dots, with an average size of 5 nm.

Y. Li et al.[17] demonstrated electrochemical prepara-tion of functional C-Dots by using cyclic voltammetryscan within ±3.0 V in Phosphate Buffer Solution. Usingfiltration-prepared graphene films as working electrode,Pt wire as counter electrode and Ag/AgCl as referenceelectrode, the as-prepared homogeneous C-Dots were fil-tered and dialyzed and re-dispersed in deionized water.The collected C-Dots were monodispersed with averagediameter of 3 to 5 nm. The LUMO level was found elec-trochemically to be ∼ 4.2-4.4 eV. The C-Dots were foundto contain hydroxyl, carbonyl and carboxylic acid groupson surface, making it soluble and functionalizable.

2. Combustion / Hydrothermal / Solvothermal Synthesis

Peng, H. et al.[18] reported synthesis of C-Dots us-ing carbohydrates via hydrothermal synthesis. The car-bohydrates were dehydrated using sulphuric acid andthen treated with HNO3, which broke down the carbona-

ceous material into smaller carbogenic nanoparticles withblue shift in emission, but with weak fluorescence. Thenanoparticles were then surface passivated with 4,7,10-trioxa-1,13-tridecanediamine, resulting in bright PL anddramatic increase in quantum yield. The carboxylicgroups of surface of bare C-Dots were converted to amidefunctional group. The C-Dots were found to comprise of57.03% C, 26.95% O, 8.51% N and 7.51% H, with thepresence of C=C, C=O, C=ONR and CON-HR bonds.TEM and XRD showed crystalline structure with latticespacings of (002) plane to be 3.2Å and 3.4Å, respectively,which matched to that of turbostatic carbon disorders.

Zhang, W. et al.[40] prepared C-Dots by organic acid,decomposing it in silane coupling agent. The formedC-Dots were mixed with commercially available epoxyoligomer. After degassing the mixture in vacuum, itwas cured by continuous heating into solid C-Dots film.The obtained C-Dots were found to be composed of sp2

graphitic core with size in the range of 2.0 to 3.6 nm,and lattice spacing of 0.2 nm. The possibility of deposit-ing C-Dots on a large and flexible substrate helped infabricating white light emission devices.

Wang, F. et al.[21] reported a "hot-injection" synthe-sis method in which citric acid was used as carbon pre-cursors was carbonized to form oil-soluble C-Dots, byusing octadecene as hot, non-coordinating solvent and1-hexadecylamine as passivating agent at 300◦C, underargon flow. They also synthesized water-soluble C-dotsby changing the solvent to glycerin and surface passi-vating agent to PEG1500N. These resulted in very highquantum yield and the change in PL with the reactiontime was attributed to the evolution of the surface of C-Dots and changing the emissive sites. The surface of bareC-Dots was found to containing carboxyl groups, whichwere converted to amide groups during passivation. TheXRD characterization reported amorphous nature of thesynthesized C-Dots.

A similar synthesis of C-Dots was presented by Bourli-nos, A. B. et al. via thermal carbonization of citric acidmonohydrate as precursor[23]. Organophilic C-Dots wereobtained by dissolving precursor in ethanol followed byadding ethanol solution containing C18H37NH2. The so-lution was dried and calcined at 300◦C. For obtaininghydrophilic C-Dots, precursor was dissolved in water anddiglycolamine (HOCH2CH2OCH2CH2NH2), followed by

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drying and heating hydrothermally. They also presentedanother route where 4-Aminoantipyrine(4AAP)30 wasdirectly calcined, and the obtained solid dissolved inCF3CH(OH)CF3 and water and washed multiple timesto obtain C-Dots. Characterization results indicatedthat hydrophilic C-Dots were more amorphous thanorganophilic, probably due to H-bonding and shortercorona. Later, they also reported thermal oxidationof ammonium carboxylate salt (NH4

+COO-), to formC-Dots with carbonaceous core and amino carboxylatefunctionalized surface[29]. Their support assisted synthe-sis is described in the later sections.

Jiang, K. et al.[22] reported the use of solvothermalmethod for synthesizing green, blue and red PL C-Dotsfrom ortho, meta and para-phenylenediamine, respec-tively, as precursors. Purification was done using silicagel chromatography. TEM and AFM measurements re-vealed monodispersion and average size of C-Dots to be8.2, 6.0, and 10.0 nm, respectively, with particle heightaround 2-3 nm. FTIR and XPS measurements showednew bonds C-H, C-N= and C=O, compared to the pre-cursors. The varying nitrogen content, 7.32%, 3.69%,and 15.57%, might be the reason for different QY.

Liu, L et al.[19] synthesized C-Dots from candle sootby hydrothermal reaction. Collected candle soot was son-icated in NaOH solution and heated at 200◦C in a poly-tetrafluoroethylene reaction kettle. The product oncecooled, resulted in a brown-yellow supernatant which wascentrifuged and then netralized with HCl, followed bydialysis. The C-Dots were sized around 3 nm, containedhydroxyl and carbonyl groups, and zeta potential mea-surements indicated negatively charged surface.

Ganiga, M. et al.[35] reported synthesis of Nitrogen richgraphitic C-Dots by simply carbonizing ethylenediamine(EDA), using P2O5 in the presence of water. The car-bonized product was diluted and supernatant dialyzedbefore further analysis. The FTIR, XPS, Raman mea-surements show the presence of high Nitrogen content,and bonds and functional groups containing Nitrogenwith Carbon and Oxygen. Li, F. et al.[41] also demon-strated that very high QY C-Dots could be produced bysynthesizing Mg/N-rich C-dots. They demonstrated hy-drothermal oxidation of citric acid to form C-Dots usingMg(OH)2 as chelation agent and EDA as surface passi-vating agent. They showed that QY of this C-Dots were

FIG. 3. Formation process of the C-Dots formed from thios-alicylic acid and EDA via hydrothermal synthesis. (Ref. [20])

reduced if Mg(OH)2 or EDA was not added during syn-thesis.

Recently, Mao, L.-H. et al.[24] reported synthesis ofwhite C-Dots by pyrolysis of poly(acrylic acid) (PAA)in glycerol, in a Nitrogen environment. The productobtained from pyrolysis was first diluted and then cen-trifuged to remove large particles, followed by dialysis toremove excess glycerol. They also showed that pyrolysisof PAA in water instead of glycerol resulted in synthesisof blue C-Dots. It was observed that, upon treatmentwith glycerol, blue C-Dots gave emission that was verysimilar to white C-Dots.

Feng, X. T. et al.[20] prepared C-Dots via one-step hy-drothermal synthesis. Thiosalicylic acid and EDA, asprecursors, were mixed with water and heated in a Teflonautoclave. The collected C-Dots solution was filtered anddialyzed. The average size of C-Dots was 2.4 nm, andcould be tuned with reaction time and temperature.

Mirtchev, P. et al.[38] used thermal dehydration of γ-butyrolactone by H2SO4, to produce C-Dots as sensitiz-ers for TiO2 solar cells. γ-butyrolactone was heated at100◦C and then concentrated H2SO4 was added. The so-lution color change from clear to yellow, to dark-brown.The solution was neutralised using Na2CO3, cooled downand filtered to obtain C-Dots solution, containing sp2

C=C groups in the core while sulfonate, carboxylate andhydroxyl functional groups at surface, enhancing it’s sol-ubility in polar aqueous and organic solvents.

Zhou, L. et al.[25] produced C-Dots by pyrolysis ofEthylenediamine-tetraacetic acid salts at 400◦C in a N2

environment. The obtained powder was dissolved in ace-tone, centrifuged, dried and re-dissolved in water to formC-Dots solution. The particles were on average 3.8 nmin size and had carboxyl and hydroxyl groups on surface,making them readily soluble.

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3. Microwave / Induction-heater assisted Synthesis

An economical and facile Microwave pyrolysis methodto synthesize C-Dots in minutes was reported by Zhu, H.et al [26]. Different amounts of PEG-200 with saccharides(glucose, fructose, etc.) were dissolved in water and thenheated in a Microwave oven at 500 W for 2-10 min. Thechange of color from colorless to yellow to light-brownindicated the formation of C-Dots. The obtained C-Dotsshowed amorphous nature, and presence of C=O, C=C,C-O-C, C-H and O-H bonds.

Wang, X. et al.[34] also reported a microwave synthesisprocedure which used carbohydrates (glucose, sucrose,glycerol, and glycol) as carbon precursors, with a verysmall amount of inorganic ion but without any passi-vating agents. These presented a green synthesis route,without the use of any sophisticated instrumentation.

Sk, M. P. et al.[28] reported synthesizing C-Dots withhigh QY using an induction coil heater. Here, citric acidand ethylenediamine (EDA) were dissolved in water in anon-sticky heating pan and then heated at 100◦C (500W)for 12 min. The obtained black-brown syrup was dis-solved in water and dialyzed to obtain C-Dots. Theyalso reported large scale synthesis procedure. They re-ported QYs of 12 other C-Dots formed by varying theprecursors and the surface passivating agents. C-Dots el-emental composition was 39.76% C, 6.31% H, 10.03% Nand 43.90% O, with sp2 as well as sp3 C present.

4. Support assisted Synthesis

Synthesizing C-Dots on the surface of NaY Zeolite(solid host) was reported by Bourlinos, A. B. et al.[29].Ion-exchange of NaY zeolite with 2,4-diaminophenol di-hydrochloride, followed by their thermal oxidation re-sulted in synthesis of NaY zeolite supported C-Dots,which showed lattice spacing similar to crystallinegraphitic carbon and presence of C=C, C=O, C=N andC-O bonds. The zeolite matrix could be easily etchedaway using hydrofluoric acid, resulting in C-Dots withaverage particle size of 4-6 nm.

Liu, R. et al.[30] presented a C-Dot synthesis methodusing surface modified silica sphere as carriers and resolsas carbon precursors. SiO2 suspension mixed with a so-

FIG. 4. Schematic of support assisted synthesis of C-Dotsover NaY zeolite. On the left, the black spheres are C-Dotswhich grow over a zeolite matrix. (Ref. [29])

lution of ampiphilic triblock copolymer F127 was cen-trifuged and re-dispersed in NaOH, followed by additionof resols. The resol/F127/SiO2 composite was heatedand stirred for polymerization, and then calcined at900◦C in Ar to result in C-Dots/SiO2 composite. Thesilica spheres were significantly etched away using NaOHsolution, followed by neutralization using HNO3. Amor-phous C-Dots were obtained with both the sp2 and sp3

carbon atoms present. The obtained C-Dots were re-fluxed with HNO3 for oxidation, followed by addition ofPEG1500N as passivating agent to attain PL emission.

FIG. 5. Schematic for synthesis of C-Dots via silica sphere assupport and resols as carbon precursor. (Ref. [30])

III. OPTICAL PROPERTIES

A. Absorbance

As mentioned previously, differently synthesized C-Dots have different optical properties, and therefore dif-ferent possible explanations of C-Dots’ absorbance. Theabsorption by C-dots is mostly observed in UV range,

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which is attributed to the core and surface states transi-tions, n-π* and π-π* C=C transitions, other functionalgroups, and/or quantum size effects. While C-Dots syn-thesized by various physical and chemical methods haveshown one or two absorbance peaks in 260-360 nm UVrange for UV-Vis spectroscopy[10, 19, 26, 28, 34], some onsurface passivation with surface passivating agents, orupon functionalization have also shown significant red-shift in absorbance peak[18, 22].

B. Photo luminescence

Differently C-Dots have been found to show a broadrange photoluminescence (PL) emission spectra. It hasbeen observed that for most of the synthesized C-dots,emission spectra dependent on excitation wavelength orsize. Laser-ablated synthesized C-dots from candle sootby Sun et al.[10] also showed emission which varied withthe wavelength of the excitation light, covering the entirevisible region. It should be pointed out that the passi-vating agents used here, PEG1500N and PPEI-EI, have nointrinsic emission in this region. Thus the PL in C-dotsis attributed to the surface energy traps which becomestabilized over surface passivation. This kind of emissionspectra is characteristically observed in various synthe-sized C-Dots, as we would be discussing in this section,and is often attributed to the size distribution of C-Dots,or the distribution of various emissive sites on the surfaceof C-Dots.

C-Dots synthesized by Liu H. et al.[14] using candle

FIG. 6. Aqueous solution of PEG1500N-passivated C-Dots (a)at λexc = 400 nm, photographed through band-pass filtersof indicated wavelengths, and (b) photographs of solutionsexcited at the indicated wavelengths. (Ref. [10])

soot by electrochemical oxidation and purification usingdenaturing polyacrylamide gel electrophoresis (PAGE),resulted in different bands of violet-blue, green-yellowand orange-red C-Dots. The different C-Dots had similarexcitation but different emission spectra. The emissionpeaks ranged from 415 nm (violet) to 615 nm (orange-red), and also the PL curves broadened for higher wave-length emissions, which could be attributed to incompleteseparation by PAGE gel, as the size spread was prettysmall.

Surface functionalized organophilic and hydrophilic C-Dots synthesized by hydrothermal carbonization of citricacid by Bourlinos, A. B. et al.[23] also showed red shiftof emission peak with increase in excitation wavelength.They also reported the same emission behavior by the4-Aminoantipyrine(4AAP)30 - C-Dots, prepared by air-pyrolysis of 4AAP. Ultrafine size of C-Dots, along withthe structure disorder, enhances the surface energy trapsite densities, and this was the possible reason stated forthe observed emission.

Multicolor PL spectra by surface passivated C-Dotssynthesized from carbohydrates by Peng, H. et al.[18] alsoshowed dependence on excitation wavelength. With in-crease in excitation wavelength, the emission peak shiftstowards longer wavelengths, decreases in intensity andalso broadens. The emission intensity also depends onthe acid reflux duration and the starting carbohydrateused for synthesis. Recombination of excitons and sta-

FIG. 7. Absorption (ABS) and the characteristic excitationdependent luminescence emission spectra (with progressivelyincreasing excitation wavelengths by 20 nm, starting from 400nm on the left) of aqueous solution of PPEI-EI C-Dots. Insetshows the normalised intensity spectra. (Ref. [10])

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bilization of surface energy traps via passivation couldbe the possible mechanism behind this kind of emissionspectra. Besides this, charge transfer between N and Ccan not be ruled out as a possible mechanism for observedemission, as this has been reported earlier in the case ofCNTs[47].

Zhu, H. et al.[26] also showed that C-Dots synthesizedvia Microwave pyrolysis method, of carbohydrate and in-organic ions, also exhibited excitation wavelength depen-dent emission. It was pointed out that the passivatingagent (PEG-200) contained no emissive chromophores inUV-visible region, and the bare C-Dots without PEG-200 showed no or very weak luminescence. Therefore,the bright, stable luminescence probably is due to thesurface-passivated C-Dots. Similarly, microwave synthe-sized C-Dots by Wang, X. et al.[34] also reported pH-independent, stable and multicolor PL emission that de-pended on excitation wavelength, choice of starting ma-terial, as well as the valency of the inorganic ion usedin synthesis. It was reported that the microwave treatedcarbohydrate and inorganic ions had no luminescence oftheir own, and therefore, the luminescence was attributedto C-Dots. The surface energy traps were believed to bestabilized by the surface functional groups, making thememissive. QY of C-Dots was reported to be increasingwith increasing valence of the ions.

Solvothermally synthesized red, green and blue C-Dotsfrom phenylenediamine isomers by Jiang, K. et al.[22]

showed PL peaks at λ = 604, 535, and 435 nm, re-spectively, in an ethanol solution (λexc = 365 nm). PLdecay lifetime measurements showed mono-exponentialdecay for red and green C-Dots, while much shorter bi-exponential decay for blue C-Dots. The PL of these C-Dots was also found to be highly stable under continu-ous excitation. Amongst these 3 C-Dots, only particlesize and Nitrogen content was found to vary, which werebelieved to be partly responsible for the different PL be-haviour.

Hydrothermally prepared C-Dots from candle soot byLiu, L et al.[19] showed absorption and emission char-acteristics at 310 and 450 nm, respectively. The largeStokes shift helped in contrast-imaging C-Dots againstbackground. The emission was excitation wavelength aswell as pH dependent, but was stable only for pH>7.The C-Dots also showed low toxicity and photostability

FIG. 8. Photographs of C-Dots’ solutions (synthesized usingmeta-PD, ortho-PD, and para-PD) dispersed in ethanol indaylight (left), and under λ=365 nm UV light (right). (Ref.[22])

with time. Here, due to the absence of any intrinsicallyemissive chromophore, the fluorescence was attributedto quantum size effect, and the short PL decay lifetimeasserted that it should be due to the radiative recombi-nation of the excitons. It was also observed that as car-boxyl groups have electron withdrawing tendecies, whilethe hydroxyl groups have electron donating tendencies,emission by C-Dots with carboxyl group covered surfacewas less than the hydroxyl group covered C-dots, becausecarboxyl groups would absorb some of the emissions.

C-Dots synthesized using using surface functionalizedsilica spheres as carriers, by Liu, R. et al.[30], showedbroad excitation-dependent emission spectra, rangingfrom violet (430 nm) to yellow (580 nm). Here too,since neither the bare C-Dots or PEG1500N were intrin-sically emissive in this range, the emission (multicolor)was attributed to the quantum size effects and energytrap states which were stabilized by passivation, via ra-diative recombination process of excitons.

Nitrogen-rich C-Dots synthesized by Ganiga, M. etal.[35] by carbonizing ethylenediamine showed character-istic electronic transitions at 230, 275, 335 and 385 nm.Peaks corresponding to 230 nm and 335 nm were at-tributed to π − π* absorption by aromatic sp2 carbonand n-π* absorption by C=O group, respectively. Thetransitions at 275 nm and 385 nm could be due to C=Ngroup, and J-type aggregation, respectively. This C-Dotsalso showed excellent photostability, which could be at-tributed to the presence of N-functional groups whichreduced the chances of quenching due to aggregation.The broad range emission spectra was excitation depen-dent as typically observed for C-Dots, and white lightemission was observed under UV excitation (365 nm).Similarly, induction heater prepared C-Dots using citricacid and ethylenediamine[28] also showed excitation de-

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pendent emission and stability with pH.Mg/N-rich C-Dots prepared by Li, F. et al.[41] also

showed non-uniform and gradual red-shift of PL emissionwith increasing excitation wavelength. The PL intensityalso decrease, and the maximum PL emission was foundat 437 nm wavelength. A blue shift in PL peak was ob-served for Mg-doped C-Dots. The PL emission showedstability with pH. The high luminescence and QY of C-Dots was attributed to the chelation by Mg and passiva-tion by EDA (N-doping).

Electrochemically synthesized C-Dots by Y. Li etal.[17] using cyclic voltammetry also exhibited excitation-dependent emission features. Under UV irradiation, thisC-Dots showed absorption peak at 320 nm and strongPL peak at 473 nm. However, these C-Dots distinctivelyshowed green luminescence under excitation by 365 nmlamp, which was attributed to the effect of size and func-tional groups.

Blue and white C-Dots prepared by Mao, L.-H. etal.[24] via pyrolysis of poly(acrylic acid) (PAA) showedstrong and stable PL in suspended state. PL emissionpeaks red-shifted with increase in excitation wavelengthwith maximum PL intensity at 420 nm. The PL mech-anism was supposed to be due to particle size confine-ment and distribution of different emissive sites over dif-ferent nanoparticles. Both PAA and glycerol showed neg-ligible PL themselves, so the PL should be attribute tothe synthesized C-Dots. Due to a number of hydrophilicfunctional groups present on white C-Dots’ surface, thesecould be dissolved in various solvents, and solvent depen-dent PL emission was demonstrated. Time resolved PLmeasurements suggested that compared to blue, whiteC-Dots had higher lifetimes. This meant less radiativeenergy traps and more number of fluorophores, due to

FIG. 9. a) UV-Vis and PL spectra (λexc=360 nm) of the Mg- EDA - C-Dots aqueous solution (Insets show photographsunder natural and UV light). b) PL spectra (λexc=360 nm)of C-Dots with different doping of Mg and EDA. (Ref. [41])

surface passivation by glycerol and higher carbonizationof white C-Dots. The white C-Dots also maintained theirPL intensity even in strong oxidative and reductive envi-ronment.

C-Dots synthesized using γ-butyrolactone by Mirtchev,P. et al.[38] showed excitation wavelength dependent PLemission without any use of a passivating agent. The dis-tribution of various emissive trap sites on C-Dots’ surfacewas supposed to be responsible for the observed PL. Re-combination of photogenerated excitons is likely to hap-pen at these sites, which also leads to lower current den-sity in fabricated solar cell.

C. Up-Conversion Photo luminescence

C-Dots show comparable up-conversion photo-luminescence (UCPL) as compared to the QDs or thecore-shell nanoparticles. Cao, L. et al. showed thatstrong PL is shown by C-Dots when they are excited bytwo photons in near-infrared (NIR) range[36], which canbe utilized for multiphoton imaging as we will discusslater. The C-Dots emitted strongly in the visible regionwhen excited by 458 nm laser (visible) and 800 nmfemtosecond pulse laser (NIR) for two-photon excitation.The process is a two photon process was confirmed bythe quadratic relationship between the luminescenceintenstiy and the laser power.

CZnS-Dots and CZnO-Dots synthesized by Sun, Y.-P. et al.[11] also showed strong photolumienscence un-der multiphoton excitation, i.e. under 800 nm femtosec-

FIG. 10. a) Absorbance (Abs), luminescence (solid line),the one-photon emission (2, 458 nm excitation) and two-photon emission (◦, 800 nm excitation) spectra of C-Dots.b) Quadratic relationship between the luminescence intensityand excitation laser power indicating that the UCPL is a two-photon process. (Ref. [36])

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ond pulse laser excitation. The luminescence was similarto that of original C-Dots (same as used by Cao, L. etal.[36]), and the luminescence obtained was similar to thatobtained under 458 nm single photon excitation. Thesesuggests the potential of doped C-Dots for one- and two-photon luminescence imaging and other applications.

Bright UCPL emission was observed by solvothermallysynthesized multicolored C-Dots using o-, m- and p-phenelynediamine precursor, under an 800 nm femtosec-ond pulse laser excitation[22]. The quadratic dependenceof intensity on laser power demonstrated a two-photonexcitation process.

D. Electrochemical Luminescence

QDs have shown to be of use as Electrochemical Lu-minescent (ECL) agents in various bioanalytical applica-tions, but their toxicity and non-biocompatibility restricttheir practical uses. Zheng, L. et al.[16] have shown thatC-Dots can also show ECL signal and can thus act asalternatives of QDs as ECL agents. These show promis-ing possibility of utilizing the ECL activity of C-Dots indevelopment of new biosensors and display devices. Fig-ure 11 shows schematically the ECL and PL mechanismin C-Dots. In the study performed by the authors, theECL behaviour of C-Dots was quite similar to that ofinorganic QDs. Here R represents C-Dots, R* is the ex-cited state of C-Dots, R•+ is the oxidized state, and R•−

is the reduced state.Zhu, H. et al.[26] reported a relatively stable ECL ac-

tivity of C-Dots, which they also found that was quitesimilar to the QDs. Here it was pointed out that thecathodic ECL intensity was smaller than the anodic, im-

FIG. 11. Schematic of the ECL and PL mechanisms in C-Dots. R•+, R•−, and R* represent negatively charged, pos-itively charged, and excited-state C-Dots, respectively. (Ref[16])

FIG. 12. Representative ECL response (a) without C-Dotsand (b) with C-Dots at an ITO electrode in 0.1 M PBS (pH7.0). (Inset: Anodic ECL response during a continuous po-tential scan, υ = 0.1 V s-1.) (Ref. [26])

plying that R•+ was less stable than R•- (figure 12).

IV. APPLICATIONS

A. Optoelectronic Devices

Wang, F. et al.[27] demonstrated for the first time thatC-Dots can be used in White Light Emitting Diodes(LEDs). The C-Dot synthesized using the method dis-cussed earlier[21] were used for the device fabricationas shown in the figure 13. At current density of 0.05mA/cm2, low external quantum efficiency of 0.058% andhigh turn-on voltage of 6V was obtained, probably duetoo surface passivation. Driving voltage independentCRI index of 82 and CIE coordinates (0.40, 0.43) wereobtained.

C-Dots prepared by Zhang, W. et al.[40] could be de-posited on a large and flexible, 0.15 mm thick polyethy-lene terephthalate substrate. This thin C-Dots film wasplaced on blue LED and this resulted in a bright whitelight emission under 460 nm excitation. The power con-version efficiency was found to be more than 60%, andthe correlated color temperature could be tuned with C-Dots film thickness.

White LEDs were also fabricated by Li, C.-X. et al.[13]

using C-Dots synthesized by plasma-induced method.Silver paste was used to attach the bottom of the con-ducting silver cup to the UV-LED chip, with λ centered

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FIG. 13. (a) Layer structure of the C-Dots’ white LED’s cross-section. (b) Suggested energy band diagram of the C-Dotswhite LEDs, relative to ITO and LiF/Al work functions. Theenergy level of the C-Dots were estimated by electrochemicalcyclic voltammetry. (c) Molecular structures of PEDOT: PSSand TPBI, and a schematic of the C-Dots. (Ref. [27])

at 380 nm. C-Dots in ethanol solution were mixed withsilicone and two kinds of CdTe QDs. The mixture wasovercoated on chip and cured at 150◦C for an hour, fol-lowing by capping the cup with optical lens and fillingvoid with transparent silicone. The LED was cured againat 150◦C for an hour. The blue-emitting C-Dots herewere mixed with the two kinds of QDs with the emis-sion at 525 (green) and 620 (red), with 380 nm UV-chipunderneath as an excitation light source. The white emis-sion obtained from the LED had a CRI value of 87.

Nitrogen-rich C-Dots by Ganiga, M. et al.[35] showedbroad range emission including emission in primary colorregions like red, green and blue, which resulted in white

FIG. 14. C, D, E: Photographs of N-rich C-Dots solution,N-rich C-Dots solid powder, and N-rich C-Dots coated over apolymeric surface, respectively, in visible light. C’, D’, E’:Corresponding photographs under UV light (365 nm) showingwhite light emission in all 3 forms. (Ref. [35])

light emission behaviour. Unlike large number of organicmolecules and inorganic QDs, this C-Dots showed whitelight emission in solution as well as solid state, implyingtheir potential for solid state device applications. Steadystate emission by C-Dots coated on PVA films is shownin figure 14.

Following Wang, F. et al.[27], fabrication of solutionprocessable color switchable LEDs, based on layeredstructures, were reported by Zhang, X. et al.[39], as shownin figure 16. Depending on the injected current den-sity, the same 3.3 nm C-Dots layer in the LED structureemitted steady white and blue, or tunable blue, cyan,magenta and white lights. Maximum brightness of 24cd/m2 for blue light and 90 cd/m2 for white light wasrecorded. Analysis of electroluminescence (EL) and timeresolved PL spectra showed that three different recombi-nation processes with different decay lifetimes were actu-ally responsible for different color of emissions at differentinjected current densities.

Mao, L.-H. et al.[24] also reported fabrication of whiteLED with C-Dots as white light converter, and a UV-backlight for LED using C-Dots as coatings. For fabrica-tion of white LED, UV-LED chip was attached at bottomof LED base, the thermocurable resin was mixed withwhite C-Dots synthesized earlier, and placed in vacuumto remove bubbles. This white C-Dots and resin mix-ture was cured at 150◦C and dispensed on the LED chip.For the backlight, concentrated solution of white C-Dotswas cast on glass or polypropylene, and kept in open atroom temperature to remove the residual solvent. Feng,X. T. et al.[20] also followed similar method to utilizehydrothermally prepared C-Dots for single white lightconverter in LEDs.

C-Dots synthesized by Mirtchev, P. et al.[38] were used

FIG. 15. Left: C-Dots-sensitized TiO2 nanoparticle and theproposed C-Dots-TiO2 bonds. Right: CurrentâĂŞvoltagecharacteristics of aqueous C-Dots-sensitized solar cell (Ref.[38])

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FIG. 16. (a) Current density (blue) and luminance (black) of the C-Dots LEDs emitting blue, cyan, magenta, and white light(Inset: the device structure comprising ITO/PEDOT:PSS (anode), poly-TPD (HTL), C-Dots (active layer), TPBi (ETL), andLiF/Al (cathode)) (b) the luminous (black) and power efficiencies (blue) vs current density plots) (c) EL spectra at 9, 8, 7 and6 V (Insets: true color photographs of blue, cyan, magenta, and white emissions) (d) CIE1931 coordinates of the blue, cyan,magenta, and white emission from the same C-Dots LEDs operated under different voltages. (Ref. [39])

FIG. 17. a) Device structure b) Energy band diagram of de-vice c) J-V characteristics of ITO/PEDOT:PSS/P3HT/Al,and ITO/PEDOT:PSS/P3HT:GQDs/Al before and after an-nealing. (Ref. [17])

as sensitizers in nanocrystalline TiO2 solar cells. Thesurface functionalized C-Dots could easily anchor to TiO2

surface. For fabrication, aqueous TiO2 paste was doctorbladed on FTO substrate. The films were calcined at450◦C and immersed in the C-Dots solution in dark. Ptcoated FTO was used as counter electrode, and was fusedwith other electrode by melting transparent thin film ofpolymer basket in between. The cell was infiltrated withsmall amount of Iodolyte. For this solar cell, Jsc wasfound to be 0.53 mA cm-2, Voc as 0.38 V, fill factor of0.64, and power conversion efficiency of 0.13%.

Organic photovoltaic cells were fabricated by Y. Liet al.[17] as shown in figure 17. PEDOT:PSS was spincoated on an ITO substrate, followed by deposition ofactive layer from P3HT and P3HT:C-Dots chlorobenzenesolution. Fabrication was then completed by thermal de-position of Al layer. The P3HT:C-Dots device was an-nealed at 140◦C for 10 min. J-V characteristics are shownin figure 17 for solar cell with P3HT as active layer with-out C-Dots doping, and for solar cell with P3HT:C-Dots(10% doped) as active layer before and after annealing.It was observed that the device performance of C-Dotsdoped solar cell was enhanced compared to a typical un-doped P3HT device. C-Dots facilitate separation of exci-tons, which lead to increased short circuit current, Isc;

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and because of the increase in surface area, the elec-tron mobility of polymer based (P3HT) device is alsoenhanced after C-Dots doping. The power conversion ef-ficiency of the annealed device reached upto 1.28%.

B. Bioimaging

1. Fluorescence Imaging

It has been well established that the commerciallyavailable CdSe/ZnS QDs are very effective as opticalimaging agents for fluorescence microscopy. Yang, S.-T. et al.[42] have showed that C-Dots are non-toxic and,indeed, competitive in performance with these core-shellQDs for fluorescence imaging. It was shown that C-Dotswere less homogeneous for the brightness of individualdots, but showed comparable or higher fluorescence in-tensity as compared to QDs.

C-Dots synthesized by Wang, X. et al.[34] via Mi-crowave synthesized method were also reported to readilyenter E. coli and 293T cells in vitro, and there multi-color luminescence showed potential use for biolabellingand bioimaging.

The first reported in-vivo study of C-Dots for opticalimaging in live mice was done by Yang, S.-T. et al.[43],where the C-Dots acted as bright fluorescent and contrastagents. C-Dots and ZnS doped C-Dots (CZnS-Dots) withPEG1500N as passivating agent were used. During theexperiments, no animals exhibited any form of toxicolog-ical effects, implying the biocompatibility and nontoxiccharacteristics of C-Dots for imaging and other biologicalapplications. This is shown in figure 18.

Multicolor C-Dots prepared by solvothermalprocess[22] showed different color bright fluorescence(λexc = 405nm) under confocal microscope when in-cubated with MCF-7 cells. The images showed thatC-Dots readily penetrated the cell membrane, with90% cell viability. Study was also reported by Ray, S.C. et al.[46] of fluorescent C-dots for cell-imaging withminimum cytotoxicity. It was found that C-Dots enteredcells readily without any required functionalization, andit was possible to track these C-Dots by fluorescenceimaging. Cells without C-Dots were colorless, butupon incubation with C-Dots gave bright blue-greenfluorescence under UV light excitation, and yellow

FIG. 18. Subcutaneous injection of (top) C-Dots and (bot-tom) CZnS-Dots in mice. (a) Photograph under bright field,(b, d) as-detected fluorescence (excitation/emission wave-lengths indicated), and (c, e) color-coded images. (Ref. [43])

FIG. 19. Top: Labelling of EAC cells using C-Dots underbright field, UV excitation, and blue excitation. Bottom:Corresponding images without C-Dots. (Ref. [46])

fluorescence under blue lights excitation. This is shownfor EAC cells in the figure 19.

Carrier-assisted synthesised PEG1500N-capped C-Dotsby Liu, R. et al.[30] also showed biocompatibility andbioimaging potential of C-Dots. C-Dots incubated E.colicells gave bright PL for a broad range of excitation wave-length. Same C-Dots were also found to enter the P19progenitor cells. These C-Dots showed photostability,with no blinking.

Fluorescence imaging of Mg/N-doped C-Dots preparedby Li, F. et al.[41] for L929 cells, incubated with C-Dotconcentration of 100 µg mL-1, was performed. They ob-served that C-Dots labelled cells gave bright and non-blinking blue, green, and red fluorescence when excitedat 405 nm, 488 nm, and 543 nm, respectively. It was alsoobserved that C-Dots distributed over cell membrane andcytoplasm, while very less C-Dots were present inside thenucleus.

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2. Multiphoton Bioimaging

Apart from fluorescence bioimaging using C-Dots,Cao, L. et al.[36] showed that C-Dots also held poten-tial for use in two-photon luminescence bioimaging fordetection of MCF-2 human breast cancer cells. CulturedMCF-7 cells incubated with C-Dots showed bright lu-minescence under fluorescence microscope upon excita-tion with 800 nm laser pulses, labelling the cell mem-brane and cytoplasm without entering the nucleus of thecells. Yang, S.-T. et al.[42] also reported the two-photonbioimaging of C-Dots were comparable to the single pho-ton fluorescence brightness of the QDs.

Red, green and blue two-photon UCPL features wereobserved under confocal microscope when C-Dots syn-thesized from o-, m- and p-phenylenediamine were incu-bated with MCF-7 cells, implying that they could be usedfor deep tissue bioimaging[22]. The UCPL was observedin both solution and Poly(vinyl alcohol) composite filmsunder a femtosecond pulse laser (λ = 800 nm).

3. Cytotoxicity Analysis

Since cytotoxicity studies are extremely important foruse of any material in biomedical applications, C-Dotshave been studied for their cytotoxicity effects in-vitroand in-vivo environments. Liu, L et al.[19] reported thatC-Dots prepared from candle soot by hydrothermal syn-thesis showed low cytotoxicity. The assertion was made

FIG. 20. Cytotoxicity analysis of C-Dots for Hela cell per-formed using CKK-8 method. (Ref. [19])

FIG. 21. Results from cytotoxicity evaluations of C-Dots(black) and PEG1500N (white). (Ref. [42])

after seeing the results of toxicity affects of these C-Dotson Hela cells studied by CKK-8 method, as shown infigure 20.

Yang, S.-T. et al.[42] also reported the toxicity evalua-tions, as shown in figure 21, based on the effects of C-Dotsand PEG1500N on proliferation, mortality and viability ofhuman breast cancer cells (MCF-7) and human colorectaladenocarcinoma cells (HT-29). The results showed thatC-Dots affected these parameters for these cells only asmuch as PEG1500N did, and also they did not imposeany significant toxic affects on mice at dosage levels evenbeyond that is used for in-vivo optical imaging applica-tions. The cell concentration and exposure time weretaken more than that used for most practical biomedicalapplications. Thus, it was concluded the C-Dots shouldbe considered as non-toxic substances.

Mg/N-doped C-Dots synthesized by Li, F. et al.[41]

also showed 90% cell viability for L929 cells using MTTassay method when incubated with C-Dots at concentra-tion levels of 250 µg mL-1 or even lesser.

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C. Chemical and Bio-Sensing

It has been reported in several studies that C-Dotscan act as efficient sensors for detecting various metalions, bio-molecules, etc. even for very small concentra-tion values. Usually, the sensing is done by observingthe quenching of the fluorescence of the C-Dots due tothe presence of the target specie. Different C-Dots havealso helped in selective sensing, where they can senseselective target species even in the presence of similarions/molecules/etc. in the ensemble.

Application of C-Dots from candle soot in detect-ing Hg2+, Cr3+, Al3+ and Fe3+ was shown by Liu, Let al.[19]. Theoretically, metal ions with low solubil-ity tend to combine with hydroxyls on C-Dots’ surface.These leads to the aggregation of C-Dots, resulting inthe quenching of their fluorescence. It was shown thatHg2+, Cr3+, Al3+ and Fe3+ quenched the fluorescence ofC-Dots while the other metal ions didn’t. Separate studyof how Cr3+ ions could be sensed by C-Dots, even in thepresence of other three ions, was also performed. Thissuggested that due to their biocompatibility and sensingcapabilities, C-Dots could be used for sensing metal ionsin human body as well.

Sk M. P. et al.[28] showed that C-Dots prepared usinginduction coil heater could be used as UV-active invisibleink as well as explosive sensors. They showed that picricacid and 2,3-dinitrophenol quenched the fluorescence ofC-Dots, and this was selective as other compounds like

FIG. 22. Plot showing selectivity of C-Dots as Hg2+ion sen-sor, amongst other metal ions. The concentration of all ionssolutions was 1 µM. (Ref. [25])

4-nitrophenol, nitrobenzene, 1,4-benzoquinonoe, and 4-methoxy benozoic acid did not significantly quench thefluorescence.

Zhou, L et al.[25] showed application of C-Dots, pre-pared via electrochemical synthesis, for simple, one-step’mix-and-detect’ sensing method of Hg2+ and biothiolsby fluorescence quenching mechanism. Hg2+ can eas-ily quench the fluorescence of C-Dots, by charge transfermechanism, while biothiols can shelter C-Dots’ fluores-cence by removing Hg2+ from C-Dots’ surface, and form-ing strong Hg-S bonds. Figure 22 shows high selectivityof C-Dots for Hg2+ ions over other metal ions.

V. OUTLOOK

Optical Properties. Even though C-Dots were firststudied only a decade ago, a lot of research has alreadybeen done on this new class of fluorescent material withthe mainstream focus on their optical property. Thoughthe reason behind the observed PL behaviour of C-Dots isyet to be understood completely, researchers have foundsome striking PL characteristics of C-Dots. As it hasbeen mentioned in this section several times, for mostof the C-Dots, synthesized using different precursors andsynthesis methods, the emission spectra showed depen-dence on excitation wavelength. The exact reason behindthis behaviour still remains unknown, but it is usually at-tributed to the quantum size effects. Since various syn-thesis methods for C-Dots usually result in a size spread,it can be understood that the band gaps correspondingto C-Dots with different sizes would be different. Forsmaller C-Dots, the band gap would be higher, and thiswould lead to a smaller wavelength emission. As the sizeincreases, the wavelength also increases because of thequantum confinement effect. The decrease in intensitycan also be explained. With the increase in size withrespect to the average size of C-Dots, the number of par-ticles decrease, and this further lead to decrease in PLintensity. It should be noted that while this size depen-dence of PL behaviour could be applicable to many ofthe synthesized C-Dots, it can not be generalized; thisis because the PL of C-Dots is not just a size depen-dent phenomenon, it can depend on several other factorslike the surface functional groups and other energy traps,

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[Ref. No.]Precursor(s) Synthesis Method λexc (nm) QY (%) Application(s)[9] Soot Arc Discharge - electrophoresis 366 1.6 -[10]Graphite + Cement Laser Ablation 400 4 - 10 -[11]CZnS-Dots & CZnO-Dots Laser Ablation 440 ∼50, ∼45 -[12]Graphite powder Laser Ablation, in 3 diff. solvent 430, 365, 410 5.0, 3.7, 7.8 -[13]Acrylamide Plasma-induced - 6 Optoelectronic devices[14]Soot Electrochemical Oxidation 366 0.8, 1.9, 0.8 Multicolor imaging[15]Soot Electrochemical Oxidation - 0.43 -[15]Soot+AgNO3/Cu(NO3)2/PdCl2 Electrochemical Oxidation - 36.7, 60.1, 33.4 -[18]Carbohydrates Hydrothermal Synthesis 360 13 -[19]Candle soot Hydrothermal 310 5.5 Metal-ion sensing[20]Thiosalicylic acid & EDA Hydrothermal 360 51.4 Optoelectronic devices[21]Citric Acid Carbonization in Octadecene 360 53 -[21]Citric Acid Carbonization in glycerin 360 17 -[22]ortho -, meta - & para - PD Solvothermal 420, 365, 510 17.6, 4.8, 26.1 Optoelectronic, UCPL bioimaging[23]Citric Acid Monohydrate Carbonization in diglycolamine 495 3.0 -[24]Poly(acrylic acid) Pyrolysis, in water & in glycerol 347 1.6 & 9 Optoelectronic devices[25]EDTA Pyrolysis - 11 Hg2+, biothiol sensing[26]Glucose + PEG-200 Microwave Assisted 340, 380 6.3, 3.1 -[27]Citric Acid Carbonization in Octadecene 360 60 Optoelectronic devices[28]Citric Acid + EDA Induction Heater 365 73.5 Invisible ink, Explosive Sensor[29](NH4

+COO-) Salt Thermal Oxidation 340 3 -[29]C6H10Cl2N2O + Zeolite Support assisted 340 0.1 -[30]Resol/F127/SiO2 composite Host(carrier)-assisted 360 14.7 Bioimaging[34]70% Glycerol + PO4

3- Salt Microwave Assisted 360 3.2 Biolabelling & imaging[34]70% Glycerol + CuSO4 Microwave Assisted 360 9.5 Biolabelling & imaging[34]70% Glycol + AlCl3 Microwave Assisted 360 5.8 Biolabelling & imaging[35]EDA Carbonization in P2O5/H2O 360 28.5 Optoelectronic devices[38]γ-butyrolactone Hydrothermal 365 0.5* Optoelectronic devices[39]Citric Acid Carbonization in Octadecene 340 40 Optoelectronic (Multicolor)[41]Citric Acid + EDA Hydrothermal, using Mg(OH)2 360 83 Bioimaging[46]Candle soot Electrochemical oxidation - 3.0 Bioimaging

TABLE I. Reported quantum yields (QY) at the respective excitation wavelengths (λexc), with the applications of C-Dotssynthesized using different synthetic routes. (*Absolute QY) [Abbrevations: Soot = Carbon soot; EDA = Ethylene diamine;PEG = Poly(ethylene glycol); PD = Phenylene diamine; EDTA = Ethylenediamine-tetraacetic acid]

the charge transfer between different groups in C-Dots,effects due to doping, emission by different functionalgroups, etc. These increases complexity of the systemand makes it difficult to give a simplified general reason-ing for the PL behaviour.

The other luminescence feature that was observed forC-Dots, in general, was the surface state stabilization bypassivating agents. Apart from a few examples, many ofthe C-Dots involve the use of a surface passivating agentfor enhancement of PL. These passivating agents, likePEG and PPEI-EI, enhance the luminescence by prevent-ing C-Dots’ agglomeration, which as a result preventsthe quenching of fluorescence because of the quantumsize effects. These passivating agents are also believedto be stabilizing the surface emissive states. In absenceof these passivating agents, the various energy traps dis-

tributed on the active surface of C-Dots would lead tonon-radiative loss of energy, decreasing the PL intensity.These passivating agents thus can be used to tune thePL behaviour of C-Dots. Thus radiative recombinationof the photo-generated exciton in these surface electronicstates is attributed as one of the reasons for PL of C-Dots.

The fluorescence emission could also be originatingfrom π-domains that are present in C-Dots. Various iso-lated π bonds lead to creation of π-electron rich sp2 hy-bridised islands. If this islands are isolated (by using,for ex., surface passivating agents), preventing the mi-gration of exciton to the non-radiative energy traps atsurface/core, the recombination from these islands couldgive enhanced emission. Thus, any site with imperfectsp2 domain can act as an energy trap, which would leadto multicolor, but weak emission.

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A lot of studies has been done to understand the ex-citation and pH dependence of emission; studies of timedependent PL lifetime measurements, with multiphotonexcitation processes, electrochemical luminescence, etc.have been performed. But to completely understand theluminescence behaviour in C-Dots even further studies,using spectroscopic techniques like Raman spectroscopy,are required. It has to be understood that C-Dots aredifferent from inorganic QDs because of their organic na-ture, because of the presence of functional groups andπ bonds, because of the structural and compositionaldishomogeneity, their amorphous and/or partially crys-talline characters, and sometimes mixed hyridization ofCarbon core structure. These makes this Carbon quan-tum dots even more complex systems than the inorganicQDs, and the only explanation using core-shell or radia-tive exciton recombination becomes insufficient.

Applications. This applications’ section briefs onhow C-Dots have come forth as potential competitors,and/or alternatives of other practically significant lumi-nescent materials like QDs, and how can they be furtherexplored in varied fields. Preliminary research on C-Dotshave shown promising possibilities for using C-Dots inoptoelectronic devices like LEDs and photovoltaic cells,for single and multiphoton bioimaging, drug delivery, de-tection by functionalization, as biocompatible sensors fordetection of metal ions, biomolecules and chemical com-pounds, as catalysts in photochemical processes, etc.

For the use of C-Dots in optoelectronic devices, fur-ther research is required for increasing the device effi-ciency and ease of fabrication process using this environ-ment friendly material. As discussed earlier, bright whitelight emission by C-Dots in solid and suspension state hasbeen shown, with appreciable external quantum efficien-cies. Also as tabulated in table 1, the QY of differentC-Dots vary with different synthesis routes and precur-sor involved. Further studies on improving the devicestructure, charge transfer properties, and thus efficienciesof this devices involving C-Dots is therefore intensivelyneeded. Studies have also been done where C-Dots wereshown as potential candidates for drug delivery and tar-geting. It has been shown that because of their non-toxicand biocompatible nature, and the possibility of func-tionalization due to their organic nature, C-Dots can befunctionalized for drug delivery, with specificity to the

target sites. They can readily enter into the target cellswith the drug, and due to their possible UCPL, deep tis-sue NIR imaging can help in developing controlled releaseof drug at the target site. Deep tissue NIR excitation ofC-Dots could also be utilised for treatments. The exist-ing therapies at many times involve use of toxic heavymetals for treatment, and C-Dots can be a good viableoptions as non-toxic and low cost alternatives. Studieshave also shown the potential use of C-Dots for SurfaceEnhanced Raman Spectroscopy.

A schematic summarizing the various useful propertiesand practical applications, discussed in this review, ofthis new fluorescent material has been presented below:

VI. CONCLUSION

In summary, it was thoroughly discussed in this re-view how different C-Dots are synthesized, using dif-ferent precursors and different synthesis techniques. Itwas shown that these synthesized C-Dots exhibit differ-ent characteristics such as structure, composition, lumi-nescence, functionalization, etc., and can be utilised indifferent applications, such as, optoelectronics, imaging,sensing, and biomedicine. Focusing on luminescence, dif-ferent opinions for the reason behind the observed PL,and how can it be tuned and used for various applica-tions have also been discussed. The various advantages,like non-toxicity and biocompatibility, that C-Dots of-fer over various existing luminescent materials like QDs,and the other interesting characteristic properties of C-Dots like up-conversion photoluminescence, electrochem-ical luminescence, quantum yield, charge transfer, func-tionalization, etc. were discussed. Finally we assert thatthese C-Dots have shown remarkable properties and thatfurther studies are required to have better understand-

17

ing of various phenomenon related to them, especiallyluminescence. Further research has to be done for bring-ing uniformity and control over synthesis methods, togain a better control over size and other properties fortheir efficient use in applications. Looking over the lastdecade since the discovery of C-Dots, and analyzing thevast range of applications and properties that have beenproposed, it can be stated that these luminescent andbio-compatible C-Dots can come forth as novel materialfor many applications in future.

VII. ACKNOWLEDGMENTS

I would like to thank Prof. Dr. Maria A. Loi for guid-ing me through the course of this project as my super-

visor; for helping me out with my queries and giving mesuggestions for making this report better. I would liketo thank Prof. Dr. Tamalika Banerjee, as my mentor,for helping me out in choosing the topic of the project. Iwould also like to thank Prof. Dr. Ryan C. Chiechi, Dr.Maxim S. Pchenitchnikov and Dr. Regis Y.N. Genglerfor conducting workshops on literature search and writingpaper. Finally, I would like to thank the Zernike Instituteof Advanced Materials and University of Groningen forproviding me this opportunity of working on this projectas a part of my program curriculum.

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