Carbon Dots: The Newest Member of the Carbon Nanomaterials Family

Carbon Dots: The Newest Member ofthe Carbon Nanomaterials Family

A. L. Himaja, P. S. Karthik, and Surya Prakash Singh*[a]

[a]Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,Uppal Road Tarnaka, Hyderabad 500007 (India), E-mail: [email protected]

Received: October 9, 2014Published online: ËË

ABSTRACT: Carbon nanomaterials have been extensively researched in the past few years owingto their interesting properties. The massive research efforts resulted in the emergence of carbondots, which belong to the carbon nanomaterials family. Carbon dots (C-dots) have garnered theattention of researchers mainly due to their convenient availability from organic as well as inorganicmaterials and also due to the novel properties they exhibit. C-Dots have been said to overcome theera of quantum dots, referring to their levels of toxicity and biocompatibility. In this review, wefocus on the discovery of C-dots, their structure and composition, surface passivation to enhancetheir optical properties, the various synthetic methods used, their applications in different areas,and future perspectives. Emphasis has been given to greener approaches for the synthesis of C-dotsin order to make them cost effective as well as to improve their biocompatibility.

Keywords: carbon, green chemistry, luminescence, nanoparticles, photocatalysis

1. Introduction

Carbon dots (C-dots) are novel nanomaterials recently discov-ered in the year 2004.[1] They are best known for their fluores-cence ability. These nanoparticles belong to the carbonnanomaterials family and have sizes less than 10 nm. C-Dotshave been gaining much more attention from researchers dueto their easy availability and simple synthesis. Their importantproperties include water solubility, chemical inertness, easyfunctionalization, photoluminescence, reduced toxicity, bio-compatibility, and resistance to photobleaching. This has madethem important in the fields of optoelectronics, bioimaging,biosensing, drug delivery, solar technology and photovoltaics.C-Dots are considered to be superior to quantum dots andorganic dyes.[2,3]

Quantum dots face the problem of the blinking effect,which can be overcome by surface passivation or core–shell

formation.[4] Bioconjugation of quantum dots increases thesize, which is undesirable and also makes their delivery intocells more difficult.[5] The composition of quantum dots isconsidered to be toxic for in vitro studies.[6] The major concernis with the bioaccumulation of these toxic materials in thebody. The use of C-dots in place of quantum dots mightovercome the above problems, as they are considered to be lesstoxic and have enhanced optical properties.

Silicon is biocompatible, nontoxic and abundantly avail-able in nature; it belongs to the same group as carbon. Similarmaterials to C-dots have been developed with silicon, known assilicon quantum dots. These silicon quantum dots also possessoptical properties similar to C-dots, and their prominent fea-tures include size tunability, fluorescence, upconversion pho-toluminescence, high water solubility and biocompatibility.

T H EC H E M I C A L

R E C O R D

Personal Account

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Hydrogen-terminated Si QDs obtained via polyoxometalate-assisted synthesis had ordered sizes of about 1–4 nm andexhibited tunable emission in the visible range, which sug-gests their potential biological application.[7] Carbon dots, onthe other hand, possess multicolor luminescence, which alsopromises their application in biology. Catalytic propertieshave been observed in silicon quantum dots,which are also a significant feature of C-dots. Similarly,silicon quantum dots find application in bioimaging, solarcells, photovoltaic systems, etc.[7–9] With the idea of thesenovel materials, research on fluorescent nanomaterials canbe improvised and newer methods of synthesis could bedeveloped.

Our research group has synthesized C-dots from kitchenwaste, which exhibited green fluorescence under UV light.[10]

We wanted to emphasize the advantages of using greenchemistry to obtain C-dots. This motivated us to explore the

different properties, synthesis techniques and applications ofC-dots, which we will discuss in this review.

1.1. Discovery of C-Dots

Basically, the origin of fluorescent carbon was the observationof surface defects in single- and multiwalled nanotubes. Also,the surface passivation of carbon nanotubes for dispersion pur-poses resulted in enhanced luminescence.[11–13] This observa-tion led to the discovery of carbon dots. C-Dots were firstobtained in 2004, accidentally during electrophoretic purifica-tion of single-walled nanotubes.

A fast-moving band of fluorescent material was observedthat exhibited different colors under UV light, as seen inFigure 1. The characterization results showed that the materialconsisted of carboxyl groups and was completely metal free.The composition was determined to be C 53.93%, H 2.56%,

Himaja A. L. is a research student atCSIR-Indian Institute of ChemicalTechnology, Hyderabad, India, in thegroup of Dr. Surya Prakash Singh. She iscurrently pursuing her Master’s degreein the field of nanotechnology atJawaharlal Nehru Technological Univer-sity, Hyderabad, India. She received herBachelor’s degree in Electronics and Communication Engi-neering from the same university. Her research interests arefocused on synthesizing nanomaterials using various tech-niques and applying them to the field of solar energy. She isalso interested in the fabrication of solar cells using cost-effective materials. She has published three research papers inthe field of nanotechnology.

Karthik P. S. is a research student atCSIR-Indian Institute of ChemicalTechnology, Hyderabad, India, in thegroup of Dr. Surya Prakash Singh. Heis currently pursuing his Master’s degreein the field of nanotechnology atJawaharlal Nehru Technological Univer-sity, Hyderabad, India. He received hisBachelor’s degree in Electrical and Electronics Engineeringfrom the same university. His research interests are focused onsynthesizing carbon nanomaterials using various techniquesand applying them to the field of solar energy. He is alsofocused on fabricating solar cells using different light-absorbing materials. He has published three research papersin the field of nanotechnology.

Dr. Surya Prakash Singh has been a Sci-entist at CSIR-Indian Institute ofChemical Technology, Hyderabad, sinceSeptember 2011. He studied chemistryat the University of Allahabad, India,and obtained his D.Phil. degree in 2005.Later, he joined CSIR-Indian Instituteof Chemical Technology, Hyderabad, asResearch Associate. After working at Nagoya Institute ofTechnology, Japan, as a postdoctoral fellow (2006–2008), hejoined Osaka University in 2008 as an Assistant Professor. Hethen worked as a researcher at the Photovoltaic MaterialsUnit, National Institute for Materials Science (NIMS),Tsukuba, Japan (2010–2011). Dr. Singh is an AssociateFellow of the Andhra Pradesh Academy of Sciences (2013),and a recipient of the NASI-Young Scientist Platinum JubileeAward (2012) from the National Academy of Sciences, India,and the Young Scientist Award from the Council of Scienceand Technology, Government of U.P. He has been involved inthe design and synthesis of materials for dye-sensitized solarcells, organic solar cells and quantum dot solar cells, pub-lished over 100 papers, reviews in peer-reviewed journals, andpatents, edited two books, and is the author of one bookchapter.

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P e r s o n a l A c c o u n t T H E C H E M I C A L R E C O R D

N 1.20%, and O 40.33%.[1] Since then, many groups havereported several methods of synthesis of C-dots.

1.2. Structure and Composition

Generally, the average size of C-dots has been reported to beless than 10 nm. This helps with the use of these nanosizedcarbon dots in biological applications. Surface passivation ofC-dots has been done to improve fluorescence emission. Thesize of the particles increases in this case. One report showedthe diameter of C-dots to be 3.1 ± 0.5 nm when they werecoated with hydroxyl groups. As seen in Figure 2, the TEMimage showed that the particles were uniformly dispersed andspherical in shape. The quantum yield increased and was about5.5%. Better fluorescence emission was observed and the par-ticles were photostable.[14] From this observation, C-dots havea better opportunity in cell imaging if the size of the particleremains under 10 nm even after surface passivation to enhanceoptical properties.

Another group reported an average size of 3 nm and theparticles were found to be spherical in shape. The elementalcomposition was found to contain carbon and oxygen, as wellas potassium due to the use of bananas as a starting material.However, potassium constituted a very minor component andthe majority of the composition was carbon and oxygen.

As seen in Figure 3, C-dots were found to have moreoxygen-containing groups, with sp3 carbon and hydroxyl-attached carbon groups observed. The interlayer spacing wasfound to be 0.42 nm, which is higher than the graphiticinterlayer spacing of 0.33 nm.[15] This leads to the conclusionthat the carbon dots were less crystalline in nature compared tographite. Also, the amorphous nature of C-dots is supported bya report that showed that C-dots synthesized from food cara-mels had no crystalline nature, as the XRD data showed abroad amorphous peak.[38] The same was observed in another

study on C-dots synthesized from orange waste peels.[16] A fewreports also showed that the C-dots were graphitic in nature. Areport on C-dots derived from graphite powder showed thatthe C-dots resembled a diamond-like structure. The selectedarea electron diffraction pattern showed the ratio of squares ofthe ring radius to be 3:8:11:16:19, wherein the rings corre-spond to the planes {111}, {220}, {311}, {400}, and {331},which belong to diamond.[3] Another report on C-dotsobtained from multiwalled carbon nanotubes (MWCNTs) byelectrochemical oxidation also showed the graphitic nature ofC-dots. The HRTEM image showed the lattice spacing to be3.3 Å, which is close to the (002) facet of graphite. Thesereports bring us to the conclusion that C-dots obtained fromdifferent starting materials can produce different structures thatare either graphitic or amorphous.

A report on the synthesis of C-dots from candle sootshowed that the composition of soot was C 91.69%, H 1.75%,N 0.12%, O (calculated) 4.36% and that of the purifiedcarbon nanoparticles was C 36.79%, H 5.91%, N 9.59%, O(calculated) 44.66%.[17] This observation shows that C-dots arehigh in oxygen content. Various functional groups likehydroxyl, carbonyl, carboxyl and epoxy groups were foundattached to the C-dots.[16] An article reported that there wasa narrow size distribution of C-dots between 2 and 7 nm.This leads us to the conclusion that carbon dots due to theirsmaller size have a larger surface area, which means greaterabsorption. This improves the applicability of C-dots to opto-electronics. Furthermore, the different functional groupspresent allow the attachment of drugs to the surface for drugdelivery systems.

Another report showed that C-dot/BiVO4 nanocomp-osites formed two different structures when attached tonanospherical and nanoplatelet-shaped BiVO4. In this case, thesize of the complex increased to 350–400 nm for nanospheresand 500 nm for nanoplatelets.[18] This leads us to the conclu-sion that C-dots can be size tuned by forming differentnanocomposites. Various shapes and sizes can be obtained inthis way, which helps in different areas of application.

1.3. C-Dot Nanocomposites

Carbon dots, being both electron accepting and electron trans-porting, provide a convenient flow of photo charge carriers,thus enhancing the photocatalytic properties. This improvedcatalytic efficiency can be obtained by making semiconductorcarbon nanomaterials. However, there has been a report on thephotocatalytic nature possessed by carbon dots without theformation of any complex. The C-dots were prepared by analkali-assisted electrochemical method. The photocatalyticactivity was tested by irradiating C-dots with near-infrared(NIR) light, which led to the oxidation of benzyl alcohol tobenzaldehyde. This experiment shows that C-dots are capable

Fig. 1. Electrophoretic profile in 1% agarose gel under 365 nm UV light. (A)Crude SWNT suspension, (B) fluorescent carbon, (C) short tubular carbon,(D,E) Further separation of (C), (F) cut SWNTs. Figure reprinted fromreference [1].



of photocatalytic activity resulting in the transformation ofalcohols to their corresponding aldehydes.[19]

ZnO is said to be a stable visible-light-driven photocata-lyst.[20] When excited by UV light, it produces electron–holepairs and initiates the formation of hydroxyls in water.[21] Thecombination of ZnO with carbon nanoparticles helps inimproving the optical properties. A report showed the loadingof C-dots with ZnO using the solution dispersion method. Thedegradation of NBB (naphthol blue black) azo dye was testedusing C-dots loaded with ZnO.

As seen in Figure 4, the NBB was completely degradedafter 45 minutes when tested with C-dots loaded with ZnO.The result was different when tested purely with C-dots, i.e.,only 4.4% of the NBB was degraded over the same duration.[16]

This shows that C-dots are photocatalytic in nature and canbe used for degrading harmful dyes. C-Dots in combinationwith other photocatalytic materials can be used in waste watertreatment.

Recent studies indicate that silver orthophosphate(Ag3PO4) has good photocatalytic ability. Although it is lesssoluble in water, it has the ability to oxidize water and degradethe organic contaminants present. One research group hasrecently worked on preparing C-dot/Ag3PO4 and C-dot/Ag/Ag3PO4 photocatalytic complex systems. The complexes wereprepared by simple dispersion of CH3COOAg and polyvinyl-pyrrolidone (PVP) in a C-dot solution and Na2HPO4 wasadded dropwise. The former was stirred at room temperaturefor 4 h in the dark and the resultant material was then dried inan oven at 50°C for 12 h.[22]

The complex was formed and then characterized usingdifferent techniques. As seen in Figure 5, the SEM imagerevealed the rhombic dodecahedral morphology of thecomplex with a size of 800–900 nm. The UV–vis spectroscopyresults showed that the complex absorbs in the range of 530–1000 nm, which leads us to estimate that these complexes showgood photocatalytic activity and C-dots might play a vital rolein absorbing sunlight.

The photocatalytic performance was tested on methylorange (MO) under visible light irradiation, as shown inFigure 6. According to the results reported, the C-dot/Ag/Ag3PO4 complex showed the highest photocatalytic activity;the MO dye was completely degraded in 10 min. In contrast,25 min were needed for the C-dot/Ag3PO4 complex. Whentested with pure Ag3PO4, it took about 55 min for the dye todegrade. This clearly indicates the higher photostability andcatalytic activity of the complexes prepared. The complexsystems also exhibited upconversion photoluminescence. Allthe above results lead us to the conclusion that C-dots, whenmade into a semiconductor complex, improve the photostabil-ity, photocatalytic activity, avoid the photocorrosion of themetal complex, act as an electron reservoir and also effectivelyharness the entire spectrum of sunlight. Additionally, a recentreport showed the formation of a C-dot@mSiO2-PEGnanocomposite by mixing the as-prepared mSiO2 with glyceroland PEG-NH2 at 230°C for 30 min. The nanocomposite wastested for drug delivery, having been particularly chosen forthis task since it acts as an efficient carrier by providing enoughspace for storing the drug molecules. The water-soluble anti-cancer drug doxorubicin (DOX) was loaded into thenanocomposite. Controlled drug delivery was achieved usingthis nanocomposite by reducing the pre-release of the drugbefore the drug gets absorbed by the target cell.[23] Anothersuch complex with improved photocatalytic activity was alsodeveloped, using monoclinic bismuth vanadate (BiVO4) as thesemiconductor material for complex formation with theC-dots. The photocatalytic ability of this complex was testedby degradation of methyl blue dye under visible light at roomtemperature.[18] C-dot/TiO2 and C-dot/SiO2 complexes weresynthesized by a different research group. These semiconductorcomplexes are another example of the photocatalytic activity ofC-dot complexes.[24] Thus, maximum light energy can be har-vested by forming semiconductor/C-dot complexes.

Improving the power conversion efficiency (PCE) is theprimary concern in photovoltaic systems. One report detailed

Fig. 2. TEM image (left) and size distribution (right) of C-dots. Figure reprinted from reference [14].



a method to synthesize a C-dot complex that acts as a semi-conductor photoelectric conversion system. C-Dots weredoped into rhodamine B (RhB)/TiO2 complex, which led toimproved photocurrent density. There was a significantincrease in the UV–vis absorption with the addition of C-dotsto the aqueous RhB solution. The doped C-dots were of greatersolubility due to the presence of oxygen-bearing functionalgroups. The size of the C-dots was <10 nm, which also helpedin greater UV–vis absorption. Investigations showed that thepresence of C-dots also helped in reducing the charge recom-bination as C-dots acted as an electron-transfer intermediate.The photoluminescence quenching was also achieved by thiscomplex system. All these improved results achieved by the

C-dot/RhB complex led us to the conclusion that high-efficiency photoelectric systems can be developed by using suchC-dot/dye/semiconductor complex systems.[25] Similarly,N-doped C-dots were synthesized and found to be successful inthe degradation of methyl orange dye. The photocatalyticactivity of such complexes will further help in environ-mental protection by degrading harmful chemicals present inwater.[26]

All of the points discussed above show that the formationof C-dot nanocomposites with different materials allows themto be applicable in several fields, such as optoelectronics, drugdelivery, waste water treatment, etc. This study on C-dotnanocomposites gives researchers a perspective on developing

Fig. 3. (a) EDX spectrum, (b) XRD pattern, and (c) FTIR spectrum of the carbon dots. Figure reprinted fromreference [15].



novel composites with these nanoparticles and making themsuitable for diverse fields of application.

1.4. Surface Passivation of C-Dots

As the surface-to-volume ratio of nanoparticles is high, theelectronic states associated with the surface have an effect onthe optical properties. The surface states that result from theunsatisfied bonds present on the surface act as temporary traps

for charge carriers. This results in the quenching of radiativerecombination and a reduction in quantum yield. Hence,surface passivation is the tool required to eliminate thesesurface states. The voids present on the surface of thenanoparticles are capped by either organic or inorganic agents,thus providing the advantages of improved photostability,water solubility, good dispersity, chemical reactivity, etc.[27]

Surface passivation is also required to improve the fluores-cence ability of C-dots. It also facilitates the integration of these

Fig. 4. Absorption intensity variations of NBB azo dye at different irradiation intervals in the presence of C-dots/ZnO. Figure reprinted from reference [16].

Fig. 5. (a) SEM and (b) HRTEM images of C-dot/Ag3PO4 complex photocatalyst. (c) SEM and (d) HRTEM imagesof C-dot/Ag/Ag3PO4 complex photocatalyst. Figure reprinted from reference [22].



nanoparticles with biological or chemical systems, and otherapplications. The water solubility of C-dots can be improvedby modifying the surface.

One group of researchers synthesized hydroxyl-coatedC-dots using candle soot. The candle soot was dispersed inNaOH solution and then refluxed at 200°C for 12 h in anoven. The resultant solution was then centrifuged to obtain asupernatant that contained a brown-yellow dispersion ofC-dots.[14]

The TEM and dynamic light scattering (DLS) analysisshowed that the particles were monodisperse and narrowlydistributed. Also, zeta potential results as seen in Figure 7showed that the C-dots were negatively charged under weaklyalkaline conditions due to ionized hydroxyls. The water solu-bility of C-dots was improved in this approach due to thepresence of hydroxyls on the surface of the nanoparticles.

Another research group reported the surface passivation ofC-dots to improve photoluminescence emission. Firstly,

C-dots were prepared by laser ablation of a carbon target(mixture of graphite powder and cement). Initially no photo-luminescence was detected from the sample. When the carbonparticles were acid treated and then surface passivated bysimple organic species, bright luminescence was observed.PEG1500N was made to react with the carbon particles forsurface passivation, as seen in Figure 8.[28]

The microscopic analysis showed that the particles werearound 5 nm in diameter. The passivated C-dots were brightlyluminescent in solution, suspension, and the solid state. Theabsorption and luminescence emission covered the entirevisible range and extended into the NIR region and it wasmainly due to the C-dots present at the core. Organic agentslike PEG1500N are considered to be water soluble and can also beeasily attached to antibodies and other bioactive molecules.This improves the applicability of C-dots in optical labelingand bioimaging applications.

C-dot-supported silver nanoparticles have been preparedto boost the performance of polymer LEDs and solar cells.C-Dots have been prepared by thermal decomposition of oli-gosaccharide α-cyclodextrin, and then surface passivated byPEG. These were then irradiated by UV light in the presence ofAgNO3, which resulted in the reduction of Ag+ ions to Agnanoparticles.

As seen in Figure 9, the yellow suspension of C-dotschanged to a brown solution, indicating the formation ofsilver nanoparticles. This results in a clustering effect of silvernanoparticles on the surface of C-dots, which leads to higherlight absorption.[29] Surface passivation of C-dots with silvernanoparticles is significant in improving the performance ofpolymer optoelectronic devices. The internal quantum effi-ciency (IQE) approached 100% in the case of PTB7:PC71BM-based polymer solar cells with C-dots+AgNO3. Thisresult is found to be highly prominent as each photon absorbedby the solar cell results in a separated pair of charge carriers.These results show the significance of C-dot-supported silvernanoparticles in enhancing the performance of polymersolar cells.

2. Synthesis Methods

Carbon dots were first discovered during the purification ofsingle-walled nanotubes obtained by the arc-discharge method.

Fig. 6. Photocatalytic activities of Ag3PO4, Ag/Ag3PO4, and the C-dot/Ag3PO4 and C-dot/Ag/Ag3PO4 complexes for MO degradation under irradia-tion with visible light (∼420 nm). Figure reprinted from reference [22].

Fig. 7. Zeta potential of C-dots in aqueous solution. Figure reprinted fromreference [14].

Fig. 8. PEG attached to the surface of a C-dot. Figure reprinted from refer-ence [28].



Since then, many researchers have discovered ways to prepareC-dots. Recent advances have shown that C-dots can be easilyprepared in bulk using low-temperature synthesis methods.Complications in physical methods have been simplified byusing low-temperature and green methods. The followingsection describes the previous methods and recent advances inthe synthesis of C-dots.

2.1. Synthesis of C-Dots from Single-Walled CarbonNanotubes (SWCNTs)/Candle Soot

In 2004, while electrophoretically purifying SWCNTs derivedfrom arc-discharge soot, a group of researchers found that thepurified soot contained two different classes of new materials.One was short tubular structures and the other was fluorescentnanoparticles.

In order to purify the arc-discharge soot, it had to besoluble in water. For this to happen, the soot was first oxidizedusing HNO3 and then extracted with water. Almost allSWCNTs and soluble particles were then extracted into theaqueous suspension. After further washings with water, TEManalysis was performed, which showed mostly graphitic sheetsand large carbonaceous particles.

Further characterization left the researchers with a surpriseas the suspension contained three different classes of materials,i.e., long nanotubes, a slow-moving dark band of nanotubes,and fast-moving highly fluorescent particles. As shown inFigure 10, the fluorescent material separated into different

components and fluoresced different colors under 365 nm UVlight.[1] This was the research that found the existence of carbondots in carbon soot. Since then, these fluorescent carbonnanoparticles have grabbed the attention of many researchersand were synthesized using several different techniques.

Additional research has been done on the purification ofcandle soot to obtain fluorescent carbon nanoparticles. Thesoot was collected by placing a glass plate over the flame of acandle. The particles present in the soot agglomerate with eachother to form microparticles. In order to allow their uniformdispersion in water, oxidative acid treatment, which is com-monly used to purify CNTs, was also used here. Purification

Fig. 9. Photographs and schematic illustrations of AgNO3 and carbon dot (CD) +AgNO3 blend solutions before(left) and after (right) UV irradiation. Figure reprinted from reference [29].

Fig. 10. Photograph of different fractions of fluorescent carbon under 365 nmillumination. Figure reprinted from reference [1].



using polyacrylamide gel electrophoresis (PAGE) then affordedthe purified fluorescent carbon nanoparticles.

The soot, which was initially hydrophobic in nature andinsoluble in water, became finely dispersed after the acid treat-ment. This was then centrifuged to obtain a light-brown super-natant that showed yellow fluorescence under UV light(312 nm), as seen in Figure 11.

Results showed that the C-dot surfaces contained OH andCO2H groups, which made the particles hydrophilic in nature.This was mainly due to the oxidative acid treatment, which wasrequired to improve the aqueous solubility, break down theagglomerated particles and improve the fluorescence. When thefluorescent material was separated by denaturing PAGE, threedifferent classes of materials were observed as seen in Figure 11,including nine fast-moving fluorescent bands, slow-movingnonfluorescent bands and insoluble agglomerates.[17] The pho-toluminescence spectra of purified carbon nanoparticles showedthat they have a broad color range with wavelengths rangingfrom 415 nm (violet) to 615 nm (orange-red).

From the above-reported results, it is evident that obtain-ing C-dots is quite a simple process and just involves purifica-tion of carbon materials. The most important features ofC-dots include their fluorescence properties and nanoscale size.C-Dots derived from carbonized materials vary in size andhence fluoresce different colors under UV light. This is aprominent feature of these novel nanomaterials and makesthem applicable in bioimaging and optoelectronics.

2.2. Electrochemical Synthesis of C-Dots

Electrochemical synthesis has been used to produce high yieldsof high-quality carbon dots. In this approach, only pure water

was used as the electrolyte and no toxic chemicals wererequired. As seen in Figure 12a, a graphite rod (99% pure) wasused as the anode and inserted into the electrolyte (ultrapurewater). Another graphite rod was placed parallel to this to serveas the counter electrode. A DC power supply of 15–60 V wasapplied between the electrodes. After 120 h of continuousstirring, the graphite rod had become eroded and a dark yellowcolored solution was obtained (Figure 12b). This solution wasthen filtered and centrifuged to remove the graphitic particlesand thus obtain a supernatant containing soluble C-dots(Figure 12c).

The resultant carbon dots were analyzed using differenttechniques, which showed that they were highly crystalline innature with high dispersibility in water and also exhibitedprominent photoluminescent properties. The yield of theC-dots was about 16.5%.The DLS results in Figure 12d showedthat the C-dots were highly dispersible in water and the particlesize was between 3 and 6 nm. The HRTEM image shown inFigure 12f revealed the lattice spacing to be 0.321 nm. XRDanalysis showed two peaks at 22.59° and 18.20°, which refer toamorphous carbon and (103) planes, respectively. The Ramanspectra showed two significant peaks at 1350 and 1600 cm−1.The XPS data indicated that the C-dots contained more oxida-tion groups. The FTIR results showed seven peaks at 3442.19(hydroxyl OH), 1710.28 (carboxyl C=O), 1444.42 (aromaticC=C), 1243.79 (epoxide/ether C–O–C), 865.59 (aromaticAr–H), and 606.55 cm−1 (aromatic Ar–H). The PL spectrashowed an upconversion property at excitation wavelengths of700, 750, 800, 850, 900 and 950.[30]

The reported findings suggest that C-dots obtained fromelectrochemical synthesis of graphite rods exhibit uniqueproperties like high crystallinity, good dispersibility andupconversion photoluminescence. These are important fea-tures for making C-dots useful as energy-transfer elements inbiological and photocatalytic applications.

2.3. Synthesis of C-Dots by Pyrolysis

This is a simple approach for the preparation of C-dots by theuse of a single carbohydrate precursor. Glycerol was usedas a solvent for the production of C-dots via a pyrolysisprocess. This approach resulted in a high yield of C-dots andwas an economical and feasible method. When the glycerol washeated at 230°C under 1 atm pressure for 30 min in thepresence of oxygen, C-dots were observed. However, C-dotswere not seen when glycerol was degassed and purged with N2

gas and heated at 230°C. This shows that C-dots can only beformed when the carbohydrate is made to react with oxygen.Also, glycerol turned acidic and underwent dehydration after30 min.

As seen in Figure 13, the microscopy results showed thatthe particles had diameters of about 5.5 ± 1.1 nm. The XPS

Fig. 11. Electrophoretic separation of fluorescent C-dots illuminated by (a)white and (b) UV light (312 nm). (c) Close-up view of the fluorescent bandsin (b). Figure reprinted from reference [17].



results showed three carbon peaks (C–C, C=O, C–O). TheXRD results revealed that the carbon was amorphous in nature.The UV–vis spectrum showed a gradual increase in the absorp-tion upon increasing the reaction time. The photolumines-cence properties were measured as a function of excitationwavelength and a red shift in the peaks was observed.[23]

These results suggest that C-dots obtained by this methodare amorphous in nature and exhibit photoluminescence athigher wavelengths. As glycerol is rich in carbohydrates, a highyield of C-dots was obtained by this method. This method canbe considered as a cost-effective way to obtain C-dots, but theoptical properties they exhibit are less significant.

2.4. Microwave-Assisted Synthesis of C-Dots

Microwave synthesis is particularly useful as it is an unfussy andflexible approach. Luminescence is very much required when aparticle has to be used in biological applications. Multicolorluminescence has been achieved by microwave synthesis ofC-dots using dextrin as a starting material. A solution contain-ing dextrin and water was prepared, to which sulfuric acid(H2SO4) was added while stirring vigorously. This solution wasthen exposed to microwaves for 2.5 min at 800 W. A light-brown supernatant was obtained along with a black precipitate.

The solution containing the C-dots was centrifuged to obtaina clear solution.

The as-prepared C-dots were characterized using differenttechniques and the luminescence spectra showed that the par-ticles attained multicolor luminescence without any require-ment for surface-passivating agents. The quantum yield wasbetween 5 and 9%.[31] This multicolor luminescence might bevery helpful in the field of bioimaging.

One research group has reported a green synthesis ofC-dots that was microwave assisted and without the use of anysurface-passivating reagents. A carbohydrate and a smallamount of inorganic ion were used as the starting materials.Glycerol was mixed with phosphate solution and subjectedto microwave heating for 14 min. The photoluminescenceresults from Figure 14 showed that the emission spectrawere broad and ranged from blue to yellow depending on theexcitation wavelength. As neither glycerol nor phosphate isluminescent in nature, this result shows the formation ofC-dots.

Upon increasing the amount of phosphate salt, theabsorption band decreased. The photoluminescence peaks redshifted and the quantum yield increased with reaction time.[32]

These results allow us to validate that inorganic ions improvethe carbonization of carbohydrates.

Fig. 12. (a) Reaction equipment for the preparation of C-dots. Digital images of C-dot solution (b) before and (c)after treatment. (d) DLS histogram of C-dots. (e) TEM and (f ) HRTEM images of C-dots. Figure reprinted fromreference [30].



The above reports indicate that C-dots obtained by themicrowave-assisted method are quite stable in nature due to thehigh temperatures involved. Also, the carbohydrate-rich sourcematerials used for synthesis result in a high yield of C-dots. Theunique property of multicolor luminescence has been observedas mentioned above. This property has been observed in C-dotswithout the need for surface passivation. It is a significantfeature to make C-dots applicable in bioimaging. Distinguish-ing various parts of the body by the use of multicolor lumines-cent C-dots will be an important bioimaging technology. Thisfeature enables easy recognition of diseased cells present atvarious locations of the body. Hence, the above reports willhelp in further research on obtaining multicolor luminescentC-dots.

2.5. Ultrasonic Synthesis of C-Dots

A simple and facile one-step approach to the synthesis ofC-dots has been reported. The starting materials used werecommercially accessible, and ultrasonic treatment was used soas to make the process straightforward to perform. Active

carbon and hydrogen peroxide were mixed in appropriateamounts to form a black suspension. This solution was thenultrasonicated at 300 W for 2 h at room temperature andvacuum filtered using a Millipore membrane to separate theinsoluble precipitate. The resulting solution was dark brown incolor, showing the presence of C-dots. For further purification,the hydrogen peroxide was evaporated and the remaining solidwas dissolved in deionized water.

The resulting fluorescent carbon nanoparticles were ana-lyzed using different techniques. TEM revealed that the diam-eter was 5–10 nm and AFM showed that the particles were welldispersed. The morphology of the particles changed withrespect to the ultrasonication time. When the solution wasultrasonicated for 30 min, large carbonaceous particles werevisible, and when ultrasonicated for 1.5 h, the particles werefinely dispersed and the shape had changed to spherical. Thesenanoparticles showed bright blue-green fluorescence when irra-diated with UV light. The photoluminescence spectrumextended to the NIR region at an excitation of 350 nm. Thesample exhibited different colors at different excitation wave-lengths when examined by a fluorescent microscope.[33]

Fig. 13. (A) TEM image, (B) XPS spectra, (C) XRD pattern, and (D) Raman spectrum of the as-prepared C-dots.Figure reprinted from reference [23].



Another group of researchers have demonstrated the syn-thesis of C-dots by acid-assisted ultrasonication. The carbonnanoparticles were obtained directly from glucose solution.Firstly, an appropriate amount of glucose was added to deion-ized water to obtain a clear solution. To the glucose solutionwas added NaOH or HCl solution and the mixture was thenultrasonicated for 4 h.[34]

Analysis of the as-prepared C-dots revealed diameters ofabout 5 nm. Photoluminescence in the entire visible to NIRrange was observed, as were upconversion luminescent prop-erties as shown in Figure 15. The C-dots were highly photo-stable and had a quantum yield of 7%. Therefore,ultrasonication can be particularly useful to improve themixing of chemicals and thus speed up the chemical reaction.Furthermore, a uniform size distribution for the C-dots isobtained by this method due to even dispersibility. Hence, thismethod of synthesis is particularly important in order to obtainuniform and smaller-sized C-dots.

2.6. Green Synthesis of C-Dots

Green synthesis has found great prominence in modernresearch. Conversion of biodegradable waste into value-added

products is an essential topic in the field of green chemistry.The toxicity concerns that have arisen due to the use of high-end chemicals paved the way for the emphasis on green chem-istry. In green synthesis, the materials used should be eco-friendly and cost effective. Almost all materials procured fromplants could be used as suitable starting materials for greensynthesis. Green synthesis has grabbed the attention ofresearchers mainly due to the fact that it is highly economical,less toxic, biocompatible, less time consuming, and requiresreduced temperatures.

Carbon, which is available in almost all organic materials,can be extracted through green synthesis. Carbon dots havebeen synthesized from many organic materials to date. One ofthe reports on the green synthesis of carbon dots demonstratedthe use of waste orange peels as the starting material. Carbon-ization of the orange peels was achieved by hydrothermal treat-ment,[16] a technique that has been practiced for a long time. Itis known to be a cheap and renewable method to producechemical compounds from raw materials. Hydrothermal syn-thesis is the most commonly used process as a good morphol-ogy of structures is obtained due to the use of high-pressureand high-temperature conditions. It is economically viable asthe solvent used is water.

The synthetic process is shown in Figure 16, and startedby first thoroughly washing the peels and then sun dryingfollowed by oven drying them at 150°C for 10 h. The car-bonized peels were then washed with H2SO4 solution andrinsed with water. These peels were then soaked in sodiumhypochlorite solution for 4 h and thoroughly washed in wateruntil the pH of the washed water reached 7. These pretreatedoxidized orange peels were hydrothermally treated in a Teflon-lined autoclave at 180°C for 12 h. The autoclave was thenallowed to cool down naturally and the resulting solution waswashed with dichloromethane. The separated solution wasthen centrifuged at 5000 rpm for 20 min to obtain a brownsolution of an aqueous suspension of C-dots. The quantumyield was 12.7% and the diameter of the particles was2–7 nm.[16]

In one article, the hydrothermal synthesis of C-dots fromsugarcane bagasse was reported. Bagasse is considered to be richin carbon, can be easily procured and is a renewable resource.To obtain the carbonaceous blocks, bagasse was first soaked inconcentrated sulfuric acid for 24 h at room temperature. It wasthen washed in hot water and alcohol until the filtrate wasneutral. The pretreated bagasse was then oven dried overnightin order to obtain carbonaceous blocks. These carbon blockswere then mixed with NaOH solution, heated for 1 h andfiltered to eliminate large particles. The solution obtained wasa dark filtrate that showed weak fluorescence. Alkaline hydro-thermal treatment was performed on the obtained solution,which then resulted in a bright yellow solution. The quantumyield was about 4.7%.[35]

Fig. 14. (a) Photoluminescence spectra of C-dots. (b) Photoluminescenceimages of C-dots in water under UV (330–385 nm), blue (450–480 nm) andgreen (510–550 nm) light excitation. Figure reprinted from reference [32].



One research group has synthesized biocompatible C-dotsfrom aqueous extracts of Trapa bispinosa peel. This was a verysimple, fast, and effective method. Trapa bispinosa peels werefirstly washed with water and then soaked in cold water for30 min. Later the peels were crushed in distilled water toobtain a light pink extract after centrifugation. The extract was

heated at 150°C for 2 h to form a greenish-brown solution,which was then centrifuged and suspended in NaOH solutionto afford a clear yellow suspension of C-dots.[36]

Carbon dots were even synthesized from sugarcane juiceby centrifugal separation. This was a very simple approachto the preparation of crystalline C-dots from highly alkaline

Fig. 15. (a) Upconversion photoluminescence spectra and (b) UV spectra of C-dots obtained from glucose. Figurereprinted from reference [34].

Fig. 16. Formation of C-dots by the hydrothermal treatment of waste orange peels. Figure reprinted from reference[16].



sugarcane juice. A few milliliters of NaOH solution were addeddropwise into the sugarcane juice, which was kept under con-stant stirring until the solution became reddish brown in color.This was then centrifuged at 5000 rpm for 15 min andchecked under UV light to observe dark green fluorescence.For separation of C-dots from the mixture, sucrose densitygradient centrifugation (SDGC) was performed.[37]

In a recent report, the synthesis of C-dots from bananajuice, which contains glucose, sucrose, fructose and ascorbicacid, was studied. The juice of a banana was extracted bysimply crushing it with a small amount of water and extract-ing the pulp-free juice. The banana juice was then mixed withethanol and heated in a glass bottle closed with a cotton corkand kept in an oven at 150°C for 4 h. The solution turnedinto a dark brown product when cooled to room temperature.This was then mixed with water and filtered to separate theresidue. The aqueous solution was mixed with ethanol andcentrifuged at 3000 rpm for 15 min at room temperature.Finally, to obtain the fluorescent C-dots, the ethanol wasevaporated completely. The yield of C-dots thus obtained was600 mg.[15]

All the above reports on greener methods for the synthesisof C-dots are of high importance in the present-day scenario.These methods result in high yields of C-dots and also presenta cost-effective approach by the use of low-cost starting mate-rials. The simplistic approach used in the above-mentionedreports will facilitate many more research groups in discoveringC-dots in cost-effective and simpler ways. Also, the fluores-cence properties that have been observed in the C-dotsobtained from eco-friendly source materials paves the way fornontoxic and biocompatible C-dots to be applicable inbioimaging techniques.

2.7. Synthesis of C-Dots from Commercially AvailableFood Products

Researchers have found that C-dots are even available from thecommercial food products that we consume in our day-to-daylife. Different caramel-containing food products like jaggery,sugar, bread, biscuits, and cornflakes are found to containamorphous C-dots. The C-dots were extracted from thebrowner part of the bread, caramelized sugar and jaggery. Thedispersions of these materials were caramel in color underwhite light but showed blue fluorescence under UV light, asshown in Figure 17. The quantum yield from bread was thehighest observed at 1.2% and jaggery had the lowest quantumyield with 0.55%.

The TEM image showed the morphology of C-dots to bespherical. The particle size was highest for bread-extractedC-dots and lowest for those extracted from sugar caramel. TheXRD analysis showed the amorphous nature of the C-dots.They were also hydrophilic in nature due to the presence of

carboxyl and alcohol functional groups.[38] All the above obser-vations led to the confirmation of the presence of C-dots inthese food products.

Carbon dots were even synthesized from coffee groundsthat had been used and dried. It was a quite simple preparationof C-dots as the coffee grounds were just heated and separated.There were four stages of preparation: dehydration, polymer-ization, carbonization and passivation. Nucleation occurred asa result of dehydration, polymerization and carbonization,which occurred due to heating. The nuclei then grew by dif-fusion of solutes towards particle surfaces.

The as-prepared C-dots were then characterized using dif-ferent techniques. The TEM results displayed in Figure 18showed that the average diameter of the particles was 5 ± 2 nm.The XRD as well as EDX data showed the presence of carbonatoms. The Raman analysis revealed the amorphous nature ofthe C-dots. The fluorescence emission spectra showed a broadrange from blue (400 nm) to red (600 nm). As the excitationwavelength was increased, the emission peak position shifted tolonger wavelengths and the intensity decreased. The strongestphotoluminescence of 3.8% occurred at 440 nm when excitedat 365 nm. The fluorescence of the C-dots could only beobserved when they were surface passivated.[39]

Hence, the above-reported observations imply that thesynthesis of C-dots also helps in waste management. Manycarbohydrate-rich food products can be used to produceC-dots in this manner. This helps in promoting scientificresearch to the next level by practicing waste management withthe help of green chemistry.

All the above-reported synthesis procedures lead us to theconclusion that preparation of carbon dots can be simplified byfollowing green chemistry. Compared to physical methods, useof green chemistry simplifies the process and also results ineco-friendly syntheses. Recent advances in the synthesis ofcarbon dots have been made using greener methods.

3. Applications of C-Dots

The novel properties of C-dots make them applicable in a widerange of areas. As C-dots are a very new area of research, notmany applications have yet been commercialized by makinguse of these nanoparticles. Indeed a lot of research has beendone on the applicability of these particles to different areas.

3.1. C-Dots for Biological Applications

Bioimaging is an essential tool in the field of biology. It isbasically necessary for targeted drug delivery and discover-ing a defective site in a body. The field of nanotechnology israpidly advancing for its application in biology. Fluorescentnanomaterials are of basic interest for the purpose of



Fig. 17. (a–c) Photographs of commercial bread, jaggery and sugar. (d–i) Excitation-wavelength-dependent emissionspectra of C-dots from bread, jaggery and sugar caramel. (j–l) Photographs of dispersions of C-dots from bread,jaggery and sugar caramel observed under white light, and (m–o) the same under UV light. Figure reprinted fromreference [38].

Fig. 18. (A) TEM and (B) HRTEM images of C-dots. Figure reprinted from reference [39].



bioimaging. Highly fluorescent semiconductor quantum dotshave facilitated the advances in bioimaging techniques. Toxic-ity issues of quantum dots are a main concern for their use inbiological systems. The discovery of C-dots has lightened thehearts of researchers as they are biocompatible in nature. A lotof research has been done on the application of C-dots in thefield of biology. An important parameter for bioimaging isphotoluminescence. This important property is exhibited byC-dots that are synthesized from various materials. C-Dots areknown for their tunable photoluminescence and alsomulticolor photoluminescence properties. This property ofphotoluminescence makes C-dots excellent candidates forapplication in bioimaging. In vitro and in vivo analysis can bedone due to improved photon tissue penetration occurringfrom the luminescent C-dots.

Owing to their nontoxicity and photostability, carbondots have been quite successful in fluorescence imaging of cells.A recent report used PEG-coated C-dots to stain bacterial cellsand oligomeric aminopolymer-functionalized C-dots oncaco-2 cells for internalization. Confocal microscopy resultsrevealed that the C-dots only showed minor penetration intothe cell nucleus; the C-dots were localized only at the cyto-plasm.[40] Another article reported in vivo imaging. A solution

of CZnS dots was injected into the back part of female mice andimaged using an in vivo imaging system. High fluorescence wasobserved in the parts where the C-dots were injected but fluo-rescence faded away after 24 h.[41,42]

As seen in Figure 19, C-dots were intravenously injectedinto the bladder of mice. The images also show fluorescenceimaging of the dissected kidneys and liver. It was also observedthat the injected C-dots were removed from the body via theurine excretion pathway.

In another study, fluorescent C-dots were synthesizedfrom several carbohydrates such as glucose, glucosamine,sucrose, dextrans of different molecular weights, cellulose andascorbic acid. The obtained nanoparticles were then incubatedwith HeLa cells for 3–6 hours and imaged using fluorescencemicroscopy.

The C-dots exhibited very low binding to the cells due totheir small size and low surface charge. As seen in Figure 20,the particles fluoresced different colors based on their size.[43]

All these findings suggest that few C-dots are capable of pen-etrating into the cell membrane and a few just help in stainingthe cells and cell nuclei. Hence, the size and surfacefunctionalization of C-dots play a major role for applications inbioimaging. Also, the results mentioned above suggest that

Fig. 19. Intravenous injection: (a) bright field, (b) as-detected fluorescence (Bl: bladder and Ur: urine), and (c)color-coded images. The same order for the images of the dissected kidneys (a’–c’) and liver (a”–c”). Figure reprintedfrom reference [41] with permission.



C-dots are nontoxic and biocompatible in nature. Thus,C-dots are considered to be prominent materials for use inbioimaging of different cells and body parts.

C-dots also find applicability in the area of targeted drugdelivery. Due to the presence of various functional groups onthe surface of C-dots, attachment of drugs for targeted drugdelivery is possible. The biocompatibility and nontoxic natureof C-dots allowed one research group to demonstrate the drugdelivery capability of C-dots. The C-dots were primarily syn-thesized from sorbitol and then functionalized with bovineserum albumin (BSA) to improve the biocompatibility anddrug loading capacity. Doxorubicin was then loaded onto theC-dots–BSA complex to further investigate the drug deliverywith folic acid being used as a medium. The drug loadingcapacity of the C-dots was found to be nearly 86%. The drugrelease profile was about 12% at a pH of 7.2 and after 60 hoursmore than 16% of the DOX was found to have been released.The cancer cells were effectively killed with the C-dot complexin comparison to free BSA. This shows that C-dots could beimportant candidates for drug delivery due to their small sizeand biocompatible nature.[44]

3.2. C-Dots as Optical Sensors

It is very essential to detect the presence of metal ions in cells.Carbon dots that have been synthesized by greener methods

can be preferred for sensing purposes. The photophysical prop-erties of C-dots are mainly dependent on the core size, chemi-cal composition and structural chemistry. The C-dots alone arenot always fluorescent in nature and functionalization isrequired to enhance their fluorescence properties. It is knownthat radiative recombination of holes results in the fluorescenceproperty of C-dots. Quantum yield is an important parameterfor fluorescence imaging or sensing. There have been reports onthe effect of photoinduced charge transfer on the photolumi-nescence intensity of C-dots. Inorganic metal ions had a greateffect on quenching or enhancement of the photoluminescenceof C-dots.[45]

In a recent report, the detection of Cu2+ using UV andNIR as excitation sources was performed. These optical sensorscan be used for selective and sensitive metal-ion detection inenvironmental as well as biological systems. The fluorescencequenching mechanism was tested in pH 4 buffer. The fluores-cence decay curves showed that they were Cu2+ sensitive. Also,other potentially interfering ions were added to test theresponse of the C-dots to Cu2+, which showed that the behaviorof the C-dots towards Cu2+ remained unchanged.[46] All theseresults indicate that C-dots can be used for detection of Cu2+ inliving cells.

Upconversion photoluminescence (UCPL) is a uniqueproperty observed for C-dots, where the emission occurs at alower wavelength than the excitation. A report on the synthesis

Fig. 20. Imaging of cells in bright field (BF) and fluorescence (FL) modes. Figure reprinted from reference [43] withpermission.



of fluorescent C-dots showed that they exhibited UCPL. TheUCPL emissions were observed in the range of 400–650 nmwhen excited in the NIR wavelength range of 650–1000 nm.[47] This distinctive property of C-dots helps them tobe applicable in biological labeling and ultrasensitive metal-iondetection. The mercury(II) or Hg2+ ion is considered to be ahighly toxic element and a dangerous pollutant that can evenpenetrate through skin, tissues and the respiratory system,leading to damage in the DNA and central nervous system.Aqueous C-dots, owing to their nondestructive nature, canhelp in the detection of Hg2+ ions based on the fluorescencequenching technique.[47,48] Hydrothermal synthesis of C-dotsfrom pomelo peel was carried out, which resulted in a quantumyield of approximately 6.9%.

For the detection of Hg2+, the experiment was carried outat room temperature in phosphate-buffered saline (PBS) solu-tion. The C-dot dispersion (3 μL) was added to PBS buffersolution (1 mL) along with Hg2+ ions. As seen in Figure 21, thephotoluminescence study showed that the C-dot solutionexhibited a strong PL peak at 444 nm, whereas the fluorescenceintensity decreased upon addition of Hg2+ to the C-dot solu-tion. Different concentrations of Hg2+ were added, which ledto a gradual decrease in the PL property with increasing con-

centrations of Hg2+. This observation shows that the fluores-cence quenching was possible due to electron or energytransfer.

The detection of Hg2+ in lake water was also tested bycollecting lake water from the South Lake of Changchun, Jilinprovince, China. The C-dot solutions containing different con-centrations of Hg2+ were added to a sample of lake water. ThePL study showed decreasing intensity upon addition of solu-tions with increasing concentrations of Hg2+ from 5 to50 nM.[48] These results indicate that the Hg2+ probe should beuseful for detection of Hg2+ in real samples upon furtherimprovements.

Apart from detecting metal ions in water samples,C-dots are also capable of detecting metal ions in biosystems.They can be used as reagents in biosensors. Carbon dots thathave been developed using the hydrothermal method havebeen reported recently. These C-dots were able to detect Fe3+

ions in biosystems. This was mainly due to the phenolichydroxy groups present on the surface of the C-dots. ThePL intensity decreased as the Fe3+ ion concentration increasedin the C-dot solution. This indicates that fluorescencequenching increased with the increasing concentration ofFe3+.[49]

Fig. 21. Illustration of the experimental procedure for detection of Hg2+ ions. Figure reprinted from reference [48].



Furthermore, C-dots are capable of detecting radicals bythe fluorescence quenching mechanism. A recent reportshowed the synthesis of highly fluorescent and ultrastableC-dots from polyvinylpyrrolidone. Detection of hydroxyl radi-cals was successful with the as-prepared C-dots based on thefluorescence quenching mechanism.[50] This research willgreatly help pave the way towards a new field of application forC-dots.

Hence, C-dots are not only capable of detecting metal ionsin the environment, but are also candidates to detect radicalsand metal ions in biosystems. In this regard, C-dots could be ofgreat use in sensor technologies by the help of the fluorescencequenching mechanism.

3.3. C-Dots in Optoelectronics

Carbon-dot-supported silver nanoparticles were reported to beused as a reducing agent and template to fabricate solution-processable polymer light-emitting diodes (PLEDs) andpolymer solar cells (PSCs). Firstly, the use of C-dots in PLEDswas investigated. The materials used were glass/indium tinoxide (ITO)/C-dot–Ag nanoparticles/PEDOT:PSS/SY/LiF/Al. These layers were spin coated onto an ITO substrate. Thecurrent efficiency and external quantum efficiency increased tomore than two times those of the device without C-dot–Agnanoparticles. Quantum efficiency is an important parameterin devising a solar cell. It is basically defined as the number ofcharge carriers produced upon irradiation of a solar cell withphotons. The device with C-dot–Ag nanoparticles showedhigh luminous efficiency, which was three times greater thanthat of the device without these nanoparticles. The investiga-tion further extended with the use of C-dot–Ag nanoparticlesin PSCs. The materials used were glass/ITO/C-dot–Agnanoparticles/PEDOT:PSS/PTB7:PC71BM/Al.

As seen in Table 1, the current efficiency and externalquantum efficiency showed improvement in the presence ofC-dot–Ag nanoparticles. The internal quantum efficiency wasabout 99% at 460 nm and only 90% in the absence of the

nanoparticles.[29] All these results show that C-dots contributeto an efficient approach to achieve high-performance PLEDsand PSCs. From this research, we can conclude that C-dotssignificantly help in the improvement of quantum efficiencyand hence can be highly useful in designing a solar cell.

3.4. Photoinduced Electron Transfer in C-Dots

Carbon dots that had been surface passivated using PEG1500N

were used to check the electron-accepting and electron-donating capabilities. The photoluminescence spectra wereobserved to be broad when excited at 425 nm. This lumines-cence emission intensity was quenched by the electronacceptors 4-nitrotoluene (−1.19 V vs NHE)[16] and 2,4-dinitrotoluene (−0.9 V vs NHE)[17] in toluene solution; Stern–Volmer quenching constants from linear regression of 38 M−1

and 83 M−1 respectively were observed, as seen in Figure 22.2,4-Dinitrotoluene was observed to be more effective than4-nitrotoluene due to its stronger electron-accepting quality.

The electron-donating capabilities were observed by reduc-tion of Ag+ to Ag. This was experimentally done byphotoirradiation of carbon dots in an aqueous solution ofAgNO3 at a visible wavelength of 450 nm. Surface plasmonabsorption occurred as a result of the increase in the amount ofAg produced due to photoreduction. The same results wereobserved at a wavelength of 600 nm. In the absence of C-dots,Ag formation was not possible.[51] Thus, C-dots are consideredto be both electron accepting and electron donating, whichhelps them in the application of light energy conversion systems.

4. Challenges

Carbon dots have gathered a lot of attention these days withrespect to the unique properties observed. Yet there are manysignificant challenges ahead. The primary issue with thesenanoparticles is stability. The C-dots prepared from organicwaste have been observed to be stable only for a few weeks.

Table 1. Device characteristics of SY-based PLEDs and PTB7:PC71BM-based PSCs with and without C-dot–Ag nanoparticles. Tablereprinted from reference [29].

PLED configurationMaximum luminance/

cd m−2 (at voltage)Current efficiency/cd A−1 (at voltage)

EQE/%(at voltage)

Luminous efficiency/lm W−1 (at voltage)

ITO/PEDOT:PSS/SY/LiF/Al 46,320 (11.6) 11.65 (7.4) 4.26 (7.4) 6.33 (4.8)ITO/C-dot–Ag nanoparticles/PEDOT:PSS/SY/LiF/Al 46,460 (9.8) 27.16 (6.4) 9.07 (5.8) 18.54 (4.0)

PSC configuration Jsc / mA cm−2 Voc / V FF PCE / %

ITO/PEDOT:PSS/PTB7:PC71BM/Al 14.41 0.75 0.70 7.53ITO/C-dot–Ag nanoparticles/PEDOT:PSS/PTB7:PC71BM/Al 16.0 0.75 0.70 8.31



Stability is a primary parameter for their application to anydevices like solar cells, LEDs, drug delivery systems, etc. Fur-thermore, the functionalization of C-dots with other materialssuch as metals and semiconductors is yet to be explored com-pletely. C-Dots should be capable of readily accepting the otherelements in order to enhance the properties for application inseveral fields. Another important challenge that could be facedin the future is their applicability in biological systems.Although it is known from several reports that C-dots arebiocompatible in nature, their applicability to human cell diag-nosis or treatment is a significant challenge yet to be faced byresearchers. As this is a very new field, there is a lot of space fornew discoveries and a lot of exploration is still needed. As timeprogresses, many challenges might be faced and at the same timelead to many more advantages that will help achieve a betterfuture.

5. Summary and Future Perspectives

In this review, we have first discussed the emergence of carbondots. Then the structural and compositional details have beengathered from earlier reports. The primary property, i.e., surfacepassivation of C-dots by organic and polymeric materials, hasbeen discussed. We emphasized the several synthetic methods

that have been researched to obtain C-dots. Starting from thephysical methods that were used initially, we then focused on thegreener methods. An observation was made that green chemistrysimplified the preparative methods for obtaining C-dots.Greener approaches were found to be more economical, lesscomplicated, and less time consuming, yielding C-dots withreduced toxicity, better biocompatibility, higher yields, a non-destructive nature and improved optical properties. Uponfurther research, C-dots were found to have excellent photolu-minescent and optical properties.The fluorescence emission wasspectacular, which improves their applicability in bioimaging.The applications of C-dots are very widespread with one majorapplication being bioimaging. Owing to their optical propertiesand nontoxicity, C-dots could be excellent candidates forbioimaging. Engineering the surface chemistry, properties andfunctionalization of C-dots provides a lot of scope for them to beincorporated in imaging, drug delivery and sensing.

There is still a lot of room for exploring the functionalityof C-dots with different elements. Use of C-dots in optoelec-tronics is also of primary interest as the production will be highand also cost effective. Electronic devices like light-emittingdiodes can be produced with improved brightness. Theiroptical properties, such as tuning of emission colors, highquantum yield, and lack of blinking, enable them to be used inthe development of nanosensors with high sensitivity. Lightenergy conversion is also possible with C-dot solutions, whichmakes them a promising candidate for use in photovoltaicsystems. Research into the detection of toxic elements presentin the environment by C-dots is also underway, relying on theirenergy-transfer capability. Their biocompatible nature allowsthem to be used in the analysis of biological samples and also inwaste water treatment, which contains toxic chemicals.

In the future, carbon dots will definitely brighten the livesof humans by paving the way to simplified bioimaging tech-niques, drug delivery systems, efficient optoelectronics, photo-voltaic systems, optical sensors and detectors.

Acknowledgements

S.P.S. thanks TAPSUN-NWP-54 for funding. The authors alsoextend their gratitude towards XII FY CSIR-INTELCOAT(CSC0114) for financial support. We acknowledge all theresearchers who contributed to the work cited in this article.

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