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Cite this: DOI: 10.1039/c2jm34690g

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Carbon nanodots: synthesis, propertie

s and applications

Haitao Li, Zhenhui Kang,* Yang Liu* and Shuit-Tong Lee*

Received 17th July 2012, Accepted 28th August 2012

DOI: 10.1039/c2jm34690g

Carbon nanodots (C-dots) have generated enormous excitement because of their superiority in water

solubility, chemical inertness, low toxicity, ease of functionalization and resistance to photobleaching.

In this review, by introducing the synthesis and photo- and electron-properties of C-dots, we hope to

provide further insight into their controversial emission origin (particularly the upconverted

photoluminescence) and to stimulate further research into their potential applications, especially in

photocatalysis, energy conversion, optoelectronics, and sensing.

1. Introduction

Carbon nanodots (C-dots) are a new class of carbon nano-

materials with sizes below 10 nm, first obtained during purifi-

cation of single-walled carbon nanotubes through preparative

electrophoresis in 2004.1 C-dots have gradually become a rising

star in the nanocarbon family, due to their benign, abundant and

inexpensive nature.2 Carbon is commonly a black material, and

until recently was generally considered to have low solubility in

water and weak fluorescence.3 The main reason why such tiny

C-dots have recently attracted wide attention is because of their

strong fluorescence, for which they are referred to as fluorescent

carbon.

Haitao Li

Haitao Li is currently a PhD

student in Institute of Functional

Nano & Soft Materials (FUN-

SOM), Soochow University.

Since 2010, he has been con-

ducting his PhD work in FUN-

SOM under the supervision of

Prof. Yang Liu, Zhenhui Kang

and Shuit-Tong Lee. His main

research interest is focused on

the study of synthesis and pho-

toluminescence of carbon dots,

as well as their catalysis and

energy conversion applications.

Institute of Functional Nano & Soft Materials (FUNSOM) and JiangsuKey Laboratory for Carbon-Based Functional Materials & Devices,Soochow University, 199 Ren’ai Road, Suzhou, 215123, China. E-mail:zhkang@suda.edu.cn; yangl@suda.edu.cn; apannale@suda.edu.cn; Tel:+86-512-65880957. Fax: +86-512-65882846

This journal is ª The Royal Society of Chemistry 2012

During the past few years, much progress has been achieved in

the synthesis, properties and applications of C-dots, as recently

reviewed by Baker et al. and Zhu et al.2,4 Nuclear magnetic

resonance (NMR) measurements showed that carbon atoms of

C-dots derived from candle soot were sp2 hybridized with no

saturated sp3 carbon atoms, indicating that C-dots are conju-

gated systems.5,6 For their strong and tunable photoluminescence

(PL) C-dots have found important and wide applications in

energy and catalysis.2,4 In particular, besides normal or down-

converted PL, C-dots were shown to possess excellent up-con-

verted PL (UCPL), which enables the design of high-perfor-

mance, complex catalyst systems based on C-dots for efficient

utilization of the full spectrum of sunlight.2,4,7–13

Compared to traditional semiconductor quantum dots (QDs)

and organic dyes, photoluminescent C-dots are superior in terms

of high aqueous solubility, robust chemical inertness, easy

functionalization, high resistance to photobleaching, low toxicity

and good biocompatibility.2,4,14–16As a result, much attention has

also been paid to their potential applications in biological

Zhenhui Kang

Zhenhui Kang is currently a

Professor in Institute of Func-

tional Nano & Soft Materials

and the Jiangsu Key Laboratory

for Carbon-Based Functional

Materials & Devices in Soochow

University, P. R. China. His

main research interests are in

the fields of synthetic and cata-

lytic chemistry of nano-

materials, polyoxometalates,

and quantum sized functional

materials, as well as the

exploring of their applications in

nanocatalysis, new energy, and

bio-imaging.

J. Mater. Chem.

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labeling, bioimaging and drug delivery.3,7,17–20 Of particular

interest and significance is the recent finding that C-dots can

exhibit PL emission in the near-infrared (NIR) spectral region

under NIR light excitation. It should be noted that NIR PL

emission of C-dots excited by NIR excitation is particularly

significant and useful for in vivo bionanotechnology because of

the transparency of body tissues in the NIR ‘‘water window’’.15,21

Interestingly, the PL from C-dots can be quenched efficiently

by either electron acceptor or electron donor molecules in solu-

tion, indicating that photoexcited C-dots are excellent electron

donors and electron acceptors. The interesting photoinduced

electron transfer properties of C-dots should offer exciting

opportunities for light energy conversion, photovoltaic devices

and related applications.22,23a C-dots can also be used as

nanoprobes for sensitive ion detection.24,25

By introducing the synthesis, structure and PL properties of

C-dots in this review, we hope to provide further insight into the

as-yet controversial mechanism of their strong emission (partic-

ularly for UCPL), as well as to stimulate further research on the

potential applications of C-dots, such as in photocatalysis, bio-

imaging, optoelectronics, sensors and surface-enhanced Raman

scattering (SERS, Scheme 1).

Scheme 1 C-dots with unique properties have great potential applica-

tions in bioimaging, optoelectronics, sensor, SERS, and photocatalysis.

Yang Liu

Yang Liu is currently an Asso-

ciate Professor in Institute of

Functional Nano & Soft Mate-

rials and the Jiangsu Key

Laboratory for Carbon-Based

Functional Materials & Devices

in Soochow University, P. R.

China. Her main research field is

dedicated to the synthesis and

assembly of morphologically

unique nanostructures and

nanocomposites, and the appli-

cations in catalysis and energy

conversion.

J. Mater. Chem.

2. Synthesis, structure, and size control

2.1. Synthetic methods

Synthetic methods for C-dots (including carbon quantum dots

(CQDs), graphene quantum dots (GQDs)) with tunable size can

generally be classified into two main groups: chemical and

physical methods. Chemical methods include electrochemical

synthesis,8,26–29 combustion/thermal/hydrothermal/acidic oxida-

tion,5,14,16,30–39 supported synthesis,30,40,41 microwave/ultra-

sonic,15,42–45 solution chemistry methods,46–53 cage-opening of

fullerene,54 and so on. Physical methods include arc discharge,1

laser ablation/passivation,7,17,18,22,55–61 and plasma treatment.62

2.1.1. Chemical methods

2.1.1.1. Electrochemical synthesis. Zhou et al. achieved

electrochemical synthesis of C-dots when they grew multiwalled

carbon nanotubes (MWCNTs) from scrolled graphene layers on

carbon paper by chemical vapour deposition (CVD).29 Zhao

et al. produced C-dots electrochemically by oxidizing a graphitic

column electrode against a saturated calomel electrode with a Pt

wire counter electrode in NaH2PO4 aqueous solution.27 Chi et al.

produced C-dots electrochemically from a graphite rod working

electrode, a Pt mesh counter electrode, and a Ag/AgCl reference

electrode assembly immersed in pH 7.0 phosphate buffer solu-

tion.28 A variety of carbon-based nanoparticles, including

C-dots, were generated by ionic liquid (IL)-assisted electro-

oxidation of graphite using the water-soluble IL 1-butyl-3-

methylimi-dazolium tetrafluoroborate [bmim][BF4] containing

up to 90 wt% water as the electrolyte.26

Kang et al. reported an alkali-assisted electrochemical method

to prepare 1–4 nm C-dots with controlled sizes.8a It can be

imagined that judicious cutting of a graphite honeycomb layer

into ultrasmall particles can lead to tiny fragments of graphite,

yielding C-dots, which may offer a straightforward and facile

strategy to prepare high-quality C-dots. Using graphite rods as

both anode and cathode, and NaOH/EtOH as electrolyte, they

synthesized C-dots with a current intensity of 10–200 mA cm�2

(Fig. 1). As a reference, a series of control experiments using

Shuit-Tong Lee

Shuit-Tong Lee is a member

(academician) of the Chinese

Academy of Sciences (CAS)

and fellow of the academy of

sciences for the developing world

(TWAS). He is a Director of

Institute of Functional Nano &

Soft Materials, Director of

College of Nano Science &

Technology at Soochow

University, and Director of

Nano-Organic Photoelectronic

Laboratory at the Technical

Institute of Physics and Chem-

istry, CAS. His major research

areas include functional nano-

materials and devices, organic electronic materials and technolo-

gies, diamond and super-hard thin film technologies.

This journal is ª The Royal Society of Chemistry 2012

Fig. 1 The schematic diagram of electrochemical fabrication of C-dots.

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acids (e.g. H2SO4/EtOH) as electrolyte yielded no formation of

C-dots. This result indicates that an alkaline environment is the

key factor, and OH� groups are essential for the formation of C-

dots by this electrochemical oxidation process.

Another facile electrochemical approach was recently reported

by the same group for the large-scale synthesis of high-quality C-

dots with high purity, using only pure water as an electrolyte

without any other chemical additives (Fig. 2).8b The obtained C-

dots feature a highly crystalline nature, excellent aqueous dis-

persibility, and remarkable down- and up-converted PL prop-

erties, and require no further purification. Moreover, they

further demonstrated that the C-dots possess high photocatalytic

activity under visible irradiation, and potential for high-effi-

ciency complex catalyst design (see part 4).

Later, Qu et al. reported a direct electrochemical synthesis of

green luminescent GQDs with a uniform size of 3–5 nm.23a They

prepared GQDs by electrochemical oxidation of a graphene

electrode in phosphate buffer solution. The oxygen-containing

groups on the surface of GQDs enabled aqueous solubility and

facilitated surface functionalization. Such GQDs produced by

electrochemical oxidation were made of 1–3 graphene layers due

to strong intergraphene attraction.46

2.1.1.2. Combustion/thermal/hydrothermal/acidic oxidation.

The preparation of C-dots using a combustion oxidation method

was reported by Mao et al. in 2007.5 In their synthesis, soot was

collected by placing a piece of aluminium foil or a glass plate atop

Fig. 2 (a) Reaction equipment for the preparation of C-dots; digital

image of C-dots solution (b) before treatment, (c) after treatment; (d)

DLS histogram of C-dots; (e) TEM, (f) HRTEM image of C-dots.

(Reproduced from ref. 8b.)

This journal is ª The Royal Society of Chemistry 2012

a burning candle, then mixed with oxidant and refluxed for 12 h

to oxidize the particle surfaces. After cooling down and collec-

tion by centrifugation or dialysis, C-dots were obtained and

further subjected to polyacrylamide gel electrophoresis (PAGE)

fractionation. Ray et al. used nitric acid oxidation of carbon soot

to synthesize C-dots.14 Size separation was performed in a

solvent mixture (water–ethanol–chloroform) via high-speed

centrifuge based separation. Tian et al. prepared C-dots by

refluxing the combustion soot of natural gas in nitric acid.31

Transmission electron microscopy (TEM) and high-resolution

TEM (HRTEM) results showed that the C-dots exhibited an

average diameter of (4.8 � 0.6) nm and the crystalline lattices

were consistent with graphitic carbons. A sample containing

C-dots was obtained by starting with the soot generated during

combustion of inexpensive paraffin oil in a flame.38

Pan et al. reported a hydrothermal route for cutting graphene

sheets into blue luminescent GQDs.32 The obtained GQDs

exhibited strong fluorescence with a quantum yield (QY) of 6.9%.

Fluorescent C-dots with diameter about 2.0 nm were also

prepared directly via a simple hydrothermal method by using L-

ascorbic acid as a carbon source.37 Water-soluble and well-

crystallized GQDs with a lateral size about 3.0 nm were also

fabricated by a hydrothermal cutting method.20 The atomic force

microscopy (AFM) observation of the GQDs deposited on a

mica substrate shows a narrow height distribution from 1.5 to

1.9 nm, suggesting that the GQDs typically consist of 2–3 gra-

phene layers.

Highly blue luminescent C-dots with PL QY of 31.6–40.6%

were prepared by a one-step pyrolytic route from ethylenedi-

amine–tetraacetic acid salts, and a unique emission strongly

dependent on pH, solvent, spin, and excitation wavelength was

observed.33

Chen et al. reported the preparation of highly photo-

luminescent C-dots in MAPO-44, a Mg-substituted microporous

aluminophosphate molecular sieve with a chabazite structure,

through thermal decomposition of the occluded template or

loaded organic molecules. The resulting composite phosphors

can be excited by a broad range of light in the ultraviolet region,

and the emission wavelength is tunable by varying the thermal

treatment condition.34

Wu et al. reported a high-yield synthesis of hydrophilic C-dots

by controlled carbonization of sucrose. Green luminescent

C-dots and non-luminous C-dots were effectively separated by

dialysis. After surface functionalization with PEG2000N, the non-

luminous C-dots emitted blue fluorescence.35

Peng et al. reported a facile one-step wet chemical synthesis of

GQDs with a resin-rich surface from acidic treatment of carbon

fibers (CF). Interestingly, the PL from the CF-derived GQDs can

be tailored by simply choosing different reaction temperatures,

which effectively produces GQDs of varying sizes.39a Similarly,

Liu and Kang et al. found that a mixed acid treatment of carbon

nanotubes/graphite could produce nearly identical C-dots of 3–4

nm (Fig. 3).39b During preparation, carbon nanotubes (CNTs)

and graphite were oxidized by a mixture of acids ((sulfuric

acid : nitric acid ¼ 3 : 1, Fig. 3A). After 24 h of refluxing, the

mixture was slowly dispersed in deionized water and filtered to

remove CNTs and graphite residues. The filtrate was then dia-

lyzed against deionized water to remove excess acids, and a tan

transparent liquid emitting bright yellow fluorescence under UV

J. Mater. Chem.

Fig. 3 Synthesis of C-dots from various carbon sources. (A) Schematic

diagram of reactions. (B) Solutions of C-dots-S, C-dots-M, and C-dots-G

under ambient (left) and UV light (right), C-dots made from MWNTs

(C-dots-M), SWNTs (C-dots-S), and graphite (C-dots-G). (C) The

stability of C-dots-M in different physiological solutions including water,

saline (0.9% NaCl), RPMI-1640 cell medium, and fetal bovine serum.

Inset: photos of C-dots-M after being stored at room temperature for 1

year taken under ambient (left) and UV light (right). (Reprinted with

permission from ref. 39b. Copyright 2012 John Wiley and Sons.)

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light (365 nm) (Fig. 3B) was obtained. The obtained solutions

were rather stable in physiological solutions; they did not contain

any sediments after being incubated in saline, fetal bovine serum

(FBS), or FBS containing cell medium for 48 h (Fig. 3C).

Notably, the C-dot solution could be stored at room temperature

for as long as one year without the formation of any precipitates

or the loss of fluorescence (Fig. 3C, inset).

2.1.1.3. Supported synthetic procedure. Supported synthetic

method has been widely adopted for the synthesis of mono-

disperse nanomaterials, involving molecular sieve, porous

carbon and so on. This method has been used to produce

nanosized C-dots. One such route was employed by Li et al., who

used surfactant-modified silica spheres as supports to localize the

growth of C-dots by blocking nanoparticle agglomeration during

high-temperature treatment.40 Giannelis and co-workers

described the synthesis of supported C-dots (4–6 nm) using

thermal oxidation of an appropriately ion-exchanged NaY

zeolite.30

Zhu et al. reported a facile approach for preparing hydrophilic

C-dots by using mesoporous silica (MS) spheres as nanoreactors

in an impregnation method.41 Without further treatment, the

resulting highly efficient photoluminescent C-dots are mono-

disperse, photostable and of low toxicity, and show excellent

luminescence properties. The overall synthetic procedure is

illustrated in Fig. 4. First, MS spheres were prepared with

N-hexadecylamine as the surfactant and tetraethoxysilane

Fig. 4 Processing diagram for the synthesis of photoluminescent car-

bogenic dots. (Reproduced from ref. 41.)

J. Mater. Chem.

(TEOS) as the precursor using ammonia as a catalyst. Subse-

quently, MS spheres were impregnated with a mixed solution of

complex salts and citric acid. Subsequent calcination and

removal of MS supports generated the nanosized hydrophilic

C-dots. The key feature of this method is the employment of MS

spheres as supports, which not only confines the C-dots to a

narrow size distribution in the pores of MS spheres, but also

prevents the aggregation of the nanosized C-dots.

2.1.1.4. Microwave/ultrasonic synthesis. Microwave/ultra-

sonic technology has become a very important process in

synthetic chemistry. Zhu et al. presented a facile and economical

microwave pyrolysis approach to synthesize fluorescent C-dots

with electrochemiluminescence (ECL) properties.42 In this

synthesis, different amounts of polyethylene glycol 200 (PEG200)

and saccharide were added to distilled water to form a trans-

parent solution. The solution was then heated in a 500 W

microwave oven for several minutes. With increasing reaction

time, the solution changed from colorless to yellow, and finally to

dark brown, which implied the formation of C-dots.

Wang et al. reported a facile and green one-step microwave

synthesis of photoluminescent C-dots.45 The preparation

requires a carbohydrate (glycerol, glycol, glucose, sucrose, etc.)

and a tiny amount of an inorganic ion, and can be completed in

just a few minutes, requiring no surface passivation reagent.

A microwave-hydrothermal synthesis of fluorescent C-dots

from graphite oxide was explored by Zheng et al.43 Kang et al.

described a simple synthesis of C-dots from glucose or active

carbon by using an ultrasonic treatment method.15a,44 Such

monodispersed water-soluble fluorescent C-dots could emit

bright and colorful PL covering the entire visible-to-NIR spectral

range (Fig. 5). Notably, the NIR emission of C-dots could be

obtained by NIR excitation. Furthermore, the C-dots had

excellent up-conversion fluorescent properties.15a

Glucose-derived water-soluble crystalline C-dots with an

average diameter as small as 1.65 nm (�5 layers) were prepared

by a facile microwave-assisted hydrothermal (MAH) method.15b

This method did not require any surface passivation agents or

inorganic additives; the sole reagent was glucose (sucrose or

fructose) and; as shown in Fig. 6; the glucose molecules were

pyrolyzed and then converted to C-dots.

Fig. 5 (a) TEM image of C-dots prepared from glucose with a diameter

of less than 5 nm; (b) and (c) photographs of C-dots dispersions in water

with sunlight and UV (365 nm, center) illumination, respectively; (d–g)

fluorescent microscope images of C-dots under different excitation: d, e,

f, and g for 360, 390, 470, and 540 nm, respectively. (Reprinted from ref.

15a, Copyright 2011, with permission from Elsevier.)

This journal is ª The Royal Society of Chemistry 2012

Fig. 6 Preparation of GQDs by MAH method. (Reprinted with

permission from ref. 15b. Copyright 2012 American Chemical Society.)

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2.1.1.5. Other chemistry methods. For chemical approaches,

solution-phase chemical methods by oxidative condensation of

aryl groups have been successfully applied to produce

GQDs.2,46–53 Although intramolecular oxidative cyclo-

dehydrogenation was useful for the synthesis of large polycyclic

aromatic hydrocarbons (PAHs) from dendritic arene precursors,

the solubility of such GQDs was unfortunately low, and the

GQDs exhibited a tendency to aggregate due to strong inter-

graphene attraction.2,63,64 Li et al. demonstrated the versatile

synthesis of large, stable colloidal GQDs with desired sizes and

structures via a new solubilization strategy.47 They found that

GQDs synthesised by multiple 20, 40, 60-trialkyl phenyl groups

covalently attached to the edges of the graphene moieties. The as-

prepared GQDs consist of graphene moieties containing 168, 132

or 170 conjugated carbon atoms, respectively. Recently, multi-

color GQDs with a uniform size of�60 nm diameter and 2–3 nm

thickness were synthesized by using unsubstituted hexa-peri-

hexabenzocoronene (HBC) as the carbon source,53 and these are

the largest GQDs reported so far. Rhee et al. developed a

synthetic method employing reverse micelles as nanoreactors to

produce highly luminescent GQDs. GQDs were prepared via

carbonization of glucose in reverse micelles followed by in situ

surface passivation, as depicted in Fig. 7. This solution-phase

chemical method offers size tunability and narrow size distribu-

tion without any impractical size separation process.65

Fig. 7 Schematic representation of synthesis of GQDs in reverse

micelles: (a) formation of water-in-oil reverse micelles, (b) hydrolytic

polymerization, (c) carbonization, and (d) in situ passivation by hex-

adecylamine. (Reproduced from ref. 65.)

This journal is ª The Royal Society of Chemistry 2012

Recently, a mechanistic approach to the synthesis of a series of

atomically defined GQDs by metal-catalysed cage-opening of

C60 was reported by Loh et al.54 The fragmentation of the

embedded molecules at elevated temperatures produced carbon

clusters that underwent diffusion and aggregation to form

GQDs.

2.1.2. Physical method

2.1.2.1. Arc discharge. Xu et al. isolated an unknown fluo-

rescent carbon nanomaterial while purifying single-walled

carbon nanotubes (SWCNTs) derived from arc-discharge soot.1

To improve the hydrophilicity of the material, they oxidized the

arc soot with 3.3 M HNO3 to introduce carboxyl functional

groups, and then extracted the sediment with an NaOH solution

(pH 8.4) to produce a stable black suspension. The suspension

was separated by gel electrophoresis into SWCNTs, short

tubular carbons, and a fast moving band of highly fluorescent

material, which was composed of C-dots.4

2.1.2.2. Laser ablation/passivation. Sun and co-workers

produced C-dots via laser ablation of a carbon target in the

presence of water vapor with argon as a carrier gas at 900 �C and

75 kPa.7,17,18,22,55–61 After refluxing in HNO3 for up to 12 h and

surface passivation by attaching simple organic species

(PEG1500N or poly(propionylethyleneimine-co-ethyleneimine

(PPEI-EI)),7,58 the acid-treated C-dots gave bright luminescence

emission. Du et al. reported synthesis of fluorescent C-dots by

laser irradiation of a suspension of carbon materials in organic

solvent (Fig. 8).56 By selecting organic solvents, the surface states

of C-dots could be modified to achieve tunable light emission.

Based on control experiments, the origin of the luminescence was

attributed to the surface states related to the ligands on the

surface of C-dots.

Li et al. reported a simple laser ablation approach to prepare

C-dots using nano-carbon materials as the starting material and

a simple solvent as the liquid medium.60 In a typical procedure,

0.02 g of nano-carbon material was dispersed in 50 mL of solvent

(such as ethanol, acetone, or water). After ultrasonication, 4 mL

of the suspension was dropped into a glass cell for laser irradi-

ation. An Nd:YAG pulsed laser with a second harmonic wave-

length of 532 nm was used to irradiate the suspension. Fig. 9

schematically illustrates the experimental setup. After laser

irradiation, the solution was centrifuged.

2.1.2.3. Plasma treatment. Gokus et al. reported that strong

PL could be induced in single-layer graphene (SLG) using an

oxygen plasma.66 The PL was spatially uniform across the flakes

and connected to elastic scattering spectra distinctly different

from those of gapless pristine graphene; Raman spectroscopy

and elastic light scattering were used to monitor the structural

Fig. 8 One-step synthesis of C-dots in PEG200N solvent. (Reproduced

from ref. 56.)

J. Mater. Chem.

Fig. 9 Schematic illustration of laser ablation experimental setup.

(Reproduced from ref. 60.)

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and optical changes.67,68 Interestingly, the GO nanosheets

possessed visible and near-infrared (vis–NIR) fluorescene.69

Jiang et al. described a ‘‘one-step’’ combined synthesis and

functionalization of C-dots using an all-in-one small submerged-

arc plasma reactor.62 They took advantage of the long-lived free

radicals generated by a submerged-arc helium atmosphere

plasma resident on the nanoparticle surfaces to supply ethyl-

enediamine to functionalize the C-dots.

2.2. Composition and structure

The chemical composition of the purified C-dots (36.8%C, 5.9%H,

9.6%N, 44.7%O) was vastly different from that of raw candle soot

(91.7% C, 1.8% H, 1.8% N, 4.4% O), having significantly higher

oxygen content due in part to the presence of carbonyl groups. 13C

NMR measurements showed three types of carbon signals; i.e.

external C]C bonds, internal C]C bonds, and C]O bonds.

Notably, no evidence for sp3-hybridized carbon was found.5

Fig. 10 (a) SAED pattern of C-dots in ref. 56. (b–g) HRTEM images of

typical C-dots with different diameters in ref. 8a (scale bar: 2 nm). (h)

Raman spectra (lex ¼ 633 nm), (i) C1s XPS spectra of graphite and

C-dots in ref. 8b (reproduced from ref. 8b and 56, reprinted with

permission from ref. 8a. Copyright 2010 John Wiley and Sons).

J. Mater. Chem.

As shown in Fig. 10a, Hu et al. reported that the selected-area

electron-diffraction (SAED) results of 3 nm C-dots obtained by a

laser ablation/passivation method56 revealed a ring pattern. The

ratio of the squares of the ring radius is 3 : 8 : 11 : 16 : 19,

implying a diamond-like structure, and the rings correspond to

the {111} {220} {311} {400} {331} planes of a diamond struc-

ture. The lattice spacings varied from 0.2 to 0.23 nm, and are

quite close to the (100) facet of graphite. The lattice fringes of the

diffraction planes of diamond-like and graphitic carbon lie very

close to one another, thus rendering unambiguous assignment

difficult without other corroborating evidence.2 Fourier-trans-

form infrared (FTIR) spectra were used to determine the surface

ligands of C-dots produced in PEG200N. Besides the vibrations of

C–OH, C–O–C and C–H bonds in PEG, the stretching vibrations

of C]O (1623 cm�1) combined with the asymmetric and

symmetric stretching vibrations of C–O–C (around 1300 cm�1

and 1200 cm�1) in carboxylate groups were also detected.56 For

C-dots in water, three peaks at 2960 cm�1, 2876 cm�1, and

1380 cm�1 are assigned to nas(CH3), ns(CH3), and ds(CH3),

respectively, which suggest that the surfaces of C-dots have

methyl groups attached. C]O and C–O stretching vibrations are

also detected, indicating that the surfaces of C-dots are partially

oxidized. Additionally, Ray et al. found that the C-dots prepared

from oxidizing candle soot had 0.208 nm lattice spacings, which

suggests sp3 diamond-like carbon or sp2 graphitic carbon.14 Chen

et al. used 13CNMRmeasurements to identify the presence of sp2

carbon with signals in the d ¼ 90–180 ppm range, while the

absence of signals in the d ¼ 8–80 ppm range showed a lack of

detectable sp3 carbon.31

C-dots produced by electrochemical oxidation of multi-wall

carbon nanotubes (MWCNTs) are graphitic in nature.29 Raman

spectra show the characteristics of both sp2 and disordered

carbon in C-dots. Zhao et al. found that the C-dots produced

from electrochemical oxidation of graphite had lattice spacings

of 3.28 �A (HRTEM results).27 Fig. 10b–g show the HRTEM

images of C-dots obtained by alkali-assisted electrochemical

method, revealing the lattice spacing of 0.32 nm, which agrees

well with the h002i spacing of graphitic carbon.8a C-dots

synthesized by electrochemical method feature a highly crystal-

line nature (Fig. 2f). Raman spectra (lex¼ 633 nm) of C-dots and

pristine graphite are shown in Fig. 10h. For graphite, three

prominent peaks appear at 1345, 1570 and 2696 cm�1, corre-

sponding to the D, G and 2D peaks, respectively. For C-dots,

only D and G peaks are displayed, with no 2D peak. The

intensity ratio of the D and G band (ID/IG) is a measurement of

the disorder extent, and the ratio of sp3/sp2 carbon. In this case,

the graphite has a weak ID/IG ratio of 0.365, but after electro-

chemical oxidation the C-dots showed a noticeable increase in

ID/IG ratio to 1.234. Graphite oxidation initially induced struc-

ture defects in C-dots, such as oxygenated groups (C–O, C]O,

O–C]O) in the sp2 carbon site, and then the vacant lattice sites

and the sp3 carbon were produced. Raman studies of C-dots

produced by the oxidation of candle soot14 and by electro-

chemical means26,29 also showed both the D and G bands. The

ratio of the intensities (ID/IG) of these characteristic bands can be

used to correlate the structural properties of the carbon. For

C-dots obtained by electrochemical treatment of MWCNTs,29

the ID/IG ratio of 2 indicates nanocrystalline graphite. Fig. 10i

shows the X-ray photoelectron spectroscopy (XPS) spectra of

This journal is ª The Royal Society of Chemistry 2012

Fig. 12 EELS spectra, STEM image, and EDS analysis of GQDs. (a)

EELS spectrum of the GQDs; the insets show the EELS spectra of C K-

edge and O K-edge of the GQDs; (b) STEM image of GQDs assembled

on Cu grid coated with ultrathin amorphous carbon film; (c) elemental C

mapping (in blue) of the image shown in panel b; (d) EDS spectrum of

GQDs; (e) elemental O mapping (in red) of the image shown in panel b.

GQDs were prepared using 11.1 wt% glucose solution and 7 min

microwave heating at 595 W. (Reprinted with permission from ref. 15b.

Copyright 2012 American Chemical Society.)

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C1s from graphite and C-dots,8b where the peak intensity

(located at about 288 eV) shows a noticeable increase, indicating

more oxidation groups (C–O, C]O) in C-dots, consistent with

Raman results.

C-dots produced by a one-step thermal decomposition reac-

tion of citrate salts showed XRD patterns of disordered carbon

and respective passivation agents.16 The XRD pattern of C-dots

made from octadecylammonium citrate shows two superimposed

broad reflections: a broad one centered at 4.3 �A and a sharper

peak at 4.14 �A, which are indicative of disordered carbon and

densely packed alkyl groups arising from the octadecyl chains.

The XRD pattern of C-dots prepared by supported method

displayed a sharp, low-intensity reflection centered at d002 ¼3.3 �A, which was very close to the graphite 002 lattice spacing

(Fig. 11a).30 However, the XRD pattern of the hydrophilic

C-dots (prepared by controlled carbonization of sucrose) in

Fig. 11b showed an (002) interlayer spacing of 4.2 �A, which is

larger than that of bulk graphite (3.3 �A), indicating poor

crystallization.35

Electron energy loss spectroscopy (EELS) is a powerful tool to

characterize the chemical composition and structure of carbon-

based materials. Fig. 12a shows the EELS spectrum of the GQDs

prepared by MAH method, and EELS spectra of C K-edge

(285 eV) and O K-edge (539 eV) in the insets. The peak at 285 eV

is attributed to the transition from 1s to p* state (1s/ p*), and

the peak at 291 eV is the transition from 1s to s* state (1s/ s*).

These are the main EELS features of sp2 bonded (C]C) carbon

in the K-edge region. The peak at 539 eV is caused by 1s/ s* of

C–O. It indicates that GQDs were passivated with O-related

functional groups. Scanning transmission electron microscopy

(STEM) and energy dispersive X-ray spectroscopy (EDS) were

employed to analyze the elemental distribution of the GQDs.

Fig. 12b shows the dark field image of the GQDs, while the

corresponding elemental C and O mappings are shown in Fig. 12

panels c and e, respectively. Evidently, the C content was greater

than the O content, and in fact the atomic ratio of C/O is 95.4/4.6,

as calculated from the EDS spectrum shown in Fig. 12d.

Based on the above discussion, we can conclude that most

C-dots consist of an amorphous to nanocrystalline core with

predominantly sp2 carbon, the lattice spacings of which are

consistent with graphitic or turbostratic carbon. However, some

C-dots showed poor crystallization (the C-dots prepared by

controlled carbonization of sucrose). In contrast to the other

C-dots, all kinds of GQDs had a graphitic structure. Unless

Fig. 11 XRD pattern of C-dots prepared by (a) supported method and

(b) controlled carbonization of sucrose. (Reprinted with permission from

ref. 30. Copyright 2008 American Chemical Society, reproduced from ref.

35 with permission from the Centre National de la Recherche Scientifique

(CNRS) and The Royal Society of Chemistry.)

This journal is ª The Royal Society of Chemistry 2012

otherwise modified, oxidized C-dots generally feature carboxylic

moieties at their surface, with overall oxygen contents ranging

from 5–50 wt%, depending upon the experimental conditions

used.2

2.3. Size control

There are few studies of size- and shape-controlled fluorescent

C-dots, although they are important for understanding quantum

phenomena, and for optoelectronics and biomedical applica-

tions.2,4,8 As mentioned above, C-dots are readily prepared by

electrochemical cutting or thermal oxidation of graphite,

however the prepared C-dots are a mixture of sizes (such as 1.2–

3.8 nm), which requires complex separation methods (e.g. dial-

ysis, chromatography, electrophoretic separation, etc) to obtain

monodisperse C-dots.5,8,35 For particle size control, Kang et al.

have developed a current density-controlled electrochemical

method, but the size distribution of the obtained C-dots

remained broad and required further separation.8

J. Mater. Chem.

Fig. 13 (a) Typical sized C-dots optical images illuminated under white

(left; daylight lamp) and UV light (right; 365 nm); (b) PL spectra of

typical sized C-dots: the red, black, green, and blue lines are the PL

spectra for blue-, green-, yellow-, and red-emission C-dots, respectively;

(c) relationship between the C-dots size and the PL properties; (d)

HOMO–LUMO gap dependence on the size of the graphene fragments.

(Reprinted with permission from ref. 8a. Copyright 2010 John Wiley and

Sons.)

Fig. 14 (A) UV/Vis absorption (Abs) and PL emission spectra (recorded

for progressively longer excitation wavelengths from 320 to 520 nm in 20

nm increments) of carbon dots surface-passivated with PEG1500N in

water. In the inset, the emission spectral intensities are normalized. (B)

Optical photograph obtained under excitation at 365 nm. (Reprinted

with permission from ref. 40. Copyright 2009 John Wiley and Sons.)

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Fig. 13a shows the optical images of C-dots of four typical

sizes, illuminated by white and UV light. The bright blue, green,

yellow, and red PL of C-dots is strong enough to be seen with the

naked eye. Fig. 13b shows the corresponding emission spectra, in

which the red, black, green, and blue lines show the PL spectra

for blue-, green-, yellow-, and red-emitting C-dots, respectively.

The PL properties vary sensitively with C-dot size (Fig. 13c), with

small C-dots (1.2 nm, center) giving UV light emission, medium-

sized C-dots (1.5–3 nm) visible light emission (400–700 nm), and

large C-dots (3.8 nm, center) near-infrared emission. To further

confirm that and explain why these strong emissions comes from

the quantum-sized graphite fragment of C-dots, Kang et al.

performed theoretical calculations to investigate the relationship

between luminescence and cluster size. Fig. 13d shows the

dependence of HOMO–LUMO gap on the size of the graphene

fragments. As the size of the fragment increases, the gap

decreases gradually, and the gap energy in the visible spectral

range is obtained from graphene fragments with a diameter of

14–22 �A, which agrees well with the visible emission of C-dots

with diameters of <3 nm. Thus, they deduced that the strong

emission of C-dots comes from the quantum-sized graphite

structure instead from the carbon–oxygen surface.8a

Kang et al. developed a polyoxometalates (POMs)-assisted

hydrothermal method for the controllable synthesis of different

carbon nanostructures.36b They show that carbon nanotubes,

nanobelts, and carbon nanoparticles (CNPs) can be directly

prepared from active carbon powder and that the POMs used are

recyclable. The POMs-assisted hydrothermal method was also

used to control the size of the obtained nanocarbons.36c Signifi-

cantly, the obtained CNPs are large-scale and monodisperse. The

functions of POMs in CNPs formation could include the

following: (1) to catalyze/promote the reaction process; (2) to

effectively control the diameters of CNPs. The POMs-assisted

J. Mater. Chem.

method is potentially an excellent approach to prepare mono-

disperse C-dots with tunable size.

Recently, Li et al. reported the synthesis of colloidal GQDs

with a uniform and tunable size through solution chemistry.47

The prepared GQDs consisted of graphene moieties containing

168, 132, and 170 conjugated carbon atoms, respectively. The

synthesis was based on oxidative condensation reactions devel-

oped by Scholl, M€ullen, and so on.50,51,70–72

The solution chemical method reported by Rhee and Kwon

offers size tunability and narrow size distribution with minimal

size separation process.65 In this approach, fluorescent GQDs

were synthesized via a facile one-step carbonization of sugar in

water-in-oil reverse micelles. The micelles are microemulsions of

water encapsulated by surfactant molecules in an immiscible oil

such as a hydrocarbon. Physically isolated from the oil phase, the

water phase inside the micelle can serve as a reactor for synthesis

of nano-sized materials, in which size can be regulated by the

water–surfactant (S) molar ratio (w ¼ [H2O]/[S]).73 Thus, this

reaction can offer size tunability and narrow size distribution.

3. Absorbance and luminescence properties

3.1. Absorbance

C-dots typically show obvious optical absorption in the UV

region, with a tail extending to the visible range (see Fig. 14A for

representative spectra).2,58 Most of the C-dots, such as those

prepared by laser-passivation,7,17,18,22 electrochemical oxida-

tion,29 microwave/ultrasonic,15,42–45 or supported method,30 have

an absorption band around 260–320 nm. However, the absor-

bance of C-dots was found to increase to longer wavelength after

surface passivation with 4,7,10-trioxa-1,13-tridecanediamine

(TTDDA)2 or organosilane (350–550 nm; 340–410 nm, 360 nm

center, respectively).

3.2. PL

The classic signature of quantum confinement is the observation

of size-dependent optical absorption or PL.74–76 From a

This journal is ª The Royal Society of Chemistry 2012

Fig. 15 PL of C-dots. (a) PL spectra of C-dots obtained at 198 mJ

pulse�1 for 30 min with different excitation wavelengths increasing from

300 nm to 480 nm in 20 nm increments. The inset is the normalized PL

spectra. (b) Relationship between excitation and emission wavelength of

PL spectra. (c) Stability tests of PL at 450 nm excited by 380 nm.

(Reproduced from ref. 60.)

Fig. 16 PL photographs of carbon dots in water under ultraviolet (330–

385 nm), blue (450–480 nm), and green (510–550 nm) light excitation.

(Reproduced from ref. 45.)

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fundamental and an application viewpoint, PL is one of the most

fascinating features of C-dots. Studies of the optical properties of

small-sized C-dots are controversial due to the exact mechanisms

of PL, which remain unsettled and require further clarification.

Nevertheless, some aspects are well recognized and established,

and are briefly reviewed here for relevance to the design of

C-based luminescent probes and photocatalysts. One interesting

feature of the PL of C-dots is the clear lex dependence of the

emission wavelength and intensity, whether it is due to differently

sized nanoparticles (quantum effect) and/or different emissive

traps on the C-dot surface, or a mechanism currently unresolved.

Similarly, the requirement for surface passivation is little

understood, but appears to be linked to the synthetic method.

For example, only after surface passivation treatment by certain

organic moieties do C-dots produced by laser ablation show

bright PL emission. The resulting PL emission spectra were

generally spectrally broad, ranging from the visible into the NIR,

and depending upon lex.2,58 Zhao et al. claimed that the depen-

dency of the C-dot PL on excitation wavelength is due solely to

size differences rather than different emissive trap sites on simi-

larly sized particles.27

The optical behavior may reflect not only effects from particles

of different sizes in the sample, but also a distribution of different

emissive sites on each C-dot.2 Mechanistically, Sun et al. attrib-

uted the PL to the presence of surface energy traps, which

become emissive upon surface passivation.58 They explained that

there must be a quantum confinement effect of emissive energy

traps on the surface in order for C-dots to exhibit strong PL upon

surface passivation, a similar effect having been observed in Si

nanocrystals.77 Sun et al. reported that C-dots coated with ZnO

or ZnS required further passivation by PEG1500N for PL to

occur.57 However, Ag, Cu, or Pd metal nanocomposites of

C-dots prepared from oxidized natural gas soot required no

further passivation for observation of PL.31

Surface passivation was essential for the C-dots prepared by

the supported method to attain PL emission40 (Fig. 14); the

C-dots also showed lex-dependent PL emission. The authors

suggested that bright and colorful PL from the C-dots might be

attributed to the presence of surface energy traps that became

emissive upon stabilization as a result of surface passivation. The

requirement of surface passivation for PL was apparently shared

by C-dots and QDs, for which the widely accepted mechanism

for luminescence was the radiative recombination of excitons.

The multicolor PL arises not only from C-dots of different sizes,

but also from a distribution of emissive trap sites. The 1.5–2 nm

C-dots also needed surface passivation to obtain a PL property,

which is lex dependent.40

Hu et al.56 and Li60 et al. claimed that carboxylate groups

formed on the particle surface were the origin of the PL

(Fig. 15a). Laser irradiation produced some carbon- and oxygen-

containing radicals, and the oxygen-containing groups on the

surface of the initial carbon starting material would be the origin

of the PL. One factor leading to tunable performance was the

different particle sizes, as similarly observed in semiconductor

nanocrystals. Another factor was the different oxygen-contain-

ing groups.

C-dots produced from MWCNTs by an electrochemical

oxidation method exhibited blue PL (when excited at 365 nm).29

These C-dots also showed lex-dependent emission, but no further

This journal is ª The Royal Society of Chemistry 2012

passivation step was required for PL to occur. Yet, another study

on electrochemically synthesized C-dots did not comment on the

lex dependence of PL, but also reported that no additional

surface passivation was required for PL to occur.28 C-dots

prepared by oxidation of candle soot contained an abundance of

carbonyl groups on their surfaces, so that no further function-

alization was needed for PL to occur.5,31 As shown in Fig. 16, the

C-dots derived from carbohydrate and inorganic ions by a

microwave method showed lex-dependent PL without further

passivation.45

Kang et al. presented definitive evidence for luminescence

arising from quantum-confinement effects and size-dependent

optical properties of C-dots prepared by alkali-assisted electro-

chemical method.8a To confirm that strong emission of C-dots

came from the quantum-sized fragment of graphite, the as-

synthesized C-dots were treated by hydrogen plasma to remove

the surface oxygen. A series of control experiments showed no

obvious change in the PL spectra of C-dots before and after

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hydrogen plasma. To further confirm that and explain why the

strong emission comes from the quantum-sized graphite frag-

ment of C-dots, they performed theoretical calculations to

investigate the relationship between luminescence and cluster size

(Fig. 13d).

C-dots produced from glucose using an ultrasonic treatment

method reported by Kang et al.15a (Fig. 17) exhibited NIR

emission upon excitation by NIR light. It should be noted that

such NIR PL emission excited by NIR excitation is particularly

significant and useful for bioapplications because of the trans-

parency of body tissues in the NIR ‘‘water window’’.

The C-dots prepared by MAH method exhibited deep ultra-

violet (DUV) emission of 4.1 eV, which was the shortest emission

wavelength among all the solution-based QDs.15b The C-dots

exhibited typical excitation wavelength-dependent properties as

expected in C-based quantum dots. However, the emission

wavelength was independent of C-dot size. The unique optical

properties of the C-dots were attributed to the self-passivated

layer on the surface of GQDs, as revealed by EELS.

The QY of C-dots varies with the fabrication method and the

surface chemistry involved.2 C-dots (�5 nm) produced by laser

ablation had QY between 4 and 10%, depending on the excita-

tion wavelength and the effectiveness of the reaction for surface

passivation.58 C-dots (7 nm) obtained by thermal decomposition

methods gave a QY of only 3%, which was virtually independent

of lex.16 Smaller (4–6 nm) C-dots made by thermal decomposi-

tion on NaY supports showed a more blue-shifted emission (as

expected based on quantum confinement effects) and even lower

QY.30 On the other hand, the QY of C-dots was found to be

dependent on surface passivation in many studies.56–58,78 C-dots

from laser ablation methods passivated with PEG1500N had

higher QY than those passivated with PPEI-EI.58 When coated

with ZnO/ZnS and PEG1500N, the QY of C-dots (lex: 440 nm)

increases to 45 and 50%, respectively.57 The enhanced QY of Zn-

based coating is attributed to a secondary, more effective surface

passivation in combination with the PEG passivation agent. The

passivation agent also can affect the QY, such as C-dots

produced in a laser ablation/passivation process.56 Interestingly,

if the C-dot has a metal-containing shell or is associated with a

metal-based nanostructure, it will have a higher QY.31,78

C-dots have been shown to have good photostability.8,27,58,78

For example, the C-dots produced by alkali-assisted electro-

chemical method show good photostability as their appearance

and PL properties remain unchanged after one year of storage in

air at room temperature.8a After several hours of continuous

Fig. 17 Photoluminescence spectra of CNPs under NIR excitation

(emission in NIR range): (a) CNPs prepared from glucose/NaOH, (b)

CNPs prepared from glucose/HCl. (Reprinted from ref. 15a, Copyright

2011, with permission from Elsevier.)

J. Mater. Chem.

exposure to excitation, the laser scanning confocal microscopy

result of C-dots showed neither blinking nor meaningful reduc-

tion in PL intensity.58 The PL intensity of C-dots (prepared by

laser passivation method) decreased by only 4.5%, even after 4 h

irradiation (Fig. 15c), whereas other fluorescent materials such as

polystyrene nanospheres would photo-bleach within 0.5 h,

indicating the good photostability of C-dots.60

Luminescence decays showed that C-dots produced by laser

ablation method have multiexponential PL decays with average

excited-state lifetimes of 5 ns for emission at 450 nm and 4.4 ns

for emission at 640 nm (excited at 407 nm).58 The multi-

exponential nature of the lifetime suggests the presence of

different emissive sites. The PL lifetime of C-dots obtained by a

one-step sodium hydroxide-assisted electrochemical method is

6.74 ns, which is much longer than most reported C-dots.79

Some researchers reported that C-dot PL intensity is depen-

dent on pH value.5,27,40,79 Zhao et al. found that PL intensity

decreased when the pH value of the solution was higher or lower

than 4.5, yet PL was totally recovered when the pH value was

adjusted back to the optimal value. Meanwhile, there is a slight

shift in the emission peak with pH variation. Liu et al. found the

QY only showed a decrease of about 3% on going from a neutral

pH value to pH 5 or 9.40 Li et al. reported that under strongly

acidic or alkaline conditions, the PL is nearly completely

quenched, whereas the C-dots showed the strongest emission at

pH 7.0.79

Generally speaking, the mechanisms of C-dot luminescence

are not fully understood. The luminescence has been tentatively

suggested to arise from excitons of carbon, emissive traps,

quantum-confinement effect, aromatic structures/oxygen-con-

taining groups, free zig-zag sites and/or edge defects. However, a

widely accepted mechanism for luminescence from C-dots

remains unavailable and needs further investigation.

3.3. Electrochemical luminescence

As ECL is extensively used to investigate the fluorescent emission

of QDs or semiconductor nanocrystals,42,80,81 so ECL studies of

C-dots have aroused interest.28,42 The ECL behavior (Fig. 18) of

C-dots is similar to that of other QDs such as CdSe and Si

Fig. 18 Representative ECL response (a) without and (b) with C-dots at

an indium–tin oxide (ITO) electrode in 0.1 M phosphate buffer solution

(pH 7.0). Inset: anodic ECL response during a continuous potential scan,

v ¼ 0.1 V s�1. (Reproduced from ref. 42a.)

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nanocrystals.42Consequently, the ECLmechanism of C-dots was

explained as follows: firstly, the oxidized state (R_+) and the

reduced state (R_�) of C-dots was formed with the potential cycle;

then the electron-transfer annihilation of the two oppositely

charged carriers (R_+ and R_�) took place to form the excited state

(R*). Finally, the excited C-dots (R*) returned to the ground

state via a radiative pathway by emitting a photon. It is inter-

esting to note that the anodic ECL intensity was larger than the

cathodic value, indicating that R� was more stable than R+.

Additionally, ECL response over time was relatively stable (inset

of Fig. 18), which implied attractive applications in ECL sensing.

Zheng et al. found C-dots prepared from the electrochemical

oxidation of graphite exhibited ECL emission as the potential

was cycled between +1.8 and �1.5 V.28 The ECL spectrum of C-

dots obtained from SWCNTs82 has also been observed. Most

ECL from semiconductor nanomaterials has been observed to

originate from surface states and is often significantly red shifted

from the PL peaks by as much as hundreds of nanometers, due to

defect states in the band gap.75,83

As ECL is mainly associated with surface-state transitions in

nanoparticles2 (whereas PL is more reminiscent of the core state

in nanoparticle), comparison between the ECL and PL of

nanoparticles is an excellent way to investigate the presence of

surface traps.2,75,84,85 Based on the ECL evidence for the presence

of surface trap states and the pervasive size- and lex-dependency

of C-dot PL, Baker et al. postulated that C-dots feature core

band gaps which are size-dependent, with the most intense PL

attributable to direct recombination of electron–holes, whereas

the less intense bands may be assigned to surface-state traps and

phonon-assisted recombination.2,75 It would also be interesting

to determine if the ECL of C-dots is size-independent.

3.4. Photoinduced electron transfer property

The PL from C-dots can be quenched efficiently by either elec-

tron acceptor or electron donor molecules in solution, namely

photoexcited C-dots are both excellent electron donors and

electron acceptors. These interesting photoinduced electron

transfer properties should offer new opportunities in using

C-dots for light energy conversion and related applications, as

well as for mechanistic elucidation.22

Kang et al. reported that C-dots obtained by an alkali-assisted

electrochemical method had good photoinduced electron trans-

fer properties.12 Fig. 19 show the luminescence decays (485 nm

Fig. 19 Luminescence decays (485 nm excitation, monitored with 550

nm narrow bandpass filter) of C-dots in (a) 2,4-dinitrotoluene and (b)

DEA. Inset: Stern–Volmer plot for the quenching of luminescence QY

(485 nm excitation) of the C-dots in (a) 2,4-dinitrotoluene and (b) DEA.

(Reproduced from ref. 12.)

This journal is ª The Royal Society of Chemistry 2012

excitation, monitored with a 550 nm narrow bandpass filter) of

C-dots, which were quenched by the known electron acceptor

2,4-dinitrotoluene (�0.9 V vs. NHE)) and electron donor N,N-

diethylaniline (DEA, 0.88 V vs. NHE), with the observed Stern–

Volmer (insets of Fig. 19a and b) quenching constants (KSV ¼sF�kq) from linear regression of 18.2 M�1 and 20.1 M�1,

respectively. The results that the broad PL spectra of C-dots were

quenched highly efficiently by either electron acceptors or elec-

tron donors clearly confirm that C-dots are excellent as both

electron acceptors and electron donors.12 Based on such prop-

erties, they designed a C-dots/Ag/Ag3PO4 (C-dots: carbon

quantum dots) photocatalyst system, which exhibit high photo-

catalytic activity (see application part).

The photocatalytic behavior is solvent-dependent and is more

efficient in polar solvents, indicating an electron transfer

quenching mechanism. There are few studies about the photo-

induced electron transfer property of C-dots, and further work is

warranted to investigate the applications of C-dots in catalysis,

solar cells, and so on.

3.5. Enhancing PL

The major drawback limiting the application of C-dots in life

sciences is their relatively low QYs (often less than 10%). Thus,

the preparation of C-dots with high QYs is still a challenge.

Passivation and doping are popular approaches to enhance the

luminescence of C-dots. Several organic polymers and diamine

compounds have been used to passivate C-dots.56,58,78The QYs of

passivated C-dots can reach 20%,18 and even 60% upon purifica-

tion.86 C-dots doped with zinc salts can give high QYs of up to

50%.9 While both strategies can significantly improve the QYs of

C-dots, problems still exist. The modification processes are often

time-consuming and impractical on a large scale,17,57,58 and the

increased size of the modified C-dots restricts their further

application.31,57New strategies, besides passivation or doping, are

required to produce luminescent C-dots with high QY.

Zheng et al. offered a simple pathway to enhance the lumi-

nescence of C-dots.87 Blue luminescent reduced-state C-dots were

prepared by reducing C-dots with NaBH4. The QYs of the

reduced-state C-dots increased from 2% to 24% and the

maximum emission wavelength shifted from 520 to 450 nm

(Fig. 20).

Fig. 20 Graphical representation of the synthesis of reduced-stateC-dots

(also called CDs) with blue luminescence from original C-dots. Inset:

photographs of aqueous solutions of C-dots (left) and reduced C-dots

(right) recorded under UV light (360 nm) (reproduced from ref. 87).

J. Mater. Chem.

Fig. 22 UCPL properties of C-dots. (Reprinted with permission from

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Interestingly, C-dots synthesized via a solvothermal route had

a sensitive effect on the ultraweak chemiluminescence (CL)

reaction of hydrogen peroxide (H2O2) and sodium bisulfite

(NaHSO3).88 It was found that the CL emission intensity of

H2O2–HSO3� was significantly enhanced by C-dots, yielding

about a 60-fold increase in CL intensity. The enhanced CL was

induced by the excited-state C-dots (CD*), which were produced

from the electron-transfer annihilation of positively charged

C-dots (CD_+) and negatively charged C-dots (CD_�).Yu et al. reported the fabrication of C-dots by laser ablation of

graphite powder dispersed in PEG200.89 Notably, via the esteri-

fication of carboxylic groups on the surface of C-dots, the

intensity enhancement and wavelength tuning of PL peak were

realized in the as-prepared C-dots in PEG200.

ref. 8a. Copyright 2010 John Wiley and Sons.)

3.6. Upconversion photoluminescence

UCPL is an attractive optical property of C-dots, which enables

many promising applications.4 Sun et al. reported that C-dots

were strongly emissive in the visible region under irradiation by

an argon-ion laser (458 nm) or a femtosecond pulsed laser for

two-photon excitation in the near-infrared range (800 nm).7

Fig. 21 shows that the one- and two-photon luminescence images

for the same scanning area of C-dots are well matched. The

representative two-photon luminescence spectrum shows that

such C-dots have UCPL properties. Results from further bio-

imaging experiments in vitro also showed the UCPL property of

C-dots. Kang et al. found that C-dots prepared by an alkali-

assisted electrochemical method possessed size-dependent PL

and excellent UCPL properties. Fig. 22 shows the PL spectra of

C-dots excited by long-wavelength light (from 500 to 1000 nm)

with the upconverted emissions located in the range from 325 to

425 nm.8a They attributed the UCPL property of C-dots to a

multiphoton active process similar to that reported above. Other

groups also reported that C-dots prepared from different

synthesis methods possessed obvious UCPL proper-

ties,15,41,44,79,90,91 and similarly attributed the properties to this

multiphoton active process.

However, Li et al. held the different view that the multiphoton

active process was insufficient to explain the UCPL properties of

C-dots.9 The PL spectrum and digital photo of GQDs excited by

a 980 nm laser showed obvious upconverted green light emission

located at about 525 nm. The upconverted emission peak shifted

from 390 to 468 nm on changing the excitation wavelength from

600 to 800 nm. Note that the energy difference between the

upconverted emission (Em) and excitation light (Ex) was almost

Fig. 21 Luminescence images (all scale bars 20 mm) of the C-dots with

(a) argon ion laser excitation at 458 nm and (b) femtosecond pulsed laser

excitation at 800 nm; (c) is an overlay of (a) and (b). (Reprinted with

permission from ref. 7. Copyright 2007 American Chemical Society.)

J. Mater. Chem.

constant at about 1.1 eV. To further confirm that and explain

how UCPL comes from the GQDs, an energy level structural

model was established, in which the UCPL spectrum could be

regarded as an anti-Stokes transition as demonstrated in

Fig. 23(d). The energy levels ofp and s orbitals were provided by

the carbene ground-state multiplicity, which is related to the

energy difference (dE) between the p and s orbitals. In this work,

the energy between the excitation light and the emission light in

the upconversion process was close to 1.1 eV (Hoffmann deter-

mined dE should be below 1.5 eV). When a bunch of low-energy

photons excite the electrons in the p orbital, the p electrons

would transition to a high-energy state such as the LUMO, and

then fall back to a low-energy state. Thus, an UCPL is emitted if

the electrons transition back to the s orbital. The electrons in the

s orbital can also be excited, they only can emit normal PL

(Fig. 23a and (b)). This explains why the difference between

upconversion excitation and emission light is a constant energy

related to the energy difference of the p and s orbitals.

The UCPL properties of C-dots may provide a new way to

design and synthesize novel photocatalysts (through the combi-

nation of C-dots and different nanospecies, such as ZnO,

Fe2O3,WO3, Au, Cu, and Pt) for photo-assisted catalytic reac-

tions, which are discussed below.92,93

4. Photocatalysis

Nano-photocatalysis is an exciting and important topic in

nanoscience and nanochemistry. Its main objective is the design

of nanocatalysts (new generation catalysts) with strong and

tunable chemical activity, specificity, and selectivity.74,94–101 A

Fig. 23 A schematic illustration of various electronic transition

processes of GQDs. (Reproduced from ref. 9.)

This journal is ª The Royal Society of Chemistry 2012

Fig. 25 Possible catalytic mechanism for TiO2/C-dots under visible light

(reprinted with permission from ref. 8a. Copyright 2010 John Wiley and

Sons).

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good photocatalyst should be able to utilize visible and/or near

UV light, and be photo-stable (against photocorrosion), inex-

pensive, and environment-friendly.94–101 Nanoscience research

can greatly impact on the development of new and more potent

catalysts via design and control of photocatalyst properties,

especially in terms of band gap energy, chemical composition,

and surface modification. In particular, size-controlled C-dots

show tunable emissions from near-infrared to blue wavelength,

making them promising candidates as photocatalysts.

Recently, Kang et al. reported the facile one-step alkali-

assisted electrochemical fabrication of C-dots with sizes of 1.2–

3.8 nm, which possess size-dependent PL and excellent upcon-

version luminescence properties.8a Exploiting the excellent

optical properties of C-dots, they demonstrated the design of

photocatalyst systems (TiO2/C-dots and SiO2/C-dots complex

systems, Fig. 24) to harness the use of the full spectrum of

sunlight (based on the upconversion luminescence properties of

C-dots). They explained the photocatalytic reaction process as

follows (Fig. 25). Upon illumination of TiO2/C-dots or SiO2/

C-dots nanocomposite photocatalyst, the C-dots absorb visible

light, and then emit shorter wavelength light (325 to 425 nm) via

upconversion, which in turn excites TiO2 or SiO2 to form elec-

tron/hole (e�/h+) pairs.8,94 The electron/hole pairs then react with

the adsorbed oxidants/reducers (usually O2/OH�) to produce

active oxygen radicals (e.g. _O2�, _OH), which cause degradation

of the dyes (methyl blue, MB).8,102–104 Significantly, when C-dots

are attached to the surface of TiO2 or SiO2, the relative position

of the band edge of C-dots permits the transfer of electrons to

TiO2 or SiO2 surface, allowing charge separation and stabiliza-

tion, but concurrently hindering recombination.105 The electrons

can be shuttled freely along the conducting network of C-dots,105

while the longer-lived holes on TiO2 or SiO2 then account for the

higher activity of the complex photocatalyst system.8,105

Based on the same principle, Kang et al. reported a series of

C-dot-based photocatalysts with excellent catalytic activity, such

as C-dots/ZnO, C-dots/Fe2O3, C-dots/Ag/Ag3PO4, and so on.11–13

For theC-dots/Fe2O3 composite,Kang et al.demonstrated that

the Fe2O3/C-dots composites have a continuous absorption band

in the range of 550–800 nm compared to the narrower band of

Fe2O3 nanoparticles, suggesting Fe2O3/C-dots have a higher

Fig. 24 (a and b) SEM image of photocatalysts for SiO2/C-dots and

TiO2/C-dots; insets show the corresponding HRTEM images; (c) rela-

tionship between MB concentration and reaction time for different

catalysts: SiO2/C-dots, TiO2/C-dots, SiO2 NPs, TiO2 NPs, and C-dots.

(Reprinted with permission from ref. 8a. Copyright 2010 John Wiley and

Sons.)

This journal is ª The Royal Society of Chemistry 2012

photocatalytic activity for the target reactions.11 The photo-

catalytic results for the degradation of gas-phase benzene and

methanol show that Fe2O3/C-dots composites exhibit much

higher activity thanFe2O3 nanoparticles alone. As nakedFe2O3 is

an inefficient photocatalyst, C-dotsmust play an important role in

the high photocatalytic activity of the Fe2O3/C-dots composites.

Kang et al. explained that C-dots play three important roles in the

Fe2O3/C-dots composite during the photocatalytic process.

Firstly, C-dots can act as an electron reservoir to trap electrons

emitted fromFe2O3particles excited by visible light, thus reducing

electron–hole recombination in the Fe2O3/C-dots composite. The

oxygen radicals (O2�) would form from the combination of

electrons with O2 adsorbed on the surfaces of C-dots. Under 24 h

visible light irradiation in argon atmosphere, the degradation

efficiency of benzene was found to be about 19% and 17%with the

assistance of Fe2O3 and Fe2O3/C-dots, respectively, much lower

than that observed in the air atmosphere (37% and 80%, respec-

tively). This phenomenon also shows that O2 and C-dots indeed

play an important role in the higher degradation efficiency of

benzene in the air atmosphere. Secondly, the C-dots possess

UCPL properties, so that they can absorb longer wavelength light

and then emit shorter wavelength light via upconversion, which

can in turn excite Fe2O3 to form electron–hole pairs (Fig. 26).8

Thirdly, for benzene degradation, the p–p interaction between

the conjugated structure of C-dots and benzene is beneficial to the

enrichment of benzene on the surface of Fe2O3/C-dots composite.

The three factors together account for the enhanced photo-

catalytic activity of the Fe2O3/C-dots composite compared to

Fe2O3 nanoparticles. Kwak and Yu reported that C-dots/meso-

porous hematite (a-Fe2O3) complex photocatalysts show similar

enhanced activity for photocatalytic reaction.106

Similarly to C-dots/Fe2O3, the C-dots/ZnO obtained by one-

step hydrothermal reaction also possess high photocatalytic

Fig. 26 Schematic model for the photocatalytic process of Fe2O3/C-dots

composites under visible light (reproduced from ref. 11).

J. Mater. Chem.

Fig. 28 (a) SEM and (b) HRTEM images of C-dots/Ag3PO4 complex

photocatalyst. (c) SEM and (d) HRTEM images of C-dots/Ag/Ag3PO4

complex photocatalyst. Insets in (b) and (d) are SEM images of typical

single particle (scale bar: 200 nm) (reproduced from ref. 12).

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activity for the degradation of gas-phase benzene and meth-

anol.13 The photocatalytic mechanism of C-dots/ZnO is shown in

Fig. 27, in which the main functions of C-dots are similar to those

in Fe2O3/C-dots. Firstly, C-dots were loaded on ZnO surface and

formed the ‘‘dyade’’ structure, providing access to photo-induced

charge transfer transitions under visible light irradiation. At the

dyade structure, the photo-induced electron is transferred to

joint charge transfer states predominately located at the C-dots,

while the separated hole stays electronically and structurally near

ZnO. This process can effectively hinder the electron–hole

recombination, and guarantee the high reactivity of photo-

generated electron and hole excited by visible light. The oxygen

radicals (_O2�) would then be formed via the combination of

electrons in the conducting network of C-dots with O2 adsorbed

C-dots surfaces (Fig. 27a). Secondly, the C-dots with UCPL

properties can convert longer wavelength light to the shorter

wavelength light, which can in turn excite ZnO to form electron–

hole pairs (Fig. 27b). Thirdly, during the degradation process of

benzene, the p–p interaction between the conjugated structure of

C-dots and benzene enhances adsorption of benzene on the

surface of ZnO/C-dots nanocomposites. The three cooperative

effects are collectively responsible for the enhanced photo-

catalytic activity of ZnO/C-dots nanocomposites compared to

bare ZnO nanoparticles.

Kang et al. reported the fabrication and catalysis activity of

Ag3PO4, C-dots/Ag3PO4, Ag/Ag3PO4 and C-dots/Ag/Ag3PO4

photocatalysts.12 Fig. 28 shows the SEM and HRTEM images of

C-dots/Ag3PO4 and C-dots/Ag/Ag3PO4 complex photocatalysts,

revealing the crystalline structure of the composites. Remark-

ably, Ag3PO4 complex photocatalysts containing C-dots feature

much enhanced photocatalytic activity and structural stability

for methyl orange (MO) photodegradation. Specifically, the C-

dots/Ag/Ag3PO4 complex photocatalyst exhibits the highest

photocatalytic activity among all the Ag3PO4-based catalyst

systems. The superior activity of the complex photocatalysts is

attributed to the following salient features. The C-dot layer on

the surface of Ag3PO4 and Ag/Ag3PO4 particles can effectively

Fig. 27 Schematic model for the photocatalytic process of ZnO/C-dots

composites under visible light (reproduced from ref. 13 with permission

from the Centre National de la Recherche Scientifique (CNRS) and The

Royal Society of Chemistry).

J. Mater. Chem.

protect Ag3PO4 from dissolution in aqueous solution. The

unique photoinduced electron transfer properties of C-dots22 can

protect Ag3PO4 from photocorrosion. The UCPL property of C-

dots make the C-dots/Ag3PO4 and C-dots/Ag/Ag3PO4 complex

systems more effective in utilizing the full spectrum of sunlight so

as to enhance the photocatalytic activity. In addition, C-dots can

act as an electron reservoir to hinder electron/hole recombination

probability. Furthermore, in the C-dots/Ag/Ag3PO4 complex

photocatalyst, the surface plasmon resonance (SPR) of Ag

particles (intense electric fields at the Ag particle surface) can

further increase the formation rate of electron/hole pairs at the

nearby surface Ag3PO4 particle (Fig. 29).

In addition, the presence of C-dots enhances the stability of the

composite photocatalysts. Kang et al. studied the crystalline

structures of Ag3PO4, Ag/Ag3PO4, C-dots/Ag3PO4 and C-dots/

Ag/Ag3PO4 after MO photodegradation experiments to evaluate

structural stability. The XRD patterns of Ag3PO4 and the related

complex photocatalysts after 10 repeats of the MO decomposi-

tion experiments show distinct diffraction peaks of metallic silver

appearing in the XRD pattern of pure Ag3PO4, indicating partial

reduction of Ag3PO4 into Ag particles during MO degradation.

However, no obvious crystalline structure changes could be

observed in the XRD pattern of C-dots/Ag3PO4, indicating that

C-dots greatly enhanced the stability of the Ag3PO4 photo-

catalyst. The XRD patterns also show no obvious structure

Fig. 29 Schematic model for the important roles of C-dots in the high

photocatalytic activity and good stability of C-dots/Ag3PO4 (reproduced

from ref. 12).

This journal is ª The Royal Society of Chemistry 2012

Fig. 31 (a) Comparison of visible light-driven photocatalytic activity of

pure TiO2, C-dots, TiO2/C-dots and P25 for photo-degradation of MB.

(b) Transient photocurrents of the as-prepared TiO2/C-dots and P25

irradiated with the wavelength larger than 510 nm. (c) Proposed forma-

tion mechanism of dyadic structure with charge transfer-type orbital

hybridizations at the surface of TiO2 and C-dots. (Reproduced from

ref. 8b.)

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changes for C-dots/Ag/Ag3PO4 due to photodegradation exper-

iments compared to the corresponding samples. Furthermore,

the C-dots/Ag3PO4 and C-dots/Ag/Ag3PO4 catalysts were

investigated by recycling them in repeated MO degradation

experiments, and they were found to remain stable during the

repeated experiments without exhibiting any significant loss of

photocatalytic activity.12

Lee et al. reported a facile ultrasonic route for the fabrication

of GQDs with excitation-independent downconversion and

UCPL behavior.107 They further designed the complex photo-

catalysts (rutile TiO2/GQD and anatase TiO2/GQD systems) to

harness the full visible spectrum of sunlight based on the

upconversion of the GQDs (Fig. 30). Significantly, the photo-

catalytic rate of the rutile TiO2/GQD complex system is ca. 9

times larger than that of the anatase TiO2/GQD complex under

visible light (l > 420 nm) irradiation in the degradation of

methylene blue. The proposed mechanism is that the upcon-

verted PL peak of GQDs is at 407 nm (3.05 eV), which is larger in

energy than the band gap of rutile TiO2 at 3.0 eV (414 nm), but

smaller than that of anatase TiO2 at 3.2 eV (388 nm). Therefore,

the upconversion PL emission of GQDs is energetically capable

of exciting the rutile TiO2 but not the anatase TiO2 to form e�/h+

pairs.

Based on the PL and light-sensitive photocatalytic properties

of C-dots produced from a water-assisted electrochemical

approach, Kang et al. designed a high-efficiency complex TiO2/

C-dots catalyst.8b They performed photo-degradation of MB and

photovoltaic cell tests to evaluate the visible light photo-catalytic

activity of pure TiO2, C-dots and TiO2/C-dots, respectively. As

shown in Fig. 31a, the as-obtained TiO2/C-dots are able to

completely degrade MB within 4 h, whereas P25 and C-dots

show much lower degradation efficiencies (17.5% and 29.4%),

and pure TiO2 show no degradation of MB under the same

irradiation conditions. Meanwhile, transient photocurrents

(Fig. 31b) of TiO2/C-dots and P25 were measured under irradi-

ation with wavelength longer than 510 nm. When irradiated with

wavelengths longer than 510 nm, TiO2/C-dots show the

highest photocurrent (�2.24 mA cm�2) compared to that of P25

(0 mA cm�2).

Fig. 30 (a) Relationship between MB concentration and reaction time

for different catalysts: rutile TiO2/GQDs, CaIn2O4, anatase TiO2/GQDs,

GQDs, rutile TiO2 NPs, and anatase TiO2 NPs. Schematic of photo-

catalytic process for (b) rutile TiO2/GQD and (c) anatase TiO2/GQD

under visible light (l > 420 nm) irradiation. (Reprinted with permission

from ref. 107. Copyright 2012 American Chemical Society.)

This journal is ª The Royal Society of Chemistry 2012

Possible mechanisms to explain the better photocatalytic

performance of TiO2/C-dots can be attributed to three features

(Fig. 31c). Firstly, hybridization of C-dots with TiO2 extends the

light response of TiO2 into the visible range of the solar spec-

trum, due to the electronic coupling between the pi states of

graphite-essential C-dots and the conduction band states of

TiO2. Such electronic coupling would induce a smaller nano-

hybrid band gap compared to pure TiO2, or new carbon energy

levels in the TiO2 bandgap also leading to a smaller bandgap

energy and a red shift. In addition, the up-conversion properties

of the C-dots will transfer longer wavelength light into shorter

wavelength emission. Consequently, light with a wider range of

wavelengths, fromUV to visible, can be used by the TiO2/C-dots.

O2 adsorbed on the surface of the C-dots can accept e� and form

O2�, and oxidize MB directly on the surface. Secondly, C-dots

can combine with TiO2 efficiently to form TiO2/C-dots, which

can generate a large amount of photoreactive species (O2� and

_OH) under visible light. These photoreactive species can diffuse

easily in reactants and products during the reaction because of

the high surface area of the TiO2/C-dots. Thirdly, when the TiO2/

C-dots photocatalysts are excited, C-dots can act as an electron

reservoir to trap electrons emitted from TiO2 under irradiation,

and thus hinder electron–hole recombination.

Water-soluble fluorescent N-doped C-dots (NCDs) were

synthesized from a one-pot ultrasonic reaction between glucose

and ammonium hydroxide. Besides strong luminescence in the

visible-to-near infrared range, NCDs displayed clear UCPL

properties. Moreover, the NCDs showed excellent photo-

catalytic properties in the photodegradation of methyl orange

under visible light.91

Kang et al. prepared C-dots/Cu2O composite with protruding

nanostructures on the surface by a facile one-step ultrasonic

J. Mater. Chem.

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treatment. They demonstrated that this photocatalytic system

could harness (near) infrared (NIR) light to enhance its photo-

catalytic activity based on the collective effect of the superior

light reflecting ability of Cu2O protruding nanostructures and

the UCPL property of C-dots (Fig. 32a and b).94b The photo-

catalytic process of C-dots/Cu2O composite under (N)IR light is

schematically illustrated in Fig. 32c. When the C-dots/Cu2O

composite photocatalyst is illuminated (Fig. 32c, (1)), the

protruding nanostructures allow multiple reflections of (N)IR

light among the vacant spaces between these protruding parti-

cles, which can make better use of the source light and therefore

offering an improved photocatalytic activity. Additionally,

C-dots can absorb (N)IR light (>700 nm), and then emit shorter

wavelength light (390–564 nm) as a result of up-conversion,

which in turn further excites Cu2O to form electron/hole (e�/h+)pairs (Fig. 32c, (2)).8a The electron/hole pairs then react with the

adsorbed oxidants/reducers (usually O2/OH�) to produce active

oxygen radicals (e.g. _O2, _OH), which subsequently cause the

degradation of organic dye (MB).92–94 Significantly, when C-dots

are attached on the surface of Cu2O, the relative position of

C-dots band edge permits the transfer of electrons from the Cu2O

surface, allowing charge separation, stabilization, and then

hindering of e�/h+ pairs’ recombination. The electrons can be

shuttled freely along the conducting network of C-dots, and the

longer-lived holes on the Cu2O then account for the higher

activity of the composite photocatalyst.

For energy saving and green environment issues, C-dots offer

great potential for a broad range of applications, including

biosensors, bioimaging, laser, and light-emitting diodes. They

may also serve as a promising candidate for a new type of fluo-

rescence marker, as well as high-efficiency catalyst design for

applications in bioscience and energy technology.

5. Optoelectronics

Graphene has attracted tremendous research interest due to its

large surface area, superior mechanical flexibility, high carrier

Fig. 32 (a) Up-converted PL spectra of C-dots; (b) energy distributions

of up-converted emissions of C-dots located in the range from 390 to

564 nm (shown as shaded areas); (c) schematic photocatalytic mechanism

for C-dots/Cu2O composite under (N)IR light irradiation. (Reproduced

from ref. 94b.)

J. Mater. Chem.

transport mobility and good thermal/chemical stability.23b In

particular, the high transport mobility and environmentally

friendly properties of graphene meet the important requirements

of optoelectronic devices.23c,d Apart from the conducting film and

transparent anode developed previously,23e–g high mobility of

graphene renders it a promising electrode and electron-acceptor

material for photovoltaic device applications.23a However, the

easy aggregation and the poor dispersion of 2D graphene sheets

in common solvents limit its optoelectronic application. Solu-

tion-processable functionalized graphenes (SPFGs) has been

developed to solve such problem,23h but their non-uniform size

and shape (on a scale of several hundred nanometers and even

micrometers) remains a daunting challenge for the fabrication of

high-quantity photovoltaic devices, with active layer thicknesses

in only nanometer scales.

To facilitate the application of graphene in nanodevices and to

effectively tune the bandgap of graphenes, a promising approach

is to convert the 2D graphene sheets into 0D C-dots. Apart from

unique electron transport properties,23i new phenomena associ-

ated with edge effects and quantum confinement are expected

from C-dots.23j Additionally, the compatible surface chemistry,

excellent photostability, good solubility in polar solvents and

extensive optical absorption throughout the visible and near-

infrared wavelength regions render C-dots potentially useful

sensitizers for photovoltaic applications.23a,108–110

Li et al. applied colloidal GQDs with green luminescence as

the electron acceptor materials in conjugated polymer, poly(3-

hexylthiophene) (P3HT)-based thin film solar cells.23a Fig. 33a

and b shows respectively the schematic and the energy level

alignment of a device configuration of ITO/poly(3,4-ethyl-

enedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/

P3HT:GQDs/Al, in which GQDs (c ¼ 10 wt%) provided an

Fig. 33 Schematic (a) and energy band (b) diagrams of the ITO/

PEDOT:PSS/P3HT:GQDs/Al device. (c) J–V characteristic curves for

the ITO/PEDOT:PSS/P3HT/Al, ITO/PEDOT:PSS/P3HT:GQDs/Al and

ITO/PEDOT:PSS/P3HT:GQDs/Al devices after annealing at 140 �C for

10 min, single log scale. (Reprinted with permission from ref. 23a.

Copyright 2011 John Wiley and Sons.)

This journal is ª The Royal Society of Chemistry 2012

Fig. 35 (a) CQD-sensitized TiO2 nanoparticle and the proposed

CQD–TiO2 bonds, (b) UV–Vis–NIR absorption spectrum of CQD thin

film on quartz substrate, (c) Current–voltage characteristics of

CQD-sensitized solar cells prepared from a MeOH CQD solution and

from an aqueous solution of C-dots refluxed in nitric acid to ensure

complete surface oxidation and (d) current–voltage characteristics of

aqueous CQD-sensitized solar cell. (Reproduced from ref. 108.)

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effective interface for charge separation and a pathway for

electron transport, as clearly evidenced by the greatly increased

photocurrent as compared to the device fabricated with P3HT

only (Fig. 33c). The device performance was further improved

after thermal annealing, with an overall power conversion effi-

ciency (PCE) of 1.28%.

Yan et al. found that GQDs have high optical absorptivity and

nearly optimized absorption in the visible and near IR region.4,51

Additionally, the calculated energy levels in GQDs suggested the

possibility of electron injection from GQDs to large bandgap

semiconductors (e.g. TiO2) upon photoexcitation and regenera-

tion of GQDs by accepting an electron from I� (Fig. 34a). Thus,

GQDs provide excellent opportunities for the development of

inexpensive, high-performance GQD-sensitized solar cells by

substituting GQDs for metal organic dyes as photosensitizers. As

a proof-of-concept example, Yan et al. exploited GQDs with 168

conjugated carbon atoms to sensitize a nanocrystalline TiO2

photoanode and obtained a photocurrent density of 200 mA

cm�2 under AM (air mass, path-length through the atmosphere

relative to the vertical thickness of the atmosphere) 1.5 illumi-

nation (Fig. 34b).51 The low performance was due largely to the

low affinity of GQDs to TiO2 surface since no chemical binding

was formed as in conventional dye-sensitized solar cells, in which

ruthenium dyes are covalently bonded to the TiO2 surface for

much higher optical absorption and faster exciton dissociation.

Zhu et al. studied the photon-to-electron conversion capability

of GQDs under different light sources (UV or near infrared

light).111 The photocurrents generated by GQDs–PEG and

GQDs on ITO electrodes were measured by a three-electrode

system. The GQDs–PEG photoelectrode generated considerable

photocurrent with both 365 nm UV light and 808 nm NIR laser,

while the pure GQD photoelectrode showed lower photocurrent

than the GQDs–PEG. So the authors thought GQDs could be a

new solar cell dopant material and that the light of the photon-

to-electron conversion may be extended from UV to near-

infrared range.4 Gupta et al. demonstrated that GQDs blended

with regioregular P3HT or poly (2-methoxy-5-(2-ethylhexyloxy)-

1,4 phenylenevinylene) (MEH-PPV) polymer resulted in a

significant improvement in solar cell efficiency or organic light

emitting diode (OLED) characteristics as compared to graphene

nanosheets (GS) blended conjugated polymers.109

Mirtchev et al. fabricated a C-dots-sensitized nanocrystalline

TiO2 solar cell.108 It is known that the most effective Ru-based

sensitizers contain a number of carboxylate ligands to enable

Fig. 34 (a) Calculated energy level alignment. (b) J–V characteristics of

colloidal GQD-sensitized TiO2 nanoparticle solar cells. (Reprinted with

permission from ref. 51. Copyright 2010 American Chemical Society.)

This journal is ª The Royal Society of Chemistry 2012

coordination of the dye to the TiO2 surface as shown in Fig. 35a,

and the anchoring scheme of sensitizer to TiO2 can be mimicked

by surface functionalized C-dots.112 The absorption spectrum of

a thin film of C-dots is shown in Fig. 35b, displaying the broad

absorption throughout the visible region. An electronic bandgap

of approximately 1.1 eV for an indirect transition and 3.1 eV for

a direct transition was calculated using Tauc relations113a with

the experimental absorption onset at approximately 800 nm, see

Fig. 35b. The current–voltage characteristics of the CQD-sensi-

tized solar cell prepared from an aqueous CQD solution under

simulated AM 1.5 irradiation are shown in Fig. 35d. A short-

circuit current density (Jsc) of 0.53 mA cm�2 and an open-circuit

voltage (Voc) of 0.38 V were produced with a fill factor (FF) of

0.64 for a power conversion efficiency of 0.13%. Cells fabricated

from MeOH CQD solutions and aqueous C-dots refluxed in

HNO3 to ensure complete surface oxidation showed similar

performance, whereas a non-sensitized nanocrystalline TiO2

gave an efficiency of 0.03%. Similar to the recent report of gra-

phene dot-sensitized nanocrystalline TiO2 solar cells, the Jscseems to the limiting factor on the efficiency of these cells, with

both the Voc and FF values comparable to those of Ru-complex

sensitized cells.51

Since C-dots exhibit relatively high QY, they may be a good

light converter for use in light emitting diode (LED) applica-

tions.15b,113b,c Lau et al. investigated the light converting property

of C-dots deriving from glucose.15b To demonstrate white light

emission via down-converting blue light, C-dots were coated

onto a commercially available blue LED. As shown in Fig. 36,

the uncoated blue LED emitted blue light centered at 410 nm.

After coating with C-dots, the intensity of the blue light weakens,

but a broad band appears at �510 nm. The corresponding LED

with and without applied voltage are shown in the left-hand side

of the inset of Fig. 36. The applied voltage and current for the

LED were 2.9 V and 10 mA, respectively. The Commission

International d’Eclairage (CIE) chromaticity coordinates for the

blue LEDs with and without C-dots coating are shown in the

J. Mater. Chem.

Fig. 36 Color converter of the GQDs. Luminescence spectra of the blue

LED with and without GQDs coating. Left inset: photographs of the

GQD coated LED without (top) and with applied voltage (bottom).

Right inset: the CIE chromaticity coordinates for the illuminating blue

LED with and without GQD layer. (Reprinted with permission from ref.

15b. Copyright 2012 American Chemical Society.)

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right-hand side of the inset of Fig. 36. Clearly, with a C-dot layer,

the CIE chromaticity coordinates of the blue LED are shifted

from (0.242, 0.156) to (0.282, 0.373), demonstrating that the C-

dots are capable of converting blue light into white light. The

water-soluble C-dots exhibit excellent light converting properties

as compared to compound-semiconductor-based QDs such as

multishell-structured CdSe/ZnS/CdSZnS and CdSe/CdS/ZnS/

CdSZnS QDs.

Choi et al. develop a simple solution method to prepare

emissive hybrid quantum dots consisting of a ZnO core wrapped

in a shell of single-layer graphene (ZnO–graphene quasi-

quantum dots, ZnO–graphene QD).94c They then use these

quantum dots to make a white LED with a brightness of 798 cd

m�2. The strain introduced by curvature opens an electronic

bandgap of 250 meV in the graphene, and two additional blue

emission peaks are observed in the luminescence spectrum of the

Fig. 37 Photoluminescence and electroluminescence transition scheme

for ZnO–graphene quasi-quantum dots. Transitions 1, 2 and 3 corre-

spond to electron transitions from the conduction band (CB) of ZnO,

LUMO + 2 and LUMO levels induced by G–Oepoxy to the valence band

(VB) of ZnO, respectively. (Reprinted with permission from ref. 94c.

Copyright 2012 Nature Publishing Group.)

J. Mater. Chem.

quantum dot. Density functional theory, calculations reveal that

these additional peaks result from a splitting of the lowest

unoccupied orbitals of the graphene into three orbitals with

distinct energy levels (Fig. 37).

6. Energy and charge transfer

Carbon-based systems play a major role in industrial and tech-

nological advances.114,115a For fuel cell development or electro-

chemical energy storage, carbon with high surface area is

instrumental in maximizing the performance of catalyst and

energy systems.115b,c Increasing the surface area of carbon by fine

particle dispersion or by using porous carbon has advanced the

development of storage batteries and electrocatalysts for fuel

cells.115c A lot of work has been devoted to the study of CNTs,

graphene/GO/reduced-GO(RGO) and fullerene-based

composite, especially on the energy and charge transfer research

of these composites.115a,d–i For example, three specific approaches

for designing nanohybrid assemblies are deposition of metal

nanoparticles, anchoring semiconductor nanoparticles, and

functionalization with photoactive molecules.115a,d–f

The interactions between graphene-based carbon nano-

structures and excited states of molecules/semiconductor nano-

particles often involve energy and/or electron transfer. The

emission of the excited molecule (or semiconductor nanoparticle)

serves as an excellent probe to monitor the interactions and thus

establish a quenching pathway. Brus and co-workers demon-

strated efficient energy transfer from individual CdSe/ZnS

nanocrystals to single- and few-layer graphene.115g The fluores-

cence intensity of single nanocrystals was quenched by a factor of

�70 on single-layer graphene, and the quenching efficiency

increased with the layer number (Fig. 38). Zhang et al. reported

that MWCNTs enhanced the separation of photoinduced elec-

tron–hole pairs produced in the ZnO-nanowires (NWs) because

of the unique three-dimensional structure of the ZnO-NWs/

MWCNTs heterostructure.115h The electron transfer process in

this heterojunction is similar to that in the semiconductor metal

composite, as indicated in Fig. 39.

Given the economical cost of graphene and the need to seek

alternate materials for next-generation electronic devices, there is

a significant drive within the scientific community to gain a

greater understanding of the properties of C-dots and explore

new applications. The potential of C-dot-based solar cells has

been explored. However, photochemical and photovoltaic

aspects of such nanocarbon structures are relatively less studied.

Fig. 38 Wide-field fluorescence image of individual CdSe/ZnS nano-

crystals on quartz and on single-layer graphene. (Reprinted with

permission from ref. 115a. Copyright 2011 American Chemical Society.)

This journal is ª The Royal Society of Chemistry 2012

Fig. 39 Schematic diagram of charge separation and transportation at

the ZnO-NWs/MWCNTs heterojunction. (Reprinted with permission

from ref. 115h. Copyright 2009 American Chemical Society.)

Fig. 40 Schematic illustration of the FRET model based on C-dots–

graphene and the mechanism of K+ determination. (Reproduced from

ref. 115k.)

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They have the same core issue: energy and charge transfer in the

related nanosystem.

There have been several reports highlighting improvements in

the photocatalysis of C-dots/semiconductor composites and the

degradation of organic compounds.8,11–13 For example, the

enhanced degradation rate seen with C-dots/TiO2 as compared

to that with TiO2 alone in these systems highlights the beneficial

role of C-dots.8 The origin of such beneficial effects is rather

indirect and not initiated by the direct absorption of light by C-

dots. In particular, the carbon nanostructure facilitates disper-

sion of catalyst particles, improved charge separation within the

C-dots/TiO2 composite, and increased concentration of organic

molecules near the catalyst surface. These arguments are

consistent with earlier reports that showed TiO2 deposited on

adsorbent supports, such as activated carbon and graphene, can

have a significant role in concentrating the pollutants from low

solution-phase concentration.114,115 In addition, C-dots

loaded on the semiconductor surface and forming the composite

structure would provide access to photo-induced charge

transfer transitions under light irradiation. At the composite

structure, the photo-induced electron is transferred to joint

charge transfer states predominately located at the C-dots, while

the separated hole stays electronically and structurally near the

wide bandgap semiconductor. This process can effectively

hinder the electron–hole recombination, and guarantee the high

reactivity of photogenerated electron and hole excited by

visible light. Then the oxygen radicals (_O2�) are formed by the

combination of electrons in the conducting network of C-

dots with O2 adsorbed on the surfaces of C-dots.8,13 Moreover,

the C-dots with UCPL properties can convert the

longer wavelength to shorter wavelength light, which can in turn

excite wide bandgap semiconductors to form electron–hole

pairs.8,11–13

Qu’s group reveal energy transfer from C-dots to graphene

and consequently construct a fluorescence resonance energy

transfer (FRET) sensor which could be used for measuring the

concentration of potassium ions (K+) with high selectivity and

tunable dynamic range (Fig. 40).115k They design a model in

which the donor (C-dots) and the acceptor (graphene) are

brought into FRET proximity through specific cation–ligand

complexation (Fig. 40). C-dots are covalently aminated, and

graphene is noncovalently functionalized with 18-crown-6 ether

(18C6E). The known tight binding of primary alkyl-ammonium

with 18C6E10 will bring C-dots and graphene into appropriate

This journal is ª The Royal Society of Chemistry 2012

proximity and hence induce energy transfer. Thereafter, the

FRET process is inhibited because of competition between K+

and ammonium for 18C6E, which has high potassium selectivity.

Notably, C-dot-based photovoltaic devices with enhanced

performance have been successfully fabricated. C-dots provided

an effective interface for charge separation and a pathway for

electron transport, as clearly evidenced by the greatly increased

photocurrent as compared to the device fabricated without C-

dots.23a,51 In particular, a C-dot-decorated PEG photoelectrode

can generate photocurrent with 808 nm NIR laser, and serve as

a new solar cell dopant material, and the light of the photon-to-

electron conversion may be extended from ultraviolet to near

infrared.111 The calculated energy levels in C-dots suggested the

possibility of electron injection from C-dots to wide bandgap

semiconductors (e.g., TiO2) upon photoexcitation and regener-

ation of C-dots by accepting an electron.51

TheMB–GQDs dispersed in the MEH-PPV polymer provided

more electrical transport paths, which resulted in an enhance-

ment of charge injection and hence an increase in carrier density,

thus achieving a lower turn-on voltage and a much higher effi-

ciency.109 The significant improvements provided by GQDs in

solar cell efficiency or organic light emitting diode characteristics

are higher than those provided by graphene nanosheets blended

conjugated polymers.

Recently, Guo et al. studied the property of electron transfer

quenching of the fluorescence of C-dots by nitroxide radicals.115j

The fluorescence of C-dots was found to be efficiently quenched

by the paramagnetic nitroxide radical. A singly occupied

molecular orbital (SOMO) facilitating electron transfer was

proposed to account for the quenching of the fluorescence of C-

dots in the C-dot@TEMPO (2,2,6,6-tetramethylpiperidine-N-

oxide free radical) conjugate, formed via electrostatic interaction

between the negatively charged C-dots and the cationic 4-amino-

2,2,6,6-tetramethylpiperidine-N-oxide free radical (4-AT).

The study of C-dot-based energy and charge transfer properties

opens up new avenues in the design of optoelectronic and light-

energy conversion devices or systems.8,15b,23a,108–110 Indeed, careful

design of hybrid assemblies is necessary to exploit the electronic

properties of C-dots. Multifunctional C-dot-based materials with

different semiconductor and metal nanoparticles can provide new

pathways to design catalyst systems for light energy conver-

sion.8,11–13,107 The abilities of C-dots in the storage and transport of

electrons has yet to be exploited fully. Carefully designed C-dots

composites have the potential to expand the capacity of next-

generation energy storage, photovoltaic devices, and

photocatalysts.

J. Mater. Chem.

Fig. 41 In vivo fluorescence imaging. (A) In vivo fluorescence images of a

C-dots-injected mouse. The images were taken under various excitation

wavelength at 455, 523, 595, 605, 635, 661, and 704 nm. Red and green

represent the fluorescent signals of C-dots and the tissue auto-

fluorescence, respectively. (B) Signal-to-background separation of the

spectral image taken under the NIR (704 nm) excitation. The fluorescence

of C-dots was well separated from the tissue autofluorescence back-

ground. (Reprinted with permission from ref. 39b. Copyright 2012 John

Wiley and Sons.)

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7. Bioimaging

Traditional QDs, such as CdTe and related core–shell nano-

particles, have been used in various in vitro and in vivo optical

imaging experiments.116–118 As the QDs contain toxic heavy

metals, their application has raised health and environmental

concerns.117Due to their excellent PL properties and low toxicity,

C-dots may be an attractive candidate for bioimaging

applications.2,4,119–129

Sun et al. and other groups have studied the use of C-dots in

bioimaging applications.7,17,18,58 They reported the potential of

C-dots passivated with PPEI-EI for two-photon luminescence

microscopy using human breast cancer MCF-7 cells.7 Zhu et al.

reported the bioimaging application of GQDs by incubating a

solution of MG-63 (human osteosarcoma) cells with suspensions

of GQDs from the stock solution with Dulbecco’s phosphate

buffer saline (DPBS).120 Chang et al. used pig kidney cell line

(LLC-PK1 cells) to test the practicality of the as-prepared C-dots

for cell imaging.127 C-dots were localized in the cell membrane

and cytoplasm of LLC-PK1 cells. The C-dots were likely inter-

nalized into the LLC-PK1 cells through endocytosis.7,129 When

excited at longer wavelengths (510–530 nm), red fluorescence was

observed from LLC-PK1 cells that had been cultured in a

medium containing 1.2 mg mL�1 C-dots for 24 h. There was no

autofluorescence from cells when excited at the same wavelength.

The strong PL intensity in the cells demonstrated the stability of

the C-dots, confirming their strong and stable fluorescence in

high ionic strength media. The cell viability was measured after

cells had been treated with different amounts of C-dots (0–2.4 mg

mL�1). The average cell viability was greater than 95% at a

C-dots concentration of up to 1.8 mg mL�1. The results revealed

that C-dots are more biocompatible than QDs.2,130

A549 human lung adenocarcinoma cells were used to evaluate

the cytocompatibility of the amino-functionalized C-dots

obtained by hydrothermal carbonization of chitosan.124 The cell

viability of the C-dots was determined by a methylthiazolteter-

azolium (MTT) assay.131 MTT assays of cell viability studies

suggested that the C-dots exhibited low cytotoxicity and posed

insignificant toxic effects. These results suggested that C-dots can

be used in high concentration for imaging or other biomedical

applications. C-dots were introduced into A549 cells to show

their bioimaging capabilities using a confocal microscopy test in

vitro. The results showed that the photoluminescent spots were

observed only in the cell membrane and cytoplasmic area of the

cell, but were very weak in the central region corresponding to

the nucleus, indicating that C-dots easily penetrated into the cell

but did not enter the nuclei. The observation is consistent with

previous studies on the interaction of living cells with nano-

materials, in which genetic disruption did not occur.132 The

results further confirmed the low cytotoxicity of C-dots.

In vivo optical imaging using C-dots produced by laser abla-

tion has been demonstrated by Sun et al.17 Recently, C-dots

made fromMWNTs were chosen for in vivo fluorescence imaging

studies by Liu and Kang et al.39b A nude mouse was subcuta-

neously administered with C-dots at three different locations.

Various excitations including blue, green, yellow, orange, red,

deep red, and NIR light with center wavelengths at 455, 523, 595,

605, 635, 661, and 704 nm, respectively, were applied during in

vivo imaging of the mouse. After spectral unmixing to

J. Mater. Chem.

differentiate the background autofluorescence (green), the

subcutaneously injected spots (red) on the mouse were seen in

these fluorescence images under all of the different light excita-

tions (Fig. 41). Compared to images acquired using blue

(455 nm) and green (523 nm) light excitation, those taken under

longer wavelength excitations (595 nm and beyond) showed

much better signal-to-background separation. Although the

fluorescence emission of C-dots is weaker at longer wavelengths,

the tissue autofluorescence background decreased even more,

resulting in an improved signal-to-noise ratio under red and NIR

excitation. In vivo optical imaging at longer wavelengths is

usually preferred owing to the improved photon tissue penetra-

tion and reduced background autofluorescence, especially in the

NIR region. The capability of C-dots for NIR in vivo fluores-

cence imaging (excitation ¼ 704 nm, emission ¼ 770 nm)

demonstrates the great potential of C-dots for use as optical

nanoprobes in biomedical imaging.

8. Sensor and surface-enhanced Raman scattering

Based on their excellent properties, C-dots can be designed for

sensors,115k,133–139 such as biosensors for DNA,133,134 nitrite

sensing,135 sensors for phosphate,136a glucose,136b a-fetoprotein136c

or metal ions.137–139 For example, Li et al. reported the use of

This journal is ª The Royal Society of Chemistry 2012

Fig. 43 SEM images of Au deposited on Si (a) and on the C-dots

honeycomb structure (b), and (c) Raman spectra of 4 � 10�6 M R6G

drop-cast on to a Si wafer (,), adsorbed on Au-coated Si (>), the

C-dots honeycomb on Si (O), the Au-deposited C-dots honeycomb on Si

(B) and the Raman spectrum of the C-dots honeycomb on Si (*). The

inset in (b) is a TEM image of Au nanoparticles deposited on the wall of

the C-dots honeycomb. Scale bars in a and b are 100 nm. (Reproduced

from ref. 148.)

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unmodified C-dots as novel and environmentally friendly fluo-

rescence probes for the sensing of Hg2+ and biothiols with high

sensitivity and selectivity. The luminescence emission of C-dots

arises from the radiative recombination of excitons, similarly to

QDs.140 On the other hand, Hg2+ can quench the fluorescence

(FL) of QDs due to facilitation of non-radiative electron/hole

recombination through an effective electron transfer process.138

The strategy inspired by these phenomena and the approach are

depicted in Fig. 42. Initially, the free C-dots showed strong FL in

aqueous solution. However, the FL of C-dot was quenched

significantly in the presence of Hg2+ through the charge transfer

process. Meanwhile, due to the strong binding preference of

biothiols toward Hg2+ by forming a Hg2+–S bond,141,142 Hg2+

was removed from the surface of C-dots and thus the FL of

C-dots could be recovered. Therefore, by taking advantage of the

observed FL change, a facile FL sensor can be fabricated to

allow sensitive detection of Hg2+ and biothiols.

Au nanoparticles have been widely used as efficient enhancers

for Raman scattering and this phenomenon is known as the

SERS effect.143 SERS effect is recognized to be due to the

excitation of surface plasmon resonance (SPR) on metal

surface, which greatly increases the local E-field near the

surface. SERS has found important applications in chemistry,

biology and material science as an ultra-sensitive detection

technique capable of identifying traces of molecules.144–146

Conventional SERS substrates are constructed by placing metal

nanoparticles on planar surfaces, which thereby have a limited

surface area. The replacement of planar surfaces by spatially

defined nanoporous surfaces with large surface areas can adsorb

more molecules for SERS signals and further improve the

Raman detection sensitivity. Indeed, Jiang and coworkers147

have demonstrated that the Raman enhancement effect can be

dramatically increased by replacing the planar surface with a

unique nanoporous superaligned carbon nanotube film with

cross stacking.

Fan et al. have assembled well-confined two-dimensional

(2D) and 3D C-dos honeycomb structures148 by electrodeposi-

tion of oxygen-rich functional C-dots within the interstitial

voids of assemblies of SiO2 nanospheres, followed by extraction

of the SiO2 cores with HF treatment. Although made from

quantum sized C-dots, the C-dot assemblies presented a solid

porous framework (Fig. 43a and b), which can be used as a

sacrificial template for the fabrication of new nanostructures

made from other functional materials. Based on the unique

honeycomb architecture of C-dots, which allows more efficient

adsorption of molecules, the Au nanoparticles formed on

C-dots honeycomb exhibit 8–11 times stronger SERS than the

widely used Au nanoparticle SERS substrate for the sensitive

detection of target molecules. This work provides a new

Fig. 42 Schematic illustration of detection mechanism of Hg2+ and

biothiols using C-dots. (Reproduced from ref. 138.)

This journal is ª The Royal Society of Chemistry 2012

approach for the design and fabrication of ultrasensitive SERS

platforms for various detections. Shi et al. also performed

similar work on C-dots or C-dots/metal composites for SERS

application (Fig. 43c).149,150

9. Outlook

In this review, we have described the recent advances in the

research on C-dots, focusing on their synthesis, surface func-

tionalization, PL properties, and applications in photocatalysis,

energy and sensor issues. A variety of synthesis techniques

already exist for producing C-dots of different characteristics.

The PL and optical properties of C-dots are both interesting and

intriguing, constituting a rich and hot research topic. The

potential of C-dots in the storage and transport of electrons

impacted by light has yet to be exploited fully. In the future, we

expect the advent of more facile and robust synthetic routes and

creative applications to better realize the potential of the

increasingly important C-dot materials. C-dots stand to have a

huge impact in biotechnological and environmental applications

because of their potential as nontoxic alternates to traditional

heavy-metal-based QDs. In addition, the unique photoinduced

electron transfer ability, as well as excellent light harvesting

capability, make C-dots an exceptional candidate for photo-

catalytic and photovoltaic applications. Carefully designed

C-dots composites have the potential to expand the capabilities

of next-generation energy-storage and photovoltaic devices,

photocatalysts and sensors. By surface and band gap modifica-

tion of C-dots via functionalization or semiconductors, we can

expect to design novel catalysts from C-dots for green chemistry

and energy issues.

J. Mater. Chem.

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Acknowledgements

This work is supported by the National Basic Research Program

of China (973 Program) (no. 2012CB825800), National Natural

Science Foundation of China (NSFC) (no. 51132006, 51072126,

21073127, 21071104, 91027041), a Foundation for the Author of

National Excellent Doctoral Dissertation of P R China

(FANEDD) (no. 200929), a grant from the Research Grants

Council of the Hong Kong Special Administrative Region,

China (Project no. CityU102010), a project funded by the

Priority Academic Program Development of Jiangsu Higher

Education Institutions (PAPD), a Suzhou Planning Project of

Science and Technology (ZXG2012028) and a project supported

by the Natural Science Foundation of the Jiangsu Higher

Education Institutions of China (Grant no. 11KJB150015). We

also thank the Innovation Program of Graduate Students in

Jiangsu Province (no: CXZZ11_0097).

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