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Ultra-High Quantum Yield of Graphene Quantum Dots:
Aromatic-Nitrogen Doping and Photoluminescence
Mechanism
Jing Sun, Siwei Yang, Zhongyang Wang, Hao Shen, Tao Xu, Litao Sun, Hao Li,
Wenwen Chen, Xingyu Jiang, Guqiao Ding,* Zhenhui Kang,* Xiaoming Xie,
and Mianheng Jiang
Fluorescent bioimaging (FBI) is one of the key aspects of the
present biomedical engineering.[1] As an ideal medical imaging
method with many attractive properties, FBI has been employed
in both fundamental biomedical research and clinical applications.[2] Light-emitting semiconductor quantum dots, which are
usually composed of elements from groups III–V, II–VI, or IV–
VI, have emerged as a class of fluorescent labels.[3] The potential cytotoxicity of semiconductor quantum dots, nevertheless,
has to be taken into account before their application to cellular
or in vivo study.[4] Nowadays, fluorescent dyes are the main
photoluminescence (PL) material used in FBI.[5] However, the
design, synthesis, and preparation of most fluorescent dyes are
complex.[6] In addition, their low stability, weak anti-jamming
ability, and cytotoxicity for organism also retard their widespread applications. Therefore, it is still imperative to develop
new PL materials with high stability, good anti-jamming ability,
and low cytotoxicity for FBI.
J. Sun, S. Yang, Prof. G. Ding, Prof. X. Xie,
Prof. M. Jiang
State Key Laboratory of Functional
Materials for Informatics
Shanghai Institute of Microsystem
and Information Technology
Shanghai 20050, China
E-mail: [email protected]
Prof. Z. Wang, H. Shen
Shanghai Advanced Research Institute
Chinese Academy of Science
Shanghai 20050, China
T. Xu, Prof. L. Sun
SEU-FEI Nano-Pico Center
Key Laboratory of MEMS of Ministry of Education
School of Electronic Science and Engineering
Southeast University
Nanjing 210096, China
H. Li, Prof. Z. Kang
Institute of Functional Nano and Soft Materials (FUNSOM)
Collaborative Innovation Center of Suzhou
Nano Science and Technology
Soochow University
Suzhou 215123, China
E-mail: [email protected]
W. Chen, Prof. X. Jiang
National Center for Nanoscience and Technology
Beijing 100190, China
DOI: 10.1002/ppsc.201400189
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The sp2-bonded graphene quantum dots (GQDs) are considered as a promising PL material for FBI stemming from their
perfect 2D structure, high stability, and biocompatibility.[7–9]
However, there are still several deficiencies of current GQDs.
The hydrothermal cutting mechanism of the GO precursor is
not clear, and as a result, different lateral size were reported
from several nanometers to a few tens of nanometers.[10] The
doping mode (lattice doping, edge doping, or just chemical
nitrogen-containing bonds) was not verified clearly although different doping elements and concentrations were reported.[9,11]
Most importantly, the quantum yield of GQDs with clear graphitic structure is no more than 0.49,[8,9] which is much lower
than that of fluorescent dyes or semiconductor quantum
dots.[3,5] Unfortunately, there is still no specific direction for the
quantum yield improvement since various factors, including
size, defects, doping, and surface modification, were reported to
be crucial factors for quantum yield.[8,9,11] The GQDs community need unambiguous evidences about the size control, the
doping, and the dominant factor for high quantum yield.
Here, we report the synthesis of 1–3 nm nitrogen
doped GQDs (N-GQDs) with high quantum yield of 0.74
(Figure S1–S4, Supporting Information) via cutting graphene
oxide (GO) precursor and in situ doping progress without further surface passivation or modification. The quantum yield
was significantly improved based on the control of the size and
nitrogen doping. The evidence of atomic resolution image was
provided to confirm that the lateral size of GQDs was determined by the remaining graphitic domains of GO precursor.
The nitrogen-doping mechanism was illustrated, and it was
experimentally demonstrated that the key factor affecting the
PL intensity is the n–π* transition between the N in aromatic
ring (Nar) and the conjugate structure of graphene. The strong
PL in solid state N-GQDs, along with their good stability and
excellent anti-jamming performance, confirm that the N-GQDs
is a better biolabeling agent for cells comparing with the traditional fluorescent dyes and semiconductor quantum dots.
Typical characterization results of N-GQDs are shown in
Figure 1. Homogeneous dots with a lateral size distribution
of 1–3 nm were found from the typical transmission electron
microscope (TEM) image (Figure 1a). The average diameter of
these dots is only 1.4 nm. The atomic force microscopy (AFM)
image confirms that the topographic height is from 0.5 to
1.0 nm (Figure S5, Supporting Information), which indicates
the 1–2 atomic layer in thickness.[10b] High-resolution TEM
image (Figure 1a inset) shows the crystallinity with lattices
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Figure 1. a) TEM image and size distribution of N-GQDs, inset: HR-TEM image with the lattice of 0.206 nm, b) spherical aberration-corrected TEM
image of a single dot, c) FFT of a single dot in (a), d) Raman spectra, and e) high-resolution N 1s spectrum of the N-GQDs.
of 0.206 nm.[12] Furthermore, spherical aberration-corrected
TEM image (Figure 1b) intuitively verifies that the product
has complete honeycomb lattice of graphene. It is noteworthy
that the fast Fourier transform (FFT) (Figure 1c) shows a high
crystalline structure and significant nonstandard sixfold symmetry. Compared with graphene, the decreased lattice spacing
of N-GQDs (Figure S6, Supporting Information) implies the
shortening and distortion of the bonds, which should be due to
N doping in the lattice.[13]
Figure 1d presents the Raman spectrum of N-GQDs. The
spectrum has peaks of D band 1315 cm−1, G band 1545 cm−1,
and 2689 cm−1 2D band. The ID/IG ratio is ca. 0.16, which is
much lower than that of N-GQDs in the previous report.[14] The
low ID/IG ratio indicates that the doping of N did not destroy
the conjugated structure of N-GQDs.[15]
The X-ray photoelectron spectroscopy (XPS) survey spectrum
of N-GQDs shows a predominant graphitic C 1s peak at ca.
284 eV, a N 1s peak at ca. 399 eV, and an O 1s peak at ca. 532 eV
(Figure S7, Supporting Information), The N/C atomic ratio was
10.06%, which is close to other heavy doped N-GQDs reported
previously.[10b,c,11] The high-resolution C 1s spectrum (Figure S8,
Supporting Information) confirmed the presence of O-rich
groups and C N bonds (285.2 eV).[10b,c,11] In addition, the highresolution N 1s spectrum of (Figure 1e) reveals the presence
of amidogen N (402.1 eV), tertiary amines N (Ar N (CH3)2,
400.2 eV), and Nar (399.4 eV),[16] with atom percentages of 2%,
4%, and 94%, respectively. The high doping concentration and
Nar/N ratio can be due to the large amount of O-rich groups
in the lattices of precursor (36.15 at%, Figure S9–12, Table S1,
Supporting Information). When the O-rich groups (such as
OH, CHO and epoxy groups) were reduced, the precursor
was cut and in situ N-doped (Figure S13). The Fourier transform infrared spectroscopy (FT-IR) and 1H NMR and 13C NMR
results also suggest the existence of these groups (Figure S14–
16, Supporting Information). What is more, these N-GQDs
have excellent solubility in water and various organic solvents
(such as: DMF, EtOH, acetone, and DMSO).
Part. Part. Syst. Charact. 2015, 32, 434–440
The UV–vis spectrum shows two absorption peaks (ca.
235 and 370 nm) (Figure 2a). The peak at ca. 235 nm can be
ascribed to the π−π* transition of benzene.[8,17] The significant
absorption peak at ca. 370 nm reveals the n-π* transition of p-π
orbit between the Nar and conjugate structure.[18] The PL spectrum shows the maximum emission wavelength is located at
500 nm with a full width at half maximum 50 nm. Furthermore,
the N-GQDs exhibit a maxima emission wavelength shift of
45 nm when the excitation wavelength increases from 340 to
510 nm (Figure S17, Supporting Information), which can be
attributed to the active groups, such as NH2, N(CH3)2,
COOH, and C O. The photoluminescence excitation (PLE)
spectrum shows three peaks at 290, 340, and 400 nm. The
optimum excitation condition is 400 nm, which corresponds
to the UV-vis peak at 370 nm.[14] The n-π* transition should
increase the PL efficiency of N-GQDs effectively since it appears
frequently in fluorescent dyes.[19] Figure 2b shows the optical
images, which indicate the N-GQDs giving bright blue-green
PL as bright as Rhodamine B (RhB) with the same mass concentration (10 µg mL−1). The quantum yield (φ) of N-GQDs is
0.74, which is higher than RhB and higher than other carbonbased “quantum dots” (Table S2, S3, Supporting Information).
The PL decay of N-GQDs is measured by a time-correlated
single photon counting technique, and fitted well with a biexponential decay as shown in Figure 2c. The lifetime (τ) is
dominant with a long decay component 12 ns (95%) plus a
small contribution from short decay 1.5 ns (5%), the weightedaverage lifetime is about 11.5 ns, which is the longest PL
lifetime observed so far in N-GQDs (Table S2, Supporting
Information). We believe that this is another characteristic feature of the aromatic-N doping dominant GQDs, because the
aromatic ring doping not only reduces the probability of n-π*
orbit coupling to the vibration of edge group and solvent, which
gives the direct channel of nonradiative decay, but also blocks
the oscillating loop of electron between the end groups, which
largely reduce the intersystem crossing from singlet to the triplet manifold according to Drexhage’s loop rule.[20] The long
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Figure 2. a) UV–vis absorption, PL (λex = 400 nm) and PLE (λex = 500 nm) spectra of N-GQDs aqueous solution, b) Photographs of N-GQDs aqueous
solution and RhB ethanol solution with the same mass concentration (10 µg mL−1) under visible light (left) and 365 nm UV light (right), c) PL decay
curves of N-GQDs measured at room temperature and excitations at 400 nm, d) PL and PLE spectra of N-GQDs in solid state, inset: Photo of bamboo
drawing with N-GQDs ink under visible light (left) and 365 nm UV light (right).
PL lifetime also explain the high quantum yield we observed
in this GQDs.
As shown in Figure 2d, the PL spectrum of N-GQDs in solid
state shows the maximum λem at 420 nm and λex at 377 nm,
which are blue-shifted compared with N-GQDs aqueous solution.[21] As shown in inset of Figure 2d (Figure S18, Supporting
Information), a picture of bamboo drawing with N-GQDs ink
is very clear under UV light (invisible under visible light). It
suggested that the n–π* transition of N-GQDs is still the main
process of PL progress, and the illuminant centre of N-GQDs
is more stable PL emission than those in previous reports (PL
of most GQDs quenched in solid state, Table S2, Supporting
Information).[8,11]
Since the above results, including the uniform size distribution, bright green PL, ultra-high quantum yield, long lifetime
and strong PL in solid state, have confirmed the excellent PL
property of N-GQDs, it is very necessary to reveal the cutting, doping, and PL mechanisms to understand and explain
the excellent PL performance. The microstructure of GO, the
precursor of GQDs, was studied through aberration corrected
TEM. Figure 3a shows a typical area of GO, and Figure 3b is
the corresponding one processed with an FET band pass filter.
It is clear that the sp2 structure was heavily damaged. Three
major features are presented: holes (indicated in purple),
graphitic regions (indicated in green), and high-contrast
disordered regions (indicated in brown), with approximate
area percentages of 4%, 19%, and 77%, respectively.[22] The
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high-contrast disordered regions split the graphitic regions
into some domains with a lateral size distribution of 1–4 nm,
which have the similar lateral size distribution with the synthesized N-GQDs. Based on the shape, the size, and the graphitic
nature of graphitic domains in GO sheet, we propose that the
continuous disordered area (purple) was highly reactive due to
the sp3 bonds with oxygen-containing groups. During the
hydrothermal process, the disordered area was gradually cut
into pieces, and at last disappeared, and the graphitic domains
became free quantum dots in the solution since they are relatively stable. Our results also indicate that the lateral size of
GQDs can be controlled by the remaining graphitic domains
after the oxidation of graphite. In order to realize uniform and
repeatable GQDs, the oxidation and intercalation of graphite
during the GO fabrication should be well controlled.
In previous literatures, the nitrogen-doping mechanism
was scarcely discussed. The doping may happen at the edge
or in the lattice with various groups. We propose the nitrogendoping mechanism during the hydrothermal reduction in
DMF in Figure 3c. The O-containing groups in these graphitic
domains play a key role for the doping. DMF resolved into
dimethylamine and further resolved into methylamine and
ammonia in hydrothermal process.[23] DMF, dimethylamine,
methylamine, and ammonia are all efficient dopants.[24] The N
atom can be easily doped into graphene lattice by or alkylation
reaction between these dopants and O-containing groups.[25] It
is clear that positions of defects, i.e., oxygen-containing groups,
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Figure 3. a) Aberration-corrected TEM image of GO. b) Image processed with an FFT band pass filter, showing holes and supporting carbon film in
purple, graphitic regions in green, and disordered regions in brown. c) A schematic diagram of the doping mechanism.
determine the nitrogen doping mode. Lattice doping can be
achieved when moderate defects in graphitic domains.
The doping position and the doping level should affect the
PL of N-GQDs. We studied the effect of N-doping and O-containing groups on the PL, respectively. N-GQDs with different
Nar/N ratios and similar total N-doping level were synthesized for PL comparison, as shown in Figure 4. In preparation process, when the solvent was DMF aqueous solution
(DMF:H2O = 1:1), the Nar/N is 71% (Figure 4a), which is lower
Part. Part. Syst. Charact. 2015, 32, 434–440
than N-GQDs prepared in pure DMF (94%). The Nar/N is
42% when DMF:H2O was 1:5 (Figure 4b). The Nar/N is only
18% when N-GQDs was prepared in NH3 aqueous solution
(NH3:H2O = 1:3) (Figure 4c, the details can be seen from Table S4,
Supporting Information). All these N-GQDs with same N/C
atomic ratio (9.5%–10%) and different Nar/N ratio show different PL properties (Figure 4d). It is clear that the φ is much
lower when the Nar/N ratio decreased. The φ is 0.51, 0.23 and
0.14 when the Nar/N ratio is 71%, 42%, and 18%, respectively.
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Figure 4. N1s spectra of N-GQDs obtained in different solution a) DMF:H2O = 1:1, b) DMF:H2O = 1:5, and c) NH3:H2O = 1:3, d) PL spectra of N-GQDs
with the same concentrations (0.1 mg mL−1), e) φ of N-GQDs with different Nar/C and O/C ratio, f) the schematic of n–π* transition in PL progress.
These results confirm that the Nar can dramatically enhance
the PL property of N-GQDs. On the other hand, the PL spectra
blue-shifted with the reducing of Nar/N ratio, the spectral shifts
can be due to the small and discontinuous conjugated systems
in N-GQDs with low Nar/N ratio.
In present system, the hydroxyl, epoxy and aldehyde groups
in GO could be easily substituted by N atoms in in situ-doping
reaction progress.[11,16b] While, the inactive O-rich groups (such
as carboxylic acid groups) are hard to be substituted, but could
be removed partially at high temperature. In the followed experiments, the O/C atomic ratio of N-GQDs (with the same N/C
atomic ratio, 9.72%–10.98%) prepared under 240 °C, 220 °C,
200 °C, 180 °C, 160 °C, and 140 °C in pure DMF are 6.82, 8.55,
10.38, 13.37, 16.88 and 19.87 at%, respectively. Figure 4e and
Table S5 (Supporting Information) show the φ values of these
N-GQDs with various O/C atomic ratios, which indicates that
there was no significant relation between φ and O contents.
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The relationship of quantum yields and Nar, quantum yields
and O-containing groups directly verifies that the Nar dominates PL mechanism and brings higher quantum yield. It is
reasonable that the aromatic nitrogen doping should bring
higher quantum yield. From the physical view, nitrogen doping
brings a new energy level in GQDs, and this new energy level
can improve the utilization of photon, as reported in ref. [11d].
From the microstructure view of GQDs, the aromatic-N doping
increases the rigidity of this 2D structure, which reduces the
energy losses in PL progress caused by the molecular vibrations
in flexible molecular structure. Based on the high efficient PL
mechanism, we further tried to introduce more Nar into the
GQDs to achieve higher quantum yields. GO with higher O/C
atomic ratio (41.5%), achieved by second oxidation, was cut and
in situ doped in different solvent (pure DMF and 80% DMF
aqueous solution, respectively). The Nar/C of these obtained
products was 0.20 and 0.25. Figure 4e shows the relationship
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performance. The size control, doping effect, and the PL mechanism were investigated. A series of comparison experiments
demonstrated that the key factor for the eminent PL properties
was the n–π* transition between the Nar and conjugate structure of N-GQDs. The N-GQDs produced by our method were
not toxic in vitro, and could be used as a practical bioimaging
material in FBI. This work will be helpful for synthesizing new
bioimaging materials with good size and doping control, high
antijamming performance, good stability, well biocompatibility,
and high quantum yield.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
Figure 5. a) Metabolic activity of HeLa cells treated with different concentrations of N-GQDs. b) Bright-field microphotographs of HeLa cells.
c) Confocal fluorescence microphotograph of HeLa cells incubated with
100 µg mL−1 N-GQDs (λex = 405 nm).
between Nar/C and φ, it is obvious that the φ decreased gradually when the Nar/C is higher than 20 at%, the φ is 0.72 and 0.64
when the Nar/C is 0.20 and 0.25, respectively. The φ decreasing
at high Nar doping concentration may attribute to the destroy
of conjugated structure with too much nitrogen atoms introduced.[26] These results further confirm that the Nar atoms act
as the illuminate centers, and the PL mechanism is related to
the n–π* transition (Figure 4f).
For a practical bioimaging application, the stability and antijamming performance of N-GQDs are very important. The
N-GQDs showed excellent photostability and anti-interference
ability under very hard conditions (Figure S19–24, Supporting
Information). Hence, We tested the in vitro cytotoxicity of
N-GQDs using the HeLa cell line. We observed metabolic
activity of HeLa cells treated with different concentrations of
N-GQDs (Figure 5a). Varied concentrations of N-GQDs were
added to the cells cultured in 96-well plates and incubated for
24 h. Subsequently, a standard assay was performed to assess
the cell viabilities after the N-GQDs treatments. No significant
reduction in cell viability was observed for cells treated with
N-GQDs even at high concentrations (up to 400 µg mL−1), demonstrating that the N-GQDs produced by our method were not
obviously toxic in vitro. Finally, to assess the prospects of the
N-GQDs as a practical bioimaging material, HeLa cells were
also used to evaluate the property of the N-GQDs. The N-GQDs
were incubated with HeLa cells to show their bioimaging ability
recorded by confocal microscopy (Figure 5b, c). Obviously,
bright green PL is observed inside the cells, which means that
the N-GQDs have been internalized by the HeLa cells and could
be used as a kind of efficient bioimaging materials.
In summary, we fabricated N-GQDs with extremely
high quantum yield (0.74), clear 2D crystalline structures,
strong, solid-state PL, good stability, and excellent antijamming
Part. Part. Syst. Charact. 2015, 32, 434–440
J. Sun and S. Yang contributed equally to this work. This work was
supported by projects from the National Science and Technology
Major Project (Grant no. 2011ZX02707), the National Natural Science
Foundation of China (Grant no. 11104303), the Chinese Academy of
Sciences (Grant no. KGZD-EW-303 and XDA02040000). The authors
acknowledge the revision from Prof. Xiaosong Liu.
Received: September 4, 2014
Revised: September 17, 2014
Published online: November 10, 2014
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