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Microwave assisted one-step green synthesis of cell-permeable multicolor
photoluminescent carbon dots without surface passivation reagents†
Xiaohui Wang,
ab
Konggang Qu,
ab
Bailu Xu,
ab
Jinsong Ren
a
and Xiaogang Qu*
a
Received 6th September 2010, Accepted 4th January 2011
DOI: 10.1039/c0jm02963g
A facile, economic and green one-step microwave synthesis route
towards photoluminescent carbon dots is proposed. The prepara-
tion requires a carbohydrate (glycerol, glycol, glucose, sucrose,
etc.) and a tiny amount of an inorganic ion, and can finish in just
a few minutes, no surface passivation reagent is needed. The
carbon dots are biologically compatible and show favorable optical
properties and have potential applications in biolabeling and
bioimaging.
Photoluminescent carbon dots have attracted growing interest in
recent years due to their great potential in biological labeling,
bioimaging, drug delivery and optoelectronic device applica-
tions.
1–8
Compared to luminescent semiconductor quantum dots,
which have known toxicity and are potentially environmentally
hazardous due to the contained heavy metals and chalcogens,
carbon based photoluminescent nanomaterials are environmen-
tally and biologically compatible.
6–9
Currently, intensive research has been concentrated on the
synthesis of these benign photoluminescent materials. Common
routes in preparing fluorescent carbon nanoparticles include the high-
energy ion beam radiation based creation of point defects in nano-
diamond particles;
1,6
the covalent modification of nanodiamond
particles with octadecylamine;
10
the oxidation of carbon nanotubes
11
or candle soot
7,12
with nitric acid; collecting nanoparticles in water
from non-sooting regions of ethylene/air laminar flames;
13
the elec-
trooxidation of carbon nanotubes
14
or graphite;
9,15,16
the laser abla-
tion of graphite;
3,4,8,17
high temperature treatment of resols tethered
on silica nanospheres;
5
the dehydration of carbohydrates by
concentrated sulfuric acid to form carbonaceous materials;
18
the
thermal oxidation of citrate acid and sodium 11-amino-undecanoate
molecular precursors at 300 Cinair;
19
the microwave pyrolysis of
glucose solution in PEG
200N
solution
20
and the laser irradiation of
a suspension of graphite powders in PEG
200N
solvent.
21
Most of these
methods require surface passivation reagents,
3–5,8,17–21
and the repor-
ted quantum yields for carbon nanoparticles without surface
passivation are relatively low (1%).
7,9,11,12
Additionally, these
methods usually involve tedious processes,
3–5,7,8,10,12,17,18
expensive
starting materials,
1,6,10,11,14
or harsh synthetic conditions,
1,5,6,13,19
which
severely limit the availability of large quantities of the luminescent
carbon nanomaterials for practical applications. Thus, the develop-
ment of simple, cost-effective and environmentally friendly method
for large scale synthesis of fluorescent carbon dots still remains
challenging. Herein, we report a facile, economic and green one-step
microwave synthesis route towards photoluminescent carbon dots.
Our preparation requires a carbohydrate (glycerol, glycol, glucose,
sucrose, etc.) and a tiny amount of an inorganic ion and can finish in
just a few minutes. A characteristic feature of this method is that no
surface passivation reagent is needed. Additionally, the photo-
luminescence intensity of the as-synthesized carbon dots do not
change at the physiological and pathological pH range of 4.5–9.5 and
show no photobleaching. Furthermore, these carbon dots enter into
cells and can be used for photoluminescence-based cell-imaging
applications.
Carbohydrates have been widely used to produce carbon mate-
rials owing to their sustainability,
18,20,22–28
but few of these materials
show strong photoluminescence.
18,20
Glycerol (70% (v/v)) was mixed
with 7.1 mM phosphate solution (pH 7.4) and heated in a domestic
microwave oven (750 W) for 14 min. As shown in Fig. 1a, the
emission spectra of the as prepared carbon dots were broad, ranging
from 430 (blue) to 525 nm (yellow), with a dependence on the
excitation wavelengths; the carbon dots exhibited blue, yellow and
red photoluminescence under ultraviolet (330–385 nm), blue (450–
480 nm) and green (510–550 nm) light excitation (Fig. 1b). AFM
was used to observe the formed carbon dots and showed the carbon
dots have height around 2.1 0.76 nm (Fig. 1c and 1d). It should
be noted that pure glycerol solution after being treated with 20 min
microwave remained clear and no apparent photoluminescence was
observed. Since neither glycerol nor phosphate is emissive in the
visible and near-UV range, the bright and colorful photo-
luminescence emission must thereforebeattributedtotheformed
carbon dots. A typical luminescence lifetime (s) was also measured
(Figure S1, ESI†), where a value of 8.00 0.07 ns was obtained.
FT-IR spectra (Fig. 2a) were used to identify the functional groups
present on carbon dots. New bands at 1743 and 1597 cm
1
were
attributed to carboxyl groups and aromatic C]C vibrations,
a
Laboratory of Chemical Biology, Division of Biological Inorganic
Chemistry, State Key Laboratory of Rare Earth Resource Utilization,
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun, Jilin, 130022, China. E-mail: xqu@ciac.jl.cn; Fax:
+86-431-85262656
b
Graduate School of the Chinese Academy of Sciences, Chinese Academy of
Sciences, Changchun, Jilin, 130022, China
† Electronic Supplementary Information (ESI) available: Experimental
details and supporting figures. See DOI: 10.1039/c0jm02963g/
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respectively. The presence of carboxyl groups offers various surface
modifications for potential applications in drug delivery and
biomedical imaging. The broad and intense peak centered at
3370 cm
1
, and the bands in the range of 1000–1300 cm
1
which
include the C–OH stretching and –OH bending vibrations imply the
existence of large numbers of residual hydroxyl groups. These func-
tional groups improve the hydrophilicity and stability of the carbon
dots in aqueous systems. As shown in Fig. 2b, the photoluminescence
intensity of carbon dots did not change with ion strength, indicating
that the carbon dot aqueous solution was stable. Unlike the carbon
dots reported by Liu et al.
12
and Zhao et al.,
9
the photoluminescence
intensity of the carbon dots synthesized here is pH independent at the
physiogical and pathological pH range of 4.5–9.5 (Fig. 2c). Addi-
tionally, the carbon dots show excellent photostability, as the pho-
toluminescence intensity did not change even after continuous
excitation with a 150 W Xe lamp (Fig. 2d). Moreover, the inhomo-
geneity of the photoluminescence of the carbon dots can be exploited
for optical labeling to allow the selection of different emission colors
with different excitation wavelengths.
17
All these features make
carbon dots good candidates for biological labeling, bioimaging or
other biomedical applications. Photoluminescence microscopic
observation (Figure S2, ESI†) showed that the carbon dots could
readily enter into E. coli and 293T cells and be used for biolabeling
and bioimaging.
To elucidate the factors influencing carbon dots formation, the
effect of microwave time was firstly investigated. As shown in
Figure S3a (ESI†) with the increase in microwave treatment time,
the glycerol solution turned brown and the UV-visible absorption
peak at around 260 nm of the resulting carbon dots shifted to
shorter wavelengths (4 min, 265.5 nm /14 min, 253.5 nm). AFM
analysis showed the mean height of carbon dots synthesized after
7 min and 14 min microwave treatment was 1.1 0.42 nm
(Figure S4, ESI†) and 2.1 0.76 nm (Fig. 1c and Figure 1d),
respectively. As indicated in Figure S3b (ESI†) the photo-
luminescence of the glycerol solution increased with reaction time,
but the normalized photoluminescence emission spectra could be
almost superposed and no apparent quantum confinement effect
was observed. These results indicate that the photoluminescence is
most likely to result from the dot surface, where emissive energy
trap sites are located.
5,17,20,21
In contrast to previous reported
methods,
3–5,8,17–21
no surface passivation agents and passivation steps
are incorporated in this green microwave synthesis route. Through
microwave pyrolysis, the carbohydrate is carbonized which results
in the formation of carbon dots; meanwhile, their surfaces have
oligosaccharide and aliphatic chains due to the partial dehydration
and condensation of the carbohydrate.
25,29
These functional groups
stabilize the surface energy traps and make them emissive. As
shown in Figure S5 (ESI†) in contrast to photoluminescence
intensity, which monotonically increased with microwave time, the
relative quantum yield of carbon dots firstly increased with
increasing of reaction time and then significantly decreased. This
result indicated that the carbon dots have strong photoluminescence
only when they are small enough, which is consistent with previous
studies.
7,12,17
Fig. 1 (a) Photoluminescence emission spectra (with progressively
longer excitation wavelengths from 320 nm to 480 nm in 20 nm incre-
ments) of carbon dots (microwave pyrolysis condition: 70% glycerol,
7.1 mM phosphate, 14 min). In the inset, the emission spectral intensities
are normalized. (b) Photoluminescence photographs of carbon dots in
water under ultraviolet (330–385 nm), blue (450–480 nm) and green (510–
550 nm) light excitation; the picture was taken by an Olympus BX-51 optical system microscope (Tokyo, Japan). (c, d) AFM characterization
of the as prepared carbon dots.
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Inorganic ions have been demonstrated to effectively accelerate
the carbonization of carbohydrates.
24,30
Therefore, we subsequently
examined the role of inorganic ions in carbon dots formation. As
shown in Figure S6 (ESI†) with the increase of phosphate salt,
the absorption band at around 260 nm of the carbon dots’
bathochromic shift decreases (7.1 mM, 262 nm /35.7 mM,
254 nm). Meanwhile, as indicated in Figure S7 (ESI†) the pho-
toluminescence intensity and quantum yield of the produced
carbon dots increased with reaction time, but the photo-
luminescence emission peaks only slightly red shifted (7.1 mM,
440 nm /35.7 mM, 450 nm). By selecting quinine sulfate as the
standard, the photoluminescence quantum yield for the carbon
dots synthesized by 70% glycerol in the presence of 35.7 mM
phosphate salt with 12 min of microwave treatment was measured
and calculated to be 3.2%. The results indicate that the phosphate
salt catalyzes the carbon dots formation. Though the amount of
phosphate salt affects the formation rate and quantum yield of
carbon dots, it does not significantly affect the photoluminescence
characteristic of the carbon dots.
We also systematically investigated the effect of different valence
cations and anions on the carbon dots formation. As shown in
Fig. 3, both the formation rate and quantum yield of carbon dots
increased with the increasing valence of the cation or anion.
Compared to monovalent ions, divalent and trivalent ions show
much greater ability to catalyze carbohydrate carbonization and
carbon dot formation. The photoluminescence emission peak of the
carbon dots blue shifts with the increase in valence of the cation,
while it slightly red shifts with the increasing of valence of anions. As
expected, we found that carbon dots catalyzed by both divalent
cation and divalent anion (CuSO
4
) catalysts show very strong
fluorescence (Figure S8, ESI†) and the quantum yield was calculated
to be 9.5%, which is much higher than that of reported carbon dots
without surface passivation,
7,9,11,12
and is comparable to that of
previous reported ligand protected cardon dots.
5,17–21
It has been
known that H
+
catalyzes carbohydrate carbonization.
24,31
As shown
in Figure S9 (ESI†) H
+
can also catalyze carbohydrate to form
carbon dots, and a dose and time dependence was observed
(Figure S10, ESI†).
Fig. 2 (a) FT-IR spectra of glycerol and carbon dots. (b) Effect of salt concentration on the photoluminescence intensity at 450 nm (l
ex
¼360 nm) of
the carbon dots. (c) Effect of pH on the photoluminescence intensity at 450 nm (l
ex
¼360 nm) of carbon dots. At pH 4.5, 5.5 and 6.5, the 10 mM
phosphate buffer was used while at pH 7.5, 8.5 and 9.5, 10 mM Tris buffer was chosen. All the values are the average of triplicate measurements. (d)
The time-dependence of photoluminescence intensity at 450 nm (l
ex
¼360 nm) of carbon dots and a commercial dye 40,6-diamidino-2-phenylindole
(DAPI).
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To investigate whether this microwave assisted photoluminescent
dots synthesis can be applicable to other systems, we also used glycol,
glucose and sucrose as the starting carbohydrate. As indicated in
Fig. 4, similar to carbon dots synthesized from the glycerol, the
carbon dots prepared from glycol, glucose or sucrose luminesce and
their emissions depended on the excitation energy. The quantum yield
for these carbon dots varied with the starting carbohydrate material
and that for carbon dots synthesized from 70% glycol in the presence
of 10 mM AlCl
3
with 14 min microwave treatment was 5.8%. In
parallel control experiments, where no inorganic ions were added, no
apparent photoluminescence was observed. Together, these results
demonstrate that this microwave assisted carbon dot synthesis
method has generality. The influence of the starting materials on the
luminescence characteristics of carbon dots was also investigated. The
results in Figure S11 (ESI†) show that photoluminescence spectrum
of carbon dots derived from glycol significantly blue shift compared
to those of carbon dots derived from glucose, glycerol and sucrose
and indicated that different starting materials resulted in the carbon
dots with different photoluminescence. Thus, it is possible to tune the
luminescence from the carbon dots by changing the starting mate-
rials.
In summary, we have developed a general and simple microwave
synthesis method to produce multicolor photoluminescent carbon
dots. In comparison to the previous reported synthesis methods, three
features become apparent: 1) our approach does not need surface
passivation regents; 2) the synthetic approach utilizes carbohydrate as
the starting materials and is a green method; 3) the carbon dots
synthesis is kitchen chemistry and does not involve special equipment
and harsh synthesis conditions. As the synthesized carbon dots are
biologically compatible and show favorable optical properties,
they are promising imaging agents in biomedical and imaging
applications.
Acknowledgements
This work was supported by NSFC (20831003, 90813001,
20833006, 90913007), 973 Project 2011CB936004 and Funds from
CAS.
Fig. 3 Absorption spectra (a, c) and photoluminescence emission spectra excited at 340 nm (b, d) of carbon dots synthesized from 70% glycerol in the
presence of 10 mM different valence cations (a, b) or anions (c, d) with 15 min microwave pyrolysis. The samples are diluted with water and recorded on
a spectrometer or fluorescence spectrometer. It should be noted that photoluminescence emission spectra (b, d) have been normalized to the absorbance
at 340 nm. The insets show the normalized photoluminescence emission spectra.
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