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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
One-step ultrasonic synthesis of water-soluble carbon
nanoparticles with excellent photoluminescent properties
Haitao Li a, Xiaodie He a, Yang Liu
Zhenhui Kang a,*
a,*
,
Hui Huang a, Suoyuan Lian a, Shuit-Tong Lee b,
a
Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices,
Soochow University, Suzhou, 215123, China
b
Centre of Super Diamond and Advanced Films, City University of Hong Kong, Hong Kong, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Monodispersed water-soluble fluorescent carbon nanoparticles (CNPs) were synthesized
Received 19 April 2010
directly from glucose by a one-step alkali or acid assisted ultrasonic treatment. The CNPs
Accepted 4 October 2010
were characterized by transmission electron microscopy, optical fluorescent microscopy,
Available online 29 October 2010
fluorescent spectrophotometry, fourier transform infrared spectrophotometry and ultraviolet-visible spectrophotometry. The results showed that the particle surfaces were rich in
hydroxyl groups, giving them high hydrophilicity. The CNPs could emit bright and colorful
photoluminescence covering the entire visible-to-near infrared (NIR) spectral range. Notably, the NIR emission of the CNPs could be obtained by NIR excitation. Furthermore, these
CNPs also had excellent up-conversion fluorescent properties.
Ó 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
Fluorescent nanoparticles have attracted increasing research
attention due to their promising applications covering electrooptics to bionanotechnology [1,2]. To date, typical photoluminescent particles have been developed from compounds of
lead, cadmium and silicon [2–6]. But these materials also have
raised concerns over potential toxicity, environmental harm
and poor photostability [3–5,7,8]. The need for photoluminescent nanostructures emitting in visible-to-near infrared (NIR)
spectral range is rapidly increasing [1,2,9,10]. Compared to
conventional measurements made in the ultraviolet (UV)visible region, spectrofluorimetry within the therapeutic window
of 700–1200 nm has many advantages, such as lower levels of
background interference and deeper penetration of living tissues [9,10]. NIR photoluminescence (PL) obtained from NIR
excitation holds great promise for in vivo uses at significant
depths in biological media and the development of noninvasive diagnostic techniques [2,5,9–11]. Consequently, an excel-
lent fluorescent nanoparticle for bioapplications should be
more stable, and less toxic as well as exhibit good NIR
emission.
Compared to traditional quantum dots (QDs) and organic
dyes, photoluminescent carbon nanomaterials are superior
in chemical inertness and likely lower toxicity [12]. The emergence of photoluminescent carbon-based nanomaterials has
presented exciting opportunities for searching for fluorescent
nanomaterials. Recently, oxygen-containing carbon nanoparticles (CNPs) with photoluminescent properties have been
prepared by laser ablation of graphite [13–15], electrochemical
oxidation of graphite [16,17] or multiwalled carbon nanotubes
(MWCNTs) [18], chemical oxidation of arc-discharge singlewalled carbon nanotubes (SWCNTs) [19], thermal oxidation
of suitable molecular precursors [20,21], vapor deposition of
soot [22,23], proton-beam irradiation of nanodiamonds [24],
microwave synthesis [25], wet chemical and alkali-assisted
electrochemical method [26,27]. These reported CNPs emit
efficient PL in the visible range, but none has been reported
* Corresponding authors: Fax: +86 512 65882846/0957.
E-mail addresses: [email protected] (Y. Liu), [email protected] (Z. Kang).
0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2010.10.004
606
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Fig. 1 – (a) TEM image of CNPs prepared from glucose with diameter less than 5 nm; (b), (c) Photographs of CNPs dispersions in
water with sunlight and UV (365 nm, center) illumination, respectively; (d-g) Fluorescent microscope images of CNPs under
different excitation: d, e, f, and g for 360, 390, 470, and 540 nm, respectively. (For interpertation of the reference to colours in
this figure legends, the reader is referred to the web version of this paper.)
to generate NIR PL. Although NIR PL under NIR excitation had
been observed in SWCNTs [28], they sufferred from the drawbacks of uncontrollable specified chirality, and expensive
preparation or separation procedures were needed.
Moreover, the up-conversion nanomaterials for biological
luminescent labels also have attracted great attentions due
to their unique luminescent properties with long lifetime
and superior photostability. In comparison with downconversion fluorescent materials, up-conversion materials have
many advantages in biological applications, such as noninvasive and deep penetration of NIR radiation, the absence
of autofluorescence of biological tissues, and feasibility of
multiple labeling with different emissions under the same
excitation [29,30]. A recent study indicated that core–shell
structured CNPs have two-photon active luminescence with
excitation in the NIR range [14]. Thus, we can expect that
carbon-based fluorescent nanoparticles may possess visible,
NIR, and up-conversion luminescence synchronously, which
are extremely important for fundamental and practical
applications.
Here, we report a one-step alkali or acid assisted ultrasonic
chemical method of synthesizing highly monodispersed
water-soluble CNPs (less than 5 nm) from glucose directly.
The CNPs can emit bright and colorful PL covering the entire
visible-NIR spectral range. Notably, the NIR emission of CNPs
can be obtained by NIR excitation. Furthermore, these CNPs
also have excellent up-conversion luminescent properties.
The fabrication details, characterization, feasibility, and performance of the CNPs are described in the following sections.
2.
Experimental section
2.1.
The synthesis and purification of CNPs
All chemicals were purchased from Sigma–Aldrich. A suitable
amount of glucose was dissolved in deionized water (50 mL)
to form a clear solution (1 mol/L). NaOH (50 ml, 1 mol/L) or
HCl (50 ml, 36–38 wt.%) solution was added into the solution
of glucose, then the mixed solution was treated ultrasonically
for 4 h.
The raw solution of CNPs obtained from glucose/HCl was
oven-dried at 80 °C for 6 h.
The other raw samples (20 mL) obtained from glucose/
NaOH were first adjusted to pH = 7 with HCl, then added
100 mL ethanol drop by drop into the solution under stirring.
After that, the solution of CNPs was treated by adding a suitable amount of MgSO4 (10 wt.%12 wt.%), stired for 20 min
and stored for 24 h to remove the salts and water.
2.2.
Characterization
All the samples were characterized by transmission electron
microscopy (TEM: Hitachi H800, acceleration voltage 200 kV),
optical fluorescent microscopy (Leica DM4500B), fluorescent
spectrophotometry (Horiba Jobin Yvon, FluoroMax 4), and
fourier transform infrared spectrophotometry (FT-IR: Varian
Spectrum GX), UV-visible spectrophotometry (Agilent 8453).
3.
Results and discussion
Fig. 1a shows the TEM image of CNPs obtained from glucose.
These small CNPs are spherical and less than 5 nm in size.
Particularly, the present CNPs can freely disperse in water
with transparent appearance without further ultrasonic dispersion, so they are called ‘‘water-soluble CNPs’’. Fig. 1b and
c show the optical images of the CNPs illuminated under sunlight and UV light (365 nm, center), respectively. The bright
blue PL of CNPs is strong enough to be easily seen with the
naked eye (Fig. 1c). Further, the CNPs were investigated under
fluorescent microscope, and different emission colors were
found in the same sample. Typical specimen for optical
microscope was prepared by placing a drop of the solution
on a cover glass and evaporating the water. Fig. 1d–g exhibit
the corresponding fluorescent microscope images of CNPs
under different excitation wavelengths in the same sample:
blue, cyan, yellow, and red. The excitation wavelengths are
360, 390, 470, and 540 nm, and the collection wavelengths
are >425 nm, >470 nm, >515 nm, 600 ± 40 nm, respectively.
To further explore their optical properties, PL of the asprepared CNPs was studied using different excitation wavelengths.
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Fig. 2 – Photoluminescent spectra of CNPs under UV and visible excitations (emission in visible range): (a) CNPs prepared from
glucose/NaOH, (b) CNPs prepared from glucose/HCl. (For interpertation of the reference to colours in this figure legends, the
reader is referred to the web version of this paper.)
Fig. 3 – Photoluminescent spectra of CNPs under NIR excitations (emission in NIR range): (a) CNPs prepared from glucose/
NaOH, (b) CNPs prepared from glucose/HCl.
In our experiment, all the spectra were obtained from the
liquid CNPs samples (water as solvent). Fig. 2 shows the
detailed photoluminescent spectra of CNPs obtained from
glucose and NaOH (Fig. 2a)/HCl (Fig. 2b). The corresponding
visible emissions of CNPs covering blue-to-red wavelength
range can all be obtained in the same sample under UV and
visible excitations. Notably, the PL emission extending into
NIR wavelength range was also observed in all the CNPs samples obtained from glucose and NaOH/HCl. Fig. S1 shows the
typical photoluminescent spectra of CNPs obtained from glucose giving visible and NIR PL emissions excited at 350 nm.
Further detailed PL study shows the NIR PL emissions can also
be obtained by NIR excitation. Fig. 3a and b show the NIR PL
emissions under different NIR excitations (700, 750, 800, and
850 nm) of the samples of CNPs obtained from glucose/NaOH
and glucose/HCl, respectively. It should be noted that this
kind of NIR PL emissions excited by NIR excitation is particularly significant and useful for bionanotechnology because of
the transparency of body tissues in the NIR ‘‘water window’’.
Remarkably, the as-prepared CNPs also exhibit excellent
up-conversion photoluminescent properties besides their
strong luminescence in visible-to-NIR range. Fig. 4a shows
the photoluminescent spectra of CNPs excited by long wave-
length light (from 700 to 1000 nm) with the up-conversion
emissions located in the range from 450 to 750 nm. This result
suggests that CNPs may be used as a powerful energy transfer
component in biology applications or photocatalyst design for
applications in environmental and energy issues.
Typical UV–visible absorption spectra of CNPs are shown
in Fig. 4b, which feature a peak in the 250–300 nm region.
These peaks represent the typical absorption of an aromatic
pi system, which is similar to that of polycyclic aromatic
hydrocarbons [31]. The red shift of the p–p* transition is due
to the extended conjugation in the structure of CNPs [32].
The FT-IR spectra (Fig. S2a, b) were used to identify the functional groups after ultrasonic treatment. The peaks around
3000 and 1600 cm 1 correspond to the C=C stretch of the
carbon skeleton of CNPs, which are consistent with the
UV-visible absorption spectra and support the concept of
aromatization of glucose during ultrasonic treatment. The
peaks in the range of 1000–1300 cm 1 include the C–OH
stretching and OH bending vibrations, implying the existence
of large numbers of residual hydroxyl groups. Partially dehydrated residues in which reductive OH or CHO groups are
covalently bonded to the carbon frameworks improve the
hydrophilicity and stability of the nanoparticles in aqueous
608
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Fig. 4 – (a) Up-conversion photoluminescent spectra and (b) UV spectra of CNPs obtained from glucose.
systems, and greatly widen their range of applications in biochemistry, diagnostics, drug delivery and so on [6,11,20].
In the present reaction system, the CNPs were prepared
from carbohydrate (glucose) and alkali or acid under ultrasonic condition. With the reaction time increasing, the solution changed from colourless to brown, indicating the
formation of CNPs. It is well known that ultrasound can generate alternating low-pressure and high-pressure waves in
solution, leading to the formation and violent collapse of
small vacuum bubbles. This cavitation causes high speed
impinging liquid jets, deagglomeration and strong hydrodynamic shear-forces. Thus the energy of ultrasonic waves lead
to the glucose polymerization, carbonization, and then the
formation of CNPs. This growing mechanism of CNPs is similar to that reported by Sun and Li [20], and this mechanism
should conform to the LaMer model [20]. Significantly, by this
one-step ultrasonic treatment, the hydrophilic and ultra
small sized CNPs with abundant photoluminescent properties were successful fabricated. The visible-NIR and upconversion PL of the as-prepared CNPs may mainly attribute to
two following reasons: (1) the surface energy traps for producing a fluorescent center into CNPs [13], (2) the polymerization
and carbonized of glucose (carbohydrate) introducing more
conjugated double bond system into CNPs [12]. Also, it can
be expected that a better carbon source candidate (e.g., sucrose or starch etc.), additive (e.g., HNO3, H2SO4, KOH, and various surfactants etc.), or suitable reaction conditions (e.g.,
ultrasonic power, reaction temperature, hydrothermal treatment) may further improve the present ultrasonic reaction
processes, which is now in progress.
It is important that the as-prepared CNPs can freely disperse in water with transparent appearance without further
ultrasonic dispersion, as well as good photostability, e.g. luminescent properties and appearance remained unchanged
after storing six months in air at room temperature. The
quantum yield of the CNPs was estimated about 7% by calibrating against Rhodamine B in ethanol [33–35]. Based on
the above experimental results, we envision the present CNPs
are superior to metal-based QDs in terms of high stability, facile use, low toxicity, environmental-friendliness, and low cost.
Consequently, good water dispersibility, strong and stable PL,
and especially strong NIR emission by NIR excitation of these
CNPs should offer great potential for a broad range of applications, including light-emitting diodes (LEDs), bio-sensors, biomedical imaging, and drug delivery.
4.
Conclusions
The one-step ultrasonic reaction process provides a green and
convenient method using natural precursors to prepare ultra
small sized CNPs by using glucose as carbon resource. The
CNPs exhibit stable (>6 months) and strong PL (quantum yeild
7%), especially two excellent photoluminescent properties:
NIR emission and up-conversion photoluminescent properties. Combining free dispersion in water (without any surface
modifications) and attractive photoluminescent properties,
CNPs should serve as a promising candidate for a new type
fluorescence marker, bio-sensors, biomedical imaging, and
drug delivery for applications in bioscience and nanobiotechnology.
Acknowledgments
This work is supported by the National Basic Research
Program of China (973 Program) (No. 2010CB934500 and
2006CB933000), Natural Science Foundation of China (NSFC)
(20801010, 20803008, 201073127 and 201071104), A Foundation
for the Author of National Excellent Doctoral Dissertation of
PR China (FANEDD) (No. 200929).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.carbon.2010.10.004.
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