Fluorescent carbon nanoparticles: Electrochemical synthesis and their pH sensitive photoluminescence properties

Article abstract of DOI:10.1039/c1nj20575g

Highly fluorescent carbon nanoparticles were synthesized directly from ethanol by a one-step sodium hydroxide-assisted electrochemical treatment. These nanoparticles could emit bright and colorful photoluminescence covering the entire visible spectral range and had excellent up-conversion fluorescent, stable ionic, long fluorescence lifetime and pH-sensitive photoluminescence properties.

Full text of DOI:10.1039/c1nj20575g

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PAPER
Fluorescent carbon nanoparticles: electrochemical synthesis and their
pH sensitive photoluminescence propertiesw
a
a
a
a
a
a
Haitao Li,z Hai Ming,z Yang Liu,* Hang Yu, Xiaodie He, Hui Huang,
a
a
b
Keming Pan, Zhenhui Kang* and Shuit-Tong Lee
Received (in Montpellier, France) 2nd July 2011, Accepted 30th August 2011
DOI: 10.1039/c1nj20575g
Highly fluorescent carbon nanoparticles were synthesized directly from ethanol by a one-step
sodium hydroxide-assisted electrochemical treatment. These nanoparticles could emit bright
and colorful photoluminescence covering the entire visible spectral range and had excellent
up-conversion fluorescent, stable ionic, long fluorescence lifetime and pH-sensitive
photoluminescence properties.
1
. Introduction
expensive precursors, high energy loss or severe synthetic
conditions, which are unlikely to be extended significantly or
not an environmental-friendly process in the near future.
Consequently, the convenient, green, low energy loss and large
scale fabrication of FCNPs is the urgent challenge in nano-
technology and nanochemistry.
In recent years, various forms of carbon nanomaterials such as
fullerenes, carbon nanotubes, nanodiamonds and other carbon
nanomaterials have been rapidly developed not only for their
1
–9
scientific interest but also for many technological applications.
The emergence of fluorescent carbon nanoparticles (FCNPs)
also has great potential application in the fields of nanobio-
Recently, we have reported water-soluble fluorescent carbon
quantum dots (CQDs) cutting from graphite rod under alkali-
30
1
0–16
technology and nanocatalysis.
Compared with traditional
assistance, which can be size-controlled and have excellent
fluorescent semiconductor quantum dots and organic dyes,
FCNPs possess excellent properties, such as chemical inertness,
high photostability against photobleaching and blinking, and
fluorescent properties. However, we find that the loss of
graphite rod after electrochemical treatment is not equal to the
amount of the as-prepared CQDs, the amount of fluorescent
carbon product is larger than the loss amount of graphite. We
speculate that the extra carbon products come from ethanol.
In this paper, we study and prove that these extra carbon
products derive from the ethanol. Meanwhile, we report a
facile and green method to prepare FCNPs by a one-step
electrochemical treatment of ethanol under sodium hydroxide
assistance, and the formation mechanism of FCNPs is also
investigated. Compared with the FCNPs or CQDs prepared using
other methods, including our own previous work, the present
approach is much simpler, more economic and easier to be
controlled or separated. Further extensive research will be carried
out on physical properties. These FCNPs also have excellent
water-solubility, strong visible emission, and up-conversion
photoluminescence (UPL), stable ionic, pH-sensitive and long
fluorescence lifetime properties.
1
1,12
can be modified easily.
In particular, brighter fluorescence
(
i.e., increased fluorescence quantum yield) is observed after
15
passivation by covalent binding of organic groups. In addition,
FCNPs are biocompatible, small in size and low in molecular
weight, and have low toxicity, making them superior to quantum
11,13
dots.
To date, a lot of routes for preparing FCNPs have
been reported, including chemical oxidation of arc-discharge
1
0,14
single-walled carbon nanotubes (SWCNTs),
5–17
laser ablation
1
of graphite, electrochemical oxidation of graphite or
1
multiwalled carbon nanotubes (MWCNTs),
8–20
thermal
2
1–24
oxidation of suitable molecular precursors,
11,26
supported
2
5
routes, vapor deposition of soot,
2
proton-beam irradiation
29
or microwave methods and so on.
7,28
of nanodiamonds
However, these methods usually involve complex processes,
a
Institute of Functional Nano & Soft Materials (FUNSOM) and
Jiangsu Key Laboratory for Carbon Based Functional Materials &
Devices, Soochow University, Suzhou, Jiangsu 215123, People’s
Republic of China. E-mail: zhkang@suda.edu.cn,
yangl@suda.edu.cn; Fax: +86 512 6588 2846;
Tel: +86 512 6588 0956
Center of Super-Diamond and Advanced Films(COSADF),
City University of Hong Kong, Hong Kong SAR, China
w Electronic supplementary information (ESI) available. See DOI:
0.1039/c1nj20575g
2. Experimental
2.1 Materials and preparation of FCNPs
All the chemicals were purchased from Sigma-Aldrich and
Beijing Chemical Reagent (Beijing, China), and were used
as-received. In a typical experiment, 8.0 g sodium hydroxide
was added to a mixed solution of 200 ml ethanol and 10 ml
deionized water to form a clear solution. Then the solution
b
1
z These authors contributed equally to this work.
2
666 New J. Chem., 2011, 35, 2666–2670
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was given an electrochemical treatment for 4 h at room
temperature (cell potential: 25–40 V). Pt rods (diameter of
about 0.2 cm) were used as an anode and a cathode, respec-
tively, the reference electrode was not used. We succeeded in
synthesizing FCNPs with a current intensity in the range of
the h111i spacing of carbon. Particularly, the present FCNPs
can freely disperse in water with transparent appearance
without further ultrasonic dispersion (Fig. 1b), so they are
called ‘‘water-soluble FCNPs’’. The FCNPs solution is very
stable, and there is no nanoparticle precipitation within 6
months. Even at a very dilute concentration, the FCNPs
solution can emit bright blue-green light when it is irradiated
using a UV lamp (365 nm). This bright blue-green fluorescence
of FCNPs is strong enough to be easily seen with the naked
eye (Fig. 1c). Furthermore, the FCNPs were investigated
under a fluorescent microscope, and different emission colors
were found in the same sample. Typical specimen for an
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 microscopy images of
CNPs at different excitation wavelengths (at 360 nm, 390 nm,
470 nm, and 540 nm excitations, respectively) in the same
sample: blue, cyan, yellow, and red. This kind of ultra small sized
CNPs with strong and stable PL would offer great potential for a
broad range of applications, including as light-emitting diodes
(LEDs) and bio-sensors, and in biomedical imaging.
ꢀ
2
2
00–260 mA cm . After that, the raw solution was given a
dialysis treatment using a semi-permeable membrane (MWCO
4 000) to remove the non-fluorescent deposit. After filter
1
treatment, the obtained solution became brown, implying
the formation of FCNPs. The conversion per pass of FCNPs
is 3–4%.
2
.2 Characterization methods
All solid samples were dried under vacuum at 60 1C for 8–10 h
before the measurement. The Fourier Transform Infrared
(
FTIR) spectrum of FCNPs was obtained with a Varian
Spectrum GX spectrometer. The PL study was carried out on
a Horiba Jobin Yvon (FluoroMax 4) Luminescence Spectro-
meter and an optical fluorescent microscope (Leica DM4500B),
while UV-visible spectra were obtained with an Agilent 8453
UV-VIS Diode Array Spectrophotometer. The transmission
electron microscopy (TEM) image was obtained with a FEI/
Philips Techal 12 BioTWIN TEM. The X-ray photoelectron
spectra (XPS) were measured by a Kratos AXIS UltraDLD
photoelectron spectroscope. The fluorescence lifetimes of the
FCNPs were measured by a FluoroLog 3-211-TCSPC.
To further explore their optical properties, the PL spectra of
the obtained FCNPs were assessed. Fig. 1h gives the typical PL
spectra of FCNPs with visible PL emission by excitation from
300 nm to 650 nm. The corresponding visible emissions of CNPs
covering the blue-to-red wavelength range can all be obtained in
the same sample under UV and/or visible excitations. The
maximum fluorescence emission intensity can be obtained when
excited at 450 nm. The fluorescent emission peaks shift to longer
wavelength with increasing the excitation wavelength.
3
. Results and discussion
3
.1 Characterization of FCNPs
The FCNPs not only have excellent PL in the visible range,
but also exhibit strong UPL property. Fig. 2a shows the PL
spectra of FCNPs excited by long wavelength light (from
Fig. 1a shows the TEM image of the obtained FCNPs, from
which we can see that these small FCNPs are well dispersed
and their diameters are in the range of 3–7 nm. The size
histogram (Fig. 1, inset) is provided to prove that the FCNPs
are well dispersed. The lower right corner of Fig. 1 shows
high-resolution TEM (HRTEM) images of FCNPs with a
lattice spacing of around 0.26 nm, which agrees well with
7
00 to 1000 nm) with the up-conversion emissions located in
the range from 400 to 700 nm. Similar to previously reported
1
6
carbon dots, this UPL property of FCNPs should be attributed
to the multiphoton active process. Additionally, the excitation
wavelength changed from 800 to 950 nm, the upconverted
emission peaks shifted from 517 to 554 nm. Remarkably,
Fig. S1 (ESIw) shows the linear relationship between Em and
Ex, and these results are similar to previous reports. The
shifting between the energy of upconverted emission light
(
Em) and excitation light (Ex) was almost unchanged, about
0
.90 eV. Compared with the carbon nanoparticles prepared
4–29
1
using other methods,
the FCNPs prepared in this paper
with an electrochemistry method are relatively inexpensive and
Fig. 1 (a) Typical TEM image of FCNPs, the inset shows the particle
size distribution of FCNPs. (b) Digital photograph of FCNPs solution
in sunlight. (c) Digital photograph of FCNPs solution with UV lamp
(365 nm, center) illumination. (d–g) Fluorescence microscopy images
of FCNPs under different excitations: for 360 nm, 390 nm, 470 nm,
and 540 nm, respectively. (h) PL spectra of FCNPs with UV and
visible light excitations.
Fig. 2 (a) The UPL spectra of FCNPs. (b) UV-vis absorption
spectrum of FCNPs, and the inset shows the enlarged graph from
200 nm to 400 nm.
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have excellent PL properties, such as strong PL in the visible
range and excellent UPL property. These results suggest that
FCNPs may be applied as a powerful energy-transfer compo-
nent in photocatalyst design for applications in energy and
environmental issues.
Fig. 2b depicts the typical UV-vis absorption spectrum of
FCNPs. The absorption band peaked at the 240–300 nm region,
and the inset shows the enlarged graph of this characteristic
absorption peak. This absorption band represents the typical
absorption of an aromatic p system, which is similar to that of
Fig. 4 (a) Fluorescence intensity of the FCNPs in NaCl aqueous
solution against ionic strength. (b) The graph of fluorescence intensity
vs. pH value.
3
1
polycyclic aromatic hydrocarbons.
Meanwhile, the XPS result indicated that the FCNPs contain
mainly carbon and oxygen (Fig. 3a), and no observable impurities
were detected. These results indicate that these small FCNPs
are pure and exhibit excellent water-solubility, strong PL
emission. Furthermore, the FTIR spectrum (Fig. 3b) was
acquired to determine the surface functional groups of the
ꢀ1
obtained FCNPs. The peaks around 3400, 3000 and 1600 cm
correspond to the vibrations of C–OH, C–H, and CQO
bonds. The asymmetric and symmetric stretching vibrations
ꢀ
1
ꢀ1
of C–O–C (B1300 cm and B1200 cm ) in the carboxylate
1
7,29
groups were also detected.
The FTIR results indicate that
the surface of FCNPs is full of hydrophilic groups: hydroxy
2
1,32
and carboxylic groups. Similar to the earlier reports,
we
speculate that these hydrophilic groups may be due to the
oxidation of the obtained carbon nanoparticles under electro-
chemical conditions, and the introduction of such functional
groups is the crucial factor for the origin of the FCNPs’ PL.
The presence of O-rich groups makes the FCNPs soluble in
aqueous medium and provides additional advantages for
further functionalization meeting various requirements. Apart
from the solubility of FCNPs in aqueous medium, the obtained
FCNPs can also be well dispersed in common organic solvents
such as acetone and acetonitrile, which facilitates the applications
in non-aqueous phase and organic devices as demonstrated
Fig. 5 Fluorescence lifetime of carbon nanoparticles prepared from
ethanol by electrochemical synthesis.
Moreover, we investigated the influence of ionic strengths
and pH on the fluorescence intensity of FCNPs (Fig. 4a and
b). The results showed that the fluorescence intensity of the
CNPs did not change even in aqueous solution with a high
ionic strength (2 M NaCl). The pH value of the solution had
some effects on the photoluminescence of the CNPs. And the
PL emission spectra (excitation at 550 nm) of FCNPs were
collected under different pH conditions at 25 1C. As shown in
Fig. 4b, under strongly acidic or alkaline conditions, the
photoluminescence is nearly completely quenched, whereas
the FCNPs had the strongest emission intensity when the
solution pH value was 7.0. Interestingly, the fluorescence
intensity decreased significantly when the pH was out of this
value. When the pH of the system was adjusted back to the
range of 6.0 to 8.0, the fluorescence intensity of the CNPs
became high again. Such a pH range (6.0–8.0) is similar to the
physiological pH environment (pH 7–8), so this result makes the
FCNPs with excellent fluorescence properties good candidates
for potential biological applications and for use as pH sensors.
The fluorescence lifetimes of the FCNPs were measured by
time-correlated single photon counting, with excitation and
emission wavelengths of 370 and 520 nm (Fig. 5). The lifetime
is 6.74 ns which is the longest lifetime among most of the
33
below for fabrication of photovoltaic cells.
The quantum yield of the FCNPs with yellow emission was
estimated to be about 4% by calibrating against fluorescein as
9
,34,35
a reference.
The as-prepared FCNPs also show good
photostability, e.g. the PL properties and appearance remained
unchanged after long time (44 days) UV illumination and
being stored for several months (46 month) in air at room
temperature. Based on the above-mentioned results, we expect
that the present FCNPs are superior to metal-based QDs in terms
of high stability, ease of use, low cost, and environmental-
friendliness. Consequently, this kind of FCNPs should offer
great potential for a broad range of applications, including in
36,37
biomedical imaging and photocatalysis, and as bio-sensors.
3
8
reported CNPs. This feature indicates that the FCNPs could
be a potential excellent fluorescent probe.
3
.2 The formation mechanism of FCNPs
Since the FCNPs were obtained directly from ethanol,
we envision that the formation mechanism of FCNPs is as
follows (see Fig. 6): (1) the decomposition of ethanol; (2) the
polymerization of carbon substances formed in the prior step;
Fig. 3 (a) XPS spectrum of the FCNPs. (b) FTIR spectrum of
FCNPs.
2
668 New J. Chem., 2011, 35, 2666–2670
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In comparison to the synthesis of FCNPs reported before, our
approach is a facile and green method, the monodisperse
FCNPs were obtained in ethanol solution, and no toxic reagents
were used. The synthetic procedure involves none of the toxic
organic solvents, initiators, or surfactants. This ensures that
the as-prepared nanoparticles are nontoxic enabling their use
in biochemistry or biodiagnostics, and so on.
4. Conclusion
In summary, the method reported herein provides a new,
convenient, low energy loss and green method to prepare a
large amount of FCNPs by one step sodium hydroxide-assisted
electrochemical treatment of ethanol, further, the present
method may provide a promising and practical approach for
the large-scale fabrication of nanosized fluorescent carbon
materials. These FCNPs exhibit stable and strong visible
emission (quantum yield B4%) and excellent UPL properties.
Combining free dispersion in water (without any surface
modifications) and attractive PL, stable ionic, pH-sensitive
and long fluorescence lifetime properties, this kind of FCNPs
should serve as a promising candidate for a new type fluorescence
marker, fluorescent probe and in related biomedical applications.
Fig. 6 Schematic growth model for FCNPs.
(
3) the growth of carbon substances formed in step (3); (4) the
oxidation process of grown carbon substances. It is known
that ethanol can be decomposed into carbon substances under
3
2
low-temperature hydrothermal conditions. An electrochemical
synthesis is achieved by passing an electric current between two
or more electrodes separated by an electrolyte. By definition,
the synthesis takes place at the electrode–electrolyte interface.
Electrochemical synthesis takes place close to the electrode
within the electric double layer, which has a very high potential
5
ꢀ1
gradient of 10 V cm . Under these conditions, the reactions
often lead to products which cannot be obtained in a chemical
synthesis. Meanwhile, an electrochemical synthesis is an oxi-
dation or a reduction reaction. By fine-tuning the applied cell
potential, the oxidizing or reducing power can be continuously
varied and suitably selected—a luxury not afforded by chemical
Acknowledgements
This work is supported by the National Basic Research Program
of China (973 Program) (No. 2010CB934500), National Natural
Science Foundation of China (NSFC) (No. 51132006, 21073127,
3
9
synthesis. So in this paper, we consider that the ethanol also
can be broken into carbon substances and these carbon
substances can be obtained after polymerization, growth and
oxidation process. In the absence of ethanol, no FCNPs were
observed, indicating that the FCNPs were formed by the
decomposition of ethanol. Additionally, similar to previous
2
1071104, 20801010, 20803008, 91027041), A Foundation for
the Author of National Excellent Doctoral Dissertation of P R
China (FANEDD) (No. 200929) and A Project Funded by the
Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD).
3
2,40
reports the carbon nanotube can be obtained from ethanol
Notes and references
and also FCNPs can be derived from carbon nanotubes after
1
0,14
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