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Facile synthesis of monodispersed silver nanoparticles on graphene oxide

sheets with enhanced antibacterial activity

Article in New Journal of Chemistry · July 2011

DOI: 10.1039/C1NJ20076C

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Cite this: New J. Chem., 2011, 35, 1418–1423

www.rsc.org/njc PAPER
Facile synthesis of monodispersed silver nanoparticles on graphene oxide
sheets with enhanced antibacterial activityw
Lei Liu, Jincheng Liu,* Yinjie Wang, Xiaoli Yan and Darren Delai Sun*
Received (in Montpellier, France) 28th January 2011, Accepted 17th March 2011
Published on 13 April 2011 on http://pubs.rsc.org | doi:10.1039/C1NJ20076C

DOI: 10.1039/c1nj20076c
Downloaded by Nanyang Technological University on 20 July 2011

Graphene oxide–Ag nanoparticle (GO–Ag) composites were synthesized through a facile

two-phase (toluene–water) process. Transmission Electron Microscopy and X-ray diffraction
analysis revealed that the Ag nanoparticles anchored on GO sheets were spherical in shape and
highly monodispersed with uniform size of 6 nm. The antibacterial activity of GO–Ag composites
was investigated against gram-negative bacteria Escherichia coli (E. coli) and showed a
remarkably enhanced antibacterial activity compared with the original Ag nanoparticles,
suggesting that the as-prepared nanocomposites may be used as effective antibacterial materials.

Introduction still insufficient. Recent research reported that GO is a relatively

biocompatible material.14 Shen et al. (2010) illustrated that Ag
Silver (Ag) nanoparticles are known to exhibit the highest nanoparticles retain good antibacterial activity on a GO
bactericidal activity and biocompatibility amongst all the sheet.15 Very recently, Das et al. (2011) reported that the
antibacterial nanomaterials.1–3 It has been reported that Ag antibacterial activity of Ag nanoparticles on GO is size and
nanoparticles and hybrid Ag nanocomposites are shown to be shape dependent, and Ma et al. (2011) reported a synergistic
effective biocides against numerous kinds of bacteria, fungi effect of GO and Ag nanoparticles.16,17 Thus, GO–Ag composites
and viruses.2 Due to their promising antibacterial capability, are supposed to be effective antibacterial materials, which
Ag nanoparticles have attracted numerous studies on their possess the specific properties of both GO and Ag nanoparticles.
application as antiseptic, disinfectant and pharmaceutical The synthesis of GO–Ag composites has been reported by
agents.4–6 Studies have shown that the toxicity of Ag nano- several researchers, mostly using a solution-based single-step
particles is size dependent and the smaller sized nanoparticles method to reduce silver ions on GO sheets.15–21 However, this
exhibit higher antibacterial activity (o10 nm), due to the high single-step approach has experienced difficulty in controlling
specific surface area and easy cell penetration.2,3 Therefore, the morphology and size of Ag nanoparticles on GO sheets,
monodispersed Ag nanoparticles with small size are desirable because Ag nanoparticles are formed on GO sheets directly
for the antibacterial control system. Nevertheless, the particle during silver ion reduction. Since the antibacterial activity of
aggregation problem and nanomaterial recovery are two big Ag nanoparticles relies heavily on its size, precise size control
challenges of using Ag nanoparticles in applications. of the Ag nanoparticles anchored on GO sheets is essential.
Graphene has been a major focus of recent research to One recent study reported a dry decoration of GO with Ag
exploit an sp2-hybrid carbon network.7 In particular, graphene nanocrystals from an arc plasma source using electrostatic
is considered as an ideal two-dimensional reinforcing component force.22 However, the use of an extra electric field limits its
for composite materials possessing superior carrier transport, broad application in reality. Our previous work successfully
high mechanical stiffness, extremely large surface area and synthesized GO–TiO2 composites by a facile two-phase self-
fine thermal/chemical stability.7–9 Graphene oxide (GO) is a assembling approach,23 and it has been proved that this
chemically modified graphene with hydroxyl and carboxyl method is suitable for the assembling of high-quality organically
groups, which has high water solubility.9 The new graphene- soluble nanocrystals on GO sheets.
based hybrids with metal nanoparticles such as Pt, Au and Ag In this work, a facile two-phase method that embeds Ag
have shown potential applications in the area of optics, nanoparticles onto GO sheets is reported. It is worth high-
electronics, catalysis and sensors.10–13 However, information lighting that this method can successfully anchor highly
regarding biological studies on graphene-based composites is monodispersed Ag nanoparticles on GO sheets at the water/
toluene interface. It is a remarkable advancement for the
School of Civil and Environmental Engineering, Nanyang existing methods of the synthesis of GO–Ag composites, and
Technological University, Block N1, Nanyang Avenue, Singapore can provide a universal approach for the synthesis of high
639798. E-mail: JCLIU@ntu.edu.sg, DDSUN@ntu.edu.sg
w Electronic supplementary information (ESI) available: Additional quality GO–metal composites. Moreover, assembling the Ag
figure, AFM of GO sheets. See DOI: 10.1039/c1nj20076c nanoparticles on large GO sheets can remarkably enhance the

1418 New J. Chem., 2011, 35, 1418–1423 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
 View Online

antibacterial activity and facilitate the recovery by simple D8-Advance Bruker-AXS diffractometer using Cu Ka irradia-
filtration, which is beneficial for the applications in environ- tion. X-ray photoelectron spectroscopy (XPS) measurements
mental engineering and other fields. were carried out by using a Kratos Axis Ultra Spectrometer
with a monochromic Al Ka source at 1486.7 eV, with a voltage
of 15 kV and an emission current of 10 mA.
Experimental
Bacterial culture
Preparation and characterization of GO–Ag composites
Escherichia coli (E. coli) K12 ER2925 (New England Biolab)
Natural graphite (SP1) was purchased from Bay Carbon
was chosen as the model pathogen for antibacterial activity
Company (USA). Sodium nitrate (NaNO3, 99%), potassium
experiments. E. coli was cultivated in Luria-Bertani nutrient
permanganate (KMnO4, 99%), hydrogen peroxide (H2O2,
solution at 37 1C for 18 h to get the exponential growth phase.
35%), concentrated sulfuric acid (36.5%), oleic acid (OLA,
The cells were harvested by centrifugation and washed with
99%), and silver nitrate (AgNO3, 99%) were purchased
saline solution (0.9% NaCl) to remove residual macromolecules.
Published on 13 April 2011 on http://pubs.rsc.org | doi:10.1039/C1NJ20076C

from Sigma-Aldrich. Toluene, acetone, ethanol (absolute),

The cells were re-suspended in a saline solution to maintain the
and tetrahydrofuran (THF) were purchased from Merk Ltd
concentration of 107–108 colony forming units (cfu mL 1). All
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(Singapore). All reagents were used without further purification.

glass apparatus and solutions used in the experiments were
GO was synthesized according to the modification of
autoclaved at 121 1C for 20 min to ensure sterility.
Hummers’ method from natural graphite, and the process
was described earlier.24,25 Oleylamine-capped Ag nanoparticles Antibacterial activity test
were synthesized by reducing AgNO3 in toluene, which was
described previously.26 GO (15 mg) and DI water (100 ml) E. coli cells were inoculated in saline solution containing 0, 20,
were added into a bottle (250 mL) and sonicated for 1 h before 50, 80, 100 mg mL 1 of GO, Ag nanoparticles and GO–Ag
use. Ag nanoparticles (45 mg) dispersed in toluene (50 mL) composites, respectively, with a final cell concentration around
were added into the GO water solution. The mixture was kept 107 cfu mL 1. The mixture was incubated with gentle shaking
stirring for 12 h at room temperature to ensure Ag nanoparticles for 2 h at 37 1C. The mixture was diluted with a gradient
coordinated with GO sheets at the water/toluene interface. method and then applied uniformly on three LB culture
The GO–Ag composites were purified with acetone and medium plates per gradient solution. These plates were incubated
centrifuged at 10 000 rpm for 10 min. The obtained GO–Ag at 37 1C for 24 h. The colony forming units were counted
composites were then washed by THF to get rid of residual and compared with control plates to calculate percentage
oleylamine on the Ag nanoparticles. The total process is of cell viability (C/C0). Meanwhile, at the concentration of
illustrated in Scheme 1a. The final GO–Ag composites were 100 mg mL 1of GO, Ag nanoparticles and GO–Ag composites,
freeze dried at 50 1C for 24 h. the mixture was taken out at reaction times of 0, 30, 60, 90 and
Atomic force microscopy (AFM) was carried out using 120 min to measure the time course of the antibacterial activity
a non-contact mode on a PSIA XE-150 scanning probe of samples.
microscope. The AFM sample was prepared by spin coating The antibacterial activity of GO–Ag composites was further
the dispersion water solution of GO onto a Si substrate verified by LIVE/DEAD BacLight bacterial viability assay
covered with 300 nm thick SiO2. Transmission electron micro- (Invitrogen, USA). SYTO 9 and propidium iodide (PI)
scopy (TEM) and high-resolution TEM (HRTEM) images stock solutions from the assay kit were combined with an
were obtained using a JEOL 2010-H microscope operating at equal volume to add to the E. coli solution after 2 h incubation
200 kV. The samples for the analysis were prepared by with 100 mg mL 1 of GO, Ag nanoparticles and GO–Ag
dropping a dilute toluene solution of oleylamine-capped Ag composites, respectively. The mixtures were incubated at
nanoparticles, GO water solution and GO–Ag water solution. room temperature in the dark for 15 min and then observed
X-ray powder diffraction (XRD) patterns were taken on a by Leica TCS SP5 laser scanning confocal microscope (Leica
Microsystems, Germany).
The morphological changes of E. coli were investigated by
scanning electron microscopy (SEM, JEOL, 6340). After
filtering the E. coli mixture with a glass filter, the cells on the
filter were quickly fixed with 2% glutaraldehyde and 1%
osmium tetroxide. Then the cells on the filter were dehydrated
with sequential treatment with 50, 70, 85, 90 and 100%
ethanol for 10 min. The filter was freeze dried at 50 1C
before testing.

Results and discussion

Preparation and characterization of GO–Ag composites
Scheme 1 Steps for the synthesis of the GO–Ag composites:
(a) schematic representation of the two-phase synthesis of GO–Ag The morphology and size of the oleylamine-capped Ag
composites; (b) photographs of the two-phase synthesis of GO–Ag nanoparticles can be well-controlled via the organic phase
composites. synthesis.26,27 Oleylamine-capped Ag nanoparticles were well

This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011 New J. Chem., 2011, 35, 1418–1423 1419
 View Online

dispersed in toluene, while hydrophilic GO sheets were well spacing of the nanoparticles is 0.236 nm, which corresponds to
dispersed in deionized (DI) water, as shown in photographs in the (111) crystal plane of Ag nanoparticles. These TEM images
Scheme 1b. The aqueous solution of GO and the toluene confirm that the highly monodispersed Ag nanoparticles with
solution of Ag nanoparticles were mixed and stirred for 12 h uniform size around 6 nm are successfully synthesized and well
to ensure the self-assembly of Ag nanoparticles on the GO located on GO sheets by the two-phase approach. Compared
sheets at the water/toluene interface. It has been reported that with the reported works, our Ag nanoparticles on GO sheets
metal nanoparticles can interact with the GO sheets through seem to be highly monodispersed with smaller uniform
electrostatic binding, physisorption and charge-transfer size.15–17
interactions.13 Meanwhile, the large GO sheets act as an To further validate the Ag nanoparticles anchored onto the
excellent support and stabilizer for the Ag nanoparticles, GO sheets and to value the mass ratio of Ag/GO, XRD and
avoiding nanoparticle aggregation. After centrifugation of XPS were applied to measurements. Fig. 2a shows the XRD
the mixture and washing off extra oleylamine, the pure patterns of GO sheets, oleylamine-capped Ag nanoparticles
GO–Ag composites can be formed and well dispersed in water and GO–Ag composites. The curve of GO sheets shows a
Published on 13 April 2011 on http://pubs.rsc.org | doi:10.1039/C1NJ20076C

as shown in Scheme 1b. The hydrophilic property of the diffraction peak at a 2y value around 11.91, which may be due
GO–Ag composites is ascribed to the carboxyl and hydroxyl to interlamellar water trapped between hydrophilic graphene
Downloaded by Nanyang Technological University on 20 July 2011

groups of the GO sheets. This two-phase method is facile and oxide sheets.31 In the curve of oleylamine-capped Ag nano-
reproducible enough to be widely used for the synthesis of particles, the clear peaks at 2y values of about 38.11, 44.31,
other GO–metal composites. 64.51 and 77.51 are assigned to the (111), (200), (220), and
AFM was used to verify the number of GO layers synthesized. (311) crystallographic planes of face-centered cubic (fcc) Ag
An AFM image of GO sheets is shown in Fig. S1.w From the nanoparticles, respectively (JCPDS No. 07-0783). The average
two line scans, the thickness of GO sheets is measured to be size of the Ag nanoparticles was calculated to be 6.6 nm from
around 1.2 nm, which is slightly larger due to the oxygen the Ag (111) peak based on Scherrer’s equation. The curve of
groups than the reported apparent thickness of single-sheet GO–Ag composites is without change in comparison with Ag
graphene.28 The measured thickness of GO assures that the nanoparticles. No obvious diffraction peaks of GO were
GO is exfoliated into single sheets in water. TEM was used observed in the GO–Ag composites, because the regular stack
to analyze the morphology of GO–Ag composites. Fig. 1a of GO was destroyed by the intercalation of Ag nanoparticles,
shows that oleylamine-capped Ag nanoparticles synthesized in which is consistent with other reported studies on GO–metal
toluene are highly monodispersed with a uniform size of 6 nm, composites.15,16,32 The crystallite size of Ag nanoparticles on
which are able to provide super antibacterial capability. GO sheets was calculated to be 7.2 nm, which is in good
Fig. 1b illustrates that the size of the GO sheets synthesized agreement with the results of TEM. The XRD results confirm
is larger than 2 mm. Since the potential cytotoxicity of the Ag nanoparticles have successfully located onto the GO
engineered nanomaterials is of significant consideration when sheets. The XPS spectrum of GO–Ag composites in Fig. 2b
they are exposed to the environment,29,30 the GO sheets with a shows the major element peaks belong to C 1s, O 1s and Ag
large size may benefit the recovery of GO–Ag composite 3d, respectively. The weight percentage of each element was
materials after disinfection by simple filtration. The GO sheets analyzed by CasaXPS software according to the peaks, as
are uniformly covered by Ag nanoparticles as is shown in shown in the inset of Fig. 2b. The weight percentage of Ag
Fig. 1c and d. The image in Fig. 1d reveals a single GO–Ag nanoparticles in GO–Ag composites is 66.27%, illustrating the
composite sheet. It can be observed clearly that Ag nanoparticles mass ratio of Ag/GO is about 2/1.
are well monodispersed on GO sheet without any aggregation.
Both the edge of the GO sheet and the nanostructure of the Ag Antibacterial activity of GO–Ag composites
nanoparticles are clearly observable in the higher magnification
image of Fig. 1e, while there are no Ag nanoparticles outside The antibacterial activity of GO–Ag composites was evaluated
the GO sheets. Fig. 1f is a high-resolution TEM image of by a colony forming count method. E. coli was chosen as the
GO–Ag composites from Fig. 1e. The measured lattice-fringe model waterborne pathogen in this experiment. Fig. 3a shows
that with the increase of the concentration of GO–Ag composites,
the number of bacteria decreases dramatically; 99% of E. coli
cells have been killed at the concentration of 80 mg mL 1.

Fig. 1 TEM images of Ag nanoparticles (a), GO sheets (b), GO–Ag Fig. 2 (a) XRD patterns of GO sheets, oleylamine-capped Ag
composites ((c), (d), (e) and (f)). Inset of (d) is the size distribution of nanoparticles and GO–Ag composites. (b) XPS spectra of GO–Ag
Ag nanoparticles on GO sheets. composites (inset table is the element weight percentage).

1420 New J. Chem., 2011, 35, 1418–1423 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
 View Online

The results demonstrate that the GO–Ag composites have To verify the reliability of the colony forming count method
high-performance antibacterial capability at relatively low in this particular study, we further examined the antibacterial
concentration. In control experiments, the antibacterial activ- activity of GO–Ag composites using a LIVE/DEAD BacLight
ity of pure GO sheets and pure Ag nanoparticles were also bacterial viability kit. With an appropriate mixture of the
tested. The pure GO sheets show low antibacterial activity to SYTO 9 and propidium iodide (PI), bacteria with intact cell
E. coli cells; 10% of E. coli cells have been inactivated at the membranes stain fluorescent green, whereas bacteria with
concentration of 80 mg mL 1, and 17% at the concentration of damaged membranes stain fluorescent red. The real images
100 mg mL 1 (Fig. 3a). This result differs from the recent of bacteria can be recorded as well with the microscope at the
published report which concludes that GO has high antibacterial same time. Fig. 5a shows that E. coli in the control experiment
activity for E. coli.14 A possible reason for this phenomenon dispersed well in saline solution, and most of the E. coli
is that GO sheets used in this study have a different oxygen- survived with green color. After incubation with GO, E. coli
containing group content compared with those they used, cells aggregated together on the GO sheets as shown in real
which may affect the interaction between GO and bacteria image of Fig. 5b. However, most of the E. coli cells were still
Published on 13 April 2011 on http://pubs.rsc.org | doi:10.1039/C1NJ20076C

cells. Fig. 3a shows that pure Ag nanoparticles have good alive to exhibit green color, which confirms that GO sheets
antibacterial activity; 86% of E. coli cells have been killed at have low toxicity to E. coli. Fig. 5c provides the images of
Downloaded by Nanyang Technological University on 20 July 2011

the concentration of 80 mg mL 1, and 96% at the concentration E. coli in the presence of 100 mg mL 1 Ag nanoparticles,
of 100 mg mL 1. This result reveals that the pure Ag nano- showing that most of the E. coli cells were dead with red color.
particles synthesized here have effective antibacterial activity, Fig. 5d shows that nearly all the E. coli cells were inactivated
and they contribute the major part of the antibacterial capability to exhibit red color at the concentration of 100 mg mL 1
of GO–Ag composites. To clearly find out the difference GO–Ag composites. The fluorescence-based assay is in good
of antibacterial efficiency between the pure Ag nanoparticles agreement with the results obtained by the colony forming
and GO–Ag composites, the real number of E. coli (initial count method. Therefore, we can conclude that GO itself has
concentration of 3  107 cfu mL 1) to be inactivated is lower antibacterial activity, and pure Ag nanoparticles have
counted according to the concentration of samples, as is higher antibacterial activity towards E. coli at the size of 6 nm.
shown in Fig. 3b. GO sheets itself shows nearly no log decrease The GO sheets and Ag nanoparticles show a synergetic effect
of E. coli cells at the concentration of 100 mg mL 1, and Ag on antibacterial activity when they combine into GO–Ag
nanoparticles shows 1.5 log decrease of E. coli cells at the same composites.
concentration. However, GO–Ag composites show 4 log Recovery of the nanomaterials is essential to avoid nano-
decrease of E. coli cells at the concentration of 100 mg mL 1. toxicity to the ecosystem. The glass filter with pore size of 0.45 mm
The results clearly illustrate that at the same concentration, was applied here for membrane filtration after disinfection.
pure GO sheets shows little antibacterial activity, while The scanning electron microscopy (SEM) was used to inves-
GO–Ag composites have much higher antibacterial activity tigate the morphology of E. coli cells after disinfection and the
than pure Ag nanoparticles. Considering that the real percen- membrane filtration process. Without GO–Ag composites, E.
tage of Ag nanoparticles of GO–Ag composites is about 66%, coli cells remained in a good state as shown in Fig. 6a.
which means that 100 mg mL 1 of GO–Ag composites is equal However, with the increasing dosage of GO–Ag composites,
to 66 mg mL 1 of Ag nanoparticles, we can conclude that the damage to the E. coli cells increased, which are illustrated
GO–Ag composites display remarkably enhanced antibacter- from Fig. 6b to d. Fig. 6b reveals that GO sheets could
ial activity compared to pure Ag nanoparticles. obviously adsorb E. coli cells together. In the presence of
The time course for the E. coli inactivation was investigated 20 mg mL 1 GO–Ag composites, most of the E. coli cells
to find out the disinfection rate of GO–Ag composites. It can gathered on the GO sheets remained in a good state.
be seen in Fig. 4 that at the same concentration, GO–Ag
composites illustrate the highest disinfection rate compared to
pure GO and pure Ag nanoparticles. This result agrees with
the results obtained above to further confirm that GO–Ag
composites have enhanced antibacterial activity compared to
pure GO and pure Ag nanoparticles.

Fig. 3 Antibacterial activity of GO sheets, Ag nanoparticles and

GO–Ag composites at different concentration with the initial E. coli Fig. 4 Time course for antibacterial activity of 100 mg mL 1 of GO
concentration of 3  107 cfu mL 1: (a) E. coli viability percentage; sheets, Ag nanoparticles and GO–Ag composites, with an initial E. coli
(b) Log E. coli viability. concentration of 3  107 cfu mL 1.

This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011 New J. Chem., 2011, 35, 1418–1423 1421
 View Online

imply the possible mechanism of antibacterial activity of

GO–Ag composites. Firstly, the water-soluble GO sheets
adsorb and gather the bacteria onto the surface, which may
enhance the interaction between bacteria and Ag nanoparticles
on GO sheets. Some research has reported that GO sheets
show high non-specific binding capability to microbes.33
Secondly, Ag nanoparticles damage the bacterial cell wall
when in contact with the bacteria. Eventually the bacteria
can be destroyed into pieces. Obviously, Ag nanoparticles play
the key role in the antibacterial process. However, pure Ag
nanoparticles show much lower antibacterial activity than
GO–Ag composites. It may be due to the aggregation of pure
Ag nanoparticles during the antibacterial process, leading to
Published on 13 April 2011 on http://pubs.rsc.org | doi:10.1039/C1NJ20076C

the reduction of active specific surface area of Ag nanoparticles.

Moreover, pure Ag nanoparticles modified with surfactant
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may prevent silver ion release, and then prevent adhesion of

Ag nanoparticles to the bacterial cell surface.2 In contrast, GO
sheets play a positive role in the adhesion of bacterial to the
surface of the GO–Ag composites, which could remarkably
increase the interaction between the Ag nanoparticles and the
bacterial surface. Since the GO–Ag composites show effective
antibacterial activity toward the bacteria, it can also be applied
as the anti-biofouling agents, as the formation of biofilm is one
of the major problems in membrane filtration processes in the
water treatment industry.34 Meanwhile, the SEM images
reveal that GO–Ag composites can be easily recovered from
treated water by filtration without leaving disinfection material
Fig. 5 Real microscopic images and fluorescence microscopic images
pollutant behind.
of bacteria after incubation with different samples in saline solution
for 2 h: (a) E. coli without samples; (b) E. coli with 100 mg mL 1 of GO Conclusions
sheets; (c) E. coli with 100 mg mL 1 of Ag nanoparticles; (d) E. coli
with 100 mg mL 1 of GO–Ag composites. In summary, monodispersed Ag nanoparticles were anchored
successfully on large GO sheets by a facile two-phase assembly
In the presence of 50 mg mL 1 of GO–Ag composite, the image method. The GO–Ag composites show remarkably enhanced
of the E. coli cells on the GO–Ag composites reveals that the antibacterial activity towards E. coli compared to the original Ag
cell walls of the bacteria have been damaged significantly, as nanoparticles. The high-performance disinfection property of
shown in Fig. 6c. At the concentration of 100 mg mL 1 GO–Ag composites may be due to the adsorption of the bacteria
GO–Ag composites, most of the bacteria in solution were by GO sheets through non-specific binding. The GO sheets
destroyed into pieces as shown in Fig. 6d. The SEM results can effectively stabilize Ag nanoparticles to prevent their aggre-
gation. The recovery of GO–Ag composites can be implemented
by a simple filtration process due to the large GO sheets. Given
the superior antibacterial activity of GO–Ag composites and the
fact that GO–Ag composites can be easily recovered, we expect
that this new composite could offer promising opportunities in
the applications of environmental engineering and other fields.

Acknowledgements
The authors would like to acknowledge the Clean Energy
Research Programme under National Research Foundation of
Singapore and the Singapore Environment & Water Industry
(EWI) Development Council for a research grant (Grant No.
NRF2007EWT-CERP01-0420 and MEWR 651/06/166) in
support of this work.

Notes and references

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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011 New J. Chem., 2011, 35, 1418–1423 1423

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