Synthesis of gold nanoparticle catalysts based on a new water-soluble ionic polymer

Article abstract of DOI:10.1021/ic200334m

A new water-soluble methyl-imidazolium-based ionic polymer was synthesized by ring-opening metathesis polymerization that was subsequently used to prepare aqueous gold nanoparticle solutions which were characterized by UV-vis spectroscopy and transmission electron microscopy (TEM). The aqueous gold nanoparticle solutions were employed as catalysts in the reduction of p-nitrophenol and in the hydrogenation of cinnamaldehyde and were found to exhibit excellent activity under mild conditions.

Full text of DOI:10.1021/ic200334m

ARTICLE
pubs.acs.org/IC
Synthesis of Gold Nanoparticle Catalysts Based on a New
Water-Soluble Ionic Polymer
Ilaria Biondi, Gꢀabor Laurenczy, and Paul J. Dyson*
Institut des Sciences et Ingꢀenierie Chimiques, Ecole Polytechnique Fꢀedꢀerale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
S Supporting Information
b
ABSTRACT: A new water-soluble methyl-imidazolium-based ionic polymer was
synthesized by ring-opening metathesis polymerization that was subsequently
used to prepare aqueous gold nanoparticle solutions which were characterized by
UVÀvis spectroscopy and transmission electron microscopy (TEM). The aqu-
eous gold nanoparticle solutions were employed as catalysts in the reduction of p-
nitrophenol and in the hydrogenation of cinnamaldehyde and were found to
exhibit excellent activity under mild conditions.
’ INTRODUCTION
manner. A limitation of this cross-linked polymer is that it has a
poorly defined structure and therefore gives rise to GNPs with a
wide size distributions and poorly defined shapes. We therefore
decided to prepare imidazolium-containing polymer stabilizers
via ROMP (see Scheme 1) using a similar synthetic methodology
to that reported previously.58
The polymer, 2, is generated in high yield from 1 using the
commercially available second generation Grubbs Catalyst and
subsequent end-capping using cis-1,4-diacetoxy-2-butene,59 since
the more commonly used vinylethylether was not efficient at
decoupling the final ruthenium complex from the terminal end of
the polymer. The polymer is insoluble in common organic
solvents, and precipitates in CH2Cl2; thus the final capping
reaction is more difficult. To ensure an efficient cross metathesis
reaction between the polymer end chains and the cis-1,4-diace-
toxy-2-butene capping agent, a small amount of degassed MeOH
was added to the reaction medium. The polymer was then
dissolved in water and the resulting uncoupled catalyst was
extracted with CH2Cl2. The final purification of the polymer
was performed by dialysis in water using a membrane tube with
molecular weight cut off of 2000.
Synthesis and Characterization of the Polymer Stabilized
GNPs. GNPs were prepared from HAuCl4, the polymer, and
NaBH4 in water with the gold/polymer ratios indicated in
Table 1. As soon as a freshly prepared aqueous solution of
NaBH4 was added to the solution containing HAuCl4 and the
polymer, the solutions turned from orange to red-brown, indicative
of the formation of GNPs of <10 nm diameter.60 GNP solutions in
water usually feature a Surface Plasmon Band (SPB), that is, a broad
absorption band in the visible region around 520 nm.60 The UVÀvis
spectra (Supporting Information, Figure S1) of GNP1À3 show a
characteristic absorption at about 520 nm, consistent with a particle
size below 10 nm, whereas in GNP4 this absorption is absent
suggesting the formation of GNPs with a core diameter of <4 nm;
Gold in the bulk metallic state is generally regarded to be
inactive in heterogeneously catalyzed reactions.1 In 1987, how-
ever, Haruta et al. discovered that small gold nanoparticles
(GNPs <5 nm) immobilized on metal oxide supports are highly
active catalysts for carbon monoxide oxidation at ambient
temperatures.2 This discovery led to considerable interest in
gold catalysts,3À5 and the range of successful catalytic applica-
tions of GNPs includes the selective oxidation of alkenes,6À11
aldehydes,12À17 alcohols,18À27 and carbohydrates,28À34 the
water-gas shift reaction,35À46 and the removal of atmospheric
pollutants, such as nitrogen oxides, by reduction.47,48 Because of
the extremely small size associated with highly active GNPs, they
are only kinetically stable and consequently stabilizers must be
used to prevent agglomeration.49 In aqueous solutions stabilizers
providing steric protection like poly(vinyl-pyrrolidone), PVP, or
poly(vinyl-alcohol), PVA, have been used for a wide range of
applications.25,26,50À55 Moreover, in the past few years polymers
providing electrosteric stabilization, that is, ionic polymers, were
also successfully employed as GNP stabilizers.56,57
This paper describes the design and synthesis of a new water-
soluble ionic polymer using ring-opening metathesis polymeri-
zation (ROMP) and the role of the polymer in the stabilization of
water-soluble GNPs. Their catalytic properties in the reduction
of p-nitrophenol showed very high performance under mild
conditions and in the hydrogenation of cinnamaldehyde they
exhibited selectivity toward the CdO bond under certain
conditions.
’ RESULTS AND DISCUSSION
It has previously been shown that imidazolium-containing
cross-linked polymers can stabilize GNPs that, depending on the
counteranion, effectively disperse in water.57 An attractive feature
of these imidazolium-containing polymers over other polymers
such as PVP is that variation of the counteranion allows the
physicochemical properties of the polymer to be varied in a facile
Received: February 17, 2011
Published: July 27, 2011
r
2011 American Chemical Society
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Scheme 1
Table 1. Composition of the GNP Solutions Used in This
Studya
Presumably the polymer layer adsorbed on the surface of GNPs
serves as a barrier, resulting in diffusion-limited growth, which
reduces the size distribution of the initial nuclei and, at high
concentrations, leads to near monosized nanoparticles.61
Catalytic Reduction of p-Nitrophenol. The catalytic reduc-
tion of p-nitrophenol by GNPs using NaBH4 in water is a well-
known reaction which proceeds through an electrochemical
mechanism, where the electron transfer occurs through the nano-
particle surface. The catalytic efficiency may be explained by the
nanoparticle redox properties relating with the particle size and
the nanoparticle potential, which has to be in the range of the
potential of the reducing agent (the donor) and the substrate
(the acceptor).62
The catalytic properties of GNP1À4 were initially evaluated in
the reduction of p-nitrophenol, chosen as a model system, as the
reaction is easily monitored by UVÀvis spectroscopy. The GNP4
solution exhibited the best performance, presumably because of
their small size and greater stability, emanating from the higher
amount of polymer stabilizer present. It is also conceivable
that the polymer facilitates diffusion of the substrate onto
the NP surface which further enhances catalytic activity.
Consequently, GNP4 was further studied at different GNP/
p-nitrophenol ratios (see Table 2); a large excess of NaBH4
was used so that the kinetics could be evaluated with respect to
p-nitrophenol consumption.
solution
ratio of polymer/Au
GNP1
GNP2
GNP3
GNP4
3:1
5:1
10:1
50:1
a All solutions were prepared with a 1:1, w/w ratio of NaBH4 to Au at a
concentration of 100 mg LÀ1 of gold.
for such small particles, the sharpness of the SPB peak decreases
because of the onset of quantum size effects.60 The GNPs exhibit
good stability although in the case of GNP4 (Supporting Informa-
tion, Figure S1) an absorption peak is observed after about 2 months
indicating that aggregation of the GNPs has occurred; however, the
diameters of the aggregates are <10 nm.
Confirmation of the GNP size and distribution was obtained
from TEM. At lower polymer/Au ratio the particle size was
found to be quite broad, whereas in the case of GNP4, which
contains the highest polymer/gold ratio of 50:1, the GNPs
showed a very narrow monodisperse distribution in the range
of 1.8 to 2.8 nm (see Figure 1).
The size and dispersity of the GNPs appear to be directly
correlated with the amount of polymer used in their preparation.
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Figure 1. TEM images and size distribution histograms for GNP1 (scale bar 50 nm), GNP2 (scale bar 50 nm), GNP3 (scale bar 20 nm), and GNP4
(scale bar 20 nm).
bleaching observed. The gradual disappearance of the absorption
peak at 400 nm and the formation of a new peak at 300 nm
indicated that the substrate had been consumed with the forma-
tion of p-aminophenol (Figure 2). The change of absorbance
values at 400 nm over time and the consequent linear relation-
ship between ln(absorbance) and time for the reactions are
indicative of pseudo first-order kinetics with respect to p-
nitrophenol consumption (Figure 3) allowing the apparent rate
constants, k, to be estimated (Table 2), in agreement with related
nanocatalysts.63À69
GNP4 exhibited high catalytic activity for the reduction of
p-nitrophenol which reached completion within a few minutes.
Although at the highest substrate/catalyst ratio (Table 2, entry 5),
the dependence between the absorbance values at 400 nm and
time is linear62 (see Supporting Information, Figure S2) and the
reduction is complete in <20 min. GNP4 compares favorably
with other GNP catalysts. For example, Haruta et al. found that
the rate of GNPs, deposited on poly(methyl methacrylate), is
7.3 Â 10À3 sÀ1 using a substrate/catalyst ratio of 1:15.64 Liu
reported GNPs that catalyzed the reduction of p-nitrophenol in
19 min at a substrate/GNP ratio of 1:10.70 Pal et al. were able to
obtain complete conversion in 4 min with GNP, but only using a
Table 2. GNP4:p-Nitrophenol Ratios and Associated Rate
Constants for the Reduction of p-Nitrophenol with NaBH4
a
entry
ratio of GNP4:p-nitrophenol
k (minÀ1
)
1
2
3
4
5
6
7
1:5
1.9905
1.573
0.766
0.732
0.5485
0.5204
0.077
1:7
1:28
1:56
1:70
1:90
1:140
a p-Nitrophenol solution (0.048 mM), GNP4 (0.5076 mM), NaBH4
(10 mg); in all experiments NaBH4/p-nitrophenol ratio is 88:1. Removal
of oxygen from the reaction mixture is required to produce reproducible
results.62,63
As confirmed by control experiments, in absence of GNP4, the
reduction of p-nitrophenol with NaBH4 did not take place, and
the reaction mixture showed only an absorption peak at 400 nm
in the UVÀvis spectrum which corresponds to the formation of
p-nitrophenate. The addition of GNP4 to the reaction mixture
caused the yellow solution to reduce in intensity with complete
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However, the ability of a catalyst to selectively hydrogenate a
CdO functionality when conjugated with a CdC double bond is
a major challenge because the hydrogenation of a CdC double
bond is kinetically and thermodynamically favored.78,79 In recent
years many supported GNP-based catalytic systems have been
developed,71,78À85 but studies on the hydrogenation of alde-
hydes and ketones show that the outcome of the reaction is
strongly related to the support material used to immobilize the
particles, making it difficult to distinguish the ability of the GNP
catalysts to selectively hydrogenate the carbonyl group. The use
of unsupported GNPs is more suitable to study their true
catalytic activity; however, there are only a few reports on the
selective hydrogenation of R,β-unsaturated aldehydes and ke-
tones with unsupported GNPs. For example, De Vos and co-
workers carried out the reaction on several R,β-unsaturated carbo-
nylic compounds with polyvinylpirrolidone-stabilized GNPs
in DMF,86 and Jin and co-workers studied the selective hydro-
genation of benzalacetone with thiolate-stabilized GNPs in a
toluene-ethanol mixture,87 but kinetic studies were not reported.
Consequently, GNP1À4 were screened as catalysts in the
hydrogenation of cinnamaldehyde, a reaction that can result in
the formation of three products, that is, cinnamyl alcohol from
selective hydrogenation of the CdO group, hydrocinnamalde-
hyde from selective hydrogenation of the CdC double bond, and
the fully hydrogenated product, hydrocinnamyl alcohol (see
Scheme 2).
The GNP4 solution showed promising activity and was
further studied in situ by NMR spectroscopy, under a H2
pressure of 100 bar in a sapphire NMR tube,88 which allowed
the reaction products to be continuously monitored. At 70 °C the
highest selectivity is obtained after 40 min with a conversion of
30% (see Supporting Information). At 40 min the cinnamyl
alcohol corresponds to 73% and the hydrocinnamaldehyde to
27% of the obtained products. Hydrocinnamyl alcohol is only
observed after 1 h and while the concentration of cinnamyl
alcohol increases rapidly during the first 4 h, the concentration of
hydrocinnamaldehyde does not change after the first hour. Both
of the partially reduced products start to decrease in concentra-
tion after 4 h, and by 14 h the hydrocinnamaldehyde is almost
completely consumed.
Under the conditions described above the GNPs selectively
reduce the CdO group at the initial stage of the reaction, when
the conversion of cinnamaldehyde is relativity low, but at higher
conversions hydrogenation of the CdC double bond is ob-
served, although the resulting hydrocinnamaldehyde is propor-
tionally smaller than the corresponding alcohols. Tsukuda89 and
co-workers demonstrated that GNPs interact (and activate)
carbonyl groups preferentially over CdC bonds which presum-
ably leads to the initial selectivity for the hydrogenation of the
CdO bond. As the broad peak in the range 5.4À6.0 ppm, that
corresponds to the CdC double bonds of the polymer, remains
unchanged at the end of the reaction it would appear that they do
not undergo hydrogenation (see Supporting Information, Figure
S4), although it has been shown that polynorbornenes can be
hydrogenated with molecular catalysts.90À92
Figure 2. (top) Successive UVÀvis spectra recorded every 2 min for
p-nitrophenol reduction (GNP4/p-nitrophenol 1:70, entry 5 in
Table 2); (middle) Kinetic curves for the reduction of p-nitrophenol
with NaBH4 in water using GNP4 obtained by plotting the absorbance
values against time: blue diamonds, 1; reddish brown squares, 2; light
green triangles, 3 Â 4; light blue asterisks, 5; yellow circles, 6. (bottom)
Estimation of rate constants by plotting ln(absorbance) against time:
blue diamonds, 1; reddish brown squares, 2; light green triangles, 3;
purple circles, 4; light blue asterisks, 5; yellow circles, 6 (numbers refer to
the entries in Table 2).
substrate/GNP ratio of 1:12 and a p-nitrophenol/NaBH4 ratio
of 1:500.62
Catalytic Hydrogenation of Cinnamaldehyde. Since GNP4
is extremely active under mild conditions for the reduction of p-
nitrophenol we decided to evaluate the GNP catalysts in the
hydrogenation of cinnamaldehyde. The selective reduction of
CdO bonds, when conjugated to a CdC bond, represents
another important application of GNP catalysts. This type of
hydrogenation reaction is of industrial interest as it provides
access to different R,β-unsaturated alcohols that are commonly
used as flavors and fragrances and are key intermediates in the
synthesis of pharmaceuticals.71 The selective reduction of CdO
bonds is usually achieved by transfer hydrogenation,72À76 or is
performed with reducing agents such as NaBH4, although the use
of molecular hydrogen is more suitable for industrial applications.77
The hydrogenation of cinnamyl alcohol and hydrocinnamal-
dehyde to hydrocinnamyl alcohol were performed separately
under the same conditions, that is, 100 bar H2 pressure at 70 °C
(see Supporting Information), indicating why the selectivity of
the reaction at 70 °C is not high. At elevated temperatures
preferential interactions between the carbonyl group and the
gold surface are presumably reduced, and indeed, at 60 °C the
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Figure 3. Kinetic curves for cinnamaldehyde hydrogenation at 60 °C and 100 bar H2 using the GNP4 solution: light blue diamonds, Cinnamaldehyde;
yellow squares, Cinnamyl alcohol; light green triangles, Hydrocinnamaldehyde; red circles, Hydrocinnamyl alcohol.
Scheme 2
hydrogenation of cinnamaldehyde exhibits higher selectivity
toward the cinnamyl alcohol product (78% after 2 h albeit at a
conversion of only 27%). However, after 3 h the formation of
hydrocinnamyl alcohol is also observed, although the overall
selectivity toward cinnamyl alcohol remains essentially un-
changed (Figure 3).
the hydrogenation of cinnamaldehyde in aqueous solutions,
demonstrating promising catalytic activity for both reactions.
’ EXPERIMENTAL SECTION
1,2-Dimethylimidazole was recrystallized from toluene before use. All
the other chemicals were purchased from Aldrich or Acros Organics and
used without further purification. Membrane dialysis tubes (MWCO
1000, MWCO 2000) were purchased from Spectrum Laboratories.
Melting points were measured on a Stuart Melting Point Apparatus
SMP3. ATR FT-IR spectra were recorded on a Perkin-Elmer Spectrum-
One instrument using freshly ground samples pressed on top of a
diamond anvil window. Sample preparation and spectral recording (in
air) was performed within 2 min. Elemental microanalysis was obtained
on a CE Instruments EA-1110 elemental analyzer. Mass spectra were
recorded using nanoelectrospray ESI techniques using a ThermoFinni-
gan LCQ DECA XP Plus quadrupole ion trap instrument on samples
diluted in acetone.74 UVÀvis spectra were recorded on a Jasco V-550
spectrometer. NMR spectra were measured on a Bruker DMX 400 using
SiMe4 as an external standard at 20 °C. For ICP-MS the samples were
digested in concentrated nitric acid for 3 h and filled to a total volume of
8 mL with water. Indium was added as an internal standard at a
concentration of 0.5 ppb. Determinations of total metal contents were
achieved on an Elan DRC II ICP-MS instrument (Perkin-Elmer,
Switzerland) equipped with a Meinhard nebulizer and a cyclonic spray
chamber. The ICP-MS instrument was tuned using a solution provided
by the manufacturer containing 1 ppb of each Mg, In, Ce, Ba, Pb and U.
External standards were prepared gravimetrically in identical matrix to
the samples (with regard to internal standard and nitric acid) with
ruthenium standard obtained from CPI International (Amsterdam, The
Netherlands).
As shown from kinetic studies, the hydrogenation of cinna-
maldehyde is a first order reaction with respect to the substrate
consumption, and the estimation of rate constants equates to
0.52 hÀ1 and 0.12 hÀ1 for the hydrogenation performed at 70 and
60 °C, respectively (see Supporting Information for details).
The selectivity is not significantly improved at lower tempera-
tures, and the rate of hydrogenation becomes very slow (after 40
h at 50 °C, 10% of the cinnamaldehyde starting material is
present). The hydrogenation studies show that the selectivity for
the reduction of the carbonyl group in cinnamaldehyde is
possible, even if subsequent competitive hydrogenation of the
CdC bond occurs, resulting in the saturated alcohol product. It
is noteworthy that ruthenium93 and palladium94 NPs have been
used for the hydrogenation of cinnamaldehyde under mild
conditions, but both show a high selectivity for the CdC bond
and hydrocinnamaldehyde is the main product. As expected, the
larger GNPs are also less active (see Supporting Information),
but the chemoselectivity does not change.
’ CONCLUSIONS
A new water-soluble ionic polymer has been prepared using
ROMP, and its application as a NP stabilizer has been studied. At
high polymer concentrations small well-defined GNPs were
obtained that are stable against aggregation, even under catalytic
conditions, and based on in situ spectroscopic studies there is no
evidence to suggest that the polymer undergoes reaction during
catalysis. Indeed, the GNPs were successfully employed as
catalysts both in p-nitrophenol reduction with NaBH4 and in
Synthesis of 3-(Bicycle[2.2.1]hept-5-en-2-ylmethyl)-1,2-
dimethyl-1H-imidazolium Methanesulfonate 1. To a solution
of bicyclo[2.2.1]hept-5-en-2-ylmethyl methanesulfonate (13.96 g) in
toluene (9 mL), a solution of 1,2-dimethylimidazole (6.76 g) in toluene
(2 mL) was added, and the reaction mixture was vigorously stirred under
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reflux under N2 for 2 days. After removal of the solvent under reduced
pressure a dark brown viscous liquid was obtained. The product was
precipitated by addition of Et2O (300 mL) and filtered to afford a highly
hygroscopic pale brown powder. Yield: 86% Mp 90À93 °C,1H NMR
(25 °C, 400.1 MHz, CDCl3), δ(ppm): 7.06 (m,CÀH), 7.35 (m, 1H),
7.23 (m, 1H), 7.22 (m, 1H), 6.34 (m, CHdCH), 6.13 (m, CHdCH),
6.04 (m, CHdCH), 4.30À4.10 (m, 2H, CH2O,) 3.97 (s, 3H, NCH3),
3.88À3.77 (m, 5H), 2.94 (m, 1H), 2.88 (m, 1H), 2.79 (m, 1H), 2.74 (s,
3H), 2.71 (s, 3H), 2.69 (s, 3H), 2.55À2.50 (m, 1H), 1.98À1.84 (m, 1H),
1.54À1.26 (m, 2H), 0.67À0.64 (m, 1H). 13C NMR (25 °C, 100.6 MHz,
CDCl3), δ(ppm): 144.09, 139.48, 137.71,135.64, 131.20, 123.17,
122.25, 121.46, 120.75, 118.75, 53.71, 52.56, 49.75, 45.11, 44.12,
42.43, 41.76, 39.52, 4, 35.70, 34.59, 30.92, 30.08, 10.99, 10.34. ESI-MS
(25 °C, acetone), (m/z) positive mode: 204, 203, 97, 59; (m/z) negative
mode 95, 62, 61, 60 [CH3SO3]. FT-IR (25 °C, solid), υ(cmÀ1): 3123,
3098, 2975, 2951, 2875, 1534, 1538, 1459,1419,1379, 1327, 1287, 1251,
1204, 1126, 1038, 911, 767, 733, 681. Anal. Calc. for C14H22N2O3S
(298.4012) C, 56.35; H, 7.43; N, 9.34%. Found: C, 53.36; H, 8.03;
N, 9.87%.
Synthesis of Poly-(3-((2,4-divinylcyclopentyl)methyl)-1,2-
dimethyl-1H-imidazolium Methanesulfonate) 2. In a glovebox
the monomer 1 (745 mg, 2.5 mmol) and (IMesH2(PCy3))(Cl)2Ru=
CHPh (85 mg, 0.1 mmol) were dissolved in dry CH2Cl2 (10 mL), and
the solution was stirred at 30 °C. After 30 min a dark brown solid started
to precipitate, and the solution was stirred for a further 7 h. After
this time a degassed solution of cis-1,4-diacetoxy-2-butene (0.5 mL,
3.14 mmol) in methanol (2 mL) was added, and the reaction stirred at
room temperature (RT) for 3 days. The solvents were then removed
under reduced pressure, bidistilled water was added (100 mL), and after
a few minutes the precipitation of a fine dark brown powder was
observed. The solution was then transferred to a membrane dialysis
tube (MWCO: 2000) and dialyzed for 1 week, after which the water was
removed under reduced pressure and a brown solid was obtained. Yield:
80% 1H NMR (25 °C, 400.1 MHz, D2O) δ(ppm): 7.26, 5.52, 5.34, 4.71,
3.99, 3.83, 3.66, 3.15, 3.02, 2.70, 2.48, 2.15, 2.01, 1.72, 1.56, 1.14. FT-IR
(25 °C, solid), υ(cmÀ1): 3434, 3135, 2931, 1644, 1588, 1538, 1453,
1421, 1328, 1176, 1037, 978, 767, 700. Ru content <0.005% (w/w).
Synthesis of the GNPs. To an aqueous solution of HAuCl4
(1 mg/mL, 3 mL, 7.62 μmol) an aqueous solution of 2 (22.86 μmol
of 2 for GNP1, 38.1 μmol for GNP2, 76.2 μmol for GNP3, 381 μmol for
GNP4) was added. Under vigorous stirring an aqueous NaBH4 solution
(0.5 mg/mL, 3 mL, 39.6 μmol) was rapidly added, and the color
immediately turned from pale yellow to red-brown. After stirring the
solution for 2 min, bidistilled water was added to obtain a final volume of
15 mL, and then the samples were dialyzed for 24 h (MWCO: 1000).
Catalytic Reduction of p-Nitrophenol. To a Schlenk flask
(250 mL) an aqueous solution of p-nitrophenol (5 mL, 0.625 mM)
was added diluted in water (65 mL), and the solution was degassed.
NaBH4 (10 mg) was added, and the solution immediately turned from
pale to intense yellow. A 1.6 mL portion of this solution was transferred
in a 2 mL quartz cuvette, GNP4 (20 μL, 0.5076 mM of Au, in this case
the GNP:substrate ratio is 1:7), and degassed water (30 μL) was added
to keep the same p-nitrophenol concentration in all experiments. The
course of the reaction was monitored by UVÀvis spectroscopy record-
ing a spectrum every 2 min. Experiments were repeated three times each
to ensure reproducibility.
employed to ensure that the concentration of H2 in the solution was in
excess compared to the substrate. The samples were prepared as D2O
solutions with 8.4% EtOH included as a cosolvent to improve the
solubility of the substrates.
TEM. GNP1À3 (0.1 mL) was diluted in bidistilled water (0.5 mL)
and immersed in a ultrasonic bath at 20 °C for 5 min. One drop of this
solution was deposited on a carbon film copper grid (200 mesh) and
dried under vacuum for 2 h prior to analysis. In the case of GNP4, the
solution (1.0 mL) was reduced in volume (0.1 mL) and ultrasonicated
for 5 min at 20 °C. One drop was deposited on a carbon film copper grid
(200 mesh) and dried under vacuum for 12 h prior to analysis. TEM
images were obtained on a PHILIPS/FEI CM20 transmission electron
microscope (200 KeV) using Quantifoil Carbon film Cu grids on 200
mesh as specimen supports. The size distribution was estimated from
200 nanoparticles.
’ ASSOCIATED CONTENT
S
Supporting Information. Further details are given in
b
Figures S1ÀS12. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: paul.dyson@epfl.ch.
’ ACKNOWLEDGMENT
We thank the EPFL and Swiss National Science Foundation
for financial support. We also thank Prof. Philippe Andrꢀe Buffat,
Dr. Marco Cantoni, and the staff at the Center Interdisciplinaire
de Microscopie Electronique (CIME) at the EPFL for assistance
and useful discussions regarding TEM and Catherine Clavel for
ICP-MS analysis.
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