Double-hydrophilic block copolymer nanoreactor for the synthesis of copper nanoparticles and for application in click chemistry

Article abstract of DOI:10.1166/jnn.2011.4355

Ultrasound (US), a nonconventional energy source, has become a very popular and useful technology in organic chemistry. Several examples of US-assisted reaction have indicated that high yields and short reaction times are reasonable, and applications of t

Full text of DOI:10.1166/jnn.2011.4355

Copyright © 2011 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 6162–6166, 2011
Double-Hydrophilic Block Copolymer Nanoreactor for
the Synthesis of Copper Nanoparticles and for
Application in Click Chemistry
Aram Kim1, Bhawana Sharma2, Byeong-Su Kim2ꢀ∗, and Kang Hyun Park1ꢀ∗
1Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Korea
2Interdisciplinary School of Green Energy and School of NanoBioscience and Chemical Engineering, Ulsan National Institute of
Science and Technology (UNIST), 100 Banyeon-ri, Ulsan 689-798, Korea
Ultrasound (US), a nonconventional energy source, has become a very popular and useful tech-
nology in organic chemistry. Several examples of US-assisted reaction have indicated that high
yields and short reaction times are reasonable, and applications of this energy transfer process
on click reactions have been published. Stable, water-soluble copper nanoparticles (Cu NPs) were
synthesized using a double-hydrophilic block copolymer as a template. The prepared Cu NPs were
effective as catalysts for the [3 + 2] cycloaddition of azides with terminal alkynes to provide prod-
ucts in good yields and with high regioselectivity. An environment-friendly process of synthesizing
regioselective triazoles was developed from a variety of azides with terminal alkynes, in which the
water-soluble Cu NPs were utilized, under 10 min sonication. The Cu NPs can be reused three
times under the present reaction conditions, without any loss of catalytic activity. Block copolymer
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nanoreactors were shown to be promising in creating a number of water-soluble catalytic metal NPs
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for a wide variety of organic transformations.
Copyright: American Scientific Publishers
Keywords: Copper (Cu), Block Copolymer, Nanoparticles (NPs), Heterogeneous, Catalyst, Click
Reaction.
1. INTRODUCTION
To date, a number of methods of preparing Cu2O NPs
with different particle sizes morphologies, and properties
have been described, including controlled thermal decom-
position, chemical reduction with the proper surfactant,
electrochemical reduction, microemulsion, and the use of
reverse micelles in a supercritical solvent. In all of these
methods, control over the particle size and morphology is
achieved through the use of either templating materials or a
capping reagent during NP growth. Block copolymers have
received much attention of late as attractive templates or
scaffolds for engineering inorganic nanostructured materi-
als to control the sizes and spatial locations of NPs.5–7
Click chemistry is a highly efficient cycloaddition
method that became a highly popular synthesis method
since its introduction in 2001 by Sharpless.8ꢀ9 The
Azide-alkyne Huisgen method, involving Cu(I)-catalyzed
cycloaddition between terminal acetylenes and azides at
room temperature, is one of the most efficient reactions
within the click chemistry philosophy.10 The reaction pro-
ceeds in variable solvents in the presence of catalysts to
yield stable triazoles with possible applications in phar-
maceuticals, DNA modification, and organic synthesis.11
The synthesis of well-defined nanoparticles (NPs) is of
special interest in nanoscience, particularly for their cat-
alytic applications due to the interesting properties associ-
ated with their nanoscale dimension. As metal and metal
oxide NPs are very stable both physically and chemically,
they have been frequently used as catalysts.1ꢀ2 In addi-
tion, their distinct characteristics as NPs with large sur-
face areas make them applicable to a wide range of fields.
Among the many metal oxide NPs in existence, copper
oxide (Cu2O, CuO) are well-known p-type semiconductor
materials with potential applications in the area of solar-
energy conversion and catalysis. For example, Tarascon
et al. recently used copper oxide NPs as an anode for
lithium ion cells,3 while Izaki et al. employed them with
ZnO for solar-cell plates, demonstrating their potential
in electrochemical applications.4 Moreover, copper oxide
NPs are nontoxic, environment-friendly, and highly stable,
and as such, recyclable.
∗Authors to whom correspondence should be addressed.
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J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 7
1533-4880/2011/11/6162/005
doi:10.1166/jnn.2011.4355

Kim et al.
Double-Hydrophilic Block Copolymer Nanoreactor for the Synthesis of Copper Nanoparticles
The commonly used catalysts in click reaction are Cu
and ruthenium. The Cu catalyst generates 1,4-regioisomer
as a product, while the ruthenium catalyst generates 1,5-
regioisomer (Eq. (1)).
2. EXPERIMENTAL PROCEDURE
2.1. General Remarks
Reagents were purchased from Aldrich Chemical Co. and
Strem Chemical Co. and used as received. The reaction
1
N
N
products were analyzed via H-NMR spectroscopy, with
spectra obtained on a Varian Mercury Plus (300 MHz).
The chemical-shift values were recorded as parts per mil-
lion relative to tetramethylsilane as an internal standard,
unless otherwise indicated, and as coupling constants in
Hertz. The reaction products were assigned by compari-
son with the literature value of known compounds. The
water-soluble Cu NPs were characterized by TEM (JEOL
2100 operated at 200 kV). Samples were prepared by plac-
ing a few drops of the corresponding colloidal solution
on carbon-coated Cu grids (Ted Pellar, Inc). The X-ray
powder diffraction (XRD) patterns were recorded on a
D8 Discover (12 kW) diffractometer. The Cu NPs were
characterized using an X-ray photoelectron spectroscope
(XPS) (Thermo Fisher, K-alpha) and a UV/Vis spectro-
scope (VARIAN, Cary 5000), and the Cu-loading amounts
were measured via inductively coupled plasma optical-
emission spectrometry (ICP-OES).
N
R1
R2
+
N
N
R2
(1)
+
N
R1
–
N
N
N
R1
R2
Ultrasound (US), a nonconventional energy source, has
become a very popular and useful technology in organic
chemistry.12 Several examples of US-assisted reaction have
indicated that high yields and short reaction times are rea-
sonable, and applications of this energy transfer process
on click reactions have been published.13
The analysis of environmental substances has become
an increasingly important issue due to the problems asso-
ciated with global environmental pollution. Performing
2.2. General Procedure for the [3+2] Cycloaddition
of Azides with Terminal Alkynes
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0.4 ml water-soluble Cu NPs (20 mM, 1 mol%), benzyl
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azide (0.1 mL, 0.80 mmol), phenylacetylene (0.13 mL,
Copyright: American Scientific Publishers
click chemistry in aqueous media is still challenging, how-
ever, due to the absence of a stable and active cata-
lyst that is water-soluble.14 Aqueous click chemistry has
the economic, environmental, and processing benefits of
both homogeneous aqueous catalysis and aqueous two-
phase catalysis. Water clearly stands out as the solvent
of choice, with its fast reaction rate, high yield, selec-
tivity, cheapness, “green” solvent nature, and environ-
mental acceptability. In this context, double-hydrophilic
block copolymers (DHBCs) were employed in the growth
control of inorganic phases, such as mineralization with
unusual structural specialty and complexity in an aqueous
solution.8 These DHBCs consist of one active ionizable
block and one neutral block. The introduced metal ions
interact with the ionizable block in solution, thus inducing
micelle formation, which promotes the stabilization of the
colloidal micelle in water. Once the formation of micelle is
induced, it acts as a nanoreactor to control the NP growth
within the micellar core. The DHBC method can provide
an easy solution-phase synthesis approach without harsh
conditions, and can create a potentially biocompatible shell
around the NPs to improve the solubility and compatibility
of the prepared particles. In this study, poly(acrylic acid)-
b-poly(ethylene oxide) (PAA-b-PEO) DHBC was used
to prepare sterically stabilized Cu NPs with controllable
sizes.
1.18 mmol), and 4.5 mL H2O were placed in a 10 mL-
pressure Schlenk tube, followed by sonication for 10 min
at 100 ꢀC, with 50% amplitude, using a Fisher Scien-
tific Sonic Dismembrator 500 (Pittsburgh, PA, USA). After
10 min, the water-soluble Cu NPs were recovered via
centrifugation, and the clean solution was analyzed at
300 MHz NMR.
2.3. Water-Soluble Cu NPs
The Cu NPs were prepared according to the follow-
ing protocols. First, double-hydrophilic block copolymer
PEO(3500)-b-PAA(7500) (Polymer Source Inc, Canada)
(21.15 mg, 0.20 mmol carboxylic-acid groups) and copper
chloride dihydrate (17.8 mg, 0.10 mmol) in 5.0 mL water
were separately prepared and mixed under vigorous stir-
ring. 1.0 M NaOH (0.10 mL, 0.10 mmol) was added to
this solution mixture, and blue precipitation was obtained.
Subsequently, 10.0 M hydrazine (0.10 mL, 1.0 mmol) was
added dropwise to the resulting suspensions under vigor-
ous stirring. As soon as hydrazine was added, the solution
turned orange. After 20 min of vigorous stirring, the reac-
tion mixture kept still for 10 min, after which the solu-
tion color changed to brownish red. The solution mixture
was centrifuged to remove the large aggregates of particles
therein (1500 rpm, 15 min), then the recovered supernatant
was used as a catalyst for click chemistry.
J. Nanosci. Nanotechnol. 11, 6162–6166, 2011
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Double-Hydrophilic Block Copolymer Nanoreactor for the Synthesis of Copper Nanoparticles
Kim et al.
3. RESULTS AND DISCUSSION
3.1. Catalyst Characterization
According to the optimized protocol presented, the pre-
pared Cu NPs exhibit a red homogeneous aqueous suspen-
sion. As surface plasmon resonance (SPR) is a well-known
phenomenon in many transition metal NPs, the absorption
peak at around 570 nm supports the nanoscale dimension
of Cu NPs.16 This value also matches that of the previously
reported SPR of Cu NPs.
The transmission electron microscopy (TEM) image
further shows the spherical shape of the Cu NPs with
an average diameter of ca. 6 nm at a relatively narrow
size distribution (Fig. 2). In addition, the highly crystalline
nature of the Cu NPs was demonstrated via high-resolution
TEM measurement with a 2.09 Å lattice spacing, which
corresponds to the primary reflection of the (111) peak in
Cu NPs. Energy-dispersive spectrometer (EDS) measure-
ment, however, indicated the presence of a small amount
of oxygen as well as elemental Cu in the prepared Cu
NPs, which may indicate the oxidized layer of Cu on the
surfaces of the particles. As such, the Cu NPs were inves-
tigated via XRD. It was observed that there were two main
peaks (ꢁ = 43.2ꢀ and 50.3ꢀꢂ assigned to the reflections of
the (111) and (200) planes in the Cu phase, together with a
small peak that corresponds to the (111) plane in the CuO
Fig. 2. (a, b) TEM images of the prepared Cu NPs. (c) The correspond-
ing EDS pattern.
(a)
phase (Fig. 3). There were also some peaks associated with
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the residual NaCl formed during the NaOH treatment.
Copyright: American Scientific Publishers
In addition, X-ray photoelectron spectroscopy (XPS)
further revealed the presence of a copper oxide layer after
the deconvolution of the obtained peaks (Fig. 3(b)). The
Cu2p3/2 peak of elemental Cu was found at 932.4 eV,
whereas Cu2+ had one main peak at 933.5 eV and shakeup
satellite peaks (characteristics of materials with a d9 con-
figuration in the ground state, such as Cu2+ꢂ at higher
bonding energies.17 Taken together, it is postulated that
the prepared Cu NPs in fact had core–shell-type struc-
tures consisting of a Cu core with a thin shell of copper
oxide. The suppression of the oxidation of metallic Cu was
(b)
Fig. 3. (a) XRD and (b) high-resolution Cu2p3/2 XPS spectra of the
prepared Cu NPs. The deconvoluted peaks represent the coexistence of
Cu and copper oxide.
Fig. 1. UV/vis spectrum of the prepared Cu NPs. (Inset) Photo image
of the Cu NPs.
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J. Nanosci. Nanotechnol. 11, 6162–6166, 2011

Kim et al.
Double-Hydrophilic Block Copolymer Nanoreactor for the Synthesis of Copper Nanoparticles
Table I. Optimization of the click reaction catalyzed by water-soluble
Cu NPs.
Entry
Cat (mol %)
Temp (ꢀC)
Time (h)
conv (%)a
1
2
3
4
5
6
7
8
1 mol% CuO-poly
1 mol% CuO-poly
1 mol% CuO-poly
1 mol% CuO-poly
1 mol% CuO-poly
0.5 mol% CuO-poly
Recovered # 4
25
50
3
3b
17
37
>99
27c
38
10 min
5 min
100
100
100
100
100
100
10 min
10 min
10 min
10 min
10 min
Scheme 1. Synthetic procedure of water-soluble copper nanoparticles.
99
100
probably due to the presence of a strongly coordinating
double-hydrophilic block copolymer in preventing excess
oxidation to pure copper oxide.
Recovered # 7
aDetermined by 1H-NMR. Yields are based on the amount of benzyl azide used.
bTraditional stirring. cConventional thermal heating at 100 ꢀC.
The original sizes and structures of the CuO NPs were
preserved during catalytic transformation. The absolute
amount of Cu that was used for click chemistry was deter-
mined via inductively coupled plasma optical-emission
spectroscopy (ICP-OES). The CuO NPs within a DHBC
shell showed excellent catalytic activity towards a wide
range of azides and acetylenes.
times without any catalytic activity loss, under the same
reaction conditions. These results confirm that the catalytic
system presented herein satisfies the conditions for het-
erogeneous catalysts (easy separation, recyclability, and
persistence).
The use of a variety of azides and terminal alkynes
in this reaction was also studied, and the results are
shown in Table II. In some cases, the electron-donating or
electron-withdrawing groups on the benzyl azides greatly
affected the reactivity. The electron-withdrawing groups
disfavored the reaction with the lower obtained yields.
Within these groups, nitrogen dioxide exhibited the largest
3.2. Reaction Test
+
N
N
N
N
(((((, H2O
N
–
N
+
Ph
Ph
Ph
Ph
Only water as solvent
withoutDorgealniivc esorlveendtsby Publishing Technology to: McMaster University
effect (Table II, entries 2 and 3).
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As an example, the reaction of the phenyl group was
Copyright: American Scientific Publishers
As shown in Table I, the reactions of benzyl azide and
phenylacetylene were studied in the presence of Cu NPs in
water. The cycloaddition reaction of benzyl azide (0.1 mL,
0.80 mmol) and phenylacetylene (0.13 mL, 1.18 mmol)
with water-soluble Cu NPs in H2O (4.5 mL), off the
shelf, afforded 1,4-disubstituted 1,2,3-triazoles as a sin-
gle regioisomer. This CuO nanostructure catalyzed a reac-
tion sequence that regiospecifically united the azides and
terminal acetylenes to give only 1,4-disubstituted 1,2,3-
triazoles. The reaction was carried out at 25–100 ꢀC, using
benzyl azide and phenylacetylene as the benchmark sub-
strates. The best results were obtained when H2O was
used as the solvent, in which 10 min sonication was exe-
cuted with 50% amplitude, at 100 ꢀC. It was observed that
the application of US irradiation significantly increased the
reaction rates and yields (5–10 min) compared with the
traditional stirring (Table I, entries 1 and 5).
When the water-soluble Cu NPs (1 mol%) were used,
1-benzyl-4-phenyl-1H-1,2,3-triazole was obtained in over
99% conversion within 10 min (Table I, entry 4). In gen-
eral, it was found that increasing the reaction temperature
and time were effective means of increasing the conver-
sion (Table I, entries 2 and 3). When the catalyst of the
water-soluble Cu NPs (0.5 mol%) was used, a 38% yield
was achieved under the same conditions (Table I, entry 6).
Remarkably, the water-soluble Cu NPs were separated via
centrifugation after the reaction, and were reused three
directly linked to the reactive azide, and the p-methoxy
group gave the expected 1-(4-methoxyphenyl)-4-phenyl-
1H-1,2,3-triazole as a single regioisomer with 100%
conversion (Table II, entry 4). In some cases, the steric
and electronic effects greatly affected the reactivity. The
sterically bulky and electron-withdrawing groups disfa-
vored the reaction, with lower obtained yields. When the
phenyl group was directly linked to the reactive azide,
its analogue with a Ph-O, p-Cl group gave the expected
1-(4-phenoxyphenyl)-4-phenyl-1H-1,2,3-triazole,
1,4-
diphenyl-1H-1,2,3-triazole, and 1-(4-Chlorophenyl)-
4-phenyl-1H-1,2,3-triazole as lower obtained yields
(Table II, entries 6 and 7). In a second series of
experiments, various terminal alkynes bearing different
groups were submitted to benzyl azide. Hydroxy-
substituted alkynes such as propargyl alcohol, and 2-
methyl-3-butyn-2-ol, also gave the expected adducts of
(1-benzyltriazol-4-yl)methanol, and 2-(1-benzyl-1H-1,2,3-
triazol-4-yl)propan-2-ol as single regioisomers in good
to high yields (Table II, entries 9 and 10). The reaction
with aliphatic alkynes, such as 1-octyne, gave high yields
(Table II, entry 11). Acetylenes conjugated with an ester
group, such as phenyl propargyl ether, reacted without
incident with the benzyl azide. The corresponding tria-
zoles 1-benzyl-4-(phenoxymethyl)-1H-1,2,3-triazole were
obtained in high yields (Table II, entry 12).
J. Nanosci. Nanotechnol. 11, 6162–6166, 2011
6165

Double-Hydrophilic Block Copolymer Nanoreactor for the Synthesis of Copper Nanoparticles
Kim et al.
Table II. US-promoted [3 + 2] cycloaddition of various azides with
synthesizing regioselective triazoles was developed from a
variety of azides with terminal alkynes, in which the water-
soluble Cu NPs were utilized, under 10 min sonication.
Water-soluble Cu NPs were shown to be useful reagents
for a wide variety of organic transformations.
terminal alkynes in the presence of water-soluble Cu NPsa.
Entry
Azide
Alkyne
Product
Conv. (%)
N
N
N
N3
Ph
Ph
Ph
1
100
N
N
N
Acknowledgment: This research was supported by
Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the
Ministry of Education, Science and Technology (2009-
0070926, 2010-0002834), and by the WCU (World-Class
University) Program, through Korea Science and Engineer-
ing Foundation, funded by the Ministry of Education, Sci-
ence, and Technology (R31-2008-000-20012-0). Specially,
Kang Hyun Park thank to the T. J. Park Junior Faculty
Fellowship.
N3
Ph
Ph
2
100
F
F
N
N
N
N3
Ph
3
4
87
O2N
O2N
N
N
N
N
Ph
Ph
100
N3
MeO
MeO
Ph
Ph
N
N
N3
5
6
7
8
89
68
26
59
References and Notes
MeO
MeO
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N
N
N
Ph
Ph
Ph
N3
N3
PhO
Cl
PhO
Cl
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Ph
N
N
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N
N
N3
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N
CopyriOgHht: American Scientific Publishers
OH
9
>99
100
56
N
N
N
N
N3
N3
N3
N
OH
10
11
12
OH
N
N
Hex
Hex
N
N
O
O
Ph
>99
13. P. Cintas, A. Barge, S. Tagliapietra, L. Boffa, and G. Cravotto,
Nature Protocols 5, 607 (2010).
Ph
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4. CONCLUSIONS
Stable, water-soluble copper nanoparticles (Cu NPs) were
synthesized using a double-hydrophilic block copolymer
as a template. The prepared Cu NPs were effective as cat-
alysts for the [3 + 2] cycloaddition of azides with termi-
nal alkynes to provide products in good yields and with
high regioselectivity. An environment-friendly process of
18. Characterization of new compound (Table II, entry 5): 1H-NMR
(CDCl3, 300 MHz): ꢃ = 8ꢄ18 (s, 1H), 7.91 (d, J = 7ꢄ2 Hz, 2H),
7ꢄ49 − 7ꢄ30 (m, 6H), 6.98 (d, J = 8ꢄ4 Hz, 1H), 3.89 (s, 3H). 13C-
NMR (CDCl3, 75 MHz): ꢃ = 160ꢄ7, 148.4, 138.1, 130.6, 130.3,
128.9, 128.5, 125.9, 117.8, 114.7, 112.4, 106.4, 55.7. HRMS for
C15H13N3O1: cacld. 251.1059; found 223.1007 (C15H13N1O1ꢂ.
Received: 14 July 2010. Accepted: 14 January 2011.
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J. Nanosci. Nanotechnol. 11, 6162–6166, 2011