Thermosensitive copolymer networks modify gold nanoparticles for nanocomposite entrapment

Article abstract of DOI:10.1002/chem.200600839

The core-shell gold nanoparticles and copolymer of N-isopropylacrylamide (NIPAM) and N,N′-methylenebisacrylamide (MBAA) hybrids (Au@copolymer) were fabricated through surface-initiated atom-transfer radical polymerization (ATRP) on the surface of gold nanoparticles in 2-propanol/water mixed solvents. The surface of citrate-stabilized gold nanoparticles was first modified by a disulfide initiator for ATRP. The slight crosslinking polymerization between NIPAM and MBAA occurred on the gold surface and resulted in the formation of core-shell Au@copolymer nano-structures that were characterized by TEM, and FTIR and UV-visible spectroscopy. Such synthesized Au@copolymer hybrids possess clearly thermosensitive properties and exhibit "inspire" and "expire" water behavior in response to temperature changes in aqueous solution. Because of this property, we enable to trap and encapsulate smaller nanoparticles by using the free space of the copolymer-network scaffold anchored at the gold surface.

Full text of DOI:10.1002/chem.200600839

DOI: 10.1002/chem.200600839
Thermosensitive Copolymer Networks Modify Gold Nanoparticles for
Nanocomposite Entrapment
Dongxiang Li,[a] Qiang He,[a, b] Yue Cui,[a] Kewei Wang,[a] Xiaoming Zhang,[a] and
Junbai Li*[a]
Abstract: The core-shell gold nanopar-
ticles and copolymer of N-isopropyla-
crylamide (NIPAM) and N,N’-methyl-
initiator for ATRP. The slight cross-
linking polymerization between
NIPAM and MBAA occurred on the
gold surface and resulted in the forma-
tion of core-shell Au@copolymer nano-
structures that were characterized by
TEM, and FTIR and UV-visible spec-
troscopy. Such synthesized Au@copoly-
mer hybrids possess clearly thermosen-
sitive properties and exhibit “inspire”
and “expire” water behavior in re-
sponse to temperature changes in
aqueous solution. Because of this prop-
erty, we enable to trap and encapsulate
smaller nanoparticles by using the free
space of the copolymer-network scaf-
fold anchored at the gold surface.
ACHTREUNG
through surface-initiated atom-transfer
radical polymerization (ATRP) on the
surface of gold nanoparticles in 2-pro-
panol/water mixed solvents. The sur-
face of citrate-stabilized gold nanopar-
ticles was first modified by a disulfide
Keywords: copolymerization · en-
capsulation · gold · nanoparticles ·
thermosensitivity
Introduction
weights and low polydispersity. This living radical polymeri-
zation from the surface of micro-or nanoparticle templates
can yield core-shell structures based on the “graft-from”
strategy.[6] Chains of the well-studied thermosensitive poly-
Much attention has been paid to the modification of gold
nanoparticles due to their potential applications in the fields
of biology and nanotechnology.[1,2] A number of methods
have been developed to decorate gold nanoparticles with
various polymer molecules for novel purposes.[3] The poly-
meric reactions occurring on the surface of gold nanoparti-
cles, such as surface-initiated living cationic polymerization[4]
and ring-opening metathesis polymerization,[5] have attract-
ed intense interest. An initiator with a thiol, sulfur, or disul-
fide group is introduced onto the gold surface through
chemical binding. The growth of the polymer chains then
progresses along the direction of initiators immobilized on
the gold surface to form the core-shell composite. The
atom-transfer radical polymerization (ATRP) is becoming a
popular method that enables good control of molecular
mer poly(N-isopropylacrylamide) (PNIPAM) undergo
a
thermally induced conformational change from the swelled,
hydrophilic state to the shrunken, hydrophobic state at
below and above the lower critical solution temperature
(LCST) of around 328C in the aqueous phase.[7] This results
in the conversion of the chemical and physical environments
among the macromolecule chains and transforms their prop-
erties.[8] These environmentally responsive polymer–nano-
particle hybrids facilitate a variety of novel technological ap-
plications, such as drug delivery, switchable microfilters,
chemical separation, and catalysis.[9] Tenhu and co-workers
have shown that the reversible-addition-fragmentation
chain-transfer polymerization can produce homogeneous
polymer layers on gold nanoparticle surfaces. Such
PNIPAM–gold nanocomposites display a low solubility in
water because of the low degree of grafting of PNIPAM li-
gands.[10] Previously, we reported that the thermosensitive
copolymer nanotubes of N-isopropylacrylamide (NIPAM)
and N,N’-methylenebisacrylamide (MBAA) by ATRP can
be fabricated within a porous membrane.[11] The pore size of
the copolymer network of PNIPAM-co-MBAA can be con-
trolled by the feed ratio of monomers in polymerization.
Herein, we report the fabrication of thermosensitive copoly-
[a] Dr. D. Li, Dr. Q. He, Dr. Y. Cui, K. Wang, X. Zhang, Prof. Dr. J. Li
Beijing National Laboratory for Molecular Sciences (BNLMS)
International Joint Lab
CAS Key Lab of Colloid and Interface Science
Institute of Chemistry, Chinese Academy of Sciences
Zhong Guan Cun, Beijing 100080 (P.R. China)
Fax : (+86)10-8261-2629
E-mail: jbli@iccas.ac.cn
[b] Dr. Q. He
Max Planck Institute of Colloids and Interfaces
14476 Golm/Potsdam (Germany)
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FULL PAPER
mer–gold nanoparticle hybrids and demonstrate their nano-
composite encapsulation by the copolymer network anch-
ored on the gold surface. The slightly cross-linked copoly-
mer chains create a porous nanostructure that is responsive
to temperature change. Therefore, such assembled gold
nanoparticle–copolymer hybrids (Au@copolymer) can be
used to trap and encapsulate other nanoparticles, biomole-
cules, dyes, or drugs by a temperature-introduced “breath-
ing” process. The whole experimental procedure of Au@co-
polymer fabrication is illustrated in Scheme 1. The gold
O is 11:1.09:2.16:0.85, which is consistent with the theoreti-
cal value (11:1:2:1) of the pure disulfide initiator. Figure 1C
shows the binding-energy increase of Au 4f within a range
of 1.3 eV (82.7 to 84.0 eV for Au 4f7/2 and 86.3 to 87.6 eV
for Au 4f5/2) after (a) and before (b) gold nanoparticle modi-
fication by disulfide initiator. This proves the formation of
[13]
À
Au S bonds on the gold surface. The FTIR spectrum of
the Au@initiator is similar to that of pure disulfide initiator,
as shown in Figure 2. The characteristic absorbance at 2922,
2850 cmÀ1 (CH2 stretching) and 1735 cmÀ1 (ester carbonyl
stretching) further verifies the
presence of disulfide initiator
molecules.
Next, the surface-initiated
ATRP of NIPAM and MBAA
monomers with the samples de-
scribed above was carried out
in protic solvents of 2-propanol
and water at room temperature.
An optimized concentration of
NIPAM monomer was 0.4m
and increasing feed ratios of
MBAA to NIPAM of 0, 0.5, 1,
and 2% were used to change
the cross-linking ratio for the
Scheme 1. Au@copolymer fabrication.
comparative studies. The Au@
copolymer hybrids obtained
were tested in both aqueous
nanoparticles were modified by disulfide initiator (Au@ini-
tiator) and the ATRP reactions of acrylamide monomers
were performed in 2-propanol/water and catalyzed by
and organic solvents and the results show that they have
good solubility in both cases. The FTIR spectra of the four
samples with different cross-linking ratios are similar, as
shown in Figure 2. The main characteristic peak assignments
N,N,N’,N’,N’’-pentamethyldiethylenetriamine
(PMDETA)
and a CuIBr catalytic system. The thickness of the polymer
shell can be controlled by the concentration of monomers
and by the reaction time. The feed ratio of the cross-linker
MBAA to NIPAM can influence the network space of the
copolymer shell.
are at 3297 cmÀ1 (secondary amide N H stretching),
À
2974 cmÀ1 (CH3 asymmetric stretching), 1645, and 1552 cmÀ1
(secondary amide C=O stretching),[11] indicating the main
contribution of PNIPAM to Au@copolymer hybrids. Nota-
bly, the contribution from the cross-linker MBAA in the
FTIR spectra may be obscured by that of PNIPAM, as the
former is present in a low amount and has a chemical struc-
ture similar to NIPAM. The TEM images in Figure 3A show
the morphology of linear PNIPAM-modified gold nanoparti-
cles. The core-shell nanoparticles are well dispersed and the
polymer shell is clearly visible. With the addition of cross-
linker (1% MBAA), the contribution of copolymer shell be-
comes more obvious, as shown in Figure 3B. The “soft” co-
polymer layer surrounding the gold nanoparticle (dark core)
(Figure 3B, inset), takes up more space. The thickness of the
copolymer layer is estimated to be about 46 nm, and that of
the linear PNIPAM layer is around 36 nm.
Results and discussion
Gold nanoparticles with diameters of 18 nm were synthe-
sized by using the conventional citrate-reduction method.[12]
The immobilization of disulfide initiator on the gold surface
was achieved through ligand exchange between disulfide
and citrate. The Au@initiator nanoparticles obtained are
more hydrophobic due to the alkyl chain of the disulfide ini-
tiator and can be easily dispersed in THF or DMF without
aggregation, as shown in Figure 1A. The elemental analysis
of as-prepared samples determined by X-ray photoelectron
Figure 4A shows the thermosensitive characterization of
the assembled Au@copolymer hybrids with different cross-
linking ratios. The surface plasmon resonance (SPR) peaks
from the UV-visible spectra of Au@copolymer hybrids
appear at 533 nm in the presence and absence of cross-
linker at room temperature. However, as the temperature
increases, the SPR peaks undergo a red shift and an abrupt
rise occurs at 32 and 348C, corresponding to the LCST value
spectroscopy (XPS) indicates the contributions of
O
(13.43%), C (58.59%), S (1.65%), Au (26.34%), and a
weak signal of Br. Figure 1B shows the detailed binding-
energy analysis of C 1s. The four peaks at 284.8, 285.5,
286.7, and 289.6 eV represent the four different carbon
atoms marked 1–4, respectively. The peak-fitting result re-
À
À
À
À
À
veals that the atom ratio of C H/C C:C O:C S/C Br:C=
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J. Li et al.
Figure 2. FTIR spectra of a) pure disulfide initiator, b) Au@initiator, and
c–f) Au@copolymer hybrids with 0, 0.5, 1, and 2% MBAA, respectively.
Figure 3. TEM images of Au@copolymer hybrids with A) 0% MBAA
and B) 1% MBAA dispersed in water. Scale bars: 100 nm. Insets: images
at a higher magnification, scale bars: 20 nm.
Figure 1. A) TEM image of Au@initiator dispersed in THF. Scale bar:
50 nm. B) C 1s spectra from XPS analysis of Au@initiator. The deconvo-
À
À
À
À
À
lution of C 1s: 1 for C C and C H, 2 for C O, 3 for C S and C Br, 4
for C=O. C) Au 4f spectra from XPS analysis after (a) and before (b)
modification by disulfide initiator.
cates that the suspension is transparent at 258C (transmit-
tance of 86%) and opaque at 388C (transmittance of about
74%). The zigzag curve further demonstrates the reversible
thermosensitive response of the Au@copolymer hybrids to
temperature changes. Notably, there is no obvious precipita-
tion observed at 388C. This is because the size of the ther-
mosensitive Au@copolymer hybrids is still on the nanoscale
(about 200–300 nm) in aqueous solution, and the tempera-
ture change will only alter the thickness of the polymer-coat
layer. Thus, no precipitation occurs at high temperature.
The change in size of the Au@copolymer hybrids at differ-
ent temperatures is proved by the dynamic light-scattering
measurements (DLS) obtained at below and above the
LCST. The results suggest that the hydrodynamic radius of
the Au@copolymer hybrids is about 160 nm at 258C and
of the samples with different feed ratios of cross-linker. A
proportion of 0.5% MBAA does not affect the LCST value,
however, as the feed ratio increases to 1 or 2%, the LCST
value increases to 348C and, thus, the effect of the cross-
linker upon the thermosensitive curves become significant.
Moreover, as a visible thermoresponse, the optical color of
Au@copolymer hybrids in aqueous suspension also dramati-
cally changes from transparent red to opaque pink as the
temperature increases from 25 to 388C, as shown in Fig-
ure 4B. Figure 4C shows the transmittance of the Au@co-
ACHTREUNG
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Thermosensitive Copolymer Networks
FULL PAPER
Figure 5. TEM image of Au@copolymer hybrids with 1% MBAA encap-
sulating 3.5-nm gold nanoparticles. Scale bar: 20 nm.
less aggregated. Thus, this procedure can be generalized for
the encapsulation of other nanoparticles or larger biomole-
cules.
The thermosensitive mechanism is commonly interpreted
as a transformation in the state of the copolymer chains
with “expire” and “inspire” water behavior, involving two
formation processes of intermolecular and intramolecular
hydrogen bonds.[8,11] Below the LCST, the formation of in-
termolecular hydrogen bonds promotes absorption of a
quantity of water, causing the polymer chains to swell; the
so-called “inspire” water process. Above the LCST, the for-
mation of intramolecular hydrogen bonds will exclude the
polymer-bonded water, causing the polymer chains to shrink
to the gold cores, forcing the water molecules out of the
Figure 4. A) Shift in peak position of surface plasmon resonance for Au@
copolymer hybrids with a) 0% MBAA, b) 0.5% MBAA, c) 1% MBAA,
and d) 2% MBAA. B) Photos of Au@copolymer hybrids with 1%
MBAA at 25 and 388C. C) Reversible transmittance curve of Au@co-
A
112 nm at 388C. The diameter of these hybrids is clearly
large below the LCST and becomes smaller above the
LCST. This result also directly reflects the thermosensitive
property of the Au@copolymer hybrids. Notably, the size of
Au@copolymer hybrids estimated by TEM is obtained by
using dried samples. Considering the significant contribution
of solvent, the dimensions of the Au@copolymer hybrids ob-
tained in aqueous solution will be larger.
Such assembled Au@copolymer hybrids can be considered
to encapsulate small molecules or nanoparticles by virtue of
the copolymer-network scaffold. For this purpose, we syn-
thesized 3.5-nm gold nanoparti-
polymer space; the “expire” water process. The alternative
G
“inspiration” and “expiration” of water imitates the “breath-
ing” process, as illustrated in Scheme 2. Thus, the breathing
process of the polymer shell as the temperature changes
promotes the flow of water molecules into and out of the
polymer network. This so-formed water flow takes gold
nanoparticles into the polymer network, accompanied by
the effect of diffusion.
cles to explore the trapping ca-
pability of the core-shell nano-
structure. We added a dilute so-
lution of the 3.5-nm gold nano-
particles into the suspension of
Au@copolymer hybrids (with
1% MBAA) under stirring and
incubated the mixture for
30 min in a still state at 258C
and then at 388C. After repeat-
ing this process three times, we
Scheme 2. The “breathing” mechanism and encapsulation process.
obtained the final samples after
washing by centrifugation.
Figure 5 shows the TEM image
Conclusions
of gold nanoparticles encapsulated by the Au@copolymer
hybrids. The dark dots located in the “shell” structures indi-
cate the existence of 3.5-nm gold nanoparticles. By compar-
ing this with the TEM image in Figure 3B, we can see that
the gold nanoparticles have been trapped within the poly-
mer network and the encapsulated gold nanoparticles are
We have demonstrated that the disulfide initiator can be
successfully immobilized onto the surface of gold nanoparti-
cles to initiate the ATRP of acrylamide monomers under
ambient conditions to construct Au@copolymer hybrids.
These Au@copolymer hybrids have a distinct core-shell
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J. Li et al.
structure and good dispersity in both the aqueous and or-
ganic phases. The copolymer “shell” has a network scaffold
that is clearly thermosensitive and that can trap nanoparti-
cles by using the “breathing” mechanism. Such assembled
Au@copolymer hybrids provide a means to potential uses in
delivery biomolecules, catalysts, and drugs.
Acknowledgements
We acknowledge the financial supports of this research by the National
Nature Science Foundation of China (No. 20404015 and 90206035) as
well as the collaborated project of the German Max-Plank Society.
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Experimental Section
Materials: 11-Mercapto-1-undecanol, 2-bromo-2-methylpropionyl bro-
mide, N-isopropylacrylamide (NIPAM), and N,N,N’,N’,N’’-pentamethyl-
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G
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repeating the cycles of centrifugation, 2-propaol/water wash, and redis-
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Thermosensitive Copolymer Networks
FULL PAPER
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Received: July 27, 2006
Published online: December 11, 2006
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