A facile preparative method for aggregation-free gold nanoparticles using poly (styrene-block-cysteine)

Article abstract of DOI:10.1002/anie.200701060

(Chemical Equation Presented) In a stable condition: The self-assembly and induced assembly of a new class of block copolymers containing polystyrene (PS) and polycysteine (PCys) is investigated (see picture). The efficiency of these "molecular chimeras"

Full text of DOI:10.1002/anie.200701060

Communications
DOI: 10.1002/anie.200701060
Gold Nanoparticles
A Facile Preparative Method for Aggregation-Free Gold Nanoparticles
Using Poly(styrene-block-cysteine)**
Sinoj Abraham, Il Kim,* and Carl A. Batt*
Recently, the synthesis and stabilization of nanoparticles with
potentially valuable properties by using various preparative
methods have been reported.[1] Stabilization of quantum dots
requires a dynamic interdisciplinary effort spanning the fields
of chemistry, materials science, biology, and electronic
engineering.[2] The possible application of gold nanoparticles
(AuNPs) in a diverse arena is the continued research
promising class of block copolymers widely termed “molec-
ular chimeras”.[16–19] Our interest in block copolymers and
their self-assembly, ability to combine various synthesis
methods, and wide application areas led us to consider
“molecular chimeras” as possible surface stabilizers of
AuNPs. Polymers with thiol functionality received great
interest as a result of the special properties imparted by
thiol groups. For instance, they can self-assemble into mono-
layers on gold substrates;[20] however, the synthesis of
polymers bearing thiol groups involves tedious chemical
routes.[21] Herein, we report the synthesis, self-assembly, and
induced assembly of a new class of amphiphilic poly(styrene-
block-cysteine) copolymers and demonstrate their ability to
conjugate with AuNPs, thus inhibiting particle aggregation.
The polystyrene was initially prepared by atom-transfer
radical polymerization (ATRP)[15] and was further modified
at the chain ends for the synthesis of polypeptide blocks.[18] S-
Carbobenzoxy-N-carboxy-l-cysteine anhydride, the mono-
mer for the extended ring-opening polymerization (ROP),
was freshly synthesized with a reversible masking at the thiol
motivation for enhancing their functional properties.[3,4]
A
wide range of encapsulation protocols have so far been
utilized for preparing aggregation-free AuNPs.[5] However,
the major challenge that still exists is the reliability and
robustness of these methods to control the material proper-
ties, especially the quantum confinement effects in their
dispersed state. Initially, small molecules, such as alkanethiols,
were used as surface stabilizers and later interest shifted
towards polymers, for example, thiol-derivatized oligosac-
charides,[6] DNA,[7] or dendrimers.[8] The utilization of block-
copolymer micelles for the synthesis and stabilization of
AuNPs was broached in the last decade with amphiphilic
poly(styrene-block-ethylene oxide)[9] and poly(styrene-block-
vinyl pyridine).[10] Later, many similar studies were reported
with different polymer systems,[11] especially poly(styrene-
block-acrylic acid).[12]
1
moieties. The H NMR spectrum of the poly(styrene-block-
cysteine) (PS100PCys20) conjugate is depicted in Figure 1
The studies on amphiphilic block copolymers as surface
stabilizers still continue as a prominent area of research
because of their readily tunable properties, such as critical
micelle concentration and kinetic stability.[13] Recent advan-
ces in controlled radical polymerization (CRP) furthered the
scope of block copolymers in diverse fields of research.[14,15]
Synthetic polymers conjugated with polypeptides lead to a
[*] S. Abraham, Prof. I. Kim
Department of Polymer Science and Engineering
Pusan National University
Busan 609735 (Korea)
Fax: (+82)51-513-7720
E-mail: ilkim@pusan.ac.kr
Homepage: http://mslab.polymer.pusan.ac.kr
Prof. C. A. Batt
Department of Food Science
Cornell University
Ithaca, NY 14853 (USA)
E-mail: cab10@cornell.edu
Figure 1. 1H NMR spectrum of poly(styrene-block-cysteine) in CDCl3.
[**] This work was supported by the Korea Research Foundation (KRF-
D00422) and the National Core Research Center Program from
MOST/KOSEF (R15–2006-022-01001-0). I.K. thanks the LG Yonam
Foundation for sabbatical support and the Center for Ultramicro-
chemical Process Systems. C.A.B. thanks the Ludwig Institute for
Cancer Research for its support.
(details in the Supporting Information). Initially, the self-
assembly of this amphiphilic block copolymer was studied by
preparing a stable micelle solution in DMF/H2O. An illus-
tration of the molecular self-assembly process is shown in
Figure 2.
A solution of the polymer in DMF (20 mgmLÀ1) was
mixed with the same amount of degassed water and DMF and
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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which implies micelle formation with polystyrene as the core
and polycysteine as the corona. Spherical micelles with
diameters of 150 to 300 nm are clearly visible in the TEM
images (Figure 3a).
However, the large size of these micelles is not in
proportion to the molecular weight of the block copolymer.[21]
The formation of multicompartment micelles could be a
reason for this observation. The thiol moieties of the hydro-
philic corona can facilitate intermicelle conjugation, thus
leading to the organization of independent micelles to form
large multicompartment micelles. Furthermore, large aggre-
gations of micelles were also observed in the TEM images, but
these aggregates were unlikely to correlate with the actual
state in solution, as the samples on TEM grids were allowed to
dry. On increasing the concentration of the block copolymer
in the stock solution to 40 mgmLÀ1, rodlike aggregates of the
micelles were formed (Figure 3b). Thus, the individual
micelles can reduce their contact area with the solvent
mixture and can exist in the lowest-energy state. Aggregates
of four to six micelles are also formed in solution, as observed
in the light-scattering spectra at low temperature (Figure 4).
Figure 2. Steps in the self-assembly of poly(styrene-block-cysteine)
(PS100PCys20) conjugated block copolymer.
heated at 1108C for 2 h and then cooled slowly at a rate of
1 KminÀ1 in an automated thermal cycler. This process
facilitates the controlled precipitation of polymer, thus
inducing molecular self-assembly and resulting in a stable
micelle solution. The micelle solution was then dialyzed
against deionized water and trace amounts of precipitated
polymer were removed by centrifugation. To understand the
mode of micellization, an NMR-scale reaction was carried out
and real-time spectra were recorded at various temperatures
(Figure 3). The resonance intensity of aromatic protons of
polystyrene decreases with decreasing solution temperature,
Figure 4. Hydrodynamic diameter of micelles by light-scattering spec-
troscopy. Number/% is the percentage number of molecules or
micelles present in the micellar solution at the particular temperature.
At any temperature, there are molecules, micelles, and micellar
aggregates in the solution.
This micellar solution of PS100PCys20 was stirred with a 20-
nm gold colloidal solution (7 ” 1011 particlesmLÀ1) for about
2 h. The solution was then centrifuged at 10000 rpm for
15 min to remove excess reactants, free micelles, and free
AuNPs. The TEM image of these hybrid micelles distinctly
shows conjugated AuNPs on the surface of the micelles (see
the Supporting Information). The thiol groups at the surface
of micelles render this S–Au conjugation, which also indicates
the orientation of the molecular self-assembly with the
polystyrene core and polycysteine corona.
Figure 3. Top: NMR spectra of the block copolymer solution moni-
tored at various temperatures, which indicate the mode of micelliza-
tion. Peaks a–c correspond to solvents and that marked ( ) corre-
*
sponds to the aromatic protons of polystyrene. Bottom: TEM images
of a) spherical micelles and b) rodlike micelles.
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Efforts have been made to encapsulate individual AuNPs
in the core of micelles by conjugating them with polymer
molecules prior to micellization.[9–12] The thiol moieties, which
are the main driving force of polymer self-assembly, as well as
gold conjugation make this a unique system. The high affinity
of the thiol groups for gold induces the organization of
polymer molecules in the reverse fashion of their self-
assembly described above. Encapsulation using this polymer-
ic system does not require any initial surface modification of
AuNPs with common aliphatic ligands or thiolipids. The
procedure is similar to the micellization protocol described
above with additionally the presence of gold colloid solution
(20 nm). A plausible scheme of AuNP functionalization is
depicted in Figure 5. The strong interaction of thiol groups
Figure 6. TEM images of block copolymer–AuNP conjugate.
the refractive index of the surrounding medium.[12c,23] More-
over, the UV absorption of these nanoparticle conjugates
after storage over a prolonged time resembled that of the
newly synthesized material (see the Supporting Information).
From all these experimental observations, it can be concluded
that the AuNPs are resistant to aggregation upon conjugation
with this class of poly(styrene-block-cysteine) polymers.
In summary, a new class of “molecular chimeras” was
successfully utilized to stabilize nanoparticles. The self-
assembly of one such block copolymer was studied in the
presence and absence of AuNPs. The polymer assembly was
reversed in the presence of AuNPs. Only a few polymer
systems have been developed for the stabilization of nano-
particles to date, and the successful use of this new class of
block copolymers as surface stabilizers with a new preparative
protocol should find wide application.
Figure 5. Encapsulation by the block copolymer coats the AuNPs with
a hydrophobic outer shell.
with the AuNPs overcomes the tendency of polymer mole-
cules to self-assemble and results in gold–polymer conjugates.
This polymer–AuNP conjugate was subjected to TEM
analysis. The polymer shells appear white in contrast to the
background, where the residue solution dried during the
preparation of TEM samples. Figure 6 shows a representative
TEM image. It is clear that AuNPs (20 nm) are covered by a
polymer shell with a total diameter of about 100 nm and
uniform thickness. Most hybrid conjugates appear to be well-
1
Experimental Section
defined and contain singly occupied AuNPs. The H NMR
Encapsulation of AuNPs: A solution of AuNPs (20 mL; 20 nm in
diameter, 7.0 ” 1011 particlesmLÀ1) was concentrated to 250 mL by
centrifugation at 14000 rpm for 30 min. The concentrated solution
was diluted with DMF (750 mL), and this solution (50 mL) was mixed
with polymer stock solution (5 mL), DMF (25 mL), and air-free water
(25 mL). Up to 96 batches at a time could be treated in the thermal
cycler (Bio-Rad). The mixture was heated to 1108C for 2 h, and then
cooled at a rate of 18CminÀ1. To remove excess reactants and free
micelles, a 5-ml batch of this solution was diluted with water (2 mL)
and centrifuged at 10000 rpm for 30 min. This step was repeated twice
more and a concentrated polymer–gold conjugate solution was
obtained.
Preparation of TEM samples: TEM grids were treated with an
oxygen plasma (from a Harrick plasma cleaner/sterilizer) for 20 s to
render their surface hydrophilic. (NH4)2MoO4 (2 mL; 2% aqueous
solution by weight), water (5 mL), and micelles or polymer–gold
conjugate (3 mL) were mixed on the surface of a plastic Petri dish to
form a small bead. A TEM grid was then floated on top of the bead
with the hydrophilic face contacting the solution. The TEM grid was
carefully removed with a pair of tweezers, wicked with a filter paper
to remove excess liquid, and then dried in air for 1 min.
spectrum of the sample is consistent with polymer adsorbed
on the AuNPs. Compared to the free polymer molecules, the
resonances are significantly broadened. The absence of sharp
resonances showed that the sample contained negligible
amounts of free thiols (see the Supporting Information).
These AuNP–polymer conjugates are stable without any
additional cross-linking, but there are free amino groups
available for cross-linking by simple carbodiimide chemistry.
Repeated centrifugation did not result in deformation or
fusion of the spherical conjugates and no ellipsoidal defor-
mations were observed in the TEM images. Possible in situ
reinforcement of the polymer shell can also take place by
inter/intramolecular coupling between the thiol groups. The
UV/Vis absorption band corresponding to the surface plas-
mon resonance (SPR) energy of this polymer–AuNP con-
jugate is red-shifted (534 nm) compared to that of free AuNPs
(522 nm) of the same diameter.[22] This observation is
consistent with the decrease in SPR energy with increase in
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Received: March 10, 2007
Revised: May 2, 2007
Published online: June 28, 2007
Keywords: block copolymers · gold · micelles · nanoparticles ·
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