Polymer-coated ferromagnetic colloids from well-defined macromolecular surfactants and assembly into nanoparticle chains

Article abstract of DOI:10.1021/ja0609147

A novel synthetic route to polymer-coated ferromagnetic colloids of metallic cobalt has been developed. Well-defined end-functional polystyrenes were synthesized using controlled radical polymerization and used as surfactants in the thermolysis of dicobal

Full text of DOI:10.1021/ja0609147

Published on Web 04/29/2006
Polymer-Coated Ferromagnetic Colloids from Well-Defined Macromolecular
Surfactants and Assembly into Nanoparticle Chains
Bryan D. Korth,† Pei Keng,† Inbo Shim,‡ Steven E. Bowles,† Chuanbing Tang,§ Tomasz Kowalewski,§
Kenneth W. Nebesny,† and Jeffrey Pyun*,†
Department of Chemistry, UniVersity of Arizona, 1306 East UniVersity BouleVard, Tucson, Arizona 85752,
Department of Nano and Electronic Physics, Kookmin UniVersity, Seoul, Korea 136-702, and
Department of Chemistry, Carnegie Mellon UniVersity, 4400 Fifth AVenue, Pittsburgh, PennsylVania 15213
Received February 15, 2006; E-mail: jpyun@email.arizona.edu
The functionalization and organization of magnetic nanoparticles
into complex mesostructured assemblies has generated considerable
interest as a novel approach to materials synthesis.1 Ferromagnetic
colloids are intriguing building blocks for self- or field-induced
assembly into one-dimensional (1-D) nanostructures due to the
dipolar associations between inorganic nanoparticles.2 Our motiva-
tion in this area is to utilize the selective dipolar associations
between ferromagnetic colloids as a novel approach for controlled
nanoparticle assembly.
A number of groups have reported the incorporation of organic
polymers to magnetic colloids using surface-initiated controlled
polymerizations,3 in situ nanoparticle reactions, or block copolymer
templates.4 The preparation of polymer-coated ferromagnetic nano-
particles is more challenging, as high-temperature annealing steps
are often required to convert superparamagnetic colloids into
ferromagnetic phases.5 A few examples of polymer-coated ferro-
magnetic nanoparticles of metallic cobalt (Co)2a,4a or iron6 have
Figure 1. TEM images of ferromagnetic pS-coated cobalt nanoparticles
(a) self-assembled by deposition from toluene dispersions onto carbon-coated
copper grids, (b) cast from toluene dispersion and aligned under a magnetic
field (100 mT), (c) self-assembled single nanoparticle chains, and (d) high-
magnification image visualizing cobalt colloidal core (dark center) and pS
surfactant shell (light halo).
been reported; however, methodologies to synthesize well-defined
nanocomposite colloids of uniform size and tunable magnetic
properties have not been extensively developed.
Herein, we describe the synthesis and characterization of
polymer-coated ferromagnetic nanoparticles that organize into
extended one-dimensional assemblies when cast onto supporting
surfaces. Controlled radical polymerization, specifically, nitroxide-
mediated polymerizations,7 enables the preparation of polymeric
surfactants which were used as shells to coat ferromagnetic cobalt
colloids and ensure uniformity of the particle size. By control of
the macromolecular structure and functionality, we demonstrate for
the first time the use of well-defined end-functional polymers to
prepare polystyrene-coated ferromagnetic cobalt colloids. The
polymer shell imparts long-term colloidal stability to magnetic
dispersions in solution, and in the solid state, forms a glassy coating
that locks in the 1-D structure of assembled nanoparticle chains
through interdigitation of polystyrene outer layers.
Scheme 1. Synthesis of Polystyrenic Surfactants and Cobalt
Nanoparticles
Polystyrene (PS)-coated cobalt nanoparticles were synthesized
by the thermolysis of dicobaltoctacarbonyl (Co2CO8) in the presence
of end-functional polymeric surfactants in refluxing 1,2-dichlo-
robenzene. Two different pS surfactants, 3 and 4 (Mn ) 5000 g/mol;
Mw/Mn ) 1.09), containing either a benzylamine or dioctylphos-
phine oxide8 end group were synthesized to mimic the small
molecule surfactant system developed by Alivisatos et al.,9 using
aliphatic amines and trioctylphosphine oxide (TOPO) (Scheme 1).10
A mixture of amine and phosphine oxide PS surfactants (4:1 wt
ratio) was then used in the thermolysis of Co2CO8 to prepare
polymer-coated cobalt nanoparticles, where the ligating end group
passivated the colloidal surface. The combination of both amine
and phosphine oxide ligands was necessary to yield uniform
ferromagnetic nanoparticles, which is in agreement with similar
studies using small molecule surfactants.5 Furthermore, ferromag-
netic cobalt colloids were obtained for pS surfactant systems despite
variation of the molar mass in the range of 2000-9000 g/mol.
However, end-functional polymers possessing Mn ) 5000 g/mol
afforded the optimal combination of uniform particle size and long-
term colloidal stability.10
These polymer-coated cobalt nanoparticles (pS-Co) were then
characterized using transmission electron microscopy (TEM),
atomic force microscopy (AFM), and magnetic force microscopy
† University of Arizona.
‡ Kookmin University.
§ Carnegie Mellon University.
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J. AM. CHEM. SOC. 2006, 128, 6562-6563
10.1021/ja0609147 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
the magnetic coercivity (Hc ) 340 Oe) was observed by aligning
nanoparticle chains under a weak magnetic field at 300 K due to
the coupling of magnetic dipole moments along the 1-D assembly.10
X-ray diffraction (XRD) of pS-coated magnetic nanoparticles
indicated the formation of the face centered cubic phase of cobalt
and the presence of a thin cobalt oxide (CoO) passivating layer
around the metal core. The polymer-coated colloids were observed
to be stable to further oxidation over a period of several weeks to
months, as confirmed from VSM and XRD.
To illustrate the potential of 1-D nanoparticle chains for bottom-
up assembly, ferromagnetic pS-cobalt colloids were blended with
silica beads (D ) 172 nm) and cast onto TEM grids. The formation
of micron-sized assemblies composed of isolated SiO2 colloids
dispersed in a matrix of pS-cobalt nanoparticle chains was imaged
using TEM (Figure 3). Nanoparticle chains were observed to
organize around larger silica inclusions. While the overall size of
the cobalt-SiO2 binary assembly was not controlled, this morphol-
ogy demonstrates that polymer-coated nanoparticle chains possess
sufficient mechanical integrity to maintain their 1-D structure when
blended and cast onto surfaces.
Figure 2. Topographical and magnetic images (size 5 × 5 µm2) of pS-
cobalt nanoparticles cast onto carbon-coated mica. AFM (a); MFM (b).
In conclusion, the synthesis of ferromagnetic cobalt nanoparticles
using well-defined polymeric surfactants is described. By control
of surfactant structure, ferromagnetic colloids possessing a poly-
meric shell were synthesized. These functional colloids are intrigu-
ing building blocks for bottom-up materials synthesis and novel
mesoscopic assemblies.
Acknowledgment. The University of Arizona (Start up funds,
J.P.) and the Information Storage Industrial Consortium (INSIC)
are gratefully acknowledged for support of this work. David Bentley
and the Arizona Research Laboratories are acknowledged for
assistance with TEM.
Figure 3. TEM image of binary assemblies composed of pS-coated cobalt
nanoparticles and SiO2 beads.
(MFM) to determine particle size and morphology of magnetic
colloids and nanoparticle chains. Low-magnification TEM images
of colloids cast onto surfaces reveal the formation of uniform
colloids organizing into extended nanoparticle chains spanning
several microns in length (Figure 1a). These chains are easily
aligned by deposition of the colloidal dispersion in the presence
of weak magnetic fields (100 mT) (Figure 1b). TEM images of
these 1-D chains at higher magnification clearly demonstrate the
presence of individual cobalt nanoparticles (Dparticle ) 15 ( 1.5
nm) surrounded by a halo of pS (shell thickness ) 2.0 nm, Figure
1c,d). The retention of the polymer coating on the cobalt colloid
was confirmed using X-ray photoelectron spectroscopy, as evi-
denced by the characteristic signature of pS with peaks at 284 and
288 eV.10
Aligned chains of nanoparticles were also clearly evident in
topographical AFM (Figure 2a) and MFM (Figure 2b) images (cf.
Supporting Information). All particles in MFM images appeared
brighter than the nearby substrate surface, and the observed contrast
did not depend on the direction of tip magnetization. Such behavior
can be viewed as an indication that nanoparticle dipole moments
were practically parallel to the surface.10
Supporting Information Available: Detailed experimental pro-
cedures on the preparation of polymers and nanoparticles and additional
TEM, AFM/MFM, XPS, XRD, and VSM data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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