Angewandte
Chemie
DOI: 10.1002/anie.201209910
Nanoparticle Assembly
Peptide-Directed Synthesis and Assembly of Hollow Spherical CoPt
Nanoparticle Superstructures**
Chengyi Song, Yang Wang, and Nathaniel L. Rosi*
Controlling the directed assembly of nanoparticles into well-
defined nanoparticle superstructures is a significant chal-
lenge,[1] and our goal is to develop a general methodology to
address this issue. The success and broad applicability of
a particular nanoparticle-assembly methodology should be
assessed according to the following important criteria:
1) diverse structural scope, 2) ability to tune and tailor
superstructure metrics (e.g., nanoparticle size, nanoparticle
shape, interparticle distances, and superstructure diameter),
and 3) diverse compositional scope. Only through rational
control of structure, metrics, and composition can diverse
collections of nanoparticle superstructures with highly spe-
cific and tailorable properties ultimately be designed and
prepared.
We recently introduced and developed a peptide-based
methodology for directing the synthesis and assembly of gold
nanoparticles.[2] This methodology relies on carefully
designed peptide-conjugate molecules that control the syn-
thesis of individual nanoparticles and direct their assembly
into complex nanoparticle superstructures. We successfully
demonstrated that this methodology addresses criteria 1 and 2
listed above. Specifically, we used this methodology to
prepare a diverse set of complex gold nanoparticle super-
structures, including double helices,[2] linear belts,[3] and
hollow spherical structures,[4] and to tune the metrics of the
superstructures.[5] In this contribution, we begin to address
criterion 3 listed above.
selected, naturally or unnaturally, to adhere to specific
inorganic surfaces.[6]
Thus far, we have only utilized a gold-binding peptide in
this methodology.[7] In principle, however, we could choose
any inorganic-binding peptide, which would allow us to target
and prepare nanoparticle superstructures of variable compo-
sition.
Here, we introduce, describe, and demonstrate how our
peptide-based methodology can be adapted to target and
prepare CoPt nanoparticle superstructures. It is established
that CoPt nanoparticles are potentially useful in nanomedi-
cine as magnetic resonance imaging (MRI) contrast agents[8]
and as electrocatalysts.[9] For these applications, individual
CoPt nanoparticles are typically employed. We sought to
assemble CoPt nanoparticles into hollow spherical sub-
100 nm superstructures, reasoning that they may ultimately
be useful as multifunctional bionanomaterials capable of
serving both as MRI imaging[8] and delivery[10] agents (e.g.,
drug, biomolecule, labeling, etc.) or as high surface area
electrocatalysts.[9] As a first step in the latter direction, we
show herein that the assembled CoPt superstructures can
serve as electrocatalysts for the oxidation of methanol.
To prepare hollow spherical CoPt nanoparticle super-
structures, we first selected the Co-binding peptide
HYPTLPLGSSTY (Co1-P10), which was isolated by Naik
et al.[11] and is hereafter referred to as PEPCo. PEPCo was used
previously by Naik et al. to prepare both Co nanoparticles
and CoPt alloy nanoparticles.[11] The reported syntheses are
straightforward and are performed in aqueous media at
a neutral pH value. In order to prepare hollow spherical
superstructures, we decided to utilize the conjugate BP-PEPCo
(C12H9CO-HYPTLPLGSSTY, BP = biphenyl; see the Sup-
porting Information for synthetic details), because our
previous results with biphenyl-based PEPAu conjugates
resulted in hollow spherical gold nanoparticle superstruc-
tures.[12] BP-PEPCo, upon dissolution and subsequent incuba-
tion in HEPES buffer (0.1m, HEPES = 4-(2-hydroxyethyl)-
piperazineethanesulfonic acid), assembles into well-defined
spherical structures ((30.9 Æ 4.5) nm), as evidenced by trans-
mission electron microscopy (TEM) studies (Figure S3 in the
Supporting Information).
Peptide conjugates are the centerpiece molecules in this
methodology. They consist of two components: an inorganic-
binding peptide and an organic moiety tethered to the peptide
terminus. The peptide portion binds to the nanoparticle
surface while the organic moiety influences the assembly of
the peptide and therefore the assembly of the nanoparticles.
Numerous peptides exist which have been evolved and
[*] C. Song, Y. Wang, Prof. N. L. Rosi
Department of Chemistry, University of Pittsburgh
219 Parkman Avenue, Pittsburgh, PA 15260 (USA)
E-mail: nrosi@pitt.edu
[**] The authors are grateful for financial support from the National
Science Foundation (DMR-0954380, NLR) and the Air Force Office
of Scientific Research (FA9550-11-1-0275, NLR). This work was also
partially supported by the US Department of Energy (Grant No.
ER46430). The authors thank the Peterson NFCF and the MEMS
Department for provision of access to TEM. The authors also thank
Tao Li for assisting with XRD measurements and Prof. David
Waldeck for thoughtful discussions and comments on the manu-
script.
Encouraged by these results, we next adapted the
reported synthesis for CoPt nanoparticles by replacing
PEPCo with BP-PEPCo to determine whether BP-PEPCo
could serve the dual purpose of directing both the synthesis
of CoPt nanoparticles and their assembly into spherical
superstructures (Scheme 1). Specifically, a 6.5:1 mixture of
cobalt acetate and BP-PEPCo was incubated for four hours at
room temperature in HEPES buffer to give a colorless
solution. Thereafter, aliquots of sodium borohydride
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2013, 52, 3993 –3995
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3993