Preparation of 1-D nanoparticle superstructures with tailorable thicknesses using gold-binding peptide conjugates

Article abstract of DOI:10.1039/c0cc02257h

We describe the preparation of new 1-D gold nanoparticle superstructures with tailorable thicknesses formed using a self-assembled gold-binding peptide conjugate template and examine how the synthesis and assembly mechanism impacts the organization of the

Full text of DOI:10.1039/c0cc02257h

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Preparation of 1-D nanoparticle superstructures with tailorable
thicknesses using gold-binding peptide conjugateswz
Leekyoung Hwang, Chun-Long Chen and Nathaniel L. Rosi*
Received 30th June 2010, Accepted 28th July 2010
DOI: 10.1039/c0cc02257h
We describe the preparation of new 1-D gold nanoparticle
superstructures with tailorable thicknesses formed using a
self-assembled gold-binding peptide conjugate template and
examine how the synthesis and assembly mechanism impacts
the organization of the superstructures.
nanoparticle superstructures were produced upon addition of
a gold salt to a solution of C12-PEPAu in HEPES buffer
(HEPES 4-(2-hydroxyethyl)-piperazineethanesulfonic
acid). HEPES serves as the principal reducing agent for
=
4a,5,6
forming the gold nanoparticles.
We found that addition
of the gold salt significantly promotes C -PEP self-assembly
Au
12
Ensembles of inorganic nanoparticles exhibit unique assembly-
dependent physical properties which depend largely on the
into nanofibers. We also discovered that filtering the
12-PEPAu/HEPES solution to remove any assembled structures
prior to adding the gold salt was important in order to
C
1
local order of the nanoparticles. To control these properties,
4a
methods must be developed to precisely control the assembly
produce gold nanoparticle double helices as product. This
indicates that dispersed, non-assembled peptide conjugates are
required at the initial stages of reaction. Collectively, these
observations suggested that nanofiber self-assembly and nano-
particle synthesis are coupled into a simultaneous process. We
note that this one-step process is markedly different from
typical peptide-based nanoparticle assembly methods which
consist of two or more independent preparative steps, including
(i) peptide self-assembly into a template and (ii) either nucleation
of nanoparticles or assembly of pre-synthesized nanoparticles
1
of nanoparticles. Short oligopeptides have emerged as
a
2
promising molecules for directing nanoparticle assembly.
However, many peptide-based assembly methods yield nano-
2a
particle superstructures with poor local nanoparticle order.
We recently introduced methodology that utilizes a new
2a,3
class of peptide
conjugate molecules designed for directing
both the synthesis and assembly of nanoparticles to yield
nanoparticle superstructures with unprecedented structural
4
complexity and excellent local nanoparticle order. We
5
2
specifically showed that C12-PEPAu, a gold-binding peptide
onto the template. We hypothesize that a simultaneous
(
PEPAu = AYSSGAPPMPPF) modified at its N-terminus
synthesis and self-assembly process is critical for the formation
of well-ordered nanoparticle superstructures.
with an aliphatic 12-carbon chain, could be used to expeditiously
prepare 1-D double-helical nanoparticle assemblies in a single
To test this hypothesis, we aimed to prepare a peptide
conjugate that, unlike C12-PEPAu, self-assembles rapidly in
the absence of gold salt; we expected that such a peptide
conjugate would direct nanoparticle assembly via a different
4a
preparative step. Further, we demonstrated that various
synthetic parameters could be systematically modified to carefully
4b
tailor the structure and metrics of these assemblies.
This unique one-step synthetic approach and the integrity
and structural complexity of the nanoparticle superstructures
it produces prompted us to explore how structural modifications
to the peptide conjugate molecule impact the synthesis and
assembly process. We reasoned that these studies would
provide insight into the mechanism of nanoparticle assembly
and illuminate specific factors which distinguish this methodology
from other peptide-based nanostructure assembly methods. In
this communication, we specifically investigate both how the
identity of the organic moiety appended to the PEPAu terminus
and how amino acid additions to the PEPAu sequence impact
the formation of nanoparticle superstructures.
mechanism than C12-PEPAu. We first prepared C12H CO-PEPAu,
9
hereafter referred to as BP-PEPAu (BP = biphenyl), reasoning
that the BP moiety would promote p–p interactions and
facilitate rapid self-assembly. BP-PEPAu was synthesized by
coupling succinimide-activated 4-phenylbenzoic acid to the
N-terminus of PEPAu (see Supporting Information for
details). We studied the self-assembly of BP-PEPAu in HEPES
buffer (pH = 7.3). However, neither transmission electron
microscopy (TEM) nor tapping-mode atomic force microscopy
(AFM) indicated formation of self-assembled BP-PEPAu
4a
structures. Our previous studies of C12-PEPAu self-assembly
suggested that the amino acids at the N-terminus of PEPAu, in
particular AYSS, played a key role in directing the assembly of
nanofibers, possibly because they promote the formation of
Several observations from our previous studies using
C
12-PEPAu serve as important precedent for the studies
7
reported herein. In those studies, 1-D double-helical
b-sheets. We therefore decided to modify the N-terminus
of PEPAu with an additional ‘AYSS’ segment which could
promote self-assembly.
Department of Chemistry, University of Pittsburgh,
219 Parkman Avenue, Pittsburgh, PA 15260, USA.
9
Accordingly, we prepared BP-AYSS-PEPAu (C12H CO-
AYSSAYSSGAPPMPPF). In contrast to BP-PEPAu, BP-AYSS-
PEPAu rapidly self-assembles (within 30 min) in HEPES buffer
to yield short 1-D fibers (Scheme 1; Fig. 1a, S3a,b) with regular
width (5.2 ꢀ 0.1 nm, Fig. S4d). Further, BP-AYSS-PEPAu
E-mail: nrosi@pitt.edu; Fax: +1-412-624-8611;
Tel: +1-412-624-3987
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Full experi-
assembles into fibers much more rapidly than C -PEP . The
mental information and additional characterization data. See DOI:
10.1039/c0cc02257h
12
Au
rapid self-assembly of BP-AYSS-PEPAu into fibers precluded
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Chem. Commun., 2011, 47, 185–187 185

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Scheme 1 Illustration of nanoparticle assembly process.
dispersion of individual BP-AYSS-PEPAu conjugates. Therefore,
we suspected that gold nanoparticle superstructures prepared
using BP-AYSS-PEPAu would form via a two step mechanism
(
Scheme 1) similar to that of typical peptide-based nanoparticle
2
assembly methods rather than the one step synthesis and
4a
assembly process observed for C -PEPAu.
12
We proceeded to prepare 1-D nanoparticle superstructures
to determine whether a two step mechanism for BP-AYSS-PEPAu
would impact the structural integrity and precision of
organization of the gold nanoparticles. BP-AYSS-PEPAu
fibers were incubated in HEPES buffer (30 min), and then
gold nanoparticle synthesis was initiated by adding a solution
4
of chloroauric acid (HAuCl ) (see Supporting Information for
details). The resulting mixture was allowed to stand at room
temperature (30 min), after which a small sample was removed
and characterized using TEM. Examination of multiple TEM
images (Fig. 1b; Fig. S4) revealed 1-D gold nanoparticle
superstructures, fibers, and free, non-assembled gold nano-
particles. The width of the superstructures (7.9 ꢀ 0.1 nm,
Fig. S4e) suggests that a single BP-AYSS-PEPAu self-assembled
fiber serves as a template for their assembly. While the super-
structures possess long-range order, their local nanoparticle
order is poor. The structures consist of irregularly positioned
nanoparticles ranging in size from B3 nm to B13 nm. The
structures do not exhibit the same structural regularity,
precision of nanoparticle placement, and narrow nanoparticle
size distribution exhibited by the nanoparticle superstructures
Fig. 1 (a) AFM height image of self-assembled BP-AYSS-PEPAu
fibers after 30 min incubation in HEPES buffer; (b) negative-stained
TEM image of 1-D gold nanoparticle superstructures formed 30 min
after adding HAuCl
4
to BP-AYSS-PEPAu fibers from (a); (c) and
(
d) TEM images of 1-D gold nanoparticle superstructures formed 24 h
4
produced using C -PEP . Rather, they more closely resemble
Au
12
4
after adding HAuCl to BP-AYSS-PEPAu fibers from (a).
structures formed using typical multi-step peptide templating
2
a
methods. The reaction was also sampled after 24 h, and,
again, the structures exhibit excellent long range order yet
poor local nanoparticle order (Fig. 1c,d).
(Fig. 2a) images reveal that BP-AYSS-PEPAu self-assembled
fibers begin to align and stack together in either 2 or 3 fiber
bundles with widths of 9.5 ꢀ 0.1 nm and 15.5 ꢀ 1.7 nm,
In order to probe the range of structures which can be
assembled using BP-AYSS-PEPAu self-assembled fibers, we
allowed BP-AYSS-PEPAu to incubate for 6 days in HEPES
buffer prior to adding gold salt. TEM (Fig. S3c–f) and AFM
respectively (Fig. S5). When a solution of HAuCl was added
4
to this mixture of fibers, we again observed 1-D gold nano-
particle superstructures, fibers, and free, non-assembled gold
nanoparticles (Fig. 2b–d; Fig. S6). A distribution of 1-D
1
86 Chem. Commun., 2011, 47, 185–187
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assembly directed by BP-AYSS-PEPAu conjugates operates by
a different mechanism than C12-PEPAu. Importantly, these
studies reveal the fundamental importance of a simultaneous
synthesis and assembly process which is necessary for producing
well-ordered nanoparticle superstructures. To produce high-
quality nanoparticle superstructures using this new methodo-
logy, the peptide conjugate should not self-assemble into a
template structure prior to the addition of gold salt, as this
precludes a one step process in which the nanoparticles and
peptide conjugates self-assemble simultaneously.
NLR acknowledges the University of Pittsburgh and and
the National Science Foundation (DMR-0954380) for funding
this work. The authors also thank the Peterson NFCF and the
MEMS Department for provision of access to AFM and
TEM, respectively.
Notes and references
1
(a) N. A. Kotov and F. Stellacci, Adv. Mater., 2008, 20, 4221–4222;
(
(
b) M. P. Pileni, J. Phys. Chem. B, 2001, 105, 3358–3371;
c) Z. H. Nie, A. Petukhova and E. Kumacheva, Nat. Nanotechnol.,
2
2
010, 5, 15–25; (d) S. Lal, S. Link and N. J. Halas, Nat. Photonics,
007, 1, 641–648; (e) W. S. Chang, L. S. Slaughter, B. P. Khanal,
Fig. 2 (a) 3-D AFM image of self-assembled BP-AYSS-PEPAu fibers
after 6 d incubation in HEPES buffer; (b)–(d) various degrees of
nanoparticle assembly and 1-D nanoparticle superstructures formed
P. Manna, E. R. Zubarev and S. Link, Nano Lett., 2009, 9,
1152–1157 (and references therein).
(a) C. L. Chen and N. L. Rosi, Angew. Chem., Int. Ed., 2010, 49,
2
3
4
1924–1942; (b) X. Y. Gao and H. Matsui, Adv. Mater., 2005, 17,
2037–2050; (c) L. S. Li and S. I. Stupp, Angew. Chem., Int. Ed.,
2005, 44, 1833–1836; (d) N. Ostrov and E. Gazit, Angew. Chem., Int.
4
15 min after adding HAuCl to BP-AYSS-PEPAu fibers which had
been incubated for 6 d.
Ed., 2010, 49, 3018–3021 (and references therein).
(a) S. R. Whaley, D. S. English, E. L. Hu, P. F. Barbara and
A. M. Belcher, Nature, 2000, 405, 665–668; (b) M. B. Dickerson,
K. H. Sandhage and R. R. Naik, Chem. Rev., 2008, 108, 4935–4978;
nanoparticle superstructures formed with widths of either
2.2 ꢀ 0.2 nm or 17.6 ꢀ 0.2 nm, suggesting that they were
templated by either the 2 or 3 fiber bundles, respectively
1
(
c) M. Sarikaya, C. Tamerler, A. K. Y. Jen, K. Schulten and
(
Fig. 2c,d, Fig. S6).
F. Baneyx, Nat. Mater., 2003, 2, 577–585 (and references therein).
(a) C. L. Chen, P. J. Zhang and N. L. Rosi, J. Am. Chem. Soc., 2008,
In conclusion, we have shown that BP-AYSS-PEPAu first
1
30, 13555–13557; (b) C. L. Chen and N. L. Rosi, J. Am. Chem.
Soc., 2010, 132, 6902–6903.
5 J. M. Slocik, M. O. Stone and R. R. Naik, Small, 2005, 1,
048–1052.
(a) A. Habib, M. Tabata and Y. G. Wu, Bull. Chem. Soc. Jpn., 2005,
8, 262–269; (b) J. P. Xie, J. Y. Lee and D. I. C. Wang, Chem.
Mater., 2007, 19, 2823–2830.
7 D. L. Minor and P. S. Kim, Nature, 1994, 367, 660–663.
assembles into a fiber template and then nanoparticles either
nucleate directly on the template or are deposited onto the
template from solution to yield 1-D nanoparticle super-
structures having tailorable widths of either 7.9 nm, 12.2 nm,
or 17.6 nm. These structures exhibit long range order but
limited local order. These results suggest that nanoparticle
1
6
7
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 185–187 187