Integrated and segregated Au/γ-Fe2O3 binary nanoparticle assemblies

Article abstract of DOI:10.1002/anie.201207469

To mix or not to mix: Integrated (left) and segregated (right) assemblies were obtained upon treating functionalized γ-Fe₂O₃ nanoparticles (NPs) with AuNPs. Their binary nature is controlled by the capping layer of the γ-Fe₂

Full text of DOI:10.1002/anie.201207469

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Angewandte
Communications
DOI: 10.1002/anie.201207469
Nanostructures
Integrated and Segregated Au/g-Fe2O3 Binary Nanoparticle
Assemblies**
Meital Boterashvili, Michal Lahav, Tanya Shirman, Dalia Freeman, and
Milko E. van der Boom*
Multicomponent materials might display synergistic effects
and possess functions not attainable with single-component
systems.[1,2] Control over the composition and structure of
such materials is not trivial because many factors are involved
(e.g., weak intermolecular forces, covalent interactions,
solvent effects).[1–4] Moreover, control of phase segregation
is essential and of high importance in material science[5–13] and
biomolecular systems.[6,14] For instance, segregation of poly-
mer blends is directly related to efficient charge-carrier
generation and transport in solar cells.[8–10] Langer et al. used
this phenomenon to develop shape-memory materials for
biomedical applications.[14] Phase-separation is also used in
the formation of Janus nanoparticles.[15] Achieving structural
control of binary nanoparticle (NP) assemblies offers a prom-
Scheme 1. Schematic representation of an integrated assembly (left)
ising route towards new materials as recently shown by
Murray and others.[16–18] Such systems consist of integrated
assemblies (IAs),[16–21] whereas segregated assemblies (SAs)
are less common and mainly originate from size-dependent
phase-separation.[17,22,23]
and a segregated assembly (right) formed upon treating 1- or 2-
capped g-Fe2O3 NPs with TOAB-capped AuNPs. The presence of free
ligand causes also some self-aggregation of AuNPs.
Herein we show the controlled and selective formation of
both integrated assemblies and segregated assemblies with
Au and g-Fe2O3 NPs (Scheme 1). These systems were
designed to comprise both the optical properties of AuNPs
and the magnetic properties of g-Fe2O3 NPs.[20,21,24,25] Bifunc-
tional ligand 1 exhibits orthogonal reactivity towards these
NPs.[26] We found that under certain reaction conditions the
N-oxide moiety can bind both NPs, whereas the pyridine
group exclusively binds to the AuNPs (Scheme S2, Table S1 in
the Supporting Information).[27–29] Using the orthogonal
reactivity of this cross-linker (1) with NPs, we succeeded to
generate integrated assemblies. Replacing the vinylpyridine
moiety of 1 with a styryl group (ligand 2) resulted in the one-
pot formation of the segregated assemblies. In addition, we
also demonstrate the stepwise formation of segregated
assemblies by using AuNP assemblies as nucleation surfaces
for domains composed of g-Fe2O3 NPs. These new assemblies
were characterized by combining UV/Vis spectroscopy, trans-
mission electron microscopy (TEM), energy dispersive X-ray
spectroscopy (EDS), and cryogenic- (cryo-) TEM.
Ligands 1 and 2 were dissolved in CH2Cl2 and added to
THF-diluted solutions of oleic acid (OA)-capped g-Fe2O3 NPs
in toluene (Scheme S2, Table S1). Then, ligand (1 or 2)-
capped g-Fe2O3 NPs were treated with tetraoctylammonium
bromide (TOAB)-capped AuNPs in solution to form inte-
grated assemblies or segregated assemblies, respectively
(Scheme 1). The addition of a 1-capped g-Fe2O3 NP light-
yellow solution to TOAB-capped AuNPs resulted in an
immediate color change. The pink AuNP solution turned
purple (Figure 1A, inset). Indeed, UV/Vis spectroscopy
showed significant changes in the surface-plasmon resonance
(SPR) band, demonstrating a 33 nm red shift, signal broad-
ening and an increase in the intensity (Figure 1A). This
behavior is indicative of the formation of AuNP-containing
aggregates.[28–30]
[*] M. Boterashvili, Dr. M. Lahav, Dr. T. Shirman, Dr. D. Freeman,
Prof. M. E. van der Boom
Department of Organic Chemistry, Weizmann Institute of Science
Rehovot-76100 (Israel)
E-mail: milko.vanderboom@weizmann.ac.il
Homepage: http://www.weizmann.ac.il/oc/vanderboom/
TEM analysis unambiguously showed the formation of
integrated binary Au/g-Fe2O3 NP assemblies, whereas the NPs
retain their original size (AuNP: 5.0 Æ 1.0 nm; g-Fe2O3 NPs:
4.5 Æ 0.5 nm) and shape (Figure 2A and Figure S3A). The Au
and g-Fe2O3 NPs are readily distinguishable as a result of their
atomic number differences (Au: 79; Fe: 26), resulting in
a brighter image for the g-Fe2O3 NPs and a darker image for
the AuNPs.[19] EDS measurements reveal the presence of both
Au and Fe (Figure 3, compare the background spectrum (a)
with spectrum (b)). In addition, cryo-TEM measurements
[**] This research was supported by the Helen and Martin Kimmel
Center for Molecular Design and the Minerva Foundation. Cryo-
TEM measurements were performed by Dr. E. Kesselman (Techn-
ion). We thank Dr. R. Popovich-Biro (WIS) for the EDS measure-
ments and Dr. R. Klajn (WIS) for the OA-capped g-Fe2O3 NPs.
M.E.v.d.B. is the incumbent of the Bruce A. Pearlman Professorial
Chair in Synthetic Organic Chemistry.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207469.
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Figure 3. EDS spectra of a) TEM grid background, b) integrated assem-
Figure 1. UV/Vis spectra for A) a) TOAB-capped AuNPs and b) the
integrated assemblies and B) a) TOAB-capped AuNPs and b) segre-
gated assemblies. The baseline is shown in black. Insets: photographs
of the corresponding solutions.
bly, c) the AuNP-domain in a segregated assembly and d) the g-Fe2O3
NP-domain in a segregated assembly. The scale of the y axis is
identical for all spectra. The arrows indicate the elemental peaks of Au
and Fe. TEM images of these aggregates and the analyzed domains
are shown in Figure S4 (integrated assembly) and in the inset of
Figure 2C (segregated assembly).
Figure 4. Photographs taken A) at t=0 and B) after 12 h of exposing
solutions containing integrated assemblies (left vial) and segregated
assemblies (right vial) to a permanent magnet.
the N-oxide group of ligand 1 to g-Fe2O3 NPs and the pyridine
moiety of this ligand to the AuNPs.[28,29]
The addition of a light yellow solution of 2-capped g-
Fe2O3 NPs to a pink solution of TOAB-capped AuNPs
resulted in only a slight color change of the AuNPs solution
(Figure 1B, inset), a small 5 nm red shift and broadening of
the SPR band (Figure 1B). These minor optical changes
indicate the formation of smaller aggregates than observed
for integrated assemblies (over 500 nm; Figure S3A). TEM
measurements confirmed the formation of these aggregates
(50–250 nm) composed of segregated Au and g-Fe2O3 NP
domains (Figure 2B and C, Figure S3B and C). High-reso-
lution TEM images of the domain interfaces of these
segregated assemblies do not show fusion or evident lattice
matching,[31] while exhibiting clear domain boundaries (Fig-
ure S7). EDS analysis confirmed that each domain consists
almost exclusively of Au or g-Fe2O3 NPs (Au or Fe atomic%
> 95; Figure 3, spectra (c) and (d), respectively). Cryo-TEM
measurements confirmed the formation of the segregated
assemblies in solution (Figure 5A and B). Applying a perma-
nent magnet to the solutions did not result in a notable color
change (Figure 4). Nevertheless, UV/Vis spectroscopy of the
remaining solution showed a decrease in intensity and a 5 nm
Figure 2. Representative TEM images of A) integrated assemblies and
(B, C) segregated assemblies. C) Inset: The white circles indicate the
areas analyzed by EDS (Figure 3); Top circle: the AuNPs domain, and
bottom circle: the g-Fe2O3 NPs domain, corresponding to EDS
spectra (c) and (d), respectively (in Figure 3). Scale bar in (A)=20 nm,
in (B,C) and (C) inset=50 nm.
show the formation of aggregates in solution (Figure S5),
ruling out drying effects. A solution containing these inte-
grated assemblies was exposed to a permanent magnet.[20,21]
A
dark precipitate started to form after about 2 h in proximity
with the magnet resulting, after approximately 12 h, in
a colorless solution (Figure 4). As expected, UV/Vis spec-
troscopy of this solution showed a significant intensity
decrease of the SPR band (Figure S6A). Almost no NPs or
assemblies thereof were observed in a TEM analysis of the
resulting solution. No precipitation or color change was
observed in the absence of a magnetic field. The formation of
the integrated assemblies can be attributed to coordination of
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controlled as a function of the capping layer, resulting in
integrated and segregated assemblies. The use of orthogonal
coordination chemistry resulted in the formation of inte-
grated assemblies, whereas the use of NP-ligand coordination
or p–p stacking resulted in well-separated domains (segre-
gated assemblies). This formation of segregated assemblies
reflects the importance and the use of hierarchical forces for
phase-separation with NPs.[29,35] Our data show that AuNP/1-
and AuNP/2-based assemblies promote the aggregation of the
corresponding 1- and 2-capped g-Fe2O3 NPs. Interestingly, the
initial state of the AuNPs (i.e., non-aggregated or aggregated)
directs the assembly mode of the NPs and can be used to form
the segregated assemblies in a consecutive manner.
Figure 5. A) and B) Representative cryo-TEM images of segregated
assemblies formed upon treating 2-capped g-Fe2O3 NPs with TOAB-
capped AuNPs. Scale bar=50 nm.
blue shift of the SPR band (Figure S6B). In agreement with
the optical data, the TEM analysis showed only some
separated AuNPs (Figure S8). No segregated assemblies
were observed.
Received: September 15, 2012
Published online: November 7, 2012
Mechanistically, the formation of the segregated assem-
blies might involve the initial formation of AuNP/2 aggregates
Keywords: aggregation · gold · iron oxide ·
nanoparticle assembly · nanostructures
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acting as
a
nucleation site for the g-Fe2O3 NPs
(Scheme 2).[32,33] This hypothesis is based on the TEM analysis
of the reaction between 2-capped g-Fe2O3 NPs (containing
free ligand 2) and TOAB-capped AuNPs. Some aggregates of
AuNP/2 were observed in addition to the segregated assem-
blies (Figure S9). This process might be driven by p–p
stacking.[34] These aggregates can also be generated directly
by treating ligand 2 with TOAB-capped AuNPs as shown by
UV/Vis spectroscopy (Figure S11B) and TEM measurements
(Figure S12B). Remarkably, the addition of 2-capped g-Fe2O3
NPs to AuNP/2 aggregates results in the stepwise formation of
segregated assemblies (Figure S14C and D). The 2-capped g-
Fe2O3 NPs do not aggregate in the absence of AuNPs
(Table S1) and no free-standing assemblies thereof were
observed in the presence of AuNP/2 aggregates. This result
implies that the modified gold surface induces the domain
formation.
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