Molecular structure-function relations of the optical properties and dimensions of gold nanoparticle assemblie

Article abstract of DOI:10.1002/anie.200906636

Figure Presented Stolen identity: The molecular geometries of a series of cross-linkers that bear between one and four pyridyl moieties are expressed in the optical properties of AuNP assemblies (see picture). TEM analysis indicates that the molecular-level structural differences of the cross-linkers are also transferred at the submicrometer level in the formation of the AuNP assemblies.

Full text of DOI:10.1002/anie.200906636

Communications
DOI: 10.1002/anie.200906636
Nanostructures
Molecular Structure–Function Relations of the Optical
Properties and Dimensions of Gold Nanoparticle
Assemblies**
Revital Kaminker, Michal Lahav, Leila Motiei, Maida Vartanian, Ronit Popovitz-
Biro, Mark A. Iron, and Milko E. van der Boom*
Angewandte
Chemie
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Structurally well-defined organic compounds have the ability
to direct the formation of large assemblies in solution and in
the solid state.[1] Control over nanoparticle (NP) assemblies
has been reported with a variety of cross-linkers (CLs),
including polymers, charged molecules, dendrimers, and
biomolecules (e.g., DNA, antigen–antibody, and avidin–
biotin) to name but a few.[2–4] In particular, the formation of
NP assemblies has been studied as a function of the size or
charge characteristics of the CL.[4] Nevertheless, designing
and predicting the properties and structure of hybrid NP-
based assemblies based on a given molecular structure is still a
difficult task. Systematic variation of the CL (2D vs. 3D
geometry, number of potential NP binding sites, size, sym-
metry, etc.) is needed to gain fundamental insight into the
factors that control the physicochemical properties of the
resulting hybrid assemblies.
Herein, we demonstrate that the molecular structure of
the organic CL, in addition to the number of available
coordination sites, has a significant impact on the formation
and optical properties of AuNP assemblies. A series of CLs
with one to four pyridyl moieties (1–6, Scheme 1)[5,6] were
reacted with solutions of citrate-capped AuNPs with a
homogeneous size distribution of (11.8 Æ 1.3) nm (Figure S1
in the Supporting Information). In particular, solutions of the
CLs in THF were added stepwise to aqueous solutions of
freshly prepared AuNPs. A distinct color change from red to
deep blue was observed with CLs 3–6, thus indicating that the
AuNPs readily formed assemblies (Figure 1, insets).[7] In
contrast, the red color of the AuNP solution remained
unchanged in the monopyridyl system (1, Figure 1) and with
pyridine. The color change for CL 2 was relatively small. Our
observations demonstrate that the color of the AuNP-based
assemblies can be controlled as a function of the structure and
concentration of the organic component (Figure 1).
Scheme 1. Molecular structures of the cross-linkers (CL)[5,6] used to
direct the AuNP–CL assembly formation and properties.
based assemblies, while the former reflects changes in the
contacting dielectric media (i.e., citrate ions versus the CLs)
on the AuNPs surfaces.[8] The optical spectra of the AuNP
The UV/Vis spectra of the assemblies formed with
compounds 2–6 show changes upon increasing the CL
concentration from 0.0 to 1.8 mm. Specifically, the intensity
of the coupled surface plasmon band at lmax = 600–635 nm
increases, while the plasmon band at lmax = 519 nm decreases.
In addition, a red shift of approximately 10 nm is observed for
the plasmon bands of compounds 3–6 (Figure 1). The change
in the latter plasmon band indicates the formation of AuNP-
[*] R. Kaminker, Dr. M. Lahav, L. Motiei, M. Vartanian,
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/
Dr. R. Popovitz-Biro, Dr. M. A. Iron
Department of Chemical Research Support
Weizmann Institute of Science
Rehovot—76100 (Israel)
Figure 1. UV/Vis spectra of the hybrid AuNP–CL-based assemblies.
The labels correspond to the molecular structures shown in Scheme 1.
Solutions of the CL (1–6; 50 mm) in THF were added (0–36 mL in 6 mL
increments) to 1 mL aqueous solutions of AuNPs to give a final
concentration of 1.8 mm. Spectra were recorded immediately after
mixing the components. High concentrations of CL 6 (1.5–1.8 mm)
resulted in a decrease in absorbance arising from precipitation of the
assemblies (traces a and b). Insets: photographs of the solutions at
each incremental addition of CL.
[**] This research was supported by the Helen and Martin Kimmel
Center for Molecular Design and by the Minerva Foundation. The
transmission electron microscopy studies were conducted at the
Irving and Cherna Moskowitz Center for Nano and Bio-Nano
Imaging at the Weizmann Institute of Science.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906636.
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Communications
solutions mixed with compounds 1 or pyridine show only
minor changes. A small shift of approximately 2 nm and
dampening of the plasmon bands implies that the stabilizing
capping layer of the AuNPs was exchanged with these ligands.
However, no coupled plasmon band is observed, thus
indicating that the AuNPs mainly exist as independent
entities for these two compounds. Interestingly, the intensity
and broadening of the coupled surface plasmon band suggest
that formation of the AuNP assemblies is related to the
number of pyridyl moieties (between one and four), where
the degree of assembly formation increases in the order:
pyridine ꢀ 1 ! 2 < 4 < 3 (Figure 2). Other structural factors
also play a dominant role in the AuNP assembly formation.
For instance, the saturated analogue of 3 (compound 5)
results in a significantly higher degree of NP assembly
formation, which is similar to that obtained with the charged
CL 6.
Figure 3. Representative TEM images of the AuNP–CL assemblies. The
labels correspond to the molecular structures shown in Scheme 1 and
the UV/Vis spectra given in Figure 1. Additional TEM images are
provided in Figure S3 in the Supporting Information.
varying dimensions. This result clearly shows that a higher
number of pyridyl units is a requirement for aggregation of
the AuNPs. TEM analyses indicate that the level of aggrega-
tion for CLs 2–4, which have a structure with between two and
four binding sites, is somewhat similar (Figure 3), although
each of these systems has unique optical properties (Figure 1).
Significantly larger and denser assemblies are observed for
CLs 5 and 6.
To provide some insight into the origins of the exper-
imental findings, density functional theory (DFT) calculations
(at the M06/SDB-pc1//M06L/SDB-pc1/DFBS level of theory,
see the Supporting Information for details) were carried out.
The organic CLs 1–5 and pyridine were coordinated to one to
four Au8 clusters, and the binding energies (BEs) were
determined (Table 1). From these DFT results, it would seem
that electronic effects are minor and the BEs for CLs 2–5 are
very similar. It is interesting to note that the BEs decrease for
each additional gold cluster coordinated to the CL. Coordi-
nation of the fourth gold cluster to CL 4 is slightly
unfavorable. The small size of the gold cluster, which is
essential to make the calculations feasible, precludes steric
interactions in the AuNP–CL structures but, nonetheless,
should reproduce trends in the electronic nature of the Au–
CL binding. If one were to consider the geometric limitations
of this system, it is clear that none of CLs considered are large
enough for them to bind to more than two NPs simulta-
neously. (A full analysis is provided in the Supporting
Figure 2. Comparison of the UV/Vis spectra of the hybrid AuNP–CL-
based assemblies in aqueous solutions with a concentration of
compounds 1–6 of 1.5 mm. Spectra were recorded immediately after
mixing the components.
Compound 6 differs from the other cross-linkers in that it
has a + 2 charge. It is known that the use of charged species as
cross-linkers results in NP assemblies formed by Coulombic
interactions.[9] When CL 6 was added to the AuNP solutions,
extensive assemblies were formed. Similar results were
obtained with [Os(bpy)3]2+ (bpy = 4,4’-bipyridyl) complexes
that bear zero or one pendant pyridyl arms (Scheme S1 in the
Supporting Information), thus indicating that NP–pyridyl
coordination is, in this case, not a requirement for NP
assembly formation. Thus, in these charged systems, the level
of control that the CL structure has over the formation and
optical properties of the NP assembly seems minimal.
Table 1: DFT binding energies (BE) of 1–4 Au8 clusters bound to CLs 1–
5.[a]
The TEM images (Figure 3 and Figure S3 in the Support-
ing Information) show increasing AuNP aggregation as a
function of the number of pyridyl units and/or the molecular
geometry, which is in good agreement with the UV/Vis
analysis (see above). Solvent-free conditions were used for
these measurements and, therefore, only the qualitative
structures of the AuNP assemblies in solution are reflected
in these images. Systems formed with CL 1 or pyridine
showed isolated spheres and small aggregates on the grid,
whereas the other systems (2–6) form larger assemblies of
CL
BE per Au8
BE(1)[b]
BE(2)[b]
BE(3)[b]
BE(4)[b]
1
2
3
4
5
À22.4
À21.6
À20.5
À20.2
À20.9
À22.4
À22.0
À21.1
À21.2
À21.8
À21.1
À20.7
À20.9
À21.0
À19.7
À19.7
À19.9
1.0
[a] Binding energies reported in kcalmolÀ1. [b] Binding energy of the first,
second, third, and fourth cluster to the CL.
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AuNPs. The saturated nature of CL 5 allows for greater
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between two AuNPs. This flexibility allows the CL to keep the
AuNPs at a larger angle, thereby reducing any possible
repulsive interactions.
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Our observations indicate that for CLs 2–5, weak
intermolecular interactions, such as p–p interactions or
dipole–dipole interactions, do not play a significant role, but
rather AuNP assembly formation results from coordination
by the pyridyl groups. The molecular geometries of a series of
pyridyl-based CLs are expressed in the formation and optical
properties of these AuNP assemblies. The structural differ-
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in the formation of the AuNP assemblies at the submicro-
meter level. Apparently, electronic factors do not seem to
play a major role in influencing assembly formation. The use
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2–4), however, results in a higher degree of aggregation.
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the degree of nanoparticle aggregation that can be achieved
by the use of neutral, organic CLs (e.g., 2–5).
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Received: November 24, 2009
Keywords: aggregation · cross-linking · gold · nanostructures
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