Template synthesis of gold nanoparticles with an organic molecular cage

Article abstract of DOI:10.1021/ja412606t

We report a novel strategy for the controlled synthesis of gold nanoparticles (AuNPs) with narrow size distribution (1.9 ± 0.4 nm) through NP nucleation and growth inside the cavity of a well-defined three-dimensional, shape-persistent organic molecular cage. Our results show that both a well-defined cage structure and pendant thioether groups pointing inside the cavity are essential for the AuNP synthesis.

Full text of DOI:10.1021/ja412606t

Communication
pubs.acs.org/JACS
Template Synthesis of Gold Nanoparticles with an Organic Molecular
Cage
Ryan McCaffrey,† Hai Long,‡ Yinghua Jin,† Aric Sanders,§ Wounjhang Park,⊥ and Wei Zhang*,†
⊥
†Department of Chemistry and Biochemistry, and Department of Electrical, Computer and Energy Engineering, University of
Colorado, Boulder, Colorado 80309, United States
‡National Renewable Energy Laboratory, Golden, Colorado 80401, United States
§National Institute of Standards and Technology, Boulder, Colorado 80305, United States
S
* Supporting Information
ABSTRACT: We report a novel strategy for the
controlled synthesis of gold nanoparticles (AuNPs) with
narrow size distribution (1.9
0.4 nm) through NP
nucleation and growth inside the cavity of a well-defined
three-dimensional, shape-persistent organic molecular
cage. Our results show that both a well-defined cage
structure and pendant thioether groups pointing inside the
cavity are essential for the AuNP synthesis.
he size-dependent optical and electronic properties of gold
T
nanoparticles (AuNPs) have long been of interest in the
context of nanoscience and nanotechnology.1 In particular,
AuNPs with diameters in the sub-nanometer to ∼2 nm range are
known to exhibit properties that are quite unique compared to
those larger than 5 nm,2 an observation that has motivated
intense interest in the design of organic architectures for the
template synthesis of 1−2 nm AuNPs that can be further used in
catalysis,3 sensor devices,4 and nanoelectronics.5 For instance,
the integration of AuNPs into electronic devices will require the
use of 1−2 nm particles.6 Since the advent of the Brust−Schiffrin
method,7 there has been a wealth of literature exploring the
structure−function relationships of ligand-stabilized NPs that
can be rationally designed for fundamental studies and practical
applications.8 To date, control over NP size, shape, and
distribution has advanced through the use of small organic
ligands, dendritic architectures, or polymers as templates or
stabilizers.1c,3b,9 Despite this recent progress, there still remain
very few examples of passivating ligands allowing for a size-
controlled synthesis of AuNPs.
Figure 1. Structures of cages 1 and 2, and side view of a fully stretched
model of cage 1. Methyl group for hexyl chain, and hydrogen for
OC16H33 and Br were used in the calculation for simplification.
conventional small organic or macromolecular ligands, which
form thick, insulating layers on the AuNP surface, a cage template
can provide a protecting shell with minimum coverage and
greater encapsulated AuNP surface accessibility. Another
potential advantage of such a “cage-template” approach is that
exterior functionalization on the cage may enable further
assembly of AuNPs in a controlled fashion as possible three-
dimensional building blocks for chemically directed hierarchical
assembly, which would provide a powerful platform for
development of novel nanostructured materials for optical or
catalytic applications. Herein, we report the first example of size-
controlled synthesis of AuNPs using a discrete organic molecular
cage as a template.
We designed trigonal prismatic cage 1 with internal cavity size
of 1.8−2.1 nm (Figure 1), the interior of which is functionalized
with three thioether groups. We chose to use thioether as the
nucleation site for the Au deposition since it has higher stability
than thiol yet is known to coordinate, albeit weakly, gold
colloids.9g,15 AuNPs of a size similar to the cage are expected to
grow inside the cavity, leading to the formation of a core/shell
structure, with AuNP as the core and the cage molecule as the
Well-defined, discrete organic molecular cages have attracted
tremendous attention10 due to their shape persistence, structural
tunability, and thermal and chemical stability. Our group has
demonstrated the modular synthesis11 of a variety of organic
molecular cages and their great potential in carbon capture12 and
fullerene separation applications.13 Furthermore, modular syn-
thesis allows for judicious external functionalization so that
molecular cages can be covalently assembled into ordered
networks through the bottom-up “cage-to-framework” ap-
proach.14 We envision that discrete rigid organic molecular
cages with multiple Au-binding sites inside the cavity can serve as
templates for controlled synthesis of AuNPs. Such a cage
template has a well-defined architecture with a spatially confined
cavity that is large enough to accommodate NPs. Compared to
Received: December 11, 2013
© XXXX American Chemical Society
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dx.doi.org/10.1021/ja412606t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 1. Synthesis of Molecular Cage 1
Figure 2. Top: TEM micrographs (scale bar 20 nm; inset scale bar 2
nm) of AuNP@1 complex (a) and AuNPs produced using 2 as a ligand
(b). Bottom: UV−vis absorption spectra of gold complexes in CH2Cl2
(c) and the size distribution of AuNP@1 complex (d).
shell. Previously reported cage 2, with the same cavity size but
lacking thioether anchoring groups, was selected as a control
compound.12a
measurements were performed in CH2Cl2 at the same
concentration (1.4 μM). In the absence of cage 1, the absorption
spectrum of tetrabutylammonium tetrachloroaurate(III) in
CH2Cl2 shows absorption peaks at λ = 250 nm, and 380 nm,
arising from the ligand-to-metal charge-transfer transition (AuIII,
red line, Figure 2c). Upon addition of the cage molecule to the
Au3+ solution, the absorption of 1 appeared as a shoulder band in
the region around 275−381 nm, and the intensity of the peak at
380 nm was decreased (AuIII@1, purple line, Figure 2c).
Complete reduction of AuCl4− to zerovalent Au and formation of
AuNPs with diameter ∼2 nm were confirmed by the
disappearance of the 250 nm band and the emergence of a
featureless broad tail extending to 700 nm after the reduction
(AuNP@1, green line).
The particle diameter and size distribution were analyzed
using TEM images. All the samples were prepared using a
solution of AuNP@1 in CH2Cl2. The solution was drop-cast
onto carbon-coated 300 mesh copper grids (CF300-Cu) and
allowed to air-dry before the measurements. The TEM image
(Figure 2a) of AuNP@1 shows the formation of well-dispersed
AuNPs with average diameters of 1.9 nm, which matches well
with the estimated cage internal cavity size of 1.8−2.1 nm.
Thermal gravimetric analysis (TGA, Figure S5) shows a mass
loss of 56% for pure cage 1 and a mass loss of 6% for AuNP@1
complex between ambient temperature and 480 °C. Based on the
above mass loss determined by TGA, we calculate that each cage
molecule contains a NP composed of roughly 150 Au atoms.
This corresponds to a 1.7 nm AuNP, which is in close agreement
with the AuNP size observed by TEM.
Cage 1 was synthesized through dynamic imine chemistry
using triamine 3 as the top and bottom panels and dialdehyde 4
with a thioether group as the three lateral edges (Scheme 1).
Triamine 3 was prepared as previously described in the
literature.12b Dialdehyde 4 was synthesized starting with 3,5-
diiodo-p-cresol 5, which was prepared from p-cresol.16 Following
alkylation of compound 5, trimethylsilyl-protected terminal
acetylenes were introduced before radical bromination at the
methyl position. The brominated intermediate is critical in that it
allows for the later introduction of pendant interior thioether
groups. Desilylation of compound 7 followed by Sonogashira
coupling with 3-bromo-5-iodobenzaldyhyde yielded the lateral
side piece 4. Imine condensation between the two building
blocks 3 and 4 and subsequent reduction led to the formation of
1
1. Cage 1 was characterized by H and 13C NMR, GPC, and
MALDI-MS.
We used a two-phase liquid−liquid approach developed by
Brust et al., with tetraoctylammonium bromide (TOAB) as a
phase-transfer reagent, to prepare cage-encapsulated AuNPs.10
A
solution of TOAB in CH2Cl2 was added to an aqueous solution
of HAuCl4 (10 equiv) and stirred until the aqueous layer was
−
colorless, indicating all AuCl4 was transferred to the organic
phase. A solution of 1 (1 equiv) in CH2Cl2 was added to the
above biphasic mixture. Upon mixing, no obvious color change
was observed in the organic phase. The mixture was subsequently
reduced with an aqueous solution of sodium borohydride (190
equiv, rt). The organic phase immediately changed color from
orange-red to dark brown without any precipitates, indicating
efficient Au3+ reduction and further stabilization of the resulting
AuNPs by cage molecule 1. The organic layer containing
AuNP@1 complex was separated, and AuNP@1 complex was
precipitated from ethanol and collected by centrifugation. The
AuNP@1 complex shows good solubility in common organic
solvents. The AuNPs were characterized by UV−vis, 1H NMR,
DOSY, TGA, and TEM.
Despite its small size, AuNP@1 complex shows excellent
stability. It is stable in solutions, with no evidence of
agglomeration, and without noticeable color change, over
periods of several months. More importantly, it can be
evaporated to dryness overnight under high vacuum and then
redissolved in common organic solvents with no signs of
aggregation. It should be noted that redispersion of AuNPs after
drying has been difficult for AuNPs with weaker binding ligands,
such as some carboxylates and multivalent thioether ligands.9h,17
Presumably, the cage shell provides good solubility as well as
effective coverage and protection of the AuNPs’ surface, thus
The UV−vis absorption spectra of the solution before and
after reduction are shown in Figure 2c. All the absorption
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dx.doi.org/10.1021/ja412606t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
preventing their aggregation. The high solubility and stability of
the AuNP@1 complex further support the notion that NPs
reside inside the cage cavity.
1
In the H NMR spectra of the above AuNP@1 complex, we
did not observe the phase-transfer agent TOAB (Figure S1).
Interestingly, substantial broadening and shifting of not only the
protons of the thioether group but also all aromatic protons of
the cage skeleton were observed. This is in great contrast to the
oligomeric ligands based on benzylic thioethers reported by
Simon and Mayor, in which no significant shifting and
broadening of proton signals of ligands were observed upon
the formation of AuNP complex.9f,15b Line broadening of
resonances is characteristic for ligands with thiol functionality,
which have the highest affinity to Au.18 Line broadening
commonly occurs due to the intrinsic heterogeneous environ-
ment for the bound ligands on the AuNPs and their restricted
mobility on the NP surface.17 The considerable line broadening
and shifting of almost all protons of the cage skeleton in the
present case therefore support the notion that the cage shell is
likely tightly wrapped around the AuNP, and experiences
restricted mobility and fast spin relaxation.
Figure 3. Energy of cage 1 as a function of encapsulated nanoparticle
radius.
Figure 4. Calculated energy-minimized structure of AuNP@1. AuNP
radius is 8.65 Å.
It has been known that multidentate thioether ligands, such as
thioether dendrimers, can stabilize AuNPs to form stable and
narrowly dispersed ligand-wrapped NPs.9f,15b,19 To further
corroborate that the particle rests inside of the cage cavity and
rule out the possibility of particle formation through aggregation
due to the long alkyl chains present around the cage exterior. The
working mechanism of this cage template is very different from
that of the previously reported dendrimer template, which
inherently relies on multipoint interactions that conform to the
surface of the particle, thus allowing it to grow until “dendritic
wrapping” of the cluster becomes favorable.9f
1
of multiple cages on the AuNP surface, H diffusion-ordered
spectroscopy (DOSY) NMR was performed on both free cage 1
and AuNP@1 under the same temperature and concentration
(Figures S3 and S4). As expected, we obtained very similar
diffusion coefficients for cage 1 and AuNP@1, 2.5 and 2.4
respectively, which indicates the similar size and shape of free
cage and AuNP@1. This study provides additional evidence
supporting that AuNP formation is a result of a single cage
encapsulation rather than intercage interactions and the simple
multivalency effect of ligands.
To further confirm the role of cage scaffold and thioether
groups in AuNP synthesis, we conducted two control experi-
ments using cage 2, which lacks thioether groups, and thioether
ligand 4, which lacks a cavity. First, HAuCl4 was reduced in the
presence of 2, a structural analogue of cage 1 but without the
three internal thioether groups. As expected, upon reduction
with NaBH4, we observed the immediate and complete
precipitation of aggregated NPs under conditions similar to
those used to form AuNP@1 complex. The TEM image of
AuNPs obtained from the control experiment with 2 (Figure 2b)
showed only shapeless agglomerates, indicating that 2 is unable
to serve as a template for the synthesis of AuNPs, presumably due
to the lack of nucleation (i.e., Au binding) sites. In the other
control experiment, the side piece 4, which bears a thioether
group, was used as the ligand. In this case, we observed similar
rapid aggregation and precipitation of AuNPs upon reduction,
suggesting the poor stabilization of AuNPs by monodentate
open ligand 4.
Computational simulation of the interaction between cage 1
and AuNPs of different radii provides theoretical support for our
experimental findings. The AuNPs were generated using a Au
crystal cubic close-packed lattice structure with a closest Au−Au
separation of 0.2884 nm. Five different AuNPs are used, with
radii of 11.54, 9.99, 8.65, 7.63, and 5.77 Å, respectively. For each
NP, we built a cage around it and bonded the three sulfur atoms
of the cage to the Au atoms at the equator of the NP, 120° apart.
The Amber 11.0 molecular dynamics program package20 was
then used to optimize the structures of the cage/NP complexes.
The force field used for the cage was the general Amber force
field (GAFF)21 with the charge parameters computed by the
AM1-BCC method,22 and the force field for gold was adopted
from Agrawal et al.23 For each optimization run, the atoms on
AuNPs were frozen, and the structure of the cage was optimized.
The cage was first minimized for 5000 steps using the conjugate
gradient method, and then it was further optimized by the
simulated annealing method for 150 ps with a time step of 1 fs.
During the simulated annealing, the system temperature was first
raised to 1000 K for 50 ps and then gradually cooled to 0 K over
another 100 ps. Finally, the annealed structure was minimized
again for another 5000 conjugate gradient steps. The total energy
of cage/cage and cage/gold interactions was calculated on the
basis of the energy-minimized structure. Figure 3 shows the cage
energy as a function of the encapsulated NP radius. The energy
first decreases when the radius increases due to the larger van der
Waals attraction between the cage and the NP. At a radius of 8.65
Å (1.7 nm diameter), the energy reaches the minimum, and the
structure of the AuNP@1 complex is presented in Figure 4. This
is in close agreement with the 1.9 0.4 nm average diameter we
observed experimentally through the TEM characterization.
In summary, we have demonstrated a novel cage-templated
strategy for the controlled synthesis of AuNPs through the use of
It is therefore tempting to conclude that both the thioether
groups and the closed cage structure itself are playing critical
roles in controlled AuNP synthesis. The thioether groups serve
as the initial nucleation sites for AuNP growth and also stabilize
the resulting NP. Once seeded, the Au nanocluster grows until it
is confined sterically within the cage, and the three thioether
groups and the six amino groups may provide stabilization
through Au surface binding. The fact that the AuNP@1
complexes themselves remain isolated from one another is likely
C
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Journal of the American Chemical Society
Communication
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* Supporting Information
Experimental procedures, GPC, TGA, TEM images, NMR
spectroscopic data, and theoretical calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
wei.zhang@colorado.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Army Research Office (W911NF-12-1-0581)
for financial support, Dr. Tom Giddings and Suehyun Cho for
TEM image analysis, and Dr. Richard Shoemaker for NMR
analysis, and Dr. Matthew Cowan for TGA study. This research
used capabilities of the National Renewable Energy Laboratory
Computational Sciences Center, which is supported by the
Office of Energy Efficiency and Renewable Energy of the U.S.
Department of Energy under Contract No. DE-AC36-
08GO28308.
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