Plasmonic vesicles of amphiphilic gold nanocrystals: Self-assembly and external-stimuli-triggered destruction

Article abstract of DOI:10.1021/ja204387w

We have developed a new class of plasmonic vesicular nanostructures assembled from amphiphilic gold nanocrystals with mixed polymer brush coatings. One major finding is that the integration of gold nanocrystals (nanoparticles and nanorods) with two types of chemically distinct polymer grafts, which are analogous to block copolymers as a whole, creates a new type of hybrid building block inheriting the amphiphilicity-driven self-assembly of block copolymers to form vesicular structures and the plasmonic properties of the nanocrystals. In contrast to other vesicular structures, the disruption of the plasmonic vesicles can be triggered by stimulus mechanisms inherent to either the polymer or the nanocrystal. Recent advances in nanocrystal synthesis and controlled surface-initiated polymerization have opened a wealth of possibilities for expanding this concept to other types of nanocrystals and integrating different types of nanocrystals into multifunctional vesicles. The development of multifunctional vesicles containing stimuli-responsive polymers could enable their broader applications in biosensing, multimodality imaging, and theragnostic nanomedicine.

Full text of DOI:10.1021/ja204387w

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Plasmonic Vesicles of Amphiphilic Gold Nanocrystals: Self-Assembly
and External-Stimuli-Triggered Destruction
Jibin Song,†,§ Lin Cheng,†,§ Aiping Liu,† Jun Yin,† Min Kuang,‡ and Hongwei Duan*,†
†School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, Singapore 637457
‡Ocean Nanotech, 2143 Worth Lane, Springdale, Arkansas 72764, United States
S Supporting Information
b
ABSTRACT: We have developed a new class of plasmonic
vesicular nanostructures assembled from amphiphilic gold
nanocrystals with mixed polymer brush coatings. One major
finding is that the integration of gold nanocrystals
(nanoparticles and nanorods) with two types of chemically
distinct polymer grafts, which are analogous to block
copolymers as a whole, creates a new type of hybrid building
block inheriting the amphiphilicity-driven self-assembly of
Figure 1. Schematic illustration of self-assembly of amphiphilic nano-
crystals with mixed polymer brushes into vesicular structures.
block copolymers to form vesicular structures and the
plasmonic properties of the nanocrystals. In contrast to
other vesicular structures, the disruption of the plasmonic
self-assembly of nanocrystals driven by the decrease in total free
vesicles can be triggered by stimulus mechanisms inherent
energy.10 Recent work by several groups has also shown that
to either the polymer or the nanocrystal. Recent advances in
nanocrystals with single-component polymer brush coatings
nanocrystal synthesis and controlled surface-initiated po-
can assemble into vesicles by taking advantage of the different
lymerization have opened a wealth of possibilities for
expanding this concept to other types of nanocrystals and
integrating different types of nanocrystals into multifunc-
tional vesicles. The development of multifunctional vesicles
hydrophobicities of the nanocrystals and polymer brushes.11
Here we report a new class of plasmonic vesicular nanostruc-
tures assembled from amphiphilic nanocrystals with mixed polymer
brush coatings, as illustrated in Figure 1. Organization of nano-
containing stimuli-responsive polymers could enable their
crystals with polymer brush coatings into well-defined assemblies
broader applications in biosensing, multimodality imaging,
and theragnostic nanomedicine.
is the subject of intense research.11,12 Our results have shown
that the integration of gold nanocrystals (nanoparticles and
nanorods) with two types of chemically distinct polymer grafts, which
are analogous to block copolymers as a whole, creates a new type
of hybrid building block inheriting the amphiphilicity-driven self-
assembly of block copolymers to form vesicular structures and
the plasmonic properties of the nanocrystals. In the vesicles, the
nanocrystals impart their intriguing properties to the assembly,
and in addition, the strong interparticle plasmonic coupling leads
to collective properties different from those in discrete units.
More interestingly, the covalently anchored “smart” polymer
grafts provide a means of modulating the interparticle coupling
through their stimuli-responsive conformational changes, mak-
ing the plasmonic vesicles potential candidates for integrated
delivery and sensing applications. This is clearly not available for
nanocrystal-loaded polymersomes (liposomes) and colloido-
somes because of the absence of responsive mechanisms and
the noncovalent linking between the vesicle shell and nanocryst-
als. The vesicles have sizes on the nanometer scale (∼200 nm), which
is similar to polymersomes but much smaller than colloidosomes
(mostly tens to hundreds of micrometers in size), although nano-
crystals form closely attached monolayers in the shells of both
assemblies. Another interesting finding is that destruction of the
elf-assembled vesicular structures with a water-filled compart-
ment enclosed by a thin shell are of considerable interest in
S
many fields ranging from materials sciences to biophysics and
nanomedicine.1 Varieties of building blocks for constructing
vesicles for the encapsulation and delivery of active ingredients
such as drug molecules and optical probes have been actively
explored.1,2 Natural and synthetic amphiphiles can assemble into
liposomes with a bilayer structure mimicking the cell membrane
through self-aggregation of their hydrophobic tails in an aqueous
medium.1b,c,3 Similarly, amphiphilicity-driven self-assembly of
block copolymers gives rise to polymersomes with tailored mechan-
ical andstructuralproperties.4,5 Advances in chemistry leading toward
well-defined polymeric amphiphiles with diverse functionalities
have enabled the development of polymersomes whose perme-
abilities are responsive to external stimuli such as pH, light, and
enzymatic degradation.6,7 Recently, the growing interest in
functional vesicles has stimulated research activities utilizing as
building blocks metal and semiconductor nanocrystals with unique
optical, electronic, and magnetic properties.8ꢀ11 Nanocrystals
have been incorporated into liposomes8 and polymersomes9
through noncovalent interactions. Alternatively, colloidosomes,
which are large microcapsules composed of a monolayer of densely
packed nanocrystals, have been obtained through the interfacial
Received: May 12, 2011
Published: June 23, 2011
r
2011 American Chemical Society
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Figure 2. TEM (a) and SEM (b) images of the plasmonic vesicles
assembled from 14 nm gold nanocrystals with mixed poly(ethylene
glycol) and poly(methyl methacrylate) brushes.
Figure 3. (a) UVꢀvis spectra of the amphiphilic nanocrystals in
chloroform and the vesicles in aqueous media: Au@PEG/PMMA
(black solid line), vesicles of Au@PEG/PMMA (black dashed line),
Au@PEG/PMMAVP containing 25% 4VP in the hydrophobic brush
(red solid line), vesicles of Au@PEG/PMMAVP at pH 7.0 (red dashed
line), and the disassembled vesicles of Au@PEG/PMMAVP at pH 5.0
(red dot line). (b, c) Dark-field microscope images of the vesicles of
Au@PEG/PMMAVP at (b) pH 7.0 and (c) pH 5.0.
plasmonic vesicles can be triggered not only by external stimuli
specific to the amphiphilic polymer grafts but also by the photo-
thermal conversion property of gold nanorods in the near-IR
(NIR) spectral window (650ꢀ 900 nm).
Amphiphilic nanocrystals with mixed polymer brushes were
synthesized through a two-step approach consisting of sequen-
tially conducted “grafting to” and “grafting from” reactions, as we
reported recently [see the Supporting Information (SI)].13 The
first step was a ligand exchange reaction in which hydrophilic
poly(ethylene glycol) (PEG) and the atom-transfer radical
polymerization (ATRP) initiator 2,20-dithiobis[1-(2-bromo-
2-methylpropionyloxy)]ethane (DTBE) were simultaneously at-
tached to gold nanocrystals through covalent AuꢀS bonds. In
the second step, the purified nanocrystals were used as macro-
initiators for surface-initiated ATRP of hydrophobic or stimuli-
response monomers. This strategy offers the possibility of inte-
grating chemically distinct polymer grafts on nanocrystals with
well-controlled physicochemical properties and the flexibility to
tune the structural parameters (ratio, molecular weight, and graft
density) of the mixed polymer brushes.14 In this study, the plasmonic
vesicles were formed using the film rehydration method, which
is commonly used for the preparation of polymersomes (see
the SI). We first examined the self-assembly of 14 nm gold nano-
particles with PEG and hydrophobic poly(methyl methacrylate)
(PMMA) grafts (Au@PEG/PMMA). The nanoparticles carry
88 PEG (Mn = 5 kDa) and 176 PMMA (Mn = 22 kDa) grafts on
average, as calculated on the basis of the molar ratio (PEG:
The surface plasmon resonance (SPR) of gold nanoparticles is
extremely sensitive to changes in the local environment sur-
rounding the particles and to the interparticle distances.15 In
UVꢀvis spectra (Figure 3a), the SPR band of the plasmonic
vesicles shows a 30 nm red shift relative to the SPR band of the
discrete nanoparticles in chloroform (530 nm) that is due to the
strong interparticle plasmonic coupling in the shell. Recent
developments in surface-initiated ATRP have made it possible
to graft well-defined polymers of functional monomers onto nano-
crystals, providing access to new building blocks with stimuli-
sensitive surface coatings.16 In a model system, we introduced
25% 4-vinylpyridine (4VP, pKa = 5.4)17 in the hydrophobic
PMMA grafts (Figure S1 in the SI), creating pH-responsive
plasmonic vesicles using the pH-sensitive nanocrystal (Au@PEG/
PMMAVP). Figure 3a shows that the SPR peak (546 nm) of the
Au@PEG/PMMAVP vesicles was also red-shifted at pH 7.0, as
observed for the vesicles without the 4VP moiety, but to a lesser
extent. This should arise from the electrostatic repulsion induced
by the partially protonated pyridine groups at pH 7.0, which
increases the interparticle distances of the gold nanoparticles and
reduces the plasmonic coupling as a result. We also found that the
degree of plasmonic coupling in the vesicles can be tuned by
changing the amount of 4VP. For example, the vesicle containing
10% 4VP had an SPR peak centered at 550 nm (Figure S2),
which lies between those of the vesicle with 100% PMMA and
the one with 25% 4VP. It should be noted that the interparticle
distances shown in the TEM images of the two vesicles
(Figure 2a and Figure S3) are not significantly different because
TEM images show the dry state under high vacuum. When
the pH was tuned from 7.0 to 5.0, the SPR peak of the vesicle ex-
perienced a sudden blue shift to 536 nm (Figure 3a and Figure S4),
and TEM observation revealed that vesicles dissociated into
single nanoparticles and small clusters of nanoparticles. This was
also manifested by the results of dynamic light scattering measure-
ments, which showed that vesicles with diameters of ∼260 nm
disintegrated into smaller particles with hydrodynamic sizes of
∼30 nm (Figure S5). This pH-triggered disruption of the vesicle
can potentially be used for intracellular delivery of therapeutic
agents by taking advantage of the acidic environment in the
endocytic pathway.18 Plasmonic nanoparticles scatter light very
strongly at the SPR wavelengths.15b We found that scattering
from individual vesicles can be easily detected using a dark-field
1
PMMA = 1:2) of the two grafts deduced from H NMR and
molecular weight information and the weight fraction (20%) of
organic polymer brushes measured by thermogravimetric anal-
ysis (see the SI).13 Transmission electron microscopy (TEM)
observation (Figure 2a) of the resultant vesicles showed a
characteristic image of hollow structures with a clear contrast
between the interior and the shell. The polymer part is not visible
in the TEM image because of its poor contrast. The shell thick-
ness was measured to be equal to the diameter of the gold nano-
particle, suggesting that the shell of the vesicle is composed of a
monolayer array of nanocrystals. Scanning electron microscopy
(SEM) (Figure 2b) further confirmed that the vesicles have a hollow
cavity enclosed by a thin shell. At higher magnification, closely
attached individual nanoparticles on the shell are clearly visible.
The vesicles retained their spherical morphology without collapse
after they were dried under vacuum, indicative of their excellent
mechanical stability. In the shell of the vesicle, it is expected that
nanocrystals are embedded in the collapsed hydrophobic PMMA
grafts, and the hydrophilic PEG chains reorganize to face the
aqueous environment on either side of the shell, as displayed in
Figure 1.
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been trapped inside the hydrophobic core of the shell. Therefore,
the orientation along the surface to form a dense monolayer
is thermodynamically more favorable for the anisotropic nano-
rods. UVꢀvis spectrometry (Figure 4b) showed that in the
vesicle, both the transverse and longitudinal plasmon resonances
of the gold nanorods were red-shifted and became broader
because of the strong plasmonic coupling of the nanorods in
close proximity. Next, we tested the light-induced deconstruction
of the vesicles by irradiating a drop of vesicle dispersion on
substrates with a 785 nm diode laser at 2 W/cm2. TEM and SEM
observations (Figure 4c,d) revealed that the vesicles lost their
spherical morphology after 2 min of irradiation and were
disrupted into flattened aggregates, confirming that the localized
heating generated by the assembled gold nanorods was sufficient
to induce collapse of the PMMA shell. Notably, the vesicles
are thermally stable enough for applications under physiological
conditions, since heating at 65 °C for 30 min did not lead to
evident changes in the hydrodynamic size distribution of the
vesicles (Figure S6). Obviously, in contrast to many other
plasmonic structures, the photothermal treatment and the che-
motherapeutic agents hosted in the large cavity could generate
a synergistic effect when the nanorod vesicles are used for
therapeutic applications.
In summary, we have presented a new type of plasmonic
vesicular structure based on amphiphilic nanocrystals with mixed
polymer brush coatings as building blocks. In three model
vesicles of plasmonic gold nanostructures, we have demonstrated
that the new building blocks consisting of a “hard” nanocrystal
core and “soft” polymeric grafts provide unprecedented oppor-
tunities for flexible tuning of the collective optical properties of
the vesicles and triggering of vesicle disruption by stimulus
mechanisms inherent to either the polymer or the nanocrystal.
Recent developments in nanocrystal synthesis and controlled
surface-initiated polymerization have opened a wealth of possi-
bilities for expanding this concept to other types of nanocrystals
and integrating different types of nanocrystals into multifunc-
tional vesicles. We envision that the development of multi-
functional vesicles containing stimuli-responsive polymers could
enable their broader applications in biosensing, multimodality
imaging, and theragnostic nanomedicine.
Figure 4. Self-assembly and NIR-irradiation-induced disruption of
AuNR@PEG/PMMA vesicles. (a) TEM and (inset) SEM images of
the vesicle. (b) UVꢀvis spectra of as-prepared gold nanorods coated
with cetyltrimethylammonium bromide (black line), AuNR@PEG/
PMMA in chloroform (red line), and the nanorod vesicles (blue line).
(c) TEM and (d) SEM images of a vesicle after 2 min of irradiation with a
785 nm laser.
microscope (Figure 3b) and that the signal completely disap-
peared after the pH decreased from 7.0 to 5.0 (Figure 3c). The
scattering cross sections of single nanoparticles are highly depen-
dent on their sizes,19 and the 14 nm gold nanoparticles used here
exhibited negligible light scattering. Obviously, the plasmonic
coupling in vesicles leads to a greatly enhanced scattering
cross section, which decreases dramatically after the vesicles
disintegrate into single nanoparticles at pH 5.0. This ON/
OFF switch of light scattering between vesicles and single
nanoparticles can be used to monitor the disintegration of the
vesicles using a dark-field microscope, which would allow the
release of payloads encapsulated in the vesicles to be traced in
real time.
Plasmonic nanostructures can rapidly convert the light ab-
sorbed at the SPR wavelengths into thermal energy to heat up the
surrounding medium.15b This photothermal conversion prop-
erty has attracted tremendous attention in the development of
plasmonic nanomaterials for photothermal therapy20 and re-
motely controlled drug delivery.21 Particularly for in vivo applica-
tions, nanostructures with SPR bands in the NIR spectral
window are highly desirable because of the smaller extent of
attenuation of light by blood and soft tissue in this spectral
range.22 Gold nanorods have emerged as promising candidates
for this purpose because their longitudinal plasmon resonance
can easily be tuned into the NIR region by variation of their
aspect ratio.20a,d Here we utilized gold nanorods (∼13 nm ꢁ
51 nm) with mixed PEG and PMMA brushes (AuNR@PEG/
PMMA) to prepare plasmonic vesicles (see the SI). Figure 4a
shows that vesicles with diameters of ∼200 nm were successfully
obtained. Interestingly, all of the anisotropic nanorods were
aligned parallel to the vesicle surface, as shown in the SEM
image (Figure 4a inset). The nanorod length is 51 nm, and the
contour length of a fully stretched PEG chain (Mn = 5 kDa) is
∼50 nm. If the nanorods had been oriented perpendicular to the
surface, a large portion of the hydrophilic PEG grafts would have
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, support-
b
ing figures, and complete ref 3a. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
hduan@ntu.edu.sg
Author Contributions
§These authors contributed equally.
’ ACKNOWLEDGMENT
H.D. thanks the Nanyang Assistant Professorship Program for
financial support.
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