Directing self-assembly of nanoparticles at water/oil interfaces

Article abstract of DOI:10.1002/anie.200460920

Finding the right angle: The contact angle of nanoparticles at the water/oil interface can be engineered close to 90° by capping with ligands containing carboxylic ester terminal groups. This drives the nanoparticles to self-assemble into close-packed films (see picture), and thus provides the opportunity to create two- or three-dimensional homo- or heterogeneous nanostructures for electronic, optoelectrical, and magnetic applications.

Full text of DOI:10.1002/anie.200460920

Angewandte
Chemie
Nanoparticles
Directing Self-Assembly of Nanoparticles at
Water/Oil Interfaces**
Hongwei Duan, Dayang Wang,* Dirk G. Kurth, and
Helmuth Möhwald
Nanoparticles (NPs) have attracted a great deal of attention
because of their promising potential and already realized
applications. Their unique electronic, magnetic, and optical
properties can be manipulated by simply varying the dimen-
[1]
sion and geometry. The integration of nanoparticles into
one-, two-, or three-dimensional (2D or 3D) structures leads
to novel collective properties that can be manipulated by the
control of the cooperative interactions between the nano-
Scheme 1. a) Schematic representation of the position of a particle at
a water/oil interface for a contact angle with the interface less than 908
(left), equal to 908 (center), and larger than 908 (right). b) Schematic
[
2]
particles. While nanoparticles may be chemically assembled
by the interaction of ligands capped on nanoparticles, the
illustration of the structures of Au@DTBE (left) and g-Fe
O @BMPA
3
2
[
3]
nanoparticles (right).
elaborate design of ligands is a prerequisite. For organic
nanoparticles, which are formed directly in organic media or
transferred from aqueous media with the aid of surfactants,
their 2D or 3D arrays can be simply realized by controlled
interfaces into bulk phases. The profile of the calculated
partition of 10-nm particles in water/oil two-phase systems
shows that only when the contact angle of the nanoparticles is
close to 908 may they reside at the interface; when the contact
angle slightly deviates from 908, the nanoparticles prefer to go
to the bulk phase, suggesting that the contact angle of 908
should play a pivotal role for the interface entrapment of
[
2b,4]
evaporation of solvents
using the Langmiur–Blodgett technique.
Thin films of nanoparticles have been produced in situ at
or at the water/air interface by
[
2a,5]
[
6]
interfaces between immiscible liquids. Most recently, Lin
and co-workers directed the assembly of hydrophobic CdSe
nanoparticles at the water/toluene interface and they ach-
ieved size-dependent interfacial entrapment of CdTe nano-
[
8a]
nanoparticles. Reincke and co-workers observed that the
addition of ethanol renders the contact angle of Au-NPs with
the water/hexane interface close to 908, which is a crucial
point for the interfacial entrapment of nanoparticles in their
[
7a]
particles.
Meanwhile, Reincke and co-workers reported
that the introduction of ethanol can pull hydrophilic citrate-
stabilized Au-NPs into the water/heptane interface, leading to
[
7b]
[7b]
a closely packed monolayer.
These two reports demon-
work.
strate a promising way to create a 2D or 3D arrangement of
both hydrophobic and hydrophilic nanoparticles at water/oil
interfaces. The use of interfaces between water/oil fluids to
trap sub- or micrometer-sized particles has been well estab-
Recent studies on phase transfer of aqueous Au-NPs show
that by using single long-chain alkylamines, one may transfer
carboxylic acid derivatized Au-NPs onto water/oil interfa-
[
9]
ces. Wei and co-workers have observed interfacial attach-
ment of the Au nanoparticles capped with a resorcinarene
[
8]
lished. Besides the particle diameter, this interfacial entrap-
[
5c]
ment is mainly determined by the contact angle (q) of the
ligand. These reports suggest that interfacial entrapment
and self-assembly of nanoparticles might be realized by
appropriate hydrophobic coating. It is well known that
substrates with carboxylic ester groups possess contact
[
8]
particle with the water/oil interface—surface wettability. As
schematically depicted in Scheme 1a, when a particle is
hydrophilic or hydrophobic it has a contact angle smaller
[
10]
(
left) or larger than 908 (right) in a water/oil system and is
angles in the range of 80–1108. Herein, we report that the
capping with ligands with a terminal 2-bromopropionate
group may lead to contact angles for aqueous or organic
nanoparticles close to 908, thus driving nanoparticles to reside
and self-assemble at water/oil interfaces. The choice of these
ligands was encouraged by the success of living free-radical
localized totally within the water or oil phase. When its
contact angle is around 908 (Scheme 1a, center) the particle
prefers to reside at the interface. As the particles approach
nanometer size, however, the thermal energy becomes com-
[
7a]
parable to the interfacial energy.
In this case, spatial
[
11]
fluctuations suffice to remove nanoparticles trapped at
polymerization on nanoparticles. The significance of our
work lies on the fact that it should provide a general protocol
for trapping and organizing nanoparticles at water/oil inter-
faces.
[*] H. Duan, Dr. D. Wang, Dr. D. G. Kurth, Prof. H. Möhwald
Max Planck Institute of Colloids and Interfaces
In this work, 5-nm and 12-nm Au nanoparticles were
synthesized by citrate reduction of chloroauric acid in the
14424 Potsdam (Germany)
Fax: (+49)331-5679202
E-mail: dayang.wang@mpikg-golm.mpg.de
[
12]
aqueous phase. Owing to their negatively charged surface,
these Au-NPs are highly hydrophilic, residing only in the
water phase. By means of ligand exchange, these aqueous Au-
NPs were capped with 2,2’-dithiobis[1-(2-bromo-2-methyl-
[
**] This work is supported by the Max Planck Society. We thank J.
Hartmann and R. Pitschke for assistance with the TEM measure-
ments.
Angew. Chem. Int. Ed. 2004, 43, 5639 –5642
DOI: 10.1002/anie.200460920
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5639

Communications
propionyloxy)ethane] (DTBE), denoted as Au@DTBE.
Scheme 1b (left) schematically represents the structure of
Au@DTBE nanoparticles. The optimal density of DTBE
capped on Au-NPs, determined by thermogravimetric anal-
ysis, is around 3 wt% in our work, corresponding to about 800
DTBE molecules per Au-NP. Addition of toluene to the
aqueous dispersions of Au@DTBE nanoparticles led to their
spontaneous transfer onto the water/toluene interface, and
their self-assembly into thin films. This interfacial entrapment
of Au@DTBE nanoparticles may be accelerated by gentle
shaking. Figure 1a is a typical photograph of the result of the
self-assembly of 12-nm Au@DTBE nanoparticles at the
water/toluene interface; one can see a giant water droplet
enclosed by a thin film of golden reflectance and blue
transmittance. This metallic luster results from the electronic
coupling of Au-NPs, suggesting the formation of closely
[
2a,9,13]
packed nanoparticle thin films.
The UV/Vis spectra
demonstrate that no Au@DTBE nanoparticles exist in water
or are transferred into the toluene phase. The films of Au@
DTBE nanoparticles formed at the water/toluene interface
can be transferred onto solid substrates by using the
Langmuir technique, which allows us to determine the contact
angle of Au@DTBE nanoparticles with the water/toluene
interface. Figure 1b reveals that Au@DTBE nanoparticles
have a contact angle of 908 with the water/toluene interface,
whereas the contact angle of citrate-stabilized Au-NPs is
about 608 (Figure 1c). After optimizing the concentration of
Au@DTBE nanoparticles and the area of the water/toluene
interface, we obtained the monolayer of Au@DTBE nano-
particles at the interface. Figure 1d shows a typical trans-
mission electron microscopy (TEM) image of a monolayer of
1
2-nm Au@DTBE nanoparticles. This monolayer does not
display long-range order, possibly because of the broad size
distribution of nanoparticles used. In such a monolayer, as
[
7b]
observed by Reincke and co-workers,
domains of close-
packed Au@DTBE nanoparticles coexist with large voids; the
latter is difficult to explain. In addition, we also achieved
interfacial entrapment and self-assembly of Au@DTBE
nanoarticles by addition of chloroform and even triethyl-
amine, that is partially immiscible with water. Besides DTBE,
we obtained similar results using 11,11’-dithiobis[1-(2-bromo-
Figure 1. a) Photograph of the self-assembled Au@DTBE nanoparticles
at the water/toluene interface in a plastic Eppendorf tube. This tube
has been titled such that the colored area corresponds to the water/
toluene interface. Owing to the reflectance from the sides, only the
blue-purple area represents a transmission image. b,c) Photographs of
a 5-mL water droplet, covered with toluene, resting on the surface of a
thin film of 12-nm Au@DTBE nanoparticles, transferred from the
water/toluene interface on a glass slide (b) and on the surface of a
thin film of 12-nm Au-NPs cast on a glass slide (c). d) TEM image of
the monolayer of 12-nm Au@DTBE nanoparticles, formed at the
water/toluene interface; inset: high-magnification TEM picture, the
scale bar is 25 nm.
2
-methylpropionyloxy)undecane] (DTBU) as capping
[
11b,d]
ligands,
suggesting that in our system surface wettability
of nanoparticles mainly depends on the terminal group of the
capping ligand, 2-bromopropionate group, rather than on the
length of the alkyl chain. Furthermore, using DTBE and
DTBU as capping ligands, we have also attached and
organized 5-nm Au-NPs into a closely packed monolayer at
the water/oil interface.
As the surface wettability of the nanoparticles is predom-
inant in the process of interfacial entrapment, our procedure
has been successfully extended to aqueous Ag-NPs. Following
the aforementioned ligand-exchange procedure, aqueous Ag-
NPs of 10–40 nm in size, prepared by reduction of silver
more, the mixture of Au@DTBE and Ag@DTBE naoparti-
cles has been attached at water/toluene interfaces, leading to
nanoalloy films (Figure 2b). Their TEM images reveal
random close-packing arrays, in which, however, one has
difficulty in identifying the chemical identity of the nano-
particles based on the contrast level in the bright field
[
14]
nitrate by sodium borohydride, were capped with DTBE
Ag@DTBE). The as-made Ag@DTBE nanoparticles display
(
interfacial behavior similar to that of Au@DTBE nano-
particles; they prefer to reside at the water/oil interface,
creating a film with metallic reflectance (Figure 2a). Further-
5
640
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Angewandte
Chemie
measuring the contact angle of these g-Fe O nanoparticles
2
3
with the water/toluene interface since the nanoparticles were
immediately removed into the toluene phase upon addition of
toluene to their thin films cast on a glass slide. The stabilizers
on the g-Fe O nanoparticles were replaced with 2-bromo-2-
2
3
methylpropionic acid (BMPA) by incubating nanoparticles
[
11c]
with BMPA for two days.
These BMPA-capped g-Fe O
2 3
nanoparticles are denoted as g-Fe O @BMPA (Scheme 1b
2
3
(
right)). Upon addition of water to a dispersion of g-Fe O @
2 3
BMPA in toluene, g-Fe O @BMPA nanoparticles were spon-
2
3
taneously transferred into the water/toluene interface; as for
the Au@DTBE nanoparticles, this process was favored by
gentle shaking. The inset in Figure 3 shows a typical picture of
Figure 3. TEM image of the thin film of 4-nm g-Fe O @BMPA nano-
2
3
particles formed at the water/toluene interface. The yellow layer
formed at the water/toluene interface is clearly visible in the photo-
graph shown in the inset.
Figure 2. Photographs of thin films of Ag-NPs (a) and of a mixture of
Au- and Ag-NPs at a molar ratio of 1:1 (b), formed at the water/tolu-
ene interface in a plastic Eppendorf tube.
the interfacial entrapment of g-Fe O @BMPA nanoparticles;
2
3
a yellow layer formed at the water/toluene interface is clearly
visible. After transferring this film onto a glass slide, we
observed that the contact angle of g-Fe O @BMPA nano-
2
3
[
4a]
image. Moreover, we observed that the transmitted and
reflected color of the resultant nanoalloy films varied with the
molar ratio of Au@DTBE to Ag@DTBE nanoparticles. The
use of interfacial entrapment of nanoparticles to construct
composite thin films is an ongoing project in our laboratory.
To gain a better insight into the effect of the terminal
group 2-bromopropionate, we conducted interfacial entrap-
ment with hydrophobic nanoparticles. We prepared 4-nm g-
Fe O nanoparticles by oxidation of iron(iii) acetylacetonate
particles with the water/toluene interface is close to 908,
similar to that of Au@DTBE nanoparticles. Unlike for the
interfacial entrapment of Au@DTBE nanoparticles, we lack
the ability to fabricate monolayers of g-Fe O @BMPA nano-
2
3
particles at the water/toluene interface. As shown in Figure 3,
both monolayers and multilayers of closely packed nano-
particles were observed. Control of the thickness of the films
of g-Fe O @BMPA nanoparticles formed at the water/tolu-
2
3
ene interface is underway.
2
3
followed by stabilization with oleic acid and oleylamine in
In summary, we have capped aqueous Au- and Ag-NPs
with DTBE and DTBU and nonaqueous g-Fe O nano-
[
15]
toluene.
As the long carbon chains of oleic acid and
2
3
oleylamine capping the g-Fe O nanoparticles are exposed
outwards, these nanoparticles are highly hydrophobic and
thus may reside only in nonpolar media. We had difficulty in
particles with BMPA. The terminal 2-bromopropionate group
of these ligands may render the contact angles of the
nanoparticles at the water/oil interface close to 908, driving
2
3
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5641

Communications
nanoparticles to the water/oil interface and to self-assemble
into closely packed arrays. Meanwhile, the possibility to form
nanoalloy films at interfaces has also been demonstrated. The
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-bromopropionate group rather than the length of the
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surface wettability of modified nanoparticles. Thus, our work
is an important step towards interfacial entrapment and
assembly of nanoparticles for the creation of 2D or 3D
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nanoparticles with ligands containing various carboxylic ester
terminal groups to generalize our concept and to establish a
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[
Experimental Section
DTBE and DTBU were prepared by acrylation of bis(2-hydroethyl)
disulfide and bis(2-hydroethyl) disulfide, obtained by employing the
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method described by Hawker and co-workers. Briefly, 2-bromo-2-
methylpropionyl bromide (14.8 mmol) was added dropwise to a
mixture of the disulfide (6.17 mmol) and triethylamine (31.5 mmol) in
dichloromethane (150 mL) at 08C under an argon atmosphere. The
solution was stirred at 08C for 1h and then for another 2 h at room
temperature. After the precipitates were filtered off, the organic
phase was extracted with 2n Na CO solution saturated with NH Cl
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2
3
4
to remove the excess bromides. Subsequent removal of dichloro-
methane yielded DTBE and DTBU.
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DTBE (4.6 mg) or DTBU (7.2 mg) was dissolved in THF (10 mL)
and the solution then added dropwise to an aqueous solution of Au or
Ag-NPs (40 mL). After incubating the mixtures for 6–12 h, followed
by removal of excess initiators by centrifugation, the initiator-capped
nanoparticles were obtained and suspended in water for further use.
A solution of 4-nm g-Fe O nanoparticles in toluene (5 mL) was
2
3
incubated with a solution of BMPA (1.67 mg) in toluene (5 mL) for
two days. The subsequent removal of excess BMPA yielded g-Fe O @
2
3
BMPA nanoparticles and they were suspended in toluene for further
use.
UV/Vis absorption spectra were recorded by using a Cary 50 UV-
visible spectrophotometer. TEM images were obtained by using a
Zeiss EM 912 Omega microscope at an acceleration voltage of
1
20 kV. Contact angle measurements were implemented with a
contact angle measuring system G10 apparatus (Krüss, Germany) at
ambient temperature.
Received: June 9, 2004
Keywords: colloids · interfaces · nanoparticles · self-assembly ·
.
surface wettability
[
[
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Angew. Chem. Int. Ed. 2004, 43, 5639 –5642