J Nanopart Res (2012) 14:826
DOI 10.1007/s11051-012-0826-4
RESEARCH PAPER
Formation of net-like patterns of gold nanoparticles
in liquid crystal matrix at the air–water interface
Jan Paczesny • Krzysztof Sozański •
_
Igor Dzie˛cielewski • Andrzej Zywociński
Robert Hołyst
•
Received: 19 January 2012 / Accepted: 9 March 2012 / Published online: 31 March 2012
Ó The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract Controlled patterning and formation of
nanostructures on surfaces based on self-assembly is a
promising area in the field of ‘‘bottom-up’’ nanomaterial engineering. We report formation of net-like
structures of gold nanoparticles (Au NPs) in a matrix
of liquid crystalline amphiphile 40 -n-octyl-4-cyanobiphenyl at the air–water interface. After initial
compression to at least 18 mN m-1, decompression
of a Langmuir film of a mixture containing both
components results in formation of net-like structures.
The average size of a unit cell of the net is easily
Electronic supplementary material The online version of
this article (doi:10.1007/s11051-012-0826-4) contains
supplementary material, which is available to authorized users.
_
J. Paczesny K. Sozański A. Zywociński
R. Hołyst (&)
Institute of Physical Chemistry, Polish Academy of
Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
e-mail: rholyst@ichf.edu.pl; robert.holyst@gmail.com
J. Paczesny
e-mail: jpaczesny@ichf.edu.pl
K. Sozański
e-mail: krzysiek.sozanski@gmail.com
_
A. Zywociński
e-mail: azywocinski@ichf.edu.pl
I. Dzie˛cielewski
Institute of High Pressure Physics Unipress,
Polish Academy of Sciences, Sokołowska 29/37,
01-142 Warsaw, Poland
e-mail: igor@unipress.waw.pl
adjustable by changing the surface pressure during the
decompression of the film. The net-like patterns of
different, desired average unit cell areas were transferred onto solid substrates (Langmuir–Blodgett
method) and investigated with scanning electron
microscopy and X-ray reflectivity (XRR). Uniform
coverage over large areas was proved. XRR data
revealed lifting of the Au NPs from the surface during
the formation of the film. A molecular mechanism of
formation of the net-like structures is discussed.
Keywords Gold nanoparticles Langmuir films
Langmuir–Blodgett films Nanostructures Net
Nanoporosity
Introduction
‘‘Bottom-up’’ lithography, also denoted as ‘‘soft’’
lithography, has been recognized as complementary to
the widely used ‘‘top-down’’ approach (Moriarty
2001). Nano- and atomic-scale manipulation by
scanning probe techniques as well as self-assembly
can possibly overcome some of the limitations of
classical methods (Whitesides et al. 1991; Kiely et al.
1998; Li et al. 1999; Pileni 2001; Fendler 2001;
Sanchez et al. 2001). A properly planned and managed
sequence of steps of ‘‘bottom-up’’ self-assembly may
allow obtaining complex systems. ‘‘Top-down’’ is
gradually approaching its limits, as device features are
down-scaled into the sub-100-nm regime. The cost of
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miniaturization associated with lithography equipment and operating facilities is likely to create an
economical barrier for development of conventional
processors and memory chips (Frank et al. 2001;
Likharev 2003). It seems that the ideal way of
preparation of future electronic devices would be
one combining these two approaches, i.e., facilitating
a scaffold, obtained via lithography or other highprecision technique, decorated with a functional
pattern of precisely located nanoscale objects
(Bjørnholm et al. 1998). Selective, controlled patterning of surfaces with nanoscale objects is therefore a
matter of significant importance and much effort has
to be directed to understanding forces governing the
self-assembly processes (Bjørnholm et al. 1999).
Among different techniques applicable for
‘‘bottom-up’’ material fabrication, Langmuir–Blodgett (LB) deposition has been found to be one of the
most promising. It requires simple equipment, yet
ensures high reproducibility and very good control
over the process and the prepared structures. Moreover, a variety of chemical building blocks for selfassembly is available. An ordered monolayer can be
easily transferred onto solid substrates with great
fidelity (Blodgett and Langmuir 1937).
One of very important topics in contemporary
research on thin films is films of periodically
organized nanoporosity (Sanchez et al. 2008). Cellular
structures were obtained in case of Au NPs (Kane et al.
2010) as well as Ag NPs (Sun et al. 2001) or Co NPs
(Petit et al. 1998). One- and two-dimensional networks
of nanoparticles are among the most applicable ones
(Srivastava and Kotov 2009). The authors described
the possible applications of unique optical (spectroscopy), magnetic (bio-imaging), and electronic (single
electron transistors) properties of such structures. 1D
array of Pd NPs was used to manufacture a gas sensor
(Favier et al. 2001). In another example, a DNA
detection sensor was based on a 2D structure of Au
NPs (Charrier et al. 2006). An interesting paper by
Kane et al. (2010) describes an Au NPs net which
exhibits electronic switching based on gating by
metabolic activity of yeast cells deposited on the
structure.
Net-like structures were also found in LB systems,
where gold nanoparticles are mixed with amphiphilic
molecules such as polymers (Hansen et al. 2008) or
phospholipids (Hassenkam et al. 2002; Mogilevsky
et al. 2010). Hassenkam et al. (2002) described a
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J Nanopart Res (2012) 14:826
mechanism of formation of a net-like structure by
hydrophobic Au NPs. The main idea is that the
nanoparticles act as impurities, which are repulsed to
the edges of domains solidifying during compression
of the Langmuir films. Mogilevsky et al. (2010)
confirmed such explanation by systematic studies on
behavior of Au NPs in LB films of a number of
phospholipids.
Some mixtures of Au NPs and liquid crystals have
already been studied. For instance, Vijayaraghavan
and Kumar (2009) described an ordered structure of
Au NPs in a matrix of a discotic LC. Our previous
work concerning bolaamphiphiles focused on preparation of surfaces of desired extent of coverage by
an ordered, densely packed monomolecular film
(Paczesny et al. 2011). The liquid crystals were
recognized as an ideal template for the ‘‘bottom-up’’
approach of preparation of more complex nanostructures. The recent review by Bisoyi and Kumar (2011)
concerning LC as an emerging avenue of soft selfassembly refers to around ninety articles showing
possibilities of utilization of LC properties.
We used the LB technique to create and transfer
onto a solid substrate the net-like structure of
controlled morphology based on amphiphilic gold
nanoparticles and 40 -n-octyl-4-cyanobiphenyl (8CB)
mixtures.
Pressure-induced layering transitions of 8CB have
been already intensively studied by means of: ellipsometry (Xue et al. 1992), Brewster angle microscopy
(BAM) (de Mul and Mann 1994, 1998; Friedenberg
et al. 1994; Modlińska et al. 2009), surface potential
measurements (Schmitz and Gruler 1995), and optical
second harmonic generation (SHG) (Guyot-Sionnest
et al. 1986). The SHG technique was also used to
confirm spontaneous organization of 8CB molecules
evaporated onto a solid substrate into multilayer stacks
(Olenik et al. 2003). Thus behavior of 8CB in LB
systems is well known and described.
Behind the collapse point 8CB does not form
disordered aggregates, yet a process of formation
of a multilayer occurs. The plateau region of the
p–A isotherm corresponds to formation of liquid
domains of a trilayer film. At the end of this plateau,
the entire film is a trilayer stack. Upon further
compression a rise of surface pressure is observed.
The second collapse point corresponds to a very close
packed trilayer film. Further decrease of available area
leads to a break in the film and formation of even
J Nanopart Res (2012) 14:826
thicker multilayer stacks (de Mul and Mann 1994,
1998; Friedenberg et al. 1994).
Experimental
Materials
8CB was obtained from Merck and used as received to
prepare a fairly dilute (1.04 mg cm-3) solution in high
purity (spectrally clean) chloroform. Gold nanoparticles (Au NPs) were synthesized according to a
procedure described elsewhere (Jana and Peng 2003)
and subsequently functionalized with TMA, so that ca.
10% of original undecanethiol ligands were substituted for TMA (Kalsin et al. 2006). Mean diameter of
Au NPs, assessed by means of SAXS measurements
and AFM images analysis, was 8.9 nm. The size
distribution was estimated to be less than 10% based
on high resolution scanning electron microscopy
(SEM) pictures. Au NPs exhibited a single, moderately narrow peak of zeta potential at ?35 mV. For the
analytical data on Au NPs, see Supporting Information. In all the experiments, 1:1 chloroform/methanol
solution of concentration of 0.5 mg(Au) ml-1 was
used. The amphiphilic Au NPs were found to form
stable Langmuir films. Upon spreading at the air–
water surface, the thiols rearrange so that the polar
groups are in contact with the water surface (Nørgaard
et al. 2004).
Methods
The LB experiments were performed with use of a
5 9 75 cm Langmuir trough (NIMA, Coventry,
England), enclosed in an acrylic glass cabinet. The
system was equipped with surface pressure and
potential sensors, a dipper and a temperature control
unit, as well as an NFT MiniBAM Brewster angle
microscope. Millipore filtered water (18.2 MX cm)
was used as a subphase. To ensure reproducibility of
achieved results, prior to every experiment the trough
was carefully cleaned with ethanol and rinsed with
water. Any remaining impurities floating on the
subphase were removed in iterated process of putting
the barriers together as close as possible and removing
the surface layer of water from in-between the barriers
with an aspirator. Rather than a traditional Wilhelmy
plate, single-use filter paper stripes were utilized in the
Page 3 of 11
surface pressure sensor. It was calibrated before every
experiment.
Solutions were applied onto the subphase with a
Hamilton microsyringe. The mixtures of two components (8CB and Au NPs) were prepared beforehand.
After 15 min needed for the solvent to evaporate, films
were compressed at a constant rate of 10 cm2 min-1.
BAM images were procured in situ, whereas for SEM
and X-ray reflectivity (XRR) measurements films had
to be transferred onto a solid substrate. The material of
choice was polished silicon of very low roughness, cut
into 2.5 9 0.8 cm wafers, purchased from the Institute of Electronic Materials Technology, Warsaw,
Poland. After cleaning with acetone and treating for at
least half an hour with ca. 30% nitric acid, wafers were
rinsed with water. The wafer was submerged vertically
in the trough prior to applying the investigated
solution onto the water surface. Film deposition was
conducted according to the LB technique at a dip
speed of 5 mm min-1, always during an upstroke.
XRR measurements were performed on a Bruker
D8 Discover diffractometer, operating at a wavelength
of 1.54 Å, with monochromatic parallel beam formation by a parabolic Goebel mirror. The system was
equipped with an Eulerian cradle and a reflectometry
sample-stage, which ensured precise sample positioning. Scintillation counter together with an automatic
absorber of primary beam allowed for linear dynamic
range better than 108 cps. For data analysis and fitting,
Leptos 4.02 software was applied.
SEM images were taken with use of a Zeiss LEO
1530 scanning electron microscope in the InLens
detection mode.
Results and discussion
A study of thin films of mixtures of Au NPs and 8CB is
hereby presented. For the sake of convenience,
concentrations of solutions used in LB experiments
are expressed in terms of volume of the solution
needed to cover 1 cm2 of the surface with a dense
monolayer (for more details see Supporting Information). The ratio of such concentrations equals in fact
the fraction of surface covered by one of the mixture
components. The reason why we use such system is
that it makes it easy to follow the evolution of patterns
with changes of the Au NPs surface coverage. For
example, a ratio of 1:1 means that both components
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Fig. 1 p–A isotherms of mixtures of Au NPs and 8CB of
different composition. The x-axis was scaled so that the amount
of Au NPs per an area unit was the same in all the plots. Only the
amount of 8CB changed
cover the same area at the interface when a monomolecular film is formed. A ratio 1:27 means that the area
covered by 8CB is 27 times greater than by Au NPs.
The influence of changes of the ratio of the
components on the obtained p–A isotherm was
investigated. The results are shown in Fig. 1. The
isotherms are scaled with respect to the area occupied
by Au NPs. We assumed that the diameter of single Au
NPs did not change significantly during the compression–decompression process and the surface coverage
J Nanopart Res (2012) 14:826
ratio is only influenced by the composition of the
mixture spread on the water surface. Therefore the
scaling, based only on the area occupied by both 8CB
and Au NPs, was linear. With changes of proportions
of the two mixture components, the shape of the
isotherm evolved from one characteristic for pure Au
NPs towards the shape obtained for pure 8CB. The
isotherm of a mixture of 1:27 ratio had a characteristic
plateau, which corresponded to the formation of a
trilayer film of 8CB. In case of the isotherm of a 1:1
sample, the character of the curve was predominantly
determined by the compression of Au NPs (for an
isotherm of pure Au NPs see Fig. S5 in Supporting
Information).
BAM images of the film were captured in real time
during the compression-decompression experiments.
In general, the brighter domains observed in the BAM
images corresponded to the more reflective regions,
populated by the Au NPs (Hoenig and Moebius 1991).
Exemplary BAM pictures taken during compressiondecompression cycle of a film of a 1:9 mixture are
shown in Fig. 2. The morphology observed during
compression did not differ from the one observed
when only Au NPs were spread at the air–water
interface (for comparison see Fig. S6 in Supporting
Information). The pictures shown in the bottom row of
Fig. 2 revealed however the formation of the net-like
structure of Au NPs in 8CB matrix during
Fig. 2 BAM pictures of a film of a mixture of Au NPs–8CB of composition 1:9. The net-like structure was visible only during the
decompression. Scale bar: 500 lm
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J Nanopart Res (2012) 14:826
Page 5 of 11
decompression. The film was initially compressed up
to at least 18 mN m-1 and then immediately decompressed. The net-like structure was visible in BAM
during the decompression starting from surface
pressure of around 15 mN m-1. It was not visible in
range of surface pressure from 18 to 15 mN m-1 due
to the limited resolution of the used equipment. Very
similar patterns featuring far smaller net unit cells
were observed with use of SEM in samples transferred
onto a solid substrate at surface pressures up to
17 mN m-1, however, still only in case of films that
were initially compressed to higher values of surface
pressure.
The SEM images of films of mixtures of different
compositions are presented in Fig. 3. The films were
transferred according to the LB method, after initial
compression to 18 mN m-1 and decompression to
surface pressure of 15 mN m-1. Then the transfer was
started after a time interval of 3 min. SEM images of
films prepared from mixtures of composition ratio 1:6
(Fig. 3c) and 1:9 (Fig. 3d) reveal Au NPs net-like
structures of very similar morphology as that observed
with use of BAM. As the amount of Au NPs in the
mixture was increased, aggregates started to appear
and the structure was strongly deformed (Fig. 3a, b).
On the other hand, in case of a film of 1:27
Fig. 3 SEM pictures of films transferred at 15 mN m-1 of Au
NPs–8CB mixtures of different compositions: a 1:1, b 1:3, c 1:6,
d 1:9, e 1:27 initially compressed to 18 mN m-1, and f 1:9
without initial compression and no time interval before the
transfer. Scale bar: 20 lm
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composition (small amount of Au NPs), the net was
not complete, i.e., the net units were open and not well
marked (Fig. 3e).
SEM picture of a film transferred at 15 mN m-1
without the initial compression step and no time
interval is shown in Fig. 3f. No net-like structure was
detected, which was in line with the BAM observations. We assumed that such film corresponded to the
structure of the film observed with use of BAM during
compression (verify with Fig. 2). Therefore, it can be
concluded that the net-like structure was not present
during the compression step up to *18 mN m-1.
In case of a film transferred immediately after
reaching the target surface pressure of 18 mN m-1,
the Au NPs were relatively densely distributed at the
surface. It was difficult to determine whether the netlike structure was present. If this was the case, the size
of a unit cell of the net was very small—in the range of
dimensions of the net ‘‘frames’’ composed of Au
NPs (see Fig. S7 in Supporting Information), so that
any hypothetical structure would be practically
undistinguishable.
Presence of organic compounds (8CB and thiols at
the Au NPs surface) might be undesirable for further
utilization of the obtained surfaces. Therefore, we
dipped the samples in the NaBH4 solution for 2 h.
Such treatment is known to reduce the S–Au bonds
(Yuan et al. 2008). This procedure resulted in a
bare gold surface without any organic residues and the
net-like patterns were preserved (See Fig. S8 in
Supporting Information).
During the decompression process, the area of the
unit cell of the net-like structure increased, as can be
noticed in Fig. 2. Therefore, the morphology could be
controlled not only by means of the composition ratio
of the 8CB–Au NPs mixtures. Also the surface
pressure during the decompression and available area
of the subphase influenced the film. We determined
the average area of the net unit cell within the film of
composition ratio 1:9 at different points during the
decompression from 18 mN m-1. The decompression
started just after reaching the target surface pressure,
and the barriers moved at a speed of 4 cm2/min (the
slowest possible). The result of analysis of unit cell
size correlated with the p–A decompression curve is
shown in Fig. 4. The first point at the plot was based on
the analysis of SEM pictures of a film transferred onto
solid substrate at 15 mN m-1. Further analysis was
based on BAM images. The increase of the average
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Fig. 4 Average size of a unit of a net-like structure versus area
of the water subphase upon decompression from 18 mN m-1of
a film of mixture of composition 1:9. Size distribution
histograms revealed increased polydispersity of the average
area of a unit cell upon decompression
size of a unit cell could be stopped at a desired value by
fixing the movable barriers’ positions.
When evolution of a single cell is concerned, it can
be noticed that in the beginning of the decompression
process it grows and the ‘‘frames’’ are thinning. At
some point the net units begin to merge. There is a
limit of the expansion of the structure. The histograms
show a significant increase of polydispersity of the
unit cells upon decompression.
XRR measurements are very sensitive for probing
film thickness, roughness and density variations in the
direction perpendicular to the surface. The method is,
therefore, very suitable for thin film investigation. Its
disadvantage is that the data simulation is nonunique
in the sense that a change of one parameter can be
compensated by other subsequent changes throughout
the model structure. Therefore it is crucial to set the
starting parameters within physically reasonable values or determine them by means of other techniques.
We used the CPK model (purely geometrical model
named from the names of Robert Corey, Linus
J Nanopart Res (2012) 14:826
Pauling, and Walter Koltun) for determination of the
simulation starting parameters based on the dimensions of 8CB and the organic shell of the Au NPs. We
kept the radius of the metallic core of the gold
nanoparticles constant and equal to the value obtained
from SAXS measurements (whole nanoparticle
radius) minus the length of the dodecanothiol molecule (organic shell). The starting parameters could
vary only in a reasonable, restricted range (±10%)
with exception for the thickness of the sublayer of the
metallic core of Au NPs. The results of XRR
measurements for a sample of composition ratio 1:9,
transferred at 15 mN m-1, are shown in Fig. 5. We
tested three different models of possible molecular
arrangement of the net-like structure: (1) the Au NPs
were in contact with the surface of the substrate, (2)
Fig. 5 a Open circles XRR profile of sample of mixture
composition 1:9 transferred at 15 mN m-1; solid curve simulated fit curve, b density profile used for XRR pattern simulation
corresponded to Au NPs located on the top of an 8CB monolayer
Page 7 of 11
the Au NPs were placed on the top of a monolayer of
8CB, (3) the Au NPs were placed on the top of a
trilayer of 8CB.
The best fit was obtained for a model that assumed
Au NPs placed on top of a monolayer of 8CB. The film
thickness (9.8 nm) was noticeably greater than the
dimensions of the nanoparticles (8.9 nm). The maximum density of the metallic core sublayer was found
at a distance of 5.4 nm from the substrate. It might be
assumed that this is where, on average, the centers of
the Au NPs were located. Therefore, the distance from
5.5 to 9.8 nm from the substrate surface (i.e., 4.3 nm)
corresponds to the radius of an Au NP. According to
SAXS measurement, this value equals 4.45 nm (half
of the diameter). Thus, the thickness of the layer
placed underneath the Au NPs was around 1.2 nm. In a
fully extended conformation, the 8CB molecule is
around 1.8 nm long. Therefore some interdigitation of
aliphatic chains of 8CB and organic shells of Au NPs
must have been observed.
The lifting of Au NPs during the net-like structures
formation was observed previously by Hansen et al.
(2008). This observation will be considered in more
detail later on in this article as a part of the discussion
on the mechanism of formation of the net-like
structures.
Moreover, we performed detailed studies on the
distribution of the nanoparticles within a composite
film. XRR is an optimal technique for such investigations due to its sensitivity to electron (and hence mass)
density variations perpendicular to the solid substrate.
As a result of fitting of XRR patterns, the density of the
Au NPs sublayer was obtained (Ruiz et al. 2005). In
case of a perfect coverage of the surface with gold, this
value should be equal to the density of gold. The value
of the simulated density of the gold sublayer corresponded to the decrease of the coverage of the surface
with Au NPs. Influence of the presence of the organic
moieties at the same z-distance from the substrate as
the metallic core on the density was neglected. The
fitted value of density of the Au NPs sublayer
(2.3 g cm-3) was compared with the expected density
for a single layer of Au NPs. The value 2.3 g cm-3
indicated that 12% of the surface was covered with Au
NPs. The coverage of the surface with Au NPs
estimated from the SEM image (see Fig. 3d) was
around 13.5 % (based on the image analysis). The
results obtained with use of SEM and fitting of XRR
patterns were in nearly perfect agreement. This
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conformity and the very good quality of the fit give
great confidence in obtained simulated parameters.
Mechanism of net-like structures formation
In case of relatively small amounts of Au NPs
incorporated within the 8CB matrix analysis of the
p–A isotherms indicated no significant alterations of
compression properties and phase transitions of 8CB.
Only in such case, the well-ordered net-like structure
was formed. Increased amount of the Au NPs in the
mixture led to a disordered film with large aggregates,
whereas too small amount of Au NPs was insufficient
to form a complete network.
The gold nanoparticles act as impurities dissolved
in a 2D solvent. The limited solubility of the particles
in the LC film may cause diffusion-limited growth of a
fractal, net-like structure (Jensen 1999; Taylor et al.
2001). Similar net-like structures were previously
found in phospholipids and polymer matrices at the
air–water interface (Hansen et al. 2008; Hassenkam
et al. 2002; Mogilevsky et al. 2010). The model
proposed by Hassenkam et al. (2002) was found to
work in case of hydrophobic Au NPs. The authors did
not observe any structure of this kind in case of
mixtures of surfactants and Au NPs of increased
hydrophilicity. It was explained in terms of competition of interactions between Au NPs themselves and
Au NPs and the water surface. For instance, the
hydrophobic Au NPs do not interact strongly with the
water surface, and therefore can be lifted and ‘‘frozen
out’’ from the domains of the condensed phase. Lifting
was confirmed by means of XRR measurements
(Hansen et al. 2008). The increased hydrophilicity of
the Au NPs resulted in different morphology of the
film, because the Au NPs should compete for the area
at the water surface with template molecules rather
than be easily lifted. This was in fact the case—
amphiphilic Au NPs (15% of OH groups at the ends of
coating molecules) were found not to create any netlike structures in a matrix of phospholipids. We agree
with such conclusion; however, our results presented
in this article indicate that it is possible for amphiphilic
Au NPs to form a net-like structure. We believe that
the model presented in the aforementioned publication
is still valid for our system, since we also observed
lifting of the Au NPs from the water surface (see
Fig. 5)—even despite the fact that the used Au NPs
were amphiphilic. Moreover, similarly as in case of
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J Nanopart Res (2012) 14:826
completely hydrophobic Au NPs, the Au NPs we used,
having 10% of polar groups in the outer shell, tended
to form rafts at the air–water interface (see Supporting
Information). Such behavior indicated that the interactions between Au NPs were stronger comparing to
interactions between Au NPs and the water surface
(Sear et al. 1999; Imperio et al. 2008). Such statement
raised a question: why Hassenkam and coworkers did
not find the net-like structure in case of amphiphilic
Au NPs and phospholipids mixtures?
It seems that the interactions between OH groups
introduced to the outer shell of the Au NPs (15% of
coating thiols) are fairly strong, comparing to the
interactions between phospholipid molecules and the
water surface. Therefore, the phospholipid could not
act as a template and Au NPs were not lifted from the
water surface during the compression of the Langmuir
film. In our case, the balance between these interactions was different, as we used the 8CB for the matrix
and only 10% of the thiols coating the Au NPs had a
polar group attached at the end of the chain. We are
planning to test this hypothesis in the future by using
the Au NPs with different amounts of OH-terminated
ligands under otherwise identical conditions.
In case of phospholipids, the 2D liquid phase
decreases in size, whereas the 2D solid phase increases
as the compression proceeds. The Au NPs are frozen
out of the liquid phase and end up at the boundaries
between the solid phase domains. The analysis of the
isotherms of the films of mixtures of 8CB and Au NPs
indicates a slightly different mechanism. The domains
of multilayer stacks, and not a 2D solid monolayer, act
as the condensed phase that causes the net-like
structure formation. The structure of Au NPs in 8CB
matrix appears upon film compression to more than
15 mN m-1. In case of compositions 1:6, 1:9, and
1:27 such value of surface pressure corresponds to a
point at the second slope of the isotherm, behind the
plateau. It is well known and proven that the plateau in
the isotherm of 8CB corresponds to formation of a
trilayer. In case of film of 1:9 composition, the
observed surface coverage upon trilayer formation
should equal 1:3 (8CB occupied thrice smaller area
than in monolayer film), i.e., around 25% of the
surface should be covered with Au NPs. However at
p = 15 mN m-1, we observed the Au NPs coverage
of around 13%, which corresponds to a coverage ratio
around 1:7. Not all of the 8CB molecules formed
densely packed trilayer domains in the observed film.
J Nanopart Res (2012) 14:826
This can be easily explained by the fact that the netlike structure was decompressed (from 18 mN m-1)
prior to the transfer (at 15 mN m-1). As a result, a
decrease in density of the packing of the molecules
was observed. Therefore, the densely packed trilayer
film should not be observed. Rather a coexistence of a
trilayer and a monolayer film of 8CB as a matrix for
the structure of Au NPs should be expected. The fitting
of XRR profile indicated that the Au NPs were lifted
from the surface by around 1.2 nm. Therefore Au NPs
were placed on the monolayer film of 8CB and the
particle-free cavities were filled with the trilayer
domains of 8CB of rather loose compaction of
molecules.
The Au NPs surface coverage in case of a sample of
composition ratio 1:9 transferred at 18 mN m-1
reached a value of around 29% (see Fig. S7 Supporting Information). As was already mentioned, in case of
formation of a trilayer film of 8CB, the Au NPs surface
coverage should equal 25%. The observed, higher
value indicates that the 8CB exhibits a layering
transition to thicker films. Such behavior of 8CB was
previously recognized (Xue et al. 1992). The second
plateau at the p–A isotherm of 1:9 mixture is not
visible because the Au NPs alter the compression
properties of the trilayer-Au NPs composite film.
Overall, the process of formation of Au NPs netlike structures requires the initial compression of the
film of Au NPs and 8CB mixture. During this process
the trilayer film of 8CB is formed and Au NPs are
‘‘frozen out’’ and appear at the domains’ border. We
found that the compression to around 18 mN m-1 is
sufficient for this process to take place. When the
initial compression was stopped at 15 mN m-1, the
process took much time—an interval of 90 min was
needed for the Au NPs to rearrange and appear at the
domains’ border (see Fig. S8 in Supporting Information). This was probably caused by the difference in
the compaction of the molecules—the higher the
surface pressure was, the more condensed phase was
formed at the air–water interface and the faster the
‘‘freezing-out’’ process was. The image analysis of the
films of composition ratio 1:9 transferred at
15 mN m-1 with varying time interval prior to
transfer (3 min to 90 min) revealed that the Au NPs
surface coverage was almost constant (see Figs. 3d, f,
S9). This confirmed 2D rearrangement (‘‘freezing
out’’) of the Au NPs during the net-like structure
formation due to condensed domains formation.
Page 9 of 11
The decompression of the film of studied mixtures
resulted in an increase of average size of the unit cell
of the structure. This was due to a layering transition
within the 8CB domains i.e. formation of a monolayer
from thicker stacks. As a result, the 8CB molecules
occupied a larger area. Due to the decompression of
the multilayer stacks, the average area of unit cell
could increase by factor smaller than 10 (depending on
the number of layers in multilayer stacks). However,
SEM and BAM observations revealed an increase of
the unit cells from around 2200 lm2 (at 15 mN/m) to
around 0.15 mm2 (at 0 mN/m). This implied an
increase of the unit cell by a factor of around 70. Such
effect was caused by further decompression of the
8CB monolayer to a 2D liquid phase. The process of
unit cells growth stopped at some point (see Fig. 4)—
probably when the 2D gaseous phase of 8CB
appeared.
Conclusions
This study presents a systematic investigation of selfassembly of amphiphilic Au NPs in a liquid crystalline
matrix of 8CB at the air–water interface. The Langmuir films of mixtures of 8CB and Au NPs form a netlike structure, wherein Au NPs aggregate around
circularly shaped Au NPs-free areas. The net-like
patterns appear due to the initial compression to at
least 18 mN m-1, as a result of formation of
condensed phase of domains of a multilayer film of
8CB. The nanoparticles act as impurities in the liquid
crystalline matrix and are ‘‘frozen out’’ to the edges of
the LC domains. At lower surface pressures such
process is very slow.
An analogous mechanism of formation of net-like
patterns was reported previously for hydrophobic Au
NPs. We confirmed its validity also in case of
amphiphilic Au NPs (with 10% of charged ligands).
The average size of the unit cell proved to be fully
controllable during film decompression. The net-like
pattern of desired average unit cell area could be easily
and effectively transferred onto a solid substrate with
use of the LB technique. SEM images indicated that
the structures were uniform over large areas.
Acknowledgments We gratefully acknowledge Prof.
E. Górecka and Dr. D. Pociecha (Department of Chemistry
Warsaw University) for their help in XRR experiments. The
123
Page 10 of 11
research was supported by the National Science Centre
according to decision number DEC-2011/01/N/ST5/02917. JP
is a scholar within Sub-measure 8.2.2 Regional Innovation
Strategies, Measure 8.2 Transfer of knowledge, Priority VIII
Regional human resources for the economy Human Capital
Operational Programme co-financed by European Social Fund
and state budget (DFS.VI.3361-4-37-033/10).
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use,
distribution, and reproduction in any medium, provided the
original author(s) and the source are credited.
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