conformational induced behaviour of copolymer-capped magnetite nanoparticles at the air/water...
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Cite this: Soft Matter, 2011, 7, 4267
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Conformational induced behaviour of copolymer-capped magnetitenanoparticles at the air/water interface
Cristina Stefaniu,*a Munish Chanana,a Heiko Ahrens,b Dayang Wang,ac Gerald Brezesinski*a
and Helmuth M€ohwalda
Received 23rd November 2010, Accepted 22nd February 2011
DOI: 10.1039/c0sm01370f
Biocompatible and stimuli-responsive copolymer-capped Fe3O4 nanoparticles (NPs) were studied at
the air/water interface, below and above their lower critical solution temperature (LCST). The NP
layers have been characterized in situ by compression–expansion isotherms, infrared reflection–
absorption spectrometry, X-ray reflectivity, and by transmission electron microscopy after being
transferred onto solid support. The data obtained highlight the different interfacial behaviour of the
NPs below the LCST (good dispersibility in the aqueous subphase) and above the LCST (lack of
dispersibility in the subphase, high affinity for hydrophobic interactions, and agglomeration at the
interface). Conformational transitions of the copolymer from pancake to brush and mushroom-like
structures are the key factors of the stimuli-induced behaviour of the NPs at the air/water interface.
These conformational changes of the copolymer shell are due to the ability of the ethylene oxide units to
form hydrogen bonds with the water molecules of the subphase or which are trapped inside the
mushroom-like structure. The red shift of the C–O–C band accompanied by the blue shift of the CH2
scissor band gives comparative information about the degree of hydration of the ethylene oxide groups
for the different conformations.
Introduction
Stimuli-responsive behavior and conformational changes1–4 of
copolymers are topics of high scientific interest for both theo-
retical and practical reasons. Understanding and controlling
these factors are important for a vast field of applications ranging
from the biomedical5–10 area to material science.7–11
Stimuli-responsive polymers containing oligo- or poly-
(ethylene glycol) are of tremendous importance for biomedical
applications due to their resistance to protein adsorption.12–14
Consequently, nanoparticles (NPs) coated with ethylene glycol
polymers are especially attractive because they can improve the
biocompatibility of the NPs and prolong their in vivo circulation
time for different biological applications.15,16
Additionally, the iron oxide nanoparticles have been inten-
sively studied in the last few years due to their increased appli-
cability in different areas. Especially in the biomedical field,17
these NPs proved to be very promising, acting as drug-delivery
systems, magnetic resonance imaging (MRI) contrast
enhancers,18,19 and useful tools for cell labeling and separation.20
aMax Planck Institute of Colloids and Interfaces, Science ParkPotsdam-Golm, Am Muehlenberg 1, D-14476 Potsdam, Germany.E-mail: [email protected]; [email protected] of Physics, University of Greifswald, Felix-Hausdorff-Strasse 6,D-17487 Greifswald, GermanycIan Wark research Institute, University of South Australia, Adelaide, SA5095, Australia
This journal is ª The Royal Society of Chemistry 2011
Fe3O4 NPs capped with catechol-terminated random copolymer
brushes of 2-(2-methoxyethoxy) ethyl methacrylate (MEO2MA)
and oligo(ethylene glycol) methacrylate (OEGMA) are able to
reversibly agglomerate in bulk solutions and even in red blood
cells (RBCs) in response to environmental temperature
changes.16 The NP agglomeration had two significant effects: it
enhanced the manipulation of the RBCs by the use of an external
magnet and it dramatically enhanced the MRI contrast of the
studied cells. Moreover, the study revealed the dependence of the
NPs lower critical solution temperature (LCST) on the molar
fractions of the two copolymers MEO2MA and OEGMA. A
higher fraction of the more hydrophilic copolymer OEGMA
leads to an increase of the LCST.16
Hence, due to the potential biological applicability of those
biocompatible and stimuli-responsive NPs, the question related
to the ability of those Fe3O4 NPs to cross biological membranes
has been raised. Consequently, in order to understand the
behavior of the NPs at biological interfaces, our study started at
the simplest hydrophilic/hydrophobic interface, the air/water
interface. Thus, the present paper belongs to a series of studies
dedicated to the interfacial behaviour of Fe3O4 NPs capped with
a biocompatible and stimuli-responsive copolymer. Previously,
we reported the ability of Fe3O4@MEO2MA90-co-OEGMA10
NPs21 to form Langmuir and Gibbs layers which are energeti-
cally and kinetically long-time stable up to the critical surface
pressure of the film. Another study, based on pressure/area (p/A)
isotherms and tensiometric measurements, was dedicated to the
Soft Matter, 2011, 7, 4267–4275 | 4267
Fig. 1 Schematic representation of the Fe3O4 @ MEO2MA NPs.
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analysis of the dependence of the NP layers’ critical surface
pressure and of the dispersibility of the NPs from the interface
into the subphase on temperature and ionic strength of the
aqueous subphase.22 That report was focused on a comparative
study of two stimuli responsive NPs: Fe3O4@MEO2MA90-co-
OEGMA10 NPs and Fe3O4@MEO2MA NPs (Fig. 1).
The current paper presents new experimental evidence of the
antagonistic interfacial behavior of the Fe3O4@MEO2MA NPs
at the air/water interface below and above the lower critical
solution temperature (LCST), highlighting the relationship
between the stimuli responsiveness and the conformational
changes of the NP copolymer shell.
Besides the Langmuir balance technique, which describes the
stability and the thermodynamic parameters of the NP layer, the
system has been characterized in situ by Infrared Reflection–
Absorption Spectroscopy (IRRAS) for understanding the
packing density of the layer and, more importantly, the confor-
mational changes of the copolymer shell. Moreover, in situ X-ray
reflectivity experiments were performed to measure electron
density profiles in order to model the adsorption layers of the
NPs at the air/water interface and to quantify the layer thickness.
Furthermore, for understanding the morphology of the NP
layers, the Transmission Electron Microscopy (TEM) technique
was employed for visualizing the NP films after their transfer
onto solid support.
A better understanding of the stimuli-responsive interfacial
behavior of the NPs was achieved by correlating the data
obtained with the above mentioned techniques and by estab-
lishing a relationship between the copolymer conformation and
their physicochemical properties.
Experimental part
Materials
The Fe3O4@MEO2MA NPs (Fig. 1) used in this study are Fe3O4
NPs (d ¼ 6.4 nm) grafted with polymer brushes of 2-(2-methoxy-
ethoxy) ethyl methacrylate (MEO2MA) (polymer shell thickness
of 4.9 nm and Mn ¼ 17 000 g mol�1). The NPs have been
synthesized and characterized as previously reported.16 Special
attention has been paid to the purity of the NP system with respect
to the free polymer as previously described.23 A weight fraction of
22% of Fe3O4 was calculated for the studied system. The NPs have
been dispersed in chloroform (HPLC grade).
Surface pressure–area isotherms
The pressure–area isotherms of the Fe3O4@MEO2MA NPs at
the air/water interface were measured with a Langmuir trough
4268 | Soft Matter, 2011, 7, 4267–4275
system equipped with one (or two) moving barrier. The setup
included a surface pressure microbalance with a filter paper
Wilhelmy plate. The results were plotted as surface pressure (p)
versus area of the trough (in cm2). The bare water surface was
proved to be clean by compression before each measurement.
The temperature of the Milli-Q Millipore water subphase was
maintained at different temperatures by using a circulating
water bath.
Different amounts (10–40 mL) of chloroform solutions of the
NPs (3 mg mL�1) were uniformly spread on the subphase by
using a microsyringe (Hamilton). The compression of the film,
at a constant rate of 10.8 cm2 min�1, was started 20 min after
spreading to ensure the complete evaporation of the solvent and
the uniform distribution of the NPs at the interface. The pres-
sure/area (p/A) isotherms were recorded during compression of
the monolayer on the computer-interfaced Langmuir trough
(R&K, Potsdam, Germany). Each measurement was repeated at
least 2 times to prove the reproducibility of the results. In order
to avoid dust contamination of the interface and to ensure
a constant humidity, the Langmuir trough was placed in
a sealed box.
TEM measurements
Transmission electron microscopy (TEM) was used for the
characterization of the transferred NP Langmuir layers. The
images were obtained using a Zeiss EM 912 Omega microscope
at an acceleration voltage of 120 kV. The samples were prepared
by Langmuir–Schaefer transfer of the NP interfacial layer on
a copper grid and dried under a flux of nitrogen.
X-Ray reflectivity
X-Ray reflectivity measurements were carried out at the undu-
lator beamline BW1 using the liquid surface diffractometer at
HASYLAB, DESY (Hamburg, Germany). The experimental
setup and evaluation procedures are described in detail else-
where.24–27
The setup was equipped with a temperature-controlled Lang-
muir trough (R&K, Potsdam, Germany), which was enclosed in
a sealed helium-filled container. The synchrotron X-ray beam
was monochromated to a wavelength of 1.304 �A by a beryllium
(002) crystal. The specular X-ray reflectivity (XR) data collection
was performed by using a NaI scintillation detector. The X-ray
reflectivity was measured with the geometry, ai¼ af¼ a, where ai
is the vertical incidence angle and af is the vertical exit angle of
the reflected X-rays. XR data were collected as a function of the
incidence angle, ai, varied in the range of 0.06–3.5�, corre-
sponding to a range of 0.01–0.6 �A�1 of the vertical scattering
vector component Qz. The background scattering from the
subphase was measured at 2qxy ¼ 0.7� and subtracted from the
signal measured at 2qxy ¼ 0.
The electron density profile has been obtained by two
complementary strategies: (i) the electron density profile is
determined with a model-independent method.28,29 From the
experimentally observed reflectivity curve, the corresponding
profile correlation function is estimated via indirect Fourier
transformation. For this profile correlation function the match-
ing electron density profile is then derived by square-root
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deconvolution. No a priori assumptions on the shape of the
electron-density profile have to be made. (ii) The slab model is
used, and individual slabs can be identified with certain struc-
tural properties of the layers. The slab model was used with the
fewest and most independent parameters.30
The criterion for a satisfactory interpretation of the experi-
mental data was the matching of the scattering profiles resulting
from the two different modeling processes.
IRRAS
Infrared reflection–absorption spectra were recorded using the
IFS 66 FT-IR spectrometer (Bruker, Germany), equipped with
a liquid-nitrogen cooled MCT detector and coupled to a Lang-
muir film balance, which was placed in a sealed container (an
external air/water reflection unit (XA-511, Bruker)) to guarantee
a constant vapor atmosphere.31,32 Using a KRS-5 (thallium
bromide and iodide mixed crystal) wire grid polarizer, the
IR-beam was polarized parallelly (p) or vertically (s) and focused
on the fluid subphase at an angle of incidence of 40�.
A computer controlled ‘‘trough shuttle system’’33 enables us to
choose between the compartment with the sample (subphase with
spread layer) and a reference compartment (pure subphase). The
single-beam reflectance spectrum from the reference trough was
taken as background for the single-beam reflectance spectrum of
the monolayer in the sample trough to calculate the reflection
absorption spectrum as �log (R/R0) in order to eliminate the
water vapor signal. FTIR spectra were collected at a resolution
of 8 cm�1 using 200 scans for s-polarized light and 400 scans for
p-polarized light.
Results and discussion
Fe3O4@MEO2MA90-co-OEGMA10 NPs (90 and 10 representing
the molar fractions of the two copolymers MEO2MA and
OEGMA) as well as Fe3O4@MEO2MA NPs are able to form
Gibbs and Langmuir monolayers.21,22 Their dual dispersibility
both in hydrophilic and hydrophobic solvents is the reason for
this special ability. The amphiphilic character of the copolymer
resides in the graft structure of the polymer’s chains (Fig. 1)
defined by an apolar carbon–carbon backbone which leads to
a competitive hydrophobic effect and multiple oligo(ethylene
glycol) side-chains of which ether oxygens form stabilizing
H-bonds with water.34 Moreover, the ethylene oxide motif can
adopt a configuration with the oxygen atoms on one side of the
molecule and with the two methylene groups on the other side,
thus giving the molecule both a hydrophilic and a hydrophobic
surface.35
Thus, of special importance for the present study is the
information that the solubility of these oligo(ethylene glycol)
copolymers in aqueous media results from hydrogen bonding
between the polymer and the surrounding water.34 This explains
why polymers containing in their structure a higher number of
ethylene glycol units have a higher LCST (43 �C of Fe3O4@
MEO2MA90-co-OEGMA10 NPs16). Therefore, MEO2MA was
used in this study instead of MEO2MA90-co-OEGMA10 because
of the lower and experimentally better accessible LCST (24 �C).
Moreover, it was previously reported that the addition of salt
additionally weakens the hydrogen bonds, thus reducing the
This journal is ª The Royal Society of Chemistry 2011
water solubility of the polymers, leading even to aglomera-
tion.16,36
Recently a comparative interfacial study of the two different
copolymer-capped NPs was performed. Their stimuli-dependent
interfacial behaviour was characterised by quantifying parame-
ters such as the critical surface pressure of the Langmuir films
and the dispersibility of the NPs into the subphase.22 Indepen-
dent of the structure of the copolymer shell, the NP layers show
high stimuli-responsiveness below the LCST with a continuous
change of the interfacial properties, whereas above the LCST,
their interfacial activity is non-variant. Those differences raised
questions concerning the conformational changes occurring in
the copolymer shell as a response to temperature changes.
Therefore, the present study provides new experimental data
which allow to establish a correlation between the NPs’ interfa-
cial properties and the conformational changes of the copolymer
shell. Hence, the Fe3O4@MEO2MA NP layers were character-
ized at two different temperatures: below and above the LCST.
The NP Langmuir films were prepared at temperatures of the
aqueous subphase of 20 �C (below the LCST) and 37 �C (human
body temperature—higher than the LCST, but chosen because of
possible future biological applications), transferred onto solid
supports and investigated using TEM. The obtained images
presented in Fig. 2B confirm the presence of dispersed NPs at the
interface at 20 �C, while at 37 �C patches of well packed NPs,
bigger than 500 nm, have been observed (Fig. 2D and E). These
results are in perfect agreement with our previously proved
scenario according to which the NPs are re-dispersed into the
subphase above the critical surface pressure (pc ¼ 28.5 mN m�1,
Fig. 2A) and below the LCST.21 The re-dispersion is favored
compared with the formation of multilayers due to the affinity of
the NP copolymer shell to the water subphase, the ether oxygens
of the oligo(ethylene glycol) side-chains forming H-bonds with
the water molecules.34 Moreover, the steric repulsion of the
copolymer chains maintains the NPs dispersed at the interface
during the lateral compression and, above the critical surface
pressure, determines the squeezing-out of the polymer-capped
NPs from the interface into the subphase. On the plateau region
(Fig. 2A), a brush conformation of the copolymer chains is
proposed for temperatures below the LCST, whereas above the
LCST, the copolymer chains are collapsed, and a change to
a mushroom conformation seems to occur. Therefore, the
polymer stabilizing H-bonds with water as well as the steric
repulsion of the polymer chains are drastically decreased in favor
of the polymer–polymer chain interactions. As a result, the
agglomeration of the NPs occurs at the interface in a 2D system,
as previously proved and used for promising biomedical appli-
cations in a 3D bulk aqueous solution and even in red blood
cells.16
Furthermore, because of the imperfection of the Langmuir–
Schaefer transfer due to the supposed stiffness of the NP inter-
facial layer and due to the drying process of the samples, the
TEM images (Fig. 2D and E) show only patches of agglomerated
NPs. Nevertheless, one of the representative puzzle-like struc-
tures (Fig. 2D) reveals by recombination of the four ‘‘puzzle-like
elements’’ a NP monolayer with a surface of approximately 0.4
mm2. Therefore, above the LCST the existence of a monolayer of
well packed NPs at the interface is proposed, and this will be
confirmed later by the other techniques. Moreover, the
Soft Matter, 2011, 7, 4267–4275 | 4269
Fig. 2 (A) Compression–expansion isotherms of the Fe3O4@MEO2MA
NP layers spread on the surface of water at 20 �C and (C) at 37 �C. The
X marks on the compression isotherms show the state where the film was
transferred to be investigated by TEM. (B) TEM images of the NP
Langmuir layer transferred on a copper grid by the Langmuir–Schaefer
technique at 20 �C and (D) and (E) at 37 �C.
Fig. 3 Compression–expansion isotherms of the Fe3O4@MEO2MA NP
layers spread on the surface of water (red line) and on the surface of 1 M
NaCl (black line) (20 �C).
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monolayer existence is in agreement with calculations taking into
account the initial surface concentration, the maximum surface
coverage and the compression ratio.
In order to confirm the TEM results of the transferred film, the
X-ray reflectivity technique and IRRAS have been used. These
techniques allow in situ investigation of the NP Langmuir films at
20 �C but not at such high temperatures as 37 �C. Therefore, for
lowering the LCST of the copolymer–NP system another
strategy was applied. Previous studies performed in aqueous bulk
solutions showed that a concentration of only 150 mM NaCl was
enough for reducing the LCST of the systems by few degrees.16
This was explained by the salting-out effect on the hydrogen
bonding between the copolymer shell and the surrounding water
molecules. Nevertheless, at the air/water interface the effect of
such low NaCl concentrations was very weak,22 and only 1 M
NaCl subphases are able to lower the LCST sufficiently. This can
be understood by the reduced accessibility and interaction of the
ions with the copolymer shell adsorbed in a pancake conforma-
tion at the air/water interface, compared to the bulk solutions.
Fig. 3 (black line) shows the compression–expansion isotherm
of the Fe3O4@MEO2MA NPs on the surface of 1 M NaCl
solution at 20 �C (above the LCST). The obtained profile
matches exactly that recorded at the air/water interface at 37 �C
(Fig. 2C). The two compression isotherms show the same lack of
4270 | Soft Matter, 2011, 7, 4267–4275
hysteresis. The limiting area (A ¼ 186.1 cm2––corresponding to
the critical surface pressure) of the NP interfacial layer is smaller
than that of the NP film recorded on the surface of water at 20 �C
(A¼ 214.4 cm2). The results indicate that the NP copolymer shell
needs above the LCST an interfacial area 13% smaller than the
area occupied by the brush-like conformation proposed for
temperatures of the subphase below the LCST. It is worthwhile
to note that above the LCST the critical pressure decreases
slightly with increasing temperature. This shows again the strong
effect of temperature on the polymer chain conformation. With
these considerations, the data obtained by using X-ray reflec-
tivity and IRRAS for the NPs at the air/1 M NaCl aqueous
solution interface at 20 �C can be directly compared with those at
the air/water interface at 37 �C.
IRRAS measurements revealed for both subphases the char-
acteristic signals of C–O and C–C stretching vibrations at 1102–
1150 cm�1 (ethylene glycol moieties),37 the stretching vibrations
of C]O (ester) at 1724 cm�1 and of the CH3 groups at 2820–2840
cm�1 (Fig. 4, 5 and 6). Moreover, the most prominent band of the
spectra is the one centered at 3600 cm�1 that arises from the OH
stretch of water and which is a characteristic feature of IRRA
spectra. The water OH stretching vibration present in the refer-
ence signal (R0) is reduced in the reflectivity signal from the
monolayer-covered surface (R) because the NP layer replaces
a water layer and masks partially the OH stretching vibration.
Therefore, this positive band is related to the compactness
(effective thickness) of the NP layer.
It is important to highlight the fact that the intensities of the
above mentioned IR bands are increasing upon compression of
the NP layer up to the critical pressure. Further compression of
the layer increases only slightly the IR band intensity (Fig. 4A
and B). These data prove once again that the NPs desorb from
the interface into the subphase only after reaching the maximum
surface coverage at the critical surface pressure of the layer.21 The
slight increase of the IR band intensity recorded for surface
pressures above the critical surface pressure agrees with the slight
increase of the surface pressures in the plateau region (Fig. 2A)
and indicates that the desorption kinetics of the NPs from the
interface is slower than the compression rate.
The ratio of the OH band intensity recorded at different
surface pressures has been plotted as a function of the surface
pressure of the layer in Fig. 4C (black stars). Due to the fact that
below the critical surface pressure (28.5 mN m�1) there is no loss
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Fig. 4 A and B) Selected regions of IRRA spectra of the Fe3O4@
MEO2MA NP layers spread on the water surface at 20 �C (black line––15
mN m�1, red line––25 mN m�1, green line––28 mN m�1, blue line––29.7
mN m�1), (C) IR signal intensity ratio (–O–H band at 3600 cm�1––black
stars) and NP surface concentration ratio (red stars) plotted versus the
surface pressure of the NP interfacial layer.
Fig. 5 A and B) Selected regions of IRRA spectra of the Fe3O4@
MEO2MA NP layers spread on the surface of 1 M NaCl at 20 �C (black
line––15 mN m�1, red line—25 mN m�1, green line––30 mN m�1, blue
line––31.6 mN m�1), (C) IR signal intensity ratio (–O–H band at 3600
cm�1––black stars) and NP surface concentration ratio (red stars) plotted
versus the surface pressure of the NP layer.
Fig. 6 A–C) IRRA spectra of the Fe3O4@MEO2MA NP layers spread
on the surface of water at 20 �C (black line––15 mN m�1, red line––25 mN
m�1, green line––28 mN m�1, blue line––29.7 mN m�1) and D–F) on the
surface of 1 M NaCl at 20 �C (black line––15 mN m�1, red line––25 mN
m�1, green line––30 mN m�1, blue line––31.6 mN m�1) (p-polarized light).
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of the NPs from the interface, the NP interfacial concentration
can be calculated.21 Thus, the ratio of the NP interfacial
concentration can also be obtained at different surface pressures
(Fig. 4C—red stars). By plotting the two sets of data versus the
surface pressure of the NP Langmuir layers, a superposition of
points is obtained (Fig. 4C) which reveals the very good corre-
lation between the ratio of the NP interfacial concentration and
the ratio of the IR band intensity characterizing the effective
packing density of the NPs.
This journal is ª The Royal Society of Chemistry 2011
On the surface of the 1 M NaCl aqueous solution (Fig.
3––black line), the NP behaviour is different due to the fact that
the subphase temperature is above the LCST of this system.
Their inability to re-disperse into the subphase after reaching the
critical surface pressure and the permanent agglomeration at the
interface is reflected in a continuous increase of the intensity of
the characteristic infrared bands (Fig. 5A and B). The band
intensity recorded at 15 and 25 mN m�1 is identical with the one
recorded on the water subphase at the same surface pressures
(same packing density). A drastic increase of the IR band
intensity has been obtained upon further compression of the NP
layer above the characteristic critical surface pressure of the layer
(30.5 mN m�1). As in the case of the water subphase, a very good
agreement was obtained between the ratio of the interfacial NP
concentration (which can be even calculated above the critical
pressure since the NPs stay at the surface) and the ratio of the IR
intensity (Fig. 5C).
The compression–expansion isotherms, TEM measurements
and IRRA spectra indicate clearly the different interfacial
behaviour of the Fe3O4@MEO2MA NPs below and above the
LCST. At surface pressures above the critical surface pressure of
the NP layers and below the LCST, the squeezing out of the NPs
from the interface into the subphase occurs, while above the
LCST, the NPs agglomerate at the interface above the critical
surface pressure.
Furthermore, the IRRAS data revealed additional informa-
tion concerning the copolymer conformation at the interface.
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Due to the fact that the interaction of the ethylene oxide units
with the aqueous phase determines the polymer conformation,
special attention was given to the characteristic bands of this
moiety. The C–O–C, C–C and the CH2 vibrational bands posi-
tion changes upon compression of the monolayer or upon the
change of the ionic strength of the subphase have been studied.
On the water surface, a red shift is observed for the band at
1113 cm�1 recorded for 15 and 25 mN m�1 which appears upon
compression (28 mN m�1) at 1109 cm�1. In the same region, the
band at 1133 cm�1 is shifted to 1128 cm�1 (Fig. 6A). Additionally,
this red shift is accompanied by a blue shift of the CH2 scissoring
band (Fig. 6B) from 1445 cm�1 (15 mN m�1) to 1447 (25 mN
m�1), 1454 (28 mN m�1) and 1458 cm�1 (29.7 mN m�1 and
minimum compression area). These two opposite shifts are
related to the formation of hydrogen bonds of the oxygen atom
of the ethylene oxide moieties with the water molecules.38 The red
shift of the C–O stretching mode results from the formation of
the hydrogen bond to the oxygen lone pair electrons which
reduces the extent of electron donation to the C–H antibonding
orbitals.39 Consequently, this leads to the strengthening of the
C–H bonds expressed by the blue shift of the characteristic IR
band, and a weakening of the C–O bond. Additionally, a blue
shift of the ns (CH3) band40 is observed (Fig. 6C). The 2820 cm�1
band recorded at 15 mN m�1 and assigned as the characteristic
C–H stretching vibration of the dehydrated methyl group is
shifted upon lateral compression to 2834 cm�1. This band is
dominant at surface pressures above the critical surface pressure
and it is assigned to the C–H stretching vibration of the methyl
group solvated by water. The blue shift of the methyl vibrational
frequencies is considered as a clear indication of increased
disorder and conformational changes41 of the ethylene glycol
moieties in contact with water.42–44
Consequently, these data indicate that at the water surface, by
lateral compression of the Fe3O4@MEO2MA NP film, more
H-bonds are formed, thus indicating an increased hydration of
the polymer chains. By correlating our data with literature data,
it seems that at low surface pressures the NP copolymer chains
adsorb at the air/water interface in an extended pancake
conformation.45,46 Therefore, below the LCST and below the
critical surface pressure, the copolymers covering the NPs adopt
a flattened conformation46 at the air/water interface, with the
majority of the ethylene oxide motifs in a weakly (or non)
hydrated38 state. Upon compression, the NPs are forced to come
closer together and, above the critical surface pressure of the NP
layer, the copolymer conformation is changed. In these condi-
tions, some of the copolymer chains are submerged into the water
subphase and the brush conformation of the chains is favored.
This is very well revealed in IRRAS by the increase of the chain
hydration. Moreover, the co-existence of both bands character-
istic for ethylene oxide groups (Fig. 6B—blue and green lines) in
the hydrated and less hydrated states (e.g. CH2 scissoring bands
at 1445 and 1458 cm�1) at surface pressures above the critical
surface pressure indicates a co-existence of pancake-like and
brush-like structures, the second one being the dominant one.
Therefore, some of the copolymer chains remain anchored at the
air/water interface in the pancake-like structure, ensuring
the attachment of the NPs to the interface and the stability of the
layer, while, due to the high interfacial density and due to the
steric repulsions of the copolymer chains, some of them are
4272 | Soft Matter, 2011, 7, 4267–4275
expanded into the subphase. The two conformations of the
copolymer capping the NPs are similar to those recently
described by Lee et al. for poly(N-isopropylacrylamide)-covered
Au-NPs.47 Therefore, in our case, for interfacial concentrations
exceeding the maximum surface coverage, the re-dispersion of
the NPs from the interface into the subphase occurs due to the
well hydrated brush conformation of the copolymer shell.
A similar analysis has been carried out in order to understand
the conformational changes of the copolymer-capped NPs on the
surface of the 1 M NaCl aqueous solution with the copolymer
chains in a collapsed state characteristic for temperatures above
the LCST. A red shift is also observed (Fig. 6D) for the C–O–C
band from 1113 to 1109 cm�1 and from 1142 to 1135 cm�1 upon
compression of the NP layer from 15 mN m�1 to surface pres-
sures above the critical surface pressure (30 mN m�1). Corre-
spondingly, the blue shift occurs for the CH2 scissoring band
(Fig. 6E) from 1443 (15 and 25 mN m�1) to 1450 (31.6 mN m�1)
cm�1. The smaller recorded shift (7 cm�1) compared to that
obtained on the water surface (13 cm�1) suggests that the degree
of H-bond formation is much lower indicating a different
conformation of the copolymer. On the other hand, even at high
surface pressures (30 mN m �1), the copolymer on the surface of
the 1 M NaCl solution shows the characteristic bands of a dry
oligo(ethylene glycol) self-assembled monolayer.38 These C–O–C
peaks (1135 and 1158 cm�1) are the ‘‘dry’’ bands corresponding
to the hydrated ones recorded on the water surface (1128 and
1143 cm�1, respectively). Similar to the air/water interface, a blue
shift (from 2819 cm�1 to 2831 cm�1) of the ns (CH3) band was
observed (Fig. 6F) upon lateral compression.
The low hydration state of the copolymer is in agreement with
the lack of dispersibility of the NPs from the interface into the
subphase. In this case, above the LCST, the copolymer chains are
in the collapsed state, weakly hydrated even at high surface
pressures of the NP layer. The conformation of the polymer can
be understood as a mushroom-like structure. In this conforma-
tion, the polymer chains are poorly hydrated, in a quasi-3-D
conformation, the few water molecules trapped in between the
chains bridging –(CH2CH2O)– segments.46 The formation of
more H-bonds upon compression explains the blue and the red
shifts observed in the IRRA spectra. Moreover, on the surface of
the 1 M NaCl solution, the bands are broader than those
recorded on the surface of water due to the allowed agglomera-
tion of the NPs at the interface. Therefore, the conformational
transition which occurs at the surface of the 1 M NaCl solution
seems to be a transition from the pancake (15 and 25 mN m�1) to
a mushroom-like structure (above 30 mN m�1).
It is important to highlight the fact that despite the high
density of the copolymer chains grafted on the surface of the
Fe3O4 NPs (0.57 chains per nm�2), the conformational changes
of the copolymer are still occurring, offering to these NPs a good
stimuli-responsivity. Our data are in perfect agreement with
previously reported data concerning the stimuli-responsivity
due to conformational changes of highly dense polymer chains
(0.26–0.4 chains per nm�2) grafted on a solid support, both for
polymers having a very similar (MEO2MA-co-OEGMA)48,49
chemical structure or a completely different one (PNIPAM)50
compared to the studied system. The question to what extent
these conformational changes occur in the copolymer shell, if
only the topmost part of the chains is sensitive to the external
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changes or are the chains reacting in the whole length, is still
open. The available experiments do not allow to give a clear
answer yet. In order to explain the interfacial behaviour of the
NPs, our model considers that the conformational changes are
taking place over the whole copolymer shell.
More information can be obtained from the IRRAS data by
analyzing the variation of the dichroic ratio of C–O–C vibrations
with the increase of the surface pressure of the NP layer. The
dichroic ratio,32,51 defined as the ratio between the band inten-
sities measured with p- and s-polarized light, gives valuable
information about the molecular conformation.52–54
For the NP layer formed on the water surface, the dichroic
ratio decreases with the increase of the surface pressure, indi-
cating that the ethylene oxide moieties change their orientation
(Fig. 7). Below the critical surface pressure, the ethylene oxide
groups are less hydrated, adsorbed at the interface in the
pancake-like structure and oriented preferentially in the plane
perpendicular to the interface. Upon lateral compression above
the critical surface pressure, the change in the conformation from
the pancake-like structure to the brush-like structure occurs. The
decrease of the Ip/Is ratio could be explained by the co-existence
of both pancake- and brush-like structures at the interface. It
seems that the ethylene oxide groups in the brush-like structure
preferentially adopt an orientation parallel to the interface.
On the surface of the 1 M NaCl solution exactly the opposite
trend with very similar values is observed. The dichroic ratio
increases with increasing surface pressure (Fig. 7). At 15 and 25
mN m�1, the ethylene oxide moieties, which are less (or similar)
hydrated as the ethylene oxide moieties on the surface of water
(at the same surface pressure), exhibit a lower dichroic ratio
showing that the transition dipole moments of the C–O–C are
oriented more parallel to the subphase. Upon compression to
and above the critical surface pressure, the ethylene oxide
moieties, which are less hydrated as explained above and in the
mushroom-like conformation, are aligned more perpendicular to
the interface.
In order to test our analytical models for the X-ray reflectivity
experiments, densely packed spherical particles close to the water
surface have been simulated. The important result of such a test
was the finding that the overall thickness of the boxes was always
slightly smaller (�10%) than the diameter of the particles. The
Fig. 7 Dichroic ratio (Ip/Is) (C–O–C band 1113–1109 cm�1) versus
surface pressure of the Fe3O4@MEO2MA NPs layer spread on the
surface of water (black squares) and on the surface of 1 M NaCl solution
(red triangles) at 20 �C.
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X-ray reflectivity data obtained for the Fe3O4@MEO2MA NP
layers formed on a water subphase and compressed to two
different pressures, below and above the critical pressure of the
monolayer, are presented in Fig. 8. Fig. 8A shows the X-ray
reflectivity curves normalized by the Fresnel reflectivity. The
reflectivity measured at 15 mN m�1 shows a broad maximum at
Qz z 0.2 �A�1 with a normalized intensity above unity, which
indicates a thin layer in the nanometre range with an electron
density greater than that of the water subphase. The shallow
minimum at Qz z 0.1 �A�1 indicates a structure of few nanometre
thickness. The contrast is obviously not good enough to observe
pronounced maxima and minima. One reason is the low particle
density in the monolayer and the smooth decay of the electron
density of spherical particles immersed into the subphase, and
the other reason is that the electron density of strongly hydrated
and in water dispersed polymer chains is close to that of water.
Therefore, mainly the less hydrated polymer chains adsorbed at
the air/water interface contribute to the reflectivity signal.
Consequently, the derived electron density profiles (Fig. 8B)
show a clear maximum at the interface. The electron density
achieves values that are close to the ones found for the crystalline
phase of poly(ethylene oxide) (PEO) (0.403 e��A�3).55 Calculating
the excess electron density in comparison with a bare water
surface with the same roughness as in the present experiment
shows that the adsorption layer is �12 �A rather thin. This result
can be taken as an indication for stretched and tightly packed
polymer chains in a pancake-like conformation. Polymethacrylic
acid (on water at pH 5.5) as well as PEO form such an adsorption
layer at hydrophobic surfaces, so it is very probable that
MEO2MA shows the same behaviour.56 The main contribution
for the second layer underneath this polymer layer comes from
the NPs which have a low number density. The whole layer has
a thickness of only 50 �A. This value is 20% smaller than the
particle diameter in agreement with our simulations and
considering the low contrast of the adsorption layer. The
hydrated and dispersed polymer chains are completely invisible.
Fig. 8 (A) Measured reflectivity (symbols) and fits (lines) of the
Fe3O4@MEO2MA NP layers on a water subphase at 20 �C (15 mN
m�1––black curve (shifted for clarity), 29 mN m�1––red curve) and (B) the
respective electron density profiles (top) and the excess electron density
profiles (bottom) of the NP layers.
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Above the critical pressure (on the plateau of the isotherm at
29 mN m�1), the maximum in the reflectivity curve has been
shifted by 0.1 �A�1 to smaller Q values. The steep increase of the
reflectivity after the critical angle indicates a few nanometre thick
structure while the absence of oscillations in the reflectivity
profile indicates a smooth decay (i.e. large roughness parameter)
of the electron density towards subphase values as can be seen in
the electron density profile in Fig. 8B. The observed layer
thickness is very similar to that observed at lower pressure. The
maximum of the electron density profile is lower, which is in
agreement with the model of losing the stretched and well-packed
conformation of the polymer chains at the surface by compres-
sion of the film. The excess electron density profile shows a higher
electron density over the whole thickness because of the higher
number density of particles in the compressed state. The polymer
chains in water are still invisible.
The results are in agreement with the IRRAS experiments
showing that the adsorption layer thickness is constant on the
plateau region.
The X-ray reflectivity data obtained for the Fe3O4@
MEO2MA NP layers formed on the 1 M NaCl subphase are
shown in Fig. 9. Adding 1 M NaCl to the subphase changes the
electron density of the subphase from 0.334 e� �A�3 (water) to
0.345 e� �A�3 (1 M NaCl).57 The reflectivity curve taken at 15 mN
m�1 shows weakly pronounced oscillations. The derived electron
density profile (Fig. 9B) is very similar to the one observed on
water but with a slightly smaller layer thickness. The high elec-
tron density close to the interface shows that the adsorbed
polymer chains are similarly stretched and packed as on the
water surface (pancake-like conformation). This is in good
agreement with the very similar IRRA spectra observed on water
and on the salt solution at lateral pressures below the critical one.
The pronounced maximum at 0.05 �A�1 and the shoulder at 0.12�A�1, observed above the critical surface pressure, can be fitted by
a two-box model. Even if the derived layer thickness is too small,
the data show clearly that the monolayer is intact and no
Fig. 9 (A) Measured reflectivity (symbols) and fits (lines) of the
Fe3O4@MEO2MA NP layers on the 1 M NaCl subphase at 20 �C (15 mN
m�1––black curve (shifted for clarity), 31 mN m�1––red curve) and (B) the
respective electron density profiles (top) and the excess electron density
profiles (bottom) of the NP layers.
4274 | Soft Matter, 2011, 7, 4267–4275
multilayers are formed at high salt concentrations. This is in
agreement with all the results obtained with the other methods
used. The drastically increased electron density in the part
underneath the polymer adsorption layer compared with that
observed on the water subphase shows that the contrast between
the subphase and the particle layer is enhanced, indicating that
the collapsed polymer chains are strictly localized between the
particles. The mushroom-like structure of the copolymer is in
agreement with these results.
Conclusions
We have shown that the critical interfacial area of the Fe3O4@
MEO2MA NPs at the air/water interface above the LCST is only
13% smaller than that below the LCST. More surprisingly, the
NP layers proved to be very similar before reaching the critical
surface pressure, independent of the subphase type (water or 1 M
NaCl) or temperature (below or above the LCST). Their simi-
larity was reflected in the compression isotherms, IRRA spectra,
and electron density profiles. Hence, below the critical surface
pressure of the layers and both below and above the LCST, the
NPs adsorb at the interface in the pancake-like conformation,
forming a densely packed layer (Fig. 10A).
In contrast, above the critical surface pressure, the Langmuir
NP layers exhibit different features below and above the LCST.
Thus, on the plateau region, below the LCST, the pancake-like
structure co-exists with the more hydrated brush-like confor-
mation (Fig. 10B). Due to this layer structure, the NP surface
concentration is kept more or less constant (desorption
kinetics—compression rate equilibrium). The further compres-
sion of the layer induces a squeezing-out of the excess NPs from
the interface into the subphase. While, above the LCST the
pancake-like structure is gradually changed upon lateral
compression of the NP Langmuir layer into a mushroom
conformation of the copolymer chains (Fig. 10C). Moreover, this
gradual shrinkage of the copolymer shell recorded above the
Fig. 10 Schematic representation of the interfacial behaviour of the
Fe3O4@MEO2MA NPs dictated by the conformational changes of the
copolymer: (A) below and above the LCST but below the critical pres-
sure, the pancake-like conformation exists on both subphases. (B) Above
the LCST on water, the pancake-like conformation transforms and
co-exists upon lateral compression with the brush conformation above
the critical pressure. (C) Above the LCST on the high ionic strength
subphase, the NP film is transformed into a mushroom-like structure by
compression to values above the critical surface pressure.
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LCST explains the only 13% smaller critical area of the NPs
compared to that recorded below the LCST. Therefore, above
the LCST, the weakly hydrated copolymer shell induces the
accumulation and agglomeration of the NPs at the interface,
allowing the formation of a well packed homogeneous mono-
layer of NPs. Additionally, the X-ray reflectivity data indicate
a less pronounced high electron density profile of the polymer
adsorption layer, which might be an indication of the presence of
the Fe3O4 cores partially above the water surface with collapsed
copolymer chains in between (Fig. 10C).
To summarize, the present work reveals that the Fe3O4@
MEO2MA NPs behave oppositely at the air/water interface
below and above the LCST due to conformational changes of the
copolymer chains. These changes occur upon variation of the
temperature or the ionic strength of the aqueous subphase and
only as a consequence of lateral compression of the film to
surface pressures above the characteristic critical surface pressure
of the NP layer.
Acknowledgements
We thank Rona Pitschke for the TEM measurements and Mandy
Meckelburg for the preparation of the NPs colloidal dispersions.
We thank HASYLAB at DESY, Hamburg, Germany, for
beamtime and excellent support. This work was supported by the
Max Planck Society.
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