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Stimuli-Responsive Magnetite Nanoparticle Monolayers Cristina Stefaniu,*,† Munish Chanana,† Dayang Wang,†,‡ Gerald Brezesinski,*,† and Helmuth M€ohwald† † ‡
Max Planck Institute of Colloids and Interfaces, Science Park Potsdam-Golm, Am Muehlenberg 1, D-14476 Potsdam, Germany Ian Wark Research Institute, University of South Australia, Adelaide, SA 5095, Australia ABSTRACT:
The stimuli-responsive behavior of copolymer-capped NPs (Fe3O4@MEO2MA90-co-OEGMA10 and Fe3O4@MEO2MA) was studied in the temperature interval between 6 and 43 °C at the air/water interface, and at 20 and 37 °C on the surface of 150 mM NaCl, 1 M NaCl, and 2% citric acid aqueous solutions. The critical surface pressure of the NP layers and the exchange of the NPs between the interface and the subphase respond clearly to temperature and ionic strength changes of the aqueous subphase. The comparative study of the two NP systems allowed the determination of the influence of the oligo(ethylene glycol) methacrylate (OEGMA) moiety on the stimuli-responsive interfacial behavior. The conformational change of the copolymer seems to dictate the interfacial behavior of the copolymer-capped NPs below and above their characteristic LCST (lower critical solution temperature).
’ INTRODUCTION We have previously reported on the ability of copolymercapped Fe3O4 nanoparticles (NPs) to form Gibbs and Langmuir layers at the air/water interface. It has been shown that the Fe3O4@MEO2MA90-co-OEGMA10 NPs are able to adsorb at the interface, to form stable films, and to redisperse on compression into the aqueous subphase1 due to their ability to interchange the hydrophilic/hydrophobic character, which is a very important aspect considering our interest in studying and understanding the ability of these NPs to cross biological membranes. The present Article is focused on a comparative study of the response to temperature and different ionic strength of the aqueous subphase of two stimuli-responsive copolymer-capped Fe3O4 NPs. The relationship between the copolymer shell structure and the response to the variation of the stimuli will be investigated mainly by analyzing and comparing the interfacial properties of the two copolymer-capped NPs. Stimuli-responsive copolymers have received a considerable attention due to their potential applicability in biotechnology,2,3 pharmaceutics,4 medicine,5 and bioengineering.6-8 Concerning the system studied in the present work, it has been previously shown that the agglomeration of Fe3O4@MEO2MAx-co-OEGMA100-x NPs can be temperature controlled in a bulk solution and that the NPs can significantly enhance the magnetic response of loaded red blood cells, enhancing the MRI contrast and allowing manipulation of the cells with an external magnet.9 r 2011 American Chemical Society
To our knowledge, only one paper has been reported on a Langmuir balance and contrast-matched neutron reflectivity study of stimuli-responsive NPs at the air/water interface,10 while many more papers have been dedicated to the study of thermo-responsive copolymers at the air/water interface.11-18 The present study was dedicated mainly to the analysis of changes recorded in compression-expansion isotherms of the NP Langmuir layers as a response to temperature and ionic strength variation of the aqueous subphase. Parameters like the stability and the critical surface pressure of the films as well as the dispersibility of the NPs from the interface into the subphase were quantified for describing the stimuli-responsive behavior of the NP layers at the air/water interface. The first system consists of Fe3O4 NPs grafted with brushes of random copolymers of 2-(2-methoxyethoxy) ethyl methacrylate (MEO2MA) and oligo(ethylene glycol) methacrylate (OEGMA). These NPs will be referred to as Fe3O4@MEO2MA90-co-OEGMA10 NPs, 90 and 10 representing the molar fractions of MEO2MA and OEGMA, respectively (Figure 1). Their interfacial properties will be compared to those of more hydrophobic NPs consisting of the same Fe3O4 core, but grafted this time only with the stimuli-responsive MEO2MA polymer. These NPs will be named Fe3O4@MEO2MA NPs. Thus, the Received: November 30, 2010 Revised: February 8, 2011 Published: March 07, 2011 5478
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Figure 1. Schematic representation of the Fe3O4 @ MEO2MAx-coOEGMA100-x.
main difference between the two copolymer-capped NP systems consists of a different hydrophilicity due to the presence or absence of the OEGMA fragment.
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barriers have been used: one barrier made of Teflon (polytetrafluoroethylene) and another one made of POM (polymethylene oxide). Because of the high hydrophobicity of the Teflon barrier (the water contact angle on Teflon is 101°), the water is repelled from the barrier surface giving the water surface a convex shape in its vicinity. This might have allowed the Langmuir layer to slip underneath the barrier. To prove that this is not happening, a barrier made of POM was used. This one is quite hydrophilic (with a water contact angle of 67°), giving a concave shape to the water surface in its vicinity. In both cases, the experiments have been performed following the same procedure and the results were exactly the same, proving that there is no loss of the NP film underneath the barrier.
’ RESULTS AND DISCUSSION
’ EXPERIMENTAL SECTION
Interfacial Behavior of NP Layers below the Lower Critical Solution Temperature. Compression isotherms of Langmuir
Materials. The Fe3O4@MEO2MA90-co-OEGMA10 (6.4 nm diameter of the inorganic core, 4.8 nm polymer shell thickness, and Mn = 39 000 g mol-1) and Fe3O4@MEO2MA (6.4 nm diameter of the inorganic core, 4.9 nm polymer shell thickness, and Mn = 17 000 g mol-1) NPs used in this study were synthesized and characterized as previously reported.9 Special attention has been paid to prove the purity of the NP system with respect to the free polymer.19 The NPs have been dispersed at different concentrations, either in Milli-Q Millipore water with a specific resistance of 18.2 MΩ cm or in chloroform (HPLC grade). Tensiometer Measurements. The dynamic surface tension measurements have been performed with a profile analysis tensiometer (PAT-1, SINTERFACE, Berlin, Germany). The value of the surface tension was determined from the shape of the pendant drops.20 The volume of the drop was kept constant by an active control loop of this instrument. For all experiments, aqueous drops with different concentrations of Fe3O4 NPs were formed in air, and the dynamic surface tension was measured over several hours to reach the equilibrium surface tension. All experiments have been performed at room temperature, (20 ( 1) °C. Surface Pressure-Area Isotherms. The pressure-area isotherms of the NPs at the air/water interface were measured with a Langmuir trough system equipped with one (or two) moving barriers. The setup included a surface pressure microbalance with filter paper Wilhelmy plate. The results were plotted as surface pressure (π) versus the 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 μL) of chloroform solutions of the NPs (3 mg/mL) 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, was started 20 min after spreading to ensure complete evaporation of the solvent and a uniform distribution of the NPs at the interface. The pressure/area (π/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 two times to prove the reproducibility of the results. To avoid dust contamination of the interface and to ensure a constant humidity, the Langmuir trough was placed in a sealed box. To prove that the NP monolayer does not slide underneath the barrier to the other compartment, two different kinds of
layers, formed by spreading different amounts of chloroform NP solutions on the water surface below the lower critical solution temperature (LCST) of the two polymers, are shown in Figure 2. For both systems, Fe3O4@MEO2MA90-co-OEGMA10 NPs and Fe3O4@MEO2MA NPs, the compression isotherms are characterized by an increase of the surface pressure until reaching a critical surface pressure (πc) of approximately 25 and 29 mN/m, respectively. At the same interfacial NP concentration, due to the higher hydrophobicity of the Fe3O4@MEO2MA NPs as compared to the Fe3O4@MEO2MA90-co-OEGMA10 NPs, the corresponding compression isotherms are shifted to higher surface pressures and higher critical pressures. The critical pressure of the pure copolymer layers is similar ((1 mN/m) to that of the NPcopolymer systems, proving that the nature of the copolymer is of utmost importance for the interfacial properties.19 Upon further compression (Figure 2A), a plateau region appears, which was previously proved to correspond to the squeezing-out of the NPs from the interface and their dispersion into the subphase.19 From the conformational point of view,21 below the LCST and below the critical surface pressure, the copolymer covering the NPs adopts at the air/water interface a flattened conformation,22 with the majority of ethylene oxide motifs adsorbed at the water surface, a pancake-like23,24 structure (Figure 2B1). Upon compression, these structures are forced to come closer together, and, above the critical surface pressure of the NP layer, a change in the copolymer chain conformation occurs. In these conditions, the brush conformation of the chains is favored due to the distribution of some of the copolymer chains into the water subphase.21 Therefore, in the plateau region, the two structures coexist (Figure 2B2): Some of the copolymer chains remain anchored at the air/water interface in the pancake-like structure, thus ensuring the attachment of the NPs to the interface and the stability of the layer, while, due to the high interfacial density and to steric repulsion of the copolymer chains, some of them are expanded into the subphase.21 This behavior is similar to that recently described by Lee et al. for PNIPAM-covered Au-NPs.10 Moreover, for interfacial concentrations exceeding the maximum surface coverage (plateau region, Figure 2B2), the dispersion of the NPs from the interface into the subphase occurs due to the well-hydrated brush conformation of the copolymer. As previously reported for the Fe3O4@MEO2MA90-co-OEGMA10 NPs, no hysteresis of the compression/expansion isotherms was observed for the interfacial film of Fe3O4@MEO2MA NPs compressed to surface pressures below the critical pressure 5479
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Figure 2. (A) Compression isotherms of NP layers prepared by spreading (starting coverage: 9.74 10-5, 2.01 10-4, and 3.89 10-4 mg/cm2) on Milli-Q Millipore water at 20 °C. (B) Dependence of the surface pressure on the interfacial concentration, with schematic representations of the conformational changes (pancake to brush structures) of the copolymer shell. Black lines = Fe3O4@MEO2MA90-co-OEGMA10 NPs, red lines = Fe3O4@MEO2MA NPs.
of the NP layer.1 This is an indication that below the critical surface pressure there is no loss of the NPs from the interface. Thus, the maximum interfacial concentration of NPs corresponding to the critical pressure can be calculated. The critical concentrations for the two systems are very similar and amount to (7.7 ( 0.6) 10-4 mg/cm2 for Fe3O4@MEO2MA90-co-OEGMA10 NPs and (6.7 ( 0.5) 10-4 mg/cm2 for Fe3O4@MEO2MA NPs. Temperature Dependence of the NPs’ Interfacial Properties. The variation of the temperature of the water subphase from 6 to 43 °C induced changes in the behavior of the NPs at the air/ water interface. Three representative compression-expansion isotherms measured at 6, 20, and 37 °C are presented in Figure 3A-C. As the isotherms reveal, the Fe3O4@MEO2MA NPs are much more temperature-sensitive in this temperature range than are the Fe3O4@MEO2MA90-co-OEGMA10 NPs. For a better understanding of the NPs interfacial behavior, the variation of two parameters (πc, the critical surface pressure, and Δπ, the difference between the initial and final surface pressures of the NP layers upon continuous compression-expansion from A = 308 cm2 to A = 19 cm2 and back) has been analyzed. For the more hydrophobic Fe3O4@MEO2MA NPs, a linear increase of the critical surface pressure is observed (Figure 3D) for temperatures of the subphase up to 25 °C. Further increase of temperature does not significantly change the value of the critical surface pressure. Concerning their dispersibility (Δπ) into the subphase, the data show that the constant value characteristic for subphase temperatures below 25 °C is drastically reduced at higher temperatures (Figure 3E). The loss of dispersibility is expressed by the lack of hysteresis of the compression-expansion isotherms (Figure 3C) and the close to zero value of Δπ (Figure 3E). Interestingly, the variation of both critical surface pressure and dispersibility is well correlated with the LCST (24 °C) of the NPs measured in aqueous bulk solution. Therefore, it is important to point out that the LCST value of these NPs can be determined as well by the use of Langmuir layers at the air/water interface. Thus, at temperatures of the subphase higher than the LCST, the critical surface pressure of the films is more or less constant, and the dispersibility of the NPs into the subphase is suppressed (Figure 3D and E). Hamley et al. recently reported similar results by studying a water-soluble thermosensitive
block-copolymer by surface tensiometry.25 Below the LCST, a strong influence of the temperature on the surface activity of the copolymer was observed, while above the LCST, the temperature dependence was very weak. Furthermore, a more detailed analysis of the compression isotherms reveals the dependence of the NP layer critical area (defined as the area corresponding to the critical surface pressure) on the temperature (Figure 3G). The decrease of the limiting area values by increasing subphase temperature, which generates an increase of the critical surface coverage (Figure 3H), suggests a conformational change in the copolymer structure. The interfacial area occupied by the copolymer-capped NPs diminishes by 20% with the increase of the subphase temperature from 6 to 25 °C. As was already reported, the surface activity of copolymer capped-NPs is based on the amphiphilic character of the copolymer shell. This (oligo ethylene glycol) methyl ether methacrylate polymer has a graft structure (Figure 1) composed of 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.26 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.27 Consequently, below the LCST and above the critical surface pressure, the increase of the subphase temperature produces a gradual dehydration of the ethylene oxide motifs (Figure 3G1, G2). Thus, the NPs adsorption to the interface is enhanced, leading to the increase of the critical surface pressure of the NP layer. Hence, the data show that the interfacial properties of these NPs are dictated by the subphase temperature, which controls the hydration level of the polymer shell and thus the conformational changes of the copolymer chains. In the first part of this study, we described conformational changes of the copolymer shell, which occur upon lateral compression of the NP layer above the critical surface pressure (Figure 2B). In this section, we are revealing conformational changes, which occur above the critical surface concentration (plateau region) upon thermal stimulation (Figure 3G and H). Thus, the brush and pancake-like structures (Figure 3H1), which coexist on the plateau region below the LCST, are thermally 5480
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Figure 3. (A-C) Compression-expansion isotherms of the NP layers spread on the surface of water at 6, 20, and 37 °C, respectively, for the Fe3O4@MEO2MA90-co-OEGMA10 NPs (black lines) and Fe3O4@MEO2MA NPs (red lines). (D) Variation of the critical pressure of the NP Langmuir layers with temperature. (E) Dependence of the dispersibility of the NPs on the temperature of the water subphase. (F) Variation of the surface pressure with the logarithmic bulk concentration of the NPs. For (D)-(F), black circles = Fe3O4@MEO2MA90-co-OEGMA10 NPs and red stars = Fe3O4@MEO2MA NPs. (G) Comparison of the variation of the critical area (blue 9) and of the critical surface pressure (red f) with the temperature for the Fe3O4@MEO2MA NP layer with a schematic representation in top view of the changes in the hydration shell (blue O = water molecules) of the NPs. (H) Comparison of the variation of the critical surface concentration (blue 9) and of the critical surface pressure (red f) with the temperature for the Fe3O4@MEO2MA NP layer with a schematic representation in side view of the conformational changes of the copolymer chains.
changed into gradually less hydrated structures of the copolymer chains (Figure 3H2). Above the LCST, the hydration layer of the copolymer shell is almost completely lost (Figure 3G3), the copolymer chains being in a collapse state (Figure 3H3), in a mushroom-like conformation.21 This state explains the shrinking of the interfacial area of the NPs by 20% at the critical surface concentration. The thermally induced change of the copolymer chain conformation at the air/water interface is similar to that reported for PEO (polyethylene oxide) containing triblock copolymers.28 To summarize, above the critical surface pressure and above the LCST, the copolymer-capped NPs occupy the smallest interfacial area, and the maximum critical concentration is reached. Being hydrophobic, the NPs lose their solubility into
the subphase, agglomerate at the interface,21 and exhibit temperature-independent interfacial properties. Similar results have been obtained for the Fe3O4@MEO2MA90-co-OEGMA10 NPs. The critical surface pressure of the interfacial NP layer increases with increasing temperature of the aqueous subphase, reaching a maximum value of approximately 27 mN/m (Figure 3D). It is interesting to note that this maximum value is reached already around 30 °C, thus markedly below the LCST (43 °C) of the copolymer-capped NPs, and is kept constant up to temperatures of 43 °C. This behavior can be explained as above for the Fe3O4@MEO2MA NPs by a conformational change of the copolymer shell. Nevertheless, it seems that there is a certain temperature range for the phase transition due to the OEGMA moieties. These findings are in 5481
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Figure 4. Compression-expansion isotherms of the NP layers spread on different subphases: water (black line), 1 M NaCl (green line), 2 wt % citric acid (blue line). (A) Fe3O4@MEO2MA90-co-OEGMA10 NP layer at 20 °C and (B) at 37 °C, and the corresponding variation of πc (E) and Δπ (F) with temperature. (C) Fe3O4@MEO2MA NP layer at 20 °C and (D) at 37 °C, and the corresponding variation of the πc (G) and Δπ (H) with temperature.
agreement with the existence of a temperature interval measured for the phase transition of a structurally similar pure copolymer by turbidimetry of an aqueous solution.26 The polydispersity of the copolymer could also contribute to a certain extent to the observed behavior. Analyzing the second parameter, the squeezing-out of the NPs from the interface into the subphase (Δπ), a rapid increase of the dispersibility is observed for temperatures of the subphase above 20 °C (Figure 3E), followed by a further slight increase of Δπ up to 35 °C. The increased ability of the NPs to leave the interface in favor of the subphase can be explained by the stretching29 of the oligo(ethylene oxide) chains into the subphase due to their increased hydration.30 By approaching the LCST of the system (43 °C), the dispersibility of the NPs into the subphase slowly decreases above 35 °C. The intermediate increase of the dispersibility with the temperature is almost imperceptible in the case of Fe3O4@MEO2MA NPs, revealing that this is purely an effect produced by the augmented hydration level of the OEGMA fragments. Because of practical limitations imposed by the Langmuir balance, no measurements have been performed above 43 °C, which corresponds to the LCST of the Fe3O4@MEO2MA90-co-OEGMA10 NPs. By comparing the two systems, Figure 3D shows that in the whole temperature range between 6 and 43 °C the πc values of Fe3O4@MEO2MA NPs are higher than those of Fe3O4@MEO2MA90-co-OEGMA10 NPs, suggesting that the more hydrophobic NPs form more stable interfacial layers below, as well as above the LCST. The stability is defined as the maximum pressure to which the monolayer can be compressed. The increase in the stability of Langmuir layers by increasing the hydrophobicity of the amphiphilic compounds is a classic and expected behavior. Surprisingly, however, the data shown in Figure 3E indicate that below 22 °C the dispersibility of Fe3O4@MEO2MA NPs is higher than that of the more hydrophilic Fe3O4@MEO2MA90-co-OEGMA10 NPs.
Moreover, tensiometry measurements performed at the air/water interface at 20 °C, using the profile analysis tensiometer, confirm the fact that at the same low concentration in bulk, the Fe3O4@MEO2MA90-co-OEGMA10 NPs are more surface active than the Fe3O4@MEO2MA NPs (Figure 3F). This interfacial behavior can be attributed to the different average molecular weight of the two copolymers: MEO2MA90-co-OEGMA10 (3.9 104 g mol-1) and MEO2MA (1.7 104 g mol-1). Our findings are similar to the results reported by Meszaros et al.31 who show that the surface activity of poly(ethylene oxide) (PEO) at the air/water interface strongly increases with the molecular weight. Moreover, it is shown that with increasing molecular weight of the PEO, the adsorption isotherms start with higher slopes and tend to smaller saturation values. This is exactly the behavior of the two NPcopolymer systems studied in the present work. Ion and Temperature Dependence of the NPs’ Interfacial Properties. Furthermore, the influence of the subphase on the interfacial properties of the NPs has been studied by measuring compression-expansion isotherms of the NP Langmuir layers spread on the surface of water, 150 mM NaCl, 1 M NaCl, and 2 wt % citric acid aqueous solutions at 20 and 37 °C (Figure 4). The data obtained on the surface of 150 mM NaCl are very similar to those obtained on the water surface; therefore, for the sake of clarity, these data are not presented. The graphs A, C and, respectively, E, G from Figure 4 indicate that at 20 °C (below the LCST) the stability of the NP layers depends on the nature of the subphase. Their stability increases in the order: 2 wt % citric acid < water < 1 M NaCl aqueous solution. The lowest stability of the NP layer recorded on the surface of citric acid aqueous solution is correlated to the enhanced formation of hydrogen bonds between the oxygen atoms of the ethylene oxide moieties and the water molecules32 of the subphase. Thus, the conformational change to the brushlike structure occurs earlier, at lower critical surface pressures. 5482
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The Journal of Physical Chemistry C The dispersion (Δπ) of the NPs from the interface into the subphase upon compression of the NP layers (Figure 4F and H) is also dependent on the nature of the subphase. The values of Δπ close to 0 recorded on a 1 M NaCl subphase at 37 °C for the Fe3O4@MEO2MA90-co-OEGMA10 NPs (Figure 4B and F) and at 20 °C for the Fe3O4@MEO2MA NPs (Figure 4C and H) show that the LCST of the two NP systems was reduced by the high concentration of NaCl. It was previously shown that the NPs’ LCST can be decreased by increasing the bulk salt concentration.9 This was explained by the salting-out effect on the hydrogen bonding between the copolymer shell and the surrounding water molecules. The studies conducted in bulk showed that a concentration of only 150 mM NaCl was enough for reducing the LCST of the systems by few degrees.9 In the present interfacial study, the effect of such low NaCl concentrations was very weak, and only a high NaCl concentration (1 M) is able to lower their LCST. This can be understood by the reduced accessibility and interaction of the ions with the copolymer shell adsorbed at the air/water interface, as compared to the bulk solutions. The isotherms measured on the different subphases are in perfect agreement below the critical pressure. The dominant conformation is a pancake-like structure in this state. Therefore, the low chain hydration is directly responsible for the low accessibility and interaction of the ions with the copolymer chains. The higher critical pressure of the Fe3O4@MEO2MA90-co-OEGMA10 NPs on the 1 M NaCl subphase (Figure 4A and E) shows that the ions in such high concentrations are able to interact to a certain extent with the polymer chains in the pancake conformation, leading to a reduction of the area requirement of the NPs on the surface, most probably due to a further decrease of the hydration level of the polymer chains. The NPs can be therefore compressed to smaller critical areas (higher critical surface concentration) explaining the higher πc values. At 20 °C, the dispersibility is surprisingly high (large Δπ values). Considering the polymer ion interactions at high ionic strength, the dispersed NPs should be in the so-called screening regime in which the binding of ions becomes increasingly unfavorable.33 The screening of electrostatic interactions leads to neutral polymer-like characteristics of PEO with again most probably reduced hydration level. The NPs should be therefore slightly more hydrophobic, which should accelerate the readsorption process. This is observed in the case of 150 mM NaCl in the subphase (results not shown). Therefore, it could be possible that the NPs readsorb very fast on the other side of the barrier.19 On the other hand, a possible aggregation of these NPs in the bulk has to be taken into account at high salt concentrations. Both scenarios could be the reason for the large Δπ values because less NPs are available for the readsorption process. The salting-out effect was observed only at temperatures close to the LCST values measured in water (20 °C as compared to 24 °C for the Fe3O4@MEO2MA NPs and 37 °C as compared to 43 °C for the Fe3O4@MEO2MA90-co-OEGMA10 NPs). The presence of ions decreases the LCST additionally9 by a synergetic effect with the subphase temperature. The formation of the low hydrated mushroom-like structures even below the LCST on water can be thus expected. Moreover, the OEGMA units are responsible for the higher dispersibility of the Fe3O4@MEO2MA90-co-OEGMA10 NPs measured at 37 °C on the surface of water and citric acid. This increased affinity for the aqueous subphase is in perfect agreement with literature data,29,30 showing that at higher temperatures (still below the LCST) the hydration of the PEO-rich
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copolymers increases (see also Figure 3E), and the chains extend in the surrounding aqueous environment.
’ CONCLUSIONS The present work reports the air/water interfacial behavior of two different copolymer-capped NPs: Fe3O4@MEO2MA90-coOEGMA10 NPs and Fe3O4@MEO2MA NPs. The comparative study was performed below and above their LCST. The ability of these stimuli-responsive NPs to react on the variation of the ionic strength and the temperature of the subphase was quantified by parameters like stability and critical surface pressure of the Langmuir films as well as dispersibility of the NPs into the subphase. The data reveal that only at surface pressures above the critical one the interfacial behavior of the NPs below the LCST is different from that above the LCST. Below the LCST, the squeezing-out of the NPs from the interface into the subphase occurs due to their hydrated polymer shell and due to the steric repulsion of the copolymer chains for which the pancake and brush-like structures coexist. In contrast, above the LCST, the NPs lack dispersion into the subphase, and agglomeration occurs at the interface. This behavior is due to the loss of the hydration shell of the NPs, the copolymer conformation being in this case that of the so-called mushroom-like structure. Thus, the NP layers show high stimuli-responsiveness below the LCST, with a continuous change of the interfacial properties, while above the LCST their interfacial activity is nonvariant. The number of ethylene glycol moieties in the structure of the polymer shell revealed itself to be a determinant factor for the surface activity, the stability of the two different NP layers, and the dispersibility of the NPs from the interface into the subphase, including the salting-out effect. Additionally, it has been shown that the Langmuir balance can be used for measuring the LCST of copolymer-capped NPs. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (C.S.); brezesinski@mpikg. mpg.de (G.B.).
’ ACKNOWLEDGMENT We thank Dr. Reinhard Miller for giving us the possibility to perform the dynamic surface tension measurements in his group, Dr. Vincent Pradines for his help and scientific discussions, and Mandy Meckelburg for the preparation of the colloidal NP dispersions. This work was supported by the Max Planck Society. ’ REFERENCES (1) Stefaniu, C.; Chanana, M.; Wang, D.; Novikov, D. V.; Brezesinski, G.; M€ ohwald, H. ChemPhysChem 2010, 11, 3585. (2) Tokarev, I.; Minko, S. Soft Matter 2009, 5, 511. (3) Motornov, M.; Roiter, Y.; Tokarev, I.; Minko, S. Prog. Polym. Sci. 2010, 35, 174. (4) Ganta, S.; Devalapally, H.; Shahiwala, A.; Amiji, M. J. Controlled Release 2008, 126, 187. (5) Brazel, C. S. Pharm. Res. 2009, 26, 644. (6) Tokarev, I.; Orlov, M.; Katz, E.; Minko, S. J. Phys. Chem. B 2007, 111, 12141. (7) Gupta, S.; Agrawal, M.; Uhlmann, P.; Simon, F.; Oertel, U.; Stamm, M. Macromolecules 2008, 41, 8152. 5483
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