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In Situ Observation of γ-Fe2O3 Nanoparticle Adsorption under Different Monolayers at the Air/Water Interface Patrick Degen,*,† Michael Paulus,‡ Michael Maas,† Rainer Kahner,† Saskia Schmacke,‡ Bernd Struth,§ Metin Tolan,‡ and Heinz Rehage† Fakulta¨t Chemie, Physikalische Chemie II, Technische UniVersita¨t Dortmund, 44227 Dortmund, Deutschland, Fakulta¨t Physik/DELTA, Technische UniVersita¨t Dortmund, 44227 Dortmund, Deutschland, and Deutsches Elektronen Synchrotron, HASYLAB, Notkestrasse 85, 22607 Hamburg, Deutschland ReceiVed July 25, 2008. ReVised Manuscript ReceiVed September 12, 2008 We studied the adsorption of γ-Fe2O3 nanoparticles from an aqueous solution under different charged Langmuir monolayers (stearic acid, stearyl alcohol, and stearyl amine). The aqueous subphase was composed of a colloidal suspension of γ-Fe2O3 nanoparticles. The average hydrodynamic diameter of the nanoparticles measured by dynamic light scattering measurements was 16 nm. The observed ζ potential of +40 mV (at pH 4) results in a long-term stability of the colloidal dispersion. The behavior of the different monolayer/nanoparticle composites were studied with surface pressure/area (π/A) isotherms. The adsorption of the nanoparticles under the different monolayers induced an expansion of the monolayers. These phenomena depended on the charge of the monolayers. After the Langmuir/Blodgett transfer on glass substrates, the nanoparticle/monolayer composite films were studied by means of UV-vis spectroscopy. The spectra pointed to increasing adsorption of the nanoparticles with increasing electronegativity of the monolayers. On the basis of these results, we studied the in situ adsorption of nanoparticles under the different monolayers by X-ray reflectivity measurements. Electron density profiles of the liquid/gas interfaces were obtained from the X-ray reflectivity data. The results gave clear evidence for the presence of electrostatic interaction between the differently charged monolayers and the positively charged nanoparticles. While the adsorption process was favored by the negatively charged stearic acid monolayer, the positively charged layer of stearyl amine prevented the formation of ultrathin nanoparticle layers.
Introduction Owing to their peculiar properties, magnetic nanoparticles with particle diameters in the range of 1 to 50 nm have attracted considerable attention during the past decade.1 Since their first appearance,2 the interest in magnetic nanoparticles and ferrofluids (stable suspensions of colloidal magnetic particles in a liquid carrier) has constantly grown, mainly because of their large number of potential applications in different fields such as magnetism, optics, rheology, biophysics and medicine.3-5 Among magnetic particles, many studies have been devoted to nanoparticles of magnetite (Fe3O4) and maghemite (γ-Fe2O3). An important task is to assemble magnetic nanoparticles in an ordered structure (e.g., layers) and maintain their isolated particle properties. These layers are of great promise as composites for utilization in optical and magnetic information storage media.6,7 Assembling thin colloid films by adsorption of nanoparticles under monolayers at a liquid/air interface represents one of the most attractive methods.8 Generally, colloidal particles * Corresponding author. † Fakulta¨t Chemie, Physikalische Chemie II, Technische Universita¨t Dortmund. ‡ Fakulta¨t Physik/DELTA, Technische Universita¨t Dortmund. § HASYLAB. (1) Bonini, M.; Wiedenmann, A.; Baglioni, P. Mater. Sci. Eng. C 2006, 26, 745. (2) Papell, S. S., U.S. Patent 3215572, 1965. (3) Berkovskyl, B.; Bashtovoy, V. G. Magnetic Fluids and Applications Handbook; Begell House, Inc.: New York, 1996. (4) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995. (5) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, 167. (6) Ao, B.; Kummerl, L.; Haarer, D. AdV. Mater. 1995, 7, 496. (7) Zhao, X. K.; Herve, P. J.; Fendler, J. H. J. Phys. Chem. 1984, 88, 716. (8) Hu, J. W.; Han, G. B.; Ren, B.; Sun, S. G.; Tian, Z. Q. Langmuir 2004, 20, 8831.
are inclined to be trapped at interfaces9,10 and there are wide ranges of investigations of nanoparticle adsorptions at these pure liquid-air interfaces.11,12 The focus of this study is to investigate the adsorption process and arrangement of maghemite (γ-Fe2O3) nanoparticles under different charged monolayers. Our investigations were based on the results of Kang et al. who reported on the adsorption of magnetite (Fe3O4) and maghemite (γ-Fe2O3) under different negatively charged monolayers such as stearic or arachidic acid.13-16 The average diameter and ζ potential were 8.3 nm and +35 mV, respectively, at acidic pH values (approximately 4). The magnetization as a function of the applied field of prepared nanocomposite Langmuir-Blodgett (LB) films showed no hysteresis loop, which indicated superparamagnetic behavior of the nanoparticles.13 The hydrosol solution of the γ-Fe2O3 nanoparticles contained no surfactants. These particles can, therefore, be incorporated into ultrathin organic films (Langmuir monolayers) without applying additional purification steps. On the basis of these first results, we used the synthesis method described in ref 13 and tested the nanoparticle hydrosol by dynamic light scattering (DLS) and ζ potential measurements. We were primarily interested in the kinetics of particle adsorption and the structure of the adsorbed nanoparticle films. In a series of experiments we investigated the adsorption process in situ utilizing the X-ray reflectivity technique and surface potential measurements. Both techniques have been described in many (9) Kralchevsky, P. A.; Nagayama, K. Particles at Fluid Interfaces and Membranes; Elsevier: Amsterdam, 2001. (10) Binks, B. P. Colloid Interface Sci. 2002, 7, 21. (11) Pieranski, P. Phys. ReV. Lett. 1980, 45, 569. (12) Onoda, G. Y. Phys. ReV. Lett. 1985, 55, 226. (13) Kang, Y. S.; Lee, D. K.; Lee, C. S. J. Phys. Chem. B 2002, 106, 9341. (14) Lee, D. K.; Kang, Y. S. J. Phys. Chem. B 2002, 106, 9341. (15) Kang, Y. S.; Risbud, S.; Rabolt, J.; Stroeve, P. Langmuir 1996, 12, 4345. (16) Kang, Y. S.; Lee, D. K.; Stroeve, P. Thin Solid Films 1998, 541, 327.
10.1021/la802394a CCC: $40.75 2008 American Chemical Society Published on Web 10/14/2008
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investigations before17-19 and allow us to study the adsorption process in detail. The principal goal of this study was to investigate the influence of the monolayer charge on the adsorption process. In different experiments we used stearic acid, stearyl alcohol, and stearyl amine, and we studied the adsorption of γ-Fe2O3 nanoparticles on these film structures. In addition, we used UV-vis spectroscopy to characterize nanocomposite LB films as already described in ref 13.
Experimental Section Materials. The chemical compounds FeCl2 · 4H2O (99+%), FeCl3 · 6H2O (99+%), stearic acid, stearyl alcohol, and stearyl amine (99+%) were obtained from Aldrich Chemical Co. and were used without further purification. The water was obtained from a pure water system (Seralpur PRO 90 CN). tert-Butylmethylether (TBME) was obtained from Aldrich Chemical Co. HPLC grade. Synthesis and Characterization of Iron Oxide Nanoparticles. To prepare the γ-Fe2O3 nanoparticles, we used the procedure described in ref 13. Volumes of 0.28 mL of 11.5 M HCl and 8.3 mL of purified, deoxygenated water (resistance of 18 MΩ, nitrogen gas bubbling for 30 min) were combined, and 2.86 g of FeCl3 · 6H2O and 1.00 g of FeCl2 · 4H2O were dissolved in the prepared solution with stirring for 20 min. The resulting solution was added dropwise into 100 mL of 1.5 M NaOH solution under vigorous stirring for 30 min. The precipitate was isolated in a magnetic field, and the supernatant was removed from the precipitate by decantation. A volume of 250 mL of purified water was added to the precipitate, and the solution was decanted after centrifugation at 5000 rpm for 10 min. After repeating the last procedure three times, 150 mL of 0.01 M HCl solution was added to the precipitate with stirring to neutralize the anionic charges on the nanoparticles. The clear hydrosol of γ-Fe2O3 at pH 4 was used as a subphase for Langmuir monolayers in a Langmuir trough. The concentration of hydrosol of solid weight fraction was determined to be 0.7 g/L. We used this hydrosol without further purification for the preparation of the Langmuir and LB layers. For the X-ray scattering measurements, we diluted the stock solution to 1:10, so the resulting concentration of the aqueous subphase was 0.07 g/L. The size and ζ potential of γ-Fe2O3 nanoparticles in the aqueous colloidal solution was measured with a Zetasizer NanoZS from Malvern Instrument Co. Optical absorption measurements: UV-vis absorption spectra of previously prepared LB layers were recorded with a Varian CARY 1E UV-vis spectrophotometer. Preparation of Langmuir and LB Layers. The LB layers were prepared using a LB-trough (611) also combined with a Wilhelmytype measuring system, both constructed by NIMA Technology. All π/A isotherm measurements started 15 min after spreading. The compression and the expansion velocities were adjusted to 0.2 nm2 molecule-1 min-1. Monolayer-spreading solutions (1 mM) were prepared by dissolving stearic acide, stearyl amine, and stearyl alcohol in TBME. To spread a surface monolayer of a surfactant, a 60 µL aliquot of the spreading solution was delivered in different locations on the surface of the pure water subphase and on the γ-Fe2O3 nanoparticle hydrosol solution at 20 °C. We prepared a single LB layer on quartz plates at a plate angle of 45° from the water surface, and the dipping velocity was adjusted to 10 mm/min. The surface pressure was fixed at 30 mN/m. X-ray Reflectivity Measurement. X-ray reflectivity measurements were carried out at the liquid surface diffractometer of beamline BW1 at HASYLAB (Hamburg, Germany).20 The compressed films (π ) 20 mN/m) inside the X-ray measuring cell were prepared in (17) Tolan, M.; X-ray Scattering from Soft Matter Thin Films, Materials Science and Basic Research, Tracts in Modern Physics; Springer: Berlin, 1999; Vol. 148. (18) Heinig, P.; Wurlitzer, S.; Steffen, P.; Kremer, F.; Fischer, T. M. Langmuir 2000, 16, 10254. (19) Demchak, R. J.; Fort, T. J. Colloid Interface Sci. 1973, 46, 191. (20) Frahm, R.; Weigelt, J.; Meyer, G.; Materlik, G. ReV. Sci. Instrum. 1995, 66, 1677.
Figure 1. Particle size distribution (intensity, volume, and number based) of the γ-Fe2O3 nanoparticle in the used hydrosol.
a custom-made Langmuir trough using the following procedure. During the measurement, the sample cell was flushed with helium in order to suppress air-scattering. The applied photon energy was 9.5 keV. One reflectivity scan took around 40 min, reaching a maximum wavevector transfer of 0.7 Å-1. All reflectivities were repeated in minimum twice in order to observe the development of the adsorption process. For reason of comparison, X-ray reflectivities of Langmuir layers prepared on pure water were recorded. Surface Potential Measurements. The surface potentials (V) were measured via the vibrating capacitor method (using a Kelvin probe Nima KP1) with an accuracy of 50 mV. We measured the timedependent adsorption process in a Teflon dish with a subphase volume of 40 mL. After setting the surface potential at a zero value for the pure water surface, we filled the Teflon dish with the same amount of γ-Fe2O3 hydrosol. The changing of the subphase had no significant influence on the surface potential (still zero). Afterward, we prepared different monolayers at the surface of the γ-Fe2O3 hydrosols and measured the surface potential versus time. The spreading among the monolayer molecules induced a surface pressure of approximately 30 mN/m. In the first 20 min after spreading, the surface potential underwent strong fluctuations (in a range of 100 mV), probably caused by the arrangement of the monolayer molecules. Therefore measurements were started 20 min after spreading.
Results Average Size and ζ Potential of the Hydrosol. The DLS measurements gave an average hydrodynamic particle diameter of approximately 16 nm with a PDI of 0.16, whereas the number distribution showed a diameter of approximately 9 nm (Figure 1a). It is interesting to note that the ζ potential of the iron oxide nanoparticles was positive (+ 40 mV) in acidic solution (pH 4). Size and ζ potential agreed with the results of literature.13 Pressure/Area Isotherms of the Monolayer. Figure 2 shows the surface pressure/area isotherms of the different lipid monolayers (stearic acid, stearyl alcohol, and stearyl amine) on hydrosols of γ-Fe2O3 (continuous line) and on pure water (dashed line). In the case of water, the π/A isotherms show the typical characteristics of such monolayers.21,22 The minima areas occupied by one molecule of approximately 0.23 nm2 are nearly identical for all investigated monolayers. The isotherms spread on γ-Fe2O3 sublayers are more expanded than those on pure water. For the stearic acid monolayer, the results agreed with previous investigations.16 As a result, we suggest that the (21) Lavigne, P.; Tancrede, P.; La Marche, F.; Max, J. J. Langmuir 1992, 8, 1988. (22) Vollhardt, D. AdV. Colloid Interface Sci. 1996, 64, 143.
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Figure 2. π/A isotherms of stearic acid (a), stearyl alcohol (b), and stearyl amine (c) spread on a subphase of pure water and on a subphase of γ-Fe2O3 nanoparticles.
Figure 3. Logarithms of the reflectivity normalized by the Fresnel reflectivity for stearyl amine (a), stearyl alcohol (b), and stearic acid (c) monolayers on water and aqueous γ-Fe2O3 hydrosol.
adsorption of γ-Fe2O3 nanoparticles below the surfactant monolayer expands the monolayer because the average diameter of the nanoparticles is much larger than the average distance between the different headgroups. The effect is stronger for the amine monolayer than for the acid monolayer. One reason can be the repulsive interactions between the positively charged amine monolayer and the positively charged nanoparticles. In contrast to the amine monolayer, the Coulomb interactions between the acid monolayer and the nanoparticles are attractive. This explanation is somewhat tentative because we ignored the influence of counterions (mainly chloride and hydroxide). These ions could also be adsorbing under the different monolayer and could influence the monolayer behavior during compression. Another, probable reason could be the slightly acid pH-value of the hydrosol. Thus the amine group was positively charged which results in an expansion of the monolayer. The better solubility
of the positively charged stearyl amine could be compensated by the size effect of the nanoparticles, which could be the reason for the expanded isotherm in the compressed state. The effect of different pH-values of the subphase on the behavior of the different monolayers should be investigated in detail in prospective research. X-ray Reflectivity Measurements. The logarithms of the reflectivities normalized by the Fresnel reflectivity are displayed in Figure 3. In a first step of data analysis the reflectivities of the Langmuir layers on the pure water subphase were refined using the effective density model17 in order to obtain the laterally averaged electron density profile of the different layers. All reflectivities were refined assuming a two-box model (headgroup and lipid tail). All refined reflectivities are displayed in Figure 3 as solid lines.
γ-Fe2O3 Nanoparticle Adsorption on Various Films
In a second step of data analysis, the reflectivity of the monolayers on the γ-Fe2O3 solution subphase were analyzed using the refined electron density profiles of the pure Langmuir layers as a starting point. A refinement using an additional homogeneous box layer, for the adsorbed γ-Fe2O3 layer was not successful for the majority of the investigated systems. Thus, the adsorbed γ-Fe2O3 layer was modulated by an iterative random displacement23 of the electron density profile using a total sample depth of 700 Å and a full with half-maximum of the displacement of 30 Å (first iterations) or 15 Å (final iterations). The resulting electron density profiles are displayed in Figure 4, respectively. As shown in Figures 3a and 4a, the reflectivity of the liquid gas interface using stearyl amine for Langmuir layer preparation can be refined without assuming an adsorbed layer of γ-Fe3O4. This is a direct consequence of the electrostatic interaction between the positively charged particles and the also positively charged monolayer, which leads to a repulsive force and prevents the formation of a particle film. Below the neutral monolayer of stearyl alcohol, a layer of γ-Fe2O3 nanoparticles adsorbs after 6 h. The small increase in the electron density points to a week adsorption process (Figure 3b and 4b). The γ-Fe2O3 nanoparticles showed very different adsorption properties in the presence of a monolayer of negatively charged stearic acid. As visible in Figure 3c, even 15 min after preparation, the reflectivity curve changed the shape drastically. The strong increase in the oscillation amplitude points to a rapid adsorption of nanoparticles below the monolayer. Because of this increased adsorption speed, the fitting procedure was not possible for the reflectivity curve after 15 min. After 240 min, the adsorption process was finished. The electron density profile displayed in Figure 4c shows a very dense layer of adsorbed nanoparticles. The layer thickness was in a range of 8 nm, which is in good agreement with the size of the nanoparticles measured by DLS measurements (number distribution). Surface Potential Measurements. Figure 5 shows the surface potential measurements of the different monolayers that were spread on the γ-Fe2O3 hydrosol (W% ) 0.7 g/L) versus time. At the beginning of the measurements in Figure 5, we observed a surface potential of approximately 200 mV for all different monolayers. This value is reasonable for a surface pressure of 30 mN and agrees with the literature.13 Using the γ-Fe2O3 hydrosol as sublayers, we observed a steep increase of the surface potential with increasing time for the stearic acid and the stearyl alcohol, whereas for the stearyl amine no surface potential increase could be observed. For the monolayer on pure water, we could not observe any changes in the surface potential versus time. We can conclude, hence, that the adsorption process of the nanoparticles causes the increase of the surface potential. The influence of the monolayer charge is about the same as that described for the reflectivity measurements. Furthermore, we get nearly the same time response of the surface potential and the X-ray reflectivity, which confirms the obtained results. UV-vis Study. The UV-vis spectra of LB layers of the different organic monolayer/γ-Fe2O3 nanoparticle composites are shown in Figure 6. The spectra show an increase of optical absorption density around 320 nm. The form and intensity of the spectra of the stearic acid/γ-Fe2O3 nanocomposite (Figure 6a) agrees with the results of previous investigations.13,16 LB films of those monolayers deposited from a water subphase (i.e., without γ-Fe2O3 nanoparticles) did not show any absorption band. (23) Sanyal, M. K.; Hazra, S.; Basu, J. K.; Datta, A. Phys. ReV. B 1998, 58, R4258.
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Figure 4. Calculated electron density profiles for stearyl amine (a), stearyl alcohol (b) and stearic acid (c) monolayers on water and aqueous γ-Fe2O3 hydrosol.
The reduced adsorption for the stearyl alcohol/γ-Fe2O3, and especially for the stearyl amine/γ-Fe2O3 nanocomposite, corresponds with a reduced adsorption of the nanoparticles under the positively charged monolayers.
Conclusion In a series of experiments, we systematically investigated the adsorption process of γ-Fe2O3 nanoparticles under different charged stearyl-monolayers. The nanoparticles were synthesized in an aqueous colloidal solution. These particles had typical diameters of approximately 9 nm (number distribution). The ζ
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Figure 5. V-t isotherms of stearic acid (a), stearyl alcohol (b), and stearyl amine (c) on the hydrosol of γ-Fe2O3 (W% ) 0.7 g/L).
potential of γ-Fe2O3 nanoparticles in hydrosol solution was on the order of +40 mV at acidic pH. Surface pressure isotherms of the different lipid monolayers demonstrated that the compressibility was dominated by differences in the particlemonolayer interactions. These differences were caused by Coulomb forces between the positively charged nanoparticles and the charged monolayers. The increase of optical absorption density around 320 nm with increasing electronegativity of the surfactant head groups (from stearyl amine over stearyl alcohol to stearic acid) could be traced back to the degree of the nanoparticle adsorption. The in situ adsorption process was investigated by X-ray reflectivity and by surface potential measurements. The time-dependent increase of the surface potential as well as the changing of the X-ray reflectivity reflects
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Figure 6. UV-vis spectra of LB-films of the stearic acid/γ-Fe2O3 nanocomposite (a), stearyl alcohol/γ-Fe2O3 nanocomposite (b), and stearyl amine/γ-Fe2O3 nanocomposite for one deposited layer (π ) 30 mN/m).
the adsorption process of the nanoparticles. The influence of the different monolayer charges could be observed in both experiments. The surface potential as well as the electron density (measured from X-ray reflectivity) increased strong for the stearic acid monolayer, weak for the stearyl alcohol monolayer, and not at all for the stearyl amine monolayer. Beside the different absolute values, the time dependence of both processes (X-ray reflectivity and surface potential) was nearly the same. On the basis of these results, we can conclude that the adsorption process of the positively charged nanoparticles was hindered by a positively charged monolayer (stearyl amine) and advanced by a negatively charged monolayer (stearic acid). LA802394A
