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Langmuir 1998, 14, 4088-4094
Electrostatic Adsorption of Polystyrene Nanospheres onto the Surface of an Ultrathin Polymer Film Prepared by Using an Alternate Adsorption Technique Takeshi Serizawa, Hiroko Takeshita, and Mitsuru Akashi* Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan Received December 12, 1997. In Final Form: April 22, 1998 We studied the adsorption of polystyrene nanospheres that have anionic charges on their surfaces from their aqueous dispersions onto the surfaces of cationic ultrathin polymer films quantitatively and kinetically by using a quartz crystal microbalance (QCM) and scanning electron microscopy (SEM). The polymer films were successively prepared by the alternate adsorption of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) in the absence or presence of suitable NaCl concentrations onto the QCM substrate so that we could observe their formation process by its frequency shifts. We observed the specific adsorption of the nanospheres by means of electrostatic interaction onto the outermost PAH surface by QCM as well. The dependence of their adsorption amounts against the NaCl concentration in polymer precursor film preparation was a sigmoid curve, indicating that there is a critical charge on/in the film for their adsorption. SEM observations showed monolayer adsorption without three-dimensional aggregation. We also studied the effects of particle size on their adsorption behavior and the possibility of the multilayering of nanospheres. This research on adsorption behavior by electrostatic interaction will be applied to the adsorption system of not only polymeric nanospheres but also other inorganic nanoparticles.
Introduction The regulated creation of colloidal mono- or multilayering is of great interest in material science. Several researchers have utilized a template surface,1 an electric field,2,3 emulsion droplets,4,5 electrostatic aggregation,6-10 solvent evaporation,11 a water surface,12-14 and other recent techniques15-18 in order to regulate colloidal assemblies in two or three dimensions. Recently, alternate layer-by-layer films that are composed of charged linear polymers and inorganic colloidal nanoparticles such as silica,19,20 TiO2/PbS,21,22 gold,23,24 and CdS25 have also been studied. Although the mono- or multilayering of their (1) Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (2) Yeh, S.-R.; Seul, M.; Shraiman, B. I. Nature 1997, 385, 57. (3) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706. (4) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2374. (5) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2385. (6) Furusawa, K.; Nagashima, K.; Anzai, C. Colloids Polym. Sci. 1994, 272, 1104. (7) Furusawa, K.; Anzai, C. Colloids Surf. 1992, 63, 103. (8) Otsubo, Y.; Edamura, K. J. Colloid Interface Sci. 1994, 168, 230. (9) Okubo, M.; Ichikawa, K.; Tsujihiro, M.; He, Y. Colloid Polym. Sci. 1990, 268, 791. (10) Okubo, M.; He, Y.; Ichikawa, K. Colloid Polym. Sci. 1991, 269, 125. (11) Demitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. (12) Horvogyi, Z.; Nemeth, S.; Fendler, J. H. Langmuir 1996, 12, 997. (13) Robinson, D. J.; Earnshaw, J. C. Langmuir 1993, 9, 1436. (14) Sheppard, E.; Tcheurekdjian, N. J. Colloid Interface Sci. 1968, 28, 481. (15) Baker, B. E.; Kline, N. J.; Treado, P. J.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 8721. (16) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (17) Pileni, M. P. Langmuir 1997, 13, 3266. (18) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr.; Schluz, P. G. Nature 1996, 382, 609. (19) Ariga, K.; Lvov, Y.; Onda, M.; Ichinose, I.; Kunitake, T. Chem. Lett. 1997, 125. (20) Lvov, L.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Langmuir 1997, 13, 6195. (21) Sun, Y.; Hao, E.; Zhang, X.; Yang, B.; Gao, M.; Shen, J. J. Chem. Soc., Chem. Commun. 1996, 2381.
inorganic particles has been successively performed, the details about monolayer adsorption (such as their kinetics or isotherms and/or the effects of precursor films before their adsorption) have not been studied. Accordingly, the general implications in regard to monolayer adsorption onto the ultrathin polymer film surface need to be understood. In an earlier study,26 we analyzed the monolayer adsorption of polystyrene nanospheres that had cationic poly(vinylamine)s grafted on their surfaces (which were synthesized by the free radical dispersion copolymerization of styrene and poly(N-vinylacetoamide) macromonomers and subsequent hydrolysis) onto the surface of an anionic polymer film that had been prepared by the alternate adsorption technique, which was recently developed by several researchers.27-32 However, their adsorption was based not only on electrostatic interaction but also on a hydrophobic one; therefore, the insights obtained were significant only in regard to their adsorption. To expand colloidal adsorption, we selected commercial polystyrene nanospheres that had no graft polymers on their surfaces. The difference in the nanospheres will lead to different adsorption behaviors. In this study, we examined their monolayer adsorption onto the surface of (22) Sun, Y.; Hao, E.; Zhang, X.; Yang, B.; Gao, M.; Shen, J.; Chi, L.; Fuchs, H. Langmuir 1997, 13, 5168. (23) Schmitt, J.; Decher, G.; Geer, R. E.; Dressick, W. J.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (24) Yonezawa, T.; Onoue, S.; Kunitake, T. Adv. Mater. 1998, 10, 414. (25) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (26) Serizawa, T.; Akashi, M. Chem. Lett. 1997, 809. (27) Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (28) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (29) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (30) Ramsden, J. J.; Lvov, Y. M.; Decher, G. Thin Solid Films 1995, 254, 246. (31) Advincula, R.; Aust, E.; Meyer, W.; Knoll, W. Langmuir 1996, 12, 3536. (32) Decher, G. Science 1997, 277, 1232.
S0743-7463(97)01366-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/25/1998
Adsorption of Polystyrene Nanospheres
Langmuir, Vol. 14, No. 15, 1998 4089 degradation of the electrical contacts when the QCM was immersed in an aqueous solution. The adsorption amount, ∆m (which could be detected by a frequency decrease of QCM, ∆F, by using Sauerbrey’s equation40) is as follows:
-∆F )
Figure 1. Schematic representation of an experimental setup.
a polymer film (which had been prepared by using an alternate adsorption technique) by using a quartz-crystal microbalance (QCM) and scanning electron microscopy (SEM). QCM can potentially detect the in situ binding of surfactants,33 biomolecules,34-39 and other chemicals onto bare or modified electrodes, which are sensitive to adsorbents with the nanogram level. In our preliminary research26 and other papers,19,20,24 the possibility of using the above technique for nanoparticle adsorption has already been demonstrated. The laterally regulated adsorption (in which a large number of the nanospheres were independently adsorbed) will also be performed by SEM observation in this paper. The experimental setup is schematically shown in Figure 1. The nanospheres were adsorbed after the alternate adsorption from cationic poly(allylamine hydrochloride) (PAH) and anionic poly(sodium 4-styrenesulfonate) aqueous solutions with a suitable salt concentration. Experimental Section Materials. PAH (Mw 8500-11 000) and PSS (Mw 70 000) were purchased from the Aldrich Co. and were used in precursor film formation without further purification. In regard to polystyrene nanospheres, we purchased Polybead Microparticles with diameters of 84, 548, and 780 nm from the Funakoshi Co., and these were used without further purification and by dilution with ultrapure distilled water. The mass of each of these nanospheres was estimated by assuming the polystyrene density to be 1.05. Their ζ potentials (which were kindly measured by Prof. H. Kitano, Toyama University) were -42.7 ( 6.4 mV, -27.8 ( 2.2 mV, and -39.7 ( 2.1 mV, respectively, indicating an anionically charged surface. The concentrations of nanospheres were estimated by an equation that has been explained in the catalog of the Funakoshi Co. Those with diameters of 780, 548, and 84 nm were 9.6 × 1010 mL-1, 2.8 × 1011 mL-1, and 7.7 × 1013 mL-1, respectively. Ultrapure distilled water was provided by MILLI-Q Labo. Quartz Crystal Microbalance. An AT-cut quartz crystal with a parent frequency of 9 MHz was purchased from USI. A crystal 9 mm in diameter was coated on both sides with gold electrodes that were 4.5 mm in diameter. The frequency was monitored by an Iwatsu frequency counter (Model SC7201) and was recorded by a PC-9801 DX. The leads of the QCM were sealed and protected by rubber gel in order to prevent the (33) Caruso, F.; Serizawa, T.; Furlong, D. N.; Okahata, Y. Langmuir 1995, 11, 1546. (34) Sato, T.; Serizawa, T.; Ohtake, F.; Nakamura, M.; Terabayashi, T.; Kawanishi, Y.; Okahata, Y. Biochim. Biophys. Acta 1998, 1380, 82. (35) Sato, T.; Serizawa, T.; Okahata, Y. Biochim. Biophys. Acta 1996, 1285, 14. (36) Sato, T.; Serizawa, T.; Okahata, Y. Biomedical Functions and Biotechnology of Natural and Artificial Polymers, Yalpani, M., Ed.; ATL Press: 1996; p 145. (37) Sato, T.; Serizawa, T.; Okahata, Y. Glycoconjugate J. 1995, 12, 590. (38) Sato, T.; Serizawa, T.; Okahata, Y. Biochem. Biophys. Res. Commun. 1994, 204, 551. (39) Bodenhofer, K.; Hierlemann, A.; Seemann, J.; Gauglitz, G.; Koppenhoefer, B.; Gopel, W. Nature 1997, 387, 577.
2F02
∆m AxFqµq
where F0 is the parent frequency of the QCM (9 × 106 Hz), A is the area of the electrode (0.16 cm2), Fq is the density of the quartz (2.65 g cm-3), and µq is the shear modulus (2.95 × 1011 dyne cm-2). This equation was reliable when measured in air, as in this study. Preparation of Precursor Films. Before immersing the QCM substrate into the nanosphere aqueous solution, we prepared precursor films on it. Both electrodes were used in this study. First, the QCM electrodes were treated with a piranha solution (H2SO4:H2O2 ) 3:1) for 1 min three times (followed by rinsing with pure water and drying with N2 gas) in order to clean its surface. According to a previous study,28 the QCM obtained was immersed into a PAH aqueous solution (0.02 unit M) for 20 min, taken out, washed thoroughly with pure water, and dried with N2 gas; the frequency decrease was then measured. Next, the QCM was immersed again into a PSS aqueous solution (0.02 unit M) for 20 min, and the same procedure was repeated. This alternate cycle was repeated at least four times for a precursor film. To obtain a PAH surface, another PAH layer was adsorbed after that procedure. The precursor films were successively prepared by the above procedure. Nanosphere Adsorption. The pretreated QCM was immersed into a nanosphere solution with a suitable concentration at 10 °C without mixing. After being immersed for a fixed amount of time, the QCM was taken out, completely washed with pure water, and dried with N2 gas; the frequency decrease was then measured. Confocal microscopic studies have shown that polystyrene nanospheres are concentrated near a substrate even if they have the same charges as the substrate.41,42 However, mixing did not affect the adsorption behaviors under the above conditions; thus, we stopped mixing. The aggregation of nanospheres in an aqueous dispersion is also well-known and may affect their adsorption behaviors. The adsorption amounts of the nanospheres gradually increased with an increase in the adsorption time after the aqueous dispersion had been prepared; for example, the adsorption amount for 60 min after 3 days of incubation resulted in it being 1.9 times larger than that where no incubation had taken place. This might have been caused by the aggregation of nanospheres in an aqueous phase. Accordingly, we used the solutions used for measurements as soon as possible after they had been prepared; thus we usually stopped the measurement after 60 min. In addition, we controlled the temperature in this measurement so that they would not become aggregated at high temperatures; this may be due to hydrophobic interaction. SEM Observation. The QCM was fixed onto a stage for SEM observation. Gold was sputtered on the QCM electrode at a thickness of approximately 20 nm after nanosphere adsorption. Next, we examined the surface with a HITACHI S-4100H.
Results and Discussion Electrostatic Adsorption of Polystyrene Nanospheres. The surface charges of the polystyrene nanospheres that we used in this study were anionic, as estimated by their ζ potentials. Therefore, these nanospheres may be adsorbed onto the surface of cationic polymers (which were deposited on the QCM substrate using the alternate adsorption technique) by electrostatic interaction. Figure 2 shows the frequency shifts against adsorption time when the QCM substrates (onto which (40) Sauerbrey, G. Z. Phys. 1959, 155, 206. (41) Ito, K.; Muramoto, T.; Kitano, H. J. Am. Chem. Soc. 1995, 117, 5005. (42) Muramoto, T.; Ito, K.; Kitano, H. J. Am. Chem. Soc. 1997, 119, 3592.
4090 Langmuir, Vol. 14, No. 15, 1998
Figure 2. Time courses for polystyrene nanosphere adsorption (diameter: 548 nm) at a nanosphere concentration of 3.8 × 1010 mL-1 at 10 °C: (a) onto a (PAH-PSS)3-PAH surface; (b) onto a (PAH-PSS)3 surface that was prepared in the presence of 2 M NaCl. (Solid smooth lines in all figures were drawn by the naked eyes.)
the precursor films were prepared by alternating PAH and PSS deposition in the presence of 2 M NaCl) were immersed into an aqueous dispersion of a polystyrene nanosphere with a diameter of 548 nm at a nanosphere concentration of 3.8 × 1010 mL-1 at 10 °C. The effects of salt on the above will be analyzed later in this study. When an outermost surface of the precursor film was adjusted to a PAH one (we actually deposited (PAHPSS)3-PAH layers), a larger frequency shift was observed, as is shown in Figure 2a. The frequency shift was saturated after 60 min, and it reached a frequency decrease of -5604 Hz, which corresponded to 6441 ng of the adsorbed nanospheres onto the QCM electrode. The mass increase corresponded to 67% coverage of the surface of the QCM electrode, assuming its hexagonal packing with a monolayer adsorption. The 9 MHz QCM that was used in this study can detect an increase in mass in more than several tens of nanograms; therefore, we could analyze the adsorption amount. On the other hand, a small frequency shift was observed on the surface of a PSS; there, we actually deposited (PAH-PSS)3 layers, as is shown in Figure 2b. The results show that the nanosphere can be adsorbed onto the surface of the precursor film by means of electrostatic interaction. It is known that an ultrathin polymer film that is prepared by the alternate adsorption technique has a regulated sheet structure, which was detected by X-ray analysis.27,30,45 It is likely that the adsorption behavior of the above is independent of an inner structure of the precursor film. The nanospheres only recognize the surface of an ultrathin polymer film in order to adsorb onto it. There has been research on kinetics of electrostatic adsorption of linear polymers,30,31,43,44 globular proteins,28 and nanoparticles25 onto the surface of charged films. The adsorption process of the polystyrene nanosphere in this study was saturated after 60 min, as was already shown in Figure 2a. Lvov et al. found that the alternate adsorption of SiO2 nanoparticles was saturated within 0.25 min.20 The saturation time was much smaller than the measurements in this study. However, the number concentration of the SiO2 nanoparticles that we estimated from the paper20 was much larger (more than 1021 times) than that of Figure 2a. That is why we obtained different (43) Aksberg, R.; Einarson, M.; Berg, J.; O ¨ dberg, L. Langmuir 1991, 7, 43. (44) Asano, K.; Miyano, K.; Ui, H.; Shimomura, M.; Ohta, Y. Langmuir 1993, 9, 3587. (45) Lvov, Y.; Decher, G.; Mohwald, H. Langmuir 1993, 9, 481.
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kinetics. The increase in saturation time with an increase in particle concentration was also observed in this study (as is discussed below) and supports the above. The kinetics of other systems for nanoparticle adsorption might also be affected by their concentrations. To compare and discuss those systems, detailed kinetic parameters (such as adsorption/desorption constants by curve fitting of adsorption curves) are needed. A cationic poly(vinylamine)-grafted polystyrene nanosphere was adsorbed onto the surface of a PSS and was stabilized on it by both electrostatic and hydrophobic interactions, as has already been noted by our research group.26 The differences in adsorption behavior between that and the polystyrene nanospheres in this study are difficult to explain completely. No significant differences between the ζ potentials were observed, though they showed opposite charges (data not shown). However, the surface structures are apparently different. Polymergrafted nanospheres have hydrated and hydrophilic polymer branches on its surface, while the polystyrene one does not. The difference in this type of steric facility for an approach between the surface of particles and that of a charged polymer film that was deposited on a substrate might lead to this type of different adsorption behavior. It is difficult to prevent certain polystyrene nanospheres (as well as those used here) from aggregating together in its dispersion at high temperatures and after the dispersion has stood for several days even at low temperatures. We controlled the adsorption temperature at 10 °C, as was described in the above Experimental Section. When the temperature was kept at 15 or 20 °C, we observed an increase of more than 10% in the adsorption amount. These SEM observations showed that the aggregation or multilayering of the nanospheres occurred on the surface of the polymer (data not shown). Furthermore, the nanosphere solution was used as soon as possible after it had been prepared; we then stopped the adsorption for 60 min. These must be the reasons why we obtained a high level of reproducibility in this study of the adsorption of polystyrene nanospheres. Based on the above, we used the same adsorption conditions as above. Effects of Precursor Films on Nanosphere Adsorption. We observed the electrostatic adsorption of polystyrene nanospheres onto the surface of an ultrathin polymer film when the film was prepared in the presence of 2 M NaCl. The authors would like to ascertain if the conditions such as the salt concentration during precursor polymer film preparation were significant in regard to nanosphere adsorption. In the next section, this will be analyzed in more detail. Figure 3 shows the time courses of nanosphere adsorption with a diameter of 548 nm at a nanosphere concentration of 3.8 × 1010 mL-1 at 10 °C onto the surfaces of (PAH-PSS)3-PAH films that were alternately deposited onto the QCM at each concentration of NaCl. The experimental errors were relatively small for all experiments. Every adsorption curve was saturated after around 60 min. Those saturated frequency shifts were plotted against the NaCl concentration, as is shown in Figure 4. The curve showed a saturation phenomenon. When the NaCl concentration was less than 0.2 M, the frequency shift tended to be obviously dependent on it; only -594 Hz (which corresponds to only 6% coverage of the surface of the QCM) was observed on the polymer film that was prepared at 0 M NaCl. Subsequently, the frequency shifts increased with an increase in the concentration of NaCl. When there was more than 0.2 M of NaCl, however, the equilibrium adsorption amounts were almost the same (reaching approximately -6000 Hz),
Adsorption of Polystyrene Nanospheres
Figure 3. Time courses for polystyrene nanosphere adsorption (diameter: 548 nm) at a nanosphere concentration of 3.8 × 1010 mL-1 at 10 °C onto (PAH-PSS)3-PAH surfaces that were prepared in the presence of NaCl: (a) 2 M; (b) 0.2 M; (c) 0.02 M; (d) 0 M.
Langmuir, Vol. 14, No. 15, 1998 4091
Figure 5. Monitoring of the ultrathin polymer film formation that was deposited by PAH and PSS in the presence of (a) 2 M, (b) 0.2 M, (c) 0.02 M, and (d) 0 M NaCl.
Figure 6. Dependence of the saturated frequency shift (diameter: 548 nm) at a concentration of 3.8 × 1010 mL-1 at 10 °C on the outermost PAH film thickness. Figure 4. Dependence of the saturated frequency shifts (diameter: 548 nm) at a concentration of 3.8 × 1010 mL-1 at 10 °C on the NaCl concentration for the ultrathin polymer film preparation.
which corresponded to about 60% coverage. These results show that nanosphere adsorption onto the surface of an ultrathin polymer film is clearly dependent on the conditions under which the ultrathin polymer films were prepared. Lvov et al. found that the presence of salt during ultrathin polymer preparation by the alternate adsorption method resulted in the formation of a thicker film without any rupturing of an ordered layer structure.30,45 Polyelectrolytes tend to form a globular structure in the presence of salt, because of the relaxation of intramolecular electrostatic repulsion. The globule seems to be electrostatically adsorbed on an oppositely charged surface, so that the salt makes the film thickness large. Accordingly, all of the charges of the adsorbed polymers do not interact with an oppositely charged film surface. This also indicates that each deposited polymer film had excess charges when the alternate adsorption was performed successively. We also observed these effects of salt on film thickness in the presence of NaCl, as is shown in Figure 5. Positive charges on/in an outermost film will affect nanosphere adsorption. Figure 6 shows the dependence of the saturated frequency shift on the thickness of the film in the outermost PAH. It was a sigmoid curve, which shows that there was a critical film thickness for nanosphere adsorption. Around a thickness of 2 Å, the frequency shifts increased steeply with a small increase
in thickness, while they became constant at a thickness that was over 4 Å. Although the cause of this phenomenon (which was observed at a certain film thickness) is complicated, the charge balance between the nanospheres and the polymer film might affect their electrostatic interaction. It seems that the thickness of an outermost film (which shows that it has an adequate amount of positive charges) is necessary for the acceleration of nanosphere adsorption. It should be noted that, when the precursor polymer film of (PAH-PSS)3 in the absence of NaCl and subsequent PAH film in the presence of 2 M NaCl was deposited, a frequency shift due to nanosphere adsorption was -5700 ( 200 Hz, which was consistent with the value onto a polymer film of (PAH-PSS)3-PAH that was thoroughly prepared in the presence of 2 M NaCl. This also shows that nanosphere adsorption is dependent on the outermost surface of the ultrathin polymer film, but not on the total charges or thickness of the film. In the following sections we deposited all of the precursor films in the presence of 2 M NaCl. Adsorption Isotherm. The isotherm for nanosphere adsorption allows one to obtain insights about a type of adsorption. Figure 7 shows the time courses for nanosphere adsorption at various concentrations onto a (PAHPSS)3-PAH surface at 10 °C. All of the curves have a high level of reproducibility and were saturated after around 60 min. The initial adsorption rates that we observed from the slope of their curves seem to be dependent on the concentration. The saturated frequency shifts were plotted against the nanosphere concentration as its adsorption isotherm, as is shown in Figure 8. The
4092 Langmuir, Vol. 14, No. 15, 1998
Figure 7. Time courses for polystyrene nanosphere adsorption (diameter: 548 nm) at 10 °C onto a (PAH-PSS)3-PAH surface prepared in the presence of 2 M NaCl at different nanosphere concentrations: (a) 19 × 1010 mL-1; (b) 9.5 × 1010 mL-1; (c) 3.8 × 1010 mL-1; (d) 0.76 × 1010 mL-1.
Figure 8. Adsorption isotherm of a nanosphere (diameter: 548 nm) onto a (PAH-PSS)3-PAH surface that was prepared in the presence of 2 M NaCl at 10 °C. The inset is the Langmuir plot.
isotherm became a saturation curve, indicating a Langmuirian type adsorption. Accordingly, we assumed the presence of a Langmuir adsorption and fitted the data to the following linear equation:
[nanosphere] 1 1 ) [nanosphere] + ∆F ∆Fmax ∆FmaxKads where ∆F is the frequency shift, ∆Fmax is the maximum estimated frequency shift, and Kads is the apparent adsorption constant. The Langmuir plot could be fitted easily (R2 ) 1), as is shown in the inset of Figure 8. The ∆Fmax that was estimated from the intercept was -8251 Hz. When the nanosphere (548 nm) was assumed to be packed in a hexagonal way with monolayer coverage, the apparent ∆Fmax was estimated to be -9682 Hz. Accordingly, we calculated that 85% of the surface of the QCM was covered with nanospheres. In this system, structural relaxation after nanosphere adsorption (which is aggregation of nanospheres in a plane and their moving two-dimensionally in a dried state possibly due to capillary force) will not occur, as the nanospheres were tightly absorbed onto the surface by electrostatic interaction. Accordingly, 15% of the space on the surface of the ultrathin polymers was not filled by the nanospheres, although using atomic force microscopy in an aqueous phase or a wet-type SEM will allow for an analysis of the
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adsorbed state in an aqueous phase. Decher at al. found that the adsorption of gold nanoparticles onto alternately deposited polymer films was caused by electrostatic interaction.23 The coverage in this study seems to be much more extensive and might be caused by many more charges in the outermost polymer film that was prepared in the presence of 2 M NaCl that was suitable for nanosphere adsorption. Coverage can be controlled by the nanosphere concentration, which might be useful for the surface modification of certain substrates. SEM Observation. Figure 9 shows the SEM images of the adsorbed nanospheres onto a (PAH-PSS)3-PAH surface at two different concentrations. The nanospheres were homogeneously absorbed two-dimensionally without three-dimensional aggregation. In Figure 9a, nearly half of the adsorbed nanospheres were adsorbed independently, possibly due to steric hindrance and/or electrostatic repulsion between the nanospheres when they were adsorbed. The amounts of adsorption amounts clearly increased with an increase in their concentrations. In the QCM measurements, we observed frequency shifts of -4950 Hz at 3.8 × 1010 mL-1 and -7689 Hz at 19 × 1010 mL-1, which corresponded to 51% and 79% coverage of the QCM electrodes, respectively. The coverage that was estimated from SEM images was consistent with those from the QCM measurements. The quantitative analysis of the polystyrene nanospheres by using the QCM was the same as the SEM observations. Here, we measured the frequency shift of the QCM in air, while in the other we sometimes had to use in an aqueous phase. In the latter, the frequency shift was frequently not proportional to the mass increase on the QCM electrode because of the water that was trapped between the absorbents. When one is discussing nanosphere adsorption in situ, we should use both the QCM and direct observation such as SEM. Nagayama studied the two-dimensional self-assembly of colloids in thin liquid films.46 The surface phenomena (such as the directional movement and assembly of fine colloid particles in thin liquid films) were significant for the hexagonally packed film formation. In our system, we believe that there is no similar structural relaxation after the electrostatic adsorption during blowing up (drying) water for washing the surface (see Experimental Section). The details have to be discussed by an in situ analysis of the surface with atomic force microscopy in an aqueous phase (without drying the surface). However, we have already studied the ordered assemblies of polystyrene nanospheres by the same procedure, in which the nanospheres were adsorbed independently and with constant distances between the edges, depending on their sizes.47 This might mean that the structural relaxation had not occurred after the nanospheres were adsorbed. On the other hand, Adamczyk et al. studied the role of hydrodynamic factors in the adsorption of colloid particles using experimental and theoretical techniques.48 However, it is difficult to discuss the formation mechanism of monolayer structure from the data described here. More detailed research on the formation mechanism is currently in progress. Adsorption of Nanospheres That Have Different Sizes. The analysis of each adsorption of polymeric nanospheres that had different sizes in this study yielded significant data about heterogeneous adsorption in a layer and of competitive adsorption from their mixture. Figure 10 shows the adsorption curves of nanospheres that had (46) Nagayama, K. Colloids Surf. 1996, 109, 363. (47) Serizawa, T.; Takeshita, H.; Akashi, M. Chem. Lett. 1998, 487. (48) Adamczyk, Z.; Siwek, B.; Szyk, L. J. Colloid Interface Sci. 1995, 174, 130.
Adsorption of Polystyrene Nanospheres
Langmuir, Vol. 14, No. 15, 1998 4093
Figure 9. SEM images of the adsorbed nanospheres onto a (PAH-PSS)3-PAH surface that was prepared in the presence of 2 M NaCl at 10 °C at nanosphere concentrations of (a) 3.8 × 1010 mL-1 and (b) 19 × 1010 mL-1 (accelerated voltage: 5 kV).
Figure 10. Time courses for polystyrene nanosphere adsorption that have three different diameters at a nanosphere concentration of 3.8 × 1010 mL-1 at 10 °C onto a (PAH-PSS)3PAH surface that was prepared in the presence of 2 M NaCl: (a) 780 nm; (b) 548 nm; (c) 84 nm.
three different sizes at a nanosphere concentration of 3.8 × 1010 mL-1 at 10 °C. The adsorption amounts apparently were different; however, the mass of their monolayer coverage on the QCM increased with an increase in their size, as the density of the polystyrene is considered to be the same. Figures 11 and 12 show the dependence of the adsorption number of the nanospheres and their surface coverage against adsorption time, respectively. We observed a reverse relationship of the effect of size between the adsorption number and the surface coverage. Although a small nanosphere can be absorbed onto the surface, the surface coverage was still small. The relationship described above is reliable, although their ζ potentials were slightly different. The size of the nanospheres (as well as the precursor film) seems to be critical in their adsorption. Further Layering of Nanospheres. The alternate adsorption technique performs multilayering of the nanospheres and ultrathin polymer film; Decher et al.23 and Ariga et al.19,20 studied the multilayering of gold and silica nanoparticles, respectively, by using this technique. In this section, we deposited more ultrathin polymer films
Figure 11. Dependence of the adsorption number on adsorption time at a nanosphere concentration of 3.8 × 1010 mL-1 at 10 °C onto a (PAH-PSS)3-PAH surface that was prepared in the presence of 2 M NaCl: (a) 84 nm; (b) 548 nm; (c) 780 nm.
Figure 12. Dependence of the surface coverage on adsorption time at a nanosphere concentration of 3.8 × 1010 mL-1 at 10 °C onto a (PAH-PSS)3-PAH surface that was prepared in the presence of 2 M NaCl: (a) 780 nm; (b) 548 nm; (c) 84 nm.
on the already adsorbed nanosphere layer as well as the second nanosphere layer. Figure 13 shows the time courses for the two-layer adsorption of the nanosphere at a nanosphere concentration of 3.8 × 1010 mL-1 at 10 °C. Regions A and B show
4094 Langmuir, Vol. 14, No. 15, 1998
Figure 13. Monitoring of two-layering adsorption (diameter: 548 nm) at a nanosphere concentration of 3.8 × 1010 mL-1 at 10 °C onto each (PAH-PSS)3-PAH surface that was prepared in the presence of 2 M NaCl: A and B regions show the first and second layers of the adsorbed nanosphere, respectively.
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ultrathin polymer film was adjusted to PAH, we observed a larger frequency shift of -9000 Hz, as is shown in Figure 13. However, we observed no large frequency shift on the surface of a PSS, as is also shown in Figure 13. This shows that we can deposit the second layer of the nanosphere by electrostatic interaction by using the alternate adsorption technique as well. The SEM image after the two-layer adsorption also showed evidence of two layering, as is shown in Figure 14. In addition, this image gives us data about the frequency shift of the second layer (-9000 Hz), which was larger than that of the first (-6000 Hz). The former seems to be formed after the void space of the first layer was filled with a nanosphere, as the -6000 Hz corresponds to about 60% of surface coverage. This is why we observed a larger frequency shift. We could also control the multilayering of the nanospheres by using this technique. Conclusion We quantitatively and kinetically studied the adsorption behavior of polymeric nanospheres onto the surface of an ultrathin polymer film by using the QCM and SEM. Anionic polystyrene nanospheres could be adsorbed onto the outermost film surface (which was deposited by using the alternate adsorption technique) by electrostatic interaction. The fine structural differences in the precursor polymer films such as their thickness (possibly the effective charges) and the nanosphere significantly affected their adsorption behavior. We also studied the possibility of structurally regulated multilayering of the nanospheres. Ideally, one can use any even or rough substrates that never dissolve or swell significantly in water in the alternate adsorption technique; accordingly, in regard to nanosphere adsorption, we can use a wide range of substrates. The surfaces that have adsorbed nanospheres that are similar to “dots”with the required distribution or height will open up new material fields. The results from this study can be expanded to other types of nanoparticle adsorption by means of electrostatic interaction.
Figure 14. SEM image of the two-layering of a nanosphere (548 nm) (accelerated voltage: 7 kV).
the monitoring of the first and second layers of the adsorbed nanospheres, respectively. The second alternate adsorption (that was composed of PAH and PSS) was also observed between regions A and B (the first layer was not shown). When the outermost surface of the second
Acknowledgment. We would like to acknowledge to Prof. H. Kitano (Toyama University) for help with ζ potential measurements and grateful discussions. This work was financially supported in part by a Grant-in-Aid for Scientific Research in the Priority Areas of “New Polymers and Their Nano-Organized Systems” (No. 277/ 09232249) from the Ministry of Education, Science, Sports, and Culture, Japan. LA9713661
