Langmuir 1997, 13, 6195-6203
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Alternate Assembly of Ordered Multilayers of SiO2 and Other Nanoparticles and Polyions Yuri Lvov,†,§ Katsuhiko Ariga,† Mitsuhiko Onda,† Izumi Ichinose,‡ and Toyoki Kunitake*,†,‡ Supermolecules Project, JST (former JRDC), Kurume Research Center, 2432 Aikawa, Kurume, Fukuoka 839, Japan, and Faculty of Engineering, Kyushu University, Fukuoka 812, Japan Received May 19, 1997X Alternate layer-by-layer assembly of colloidal SiO2 particles with polycations has been investigated by quartz crystal microbalance (QCM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). QCM measurement confirmed the high regularity and reproducibility of the assembling process that depends on particle concentration, particle size, and ionic strength. The individual adsorption step was completed within 15 s. The thickness of adsorbed layers increased with increasing SiO2 concentrations at the three particle sizes used (45, 25, and 78 nm in diameter), unlike the case for other polyion assemblies. It also increased with increasing ionic strength of aqueous SiO2 dispersions. According to SEM observation, the assembled film possessed surprisingly flat surfaces at optimized ionic strengths. AFM observation revealed that SiO2 particles were not closely packed. The neutralization ratio of SiO2 and PDDA was estimated by turbidity measurement. Comparison of turbidity and QCM data indicated that the positive charge on PDDA was not completely neutralized by the negative charge on SiO2 particles in the course of alternate adsorption. This is apparently caused by a large difference in rigidity and charge density between SiO2 and PDDA. Since the charge density on PDDA is significantly larger than that on SiO2, all of the former charges cannot form short-distance ion pairs with surface charges of rigid SiO2 particles. The formation of long-distance charge pairs between SiO2 and PDDA, unlike the case for oppositely charged pairs of linear polyions, appears to be the origin of the remarkable dependence of film thickness and surface morphology on ionic strength and particle concentration. The other nanoparticles (CeO2 and TiO2) were similarly assembled with oppositely-charged linear polyions. Further modifications of the film structure were demonstrated by assembly between particles with different sizes and that between SiO2 and enzyme and by taking advantage of premixing components.
Introduction Construction of organic-inorganic nanostructured materials is an important target of modern materials research. Formation of crystalline CdS and PbS nanoparticles under Langmuir monolayers has been studied to prepare ordered particle layers “sandwiched” by amphiphile bilayers.1-5 A more convenient method for preparation of nanocomposite films is solvent casting of mixtures of metal oxide particles and amphiphile molecules: for example, alternate multilayers of alumina particles and amphiphile bilayers,6 and Li or Al doublemetal hydroxide and organic acids.7 Formation of ordered layers was reported for monodisperse nanoparticles of latex, silica, gold and silver.8-11 All these films are * To whom correspondence should be addressed at Kyushu University. † Supermolecules Project, JST. ‡ Kyushu University. § Current Address: Chemistry Department, University of Connecticut, Storrs, CT 06269-4060. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) (a) Fendler, J. H. In Thin Films, Vol. 20. Organic thin films and surfaces: directions for the nineties; Ulman, A., Ed.; Academic Press: San Diego, New York, Boston, 1995; pp 11-40. (b) Fendler, J. H. Chem. Rev. 1987, 87, 877. (2) Ozin, G. Adv. Mater. 1993, 4, 612. (3) Smotkin, E. S.; Lee, C.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. Chem. Phys. Lett. 1988, 142, 265. (4) Moriguchi, I.; Shibata, F.; Teraoka, Y.; Kagawa, S. Chem. Lett. 1995, 761. (5) Scoberg, D. J.; Grieser, F.; Furlong, D. N. J. Chem. Soc., Chem. Commun. 1991, 515. (6) (a) Ichinose, I.; Kimizuka, N.; Kunitake, T. J. Phys. Chem. 1995, 99, 3736. (b) Tsutsumi, N.; Sakata, K.; Kunitake, T. Chem. Lett. 1992, 1465. (c) Isayama, M.; Sakata, K.; Kunitake, T. Chem. Lett. 1993, 1283. (7) Dutta, P.; Robins, D. Langmuir 1994, 10, 1851. (8) Trau, M.; Saville, D.; Aksay, I. Science 1996, 272, 706. (9) Andres, R.; Bielefeld, J.; Henderson, J.; Janes, D.; Kolagunta, V.; Kubiak, C.; Mahoney, W.; Osifchin, R. Science 1996, 273, 1690.
S0743-7463(97)00517-9 CCC: $14.00
composed of uniformly ordered particles in two or three dimensions; however, it was not possible in the solution casting to create multilayers with precisely known numbers of layers and with predetermined alternation of different layers. Recently, alternate adsorption of oppositely-charged macromolecules is attracting much attention as a new technique of multilayer formation. This principle had been proposed for oppositely-charged colloidal particles in the pioneering work of Iler.12 Decher and co-workers13 extended this technique to a combination of linear polycations and polyanions (Figure 1a). Immersion of a charged substrate into a solution of oppositely-charged polyions leads to charge neutralization followed by charge reversal due to excessive polyion adsorption. The latter step is spontaneously terminated upon charge resaturation to produce a new polymer layer with controlled thickness. By repeating adsorption of oppositely-charged polyions, alternate layer-by-layer assembly becomes obtainable. The alternate assembly is now extended to conductive polyions,14,15 globular proteins (Figure 1b),16,17 clay microplates (Figure 1c),18,19 bola-amphiphiles,20-23 lipid bilayers,24 biospecific complexes,25 and organic dyes.22,26-29 (10) Kondo, M.; Shinozaki, K.; Bergstro¨m, L.; Mizutani, N. Langmuir 1995, 11, 394. (11) Kim, E.; Xia, Y.; Whitesides, G. Adv. Mater. 1996, 8, 245. (12) Iler, R. J. Colloid Interface Sci. 1966, 21, 569. (13) (a) Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (b) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831. (c) Lvov, Y.; Decher, G.; Mo¨hwald, H. Langmuir 1993, 9, 481. (d) Korneev, D.; Lvov, Y.; Decher, G.; Schmitt, J.; Yarodaikin, S. Physica B 1995, 213/214, 954. (14) (a) Ferreira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 806. (b) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115. (c) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501. (15) Onoda, M.; Yoshino, K. Jpn. J. Appl. Phys. 1995, 34, L260.
© 1997 American Chemical Society
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Figure 1. Schematic illustrations of alternate layer-by-layer assembly: (a) polyion-polyion assembly process; (b) assembled film of globular protein and polyion; (c) assembled film of clay microplate and polyion.
Iler’s pioneering work was concerned with alternate assembly of negative and positive colloidal particles such as silica and alumina, and he estimated the layer thickness qualitatively by interference color. Krozer et al.30 repeated the work of Iler with negatively- and positively-charged silica particles by using a quartz crystal microbalance (QCM). Recently we reported difficulty in direct assembly of conformationally rigid components such as proteins and clay, and we pointed out the importance of intermediate layers of flexible polyions as an electrostatic glue.16a,31 Unlike the case for earlier approaches, we,32 Fendler et al.,33 and Claus et al.34 have found successful assembly of colloidal nanoparticles alternately with flexible polyions. (16) (a) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (b) Lvov, Y.; Ariga, K.; Kunitake, T. Chem. Lett. 1994, 2323. (c) Onda, M.; Lvov, Y.; Ariga, K.; Kunitake, T. Biotechnol. Bioeng. 1996, 51, 163. (d) Onda, M.; Lvov, Y.; Ariga, K.; Kunitake, T. J. Ferment. Bioeng. 1996, 82, 502. (e) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Thin Solid Films 1996, 284/285, 797. (17) Kong, W.; Zhang, X.; Gao, M.; Zhou, H.; Li, W.; Shen, J. Macromol. Rapid Commun. 1994, 15, 405. (18) Kleinfeld, E.; Ferguson, G. Science 1994, 265, 370. (19) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 3038. (20) Sellergren, B.; Swietlov, A.; Amebrant, T.; Unger, K. Anal. Chem. 1996, 68, 402. (21) (a) Mao, G.; Tsao, Y.; Tirrell, M.; Davis, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1995, 11, 942. (b) Mao, G.; Tsao, Y.; Tirrell, M.; Davis, H. T. Langmuir 1993, 9, 3461. (22) Zhang, X.; Gao, M.; Kong, X.; Sun, Y.; Shen, J. J. Chem. Soc., Chem. Commun. 1994, 1055. (23) (a) Saremi, F.; Tieke, B. Adv. Mater. 1995, 7, 378. (b) Saremi, F.; Maassen, E.; Tieke, B. Langmuir 1995, 11, 1068. (24) Ichinose, I.; Fujiyoshi, K.; Mizuki, S.; Lvov, Y.; Kunitake, T. Chem. Lett. 1996, 257. (25) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Chem. Soc., Chem. Commun. 1995, 2313. (26) Cooper, T.; Campbell, A.; Crane, R. Langmuir 1995, 11, 2713. (27) Yoo, D.; Lee, J.-K.; Rubner, M. F. Mater. Res. Soc. Symp. Proc. 1996, 413, 395. (28) Araki, K.; Wagner, M. J.; Wrighton, M. S. Langmuir 1996, 12, 5393. (29) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (30) Krozer, A.; Nordin, S.-A.; Kasemo, B. J. Colloid Interface Sci. 1995, 176, 479. (31) Ariga, K.; Onda, M.; Lvov, Y.; Kunitake, T. Chem. Lett. 1997, 25. (32) Ariga, K.; Lvov, Y.; Onda, M.; Ichinose, I.; Kunitake, T. Chem. Lett. 1997, 125.
However, fundamental aspects of the assembly process of polyions and colloidal nanoparticles have not been fully explored. Charge density and conformational rigidity are very different between colloidal particles and polyions. The assembly of colloidal particles would have characteristics different from those of previously reported assemblies of flexible polyions. We have carried out systematic studies on multilayer assembly of charged nanoparticles of SiO2, TiO2, and CeO2 with polyions by employing a quartz crystal microbalance (QCM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Useful modification of the assembling process was demonstrated by taking advantage of premixed components. Experimental Section Materials. Poly(diallyldimethylammonium chloride) (PDDA, Aldrich, medium MW), sodium poly(styrenesulfonate) (PSS, Aldrich, MW 70 000), and poly(ethyleneimine) (PEI, Wako, MW 60 000) were commercially available and used without further purification at a concentration of 1.5-3 mg‚mL-1 (Chart 1). PDDA is a linear quaternary ammonium polymer and PEI (pKa 11) is a highly branched protonated polyamine. PSS (pKa 1) is a linear polyanion. The pH of the solutions was adjusted by adding aqueous NaOH or HCl. Glucose oxidase, Aspergillus Niger (GOD, Wako) was used in water at 2 mg‚mL-1. Deionized water with a specific resistance better than 18 MΩ‚cm was obtained by reverse osmosis followed by ion exchange and filtration (YamatoWQ500, Millipore, Japan). SiO2 colloidal dispersions in water (231 mg‚mL-1, Nissan Kagaku, Japan) were diluted to provide concentrations of 100, 10, 1, and 0.1 mg‚mL-1 at pH 10. The diameters of SiO2 particles were 25 ( 5, 45 ( 5, and 78 ( 5 nm, according to Nissan Kagaku. Aqueous colloids of cationic CeO2 and anionic TiO2 were obtained from Taki Chemical, Japan. Quartz Crystal Microbalance Technique. The quartz crystal microbalance (QCM) technique is used for detection of mass change during the assembling process. The QCM frequency decreases proportionally upon mass increase.35,36 The QCM resonators used (USI System, Japan) were covered by vapordeposited silver electrodes on both faces. The resonance frequency was 9 MHz (AT-cut). The QCM resonator was immersed for a given period of time in a polyelectrolyte solution and dried in a nitrogen stream, and frequency changes were measured. The QCM frequency in air was stable within (2 Hz during 1 h. All experiments were carried out in an air-conditioned room at ca. 22 °C. In the first stage, a well-defined precursor film with a thickness of ca. 10 nm was assembled with PDDA and PSS onto resonators or mica. The precursor films contained several polyion layers in the alternate mode (PDDA/PSS), and the terminal layer was “positive” PDDA. Then a substrate was alternately immersed for 15 min in aqueous dispersions of SiO2 and in aqueous PDDA or PEI with intermediate water washing. This process was periodically interrupted to measure the QCM resonance frequency. (33) (a) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (b) Kotov, N. A.; Dekany, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637. (34) Liu, Y.; Wang, A.; Claus, R. J. Phys. Chem. B 1997, 101, 1385. (35) Sauerbrey, G. Z. Phys. 1959, 155, 206. (36) (a) Ebara, Y.; Okahata, Y. J. Am. Chem. Soc. 1994, 116, 11209. (b) Ebara, Y.; Itakura, K.; Okahata, Y. Langmuir 1996, 12, 5165.
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The following relationship is obtained between adsorbed mass M (g) and frequency shift ∆F (Hz) by taking into account the characteristics of the quartz resonators used34
∆F ) (-1.83 × 108)M/A
(1)
where A is the cross-sectional area of the quartz microbalance placed between QCM electrodes and is 0.16 ( 0.01 cm2 in our system. Then, one finds that a 1 Hz change in -∆F corresponds to 0.9 ng in mass. It was necessary to calibrate the surface roughness of the resonator.16a Therefore, the thickness of the alternate layer corresponding to the total QCM frequency shift was experimentally determined by SEM observation of the crosssection of severed resonators coated with SiO2/polycation films to give the following relationship with (5% reliability.
d (nm) ) 0.022(-∆F (Hz))
(2)
Other Measurements. Resonators with assembled films were cut and coated with 20 Å thick Pt by use of an ion coater (Hitachi E-1030 ion sputter, 15 mA/10 Pa) under argon atmosphere. Their scanning electron micrographs were obtained with a Hitachi S-900 instrument at an acceleration voltage of 25 kV. Atomic force microscopy (AFM) was conducted by using a TMX2000 SPM System (TopoMetrix) in noncontact mode with a 2 µm scanner. Measurements were carried out in the air. Silicon nitride cantilevers with spring constants of 38-66 N‚m-1 were used as purchased. The resonance frequency of the cantilever was in the range 130-150 kHz. The scanning range was 1000 nm × 1000 nm. Scanning rates were 3 µm‚s-1. Turbidity was measured as follows. Equal volumes of aqueous SiO2 (5 mg‚mL-1) and aqueous PDDA (0.02-20 mg‚mL-1) were mixed, and 5 min later, UV absorption was measured at 550 nm, where there are no characteristic absorptions of SiO2 and PDDA, and at 25 °C with a JASCO UV/vis/NIR spectrometer equipped with a JASCO EHC-441 temperature controller.
Figure 2. Influence of the particle concentration in the alternate adsorption of SiO2 and PDDA at 22 °C and an adsorption time of 15 min: even-number steps, SiO2 adsorption; odd-number steps, PDDA adsorption. SiO2: particle diameter, 45 nm; dispersed at pH 10. PDDA: 3 mg‚mL-1.
Results and Discussion Regular Assembly of SiO2 Particles in Alternation with Polycations. Alternation with oppositely-charged polycations is essential for successful multilayer assembly of anionic SiO2. SiO2 particles were not adsorbed onto the anionic PSS surface, because SiO2 particles are negatively-charged at pH 10. Regular assembly was not possible by repetition of SiO2 adsorption onto a preformed SiO2 layer. In the latter case, the mass increase of a film due to the second SiO2 adsorption was only 50% of the first step, 20% at the third step, and 5% at the fourth step. Figure 2 shows QCM frequency changes in the assembly of 45-nm SiO2 at pH 10 in alternation with the linear polycation PDDA. One can see a linear increase of film mass (-∆F is proportional to mass M) for all the SiO2 concentrations. Larger and smaller steps correspond to adsorption of SiO2 particles and PDDA, respectively. An earlier report33a suggested that the subsequent washing of the film in water induced desorption of the colloidal particle. However, we found highly reproducible adsorption under all conditions, independent of water washing. Desorption upon water washing was not significant in our case. The magnitude of the growth step increases with SiO2 concentrations. Analogous concentration dependency was found for adsorption of SiO2 particles with different sizes (Figure 3). The -∆F values of the growth step increase with the concentration at all the SiO2 sizes. The obtained -∆F values for single SiO2 adsorption steps at 10 mg‚mL-1 are 520, 910, and 1350 Hz for 25-nm, 45-nm, and 78-nm particles, respectively. These values can be converted to thicknesses of 11, 20, and 30 nm, respectively, by eq 2. These thicknesses are clearly smaller than the diameters of the adsorbed SiO2 particles. Surface coverage can be defined as the ratio of the experimentally estimated layer thickness of a single
Figure 3. Effects of the size and concentration of SiO2 particles in alternate assembly with PDDA: 22 °C; adsorption time, 15 min. SiO2: particle diameter, 25, 45, and 78 nm; dispersion, pH 10. PDDA: 3 mg‚mL-1.
adsorption step and the particle diameter. The surface coverage is 100%, if the PDDA surface is wholly covered by a hexagonally packed layer of SiO2 particles. The QCM thickness is given by the mass increase due to particle adsorption corrected for the shape factor (hexagonal packing of spherical particle) of 0.63. The surface coverage estimated from eq 2 is ca. 40% (based on simple mass increase) for three sizes of SiO2. Thus, the extent of surface coverage by SiO2 particles becomes ca. 70% (40%/0.63). Therefore, the PDDA surface is not covered completely by SiO2 particles. The extent of the surface coverage in one adsorption step depends on the particle concentration. It is ca. 70% when the 45-nm SiO2 is assembled at a concentration of 100 mg‚mL-1. The coverage is much smaller at 0.1 and 1 mg‚mL-1. In particular, the average layer thickness at 0.1 mg‚mL-1 is estimated to be less than 2 nm, irrespective of particle sizes. This means that SiO2 particles are only sparsely placed on the surface. Saturation of the adsorption step is attained very quickly, as can be seen from the QCM data of Figure 4,
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Figure 4. Influence of the adsorption time of the SiO2 adsorption process in the alternate SiO2-PDDA adsorption at 22 °C. SiO2: particle diameter, 45 nm; dispersion, 10 mg‚mL-1 and pH 10. PDDA: 3 mg‚mL-1. Adsorption time: steps 1-5, 15 min (PSS-PDDA precursor film); odd number steps for steps 7-31, 15 min (PDDA); steps 6 and 8, 20 min (SiO2); step 10, 10 min (SiO2); steps 12 and 14, 5 min (SiO2); steps 16 and 18, 2.5 min (SiO2); steps 20 and 22, 1 min (SiO2); steps 24 and 26, 0.5 min (SiO2); steps 28 and 30, 0.25 min (SiO2).
in which the adsorption time for SiO2 particles was successively lessened to half of the previous adsorption time. The film growth remained exactly the same: -∆F ) 980 Hz for the SiO2/PDDA cycle, as the adsorption time was changed from 20 min to 15 s. Adsorption of linear PDDA is saturated in 10-12 min, as studied previously.16a This is surprising, since an earlier report33a used an immersion time of 24 h for adsorption of colloidal particles. Assembly of SiO2 particles with another polycation, PEI, gave similar kinetic parameters. The growth step for a SiO2/PEI multilayer was slightly less than that for SiO2/ PDDA films (see Table 1). Effect of Ionic Strength on the Multilayer Growth. The electrostatic interaction during alternate adsorption of anionic silica and polycation would be affected by the ionic strength of the solutions. Figure 5 gives QCM monitoring of a 45-nm SiO2 assembly in alternation with two polycations (PDDA or PEI) at different NaCl concentrations. The highest NaCl concentration we can use was 0.3 M, because the particle precipitated above this concentration. Regular film growth was found for all cases, but the magnitude of the growth step increased with increasing ionic strength. Table 1 summarizes the assembly parameters of the 45-nm particle at various ionic strengths. Thicknesses for single SiO2 adsorption are 20, 31, 95, and 143 nm at 0, 0.01, 0.1, and 0.25 M NaCl, respectively. Obviously, more than one SiO2 layer are adsorbed at every dipping at higher ionic strengths. A SiO2 layer of 95-nm thickness was deposited in one adsorption cycle at 0.1 M NaCl. It corresponds to ca. 2 layers of SiO2 particles, and the adsorption was saturated at this stage. Intermediate drying of the film is not required for regular multilayer assembly, as demonstrated by cycles 16-20 of Figure 5a, where alternate adsorption was performed without interruption for drying. Figure 6 gives the dependence of the growth step on ionic strength. The growth step is enhanced by similar extents for both SiO2-PDDA and SiO2-PEI pairs at low ionic strengths. However, at 0.1-0.25 M NaCl, the growth step of SiO2-PDDA becomes much greater than that of
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SiO2-PEI. This difference may be originated from the state of protonation, as discussed later. Assembly of Other Metal Oxide Particles. Alternate assembly of other charged nanoparticles should be possible in combination with oppositely-charged polyions. For this purpose, we assembled multilayers of cationic CeO2 nanoparticles at pH 3.5 by sequential adsorption with anionic PSS (Figure 7). The assembly process was regularly repeated. The growth step increased from -∆F ) 200 Hz at the CeO2 concentration 2 mg‚mL-1 to -∆F ) 480 Hz at 150 mg‚mL-1. The mass increase of PSS was ca. 10% of the total. The combined mass increase at these two CeO2 concentrations corresponds to 4-nm and 9.6-nm thickness, respectively. An assembly of anionic TiO2 particles at pH 10 (from 2 mg‚mL-1 solution) in alternation with PDDA or PEI also gave linear film growth with -∆F ) 400 Hz for one cycle (thickness corresponds to ca. 8 nm). Formation of Complex Layer Architectures. It is of interest to prepare more elaborate layer structures based on the preceding results and our previous experience for protein assembly.16 Figure 8a demonstrates consecutive assembly of two kinds of layers. The first four layers were made from 45-nm SiO2, and the following six layers were assembled from 25-nm SiO2. Growth steps for the two types of SiO2 particles are 1360 and 540 Hz, respectively, and the adsorption of the PDDA layer results in 80 Hz frequency shifts in both cases. Another example is formation of a four-unit layer that is made of repetition of the {45-nm SiO2/PDDA/25-nm SiO2/PDDA} layer (Figure 8b). Individual growth steps in these heterogeneous films are identical to the respective growth steps of their single-component films. Adsorption parameters of single components are preserved even when they are used in the complex architecture. Interestingly, it was possible to assemble a film from 1:1 mixtures of 45-nm and 25-nm silica particles in alternation with PDDA. The growth step of SiO2 in this instance was equal to the mean value of the growth steps of the two separate multilayers. It was also possible to prepare a multilayer assembly with alternation of 45-nm SiO2 and anionic glucose oxidase (GOD) with an intermediate PDDA layer: {SiO2/PDDA/ GOD/PDDA}n, n ) 1-5. We found precise repetition of -∆F ) (1400 + 70 + 500 + 70) Hz for one cycle of the multilayer assembly. Again, the frequency shifts for the individual components including GOD were identical to their respective -∆F values in single-component films. Preservation of the enzymatic activity in this case was confirmed by the method described previously.16c Modification of SiO2 Particles by Complexation with Polycation. SiO2 particles are negatively-charged at pH 10. However, these surface charges can be altered by complexation with polycations.37,38 Formation of watersoluble polyion complexes has been intensively studied in the past. Aqueous complexes of oppositely-charged polyelectrolytes are usually dispersible at stoichiometries far from neutralization. The maximum flocculation is found close to the neutralization point, and this value can be used for estimation of the surface charge of particles. This approach has been applied to globular proteins by Kabanov et al.38 Figure 9 gives a turbidity curve of SiO2-PDDA mixtures. Maximum flocculation occurs at [SiO2]/[PDDA] ) 64 (g/g). At the left and right sides of the maximum, the dispersions were slightly milky, though transparent and homogeneous. We suppose that this ratio is the point of neutrality. At this point, one SiO2 particle with a 45-nm (37) (a) Kabanov, V.; Zezin, A. Pure Appl. Chem. 1984, 56, 343. (b) Kabanov, A. Polym. Sci. 1994, 36, 143. (38) Park, J.; Muhoberac, B.; Dubin, P.; Xia, J. Macromolecules 1992, 25, 290.
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Table 1. Assembly Parameters for Multilayers of SiO2-PDDA and SiO2-PEIa NaCl concn in silica dispersion pure water
0.01 M
0.1 M
0.25 M
growth step of SiO2 + PDDA in QCM (Hz) thickness of SiO2 + PDDA from QCM data (nm) mass ratio, SiO2/PDDA (g/g) total film mass (µg) total film thickness from SEM observationb (nm) film densityc (g‚cm-3)
SiO2/PDDA 910 + 70 20 + 2 13 8.1 200 (10) 1.26
1400 + 90 31 + 2 16 13 320 (10) 1.23
4300 + 140 95 + 3 31 30 700 (7) 1.30
6500 + 220 143 + 5 30 47 1200 (8) 1.22
growth steps of SiO2 + PEI in QCM (Hz) thickness of SiO2 + PEI from QCM data (nm) mass ratio, SiO2/PEI (g/g)
SiO2/PEId 700 + 50 15 + 1 14
1320 + 80 29 + 2 17
1820 + 100 40 + 2 18
a SiO : particle diameter, 45 nm; dispersion, 10 mg‚ml-1 and pH 10. PDDA and PEI: 3 mg‚ml-1. b The values represent the total 2 thickness of the precursor film and the (SiO2/PDDA)n film. The number of the adsorption cycle, n, is given in parenthesis. c Error is ca. d 5%. Reliable estimation of the film thickness is difficult because the film surface is very rough.
Figure 6. Averaged frequency shifts of SiO2 adsorption plotted as a function of NaCl concentration of aqueous SiO2 dispersions: 22 °C; adsorption time, 15 min; O, SiO2-PDDA; 9, SiO2PEI. SiO2: particle diameter, 45 nm; dispersion, 10 mg‚mL-1 and pH 10. PDDA: 3 mg‚mL-1. PEI: 1.5 mg‚mL-1.
Figure 5. Effect of NaCl concentration of aqueous SiO2 dispersion in the alternate adsorption of (a) SiO2-PDDA and (b) SiO2-PEI at 22 °C and an adsorption time of 15 min. SiO2: particle diameter, 45 nm; dispersion, 10 mg‚mL-1 and pH 10. PDDA: 3 mg‚mL-1. Although steps 17-19 are conducted without intermediate drying, regular mass increases are clearly observed.
diameter and a 6.4 × 103 nm2 surface area is bound with 8 × 103 monomer units of PDDA. The surface charge of the complexed particle is reversed from negative in the right region to positive in the left region. It is possible to use not only “pure” SiO2 particles but also their complexes with polycations as negativelycharged particles (with less PDDA) or positively-charged particles (with excess PDDA). Assembly of the original 45-nm SiO2 and PDDA gives linear growth of 470 and 50 Hz, respectively, at a SiO2 concentration of 1 mg‚mL-1. In a first trial, we used a positively-charged SiO2-PDDA complex (2 mg‚mL-1 of SiO2 mixed with an equal volume of 1 mg‚mL-1 of PDDA). The complex was not adsorbed onto the cationic PDDA surface, but it was readily adsorbed onto an anionic PSS surface with -∆F ) 200 Hz. This is much less than a typical multilayer growth of bare SiO2 (470 Hz) under similar conditions, and the assembly did
Figure 7. QCM frequency shifts due to alternate CeO2-PSS adsorption at 22 °C and an adsorption time of 15 min. CeO2: 1-150 mg‚mL-1 and pH 3.5. PSS: 3 mg‚mL-1.
not proceed further. It appears that only the PDDA component is adsorbed onto the PSS surface from the SiO2-PDDA dispersion. In contrast, assembly of a negatively-charged complex ([SiO2]/[PDDA] ) 100) with
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Figure 9. Relative turbidity of aqueous mixtures of SiO2 (45 nm) and PDDA as a function of the ratio of SiO2 to PDDA (g/g). Turbidity was estimated by UV absorption at 550 nm (22 °C and pH 10). The mixing ratio was adjusted by mixing equal volumes of an aqueous SiO2 dispersion (2 mg‚mL-1, constant) and aqueous PDDA (varied concentration). The turbidity is normalized by taking the maximum value as unity.
every step for adsorption of bare SiO2 and 50 Hz for that of SiO2-PDDA. Excess PDDA effectively acts to bind successive silica layers. Structure of SiO2-Polycation Multilayers. In order to investigate the structure of the SiO2-PDDA assembly, the multilayer films were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). A best quality film with a smooth surface and constant thickness was obtained from 10 mg‚mL-1 of aqueous SiO2 with 0.1 M NaCl. One can readily see the internal structure of the film in the cross-sectional image (Figure 10a). The SiO2 spheres are closely packed in the layer, but long-range ordering is not found. The top view of Figure 10b shows well-packed particles in the twodimensional plane. Partial shielding of SiO2 surface charge may give more ordered films. Lower quality SiO2PDDA films (loose SiO2 packing within layers) were obtained for the assembly at 0 and 0.25 M NaCl. Multilayers of SiO2-PEI were less ordered than the ones of SiO2-PDDA. The former dispersion showed a tendency to form aggregates that lessen the flatness of films. In order to estimate particle packing more quantitatively, the surface morphology was investigated by atomic force microscopy (AFM) in the noncontact mode. Figure 11 shows AFM images of the surface of the assembled films. An AFM image of the PSS-PDDA precursor film (Figure 11a) has a surprisingly flat surface with a height difference of only 1 nm over 500 nm in the horizontal distance. This observation is consistent with our former finding on the PSS-PEI assembly.39 The obtained flatness is superior to those of LB films40 (height difference: a couple of 10 nm in a usual LB transfer and a few nanometers upon their multistep creeping treatment) and ultrathin films of amorphous polystyrene spread on water (a few nanometers).41 In contrast, individual SiO2 particles are seen in the AFM image of a SiO2-PDDA film (Figure 11b). As seen in the SEM image, the film surface has an array of packed silica particles, but it also contains some structural defects. Height profiling of the image shows a height difference of 10-20 nm, in reasonable agreement with the actual particle radius of 23 nm. However, the horizontal profile displays a round shape of the particles with the diameter along the horizontal axis 60-70 nm. This value is larger than the actual particle size (45 nm in diameter). A similar horizontal profile was observed even when we used a new cantilever. Therefore, the larger horizontal size is reproducible and may be explained by rolling of SiO2 particles during the AFM scanning. If this happens, the top view would contain ellipsoidal shapes along the scanning direction. However, this is not the case, since spherical particle shapes are retained. Instead, the larger size may be explained by assuming that the probe tip does not follow precisely the surface morphology. It was reported that the observed size of isolated ferritin molecules was larger than their actual size due to blunt scanning tips.42 The SiO2 particle is not close packed, and the apparent size becomes greater than the actual size. We can estimate the film density from layer mass and thickness as measured by QCM and SEM, respectively. It is 1.25 ( 0.05 g‚cm-3 for the samples prepared from
PDDA gives linear growth with steps of 460 and 50 Hz for the complex and PDDA, respectively. Therefore, the presence of a small fraction of PDDA attached to silica particles does not change the assembly mode. We also tested alternate assembly of bare SiO2 particles and positively-charged SiO2-PDDA (excess PDDA, [SiO2]/ [PDDA] ) 2). Surprisingly, regular growth was observed with a mass increase corresponding to -∆F ) 460 Hz at
(39) Onda, M.; Lvov, Y.; Ariga, K.; Kunitake, T. Submitted to Jpn. J. Appl. Phys. (40) Kuri, T.; Honda, N.; Oishi, Y.; Kajiyama, T. Chem. Lett. 1994, 2223. (41) Shuto, K.; Oishi, Y.; Kajiyama, T.; Han, C. C. Trans. Mater. Res. Soc. Jpn. 1994, 15A, 615. (42) Ohnishi, S.; Hara, M.; Furuno, T.; Sasabe, H. Biophys. J. 1992, 63, 1425. (43) Handbook of Chemistry and Physics; Weast, R., Ed.; CRC Press: Boca Raton, FL, 1988; p F-8.
Figure 8. Alternate assemblies by using two kinds of SiO2 particles: 22 °C; adsorption time, 15 min; (a) frequency changes upon 45-nm SiO2 adsorption (steps 4-12) and 25-nm SiO2 adsorption (steps 13-24); (b) frequency changes upon 45-nm SiO2 adsorption (steps 6-9) and three cycles of {(45-nm SiO2)/ PDDA/(25-nm SiO2)/PDDA} assembly (steps 10-21). SiO2: 10 mg‚mL-1, pH 10, and 0.01 M NaCl. PDDA: 3 mg‚mL-1.
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Figure 10. Scanning electron micrographs of {(PDDA/PSS)2 + PDDA + (SiO2/PDDA)8} film on a silver electrode of a QCM resonator: (a) cross-section; (b) top view. Preparation conditions: SiO2, 45 nm, 10 mg‚mL-1, 0.1 M NaCl, and pH 10; PDDA, 3 mg‚mL-1; adsorption time, 15 min.
Figure 11. Noncontact mode AFM images of the surface of the assembled films: (a) mica + (PDDA/PSS)4 + PDDA film; (b) mica + (PDDA/PSS)4 + (PDDA/SiO2)2 film. Preparation conditions: SiO2, 45 nm, 10 mg‚mL-1, 0.1 M NaCl, and pH 10; PDDA, 3 mg‚mL-1; adsorption time, 15 min. Upper figures show raw AFM images (500 nm × 500 nm), and lower ones represent height profiles along the line in the upper image. In the latter profiles, Z data represent the relative height against the lowest point of the whole two-dimensional image.
45-nm SiO2 in 0.1 M aqueous NaCl (Table 1). From the ratio of this value to the silica density (2.2 g‚cm-3),43 one obtains a packing coefficient of 0.56 ( 0.05 in the film. This value is smaller than the packing coefficient of 0.63 that is estimated for three-dimensionally close-packed solid spheres. The mass increase due to polycation adsorption was less than 8% of the combined mass increase in one cycle of SiO2/PDDA adsorption. The void volume in a close-packed layer is not less than 37%, and the mass
of the polycation is not sufficient to fill all the pores among silica particles. Water is not readily trapped in the void, since the film mass does not decrease even after 2 h of pumping out at 22 °C and heating for 2 h at 80 °C and an additional 2 h at 110 °C. Therefore, the void volume is preserved in the film, in agreement with the observations by SEM and AFM. Adsorption Stoichiometry and Assembly Mechanism. The process of SiO2-PDDA assembly is signifi-
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cantly influenced by ionic strength and SiO2 concentration. These influences are much more pronounced than those observed for alternate assemblies of polyion-polyion and protein-polyion. The thickness of the SiO2 layer at 0.25 M NaCl is ca. 7 times as large as that at 0 M NaCl, while that of the poly(vinyl sulfate)-poly(allylamine) layer was reported to change only from 13 to 34 Å with increasing NaCl concentrations from 0 to 0.9 M.13e In the proteinpolyion assembly, charged globular proteins formed molecular monolayers independent of protein concentrations. Most proteins are adsorbed as monolayers under any concentration tested.16e The concentration dependence of the SiO2 assembly may come from the unique characteristics of colloidal SiO2 particles. Table 1 summarizes the parameters of the multilayer assembly. QCM frequency shifts and layer thicknesses of the two components are given for both SiO2-PDDA and SiO2-PEI multilayer films. In all the cases, SiO2 layers are much thicker than polycation layers. With increasing ionic strength, the mass ratio of SiO2 to polycation in the films increases from 13 to 31. It is interesting to compare these mass ratios with the SiO2/ PDDA ratio of 64 obtained as the neutralization ratio in bulk solution. Clearly, excess PDDA is contained in the film. The excess charge on PDDA must be neutralized by small counterions such as Cl-. This charge unbalance appears to be originated in the fixed charge distribution on SiO2 particles and the large difference in charge densities between SiO2 and polycation. Fendler et al.33a already discussed that PDDA has a significantly larger charge density than those in colloidal particles such as CdS, PbS, and TiO2. In our case, adsorption of rigid SiO2 particles with lower charge density onto highly charged PDDA necessarily leaves free cationic sites on PDDA molecules. The stoichiometric complexation will be more readily accomplished in PDDA-SiO2 interaction in bulk solution. We also have to consider the size difference between SiO2 particles and polyions. In the previously reported protein-polyion assembly, the polycation molecule is ca. 100 nm in extended length, as estimated from its molecular weight, and one polyion molecule can cover and bridge tens of protein molecules.16a On the other hand, few bridges would be formed by the polycation among much larger SiO2 particles with diameters of 25-78 nm. One step of PDDA adsorption gives 60-150 Hz in QCM frequency shifts, and this mass increase corresponds to a 1-2 nm depth of the polycation layer. Therefore, the preadsorbed PDDA chain would form contact ion pairs only at the lower surface of the incoming SiO2 particle. Contact ion pairs will be formed mainly at the top of SiO2 particles on the subsequent adsorption of PDDA. Thus, we may assume that SiO2 and PDDA give rise to some fractions of long-distance charge pairs in contrast to exclusive formation of contact ion pairs expected for polyion-polyion assembly. This situation is schematically given in Figure 12. In the alternate assembly of polyion-polyion44 and polyion-bolaamphiphile,21a increases in ionic strength caused certain increases in film thickness. Linear polyion chains are extended in salt-free water due to charge repulsion but assume coil-like structures at higher ionic strength. The film thickness is enhanced when random coils, rather than extended chains, are adsorbed. The effect of ionic strength is more pronounced in the case of SiO2-PDDA assembly, and an increase in ionic strength does enhance not only the extent of adsorption but also the SiO2/PDDA ratio (Table 1). Charge shielding by (44) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160.
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Figure 12. Schematic representation of long-distance charge pairs of SiO2 and PDDA.
increased ionic strength may explain this effect. The outermost SiO2 particles should contain a large amount of effective negative charge due to excessive adsorption and the presence of long-distance charge pairs, as illustrated in Figure 12. The repulsive force among the effective negative charge would be suppressed by increased ionic strength more efficiently than in the case which involves short-distance charge pairs of lessened effective charge. This leads to enhanced adsorption of SiO2 particles. The salt effect operates more strongly with PDDA than with PEI. The smaller salt sensitivity of the SiO2-PEI film may be related to reversible protonation and chain flexibility of PEI. PEI has a pKa value of ca. 10.8, and protonation occurs only partially under the experimental conditions used (pH 10). The charge density of PEI is not so high as that of PDDA, and the extent and site of protonation are variable, unlike those of permanently and densely charged PDDA. Thus, short-distance charge pairs that are more stable will be formed in the SiO2-PEI film, and it becomes less sensitive toward ionic strength. As another possibility, the extent of protonation may be increased at higher ionic strengths. This factor can compensate the suppressive influence of ionic strength on Coulombic repulsion, leading to lessened sensitivity toward ionic strength. The adsorption of SiO2 particles is strongly dependent on their concentration, as described in Figures 2 and 3. These results cannot be explained by simple shifts of partition of SiO2 particles between bulk medium and surface. This is supported by the fact that subsequent washing of the film in water does not cause significant removal of the particles from the film. We can present two possibilities for the concentration effect. One explanation is based on the increase in ionic strength at high SiO2 concentrations, which induces enhanced adsorption. Another explanation is related to the state of dispersion of SiO2 particles. As shown in Figure 4, adsorption of SiO2 particles on PDDA is saturated within only 15 s. This fast kinetics implies that the adsorption is a nonequilibrium process. It is possible, then, that large clusters of SiO2 particles are formed at high SiO2 concentrations and are preferentially adsorbed. We cannot discriminate these two possibilities at the moment. In spite of these complex situations, it is clear that the adsorption process is highly regular and reproducible. Conclusion In this paper, we have demonstrated highly regular, reproducible assembly of SiO2 nanoparticles and linear
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polycations. The alternate films obtained have unit layers with molecular thickness and very flat surfaces. The large difference in size, morphology, and charge density between SiO2 and PDDA appears to cause formation of longdistance charge pairs; thus, the assembly is strongly dependent on ionic strength and particle concentration. This feature is different from that of the conventional polycation-polyanion and protein-polyion assemblies that would form short-distance charge pairs. The versatility of the present methodology is remarkable. First of all, a large variety of charged nanoparticles can be assembled in combination with linear polyions. We showed in this study that nanoparticles of TiO2 and CeO2 are similarly subjected to alternate assembly. Fendler and others33 used CdS and TiO2 particles. Multicomponent assembly is an interesting direction to pursue. Alternate assembly can be performed for nanoparticles, inorganic microplates, and globular proteins when they are combined with linear polyions. Then, macroions involved in the individual adsorption step may be either one of them, as illustrated in Figure 13A, and the kind of adsorbing species as well as the order of adsorption can be flexibly modified. Another interesting proposition is the formation of patterned layer structures. By assembling nanoparticles of different sizes in certain sequences, the size gradient and size alteration can be
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Figure 13. Versatile motifs of the alternate layer-by-layer assembly.
readily created (Figure 13B). The premixing assembly can provide yet another layer pattern (Figure 13C). These versatile procedures are inseparably related to novel functions in physical, chemical, biochemical, and material applications. We have shown that sequentially adsorbed layers of glucoamylase and glucose oxidase yielded a unique multienzyme microreactor.16d The variety of assembly illustrated in Figure 13 promises a much greater potential of these films. LA970517X
