Langmuir 2002, 18, 7669-7676
7669
Investigation of the Factors Influencing the Formation of Dendrimer/Polyanion Multilayer Films Ajay J. Khopade and Frank Caruso* Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany Received March 12, 2002. In Final Form: June 28, 2002 Poly(amidoamine) dendrimer (PAMAM)/poly(styrenesulfonate) (PSS) multilayer films were prepared by an iterative electrostatic self-assembly process, viz., the layer-by-layer deposition of PAMAM and PSS onto planar supports. Multilayer film growth was monitored by UV-vis spectrophotometry, by use of a quartz crystal microbalance (QCM), and by ellipsometry and atomic force microscopy (AFM). The UV-vis data revealed an adsorption-desorption phenomenon that was sensitive to the pH and ionic strength of the PSS and dendrimer solutions, the dendrimer generation (2G, 3G, or 4G) and concentration, and the PSS molecular weight and concentration. This behavior was due to the different nature of interaction and complexation between the hierarchical (PAMAM) and linear (PSS) polyelectrolytes, both at the filmsolution interface and in bulk solution. QCM experiments provided evidence for multilayer film growth, but the adsorption-desorption behavior precluded quantitative evaluation of the amount of polyelectrolyte deposited. The use of “fresh” polyelectrolyte solutions for deposition of each layer was found to facilitate the regular formation of PSS/PAMAM multilayers. For PSS/4G PAMAM films prepared in this way, the bilayer thickness was determined to be 3.9 ( 0.2 nm from ellipsometry, while AFM showed that 10-layer films were rather smooth, with a root-mean-square roughness of ca. 0.6 ( 0.1 nm. Investigation of the formation of thin films from two different classes of polyelectrolytes (hierarchical and linear) provides insights into their interactions and opens the possibility of constructing complex films based on dendrimers that can be exploited as nanoreactors and nanoreservoirs.
Introduction Over the past decade, dendrimers have attracted significant interest for employment as sensors and catalysts, as well as gene delivery agents and drug carriers for controlled release and site-specific delivery.1a-c They are unique in that they comprise a discrete number of functionalities and a high local density of active groups and because the dendritic core microenvironment can be exploited for host-guest chemistry.1,2 Recently, investigations have also focused on the properties and potential applications of dendrimer-based ultrathin multilayer films, prepared either by nonelectrostatic3-6 or electrostatically driven self-assembly processes.7-13 * To whom correspondence should be addressed. Fax: +49 331 567 9202. E-mail:
[email protected]. (1) (a) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665. (b) Esfand, R.; Tomalia, D. A. Drug Discovery Today 2001, 6, 427. (c) Florence, A. T.; Hussain, N. Adv. Drug Delivery Rev. 2001, 50, S69. (2) Liu, M.; Frechet, M. J. PSTT 1999, 10, 393. (3) Watanabe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 8855. (4) Zhong, H.; Wang, J.; Jia, X.; Li, Y.; Qin, Y.; Chen, J.; Zhao, X. S.; Cao, W.; Li, M.; Wei, Y. Macromol. Rapid Commun. 2001, 22, 583. (5) (a) Anzai, J.; Kobayashi, Y.; Nakamura, N.; Nishimura, M.; Hoshi, T. Langmuir 1997, 15, 2221. (b) Anzai, J.; Nishimura, M. J. Chem. Soc., Perkin Trans. 2 1997, 1887. (6) (a) Liu, Y.; Breuning, M. L.; Bergbreiter, D. E.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2114. (b) Zhao, M.; Liu, Y.; Crooks, R. M.; Bergbreiter, D. E. J. Am. Chem. Soc. 1999, 121, 923. (7) (a) Yoon, H. C.; Kim, H. S. Anal. Chem. 2000, 72, 922. (b) Yoon, H. C.; Hong, M. Y.; Kim, H. S. Anal. Chem. 2000, 72, 4420. (8) (a) Decher, G.; Hong, J. D. Makromol. Chem. 1991, 46, 321. (b) For a review, see: Decher, G. Science 1997, 277, 1232. (9) (a) Tsukruk, V. V. Adv. Mater. 1998, 10, 253. (b) Tsukruk, V. V.; Rinderspracher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (c) Tsukruk, V. V.; Rinderspracher, F.; Bliznyuk, V. N. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1997, 37, 571. (10) Cheng, L.; Cox, J. A. Electrochem. Commun. 2001, 3, 285. (11) He, J. A.; Valluzzi, R.; Yang, K.; Dolukhanyan, T.; Sung, C. M.; Tripathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A. Chem. Mater. 1999, 11, 3268. (12) Wang, J. F.; Chen, J. Y.; Jia, X. R.; Cao, W. X.; Li, M. Q. Chem. Commun. 2000, 6, 511.
The nonelectrostatic principles used for dendrimer multilayer formation include coordination chemistry, H-bonding, biospecific recognition (avidin-biotin), and covalent bonding chemistry.3-6 Regen and Watanabe3 exploited the principles of coordination chemistry to construct multilayer poly(amidoamine) (PAMAM) dendrimer-based films. Using phenolic shell-modified dendrimer and diazoresin building blocks, Cao and co-workers prepared multilayer films via H-bonding.4 The alternating sequential deposition of fluorescently labeled avidin- and biotin-functionalized PAMAM dendrimer via the wellknown avidin-biotin interaction was reported by Anzai and co-workers.5a,b Bergbreiter and co-workers6a developed an approach for the multistep sequential deposition of a functional polymer (Gantrez) and PAMAM dendrimer on glass and silicon substrates. Each step followed a covalent bonding process between the anhydride groups of Gantrez and the amino groups of dendrimer. This method was extended to fabricate patterned PAMAM-Gantrez composite multilayer films.6b Yoon and Kim7a,b recently reported the formation of glucose oxidase/PAMAM multilayers by using aldehyde-activated glucose oxidase that reacts with the amine groups of dendrimers to form imine bonds. On the basis of the technique of electrostatic layer-bylayer (LbL) self-assembly of oppositely charged polyelectrolytes, originally developed by Decher,8 Tsukruk and co-workers9a-c fabricated multilayer films comprising oppositely charged PAMAM dendrimers. The LbL deposition technique was also used to construct multilayer films of polyoxometalates and fourth generation (4G) PAMAM dendrimers on quartz and gold substrates.10 In contrast to films typically prepared by the LbL method, in the above studies no polyelectrolyte interlayers were used for promoting film growth. Tomalia and co-workers,11 how(13) Tanaka, Y.; Nemoto, T.; Naka, K.; Chujo, Y. Polym. Bull. 2000, 45, 447.
10.1021/la020251g CCC: $22.00 © 2002 American Chemical Society Published on Web 08/29/2002
7670
Langmuir, Vol. 18, No. 20, 2002
ever, have reported the formation of dendrimer-stabilized gold nanoparticle/polyelectrolyte nanocomposite multilayer films, where the polyelectrolyte was used as a spacer layer. The focus of that study was mainly on the formation of gold nanoparticle films. In an investigation where the covalent bonding process was the primary objective,12 a combination of electrostatic and covalent bonding interactions was employed to prepare robust multilayer films from PAMAM dendrimers and nitro-containing diazoresin polyelectrolytes via the LbL technique. The dendrimer carboxyl groups interacting with the diazonium moieties on the polymer forming ionic linkages were converted to covalent bonds by UV irradiation. In yet another recent report, calcium carbonate-poly(ethyleneimine) composite/ anionic PAMAM dendrimer (G ) 3.5) films were prepared.13 Although the importance of dendrimer for multilayer buildup was specifically highlighted in that work, the main focus was on the formation and characterization of CaCO3 in the film. By use of a different class of amineterminated dendrimers (poly(propylenimine)), polyelectrolyte multilayers were built in alternation with a polyanion for second harmonic generation studies.14 Despite the aforementioned studies on the formation of dendrimer/polyelectrolyte multilayer films,11-14 a detailed investigation concerning the factors that affect multilayer formation is lacking in the literature. Such a study is important because dendrimer-linear polyelectrolyte complexes, formed by electrostatic interaction, are qualitatively different from complexes formed by two oppositely charged linear polyelectrolytes in solution.15 Therefore, it can be assumed that the interaction between such molecules is also different when forming multilayer films on solid supports. By control of the dendrimer-polyelectrolyte interaction, the dendrimer host-guest properties can be retained, which can then be harnessed for a variety of purposes, including drug delivery, chemical separations, and bioreactions. Following our preliminary study,16 herein we report a detailed investigation of the formation of dendrimer and PSS multilayer films prepared by the LbL technique. We examine the influence of reusing the same two solutions and using “fresh” polyelectrolyte solutions, and the influence of pH, ionic strength, concentration of the polyelectrolyte solutions (i.e., both dendrimer and PSS), and polyelectrolyte molecular weight (from ∼13 000 to ∼70 000 for PSS and from 3256 (2G) to 6909 (3G) to 14215 (4G) for dendrimer) on the buildup of the multilayer films. The step-by-step formation of these films is examined by UV-vis spectrophotometry. Additionally, the 4G PAMAM was used as a model dendrimer, and its LbL film growth, when deposited in alternation with PSS, was followed by using a quartz crystal microbalance (QCM), ellipsometry, and atomic force microscopy (AFM). On the whole, the current study provides an understanding of the factors that govern the assembly of dendrimer/polyelectrolyte multilayers, which is essential for the creation of tailored, ultrathin dendrimer-based films for various applications.16 Experimental Section Materials. Poly(sodium 4-styrenesulfonate) (PSS), molecular weight (Mw) ∼ 70 000, ∼47 000, ∼32 000, ∼17 000, and ∼13 000, (14) Casson, J. L.; McBranch, D. W.; Robinson, J. M.; Wang, H.-L.; Roberts, J. B.; Chiarelli, P. A.; Johal, M. S. J. Phys. Chem. B 2000, 104, 11996. (15) (a) Welch, P.; Muthukumar, M. Macromolecules 2000, 33, 6159. (b) Ghosh, P.; Lackowski, W. M.; Crooks, R. M. Macromolecules 2001, 34, 2131. (c) Miura, N.; Dubin, P. L.; Moorefield, C. N.; Newkome, G. R. Langmuir 1999, 15, 4245. (d) Kaname, T.; Keigo, A.; Masahiko, O. Polym. J. 2000, 32, 107. (e) Tsutsumiuchi, K.; Aoi, K.; Okada, M. Polym. J. 2000, 32, 107. (16) Khopade, A. J.; Caruso, F. Nano Lett. 2002, 2, 415.
Khopade and Caruso and poly(ethyleneimine) (PEI), Mw ∼ 15 000, were obtained from Fluka. Poly(allylamine hydrochloride) (PAH), Mw ∼ 25000, and second-, third-, and fourth-generation (2G, 3G, and 4G, respectively) PAMAM dendrimers were purchased from Aldrich Chemical Co. All chemicals were used without further purification. Sodium chloride and hydrochloric acid were obtained from Merck. Ultrapure water (Millipore) with a resistivity of more than 18 MΩ cm was used in all experiments. Experiments were carried out at room temperature (25 °C). Substrate Preparation. Quartz slides and silicon wafers were sonicated for 5 min in a 1:1 isopropyl alcohol/water mixture. They were then immersed in 5:1:1 (vol %) H2O/H2O2/NH3 at 70 °C for ca. 15 min, followed by thorough rinsing with Millipore water. After this cleaning procedure, the slides were hydrophilic. The slides were kept immersed in Millipore water until used. QCM electrodes were cleaned by dropping 30 µL of a H2O2/H2SO4 (3:1 vol %) mixture (piranha solution; CAUTION: Piranha solution should be handled with extreme care, and only small volumes should be prepared at any one time) on the gold surface for 1 min, followed by extensive rinsing with Millipore water. Polyelectrolyte Multilayer Formation. Substrates (quartz, silicon, and gold) were coated with a priming layer of PEI by immersing them in a 1 mg mL-1 PEI solution (containing 0.154 M NaCl,16 physiological salt concentration) for 30 min, followed by dip washing (three times) and nitrogen drying. All polyelectrolyte solutions (PSS or dendrimers) were prepared at a concentration of 1 mg mL-1 and contained 0.154 M NaCl. The pH and ionic strength of the solutions prepared were not adjusted unless otherwise stated (PSS, pH 6.5 ( 0.3; 2G, 3G, and 4G PAMAM, pH 10.3 ( 0.2). Polyelectrolyte adsorption was performed as follows: substrates were sequentially dipped in the polyelectrolyte solutions for 30 min, after which time they were rinsed with Millipore water by dipping (1 min each) in three different vials containing water and finally nitrogen dried. Analysis of the films was undertaken after each adsorption step. UV-Vis Spectrophotometry. UV-vis spectra were recorded with an Agilent 8453 spectrophotometer. The quartz slides were placed perpendicular to the beam using a slide holder to maintain the same positioning during each measurement. Quartz Crystal Microbalance. An in-house-built QCM device was used. The QCM frequency was stable within 5 Hz (in air) for several hours. The gold-coated QCM electrodes had a resonance frequency of ca. 9 MHz. The contact wires were insulated by silicone paste to avoid corrosion during the experiments with high salt concentration solutions. Typically, the QCM crystals were immersed in the polyelectrolyte solution and, following adsorption of the species, then were removed, rinsed with Millipore water several times, and dried by a gentle stream of nitrogen. The difference in the QCM resonance frequency before and after polyelectrolyte adsorption was used to follow multilayer film growth. Ellipsometry. Ellipsometry was performed using an optical null ellipsometer (Multiskop Ellipsometer, Optrel GmbH) with a He-Ne laser (632.8 nm). The effective film thickness was calculated using a computer program assuming a film refractive index of 1.45, a value that is similar to that typically used for multilayers of linear polycations and polyanions (1.47).17 Atomic Force Microscopy. AFM experiments were conducted in air by using a Nanoscope IIIa microscope, Digital Instruments, Santa Barbara, CA, in Tapping Mode. 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 of 130-150 kHz. The scanning rate was 1.5 µm s-1. Samples were prepared on precleaned silicon wafers (as described earlier).
Results and Discussion Our initial experiments concentrated on the LbL formation of PSS/PAMAM multilayers on solid supports by repeatedly using the same two solutions (one for PSS and one for PAMAM) for deposition of each alternate layer. (17) (a) Ramsden, J. J.; Lvov, Y, M.; Decher, G. Thin Solid Films 1995, 254, 246. (b) Ruths, J. Ph.D. Thesis, University of Mainz, Mainz, Germany, 1996; p 142. (c) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317.
Formation of Dendrimer/Polyanion Multilayer Films
Langmuir, Vol. 18, No. 20, 2002 7671
Figure 1. UV-vis spectra showing the LbL growth of PSS/4G PAMAM multilayers on quartz slides. The absorbance at 227 nm originates from the phenyl group in PSS. The solid and dotted lines correspond to the spectra measured following PSS and 4G PAMAM adsorption, respectively. The spectra clearly show the differences between the PSS and 4G PAMAM adsorption steps, indicating large PSS removal. Deposition of consecutive bilayers systematically yielded larger absorbances, showing multilayer film growth. 4G PAMAM and PSS concentration ) 5 mg mL-1.
This experimental protocol is typically employed for the LbL preparation of multilayers from polycation and polyanion solutions,8 since in most cases very little desorption of the adsorbed layers occurs with subsequent layer depositions18 and hence solution cross-contamination is minimal. In the current work, we show that by following this “standard” procedure for the preparation of dendrimer-polyelectrolyte films, there exists a complicated adsorption-desorption phenomenon that dominates film growth. This behavior is largely attributed to the significant removal of one adsorbed species with deposition of subsequent layers. In a second series of experiments, to avoid cross-contamination of solutions due to desorption of the species, fresh polycation and polyanion solutions were used for deposition of each PSS and PAMAM layer. The growth of films prepared by this latter method was followed, step-by-step, by various techniques, and the influence of factors such as solution pH and ionic strength, and polymer molecular weight and concentration on multilayer formation was studied. A. Deposition of Layers from Two Single Polycation and Polyanion Solutions. Typical UV-vis spectra obtained for the PSS/4G PAMAM films after each adsorption step are shown in Figure 1. The absorption peak at 227 nm, due to the phenyl ring present in PSS, decreases after exposure to the 4G PAMAM solution, indicating the removal of PSS from the film. (PSS removal from the film into the deposition solution was confirmed by UV-vis spectrophotometry.) Film growth is observed (with deposition of additional layers), suggesting that dendrimer adsorption also occurs at this step in this system (see later). Similar adsorption-desorption behavior has been reported for LbL-prepared polyelectrolyte-based films where one of the species used is a low molecular weight compound (e.g., a dye molecule),19a-c but such a trend is not often observed for polycation/polyanion multilayer films from linear polyelectrolytes.18 (18) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Colloids Surf., A 1999, 146, 337. (19) (a) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (b) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (c) Linford, M.; Auch, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 178.
Figure 2. LbL adsorption of PSS and PAMAM on quartz slides, plotted as film absorbance at 227 nm (peak) as a function of layer number. Solid squares and circles represent absorbance values at 227 nm after PSS and PAMAM adsorption, respectively. The lines connecting these data points show (a) divergent, (b) convergent, and (c) sigmoidal behavior. The dotted lines connecting the squares and circles illustrate the absorptiondesorption phenomena. PSS: Mw ∼ 70 000, 1 mg mL-1 solution containing 0.154 M NaCl, pH 6.5 ( 0.3; 2G, 3G, and 4G PAMAM: 1 mg mL-1/0.154 M NaCl, pH 10.3 ( 0.2.
The absorption maxima (227 nm) were plotted as a function of layer number for the different dendrimer (4G, 3G, 2G) systems to illustrate the adsorption-desorption behavior (Figure 2). For the PSS/4G PAMAM combination, the solid lines connecting the data points before and after dendrimer adsorption diverge with increasing layer number (Figure 2a), whereas for the PSS/2G PAMAM system the solid lines converge from the beginning (Figure 2b). These data indicate that for the 4G and 2G PAMAM films, the amount of PSS adsorbed and removed (with
7672
Langmuir, Vol. 18, No. 20, 2002
each subsequent deposition step) increases and decreases, respectively. For the sequential adsorption of 3G PAMAM and PSS, the first 6 layers are divergent and then the data converge up to 16-18 layers (Figure 2c): the data for the dendrimer adsorption steps fit a sigmoidal curve. The convergent-divergent behavior observed suggests an interplay between two processes: (i) PSS/PAMAM interaction at the surface that may cause a change in roughness and hence surface area of the film with each deposition step; and (ii) PSS-PAMAM complexation in the deposition solutions, due to their removal from the interface, leading to changes in the charge density and conformation of the depositing molecules. AFM studies of the films show that they are rather smooth with rootmean-square roughness values (0.5-0.9 nm for a 6-8 layer film) similar to those for films comprised of linear polycations and polyanions (e.g., PAH and PSS20), largely ruling out the first possibility. Therefore, the influence of PSS-PAMAM complexation in the deposition solutions is herein discussed to describe the extent of PSS deposition and removal observed. A likely explanation for the diverging behavior observed in Figure 2a, in addition to the PSS removal by the dendrimer solution (see earlier), is the removal of 4G PAMAM from the film to PSS solution upon the PSS deposition steps. (4G PAMAM desorption could not be directly discerned from the UV-vis measurements because the dendrimer does not absorb within the range studied.) The desorbed dendrimer would then be complexed by PSS in solution, increasing the PSS coiling (complexation-induced collapse) due to typical dendrimerlinear polyelectrolyte complexation.15a The adsorption of coiled PSS, the amount of which could increase with each additional PSS deposition step, would then occur along with that of uncomplexed PSS. In analogy, the amount of PSS deposited as a result of coiling induced by salt also increases when using various linear polyelectrolyte combinations to form multilayers.21 As the layering process continues, more PSS is adsorbed from the PSS solutions but more is also removed upon exposure to the 4G PAMAM solution, leading to the divergence observed. The converging effect observed (e.g., for the 2G PAMAM case, Figure 2b) is likely to be due to the removal of adsorbed PSS from the film by the dendrimer solution and the simultaneous adsorption of 2G PAMAM-PSS complexes formed in the dendrimer solution. The effect becomes more apparent when a growing number of dendrimer-polyelectrolyte complexes are formed in the dendrimer solution. It was verified that multilayer films could be formed from positively and negatively charged PAMAM-PSS complexes (PAMAM-PSS 5:5 w/w, negative; 9:1 w/w, positive) and an oppositely charged polyelectrolyte, lending support that complexes can compete with adsorption of the single species. Films prepared in this way were found to grow regularly (i.e., a linear increase in PSS absorbance was observed). In contrast to the 4G and 2G PAMAM cases, the data for the PSS/3G PAMAM system (Figure 2c), which shows both converging and diverging effects, could be best fitted by a linear curve for the PSS adsorption steps and a sigmoidal curve for the dendrimer steps. The linearity indicates that the amount of adsorbed PSS increases. The sigmoidal curve suggests that the overall amount of PSS deposited with each step is facilitated by the presence of (20) Caruso, F.; Furlong, D. N.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1998, 14, 4559. (21) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893.
Khopade and Caruso
an optimum amount of PSS in the dendrimer solution: there is increased PSS desorption below this optimum PSS amount, and growth does not occur above this PSS quantity. The differences observed in the data (Figure 2a-c) may, in part, be attributed to the number of charges on the dendrimers: assuming that only the surface amine groups are charged at pH 10.3, the number of reactive amines available for 3G and 2G PAMAM are 50% and 25%, respectively, compared with 4G PAMAM. To investigate the effect of charges and to rule out the possible influence of shape and size, 2G and 3G PAMAM were adsorbed alternately with low molecular weight PSS to keep the charge ratio (per molecule) equal or close to that of the PSS (Mw ∼ 70 000)/4G PAMAM system (ca. 5.3). Thus, 2G PAMAM was adsorbed alternately with PSS of Mw ∼ 17 000 (charge ratio, ca. 5.1), and 3G PAMAM was adsorbed with PSS of Mw ∼ 32 000 (charge ratio, ca. 4.8). Plots of the absorbance (227 nm) versus layer number for the three different systems showed almost the same behavior (diverging or almost parallel lines for at least the first 16 layers, data not shown). This shows that the relative charge ratio of PSS and dendrimer impacts the film growth behavior. Thus, it may be summarized that the divergent, convergent, and sigmoidal effects are a result of the removal of PSS from the films into PAMAM solution during dendrimer adsorption and vice versa. The degree of each effect depends on the number of charges each species bears and the composition of the dendrimerpolyelectrolyte complexes. These effects highlight the dominance of adsorption-desorption phenomena present in the formation of dendrimer/polyelectrolyte multilayers prepared in the conventional LbL process by using two dipping solutions,8 with no exchange of the solutions between layer depositions. Previous studies on the preparation of dendrimer/polyelectrolyte multilayer films have not underlined these effects, which can significantly impact film formation. B. Deposition of Layers from Fresh Polycation and Polyanion Solutions. The LbL experiments were conducted using new (fresh) solutions for each subsequent adsorption of PSS and dendrimer. With the PSS/4G PAMAM combination as a model system, PSS removal was still observed during film buildup but the lines connecting the data points before and after dendrimer adsorption are now almost parallel, indicating a constant amount of PSS removal after each adsorption step (Figure 3). This result is similar to that obtained with LbLconstructed dye-polyelectrolyte multilayers.19a The films prepared were characterized by QCM, ellipsometry, and AFM. QCM measurements show regular growth of the multilayer films (Figure 4a): average frequency changes of -37 ( 9 and -140 ( 20 Hz for each 4G PAMAM and PSS layer, respectively, were obtained. The dendrimer value is slightly lower than that expected theoretically for a close-packed monolayer of 4G PAMAM (-54 Hz). LbL growth of the PSS/4G PAMAM multilayers, as measured by ellipsometry, is shown in Figure 4b. 4G PAMAM has a diameter (thickness) of 4.5 nm, and polyelectrolyte layers in LbL-assembled films are ca. 1-2 nm thick.20 Hence, an average thickness per bilayer of ca. 5-6 nm was expected. The average effective thickness obtained from the ellipsometric measurements is 3.9 ( 0.2 nm per bilayer, which, like the QCM data, suggests that less than the equivalent of a monolayer of dendrimer was finally retained in the film for each PSS/dendrimer adsorption cycle. This could be because less than a monolayer of dendrimer was adsorbed and/or because PSS
Formation of Dendrimer/Polyanion Multilayer Films
Figure 3. LbL deposition of PSS and 4G PAMAM on quartz slides, plotted as film absorbance at 227 nm (peak) against layer number. Fresh solutions were used for each layer deposition. Solid squares and circles represent absorbance values at 227 nm after PSS and 4G PAMAM adsorption, respectively. The adsorption and desorption amounts with each deposition step are approximately the same.
Langmuir, Vol. 18, No. 20, 2002 7673
Figure 5. AFM image of a 10-layer PSS/4G PAMAM multilayer film. The first layer deposited was PEI, followed by alternating layers of PSS and 4G PAMAM. The image size is 1×1 µm2.
chains shrink in their radial dimension upon complexation.15a The morphology of the PSS/4G PAMAM films was examined by using AFM in tapping mode. The films were smooth and uniform, with a root-mean-square roughness of ca. 0.6 ( 0.1 nm (over a 1 µm2 area) and a grain size of ca. 60 ( 10 nm for 10-layer films (Figure 5). C. Factors Affecting Multilayer Growth. The influence of various parameters (e.g., dendrimer generation, PSS molecular weight, pH, solution ionic strength, and polyelectrolyte concentration) on PSS adsorption and removal was studied, as this was used as a measure to follow PSS/4G PAMAM multilayer film formation. In the following, the amount of PSS removed was calculated as a percentage (PSSR) from the equation
PSSR )
Figure 4. (a) LbL adsorption of PSS and 4G PAMAM on goldcoated QCM electrodes. The first layer deposited was PEI, followed by alternating layers of PSS and 4G PAMAM. (b) Effective thickness of PSS/4G PAMAM multilayers on silicon as a function of the number of layers deposited, as determined by ellipsometry. The thickness of the priming PEI layer is subtracted; hence the odd layer numbers represent PSS deposition and the even layer numbers represent 4G PAMAM adsorption.
removed some of the adsorbed dendrimer. Additionally, deformation of the dendrimer could have occurred upon adsorption and polyelectrolyte complexation. Molecular modeling shows that dendrimers and polyelectrolyte
An(PSS) - An+1(4G PAMAM) An(PSS) - An-1(4G PAMAM)
× 100
where A(PSS) and A(4G PAMAM) are the absorbance values at 227 nm for the PSS and 4G PAMAM adsorption steps, respectively, and n is the layer number. The average value of at least three experiments was taken. 4G PAMAM and PSS Mw ∼ 70 000 were used, unless otherwise stated. Polyelectrolyte Molecular Weight. (a) Dendrimer Generation. The molecular weights of the 2G, 3G, and 4G PAMAM dendrimers are 3256, 6909, and 14215. The LbL multilayer growth of PSS and 2G, 3G, or 4G PAMAM films, as monitored by UV-vis spectrophotometry, is shown in Figure 6a. The amount of PSS adsorbed increases with increasing dendrimer generation as a result of the increased number of charges on the dendrimers: 2G, 3G, and 4G PAMAMs have 16, 32, and 64 terminal (surface) amino groups, respectively, and additionally they have 14, 30, and 62 tertiary amine groups. At the pH of the solutions used (∼10.3), only the terminal amine moieties are likely to take part in the interaction.22 PSS removal was observed in all three systems and was found to increase with decreasing dendrimer generation (data not shown). Nevertheless, a sufficient quantity of PSS (average of 57 ( 3, 44 ( 2, and 30 ( 3% for 4G, 3G, and 2G PAMAM, respectively) remains in the film to promote further multilayer growth. (b) PSS Molecular Weight. The effect of PSS molecular weight on the formation of PSS/4G PAMAM multilayers is shown in Figure 6b. The amount of PSS deposited per layer increases with increasing PSS molecular weight. (22) Chen, W.; Tomalia, D. A.; Thomas, J. L. Macromolecules 2000, 33, 9169.
7674
Langmuir, Vol. 18, No. 20, 2002
Khopade and Caruso
Figure 6. (a) LbL adsorption of PSS and different PAMAM generations on quartz slides. The plot shows the film absorbance at 227 nm (peak) versus the bilayer (PSS/PAMAM) number. The amount of PSS adsorbed increases with increasing dendrimer generation. (b) LbL deposition of PSS of different molecular weights and 4G PAMAM on quartz slides, plotted as film absorbance at 227 nm (peak) versus bilayer (PSS/4G PAMAM) number. The amount of PSS adsorbed increases with increasing molecular weight of PSS.
Figure 7. (a) PSS/4G PAMAM multilayer growth dependence on pH of the deposition solutions, plotted as film absorbance at 227 nm (peak) versus bilayer (PSS/4G PAMAM) number. (b) Removal of adsorbed PSS from an 8-bilayer PSS/4G PAMAM film as a function of pH of the deposition solutions. The pH of the polyelectrolyte solutions was adjusted using 0.01 N NaOH or HCl. The percentage removal was calculated according to the formula given in the text.
(The absorbance is directly related to the number of phenyl groups (molecules) deposited per unit area.) In contrast, the trend in the percentage of PSS removed was the reverse, with minimum removal for PSS Mw ∼ 32 000 (ca. 31% removal). The removal was as high as 61% for PSS Mw ∼ 13 000 and about 40% for PSS Mw ∼ 43 000 and Mw ∼ 70 000. A possible scenario to explain these data is as follows. PSS removal from the substrate depends on the conformation (e.g., loops, trains, and tails) of PSS adsorbed on a dendrimer layer and more specifically on the number of contact points (electrostatically interacting groups) per PSS chain and the number of PSS molecules adsorbed per unit area. The minimum number of contact points between the PSS chains and a PAMAM layer depends on the PSS molecular weight. For example, for similar surface coverages, ∼5 chains of PSS Mw ∼ 13 000 would be required for each PSS of Mw ∼ 70 000 (assuming identical effects on the conformation/radius of gyration at given salt concentration21). The number of contact points for each PSS Mw ∼ 13 000 chain would be less than those for each PSS Mw ∼ 70 000 chain. Thus, the probability of removal should be larger for the smaller molecular weight PSS. This suggests that there should be a minimum number of contact points to prevent PSS desorption, and when the minimum is exceeded for a given PSS molecular weight, PSS desorption should in turn depend on the interaction of the adsorbed PSS loops or tails (conformation) extending away from the surface and 4G PAMAM in solution.
Muthukumar and co-workers have reported a molecular weight dependence of PSS conformation in solution,23 which is likely, in part, to be responsible for the conformation of PSS deposited on flat substrates and its subsequent removal. The data (see earlier) show qualitatively that the balance between the number of contact points and PSS conformation is optimal for PSS Mw ∼ 32 000, leading to minimum PSS removal. pH. LbL film growth was observed when PSS and 4G PAMAM solutions with pH ranging from 2.5 to 10.5 were used (Figure 7a). Data for the two extreme (2.5 and 10.5) and one intermediate (6.5) pH values are shown. The data for pH 6.5 are representative of those observed for the pH range 4.5-8.5, as a strong interaction occurs between PSS and dendrimer in this region (see later). The differences in the growth behavior are due to variations in the ionization of the polyelectrolyte species (see later). As shown in Figure 7b, the amount of PSS removed was in the range of 42-32%. There was minimum loss of PSS at a pH range from 4.5 to 8.5. This may be explained on the basis of protonation of either of the polyelectrolytes. Below pH 4.5, the degree of protonated PSS increases, leading to a weaker interaction with dendrimer. Below pH 8.5, the 4G PAMAM is almost completely protonated (both primary and tertiary amines).22 The number of charges (23) Prabhu, V. M.; Muthukumar, M.; Wignall, G. D.; Melnichenko, Y. B. Polymer 2001, 42, 8935.
Formation of Dendrimer/Polyanion Multilayer Films
Figure 8. (a) PSS/4G PAMAM multilayer growth dependence on ionic strength of the deposition solutions, plotted as film absorbance at 227 nm (peak) versus bilayer (PSS/4G PAMAM) number. (b) Percentage of PSS removal from an 8-bilayer PSS/ 4G PAMAM film as a function of ionic strength. The ionic strength was adjusted to 0.025, 0.05, 0.154, 0.5, and 1.0 M with NaCl.
per dendrimer molecule interacting with PSS should increase below this pH and may lead to strong adsorption and less removal of PSS. At a pH greater than 8.5, only the surface amine groups that are protonated (they are only partially protonated at pH 10.5)22 participate in PSS adsorption. The loosely bound PSS on the surface may thus be easily removed in the next adsorption step at pH 10.5. Between pH 4.5 and 8.5, the charge density on both PSS and dendrimer is high enough to allow strong interaction, while at the extreme pH values studied the charge density on either molecule is suppressed, leading to weakened interactions.15d Solution Ionic Strength. Polyelectrolyte solutions (both 4G PAMAM and PSS) were prepared containing varying concentrations of NaCl, and films were constructed using these solutions. Figure 8 shows that the degree of PSS removal increases with increasing salt concentration and becomes constant after about 0.5 M. The amount of PSS adsorbed at higher salt concentrations is larger (for the PSS adsorption step) due to the coiled conformation of PSS in high ionic strength solutions.22 The amount of PSS deposited per bilayer (after the dendrimer adsorption step), however, initially increases with increasing salt concentration and then decreases at high salt concentration (1 M NaCl) (Figure 8a), which is in contrast to what is observed for multilayer films composed of two linear polyelectrolytes. The amount of PSS remaining in the film when the multilayers are deposited from 1 M NaCl is almost the same as that for the same films prepared
Langmuir, Vol. 18, No. 20, 2002 7675
Figure 9. Influence of polyelectrolyte concentration on the multilayer growth of PSS/4G PAMAM films. (a) Plot of the absorbance of an 8-bilayer PSS/4G PAMAM film versus the polyelectrolyte concentration. The concentration was varied from 1 to 5 mg mL-1 for one species, while keeping the other species constant at 1 mg mL-1. The triangles (curves 1 and 1′) correspond to a constant PSS concentration and varying 4G PAMAM concentrations, while the squares (curves 2 and 2′) represent a constant 4G PAMAM concentration and varying PSS concentrations. The solid squares and triangles show the absorbance for an 8-bilayer film after the PSS adsorption steps, and the open squares and triangles represent values obtained after the 4G PAMAM adsorption steps. (b) Plot showing the percentage of PSS removal as a function of PSS and 4G PAMAM concentration, derived from (a). The symbols correspond to the same data as in (a).
without adding salt to the solutions (Figure 8a), even though the film growth behavior is different (Figure 8b) (i.e., at high ionic strengths more PSS is deposited but more PSS is removed). This is likely to be due to (i) increased screening of the electrostatic interactions at high ionic strength and (ii) the more coiled conformation21 of PSS at high salt concentration (due to screening of charge), thus giving a lower number of contact points. Polyelectrolyte Concentration. The effect of 4G PAMAM and PSS concentrations on the LbL growth of PSS/4G PAMAM multilayers is shown in Figure 9. Fixing the PSS concentration at 1 mg mL-1 and increasing the 4G PAMAM concentration from 1 to 5 mg mL-1 systematically adsorbed and removed more PSS (plotted for the average of an 8-layer film, curves 1 and 1′). Increasing the PSS concentration increased the amount of PSS adsorbed (Figure 9a, curve 2) due to the concentration-dependent conformation of the PSS solution.24 The amount of PSS removal also increased at constant dendrimer (curve 2′) concentration (with increasing PSS concentration), but
7676
Langmuir, Vol. 18, No. 20, 2002
the percentage of PSS removal remained almost constant (Figure 9b). Figure 9a shows that the lines connecting the data points for the absorbance at 227 nm for an 8-layer film diverge with increasing polymer concentration. This is consistent with the earlier observation (divergence behavior), suggesting that the factors which allow increased PSS adsorption amounts at the PSS adsorption step also lead to increased PSS removal after 4G PAMAM adsorption. The net amount of PSS deposited per bilayer remained almost constant for both increasing PSS and 4G PAMAM concentrations (curves 1′ and 2′). The data show that the dendrimer concentration predominantly governs the growth of the multilayer films. Conclusions The formation of PSS/PAMAM multilayer films was demonstrated, and the factors that govern their assembly were examined. During formation of the layers, the deposition of one polyelectrolyte causes removal of the other from the film, but a sufficient amount of each species remains to permit multilayer film growth. When the same two polyelectrolyte solutions were used to deposit multiple layers, the PSS removal profile showed divergence, (24) Takahashi, Y.; Matsumoto, N.; Iio, S.; Kondo, H.; Noda, I.; Imai, M.; Matsushita, Y. Langmuir 1999, 15, 4120.
Khopade and Caruso
convergence, and sigmoidal behavior, depending on the PSS/dendrimer charge ratio. This behavior was due, in part, to competition adsorption between the pure polyelectrolyte species and the PAMAM-PSS complexes formed in bulk solution. The use of fresh solutions for each deposition step led to the same amount of PSS being removed with each deposition step. The growth and removal of PSS during film formation depended upon the PAMAM generation and concentration, PSS molecular weight and concentration, and the pH and ionic strength of the deposition solutions. The results indicate that the PSS/PAMAM interactions at the surface are weaker than the corresponding interactions in the bulk. These significantly influence PSS/PAMAM multilayer growth. Studies on the stability and covalent modification of polyelectrolyte/dendrimer multilayers on flat and spherical substrates are currently under way, with the view of using such films as nanoreservoirs16 and nanoreactors. Acknowledgment. A. J. Khopade acknowledges the Alexander von Humboldt Foundation for a research fellowship. This work was funded by the BMBF. B. Scho¨ler is thanked for assistance with AFM measurements, T. Cassagneau for fruitful discussions, and H. Mo¨hwald for supporting the work within the MPI Interface Department. LA020251G
