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Morphology of Amphiphilic Gold/Dendrimer Nanocomposite Monolayers Young-Soo Seo, Kwang-Sok Kim, Kwanwoo Shin,† Henry White, Miriam Rafailovich,* and Jonathan Sokolov Department of Material Science and Engineering, SUNY at Stony Brook, Stony Brook, New York 11794
Binhua Lin Center for Advanced Radiation Sources and James Franck Institute, The University of Chicago, Chicago, Illinois 60637
Hyung Jung Kim Argonne National Laboratory, Argonne, Illinois 60439
Chunxin Zhang and Lajos Balogh* Center for Biologic Nanotechnology, University of Michigan, Ann Arbor, Michigan 48109-0533 Received January 3, 2002. In Final Form: May 7, 2002 We have synthesized amphiphilic gold/poly(amidoamine) (PAMAM) dendrimer nanocomposites (DNCs) using hydrophobically modified ethylenediamine (EDA) core generation-2 and -4 dendrimers as templates. Monolayers of gold DNCs were spread at the air/water interface of a Langmuir trough, and X-ray reflectivity was performed both in situ and on transferred LB layers. From the scattering length density and the specular reflectivity profiles, we find that the dendrimer layer is hydrated and the Au is uniformly distributed within the dendrimer body. Diffuse scattering shows no distinct features in agreement with TEM images, which show that the Au is disordered. The second-generation dendrimer was spherical on the water surface, whereas the fourth-generation became oblate at high pressures. The isotherms were also measured as a function of the pH of the water subphase. The results show that, at high pH, the dendrimers become “soft” and the films collapse at smaller areas. For lower pH values, a distinct plateau is observed that is postulated to arise from crystallization of the carbon side chains. The presence of Au in the dendrimer appears to amplify the effects of pH on the isotherms.
Introduction Dendrimers are monodisperse macromolecules built from connectors and symmetric branching units around a small molecule or a linear polymer core.1-4 A high level of synthetic control makes possible the synthesis of a narrow molecular weight range of well-defined and highly symmetrical polymer molecules containing a large number of regularly spaced internal and external (terminal) functional groups. As a result, the interior of amphiphilic dendrimers can be hydrophilic and the exterior hydrophobic, or vice versa, depending on their design and synthesis. The interactions of dendrimer macromolecules * Corresponding authors. For measurements: Miriam Rafailovich, Department of Material Science and Engineering, SUNY at Stony Brook, Stony Brook, NY 11794. E-mail: Miriam.rafailovich@ sunysb.edu. For materials: Lajos Balogh, The University of Michigan Center for Biologic Nanotechnology, 4010 Kresge Research Building II, 200 Zina Pitcher Place, Ann Arbor, MI 481090533.Phone: (734)615-0623.Fax: (734)615-0621.E-mail:
[email protected]. † Present address: Department of Materials Science and Engineering, K-JIST, Kwang-ju, Korea. (1) Tomalia, D. A.; Dewald, J. R.; Hall, M. J.; Martin, S. J.; Smith, P. B. In First SPSJ International Polymer Conference; The Society of Polymer Science, Japan: Tokyo, Japan, 1984; p 65. (2) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, M.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. (Tokyo) 1985, 17, 117. (3) Newkome, G. R.; Yao, Z.-Q.; Baker, G. R.; Gupta, V. K. J. Org. Chem. 1985, 50, 2003. (4) Hawker, C. J.; Frechet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638.
adsorbed on a surface will therefore depend on the ability of the different functional groups to interact with the surface. For example, for low generations, the dendrimers are known to be flexible; thus, both the inner and the outer groups can come into contact with the surface. This type of interaction is expected to produce distortions of the dendrimer molecule, depending on the nature of the surface interactions. Numerous studies have been performed on poly(amidoamine) (PAMAM) type dendrimers because the hydrophobicity of their surface can be controlled by grafting different functional groups. Sui et al.5 performed in situ surface dipole moment measurements at the air/water interfaces on fourth-generation 12-hydroxydodecanoic acid functionalized PAMAM dendrimer (HA-PAMAM), where hydrophilic -OH group are attached to the end of each hydrocarbon chain (C12). They found that the dendrimer molecules formed an edge-on disk-shaped structure at the air/water interface, which they explained as being due to strong adsorption of the -OH groups at the air/water interface. Tomalia and coworkers6 studied hydrocarbon-modified PAMAM dendrimers and concluded from surface pressure-area (ΠA) isotherms that these dendrimers retained their morphology when spread at the air/water interface. (5) Sui, G.; Micic, M.; Huo, Q.; Goger, M.; Leblanc, R.M. Langmuir 2000, 16, 7847-7851. (6) Sayed-Sweet, Y.; Hedstrand, D. M.; Spinder, R.; Tomalia, D. A. J. Mater. Chem. 1997, 7, 1199-1205.
10.1021/la025504k CCC: $22.00 © 2002 American Chemical Society Published on Web 06/21/2002
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Table 1. Characteristics of Amphiphilic Au/PAMAM DNCs
{Au1.14E2.R} {Au9.93E4.R}
hydrocarbon coverage
no. of gold atoms
Mn without Au
Mn with Aua
14 C12 out of 16 61 C12 out of 64
1.14 9.93
5890 25 492
6117 27 469
a Calculated as the average mass of nanocomposite units composed of the template and the encapsulated gold atoms.
Here, we apply X-ray reflectivity to measure the in situ conformation of dendrimers at both the air/water and air/ solid interfaces. Because the X-ray scattering contrasts between the dendrimers and water is weak, we chose to use amphiphilic C12-PAMAM dendrimers that have been modified with Au nanocomposites. A major advantage of the dendrimer nanocomposite is the large X-ray scattering contrast provided by the metal particles, which enables one to observe directly the distribution of the dendrimers at the air/water or air/solid interface as a function of surface pressure, pH, and other factors. We report on a study of the behavior of second- and fourth-generation amphiphilic gold/PAMAM dendrimer nanocomposites in which we examine conformation as a function of pressure and area when spread in a Langmuir trough at the air/water interface. The results are then compared with Langmuir-Blodgett (LB) films lifted at the same pressures onto silicon substrates. Experimental Section Materials. 1,2-Epoxy dodecane (95%) was purchased from Aldrich and used without further purification. PAMAM dendrimers in methanol solution were purchased from Dendritech and were used without further purification. The respective amphiphilic nanocomposite templates were synthesized from EDA-core generation-2 and generation-4 PAMAM dendrimers with 1,2-epoxy dodecane by a slightly modified literature procedure.7 The degree of substitution of the terminal amines was measured by 1H NMR spectroscopy and was found to be 89.5 and 95.6%, respectively. Gold-dendrimer nanocomposites {Au(0)1.14-PAMAM•E2.C12} and ({Au(0)9.93-PAMAM•E4.C12} (or {Au1.14E2.R} and {Au9.93E4.R}, respectively, for short; {Aun0-PAMAM.R} for general notation) with hydrophobic terminal substituents were synthesized. First, gold-dendrimer complexes were prepared by mixing dilute solutions of PAMAM dendrimers with a solution of HAuCl4. In this step, a complex salt formed between the tertiary nitrogen branching centers of the dendrimer and the tetrachloroaurate anions.7-9 The reduction then was completed by the addition of a slight excess of hydrazine. DNC solutions were deep red, indicating reduction of the complex salts into metallic gold and the formation of a nanocomposite structure. The starting materials and the obtained products were carefully characterized by UV-vis and 1H and 13C NMR spectroscopies, size-exclusion chromatography (SEC), and high-resolution transmission electron microscopy (HRTEM). Details of the experimental procedure and analytical techniques can be found in a previous study.7 The comparative properties of the amphiphilic {Aun0-PAMAM.R} DNCs used are summarized in Table 1. Measurements. Calculated volumes of the DNC solution (concentration fixed at 1 mg/mL in chloroform) were spread on the surface of aqueous subphase with controlled pH using a microsyringe at 25 °C. A computer-controlled KSV3000 doublebarrier LB trough was used. Measurement of Π-A isotherms was followed by the transfer of monolayers (LB film) onto (7) Balogh, L.; Valluzzi, R.; Laverdure, K. S.; Gido, S. P.; Hagnauer, G. L.; Tomalia, D. A. J. Nanopart. Res. 1999, 1 (3), 353. (8) He, J.-A.; Valluzzi, R.; Yang, K.; Dolukhanyan, T.; Sung, C. M.; Kumar, J.; Tripathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A. Chem. Mater. 1999, 11, 3268. (9) Gro¨hn, F.; Bauer, B. J.; Akpalu, Y. A.; Jackson, C. L.; Amis, E. J. Macromolecules 2000, 33 (16), 6042.
Figure 1. Π-A isotherms of amphiphilic gold/PAMAM DNCs monolayers: (a) generation-2 and (b) generation-4. Each DNC solution concentration equals 1 mg/mL. Ao (the onset point obtained from the derivatives) and Ac (the collapse point) indicate the areas per molecule at which the films begin to interact with each other and collapse, respectively. hydrophilic silicon wafers. The barrier and film transferring speed were 5 and 2 mm/min, respectively. Distilled and deionized water was used as the subphase. In situ X-ray reflectivity measurements were performed at the Brookhaven National Laboratory (BNL) National Synchrotron Light Source (NSLS) X19C beamline (wavelength, λ ) 1.54 Å).10 The LB trough was attached to the sample stage of a liquid reflectometer. The hydrophobic gold/PAMAM DNC solution was spread at the air/water interface, and the reflectivity was measured at various surface pressures. The incident beam was perpendicular to the direction of the moving barrier of the film balance. The barrier and a Wilhelmy plate were placed far away from the beam footprint to avoid problems with meniscus effects on the flat water surface. The reflectivity of the LB film deposited on the silicon substrate was measured at the NSLS/BNL X10B beamline (λ ) 1.13 Å). Both specular and diffuse scattering measurements were obtained. Specular measurements yield the electron density profile perpendicular to the film layer, and diffuse measurements yield the in-plane information. The specular reflectivity is defined as R ) |r|2, where r is the reflectance, measured as a function of scattering wave-vector transfer, qz ) (4π/λ) sin(θ), where θ is an incident angle. In situ diffuse scattering measurements were taken along the x direction at θ ) 0.13 fixed as a function of scattering wave-vector transfer, qx ) (2π/λ)(cos β - cos θ), where β is an outgoing angle to the x direction.
Results and Discussion Π-A Isotherm Analysis. Figure 1a and b depicts the Π-A isotherms of second- and fourth-generation, respectively, hydrophobic (C12) Au/PAMAM DNC monolayers spread at the air/water interface. From this figure, we can see that the onset pressure (Ao) occurs at an area of (10) Schlossman, M. L.; Synal, D.; Guan, Y.; Mero, M.; SheaMcCarthy, G.; Huanhg, Z.; Williams, S. M.; Rice, S. A.; Viccaro, P. J. Rev. Sci. Instrum. 1997, 68, 4372.
Amphiphilic Gold/Dendrimer Nanocomposites
1410 Å2/molecule for the generation-2 dendrimer, much smaller than the value for the generation-4 dendrimer. A classic gas-phase Π-A isotherms is observed for the generation-2 dendrimer. A discontinuity occurs at Ac ) 460 Å2/molecule, where optical examination indicates that the film begins to buckle. Hence, this pressure is assumed to correspond to hard-core collapse of the film. A similar hard-core collapse is observed for the generation-4 film at a higher area per molecule of Ac ) 1550 Å2/molecule. On the other hand, the isotherm of the generation-4 film clearly shows a plateau at ∼2700 Å2/molecule, which indicates that another phase transition might be occurring at that pressure. We can estimate the area of the individual nanocomposite from the values at Ac, 460 and 1550 Å2/ molecule for {Au1.14E2.R} and {Au9.93E4.R}, respectively, if we assume that the particles form a close-packed “faceon” hexagonal 2D array. From the values of Ac, we obtain diameters for the nanocomposites of approximately 23 and 42 Å for the second and fourth generations, respectively. The value for the fourth generation is in good agreement with 34 Å, which was reported in ref 11 for the PANAM dendrimer without Au and hydrocarbons. No direct measurements exist for the second generation, but our value is consistent with 30 Å, the value inferred from the data of ref 6 for the same dendrimers without Au. At the Air/Water Interface. In Figure 2a and b, we plot the in situ X-ray reflectivity as a function of qz for the second- {Au1.14E2.R} and fourth- {Au9.93E4.R} generation dendrimers, respectively, at the air/water interface. From this figure, we can see that the scattering contrast is quite good and distinct oscillations are observed for both types of films. In both cases, the period of oscillation is seen to increase gradually with surface pressure up to π ) 40 dyn/cm. A sharp increase in the period is then seen for π ) ∼45 dyn/cm (after the collapse point). The model used to fit the data is shown as an inset in both figures. The parameters of the fit were the scattering length density (reF ) 2πδ/λ2, where re is the classical electron radius, equal to 2.814 × 10 - 5 Å; F is the electron density; and δ is the dispersion coefficient), the film thickness, and σ1 and σ2 (roughness at the air and water interfaces, respectively). The values at 40 dyn/cm are tabulated in Table 2. The reF values obtained, 1.08 × 10-5 and 1.32 × 10-5 Å-2, for the second and fourth generations, respectively, correspond to relative Au weight fractions of 3.7 and 7.2% which are agreement with the stoichiometric amounts of Au in the nanocomposite. In Figure 3, we plot the thickness of {Au1.14E2.R} and {Au9.93E4.R} layers as a function of surface pressures. From this figure, we can see that the second generation forms a layer approximately 16 Å thick at π ) 10 dyn/cm, which increases gradually to 23 Å thick at π ) 40 dyn/cm. Similarly, the fourth generation forms a layer 17 Å thick at the lowest pressure, which increases to 25 Å at π ) 40 dyn/cm. In both cases, the thickness of the film doubles abruptly for pressures larger than ∼45 dyn/cm, where the film collapses. The lateral sizes of the DNCs were determined from the previous collapse point analysis in the Π-A isotherms where the corresponding surface pressures were 44 dyn/ cm for {Au1.14E2.R} and 46 dyn/cm for {Au9.93E4.R}. Orthogonal dimensions of the DNCs were obtained from the thicknesses of the Langmuir films at 40 dyn/cm, where the corresponding area per molecule values were found to be 450 Å2 for {Au1.14E2.R} and 1700 Å2 for {Au9.93E4.R}. If we neglect the small discrepancy between the area per molecule for measurements in the lateral and orthogonal (11) Prosa, T. Y. J.; Bauser, B. J.; Amis, E. J.; Tomalia, D. A.; Scherrenberg, R. J. Polym. Sci. B 1997, 35, 2913.
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Figure 2. In situ X-ray reflectivity measurement at the air/ water interface: (a) {Au1.14E2.R} and (b) {Au9.93E4.R}. The solid line is the best fit for each data set. The inset depicts a corresponding scattering length density (reF) profile on the basis of a single-layer model. All curves are shifted for clarity. Table 2. Fitting Parameters of Amphiphilic Au/PAMAM DNCs Layers at 40 dyn/cm at the Air/Water Interface for in Situ X-ray Reflectivitya
{Au1.14E2.R} {Au9.93E4.R}
σ1 (Å)
reFDNC (Å-2)
thickness (Å)
σ2 (Å)
7.1 9.1
1.08 × 10-5 1.32 × 10-5
23 25
6.9 6.1
a The scattering length density of the water subphase (r F e water ) 0.95 × 10-5 Å-2) is fixed, and σ1 and σ2 are the roughnesses at the water/DNC interface and at the DNC/air interface, respectively.
dimensions, we find that for the second generation the dendrimers were spherical i.e., the measured thickness of 23 Å is exactly equal to the diameter. In the case of the fourth generation, the thickness, 25 Å, is slightly larger than one-half of the diameter of 42 Å. Hence, we conclude that the fourth generation remains “pancake-shaped” even at the highest pressure. The sizes of the DNCs are summarized in Table 3. These observations are in agreement with previous theoretical models that predict an oblate deformation when strong interactions exist between the groups on the dendrimer and the substrate.12,13 In (12) Mansfield, M. L. Polymer 1996, 37, 3835.
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Figure 3. Summarized results of the thickness (solid line) of the film and the roughness (σ2) (dotted) at the film/air interface of {Au1.14E2.R} (squares) and {Au9.93E4.R} (circles) from Figure 2. Table 3. Summary of Amphiphilic Au/PAMAM DNC Size (Å) from Different Measurements
{Au1.14E2.R} {Au9.93E4.R}
Π-A isothermsa
Langmuir filmb
LB filmb
23 42
23 25
20 23
a Lateral size calculated from the collapse point. b Orthogonal size of the Langmuir film and the LB film at 40 dyn/cm.
this case, the hydrocarbon PAMAM exterior is hydrophobic, and the distortion must arise from interactions with the inner hydrophilic groups. The roughness of the films at the air interfaces, σ2 ≈ 6 Å, is plotted as a function of the surface pressure in Figure 3. From this figure, we can see that the roughness for both types of films remains constant up to π ) 40 dyn/cm and then increases abruptly to ∼10 Å. This value is also relatively small and indicates that the film is still fairly flat, even after multilayers have formed. If we compare the isotherms of the second and fourth generations (Figure 1a and b), we find that they are basically similar, except for a pronounced plateau that occurs only in the fourth generation film, indicating the existence of a phase transition. Upon comparison with the measured thickness vs surface pressure curves in Figure 3, we see no correlation between the thickness and the phase transition. The presence of this “shouldering” might be explained by interactions between the substituted hydrocarbon (C12) shell of the DNC particles on the water. C8-substituted generation-4 PAMAM dendrimer did not show shouldering in the isotherm,6 which suggests that it is necessary to have minimum substituted hydrocarbon chain length to represent the interaction in the isotherm. This situation is similar to that observed for fully quaternized PS260-b-P(4VP/CnBr)240 block copolymers spread at the air-water interface. These polymers selfassemble into surface micelles, with a hydrophobic core and hydrophilic corona pinned to the water surface by the hydrophobic quaternizing chains. In that system as well, the Π-A isotherms exhibit strong plateaus for side chains with n g 10 without any change in the total thickness of the films. In situ FTIR measurements showed that alkyl side chains formed a partially crystalline structure between the micelles at the observed phase transition in (13) Tsukruk, V. V. Adv. Mater. 1998, 10 (3), 253.
Figure 4. Diffuse scattering intensity profile of {Au1.14E2.R} at the air/water interface as a function of qx as surface pressure increases. All curves are shifted for clarity.
the Π-A isotherms.14 The crystalline structure formed only when the side chains were long enough to overlap. We therefore propose that a similar overlap mechanism occurs in the dendrimers. The area per molecule at the shoulder in Figure 1b corresponds to 2700 Å2/molecule for {Au9.93E4.R}, which yields a diameter is 56 Å based on a hexagonal geometry. The decrease in diameter upon further compression to 42 Å indicates that interpenetration of the dendrimers has occurred over a distance of 7 Å, which is comparable to ∼16 Å, the molecular length of attached hydrocarbon (-C11H22CH3). It is strongly suggested that C12 hydrocarbon shells interact with each other to form an interdigitated structure in which the interparticle distance approaches the shoulder point and the effect is stronger for higher generations. In-plane diffuse scattering was also obtained from the Langmuir films at different pressures. The scattering intensity data for the second generation are plotted as a function of qx in Figure 4 for different pressures. From this figure, we see that the spectra appear capillary-like with no distinct features. This indicates that no unique wave vector can be associated with the distribution of the Au in the dendrimers.15 This randomness is consistent with the TEM micrographs shown in ref 7. LB Film Analysis. To examine the effect of hydration on the dendrimer nanocomposites, the equilibrated monolayer of {Aun0-PAMAM.R} DNC was transferred onto a hydrophilic silicon wafer at surface pressures of 10, 20, and 40 dyn/cm. The thickness and roughness were then measured by specular X-ray reflectivity. The data are shown in Figure 5 for both {Au1.14E2.R} and {Au9.93E4.R}. From this figure, we can see that the film exhibits welldefined oscillations whose period decreases with increasing surface pressure. If we assumed that the Au were uniformly distributed within the dendrimer, a two-layer model consisting of a DNC film on top of a native silicon oxide wafer could be constructed. If the Au were not uniformly distributed and segregation by the outer groups occurred at either the air or the Si interface, a third lowdensity layer would have to be introduced to fit the experimental data (inset in Figure 5). The thickness of the silicon oxide layer was almost constant, whereas the (14) Shin, K.; Rafailovich, M. H.; Sokolov, J.; Chang, D. M.; Cox, J. K.; Lennox, R. B.; Eisenberg, A.; Gibaud, A.; Huang, J.; Hsu, S. L.; Satija, S. K. Langmuir 2001, 17, 4955. (15) The scattering intensity profile observed is only from the Au particles. We cannot observe scattering from the dendrimers in the q range accessible in our geometry. To observe the in-plane distance between dendrimers, which have a diameter of approximately 40 Å, we would require a q range in excess of 0.2 Å-1.
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Figure 7. pH dependence of Π-A isotherms of {Au9.93E4.R} (filled circles, pH 4; filled squares, pH 7; filled triangles, pH 10) and {E4.R} (open squares, pH 7; open triangles, pH 10). Dendrimer concentration (1 mg/mL) and spreading volume (55 µL) are fixed for each sample.
Figure 5. X-ray reflectivity measurement of the LB film: 2nd stands for {Au1.14E2.R} and 4th for {Au9.93E4.R} for different surface pressures. The solid line is the best fit for each data set. The inset depicts corresponding scattering length density (reF) profiles of the {Au1.14E2.R} and {Au9.93E4.R} films at 40 dyn/ cm on the basis of a two-layer model. All curves are shifted for clarity.
Figure 6. Summarized results of the thickness (solid line) of the film and the roughness (σ2) (dotted) at the film/air interface of {Au1.14E2.R} (squares) and {Au9.93E4.R} (circles) for the LB films from Figure 5.
DNC layer thickened substantially. For {Au1.14E2.R}, a uniform layer, 14 Å thick, was formed at 10 dyn/cm. The film thickness increased to 20 Å at 40 dyn/cm before the collapse pressure (Figure 6). For {Au9.93E4.R}, the film thickness increased from 16 Å at 10 dyn/cm to 23 Å at 40 dyn/cm. In Figure 6, we plot the roughness of the films as a function of surface pressure, and we find that the roughness, in fact, decreases slightly with increasing surface pressure. The absolute value of the roughness, approximately 5 Å, indicating that the films lifted on Si are slightly smoother than those spread on the water, possibly because of higher interactions with the Si substrate. The scattering length density of the films on Si, reF ) 1.33 × 10-5 and 1.44 × 10-5 Å-2 for the second and fourth generations, respectively, is ∼10% higher than for the films on the water. This indicates that, on the water, there is significant hydration of the DNC, as water is adsorbed into the hydrophilic core. The data show that, for films lifted near the collapse point (at 40 dyn/cm), the orthogonal dimensions of the
films are 20 Å for {Au1.14E2.R} and 23 Å for {Au9.93E4.R} (Table 3) which corresponds to aspect ratios of 0.87 and 0.51 for the second and fourth generations, respectively. Hence, when lifted from the water both LB films have very oblate conformations. This deformation might be partly due to drying of the DNC, followed by adsorption of the inner polar segments to the silicon oxide surface. As expected, the larger dendrimer, {Au9.93E4.R}, that has more polar groups is more highly deformed. This behavior appears to be similar to previously reported results for PAMAM dendrimers without Au, which are also highly deformed when adsorbed onto a silicon surface from water.13,16 pH Effects on the Π-A Isotherms. Because the X-ray reflectivity data show significant penetration of water into the DNC films, we also investigated the effect of the pH of the water subphase on the Π-A isotherms. The Π-A isotherms for different pH values are shown in Figure 7 for {Au9.93E4.R}. From this figure, we see that the isotherms for pH 7 and 4 are similar. Both exhibit the plateau described previously, and they collapse at areas of 1540 and 1650 Å2/molecule, respectively. The isotherm for pH 10 is quite different. The plateau has disappeared, and the film collapses at a significantly smaller area, corresponding to 1190 Å2/molecule. If the pH is ∼10, the interior tertiary amine groups are deprotonated, and internal nitrogens no longer repel each other. Consequently, the DNCs have a “softer” structure and are more compressible. In contrast, a “harder” type of DNC exists at lower pH (pH 7 and 4) where partial or almost full protonation occurs, leaving the backbone amine groups highly charged. The repulsion causes the backbone to be very rigid, possibly inducing crystallization of the side chains at a temperature well above the melting temperature (Tm ) -12 °C). This effect of backbone rigidity on Tm was previously reported in ref 14, where a similar plateau in the isotherm was ascribed to crystallization of the side chains in quaternized ionomeric block copolymers adsorbed rigidly at the air/water interface. Here, we postulate that crystallization occurs only when the dendritic structure is rigid. At high pH, the DNCs are no longer charged, causing the crystallinity to disappear above Tm as the underlying structure becomes soft. In Figure 7, we also show the isotherms for the dendrimer without Au ({E4.R}) at pH 7 and 10. From this figure, we see that, in the absence of Au, the effect of pH (16) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13 (8), 2171.
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is less pronounced. The plateau is smaller and is present at both values of the pH. At pH 7, we find that the DNCs are somewhat harder than {E4.R}, with a corresponding collapse point of 1450 Å2/molecule. This is intuitively expected as the inorganic inclusions increase the stiffness of the dendrimer. However, at pH 10, the {E4.R} film collapses at 1360 Å2/molecule, which is higher than the value obtained at the corresponding pH when Au is present. This implies that the deprotonation process is affected by the presence of Au. We therefore postulate that the Au atoms, being hydrophilic, somehow modify the concentration of water within the dendrimer, which, in turn, affects the local pH and, consequently, the stiffness. More work is in progress to understand the specific chemistry within the dendrimer when Au is present. Conclusion We have synthesized {Au(0)1.14-PAMAM•E2.C12} and {Au(0)9.93-PAMAM•E4.C12} amphiphilic dendrimer nanocomposites using hydrophobically modified EDA-core generation-2 and generation-4 PAMAM dendrimers as templates. Monolayers of gold DNCs were spread at the air/water interface of a Langmuir trough, and in situ X-ray reflectivity and Π-A isotherms were measured. DNC layers were transferred onto hydrophilic silicon wafers at
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various surface pressures. The morphology of the amphiphilic gold/PAMAM nanocomposites was investigated by X-ray reflectivity measurements. From the scattering length density magnitude and profile, we find that the dendrimer layer is hydrated and the Au nanocomposite is uniformly distributed within the dendrimer body. The second-generation dendrimer was spherical on the water surface, whereas the fourth generation became oblate at high pressures. From the reflectivity measurements, we find that the thickness of the layers is smaller when transferred on Si than at the air-water interface because of bonding between the DNC groups and the substrate. The isotherms were also measured at different pH values for the subphase. The results show that dendrimers become soft and collapse at high pH for smaller areas. For pH 7 or less, a distinct plateau is seen to form that is postulated to arise from crystallization of the carbon side chains. The addition of Au into the dendrimers is also shown to make them more sensitive to changes in pH. Acknowledgment. This work was supported by the NSF-MRSEC Program, the Department of Energy under Grant FGO1-00NE22943, and the Postdoctoral Fellowship Program of the Korea Science & Engineering Foundation (KOSEF). LA025504K
