Supramolecular Complex Induced by the Binding of Sodium Dodecyl

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Langmuir 2007, 23, 1635-1639

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Articles Supramolecular Complex Induced by the Binding of Sodium Dodecyl Sulfate to PAMAM Dendrimers Chang Wang,† Evan Wyn-Jones,‡ Jagraj Sidhu,‡ and Kam Chiu Tam*,† Singapore-MIT Alliance, School of Mechanical and Aerospace Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798, and School of Computing Science and Engineering, UniVersity of Salford, Newton Building, Salford M5 4WT, United Kingdom ReceiVed September 4, 2006. In Final Form: NoVember 13, 2006 Isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) were employed to study the spontaneous supramolecular complexation of amine terminated PAMAM dendrimer (G3[EDA] PAMAM-NH2) induced by the binding of an anionic surfactant, sodium dodecyl sulfate (SDS). At pH e 2, SDS molecules bound to protonated amines on the outer rims of G3[EDA] PAMAM-NH2 driven by electrostatic interaction, which induced the formation of PAMAM/SDS supramolecular complex via hydrophobic association between bound SDS molecules. The complex with radius of ∼37 nm was observed at SDS concentration as low as 0.02 mM (in 0.2 mM PAMAM). The size of the complex increased progressively with increasing SDS concentration and precipitated when the SDS concentration exceeded 15 mM. At pH of ∼7.4, the formation of PAMAM/SDS complex was observed at higher SDS concentration (0.1 mM in 0.2 mM PAMAM), and it resolubilized with further increase of SDS concentration to ∼18 mM due to weaker electrostatic interaction at higher pH. At pH g 10, the electrostatic binding ceased because the deprotonated PAMAM dendrimer was uncharged, and hence the surfactant-induced supramolecular assembly could not be formed.

Introduction Poly(amidoamine) (PAMAM) dendrimers have attracted increasing attention in recent years because of their unique structure, interesting properties, as well as their potential applications in medicine, catalysis, gene therapy, and nanoreactor systems.1-8 PAMAM dendrimers are monodisperse, highly branched polyelectrolytes with ammonium functional groups on the surface (primary amine) and at the branch points in the interior (tertiary amine). The PAMAM dendrimers can be hydrated and expanded when prontonated in aqueous medium where the degree of protonation depends on pH.5,8-11 In general, the charging mechanism of PAMAM dendrimers is described as a multistep protonation; starting from neutral (uncharged) dendrimer at pH ∼10-11 and with decreasing pH, the primary amines on the surface (periphery) first protonate at pH ∼7-8 followed by tertiary amines within the interior core of dendrimer, which protonate * Corresponding author. Telephone: +65-6790 5590. Fax: +65-6791 1859. E-mail: [email protected]. † Nanyang Technological University. ‡ University of Salford. (1) Kim, Y. H.; Webster, O. W. J. Am. Chem. Soc. 1990, 112, 4592. (2) Newkome, G. R.; Moorefield, C. N.; Baker, G. R.; Johnson, A. L.; Behera, R. K. Angew. Chem., Int. Ed. Engl. 1991, 30, 1176. (3) Naylor, A. M.; Goddard, W. A., III; Kiefer, G. E.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 111, 2339. (4) Haensler, J.; Szoka, F. Bioconjugate Chem. 1993, 4, 372. (5) Tomalia, D. A. Prog. Polym. Sci. 2005, 30, 294. (6) Chen, W.; Tomalia, D. A.; Thomas, J. L. Macromolecules 2000, 33, 9169. (7) Sun, L.; Crooks, R. M. J. Phys. Chem. B 2002, 106, 5864. (8) Cakara, D.; Kleimann, J.; Borkovec, M. Macromolecules 2003, 36, 4201. (9) Rangarajan, B.; Coons, L. S.; Scranton, A. B. Biomaterials 1996, 17, 649. (10) Prosa, T. J.; Bauer, B. J.; Amis, E. J. Macromolecules 2001, 34, 4897. (11) Ottaviani, M. F.; Mondalti, F.; Romanelli, M.; Turro, N. J.; Tomalia, D. A. J. Phys. Chem. B 1996, 100, 11033. (12) Ottaviani, M. F.; Turro, N. J.; Jockusch, S.; Tomalia, D. A. Colloids Surf., A 1996, 115, 9.

fully at pH 2.8 Chen and co-workers also reported that essentially all charges are on the primary amines at pH > 8.3, and further reduction in pH led to the protonation of tertiary amines in the dendrimer core.5 The amphiphilic nature, well-defined size, shape, and architecture, as well as the chargeable amine groups on the surface and interior of PAMAM dendrimers make them good hosts for small guest molecules, especially surfactants. In principle, both the periphery and the internal core of dendrimer can interact with other molecules, giving them a dual role as hosts.12-16 The macromolecule/surfactant host-guest system is of great scientific and industrial importance because of its complex behaviors and potential applications in pharmaceutical, cosmetic, agricultural formulations, and food processing.17-23 A number of studies have focused on the interaction and aggregation behaviors between the PAMAM dendrimers and surfactants in aqueous solution.11-16,24-31 It was observed that the interaction is strongly (13) Watkins, D. M.; Sayed-Sweet, Y.; Klimash, J. W.; Turro, N. J.; Tomalia, D. A. Langmuir 1997, 13, 3136. (14) Chechik, V.; Zhao, M.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 4910. (15) Bakshi, M. S.; Kaura, A.; Miller, J. D.; Paruchuri, V. K. J. Colloid Interface Sci. 2004, 278, 472. (16) Bakshi, M. S.; Kaura, A.; Mahajan, R. K.; Toshimura, T.; Esumi, K. Colloids Surf., A 2004, 246, 39. (17) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; Chapter 4. (18) Hayakawa, K.; Kwak, J. C. J. Phys. Chem. 1982, 86, 3866. (19) Fundin, J.; Hansson, P.; Brown, W.; Lidegran, I. Macromolecules 1997, 30, 1118. (20) Bloor, D. M.; Wan-Yunus, W. M. Z.; Wan-Badhi, W. A.; Li, Y.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (21) Wang, C.; Tam, K. C. Langmuir 2002, 18, 6484. (22) Wang, C.; Tam, K. C. J. Phys. Chem. B 2004, 108, 8976. (23) Wang, C.; Ravi, P.; Tam, K. C. Langmuir 2006, 22, 2979. (24) Jockusch, S.; Turro, N. J.; Tomalia, D. A. Macromolecules 1995, 28, 7416.

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dependent on the nature of surfactant head groups, generation and terminal groups of dendrimers, as well as ionic strength and pH of the solution.11-16,26 In the case of the PAMAM dendrimer/ SDS system, the interaction is clearly observable because of strong electrostatic attraction between protonated amines and anionic surfactant head groups, which induces the precipitation of the complex, a phenomenon which is well-known following the interaction between oppositely charged polyelectrolytes and surfactants.13-16,24,25,27-31 Mizutani and co-workers observed a maximum in the turbidity and particle size corresponding to the complexation of SDS with alkyl-modified PAMAM dendrimer, but no details on the dendrimer/SDS aggregation were provided.30 Wyn-Jones et al. showed that neutral -NH2- and -OH-terminated dendrimers interact with surfactant much in the same way as neutral linear polymers; that is, binding of SDS is accompanied by the formation of a dendrimer/micelle complex. At the binding limit, the electromotive force (EMF) binding data were complemented by small angle neutron scattering measurement to determine the dendrimer/bound micelle ratio.28,32,33 Other dendrimer/aggregated surfactant complexes have been proposed by Ottavani et al.11,12 Often, the case of the binding at extremely low surfactant concentration is difficult to monitor by conventional techniques. Furthermore, when precipitation takes place, it is difficult to obtain information on the particle size. As a result, a comprehensive understanding on the mechanism and nanostructure of the supramolecular assembly of PAMAM dendrimer with anionic surfactant at very low to high surfactant concentrations is lacking. This article describes a detailed and quantitative study on the SDS induced supramolecular complexation of generation 3 ethylene diamine (EDA) core, amine-terminated PAMAM dendrimer (designated as G3[EDA] PAMAM-NH2), focusing on the detailed mechanism for the complex formation at different pH’s. The investigation involved using the extremely sensitive isothermal titration calorimetric (ITC) technique, which can monitor the binding even when precipitation occurs, complemented by dynamic light scattering (DLS), which gives information on the particle size. In this study, we have selectively controlled the amino periphery groups and the core by varying the pH of the solution. This study provides new insights into the formation mechanism and nanostructure of PAMAM/SDS complex, as well as a simple and reversible route of hydrophobic modification of the hydrophilic surface of PAMAM dendrimers based on electrostatic interaction. Experimental Section Materials. The amine-terminated, ethylene diamine core, generation 3 poly(amidoamine) dendrimer (designated as G3[EDA] PAMAM-NH2, >99% purity) was purchased from Sigma-Aldrich and used without further purification. It has 32 primary amines on the surface and 90 tertiary amines at branch points within the core. (25) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515. (26) Schenning, A. P. H. J.; Elissen-Roman, C.; Weener, J.; Baars, M. W. P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. (27) Miyazaki, M.; Torigoe, K.; Esumi, K. Colloids Surf., A 2001, 179, 103. (28) Ghoreishi, S. M.; Li, Y.; Holzwarth, J. F.; Khoshdel, E.; Warr, J.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1999, 15, 1938. (29) Esumi, K.; Kuwabara, K.; Chiba, T.; Kobayashi, F.; Mizutani, H.; Torigoe, K. Colloids Surf., A 2002, 197, 141. (30) Mizutani, H.; Torigoe, K.; Esumi, K. J. Colloid Interface Sci. 2002, 248, 493. (31) Bakshi, M. S.; Kaura, A. Colloids Surf., A 2004, 244, 45. (32) Li, Y.; McMillan, C. A.; Bloor, D. M.; Penfold, J.; Warr, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 7999. (33) Sidhu, J.; Bloor, D. M.; Couderc-Azouani, S.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2004, 20, 9320.

Wang et al. It is deprotonated and fully protonated at pH ∼10 and ∼2, respectively, and in the pH range between 7 and 8, the primary amines on the periphery are protonated. The anionic surfactant sodium dodecyl sulfate (SDS) was obtained from BDH Lab Supplies (>99% purity) and used as received. Isothermal Titration Calorimetry. The microcalorimetric measurements were carried out using the Microcal isothermal titration calorimeter (ITC). This power compensation, differential instrument was described in detail by Wiseman et al.34,35 It has a reference and a sample cell of approximately 1.35 mL, and both cells are insulated by an adiabatic shield. The titration was carried out at 25.0 ( 0.02 °C, by injecting 200 mM SDS solution from a 250 µL syringe into the sample cell filled with the aqueous solution of PAMAM dendrimer. The syringe is tailored-made such that the tip acts as a blade-type stirrer to ensure an optimum mixing efficiency at 400 rpm. The heat evolved or absorbed by each injection in the course of titration is directly measured, producing the raw heat signal, also known as cell feedback (CFB). Integration of the CFB gives the differential enthalpy curve. Dynamic Light Scattering. Dynamic light scattering (DLS) studies were conducted using a Brookhaven BI-200SM goniometer and BI-9000AT digital correlator equipped with an argon-ion laser. The time correlation function of the scattered intensity G2(t), which is defined as G2(t) ) I(t)I(t + ∆t), where I(t) is the intensity at time t and ∆t is the lag time, was analyzed using the inverse Laplace transformation technique (REPES for our case) to produce the distribution function of decay rates. Thus, the apparent hydrodynamic radius Rh can be determined from the decay rate via Stokes-Einstein equation, Rh ) kTq2/6πηΓ, where k is the Boltzmann constant, q is the scattering vector (q ) 4πn sin(θ/2)/λ, where n is the refractive index of solvent, θ is the scattering angle, and λ is the wavelength of the incident laser light in vacuum), η is the solvent viscosity, and Γ is the decay rate. Several measurements were performed for a sample to obtain an average hydrodynamic radius, and the variation in the Rh values was found to be small. The angular dependence measurement (from 60° to 110° at 10° interval) of relaxation time distribution functions for the dendrimer/ SDS mixture showed that the decay rate Γ exhibited good linear relationship with q2, confirming that the distribution functions were caused by translational diffusion of particles.

Results and Discussion The differential enthalpy curves obtained from titrating 200 mM SDS into different concentrations of G3[EDA] PAMAMNH2 at pH ) 2, together with the dilution curve of SDS, were plotted in Figure 1a. Comparison of the binding curves and dilution curve indicated that SDS interacted strongly with G3[EDA] PAMAM-NH2. The thermogram possessed a large exothermic enthalpy upon the first addition of SDS (0.59 mM), suggesting that electrostatic binding took place at extremely low SDS concentration. The pronounced exothermic maximum corresponded to the binding of anionic head groups of SDS to the positively charged amines on dendrimers driven by electrostatic interaction. With further addition of SDS, the enthalpy remained essentially constant over a defined range of SDS concentration, characterizing the continuous binding of SDS molecules to amine groups on the dendrimers. The instantaneous binding (an absence of a critical aggregation concentration) and the enthalpic plateau displayed by the enthalpy curve reflected the uncooperative nature of the electrostatic interaction between individual SDS molecules and dendrimers. At the end of strong electrostatic binding, the enthalpy rapidly approached the dilution curve and eventually merged with the dilution curve at ∼18-24 mM, suggesting the end of binding. The electrostatic binding was exothermic, and the negative enthalpy (-11 kJ/mol dendrimer) was attributed to (34) Wiseman, T.; Williston, S.; Brandts, J. F.; Lin, L. Anal. Biochem. 1989, 179, 131. (35) Jelesarov, I.; Bosshard, H. R. J. Mol. Recognit. 1999, 12, 3.

Complex from Binding of SDS to PAMAM Dendrimers

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Figure 2. Differential enthalpy versus corresponding titrations of SDS into 0.2 mM solutions of G3[EDA] PAMAM-NH2 at pH ) 2 and pH ) 7.4: (0) pH ) 2; (]) pH ) 7.4; ([) dilution curve of SDS.

Figure 1. Differential enthalpy versus (a) concentration of SDS and (b) sulfate/amine charge ratio for fully protonated (pH ) 2) G3[EDA] PAMAM-NH2 of different concentrations: (0) 0.2 mM; (]) 0.15 mM; (4) 0.1 mM; (O) 0.05 mM; (9) 0.025 mM; ([) dilution curve of SDS.

the formation of ion-pair between sulfate and protonated amine. This is in agreement with the exothermic electrostatic interactions observed in other oppositely charged macromolecular systems such as DNA/surfactant, antibiotic/serum albumin, and peptide/ phospholipid membrane systems.36-38 Figure 1a also demonstrated that the binding regime in terms of SDS concentration is proportional to the concentration of G3[EDA] PAMAM-NH2, indicating the stoichiometric characteristic of the site-to-site binding driven by electrostatic force. In Figure 1b, the enthalpy curves obtained at different G3[EDA] PAMAM-NH2 concentrations were plotted against the charge ratio of sulfate to amine groups ([∼SO4-]/[∼N+]), where all of the enthalpy curves fall on a master curve and the dendrimer was saturated by SDS at a charge ratio of 0.62. This suggested that only 62%, which is approximately 76 amine groups of G3[EDA] PAMAM-NH2, were bound to negatively charged SDS molecules, even though all of the 122 amines were protonated at pH of 2. Thus, we concluded that electrostatic binding only took place on approximately 80 amine groups on the outer rim of G3[EDA] PAMAM-NH2, which are the 32 outermost primary amines on the surface, the first shell of 32 tertiary amines (third generation), (36) Eastman, S. J.; Siegel, C.; Tousignant, J.; Smith, A. E.; Cheng, S. H.; Scheule, R. K. Biochim. Biophys. Acta 1997, 1325, 41. (37) Tang, K.; Qin, Y. M.; Lin, A. H.; Hu, X.; Zou, G. L. J. Pharm. Biomed. Anal. 2005, 39, 404. (38) Austin, R. P.; Barton, P.; Davis, A. M.; Fessey, R. E.; Wenlock, M. C. Pharm. Res. 2005, 22, 1649.

and the second shell of 16 tertiary amines (second generation), respectively. These sites are more readily accessible by small guest molecules. The absence of binding between SDS and the remaining amine groups within the dendrimer interior is mainly attributed to the strong steric hindrance caused by highly branched structure of the dendrimer. Moreover, the formation of sulfate/ amine ion-pair reduced the amount of positive charges on dendrimer, which shrank the dendrimer into a compact conformation, and this prevented SDS molecules from interacting with the amine groups within dendrimer core. The schematic drawing of the electrostatic binding sites of G3[EDA] PAMAM-NH2 (filled cycle) is shown in the inset of Figure 1b. Figure 2 shows the differential enthalpy curves for the titration of 200 mM SDS into 0.2 mM G3[EDA] PAMAM-NH2 at pH 2 and 7.4, respectively. The height and the breadth of the endothermic curve corresponding to the binding of SDS decreased significantly at pH 7.4, indicating that the binding is weakened when the pH was increased from 2 to 7.4. At pH 7.4, only the primary amines on the periphery of dendrimer are protonated, while the tertiary amines within the interior core remained uncharged; thus these dendrimers possessed lesser positively charged sites for SDS molecules to bind electrostatically. It was also noted that the enthalpy curve at pH 7.4 exhibited a broad shoulder, which may characterize a structural transformation of dendrimer/SDS complex in moderate pH. This structural transformation, which is a resolubilization of precipitated dendrimer/SDS complex in excess amounts of SDS, was further confirmed by the dynamic light scattering study described below. Further confirmation of the strong binding and its induced supramolecular assembly is clearly visible as depicted by the pictures shown in Figure 3, which demonstrated the phase behavior of 0.2 mM G3[EDA] PAMAM-NH2 in the presence of different amounts of SDS at pH of 2, 7.4, and 10, respectively. At pH of 2, the G3[EDA] PAMAM-NH2 solution became slightly cloudy at SDS concentration as low as 1 mM, signaling the formation of dendrimer/SDS complex driven by electrostatic binding. The complex continued to evolve to a larger size and higher density, resulting in a cloudy solution at SDS concentration of 5 mM, and thereafter white precipitates appeared when the SDS concentration exceeded 10 mM. At pH of ∼7.4, the solution turned cloudy and the mixture precipitated at a higher SDS concentration (>2 mM) as compared to pH of 2. It redispersed when SDS concentration exceeded 15 mM, suggesting that the complex underwent a structural reorganization in excess amounts

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Wang et al.

Figure 3. Pictures depicting the solutions of 0.2 mM G3[EDA] PAMAM-NH2 at pH 2, pH 7.4, and pH 10 mixed with different amounts of SDS.

Figure 4. Dependences of hydrodynamic radius Rh and scattering intensity on the SDS concentration in 0.2 mM G3[EDA] PAMAMNH2 at pH ) 2, pH ) 7.4, and pH ) 10: (]) Rh at pH ) 2; (O) Rh at pH ) 7.4; (4) Rh at pH ) 10; ([) intensity at pH ) 2; (b) intensity at pH ) 7.4.

of SDS. At pH of 10, the amines of dendrimer are deprotonated, and hence electrostatic binding with SDS is absent. The solution remained clear and did not show any sign of complexation at all SDS concentrations (from 0 to 15 mM). Dynamic light scattering was carried out to further examine the complexation mechanism and structural transformation during the course of binding. Contrary to previous studies that focused on the complexation behavior at high SDS concentration (g20 mM),13,15,28,32 the current study attempted to monitor the gradual formation of the complex over a concentration regime ranging from an extremely low SDS concentration (18 mM). The dependence of hydrodynamic radius (Rh) and scattering intensity on the SDS concentration for 0.2 mM G3[EDA] PAMAM-NH2 at pH of 2, 7.4, and 10 was plotted in Figure 4. The Rh of G3[EDA] PAMAMNH2 in the absence of SDS was approximately 2 nm, characterizing the individual dendrimer molecule in aqueous solution. The particle size at pH 2 was slightly higher than that at pH 7.4 and 10, reflecting the swollen conformation of protonated dendrimer molecule at low pH. Therefore, any particle with a size from ∼5 to 400 nm will involve the continuous aggregation of individual dendrimer/surfactant complexes, leading to the formation of three-dimensional networks. We will now discuss how these aggregates are formed when the dendrimer is fully protonated (core plus interior) and when it is selectively protonated at the periphery.

At pH of 2, the Rh increased from 6 to 38 nm as the SDS concentration was increased from 0.002 to 0.01 mM (open diamonds), signaling the commencement of complexation induced by electrostatic binding, which can be described as a formation of ion-pair between sulfates of SDS and potonated amines of G3[EDA] PAMAM-NH2. The SDS molecules adsorbed on the surface of dendrimer with their hydrophobic tails pointing outward, and this promoted the self-association of dendrimers to produce larger aggregates. Complexation occurred at SDS concentration as low as 0.01 mM in 0.2 mM G3[EDA] PAMAMNH2 solution, where the ratio of SDS monomer to dendrimer is 1:20, and at such low SDS concentration, spherical SDS micelles are unlikely to form on the dendrimer. The electrostatic binding of SDS molecules at such low SDS concentration was confirmed by the titration calorimetric data described in Figures 1 and 2. We believed that the binding and complexation took place well before the formation of SDS micelles, and the hydrophobic association between neighboring SDS molecules bound with dendrimer was responsible for the aggregation, as reflected by the gradual increase of particle size. With further increase in SDS concentration, the Rh increased steadily from 38 to 101 nm at 10 mM of SDS. Similarly, the scattering intensity also increased from 14 to 1800 kcps as reflected by the enhanced turbidity of the solution. These features corresponded to the continuous binding and assembly of SDS-bound dendrimer complex. When SDS exceeded 15 mM, Rh increased sharply to 423 nm and the complex precipitated as shown in Figure 3, representing a structural transformation of the dendrimer/SDS complex from a large aggregate to an unstable three-dimensional network structure. A similar phenomenon was observed for the binding of ionic surfactant to oppositely charged polymer of sufficiently high charge density.21,22 At pH of 7.4, the dependence of Rh and scattering intensity on SDS concentration exhibited trends similar to those observed at pH 2. However, the commencement of the complexation signaled by an increase of Rh from 3 to 53 nm was observed at higher SDS concentration (∼0.1 mM). The number of electrostatic binding sites on the G3[EDA] PAMAM-NH2 is significantly reduced at pH of 7.4 because only 32 primary amines on the surface are protonated;5,8 thus more SDS is required to initiate the binding and to induce inter-particle association. With further increase of SDS concentration to 2 mM, the Rh and scattering intensity increased progressively from 53 to 70 nm and from 14 to 650 kcps, respectively, characterizing the progressive formation of the complex via the association of bound SDS molecules. When the SDS concentration exceeded 4 mM, the system became

Complex from Binding of SDS to PAMAM Dendrimers

opaque, where Rh increased to 167 nm and the scattering intensity soared to 1300 kcps, indicating the formation of precipitated dendrimer/SDS complex. In contrast to the dendrimer/SDS mixture at pH 2 (where the precipitation is persistent even at high SDS concentration >20 mM), the insoluble dendrimer/ SDS complex at pH of 7.4 was redispersed, and the system became transparent when the SDS concentration exceeded 15 mM. The redispersion corresponded to a structural reorganization at higher SDS concentration. At pH of 7.4, one G3[EDA] PAMAM-NH2 molecule is able to host a maximum of 32 SDS molecules on its surface, which is significantly lower than the dendrimer at pH 2, which can host 80 SDS molecules on its surface and interior of the dendrimer. The lower number of bound SDS molecules is not adequate to physically cross-link the dendrimers to yield a three-dimensional network structure at pH of 7.4. On the other hand, spherical SDS micelles are formed when the SDS concentration is sufficiently high (>saturation concentration), which can stabilize dendrimer on its surface via electrostatic interaction. The existence of spherical SDS micelles on PAMAM dendrimer surface beyond the saturation concentration was confirmed by Wyn-Jones and co-workers based on the SANS results.32 It is noted that the redispersion was encountered at SDS micelle concentration of 0.25 mM (under the assumption that the aggregation number of SDS is 60),39 which equals approximately the concentration of G3[EDA] PAMAM-NH2, suggesting that at least one SDS micelle is required to stabilize one dendrimer molecule. This finding is in agreement with the previous study on the structure of poly(1,4-diaminobutane) dendrimer/SDS complex.32 The interior of G3[EDA] PAMAMNH2 is fairly hydrophobic as the amines are not protonated, SDS (39) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988, 92, 5580.

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will form mixed micelles within the interior with their hydrophilic head groups pointing outward, and this enhanced the solubility of the complex. At pH of 10, Rh remained essentially constant at 2.5 nm over the entire range of SDS concentration (from 0 to 15 mM), indicating the absence of complexation because the electrostatic binding was absent.

Conclusions We observed strong electrostatic binding of SDS to amineterminated PAMAM dendrimer when pH e 7.4. The binding induced physical hydrophobic modification on the exterior of PAMAM dendrimer, which promoted the formation of PAMAM/ SDS supramolecular complex via the association between the bound SDS molecules. At pH e 2, SDS only bound to the outermost 80 out of the total 122 protonated amines of G3[EDA] PAMAM-NH2. The complex induced by the binding was produced at SDS concentration as low as 0.02 mM (in 0.2 mM PAMAM) with a radius of ∼37 nm, and it grew continuously to ∼100 nm until the SDS concentration reached ∼10 mM, beyond which the complex precipitated. At pH ∼7.4, SDS bound to 32 protonated primary amines on the surface of the dendrimer. The complex was formed at higher SDS concentration (0.1 mM in 0.2 mM PAMAM), and the precipitated complex redispersed at higher SDS concentration (>15 mM), which is attributed to the reduced electrostatic interaction between dendrimer and SDS at higher pH. At pH g 10, electrostatic binding did not exist because the dendrimer was not protonated. Acknowledgment. We are grateful for the financial support provided by Nanyang Technological University and the Singapore-MIT Alliance. LA0625897