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Langmuir 2008, 24, 12398-12404
Naphthalene Sulfonate Functionalized Dendrimers at the Solid-Liquid Interface: Influence of Core Type, Ionic Strength, and Competitive Ionic Adsorbates Timothy J. Barnes, Igor Ametov, and Clive A. Prestidge* Ian Wark Research Institute, ARC Special Research Centre for Particle and Material Interfaces, UniVersity of South Australia, Mawson Lakes, SA 5095, Australia ReceiVed July 3, 2008. ReVised Manuscript ReceiVed September 1, 2008 The adsorption of naphthalene disulfonic acid surface-functionalized dendrimers (generation 4) on to colloidal alumina particles is reported, considering the role of dendrimer core type (ammonia vs benzylhydrylamine-polylysine) and electrolyte addition on the adsorption affinity and interfacial packing and competitive adsorption. Irrespective of the dendrimer core type, the maximum adsorbed amount increased with increasing ionic strength. The adsorption affinity of a benzylhydrylamine-cored SPL-7013 increased with increasing ionic strength, whereas a decrease was observed for the ammonia-cored SPL-2923. At high ionic strengths (g10-1 M NaCl) dendrimers close pack at the interface as an array of equivalent hard spheres, whereas at lower ionic strengths both dendrimers occupy a lower area than theoretically predicted for either cubic or hexagonal close packing, based on double layer repulsion. The additional attraction between dendrimers is attributed to the intercalation of the neighboring dendrons. Adsorption of SPL-2923 is enhanced by the presence of Ca2+ ions and depressed by the presence of HCO3- and HPO42- ions, whereas SPL-7013 adsorption is only depressed by the presence of HPO42- ions, suggesting a dendrimer-specific competitive adsorption process. This work clearly demonstrates the role of dendrimer architecture on adsorption at an interface, a process of fundamental importance to a wide range of dendrimer applications.
Introduction Dendrimers form a unique class of synthetic polyelectrolytes, being highly monodisperse, polyvalent macromolecules with a well-defined molecular architecture.1-3 Significant control over dendrimer structure is possible through choice of the core molecule, generation number, and terminal surface functionality, and hence the considerable interest in dendrimers for use in a range of bionano applications, including nanoparticle synthesis,4-6 drug delivery,7-11 gene delivery,12,13 and medical imaging.14,15 One dendrimer system that has received particular attention in recent years is naphthalene disulfonate functionalized * Corresponding author. Phone: +61 8 8302 3569. Fax: +61 8 8302 3683. E-mail:
[email protected]. (1) Tomalia, D.; Naylor, A.; Goddard, W. Angew. Chem., Int. Ed. 1990, 29, 138–175. (2) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515–8522. (3) Roberts, J. C.; Bhalgat, M. K.; Zera, R. T. J Biomed. Mater. Res. 1996, 30, 53–65. (4) Wu, H.; Liu, Z.; Wang, X.; Zhao, B.; Zhang, J.; Li, C. J. Colloid Interface Sci. 2006, 302, 142–148. (5) Ujihara, M.; Mitamura, K.; Torikai, N.; Imae, T. Langmuir 2006, 22, 3656– 3661. (6) Gu, Y.; Xie, H.; Gao, J.; Liu, D.; Williams, C. T.; Murphy, C. J.; Ploehn, H. J. Langmuir 2005, 21, 3122–3131. (7) Gurdag, S.; Khandare, J.; Stapels, S.; Matherly, L. H.; Kannan, R. M. Bioconjugate Chem. 2006, 17, 275–283. (8) Boyd, B. J.; Kaminskas, L.; Karellas, P.; Krippner, G.; Lessene, R.; Porter, C. J. H. Mol. Pharm. 2006, 3, 614–627. (9) Dhanikula, R. S.; Hildgen, P. Bioconjugate Chem. 2006, 17, 29–41. (10) Vandamme, T. F.; Brobeck, L. J. Controlled Release 2005, 102, 23–38. (11) Patri, A. K.; Kukowska-Latallo, J. F.; Baker, J.; James, R. AdV. Drug DeliVery ReV. 2005, 57, 2203–2214. (12) Su, C. J.; Liu, Y. C.; Chen, H. L.; Li, Y. C.; Lin, H. K.; Liu, W. L.; Hsu, C. S. Langmuir 2007, 23, 975–978. (13) Guillot-Nieckowski, M.; Joester, D.; Stohr, M.; Losson, M.; Adrian, M.; Wagner, B.; Kansy, M.; Heinzelmann, H.; Pugin, R.; Diederich, F.; Gallani, J. L. Langmuir 2007, 23, 737–746. (14) Barrett, T.; Kobayashi, H.; Brechbiel, M.; Choyke, P. L. Eur. J. Radiol. 2006, 60, 353–366. (15) Haba, Y.; Kojima, C.; Harada, A.; Ura, T.; Horinaka, H.; Kono, K. Langmuir 2007, 23, 5243–5246.
poly-L-lysine dendrimer (SPL-7013) developed by Starpharma Holdings Pty Ltd., which has shown significant antiviral activity against both herpes simplex and human immunodeficiency virus.16-18 Fundamental to these applications is the need for understanding of dendrimer interfacial activity. Of particular interest is in the role of core geometry which determines the dendrimer’s three-dimensional (3D) structure in solution and the arrangement of the surface functional groups. The adsorption of PAMAM (polyamidoamine) dendrimers has been reported on various solid-liquid interfaces, e.g., hemeatite,19 silica,20-23 alumina,22-26 and polystyrene latex.27 Cahill et al.20 recently reported on the role of ionic strength and pH in controlling the adsorption density of late generation (G8 and G10) amine-terminated PAMAM dendrimers on a silica substrate. Initially, the adsorption was observed to follow a firstorder, transport-limited process, whereas at a high dendrimer surface coverage the adsorbed dendrimers form loose, albeit organized, structures. McCain et al.21 investigated the influence of dendrimer generation and functionalization (amine vs car(16) McCarthy, T. D.; Karellas, P.; Henderson, S. A.; Giannis, M.; O’Keefe, D. F.; Heery, G.; Paull, J. R. A.; Matthews, B. R.; Holan, G. Mol. Pharm. 2005, 2, 312–318. (17) Gong, E.; Matthews, B.; McCarthy, T.; Chu, J.; Holan, G.; Raff, J.; Sacks, S. AntiViral Res. 2005, 68, 139–146. (18) Patton, D. L.; Cosgrove Sweeney, Y. T.; McCarthy, T. D.; Hillier, S. L. Antimicrob. Agents Chemother. 2006, 50, 1696–1700. (19) Pan, Z.; Somasundaran, P.; Turro, N. J.; Jockusch, S. Colloids Surf., A 2004, 238, 123–126. (20) Cahill, B. P.; Papastavrou, G.; Koper, G. J. M.; Borkovec, M. Langmuir 2008, 24, 465–473. (21) McCain, K. S.; Schluesche, P.; Harris, J. M. Anal. Chem. 2004, 76, 930– 938. (22) Esumi, K.; Goino, M. Langmuir 1998, 14, 4466–4470. (23) Ottaviani, M. F.; Turro, N. J.; Jockusch, S.; Tomalia, D. A. J. Phys. Chem. B 2003, 107, 2046–2053. (24) Esumi, K.; Fujimoto, N.; Torigoe, K. Langmuir 1999, 15, 4613–4616. (25) Goino, M.; Esumi, K. J. Colloid Interface Sci. 1998, 203, 214–217. (26) Esumi, K.; Sakagami, K.; Kuniyasu, S.; Nagata, Y.; Sakai, K.; Torigoe, K. Langmuir 2000, 16, 10264–10268. (27) Lin, W.; Galletto, P.; Borkovec, M. Langmuir 2004, 20, 7465–7473.
10.1021/la8020996 CCC: $40.75 2008 American Chemical Society Published on Web 10/04/2008
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Table 1. Dendrimer Core Type, Generation, Surface Group Type, and Number and Molecular Weight code
chemical formulas
SPL-2923 AM[Lys]45-NDS24 SPL-7013 BHA-Lys[Lys]32-NDS32
number of generation NDS groups Mw g/mol 4 4
24 32
14 498 16 581
boxylic acid) on adsorption at the negatively charged silica-water interface. Electrostatic interactions governed the adsorption, adsorption affinity increased with increasing dendrimer generation, and no adsorption was observed with the conversion of the positive surface primary amines to the negatively charged carboxylate groups. Similarly, Ottaviani et al.23 reported that the high surface charge density of amine-terminated G6 PAMAM dendrimers resulted in enhanced electrostatic interaction with basic alumina particles. In contrast, G2 PAMAM dendrimers exhibited preferential hydrophobic bonding with the low-polarity Si-O-Si surface of silica particles due to their more open structure and lower surface charge density. Lin et al.27 investigated the impact of adsorbed PAMAM dendrimers on the colloidal stability of polystyrene latex particles; the maximum aggregation rate was observed at the isoelectric point (IEP) of the system, while increasing ionic strength resulted in a broadening of the region of maximum aggregation rate. Previously, we have demonstrated the relationship between solution conditions (ionic strength and pH) and the adsorption of the naphthalene disulfonate functionalized, poly-L-lysine based dendrimer (SPL-7013) on alumina particles;28 the findings were in agreement with other studies that have established the role of electrostatic forces in controlling the adsorption of negatively charged PAMAM dendrimers on alumina particles.22,24,25 To date, however, the role of dendrimer architecture in controlling adsorption and competitive adsorption at a solid-liquid interface has yet to be established. In this paper we elucidate the influence of dendrimer molecular architecture on the adsorption of G4 naphthalene sulfonate functionalized dendrimers with either ammonia (AM)- or benzylhydrylamine-lysine (BHA-Lys)-cored dendrimers on alumina particles. The interfacial packing of the dendrimers is compared to theoretical calculations based on both cubic- and hexagonal-packed arrays. Dendrimer adsorption in the presence of simulated body fluid was also investigated to examine the influence of a complex ion mixture, and in particular the incidence of competitive adsorption for different dendrimer architecture is discussed.
Materials and Methods Materials. Naphthalene disulfonate (NDS) functionalized lysine (Lys) based dendrimers were used as supplied by Starpharma Holdings Pty Ltd. (Australia), without further purification. The dendrimer specifications are provided in Table 1, and their structural formulas are provided in Figure 1. Hydrated alumina, γ-Al(OH)3 crystals (Hydral 710, Alcoa Australia Ltd.), was used as supplied, with a d32 of 0.62 µm and BET surface area of 7.2 m2/g.29 All salts used (NaCl, CaCl2, MgCl2, NaHCO3, Na2HPO4, Tris) were AR grade or higher, and all experiments were carried out in Milli-Q water. The pH was adjusted by the addition of small quantities of NaOH and HCl solutions. Methods. Dendrimer Adsorption Isotherms. Dendrimer adsorption onto the γ-alumina particles was monitored using a depletion method at room temperature (∼22 °C). Alumina suspensions (10 g/L) were prepared in a range of NaCl solutions (10-3-10-1 M) at a pH () (28) Barnes, T. J.; Ametov, I.; Prestidge, C. A. Asia-Pac. J. Chem. Eng. 2008, 3, 13–17. (29) Addai-Menash, J.; Dawe, J.; Hayes, R.; Prestidge, C. A.; Ralston, J. J. Colloid Interface Sci. 1998, 203, 115–121.
6.5) which remained constant for the duration of the experiment. It was established that adsorption equilibrium was achieved in less than 3 h. After equilibration in the presence of dendrimers, the alumina particles were removed from the solutions by centrifugation (18 000 rpm). The dendrimer concentration remaining in the supernatant was then determined by UV spectrophotometry (Cary 1, Varian Australia Pty Ltd.) from the absorbance at 331 and 232 nm (SPL7013 and SPL-2923), respectively. The equilibrium amount of dendrimer adsorbed (qe) was calculated and the adsorption isotherms by plotting qe versus the equilibrium concentration (Ce). Dendrimer adsorption isotherm data was analyzed using the Langmuir model,30,31 with good agreement observed between experimental and theoretically calculated values of qe.
qe )
Q0bCe 1 + bCe
(1)
Adsorption parameters were calculated from the Langmuir model including the plateau adsorbed amount (Q0) and adsorption affinity constant (b). The free energy of adsorption (∆Gads) was calculated from b, the universal gas constant (R), and the absolute temperature (T ).
∆Gads ) -RT ln(55.55b)
(2)
A Malvern Zetasizer Nano (Malvern Instruments, U.K.) was used to measure the ζ-potential in 10-3 M NaCl of γ-alumina particles in the presence and absence of dendrimers. Electrophoretic mobility data were obtained using the M3-PALS technique, which is a combination of laser doppler velocimetry and phase analysis light scattering (PALS). The electrophoretic mobility was converted to a ζ-potential (ζ) using the Smoluchowski equation, since κa . 1.32 Dendrimer Interfacial Arrangement. The theoretical maximum adsorbed amount (Q0theory) was calculated assuming either hexagonal or cubic close-packed two-dimensional arrays of spherical dendrimer molecules on the alumina surface. The equivalent spherical radius (aeff) of each dendrimer was calculated based on both the physical radius (a), as well as the inverse Debye length (κ-1)sat low ionic strengths the repulsive electrical double layer (EDL) forces prevent the close approach of neighboring dendrimers, i.e., aeff ) a + (1/κ). This approach has previously been successfully used in calculating the coverage of silica nanoparticles adsorbed on PDMS droplets over a wide range of salt concentrations.33,34 The experimental area per adsorbed dendrimer molecule (Aexp) at the alumina-water was calculated from Q0, Avogadro’s number (Na), and the specific surface area (SSA):
A)
1 Q0Na(SSA)
(3)
Results and Discussion Dendrimer Adsorption and the Influence of Ionic Strength. Adsorption Isotherms. Equilibrium adsorption isotherms for SPL2923 and SPL-7013 on alumina in a range of ionic strengths (10-3-10-1 M NaCl), including Langmuir isotherm fits, are presented in Figure 2; the associated thermodynamic parameters are presented in Table 2. In general, good agreement was observed between the experimental qe data and the Langmuir model (correlation coefficients, r2 > 0.98), indicating that the underlying assumptions of the model (i.e., homogeneous, monolayer adsorption) are applicable in this system. Isotherm shape, maximum adsorption, and the ionic strength dependence is clearly dependent on the dendrimer type. (30) Kapsabellis, S.; Prestidge, C. A. J. Colloid Interface Sci. 2000, 228, 297– 305. (31) Prestidge, C. A.; Barnes, T. J.; Simovic, S. AdV. Colloid Interface Sci. 2004, 108-109, 105–118. (32) Hunter, R. J. Foundations of Colloid Science; Clarendon: Oxford, 1987; Vol. 1. (33) Simovic, S.; Prestidge, C. A. Langmuir 2003, 19, 3785–3792. (34) Simovic, S.; Prestidge, C. A. Langmuir 2003, 19, 8364–8370.
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Barnes et al.
Figure 1. Structural formulas of dendrimers: (a) SPL-2923 and (b) SPL-7013. Note: the colors highlight the structural features of each dendrimerscore (red), generations (green, pink, brown, black), and surface functional NDS groups (blue).
Naphthalene Sulfonate Functionalized Dendrimers
Figure 2. Dendrimer adsorption on γ-alumina as a function of ionic strength, 10-3 M (O), 10-2 M (∆), and 10-1 M (0) NaCl, at pH 6.7, for (a) SPL-2923 and (b) SPL-7013, including Langmuir fits (line).
For the AM-cored SPL-2923, Q0 increased with increasing ionic strength (10-3 and 10-2 M NaCl), with Q0 values of 0.37 and 0.56 mg/m2, respectively; however, as the ionic strength was increased further to 10-1 M NaCl, no further increase in Q0 was observed. Corresponding to these values of Q0, the area occupied per molecule (Aexp) decreased from 51 to 32 nm2/molecule, with increasing NaCl concentration (10-3 to 10-1 M, respectively). In contrast, for the BHA-Lys-cored SPL-7013, Q0 was observed to increase from 0.38 to 0.79 mg/m2, which corresponded with a decrease in Aexp from ∼70 to ∼30 nm2/molecule with increasing ionic strength (10-3 to 10-1 M). The values of Q0 and Aexp obtained in this study were lower than those previously observed for the adsorption of generation 1.5 and 5.5 carboxylate-terminated PAMAM dendrimers on alumina, i.e., Q0 values of 0.9 and 1.45 mg/m2 which corresponds to area per molecule values of 6.3 and 44.4 nm2/molecule.24 It is interesting to note that although in 10-1 M NaCl both SPL-2923 and SPL-7013 occupied an equivalent surface area (∼30 nm2/molecule), in 10-3 M NaCl, SPL-7013 occupies ∼40% more alumina surface area per molecule, suggesting a difference in their interfacial arrangement, resulting from their differing core geometry. For the adsorption of SPL-2923 on the alumina particles, ∆Gads values decreased from -51.6 to-47.0 kJ/mol, whereas ∆Gads for SPL-7013 increased from -46.3 to -53.4 kJ/mol upon increasing ionic strength (10-3 to 10-1 M), respectively. This suggests a strong electrostatic driving force for the adsorption ofbothdendrimersandreflectingthemultivalentdendrimer-alumina interaction. Previous studies21 have reported significantly lower ∆Gads values in the range of -26 to -36 kJ/mol for the adsorption of amine-terminated (G3 to G7) PAMAM dendrimers onto a fused-silica surface from a methanol/water solution, highlighting a increased NDS-alumina affinity, compared to amine-silica.
Langmuir, Vol. 24, No. 21, 2008 12401
The change in ζ-potential as a function of dendrimer adsorbed amount in 10-3 M NaCl is shown in Figure 3. For both SPL2923 and SPL-7013, charge reversal was observed, with increasing dendrimer concentrations, in agreement to previous observations for the adsorption of PAMAM dendrimers on latex particles.27 In the case of SPL-2923, a linear decrease in ζ-potential occurred with increasing qe, and the observed IEP corresponds to the onset of the adsorption plateau (Q0) ∼0.3 mg/m2. At higher equilibrium dendrimer concentrations (i.e., further along the adsorption plateau), the ζ-potential decreased to ∼ -20 mV. This suggests that the interfacial arrangement of the AM-cored SPL-2923 occurs, resulting in the exposure of additional NDS groups, not unexpected given that these experiments were carried out under equilibrium conditions. In contrast, for the BHA-Lys-cored SPL-7013, a linear decrease in ζ-potential was observed, and the IEP corresponded to qe ∼ 0.1 mg/m2, i.e., significantly below monolayer coverage. This suggests that a greater proportion of the NDS surface groups of SPL-7013 remain exposed to the solution rather than adsorbing to the alumina surface. At monolayer coverage, the ζ-potential of SPL-7013-coated alumina particles was more negative than was observed for SPL-2923, which corresponds to a greater number (i.e., density) of exposed NDS groups the alumina particles EDL. Dendrimer Arrangement at the Alumina-Water Interface. To further elucidate the adsorbed conformation of SPL-2923 and SPL-7013 at the alumina surface, both experimental and theoretical values for the plateau adsorbed amount (Q0) as well as the corresponding area per molecule are plotted as a function of ionic strength in Figures 4 and 5, respectively, and the values are summarized in Table 2. By assuming both two-dimensional cubic and hexagonal close packing of spheres of an equivalent spherical volume (note: hexagonal close packing is approximately 27% denser than cubic packing), theoretical values of the plateau adsorbed amount (Q0theory) and area per molecule (Atheory) were calculated. In these calculations, the effective dendrimer diameter was calculated based on the hydrodynamic diameter (∼4 nm as determined by dynamic light scattering (DLS)) of the dendrimer as well as the thickness of the electrical double layer (κ-1) at the corresponding ionic strength. From the data in Figures 4 and 5, we can draw several conclusions as to the adsorbed conformation of the dendrimers considering both high (10-1 M NaCl) and low ( HCO3- > SPL-2923 . Ca2+, Mg2+, Tris, physiological saline.
Conclusions The role of core type (ammonia vs benzylhydrylaminepolylysine) on the adsorption of NDS-functionalized dendrimers
2+
Ca
0.80 2.8 -46.8 0.80 2.9 -46.8
Mg2+
HCO3-
HPO42-
Tris
0.60 3.1 -47.0 0.75 8.2 -49.4
0.23 2.8 -46.7 0.78 3.8 -47.5
0.45 0.06 -37.2 0.96 0.21 -40.3
0.45 18 -51.4 0.73 3.2 -47.1
onto γ-alumina was investigated. At high ionic strengths (0.1 M NaCl) both dendrimers were found to occupy equivalent area per molecule values corresponding to the theoretical values calculated assuming cubic close packing, indicating hard sphere behavior due to the collapse of the dendrons in the absence of sufficient electrostatic repulsion. In contrast, at lower ionic strengths Q0exp > Q0theory and Aexp < Atheory, suggesting a significantly higher adsorption density than predicted theoretically based on the dendrimer hydrodynamic radius and the thickness of the electrical double layer. In the presence of SBF, the adsorption both SPL-2923 and SPL-7013 was significantly depressed, due to the competitive adsorption of HCO3- and HPO42- anions. SPL-7013 exhibits a higher binding affinity for the γ-alumina surface than SPL-2923; this is attributed to differences in molecular architecture resulting in changes to the adsorbed conformation of each dendrimer. Acknowledgment. The authors gratefully acknowledge Starpharma Holdings Pty Ltd. for providing the dendrimers used in this work. Starpharma Holdings Pty Ltd. and the Australian Research Council (ARC) Linkage Grant Scheme (LP0455268) are acknowledged for funding this work. LA8020996
