Transition from Completely Reversible to Irreversible Adsorption of

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Langmuir 2009, 25, 2928-2934

Transition from Completely Reversible to Irreversible Adsorption of Poly(amido amine) Dendrimers on Silica Re´mi Longtin, Plinio Maroni, and Michal Borkovec* Department of Inorganic, Analytical and Applied Chemistry, UniVersity of GeneVa, 30 Quai Ernest-Ansermet, 1211 GeneVa ReceiVed NoVember 24, 2008. ReVised Manuscript ReceiVed January 6, 2009 The adsorption and desorption behavior of poly(amido amine) (PAMAM) dendrimers at the water-silica interface was investigated by optical reflectometry. Polymer desorption upon dilution was studied as a function of generation (i.e., molecular mass), solution pH, and ionic strength. Three distinct adsorption regimes upon flushing with dendrimerfree solutions were identified. (i) Completely reversible adsorption refers to rapid and complete desorption. (ii) Partially reversible adsorption is characterized by rapid but partial desorption and a remaining irreversibly bound fraction. (iii) Irreversible adsorption refers to the case where there is no detectable change in the adsorbed mass. The system tends to be completely reversible for low generations, low pH values, and high ionic strengths, while it tends to be irreversible for high generations, high pH values, and low ionic strengths. The parameters for which these regimes are found are summarized in corresponding adsorption maps.

Introduction Adsorption of polyelectrolytes on oppositely charged solid surfaces is relevant in many industrial applications. For example, polyelectrolytes are used as retention aids in papermaking or as flocculants in water purification.1,2 For this reason, these adsorption processes were experimentally investigated in detail, and numerous authors have highlighted the importance of electrostatic forces, which are most clearly evidenced by the charge reversal phenomenon and by the increase of the adsorbed amount with increasing salt concentration.3-8 The latter aspect can be explained by the progressive screening of the repulsive electrostatic interactions between the adsorbing polyelectrolyte chains. In some systems, a sharp decrease in the adsorbed amount is observed at high salt levels, which is interpreted as the screening of the attractive electrostatic interactions between the adsorbing polyelectrolyte chains and the substrate.9,10 These effects have been rationalized with theoretical models that invoke concepts such as charge regulation, overcharging, and ion correlations.11-15 * To whom correspondence should be addressed. Telephone: ++41 22 379 6405. E-mail: [email protected]. (1) Retention Aids, 2nd ed.; Horn, D., Linhart, F., Eds.; Blackie Academic and Professional: London, 1996. (2) Bolto, B.; Gregory, J. Water Res. 2007, 41, 2301–2324. (3) Cohen Stuart, M. A.; Hoogendam, C. W.; deKeizer, A. J. Phys.: Condens. Matter 1997, 9, 7767–7783. (4) Bauer, D.; Buchhammer, H.; Fuchs, A.; Jaeger, W.; Killmann, E.; Lunkwitz, K.; Rehmet, R.; Schwarz, S. Colloids Surf., A 1999, 156, 291–305. (5) Kleimann, J.; Gehin-Delval, C.; Auweter, H.; Borkovec, M. Langmuir 2005, 21, 3688–3698. (6) Bouyer, F.; Robben, A.; Yu, W. L.; Borkovec, M. Langmuir 2001, 17, 5225–5231. (7) Rojas, O. J.; Ernstsson, M.; Neuman, R. D.; Claesson, P. M. Langmuir 2002, 18, 1604–1612. (8) Popa, I.; Cahill, B. P.; Maroni, P.; Papastavrou, G.; Borkovec, M. J. Colloid Interface Sci. 2007, 309, 28–35. (9) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 133–145. (10) Hansupalak, N.; Santore, M. M. Langmuir 2003, 19, 7423–7426. (11) Linse, P. Macromolecules 1996, 29, 326–336. (12) Dobrynin, A. V.; Deshkovski, A.; Rubinstein, M. Macromolecules 2001, 34, 3421–3436. (13) Grosberg, A. Y.; Nguyen, T. T.; Shklovskii, B. I. ReV. Mod. Phys. 2002, 74, 329–345. (14) Shafir, A.; Andelman, D. Phys. ReV. E 2004, 70, 061804. (15) Netz, R. R.; Joanny, J. F. Macromolecules 1998, 31, 5123–5141.

Similarly, understanding adsorption of proteins on solid substrates is essential for the life sciences, for example, in the context of prevention of plaque on teeth or biocompatible surgical implants.16-18 This topic was investigated experimentally in detail, and attention was mostly focused on kinetic aspects.16,19-21 The random sequential adsorption (RSA) model, which describes the time-dependence of irreversible adsorption processes, was initially developed to understand protein adsorption and particle deposition.22-24 Other relevant aspects are changes in protein orientation and conformation after the adsorption event, as evidenced by atomic force microscopy (AFM) or fluorescence spectroscopy.16,19,25-27 Various studies have revealed that nonelectrostatic forces play an essential importance, and several authors stressed the role of counterion evaporation and of hydrophobic interactions.19,20,28,29 The role of electrostatic forces in protein adsorption are less obvious, and these effects are complicated by charge regulation and its nonuniform lateral distribution.21,30 The question of adsorption reversibility for polyelectrolytes and proteins is particularly pertinent in the context of fabricating stable surface coatings or adsorbent layers for which the ease of regeneration is a key issue.31 However, a relatively limited number (16) Ramsden, J. J. Chem. Soc. ReV. 1995, 24, 73–78. (17) Halthur, T. J.; Claesson, P. M.; Elofsson, U. M. Langmuir 2006, 22, 11065–11071. (18) Kenausis, G. L.; Voros, J.; Elbert, D. L.; Huang, N.; Hofer, R.; RuizTaylor, L.; Textor, M.; Hubbell, J. A.; Spencer, N. D. J. Phys. Chem. B 2000, 104, 3298–3309. (19) Wahlgren, M.; Arnebrant, T. Trends Biotechnol. 1991, 9, 201–208. (20) van der Veen, M.; Stuart, M. C.; Norde, W. Colloids Surf., B 2007, 54, 136–142. (21) van der Veen, M.; Norde, W.; Stuart, M. C. Colloid Surf., B 2004, 35, 33–40. (22) Feder, J. J. Theor. Biol. 1980, 87, 237–254. (23) van Tassel, P. R.; Guemouri, L.; Ramsden, J. J.; Tarjus, G.; Viot, P.; Talbot, J. J. Colloid Interface Sci. 1998, 207, 317–323. (24) Ko, C. H.; Bhattacharjee, S.; Elimelech, M. J. Colloid Interface Sci. 2000, 229, 554–567. (25) Santore, M. M.; Wertz, C. F. Langmuir 2005, 21, 10172–10178. (26) Agnihotri, A.; Siedlecki, C. A. Langmuir 2004, 20, 8846–8852. (27) Wertz, C. F.; Santore, M. M. Langmuir 2002, 18, 706–715. (28) Fleck, C.; von Grunberg, H. H. Phys. ReV. E 2001, 63, 061804. (29) Haynes, C. A.; Norde, W. Colloids Surf. B 1994, 2, 517–566. (30) Biesheuvel, P. M.; van der Veen, M.; Norde, W. J. Phys. Chem. B 2005, 109, 4172–4180. (31) Dabrowski, A. AdV. Colloid Interface Sci. 2001, 93, 135–224.

10.1021/la8038818 CCC: $40.75  2009 American Chemical Society Published on Web 01/27/2009

Adsorption/Desorption of PAMAM Dendrimers on Silica

of studies have addressed reversibility from the fundamental point of view. Polyelectrolytes have been reported to hardly desorb upon dilution.32 Their desorption can be more easily achieved by changing the solution pH, electrolyte concentration, or by adding displacer molecules, which lead to surface exchange processes.7,32-34 Fullerene nanoparticles were reported to desorb from a silica substrate in basic conditions.35 The question of reversibility has been addressed in the protein literature in detail but is far from being resolved.16,19,20 Several authors have observed that a fraction of the adsorbed protein often desorbs rapidly upon dilution, while the remaining fraction remains adsorbed irreversibly. This phenomenon was related to postadsorption conformational transitions of the protein in the adsorbed state or its changes in orientation with respect to the surface. A protein, which had more time to relax and to find anchor points on the surface in its adsorbed state, will desorb more slowly than a protein that just came in contact with the surface. While adsorbed proteins do not readily desorb upon dilution, they do yield their place to other, more strongly adsorbing, proteins.16,19 Dendrimers represent a relatively new class of branched charged macromolecules, which interact strongly with interfaces.36-40 They represent an interesting model system to study the adsorption of macromolecules at interfaces due to their relative monodispersity, regular structure, and the availability over a wide range of generations (i.e., molecular masses). Poly(amido amine) (PAMAM) dendrimers have already been investigated in this context in some detail.38,41 These dendrimers are positively charged, and therefore strongly adsorb to negatively charged substrates. Dendrimer adsorption on planar substrates has been studied by AFM, reflectivity, and fluorescence correlation spectroscopy,42-47 and on colloidal particles by electrophoresis and depletion experiments.48,49 On the basis of AFM studies, it has been shown that PAMAM dendrimers substantially flatten upon adsorption.45,50-53 Computer simulations further suggest that this deformation becomes more pronounced with increasing attraction between the dendrimer segments and the substrate.52,54 The dendrimer adsorption process was interpreted in terms of the RSA model.45,47 These studies have demonstrated that dendrimers adsorb in incomplete monolayers with a frozen, liquid(32) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 146–157. (33) Meszaros, R.; Varga, I.; Gilanyi, T. Langmuir 2004, 20, 5026–5029. (34) Naderi, A.; Olanya, G.; Makuska, R.; Claesson, P. M. J. Colloid Interface Sci. 2008, 323, 223–228. (35) Chen, K. L.; Elimelech, M. Langmuir 2006, 22, 10994–11001. (36) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Macromolecules 1986, 19, 2466–2468. (37) Tully, D. C.; Frechet, J. M. J. Chem. Commun. 2001, 1229–1239. (38) Tomalia, D. A. Mater. Today 2005, 8, 34–46. (39) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219–227. (40) Ballauff, M.; Likos, C. N. Angew. Chem., Int. Ed. 2004, 43, 2998–3020. (41) Tsukruk, V. V. AdV. Mater. 1998, 10, 253–257. (42) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171–2176. (43) van Duijvenbode, R. C.; Rietveld, I. B.; Koper, G. J. M. Langmuir 2000, 16, 7720–7725. (44) van Duijvenbode, R. C.; Koper, G. J. M.; Bohmer, M. R. Langmuir 2000, 16, 7713–7719. (45) Pericet-Camara, R.; Papastavrou, G.; Borkovec, M. Langmuir 2004, 20, 3264–3270. (46) McCain, K. S.; Schluesche, P.; Harris, J. M. Anal. Chem. 2004, 76, 930– 938. (47) Cahill, B. P.; Papastavrou, G.; Koper, G. J. M.; Borkovec, M. Langmuir 2008, 24, 465–473. (48) Esumi, K.; Goino, M. Langmuir 1998, 14, 4466–4470. (49) Lin, W.; Galletto, P.; Borkovec, M. Langmuir 2004, 20, 7465–7473. (50) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323–5324. (51) Betley, T. A.; Banaszak Holl, M. M.; Orr, B. G.; Swanson, D. R.; Tomalia, D. A.; Baker, J. R., Jr. Langmuir 2001, 17, 2768–2773. (52) Mecke, A.; Lee, I.; Baker, J. R.; Holl, M. M. B.; Orr, B. G. Eur. Phys. J. E 2004, 14, 7–16.

Langmuir, Vol. 25, No. 5, 2009 2929 Table 1. Properties of PAMAM Dendrimers

generation

molecular massa (g/mol)

number of primary amine groupsa

hydrodynamic radiusb (nm)

0 1 2 3 4 6 8 10

517 1430 3256 6909 14 215 58 048 233 383 934 720

4 8 16 32 64 256 1024 4096

0.67 1.04 1.3 1.82 2.08 3.35 4.85 6.75

a

Data for ideal structures provided by the manufacturer. b From ref 57.

like structure at sufficiently low salt concentrations. The adsorbed amount increases strongly with increasing ionic strength due to progressive screening of the repulsive interactions between the dendrimers. Moreover, the adsorbed amount further increases with increasing charge of the substrate, since the counterions in the diffuse layer contribute to the screening as well.47 The latter phenomenon explains the increase of the adsorbed amount of PAMAM dendrimers on silica with increasing pH. While the adsorption process of dendrimers on oppositely charged substrates is reasonably well understood, the knowledge of their desorption behavior is limited. Smaller generation PAMAM dendrimers were reported to desorb rapidly,46 while higher generation dendrimers do not desorb at all.47 Better insight into such processes is essential to assess the stability of adsorbed dendrimer films, which are becoming increasingly relevant for applications such as surface-based sensors or surface nanopatterning.37,39,55 In this article, we show that concepts borrowed from protein literature can be used to rationalize the transition from reversibility to irreversibility of PAMAM dendrimer adsorption at the water-silica interface. The adsorption process is investigated by in situ reflectometry, and we show that it is strongly influenced by dendrimer generation and solution composition. The observed behavior was classified as completely reversible, partially reversible, and irreversible. Finally, we present adsorption maps that are invaluable tools to represent layer stability.

Experiment Materials. PAMAM dendrimers of generations G0 to G10 were obtained as aqueous solutions from Dendritech (Midland, MI). Their concentrations were checked by carbon and nitrogen analysis. Solutions containing 5.6 mg/L of polymer were prepared by dilution with Milli-Q water, which was used throughout. The solution pH was adjusted with HCl and KOH, and the ionic strength was adjusted with KCl. The refractive index increment dn/dc was measured to be 0.194 mL/g and was independent of dendrimer generation.47 Below pH 4, basically all amine groups are protonated, making the PAMAM dendrimers highly positively charged. With increasing pH, their charge decreases gradually. At pH 8, about half of the groups still remain protonated, while only at pH > 11 do they become neutral.56 Table 1 reports on other dendrimer properties such as molecular mass and hydrodynamic radius.57 Polished silicon wafers (p-type, boron-doped, Silchem GmbH, Germany) were thermally oxidized in a furnace at 1000 °C for several minutes depending on the desired oxide thickness. Measurements (53) Muller, T.; Yablon, D. G.; Karchner, R.; Knapp, D.; Kleinman, M. H.; Fang, H.; Durning, C. J.; Tomalia, D. A.; Turro, N. J.; Flynn, G. W. Langmuir 2002, 18, 7452–7455. (54) Mansfield, M. L. Polymer 1996, 37, 3835–3841. (55) Pericet-Camara, R.; Cahill, B. P.; Papastavrou, G.; Borkovec, M. Chem. Commun. 2007, 266–268. (56) Cakara, D.; Kleimann, J.; Borkovec, M. Macromolecules 2003, 36, 4201– 4207. (57) Fritzinger, B.; Scheler, U. Macromol. Chem. Phys. 2005, 206, 1288– 1291.

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Longtin et al.

Figure 1. Typical transients of the adsorbed mass obtained by reflectometry. The cell is flushed with an electrolyte solution to obtain a signal baseline, and at t ) 0 the dendrimers in the same electrolyte solution are injected. The adsorption proceeds until a maximum plateau at Γmax is reached. At t ) t1, the cell is rinsed with a dendrimer-free electrolyte solution, and the decrease of the adsorbed mass is monitored until one reaches the residual mass Γres. The nature of this decrease is characteristic for the three different regimes of (a) complete reversibility, where Γres ≈ 0, (b) partial reversibility, where Γres < Γmax, and (c) irreversibility, where Γres ≈ Γmax.

of the silica layer thickness, typically ranging from 6 to 20 nm, were done by null ellipsometry in air (Multiskop, Optrel, Germany). The oxidized wafers were then cut in 5 × 15 mm2 pieces. They were cleaned in a mixture of 25% NH3, 35% H2O2, and water in a ratio of 1:1:5 by volume at 80 °C for 20 min. After extensive rinsing, the wafers were stored in water. Reflectometry. The home-built reflectomer uses a goniometer with a horizontal axis and two adjustable arms. A He-Ne laser emitting at 632.8 nm is attached on one arm, while the other carries the detection electronics. A stagnation-point flow cell is mounted in the rotation axis. The laser beam is aimed at the stagnation point through a 60° truncated prism under which the oxidized wafers are inserted. As a result of light refraction in the prism, the actual angle of incidence is 71.3°. The solutions are injected perpendicularly to the silica surface through a bore hole with a peristaltic pump. All experiments were performed at a temperature of about 23 °C. Further details on the cell geometry and flow conditions are given elsewhere.47,58 The incident laser beam is first polarized with a standard polarizer. The angle of polarization is rotated by a half-wave plate to equalize the intensities of the polarization components of the incoming beam. The reflected beam is separated in its two components by a polarizing beam-splitter. The intensities of the parallel and perpendicularly polarized components are proportional to the respective reflectances R(p) and R(s). Their ratio can be measured (R ) CR(p)/ R(s)) up to an undetermined instrumental constant C. When one measures this ratio as a function of time t, this constant can be eliminated by normalizing the ratio to its initial value. The signal measured is thus

S(t) )

R(t) - R(0) R(0)

(1)

where t ) 0 denotes the instant of polymer injection. The signal is directly proportional to the adsorbed mass per unit area given by Γ(t) ) A- 1S(t) where the sensitivity factor A is calculated from a standard homogeneous slab model that describes the optical response of the dendrimer covered surface.58,59 Typical values of the sensitivity factor are 0.2-0.4 m2/mg. The signal-to-noise ratio was improved by averaging 1.5 × 105 points. The experimental detection limit of this setup is 2 µg/m2. The experimental deviation between two consecutive measurements on separate silica substrates was below 15%. The point of zero charge (PZC) of the oxidized silicon wafers was determined by measuring the deposition of small colloidal particles of opposite charge.24,60 Positively charged amidine and negatively charged sulfate latex particles (Interfacial Dynamics Corporation, (58) Kleimann, J.; Lecoultre, G.; Papastavrou, G.; Jeanneret, S.; Galletto, P.; Koper, G. J. M.; Borkovec, M. J. Colloid Interface Sci. 2006, 303, 460–471. (59) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. AdV. Colloid Interface Sci. 1994, 50, 79–101. (60) Kallay, N.; Torbic, Z.; Golic, M.; Matijevic, E. J. Phys. Chem. 1991, 95, 7028–7032.

Portland, OR) of diameters of 52 and 53 nm, respectively, were used. The initial deposition rate of these particles was measured by reflectometry in 10 mM KCl electrolyte as a function of pH. The deposition rate of the negatively charged particles decreases with increasing pH, while this rate increases for the positively charged particles. The PZC was identified as the locus where both rates were the same. We have found a PZC of 2.6 ( 0.4, which is slightly higher than the values of 2.0 ( 0.3 reported in literature.61

Results and Discussion The adsorption of positively charged PAMAM dendrimers on negatively charged silica substrates was studied by reflectometry. Reversibility or irreversibility of the adsorption process was assessed by first adsorbing dendrimers from an electrolyte solution and then by following their desorption upon rinsing the surface with an identical medium, but devoid of dendrimers. The dendrimer generation, the solution pH, and the ionic strength were varied systematically, as all of these parameters strongly influence the reversibility of the adsorption process. Typical Adsorption-Desorption Transients. In a typical adsorption experiment, a constant signal baseline is obtained by flushing the cell with a background electrolyte solution of given pH and ionic strength. A dendrimer solution having the same pH and ionic strength as the background electrolyte solution is injected at t ) 0. At this point, the adsorbed mass starts to increase until the maximum adsorption value Γmax is reached. The time necessary to reach this plateau depends on the dendrimer concentration used, but, in the present study, about 500 s was normally sufficient. After the plateau has been reached, the surface with adsorbed dendrimers is rinsed with a dendrimer-free background electrolyte solution at time t ) t1. This rinsing solution is identical to the solution used to obtain the baseline prior to adsorption and has the same pH and ionic strength as the dendrimer solution. By further monitoring the adsorbed mass, the desorption transient provides an important characteristic of the reversibility or irreversibility of adsorption behavior. We distinguish three cases concerning the reversibility of the adsorption: (i) completely reVersible, (ii) partially reVersible, and (iii) irreVersible. These cases are depicted schematically in Figure 1. We refer to completely reversible adsorption when desorption of the adsorbed dendrimers is rapid and complete. However, one often faces the situation of partially reversible adsorption, whereby a certain fraction of the adsorbed dendrimers desorbs rapidly, while the remaining fraction will not desorb within the experimental time window. Finally, we refer to irreversible adsorption when no decrease of the adsorbed mass (61) Hiemstra, T.; Venema, P.; VanRiemsdijk, W. H. J. Colloid Interface Sci. 1996, 184, 680–692.

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Langmuir, Vol. 25, No. 5, 2009 2931

is observed. Hoogeveen et al.32 proposed an analogous classification in the context of adsorption of linear polyelectrolytes on solid substrates. These three cases can be distinguished experimentally by comparing the residual irreversibly adsorbed mass Γres normalized to the maximum plateau value Γmax. The ratio of these two quantities will be referred to as the irreversibly bound fraction

x)

Γres Γmax

(2)

Because of instrumental noise and signal instabilities, we allow for a measurement uncertainty in this quantity of 0.02. Completely reversible adsorption is characterized by full desorption with x e 0.02 (see Figure 1a). The adsorption is partially reversible for a detectable residual adsorbed mass with 0.02 < x e 0.98 (see Figure 1b). Irreversible adsorption is typified by the lack of desorption with x > 0.98 (see Figure 1c). In most cases, the fast desorbing fraction can be clearly distinguished from the irreversibly bound fraction. Normally, desorption takes place within a few minutes. After this transient, the adsorbed mass remains basically constant during the time scale of the measurement of about 15-20 min. While one can often observe a very slow decrease of the residual adsorbed mass, the precise characterization of this process is hampered by instrumental instabilities. For this reason, meaningful results concerning the long-term desorption kinetics on the time scale of several hours or longer are currently not available. Effect of pH and Surface Charge Density. Experimental adsorption transients for different dendrimer generations in 0.1 M KCl electrolyte are shown in Figure 2. By comparing top and bottom panels, one observes that the adsorption process becomes progressively reversible with decreasing generation and decreasing pH. Completely reversible adsorption is observed for G0 at pH 2 and several adsorption-desorption cycles can be realized. Under the same conditions, G10 is only partially reversible. One can see that G0 becomes partially reversible at pH 6, and irreversible adsorption is observed for G10 at pH 6. The maximum adsorbed amount increases with increasing generation and pH. This trend was already reported for the adsorption of PAMAM dendrimers of generations G8 and G10 on silica by Cahill et al.47 Furthermore, a similar increase of the adsorbed amount for generations G0 to G5 was equally reported earlier.48 The characteristic dependence of the adsorbed mass on pH was shown to be of electrostatic origin.47 With increasing pH, silica develops a negative surface charge, which is mainly neutralized by cations adsorbed in the diffuse layer. These cations screen the electrostatic repulsion between the dendrimers, leading to weaker lateral repulsion and higher adsorbed mass. Let us now focus on the transition between completely reversible and irreversible adsorption (see Figure 2). An example of complete reversibility is G0 at pH 2 shown in Figure 2a. On the other hand, irreversible adsorption is shown for G10 at pH 6 in Figure 2b. The adsorption of all other generations is partially reversible. The irreversibly bound fraction x is plotted as a function of pH for the different generations in Figure 3a. This fraction increases with increasing pH and with increasing generation. A similar trend with molecular mass (i.e., generation) was found for desorption of polyvinylpyridine molecules adsorbed to a titania surface.32 Furthermore, one observes that the transition between complete (x e 0.02) and partial reversibility (x > 0.02) is rather abrupt and can be easily located for G0, G1, and G2. The transition between partial reversibility (x e 0.98) and irreversibility (x > 0.98) is much more gradual, but can be still clearly located in most cases.

Figure 2. Time dependence of the adsorbed mass for different PAMAM dendrimer generations observed by reflectometry at constant ionic strength of 0.1 M KCl at pH 2 (top panel) and pH 6 (bottom panel). The vertical line indicates when the flushing with the dendrimer-free solution started. The complete reversibility for G0 at pH 2 is illustrated by several adsorption and desorption cycles.

The increase of the irreversibly bound fraction with increasing dendrimer generation can be explained by the increasing number of contact points between the dendrimer branches and the substrate. The trend with pH can be similarly understood in relation to the magnitude of the attractive electrostatic forces between the positively charged dendrimers and the negatively charged silica. Indeed, with increasing pH, the magnitude of the surface charge increases61,62 leads to stronger binding, and thus to a larger irreversibly bound fraction. Although the positive charge of the dendrimers decreases somewhat upon increasing the pH, they maintain a high positive charge.56 However, the adsorbed mass remains substantial at pH 2, where silica is neutral or slightly positively charged (PZC = 2.5). This observation indicates that nonelectrostatic forces also contribute to the attractions between the dendrimers and the surface. These additional forces probably originate from hydrophobic interactions or hydrogen bonding between the terminal amines and the silanol groups.46 Another characteristic of the desorption process is the initial desorption rate coefficient kdes defined by the relation

dΓ ) -kdesΓ dt

(3)

Clearly, one has kdes ) 0 for an irreversible process. However, for partially and completely reversible processes, one can estimate (62) Kobayashi, M.; Juillerat, F.; Galletto, P.; Bowen, P.; Borkovec, M. Langmuir 2005, 21, 5761–5769.

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the desorption rate coefficient from the initial decrease of the adsorbed mass. Desorption rate coefficients as a function of generation for several pH values are presented in Figure 3b, and one can see that this quantity decreases with increasing generation and with increasing pH. McCain et al.46 observed the same dependence of the desorption rate coefficient with generation by total internal reflection-fluorescence correlation spectroscopy for G3, G5, and G7 PAMAM dendrimers adsorbed on silica. While the desorption rate coefficients presented here are about 3 orders of magnitude smaller than those reported in the study by McCain et al.,46 this discrepancy can be explained by the fact that the mentioned study used a mixture of methanol and water as solvent. The lower polarity of this solvent leads to a higher solubility of the dendrimers, and consequently to weaker adsorption. Effect of Ionic Strength. Figure 4a shows the irreversibly bound fraction for G2 as a function of pH for different ionic strengths. Increasing the ionic strength has a similar effect as decreasing the dendrimer generation. For example, G2 dendrimers are partially reversible at 0.01 M and below, while they become reversible at 0.1 M and above. The effect can be qualitatively explained by electrostatic screening. At low salt, there is a strong attractive electrostatic interaction between the positively charged dendrimers and the negatively charge surface. With increasing ionic strength, the attraction gets weaker due to screening, and thus the adsorption becomes more reversible. However, changing the salt concentration is not as effective as changing the generation to induce the transition in the desorption behavior. A similar effect is observed for desorption rate coefficients. The rate coefficient increases with increasing ionic strength. This trend reflects again the screening of electrostatic attractions with increasing salt level. Figure 3 further illustrates that the rate coefficients also decrease with increasing pH. This effect was already discussed above.

Figure 3. Desorption characteristics of PAMAM dendrimers for different dendrimer generations and for different pH values in 0.1 M KCl electrolyte: (a) the irreversibly bound fraction x, and (b) desorption rate constant kdes. The lines serve as eye guides only.

Longtin et al.

Figure 4. Desorption characteristics of PAMAM G2 dendrimers for different pH values and different KCl electrolyte ionic strengths: (a) the irreversibly bound fraction x, and (b) desorption rate constant kdes. The lines serve as eye guides only.

Interpretation of the Desorption Process. The existence of partial reversibility clearly points toward a heterogeneous population of adsorbed dendrimers at the surface. The existence of a rapidly desorbing fraction and an irreversibly bound fraction suggest the existence of at least two populations of adsorbed dendrimers. However, it is equally conceivable that the distribution of desorption rate constants is continuous, but very wide, and only two distinct populations are apparent because of experimental limitations. A heterogeneous population of adsorbed dendrimers could be due to the following effects: (i) dendrimer heterogeneity, (ii) dendrimer aggregation, (iii) surface heterogeneity, and (iv) conformational changes in the adsorbed state. In the following, we will first argue against explanations (i) through (iii) as they are unlikely, and finally we show that explanation (iv) is the most likely one. PAMAM dendrimers are chemically homogeneous and relatively monodisperse with a polydispersity index of 1.3 for G10.45,51 While defects in their architecture may exist, the highly heterogeneous dendrimer population observed in the desorption experiment can hardly result from the modest heterogeneity introduced by these defects. The dendrimers are also highly protonated within the pH range studied, and therefore the interactions between them are expected to be strongly repulsive. For this reason, we suspect that aggregation of dendrimers is unimportant. An oxidized silica wafer is not atomically flat, and substrate heterogeneity could contribute to this heterogeneity. From AFM studies, one knows that the root-mean-square roughness of such substrates is about 0.2 nm.63,64 While these values are comparable with those of smaller dendrimers, they are much smaller than that of the largest dendrimer studied (see (63) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831– 1836. (64) Rentsch, S.; Pericet-Camara, R.; Papastavrou, G.; Borkovec, M. Phys. Chem. Chem. Phys. 2006, 8, 2531–2538.

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Figure 5. Schematic representation of the suggested mechanism responsible for the different desorption behavior. The dendrimers adsorb initially at the interface in a nondeformed state. Once adsorbed, they relax to a flattened conformation. The nondeformed dendrimers desorb reversibly, while the deformed ones desorb only slowly or not at all.

Table 1). If the effect of the substrate heterogeneity is important, one would expect that the adsorption of smaller dendrimers should be promoted by the presence of these asperities, while the larger dendrimers should be affected to a lesser extent. As a consequence, the regime of partial reversibility would be expected to shrink with increasing generation. Since this trend is not observed, we conclude that substrate heterogeneity may play a role but is negligible in this context. On the basis of AFM imaging of larger generation dendrimers, one knows that adsorbed dendrimers flatten substantially and spread laterally upon adsorption.45,51 This type of deformation is not surprising since a dendrimer is a soft object, and the attractive interactions between the branches of the dendrimer and the surface will lead to its flattening. Computer simulations further indicate that dendrimers assume in the adsorbed state an equilibrium shape resembling a sphere-cap, whose height decreases with increasing strength of dendrimer-surface attractions.52,54 However, this postadsorption deformation process will not be instantaneous, and one expects that the surface area occupied by the adsorbed molecules increases with time. For this reason, we conjecture that dendrimers adsorbed during the initial stages of the experiment had time to fully deform, while the latecomers to the surface remain in a more native configuration. With increasing contact area one expects the desorption rate to decrease, and the residence time of the dendrimer on the surface will increase. On the other hand, a dendrimer having a smaller contact area will desorb more quickly. Simplistically, one may consider two extreme cases. The irreversibly bound fraction corresponds to the fully deformed dendrimers, while the rapidly desorbing fraction corresponds to freshly adsorbed, nondeformed dendrimers. This process is schematically depicted in Figure 5. Obviously, the deformation and adsorption kinetics will be coupled, and the details of these processes are not entirely obvious to us at this point. The importance of similar postadsorption conformational transitions has been discussed in protein adsorption.16,23,65 In particular, desorption transients similar to the ones reported here in the partially reversible regime were observed for proteins.20,21,66 The heterogeneity of adsorbed proteins was equally interpreted in terms of different binding strength of various conformations in the adsorbed state. Proteins may equally flatten in the adsorbed state, and the kinetics of this flattening process was studied by AFM.26 Since proteins have asymmetric shapes, one may equally argue that orientational transitions could be responsible for the irreversibility of the adsorption process. However, this effect is probably of little relevance for PAMAM dendrimers. Clearly, such conformational transitions are certainly much richer for proteins than for dendrimers. Adsorption Maps. To provide an overview of the transition between reversibility and irreversibility, adsorption maps were (65) Van Tassel, P. R.; Viot, P.; Tarjus, G. J. Chem. Phys. 1997, 106, 761–770. (66) Calonder, C.; Tie, Y.; VanTassel, P. R. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 10664–10669.

Figure 6. Adsorption maps summarize the reversibility of the PAMAM dendrimer adsorption for different generations and pH. Three different regimes can be distinguished, namely (i) completely reversible, (ii) partially reversible, and (iii) irreversible. The maps are shown for three different ionic strengths in KCl background electrolyte: (a) 1 M, (b) 0.1 M, and (c) 0.01 M.

constructed in the generation-pH space for three different ionic strengths. For each generation, the pH value transitions from completely reversible to partially reversible regime as well as from partially reversible to irreversible regimes are shown in Figure 6. Two boundaries are obtained, and they separate the parameter space in three regions, namely (i) completely reversible, (ii) partially reversible, and (iii) irreversible.

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Figure 6 illustrates that the system is reversible for low generations and low pH, while it is irreversible at high generations and high pH. The trend with the dendrimer generation can be rationalized since the adsorption strength increases with increasing number of surface-dendrimer contact points. The trend with pH is related to the fact that the surface charge of silica increases with increasing pH, which leads to increasing strength of the electrostatic interaction between the positively charged dendrimers and the negatively charged surface. These maps further confirm that an increase in the salt concentration makes the adsorption more reversible. In Figure 6c, the reversible region (i) is absent, and the irreversible region (iii) dominates. In Figure 6b, region (i) emerges, and the extent of region (iii) is reduced. In Figure 6a, region (i) is extended, while the extent of region (iii) is further reduced. While the transition between regions (i) and (ii) resembles a straight line, the transition between regions (ii) and (iii) is strongly curved. The latter trend is most obvious at low salt (see Figure 6c). At high generations, the transition between regions (ii) and (iii) is located at pH 2-3, and depends only weakly on the pH. For lower generations, however, one observes a transition between these two regions between G3 and G4. A strikingly similar map of dendrimer adsorption was obtained from computer simulations by Mansfield.54 The map obtained from simulations is presented as a function of the dendrimer generation, but the pH used here is replaced by a short-range interaction of strength A between dendrimer branching points and the surface. For small dendrimer generations and small values of A, the regime of complete reversibility, which is referred to as the “desorption regime”, is predicted. For larger values of these parameters, one finds the regime of partial reversibility, which is referred to as the “weak adsorption regime”. However, the transition from partial reversibility to irreversibility appears more complex. For lower generations, conformational states in which all three branches of the dendrimer (i.e., dendrons) are adsorbed seem to be favored, while states with only two branches of the dendrimer are adsorbed and the others are dangling into the solution appear to be important for higher generations. While it is impossible to make any conjectures concerning kinetic phenomena from a Monte Carlo simulation, these results suggest that the experimentally observed spreading behavior is related to the strength of the interactions with substrate. Note that the dendrimers considered in Mansfield’s study54 have three dendrons, while the PAMAM dendrimers studied here have four, which opens up the possibly of additional regimes to be considered.

Longtin et al.

Conclusion The transition from reversible to irreversible adsorption of PAMAM dendrimer at the silica-water interface is studied. The dendrimers are adsorbed from an electrolyte solution of fixed pH, and desorption is initiated upon rinsing with a dendrimerfree solution of identical composition. Three regimes were identified. The completely reversible regime is found only for low generations, low pH, and high ionic strength, and is characterized by rapid and complete desorption. The irreversible regime is found for high generations, high pH, and low ionic strength, and is characterized by no observable desorption within the experimental time window. The partially reversible regime is characterized by a rapidly desorbing fraction and an irreversibly bound fraction. The latter fraction probably desorbs too, but does so extremely slowly. The experimental conditions at which these behaviors were observed are summarized in three adsorption maps. The overall structure of the adsorption maps was found to strongly resemble the predictions obtained from previously published computer simulations by Mansfield.54 Our results suggest that the dendrimers adsorb in a distribution of surface states, which result in different desorption rates. To provide a definitive molecular interpretation of this adsorption heterogeneity is difficult at this point. However, we suspect that the main reason for this heterogeneity is that adsorbed dendrimers undergo conformational transitions at the surface, and that the different desorption rates correspond to the extent of spreading of the dendrimers on the surface. In particular, weakly deformed dendrimers with few contact points will desorb quickly, while strongly deformed and flattened dendrimers with many contact points will desorb slowly. A similar spreading effect is well documented in the protein adsorption literature, and it appears that the desorption process of dendrimers resembles the one of globular proteins.20,21,66 Such postadsorption events could be equally responsible for slow desorption of viruses from mineral surfaces.67 Acknowledgment. This work was supported by the Swiss National Science Foundation and the University of Geneva. Partial support by the COST Action D43 and the Swiss Federal Office for Education and Science within the same Action is equally acknowledged. LA8038818 (67) Ryan, J. N.; Harvey, R. W.; Metge, D.; Elimelech, M.; Navigato, T.; Pieper, A. P. EnViron. Sci. Technol. 2002, 36, 2403–2413.