Manipulation of Micrometer-Scale Adhesion by Tuning Nanometer

Langmuir 2006, 22, 1135-1142

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Manipulation of Micrometer-Scale Adhesion by Tuning Nanometer-Scale Surface Features Natalia Kozlova and Maria M. Santore* Department of Polymer Science and Engineering, UniVersity of Massachusetts, Amherst, Massachusetts 01003

Downloaded by UNIV OF CAMBRIDGE on September 12, 2015 | http://pubs.acs.org Publication Date (Web): December 21, 2005 | doi: 10.1021/la0515221

ReceiVed June 8, 2005. In Final Form: NoVember 14, 2005 This article demonstrates how the adhesion rates of micrometer-scale particles on a planar surface can be manipulated by nanometer-scale features on the latter. Here, ∼500-nm-diameter spherical silica particles carrying a substantial and relatively uniform negative charge experienced competing attractions and repulsions as they approached and attempted to adhere to a negative planar silica surface carrying flat 11-nm-diameter patches of concentrated positive charge. The average spacing of these patches profoundly influenced the particle adhesion. For dense positive patch spacing on the planar collector, the particle adhesion was rapid, and the fundamental adhesion kinetics were masked by particle transport to the interface. For patch densities corresponding to a planar surface with net zero charge, particle adhesion was still rapid. Adhesion kinetics were observably reduced for patch spacings exceeding 20 nm and become slower with increased patch spacing. Ultimately, above a critical or threshold average patch spacing of 32 nm, no particle adhesion occurred. The presence of the threshold average patch spacing suggests that more than one positive surface patch was needed for particle capture under the particular conditions of this study. Furthermore, at the threshold, the length scales of the patch spacing and of the interactive surface area between the particle and the surface become similar: The concept of adhesion dominated by the matching of length scales is reminiscent of pattern recognition, even though the patch distribution on the collector is random in this work. Indeed, fluctuations play a critical role in these adhesion dynamics, hence the current behavior cannot be predicted by a mean field approach.

Introduction In the past few years, demands on scientists to design and fabricate surfaces with selective properties have arisen on all sides: tissue scaffolds, sensors, smart adhesives, and separation media. This selectivity is most often achieved, borrowing directly from biology, by the covalent attachment of biomolecular fragments (both polypeptides1-7 and DNA8-11) and the passivation of remaining surface area. The resulting surfaces bind target molecules, often in micrometer and slightly submicrometer patterns1,4,6,11,12 that form the basis for array devices. Although there is much to be gained by the direct incorporation of biomolecules, entirely synthetic systems displaying adhesion specificity offer technical breadth. To this end, the current article, inspired by biological ligand-receptor interactions, explores related behavior in an entirely artificial interfacial system. In nature, the biofunction of ligand-receptor interactions transcends simple pairing of partner molecules:13 the association (1) Blawas, A. S.; Oliver, T. F.; Pirrung, M. C.; Reichert, W. M. Langmuir 1998, 14, 4243-4250. (2) Williams, R. A.; Blanch, H. W. Biosens. Bioelectron. 1994, 9, 159-167. (3) Scouten, W. H.; Luong, J. H. T.; Brown, R. S. Trends Biotechnol. 1995, 13, 178-185. (4) Houseman, B. T.; Huh, J. H.; Kron, S. J.; Mrksich, M. Nat. Biotechnol. 2002, 20, 270-274. (5) Roberts, C.; Chen, C. S.; Mrksich, M.; Martichonok, V.; Ingber, D. E.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 6548-6555. (6) Lee, K. B.; Park, S. J.; Mirkin, C. A.; Smith, J. C.; Mrksich, M. Science 2002, 295, 1702-1705. (7) Chen, C. S.; Mrksich, M.; Huang, S.; Whitesides, G. M.; Ingber, D. E. Biotechnol. Prog. 1998, 14, 356-363. (8) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607-609. (9) Liu, Q. H.; Wang, L. M.; Frutos, A. G.; Condon, A. E.; Corn, R. M.; Smith, L. M. Nature 2000, 403, 175-179. (10) Jordan, C. E.; Frutos, A. G.; Thiel, A. J.; Corn, R. M. Anal. Chem. 1997, 69, 4939-4947. (11) Taton, T. A.; Mirkin, C. A.; Letsinger, R. L. Science 2000, 289, 17571760. (12) Kodadek, T. Chem. Biol. 2001, 8, 105-115. (13) Bell, G. I.; Dembo, M.; Bongrand, P. Biophys. J. 1984, 45, 1051-1064.

(binding) and disbonding reactions follow well-defined dynamic time scales, with the rate constants sensitive to applied force.14,15 Indeed, the function of adhesive biomolecules demands both specificity and dynamic response. As an example of the importance of the latter, the initial contact of neutrophils (a type of white blood cell) on the endothelium near the site of an injury is by a rolling motion, governed by hydrodynamic forces and the forming and breaking of lectin-selectin associations.16-19 Only once neutrophil velocity is reduced by the selectins, from that of the bulk fluid to rolling along the endothelium wall, can integrin receptors bring these cells to complete arrest.20,21 Scientists have yet to capture and exploit such dynamic features at completely synthetic interfaces. Another important feature of natural ligand-receptor bonds, motivating the current study, is their exploitation of pattern recognition: precisely positioned groups exert hydrophobic, van der Waals, electrostatic, polar, and acid-base interactions that contribute angstrom- and nanometer-scale repulsions and attractions at the ligand-receptor junction, depending on molecular orientation and separation. Targeted ligands navigate this complex energy landscape during their approach to receptors, ultimately achieving specificity via pattern recognition. Molecules other than the target are rejected. The current article considers a simplified system of mating interfaces with competing attractions and repulsions and tunable length scales: in the experimental scenario of Figure 1a, a (14) Evans, E. Faraday Discuss. 1998, 1-16. (15) Dembo, M.; Torney, D. C.; Saxman, K.; Hammer, D. Proc. R. Soc. London, Ser. B 1988, 234, 55-83. (16) Alon, R.; Hammer, D. A.; Springer, T. A. Nature 1995, 374, 539-542. (17) Hammer, D. A.; Lauffenburger, D. A. Biophys. J. 1987, 52, 475-487. (18) Brunk, D. K.; Hammer, D. A. Biophys. J. 1997, 72, 2820-2833. (19) Brunk, D. K.; Goetz, D. J.; Hammer, D. A. Biophys. J. 1996, 71, 29022907. (20) Lawrence, M. B.; Springer, T. A. Cell 1991, 65, 859-873. (21) Vonandrian, U. H.; Chambers, J. D.; McEvoy, L. M.; Bargatze, R. F.; Arfors, K. E.; Butcher, E. C. Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 7538-7542.

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Figure 1. (A) A negative micron-scale sphere interacts with a negative surface carrying patches of multiple positive charges, where the patches are substantially smaller than the sphere. At any position, the sphere experiences both attractions and repulsions at different points on its surface. The sphere is currently in an attractive well, and translation of the sphere over the surface results in a height-dependent spatially varying potential. (B) Now small multicharged positive patches are present on both the sphere and planar surface. The sphere is currently in a repulsive maximum, and translation or rotation will cause the potential to vary. If the patch distribution on the sphere were out of registry with that on the plane, then a more complicated potential with weaker attractions would result.

negatively charged particle interacts with an electrostatically patchy surface presenting attractive (positive) and repulsive (negative) domains that are small relative to the particle size. Lateral variations in surface charge (the patches) attract or repel the sphere, depending on its position in the x, y, and z directions. Thus, the approaching sphere experiences a potential energy landscape analogous to that experienced by approaching receptors and ligands. In Figure 1a, the particle experiences this landscape by translating across the surface. This situation is similar to the more classical pattern-recognition situation in Figure 1b where, with a pattern on both sides of the interface, attractions and repulsions result from particle translation and rotation. In this article, we demonstrate how tuning the interfacial features of Figure 1a allows the manipulation of adhesion dynamics, including an adhesion threshold. Adhesion occurs with pattern feature spacing above the threshold but not below it. Hence, we observe a selective behavior for the system in Figure 1a typically associated with the classical pattern recognition scenario in Figure 1b. The experimental model of Figure 1a, although capturing some features of ligand-receptor systems, also reflects real surfaces that display chemical heterogeneity and, on some length scales, roughness. Indeed, even without invoking pattern recognition as part of the explanation, interesting behaviors (for instance, reduced adhesion kinetics) have been attributed to roughness22-24 and chemical heterogeneity.25-29 It is also worth noting that patterned or heterogeneous surfaces in nature (e.g., patchy mineral surfaces30-40 and rafts on the surfaces of cells39,41-44) may not (22) Huang, Y. W.; Gupta, V. K. Macromolecules 2001, 34, 3757-3764. (23) Huang, Y. W.; Gupta, V. K. Langmuir 2002, 18, 2280-2287. (24) Levins, J. M.; Vanderlick, T. K. J. Phys. Chem. 1995, 99, 5067-5076. (25) Huang, Y. W.; Chun, K. Y.; Gupta, V. K. Langmuir 2003, 19, 21752180. (26) Chun, K. Y.; Huang, Y. W.; Gupta, V. K. J. Chem. Phys. 2003, 118, 3252-3257. (27) Tagawa, M.; Yasukawa, A.; Gotoh, K.; Ohmae, N.; Umeno, M. J. Adh. Sci. Technol. 1992, 6, 763-776. (28) Taylor, A. M.; Watts, J. F.; Bromleybarratt, J.; Beamson, G. Surf. Interface Anal. 1994, 21, 697-702. (29) Litton, G. M.; Olson, T. M. Colloids Surf., A 1996, 107, 273-283. (30) Gun’ko, V. M.; Leboda, R.; Turov, V. V.; Charmas, B.; SkubiszewskaZieba, J. Appl. Surf. Sci. 2002, 191, 286-299. (31) Duval, F. P.; Porion, P.; Faugere, A. M.; Van Damme, H. J. Colloid Interface Sci. 2001, 242, 319-326.

approach the spatial regularity of the beautiful interfaces now being synthesized in the laboratory on the micrometer and moderately submicrometer length scales, yet they perform unique functions all the same. This fact argues that programs developing smart surfaces and studies aiming to explain the consequences of heterogeneous surfaces (such as dynamic pattern recognition) can tolerate pattern imperfections. Also worth noting is the relevance of the experimental geometry in Figure 1a to experimental probes such as CFM (colloidal force microscopy),45,46 TIRM (total internal reflectance microscopy),47,48 and AFM (atomic force microscopy)49-51 studies. Here colloidal particles, or an AFM tip with finite curvature, sample a finite surface region containing multiple chemical groups and competing (32) Gun’ko, V. M.; Leboda, R.; Turov, V. V.; Villieras, F.; SkubiszewskaXieba, J.; Chodorowski, S.; Marciniak, M. J. Colloid Interface Sci. 2001, 238, 340-356. (33) Medout-Marere, V.; El Ghzaoui, A.; Charnay, C.; Douillard, J. M.; Chauveteau, G.; Partyka, S. J. Colloid Interface Sci. 2000, 223, 205-214. (34) Rudzinski, W.; Charmas, R.; Piasecki, W.; Prelot, B.; Thomas, F.; Villieras, F.; Cases, J. M. Langmuir 1999, 15, 5977-5983. (35) Mather, R. R.; Taylor, D. Dyes Pigm. 1999, 43, 47-50. (36) Frink, L. J. D.; Salinger, A. G. J. Chem. Phys. 1999, 110, 5969-5977. (37) Drach, M.; Lajtar, L.; Narkiewicz-Michalek, J.; Rudzinski, W.; Zajac, J. Colloids Surf., A 1998, 145, 243-261. (38) Luthi, Y.; Ricka, J.; Borkovec, M. J. Colloid Interface Sci. 1998, 206, 314-321. (39) Karamushka, V. I.; Ul’berg, Z. R.; Gruzina, T. G.; Stepura, L. G. Colloid J. 1998, 60, 301-306. (40) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294. (41) Kasahara, K.; Watanabe, K.; Takeuchi, K.; Kaneko, H.; Oohira, A.; Yamamoto, T.; Sanai, Y. J. Biol. Chem. 2000, 275, 34701-34709. (42) Gomez-Mouton, C.; Abad, J. L.; Mira, E.; Lacalle, R. A.; Gallardo, E.; Jimenez-Baranda, S.; Illa, I.; Bernad, A.; Manes, S.; Martinez-A, C. Proc.e Natl. Acad. Sci. U.S.A. 2001, 98, 9642-9647. (43) Krauss, K.; Altevogt, P. J. Biol. Chem. 1999, 274, 36921-36927. (44) Harder, T.; Scheiffele, P.; Verkade, P.; Simons, K. J. Cell Biol. 1998, 141, 929-942. (45) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 18311836. (46) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239241. (47) Prieve, D. C. AdV. Colloid Interface Sci. 1999, 82, 93-125. (48) Prieve, D. C.; Walz, J. Y. Appl. Opt. 1993, 32, 1629-1641. (49) Drelich, J.; Laskowski, J. S.; Pawlik, M.; Veeramasuneni, S. J. Adh. Sci. Technol. 1997, 11, 1399-1431. (50) Estel, K.; Kramer, G.; Schmitt, F. J. Colloids Surf., A 2000, 161, 193202. (51) Nag, K.; Munro, J. G.; Hearn, S. A.; Rasmusson, J.; Petersen, N. O.; Possmayer, F. J. Struct. Biol. 1999, 126, 1-15.


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forces. The situation in Figure 1a is also relevant to the deposition of colloidal particles on flat patterned surfaces, particularly at the edges of patterns.52 The number of experimental studies using well-defined systems approaching the case in Figure 1a is limited. In one interesting paper,53 Kokkoli probes micrometer-scale hydrophobic and hydrophilic stripes with a hydrophobic silica sphere on an AFM tip. She reports, in addition to images, potentials at different lateral positions perpendicular to the surface stripes, finding nonadditive behavior perhaps as a result of the different range of the hydrophobic and polar interactions. The surface stripe to particle size ratio approached unity more closely than in the current work; however, the observed repulsive barriers should fundamentally relate to adhesion rate constants such as those measured in the current work. In a separate study of micrometerscale particles flowing over micrometer-scale electrostatic stripes, the particle adhesion rate was found to be nonlinear in the nominal attractive area of the surface, which is another example of nonadditivity.54 This article describes our study of the interactions between micrometer-scale (460 nm) particles and planar surfaces containing randomly arranged 11 nm electrostatically adhesive patches of varied density (with the remaining surface area being electrostatically repulsive toward the particles). Rather than AFM, colloidal probe, or surface forces methods, we employ particle deposition from gently flowing solution to assess the interactions from a practical perspective and to demonstrate patternrecognition-like features of the interactions. We report adhesion rates as a function of the patch density and find a critical surface condition needed for adhesion, much like the concept of interfacial pattern recognition.

Technical Background: Design of an Electrostatically Patchy Collector Surface The patchy planar surfaces in this work were made by adsorbing controlled amounts of a cationic polyelectrolyte, poly(dimethylaminoethyl methacrylate), pDMAEMA, onto the negative surface of an acid-washed microscope slide. We envision the patches roughly as round with diameters on the order of 10 nm but flat to the surface55 and randomly placed on the basis of a large bank of data collected in other studies.56-59 First, roundish blobs (rather than substantially elongated or extended chains) are expected in the limit where small amounts (isolated chains) of pDMAEMA adsorb to the silica surface because under the adsorption conditions (which include an ionic strength of 0.026 M and gentle shearing flow) pDMAEMA is a coil rather than an extended chain in free solution.60-62 The fast adsorption rate63,64 and the strong (irreversible) adsorption energetics58 minimize the potential for substantial reconformations (52) Chen, K. M.; Jiang, X. P.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825-7834. (53) Kokkoli, E.; Zukoski, C. F. Langmuir 2001, 17, 369-376. (54) Elimelech, M.; Chen, J. Y.; Kuznar, Z. A. Langmuir 2003, 19, 65946597. (55) Borisov, O. V.; Boulakh, A. B.; Zhulina, E. B. Eur. Phys. J. E 2003, 12, 543-551. (56) Shin, Y.; Roberts, J. E.; Santore, M. M. Macromolecules 2002, 35, 40904095. (57) Shin, Y. W.; Roberts, J. E.; Santore, M. M. J. Colloid Interface Sci. 2002, 247, 220-230. (58) Hansupalak, N.; Santore, M. M. Macromolecules 2004, 37, 1621-1629. (59) Hansupalak, N.; Santore, M. M. Langmuir 2003, 19, 7423-7426. (60) Degennes, P. G.; Pincus, P.; Velasco, R. M.; Brochard, F. J. Phys. 1976, 37, 1461-1473. (61) Dobrynin, A. V.; Colby, R. H.; Rubinstein, M. Macromolecules 1995, 28, 1859-1871. (62) Irurzun, I. M.; Matteo, C. L. Macromol. Theory Simul. 2001, 10, 237243. (63) Hansupalak, N. Ph.D. Thesis, Lehigh University, 2003.

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of segments after they contact the surface so that the free coil conformation should be reflected at least roughly by the adsorbed footprint size. Our particular pDMAEMA sample had a molecular weight of 31 300 (200 monomers) and a polydispersity of 1.1. A hydrodynamic coil radius, RH, of 4.5 nm measured by dynamic light scattering63 approximates the radius of gyration, Rg, to first order for these coils in solution. The adsorbed chain footprint is expected to be similar, with the patch diameter approximated by the free coil end-end distance, 61/2Rg, of 11 nm. Because pDMAEMA is a weak polyelectrolyte, its protonation is pH-dependent: at a pH of 6.1 in this study, pDMAEMA is 70% protonated in free solution,57,65 but because of the relatively dense spacing of these underlying backbone charges, counterion condensation reduces the net backbone charge to a spacing approaching the Bjerrum length of 0.7 nm, or 74 net positive charges/ chain. We also expect the individually adsorbed DMAEMA chains to produce relatively flat (on the order of 1 nm thick) patches, per conventional wisdom for the adsorption of substantially charged polyelectrolytes.55,66 Indeed, NMR studies, under conditions relevant to this study, found adsorbed pDMAEMA chains to lie flat to the surface:56 at pH values of 7 and below, saturated layers (with coverages of 0.4-0.6 mg/m2) are 80% trains (the part of the chain that contacts the surface) and no more than 20% loops and tails. At coverages below 0.1 mg/m2, where interesting adhesion kinetics of silica particles were found in the current work, the adsorbed polymer is 100% trains within detectable limits.56 This means that the polymer patches are relatively flat to the surface, extending only the thickness of the backbone (on the order of 1 nm) into solution. The patch arrangement on the collector surfaces is expected to be random, especially at low patch coverages when chain spacing exceeds the Debye length. Aggregation or clustering of adsorbed chains is improbable because positive backbone charge causes interchain repulsion. Additionally, we observe no aggregation in the bulk at pH 6 over a broad range of ionic strength. A concern in using adsorbed polymer as a component of random surface patterning is that it stays adsorbed to the underlying planar surface. In this work, pDMAEMA was employed at pH 6.1, where we have shown that it does not desorb or exchange with material from solution over a period of days.58 Although salt often facilitates polyelectrolyte desorption,66 in our system at pH 6 or 7 it has almost no effect on the adsorbed amount because of the dense backbone charging.59 Because pDMAEMA chains are so immobile against desorption or self-exchange with other pDMAEMA chains,58 we also expect a low lateral mobility of these chains so that they should stay put when encountered by colloidal particles. In this study, controls employing fluorescent pDMAEMA demonstrated complete patch retention on exposure to particles. The final and central feature of the patchy polymer surface is its electrostatic landscape. It is well known that saturated layers of polyelectrolytes with sufficient backbone charge can reverse the underlying substrate charge.66 This behavior makes possible multilayer structures containing negatively and positively charged polyelectrolytes.67-69 Indeed, saturated pDMAEMA layers on (64) Hoogeveen, N. G.; Stuart, M. A. C.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 133-145. (65) Shin, Y. W.; Roberts, J. E.; Santore, M. J. Colloid Interface Sci. 2001, 244, 190-199. (66) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: New York, 1993. (67) Hoogeveen, N. G.; Stuart, M. A. C.; Fleer, G. J.; Bohmer, M. R. Langmuir 1996, 12, 3675-3681. (68) Ferreira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 806-809.

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Figure 2. Anatomy of a patch. The representation on the left shows the adsorbed polycation, and that on the right emphasizes the charge distribution and downplays the backbone conformation.

silica, in the pH range of the current article, are known to reverse the underlying silica charge completely.57 Although the current work employs sufficiently low pDMAEMA coverages to produce isolated adsorbed coils, we still expect the region in the vicinity of each coil to be positively charged. Indeed, the concept that single chains can contribute electrostatic patches is well established in the cationic flocculant literature.70-76 When adsorbing chains are too short to induce bridging flocculation, the addition of small amounts of densely positively charged polymer to negatively charged colloids can induce flocculation by the electrostatic attraction of the positive patches adsorbed on one sphere to the negative bare surface of a colliding particle. In a previous work, the electrophoretic mobility of pDMAEMA adsorbed on silica spheres (a model approximating the planar surfaces in the current work) was linear in the adsorbed amount of pDMAEMA over the full range of pDMAEMA coverage from bare silica to saturated pDMAEMA layers.57 That observation indicates that each adsorbing chain, from the first chain to adsorb onto a bare silica surface to the last to incorporate into a saturated layer, brings the same net charge to the interface. In the dilute surface limit of just a few pDMAEMA chains on a relatively bare silica surface, positive charge should be localized in the vicinity of the adsorbed chains. Measurements of the electrophoretic mobility and titrations of the charge in the polymer solutions and colloidal dispersions led to calculations of the charge associated with each adsorbed chain.57 We determined that at pH 6 adsorbing pDMAEMA releases sodium ions from silica’s double layer and promotes further ionization of surface silanols, which locally increases the underlying negative charge on the silica (releasing protons). Even with this charge regulation, however, we calculated that each adsorbing pDMAEMA chain brings +28 charges to the surface (through the combined processes of adsorption, counterion release, and silica charge regulation). (The electrokinetically measurable charge is lower because of counterions inside the shear plane.) By comparison, the bare silica surface charge density is 0.16 - Ve charges/nm2 at pH 6. Hence our model surface is described by the scenario in Figure 2, which includes ∼11 nm patches with 28 new positive charges each, on top of a negative surface whose underlying charge density is -0.16/nm2. (There are 14 original negative charges beneath the 28 new positive charges appearing when a chain adsorbs so that at saturation the underlying silica surface is almost exactly overcompensated by the adsorbing polymer and the surface regulation that occurs with adsorption.) In this work, the patch density is varied to give interesting adhesion with particles in solution. Experimental Materials and Methods PDMAEMA with a molecular weight of 31 300 and a polydispersity of 1.1 was a gift from DuPont, supplied in a THF solution. Rotary evaporation was used to replace the original THF solvent (69) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831835. (70) Leong, Y. K. Colloid Polym. Sci. 1999, 277, 299-305. (71) Bohm, N.; Kulicke, W. M. Colloid Polym. Sci. 1997, 275, 73-81. (72) Petzold, G.; Buchhammer, H. M.; Lunkwitz, K. Colloids Surf., A 1996, 119, 87-92. (73) Durandpiana, G.; Lafuma, F.; Audebert, R. J. Colloid Interface Sci. 1987, 119, 474-480. (74) Mabire, F.; Audebert, R.; Quivoron, C. J. Colloid Interface Sci. 1984, 97, 120-136.

with water. The final sample purity was confirmed via 1H NMR spectroscopy. A handful of control runs tracking the presence of pDMAEMA surface patches employed a lightly rhodamine B-tagged version of pDMAEMA. The labeling procedure has been described previously,58 and in the current study, the labeling density was 1 label/13 chains. Of note is that rhodamine was found to be noninvasive in mobility studies of pDMAEMA on silica.58 The planar substrates were the surfaces of microscope slides (Fisher Scientific, Pittsburgh, PA) that had been treated with sulfuric acid and rinsed in a sealed flow cell to produce a pure silica surface that was free from contamination by airborne organics. Previous XPS studies confirmed the removal of sodium, calcium, and other salts from the region near the surface, leaving the exposed silica layer.77,78 This silica layer, measured optically, is on the order of 10 nm thick, with a refractive index of 1.49.79 pH 6.1 ((0.05) buffer solutions (with I ) 0.026 M) were made using 0.0234 M KH2PO4 and adding a very small amount of 0.000267 M NaOH. This solution was diluted as necessary to achieve the dilute buffer concentration (I ) 0.005 M) in this work. We found negligible effects of buffer concentration on pH in this range and therefore did not add acid or base for further pH adjustment. Chargewise-patchy planar surfaces were generated by adsorbing varied amounts of pDMAEMA onto these silica surfaces from a 20 ppm flowing pH 6.1 buffered solution (at ionic strength I ) 0.026 M) using a laminar slit flow cell with a 10 × 40 mm2 slit machined into a black Teflon block and sealed against the microscope slide substrate using an O-ring. Continuous gentle flow (with wall shear rates in the range of 10-50 s-1) maintained a constant bulk solution concentration and defined the mass-transport conditions. Saturated (completely positive and overcompensating the underlying silica substrate) surfaces were generated by allowing the pDMAEMA to adsorb for a few minutes longer than needed to saturate the surface. To generate patchy surfaces, adsorption from gentle shearing flow was allowed to proceed only for a few seconds (a time that was systematically varied to tune the patch density) before the flow was switched back to pure buffer. Adsorbed pDMAEMA chains were aged in this buffer for 10 min before exposure to silica particles in adhesion tests, a time period found to yield fully relaxed pDMAEMA layers (eliminating any potential history dependence). To synthesize colloidal silica particles with fluorescent cores, we modified a procedure80 that was based on the Stober process.81 Tetraethoxysilane (TEOS, Sigma) was freshly distilled before the synthesis to remove all of the aggregates; (3-aminopropyl)triethoxysilane (APTS, Aldrich) was used without purification. Ethanol (200 proof, VWR) and ammonium hydroxide (SigmaAldrich, 25%, analytical reagent quality) were filtered through 0.2 µm filters to remove dust prior the synthesis. In the first step, which produced a “dye precursor,” rhodamine B isothiocyanate (Sigma) was covalently attached to APTS by mixing the two in anhydrous ethanol (with a 25% excess of APTS) and allowing them to react under the nitrogen flow for at least 12 h. In the second step, seed formation, freshly made dye precursor was added to an ammoniaethanol mixture along with TEOS and stirred gently for 10 h. Four steps of growing were then carried out, adding same amount of TEOS and allowing the mixture to react for at least 8 h for each step to place shells of untagged silica progressively over the core. After that, particles were gently centrifuged to remove supernatant with (75) Gregory, J. J. Colloid Interface Sci. 1976, 55, 35-44. (76) Gregory, J. J. Colloid Interface Sci. 1973, 42, 448-456. (77) Rebar, V. A. Ph.D. Thesis, Lehigh University, 1995. (78) Rebar, V. A.; Santore, M. M. J. Colloid Interface Sci. 1996, 178, 29-41. (79) Fu, Z. G.; Santore, M. M. Colloids Surf., A 1998, 135, 63-75. (80) Vanblaaderen, A.; Vrij, A. Langmuir 1992, 8, 2921-2931. (81) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-&.



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unreacted dye and APTS and washed with ethanol two times. Next, rinsed particles were resuspended in an ethanol-ammonia mixture, and another four growth steps were performed. Particles were then washed four times with ethanol and seven times with water. Particles size was characterized using SEM and DLS. Both methods gave similar resultssa diameter of about 460 nm and a low polydispersity of 1-2%. Total internal reflectance fluorescence was used to measure the deposition of the fluorescent-core silica spheres and, in control studies employing fluorescent pDMAEMA, polymer adsorption. With this method, the total internal reflection of a laser inside the planar substrate generates an evanescent wave whose intensity decays normal to the surface, providing an excitation source for fluorophores near the interface. This study employed a TIRF cell inside a Spex Fluorolog II spectrometer, as previously described.58 Excitation light was at 553 nm, and emissions were measured at 573 nm. It is worth noting that an evanescent penetration depth near 100 nm, in our instrument, easily excites labels on a nanometer-thin layer of adsorbed polymer but it also is effective to excite the cores of 460 nm silica particles. Though an evanescent wave with a decay length of 100 nm would appear not to be able to reach into the core of an interfacially adhesive 0.5 µm silica sphere, evanescent light can traverse a thin gap of low refractive index to tunnel into a higher refractive index medium. The evanescent wave tunnels into the sphere and scatters, exciting the rhodamine-containing core. Indeed, the fluorescence signal from interfacially adhesive rhodamine-core silica particles was one of the largest in our experience with TIRF.

Results Particle Adhesion on a Uniformly Positive Attractive Surface. As a reference point in characterizing the adhesion rates of colloidal particles, we examined the deposition of silica particles on microscope slides carrying saturated (full) layers of pDMAEMA with a coverage of 0.45 mg/m2. These polymer layers were adsorbed at pH 6.1, I ) 0.026 M, and under these conditions, the underlying silica surface charge is completely reversed.57 Such positively charged surfaces should be strongly adhesive toward negatively charged particles in solution, and indeed, several laboratories have shown the deposition of silica particles onto adsorbed layers of other polycations.52,82,83 This section demonstrates the transport-limited adhesion of silica spheres to full pDMAEMA layers, confirming the absence of an energy barrier between the negative particles and the relatively uniform positive planar surface. In Figures 3 and 4, the nonfluorescent layers of pDMAEMA have been deposited prior to the introduction of fluorescent silica particles at time zero. Control studies with fluorescent pDMAEMA were performed to confirm that no polymer was removed during the interaction of the layer with particles from solution. Figure 3a presents TIRF data for the adhesion of the 460 nm fluorescent-core silica spheres on the fully adhesive (saturated pDMAEMA) surface. With a bulk concentration of 0.1 wt % and a wall shear rate of 39 s-1, particle adhesion is initially linear in time but levels off within 5 h. Toward the end of the run, pure pH 6.1 buffer was injected, and the particles were not rinsed off the surface, indicating irreversible (for practical purposes) adhesion. Particles could not be removed even when the ionic strength was raised to 1 M or DI water was introduced. Figure 3b further explores the initial particle adsorption kinetics onto saturated pDMAEMA layers, focusing on the influence of particle concentration. In each of these runs, buffer is injected at the arrows, and the lack of signal drop indicates particle retention (82) Kim, D. W.; Blumstein, A.; Tripathy, S. K. Chem. Mater. 2001, 13, 1916-1922. (83) Mamedov, A.; Ostrander, J.; Aliev, F.; Kotov, N. A. Langmuir 2000, 16, 3941-3949.

Figure 3. TIRF data for the adhesion of 460 nm silica spheres onto a planar surface made positive by a saturated layer of pDMAEMA. (A) Continued particle deposition for several hours until the particle deposition rate is substantially reduced. The bulk particle concentration is 0.1 wt %, and the wall shear rate is 39 s-1. (B) Initial deposition kinetics at the same wall shear rate and different bulk particle concentrations. The inset shows the explicit dependence of the deposition rate on the particle concentration. (C) Control runs showing that the particle adhesion rate is independent of bulk ionic strength in the range of 0.005-0.026 M and also that there is no particle adhesion onto bare silica for this batch of 460 nm spheres. Here, buffer is reinjected after 20 min of particle deposition.

on these surfaces. In the inset, it is clear that the initial adhesion rate increases in linear proportion to the bulk particle concentration, one of the signatures of transport-limited deposition. Figure 3c demonstrates that the initial particle deposition rate is independent of ionic strength (in the range of 0.005-0.026 M). (This was also true of the initial pDMAEMA coverage, which is not shown.) Also in Figure 3c, another critical control experiment is shown: the silica particles do not adhere to acidetched slides not carrying pDMAEMA. This confirms the repulsive interaction between a bare collector surface and the silica particles. (This result is nontrivial because we examined several batches of silica particles from commercial sources that failed this critical test.)

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Figure 5. Initial particle adhesion kinetics at three different positive patch densities (10, 14, and 100%) on the planar collector. (The pDMAEMA deposition steps, which were done before the particle adhesion steps, are not shown.) The ionic strength is 0.005 M. Figure 4. Optical micrographs of 460 nm particles adhering to fully positive collectors for runs such as that in Figure 3a, but interrupted at different times for optical microscopy: (A) 3, (B) 10, and (C) 500 min.

Figure 4 presents optical micrographs of surfaces generated in runs such as that in Figure 3A. After in-situ cleaning and rinsing of the microscope slide in the flow cell, a saturated pDMAEMA layer was deposited from a bulk solution of 85 ppm and a wall shear rate of 39 s-1 (at pH 6.1 and an ionic strength of 0.026 M). After 10 min, the layer was exposed to pure buffer for another 10 min. Next, a 0.1 wt % dispersion of 460 nm silica spheres was passed over the surface for the times indicated in parts A-C of Figure 4, after which the flow was switched back to buffer to remove the free particles from the bulk solution. During this final buffer rinse, no particles were removed from the surface, as evidenced by a lack of fluorescence signal drop. At this point, the TIRF cell was carefully dismantled, and the microscope slide was dried and moved to an optical microscope to record the image at the central observation point (corresponding to the mass transfer conditions in the active TIRF area.) Figure 4 therefore shows the adherent particles for different times in the run of Figure 3A. In Figure 4A-C, it is clear that the silica particle coverage increases substantially with increased particle deposition time, as expected. Also of note is the random arrangement of particle deposition and lack of colloidal ordering, though some doublets are present. (Many of these doublets come from the bulk solution though a few result when an incoming particle collides with one already adhered to the surface, as seen in our microscope). At long times (in Figure 4C), there is no evidence for multilayer formation, and indeed none would be expected because the flow cell is oriented vertically so that gravity does not aid deposition. At short times (i.e., Figure 4A), it was possible to count the adherent silica particles. For the run at 3 min, we counted a total of 236 particles per 40 × 50 µm2 (40 × 50 µm2 is the full field, only close-ups are shown in Figure 4), which corresponds to a silica particle coverage of 13.2 mg/m2. For interfacially adherent objects that act as Brownian diffusers in free solution, the Leveque equation84 describes the transportlimited accumulation of these objects on a collector, dΓ/dt, from the steady-state solution to the convection-diffusion equation in a slit shear cell:

dΓ γ 1/3 1 ) DCo 1/3 dt Γ(4/3)9 DL

( )

(1)

Here, Co is the bulk solution concentration, γ is the wall shear rate, D is the bulk solution diffusivity, and L is the distance from the entrance to the point of observation in the cell. On the right side of eq 1 (and only there), Γ(4/3) is the gamma function evaluated for an argument of 4/3. We have successfully applied eq 1 to describe the adsorption kinetics of polymers and proteins in our shear cells.79,85-87 Applying it to our 460 nm silica particles (with a Stokes-Einstein diffusivity of 1.062 × 10-8 cm2/s), we anticipate a deposition rate, dΓ/dt, of 0.0712 mg/m2s for the linear regime of Figure 3A. At 3 min, this corresponds to a coverage of 12.8 mg/m2, which is in excellent agreement (less than 5% error) with the particles counted in the micrograph of Figure 4A, 13.2 mg/m2. Hence, we have every indication that the adhesion rate of 460 nm particles on the positive collector is the transport-limited rate corresponding to the lack of energy barrier between the particle and the surface. The observations also confirm that the 460 nm particles are too small to exhibit hydrodynamic lift88,89 at the interface, which would tend to retard their adhesion. Using the transport-limited regime as a measure of coverage, we note that the ultimate coverage level on the plateau of Figure 3A is approximately 0.42 g/m2, which is substantially less than the coverages of 0.53 and 0.61 g/m2 that correspond to square and hexagonal packing of 460 nm silica spheres on a planar surface, respectively. The coverages corresponding to the initial deposition kinetics are far below these levels, and in the patchy surface studies of the next section, the initial adsorption kinetics characterize the bare interactions between the particles and the patchy collector surfaces. Adhesion on Charge-Patchy Surfaces. Figure 5 shows the initial adhesion kinetics of the silica particles on planar surfaces carrying different amounts of positive patches: 10, 14, and 100%. Here, the % patch loading is arbitrarily defined relative to the saturated pDMAEMA layer of 0.45 mg/m2 and reports how much of the original silica charge is overcompensated for by the adsorbing pDMAEMA. Because the interfacial charge is linear in the amount of pDMAEMA deposited and at pH 6 the saturated pDMAEMA surface almost exactly overcompensates for the underlying silica charge, 50% patches corresponds to a planar surface with net zero charge, even though the substantial charge (84) Leveque, M. A. Ann. Mines 1928, 13, 284. (85) Fu, Z. L.; Santore, M. M. Macromolecules 1998, 31, 7014-7022. (86) Wertz, C. F.; Santore, M. M. Langmuir 1999, 15, 8884-8894. (87) Wertz, C. F.; Santore, M. M. Langmuir 2001, 17, 3006-3016. (88) King, M. R.; Leighton, D. T. Phys. Fluids 1997, 9, 1248-1255. (89) Goldman, A. J.; Cox, R. G.; Brenner, H. Chem. Eng. Sci. 1967, 22, 637-.



Figure 6. Adhesion rates of 460 nm silica particles as a function of patch density for I ) 0.005 M.

is distributed between polymer-rich and silica-rich regions of the surface. We also note that the “percent” description of patch loading is not defined in terms of surface area though the result is actually close. At a saturation coverage of 0.45 mg/m2, the apparent footprint of each chain is 116 nm2 (corresponding to a disk with a diameter of 12.1 nm). This footprint diameter is in good agreement with the 11-nm-diameter light scattering estimate from the free coil size. Whereas in reality adsorbed polymer layers are typically thought of as entangled carpets of chains, as the surface is made more dilute, individual coils should ultimately become distinguishable (by the right experimental probes.) The results of this section are most interesting in the dilute limit where the question of adsorbed chain identity is not blurred by the possibility of chain overlap on the surface. In Figure 5, the particle deposition at 100% patches (full pDMAEMA layer) proceeds at the transport-limited rate per the previous section, indicating fundamentally fast underlying adhesion kinetics between the spheres and the collector. With 14 and 10% patches, however, the deposition rate becomes increasingly slower, indicating a reduction in the fundamental sticking rate of particles as they approach the interface. The finite adhesion rate constants associated with these patchy surfaces reflect an energy barrier resulting from competing attractive and repulsive forces acting on a single particle and the fact that particles approaching the interface in a locally negative region of the collector will have a greater probability of rejection than those more directly approaching the positive patches. Figure 6 summarizes the data in Figure 5 and other runs like them for a bulk solution concentration of 0.1 wt % particles and an ionic strength of 0.005 M. The x axis is linear in the pDMAEMA loading on the collector, represented as “% patches”; and a second x axis shows the average spacing between the centers of the pDMAEMA coils. The y axis shows the particle adhesion rate. Several interesting features appear, starting on the right-hand side of the graph (high or dense patch loading on the collector) where the transport-limited particle deposition rate persists for a substantial range of relatively high patch loadings: even when the surface has substantial negative regions, particles approaching the planar collector can quickly find positive regions of the surface where they adhere. This is the case at 50% patch loading, where transport-limited deposition occurs even though the surface is net neutral, and down to about 25% patch loading, where the surface carries a substantial net negative charge and the average center-center patch spacing is about 19 nm. In this large transport-limited regime of particle deposition, from 25 to 100% patch loading, we observed the adhesion of the 460 nm

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silica particles to be irreversible. That is, like the data in Figure 3, the particles were not displaced from the surface during buffer flow after their initial deposition. The particle adhesion rate becomes noticeably slow (relative to the transport limit) only for average patch spacings of 20 nm or more. As the average spacing between the positive patches is increased and as the surface becomes increasingly net negative, the particle adhesion rate decreases. Ultimately, the particle adhesion rate approaches zero, at a finite value of the average patch spacing, near 32 nm. It is notable that the sloping branch of the data in Figure 6 does not intersect the origin but rather defines a threshold interfacial condition for adhesion. This observation of a threshold length scale is reminiscent of expectations for systems exhibiting pattern recognition. From another perspective, the fact that the data miss the origin suggests that one patch alone on a negative surface (with the charge densities and potentials in the current study) is not sufficient to trap and hold a particle (otherwise there would be finite particle accumulations and measurable rates for all patch loadings greater than zero). Hence the adhesion of particles in our system relies on spatial fluctuations in the patch placement on the surface: particles tend to adhere selectively to regions of the surface containing a higher than average density of patches. Another interesting observation for the data in Figure 6 was that that in the limit of slow particle adhesion (for average patch densities of about 15% and below) the particle adhesion was reversible. That is, for the most negative surfaces to which particles adhered, they could be washed off in a matter of minutes simply by flowing pH 6 buffer (with I ) 0.005 M). This observation is consistent with the concept of patch-density-dependent adhesion and de-adhesion rate constants for the particles: particle adhesion onto more positive surfaces is characterized by a relatively large adhesion rate constant and apparently irreversible adhesion (slow de-adhesion rates). The more negative the surface, the slower the forward adhesion reaction and the more accessible the deadhesion kinetics (the subject of a future work). The distinct on-off rate processes parallel the discussion of cell adhesion in the Introduction. Following up on the concept of a threshold surface length scale, we illustrate in Figure 7 the critical consideration of determining whether a particle will adhere to a patchy surface: the amount of lateral area on the collector a particle can see as it approaches a surface. Figure 7 proposes to estimate the maximum size of this “zone of influence” (on the collector) based on electrostatics. The particle is placed in hard contact with the surface, and the zone of influence is defined as the area on the planar surface that is intersected by an imaginary shell around the particle, corresponding to the Debye length. For a particle size of 460 nm and a Debye length of 4 nm (corresponding to I ) 0.005 M buffer) Rzi ) 43 nm. Alternately, one might choose to include the Debye layer near the planar surface in the calculation: if once considers the intersection of the Debye layers of the sphere and the planar surface, one finds Rzi ) 62 nm. The two estimates are always the same within a first order multiplicative constant. Regardless of the details of how one chooses to define the zone of influence, above, its radius approaches the average spacing between the patches at the adhesion threshold (Rzi ) 40-60 nm, average patch spacing 30 nm), further supporting the concept that spatial fluctuations in the patch arrangement are key to particle adhesion. If one considers a circle of radius Rzi (43 nm as an example) on the collector, then one calculates an average of 5.5 patches in this zone at the adhesion threshold of 11% patches. Each patch carries a net 14 positive charges (28 new positive


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Figure 7. Definition of the zone of influence and its radius, Rzi. Here a sphere of radius Rp contacts the patchy planar surface under conditions that give a Debye length, κ-1. Rzi is calculated according to right triangles: Rzi2 + Rp2 ) (Rp + κ-1)2.

charges from adsorption minus 14 original negative silica charges beneath the patch). (This calculation does not include any perturbations of the patch charge or counterions during the approach of the sphere, compressing the gap.) Despite the presence of these 5 or 6 patches in a 1500 nm2 zone of influence, the average surface character is substantially net negative. In fact, 25 patches would be required to neutralize the zone of influence completely. If one assumes that particle adhesion requires at least a net neutral region of the surface, then at the adhesion threshold particles adhere to localized regions with more than 2 to 3 times the average positive patch density. As the radius of the zone of influence approaches the average interpatch spacing (at the adhesion threshold), such fluctuations become more probable compared to the situation, for instance, when the zone of influence is much larger or smaller than the average patch distance. The details of how surface fluctuations affect adhesion will be the topic of an upcoming paper.

Discussion and Summary This work demonstrates how nanometer-scale surface features can be tuned to manipulate the adhesion of micrometer-scale objects: the adhesion of 0.5 µm silica particles was a strong function of the surface density of 11 nm positive patches on a negative collector surface. Whereas other works in the literature, particularly that of Kokkoli,53 probe the impact of surface features on a sphere-plate potential, the current work differs in that our particle size and surface features are on a dramatically different relative scale.

KozloVa and Santore

In this work, we argue that the critical length scale to be considered is not the particle size but the size of the interactive zone between the sphere and the plate, compared with the scale of the features on the planar surface. For the electrostatic system studied here, these length scales are well defined and approach each other at conditions critical for adhesion. Indeed, it was the presence of the adhesion threshold that comprised the major finding in this work because it gives rise to potentially selective adhesive behavior. Although we were able to estimate the important length scales in this adhesion problem, we did not quantitatively address the local electrostatics of a negative sphere approaching a chargewisepatchy surface or, for that matter, the simpler case of a negative sphere approaching a single positive patch on a negative plane. Estimates of charge were based exclusively on the single interface scenario: we do know the negative charge on individual silica surfaces and have a good idea of the charge in the local vicinity of each patch before a sphere approaches. From this perspective, we find rapid adhesion of negative spheres to neutral and even net negatively charged surfaces. This simplified view, however, does not account for the possibility of changes in local charge and potential as the sphere approaches the plate. Whereas one might expect adjustments in counterions to reduce repulsions, it is not clear that a surface with 25% positive patches would be attractive (in a mean field sense) toward negative spheres. Indeed, in the limit of sparse positive patch density, the repulsive silicasilica potential will prevent adhesion, so interesting physics occur at the adhesion threshold where the potential begins to deviate from that of well-established electrostatic repulsion. At the adhesion threshold, spheres begin to stick to a surface, which is substantially net negative. With about 30 nm between positive patches and a net positive charge of +14 per patch, even adjustments in counterions are unlikely to produce an attractive potential in the mean field sense. We therefore strongly believe that fluctuations in patch density, which must exist because the patches were deposited in a random fashion, must be important at the threshold. This hypothesis is consistent with the observation that the average patch spacing approaches the size of the zone of influence at the threshold: under these conditions, the number of patches seen by an approaching sphere will be widely varied about the average. By contrast, one might imagine a large zone of influence that samples more of an average surface character, minimizing the influence of such fluctuations. The quantitative length-scale arguments in this work have motivated additional studies in our laboratory to probe systematic variations in the size of the zone of influence, quantitative predictions of adhesion from simple fluctuation theory, and the selective nature of adhesion to these surfaces. Acknowledgment. This work was supported by NSF grants CTS0242647 and CTS-0428455. pDMAEMA was generously supplied by H. Spinelli and M. Wolfe of DuPont. LA0515221

Manipulation of Micrometer-Scale Adhesion by Tuning Nanometer

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