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Selective Self-Organization of Colloids on Patterned Polyelectrolyte Templates Kevin M. Chen,† Xueping Jiang,† Lionel C. Kimerling, and Paula T. Hammond* Department of Materials Science and Engineering and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received February 26, 2000. In Final Form: June 9, 2000 Submicron-sized colloidal particles have been self-organized into patterned arrangements on a substrate using a novel technique. At the substrate, a polyelectrolyte multilayer film has been deposited onto a chemically patterned surface; subsequently, the polyelectrolyte surface is immersed in an aqueous colloidal suspension of bare SiO2 microspheres or functionalized polystyrene latex particles. The colloids selforganize at the surface, driven by the spatially varied electrostatic and secondary interactions between the colloid and the substrate. The polyelectrolyte platform provides a strong bond to the colloids, imparting mechanical robustness which enables postprocessing of the patterned assemblies. An important advantage to this approach is that the use of a polyelectrolyte multilayer platform opens up the possibility of introducing functionality into the underlying layers. We have demonstrated control over the density and selectivity of particle adsorption. Three mechanisms have been used to control adsorption: (i) pH of the colloid suspension, which determines the ionization of the uppermost surface of the polyelectrolyte multilayer; (ii) ionic strength of the suspension, which determines the extent of charge screening about the colloid and polyelectrolyte; and (iii) concentration of added surfactant, which causes charge screening and introduces hydrophobic interactions between the surfactant and polyelectrolyte. Finally, an energy adsorption model is presented.
Introduction The ability to harness intrinsic interactions between surfaces leads to novel and elegant methods for selforganized deposition of particles. The driving forces that direct self-organization include electrostatic interactions, surface tension, van der Waals forces, steric interactions, and capillary forces. Recently, there has been strong interest in the self-assembly of particles utilizing these forces to create ordered structures in two and three dimensions. Electrostatic interactions have been widely employed in the assembly of particles ranging from microto nanoscale dimensions.1-5 Capillary forces and flow fields have been used to organize microspheres in close-packed arrays;6,7 extensive ordered arrays have been demonstrated by Xia et al.8 utilizing forced flow through a cell followed by evaporation. Other techniques include sedimentation and crystallization.8-12 Such systems have a range of potential applications: functional templates and catalysts for chemical and biological processes, sensor arrays, masks for nonlithographic patterning of deposited species, and novel optical materials and photonic crystal * To whom correspondence should be addressed. † These authors contributed equally to this work. (1) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 53495355. (2) Serizawa, T.; Takeshita, H.; Akashi, M. Langmuir 1998, 14, 4088. (3) Schmitt, J.; Machtle, P.; Eck, D.; Mohwald, H.; Helm, C. A. Langmuir 1999, 15, 3256. (4) Yonezawa, T.; Onoue, S.; Kunitake, T. Chem. Lett. 1998, 689. (5) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R. E.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (6) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26. (7) Kim, E.; Xia, Y.; Whitesides, G. M. Nature 1995, 376, 581. (8) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1999, 10, 1028-1032. (9) Donselaar, L. N.; Philipse, A. P.; Suurmond, J. Langmuir 1997, 13, 6018-6025. (10) Miguez, H.; Meseguer, F.; Lopez, C.; Mifsud, A.; Moya, J. S.; Vazquez, L. Langmuir 1997, 13, 6009-6011. (11) Rakers, S.; Chi, L. F.; Fuchs, H. Langmuir 1997, 13, 71217124. (12) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303-1311.
devices. Fabrication of photonic crystals operating at visible wavelengths, for example, benefits from the submicron size scale accessible using self-assembled systems. Moreover, cleverly designed fabrication processes based on colloidal self-organization may lead to photonic crystals that enable waveguiding or optical cavities. Selfassembly is thus a tool of great interest based on the need for an inexpensive approach to controlling the arrangement of small objects over large areas. For many of the above-named applications, it is desirable to control the placement of colloidal spheres on surfaces to obtain geometries of interest for sensors and devices. In the case of colloidal self-organization onto a substrate, the use of patterned self-assembled monolayers (SAMs) for directed self-assembly has been presented as an effective means for creating patterned arrays of micronsized objects at the substrate. In particular, this control was demonstrated by Tien et al.1 using 10 µm Au platelets on 100 µm scale patterned SAMs. This work and others like it13 demonstrate that alternately charged regions of a surface can be used to selectively adsorb charged particles. The stability and adhesion of colloids adsorbed to a molecular monolayer can vary to a large extent, depending on the specific interactions of the particle with the surface and the nature of the monolayer. Adhesion could be controlled by the introduction of underlying polymeric layers between the colloid and the solid substrate. Further, a number of new applications for ordered colloidal arrays could be realized by incorporating functionality into these underlying polymer layers. This important advantage would allow for the creation of new devices, sensors, and other multicomponent systems. As an ideal approach, we present the use of patterned layer-by-layer thin films as functional templates for the (13) Brandow, S. L.; Dressick, W. J.; Dulcey, C. S.; Koloski, T. S.; Shirey, L. M.; Schmidt, J.; Calvert, J. M. J. Vac. Sci. Technol., B 1997, 15, 1818-1824.
10.1021/la000277c CCC: $19.00 © 2000 American Chemical Society Published on Web 09/08/2000
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assembly of particles on the polymer surface. In the electrostatic layer-by-layer assembly process, thin films are formed via the alternate adsorption of oppositely charged polyelectrolytes, as first demonstrated by Decher et al.14-17 Polymer film functionality can be varied through the use of electrically or optically active polyelectrolyte systems. Integration of conducting18-21 or luminescent polyelectrolyte multilayer films,22-26 nonlinear optical dyes,27,28 or electrochemical mediators29-31 with patterned colloidal assemblies might lead to mesoscale components in optical or photonic devices or sensors. The multiple charged nature of the polyelectrolyte chain segments and the interpenetrated, polymeric nature of the film can be used to impart mechanical stability and strong adhesion to oppositely charged particles. Tailoring of the polyelectrolyte surface properties affords control over colloid adhesion. For example, the Rubner research group has demonstrated that surface wettability and density of ionic functional groups in weak polyelectrolyte multilayers are strongly influenced by pH.32,33 On the basis of this understanding, adsorption parameters such as pH and ionic strength might be used as “tuning knobs” to modify the degree of adhesion and packing density of charged particles on the polyelectrolyte film surface via shielding and secondary interactions. Interesting studies have been done on continuous films containing multiple layers of microparticles formed via layer-by-layer assembly of charged colloidal systems34 to create multilayer particle films. Adsorption of single layers of colloidal Au3, SiO2,4,35 and latex particles2 onto continuous polyelectrolyte surfaces has also been accomplished. In these cases, long range dimensional periodic order and (14) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321-327. (15) Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430-1434. (16) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (17) Decher, G. Science 1997, 277, 1232. (18) Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994, 244, 985-989. (19) Fou, A. C.; Onitsuka, O.; Ferreira, M. S.; Howie, D.; Rubner, M. F. Self-Assembled Multilayers of Electroactive Polymers: From Highly Conducting Tranparent Thin Films to Light Emitting Diodes. ACS Meeting Proceedings, Anaheim, CA, Spring 1995; American Chemical Society: Washington, DC, 1995. (20) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115. (21) Liu, Y. J.; Wang, Y. X.; Claus, R. O. Chem. Phys. Lett. 1998, 298, 315-319. (22) Ferreira, M. S.; Rubner, M. F.; Hsieh, B. R. Luminescence Behavior of Self-assembled Multilayer Heterostructures of Poly(phenylene-vinylene); MRS Symposium Proceedings, San Francisco, CA, Spring 1994; Materials Research Society: Pittsburgh, PA, 1995; Vol. 328, pp 119-124. (23) Gao, M.; Richter, B.; Kirstein, S. Adv. Mater. 1997, 9, 802-805. (24) Hong, H.; Davidov, D.; Avny, Y.; Chayet, H.; Faraggi, E. Z.; Neumann, R. Adv. Mater. 1995, 7, 846-849. (25) Hong, H.; Davidov, D.; Tarabia, M.; Chayet, H.; Benjamin, I.; Faraggi, E. Z.; Avny, Y.; Neumann, R. Synth. Met. 1997, 85, 12651266. (26) Balanda, P. B.; Ramey, M. B.; Reynolds, J. R. Macromolecules 1999, 32, 3970-3978. (27) Lenahan, K. M.; Wang, Y. X.; Liu, Y. J.; Claus, R. O.; Heflin, J. R.; Marciu, D.; Figura, C. Adv. Mater. 1998, 10, 853. (28) Li, D.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 1789-1790. (29) Bergstedt, T. S.; Hauser, B. T.; Schanze, K. S. J. Am. Chem. Soc. 1994, 116, 8380-8381. (30) Hanken, D. G.; Corn, R. M. Isr. J. Chem. 1997, 37, 165-172. (31) Wu, A. P.; Lee, J.; Rubner, M. F. Thin Solid Films 1998, 329, 663-667. (32) Shiratori, S.; Rubner, M. F. Macromolecules 2000, 33, 42134219. (33) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309-4318. (34) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (35) Kampes, A.; Tieke, B. Mater. Sci. Eng., C: Biomimetic Supramolecular Systems 1999, 8-9, 195-204.
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regular packing of particles were not achieved. Thus far, patterned arrays of particles have not been demonstrated using the electrostatic multilayer assembly techniques. We have recently found that chemically patterned surfaces may be used to selectively direct the deposition of polyelectrolyte multilayers to specific regions of a surface, enabling creation of patterned polymeric microstructures.36-38 By tuning interactions between polyelectrolyte and surface through the manipulation of pH as well as ionic strength, we were able to control the preferred region of polyelectrolyte deposition.38-41 By exploring the deposition of microparticles on these patterned polyelectrolyte thin films, we expand the possibilities for construction of three-dimensional, complex structures and devices to include micro- to nanoparticle arrays in specific regions within the material. In this paper, we begin an initial examination of charged particle adsorption and assembly on patterned polyelectrolyte surfaces and demonstrate that assembly can be controlled by tuning interactions between the colloid, the underlying patterned polyelectrolyte surface, and an oligoethylene glycol-terminated SAM “resist” surface. We examine and discuss the factors which determine the selectivity and density of colloid adsorption on the patterned polyelectrolyte surfaces: the surface charge density of the particle and the polyelectrolyte surface, and the hydrophobic or hydrophilic nature of these surfaces when adsorbed from aqueous solution. These factors are controlled using three different mechanisms, shown schematically in Figure 1. First, the pH of colloidal dispersions of SiO2 microspheres is adjusted to change the degree of ionization of the weak polyelectrolyte surface (Figure 1a). Second, the ionic strength of SiO2 microsphere dispersions is increased to screen charges on the particles and at the surface (Figure 1b). Finally, the addition of surfactant to charged hydrophobic and hydrophilic polystyrene latex spheres is used to shield charge while increasing secondary interparticle interactions (Figure 1c). In this last case, the role of secondary interactions as well as electrostatic interactions is addressed, and analogies are found between the importance of hydrophobic and electrostatic interactions in the particle/polyelectrolyte systems and in the polyamine/polyacid systems examined in previous work.39,42 Experimental Section Materials. Weak polyelectrolytes; linear poly(ethylene imine) (LPEI; Polysciences; MW ) 25 000) and poly(acrylic acid) sodium salt (PAA; Polysciences; MW ) 20 000). Strong polyelectrolytes: poly(diallyldimethylammonium chloride) (PDAC; Aldrich; MW ) 150 000) and sulfonated polystyrene (SPS; Polysciences; MW ) 35 000). Selfassembled monolayers (SAMs): 16-mercaptohexadecanoic acid (COOH; Aldrich) and 11-mercaptoundecanoic triethyleneglycol (EG; synthesized according to ref 43). Colloids: an aqueous slurry of SiO2 microspheres (0.71 ( 0.05 µm diameter) containing no surfactants was used as (36) Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromolecules 1997, 30, 7237. (37) Clark, S. L.; Montague, M. F.; Hammond, P. T. Supramol. Sci. 1997, 4, 141. (38) Clark, S. L.; Hammond, P. T. Adv. Mater. 1998, 10, 1515. (39) Clark, S. L.; Handy, E. S.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 1999, 11, 1031. (40) Clark, S. L.; Montague, M. F.; Hammond, P. T. ACS Symp. Ser. 1998, 695, 206-219. (41) Jiang, X.-P.; Hammond, P. T. Polym. Prepr. 1999, August. (42) Clark, S. L.; Hammond, P. T. Langmuir, to be submitted. (43) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 583.
Colloids on Patterned Polyelectrolyte Templates
Figure 1. Three mechanisms for regulation of charged colloid adsorption on an oppositely charged substrate region: (a) surface charge density regulated by pH; (b) charge screening by electrolytes; and (c) charge shielding by surfactant.
purchased from Duke Scientific (Palo Alto, CA). Surfactant-free, functionalized polystyrene latex particles were purchased from Interfacial Dynamics Co. (Portland, OR): sulfate latex spheres (type “1-500”, 0.530 ( 0.027 µm diameter) and carboxyl and amide latex spheres (type “91000”, 1.000 ( 0.049 µm diameter). Surfactant: dodecyltrimethylammonium bromide (DTAB, Aldrich). Poly(dimethylsiloxane) (PDMS; Ellsworth Adhesives, Germantown, WI) from a mixture kit was used to form stamps for microcontact printing. Characterization. Atomic force microscopy (AFM) was done using a Nanoscope IIIa in D3000 tapping mode. Scanning electron microscopy (SEM) was done using a JEOL JSM6400. The samples were coated with a ∼200 Å Au film by evaporation. Optical micrographs were taken using a digital camera mounted on the optical microscope. Methods. Figure 2 illustrates the sample fabrication process employed to form colloidal arrays on patterned polyelectrolyte surfaces. Substrate Preparation. A patterned surface with adhesion-promoting and adhesion-resisting regions was prepared as follows. A 1000 Å Au layer was evaporated onto a Si(100) wafer with 100 Å Cr as an adhesion layer. Mercaptohexadecanoic acid (COOH), an acid-terminated SAM, was then stamped directly onto the Au-coated substrate utilizing the microcontact printing method (µCP),44,45 and the sample was immersed in a dilute solution containing mercaptoundecanoic triethylene glycol (EG) to cover the unstamped Au surfaces with a hydrophilic resist SAM. The oligoethylene glycol prevents the (44) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (45) Wilbur, J. L.; Kumar, A.; Biebuyck, H. A.; Kim, E.; Whitesides, G. M. Nanotechnology 1996, 7, 452-457.
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Figure 2. Schematic of sample fabrication process: substrate patterning, polyelectrolyte adsorption, and colloid deposition.
adsorption of charged polyelectrolytes in solution due to the hydrated nature of the monolayer.46-49 Polyelectrolyte multilayer films were deposited on the patterned Au surfaces via alternate adsorption of a polyanion and polycation. The polyelectrolytes used here include poly(diallyldimethylammonium chloride) (PDAC), a strong polycation, and sulfonated polystyrene (SPS), a strong polyanion. Weak polyelectrolyte multilayers were formed from linear poly(ethylene imine) (LPEI), a weak polycation, and poly(acrylic acid) (PAA), a weak polyanion. The polyelectrolytes were deposited from 0.01 M solutions of LPEI, PAA, and SPS and a 0.02 M solution of PDAC. Five and one-half polyelectrolyte pairs were deposited for each multilayer, such that each stack is terminated by the polycationic species. A net positive surface charge is thus presented to the microspheres. The adsorption process was aided by use of a programmable slide stainer. A detailed procedure for polyelectrolyte deposition can be found in previous papers.36,37 Solution pH and ionic strength were adjusted to achieve optimal selective deposition of the polyelectrolyte films. For the LPEI/PAA system, the optimum selectivity of polyelectrolyte adsorption onto the patterned SAM surface occurs at pH ) 4.8 for these weak polyelectrolytes,39 as seen by AFM (Figure 3a). For the PDAC/SPS system, the addition of NaCl electrolyte causes partial screening of charges for the strong polyelectrolyte, resulting in thick, (46) Mrksich, M.; Whitesides, G. M. Using self-assembled monolayers that present oligo(ethylene glycol) groups to control the interactions of proteins with surfaces. ACS Symp. Ser. 1997, 680, 361-373. (47) Mrksich, M.; Dike, L. E.; Tien, J.; Ingber, D. E.; Whitesides, G. M. Exp. Cell Res. 1997, 235, 305-313. (48) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714-10721. (49) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426-436.
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trolyte surface into a dilute latex suspension. The samples were immersed for 1 h, rinsed by agitation in DI water, and dried in air. The original latex suspension was diluted by either DI water or surfactant solution to 0.5 g/100 mL. This combination of colloid concentration and immersion time results in monolayer adsorption saturation. A concentration that is too high can give rise to multilayer deposition. Results and Discussion
Figure 3. AFM images of patterned polyelectrolytes for the (a) weak polyelectrolyte (LPEI/PAA)5LPEI and (b) strong polyelectrolyte (PDAC/SPS)5PDAC multilayer systems.
loopy films and optimal selective adsorption onto the COOH regions36 (Figure 3b). In Figure 3, polyelectrolytes deposited on the 5 µm stripes measure approximately 40 nm thick on the stamped COOH surfaces with minimal deposition onto the EG resist surfaces. We note that the straight edges of the stamped linear patterns are preserved during polyelectrolyte deposition, forming well-defined walls as the polyelectrolyte thickness grows. The different surface morphologies between the weak and strong polyelectrolyte multilayers are attributed to the effect of ionization on polyelectrolyte chain conformations under the conditions used here. Colloid Adsorption. SiO2 colloid adsorption was accomplished by applying a droplet of the SiO2 microsphere slurry directly onto the patterned polyelectrolyte surface. For pH adjustment, substrates coated by the patterned LPEI/PAA weak polyelectrolyte system were used. A droplet of pH-adjusted DI water was placed first on the sample. Another droplet containing the microspheres (which were pH neutral) was then added to the pHadjusted water droplet using a syringe, roughly doubling the volume. The pH of the resulting droplet mixture could then be calculated. An adsorption time of 10 min was allowed for the slurry, which is sufficient to allow sedimentation of a colloid monolayer. The sample was given an agitated rinse in DI water for 1 min and then dried in a stream of compressed air. For ionic strength adjustment, substrates coated by the patterned PDAC/ SPS strong polyelectrolyte system were used. Similarly, several droplets of an ionic solution were placed first on the sample, followed by the colloid slurry. The sample was rinsed and dried in the same manner. Polystyrene latex particle adsorption resulted from immersion of the patterned PDAC/SPS strong polyelec-
It is well-known50 that the SiO2 surface reacts with water to form a layer of silanol (SiOH) groups, thereby giving it a net negative surface charge. When in the presence of an oppositely charged surface, such as that presented by the cationic LPEI and PDAC, the SiO2 spheres undergo an adsorption process driven by electrostatic attraction. If the colloidal slurry is allowed to slowly evaporate on the pattern surface, the thinning liquid layer introduces interparticle capillary forces that overpower the electrostatic attractions between colloidal particles and the surface.6 These capillary forces enable formation of wellpacked colloidal arrays. However, we have observed that, under slow evaporation, a patterned substrate exerts no influence on colloid placement, which results in nonselective deposition. To avoid this problem, the onset of capillary forces is bypassed by agitating the sample under DI water while the colloidal slurry was still contacting the sample surface in droplet form. The agitated rinse removes colloids from the EG-covered surface and leaves those spheres that have adhered electrostatically to the polyelectrolyte. On control samples coated by a continuous EG SAM resist, no deposition was observed. Similarly, for the negatively charged particles, no deposition was observed on multilayers terminated by an uppermost polyanionic species. Effect of pH. By adjusting the pH of the colloidal slurry, it is possible to alter the strength of the electrostatic attraction between the colloid and the surface in weak polyelectrolyte systems. Adjusting the pH alters the equilibrium balance of the ionization reactions COOH T COO- + H+ and NH2+R T NHR + H+ that occur on the polyelectrolyte backbone. Experimentally, a pH-adjusted DI water droplet is placed first on the polyelectrolyte surface to allow an equilibrium degree of ionization. An equal amount of the colloid slurry is mixed into the droplet thereafter. Figure 4 shows the adsorption of SiO2 spheres 700 nm in diameter onto the patterned (LPEI/PAA)5LPEI surface at different pH values. Because the ionization fraction is highly sensitive to pH when near the pKa of weak polyelectrolytes, it is possible to see drastically different adsorption behavior with small changes in pH. At values below the pKa of LPEI (pKa ∼ 6.0-7.0), the polyelectrolyte is highly ionized. When pH ) 6.8 (Figure 4a), there is strong adsorption of the colloids onto the pattern, as manifested by the formation of multilayer SiO2 “clumps” on the striped pattern. Clumping occurs because the highly ionized underlying polyelectrolyte pattern is able to exert an attractive force on colloids located more than one particle diameter above and adjacent to the pattern. These clumps also form due to increased interparticle attraction at this pH. When pH ) 8.1 (Figure 4b), the degree of ionization is lower; the adsorption is thus weaker, characterized by sparser, submonolayer deposition of the colloids. At this pH, there is good separation of spheres between regions with and without the polyelectrolyte due to a reduced attraction to the surface and electrostatic repulsion between the particles. At pH values greater than (50) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.
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Figure 4. SEM micrographs of SiO2 microspheres bound by electrostatic attraction onto underlying (LPEI/PAA)5LPEI at (a) pH ) 6.8 and (b) pH ) 8.1.
the pKa, the ionization fraction diminishes. When pH ) 9.9, colloid adsorption does not occur anywhere on the polyelectrolyte surface (picture not shown). Therefore, by adjusting the degree of polyelectrolyte ionization, it is possible to move between regimes of weak to strong colloid adsorption by electrostatic attraction. Effect of Ionic Strength. For the PDAC/SPS strong polyelectrolyte system, adjustment of electrolyte concentration in the colloid slurry results in varying strengths of adsorption. Figure 5 shows adsorption of SiO2 spheres onto the (PDAC/SPS)5PDAC stripes at different concentrations of NaCl. For reference, Figure 5a illustrates the case when no electrolyte is used. Under these conditions, there is a strong electrostatic attraction between the polyelectrolyte surface and the SiO2 spheres, such that good adsorption selectivity results. When 0.01 M NaCl is added, the counterions form a double layer that screens surface charges from the colloids. The double-layer screening also reduces interparticle repulsion, so that the SiO2 spheres are more likely to form multiple layers. As the electrostatic repulsion between spheres is decreased, the relatively neutral EG surface also becomes a more likely site for deposition of particles on the basis of secondary interactions (e.g., H-bonding between Si-OH and the ether oxygens of the EG monolayer). The net result is decreased selectivity, as shown in Figure 5b for 0.01 M NaCl. Increasing the salt concentration to 0.1 M provides additional surface charge screening. The PDAC/SPS surface still exerts a weak attraction with the SiO2 spheres at this ionic strength, as evidenced by the continued adsorption on the multilayer regions; however, adsorption selectivity is further reduced, as shown in Figure 5c.
Figure 5. SEM micrographs of SiO2 microspheres bound by electrostatic attraction onto underlying (PDAC/SPS)5PDAC in the presence of (a) no NaCl, (b) 0.01 M NaCl, and (d) 0.1 M NaCl.
Studies of sequential adsorption of polyelectrolyte layers show that strong surface screening effects can be observed at 0.4 to 1 M NaCl,36,51 which result in very small adsorbed amounts on the surface. In general, similar observations on deposition are expected upon introducing salt in greater concentrations to colloidal solutions. To the contrary, at 0.2 M NaCl, very dense, nonselective deposition of multiple layers of colloids is observed. Aggregation effects may explain this large-scale deposition onto all available (51) Clark, S. L.; Montague, M. F.; Hammond, P. T. ACS Symp. Ser. 1998, 695, 206.
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surface regions as the suspension aggregation limit is approached upon increasing electrolyte. It is therefore not possible to go to higher ionic strengths. For this reason, manipulation of the pH appears to be a more effective means of controlling the selectivity and density of adsorption in these systems. The degree of ionization of the surface can be altered almost independently of SiO2 surface charge, and the stability of the colloidal suspension is maintained. For both systems of polyelectrolytes, deposition occurred uniformly over the entire surface exposed to the colloidal slurry (∼1 cm2). The deposition process was also repeatable. After the samples had been dried, the adhered colloids withstood repeated dipping cycles in DI water and blowing with compressed air, remaining adsorbed to the surface. Moreover, the colloids could not be stripped off by use of common adhesive tape. Thus, the use of polyelectrolyte multilayers results in well-adhered, stable films of patterned colloidal particles, as suggested previously. Interesting comparisons can be drawn between the systems reported here and the mesoscale charged Au disk systems studied by Tien et al.1 Tien et al. reported that neutral surfaces with low surface energies were effective in preventing adsorption of the charged disks in solution. Different solvent systems were used, with ethanol as the solvent yielding the most selective deposition; in fact, water was found to be a relatively ineffective solvent. It was reasoned that the high dielectric medium of water shielded some of the electrostatic effects of the deposition. We have found that aqueous solutions are very effective for adsorption to charged polyelectrolyte surfaces when appropriate pH and ionic strength adjustments have been made to control surface charge density and minimize particle shielding. Some of these differences may also be attributed to the very high surface charge densities of polyelectrolytes versus those of the SAMs. Importantly, the strong adhesion of colloids to the patterned polyelectrolyte surface, coupled with adhesion tailorability, offers a robust and flexible platform for subsequent processing absent when SAM underlayers are used exclusively. Unlike other systems reported, the EG surface serves as a high-energy (rather than a low-energy) surface, yielding a favorable low interfacial free energy with water that prevents deposition. Interestingly, Tien et al. report that the use of an EG SAM resulted in deposition of positively charged particles and loss of selectivity. This result may be due to specific interactions between EG and certain polyamines, as observed by the authors in polyelectrolyte multilayer work;39 however, these effects were not evident with the negatively charged SiO2 colloidal systems. The use of slow evaporation may have enabled capillary forces to enhance the density of packing on the surface, as compared to the particle density reported in other papers. Tien et al. suggested that surface mobility of particles is one barrier to forming dense, well-packed arrays using electrostatics alone; they suggested the use of capillary forces in conjunction with electrostatics. To optimize the packing of particles, capillary forces might be harnessed or even replaced with packing induced by secondary interactions that exist in surfactant systems. For this reason, colloidal adsorption has also been investigated with polystyrene latex particles treated with surfactants. Effect of Surfactant: Polystyrene Latex Particle Arrays. The deposition of charged particles on oppositely charged surfaces can be adjusted by altering pH and salt concentration in the aqueous phase; however, in most cases, close-packed order between colloidal particles is
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absent. It has been shown that cationic surfactants such as DTAB can be added to negatively charged latex particle suspensions to achieve a higher degree of ordering between particles.52-54 To explore the effects of secondary hydrophobic interactions on particle assembly, we have added a cationic surfactant, DTAB, to solutions of negatively charged particles. In this process, the latex particles are slowly “titrated” with hydrocarbon chains, as shown in Figure 1c, ultimately resulting in a hydrophobic layer of surfactant surrounding the particle surface. As the amount of DTAB is increased, the overall surface charge density is lowered, resulting in shielding effects. The interactions between the hydrocarbon tails of DTAB may induce ordered packing between particles. On the other hand, the decrease in particle surface charge density upon addition of DTAB can lower the adhesion strength of particles on positively charged regions and reduce the selectivity due to shielding and secondary interactions with the EG surface. On the basis of this understanding, it is anticipated that there will be an optimal DTAB concentration for which selectivity is maintained but particle ordering is gained. In any case, the influence of surfactant on the polyelectrolyte multilayer platform can be neglected, as ionically cross-linked polyelectrolyte multilayers are highly stable and are not thought to be susceptible to most aqueous, salt, or solvent conditions. Further, the outermost surface of the multilayer is always of like (positive) charge as the surfactant, such that electrostatic repulsion of the surfactant prevents adsorption and diffusion into the ionically cross-linked polyelectrolyte multilayer. Polystyrene latex particles are widely used and can be functionalized with a variety of moieties. Via surface chemistry, one is able to synthesize various functional groups at the surface and control their density to modify the nature of the particle surface. The polar nature of the particles can also be easily modified to make the particle surface hydrophobic or hydrophilic. For this reason, polystyrene latex particles were selected for this portion of the study. We have chosen two types of negatively charged polystyrene latex particles to enable us to examine the role of secondary interactions between particles. The original latex suspensions are surfactant-free. The first type, denoted as “1-500”, are 500 nm in diameter and functionalized with sulfate and hydroxyl surface groups. Because the pKa is less than 2.0 for the sulfate groups, these particles remain fully charged over a wide pH range. However, due to their hydrophobicity, the particles undergo aggregation at low concentrations of divalent cations. The second particle type, denoted as “9-1000”, are 1000 nm in diameter and very hydrophilic, due to the amide and carboxylic acid surface groups which exist along the backbone of the polymer chains which act as a coating at the surface. The effective surface charge density (from the ionization of carboxylic acid groups) depends on solution pH; however, the particle suspension remains stable over a wide pH range because of the hydrophilic surface. Figure 6 illustrates these two types of latex spheres. The 1-500 particles were used to demonstrate interactions between particles with an inherently hydrophobic surface. The undiluted 1-500 particle suspension contained 8.2 ( 0.1 g of solid/100 mL, which corresponds to 1.0E+12 (52) Velikov, K. P.; Durst, F.; Velev, O. D. Langmuir 1998, 14, 11481155. (53) Denkov, G. S.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (54) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2374 and 2385.
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Figure 6. Functionalized polystyrene latex spheres denoted as (a) “1-500” and (b) “9-1000.”
particles/mL. There are 2.1E+05 charged groups/particle (from manufacturer). The suspension was diluted to 0.5 g/100 mL by DI water and used as a control; calculations indicate a charge density of 1.31E+16 charge groups/mL. The suspension was also diluted to 0.5 g/100 mL using 1E-05, 1E-04, 5E-04, and 1E-03 M DTAB aqueous solutions, corresponding to 6.02E+15, 6.02E+16, 3.01E+17, and 6.02E+17 charge groups/mL, respectively. (The concentrations are far below the critical micelle concentration of DTAB, which is cmc ) 0.016 M.55) It is reasonable to assume that all sulfate groups are charged at close to neutral pH conditions. Parts a-d of Figure 7 show SEM pictures and corresponding optical micrographs as insets of patterned polyelectrolyte (PDAC/SPS)5/PDAC/latex sphere films at zero, 1E-05, 1E-04, and 5E-04 M DTAB concentrations, respectively. Here, we examined patterns with broader EG stripes and narrower polyelectrolyte stripes to distinguish positive or negative deposition. Positive, or normal, deposition refers to the case when negatively charged particles preferentially adsorb onto the positively charged PDAC surface; negative deposition denotes the preferential adsorption of particles to the EG SAM resist layer. As can be seen, selectivity and particle density vary greatly with the introduction of surfactant molecules. In the absence of DTAB, submonolayer adsorption of latex particles on the PDAC surface was obtained (Figure 7a). The neighboring particles at this point are fully charged and do not undergo interparticle contact due to electrostatic repulsion. Upon shielding 46% of the charge on the 1-500 particles at 1E-05 M DTAB, slightly denser deposition was achieved while positive deposition on the polyelectrolyte surface is maintained (Figure 7b). This results from increased particle-particle interactions due to the lowered effective charge density as well as increased hydrophobic interactions between particles. Interestingly, even higher selectivity is obtained in this case, as can be seen by comparing the optical micrograph insets in Figure 7a and b; very few particles adsorb to the EG surface in the partially shielded example. When the extent of shielding is increased to 230% of the particle charge density at 5E-05 M, no selectivity is observed at all (image not shown). In this case, particle deposition occurs on both surfaces in approximately equal amounts. When the particles are completely shielded by the DTAB surfactant, they act as uncharged, hydrophobic particles and exhibit equal affinities for the PDAC and the EG surfaces solely on the basis of secondary interactions. In contrast, when the DTAB concentration is (55) Rosen, M. J. Surfactant and Interfacial Phenomena; John Wiley & Sons: New York, 1989.
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increased to 460% of the particle charge density at 1E-04 M, negative selectivity is observed, as shown in Figure 7c. Particles now preferentially adsorb onto the EG surface, forming a nonuniform layer consisting of locally ordered clusters. As the surfactant concentration is continually increased beyond the point of 100% of the charged groups on the particle surface, additional surfactant molecules present in solution may form a second layer of surfactant around the first. This surfactant bilayer may result in a net positive charge on the particle, which prevents its deposition on the positively charged PDAC surface. At a still higher DTAB concentration of 5E-04 M, adsorption onto the EG surface formed a denser and more uniform layer (Figure 7d). Larger ordered clusters appear. In going from 460% to 2300% of the latex particle charge density, the packing density of particles on the neutral EG surface is gradually increased under conditions of negative deposition. It appears that the additional surfactant assists in the packing of these particles on the neutral EG surface by increasing favorable interactions between particles. When the particles are completely shielded, they are covered with a hydrophobic alkyl shell and undergo favorable interactions with the EG surface. Such results suggest that there are strong dispersion interactions between the DTAB-coated polystyrene latex particles and the EG surface when the charges on polystyrene latex particles are completely shielded by DTAB. Similar sorts of attractions with the EG SAM have been observed for hydrophobic polyamines such as poly(allylamine hydrochloride) in previous studies.39 Partial positive charge from a surfactant double layer could also encourage deposition of both surfactant chains and surfactant shielded particles onto the EG surface, which is known to undergo favorable interactions between its crown ether oxygens and various cations. The SEM pictures show the details of particle packing under different DTAB concentrations. The type and degree of deposition selectivity of these particles depend on the competition between the electrostatic and hydrophobic interactions at the particle/ substrate interface. Additionally, the packing density and order are influenced by the electrostatic repulsion and hydrophobic interactions among particlessinteractions that are governed by surfactant concentration. Due to the hydrophobic nature of the 1-500 particles, reverse deposition was induced when higher concentrations of DTAB were used to promote ordering. We thus turn our attention to particles with a hydrophilic surface. As mentioned earlier, the carboxyl functional surface groups and polyamide surface coating on the 9-1000 particles impart a hydrophilic nature to the surface and minimize nonspecific binding. The undiluted 9-1000 suspension contained hydrophilic latex spheres at a concentration of 4.1 ( 0.1 g of solid/100 mL, which corresponds to 7.4E+10 particles/mL. There are 2.8E+07 charged groups/particle (from manufacturer). The suspension was diluted by DI water to 0.5 g/100 mL, which results in 2.59E+17 charge groups/mL. Assuming pKa ) 5.0 (using the pKa for PAA56) for the carboxyl groups on the latex particles, 60% ionization is expected in a pHneutral environment. There are thus 1.55E+17 charge groups/mL in the diluted suspension; the charge density of the 9-1000 particles is higher than that of the 1-500 particles by 1 order of magnitude. The suspension was diluted to 0.5 g/100 mL by 1E-04, 5E-04, 1E-03, and (56) Tsuchida, E.; Abe, K. Interaction Between Macromolecules in Solution and Intermacromolecular Complexes; Springer-Verlag: Berlin, 1982; Vol. 45.
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Figure 7. SEM micrographs of patterned (PDAC/SPS)5PDAC multilayers immersed in the “1-500” polystyrene latex sphere suspension with (a) 0 M, (b) 1E-05 M, (c) 1E-04 M, and (d) 5E-04 M DTAB surfactant. The insets are optical micrographs.
6E-03 M DTAB solutions, which correspond to 6.02E+16, 3.01E+17, 6.02E+17, and 3.61E+18 charge groups/mL, respectively. The results of adsorption to an PDAC/SPS-patterned electrolyte surface are shown for each case in Figure 8. Excellent selectivity was obtained from the dilute hydrophilic 9-1000 particle suspension without added surfactant (Figure 8a), which indicates that the EG surface acts as a highly effective resist for these hydrophilic particles. The packing is denser than that observed for the similar case with 1-500 particles. Short range ordering exists within clusters, which is unusual considering the electrostatic repulsion among particles with high surface charge density. It is possible that hydrogen bonding between amide groups on the surface layer of the 9-1000 particles is responsible for more ordered packing, since amide groups are excellent hydrogen bond donors and acceptors. At 1E-04 M DTAB (∼39% shielding), the packing density is observed to decrease (Figure 8b). Here, the DTAB surfactant chains may be present in large enough quantities on the particle surfaces to impede hydrogen bonding between amide groups on neighboring particles and ionic interactions between the particles and the PDAC surface. At the same time, there may be insufficient coverage by surfactants to introduce strong hydrophobic interactions between particles. Much of the surface still consists of hydrophilic groups such as amides and both ionized and un-ionized carboxylic acid groups not shielded by DTAB, so that the hydrophilicity is maintained and the selectivity remains positive. At higher DTAB con-
centrations, packing density and ordering increase slightly, at the expense of selectivity, due to the growing hydrophobic nature of the particles (Figure 8c and d). Note that, at these higher amounts of shielding, corresponding to 195% and 390% shielding of the particle surface charge, dense deposition is caused by hydrophobic interactions among particles as the charges become completely shielded. The hydrophobic interactions are observed as reduced selectivity; occasional particles are found adsorbed to the EG surface at the higher DTAB concentrations. Because the hydrophobic shielding groups do not overcome the hydrophilic nature of the surface due to the high density of uncharged amide and acid groups at the surface, selectivity is observed to be positive at all DTAB concentrations. In comparing the results from the 1-500 and 9-1000 particle suspensions, we find that the hydrophilic nature of the particle as well as its surface charge is of great importance for positive selective adsorption. The highly hydrated surface of 9-1000 particles serves to maintain positive deposition over a wide range of surfactant concentrations. Also, 9-1000 particles have a much higher surface charge density (142.3 µC/cm2) than 1-500 particles (3.9 µC/cm2), which strengthens the adsorption of 9-1000 particles on the oppositely charged PDAC surface. Therefore, it is possible to maintain positive selectivity for particles that are hydrophilic in nature while increasing packing density and order by adding surfactant. Under carefully controlled conditions, it may also be possible to achieve highly ordered arrays of patterned particles. It should be noted that the use of polystyrene latex particles
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Figure 8. Optical micrographs of patterned (PDAC/SPS)5PDAC multilayers immersed in the “9-1000” polystyrene latex sphere suspension with (a) 0 M, (b) 1E-04 M, (c) 5E-04 M, and (d) 1E-03 M DTAB surfactant.
Figure 9. SEM micrograph showing single-particle chains of polystyrene latex particles. The particles are 1 µm in diameter, and the width of the underlying polyelectrolyte stripe is 2 µm.
does appear to lower adhesion of the colloids to the surface, as compared to that for the silica colloid systems, when adhesion is tested with an adhesive tape test. It may be possible to improve adhesion through appropriate choice of surfactant, particle, and platform; this concept will be explored in future work. Surface confinement might also provide effects that assist in this ordering. For example, the 9-1000 particles shown in Figure 9 were adsorbed onto striped regions in which the stripe size is of similar dimensions as the particle diameter, resulting in regions with rows of single particles
closely packed on the surface. Such arrangements could provide a route to new materials synthesis of electrical and optical devices. Sequential Adsorption Model. On the macroscopic level, we have demonstrated organization of colloids over large patterned regions. On a smaller scale, however, we find that there is a lack of short range colloidal order, which suggests that the deposition process can be described to the first order by the random sequential adsorption (RSA) process.57 In the basic RSA model, a particle in proximity to a surface can either immediately adsorb to it or bounce away, depending on the availability of adsorption sites. As deposition progresses, the surface continues to be occupied at random sites by the incoming particles until it reaches a saturation point when the available surface sites have been exhausted. The surface “jamming limit” has been computed to be θ ) 0.547,58 where θ is the unit projected surface area of adsorbed particles. In depositions of SiO2 on unscreened, fully charged PDAC/SPS multilayers, we measure colloid surface coverage to be θ ≈ 0.7, which exceeds the computed jamming limit under the above model. The measurement is obtained by counting the number of particles over a unit surface area. The discrepancy between the measured and predicted values of surface coverage demonstrates that colloids are able to pack more closely than predicted under immediate (kinetic) adsorption conditions. We suggest that there is an additional component of colloid surface diffusion following the initial adsorption which allows the colloids (57) Evans, J. W. Rev. Mod. Phys. 1993, 65, 1281. (58) Feder, J. J. Theor. Biol. 1980, 87, 237.
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to migrate laterally before becoming fully adhered near other colloids. Drawing from the classical atomic model of thin film epitaxy,59 we can conceptualize colloid adsorption as occurring on a potential energy landscape. In this framework, an energetic, adsorbed colloid faces a potential energy barrier to lateral diffusion that is characterized by the binding potential, Eb. Since we have modulated the electrostatic interactions between the colloids and the surface in our experiments, we suggest that the potential energy barrier to colloidal diffusion has been reduced to E′b We have also applied mechanical energy to the adsorbed colloids via sonication, Es, which would impart a lateral jump attempt frequency, f, to the colloids. These parameters can be cast in a form relating surface diffusivity to the Boltzmann term f exp[-(E′b Es)/kT]. This model of the observed colloid behavior enables process optimization for colloid lattice perfection. In addition to electrostatic attraction at the surface, another obstacle to surface diffusion is polyelectrolyte morphology. Macroscopic lateral surface undulations on the order of microns exist for the LPEI/PAA system, dimensions that are on the scale of the colloid diameter. On the other hand, the PDAC/SPS system deposits evenly, without noticeable long range surface features. The colloid adsorption behavior onto the two systems is contrasted by the SEM micrographs (Figures 4 and 5). By inspection, the LPEI/PAA samples show SiO2 microspheres that are clumped together, following the underlying polyelectrolyte morphology. Microspheres on the PDAC/SPS samples, however, show a tendency to nestle closer to each other in-plane. Ultimately, achievement of large-area closepacked arrays of colloids on a patterned polyelectrolyte can lead to a hierarchy of novel microstructures. (59) Venebles, J. A.; Price, G. L. Epitaxial Growth; Academic: New York, 1975.
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Conclusions The intrinsic interactions between surfaces offer a rich toolbox for self-assembly of submicron particles. Using a combination of both electrostatic and hydrophobic interactions, we have demonstrated a new technique for the self-organization of SiO2 and polystyrene latex colloids onto a stable, adhesive, patterned polyelectrolyte substrate. The resulting patterned colloidal assemblies are robust to subsequent processing steps. The modulation of surface charge density by means of pH, ionic strength, and surfactant leads to a range of adsorption strengths and deposition selectivities. A significant finding is that, by simply changing adsorption conditions such as pH or ionic strength, the preferred region of adsorption can change drastically; these adsorption parameters can be used as tools in the construction of complex particle/ polymer assemblies. The self-organization technique reported here expands the possibility of fabricating complex micro- and nanostructures by the manipulation of submicron particles via an elegant, self-driven process. Acknowledgment. The authors wish to thank Prof. Michael F. Rubner and Dr. Peter W. C. Wan for insightful conversations regarding the role of pH in controlling polyelectrolyte film thickness, charge density, and conformation. P.T.H. and X.J. wish to acknowledge the MIT CMSE, a center funded by the National Science Foundation, as well as the Office of Naval Research, which provided funding for this work. K.M.C. gratefully acknowledges support by a National Defense Science and Engineering Graduate (NDSEG) Fellowship. LA000277C
