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Directed Patterned Adsorption of Magnetic Beads on Polyelectrolyte Multilayers on Glass Bethany F. Lyles, Marianne S. Terrot, Paula T. Hammond, and Alice P. Gast* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received February 27, 2004 Here we present a study of patterning micrometer-sized features on glass surfaces with magnetic colloids. We employ a polyelectrolyte foundation composed of five bilayers on the surface of a glass slide. The directed assembly of carboxylate-modified superparamagnetic particles is accomplished by creating patterned regions of opposite charge via polymer-on-polymer stamping. Dot patterns created in a two-dimensional square array produce colloidal clusters with diameters ranging from 1 to 12 µm. Individual magnetic particles were patterned on the glass surface with micrometer-scale precision. Striped patterns also produced lines of magnetic particles varying in spacing and width from 1 to 7 µm.
Introduction Magnetorheological (MR) fluids consist of micrometersized superparamagnetic particles dispersed in a nonmagnetic medium. In the presence of an external magnetic field, these magnetic beads acquire a dipole moment that is proportional to the magnetic field strength. The dipoledipole interactions induce aggregation of the particles, which form chains and cross-linked structures. The rapid structural response of these particles makes them of scientific interest.1-5 The structural changes in MR fluids are accompanied by significant changes in their rheological properties, making MR suspensions of interest for incorporation into such devices as audio speakers and inertial dampers in motors.6 Magnetically controlled suspensions also have potential biomedical applications including ophthalmic surgical aids in the repair of detached retinas and ventricular assist devices in blood pumps.7,8 We have studied a superparamagnetic suspension comprising polymer-encapsulated beads with a magnetite (Fe2O3) domain dispersed in an aqueous medium. In the presence of a magnetic field, these iron oxide domains acquire a dipole moment µ ) 4/3πr3µ0χH, where r is the particle radius, µ0 is the magnetic permeability in a vacuum, χ is the magnetic susceptibility, and H is the external field. The interaction energy between two magnetic beads is U(r, θ) ) (µ2/4πµ0)[1 - 3 cos2(θ)]/r3, where θ is the angle between the applied field and the axis between the bead centers. When the interaction energy of the induced dipoles overcomes the thermal energy, kT, the particles aggregate via dipole-dipole interactions to form magnetic chains. It is of interest to control the arrangements of such chains through their manipulation at surfaces; one means of achieving this goal is through the use of chemically patterned surfaces designed to guide the formation of magnetic particle arrays. * To whom correspondence should be addressed. E-mail: gast@ mit.edu. (1) Promislow, J. H. E.; Gast, A. P. Langmuir 1996, 12, 4095. (2) Promislow, J. H. E.; Gast, A. P. Phys. Rev. E 1997, 56, 642. (3) Hagenbuchle, M.; Liu, J. Appl. Opt. 1997, 36, 7664. (4) Furst, E. M.; Gast, A. P. Phys. Rev. E 2000, 61, 6732. (5) Furst, E. M.; Gast, A. P. Phys. Rev. E 2000, 62, 6916. (6) Raj, K.; Moskowitz, B.; Casciari, R. J. Magn. Magn. Mater. 1995, 149, 174. (7) Dailey, J. P.; Phillips, J. P.; Li, C.; Riffle, J. S. J. Magn. Magn. Mater. 1999, 194, 140. (8) Nethe, A.; Schoppe, T.; Stahlmann, H.-D. J. Magn. Magn. Mater. 1999, 201, 423.
Microcontact printing (µCP) is a powerful technique forming the basis for a revolution in self-assembly-driven soft lithography.9,10 One application of µCP gaining recent attention is the directed deposition of colloids on printed templates carrying specific interaction chemistries.11,12 We have used a combination of directed polyelectrolyte adsorption and chemical templating to guide colloid assembly on polymer templates.13-16 This approach has been used to pattern magnetic materials on gold or SiO2 substrates; in particular, magnetite was deposited on goldcoated SiO2 using selective wetting.17 Also, various magnetic particles including Co, Ni, Fe, and some ferrite complexes were adsorbed onto patterns on oxidized Si wafers.18 Our approach differs in that we deposit charged composite superparamagnetic particles on polyelectrolyte multilayer templates on glass with micrometer precision; polymer-on-polymer stamping (POPS) yields more selective and stable adhesion sites than conventional µCP of self-assembled monolayers. Superparamagnetic particles will only acquire a dipole moment upon the application of an external field; thus, they behave as inert surface layers until needed as anchors to aggregate magnetic particles or control magnetic suspension flow. The construction of polyelectrolyte multilayers on glass patterned by POPS or through selective deposition of multilayers has been demonstrated,19,20 and this approach provides a functional template for the deposition of magnetic particles because a number of electroactive or optically functional systems can be incorporated within the layers. Features as small as 300 nm and as large as (9) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 20022004. (10) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576. (11) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349. (12) Aizenberg, J.; Braun, P. V.; Wiltzius, P. Phy. Rev. Lett. 2000, 84, 2997. (13) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825. (14) Zheng, H.; Lee, I.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 2002, 14, 569. (15) Lee, I.; Zheng, H.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 2002, 14, 572. (16) Zheng, H.; Rubner, M. F.; Hammond, P. T. Langmuir 2002, 18, 4505. (17) Palacin, S.; Hidber, P. C.; Bourgoin, J.-P.; Miramond, C.; Fermon, C.; Whitesides, G. M. Chem. Mater. 1996, 8, 1316. (18) Zhong, Z.; Gates, B.; Xia, Y.; Qin, D. Langmuir 2000, 16, 10369. (19) Jiang, X.; Hammond, P. T. Langmuir 2000, 16, 8501. (20) Jiang, X.; Zheng, H.; Gourdin, S.; Hammond, P. T. Langmuir 2002, 18, 2607.
10.1021/la049486d CCC: $27.50 © 2004 American Chemical Society Published on Web 03/20/2004
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Langmuir, Vol. 20, No. 8, 2004 3029 Table 1. Parameters for Patterned Magnetic Dots figure
dot diameter (µm)
av number of beads per dot
2a 2b 2c 2d 2e 2f
1 2 3 5 8 12
0.8 2.1 6.9 12.5 35 60
Table 2. Parameters for Patterned Magnetic Stripes
Figure 1. Schematic diagram illustrating directed adsorption of magnetic colloids.
50 µm within regular patterns covering 1-5 cm2 of a polymeric surface have been created via POPS. Compared to traditional µCP approaches, POPS allows greater flexibility in the choice of substrate. The controlled adhesion of charged magnetic microparticles on a polymeric substrate provides a reproducible means for preparing ordered arrays of paramagnetic colloids on an optically transparent foundation. Finally, we show that individual magnetic colloids can be deposited in an ordered array with micrometer precision. In the work reported here, a polyelectrolyte multilayer platform capped by a polyanionic surface was created through layer-by-layer assembly21 and stamped with a polycationic pattern using POPS, to form docking sites for the negatively charged magnetic beads. The resulting polycationic patterns consisted of two-dimensional arrays of dots, with dot diameters ranging from 1.3 to 10.0 µm or stripes of widths ranging from 1.3 to 16 µm. The magnetic colloids deposited in round clusters of average diameters from 1 to 12 µm on the dot patterns and in lines of average widths from 1 to 7 µm on the stripe patterns. Such a magnetizable layer can then be used as the foundation to grow magnetic chains in a microfluidic flow. Experimental Section The polyelectrolyte substrate was prepared on a glass slide treated under air plasma (Harrick Scientific PDG-32 plasma cleaner). Aqueous solutions of 20 mM poly(diallyldimethylammonium chloride) (PDAC, Aldrich, MW ) 100 000-200 000 g/mol) and 10 mM sulfonated polystyrene (SPS, Aldrich, MW ) 70 000 g/mol) in Milli-Q water at 0.1 M NaCl were prepared and passed through a 0.22-µm filter. Polyelectrolyte molarities are calculated on a repeat-unit basis. The multilayer assembly was created by sequential immersion of the glass substrate into the polyelectrolyte solutions for 20 min each. Two 1-min water rinses were used after each polyelectrolyte adsorption step, and each bilayer addition cycle was concluded by a 3-min immersion in an ultrasonication bath. A modified slide stainer (Carl Zeiss) equipped with an ultrasonic bath (Advanced Sonic Processing) was used to automate bilayer addition. Five bilayers were added in this manner, ending with the polyanionic (SPS) layer. POPS was used to create a pattern of polycationic PDAC on the polyanionic SPS surface (Figure 1). Chromium photo masks (Advanced Reproductions, Andover) and corresponding silicon masters (Microsystems Technology Laboratory, MIT) were prepared as templates for the poly(dimethylsiloxane) (PDMS, Sylgard 184, Dow Corning) stamps. Two stamp designs created for a previous study14,15 provided an array of dots of different diameters and an arrangement of stripes of various thicknesses and center-to-center spacings. The PDMS stamp was washed with soap and water, rinsed with Milli-Q water, dried under (21) Decher, G. Science 1997, 277, 1232.
figure
av stripe width (µm)
av beads per stripe width
3a 3b 3c 3d
0.9 2.8 4.7 6.9
1 3.5 5 6
nitrogen flow, and treated by air plasma for 20 s before use. The stamp was inked with a 0.25 M PDAC solution at 0.1 M NaCl in a mixed solvent of 75:25 ethanol/water using a cotton-tipped applicator. The stamp was dried under a N2 stream until the solvent had evaporated and then placed on the multilayer surface with slight pressure for 45 s to 1 min. The patterned surface was rinsed with water and dried under nitrogen. Charged magnetic beads were deposited on the patterned surface as illustrated in Figure 1. Carboxylate-modified superparamagnetic microparticles (Seradyn) of diameter d ) 0.8 µm were prepared in a 0.5 wt % solution. The bead solution was placed on the polyelectrolyte surface for a minimum of 1 h. The sample was then thoroughly rinsed with Milli-Q water and dried under nitrogen. To distinguish the beads involved in chain growth from those comprising the patterned surface, chain growth was accomplished using fluorescent magnetic beads. Carboxylate-modified fluorescent superparamagnetic microspheres (Bangs Labs, d ) 0.96 µm), internally labeled through polymeric entrapment with a Dragon Green fluorophore (480-nm excitation, 520-nm emission wavelength), were employed in this study. A mercury arc lamp was used as an excitation source, providing excitation wavelengths from 250 to 800 nm. The fluorescent beads were prepared in a 0.01 wt % solution that was pipetted over the patterned magnetic array and capped with a glass coverslip. The magnetic field was applied using a block neodymium iron boron magnet (Indigo Instruments, 33 mT). Optical micrographs were taken using an analogue chargecoupled device camera mounted to an optical microscope. Colloidal patterns were observed in both reflectance and transmittance mode. Images were digitized and processed using Scion Image for Windows (Version Beta 4.0.2).
Results and Discussion Magnetic colloids were deposited on an ordered square array of printed dots. The horizontal and vertical average separation distances between dots were measured from the center-to-center spacing of the patterned dots. Figure 2 shows images of the magnetic beads deposited in colloidal clusters of controlled diameter ranging from 1 to 12 µm. The image parameters for Figure 2 are delineated in Table 1. We note that the surface selectivity is very high with few particles adsorbed outside the patterns. Magnetic colloids have a large Hamaker constant and tend to adhere even to negatively charged glass surfaces; often surfactants or bovine serum albumin must be added to prevent adsorption. In this work, the multilayer polyelectrolyte surface providing the surface coating is effective at preventing this nonspecific adsorption and may be useful for other situations where particle adsorption is not desired. Figure 2a represents an image with nominally one bead per patterned site. Thus, using a sufficiently small cationic docking site, single magnetic colloids can be made to undergo directed adsorption in a two-dimensional array
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Figure 2. Magnetic colloids deposited on the PDAC dot template (a-f).
on glass. As the patterned site diameter increases, the average number of beads per cluster increases. When the cluster dot diameter becomes larger than 8 µm, as in Figure 2e-f, beads tend to be deposited in a stacked pseudodouble layer to improve packing density. The result is that the average number of beads per site becomes more difficult to evaluate accurately. With a small dot diameter (D < 3 µm), the ratio of the dot diameter (D) to the diameter of the magnetic bead (d), D/d, is approximately equal to the average number of beads per dot. With D > 3 µm, D/d is less than the average number of beads per dot. This trend shows that with small D, beads tend to self-assemble linearly along the deposited dot axis. As D increases, beads span the magnetic dot area to increase packing density. For all dot diameters, the number of adsorbed beads scales with the area of the patterned dot. Adjusting the pH of the colloidal suspension or ionic strength of the polyelectrolyte solutions allows for tuning of the packing density of the beads on the cationic docking sites and may improve the stability and adhesion of the self-organized colloidal patterns.13 The directed adsorption of superparamagnetic carboxylate-functionalized beads into stripes is described in Table 2 and Figure 3. The average stripe width was taken as the full width at half-maximum value from an intensity profile of the stripe. The average spacing between stripes was measured as the stripe midpoint-to-midpoint distance. Figure 3 shows images of patterned stripes of different widths and spacing. The parameters associated with these stripe patterns are summarized in Table 2. Stripes range in thickness from 1 to 7 µm. Stripes of single-bead width are shown in Figure 3a, demonstrating our ability to create linear arrays of magnetic colloids with micrometer-scale precision. In areas patterned with broader PDAC stripes, solid patterning and a high packing density are observed (Figure 3b-d). Patterned magnetic beads have potential applications in microfluidic devices. Under an applied magnetic field, paramagnetic particles will form chains in suspension.1-5 Recently, we have shown how rotating chains of magnetic beads can serve as micromixers in a microfluidic flow.22 To use chains of magnetic particles in a microfluidic device, it is useful to be able to capture them onto specific points on the surface. The patterned superparamagnetic colloidal (22) Biswal, S. L.; Gast, A. P. Phys. Rev. E 2003, accepted for publication.
Figure 3. Magnetic colloids deposited on the PDAC stripe template (a-d).
Figure 4. Chain of fluorescently labeled superparamagnetic particles grown from a 6-µm magnetic cluster under (a) white light and (b) a Hg arc lamp.
arrays serve as nucleation sites for magnetic chains, as illustrated in Figures 4 and 5. Figure 4a shows a magnetic chain aligned in a magnetic field composed of fluorescently labeled magnetic beads in an aqueous solution attached to a 5.6-µm patterned magnetic dot. Figure 4b shows the system illuminated by a mercury arc lamp such that the fluorescent particles are clearly distinguished from the patterned surface and the free beads in solution. Magnetic chains grown in this arrangement may act as molecular speed bumps in flow cells, to allow cross-flow separation
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Langmuir, Vol. 20, No. 8, 2004 3031
anchored to the patterned spots and respond to fluid flow and applied magnetic fields as shown in Figure 5. This creation of permanently anchored magnetic chains opens the way for the formation of interesting microfluidic devices carrying magnetic chains capable of magneticfield-driven mixing, flow actuation, or sieving. Conclusion
Figure 5. Permanently linked chains of superparamagnetic particles anchored to the patterned surface.
of magnetic beads on the basis of size and magnetic susceptibility. One of the exciting applications of patterned surfaces is the ability to grow magnetic chains from them in situ and link them together permanently. We show an example of chains grown by placing a suspension of 0.1 wt % carboxylate-modified superparamagnetic beads over the pattern, applying a magnetic field, and adding PAHg-PEG at a 10 mM concentration. These chains remain
In closing, we show that polymer-on-polymer µCP can be used to pattern micrometer-sized features with superparamagnetic particles on glass surfaces. Dot and stripe geometries show both the ability to tailor the number of magnetic beads per spot and the precision of their placement. These magnetic patterns are useful as foundations for separation apparatuses in microfluidic devices. Acknowledgment. This work was funded by NASA Microgravity Sciences Program Grant NAG3-2832, NSF Grant CTS-9980860, and NSF MRSEC Grant DMR 02-13282. LA049486D
