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Size-Selective Deposition and Sorting of Lyophilic Colloidal Particles on Surfaces of Patterned Wettability Fengqiu Fan Department of Chemical & Biomolecular Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218
Kathleen J. Stebe* Department of Chemical & Biomolecular Engineering, Department of Materials Science & Engineering, Department of Mechanical Engineering, and Department of Biomedical Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218 Received August 27, 2004. In Final Form: December 17, 2004 A highly parallelizable means of positioning or sorting particles in a size-selective manner into arrays is demonstrated based on the placement of particle suspensions on surfaces of patterned wettability and the subsequent evaporation of the suspending solvent. The method relies on creating lyophilic features of dimensions similar to or greater than those of the particles to be arrayed and smaller than those of the particles to be excluded. As the contact line recedes, it fills lyophilic features, creating discrete fluid elements that mimic the underlying lyophilic pattern. The fluid elements have aspect ratios dictated by the contact angle. By adjusting the size of the lyophilic features, the heights of the fluid elements can be adjusted to sequester or exclude particles based on their diameter. The principal interest of this work is its broad applicability. No prior understanding of the particle properties is needed except for the size of the particle and its ability to be suspended in a solvent.
Patterned media created from particle assemblies with periodic spatial variations in complex two- and threedimensional structures have attracted interest because of their potential applications in optics, electronics, biochip devices, and sensors. A number of methods have been reported for preparing such structures, including electrostatically guided deposition of particles on patterned substrates,1-5 flow-induced packing into cavities of controlled dimensions and shape,5 deposition by the Langmuir-Blodgett (LB) technique,6 deposition by gravitational sedimentation,7 and electrophoretic deposition.8 Evaporation of particle suspensions can also be used to convect and assemble particles into ordered structures.9-15 This idea has been extended to include evaporation on surfaces with patterned lyophobicity.16-18 * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Aizenberg, J.; Braun, P. V.; Wilzius, P. Phys. Rev. Lett. 2000, 84, 2997. (2) Kruger, C.; Jonas, U. J. Colloid Interface Science 2002, 252, 331. (3) Jonas, U.; Campo, A.; Kruger, C.; Glasser, G.; Boos, D. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 8. (4) Zheng, H.; Lee, I. L.; Rubner, M.; Hammond, P. Adv. Mater. 2002, 14 (8), 569. (5) Mamak, M.; Coombs, N.; Ozin, G. A. J. Am. Chem. Soc. 2000, 122, 8932. (6) Guo, Q.; Teng, X.; Rahman, S.; Yang, H. J. Am. Chem. Soc. 2003, 125, 630. (7) Pusey, P. N.; Vanmegen, W. Nature 1986, 320, 340. (8) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Nature 2000, 404, 56. (9) As reviewed in: Nguyen, V. X.; Stebe, K. J. Phys. Rev. Lett. 2002, 88, 164501. (10) (a) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (b) Deegan, R. D. Phys. Rev. E 2000, 61, 475. (11) Maenosono, S.; Dushkin, C. D.; Saita, S.; Yamaguchi, Y. Langmuir 1999, 15, 957. (12) Conway, J.; Korns, H.; Fisch, M. Langmuir 1997, 13, 426. (13) Uno, K.; Hayashi, K.; Hayashi, T.; Ito, K.; Kitano, H. Colloid Polym. Sci. 1998, 276, 810. (14) Adachi, E.; Dimitrov, A. S.; Nagayama, K. Langmuir 1995, 11, 1057.
Two challenges in nanotechnology are the creation of particle arrays of two or more objects within a matrix and the sorting of objects. Furthermore, methods to create these arrays or to sort particles are needed that work for heterogeneous objects (proteins, particles functionalized with surfactants or proteins, compound structures, etc.) with complex distributions of surface charge or surface energy. Patterns of heterogeneous colloidal particles have been formed on patterned surfaces by using Coulombic interactions to selectively pattern charged particles.4 Other particle separation techniques include field-flow fractionation (FFF)18,19 and techniques relying on magnetic particles.20,21 There is a growing need in industry and health sciences for the separation of particulate material whose components may include various kinds of macromolecules, including DNA, synthetic polymers, and micronsized particles including biological cells, polymeric or metallic particles, environmental particles, industrial powders, crystallization products, abrasives, etc. In this paper, we present a method that can distribute and sort lyophilic particles by size on surfaces of patterned lyophilicity requiring no a priori knowledge about the particle except that it can be placed in suspension in a solvent that can be removed by evaporation. (15) (a) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H. Nature 1993, 361, 26. (b) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H. Langmuir 1992, 8, 3183. (c) Dushkin, C. D.; Yoshimura, H.; Nagayama, K. Chem. Phys. Lett. 1993, 204, 455. (16) Masuda, Y.; Tomimoto, K.; Kuomoto, K. Langmuir 2003, 19, 5179. (17) Fustin, C.-A.; Glasser, G.; Spiess, H. W.; Jonas, U. Adv. Mater. 2003, 15 (12), 1025. (18) Fan, F.; Stebe, K. J. Langmuir 2004, 20, 3062. (19) Chemela, E.; Tijssen, R.; Blorn, M. T.; Gardeniers, H. J. G. E.; Berg, A. Anal. Chem. 2002, 74, 3470. (20) Siiman, O.; Burshteyn, A.; Maples, J. A.; Whitesell, J. K. Bioconjugate Chem. 2000, 11, 549. (21) Relle, S.; Grant, S. B. Langmuir 1998, 14, 2316.
10.1021/la047856s CCC: $30.25 © 2005 American Chemical Society Published on Web 01/19/2005
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Figure 1. (a-c) A schematic illustration of the selective deposition of particles by size from a suspension containing a mixture of particles on a patterned hydrophilic/hydrophobic surface. (a) A drop containing large and small particles is placed on the surface and evaporates there. (b) Small fluid elements form as the contact line recedes on the hydrophilic regions which sequester only the smaller particles. (c) Only the smaller particles are deposited once the fluid elements dry. (d-f) A schematic of particle sorting (or 2-D sieving) on a patterned surface. (d) A drop containing a suspension of three different sized particles (a, b, and c) is deposited on a surface patterned with three different sized features (A, B, and C). (e) As the contact line recedes, the particles are sorted by size into 2-D bins. (f) The residue of particles left after the drop has evaporated, with only a in A, both a and b in B, and a, b, and c in C.
In recent work, we demonstrated that surfaces of patterned wettability provide templates to deposit particles from an evaporating drop.18 Monodisperse particles were deposited from an evaporating drop on a surface of patterned wettablility and spontaneously arranged in arrays that mimicked the underlying lyophilic features. As the contact line of the drop receded, the water layer became discontinuous, pulling back rapidly from the nonwet regions and filling the wet regions with fluid elements whose heights h were dependent on the lateral dimensions of the feature 2L and the receding contact angle of the parent drop θ according to
tan
θ h ) 2 L
The fluid elements contained particles if the particle diameter was less than the height. If, however, the particles were larger than the height of the fluid element, they were excluded from the wet patch and were pulled backwards with the parent drop. In this work, these ideas are exploited to deposit selectively particles from a suspension of mixed particles and to sort mixed particles into 2-D bins by size. Figure 1a-c illustrates the concept of selectively depositing particles. A drop containing particles of two different sizes is placed on a substrate with patterned lyophilic features (stripes in this schematic) of a given size. The drop is allowed to evaporate. As in the discussion above, as the contact line recedes, it fills the lyophilic features with liquid creating fluid elements of a given height determined
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by the lateral dimension of the feature and the contact angle. In this example, the height of the fluid element is engineered to be intermediate to the diameters of the two particles. Thus, only the small particles enter the fluid elements, while the larger particles are excluded from them and recede with the parent drop. Dip-coated films that are subsequently evaporated or films that are continuously deposited and evaporated can also be used to selectively deposit particles by size. Figure 1d-f presents a schematic of the procedure used to sort particles by size by deposition from a mixed suspension using control over the aspect ratio of the fluid elements. Using microcontact printing, a surface with a series of lyophilic features of different size (from A to C) is formed, surrounded by a lyophobic matrix. A drop of a suspension containing different sized particles (from a to c) is placed on the substrate. As the contact line recedes, the patterned surface acts like a series of two-dimensional sieves. The smallest fluid elements containing only the smallest particles (a) are left on the smallest features (A). Intermediate-sized fluid elements form in the intermediate-sized features (B), containing the particles small enough to be sequestered in this element (a mixture of a and b). The largest fluid elements form on the largest features (C) and contain all of the particles present in the parent suspension (a, b, and c). As these fluid elements evaporate, the particles a, the particles a and b, and all three of the particles in the parent suspension deposit on features A, B, and C, respectively. Figure 2 shows an example of selective deposition of small particles from an evaporating drop placed on a substrate that had been patterned with alternating wet (-COOH terminated)/nonwet (-CH3 terminated) stripes 1 µm wide by soft lithography on a gold surface. A drop containing an aqueous suspension of both 210 nm particles and 810 nm amidine-functionalized polystyrene particles was placed on the substrate. The drop diameter was large compared to the dimensions of the underlying pattern (further experimental details are reported in the Supporting Information). The drop spread to reach its advancing contact angle and evaporated with a fixed contact line until it reached its receding contact angle. During that time, a coffee ring consisting primarily of 810 nm particles formed along the drop contact line (see region I in Figure 2A and enlarged view in Figure 2B). Thereafter, the contact line depinned from this original location, receding across the surface (toward the upper left-hand corner in Figure 2A) at its receding contact angle. When the contact line encountered a wet stripe, it became pinned there, filled it with the particle suspension, then receded backward to the next wet stripe, repeating the process. The height of the fluid element was intermediate to the diameters of the two particles, so only the small particles were deposited (see region II in Figure 2A and the corresponding SEM image in Figure 2C). The receding parent drop was enriched with 810 nm particles, which deposited in a disordered residue after the drop had dried (see region III in Figure 2A and the magnified view in Figure 2D). The height h of the fluid element was estimated to be roughly 180 nm, comparable to the small particle diameter, as inferred from a cylinder of radius L ) 0.5 µm, using the measured value for θ of 40° measured by considering the parent drop as a spherical cap as it began to recede across the surface. A surface with the same pattern was also vertically lifted out of this suspension at a controlled, slow speed (100 µm/min) parallel to the stripes. As the contact line of the suspension receded, fluid elements formed in the hydrophilic regions, sequestering particles small enough
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Figure 2. Optical microscope images of (A) an overview of residue left from an evaporated drop that contained a mixture of 810 and 210 nm polystyrene particles on a surface patterned with 1 µm wide alternating hydrophilic/hydrophobic stripes. (B) A magnified view of region I, the coffee ring region, which is enriched with 810 nm particles. (C) A magnified view of region II, showing that the 210 nm particles are deposited on the hydrophilic stripes (inset: SEM image from another drop under similar conditions). (D) A magnified view of region III, the residue left from the receding parent drop, enriched with 810 nm particles.
to fit them. Once again, only 210 nm particles deposit, forming an array on the striped patterned surface, as shown in Figure 3A and inset. When the same experiment is performed with wider stripes (5 µm), the fluid element height increases, becoming large enough to sequester 810 nm particles as well (see Figure 3B). (The receding contact angle on this surface was 50°; the corresponding height of the fluid elements was estimated to be 1.2 µm.) Similar results can be obtained regardless of the orientation of the pattern to the dip-coating direction (results not shown). This sieving effect can be used to sort particles into 2-D bins by size. In Figure 4A, residue left from an evaporating aqueous drop containing a mixture of particles (0.1 vol % suspension of 810 nm and 2.1 µm amidine-functionalized polystyrene spheres (in a 1:3 ratio)) is placed on a surface patterned with 5 µm and 25 µm square lyophilic patches. Only the small particles deposit on the 5 µm squares forming the clusters, while both the small and large particles deposit on the 25 µm squares to form a coffee ring. These results are of particular interest in biosensor applications when different proteins or other biologically functional materials are to be attached to the various particles.22,23 To illustrate this concept, a mixture of polystyrene particles functionalized with streptavidin (with diameters of 900 nm and 5.46 µm) were deposited. Two sets of experiments were performed. As a control, particles which were not exposed to biotinylated fluorescent dyes were deposited to form a sorted array. The surface was exposed to the excitation wavelengths of the probes, and no fluorescence was observed. Thereafter, the 900 nm spheres were bound to a biotinylated dye (biotin4-fluorescein), which emits green light upon excitation, (22) Ali, M. F.; Kirby, R.; Goodey, A. P.; Rodriguez, M. D.; Ellington, A. D.; Neikirk, D. P.; McDevitt, J. T. Anal. Chem. 2003, 75, 4732. (23) Ferguson, J. A.; Steemers, F. J.; Walt, D. R. Anal. Chem. 2000, 72, 5618.
Figure 3. (A) An optical micrograph of 210 nm spheres deposited by dip-coating a surface patterned with alternating 1 µm hydrophilic/hydrophobic stripes from a mixed suspension of 210 and 810 nm polystyrene particles. Inset: corresponding SEM image. (B) An optical micrograph of both 210 and 810 nm particles assembled on 5 µm stripes; both large and small particles are sequestered on the hydrophilic stripes.
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when excited at the appropriate wavelength, confirming that small particles went to the small features (Figure 4B) and also formed coffee ring structures on large pattern regions. The red fluorescence was only observed in the center on large square features, indicating that the particles had indeed been sorted by size, with the larger particles being excluded from the small features. In conclusion, we have demonstrated that particles can be selectively deposited or sorted into 2-D bins by size in a spontaneous and highly parallelizable fashion. The particles need only be able to be suspended in the solvent from which they are deposited. This approach employs surfaces with patterned lyophilic and lyophobic regions. By adjusting the size of lyophilic patterns, the heights of the discrete fluid elements can be adjusted to sequester or exclude particles based on their diameter, allowing the surface to act as a 2-D sieve. Particles with diameters larger than the height of the fluid elements are excluded from the features, receding backward with the parent drop. Using this method, particles can be deposited and sorted on different regions of a surface by dip-coating or by depositing a drop of the suspension on the patterned surface. The key advantage of this method is its broad applicability. Because the sorting mechanism is based on interactions of the suspending solvent with the substrate, no prior understanding of particle properties is needed beyond their ability to be suspended in a given solvent. The technique is independent of particle details including whether the particles present a heterogeneous surface, the net charge or charge distribution on the particle, or the particle response to applied (e.g. magnetic) fields. This method should be applicable to the creation of arrays for biosensors and in microphotonics.22,23 Figure 4. (A) An optical micrograph of 2.1 µm and 810 nm particles deposited on a surface patterned with alternating rows of 5 µm squares and 25 µm squares. Inset: corresponding SEM image. (B) An overlay of fluorescence images obtained by exciting fluorophores bound to 900 nm particles and 5.46 µm particles deposited on a surface with 5 µm and 25 µm square hydrophilic regions.
Acknowledgment. The authors gratefully acknowledge Ms. Jennifer Maldarelli and Mr. Noshir Pesika for help with sample preparation, the JHU-MRSEC for the use of the SEM, and the support of the Petroleum Research Fund (ACS PRF# 36382-AC9) and the National Science Foundation (NSF-CTS 0244592).
and the 5.46 µm spheres were bound to another biotinylated dye (Alexa-fluor 594 biocytin) that can be excited to emit red light. When this mixture was deposited on the substrate, all elements in the pattern fluoresced green
Supporting Information Available: Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org. LA047856S
