Langmuir 1999, 15, 8565-8568
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Tailored Surfaces Using Optically Manipulated Colloidal Particles C. Mio and D. W. M. Marr* Chemical Engineering Department, Colorado School of Mines, Golden, Colorado 80401 Received May 18, 1999 Optical trapping techniques have been used extensively to manipulate biological objects and micrometersized colloids in a wide variety of investigations. We have used an extension of this technique, scanning laser optical trapping, to simultaneously trap multiple colloids in a designed pattern and have locked-in this artificially created structure through photopolymerization of the monomer-containing solvent. This technique can be used as a means of constructing templates for lithography or as a starting point for creation of larger three-dimensional colloidal structures.
Introduction A number of years ago, Ashkin and co-workers experimentally demonstrated that small particles (25 nm to 10 µm) in solution could be trapped by a single focused laser beam.1 Following their work, many researchers have used optical trapping techniques (also known as optical tweezers) to manipulate micrometer and sub-micrometer sized objects (see, for example, refs 2-4). Because of their nondestructive, sterile nature, optical tweezers have found great success in manipulating biological systems, including bacteria,5,6 viruses,7 chromosomes,8,9 and DNA.10,11 For example, researchers have measured the compliance of bacterial flagella,12 the force generated by RNA polymerase enzyme,13 and the force of molecular motors responsible for cell motility.14,15 Reviews on such biological applications of optical forces can be found in refs.16-18 Optical manipulation techniques have also been used in a variety of applications in other areas of physics and chemistry, such as the measure of colloidal dynamics and interactions, polymer elasticity, and physical properties of membranes and vesicles (see ref 19 and references therein). (1) Ashkin, A.; Dziedzic, J. M.; Bjorkholm, J. E.; Chu, S. Optics Lett. 1986, 11, 288. (2) Ackerson, B. J.; Chowdhury, A. Faraday Discuss. Chem. Soc. 1987, 83, 309. (3) Burns, M. M.; Fournier, J. M.; Golovchenko, J. A. Science 1990, 249, 749. (4) Smith, S. P.; Bhalotra, S. R.; Brody, A. L.; Brown, B. L.; Boyda, E. K.; Prentiss, M. Am. J. Phys. 1999, 67, 26. (5) Mitchell, J. G.; Weller, R.; Beconi, M.; Sell, J.; Holland, J. Microb. Ecol. 1993, 25, 113. (6) Ashkin, A.; Dziedzic, J. M.; Yamane, T. Nature 1987, 330, 769. (7) Ashkin, A.; Dziedzic, J. M. Science 1987, 235, 1517. (8) Berns, M. W.; Wright, W. H.; Tromberg, B. J.; Profeta, G. A.; Andrews, J. J.; Walter, R. J. Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 4539. (9) Seeger, S.; Monajembashi, S.; Hutter, K. J.; Futterman, G.; Wolfrum, J.; Greulich, K. O. Cytometry 1991, 12, 497. (10) Shivashankar, G. V.; Libchaber, A. Appl. Phys. Lett. 1997, 71, 3727. (11) Wang, M. D.; Yin, H.; Landick, R.; Gelles, J.; Block, S. M. Biophys. J. 1997, 72, 1335. (12) Block, S. M.; Blair, D. F.; Berg, H. C. Cytometry 1991, 12, 492. (13) Wang, M. D.; Schnitzer, M. J.; Yin, H.; Landick, R.; Gelles, J.; Block, S. M. Science 1998, 282, 902. (14) Tadir, Y.; Wright, W. H.; Vafa, O.; Ord, T.; Asch, R. H.; Berns, M. W. Fertil. Steril. 1990, 53, 944. (15) Block, S. M.; Goldstein, L. S. B.; Schnapp, B. J. Nature 1990, 348, 348. (16) Svoboda, K.; Block, S. M. Annu. Rev. Biophys. Biomol. Struct. 1994, 23, 247. (17) Sato, S.; Inaba, H. Opt. Quantum Electron. 1996, 28, 1. (18) Ashkin, A. Proc. Natl. Acad. Sci USA 1997, 94, 4853. (19) Grier, D. G. Curr. Opin. Colloid Interface Sci. 1997, 2, 264.
The principle of optical trapping is based on the transfer of momentum from laser photons reflected and refracted by a trapped particle to the particle itself. The net effect is that high-index colloids are pushed toward regions of highest light intensity and a single, well-focused laser beam can trap particles in solution in three dimensions. Optical forces applied to trapped particles are usually divided into two types: the scattering force and the gradient force.1,16 The scattering force is proportional to the light intensity and acts in the direction of the light propagation. The gradient force is proportional to the spatial gradient of the light intensity and acts in the direction of the intensity gradient. Trapping is achieved when the gradient force is larger than the scattering force. This condition occurs only with very steep light gradients, which can be produced by a microscope objective of high numerical aperture (NA). Recently, the simple single-beam trap has been improved to allow simultaneous trapping of several particles in arbitrary patterns by repeatedly scanning the laser at high speed along the desired pattern.20,21 To trap multiple colloids with the same beam, the repetition scanning rate must be faster than the time scale of the particle Brownian motion. Assuming neutral buoyancy, a characteristic time for particle diffusion due to Brownian motion can be estimated using the normalized one-coordinate (x) probability distribution function, f(x,t), derived from the Langevin equation:22
f(x,t) )
exp(-x2/4D0t) (4πD0t)1/2
(1)
where t is time and D0 is the particle diffusion coefficient. We define a characteristic time τ that describes how often one must retrap a particle before it diffuses too far away from its original position. Let a be the particle radius, n the number of radii that particles are allowed to diffuse, and γ the fraction of particles that must remain within na of the origin at time τ. Solving na f(x,τ) dx ) γ ∫-na
(2)
for τ leads to
ν ) τ-1 )
2kT(erf-1(γ))2 3πµa3n2
10.1021/la990610g CCC: $18.00 © 1999 American Chemical Society Published on Web 11/17/1999
(3)
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Figure 1. Video image of 1-µm particles in pure water trapped in rectangular and triangular patterns (scale bar, 10 µm).
using the Stokes-Einstein relation for the diffusivity of an isolated sphere (ν is the scan rate and µ is the solvent viscosity)
D0 ) kT/6πµa
(4)
Equation 3 illustrates the strong dependence of the required scan rate on the particle radius. Using γ ) 0.9 and n ) 0.01, for example, one finds that micrometersized particles require scan rates on the order of 100 Hz to keep them strongly fixed while smaller particles require significantly faster scan rates. We illustrate the ability to trap multiple particles using a single scanning beam in Figure 1 where, using a scan rate of 500 Hz, 1-µm polystyrene colloids (Interfacial Dynamics Corp., Portland, OR) in water are trapped in rectangular and triangular patterns. The ability to reversibly manipulate colloids provides not only an elegant means of conducting thermodynamic investigations but also a new route to the creation of novel structures. Using multiple beams, Misawa et al.23,24 joined five 3-µm particles in a line one at a time by photopolymerizing the interfacial layer between the individual colloids, thus creating a free-floating linear aggregate. Using a single laser and the ability to simultaneously manipulate many smaller colloids however, we can create significantly more complicated colloidal structures and lock-in their morphology to construct tailored surfaces all in one step. We do this by positioning micrometer-size colloids with optical trapping and then freezing the particle pattern through polymerization of the surrounding solvent. This capability may enable the synthesis of new patterned colloidal structures for applications such as materials for photonics, lithography, ceramics, and biochemical sensors.25 Colloid-patterned surfaces could also be used as templates (nucleation seeds) for threedimensional (3D) colloidal crystallization. Other researchers26-29 have relied on the self-assembly of charged colloids to make submicrometer periodic (20) Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Jpn. J. Appl. Phys., Part 2 1991, 30, L907. (21) Visscher, K.; Brakenhoff, G. J.; Krol, J. J. Cytometry 1993, 14, 105. (22) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: New York, 1995. (23) Misawa, H.; Sasaki, K.; Koshioka, M.; Kitamura, N.; Masuhara, H. Appl. Phys. Lett. 1992, 60, 310. (24) Misawa, H.; Sasaki, K.; Koshioka, M.; Kitamura, N.; Masuhara, H. Macromolecules 1993, 26, 282. (25) Dinsmore, A. D.; Crocker, J. C.; Yodh, A. G. Curr. Opin. Colloid Interface Sci. 1998, 3, 5. (26) Asher, S. A.; Holtz, J.; Liu, L.; Wu, Z. J. Am. Chem. Soc. 1994, 116, 4997. (27) Asher, S. A.; Holtz, J.; Weissman, J.; Pan, G. MRS Bull. 1998, October, 44. (28) Jethmalani, J. M.; Ford, W. T.; Beaucage, G. Langmuir 1997, 13, 3, 3338.
materials that will eventually find applications as chemical sensors, optical filters, and optical switches. These periodic materials are composed of body-centered cubic (bcc), facecentered cubic (fcc), or random hexagonal stacked arrays of particles. After their equilibrium structure is achieved, the ordered arrays are permanently locked in place by introducing a monomer in the medium of the colloidal suspension and by polymerizing it into a solid matrix. In addition to controlling equilibrium structure by manipulating colloidal interactions, specific colloidal arrays can be grown via templates. Recently, van Blaaderen and coworkers30 used a two-dimensional (2D) patterned substrate to direct the crystallization of colloidal crystals. Using a template constructed from a polymer layer with holes made with electron beam lithography, they exerted some control over the lattice structure and the orientation of the subsequent crystals formed. Although easy in principle, self-assembly can be difficult to achieve; for example, the colloids must be quite monodisperse and the suspension free of impurities. In addition, lattice morphology is controlled only indirectly by changing the particle concentration and through manipulation of the interactions between individual colloidal particles. These drawbacks can be overcome, however, by using optical trapping to manipulate and move colloids instead of relying solely on their self-assembly capabilities. This Letter illustrates the flexibility of this approach. In this, we have created templates for desired designs by manipulating particles using scanning laser optical trapping (SLOT) and have locked in these artificial patterns by photopolymerization of the surrounding medium. Experimental Details To create the optical traps, we used a continuous-wave 532-nm green laser (frequency-doubled diode-pumped 1064-nm neodymium yttrium vanadate (Nd:YVO4) solidstate laser, Millennia V, Spectra Physics, Mountain View, CA). The beam was Gaussian (TEM00 mode), of diameter 2.5 mm, vertically polarized, and its power can be adjusted from 0.2 to 5 W. During our experiments, however, the laser was never used above 0.6 W to avoid melting the particles. A liquid-crystal variable retarder (LCVR) was used in combination with a horizontal polarizer to further attenuate the laser light power that reached the sample cell. Figure 2 illustrates the optical trap setup where the beam steering and scanning were achieved with a piezoelectric mirror (model S-315.10, Physik Instrumente, Waldbronn, Germany) and two lenses (lens A, focal length ) 500 mm; lens B, focal length ) 125 mm). The lens spacing and mirror placement were set such that constant beam power was maintained during two-dimensional (x-y) beam steering.31 The piezoelectric mirror was controlled by a Macintosh computer, a data acquisition board (PCI-MIO-16E-4, National Instruments, Austin, TX), and software written in Labview (Labview 5.0, National Instruments, Austin, TX), which translated the desired x-y coordinates into voltages used to control the piezomirror. Due to the size of the colloids used in these investigations, typical scanning frequencies ranged between 300 and 700 Hz. The size of the scanned area in the sample was approximately 20 × 20 µm2. The laser beam entered the microscope (Optiphot 150, Nikon, Melville, NY) from the top and was deflected by (29) Sunkara, H. B.; Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1994, 6, 362. (30) vanBlaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (31) Fa¨llman, E.; Axner, O. Appl. Opt. 1997, 36, 2107.
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Figure 2. Experimental apparatus: (1) laser, (2) liquid-crystal variable retarder, (3) horizontal polarizer, (4) piezomirror, (5) lens A, (6) lens B, (7) gold mirror, (8) band rejection filter, (9) CCD camera, (10) microscope objective, (11) sample cell, (12) microscope stage, (13) microscope illumination, visible or UV light.
a 45° gold-coated partial mirror down to the sample cell. The gold mirror partially transmitted the illumination light coming from the bottom of the microscope to a monochrome videocamera (Sony XC-75, Edmund Scientific, Barrington, NJ) placed on top of the microscope. The beam was focused in the sample by an oil-immersion microscope objective, NA ) 1.3 (CFN plan fluor 100×, Nikon, Melville, NY). A 532-nm narrow-band filter above the gold mirror protected the videocamera/eyepiece from the laser beam. A 400-W mercury-vapor lamp (S-363 Sperti Sunlamp, Cooper Hewitt Electric Corp., Enanger, KY) was used to initiate photopolymerization in the sample. The ultraviolet (UV) light was focused in the sample plane from the bottom of the microscope by the same condenser used for the sample visible light illumination. The two types of colloidal particles used for the trapping experiment were monodisperse 3-µm polystyrene particles with sulfate surface groups and surface charge density of 7.5 mC/cm2, and 2.1-µm polystyrene particles with 5.7 × 107 carboxyl surface groups per particle (Interfacial Dynamics Corp., Portland, OR). The solvent was an aqueous solution of monomer (acrylamide, 1.56 M, electrophoresis grade, purity ) 99+%, Aldrich, Milwaukee, WI), cross-linker (methylene bisacrylamide, 0.038 M, purity ) 99%, Aldrich, Milwaukee, WI), and photoinitiator (Darocur 1173, 2-hydroxy-2-methyl-1-phenyl-1-propanone, 0.2 M, purity ) 95-100 wt %, Ciba Specialty Chemicals, Tarrytown, NY). The absorption maximum of Darocur 1173 is 320 nm. All chemicals were used as received without further purification. Experimental Procedure and Results A drop of the colloidal suspension was placed between two round glass cover slips (12 mm in diameter) on a common 1 in. × 3 in. glass microscope slide in the microscope stage. The cover slips were small to allow an easy fit in the AFM sample holder (15 mm in diameter) for subsequent characterization. The bottom cover slip was treated with an octadecylsilicone coating (Glass Clad 18, United Chemical Technologies, Inc., Bristol, PA) to facilitate the removal of the cover slip for sample characterization. During the trapping procedure, the laser beam power and the microscope illumination light were kept as low as possible to prevent photopolymerization from starting. At laser output power 0.2 W (minimum output power),
Figure 3. AFM three-dimensional height image of 2.1-µm particles (height scale 1.6 µm).
Figure 4. AFM three-dimensional height image of 2.1-µm particles (height scale 2 µm).
the laser light reaching the sample was approximately 35 mW, a power which could be decreased with the LCVR. After the particles were trapped in the desired pattern using SLOT, the UV lamp was turned on. Photopolymerization of the solution around the trapped particles occurred very quickly. In fact, after 5-10 s the laser beam could be turned off without the particles diffusing away. The UV was left on for additional 15-30 min to ensure that the entire sample solution polymerized. To facilitate subsequent characterization via atomic force microscopy (AFM), the trapping beam focus was maintained near the bottom of the cell to freeze the system close to the bottom-cover-slip/gel interface. The height of the laser beam focus in the cell (and thus the height of the trapped particles) was changed by moving lens A along the optical path. By simple paraxial theory, a 10-mm change in the distance between the two lenses corresponds approximately to a change of 2 µm in focus height within the sample plane. Following solvent polymerization and removal of the bottom cover slip, the locked-in structures were characterized using tapping mode AFM (Nanoscope IIIa, Digital
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Figure 5. AFM three-dimensional height image of 2.1-µm particles (height scale: 2 µm).
Instrument, Santa Barbara, CA). Figure 3 shows the 3D AFM height image of 2.1-µm particles in a hexagonal pattern after trapping and polymerization of the surrounding solvent. Figures 4 and 5 illustrate other example patterns we have constructed. In these, the particles are not perfectly monodisperse, but this does not prevent the laser from building the patterns, since monodispersity or solution purity is not an issue with optical trapping techniques. The number of particles that we can trap is limited by the scan area (related to the optics setup) and by the size of the colloids, but not currently by the laser power. We
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can now simultaneously trap approximately 50 particles; but by improving our technique, it should be possible to polymerize the solution only locally where the particles are trapped, move to another region, and repeat the procedure as many times as desired. In conclusion, this Letter reports a technique to construct and lock-in nonequilibrium colloidal structures. We first used SLOT to assemble the colloids in a chosen geometry, and then photopolymerized the suspension medium, freezing the particle pattern. The ability to control the assembly of micrometer-sized particles with this “trap-and-lock” technique creates a new means for the synthesis of novel structures and composite materials. Such 2D colloidal arrays can be used as lithographic or etching masks to structure surfaces. Another approach involves their use as nuclei to induce 3D crystal growth. With change of template pattern and particle spacing, a variety of useful array geometries can be created for use in fundamental investigations on heterogeneous crystallization. In addition, template-grown 3D crystalline structures could find practical application as photonic materials for tunable filters, optical switches, or diffraction gratings. Because of the flexibility of SLOT to arrange particles in any desired pattern, the variety of possible structures one could create with this “trap-and-lock” technique is truly unlimited. Acknowledgment. We thank the NSF for support of this research under CAREER Award CTS-9734136. C. Mio acknowledges the “Fondazione Ing. A. Gini” and the “Universita´ di Padova” for their support. In addition, we thank Alex Terray for his assistance and design of the laser control software. LA990610G
