Anal. Chem. 2008, 80, 2628-2630
Cross-Type Optical Particle Separation in a Microchannel Sang Bok Kim, Sang Youl Yoon, Hyung Jin Sung,* and Sang Soo Kim*
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea
A continuous, real-time optical particle separation, which was previously delineated theoretically, is successfully implemented experimentally for the first time. In this method, particles suspended in a flowing fluid are irradiated with a laser beam propagating in a direction perpendicular to direction of fluid flow. Upstream of the laser beam, the particles move parallel to the direction of fluid flow. When the particles pass through the laser beam, the scattering force pushes them in the direction of laser beam propagation, causing the particles to be displaced perpendicular to the fluid flow direction. This displacement, known as the retention distance, depends on the particle size and the laser beam parameters. Finally, the particles escape from the laser beam and maintain their retention distances as they move downstream. In the present work, the trajectories and retention distances of polystyrene latex microspheres with three distinct diameters were monitored and measured using cross-type optical particle separation. The measured retention distances for different-sized particles were in good agreement with theoretical predictions. Since the first observation of radiation forces on micro-objects by Ashkin,1 such forces have been successfully applied in many research fields.2-4 The best-known application of radiation forces is optical tweezers, which are now widely used in biological cell manipulation.4 Optical tweezers are also used to separate particles; however, although optical tweezers can manipulate single particles, an additional automatic laser beam stirring system5 and holographic optical tweezers are required to separate many particles in a single experiment.6 Other methods have been used to sort particles based on particle size and properties.7-9 Imasaka * To whom correspondence should be addressed. E-mail:
[email protected] (H.J.S.);
[email protected] (S.S.K.). (1) Ashkin, A. Phys. Rev. Lett. 1976, 24, 156-159. (2) Ashkin, A.; Dziedzic, J. M.; Bjorkholm, J. E.; Chu, S. Opt. Lett. 1986, 11, 288-289. (3) Gauthier, R. C. Opt. Laser Technol. 1997, 29, 389-399. (4) Ashkin, A.; Dziedzic, J. M. Science 1987, 235, 1517-1520. (5) Wang, M. M.; Tu, E.; Raymond, D. E.; Yang, J. M.; Zhang, H.; Hagen, N.; Dees, B.; Mercer, E. M.; Forster, A. H.; Kariv, I.; Marchand, P. J.; Butler, W. F. Nat. Biotechnol. 2005, 23, 83-87. (6) MacDonald, M. P.; Spalding, G. C.; Dholakia, K. Nature 2003, 426, 421424. (7) Murthy, S. K.; Sethu, P.; Vunjak-Novakovic, G.; Toner, M.; Radisic, M. Biomed. Microdev. 2006, 8, 231-237. (8) Das, C. M.; Becker, F.; Vernon, S.; Noshari, J.; Joyce, C.; Gascoyne, P. R. C. Anal. Chem. 2005, 77, 2708-2719.
2628 Analytical Chemistry, Vol. 80, No. 7, April 1, 2008
et al. developed a simple optical particle separation method, termed optical chromatography,10 which can separate many particles simultaneously in a single experiment. In optical chromatography, the scattering force, a radiation force that acts in the direction of laser beam propagation, and the fluid drag force acts on particles in the opposite direction. Thus, when the scattering and drag forces are equal, the particles remain stationary. Because the scattering and drag forces depend on the particle size, particles can be separated according to their size.10 Furthermore, the scattering force also depends on the optical properties of particles; hence, particles of the same size can be separated according to their optical properties.11 For biological cells, optical chromatography is preferable to other optical tweezers because it uses a loosely focused laser beam, thereby minimizing radiation-induced damage to cells. Hart et al. showed that even closely related bacteria can be separated by optical chromatography.12 In optical chromatography, the separated particles remain stationary and other procedures are required to further analyze the separated particles or biological cells. To improve the conventional optical chromatography technique, Kim et al. devised a new technique known as cross-type optical separation and tested this technique in a theoretical study.13 In cross-type optical separation, a loosely focused laser beam propagates perpendicular to the liquid flow direction in order to separate particles and deliver the separated particles to the analysis zone continuously. Kim et al. calculated the trajectories of particles in cross-type optical separation according to particle size and optical properties and predicted the retention distance of particles in an analytical manner. Here we present the first ever experimental demonstration of cross-type optical separation. The cross-type optical separation instrument was fabricated using microchannel and fiber optic components. The retention behaviors of particles in the cross-type optical separation experiments were recorded using a CMOS camera, and the measured retention distances of different-sized particles were compared to theoretical predictions. (9) Rodriguez, M. A.; Armstrong, D. W. J. Chromatogr., B 2004, 800, 7-25. (10) Imasaka, Y.; Kawabata, Y.; Kaneta, T.; Ishidzu, Y. Anal. Chem. 1995, 67, 1763-1765. (11) Hart, S. J.; Terray, A. V. Appl. Phys. Lett. 2003, 83, 5316-5318. (12) Hart, S. J.; Terray, A.; Leski, T. A.; Arnold, J.; Stroud, R. Anal. Chem. 2006, 78, 3221-3225. (13) Kim, S. B.; Kim, J. H.; Kim, S. S. Appl. Opt. 2006, 45, 6919-6924. 10.1021/ac8000918 CCC: $40.75
© 2008 American Chemical Society Published on Web 02/15/2008
Figure 1. Schematics of the optical particle separation.
THEORY In their theoretical study of cross-type optical separation, Kim et al. calculated the retention distance using a ray-optics model, assuming a spatially averaged constant scattering force. They additionally assumed that the laser beam width is larger than the particle size, allowing the gradient force to be ignored.13,14 The spatially averaged constant scattering force, for a laser beam with a Gaussian intensity profile, can be expressed as
F* )
()
n0P dp 2 Q* 4c ω0
xπ2 erf(x2)
(1)
where n0 is the refractive index of the medium, P is the power of the laser beam, c is the speed of light in a vacuum, dp is the particle diameter, ω0 is the width of the laser beam, Q* is a constant that depends on the refractive indices of the particle and medium, and “erf” denotes the error function. The particle dynamic equation in cross-type optical separation can be written as
d2z dz mp 2 + 3πµdp ) F* dt dt
(2)
y ) Ut
(3)
where mp is the particle mass, µ is the dynamic viscosity of the fluid, U is the uniform fluid velocity, and z and y are the particle positions in the z and y directions, respectively. Because the scattering force was assumed to be constant, the following analytic expression is obtained for the retention distance:
z)
noP dp Q* 6πµUc ω0
xπ2 erf(x2)
(4)
In eq 4, the retention distance is proportional to the particle diameter and there is no minimum size of particle that can be separated. However, electromagnetic theory should be used for particles with diameters smaller than the wavelength of the illuminating laser light. EXPERIMENTAL SECTION Figure 1 shows a schematic diagram of the cross-type optical separation method. Initially, the particles suspended in the fluid (14) Kim, S. B.; Kim, S. S. J. Opt. Soc. Am. B 2006, 23, 897-903.
Figure 2. Experimental setup of the optical particle separation.
move parallel to the direction of fluid flow. As the particles pass through the laser beam, however, the scattering force pushes them in the direction of laser beam propagation, which is perpendicular to the fluid flow direction, and the particles shift their position within the plane perpendicular to the fluid flow. After passing through the laser beam, the particles flow downstream while maintaining their retention distance in the laser beam propagating direction. Figure 2 shows a schematic diagram of the experimental setup. The PDMS microchannel, which was fabricated using conventional soft lithography, had a channel width and height of 210 and 100 µm, respectively. A Nd:YAG continuous wave (CW) laser operating at wavelength 532 nm was used to exert the radiation force; the laser beam was delivered into the microchannel through an optical fiber (MMJ-3l-IRVIS-50/125, NA ) 0.11, Oz Optics). Because the output width of the laser beam from the optical fiber is small enough to exert the scattering force on particles, no focusing is required. With the use of the CMOS camera (pco. 1200hs), an objective lens (20×), and a mirror, an inverted microscope was set up as shown in Figure 2. To obtain clear images of the particle trajectories, scattered light from the laser was removed using an interference filter (F10-632.8-4-2.00, CVI Optics) and a red LED was used as a light source for image capture. The flow system consisted of a 1 mL gastight syringe (81320, Hamilton), a syringe pump (pump 11, Harvard apparatus), and a PTFE tube. Polystyrene latex microspheres (PSL, Duke Scientific Corp.) of three distinct diameters were used: 2.0 ( 0.02, 5.0 ( 0.05, and 10.0 ( 0.09 µm. All PSL particles had a refractive index of 1.59. The particles were suspended in water and introduced into the microchannel. RESULTS AND DISCUSSION In the experiments, the flow velocity was 250 µm/s, the laser power was 1 W, and the laser beam radius was 40 µm. Because the laser beam radius is much larger than the particles employed in the experiments, the assumptions used in the derivation of the theoretical retention distance are satisfied. Figure 3 shows snapshots of the particle trajectories. Initially, the particles move downstream with a straight trajectory and then deviate on entering the laser beam, with larger particles showing a larger deviation. After exiting the laser beam, the particles maintain their retention distance. These observations are in good agreement with the theoretical prediction in eq 4. Analytical Chemistry, Vol. 80, No. 7, April 1, 2008
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Figure 3. Trajectories of the 5 and 10 µm particles. The flow velocity is 250 µm/s.
The cross-type optical separation technique established in the present study has many advantages. It can be used to separate biological cells, which range in size from 1 to 100 µm, without the need for bead attachment. For smaller objects such as DNA, cross-type optical separation can be used in conjunction with the conventional bead attaching technique based on antibody-antigen reactions. Cross-type optical separation does not require an additional transport device to transport the separated particles because the separated particles can be moved to the analysis zone by the fluid flow. Furthermore, the separation time is short compared to that of conventional optical chromatography. In conventional optical chromatography, particles are decelerated by radiation forces until they become stationary. In the present crosstype optical separation technique, by contrast, the particles are transported at constant velocity. CONCLUSIONS The cross-type optical separation method has been verified experimentally for the first time. Retention behavior was observed, and the measured retention distances of different-sized particles were in good agreement with theoretical predictions. In crosstype optical separation, particles are separated according to their sizes and move downstream continuously, obviating the need for additional devices to transport the separated particles to an analysis zone. Because the particles are not decelerated in the direction of fluid flow, the separation time is shorter compared to the conventional optical chromatography technique.
Figure 4. Comparison between experimental measurements and theoretical predictions of the retention distances of different particle sizes.
The measured retention distances of the particles with diameters of 2, 5, and 10 µm were 10 ( 1.3, 26 ( 4.7, and 58 ( 6.4 µm, respectively. Thus, the retention distance of the particles increases approximately linearly with particle size, consistent with eq 4. Figure 4 shows a comparison between the experimental measurements and theoretical predictions of the retention distance for different particle sizes. The measured retention distances agree well with the theoretical predictions.
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Analytical Chemistry, Vol. 80, No. 7, April 1, 2008
ACKNOWLEDGMENT The authors express their gratitude for support through a grant from the Brain Korea 21 Program of the Ministry of Education. SUPPORTING INFORMATION AVAILABLE Video of optical particle separation and description. This material is available free of charge via the Internet at http:// pubs.acs.org. Received for review January 14, 2008. Accepted February 6, 2008. AC8000918
