Anal. Chem. 1999, 71, 3441-3445
Dielectrophoretic Characterization and Separation of Antibody-Coated Submicrometer Latex Spheres Michael P. Hughes† and Hywel Morgan*
Bioelectronics Research Centre, Department of Electronic Engineering, University of Glasgow, Glasgow G12 8QQ, U.K.
The dielectrophoretic behavior of carboxylated 216-nmdiameter latex spheres has been characterized as a function of both medium conductivity and applied field frequency. Dielectrophoretic crossover measurements and analysis were used to characterize the dielectric properties of the particles. The particles were functionalized with antibodies using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDAC)-based coupling. Measurements indicated that the surface conductance of the native particles was 1.2 nS and that this reduced to a value of 0.7 nS after EDAC treatment and 0.25 nS after antibody coupling. Changes in the dielectrophoretic spectrum of the particles were exploited to demonstrate the principle of separation of unlabeled and protein-labeled particles. This demonstrates the potential for the development of new affinity separation systems based on ac electrokinetic methods. When a dielectric particle is suspended in a spatially nonuniform electric field, the interaction of the applied field and the induced dipole moment generates a force on the particle. This force, termed dielectrophoresis (DEP),1,2 has been demonstrated to be an effective means of manipulating particles in solution. Recent advances in electrode fabrication methods, such as the use of electron beam lithography, means that DEP can now be used to manipulate submicrometer particles such as viruses,3-5 latex spheres,6,7 and macromolecules.8,9 The magnitude and direction of the dielectrophoretic force is governed both by the electric field and by the complex permittivity of the particle and suspending medium. These depend on the frequency of the applied field, so that the magnitude and/or * Corresponding author: (Tel) +44 141 330 5237; (fax) +44 141 330 4907; (e-mail)
[email protected]. † Present address: European Institute of Health and Medical Sciences, University of Surrey, Guildford, Surrey GU2 5XH, U.K. (1) Pohl, H. A. Dielectrophoresis; Cambridge University Press: Cambridge, 1978. (2) Jones, T. B. Electromechanics of particles; Cambridge University Press: Cambridge, 1995. (3) Mu ¨ller, T.; Fiedler, S.; Schnelle, T.; Ludwig, K.; Jung, H.; Fuhr, G. Biotechnol. Tech. 1996, 10, 221-226. (4) Morgan, H.; Green, N. G. J. Electrostatics 1997, 42, 279-293. (5) Hughes, M. P.; Morgan, H.; Rixon, F. J.; Burt, J. P. H.; Pethig, R. Biochim. Biophys. Acta 1998, 1425, 119-126. (6) Mu ¨ ller, T.; Gerardino, A.; Schnelle, T.; Shirley, S. G.; Bordoni, F.; De Gasperis, G.; Leoni, R.; Fuhr, G. J. Phys. D: Appl. Phys. 1996, 29, 340349. (7) Green, N. G.; Morgan, H. J. Phys. D: Appl. Phys. 1997, 30, 2626-2633. (8) Washizu, M.; Kurosawa, O. IEEE Trans. Ind. Appl. 1990, 26, 1165-1172. (9) Washizu, M.; Suzuki, S.; Kurosawa, O.; Nishizaka T.; Shinohara T. IEEE Trans. Ind. Appl. 1994, 30, 835-843. 10.1021/ac990172i CCC: $18.00 Published on Web 06/30/1999
© 1999 American Chemical Society
direction of force exerted on a particle can vary with the frequency. The sign of the force denotes whether the particle is attracted to regions of high electric field or repelled from them, modes of behavior termed positive and negative DEP, respectively. Since the direction of force depends on a particle’s specific dielectric properties, particles with differing properties will experience different forces. It has been demonstrated that, under the appropriate conditions (e.g., medium conductivity and field frequency), particles of different types experience forces in different directionssacting toward or away from regions of high electric field. Thus, dielectrophoretic methods have been used to separate cells such as cancerous and normal blood cells,10 different strains of bacteria,11 and different types of viruses.12 Recent advances in the field of DEP separation have shown that fluid flow (Stoke’s force), gravitational, and DEP forces can be combined to achieve particle separation in a DEP-gravitational field-flow fractionation system.13-15 As has been shown by Green and Morgan16,17 the DEP behavior of submicrometer particles is governed largely by surface properties. This means that DEP could be used to detect and measure changes in the composition of a particle’s surface, e.g., changes following binding of an antigen to a surface-immobilized antibody. Such changes are expected to give rise to variations in the dielectric properties of the particles, resulting in differences in the magnitude and/or direction of the DEP force. Thus, a DEPbased separation technology could be developed for submicrometer particles as a new type of affinity fractionation. In this paper, we demonstrate that the dielectrophoretic behavior of 216-nm-diameter fluorescent latex spheres depends on the chemical composition of the surface. The dielectrophoretic properties of the spheres was measured for three cases: (i) beads as supplied by the manufacturer; (ii) after surface activation (for protein attachment) with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDAC); and (iii) after immobilization of a monolayer of protein. It is shown that the DEP force on the spheres varies with (10) Gascoyne, P. R. C.; Wang, X. B.; Huang, Y.; Becker, F. F. IEEE Trans. Ind. Appl. 1997, 33, 670-678. (11) Markx, G. H.; Pethig, R. Biotechnol. Bioeng. 1995, 45, 337-343. (12) Morgan, H.; Hughes, M. P.; Green, N. G. Biophys. J. 1997, 77, 516-525. (13) Markx, G. H.; Rousselet, J.; Pethig, R. J. Liq. Chromatogr. Rel. Technol. 1997, 20, 2857-2872. (14) Huang, Y.; Wang, X. B.; Becker, F. F.; Gascoyne, P. R. C. Biophys. J. 1997, 73, 1118-1129. (15) Wang, X. B.; Vykoukal, J.; Becker, F. F.; Gascoyne, P. R. C. Biophys. J. 1998, 74, 2689-2701. (16) Green, N. G.; Morgan, H. J. Phys. Chem. 1999, 103, 41-50. (17) Green, N. G.; Morgan, H. J. Phys. D: Appl. Phys. 1997, 30, L41-L44.
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surface functionality and that the spatial separation of labeled and unlabeled spheres can be achieved using appropriate microelectrodes. THEORY The dielectrophoretic force, FDEP, acting on a homogeneous, isotropic dielectric sphere, is given by1,2
FDEP ) 2πr3mRe[K(ω)]∇E2
(1)
where r is the particle radius, m the permittivity of the suspending medium, ∇ the del vector operator, E the rms electric field, and Re[K(ω)] the real part of the Clausius-Mossotti factor, given by
K(ω) ) (/p - /m)/(/p + 2/m)
(2)
where /m and /p are the complex permittivities of the medium and particle, respectively, and * ) - j(σ/ω) with σ the conductivity, the permittivity and ω the angular frequency. The frequency dependence of Re[K(ω)] implies that the force on the particle also varies with the frequency. The magnitude of Re[K(ω)] depends on whether the particle is more or less polarizable than the medium. If Re[K(ω)] is positive, then particles move to regions of highest field strength (positive dielectrophoresis); the converse is negative dielectrophoresis where particles are repelled from these regions. Under certain conditions, and at a specific value of frequency, the force on the particle goes to zero when Re[K(ω)] ) 0. Measurements of the variation of this zero-force frequency with medium conductivity can be used to characterize the dielectric properties of the particles (e.g., refs 4 and 18). It has been demonstrated19 that the effective conductivity of a latex particle can be written as a combination of a component of conductivity through the particle (the bulk conductivity σpbulk) and a component of conductivity around the particle due to the surface conductance Ks. The effective conductivity of the particle, σp is then the sum of these two components19 according to
σp ) σpbulk + 2Ks/r
(3)
The intrinsic conductivity of a latex sphere is negligible, so that that the effective conductivity of the sphere is dominated by surface conductance, Ks. Measurements of the zero-force frequency as a function of suspending medium conductivity can be used to characterize the dielectric properties of particles such as cells18 and submicrometer spheres.16 For a solid homogeneous dielectric sphere, with a single relaxation time, a typical zero-force spectrum is plotted in Figure 1. This plot was calculated from eqs 2 and 3, with the following parameters for the particle: p ) 2.55, Ks ) 1.2 nS. It can be seen that up to a medium conductivity of 20 mS m-1 the zero-force frequency is independent of conductivity. However, for conductivi(18) Gascoyne, P. R. C.; Pethig, R.; Satayavivad, J.; Becker, F. F.; Ruchirawat, M. Biochim. Biophys. Acta 1997, 1323, 240-252. (19) Arnold, W. M.; Zimmermann, U. J. Electrostat. 1988, 21, 151-191.
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Figure 1. Theoretical plot showing the range of electrolyte conductivities and frequencies over which a homogeneous dielectric particle experiences positive or negative dielectrophoresis. The plot was calculated from eq 2, with the following parameters for the particle: p ) 2.55, Ks ) 1.2 nS, and with m ) 780.
ties greater than 20 mS m-1 the particle is always less polarizable than the medium and only experiences negative DEP. MATERIALS AND METHODS Latex Spheres: Protein Coupling. Carboxylate-modified latex spheres, 216 nm diameter, were purchased from Molecular Probes. The spheres were obtained preloaded with fluorescent dye emitting either in the yellow-green (515 nm) or red (575 nm) spectrum. The spheres were activated for protein coupling according to a modification of a protocol supplied by Molecular Probes and Bangs Labs. The protocol was as follows: 250 µL of 2% (w/v) spheres were mixed with 500 µL of 10% (w/v) EDAC (Sigma Chemical Co.) in water together with 250 µL of 50 mM KH2PO4, and incubated for 30 min at room temperature. This solution was then dialyzed using 1000 MWCO dialysis tubing (Pierce) in 100 mM Borax for 1 h. For sample ii, the coupling procedure was stopped at this point in order to evaluate the effect of surface activation on the dielectrophoretic properties of the spheres. Protein coupling was performed as follows: to a solution of EDAC-activated spheres (in 100 mM Borax) was added 250 µL of 10 mg/mL rabbit anti-goat IgG (Sigma Chemical Co.) and the mixture incubated at room temperature for 4 h with agitation. The reaction was quenched by the addition of 100 µL of 1 M glycine. Particles were washed twice in H2O followed each time by centrifugation at 13000g in a microfuge. Particles were sonicated between spins to ensure monodispersity. Sphere concentration was estimated using light scattering. The light transmission through a solution of spheres was measured (with a Hitachi U2000 spectrophotometer) at a wavelength of 700 nm using a cell with a 1-cm path length. A calibration curve was obtained by measuring the light transmission as a function of sphere concentration over the range 10-2-10-5 volume fraction (from serial dilution of the stock solution). Over this range of particle concentration, the transmission was linearly related to concentration. Using this method, the volume fraction of the labeled spheres was estimated to be 5 × 10-4, equivalent to ∼1011 spheres/mL.
Figure 2. SEM photograph showing part of the castellated electrodes used for the manipulation of the spheres. The upper image shows a large area of the electrode array, while the lower image is a 10× magnification of the section outlined by the bounding box (top image). The electrodes have a feature size of 10 µm along each edge and were fabricated from a 100-nm-thick Au layer sandwiched between two 10-nm-thick Ti layers.
The effectiveness of the protein labeling was analyzed using a bicinchroninic acid (BCA) protein assay kit (Pierce). This method is used to measure protein concentrations spectrophotometrically at a wavelength of 562 nm. After labeling the spheres, the protein concentration of a solution of spheres was determined according to the manufacturer’s protocol. For a solution of spheres at a concentration of 1011/mL, the protein content was estimated to be 30 ( 10 µg/mL, equivalent to ∼1200 IgG molecules/sphere (for a perfectly smooth surface). Assuming that the proteins are evenly distributed, then this indicates that each IgG molecule occupies an area of 10 nm × 10 nm, which is close to the average area of a single IgG molecule as estimated using scanning tunneling microscopy.20 Thus it can be concluded that the labeling procedure was successful in producing spheres coated with on average a monolayer of protein molecules. Microelectrodes. The electrodes used for crossover frequency measurements and for separation experiments were of the “castellated” design21 with square dimensions of 10 µm along all faces. An SEM of the electrodes is shown in Figure 2. Alternating castellations on the array were displaced by 10 µm so that the castellations of adjacent electrodes are either facing each other or offset as shown in the figure. Electrodes were manufactured on glass microscope slides using direct-write electron beam lithographic techniques and consisted of a layer of 100-nm Au sandwiched between two layers of 10-nm Ti. (20) Leatherbarrow, R. J.; Stedman, M.; Wells, T. N. C. J. Mol. Biol. 1991, 221, 361-36. (21) Burt, J. P. H.; Alameen, T. A. K.; Pethig, R. J. Phys. E: Sci. Instrum. 1989, 22, 952-957.
Figure 3. Photographs showing positive dielectrophoresis (a) and negative dielectrophoresis (b) of 216-nm-diameter fluorescent spheres. The particles were suspended in ultrapure water (conductivity 0.25 mS m-1) and exposed to an ac electric field of 5 V peak to peak. The frequencies of the applied field in the photograph were (a) 0.5 and (b) 10 MHz.
Experimental Details. Electrodes were powered using a Hewlett-Packard signal generator providing 5 V peak-to-peak sinusoidal signals over the frequency range 1 Hz-20 MHz. Particle motion was observed using a Nikon Microphot microscope with camera attachment, and photographs were recorded on Konica 3200ASA color film. An aliquot of particles was suspended in KCl solution of a range of conductivities (measured with a Hewlett-Packard 4192A impedance analyzer) at a concentration of ∼108 particles/mL. Approximately 5 µL of the particle solution was micropipetted into the electrode chamber and the assembly sealed with a cover slip. RESULTS AND DISCUSSION Dielectrophoretic Behavior. For any medium of conductivity less than 10 mS m-1, an electric field of peak amplitude 5 V, and frequency in the range 100 kHz and 1 MHz, all the spheres exhibited positive DEP (unlabeled, EDAC-activated, and proteinlabeled). This meant that they collected at regions of high electric field at the tips of the electrodes as shown in Figure 3a. Below 100 kHz, particle motion was influenced by electric field-induced fluid flow as reported elsewhere.22 For frequencies above 10 MHz, all the spheres exhibited negative DEP and were observed to collect in regions of low electric field. This is shown in Figure 3b where the small triangular-shaped aggregates of particles are characteristic of negative DEP trapping in castellated electrode (22) Green, N. G.; Morgan, H. J. Phys. D: Appl. Phys. 1998, 31, L25-L30.
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arrays.23 For conductivities in the range 20-50 mS m-1, a step reduction in the crossover frequency (of approximately one decade) was observed for all three types of spheres. The experimental zero-force data for the three types of spheres is plotted in Figure 4, together with the best theoretical fit to the data (see below). For each electrolyte, the average response of several hundred particles was observed. The range over which a transition from positive to negative DEP or negative to positive DEP occurs is indicated by the upper and lower bounds of the data. In the case where no line is shown, the measured crossover frequency occurred within a narrow window, approximately the size of the data point or less. The experimental data show some similarity to the theoretical prediction of Figure 1, with an approximately constant zero-force frequency up to particular medium conductivity and a transition point where the zero-force frequency drops. Up to this transition conductivity, the zero-force frequency is in the range 4-9 MHz for the unlabeled spheres (Figure 4a). For the EDAC-activated spheres the range is 2.5-5 MHz (Figure 4b) and for the IgGfunctionalized spheres 1-2 MHz (Figure 4c). The conductivity at which the transition in zero-force frequency occurs is approximately 50, 40, and 20 mS m-1, respectively for the three types of spheres. Although the absolute value of the dielectrophoretic force on the spheres was not measured, observations of the rate of movement of the particles indicated that the dielectrophoretic force on the IgG-functionalized spheres was noticeably less than that on the other spheres. This resulted in a less accurate measurement of the zero-force frequency for these spheres. Figure 5 shows the data for the unlabeled spheres together with two limiting theoretical responses obtained using eqs 2 and 3, dotted lines. The two lines are plotted for upper and lower limits of surface conductance and calculated using the following parameters: medium relative permittivity m ) 78, particle relative permittivity p ) 2.55, particle bulk conductivity σpbulk e 10-4 S m-1. Two values of surface conductance were used corresponding to Ks ) 1.25 nS (lower line) and Ks ) 3 nS (upper line). It can be seen that the experimental data deviate from theory in both cases, indicating that as the conductivity of the electrolyte increases the effective value of Ks must also increase. Both theoretical fits also predict an asymptotic crossover frequency. This is not observed experimentally due to the presence of the low-frequency relaxation associated with the double-layer polarization.5,7 As the broken lines in Figure 5 show, the variation in crossover frequency with medium conductivity cannot be predicted simply by treating the particle as a solid homogeneous dielectric with constant surface conductance. An empirical fit to the data can be obtained through the addition of a surface conductance that is a function of the suspending medium conductivity. Equation 3 can then be modified as follows:
σp ) σpbulk +
2Ks 2σm + r xκr
(4)
where κ-1 is the Debye length. For the unlabeled spheres, the (23) Gascoyne, P. R. C.; Huang, Y.; Pethig, R.; Vykoukal, J.; Becker, F. F. Measure. Sci. Technol. 1992, 3, 439-445.
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Figure 4. Experimental measurement of the crossover frequencies as a function of electrolyte conductivity for 216-nm spheres, together with the best fit obtained from eqs 2 and 4, (solid line). Unmodified particles are shown in (a) with a best-fit surface conductance, Ks ) 1.2 nS; EDAC-activated spheres are shown in (b) with Ks ) 0.7 nS and IgG-labeled in (c) with Ks ) 0.25 nS.
best fit to the data obtained with this function is with Ks ) 1.2 nS, and the predicted response is shown by the solid line in Figure 4a. It can be seen that this fits the experimental data in terms of both crossover frequency and transition conductivity. Using eq 4, best fits to the experimental data for the EDAC-activated and IgG-labeled spheres are plotted in parts b and c of Figure 4 (solid
Figure 5. Plot of the experimental crossover frequency for unlabeled spheres together with theoretical plots obtained from eqs 2 and 3 (dotted lines) and eq 4 (solid line). The two dotted lines represent the upper and lower limits of surface conductance that bound the data with Ks ) 1.25 nS (lower line) and Ks ) 3 nS (upper line). The solid line was calculated with Ks ) 1.2 nS.
lines) with Ks ) 0.7 and 0.25 nS, respectively. This indicates that the surface conductance of the spheres is reduced by a factor of 5 after surface modification with a monolayer of protein. DEP Separation of Sphere Types. It has previously been shown that submicrometer latex particles (of identical size) can be separated into subpopulations by dielectrophoretic methods on the basis of a distribution in the particle surface charge density.17 The separation of protein functionalized latex spheres from unmodified spheres can similarly be demonstrated. A mixture of equal quantities of unlabeled and IgG-labeled spheres was resuspended in ultrapure water (measured conductivity 250 µS m-1) and micropipetted onto an electrode array. To aid observation, red fluorescing spheres were labeled with IgG while unlabeled spheres that emitted a green fluorescence were used. When an applied potential of peak amplitude 5 V at a frequency of 5 MHz was applied to the electrodes, both types of spheres experienced DEP, but in opposite directions so that spatial separation of the particles was observed. The unlabeled spheres were attracted to the electrodes under positive DEP forces, while simultaneously the IgG-labeled spheres were repelled away from the electrodes into the solution by negative DEP forces. When the frequency was reduced to 1.0 MHz, the labeled spheres also collected at the electrodes under positive DEP. However, in this case, the DEP force was lower that for the unlabeled spheres, so that they formed pearl chains between the electrode tips, while the more strongly attracted, unlabeled spheres collected at the electrode edges. This effect is shown in the photograph of Figure 6.
Figure 6. Photograph showing the separation of unlabeled spheres and IgG-labeled spheres. The protein-labeled spheres (gray in the photograph) are only weakly attracted to the electrodes and form pearl chains between opposite castellations, while the unlabeled spheres (white in the photograph) are strongly attracted to the electrode tips. For this experiment, the spheres were suspended in ultrapure water (conductivity 0.25 mS m-1) and exposed to an ac signal of 5 V peak to peak at a frequency of 1 MHz.
CONCLUSIONS The dielectrophoretic behavior of 216-nm-diameter latex spheres has been characterized by measurements of the zero-force or crossover frequency of the particles as a function of medium conductivity. The dielectric properties of unlabeled spheres and EDAC-activated and IgG-labeled spheres have been obtained, and the data indicate that coupling of IgG to the surface of the spheres results in a 5-fold reduction in surface conductance. This gives rise to significant changes in the DEP spectrum of the spheres, so that the protein-labeled spheres can be separated from the unlabeled spheres using DEP. These results demonstrate the potential for the development of new ac electrokinetic-based affinity separation systems. ACKNOWLEDGMENT The authors thank Ms. Mary Robertson for the electrodes and Dr. Nicolas Green for valuable discussions. This work was supported by the Biotechnology and Biological Sciences Research Council (U.K.) Grant 17/T05315.
Received for review February 15, 1999. Accepted May 6, 1999. AC990172I
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