Dielectrophoretic Coassembly of Binary Colloidal Mixtures in AC

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Dielectrophoretic Coassembly of Binary Colloidal Mixtures in AC Electric Fields Saurabh Jain‡ and Shalini Gupta* Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi, India 110016 S Supporting Information *

ABSTRACT: We report the novel use of dielectrophoresis (DEP) for fabricating a new class of composite structures composed of binary mixtures of micrometer-sized colloidal particles. Latex−latex and latex−yeast cells have been coassembled in a combinatorial manner into one- (1D) and two-dimensional (2D) architectures in spatially varying external alternating current (AC) electric fields using two different electrode geometries. The effects of voltage, frequency, particle size, particle concentration, and particle type are investigated in detail to determine how the relative polarizabilities of the particles can be tuned to influence the overall coassembly process. Our observations reveal key differences in the latex−latex and latex−yeast cell assembly behavior especially in the case of 2D structure formation arising mainly due to the intrinsically high polarizability and polydispersity of the cells. This study is useful for making a potpourri of new hybrid structures with advanced functionalities for photonic and biosensing applications.

1. INTRODUCTION The technological advances in the materials field have led to the discovery of new affordable methods that can enable the realization of advanced structures with applications in photonics,1−6 electronics,7,8 sensing9 and drug delivery.10 One such technique that has been extensively used as an efficient tool in the last two decades for rapidly assembling colloidal particles on a chip is dielectrophoresis (DEP).11 Much of the DEP-related research effort has until now focused on the assembly of single type of colloidal particles into a rich variety of one- (1D), two- (2D), and three-dimensional (3D) and supramolecular architectures. This is typically achieved using synthetic particle components such as gold and silver nanocrystals,12,13 quantum dots,14 silica and latex microspheres,15−19 carbon nanotubes,20,21 microgel beads,22 colloidal ellipsoids,23 or even more complex entities such as Janus particles,24,25 live cells,26 and metallic nanowires.8,27 However, in spite of the fabrication of this wide spectrum of material types and our supposedly good understanding of the underlying physical mechanisms of particles’ response to a spatially varying external electric field, there is little reported literature on the DEP-driven coassembly of colloidal mixtures.15,18,19 This has greatly limited the potential of the DEP technique for making superior hybrid structures with enhanced functionality. Manipulation of particles using AC electric field-induced DEP has a key advantage for colloidal assembly over other methods because of the wide range of tunable forces that can be selectively applied on any particle of choice by merely controlling the external field conditions remotely.28,29 The © 2013 American Chemical Society

time-averaged dielectrophoretic force exerted on a particle (p) of radius r and complex permittivity ε̃p = εp − iσp/ω, suspended in a medium (m) of ε̃m = εm − iσm/ω, is given by the classical Maxwell−Wagner theory as ⟨FDEP⟩ = 2πεmr 3Re|K (ω)|∇Erms 2

(1)

This equation shows that the DEP force is directly proportional to the gradient of the square of the root-mean-square (rms) electric field and scales with the volume of the particle.28,30−34 Here, ω is the frequency, ε is the relative permittivity, σ is the conductivity, and K(ω) is the dipolar Clausius−Mossotti function described by (εp̃ − εm̃ )/(εp̃ + 2εm̃ ) whose value dictates whether the particles show positive (Re|K(ω)| > 0) or negative (Re|K(ω)| < 0) DEP behavior in solution. In the case of positive DEP, the particles being more polarizable than their surrounding medium get drawn toward the electrodes (or wherever the electric field gradient is the highest) as well as get dielectrophoretically attracted to each other through multipolar interactions that arise due to the local field distortions created by the particles. In addition, attractive chaining forces between induced dipoles further contribute in the alignment of the particles along the electric field direction. All this happens assuming that the chaining and DEP forces are large enough to overcome the dispersion due to Brownian motion which is mostly the case for most micrometer-sized Received: August 29, 2013 Revised: November 27, 2013 Published: December 9, 2013 16105

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aggregation of the particles. Dried, active baker’s yeast (Saccharomyces cerevisiae) was procured from a local vendor and used at varying concentrations after dispersing in 0.1 w/v % Tween 20 in DI water of conductivity 2 μS cm−1 at 25 °C. The solution for higher conductivity experiments was prepared by dissolving 100 mg of NaCl in 200 mL of DI water, and its conductivity was measured as 1020 μS cm−1 using the S30 SevenEasy conductivity meter (India). 2.2. Experimental Setup. Standard microscope glass slides were cleaned by dipping overnight in a 0.95 w/v % sodium persulfate solution in concentrated sulfuric acid followed by excessive washing with DI water. The coplanar gold electrodes were prepared by covering the clean, dried glass slides with 2−3 mm wide Teflon tape and sputtering them with 10 nm chromium followed by 100 nm of gold (DC magnetron sputtering system, India) (Figure 1a). The

particles; both the gravitational and dielectrophoretic forces are several times higher than kT (see the Supporting Information). The frequency where the particles stop responding to the electric field (Re|K(ω)| = 0) is known as the crossover frequency, ωc =

1 2π

(σp − σm)(σp + 2σm) (εm − εp)(εp + 2εm)

(2)

It is now well established that latex colloidal particles display positive DEP in water (εm ∼ 78.5) in spite of having an intrinsically low permittivity (εp ∼ 2.55) and poor internal conductivity (σint ∼ 10−15 S m−1). This is attributed to the high surface conductance of the counterionic diffuse layer dominating the dielectric response of the latex particles up to very high frequencies (typically lower MHz range). The overall particle conductivity (σp) is inversely proportional to the particle size and is given by σp = σint + A

2σ ′μ r

(3)

where σ′ is the counterionic surface charge density in the diffuse layer, μ is the counterionic charge mobility, and A is a scaling factor.16,35 Depending on the medium conductivity, particle charge density, and size, σp can be modulated to give a different dielectrophoretic response in the same electric field. For two different sized colloidal particles in a mixture, the dielectrophoretic coassembly (or segregation) can be expected to be a function of their relative polarizabilities, and since the strength of the dielectrophoretic polarizability is itself dependent upon multiple experimental parameters such as AC field voltage, frequency, medium conductivity, and electrode geometry, one can envision a rich variety of composite structures to evolve under the influence of external electric fields. The morphology of these structures may be further tuned by adjusting the relative concentrations of the two particle types or by taking particles of two different compositions. In this study, we investigate the phase behavior and assembly patterns of binary colloidal mixtures under DEP. We start by examining the arrangement of single particles into 1D and 2D structures and how their morphology and upper frequency limits for DEP are affected by the external field frequency, particle concentration, and size. Next, we illustrate the fielddependent 1D coassembly of colloidal mixtures looking specifically at the effects of relative particle sizes and particle type (latex−latex vs latex−yeast). In the last section, we demonstrate the formation of 2D composite arrays highlighting the critical role of colloidal size polydispersity in the packing and long-range ordering of the particles. All the results reported here are for pseudoequilibrium structures formed under external AC fields. The dynamics of assembly are not discussed in this paper.

Figure 1. Schematics of the two electrode configurations used in our experiments for carrying out the DEP-induced colloidal assembly.

electrodes were washed with acetone (Fisher Scientific) and DI water, dried in a purge of nitrogen, and then encapsulated in a 120 μm thick paraffin chamber before each experimental run. The four-point electrodes were prepared by sandwiching four orthogonally arranged aluminum wires between a clean glass slide and a paraffin chamber (Figure 1b). 2.3. Method. Binary colloidal suspensions were prepared by mixing 5 μm latex or yeast cells with smaller sized latex in the volumetric ratios of rsmall:2rlarge. The samples were homogenized by gentle shaking of hand and then injected into the chamber and covered with a microscopic glass coverslip to minimize evaporation. DEP-induced assembly of chains and 2D arrays was carried out by applying 4−16 V mm−1 electric fields to the electrodes in the 10 Hz to 1 MHz frequency range using a sinusoidal wave-field function generator (Aplab DDS30 30 MHz) connected in series with a voltage amplifier (Trek model 603). A 1 μF capacitor was included in the series circuit to filter out any direct current. The strength of the applied electric field was measured using a multimeter. The assembly process was continuously monitored at 10× or 40× objective magnification using an Olympus BX-53 optical microscope. Once the assembly process reached saturation such that the structures no longer changed significantly with time (Figure S1, Supporting Information), images were taken using a CCD camera (pixelfly, PCO, Germany) and analyzed with the ImageJ software. Each data point corresponds to a fresh sample to ensure no effect of residual dipole moment remaining in the particle after its subjection to the electric field. Error bars for each data point were obtained after analyzing three representative images containing at least 100 particles

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Carboxylate-modified polystyrene particles of sizes 1, 2, 3, 4, and 5 μm having surface charge densities of 0.16, 1.10, 0.39, 1.31, and 0.97 C m−2, respectively, were purchased from IDC Invitrogen. The latex particles were washed thrice via repeated centrifugation to remove any surfactants, electrolytes, or preservatives from the media and redispersed in DI water of conductivity 2 μS cm−1 containing 0.1 w/v % of Tween 20. The nonionic surfactant was added to each sample to minimize nonspecific 16106

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each. The average particle chain length was calculated by dividing the total number of particles by the total number of chains.

observed that the average chain length reduced considerably as compared to latex as the cells had greater susceptibility for lateral chain attraction which lead to the formation of wavelike patterns throughout the chamber (Supporting Information Figure S3). The overall structural heterogeneity resulted from the fact that cells are largely nonspherical and nonuniform in shape. In fact, yeast is intrinsically one of the most polydisperse cell systems in nature. The cells are also highly polarizable with their overall conductivity being a combination of the cytoplasm and cell wall conductivities both of which are significantly high at 0.5 S m−1 and 0.1 S m−1, respectively.28 These characteristics make the mutual polarization and interparticle attraction behavior in cells more complex than latex particles. Later in this paper, we illustrate how the polydispersity of the cells plays a critical role in the coassembly of the 2D composite structures. 3.1.2. Upper Frequency Limits. We carried out experiments to determine the upper frequency limits for positive DEP in different sizes of latex particles. This upper frequency limit was defined as the terminal frequency where the mutual force of attraction between the particles became so small that their effective mobility in the direction of the electric field gradient as observed through the microscope approached zero. The particle mobility36 in a medium of viscosity η depends on the surface area of the particle and its real component of polarizability, and is defined as μDEP = εmr2Re|K(ω)|/3η. In principle, the terminal frequency should be no different from the crossover frequency but practically this is not what we observe as the transition in the Re|K(ω)| value from a positive to negative takes place over a broad range of frequency instead of being a unique value as predicted by theory. This implies that the particles can appear to stop responding to the electric field much below the anticipated crossover frequency value. The effect is more pronounced in the case of micrometer sized particles. For example, the theoretically estimated crossover frequency for 4 μm latex particles is 11 MHz (see the Supporting Information for calculations), whereas the experimentally measured particle mobility becomes zero much lower at around 0.57 MHz. Even when the frequency is increased up to 30 MHz (the maximum allowable limit in our lab instrument), the particles do not respond to the electric field suggesting that the effect of negative DEP too is absent at these conditions or at least its value is offset by the relatively large weight of the particles. The negative DEP range for 5 μm latex has been reported to be around 100 MHz in the literature.37 We collected the terminal frequency data for different sizes of latex at two separate voltages and medium conductivities (Figure 3). The terminal frequency values always increased monotonically for all the latex and interestingly scaled with medium conductivity proportional to the surface charge densities of the particles. Only the 1 μm particles responded to the electric field selectively up to 250 Hz frequency at 2 μS cm−1 and at no other condition. According to the literature, the crossover frequencies of particles with the same surface charge densities should vary inversely with particle size and medium conductivity which is opposite to what we observe with the terminal frequency data.18,16 The reason for this conflict is due to the dependence of mobility on particle size which actually reverses its behavior below the crossover frequency as shown in the plot of normalized μDEP vs frequency in Figure 4. These results further corroborate with our initial hypothesis that what we measure is not the true crossover frequency. The terminal frequency values in Figure 3 increase with voltage as voltage is the main driving force for dipole formation.

3. RESULTS AND DISCUSSION 3.1. Single Particle Assembly via DEP in AC Field. 3.1.1. Chain Formation. The colloidal particles were initially uniformly distributed in the chamber due to Brownian motion. Following the application of voltage to the electrodes, particles rapidly aligned into chains along the electric field direction and were simultaneously pulled toward the bottom of the substrate where the electric field gradient was the highest. The response time for chaining was particle size-dependent. We began our investigation by studying the affect of frequency on the chaining process. Suspensions containing low particle volume fractions of 5 μm latex were taken in varying concentrations from 0.01 to 0.05 w/v % and subjected to 6 min of constant AC field voltage across coplanar electrodes with increasing frequencies. A wide distribution of particle chains was obtained whose average length reduced systematically with frequency until the chains stopped forming altogether at ∼0.53 MHz (Figure 2 and

Figure 2. Frequency-dependent average chain length of 5 μm latex 1D assemblies at 20 V and increasing particle concentrations.

Supporting Information Figure S2). The average chain length increased, on the other hand, with particle concentration until saturation was reached at 0.03 v/w %. Increasing the concentration beyond this point lead to the onset of chaining in the lateral direction but this happened only below a threshold frequency, the value of which too varied in a concentration-dependent manner. The reason why frequency plays an integral role in governing the particle chain length is because the polarizability of latex is a strong function of the conductance of the counterions in the double layer (mainly Na+ in our case). At low frequencies, the distortion of the double layer is such that it causes strong dipoles to form which result in the formation of longer chains due to positive DEP. As the frequency is increased, the ions in the double layer get lesser and lesser time to redistribute and align with the alternating electric field direction thereby weakening the overall dipole. In other words, the value of Re|K(ω)| drops, yielding smaller chains. At higher particle volume fractions, longer chains are formed mainly due to faster kinetics of assembly because of easier availability of particles. Along with frequency, the type of particle used is another critical parameter in the assembly process. When we repeated our chaining experiments with yeast cells (also ∼5 μm), we 16107

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response of the particles becomes zero. The 1 μm latex also does not respond to the electric field above 250 Hz at 2 μS cm−1, likely because they are below the threshold voltage required for the onset of chaining. 3.1.3. 2D Array Formation. The 2D assembly of particles was carried out at large volume fractions (∼1.2 w/v % commensurate to form a monolayer) using the four-point electrodes as described previously.19,26 Briefly, the electric field was applied in two perpendicular directions alternately for 30 s each using four orthogonal electrodes. The orthogonal setup enabled overcoming the problem of poor packing efficiency, caused by the high polydispersity in size, by increasing the effective interaction between the adjacent neighboring particles. Thus, both the latex and yeast particles could be well organized into hexagonally closed packed arrays with long-range 2D ordering (Supporting Information Figures S4 and S5). 3.2. DEP-Driven 1D Assembly in Binary Mixtures. Both the chaining and DEP forces depend on particle size and particle type which can significantly affect the outcome of the cocrystallization process. We examined the effect of DEP-driven coassembly on the structural evolution of colloidal mixtures by taking low volume fractions of binary suspensions of latex− latex particles and latex particle−yeast cells in the size ratios of 1:5 to 4:5 (Figure 5). The experiments were performed at 40 V and 100 Hz keeping the concentration of the larger particles fixed at 0.05 w/v % and varying the amounts of smaller sized latex according to the ratio rsmall:2rlarge. The response and travel time for the dielectrophoretic movement of the particles was found to be inversely

Figure 3. Terminal frequency values for different sizes of latex measured at different field strengths and medium conductivities. The particle concentration was 0.05% w/v in every experiment.

Figure 4. Theoretical plot predicting frequency-dependent normalized DEP mobility of different sizes of latex particles. The calculations are made using the same surface charge densities for all the particles knowing that they have little influence on the terminal frequency values as demonstrated in Figure 3.

The dependence on medium conductivity, however, is more complex. The monotonic increase in terminal frequency values with particle size, in spite of the large variation in their intrinsic surface charge densities (varying as much as an order of magnitude in some cases; refer to the Experimental Section), suggests that the counterionic charges in the diffuse layer must nearly be the same in all the different latex sizes. This is only possible if we assume that the counterionic atmosphere at 2 μS cm−1 medium conductivity is well below the saturation limit for even the particle with the lowest intrinsic surface charge density (1 μm in our case). When the medium conductivity is increased to 1020 μS cm−1, the surface conductance of the particles also rises because of the decrease in the debye length from 71 to 3.3 nm which results in greater polarization of the particles and thus, an increase in the terminal frequency values. At this high salt concentration, however, not only is the counterionic saturation limit for the 1 μm latex reached, it becomes comparable to the medium conductivity which is why the DEP

Figure 5. DEP assembly of composite chains from binary mixtures of colloidal particles aligned using 40 V and 100 Hz AC electric field. The columns show assembly of latex−latex and latex−yeast cells. The rows display the relative sizes of the two particle types made by mixing 1, 2, 3, or 4 μm latex to the 5 μm latex or yeast particle. 16108

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proportional to their size. Contrary to our expectation, no sizeinduced phase separation due to the incompatibility of the lateral interactions of the dipoles in the chains of different sized particles was observed as suggested elsewhere.15 In the 1:5 and 2:5 latex−latex mixtures, the 5 μm particles aligned first within 5 s, forming the long chain backbone, and subsequently the 1 and 2 μm particles pulled into the adjoining gaps between their interparticle junctions. Some of the 2 μm particles also formed chains of their own. In the 3:5 latex mixtures, the 3 μm particles were sandwiched between the 5 μm latex either in the form of a small chain or as a pair aligned perpendicular to the electric field direction. Finally, in the case of 4:5 size ratios, the chains became fairly homogeneous, rendering the two latex types indistinguishable from each other. The outcome with yeast cells showed similar structural patterns except for a few noticeable differences: (1) the number of small particles trapped between the cells exceeded those between the latex (as also evident by the lower number density of the smaller latex in the background), (2) there was a prominent reduction in the average chain length of the assemblies, and (3) the chains formed were highly irregular in shape. Our understanding of the mixed system from a previous study suggests that larger particles being more polarizable tend to have the highest field intensities at the top and bottom of their dipoles.19 These high intensity zones create local electric field gradients which act as traps for the small particles attracting them toward the interstitial spaces between the larger ones. The process happens more readily in the case of highly conductive yeast cells where the reduced axial dipolar interactions between cells (due to shape irregularities) further facilitates the collection of the smaller particles. Figure 5 suggests that the overall interaction and final colloidal arrangement between dissimilar particles is not only governed by the relative contributions of the size-dependent strength of dipolar interactions but also by the mutual particle size compatibility. It is important to note that in no case do we observe phase separation due to unfavorable interactions because of shape mismatch between the dipoles in two chains of particles of different sizes. Also, if DEP forces between dissimilar particles are large, the size incompatibility becomes less important as seen in the case of yeast cells. To quantifiably compare the extent of small particle trapping and how it varies with colloidal size and frequency, we classified the composite chain structures into three categories based on the configuration of the small particles: (i) Mixed chains: containing N1 number of small particles captured in the chains of larger particles. (ii) Individual chains: containing N2 number of small particles forming chains of their own. (iii) No chains: containing N3 number of small particles not involved in any chain formation. The fraction of small particles in each of these three configurations was reported as n1 = N1/(N1 + N2 + N3), n2 = N2/(N1 + N2 + N3), and n3 = N3/(N1 + N2 + N3), respectively. The variations in the values of n1 and n2 were measured against frequency in 2:5, 3:5, and 4:5 latex−latex mixtures as shown in Figure 6. The 1:5 latex mixtures were not analyzed due to insufficient response of the 1 μm particles above 250 Hz. The value of n1 was highest for the 4 μm latex and decreased continuously with frequency for all sizes as anticipated. On the other hand, the value of n2 decreased continuously only for the 2 μm latex and exhibited nonmonotonic behavior for the 3 and 4 μm particles. The fall in the value of n1 is a direct manifestation of how frequency impacts particle polarizability

Figure 6. Frequency-dependent coassembly of 2, 3, and 4 μm particles with 5 μm latex in mixed binary suspensions of concentration 0.05% at 20 V: (a) fraction of smaller particles in mixed chains and (b) fraction of smaller particles in individual chains.

in a size-dependent manner. The drop in polarizability with frequency modification is higher in larger size particles as compared to small ones and so, as the frequency is increased, the field intensity at the dipolar ends of the 5 μm latex decreases more and the small particles are released from the mixed chains into the solution. This increase in small particle concentration in solution leads to the rise in their n2 value, as seen in the case of 3 and 4 μm latex, unless the DEP forces are so weak that they are unable to align even the excess particles into individual chains, as is the case with 2 μm latex. The increase is sharper in the case of 3 μm particles as compared to 4 μm because a larger number of 3 μm latex reside around the 5 μm latex due to their size compatibility relationship. The strength of the dipoles eventually decreases in all the particles. 3.3. 2D Composite Arrays from Binary Suspensions Using DEP. Particles in mixed systems are expected to form closed pack arrays if the colloidal size ratio is such that the smaller sized particles can be confined in the interstitial pockets of the crystal lattice formed by the larger ones. Otherwise, the smaller particles may prevent the large particles from crystallizing, or the entire system may undergo some sort of interaction-induced phase separation. We performed experi16109

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ments in our four-point electrode setup to check the disposition of binary colloidal suspensions for making 2D composite arrays under the direction of external AC fields for a broad range of particle size ratios. Colloidal mixtures were taken in rsmall:2rlarge concentration ratios at high volume fractions commensurate to realize a monolayer. The camera was focused 50 μm above the bottom surface and images of the particles in suspension were taken just before (t = 0) and twice after the application of the AC field, first at 35 s corresponding to the formation of chains at the bottom surface and then at 180 s corresponding to the formation of a 2D layer (Supporting Information Figure S6). The results are discussed for the 3:5 latex−latex and latex− yeast case at two different frequencies as shown in Figure 7. The graphs show time-dependent concentrations of 3 and 5 μm particles measured at the 50 μm focal plane. The number of 5 μm particles decreased monotonically in all the cases, whereas the 3 μm latex concentration first reduced and then increased at 100 Hz in the latex−latex mixtures but remained constant at 1 kHz. In the latex−yeast mix, their concentration decreased initially and then remained constant at 100 Hz. Similar trends were observed with the 2:5 particle size ratio (Supporting Information Table S1). What likely happens is that in the first stage of assembly where particle chaining takes place, the 3 μm particles readily coalign with the larger colloids as illustrated in Figure 5. This happens equally effectively in both latex and yeast cells at the low frequency. At the higher frequency, the DEP response of 3 μm latex is negligible and so their concentration in suspension remains almost constant. In the second stage of assembly where lateral compression takes place, the 3 μm latex is unable to cocrystallize with the 5 μm latex due to particle size incompatibility, and thus, phase segregation takes place because of the lowering of synergistic dipolar interactions. Simple theoretical calculations made to predict the interstitial void space between 5 μm spheres in a hexagonally closed packed (hcp) monolayer yield a value of 0.77 μm (see the Supporting Information) which shows that size mismatch is indeed the reason why the formation of compositely closed-pack lattices is entropically unfavored. A phase separation is, however, not observed in the case of yeast cells, which again is not surprising. The high polydispersity of the cells makes their interstitial pockets so widely heterogeneous that it enables overcoming the limitations due to size dissimilarity for efficient entrapment of the smaller particles (Supporting Information Figure S7). The strong DEP attractive forces in cells further strengthen the coassembly thermodynamics. The above-mentioned phenomenon is of great practical importance as it could be applied for quantifying the polydispersity indices of colloidal suspensions which is currently achieved using optical scattering techniques. The DEP method could also be applied for the segregation of particle mixtures by choosing appropriate frequency ranges where the mixtures exhibit field-induced phase separation. The ability to generate 2D lattices of mixed particle types and sizes is itself a step forward as it can be used for fabricating complex photonic materials of desired morphology. The concept may be further extended to tertiary or higher order systems for the formation of various stable and metastable crystalline phases which is otherwise very hard to achieve. Another potential manifestation of our assembly approach could be in the tissue engineering field for making novel biomaterials for prosthetic applications. We have already previously demonstrated how submicrometer bioconjugated magnetic particles can be

Figure 7. Coassembly of latex−latex and latex−yeast cell binary suspensions at 20 V. The y-axis values are obtained by counting the number of particles 50 μm above the bottom surface to capture their coassembly behavior at the bottom surface. (a) 3 and 5 μm latex particles at 100 Hz, (b) 3 and 5 μm latex particles at 1 kHz, and (c) 3 μm latex and yeast cells at 100 Hz.

coassembled with yeast and mammalian cells to fabricate magnetically responsive chains and membranes with potential applications in the biosensing and biomedical fields.26 We 16110

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further hope to explore and expand these avenues in our future research.

4. CONCLUSIONS We have demonstrated how DEP can be used as a facile technique to manipulate and organize colloidal particles into composite structures. Colloidal suspensions of synthetic particles and yeast cells were arranged separately and together into 1D chains and 2D arrays using two different electrode geometries. The assembly process was driven by frequencydependent field- and mutually induced dipolar interactions. The phase behavior of single particles showed that they stop responding to the electric field much below the crossover frequency at what we term the terminal frequency. Unlike the crossover frequency, the terminal frequency scales directly with particle size, voltage, and medium conductivity. DEP assembly of binary colloidal suspensions of latex−latex and latex−yeast yielded hybrid composite structures. The major guiding factors for coassembly were particle size compatibility and colloidal polarizability. In the case of 2D arrays, incompatible particle sizes could only pack into hybrid structures if the latticeforming particles were polydisperse in nature. Otherwise, the system underwent partial phase separation. This phase segregation phenomenon was, however, never observed in the case of 1D assembly. We hope our study can serve as a broad guideline to prepare novel structures for a wide variety of applications. One such application is for the fabrication of functional hybrid biomaterials from a mixture of biological cells which can be advantageous for the tissue engineering field. Electric fields can also be used to form live cell monolayer scaffolds for pharmaceutical testing and biosensing. Future research is needed in these areas, and DEP opens a new pathway for making these novel materials.

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ASSOCIATED CONTENT

S Supporting Information *

Comparision of gravitational, dielectrophoretic, and thermal forces; crossover frequency calculations for 4 μm latex particles; calculation of the void space between hexagonally close-packed 5 μm particles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 91-11-2659-1070. Present Address ‡

S.J.: Reliance SEZ Refinery, Jamnagar, Gujarat, 361142.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the funds provided by the Council of Scientific and Industrial Research (CSIR) (Project No. 22(0637)/13/EMR-II).

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REFERENCES

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