Directed Assembly of Janus Particles under High Frequency ac

Aug 27, 2012 - Author Present Address. Cabot Corporation Business and Technology Center, 157 Concord Road, Billerica, MA 01821, USA, ...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Directed Assembly of Janus Particles under High Frequency acElectric Fields: Effects of Medium Conductivity and Colloidal Surface Chemistry Lu Zhang† and Yingxi Zhu* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: The manipulation and assembly of polystyrenebased Janus particles of varied surface chemistry on one hemispherical particle surface under high frequency nonuniform ac-electric fields is examined experimentally by in situ microscopic observation. Strong effects of ac-field frequency, medium conductivity, and particle surface chemistry on the structure of Janus colloidal assembly are observed. At low medium conductivity, σm from 0.0007 S/m to 0.0153 S/m, pearl chains of Janus particles are observed over the ac-frequency range from 25 kHz to 20 MHz, indicating the dielectrophoresis (DEP)-directed assembly. In contrast, the chaining of Janus particles is disrupted in a certain frequency range at high σm from 0.0153 S/m to 0.116 S/m, suggesting the combining effects of both induced-charge electrophoresis (ICEP) and DEP. The critical transition frequency for the onset of the fractal aggregation at high σm from 0.0153 S/m to 0.116 S/m is experimentally determined, showing a good agreement with the theoretically predicted upper ICEP frequency limit. Additionally, it is demonstrated that by using zwitterionic Janus particles, the assembled structure of Janus particles under ac-fields can be modified by the chemical coatings on each hemispherical surface of Janus particles.

■

INTRODUCTION There is burgeoning interest in the assembly of anisotropic molecules and particles into hierarchically structured materials for various applications in recent years. Among them, the Janus particle (JP),1 which possesses two distinct surface chemical compositions or properties on two sides of the particle, has been mostly explored2−11 since it was called to attention by de Gennes in his Nobel Prize address.1 Because of its anisotropic nature, a wide variety of novel material properties as well as intriguing interparticle interactions and the resulting assembled structures of JPs have opened up a great diversity of conceivable applications ranging from self-assembly into one-, two-, and three-dimensional superstructures with different length scales, multiple-functionalized optical, electronic, and sensor devices, to dual-functionalized carriers for catalysis, biosensing, and drug delivery.1−14 The anisotropic feature of JPs with distinct surface chemical composition on each hemisphere can introduce asymmetric interparticle interactions in contrast to homogeneous particles, leading to the assembly of complex hierarchical structures.6,10 For example, Hong et al.6 investigated the self-assembly of zwitterionic JPs with opposite surface charges on each hemispherical surface in aqueous suspensions and demonstrated that colloidal clustering of distinct colloidal packing configuration, instead of a simple colloidal string formation, can be formed and controlled. Bradley et al.15 studied the reversible and irreversible clustering of stimuli responsive Janus microgels as a function of pH, temperature, and electrolyte concentration. Nevertheless, much prior work on the assembly of JPs has © 2012 American Chemical Society

focused on passively tuning the interparticle interactions in suspensions by JP surface chemistry or varying suspension conditions such as ionic strength and pH. Approaches to actively manipulate the JPs with external force fields have been few. In our previous work16 we examined the dynamics of polystyrene (PS)-based JPs with gold and alkanethiol coatings on one side of the particle under nonuniform ac-electric fields in the ac-frequency range of 25 kHz to 20 MHz, which results in a dielectrophoresis (DEP) force exerted on the ac-polarized colloidal particle to direct colloidal motion and assembly toward the high or low ac-field regions, depending on its induced dipole with respect to the applied field.16−19 It is observed that methyl-terminated alkanethiol monolayer treatment on the gold-coated hemisphere of the JP can effectively alter the DEP response with a negative (n-DEP)-to-positive DEP (p-DEP) transition as increasing ac-frequency, opposite to the p-DEP-to-n-DEP transition observed for the homogeneous precursor particle; additionally, the n-DEP-to-p-DEP crossover frequency of alkanethiol-treated JPs exhibits a strong dependence on alkanethiol thickness.16 In this work, we further investigate the ac-field directed assembly of JPs and examine the dependence of JP assembly on ac-field frequency, medium conductivity, and JP surface chemistry. Received: July 6, 2012 Revised: August 24, 2012 Published: August 27, 2012 13201

dx.doi.org/10.1021/la302725v | Langmuir 2012, 28, 13201−13207

Langmuir

Article

Figure 1. (a) SEM images (i,ii) of carboxyl-end functionalized PS particles of d = 3.8 μm coated with a thin gold layer of 30 nm thick on one hemispherical surface; the red circle shown in panel (ii) is a guide to eye for the illustration of the contour shape of the entire spherical particle. (b) Schematic diagram of experimental setup using a pair of 40 μm × 40 μm square gold microelectrodes separated by a distance, L = 50 μm, whose dimensions are not proportionally drawn in this schematic illustration. (c) Computed profile of ac-electric field strength between two coplanar microelectrodes at L = 50 μm at f = 100 kHz by using Poisson’s equation with COMSOL.

■

to be −5.73 mV, indicating nearly net charge neutrality of the zwitterionic JP and thereby also suggesting nearly 50% surface coverage of the gold film on a PS particle. It should be also noted that based on both SEM and particle size measurements, the effect of the 30-nm-think gold coating and 1−2-nm-thick alkanethiol coating on the overall particle size is negligible. NaCl (Aldrich, > 99%) is added to the aqueous suspensions to vary the medium conductivity, σm, from 0.0007 S/m to 0.116 S/m (corresponding to salt molar concentration from ∼10−5 M to 0.01 M) as determined by a conductivity meter (Cole-Parmer, model #148190). All the experiments are conducted in aqueous media of constant pH = 5.5. Experimental Setup and Characterization. As schematically illustrated in Figure 1b, a pair of 40 μm × 40 μm coplanar gold microelectrodes of 170 nm thickness, which are deposited on a titanium layer of 30 nm thickness on the glass coverslip to enhance the bonding between gold and glass, separated by a distance of 50 μm, are fabricated onto a clean glass coverslip by photolithography and metal deposition.16 The fabrication is completed by photoresist dissolution and metal liftoff in acetone. A fluid cell, using a sterile coverglass chamber (Fisher Scientific, Lab-Tek*), is assembled onto the microelectrode-fabricated coverslip using UV glue (Nolan 80) to provide an enclosed microchamber environment for the experiments under applied ac-fields to minimize solvent evaporation. In this work, ac-electric potentials of constant peak-to-peak ac-voltage, Vpp =10 V and varied ac-frequency, f = 25 kHz − 20 MHz are applied across the two microelectrodes by a function generator (Agilent 33220A) via copper tapes. The resulting ac-electric field gradient near the coplanar microelectrodes is computed as illustrated in Figure 1c. The motion and field-induced assembly structure of JPs in aqueous media are examined in situ by confocal laser scanning microscopy (CLSM) (Zeiss LSM 5 Pascal) using the reflection mode with 20× and 63× objective lenses, thanks to the gold coating on one JP hemisphere. Time-series micrographs are taken to examine the evolution of colloidal assembly in response to nonuniform ac-fields. The structure of JP assembly is also verified by using an inverted optical microscope (Olympus IX-71) under the same ac-fields.

EXPERIMENTAL SECTION

Materials and JP Preparation. Plain PS particles of diameter d = 3.8 μm stabilized with carboxyl end-functionality (polydispersity < 3%, Duke Scientific) are used for the preparation of metallic JPs with a thin gold layer coating on one hemisphere.16 Briefly, PS particles suspended in a mixture of 80% ethanol (Aldrich) and 20% deionized water (Barnstead Nanopure II) are spun-coated at a speed of 500 rpm for 1 min and adhered to a 25 mm × 75 mm glass coverslip (Fisher Scientific) that is precleaned with UV Ozone Cleaner (Jelight, Model 144AX) to form a monolayer of PS particles. The top side of dried PS particle monolayer is then deposited with a thin gold layer of 30 nm thick using a sputter coater (Emitech K675) with the deposition beam perpendicular to the normal direction of the dried PS particle layer, resulting in the nearly 50% surface coverage of the gold coating on the precursor PS particle (see Figure 1a). The surface chemistry of gold JPs (Au-JPs) can be furthered modified by a self-assembled monolayer (SAM) of alkanethiols on the gold thin film before the dry Au-JP monolayer is sonicated in deionized water. Methyl-terminated 1-octanethiol (CH3) and amineterminated 5-amino-1,3,4-thiadiazole-2-thiol (NH 2) (both from Sigma-Aldrich, purity ≥98%) are used in this work to further treat Au-JPs with different surface chemistry on the gold-coated hemisphere. The monolayer of CH3 or NH2 thiols is formed via selfassembly by placing a large droplet of 0.5 M CH3 or NH2 thiol solution in ethanol on the dry Au-JP monolayer for 30 min, and then rinsing the ethanol to remove excess alkanethiols from the particle surface. After the gold film deposition is followed with or without alkanethiol chemical treatment, the JPs are then redispersed in deionized water with mild sonication and are repeatedly washed with deionized water before experiment. Au-JPs are characterized by scanning electron microscopy (SEM) (Jeon JXA-8600) to examine the gold coating. As shown in Figure 1a, the bright region on the spherical particle confirms the coating of a gold thin layer on one hemispherical cap of a PS particle, indicating roughly 50% surface coverage of the gold film on a PS particle. Furthermore, the zeta-potential, ζ, of CH3SAM-treated JPs is measured by light scattering (Brookhaven ZetaPlus) to be −36.41 mV, in comparison to ζ = −52.37 mV for the precursor PS particle, suggesting that one hemisphere is well coated with uncharged CH3-SAM at an equivalent 30−40% overall surface coverage. For NH2-SAM-treated zwitterionic JPs, ζ is measured

■

RESULTS AND DISCUSSION We start with examining the response of Au-JPs in a salt-free aqueous suspension of σm = 0.0007 S/m to applied ac-fields by 13202

dx.doi.org/10.1021/la302725v | Langmuir 2012, 28, 13201−13207

Langmuir

Article

Figure 2. Representative micrographs (a−d) taken in time series, demonstrating the typical DEP driven assembly of Au-JPs in an aqueous suspension of σm = 0.0007S/m under applied nonuniform ac-field of frequency, f = 5 MHz, and peak-to-peak voltage, Vpp = 10 V. All the micrographs are taken by CLSM with the reflection mode. (e) The blowout of the structures of Au-JP assembly in the red boxes of panel d. (f) The pearl chain formation resulting from p-DEP on Au-JPs is also confirmed by optical microscopy.

colloidal particles can align into pearl chains.17−19,24,25 For AuJPs, we have derived previously16 that the effective CM factor can be approximately the average of the corresponding CM factors of a dielectric PS sphere and a PS particle with a uniform gold outer shell as Re[f CM,JP(f)] = (Re[f CM,PS(f)] + Re[f CM,PS‑Au( f)])/2. At applied Vpp = 10 V across the microelectrode separation distance of 50 μm that yields Erms,max = 1.44 × 105 V/m, Re[f CM,JP( f)] is estimated to vary from 0.282 to 0.864 corresponding to f = 1 kHz to 50 MHz and σm = 0.0001−0.1 S/m (see Supporting Information Figure 1). It is thus indicated that the Au-JPs exhibit p-DEP at applied f = 25 kHz to 20 MHz in this work, in contrast to the typical p-DEP to n-DEP crossover for the same sized PS particles with increasing ac-frequency.19 For the formation of pearl chains at Udipole > kT, Re[f CM,J( f)] has to be greater than 5.19 × 10−2 at interparticle separation, r < 300 μm.17−19,24,25 Hence, according to our estimation shown in Supporting Information Figure 1, the formation of pearl chains of Au-JPs is expected to occur readily when Au-JPs are driven to become concentrated near microelectrodes under imposed p-DEP. Next we examine the effect of σm on ac-field induced Au-JP assembly in aqueous suspensions added with NaCl. We observe in Figure 3 that as σm is increased to be greater than 0.0153 S/ m; applied ac-frequency exhibits a significant influence on the motion and assembly of JPs in salted suspensions. At σm > 0.0153 S/m, apparent n-DEP is observed when f exceeds a critical frequency that is σm dependent, where reduced or no accumulation of Au-JPs is observed as indicated in the redboxed micrographs in Figure 3. For instance, at σm = 0.06 S/m, we observe a critical frequency, f ∼ 500 kHz, below which JPs are driven away from the high field region near microelectrodes to form small random aggregates, instead of pearl chains, in bulk suspension. The n-DEP-like observation at high frequency

CLSM under the reflection mode. Under applied ac-fields of f = 5 MHz and Vpp = 10 V, we observe the field-driven motion of Au-JPs toward the edge of microelectrodes, which corresponds to the high field intensity region as indicated in Figure 1c, indicating a p-DEP response of Au-JPs. As the micrographs in time sequence are shown in Figure 2a−d, the originally welldispersed Au-JPs are driven to move toward the electrodes and gradually align along the field gradient lines to form pearl chains in the suspension and at the edge of microelectrodes shortly after the nonuniform ac-field application. Over time, the pearl chains are also attracted to each other and form colloidal bundles or large assemblies in the high field regions near the microelectrodes. As shown in Figure 2e, a closer look under high magnitude confocal microscopy indicates that the Au-JP assembly indeed exhibits a crystalline structure, consistent with a previous report in the literature.17 Similar pearl chains of AuJP assembly are also confirmed by optical microscopy as shown in Figure 2f. Similar p-DEP induced chaining and assembly of Au-JPs in salt-free aqueous suspension is observed at f = 100 kHz to 20 MHz, confirming the p-DEP response of Au-JPs thanks to the strong polarization of the gold coating on one hemispherical surface of PS particles. The formation of the pearl chains results from the attraction and alignment of induced particle dipoles exceeding particle thermal fluctuation.17−27 We estimate the interparticle interaction energy, Udipole of polarized JPs, when their induced dipoles are aligned at a center-to-center interparticle distance, r, in the suspension of permittivity, εm, due to imposed p-DEP. According to the classical Clausius−Mossotti (CM) theory, Udipole(r) = −4πεm(d/2)3Re[f CM(f)]2▽|Erms|2(d/2r)3, where Re[f CM( f)] is the real part of the frequency-dependent CM factor, and E rm s is the root-mean-squared ac-field strength;18,19,25 when Udipole exceeds the thermal energy, kT, 13203

dx.doi.org/10.1021/la302725v | Langmuir 2012, 28, 13201−13207

Langmuir

Article

Figure 3. Micrographs of Au-JP assembly in aqueous suspensions of varied σm in response to applied nonuniform ac-fields of constant Vpp = 10 V and varied f. All images are taken with CLSM with reflection mode after ∼20 min of the application of ac-fields.

Figure 4. Experimental micrographs (top) taken by CLSM and reconstructed micrographs (bottom) of Au-JP assembly in aqueous suspensions of σm = 0.0007, 0.0021, 0.0153, and 0.06 S/m under applied nonuniform ac-fields of f = 1 MHz and Vpp = 10 V.

13204

dx.doi.org/10.1021/la302725v | Langmuir 2012, 28, 13201−13207

Langmuir

Article

we estimate the effective f range, in which the ICEP-induced JP motion becomes significant. The lower frequency limit, f ICEP,L of ICEP near a particle of R in radius is predicted to nearly overlap with the effective AC-EO frequency range and approximately to be below 100 kHz as σm varies from 0.0007 S/m to 0.116 S/m, respectively. The upper frequency limit, f ICEP,U, is determined by the characteristic “RC time”, which scales with f ICEP,U ∼ D/λR, and could approach up to 1 MHz at relatively high σm (= 0.116 S/m). The theoretically calculated lower and upper ICEP effective frequency range in this work is also summarized in Figure 5a.

in salted suspensions clearly contradicts the theoretical prediction of p-DEP for Au-JPs over the f range of 1 kHz to 20 MHz. Furthermore, the structure of Au-JP assembly also exhibits a strong dependence on σm at high ac-frequency. As shown in Figure 4, at f = 1 MHz, shorter pearl chains are observed with increasing σm, suggesting weakened DEP force imposed on AuJPs. Upon further increasing σm ≥ 0.06 S/m, random aggregates are observed as indicated by the rendering of particle aggregation using the software of Audodesk 3ds Max. Clearly, the DEP force does not appear to be the dominant one to direct the dynamics and assembly of Au-JPs in highly conductive aqueous medium. It should be noted that the DEPdirected crystalline assembly of precursor PS particles is confirmed under the same nonuniform ac-fields.28 Hence we consider other possible ac-electrokinetic effects, such as acelectroosmosis (AC-EO), induced-charge electrophoresis (ICEP) and electrothermal effects,18,19,26−28 all of which exhibit strong dependence on σm and f and have the opposite effect from DEP on particle assembly. First, we exclude the contribution from AC-EO flow, which is produced between two polarized microelectrodes to form an ac-field-induced double-layer and cause the fluid flow, simply because the frequency range applied in this work is far greater than the optimal AC-EO frequency range at which the AC-EO flow is the most effective on manipulating particles. According to the capacitive charging theories, 26,27 the optimum AC-EO frequency, fAC‑EO scales with D/λL, where D is ion diffusivity and is ∼1.31 × 10−9 m2/s for Na+, λ is the Debye length, and L is the spacing between two electrodes and equal to 50 μm in this work. As λ decreases from ∼100 nm to ∼3 nm corresponding to the increased NaCl concentration from 10−5 to 0.01 M in this work, respectively, fAC‑EO is in the range of ∼260 Hz to 8.7 kHz, which is far below the acfrequency range we apply in this work; therefore, the AC-EO effect is negligible in this work. With increased σm, electrothermal flow and the Joule heating effect might arise to affect the colloidal assembly in ac-fields. The nonuniform temperature field created by local ac-Ohmic heating is predicted to scale with σmVrms2/κ, where κ is the thermal conductivity and Vrms is the root-mean-square voltage across the electrodes.19,28,29 For experiments conducted in aqueous media of low conductivity, i.e., σm < 0.05 S/m, electrothermal flow is estimated to be negligible.19,28,29 At the highest σm = 0.116 S/m in this work, the temperature difference across the suspension is estimated to be no more than 5 °C. Yet experimentally we detect that the temperature difference across the Au-JPs in aqueous suspension is nearly zero at σm = 0.116 S/m so that the electrothermal contribution to the observed Au-JP aggregation is also excluded. Accordingly, for asymmetric colloids such as the Au-JPs used in this work, the ICEP effect has been predicted to primarily direct the motion of JPs perpendicular to the applied ac-field lines, resulting from an imbalanced inducedcharge electroosmosis (ICEO) flow on the two hemispheres of the JP.17,30 For our Au-JP particles, the polarizability on the gold hemispherical side is much stronger than that on the PS side, leading to a considerable difference in the induced-charge distribution on each hemisphere and thereby imposing ICEP on Au-JPs when applied ac-frequency is low. Under ICEP, the Au-JP is driven toward the direction of the untreated PS hemisphere perpendicular to the electric field, yielding an effect opposite to that of DEP-induced chaining of particles. Hence,

Figure 5. (a) Calculated ICEP upper and lower frequency limits for Au-JPs against σm. (b) Experimentally determined critical transition frequency for the onset of random JP aggregation, in comparison to the theoretically estimated ICEP upper frequency limit, against σm.

Apparently, as the f ICEP,U approaches 1 MHz at σm = 0.116 S/m, it is suggested that the ICEP becomes a dominant driving force to direct the motion and assembly of Au-JPs in the direction perpendicular to the nonuniform ac-field line. While the DEP force imposed on JPs is conversely weakened with increasing σm as theoretically suggested in Supporting Figure 1, the combined ICEP and DEP effects on Au-JPs could result in a random aggregation structure as observed in Figures 3 and 4 for the case at high σm and high f. To further examine the predominant contribution of the ICEP effect to the repellent motion and random aggregation of Au-JPs in the suspensions of high σm, we tentatively calculate the critical transition frequency, fcr from p-DEP to apparent n-DEP, at which the pearl chain structure of Au-JP assembly commences to vanish. Surprisingly, the experimentally measured fcr shows a good agreement with the estimated f ICEP,U at σm = ∼0.06−0.116 S/m 13205

dx.doi.org/10.1021/la302725v | Langmuir 2012, 28, 13201−13207

Langmuir

Article

Figure 6. (a) Schematic illustration of zwitterionic JPs coated with NH2-SAM on the gold-treated hemisphere. (b) Micrograph of zwitterionic JPs in deionized water in the absence of applied ac-field, exhibiting small aggregates formed in the bulk suspension. (c,d) Micrographs demonstrating that zwitterionic JPs with opposite surface charges on each hemisphere can rapidly form irreversible crystalline assembly in deionized water under applied nonuniform ac-fields of f = 1 MHz and Vpp = 10 V.

hemisphere, assuming the surface density of different alkanethiol SAMs on Au-JPs is essentially similar. However, as shown in Figure 6, the DEP-induced assembly of zwitterionic JPs is drastically different from that of Au-JPs or JPs capped with CH3-SAM. Irreversible crystallization of zwitterionic JPs is rapidly formed by imposed DEP under applied nonuniform acfields, exhibiting no dependence on f and σm; it is mainly due to the alignment of the ac-field induced dipoles on zwitterionic JPs and strong electrostatic attraction between opposite charged surfaces. It should be noted that in the absence of ac-fields, only small crystalline aggregates are observed as shown in Figure 6b. By contrast, under applied f = 100 kHz to 20 MHz, imposed DEP on JPs leads to rapidly concentrating both dispersed zwitterionic JPs and their small aggregates to form large random close packed crystalline assembly as shown in Figure 6c−d.

as shown in Figure 5b, suggesting that the ICEP force becomes predominantly stronger than the DEP force at f ranging from ∼100 kHz to ∼1 MHz at high σm. As a result, the Au-JPs are driven away from the high-field region near the microelectrodes and aggregate to form small clusters, instead of pearl chains. In addition to JPs with bare gold coating on one hemisphere, we have also conducted similar experiments with the JPs capped with CH3−SAM or NH2−SAM on the gold-coated hemisphere under nonuniform ac-fields. For the JPs capped with CH3-SAM, we have observed a similar DEP-induced pearl chain structure in salt-free aqueous medium as the results show in Figure 3 for Au-JPs. Consistently, the σm effect also plays a significant role on the dynamics and assembly of JPs capped with CH3-SAM in salted aqueous media. Interestingly, we observe a distinct n-DEP to p-DEP at f ranging from ∼105 Hz to ∼107 Hz with increasing σm from 10−4 S/m to 0.1 S/m (see Supporting Figure 2). The dependence of the n-DEP to p-DEP crossover frequency on σm and alkanethiol chain length has been reported previously in detail.16 In addition, we have also examined the response of JPs capped with NH2-SAM to applied nonuniform ac-fields. As the purchased precursor PS particles are end-functionalized with carboxyl groups for colloidal stability, the NH2-SAM coating on the gold hemisphere of Au-JP indeed leads to a zwitterionic JP carrying opposite charges on each hemisphere.6 The polarization and DEP dynamics of zwitterionic JPs is expected to be similar to that of Au-JPs and JPs capped with CH3-SAM on one

■

CONCLUSIONS In summary, we have examined the assembly of JPs of varied surface chemistry on one hemispherical surface in aqueous suspensions under high frequency nonuniform ac-electric fields. Au-JPs experience p-DEP over the entire ac-frequency range from 25 kHz to 20 MHz at σm ranging from ∼0.0007 S/m to ∼0.116 S/m. However, the ac-field induced assembly of Au-JPs show a strong dependence on σm. Pearl chains of Au-JPs are observed at low σm from 0.0007 S/m to ∼0.0153 S/m; as increasing σm to the range of 0.0153−0.116 S/m, only small 13206

dx.doi.org/10.1021/la302725v | Langmuir 2012, 28, 13201−13207

Langmuir

Article

(6) Hong, L.; et al. Clusters of charged Janus spheres. Nano Lett. 2006, 6, 2510−2514. (7) Nie, Z.; et al. Janus and ternary particles generated by microfluidic synthesis: Design, synthesis, and self-assembly. J. Am. Chem. Soc. 2006, 128, 9408−9412. (8) Nisisako, T.; et al. Synthesis of monodisperse bicolored Janus particles with electrical anisotropy using a microfluidic co-flow system. Adv. Mater. 2006, 18, 1152−1156. (9) Walther, A.; Muller, A. H. E. Janus particles. Soft Matter 2008, 4, 663−668. (10) Hong, L.; Jiang, S.; Granick, S. Simple method to produce Janus colloidal particles in large quantity. Langmuir 2006, 22, 9495−9499. (11) Paunov, V. N. Novel method for determining the three-phase contact angle of colloid particles adsorbed at air−water and oil−water interfaces. Langmuir 2003, 19, 7970−7976. (12) Perro, A.; et al. Design and synthesis of Janus micro- and nanoparticles. J. Mater. Chem. 2005, 15, 3745−3760. (13) Nonomura, Y.; Komura, S.; Tsujii, K. Adsorption of disk-shaped Janus beads at liquid−liquid interfaces. Langmuir 2004, 20, 11821− 11823. (14) Roh, K.-H.; Martin, D. C.; Lahann, J. Biphasic Janus particles with nanoscale anisotropy. Nat. Mater. 2005, 4, 759−763. (15) Bradley, M.; Rowe, J. Cluster formation of Janus polymer microgels. Soft Matter 2009, 5, 3114−3119. (16) Zhang, L.; Zhu, Y. Dielectrophoresis of Janus particles under high frequency ac-electric fields. Appl. Phys. Lett. 2010, 96, 141902. (17) Gangwal, S.; Cayre, O. J.; Velev, O. D. Dielectrophoretic assembly of metallodielectric Janus particles in AC electric fields. Langmuir 2008, 24, 13312−13320. (18) Pohl, H. A. Dielectrophoresis; Cambridge University Press: Cambridge, UK, 1978, (19) Morgan, N.; Green, N. G. AC Electrokinetics: Colloids and Nanoparticles, 1 ed.; Research Studies Press: Hertfordshire, UK, 2003. (20) Kretschmer, R.; Fritzsche, W. Pearl chain formation of nanoparticles in microelectrode gaps by dielectrophoresis. Langmuir 2004, 20, 11797−11801. (21) Lumsdon, S. O.; Kaler, E. W.; Velev, O. D. Two-dimensional crystallization of microspheres by a coplanar AC electric field. Langmuir 2004, 20, 2108−2116. (22) Jones, T. B. Electromechanics of Particles; Cambridge University Press, Cambridge, U.K., 1995. (23) Kadaksham, J.; Singh, P.; Aubry, N. Dielectrophoresis induced clustering regimes of viable yeast cells. Electrophoresis 2005, 26, 3738− 3744. (24) Sharma, A.; Bakis, C. E.; Wang, K. W. A new method of chaining carbon nanofibers in epoxy. Nanotechnology 2008, 325606. (25) Lele, P. P.; Mittal, M.; Furst, E. M. Anomalous particle rotation and resulting microstructure of colloids in AC electric fields. Langmuir 2008, 24, 12842−12848. (26) Castellanos, A.; et al. Electrohydrodynamics and dielectrophoresis in microsystems: Scaling laws. J. Phys. D: Appl. Phys. 2003, 36, 2584−2597. (27) Gonzalez, A.; et al. Fluid flow induced by nonuniform ac electric fields in electrolytes on microelectrodes. II. A linear double-layer analysis. Phys. Rev. E 2000, 61, 4019−4028. (28) Hoffman, P. D.; Sarangapani, P. S.; Zhu, Y. Dielectrophoresis and AC-induced assembly in binary colloidal suspensions. Langmuir 2008, 24, 12164−12171. (29) Gagnon, Z.; Chang, H.-C. Aligning fast alternating current electroosmotic flow fields and characteristic frequencies with dielectrophoretic traps to achieve rapid bacteria detection. Electrophoresis 2005, 26, 3725−3735. (30) Gangwal, S.; et al. Induced-charge electrophoresis of metallodielectric particles. Phys. Rev. Lett. 2008, 100, 058302.

random aggregates are observed at low f, due to the combined ICEP and DEP effects. The experimentally determined fcr for the random colloidal aggregation shows good agreement with the theoretically predicted upper ICEP frequency limit, f ICEP,U. Additionally, the similar behavior of DEP-induced assembly is confirmed with the JPs coated with CH3-SAM on the goldtreated hemisphere. In contrast, zwitterionic JPs with NH2SAM coating on the gold-treated hemisphere can readily assemble into random close-packed crystals due to strong electrostatic attraction that is further enhanced by the alignment of the induce dipoles on JPs under high-frequency ac-fields. Hence, our results demonstrate that the assembly structures of JPs can be rapidly and actively controlled by acfrequency in nonuniform ac-fields, medium conductivity, and JP surface chemistry, which can become useful for guiding the rapid synthesis of highly hierarchical structured materials from anisotropic building blocks.

■

ASSOCIATED CONTENT

S Supporting Information *

Figure 1: The calculated value for the real part of the effective CM factor of the JPs of d = 3.8 μm coated with a thin gold layer of 30 nm thickness on one hemisphere as a function of the applied frequencies at varied medium conductivity. Figure 2: (a) The calculated real part of the effective CM factor of JPs of d = 3.8 μm with a 1-octanethiol SAM on a gold-coated hemisphere as a function of the applied frequencies at varied medium conductivity, and (b) experimentally obtained crossover frequency as a function of the medium conductivity for JPs of d = 3.8 μm with a 1-octanethiol SAM on a gold-coated hemisphere in comparison to the theoretically calculated crossover frequencies.This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

† Cabot Corporation Business and Technology Center, 157 Concord Road, Billerica, MA 01821, USA,

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Dan Ho for help in computing the profile of ac-field strength near microelectrodes. We are grateful to the fruitful discussion with Prof. H.-C. Chang at the University of Notre Dame. This work is supported by the National Science Foundation (CMMI-1129821).

■

REFERENCES

(1) de Gennes, P. G. Soft matter. Rev. Mod. Phys. 1992, 64, 645. (2) Casagrande, C.; et al. “Janus beads”: Realization and behaviour at water/oil interfaces. Europhys. Lett. 1989, 9, 251. (3) Cayre, O.; Paunov, V. N.; Velev, O. D. Fabrication of asymmetrically coated colloid particles by microcontact printing techniques. J. Mater. Chem. 2003, 13, 2445−2450. (4) Cayre, O.; Paunov, V. N.; Velev, O. D. Fabrication of dipolar colloid particles by microcontact printing. Chem. Commun. 2003, 2296−2297. (5) Erhardt, R.; et al. Amphiphilic Janus micelles with polystyrene and poly(methacrylic acid) hemispheres. J. Am. Chem. Soc. 2003, 125, 3260−3267. 13207

dx.doi.org/10.1021/la302725v | Langmuir 2012, 28, 13201−13207