Electric Field Assembly of Colloidal Superstructures - ACS Publications

Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 4437−4443

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Electric Field Assembly of Colloidal Superstructures Ahmet F. Demirörs* and Lauriane Alison Complex Materials, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland

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S Supporting Information *

ABSTRACT: The assembly of materials from building blocks has been in the core of a wide range of applications from catalysis to photonics and electronics. External electric fields enable the interactions between building blocks to be controlled via induced dipoles. Dipolar interactions were used so far to obtain one-dimensional chains or three-dimensional non-close-packed lattices. However, complex colloidal assemblies and clusters of simple spherical particles are rare. Here we demonstrate a novel selfassembly approach enabling the formation of regular axially symmetric clusters, an array of colloidal assemblies as per design of posts, and hierarchical complex assemblies by using posts and dipolar interactions or combining them. Regulating the polarization of the particles from positive to negative allows us to control the interparticle interactions from attractive to repulsive at the poles or equator of the particles. Therefore, such particle−particle interactions enable switching between Saturn ring-like and candleflame-like axially symmetric assemblies, which could potentially be exploited for display applications.

S

as per design of posts, or (iii) hierarchical complex assemblies when posts and dipolar interactions are employed together. When an AC electric field is applied, colloidal particles are expected to polarize and acquire a dipole moment.28 Such a spherical particle having a radius a with a volume fraction of φ is polarized under an alternating electric field of amplitude E and angular frequency ω. An induced dipole, μ⃗, defined as μ⃗ = vpε0εef f(ω)E⃗ , is then created, where vp is the volume of the particle and ε0 is the vacuum dielectric permittivity. Hereby, εeff is the effective relative permittivity that depends on φ, and29

elf-assembly of complex structures based on simple colloidal building blocks is of practical interest for fabricating materials with unique photonic1 and electronic2,3 properties. It is also of fundamental importance as a model system to understand self-assembly processes at different length scales from molecules to macroscopic objects.4−7 A wide variety of interparticle interactions (e.g., attractive, repulsive, and DNA sticky) have been exploited over different interaction ranges to assemble a diverse set of colloidal structures, including particle clusters, colloidal strings,8−10 colloidal arrays, or 2D11−13 and 3D lattices.14−16 In addition to interparticle interactions, templated self-assembly was also employed to direct the structure from close-packed colloidal crystal and clusters to icosahedral clusters. Furthermore, electric and magnetic fields or templates were also employed to direct the assembly structure from one-dimensional chains8,10,17,18 and two-dimensional arrays18−21 to threedimensional clusters22−24 or to template close-packed crystals25,26 or clusters.4,27 Despite all these recent advances in colloidal assembly, there have been relatively few examples of control on the interactions to obtain complex preprogrammed cluster structures via electric fields. The existing dipolar assemblies are limited to electric fields at low frequency range, which leads to unwanted electrohydrodynamic flows.21 Using high field frequencies while being able to switch the polarity of colloid mixtures is expected to provide more stable, reversible, and complex assemblies from simple building blocks. Here, we demonstrate electric-field-induced reversible interactions, which were tuned from similar polarity (repulsive) to opposite polarity (attractive) by changing the dispersion medium. This allows us to assemble (i) complex, axially symmetric, regular colloidal clusters in a Saturn ring-like or candle-flame-like fashion, (ii) an array of colloidal structures © 2018 American Chemical Society

εeff (ω) = 3εmℜ{fCM (ω)/[(1 − fCM (ω)φ)2 ]}

(1)

is written with εm the relative permittivity of the medium and f CM(ω) the Clausius-Mossotti function. The polarization mechanism described by the Maxwell− Wagner−O’Konski (MWO) relaxation28,30 assumes the polarizability to be frequency dependent, whereas it assumes the permittivity and conductivity of the particles and medium to be frequency independent. It stems from the difference of dielectric properties between particles and dispersing medium captured by f CM(ω) = (ε*p − ε*m)/(ε*p + 2ε*m). The frequency dependence is accounted through the complex permittivity term, εp,m * = ε0εp,m − iσp,m/ω, where the subscripts p and m refer to the particle and medium and ε and σ to their relative permittivity and conductivity, respectively. Let us assume that induced dipoles further interact with other dipoles where the dipolar interactions are expressed within a point dipole approximation as Received: May 16, 2018 Accepted: July 20, 2018 Published: July 20, 2018 4437

DOI: 10.1021/acs.jpclett.8b01538 J. Phys. Chem. Lett. 2018, 9, 4437−4443

Letter

The Journal of Physical Chemistry Letters

Figure 1. Finite element analysis and sketches of assembly configurations. A colloidal particle in the proximity of one of the electrodes changes the local electric field strength around itself. (a) Electric field strength modulations of a sedimented colloid given in the xz plane. Insets show the cross sections in the xy planes on the top and bottom of the colloid. Field strength on the top and bottom of the particle dramatically drops forming a potential trap for other particles. Electric field strength modulation and polarization of a colloidal particle is analyzed for cases when (b) the colloid is less polarizable than the medium (ℜ{fCM (ω)} < 0) and (c) the colloid is more polarizable than the medium (ℜ{fCM (ω)} > 0). (d) Interactions of dipolar particles for repulsive and attractive configurations. Large and small particles, with similar polarity, assemble at a head-to-toe configuration as in (e); however, assembly for a sedimented (1) and an elevated (2) large (red) particle is expected to slightly differ. Opposite polarity of large and small particles induce attraction resulting in a Saturn ring-like assembly for binary-sized particles as in (e3). (f) Possible configurations of assembly when large and small particles are brought together on electrodes with posts.

Udip(r ) = μ2 (1 − 3 cos2 θ )/(4πε0εeff r 3)

ability to design such posts allows us to control the assembly of colloids into 2D arrays. Ultimately, one can combine a binary colloidal system with opposite polarization and an array of posts to arrive at complex hierarchical assemblies. In principle, the particle polarization and, hence, the field gradients around the particle can be altered via changing the applied field frequencies. However, low frequency electric fields usually cause electrohydrodynamic flows, destabilize the system, and thus the formed structures.31 Therefore, a 1 MHz AC field was applied to avoid such unwanted effects. For a binary mixture of colloids, interparticle dipolar interactions and dielectrophoretic forces can be used to control (i) the microstructure of particle assembly and (ii) the spatial distribution of the particles. Figure 1 shows finite element calculations (Comsol Multiphysics 5.2) of electric field strength modulations caused by a spherical dielectric colloid placed between two electrodes. Electric field strength at different cross sections of the sample cell are shown in Figure 1a for a colloid, sedimented on one of the electrodes. The field strength in the xz plane is displayed in Figure 1a, and the cross sections in the xy plane above and below the colloid are given as insets. Polarization of the particle and therefore the dipole of the colloids depend on the suspending medium. If the colloid is less polarizable than the medium (ℜ{fCM (ω)} < 0), the field strength modulation and the dipole orientation is opposite to the field direction, as given in Figure 1b. However, the field strength modulation and polarization of a colloidal particle that is more polarizable than the medium (ℜ{fCM (ω)} > 0) is reversed and stays along the field direction, Figure 1c. Because dipolar interactions are angle dependent (see eq 2) interactions of dipolar particles can be repulsive or attractive depending on the orientation and the angle between the dipoles, as shown in Figure 1d. For a binarysized-colloid mixture with different polarities, some possible configurations of assembly are given in Figure 1e. Particles with similar polarities are expected to assemble at a head-to-toe

(2)

where r is the center-to-center distance and θ is the angle between vectors r⃗ and E⃗ . The maximum interactions are observed for θ = 0° when the two particles are at (head-to-toe) contact. Note that here we consider only the real part of the f CM(ω) and neglect its imaginary part; see Ristenpart et al.21 for the potential including the imaginary part. In addition to dipolar interactions, if the electric field is nonuniform, particles experience dielectrophoretic (DEP) forces defined as FDEP = vpε0εeff ∇E2 /2

(3)

The sign of FDEP is then given by the value of εef f that depends on ℜ{fCM (ω)}. When particles are more polarizable than the medium (ℜ{fCM (ω)} > 0), they are displaced toward high field regions also referred to as positive dielectrophoresis. Conversely, when particles are less polarizable than the medium (ℜ{fCM (ω)} < 0), they move to low field regions due to negative dielectrophoresis. Such a technique allows the distribution of suspended colloidal particles to be spatially controlled using electrical field gradients to achieve particle manipulation, separation, and assembly. Interactions between colloids due to their polarizations can be used to assemble binary mixtures of colloids. A spherical colloid that is less polarizable than the medium (ℜ{fCM (ω)} < 0) weakens the electric field strength above and below itself (at the poles) while increasing the field strength around itself (at the equator). This field alteration eventually forms a potential that interacts with other particles, and it can potentially be used to form complex particle assemblies. In addition, one can also generate dielectric posts via photolithography on the electrode surface and induce field modulations by using arrays of such dielectric posts. The 4438


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Figure 2. Clausius-Mossotti factor of colloids at frequency range in DMSO and water. Real (black, square) and imaginary (red, round) parts of the Clausius-Mossotti factor are plotted here. For polarizations the real part (ℜ{fCM (ω)}) is taken into account. By altering the solvent of the dispersion from DMSO to water, ℜ{fCM (ω)} changes sign for the polystyrene (PS) particles being ℜ{fCM (ω)} = − 0.44 in DMSO and ℜ{fCM (ω)} = 0.62 in water at 1 MHz. (a) Frequency dependence of the f CM(ω) for PS particles in DMSO. (b) Frequency dependence of the f CM(ω) factor for PS particles in deionized water. (c) Frequency dependence of the f CM(ω) for silica particles in water (gray) and DMSO (black).

and from thereof the surface conductivity in a similar fashion as reported by de la Cotte et al.33 Because the bulk conductivity of PS is negligible, its surface conductivity has the main contribution to the conductivity σPS in the complex permittivity term, ε*p = ε0εp − iσp/ω. For PS colloids, the Zetapotential values were measured in water and DMSO, as −33.7 mV and −2.8 mV, respectively (see Supporting Information Table S1). By using these Zeta potentials we estimated the conductivity of the particle as 1.68 × 10−2 S m−1 in water and as 5.20 × 10−5 S m−1 in DMSO. When we plugged in these numbers33 and calculated the Clausius-Mossotti functions for PS, we observed that real part of the Clausius-Mossotti factor reads negative, ℜ{fCM (ω)} = −0.44 , in DMSO and switches to read positive, ℜ{fCM (ω)} = 0.62 , in water at 1 MHz; see Figure 2a,b. These values agree with the experimental observations and prove that the surface charges were responsible for the switching behavior. Table 1 also shows

configuration, but due to the size mismatch, a large (red) particle assembles several small particles at its poles. Note that, assembly for a sedimented (e1) and an elevated (e2) large (red) particle is expected to slightly differ as shown in Figure 1e. Particles with opposite polarity attract each other at the equators resulting in a Saturn ring-like22 assembly as shown in Figure 1e3. When an array of dielectric posts is fabricated on the electrode surface, posts also modify the electric field strength around them allowing us to design complex assemblies and control the spatial distribution of colloidal particles. Figure 1f shows some of the possible configurations of assembly when two types of particles are assembled on electrodes with lithographic posts. The Clausius-Mossotti factor f CM(ω) in eq 1 is the main parameter that dictates the sign of polarization. This factor depends on parameters like the frequency of the field, the dielectric constants, and the conductivities of the medium and the particle. Typically, polystyrene is known to exhibit both positive and negative polarity at different frequencies.21 However, the change of sign in Clausius-Mossotti factor happens around an electrohydrodynamically less stable frequency of ∼100 kHz. To our surprise, the polystyrene (PS) particles with iron oxide content (microparticles GmbH, PS-MAG-RhB-S2538, 4.69 μm in size) exhibit positive polarity, ℜ{fCM (ω)} > 0, at frequencies as high as 1−2 MHz. When the particles are dispersed in dimethyl sulfoxide (DMSO) the behavior switched to negative polarity, ℜ{fCM (ω)} < 0 at 1 MHz and stayed negative over a range of frequencies. Silica colloids, 1 μm in size and synthesized according to van Blaaderen et al.,32 exhibit negative polarity regardless of the media at 1 MHz. The reason for the unexpected polarity switch of PS in the two different media is the change of particle conductivity. From the parameters like the field frequency (always kept at 1 MHz), the dielectric constant, and the conductivity influencing the Clausius-Mossotti factor, particle conductivity is the only one that significantly differs when the dispersing medium is changed from DMSO to water. Note also that, from these three parameters, the difference in dielectric constants of silica (εSilica = 3.8) and PS (εPS = 4) are minimal, which leaves us with the conductivity term as a possible reason for the polarity change observed for PS but not for silica colloids. Therefore, we looked into the Zeta-potential of the PS particles in both media to assess the particle conductivity. We measured the Zeta potential of PS colloids to estimate the surface charges

Table 1. Parameters Used To Estimate the f CM(ω) Function dielectric constant particle, εp PS in water PS in DMSO silica in water silica in DMSO

dielectric constant medium, εm

particle conductivity, σp [S m−1]

frequency [MHz]

4

87

1.68 × 10−2

1

4

47

5.20 × 10−5

1

3.8

87

4.80 × 10−3

1

3.8

47

8.0 × 10−4

1

the comparison of the parameters influencing the f CM(ω) function and clearly demonstrates that the particle conductivity for PS differs significantly between the two media in comparison to the difference silica colloids exhibit. Although the particle conductivity also drops for silica particles from 4.8 × 10−3 S m−1 in water to 8 × 10−4 S m−1 in DMSO, this difference is not as drastic as the PS case and does not yield a polarity reversal. The plots for silica also agree with the experimental observations with negative Clausius‑Mossotti values: ℜ{fCM (ω)} = −0.41 for DMSO and ℜ{fCM (ω)} = −0.44 for water; see Figure 2c. Thus, the particle and media combinations provided an easily accessible alteration of interparticle interactions, and more importantly this was allowed at a high frequency avoiding unwanted electrohydrodynamic effects. 4439


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silica, used in our experiments) to move toward the posts.34,35 When an array of rod-shaped posts was deposited on the electrode to assemble colloidal rods, we observed that rods aligned along the field and formed assemblies of standing rods on these posts; see Experimental Methods. Because the induced dipole of the rods would favor aligning along the field direction, we obtain standing rods on the electrode. At the same time, the dipolar and dielectrophoretic interactions (see Supporting Information) between the posts and these rods drive the rods at the low field regions, which are above the lithographic posts. Therefore, assemblies of standing rods on rod-shaped lithographic prints form, Figure 3d. Figure 3d is a 3D reconstructed image of confocal microscopy images, where rods stood on the line-shaped fixed posts. Combining the toolbox of tunable dipolar interactions with a binary mixture of particles with different size and polarity should allow for more complex and reversible colloidal assemblies. The ability to control the dipolar polarity of a colloid allows us to program the structure of other particles assembled around it. In a binary particle system of 4.69 μm polystyrene (PS) particles and 1 μm silica particles in DMSO, both particles are less polarizable than the medium. Thus, the two types of colloids acquire a similar polarity, Figure 1e. As a result, a sedimented PS particle attracts the silica colloids to its poles, as shown in Figure 4a. To determine the assembly of smaller particles at the two poles, confocal microscopy images of the binary assembly in DMSO was taken at two elevations, one above and one below the PS colloid, Figure 4c. Here the assembly of binary particles forms a candle-flame-like assembly as shown in the 3D reconstruction of series of confocal images in z-axis, given in Figure 4e. By changing the dispersing medium from DMSO to water, it was possible to reverse the polarity of the central PS colloids, while the polarity of the silica remained unchanged. Therefore, interactions between PS and silica were altered, and silica particles were attracted to the equator of the PS particle as displayed in the experimental 3D reconstruction in Figure 4b. The confocal microscopy image of the assembly in water is given in the xy-plane (and xz-plane, inset) in Figure 4d. A 3D reconstruction of a series of images of the assembly in water is given in Figure 4f. The 3D reconstruction clearly displays the Saturn ring-like superstructure of the assembly. One of the tunable parameters to explore forming more structures out of binary assemblies of colloids would be to tune the size ratio between the colloidal particles and monitor the influence of this for the structure of the assemblies. To address the influence of the size ratio between the colloidal particles, we calculated the electric field strength around assemblies of inversely polarized particle clusters for a range of size ratios from 1:4 to 4:4. From the electric field strength calculations shown in Supporting Information Figure S2 we can see that the field strength distribution around the colloids remains unchanged irrespective of the size ratio. For evaluating the field strength distribution around the particle we determined the field strength at a distance a quarter of the particle size away from the particle for all sizes. The comparison of the field strengths around the colloidal particles was plotted in Supporting Information Figure S2a for the positively and negatively polarized particles, which form the assembled Saturn clusters. Similarity of the electric field around the particles means that all parameters in the dipole moment formula μ⃗ = vpε0εef f(ω)E⃗ besides the vp, volume term, will read equivalent.

Instead of using Brownian colloidal particles, polymer posts lithographically printed on the electrode can be used to generate control on spatial distribution of assemblies by designing and exploiting colloid−colloid and colloid−post dipolar interactions. Freely designed and lithographically printed dielectric polymer posts provide a simple way to program the position of colloids and their assemblies. Similar to colloids, dielectric posts printed on the surface of an electrode polarize and alter the field strength around them. These dielectric posts are fixed to the surface and can be easily designed in different sizes and shapes. Consequently, such posts can be used to control the locations of the colloidal particles and their clusters. We demonstrate this by depositing permanent 500 nm thick polymer discs of different sizes on the ITO electrode and by examining the assembly of colloids with such an electrode. When an electric potential is applied, particles with a similar polarity started to accumulate over these posts. Increasing the field to 0.1 V μm−1, the colloid assembled firmly on them; see Figure 3a. On round posts with 1 μm diameter, which is

Figure 3. Fixed dielectric patterns to trap and assemble particles. (a) Confocal microscopy image of 1 μm fluorescent silica colloids trapped over 1 μm wide and 500 nm thick photoresist discs on an ITO electrode under an electric field of 0.1 V μm−1. (b) Few particle clusters are assembled on 2 μm sized photoresist posts under same conditions as (a). Insets show SEM images of the posts. (c) Finite element simulations showing the electric field strength above the microfabricated posts given in (a) and (b). (d) A reconstruction image of confocal microscopy images where permanently fixed rod shaped posts trap standing rods. Inset shows an SEM of the ITO electrode with rod-shaped photoresist posts.

similarly sized with the colloids, we obtained a single particle array of colloids. Increasing the size of the posts to 2 μm provided more space for the particles and resulted an array of particle clusters over the posts; see Figure 3b. Insets show SEM images of the ITO electrodes with microfabricated polymer posts. The electric field strength over the posts on the ITO electrode was calculated by finite element analysis. Field strength above the posts in the xy plane given in Figure 3c displays the decrease of the field strength at the post locations. This decrease in the field strength makes it dielectrophoretically more favorable for the less polarizable particles (e.g., 4440


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Figure 4. Colloidal superstructures and hierarchical assemblies. Representations of the dipolar interactions observed in (a) DMSO and (b) water dispersions shown together with the experimental 3D microscopy reconstructions of the assemblies. (c) Electric field induced assemblies of silica (green, 1 μm) and polystyrene (red, 4.69 μm) particles in DMSO at a field strength of 0.1 V μm−1 imaged above and below the focal plane of larger polystyrene particles. (d) Assembly of the same particles under similar conditions in a water dispersion. Inset is an xz-scan of one of the assemblies, demonstrating the assembly of small particles at the equator of the large one. (e) Experimental 3D reconstructed image of assemblies in (c) displaying the flame-like superstructure of colloids. (f) 3D reconstruction of the assembly in (d) displaying the Saturn ring superstructures. (g) Sketches of the dipoles that construct the hierarchical assembly of particles on dielectric posts printed on an electrode. (h) Microscopy image of the hierarchical assembly in DMSO. (i) Hierarchical assembly observed in water. Scale bars are 5 μm.

Therefore, the magnitude of the dipole will be dictated by the size of the particle. For our system typical dipolar forces for a 0.1−0.25 V/μm electric field strength and for a 1 μm colloid are about 2−10 pN. For such forces on a 1 μm colloid, electric potential energies result on the order of ∼102 kT. Thus, assemblies of 1 μm or larger colloids with any size ratio are expected to be energetically favorable. However, the size ratio between the assemblies will determine the space per colloid and therefore dictate how many colloids will fit into the available volume. The electric field strength will also tune the magnitude of the dipole moment and therefore the volume of the attraction zone around a central colloidal particle and may induce multilayers of assembly with increasing field strength

and size ratio; see Supporting Information Figure S2b−g. In addition, when the size of the satellites (3 μm) is increased in comparison to the central colloid (10 μm), multiple-layer assemblies of satellite particles become suppressed and we obtain a single layer of satellites around the central one (E = 0.1 V/μm, Supporting Information Figure S2h). Furthermore, if a combination of dielectric posts and binary particle assemblies were used, more complex assemblies could be achieved. These assemblies are expected to behave differently depending on the dispersing media as sketched in Figure 4g. In DMSO, PS and silica beads acquire a similar dipole; as a result, both PS and silica particles assemble on the posts as shown in Figure 4h. When the particles are sharing a 4441


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The Journal of Physical Chemistry Letters post, the large one sits in the center while being surrounded by the small ones. In water, PS particles acquire an induced-dipole opposite to the posts. Therefore, PS particles are repelled from the posts while the silica colloids are attracted on them. However, PS particles are expected to attract the silica beads on their equator; accordingly, the hierarchical assembly given in Figure 4i occurs. Lithographic posts offer a variety of novel superstructures and ability to form ordered structures at another scale, thus enabling hierarchical ordered structures to be constructed. However, the method used for post-fabrication allows for a wafer-scale production, and while being reusable, it is relatively costly, especially when compared with particle lithography techniques that use colloidal self-assembly.36 In addition, these posts allow the location of cluster assemblies to be dictated and may potentially be beneficial for programmed assembly of functional colloidal clusters at precisely known locations for device applications. Although colloids in electric fields have been studied for decades, here we demonstrated a novel reversible assembly technique by using binary mixtures of colloids to create a variety of complex assemblies, including Saturn ring-like and candle-flame-like clusters. When we used dielectric posts in addition to particles, this allowed us to obtain hierarchical arrays of clusters. The variety in assemblies was achieved by tuning the dipolar interactions between the building blocks via changing the polarization of the particles, which helped us to control and program the pathway of the assembly. We expect that the lithographic post geometries and arrangements can be changed by modifying the lithographic patterns, and this can open the door to design more complex patterns and superstructures. In addition, an even richer variety of different particle configurations could be possible by using different particles with diverse size, type, and degree of polarization; e.g., see Supporting Information Figure S1 for a ternary assembly of colloids.

coverslip to a thickness of 500 nm. This resist was exposed to UV to cross-link some regions, where the rest was dissolved with developer. The coverslip was then washed with deionized water and further used.39 These fabrications were performed at the ETH clean room facility FIRST. Finite Element Analyses and f CM(ω) Calculations. Finite element analyses were conducted by using Comsol 5.2 to estimate the electric field strength modulated by the PS particle and the dielectric posts. Such simulations are semiquantitative and do not include the electrical conductivity and the frequency dependence of the dielectric response of the colloid. These calculations provide useful information on how the field strength changes in 3D for a given potential applied between the electrode (0.1 V μm−1 in our case). Taking the polarity of the particles that is either a negative or a positive, such analyses help to predict several features of the cluster assembly (see main text). The f CM(ω) plots were calculated by using experimental values from literature or our measurements. For calculating the f CM(ω) one needs the conductivity and dielectric susceptibility values of the media and the colloids (see the f CM(ω) equation in main text). The particle conductivities for the particles were estimated by the approach of de la Cotte et al.33 This approach uses the Zeta potential of the particles for estimating the surface conductivity of the particles which has the dominant contribution to the particle conductivity because the bulk conductivity of PS is negligible. Microscopy Setup. Our imaging experiments were performed using a Leica SP2 inverted confocal scanning laser microscope with a PL APO 63x/1.40 NA objective.

EXPERIMENTAL METHODS Colloids. The spherical core−shell silica particles used as colloidal cargo had a fluorescent core of diameter d ≈ 500 nm, surrounded by a nonfluorescent shell with a thickness of 250 nm. Such particles were synthesized using a protocol proposed by van Blaaderen et al.32 Suspensions were prepared by mixing these particles with 4.69 μm superparamagnetic colloids purchased from Microparticles GmbH (PS-MAG-RhBS2538). Rod shaped silica particles with a length of 4.65 μm ± 0.4 μm and width of 752 ± 117 nm were synthesized according to Kuijk et al.37 Electrode and Electric Field Sample Cell Fabrication. Transparent and conductive indium tin oxide (ITO) coverslips were used as electrodes for applying the AC electric field at 1 MHz frequency while keeping the conductive faces parallel to one another. The sample cells were fabricated by sandwiching two ITO coverslips separated by spacers with thicknesses of 0.09 mm−0.12 mm (#0 coverslips) or 0.06 mm−0.08 mm (#00 coverslips). The electric field sample cell composed of two parallel ITO electrodes facing each other38 allows for a uniform 1D electric field, and in such a setup the real part of the f CM(ω) determines the polarization and the interparticle forces; therefore, we ignored the influence of the imaginary part of the Clausius-Mossotti factor in our calculations. Deposition of lithographic post on the ITO electrodes was performed by standard photolithography. A commercial photoresist SU8 2000.5 (MicroChem, U.S.A.) was spun on a

Figures showing the images of ternary assemblies, calculations of the interactions for different size ratios, a table of particle Zeta potential measurements, and a description on dielectrophoretic interactions (PDF)

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01538.

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AUTHOR INFORMATION

Corresponding Author

*(A.F.D.) E-mail: [email protected]. ORCID

Ahmet F. Demirörs: 0000-0002-2748-1516 Lauriane Alison: 0000-0001-8142-6764 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.F.D. acknowledges Dr. Hanumantha Rao Vutukuri for discussions, Prof. André R. Studart for discussions and support, and the Swiss National Science Foundation (Ambizione grant, Number P Z00P 2_148040) for partially financing this work.

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REFERENCES

(1) Xia, Y.; Gates, B.; Li, Z.-Y. Self-Assembly Approaches to ThreeDimensional Photonic Crystals. Adv. Mater. 2001, 13 (6), 409−413. (2) Dinsmore, A. D.; Crocker, J. C.; Yodh, A. G. Self-Assembly of Colloidal Crystals. Curr. Opin. Colloid Interface Sci. 1998, 3 (1), 5−11.

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Electric Field Assembly of Colloidal Superstructures - ACS Publications

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