Controlled Levitation of Colloids through Direct Current Electric Fields

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1 Supplemental Information Controlled Levitation of Colloids through Direct Current Electric Fields Carlos A. Silvera Batista*, Hossein Rezvantalab, Ronald G. Larson and Michael J. Solomon Department of Chemical Engineering and Biointerfaces Institute, University of Michigan, Ann Arbor, MI, *Corresponding Author. Materials and Methods Synthesis of JP-PS particles. JP-PS were synthesized as described generally in reference (1) and specifically in ref. (2), with minor modifications in particle recovery. Fluorescent carboxylate-modified polystyrene spheres (CB-PS) with diameter of 0.5µm and 1 µm ( F8812, F8821 from Life Technologies) were washed with water and re-suspended in ethanol at ~0.5 % volume. 400 µl of particle suspension in ethanol is spin-coated onto a glass microscope slide (75 mm 50 mm 1.0 mm, Fisher Scientific) using a Laurell spin coater at 300 rpm for 20 s, and then at 3000 rpm for 20 s, to create a monolayer. The glass microscope slides were cleaned by soaking in a 0.1 N potassium hydroxide solution (319325, Sigma- Aldrich) for 20 min. Layers of Cr (2.5 nm) and Au (13.5 nm) were deposited on the particle monolayers using a vacuum deposition system (01410, Angstrom Engineering). Atomic force microscopy was used to verify the thickness of the metal layer on a silicon chip that was placed in the evaporation chamber alongside the glass slides. The measurements are performed on height steps that were created by covering half of the silicon chip with silver epoxy prior to metal deposition. The silver epoxy layer was removed before the measurements by brief sonication, first in acetone and then in isopropanol. After evaporation, the gold side of the particles was functionalized with a self-assembled monolayer of 16-mercaptohexadecanoic acid (MHA) (448303, Sigma Aldrich) to stabilize the particles against aggregation and reduce irreversible adsorption to the electrodes. Functionalization was achieved by immersing the glass slides, with the JP-PS particles still attached, for 24 hr in MHA solutions (2mM MHA in 200 proof ethanol). After functionalization, particles were dislodged from the glass slides through bath sonication in 100 ml of ultrapure water (18 MΩ). Particles were recovered after centrifugation using Teflon tubes, resuspended in 10 ml of water and tip sonicated (60 s, 100 Watts, 1/8 diam, Cat. # UX ). Sometimes large aggregates of particles and flakes of metal remained despite (or potentially because of) the tip sonication. In those cases, passing through a 5 µm gravity filter from Millipore (Cat. # SVLP02500) was performed. Finally, the suspension was centrifuged and the water exchanged once for dimethyl sulfoxide (DMSO) (Anhydrous 99.9 %, Sigma Aldrich), with conductivity in the range of ms/m. Particles were analyzed by scanning electron microscopy (SEM) using a Nova FIB instrument. Figure 1A shows SEM images of the particles, with characteristic metallic and polymer halves. The volume fraction for experiments was about 0.001%, varying between % and %. The volume fractions of particles were measured using a disposable hemocytometer from INCYTO (c-chip DHC-N1). 1

2 Synthesis of PEG-PS Particles. The crosslinking of the PEG chains to the carboxyl groups on the CB-PS particle to make the PEG modified particles (PEG-PS) was performed through the EDC/NHS crosslinking of carboxylates to primary amines. First, 50 µl of the CB-PS (2%) stock suspension were added to a 2 mm solution of EDC (1-ethyl-3-[3- dimethylaminopropyl]carbodiimide, Thermo Scientific) in MES buffer (0.2 M, ph 6.0, Cat. # BB-10, Boston Bioproducts). After vortexing, the reaction was allowed to proceed in a rotator for 10 min. Then, sulfo-nhs in MES buffer is added to the suspension containing the particles. We used the premeasured, no-weight format of Sulfo-NHS (24520, Thermo Scientific). 40 µl of MES buffer was added to an individual tube containing 2 mg of Sulfo-NHS and then 22 µl of this solution was added to the reacting mixture to a final concentration of 5 mm in Sulfo-NHS. After vortexing, the reaction was allowed to proceed for 10 min in a rotator. After centrifugation, the medium was exchanged for 1 ml of PBS buffer (ph 7.4, Cat. # , Life Technologies). Tip sonication was briefly performed to redisperse the particles. Then, 10 mg of the 30 kda amine terminated PEG ( 30 kda NH2-PEGs from Laysan Bio Inc) was added to the activated particles in PBS. The same procedure was performed to attach PEG polymers of other molecular weights (NH2-PEG, 20 kda, 10 kda, 5 kda and 2 kda from Laysan Bio Inc). The final reaction coupling the NH2-PEG to the activated particles proceeded for 2 hr in a rotator. The centrifuge tubes were covered with foil. Afterwards, the particles were centrifuged and wash three times with water. By using PEG polymers of different molecular weights we systematically modified the zeta potential of the particles. Table 1 shows the particles used to build the phase diagram in Figure 3 of the main text. The sample 30 kda PEG-PS_2 was obtained by using 1 mg 30 kda PEG, instead of 10 mg, in the synthesis of the particles. Other PS Particles. The uniform PS particles for experiments shown in Figure 4 were from Bangs Laboratories (Dragon Green, 0.52µm, Cat. # FS03F). Table S1. Zeta potential of 1.0 µm PS particles functionalized with PEGs of different molecular weights. Sample ζ (mv) 30 kda PEG- PS ± kda PEG- PS ± kda PEG- PS ± kda PEG- PS_ ± 0.3 CB- PS ± 4.0 2

3 Assembly of Devices. The electric fields were applied in devices prepared as in references (3, 4). The devices consisted of two transparent electrodes separated by a non-conducting spacer of nominal thickness 120 ± 10 µm, 100 ± 10 µm, 70 ± 10 µm. The electrodes were microscope cover glass slides ( from Fisher) coated with a 10 nm layer of indium tin oxide (ITO) (ZC&R Coatings for Optics). Cells were assembled with the ITO surface in contact with the solvent. To reduce irreversible adsorption, the slides were precleaned by sequentially sonicating in acetone, isopropanol and DI water for 10 min in each solvent. Then, right before assembly of the devices, the slides were exposed to a UV-Ozone treatment (Jelight UVO Cleaner) for 20 min. Auto adhesive SecureSeal TM spacers (Grace Bio-Labs) were employed to maintain a clean surface. The spacers were square plastic sheets with a total area of mm 2, while the circular area which confines the liquid was 13 mm in diameter. The spacers had a cavity of 13 mm in diameter, a total area of mm 2 and were 120 ± 10 µm, 100 ± 10 µm and 75 ± 10 µm thick. The 120 ± 10 µm spacer is a standard product (Cat. # ), while the 100 ± 10 µm and 70 ± 10 µm spacers were custom ordered. The devices were assembled by first fixing the bottom electrode with double-sided tape to a platform that facilitated handling and placing on microscope stage. The platforms were 4.5 cm 6.5 cm glass rectangles, made from 1 mm thick glass slides. A circular area of 5.72 cm 2 (2.7 cm diam) was removed from the slide to allow imaging of the bottom electrode. Leads (~3 cm long wires, 40 Gauge) were glued to the ITO side of the slides using conductive tape from Ted Pella (Cat. # ). Once the top electrode was in place, the cavity was filled with ~20 µl of liquid through a hole (~1 mm) that was lined up to the edge of the cavity. The holes were drilled on the top electrode using a Dremel tool equipped with diamond bit. The holes were drilled prior to solvent cleaning. Applying Electric Fields, Visualization and Image Analysis. DC electric fields were generated by means of an Autolab PGSTAT 128N potentiostat/galvanostat. Electric fields were applied at constant current (galvanostatic mode), see Fig. S2. The strength of the electric field was computed through E = i/λ!, where i is the current density and λ! is the conductivity of the suspension. In these experiments we applied currents in the range of 3 70 µa. Particles and their motion were imaged using a Nikon A1R confocal laser scanning microscope (CLSM) with a 60, 1.4 NA, oil immersion objective lens. The polymer side of the particles was imaged in fluorescence mode by excitation with a 561 nm laser and an emission window from 570 to 620 nm. The gold side was imaged in reflection mode at 488 nm. Fig1b shows 2D confocal images of the particles under quiescent conditions. The green color represents the gold half, whereas the red color represents the polymer side. During the course of an experiment, 512 pixels 512 pixels images were taken every 2 µm in the z-direction, from the bottom to the top electrode. The 2 µm step provided an adequate time resolution; for example, a scan with 75 steps was completed in 5.0 s. Qualitative information on the particle motion was readily obtained by observing time sequences of volumes during the course of an experiment. The volumes or 3D images were constructed from the stacks of 2D images using the Elements Nikon Software. Quantitative 3

4 information was extracted by tracking the motion of particle ensembles rather than a single particle. The fact that particles generally moved in unison, and as a band, favored this approach. The ensemble analysis started by calculating the average intensity of every slice in a stack using the Time Series pluggin in FIJI ( Therefore, the stacks of 2D images could be represented as a curve displaying the average intensity at every z-position (Fig. 2). Therefore, the 2D stacks were used to construct 2D images to create curves of average intensities. By constructing similar curves at every time, it is possible to track the trajectory of particles as shown in Fig. S6. Measurement of Zeta Potential. The zeta potential of CB-PS and PEG-PS was measured using a Malvern Zetasizer Nano instrument. The zeta potential of 1 µm particles was calculated using the Helmholtz-Smoluchowski expression. For the 0.5 µm particles, Henry s expression was used. The Debye length in our samples is ~50 nm. The ion concentration was estimated to be ~ 0.02 mm from the values of conductivity, assuming an ion diffusion coefficient equal to m 2 /s. The zeta potential of JP-PS was measured using microelectrophoresis in which particle velocities were directly quantified through microscopic observations as per references (5, 6) and using the analysis of Bowen (7) to determine the slip velocity at the wall of the capillary as well as electrophoretic velocity of particles. The microelectrophoresis devices were built by gluing a 50 mm long glass capillary (height = 0.2 mm, width = 2 mm, from VitroCom, Cat. # ) to the same glass platform used to assemble the electric field devices. The capillary was filled with a suspension of particles containing ~ % and % in volume fraction of JP-PS and CB-PS. After inserting the leads, the device was sealed with vacuum grease and UV glue. The leads were gold wires (3 cm long, 0.5 mm diameter, 99 %) from Alfa Aesar. The wires were flattened at the tip so that they could fit tightly inside the capillary. Before a measurement, the wires were cleaned by sequentially sonicating in acetone, ethanol and water for 5 min in each solvent. The device was mounted on the microscope stage and connected to the potentiostat. The microelectrophoresis experiment proceeded by first placing the objective at the longitudinal center of the capillary. Then, the mid plane of the capillary was located using lower magnification objectives so that measurements were performed at approximately (x = 0, see Fig. S3 for cross section of capillary). An electric field of 143 V/m was applied and the movement of the particles was observed using a 60 objective and recorded for further image analysis. The measurements were repeated at different heights, starting from the bottom surface of the capillary and moving up in intervals of 10 µm towards the top surface (~ 185 µm). Since JP-PS particles sediment rapidly, they were only observed within 5 µm of the bottom of the capillary. Figure S4 shows the velocity distribution of the CB-PS particles. The profile shows the typical parabolic profile of charge colloids in close capillaries. The electrophoretic velocity for the CB- PS particles was 1.0 µm/s (Fig. S4, at the stationary height) which translates into a zeta potential of 35 ± 5 mv. This value of zeta potential is similar to the values measured through light scattering ( 37 ± 2 mv). The electrophoretic velocity of JP-PS particles was calculated consistently by subtracting the slip velocity at the wall of the capillary or through direct comparison with the CB-PS particles located at the same plane of view within the channel. The electrophoretic velocity and zeta potential of JP-PS particles were 0.66 µm/s and -23 ± 4 mv mv, respectively. 4

5 Figure S1. (A) SEM images of JP-PS particles. The inset is an image of a single particle at higher magnification. The scale bars represent 10 µm (Fig1A) and 1 µm (inset). (B) 2D confocal image of an ensemble of JP-PS particles. The red and green colors represent the polymer fluorescence and the reflection from gold, respectively. Scale bar is 10 µm. Figure S2. Transient potential curves as fixed currents of (A) 30 µa and (B) -30 µa are applied through the devices. 5

6 Figure S3. Cross section of the capillary. Figure S4. Velocity of CB-PS particles at different heights above the bottom of the capillary. The stationary height is at 39 µm (61 µm from the center of the capillary). 6

7 Figure S5. 3D confocal images of JP-PS (A-C) and uniform 1µm CB-PS particles (D-F) under DC electric fields. JP-PS particles: (A) before a field is applied; (B) and (C) 60 s after applying 2.1 kv/m in the positive and negative z direction, respectively. CB-PS particles: (D) before a field is applied; (E) and (F) 60 s after applying 0.83 kv/m in the positive and negative z direction, respectively. The images report volumes of cross section area µm2 and inter-electrode gap 148 µm (A-B) and 164 µm (C); 168 µm (D-E), 148 µm (F). 7

8 Figure S6. Behavior of the steady-state levitation height (H s ) as a function of field magnitude, E for JP-PS particles. In these experiments the field points upwards (+ Bottom) or downwards (- Bottom). These data points represent an experimental replicate of the data in Figure 2D of the main text using a different sample from the same batch of particles. Notice the similarities between the trends in this figure and Figure 2D, although the data points are shifted relative to Figure 2D. 1. A. B. Pawar, I. Kretzschmar, Patchy particles by glancing angle deposition. Langmuir. 24, (2008). 2. O. Shemi, M. J. Solomon, Effect of surface chemistry and metallic layer thickness on the clustering of metallodielectric Janus spheres. Langmuir. 30, (2014). 3. A. A. Shah et al., Liquid crystal order in colloidal suspensions of spheroidal particles by direct current electric field assembly. Small. 8, (2012). 4. A. A. Shah, M. Ganesan, J. Jocz, M. J. Solomon, Direct current electric field assembly of colloidal crystals displaying reversible structural color. ACS Nano. 8, (2014). 5. M. N. van der Linden et al., Charging of Poly(methyl methacrylate) (PMMA) Colloids in Cyclohexyl Bromide: Locking, Size Dependence, and Particle Mixtures. Langmuir. 31, (2015). 6. S. Das et al., Boundaries can steer active Janus spheres. Nat. Commun. 6, 8999 (2015). 7. B. D. Bowen, Effect of a finite half-width on combined electroosmosis-electrophoresis measurements in a rectangular cell. Journal of Colloid And Interface Science. 82,

9 (1981). 9