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Monodisperse Magnetic Silica Hexapods JAEHYUN KIM, Hye Jeong Hwang, Joon Suk Oh, Stefano Sacanna, and Gi-Ra Yi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05128 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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Journal of the American Chemical Society

Monodisperse Magnetic Silica Hexapods Jae-Hyun Kim a, Hye Jeong Hwang a, Joon Suk Ohb, Stefano Sacannac and Gi-Ra Yia* a

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School of Chemical Engineering, Sungkyunkwan University, Suwon, 16419, Republic of Korea, Center for Soft Matc ter Research, Department of Physics, New York University, New York, NY10003, USA, Molecular Design Institute and Department of Chemistry, New York University, New York, NY10003, USA

ABSTRACT: A simple yet versatile solution-based process to produce colloidal silica hexapods is developed in which various shapes of silica rods are grown on the faces of cubes in a controlled manner. In the presence of hematite cubic particles, water droplets nucleate on the surface of hematite by phase separation in pentanol. By adjusting the water concentration, six droplets can form on each face of the hematite cube. Silica precursor is then administered into the system which gradually diffuses into the water droplets through the oil phase. Within the droplets, hydrolysis and condensation of the precursors take place, leading to formation of silica rods. As a result, silica hexapods on a magnetic hematite cubic seed are produced. Furthermore, when the emulsions are aged at 60 oC prior to the silica growth, the water content in the solution are decreases gradually due to evaporation and spiky sharp hexapods produced. On the other hand, when organosilane precursor is added together, pancake-like hexapods are formed due to the reduction of interfacial tension. These colloidal hexapods can further be utilized as new building blocks for self-assembly for constructing functional materials or as a model system for understanding collective behaviors.

INTRODUCTION Anisotropic colloids have attracted considerable interest due to their potential uses as building blocks for designed self-assembled structures1-2 as well as in various practical applications including light diffusion films, porous membranes for separation, composite polymer films and functional coatings3-5. Recently, solution-based template growth of non-spherical anisotropic silica particles has been investigated intensively for mass production. For instance, silica particles can be grown on spherical polymer particles to obtain bowl-like or dimpled spherical particles6-8. Similarly, emulsion droplets have been utilized as a template to produce rod-like silica colloids9-10. Due to their non-spherical morphology and high aspect ratio, colloidal silica rods have been used to understand new colloidal phases such as plastic colloidal crystals or liquid crystals as well as the interaction with cells or other biological systems11,12. More recently, multiple silica rods can be grown from solid particles to generate urchin-like colloidal particles13, which could be of great importance due to its potential implication for numerous applications. However, it is still challenging to develop a method for determining a priori the exact number of silica rods growing from a solid seed. In this report, we have utilized uniform colloidal hematite cubes as a seed to selectively grow silica rods on each face of the cube, resulting in uni-

form hexapods. Furthermore, we tweak the method to tune the length, chemistry and morphology of hexapods. These uniform colloidal hexapods can be further utilized to form complex colloidal superstructures and also used as a model system for understanding dynamic selfassembled structures. Due to unique shape and magnetism, they could also find practical application as drug delivery carriers or anti-bacterial coating RESULTS AND DISCUSSION Uniform silica rods can be grown by sol-gel reaction of tetraethylorthosilicate (TEOS) inside water droplets containing polyvinylpyrrolidone (PVP) and ammonia in pentanol9. When spherical inorganic particles such as silica or titania are introduced into a solution, water droplets form on each particle as shown in Figure 1a13. Silica precursors from the oil phase were dissolved into the water droplets through hydrolysis and deposited on the particles within the droplets by condensation. Thereby, silica rods can be grown from water droplets on spherical titania or silica particles as depicted in Figure 1a and S1. As shown in the SEM image of Figure 1c, resulting matchstick-like silica particles on titania spheres were fairly uniform. Notably, as shown by the STEM-EDS spectra in Figure 1b, titania spheres were covered with a thin layer of silica, which we believe, is formed due to the hydrated surface of titania or PVP coating layers.

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enhance the catalytic activity by aging in sodium hydroxide solution (0.1 M) for 2 hours. In addition, hematite cubes can be manipulated by external magnetic fields because of their permanent magnetic dipole moment.18 It provides us with an option to control the direction of motion of the matchsticks (See Figure S2c and S2d and supplementary movie clip 1). l=570nm

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Figure 1. (a) Schematic showing the mechanism of silica-rod formation on a titania spheres. (b) TEM and (c) SEM image of the resulting silica-titania matchstick-like particles. (d) SEM image of matchstick-like particle with PVP aggregate and (e) TEM image. (f) Separation of magnetic matchstick particles by sedimentation with strong magnet on the side. Scale bars are 1 µm.

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Similarly, on the hematite cubic seeds , silica rods were successfully grown uniformly forming matchstick-like particles in bulk (Figure 1d). The TEM image in Figure 1e clearly shows PVP aggregates of the matchstick-like particles which were dried from a water droplet at the end of the silica rods. We also found that a thin layer of silica is present on the entire surface of the hematite seed. Fortunately, we could selectively separate the silica-hematite matchstick-like particles from the mixture of silica rods and matchstick-like particles by repeated sedimentation with a magnet as shown in Figure 1f. Furthermore, the hematite-silica matchstick-like particles can be used as active colloids due to the anisotropic structure and photocatalytic activity of hematite particle, decomposing hydrogen peroxide under ultraviolet or blue light15-17. When added into suspension of matchstick particles, hydrogen peroxide can be decomposed only on hematite surface under light, which reduces the concentration near hematite surface so that matchsticks selfpropel into hydrogen peroxide-rich area on substrate by diffusiophoresis (See Figure S2a and S2b and movie clip 1). It should be informed that we partially removed the thin silica layer around hematite of matchsticks prior to the experiment to

Figure 2. (a-c) Schematic illustration of the nucleation of water droplets on the hematite seed under different conditions and subsequent growth of silica rods. (a,d,g) At low concentration of water in oil (2.09 M), small droplets are randomly nucleated, from which small rods are grown. Two different sized hematite cubes are used (edge length: 570 and 950 nm). (b,e,h) At medium concentration (3.00 M), six droplets are nucleated on each face of hematite, from which six silica rods are grown forming hexapods. (c,f,i) At high concentration (3.44 M), a single droplet engulfs the hematite, from which thick rod are grown forming matchstick-like particles. Scale bars are 1 µm. (j-k) Lowmagnification SEM images of colloidal hexapods with (j) 570-nm and (k) 950-nm cubic seeds. Scale bars are 5 µm. (l-o) SEM images of colloidal hexapods on hematite cubes grown for (l) 20 min, (m) 1 h, (n) 3h, and (o) 6 h. Scale bars are 1 µm. (p-r) (p) STEM image of a colloidal hexapod with hematite cube and corresponding EDS mapping images for (q) iron and (r) silicon. Scale bars are 1 µm.

We demonstrated that water droplet formed on the surface of seed particle act as a confined microreactor for the formation of silica rods. For obtaining multiple silica


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rods instead of a single silica rod, we need to control the number of droplets controlled on hematite cubic seeds. Interestingly, we found that multiple smaller water droplets were formed at lower concentration of water in the system in the presence of hematite cubes in Figure S313. Typically, for water-in-pentanol emulsion, PVP polymers (10 wt.%) are first dissolved in pentanol and then ethanol and water are mixed, which yields a homogeneous solution. Then, when an additional water with sodium citrate (0.18 M) is mixed with the homogeneous solution, it becomes slightly opaque which indicates that water-inpentanol emulsion droplets were formed by phase separation. (see Figure S4) When water concentration is low (2.09M) comparing with one (3.44M) for matchstick-like particles (Figure 2c), smaller water droplets are formed by phase separation, which can no longer engulf the entire hematite cubes but multiple smaller droplets were formed on the particle surfaces as shown in Figure 2a and S3a. At medium water concentration (3.00 M), the water droplets and hematite cubes are comparable in size. Therefore, each face of the cube has a single droplet as shown in the TEM image of Figure S3b, from which a silica rod can be grown uniformly to yield a colloidal hexapod as depicted in Figure 2b and shown in the TEM image of Figure S3e. We surmise that a single emulsion droplet covered a cubic seed initially, but it could be divided into six small droplets due to the increase of interfacial tension when ammonia are introduced. We have observed a significant increase in the contact angle of the water droplet on the substrate upon the addition of ammonia as shown in Figure S5 and movie clip 2. SEM images in Figure 2(d to i) clearly show the effect of the water concentration on the final shapes of particles, where two different sized hematite cubes are used (edge length: 570 and 950 nm). Lowmagnification SEM images in Figure 2j and 2k show that both colloidal hexapods from hematite are fairly uniform in shape as well as length of silica rods. Notably, since the droplets are formed both in bulk and on the surface of cubic particles, we obtained also non-magnetic silica rods in the resulting products, which were purified as shown in Figure S6b and S6c by same method as performed before for matchstick-like particles. We can also control the length of the silica rods on hematite by simply adjusting the reaction time. During the growth of magnetic hexapods, samples at different reaction times were collected and observed under scanning electron microscopy as shown in Figure 2l-o, which confirmed that silica rods were grown uniformly as precursors are supplied from the oil phase. The growth rate of silica rods on the cubic seed was approximately 220 nm per hour. On the other hand, a silica shell was also formed around the hematite cubes as shown in STEM images and their EDS chemical mapping images of Figure 2p-r. Interestingly, due to magnetic cubes in the cores, colloidal hexapods with relatively short silica rods could be aligned forming rigid chains as shown in fluorescent microscopic images of Figure S7a and S7c. Those chain

structures were also observed in the SEM images of Figure S7b and S7d which were prepared by drying the suspensions under a magnetic field. For fluorescent magnetic colloidal hexapods, we have added 1 vol. % of (3Aminopropyl) triethoxysilane (APTES) in TEOS for growing silica rods with amine groups, on which FITC dyes were coupled in ethanol19-20. Furthermore, we could prepare hollow hexapods by etching out hematite cores in hydrogen chloride solution (5 M) at 60 oC as shown in TEM images of Figure S8a. Alternatively, as shown in Figure 8b, we were also able to produce hollow hexapods by growing silica rods from hollow silica cubes, which were prepared by coating silica on hematite cubes and chemically etching out the hematite cubes20.

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Figure 3. (a) Schematic illustration of sharp hexapod formations. (b-c) High-magnification SEM images of sharp hexapods with (b) short silica rods (l =2.76 μm) and (c) long ones (l =4.40 μm). Scale bars in (b-c) are 5 µm. Inset in (c) shows rounded end of silica rods on cube core. (d) Schematic illustration of tack-like hexapod formations. (e-f) SEM images of (e) pancake-like and (f) tact-like hexapods, which were grown using mixture of TEOS (99 vol%) and GPTMS (1 vol%) as precursors at 5°C and 10°C for (e) pancake-like and (f) tact-like hexapods, respectively. Inset shows longer tack-like hexapods grown at room temperature. (g) Segmented hexapods through two-step growth with TEOS and mixture of TEOS and GPTMS at 5°C of growth reaction temperature. (h) SEM images of simple short hexapods by two-step growth with TEOS at 5°C of growth reaction temperature. Scale bars in (e-h) are 1 μm.


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In addition to the length of silica rods on cubes, we could also control their diameter since the diameter of the growing rod depends on the interfacial area between the water droplet and the end of the tip21-22. Once one droplet was formed on each face of the cubic seeds at medium water concentration as described above, solution was annealed at 60 oC for 5 min to reduce the droplet size in pentanol and silica precursors are introduced at room temperature for growing silica rods. We expected to obtain relatively thinner rods with a uniform diameter. However, surprisingly, as shown in Figure 3b, we obtained spiky hexapods, in which the diameter of silica rods was gradually reduced. The formation of these spiky sharp hexapods may be ascribed to gradual diffusion of water into the pentanol phase during the growth reaction of silica. We suppose that water dissolved in pentanol was evaporated out first during short thermal annealing and then the water in the droplets on cubes may be gradually diffused out into pentanol later. When initial concentration of water in pentanol was slightly greater by 0.2M, silica rods become noticeably longer than that shown in Figure 3b. The inset image clearly shows that the tip of silica rod is round, which is quite different from the case in Figure 2. SEM image in Figure S9 shows those sharp colloidal hexapods in bulk after the purification. The diameter of the silica rods could also be controlled by changing the reaction temperature since the diameter of water droplets on silica rods can be reduced at higher temperature. Therefore, segmented silica rods in previous reports13,23-24, were prepared by programming the growth reaction temperature. Alternatively, the thickness of the silica rods can be controlled by changing the wettability of water droplets on silica. For thicker silica rods, we have added a small amount of hydrophobic silanes in precursor for reducing the contact angle of water droplet on silica. Typically, 1 vol.% of epoxy-silane (GPTMS, glycidoxypropyl trimethoxysilane) in TEOS was added into silica precursor at a low temperature (5 °C), in which TEOS was consumed at the initial stage forming a small neck but then GPTMS was consumed later which formed large plates producing pancake-like structures as shown in Figure 3e. At 10 oC, silica rods were grown faster and formed slightly longer silica rods resulting in tack-like structures on each face (Figure 3f). At room temperature, longer and peculiar silica rods were formed as shown in the inset of Figure 3f. When silica rods were grown in two steps with TEOS and mixture of TEOS and GPTMS, segmented hexapods were obtained as shown in Figure 3g. By contrast, when TEOS solutions were injected twice at low temperature (5 oC), short colloidal hexapods were obtained (See Figure 3h). The epoxy functional groups on the outer surfaces of these pancake-like silica structures can be further modified via ring-opening reaction with sodium azide (NaN3). Then, resulting azide group can be coupled with dibenzocyclooctyl(DBCO)-functionalized DNA strands via strain-

promoted alkyne-azide cycloaddition (SPAAC). As shown in Figure S10, when DNA sticky ends are selfcomplementary, DNA-modified colloidal hexapods were assembled with each other forming a colloidal chain via DNA-mediated assembly. These DNA-coated hexapods may be further utilized for building up complex structures.

Figure 4. (a-b) Fluorescent microscopic images of colloidal hexapods on magnetic hematite cubes under rotating magnetic field (Supplementary movie clip 3). Scale bars are 5 μm. (c) Timelapse images of rotating colloidal hexapods and schematic images of their configuration in supplementary movie clip 4. Scale bar is 1 μm. A blue dot is marked on one of the tips for better visualization of the rotating direction. (d) Schematic image of rotation (dotted pink circle) and translation (dotted black line) of colloidal hexapod under rotating magnetic field.

On the other hand, as observed in the case of colloidal spheres with hematite cubic cores does in a previous report25, our magnetic colloidal hexapods could rotate, under a rotating magnetic field, for which a magnet (6,000 Gauss) was installed on a rotating motor (Misung Scientific Co., MSBL610D) next to the microscope objective. In concert with magnetic rotation (35 ~ 85 rpm) as marked in Figure 4, colloidal hexapods were rotated in the same direction and translated as marked in dotted black line which is consistent with previous report on colloidal rollers25. In Figure 4c and S11, time-lapse images of rotating colloidal magnetic hexapods were obtained under rotat-


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ing magnetic field (35 rpm) in which one rotational cycle takes approximately 1.5 s. CONCLUSIONS In summary, we have prepared monodisperse colloidal hexapods of silica rods on hematite cubes by using the sol-gel method. We found that the concentration of water in the system is crucial to determine the number of silica rods on cubic particles. At low concentration of water, thin irregular silica rods are randomly grown on the surface of cubes. At intermediate concentration, we could grow six uniform silica rods on each face of the cubes. At high concentration, we obtained matchstick-like particles. Furthermore, sharp colloidal hexapods were also obtained in bulk by introducing an aging step at 60 oC for 5 min before injecting precursors, in which water in the solvents was initially removed by evaporation and then the water in droplets on hematite diffused out gradually into the solvent as silica rods grew. In addition, we could further adjust the shape of silica rods by introducing a small amount of hydrophobic silanes into the TEOS precursor solution. At low temperature, with 1 vol.% of GPTMS are mixed with TEOS, tack-like colloidal hexapods were obtained. We further changed the shape of colloidal hexapods by injecting precursors in multiple times during the growth reaction. These uniform colloidal hexapods can be further utilized for complex colloidal superstructures and also model systems for understanding dynamic self-assembled structures because of their magnetic cores and unique shape. They could also be useful for drug delivery26-28 or anti-bacterial coating29-30 due to their unique shape-dependent interaction with bacteria or cells. EXPERIMENTAL SECTION Materials. Polyvinylpyrrolidone (PVP, Mw = 40,000), 1pentanol (>99%) and ammonium hydroxide solution (28 wt. % in water) were purchased from Samchun Pure Chemical Co. Anhydrous ethanol (>99.5%), sodium citrate tribasic dihydrate (ACS reagent, >99%), tetraethyl orthosilicate (TEOS, >99%), 3-aminopropyl triethoxysilane (APTES, >98%), 3-glycidyloxypropyl trimethoxysilane (GPTMS, >98%), fluoresecin isothiocyane isomer 1 (FITC, >90%), sodium dodecyl sulfate (SDS, ACS reagent, >99%,), hydrogen peroxide solution (30 wt. % in water), trimethylphenylammonium hydroxide solution (TMAH, 25 wt. % in H2O), iron (III) chloride (FeCl3) hexahydrate and sodium hydroxide (NaOH, anhydrous) were purchased from Sigma-Aldrich. Synthesis of hematite cubes. Monodisperse colloidal hematite cubes were prepared from condensed ferric hydroxide gel as described in a previous report22. Briefly, FeCl3 · 6H2O (2M, 100 mL) and NaOH (6M, 90 mL) were mixed in water (10 mL) thoroughly, which were aged at 100 °C for 8 days in a sealed 250-ml Pyrex bottle. The final

hematite cubes were isolated by repeated sedimentation and re-suspension in deionized water. Growth of silica rods on hematite cubes. For hexapods on hematite cubes with 950 nm of edge length, PVP (1.0 g, Mn = 40 kg/mol) powders were dissolved in 1pentanol (10 mL) by magnetic stirring at 80 °C for 12 hours. Once the PVP powders were completely dissolved, anhydrous ethanol (1 ml), distilled water (200 µl), hematite cube suspension (2.5wt. %, 200 µl) and sodium citrate aqueous solution (0.18 M, 100 µl) were added into 1pentanol solution with PVP, which were mixed for a minute. Then, ammonia (28 wt.% in water, 200 µl) was mixed by hand shaking and TEOS (100 µl) was introduced into the mixture. After mixing all ingredients by hand shaking, the bottle was left to age for 12 hours. Then, powders were collected by ultracentrifuge at 3,000 rpm for 30 minutes and re-dispersed in ethanol, which was repeated twice. For matchstick-like particles, we repeated same experimental procedures but with more distilled water (200 µl). For matchstick-like particles with 570-nm hematite cube, more hematite cubes (4 wt. %) were used as seeds. For sharp ends of colloidal hexapods, the reaction samples were aged at at 60°C for 5 minutes and then silica rods were grown at room temperature after injecting TEOS. For pancake-like and tack-like colloidal hexapods, the reaction temperature was 5 oC or 10 oC, respectively, and a mixture of TEOS (99 vol. %) and GPTMS (1 vol. %) was used for precursors. For diblock hexapods, we hydrolyzed GPTMS (30 vol. %) in water (70 vol. %), which is then introduced into the colloidal hexapods. For fluorescent magenetic colloidal hexapods, a mixture of TEOS (99 vol. %) and APTES (1 vol. %) was used as precursor for growing silica rods with amine groups. Then, amine-functionalized hexapods in ethanol (0.5 wt. %) were mixed with 5 mL of ethanol with FITC (25 mg), which were stirred for 12 hours. Unreacted dyes and other chemicals were washed by centrifugation and redispersion by sonication in ethanol and water. Light-activated self-propulsion of matchstick particles. Matchstick-like particles were washed by centrifugation and re-suspended in a sodium dodecylsulfate (SDS) solution (4 mM). A buffer solution with hydrogen peroxide was prepared as fuel solution. Typically, 1 µL of matchstick-like particles was mixed into 75 µl of fuel solution. After strong vortex mixing, samples were filled in a borosilicate capillary (Vitrotubes, 100 µm × 2 mm) plasma-cleaned with the solution and sealed it with capillary wax (Hampton Research). The sample was then attached to a glass slide (thickness, 1 mm) and ready for observation under a microscope. All the observations were performed on an inverted microscope (Nikon, Eclipse Ti-U) with oil immersion, high numerical apertures objectives (60×, N.A.=1.4 and 100×, N.A.=1.4). A fluorescent metal halide lamp (Nikon Intensilight) was filtered with a



bandpass filter (Semrock, FF01-460/60-25) resulting in a blue-violet light (λ~ 430-490nm). DNA modification of tack-like colloidal hexapods. To functionalize the surface of hexapods with the azide group, tack-like hexapods with epoxy functional groups in 100 mM were mixed in sodium azide solution with 0.1% of Tween 20, which were aged for 24 h at 70 °C. Then, the azide-functionalized hexapods were thoroughly washed with DI water several times and re-dispersed in 190 ߤl of PBS buffer with 0.1% Tween 20 and additional NaCl to make the salt concentration to 500 mM. Finally, for strain-promoted alkyne-azide cycloaddition (SPAAC), also known as copper-free click chemistry, dibenzocyclooctyl (DBCO)-functionalized DNA solution (70 ߤM, 10 ߤl) was added to the suspension of azide-functionlized tack-like hexapods, which was agitated on a horizontal shaker at 1000 rpm for 24 h at room temperature. After the DNA coating, hexapods were washed with DI water several times. For confocal imaging, fluorescent dyes (Cy5) were incorporated within the DNA strands. The structure of the DNA strand waa 5ʹ-DBCO-Cy5T56GCGC-3ʹ. Characterizations. Colloidal particles were observed under SEM (Hitachi S-4300, Carl Zeiss Merlin) and TEM (JEOL LTD, JEM-2100F) and their chemical compositions are analyzed with EDS. SEM samples were prepared by depositing the washed sample on a silicon wafer and gold sputtering coating is applied on the deposited sample. TEM samples were prepared by depositing the washed sample on a carbon film coated mesh copper grid. The grid is purchased from Electron microscopy sciences (CF200-CU). The fluorescent particle and matchstick-like particle in solution were observed under optical microscope (Nikon, Eclipse Ti-U). The drop shape analyzer (KRUSS, DSA25) was used to observe change of contact angle of droplets on silicon substrate.

We would like to thank Mena Youssef for helping us improving the shape of hematite cubes and Prof. David J. Pine and Dr. Sung H. Lim for his helpful discussion and critical reading. This work was supported by National Research Foundation of Korea (2014-M3A9B8023471, 2017M3A7B8065528, and 2017R1A5A1070259). S.S. acknowledges support from the NSF through MRSEC DMR-1420073)

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ASSOCIATED CONTENT Supporting Information.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.xxxxxxx. Characterization of hexapods and hallow hexapods. TEM data of various wetting type of emulsion on hematite cube depends on water amount. Contact angle of water-inpentanol emulsion on silicon wafer. SEM data of purified hexapods. Fluorescent microscopic and SEM image of chains of magnetic hexapods. Movie clips of light-activated active motion of matchstick-like particles under magnetic field and magnetic hexapods under rotating magnetic field.

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AUTHOR INFORMATION

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Corresponding Author *[email protected]

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ACKNOWLEDGMENT

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Monodisperse Magnetic Silica Hexapods - Journal of the American

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