Fabrication and assembly of magneto-responsive, anisotropic, and hybrid microparticles of variable size and shape.

Magnetic particles that can be remotely manipulated by an external magnetic field have attracted a wide range of interests for applications, including bioimaging, biosensing, multiplexed detection, photonic crystals, microdisplay, and separation, transport, and sorting in microfluidic devices. While most studies have focused on spherical magnetic particles, it has been reported that magneto-rheological and electro-rheological properties can be enhanced using anisotropic particles. Meanwhile, new types of self-assembled structures (e.g. chiral cluster and symmetric dimers) have been created by controlling the shape and interactions between colloidal particles under a magnetic field. Magnetic microparticles are typically synthesized by oilin-water emulsion, seeded polymerization, and microfluidic techniques. Using the emulsion method, spherical magnetic microparticles of variable sizes (from tens to hundreds of micrometers in diameter) can be synthesized. Using magnetic nanoparticles as seeds and subsequent polymerization, magneto-polymer composite particles can be formed, typically also in a spherical shape. However, it is challenging to obtain uniform sized particles by these methods. In comparison, microfluidic devices have been particularly successful in producing magneto-responsive microparticles of uniform size and surface chemistry. Yin et al. have fabricated Janus supraballs from magnetic nanoparticles and quantum dots, and demonstrated a magneto-responsive fluorescent switch for flexible bead displays. Kim and coworkers also prepared magnetic Janus microparticles with nanoscopic surface textures using microfluidic systems and demonstrated the remote controlled locomotion. However, the microfluidic techniques impose limitations on particle size (typically several hundreds of micrometers) and shape (mostly spherical), as well as constituent materials (UV curable acrylate resins), as a result of the required complex design, fabrication process, and the preparation of materials in advance. Recently, some progress has been made in preparing magnetic particles with precisely controlled size and shape by top-down approaches, including templating, photolithographic process, and stop-flow lithography. Nevertheless, questions remain in synthesizing highly responsive magnetic particles, which exploit particle size and shape, and shape anisotropy for the dynamic tuning of particle assemblies. Herein, we present a facile and versatile method to fabricate monodispersed hybrid microparticles of variable size and shape, and investigate their magneto-responsive behaviors in aqueous dispersions with and without geometric confinement. The key feature of our method involves coassembly of silica colloidal particles and magnetic iron oxide nanoparticles in a poly(dimethylsiloxane) (PDMS) mold with a periodic micron-sized hole array, followed by infiltration and UV curing of epoxy resin to lock the particles, and releasing the microstructures from the mold and the substrate. Compared to other methods, the replica molding method allowed us to fabricate smaller, non-spherical shaped magnetic microparticles (a few microns in diameter and aspect ratio up to 10). The fabrication was versatile and high throughput: approximately 10 particles/cm could be prepared in a single batch and particle size and shape could be varied by simply using different molds. Importantly, the particles were highly uniform and responsive to the magnetic field: they could be rotated and flipped freely and reversibly, and assembled into one-dimensional (1D) chain structures and a two-dimensional (2D) hexagonal array. The high responsiveness was attributed to the use of silica colloids, which not only stabilized the aqueous dispersion of the magnetic microparticles, but minimized possible cancellation of the magnetic moment by sequestering the iron oxide nanoparticles to the interstitial space between highly ordered silica colloids organized within the PDMS mold. Further, we demonstrated the possibility to create isolated magnetic inclusions in the elastomeric PDMS matrix, where the motion of particles can be exquisitely controlled within the confined geometry. First, the square-array PDMS mold with cylindrical holes (10 mmdiameter, 20 mmdepth, and 15 mmpitch) was prepared by replica molding from 2D SU8 micropillar arrays, which were microfabricated by photolithography. As illustrated in Figure 1, an ethanol suspension consisting of silica colloidal particles (500 nm in diameter) and iron oxide (a-Fe2O3) nanoparticles (under 50 nm in diameter) was dropped onto PDMS mold. Epoxy resin was then infiltrated into the PDMS mold by capillary force and cured under UV light while applying the magnetic field along the film thickness to induce the alignment of the magnetic moment of the iron oxide nanoparticles. A UV-curable epoxy resin with high elastic modulus (ca. 3 GPa) was chosen to prepare robust microparticles with the desired shape and high aspect ratio (length/ [*] Dr. S. Y. Lee, Prof. S. Yang Department of Materials Science and Engineering University of Pennsylvania 3231 Walnut Street, Philadelphia, PA 19104 (USA) E-mail: shuyang@seas.upenn.edu