Janus Supraparticles by Induced Phase Separation of Nanoparticles in Droplets

Adv. Mater. 2009, 21, 1949–1953 2009 WILEY-VCH Verlag G IO N Janus particles are non-centrosymmetric particles with two distinct sides. The two sides may differ in surface wettability or in optical, electrical, or magnetic properties. Depending upon the anisotropic properties, these particles can be used as emulsion stabilizers, analogous to surfactant molecules, optical probes for chemical, biological, or rheological measurements, or building blocks for electronic paper or displays based on particle-actuation. The early and most intuitive methods for making Janus particles involved differential surface modification of the two sides of solid particles. However, uniform surface modification of one side of the particles without affecting the surface of the other side can be challenging. Also, such cosmetically functionalized particles are not truly bicompartmental as they possess a homogeneous core. Bicompartmental Janus particles can be made by spinning disk techniques wherein, a bilayered jet of two dissimilar molten polymers, ejected off the edge of a spinning disk, breaks down into Janus droplets that are cooled and solidified in air. Such particles can also be made by the simultaneous electrohydrodynamic jetting of parallel polymer solutions under the influence of an electrical field. A significant limitation of these two techniques is that Janus particles produced using them exhibit a high degree of polydispersity. Alternatively, microfluidic techniques can be used to fabricate bicompartmental Janus particles of different shapes and chemical compositions. These techniques entail co-flowing two dissimilar monomer streams in parallel at low Reynolds numbers. Janus particles are formed within the microfluidic channels by photopolymerizing the droplets formed by the controlled breakup of these parallel streams, or by photopolymerizing across these streams using continuous flow lithography with a mask-based template. An advantage of the microfluidic techniques is the highly monodisperse Janus particles that can be made with relative ease in one step. However, since these particles are produced one at a time, a single device can generate only a few tens of grams of particles per day. Also, the microfluidic device must be shielded from any perturbations that disturb the laminar flow of the co-flowing streams or cause convective cross-mixing of the monomers within the droplets, since, these effects can lead to formation of particles with undesirable internal morphologies. Moreover, most current fabrication processes offer very limited flexibility in adjusting the relative volumes of the two phases of the Janus particles. Thus, a robust and scalable technique for producing Janus particles with a tunable internal morphology is strongly desirable, as such a technique would expand the portfolio of the properties of such particles and potentially lead to new commercially viable applications. In this paper, we describe a versatile and robust technique to fabricate Janus particles with a novel, highly anisotropic, and finely tunable internal architecture. We generate microparticles with one side composed of a hydrogel and the other side composed predominantly of aggregated colloidal nanoparticles. The creation of Janus particles with such a unique internal morphology is facilitated by the induced phase separation of colloidal nanoparticles in droplets. By using microfluidic devices, this technique can be used to make extremely monodisperse particles; moreover, this technique can also be combined with bulk emulsification methods, such as membrane emulsification, to produce Janus particles in large quantities for more commercially viable applications. We demonstrate the technique by forming Janus particles with polyacrylamide (PAAm) as the hydrogel and poly(N-isopropylacrylamide), PNIPAm, microgels as the nanoparticles. The thermosensitive nature of the PNIPAm microgels offers a means of control for precisely tuning the relative volumes of the two phases. The functional dichotomy of the Janus particles can be further enhanced by embedding different functional materials selectively into any of the two sides of the particles as illuistrated by the incorporation of magnetic nanoparticles in the microgel-rich phase of the particles. We begin with N-isopropylacrylamide, NIPAm, monomer which is subsequently polymerized by precipitation polymerization in an aqueous phase to yield an aqueous suspension of 500–nm-diameter PNIPAm microgels. Allylamine (5mol%) is copolymerized along with the NIPAm monomer to incorporate reactive amine groups into and onto the microgel particles. The cationic nature of the microgels was confirmed by measuring the electrophoretic mobility of the particles which was found to be 0.9 10 m v s . To this microgel suspension we add a small amount of an oppositely charged water soluble polymer, a high molecular weight polyacrylic acid (PAAc), to induce clustering of the microgels by electrostatic interactions between the ammonium ions of the microgels and the carboxyl groups of PAAc. We also dissolve 10wt% acrylamide in the microgel suspension along with a crosslinker (methylene-bis-acrylamide) and a