Microfluidic Methods in Janus Particle Synthesis

Abstract

Janus particles have been at the center of attention over the years due to their asymmetric nature that makes them superior in many ways to conventional monophase particles. Several techniques have been reported for the synthesis of Janus particles; however, microfluidic-based techniques are by far the most popular due to their versatility, rapid prototyping, low reagent consumption and superior control over reaction conditions. In this review, we will go through microfluidic-based Janus particle synthesis techniques and highlight how recent advances have led to complex functionalities being imparted to the Janus particles.

Keywords: anisotropy; asymmetric; droplet-based; multi-functionality; nanoparticles.

Figure 1

An example of a co-flowing microfluidic method to generate Janus droplets. (A) Schematic of the droplet generation process and subsequent polymerization. A mixture of an acrylamide, a cross-linker and a photoinitiator is injected from inlet 1 and 2. The fluids injected from inlet 1 and 2 contain different fluorescent materials. Mineral oil is injected from inlet 3. (B) Chart showing the variety of particles synthesized using this technique. (C) Fluorescent image of the parallel flowing streams showing the maintenance of a clear interface between the two streams. (D) Micrograph of the droplet generation process.

Figure 2

Janus particle generation in air microfluidics. Hydrogel particles with a paramagnetic compartment were synthesized at high throughput compared to channel based microfluidic systems. The size of these particles could be tuned, and the technique allowed monodiperse particle synthesis. Enzymes were encapsulated inside these particles and they were proposed as enzymatic bioreactors. (A) Concept of the device, (B) formation steps of the of the Janus particles, (C and D) real-time images during particle synthesis, (E) synthesized Janus particles under the fluorescent microscope, characterization results of the particles show that (F) the particles are monodisperse and that (G) there is a clear distinction between the two compartments.

Figure 3

Microfluidic device that uses immiscible co-flowing streams to generate Janus and ternary microparticles. (A) Device schematic, where M1 is methacryloxypropyl dimethylsiloxane, M2 is a mixture of pentaerythritol triacrylate, poly(ethylene glycol) diacrylate and acrylic acid, and W is aqueous SDS (sodium dodecyl sulfate) solution. (B) Droplet generation from co-flowing streams, (C–F) optical and fluorescent (inset) microscopy images of the synthesized particles with varying ratios of monomers. (parts of two figures were merged from the original paper). Adapted with permission from Nie Z, Li W, Seo M, et al. Janus and ternary particles generated by microfluidic synthesis: design, synthesis, and self-assembly. J Am Chem Soc. 2006;128(29):9408–9412 . Copyright (2006)American Chemical Society.

Figure 4

(A) Schematic showing the formation of magnetically anisotropic and highly porous polymeric particles due to the two step phase separation between polystyrene and poly(vinyl acetate). (B) Illustration of the two step synthesis of highly porous isotropic magnetic polymer particles. DCM: dichloromethane; PS: polystyrene; PVAc: poly(vinyl acetate), Fe₃O₄: magnetic nanoparticles. Reprinted with permission from Al Nuumani R, Smoukov SK, Bolognesi G, et al. Highly porous magnetic Janus microparticles with asymmetric surface topology. Langmuir. 2020;36(42):12702–12711. Copyright 2020 American Chemical Society.

Figure 5

(A) Schematic of a double emulsion microfluidic device and droplets formed with one core. (B) Optical image of double emulsion Janus droplet formation in a PDMS device. (C) Optical image of the collection reservoir where the double emulsion droplets are collected and exposed to UV radiation to initiate polymerization. Reprinted with permission from Chen CH, Shah RK, Abate AR, et al. Janus particles templated from double emulsion droplets generated using microfluidics. Langmuir. 2009;25(8):4320–4323. Copyright (2009) American Chemical Society.

Figure 6

Schematic of the microfluidic device used to synthesize dimer like Janus particles. Reprinted with permission from Lu AX, Jiang K, DeVoe DL, et al. Microfluidic assembly of Janus-like dimer capsules. Langmuir. 2013;29(44):13624–13629. Copyright (2013) American Chemical Society.