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Janus Particles – Properties and Applications

What are Janus Particles?

Bichromal (half-white half-black or any other color) Microspheres, Janus Particles
Bichromal (half-white half-black or any other color) Microspheres, Janus Particles

Named after two-faced Roman god Janus who looks to both the future and the past, Janus particles are typically microscopic spheres composed of two different regions.

Originally the term “janus particles” was used in reference only to particles that had a hydrophobic surface on one hemisphere and a hydrophilic surface on another.

The first “Janus beads” reported in literature were prepared glass spherical particles, with one hemisphere hydrophilic, and the other one hydrophobic. These objects were considered as amphiphilic solids.((Casagrande C, Fabre P, Raphaël E, Veyssié M. “Janus Beads”: realization and behaviour at water/oil interfaces. EPL Europhys Lett. 1989;9(3):251. doi:10.1209/0295-5075/9/3/011))

However, over the last decade the definition of “janus particles” expanded to include two hemispheres that vary in any physical, chemical, or mechanical properties.

The two hemispheres can vary in color, optical properties, surface tension, electrostatic charge, magnetic response, conductivity, fluorescence, reflectivity and more.

Fabrication of Janus Particles

There are several manufacturing techniques that create particles with such dual-functionality of janus particles. There has been a tremendous amount of research with a focus on developing novel fabrication methods. Typically the functionality is achieved with proprietary and patented process that allows extremely precise coating on one hemisphere without affecting the other. Several synthesis methods have been developed including surface coating, layer-by-layer self-assembly, polymerization, Pickering emulsion, and electrified jetting over the past two decades. This has led to the generation of various kinds of Janus beads made from a range of materials including polymer–polymer, polymer–metal,metal–metal, glass-polymer, glass-metal, ceramic-polymer, ceramic-metal and more.

Applications of Janus Particles

Optically-anisotropic janus particles with magnetic half-shells have numerous applications in diagnostics, medical research, microscopy and biotechnology, as well as electronics, due to their ability to orient themselves in response to electromagnetic field and show a visual response.

Janus Particles in Digital Displays (Electronic Paper)

The functionality of janus particles rotating in electromagnetic field is achieved by making spheres both bipolar and bichromal, with dipole precisely aligned with two differently colored hemispheres. As the spheres align themselves, the viewer will observe the color of one hemisphere, while the other hemisphere will be hidden from view, providing an obvious strong visible indication of the presence of the field or other stimuli. In alternating electromagnetic field, these microspheres have been proven to spin at hundreds of times per second.

Janus particles were originally developed for very high tolerance electronic digital displays where functionalized microspheres are used to create an image that appears to the viewer. To achieve high resolution in display applications it is critical that every single sphere responds to electromagnetic field in the same way at the same time and aligns precisely with the other spheres. It is also critical that there are no color gradients in the display.

Cospheric offers unique capability to manufacture bichromal Janus particles and microparticles with partial coatings and dual functionality. Currently half-shell or hemispherical coatings can be applied to any sphere (glass, polymer, ceramic) in sizes 45 micron in diameter and higher. Coatings can be customized for any color and coverage of between 20% to 60% of the sphere. Each coating is custom formulated for color, charge, magnetic, electric, and surface properties, and solvent resistance per customers’ needs.

Hemispherical coatings of less than 1 micron with tolerances as low as 0.25 micron have been routinely demonstrated. Color combinations are truly unlimited. White, black, silver, blue, green, red, yellow, brown, purple as well as transparent microspheres have been made. Sphericity of greater than 90% and custom particle size ranges are offered.

Janus Particles in Biotechnology

As a biomedical device, Janus particles offer opportunities to incorporate therapeutics, imaging, or sensing modalities in independent compartments of a single particle in a spatially controlled manner. This may result in synergistic actions of combined therapies and multi-level targeting not possible in isotropic systems. Janus particles can be used as therapeutic delivery carriers, in vivo imaging probes, and biosensors.((Le TC, Zhai J, Chiu WH, Tran PA, Tran N. Janus particles: recent advances in the biomedical applications. Int J Nanomedicine. 2019;14:6749-6777. Published 2019 Aug 23. doi:10.2147/IJN.S169030))

Janus particles have been shown to have advantages in the applications of dual drug release, combined therapy such as chemo-photothermal therapy, multimodal imaging, and cellular targeting because of the combination of at least two distinct materials and properties in a single particle. Moreover, the asymmetric structures of Janus beads offer compartmentalization of materials with opposite or complementary properties, each imparting a different functionality. For example, Janus beads can be designed as such that one compartment of the Janus houses a hydrophilic drug molecule whilst the second compartment encapsulates a hydrophobic agent, enabling dual release or combined therapy.((Le TC, Zhai J, Chiu WH, Tran PA, Tran N. Janus particles: recent advances in the biomedical applications. Int J Nanomedicine. 2019;14:6749-6777. Published 2019 Aug 23. doi:10.2147/IJN.S169030))

Especially, individual Janus particles (JPs) with both chemical and geometrical anisotropy as well as their assembled layers provide considerable advantages over the conventional monofunctional particles or surfactant molecules offering((Marschelke, C., Fery, A. & Synytska, A. Janus particles: from concepts to environmentally friendly materials and sustainable applications. Colloid Polym Sci 298, 841–865 (2020). https://doi.org/10.1007/s00396-020-04601-y))

  • a high surface-to-volume ratio;
  • high interfacial activity;
  • target controlling and manipulation of their interfacial activity by external signals such as temperature, light, pH, or ionic strength and achieving switching between stable emulsions and macro-phase separation;
  • recovery and recycling;
  • controlling the mass transport across the interface between the two phases;
  • tunable several functionalities in one particle allowing their use either as carrier materials for immobilized catalytically active substances or, alternatively, their site-selective attachment to substrates keeping another functionality active for further reactions.

Janus Particles in Other Applications

Fluorescent Red Coating on Silver Coated Glass 50um
Fluorescent Red Coating on Silver Coated Glass Spheres 50um

The custom coating capability offers customers the ability to create fluorescent glass micro-spheres or colored ceramic janus particles of the specific size and emission/excitation needed. As the microspheres and coating are solvent resistant they work ideally as fluorescent tracers or highly visible targets.

Researchers looking for brightly colored glass or ceramic particles might not be looking specifically for janus functionality. The benefits that janus particles offer them is the combination of high visibility of colored and/or fluorescent coatings with high temperature, solvent resistance, and general durability of materials such as glass or ceramic spheres which are not typically available in bright colors or with a fluorescent response.

Cospheric can overcoat clear glass or silver coated glass for the effect needed.