Marla D. McConnell, Matthew J. Kraeutler, Shu Yang, and Russell J. Composto

Janus particles are traditionally composed of two chemically distinct regions, making them suitable for applications as biological sensors, nano-motors, anti-reflection coatings, optical sensing devices, two-phase stabilizers, as well as for fundamental studies in asymmetric particle assembly. Recently, interest has emerged in the synthesis of complex, patchy- and multi-region Janus particles. Such particles have more chemical surface variation than two-region Janus particles, which results in an increased number of available assembly mechanisms and morphologies. Because metal nanoparticles have dimensionally-dependent surface plasmon resonances that absorb radiation in the visible region of the spectrum, there has been much focus on manipulating their size, shape, and architecture. The formation of gold nanoshells around a different core material has garnered much attention, because these hierarchical structures allow the plasmon resonance to be tuned into the near-IR.  Janus particles that incorporate optically active species such as gold are attractive for many applications, including as sensors.

            Here we develop a new approach to create self-assembled, optically tunable, multi-region and patchy, gold-on-silica Janus particles. Multi-region particles with gold caps on the top surface, gold patches around the particle equator and silica bottoms, as well as Janus particles with tunable gold patch sizes, can be synthesized on the nano- and sub-micron scale. The multi-region and patchy Janus particles can both be synthesized from a single, hierarchical self-assembly method. Nano- and sub-micron silica particles were selectively protected on their bottom surfaces by covalent attachment to a copolymer film, then gold nanoparticles were assembled on their top surfaces. The morphologies of the gold particle layer, and the resulting optical properties of the Janus particles, were tuned by changing the surface energy between the silica and gold particles, followed by thermal annealing.