Flow Synthesis of Plasmonic Nanoshells using a Microreactor
Satoshi Watanabe, Junji Hosokawa, and Minoru T. Miyahara
Department of Chemical Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510,
Japan
Silica@gold core-shell nanoparticles consist of silica cores and gold nanoshells.
Because of their plasmon resonance absorbance of near-infrared rays, they can findapplications in a wide range of fields such as medical imaging and photothermal therapy. The
absorption peak depends on the ratio of shell thickness to core size. Controlling the structureof silica@gold core-shell nanoparticles is of crucial importance. A widely-used preparation
method of Au nanoshells is the seed-mediated growth, which is composed of three steps asfollows: the surface modification of silica cores to increase affinity with Au, producing theformation of core-shell clusters (CSCs), in which silica cores are decorated with Au
nanoparticle seeds, and the shell growth by reducing Au ions on the Au seeds.[1] This method
allows high coverage of gold seeds and good control in the shell thickness, although it istypically operated in a batch type process which takes about 1 week to complete the procedure.
Hence, it is required to establish a facile and versatile method.
Here we developed a flow process using a microreactor to synthesize CSCs andsilica@Au core-shell nanoparticles. The microreactor we applied is central collision type,which realizes quick and homogeneous mixing by intensively bombarding two inlet fluids.[2]In the process to synthesize CSCs, we mixed a surface-modified silica suspension containing
Au ions with a strong reducing agent, NaBH4, in the microreactor to in-situ reduce Au ions oncore silica surfaces. Fig.1a shows a typical TEM image of resultant particles, in which coresilica particles are uniformly decorated with monodispersed Au particles. Because a batchsynthesis with the same procedure resulted in nonuniform decoration of polydispersed Auparticles and also produced unattached Au nanoprticles, high mixing performance provided by
the microreactor enabled one-step synthesis of CSCs. The microreactor is effective to produce
Au nanoshells as well. We successfully synthesizeduniform Au nanoshells by mixing CSCs containing Au
ions with a mild reducing agent, ascorbic acid, as shownin Fig.1b. By taking advantages of the microreactor, weestablished the flow process to synthesize CSCs andsilica@Au core-shell nanoparticles, which dramaticallyshorten the synthetic period compared to batchprocesses.[3]As an application of this process, we examined silica@Pt core-shell nanoparticles.
Because the wide use of Pt as a catalyst is prevented because of its preciousness, producing Pt
nanoshells can reduce the cost and promote the utility. We applied the above-mentionedprocedure and synthesized silica@Pt core-shell nanoparticles with the shell thickness of 15 ~20 nm.
References
1. S.J. Oldenburg, R.D. Averitt, S.L. Westcott, and N.J. Halas, Chem. Phys. Lett., 288, 243(1998)
2. H. Nagasawa, N. Aoki, and K. Mae, Chem. Eng. Technol., 28, 324 (2005)
3. S.Watanabe, T. Hiratsuka, Y. Asahi, A. Tanaka, K. Mae and Minoru T. Miyahara, Part. Part.
Syst. Charact.,32, 234 (2015).