The optical properties of gold nanoparticles, such as surface plasmon resonance, depend critically on the shape and size of the particles. Researchers from the US and China have devised a method to prepare spherical, single-crystalline gold nanoparticles with controlled size.

In their variation of seed-mediated growth, Younan Xia, Georgia Institute of Technology, USA, and collaborators used gold clusters capped with cetyl trimethylammonium bromide (CTAB) as seeds. They then added the analogous chloride, CTAC, HAuCl₄ as gold precursor, and ascorbic acid as reducing agent. The rapid reduction of the gold precursor led to the formation of the desired spherical particles with sizes of 5 to 17 nm. Fewer seeds led to larger particles, while more seeds resulted in smaller particles. For larger particles up to 30 nm, the as-prepared 10 nm particles were used as seeds.

Dilution of aqueous suspensions of the as-prepared nanoparticles led to aggregation. Dilution shifts the equilibrium between bound and free capping agent, thus releasing capping agent from the particle surfaces and allowing the particles to form first dimers and trimers and then, with increasing dilution, wavy gold nanowires. The authors comment that these uniform gold nanospheres will find immediate use in the syntheses of bimetallic core–shell nanostructures and in the investigation of the mechanisms that are involved in seed-mediated growth.

• Seed-Mediated Synthesis of Single-Crystal Gold Nanospheres with Controlled Diameters in the Range 5–30 nm and their Self-Assembly upon Dilution,
  Yiqun Zheng, Yanyun Ma, Jie Zeng, Xiaolan Zhong, Mingshang Jin, Zhi-Yuan Li, Younan Xia,
  Chem. Asian J. 2013.
  DOI: 10.1002/asia.201201105