Certain nanoparticles can serve as hot spots, e.g., in surface-enhanced Raman scattering (SERS) and generate large local electromagnetic fields. However, single isolated hot spots usually produce heterogeneous EM fields with poor spectral reproducibility. Single, isolated hot spots with uniform and reproducible electromagnetic fields are, thus, interesting research targets.

Xidong Duan, Jiawen Hu, Hunan University, Changsha, China, Yue-Jiao Zhang, Jian-Feng Li, Xiamen University, China, have developed a method for the large-scale synthesis of monodisperse, highly spherical, single crystalline gold nanospheres with tunable sizes (38–92 nm) that can be used to create reproducible hot spots. The team used HAuCl₄ as a gold source, nonvolatile FeCl₃ as an oxidative etchant, AgNO₃ as a growth agent, and poly(diallyldimethylammonium) chloride (PDDA) as a surface ligand to prepare the nanospheres in ethylene glycol at 190 °C.

The resulting nanospheres are highly spherical and uniform, which was confirmed using transmission electron microscopy (TEM). Their sizes could be tuned by varying the concentrations of FeCl₃ and AgNO₃, as well as the reaction temperature. The researchers propose a growth mechanism that involves the initial reduction of HAuCl₄ to give gold nano-octahedra, followed by further gold deposition, the etching and deposition of silver along the edges and at vertices, and the selective deposition of the remaining gold on the “free” facets to form the nanospheres.

The prepared gold nanospheres were used to create reproducible hot spots in a nanosphere-on-mirror configuration on a gold film. Due to the spherical shape and uniform size of the product, single nanosphere-on-mirror hot spots generate uniform and reproducible electromagnetic fields, which was confirmed by SERS experiments.

• Monodisperse, Highly Spherical, Single Crystalline Au Nanospheres for Uniform and Reproducible Hot Spots in Surface-Enhanced Raman Scattering at the Single-Particle Level,
  Huan Cheng, Kun Wang, Renxian Gao, Shi-Ying Fu, Xiao-Ting Wang, Jia-Sheng Lin, Yingying Liu, Xuguang Sun, Zhilin Yang, Xidong Duan, Yue-Jiao Zhang, Jiawen Hu, Jian-Feng Li,
  Nano Lett. 2024.
  https://doi.org/10.1021/acs.nanolett.4c04951