Size and Shape Homogenization of Ag Nanoparticles by Laser Irradiation

Noble metal nanoparticles have been a subject of great interest in nanochemistry due to their unique optical properties and a wide range of applications. Collective oscillation of conduction electrons, referred to as surface plasmon, renders noble metal nanoparticles distinguished from other nanoscale materials. Resonance of the surface plasmon with the frequency of incident light gives rise to strong absorption and scattering of light by the nanoparticles (e ~ 10 Mcm). The electromagnetic fields near the nanoparticles induced by the surface plasmon resonance (SPR) increase Raman scattering cross-sections of molecules in close proximity by 6 to 14 orders of magnitude, which is called the surface-enhanced Raman scattering (SERS). SERS enables detection of single molecules and has been used as a major detection scheme of biosensors. Relaxation of the excited SPR via localized heating of the surroundings provides a basis for photothermal therapy. The SPR of noble metal nanoparticles that makes all these possible varies with the size and shape of the nanoparticles. Therefore, nanoparticles with a broad distribution of size and shape have poorly defined SPR characteristics. In this regard, preparing noble metal nanoparticles in a highly uniform size and shape distribution is of paramount importance. Noble metal nanoparticles are typically synthesized by the reduction of precursor metal ions. Citrate-reduction of hydroauric acid (HAuCl4) generally yields very homogeneous gold nanoparticles (AuNPs) with a monodisperse distribution of the size and shape. In this scheme, known as the Turkevich method, citrate functions as a surface stabilizer as well as a reductant. In marked contrast to AuNPs, citratereduction of silver ions produces extremely diverse silver nanoparticles (AgNPs) in sizes and shapes. This broad distribution of citrate-capped AgNPs impedes not only studies of size-dependent properties of AgNPs but also comparative studies between gold and silver nanoparticles with identical sizes and capping agents. In this Note, we report a facile method of preparing uniform AgNPs using laser irradiation. Extensive studies have been done on the interactions of laser with AuNPs. Resonant excitation of the surface plasmon of AuNPs at 532 nm leads to fragmentation and melting of AuNPs. Relatively fewer reports are available on the laser irradiation of AgNPs. Notably, Pyatenko and coworkers found that at high laser powers (532 nm, 200 mJ/cm) AgNPs vaporize, producing Ag atoms, and then form core particles, which grow into 8 nm-particles. They report that AgNPs remain unchanged at lower laser power. Here we recast the laser irradiation of AgNPs with a focus on preparing AgNPs with a monodisperse size distribution. Laser irradiation of a synthesized inhomogeneous mixture of AgNPs at 532 nm, 80 mJ/cm yields homogeneous spherical AgNPs with a diameter of 11.5 nm, comparable to the size of AuNPs typically prepared by the Turkevich method; thus, these AgNPs can be used for comparative studies of noble metal nanoparticles with a similar size and surface characteristics, but differing in materials. As a demonstration, AgNPs, homogenized by the laser irradiation, are coated with silica (Ag@SiO2) and compared with Au@SiO2.