Mesoscopic Au “Meatball” Particles

In this Communication we report the synthesis and optical properties of mesoscopic Au particles with a novel, highly roughened, “meatball-like” morphology, and the performance of these particles as substrates for surface-enhanced Raman spectroscopy (SERS). Sub-micrometer metallic particles of Au and Ag have unique optical properties in the visible and near infrared (NIR) regions of the spectrum that are highly useful for a variety of applications, such as nanoscale optical components or devices, chemical and biomolecular sensing, medical imaging and photothermal therapy, and surface-enhanced spectroscopies. The optical properties of metallic nanoparticles are dominated by the collective oscillations of free electrons in the metal, known as surface plasmons. The plasmon resonance frequencies of a metallic nanoparticle are dependent on the particle’s size, shape, and surface topography because the oscillations of free electrons are controlled by the particle geometry. For spherical Au or Ag nanoparticles much smaller than the wavelength of light, only the dipole plasmon resonance is excited. This size regime is known as the “dipole” or “quasistatic” limit, since all the electrons of the particle experience the same phase of the incident electromagnetic field. As the particle size is increased, higher-order multipolar plasmon modes can also be excited, with the multipolar plasmon modes appearing at shorter wavelengths than the dipole plasmon feature in the particle’s overall extinction spectrum. Since the excitation of higher-order modes occurs when the phase of the optical field varies across the spatial extent of the particle, they are also known as phase retardation effects. Phase retardation also gives rise to significant red-shifting and broadening of the dipole resonance. Higher-order multipolar plasmon modes have been theoretically predicted and experimentally observed for large Au or Ag quasispherical particles with diameters larger than 100 nm and spherical metallic nanoshells. For particles with reduced symmetry, or in an anisotropic dielectric environment, the nature of the multipolar plasmon modes becomes more complex. Metallic nanoparticles with nonspherical shapes, such as nanorods, nanocubes, triangular nanoprisms, and nanopolyhedrons, have also been shown to support higher-order multipolar plasmon modes. In addition to particle size and shape, the surface topography of a particle can also significantly influence its optical properties. For example, recent studies have shown that the introduction of nanoscale texturing or the presence of defects on the nanoparticle surface can result in interesting changes to both the far-field and near-field properties of metallic nanoshells. On macroscopic metallic surfaces or films, surface roughness and defects have long been known to relax the boundary conditions that prevent the direct excitation of surface plasmon waves. For mesoscopic particles, already in a size regime where direct optical excitation of dipole and higher-order multipole modes is possible, it is observed that the introduction of roughness onto the particle surface preferentially dampens the higher-order modes relative to those of a smooth spherical particle. In general, roughened subwavelength particles are of great interest in light scattering studies, since a wide variety of naturally occurring particles, such as biological structures or atmospheric dust, have surfaces with nanoscale roughness. For metallic particles with rough surfaces, the enhanced local field is one mechanism that can contribute to the signal strength in surface-enhanced spectroscopies such as SERS. Here we investigate the optical properties of sub-micrometer Au spheres with nanoscale surface roughness. These meatball-like Au particles were fabricated through controlled reduction of chloroauric acid by ascorbic acid in aqueous solution at room temperature. This approach is a modified protocol based on the fabrication procedure developed by Matijevic for polycrystalline Au microspheres and by Van Blaaderen for large Ag colloids. The colloidal meatball-like particles fabricated by this method exhibit nanoscale surface roughness (Fig. 1A), and also appear quite monodisperse, given their irregular surfaces. The size distribution reported in a histogram (Fig. 1B) was obtained from scanning electron microscopy (SEM) images of over 200 particles. The average particle size is 430 nm with a standard deviation of 38 nm. In Figure 1C, a SEM image with higher magnification is shown, revealing the surface topography of several individual particles. The surface of each particle is composed of a large number of randomly arranged, irregular nanoscale protrusions approximately 20–50 nm in size. Each particle appears to consist CO M M U N IC A TI O N