Size-Tunable Polymer Colloids as Substrates for Plasmonic Nanoparticles

Size-tunable polymer colloids, particles that are stable in solution, are synthesized via an emulsion polymerization with varying ratios of vinylpyridine and styrene monomers. The pyridine monomers act as a base, allowing the polymer to accept protons under acidic conditions resulting in positively charged pyridinium ions within the copolymer. Due to electrostatic repulsion of these positive charges, the ions cause the polymer nanospheres to expand to larger sizes. By adjusting the ratio of the vinylpyridine and styrene monomers within the copolymer, the swelling properties of the colloidal nanospheres can be tuned. These size-tunable colloids have potential applications in a number of areas of research as size-tunable substrates for plasmon-active metallic nanoparticles. Surface plasmons, the collective oscillation of surface electrons, can occur on metallic nanoparticles of appropriate composition, size, and shape, creating enhanced electromagnetic fields around the nanoparticles and exciting electrons for reduction reactions. Monodisperse colloids can form colloidal crystals upon removal of the solvent, self-assembling into periodic nanostructures which behave as photonic crystals. Photonic crystals can slow down light within the material, and the photons that are slowed can be selected for by the diameter of the colloids to match the peak plasmon absorptions of the incorporated nanoparticle. In doing so, the chances of photon absorption can be greatly increased and the benefits of both plasmon-active nanoparticles and photonic crystals can be paired to make a synergistic nanocatalyst. Another application includes incorporating metallic nanoparticles onto the surface of the colloids in solution, where the proximity of these nanoparticles to one another can be tuned based on pH for surface-enhanced Raman spectroscopy (SERS). For SERS, nanoparticles are typically irreversibly aggregated in solution to increase the surface area, which serves to increase the enhancement factor of Raman measurements. Hosting the nanoparticles onto a size-tunable substrate would allow for pH-dependent enhancement factors for SERS measurements