Metal nanostructures: Growth, optical response, and bio-applications

Unique properties emerge for nanometer sized metal particles and of particular interest and importance is their surface plasmon resonance (SPR). Under resonant electromagnetic fields, the surface bound electrons oscillate, producing a prominent absorption in the UV-visible range for gold and silver nanoparticles. This optical property is suitable for many applications such as surface enhanced Raman scattering (SERS), which has much potential in molecular detection due to its high sensitivity and specificity. Hollow gold nanospheres, which exhibit a tunable SPR and consistent SERS output, were developed and demonstrated for potential use as intracellular pH nano- probes. Silver was coated on the hollow gold nanospheres to improve the SERS activity of this original substrate. Silver is more SERS active than gold while hollow gold nanospheres exhibit higher uniformity than silver nanoparticles. Combining both properties of uniformity and high SERS activity is desirable for sensitive and accurate sensing devices based on SERS. Metal nanoparticle properties including optical, electronic, and catalytic abilities are very sensitive to the shape and geometry. An in-situ atomic force microscope (AFM) and scanning electron microscope were used to investigate the crystal growth of silver in the presence and absence of polyvinylpyrolidone (PVP) which is a common surfactant utilized for directing specific facet growth resulting in distinct silver shapes. Future direction of nanomaterials in biological applications by bioconjugation methods will also be discussed.