Optical Properties of Silver/Gold Nanostructures Fabricated by Shadowing Growth and Their Sensing Applications

Various Ag and Au nanostructured films such as Ag nanoparticle (NP) films, Au NP films, and Au NP/TiO2/Au NP sandwich structures are fabricated by oblique angle deposition (OAD) and glancing angle deposition (GLAD) methods. Their optical absorbance properties and localized surface plasmon resonance (LSPR) have been studied systematically for samples prepared at different deposition conditions. Under the same deposition conditions, the Ag or Au NP substrates produced by GLAD method are more uniform and reproducible. The LSPR wavelength of Ag or Au NP substrates can be easily tuned by changing the film thickness, the deposition angle, and the coating of dielectric layer. The ability of the nanoparticle films as a chemical and biological biosensor has been explored by sensing the biomolecule NeutrAvidin and the bacterium Salmonella. Those NP films are very sensitive to chemical detection but are insensitive for bacteria detection. Based on Mie theory and effective medium theory, this is due to the small contact area between the nanoparticle and the bacteria, and the short range interaction of the local electric field. Our results demonstrate that shadowing based growth is a very versatile fabrication technique to produce reproducible and fine-tuned LSPR substrates.