Nanoplasmonic Structures for Bio/Chemo Sensing

Doctoral thesis, 2006

Localized surface plasmon resonances (LSPR's) in noble metal nanostructures are sensitive to refractive index changes close to the metal surface, and the LSPR's can therefore be utilized to sense molecular binding reactions. This thesis focuses on fundamental properties and proof-of-principle sensing studies of nanoplasmonic structures relevant for molecular detection. A sensor based on avidin-coated colloidal gold particles was tested and modeled using Mie theory for coated spheres. A good quantitative agreement between the experimental results and theory was achieved when realistic values for the thicknesses and refractive indices of the molecular layers were used. A new sensing platform - single nanometric holes in thin gold films - was assessed experimentally and theoretically. The results showed that the hole resonance can be assigned to a dipolar LSPR that couples into the so-called anti-symmetric bound surface plasmon polariton (SPP) mode of the thin Au film. Using elastic scattering and extinction spectroscopies, single and short-range ordered nanometric holes were evaluated for molecular detection purposes. The results showed that nanoholes can be used to detect biotin modified lipid vesicles binding on SiO2 as well as subsequent neutravidin adsorption. Experiments with alkanethiol layers of different thicknesses indicated that the field decay length around a single nanohole is of the order δ≈10-20 nm. However, experiments on short-range ordered hole and disk arrays coated with Langmuir-Blodgett films of increasing thickness d revealed an LSPR oscillation as a function of d even for d<<δ. This phenomenon indicate that it might be possible to detect molecular binding events even hundreds of nanometers away from the metal nanostructure surface. Finally, the LSPR properties of strongly interacting metal nanoparticles, specifically dimers and chains of silver nanodisks, are discussed. Such nanoparticle assemblies can be interesting for sensing applications because of electromagnetic field confinement at small inter-particle distances, or narrow LSPR line shapes generated through diffractive coupling at distances comparable to the LSPR wavelength. The electromagnetic interactions were modeled using the coupled dipole approximation and good agreement between experiment and theory was found.