Au nanoparticles decoration of silica nanowires for improved optical bio-sensing

In this work we study the potential of silica nanowires (NWs) decorated with Au nanoparticles (NPs) as novel optical platform for plasmonic biosensing. These composite materials combine the high white-light scattering of silica NWs with the selective absorption of the localized surface plasmon resonances (LSPRs) of the metal NPs, generating efficient light trapping just at the LSPR wavelengths. In addition, the NWs offer a macroporous framework for a three-dimensional (3D) distribution of the Au NPs, easily accessible by biomolecules. These optical and morphological pecularities, make the decoration of silica NWs with metal NPs a promising strategy for improved plasmonic biosensing. We fabricated silica NWs by thermal oxidation of silicon (Si) NWs, grown by plasma enhanced chemical vapor deposition (PECVD). The silica NWs were decorated with Au NPs by evaporating and then de-wetting via thermal annealing Au thin films. The biosensing ability of these materials was studied and compared with the performances of similar Au NPs deposited on glass substrate, forming a planar (two-dimensional, 2D) distribution. A careful morphological analysis about the NWs and their Au NP coverage, including their vertical distribution across the NW film, was performed by high resolution scanning electron microscopy (HR-SEM) and energy dispersive X-ray spectroscopy (EDS) experiments. The morphological information was used for building a theoretical model through 2D finite element modeling (FEM) able to describe the optical behavior of the new transducers and their functional properties, as well as to predict the evolution of the sensing performance of the transducer when passing from a 2D to a 3D distribution of plasmonic NPs. The Au NPs decorated NWs were finally modified with a self-assembled monolayer of BSA (bovin serum albumin) and the specific binding with the related antibody monitored during the time.