Spectroscopic ellipsometry combined with localized surface plasmon resonance (LSPR) of gold nanoparticles (Au NPs) is exploited to design label-free bionsensors. We demonstrate that the size of Au NPs significantly affects the sensitivity of the ellipsometry analysis. Additionally, functionalizing Au NPs of different sizes with molecules/proteins of different sizes and shapes, such as dodecanethiol, hemin, human albumin and its antibody, we show that the size of nanoparticles can strongly influence the binding activity of adsorbed proteins and, consequently, the sensor functioning. Specifically, Au NPs with a diameter in the range 30-50 nm exhibit higher sensitivity to the change in the optical properties, and the variations of the ellipsometric parameter Psi allow discerning phenomena of aggregation of Au NPs of the sensor, of detachment of Au NPs and of protein chemisorption on Au NPs. The data are discussed in terms of two main factors affecting the ellipsometry sensitivity, i.e., the dependence of the LSPR electromagnetic enhancement on the Au NP size, and the strength of the interaction of the functionalizing molecule with Au NPs. (C) 2013 Elsevier B.V. All rights reserved.