Abstract: Gold nanoparticles (GNPs) are very attractive materials due to their unique properties of small size, large surface area to volume ratio, high reactivity to the living cells, stability over high temperatures. These properties along with the evidence that GNPs are amenable to the attachment of biomolecules or ligands through well-known thiol and amino chemistry or simply by electrostatic interactions have led to a wealth of nanoparticle-based bio-devices for many teranostic applications and the research effort in the field is still huge. In the present work absorption spectroscopy, static and dynamic light scattering, Fourier-Transform infrared (FT-IR) microspectroscopy and TEM microscopy have been used to characterize different sized bare and biotinylated GNPs (from 20 to 70 nm diameter). These experimental techniques have been also applied to investigate the aggregation process of the biotinylated particles induced by addition of neutravidin at various concentrations in the nanomolar range. In particular, optical visible techniques have been used for estimating the size of particles before and after the biotin capping procedure. FT-IR microspectroscopy has allowed us to investigate the biochemical changes occurring in GNPs after interaction processes while TEM microscopy has enabled to observe the relative morphological modifications. Moreover, the outlined modifications in the scattering properties have been related to changes in the size also thanks to numerical evaluation of scattering cross section by using Mie theory. The complete characterization of these processes is of fundamental importance for further manipulations required for GNPs teranostic applications.