This work is focused on studying the grafting of gold nanoparticles (Np) on a cystamine self-assembled monolayer on gold, in order to build sensitive immunosensors. The synthesis and deposition of gold nanoparticles, 13 and 55 nm sizes, were characterised by combining Polarisation Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRRAS), X-ray Photoelectron Spectroscopy (XPS) Surface Enhanced Raman Scattering (SERS), and Atomic Force Microscopy (AFM) which all indicated the formation of a dispersed layer of nanoparticles. This observation is explained by the compromise between the high reactivity of amine-terminated layers towards gold, and interparticle repulsions. Nps were then functionalised with antibody probes, and the recognition by an anti-rIgG was assayed both on planar and Np gold surfaces. The important result is that nanoparticles of 55 nm are preferable for the following reasons: they enable to build a denser and well dispersed layer and they increase both the number of receptors (IgGs) and their accessibility. Beside these geometric improvements, a net enhancement of the Raman signal was observed on the 55 nm nanoparticle layer, making this new platform promising for optical detection based biosensors.