Stability of gold nanoparticles (AuNPs) is often compromised in physiological conditions. The loss of colloidal stability might lead to undesired biological responses for drug delivery nanosystems. Here a methodology to confer additional stability to the AuNPs by the addition of PEG is presented. Also, protocols to prepare and characterize the composition and conformation of mixed layers with PEG and alkanethiols are described here. Finally, methods to assay the stability of the link between the NP conjugates and a model drug are shown.