Colloidal stability of nanoparticles in biological media is crucial to preserve their utility as aggregation often leads to undesirable biological response. A quantitative measurement of nanoparticles aggregation in solution would provide a valuable assessment of colloidal stability of bio-nano interfaces after surface functionalization. Here, we develop a quantitative technique based on optical absorption to assay the colloidal stability of plasmonic nanoparticles functionalized with different amphiphilic surface ligands in biologically relevant media.