Ratiometric surface enhanced Raman quantification of ligand adsorption onto a gold nanoparticle.

Surface modification is essential in biomedical applications and nanotechnological developments. We report a novel method using surface-enhanced Raman spectroscopy (SERS) for quantifying ligand adsorption onto gold nanoparticles (AuNPs). After centrifugal or settlement removal of the AuNPs-ligand complex, the amount of unbound ligand in the supernatant was determined ratiometrically with an isotope-encoded SERS reference method where known amounts of isotope-substituted ligand were added to the supernatant as an internal reference. Not only is this ratiometric method robust and accurate but it is also very easy to perform. Using this technique, the binding constant and packing density of the model ligand mercaptobenzimidazole (MBI) on a AuNP was determined for the very first time.