Quantifying the coverage density of poly(ethylene glycol) chains on the surface of gold nanostructures

The coverage density of poly(ethylene glycol) (PEG) is a key parameter in determining the efficiency of PEGylation, a process pivotal to in vivo delivery and targeting of nanomaterials. Here we report four complementary methods for quantifying the coverage density of PEG chains on various types of Au nanostructures by using a model system based on HS-PEG-NH(2) with different molecular weights. Specifically, the methods involve reactions with fluorescamine and ninhydrin, as well as labeling with fluorescein isothiocyanate (FITC) and Cu(2+) ions. The first two methods use conventional amine assays to measure the number of unreacted HS-PEG-NH(2) molecules left behind in the solution after incubation with the Au nanostructures. The other two methods involve coupling between the terminal -NH(2) groups of adsorbed -S-PEG-NH(2) chains and FITC or a ligand for Cu(2+) ion, and thus pertain to the "active" -NH(2) groups on the surface of a Au nanostructure. We found that the coverage density decreased as the length of PEG chains increased. A stronger binding affinity of the initial capping ligand to the Au surface tended to reduce the PEGylation efficiency by slowing down the ligand exchange process. For the Au nanostructures and capping ligands we have tested, the PEGylation efficiency decreased in the order of citrate-capped nanoparticles > PVP-capped nanocages ≈ CTAC-capped nanoparticles ≫ CTAB-capped nanorods, where PVP, CTAC, and CTAB stand for poly(vinyl pyrrolidone), cetyltrimethylammonium chloride, and cetyltrimethylammonium bromide, respectively.