AFM study of organic ligand packing on gold for nanoparticle drug delivery applications

Gold nanoparticles (AuNPs) functionalized with polyethylene glycol (PEG) and peptide ligands are promising drug delivery vectors for use in cancer therapy. However, to control the interaction of the ligands with blood proteins and specific cell receptors, more needs to be known about the arrangement and properties of these grafted ligands. This issue is examined here principally with atomic force microscopy (AFM) for ligands grafted to both gold films and gold nanoparticles. Macro-level sessile drop measurements on the films indicate that, in dilute solutions, the PEG and peptide ligands exhibit similar affinity to the gold surface. However, at the nano-level, AFM experiments show differences in the morphology and cohesion of the adsorbed ligand films. Differences in tip/surface adhesion are also measured, indicative of variations of hydrophilicity for the top surface of these ligand films. Analysis of force-separation curves suggests a low packing density of PEG ligands. AFM investigations of functionalized AuNPs shows that the ligands improve dispersion and modify the tip/surface adhesion behaviour on the nanoparticles. However, AFM images of as-synthesised citric-capped and functionalized AuNPs gave similar nanoparticle diameters. Analysis of the AFM tip tapping on the AuNPs suggests that Tapping-AFM is not suitable for detection of low packing density ligands.