This example illustrates how a ligand’s binding
affinity increases proportionally to the size of a nanoparticle owing to a higher protein density
on the nanoparticle surface. However, when viewed in terms of the downstream signaling via the
ErbB2 receptor, 40–50-nm gold nanoparticles induced the strongest effect, suggesting that other
factors beyond binding affinity must be considered. Nevertheless, several studies have shown that
nanoparticle design can cause differential cell signaling when compared with free ligand in solution.
For example, the aforementioned 40–50-nm Herceptin-coated gold nanoparticles altered
cellular apoptosis by influencing the activation of caspase enzymes