This phenomenon is observed in antibodies that possess at
least two antigen-binding sites. Similarly, on nanoparticle surfaces, the presence of ligands at a
given density over a specific curvature will contribute to the overall avidity of the nanoparticlelinked
ligands for available cell receptors. To illustrate this phenomenon, the binding affinity of
Herceptin to the ErbB2 receptor is 10−10 Min solution, 5.5 × 10−12 Mon a 10-nm nanoparticle,
and 1.5 × 10−13 M on a 70-nm nanoparticle