When nanoparticles enter a biological environment, they are immediately surrounded by a variety of proteins and other biomolecules. These molecules adsorb onto the nanoparticle surface via various forces, including van der Waals forces, electrostatic interactions, and hydrophobic effects. Initially, a "soft corona" of loosely bound proteins forms, but over time, more stable, higher-affinity proteins replace the loosely bound ones, creating a "hard corona."
