As drugs/drug carriers, upon encountering physiological fluids, nanoparticles adsorb biological molecules almost immediately to form a biocorona, which is often simply called a corona. Once the corona is formed, it dictates the subsequent fate of the drug nanoparticle as a therapeutic agent. Protein adsorption on micron-size or even bigger particles was originally described by the Vroman effect. It has served as a useful framework to understand the corona formation. Proteins that are irreversibly adsorbed on nanoparticles form what is called a hard corona. Beyond that is the exchangeable population of proteins, which constitute the dynamic structure called a soft corona. More than the abundance, the affinity of the proteins toward the nanoparticles decides which ones end up in the corona. For example, the more common serum albumin, which is deposited initially, is displaced by fibrinogen, which has a higher affinity for gold nanoparticles. The curvature of the particle is a crucial parameter with bigger particles generally able to bind a more diverse population of proteins from the physiological milieu. The earlier perception of the corona formation being a challenge for drug targeting, etc. has been turned into an opportunity by engineering corona to manipulate properties like circulating half-lives, capacity to evade the immune system, and targeting or even overcoming the blood-brain barrier. The most commonly used techniques for particle characterization, including dynamic light scattering (DLS), differential sedimentation centrifugation, transmission electron microscopy (TEM), and SDS-PAGE, have been adopted to study corona formation in the past. Many newer tools, for example, a combination of capillary electrophoresis with mass spectrometry, are being used to study the corona composition. The comparison of interlaboratory results is a problem because of the lack of standard protocols. This has hindered the ability to obtain more precise information about the corona composition. That, in turn, affects our prospects to use nanoparticles as drugs/drug carriers. This overview is an attempt to assess our understanding of corona formation critically and to outline the complexities involved in gaining precise information. The discussion is largely focused on findings of the last year or so.
Keywords: Vroman effect; biocorona; drug delivery; drug targeting; nanoparticles; polyethylene glycol; protein adsorption.