Analyzing the Biological Entity of Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 61

To summarize, for a detailed understanding of such nano-bio interactions occur-
ring in vitro and in vivo and therefore also for interpreting toxicity results, a complete
in situ characterization of each nanomaterial under the specific test conditions is
absolutely essential, which has been widely recognized nowadays (Warheit 2008;
Sayes and Warheit 2009; Montes-Burgos et al. 2010).
In this chapter, we will highlight important issues to consider when characteriz-
ing nanomaterials in situ , which here refers to the conditions under which in vitro or
in vivo experiments are performed. First of all, we will focus on general aspects of
characterizing nanomaterials in biological fluids, and discuss parameters influenc-
ing the interactions with biomolecules as well as methods suitable to study them.
Furthermore we will explain nanomaterial interactions with serum, with lung lining
fluid, and with gastric juices as examples.
4.1.2 i nteraCtion of n anoPartiCles With P roteins and i influenCing f aCtors
The fact that proteins bind to materials immediately after they have been introduced
to the body is actually a common phenomenon, and this topic has been extensively
investigated in the last 30-40 years, mainly with respect to implants and other medi-
cal devices (Rabe, Verdes, and Seeger 2011). Although some principal rules could be
established and a few strategies such as PEGylation preventing such interactions (e.g.,
antifouling) are available (Walkey and Chan 2012; Morra 2000), protein adsorp-
tion to surfaces is still a complex topic and not yet fully understood (Nakanishi,
Sakiyama, and Imamura 2001). Protein adsorption to nanomaterials can in principle
be considered as a special case of interactions of proteins with surfaces in general
such that some of the known principles may be applied here as well (Walkey and
Chan 2012; Gray 2004). However, there are some unique features of nanomateri-
als, which may lead to important differences. For instance, nanomaterials typically
have a very large specific surface area often combined with high surface curva-
ture (leading to a higher probability of defects in crystal structure on the surface),
which in combination may cause a higher surface reactivity toward biomolecules.
Furthermore, nanoparticles in contrast to medical implants are mobile, that is, they
can translocate to different physiological compartments and distribute within the
body after entry (Walkey and Chan 2012). With this migration of nanomaterials and
consequently the exposure to different physiological compartments, the composition
of the protein corona will change over time. This has been referred to as “evolution”
of protein corona (Dell'Orco et al. 2010; Lundqvist et al. 2011).
The pattern of adsorbed proteins depends primarily on the physicochemical prop-
erties of the nanomaterial, such as chemical composition, size, shape, roughness,
effective surface charge (i.e., zeta-potential), and hydrophobicity. The sum of these
properties drives and defines the interplay at the so-called “nano-bio interface.” The
forces governing the adsorption of protein to nanomaterials are both long range van
der Waals and electrostatic interactions, as well as short range interactions such as
steric, hydrophobic, or charge effects (Nel et al. 2009). The size and shape of nano-
materials—both determining their surface curvature—can largely affect the identity
of adsorbed proteins (Lundqvist et al. 2008; Tenzer et al. 2011). The binding of some
proteins (e.g., fibrinogen) for instance depends on the space available for a protein