The ways in which nanoparticles may enter living cells and the toxic responses of those cells are currently of interest. The commercial interest in nanoparticle technology for optoelectronics, composites and energy capture and storage as well as medical uses suggests an eventual wide dispersion of nanoparticulate material in the environment. It has been proposed that cell entry is facilitated by body proteins forming a ‘‘corona’’ around an exogeneous nanoparticle upon entry to the lungs or blood stream. The corona is supposed to facilitate cell entry by compatiblizing the nanoparticle to living tissue. The phenomena may be more general, however, since it has been demonstrated that entry of foreign bodies into membranes and whole cells can be promoted by a variety of peptide and macromolecular ‘‘opsonins’’, or hindered by ‘‘dysopsonins’’. Previous work by others have identified a wide range of biomolecules in human plasma that bind to polystyrene nanoparticles forming a nanoparticle–protein corona. The identity of the corona was found to be significantly affected by the size and surface chemistry of the nanoparticles. We report in this paper the difference in organization and structure of the nanoparticle–protein corona depending on the method of sample preparation (free protein in solution and oriented protein at the air–water interface). Nanotoxicology may be a problem in the future unless the causes and prevention of toxic responses can be identified. The exploration of these types of response is the focus of our current work.
Here we show the extent to which nanoparticle–protein binding can be studied quantitatively on the nanoscale by X-ray and neutron reflectometry (using isotopic contrast variation) at the air–water interface. The method allows structural changes in ‘‘wet’’ conditions to be followed in real time. The structures produced by the interaction of hydrophilic silica nanoparticles of diameter about 8 nm with β-casein are described for the proteins held at the air–water interface, as isolated monolayers and at this interface after binding to the silica in solution.