Protein Adsorption onto FePt Nanoparticles Investigated by Dual-Focus Fluorescence Correlation Spectroscopy

Upon exposure to biological fluids such as blood plasma, nanoparticles (NPs) adsorb proteins and other biomolecules from the fluid. The formation of the so-called ‘protein corona’ is a dynamic process: The composition of the corona depends on the respective concentrations of the proteins in the fluid and on their affinities to the NP. Understanding the structural and dynamic properties of the corona at the molecular level is a prerequisite to the safe use of NPs in industrial or medical applications.By using dual-focus fluorescence correlation spectroscopy (2fFCS), we have studied the adsorption of human blood serum proteins onto polymer-coated, fluorescently labeled FePt NPs (∼12 nm diameter) carrying negatively charged carboxyl groups on their surfaces. For all proteins, a step-wise increase in hydrodynamic radius with protein concentration was observed, strongly suggesting the formation of protein monolayers. Indeed, the absolute increase in hydrodynamic radius can be correlated with the molecular dimensions of the proteins. The apparent dissociation coefficients measuring the affinity of the proteins to the nanoparticles differed by about four orders of magnitude. These variations can be explained in terms of the electrostatic properties of the protein surfaces.