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The protein corona on nanoparticles as viewed from a nanoparticle‐sizing perspective

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Most surfaces of engineered nanoparticles (NPs) are reactive toward biomolecules. Therefore, whenever NPs become immersed in biological fluids, proteins and other biomolecules bind to the NP surface, forming an adsorption layer (biomolecular corona) that modifies the NPs’ physicochemical properties and subsequent interactions with living systems. Its exploration is a formidable endeavor owing to the enormous diversity of engineered NPs in terms of their physicochemical properties and the vast number of biomolecules available in biofluids that may bind to NPs with widely different strengths. Significant progress has been made in our understanding of the biomolecular corona, but even very basic issues are still controversially debated. In fact, there are divergent views of its microscopic structure and dynamics, even on physical properties, such as its thickness. As an example, there is no agreement on whether proteins form monolayers or multilayers upon adsorption. In our quantitative studies of NP–protein interactions by in situ fluorescence correlation spectroscopy (FCS) with highly defined model NPs and important serum proteins, we have universally observed protein monolayer formation around NPs under saturation or even oversaturation conditions. Here, we critically discuss biomolecular corona characterization using FCS and dynamic light scattering and identify challenges and future opportunities. Further careful, quantitative experiments are needed to elucidate the mechanisms of biomolecular corona formation and to characterize its structure. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology
Nanoparticle (NP)–biomolecule interactions. In a biological fluid, proteins may be considered as a cloud around the NP. Some will adsorb reversibly to the NP surface and form a monolayer. Adsorption may be accompanied by (partial) unfolding, which could expose hydrophobic moieties, onto which further proteins may possibly adsorb as a second layer. NPs covered with unfolded proteins may also undergo protein‐mediated aggregation. Box: reversible protein binding.
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Fluorescence correlation spectroscopy (FCS) and dynamic light scattering (DLS) of transferrin (TF) binding to fluorescent PS NPs. (a, b) Normalized FCS autocorrelation curves of (a) 0.7 nM PS‐COOH NPs and (b) 0.7 nM PS‐OSO3H NPs without (black) and with 4 μM TF (magenta). (c, d) Intensity time traces and depictions visualizing the observed particle size increase due to TF adsorption and particle clustering, respectively. (e, f) DLS profiles showing intensity‐based RH distributions of the NPs.
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Fluorescence correlation spectroscopy (FCS). (a) Schematic depiction of the experiment. Individual fluorophores diffusing through the observation volume emit brief bursts of photons that are recorded in the intensity time trace. (b) From these data, normalized autocorrelation curves, G(τ), are calculated to yield the diffusional correlation times, τD, from which the hydrodynamic radii, RH, of the diffusing entities are obtained. The arrow indicates increasing transferrin (TF) concentration. (c) Hydrodynamic radii, RH, of small carboxyl‐functionalized FePt NPs (black, ΔRH = 7.0 ± 0.4 nm) and large PS‐COOH NPs (magenta, ΔRH = 9.4 ± 1.0 nm) are plotted as a function of the concentration of apo‐TF proteins freely diffusing in the solution. The solid lines are fits with a binding curve, giving the midpoint concentration, K’D, as a measure of the binding affinity.
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