Recent surveys have shown that the number of nanoparticle-based formulations actually used at the clinical level is significantly lower than expected a decade ago. One reason for this is that the nanoparticle physicochemical properties fall short for handling the complexity of biological environments and for preventing nonspecific protein adsorption. In this study, we address the issue of the interactions of plasma proteins with polymer coated surfaces. To this aim, we use a non-covalent grafting-to method to functionalize iron oxide sub-10 nm nanoparticles and iron oxide flat substrates, and compare their protein responses. The functionalized copolymers consist in alternating poly(ethylene glycol) (PEG) chains and phosphonic acid grafted on the same backbone. Quartz Crystal Microbalance with dissipation was used to monitor the polymer adsorption kinetics and to evaluate the resistance to protein adsorption. On flat substrates, functionalized PEG copolymers adsorb and form a brush in the moderate or in the highly stretched regimes, with density between 0.15 and 1.5 nm-2. PEG layers using phosphonic acid as linkers exhibit excellent protein resistance. In contrast, layers prepared with carboxylic acid as grafting agent exhibit mitigated protein responses and layer destructuration. The present study establishes a correlation between the long-term stability of PEG coated particles in biofluids and the protein resistance of surfaces coated with the same polymers.