The adsorption of serum proteins on the surface of nanoparticles (NPs) delivered into a biological environment has been known to alter NP surface properties and consequently their targeting efficiency. In this present article we use random copolymer (p(HEMA-ran-GMA))-based NPs synthesized using 2-hydroxyethyl methacrylate (HEMA) and glycidyl methacrylate (GMA). We show that serum proteins bind to the NP and that functionalization with antibodies and peptides designed to facilitate NP passage across the blood brain barrier (BBB) to bind specific cell types is ineffective. In particular, we use systematic in vitro and in vivo analyses to demonstrate that p(HEMA-ran-GMA) NPs functionalized with HIV-1 transactivating transcriptor (TAT) peptide (known to cross the BBB) and α neural/glial antigen 2 (NG2) (known for targeting oligodendrocyte precursor cells (OPCs)), individually and in combination, do not specifically target OPCs and are unable to cross the BBB, likely due to the serum protein binding to the NPs.