Ultra-dense and long-lasting shells for inorganic nanoparticles are based on cyclic polymer brushes

The application of functional, inorganic nanoparticles (NPs) within physiological environments is in the first place determined by the capability of their organic shells to protect them from any unspecific physico-chemical interaction with the surrounding medium. The most common strategy to meet this prerequisite relies on the entropic and enthalpic stabilization of NPs mediated by hydrophilic and neutral end-tethered polymers forming dense “brush” shells around their inorganic cores. Concentrating on the applicability of polymer stabilizers for NPs beyond chain linearity, we demonstrate that cyclic poly-2-ethyl-2-oxazoline (PEOXA) ligands, applied on superparamagnetic Fe3O4 NPs provide enhanced colloidal stability and bioinertness in physiological media. When linear PEOXA brush shells fail in providing colloidal stabilization to NPs, the cyclic ones assure long lasting dispersions. While the thermal-induced de-hydration of linear PEOXA shells cause irreversible aggregation of the NPs due to the insufficient screening of their inorganic cores, the collapse and subsequent re-hydration of similarly grafted cyclic brushes, allow the full recovery of individually dispersed NPs. Although linear PEOXA ligands are densely grafted on Fe3O4 cores, a small plasma protein like bovine serum albumin (BSA) still physisorbs within their shells. In contrast, the impenetrable entropic shield provided by cyclic brushes efficiently prevents nonspecific interaction with proteins. All the unique properties of cyclic polymer brush shells suggest the next-generation design for the development of bioimaging and drug-delivery systems based on inorganic NPs.