Divalent transition metal coatings for an effective protein-nanoparticle conjugation

Magnetic nanoparticles (MNPs) are widely used for immobilizing complex biomolecules such as proteins due to their high loading capacity, biocompatibility, and easy magnetic harvesting. However, there is not a universal solution to achieve the oriented immobilization of the protein of interest. One common strategy involves the selective binding of poly-histidine-tagged (His-tag) proteins to divalent transition metal centers (TM2+) chelated by organic molecules like nitrilotriacetic acid (NTA). This article describes an innovative procedure to functionalize MNPs by simply growing of a thin layer of TM2+ hydroxides on the MNP surface, which increases the number of coordination centers and enhances the surface interaction with the protein surface. The immobilization yields of two sets of MNPs coated with Co2+, Ni2+ and Cu2+ hydroxides were compared to the conventional coating using the same inorganic cores but coated and functionalized with NTA-TM2+ moieties. It is shown that the one-step Co coated MNPs showed the highest selectivity and specificity towards His-tags. Thus, this coating not only reduce the functionalization steps to drive an oriented protein binding but also achieve an increment of immobilization yields over 1.5-fold respect to NTA-Co2+ analogous for his-tagged proteins with different 3D complexity while preserving similar or even higher enzymatic activity.