Size-controlled synthesis and characterization of CoPt nanoparticles using protein shells

Nanostructured magnetic materials such as iron oxide and bimetallic nanoparticles can be potentially applied to a variety of fields, including electronics and nanomedicine. To develop these applications, it is important to control their particle size which affects their magnetic properties. In particular, it is a major challenge to synthesize small-sized nanoparticles with high reproducibility. In this study, we synthesized cobalt-platinum nanoparticles (CoPt NPs) in an ambient solution phase using PepA, a bacterial aminopeptidase, as a protein shell, and investigated the physicochemical and magnetic properties of NPs with and without encapsulating proteins. The size of CoPt NPs encapsulated by PepA was stringently controlled from 1.1 to 2.8 nm, and their magnetic property was related to the size. The CoPt NPs with the diameter of 1.1 nm showed a superparamagnetic behavior only at low temperatures, while 2.1 and 2.8 nm CoPt NPs were ferromagnetic below the blocking temperature. PepA had no deleterious effects on the coercivity of CoPt NPs, as evidenced by the marginal effect of PepA on the coercivity of CoPt NPs. This study demonstrated that the particle size and magnetic property of CoPt NPs can be controlled by using PepA as a protein shell. Encapsulation by PepA will aid the development of multifunctional magnetic materials, since the biocompatibility and modification capability of PepA can be synergistically combined with the advanced functionalities of CoPt NPs.