Formation of Fe3O4@SiO2@C/Ni hybrids with enhanced catalytic activity and histidine-rich protein separation

Abstract

In this paper, we have developed an extended Stöber method to construct a Ni²⁺–polydopamine (PDA) complex thin coating on Fe₃O₄@SiO₂ spheres, which can be carbonized to produce hybrid composites with metallic nickel nanoparticles embedded in a PDA-derived thin graphitic carbon layer (named Fe₃O₄@SiO₂@C/Ni). Interestingly, by introducing a thin SiO₂ spacer layer between PDA–Ni²⁺ and Fe₃O₄, the reverse electron transfer from PDA to Fe₃O₄ is probably able to be suppressed in the calcination process, which leads to the in situ reduction of only Ni²⁺ by PDA instead of Fe₃O₄ and Ni²⁺. Consequently, the size and density of nickel nanoparticles on the surface of SiO₂@Fe₃O₄ can be finely adjusted. Moreover, it is found that the ability of tuning nickel nanoparticles is mainly dependent on the thickness of the spacer layer. When the thickness of the SiO₂ spacer is beyond the electron penetration depth, the size and density of nickel nanoparticles can be exactly tuned. The as-prepared Fe₃O₄@SiO₂@C/Ni was employed as the catalyst to investigate the catalytic performance in the reduction of 4-nitrophenol (4-NP); furthermore, nickel nanoparticles decorated on Fe₃O₄@SiO₂@C spheres display a strong affinity to His-tagged proteins (BHb and BSA) via a specific metal affinity force between polyhistidine groups and nickel nanoparticles.