Functionalization Platform of Magnetic Nanoparticles Through Silica Shell Formation

Magnetic nanomaterials enable transduction of weak magnetic fields into biological signals including force, heat, and force or heat-mediated chemical release. However, biochemically benign ferrites, including iron oxide, pose challenges to surface modification offering limited surface functionalization options for cell targeting. We hypothesized that functionalization of magnetic nanomaterials with silica, for which numerous surface chemistries are established, would offer a versatile and robust route to their surface engineering. Although prior work suggested the use of the Stöber and reverse microemulsion methods to functionalize magnetic nanomaterials with silica, formation of thin uniform shells necessary to preserve the particles’ functional properties remained challenging. Furthermore, during functionalization reactions multiple nanoparticles are readily encapsulated in a single silica shell, which similarly compromises performance. To overcome these challenges, we develop a novel, robust protocol for a reverse microemulsion method to create uniform shells with tunable thickness on magnetic nanoparticle cores. Our protocol uses oleic acid as a secondary surfactant to prevent the formation of multi-core particles and control the rate of silica shell growth. We demonstrate that our protocol is adaptable to a wide range of particles of different shapes/sizes and compositions such as spherical nanoparticles with high heat-dissipation rates in magnetic fields, hexagonal ferrite nanodiscs (>200 nm in diameter), and semiconductor quantum dots. Additionally, the surface chemistry of our coated nanoparticles can be readily modified with desired functional groups. Our protocol can be used to form uniformly coated core-shell nanoparticles of a desired thickness as a platform for applications in materials research, chemistry, and biomedicine.