Polymer-nanoparticle thin scaffolds with any-shape magnetic field gradients

This study investigates the self-assembly of magnetic nanoparticles into a flat nanocomposite surface with concealed magnetic patterns. The application of a multilayer stamping medium and the presence of a magnetic field gradient would enable the attraction and organization of superparamagnetic iron oxide nanoparticles (SPIONs) onto predefined areas, corresponding to a pre-fabricated magnetic layer. The experimental approach involved preparing patterned surfaces by applying a multilayer of poly(ethyleneimine) (PEI) and poly(styrene sulfonate) (PSS) as a stamping medium. The SPIONs selectively adhered to the surface, mirroring the underlying magnetic layer shape. Subsequently, these SPION were subjected to an 80 °C treatment in the presence of an external magnetic field to enhance their magnetic properties, and then covered with a polystyrene layer. Using atomic force microscopy, magnetic force microscopy and dark field mode optical microscopy we demonstrated that obtained patterned magnetic field gradients are strong enough to attract and organize magnetic material on top of them, even when being covered with polymeric layers. Moreover, our findings show that the oriented deposition of magnetic nanoparticles is primarily determined by the strength of magnetic interactions between the pattern and nanoparticles, independently of surface topography (with roughness of 0.21 ± 0.06 nm) or Brownian motions.