Multiple length scale patterning of magnetic nanoparticles by stamp assisted deposition

Control of size and spatial distribution of materials at multiple length scales is one of the most compelling issues in bio-nanotechnology research. Breakthroughs are expected in the patterning of biologically active substances coupled to magnetic particles, the functional capping and interface engineering of the device, the design and use of the microfluidics as a tool for the device fabrication as well as for the experimental control from single molecules to cells [1]. Several approaches have been used to control the organization of molecules and nanoclusters on a solid surface, mostly with the purpose to align the stretching direction of individual molecules. These include deposition of a nanoparticles solution on PDMS (polydimethylsiloxane) stamp followed by transfer printing on mica[4]; molecular combing[5] by capillary flow; spin stretching[6]; casting solutions on a surface pre-patterned with PDMS[7], lithographically controlled wetting [8]. To overcome the problem of the placement of nanoparticles, we applied micro molding in capillaries (MIMIC)[9] to confine the solution within sub-micrometric channels defined by the stamp protrusions in intimate contact with the surface. When the solution is placed at an open end of the cavity, the solution flows inside driven by capillary forces and surface tension with the boundary walls. Self-organization of the molecular solute occurs at the later stages of solvent shrinkage. We report a multiple length scale patterning of pure fine magnetic particles as well as biocompatible magnetic particles with various surface modifications (such as Dextran, Polyethylene glycol, Lauric acid and so on) based on a printing technique and dewetting. The magnetic particles were prepared by well-known cooprecipitation technique of ferric and ferrous salts in alkali medium. We demonstrate that the morphology of nanoparticles deposit can be controlled by simply control the concentration of the solution (Fig.1). The approach yields arrays of nanodots on mm2 area, where each dot consists of a few particles. Our method exploits the self-organization of the nanoparticles in a solution confined between a stamps and different surfaces. The stamp imposes the larger length scale, and the self-organization introduces the smaller characteristic length scale. The shape, size and spacing of nanostructures can be modulated by the choice of stamp features and the concentration of the solution, via the control of the wetting regime. Dewetting is demonstrated as a viable route to the patterning of arrays of nanoparticles across large area. Our results hint to the possibility of patterning of nanoparticles for many applications using a stamp-assisted deposition from a solution on a substrate in a regime of partial wetting or dewetting. Our method is suitable for upscaling to a large area fabrication. The control of multiple length scales by exploiting confinement and competing interactions between the adsorbate and the substrate represents a remarkable example of integrated top-down/ bottom-up process. This work was supported by the EU- Project BIODOT (NMP4-CT-2006-032652), ESF-EURYI project DYMOT and VEGA 6166 and the Slovak Research and Development Agency under the contracts APVV-26-026505 and 51-027904.