Magnetic Layer-by-Layer Assembly: From Linear Plasmonic Polymers to Oligomers

One-dimensional nanostructures with controllable aspect ratios are essential for a wide range of applications. An approach for magnetic superparticle (SP) assembly over large areas (55 mm × 25 mm) is introduced via co-assistance of electrostatic and magnetic fields, so-called magnetic layer-by-layer assembly, on an arbitrary hydrophilic substrate within minutes. The SP structures [diameter (d) = 120-350 nm] of Fe₃O₄ or Ag@Fe₃O₄ composites composed of hundreds of magnetite nanocrystals (d = 10-20 nm) are used as colloidal monomers to fabricate arrays of high aspect ratio (up to 10²) linear nanochains, viz. colloidal polymers, where thermal disturbances were minimized. The arrays of colloidal polymers exhibit strong optical polarization effects owing to their geometrical anisotropy, which can be used as a simple optical filter. Furthermore, by using the binary colloidal mixture of different magnetic colloids, including different sized Fe₃O₄ and magnetoplasmonic Ag@Fe₃O₄, low aspect ratio (2-15) colloidal chains, viz. magnetic/plasmonic oligomers, with tunable lengths were fabricated, affording a facile but an effective approach to modulate the optical properties of the chains. The scalable fabrication of well-aligned, linear colloidal polymers and oligomers opens up appealing opportunities for the development of sensors, subwavelength waveguides, optical tweezers, and enhanced solar harvesting devices.