Magnetic Field-Assisted Fast Assembly of Microgel Colloidal Crystals

Compared with the colloidal crystals (CCs) of hard spheres, large-scale, high-quality CCs of soft microgel spheres are easier to be assembled because they are more tolerant to defects. However, to assemble microgel CCs, a microgel dispersion should first be concentrated and then allowed to crystallize, which is tedious and time-consuming. Herein, we demonstrated that a magnetic poly(N-isopropylacrylamide) (PNIPAM) microgel with an Fe₃O₄ core and a PNIPAM shell can be assembled into CCs quickly by simply applying an external magnetic field to the diluted microgel dispersions. The resulting CCs are highly ordered as revealed by their iridescent color, laser diffraction pattern, and confocal characterization. They display a sharp Bragg peak on their reflection spectra, which shifts to lower wavelength when heated because of the thermosensitivity of the PNIPAM shell. The magnetic assembly is not only simple and fast but also allows control of the CC structure in both horizontal and vertical directions. Using spatially varying magnetic fields, patterned microgel CCs were facilely assembled. More importantly, magnetic microgel spheres with different sizes can be assembled in a layer-by-layer manner by adding them sequentially, and the thickness of each layer can be simply controlled by the amount of spheres added.