posted on 2022-05-03, 08:03authored byYafei Wang, Yan Zhang, Ying Guan, Yongjun Zhang
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.