Magnetic Field-Assisted Fast Assembly of Microgel Colloidal Crystals

Abstract

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 Fe 3 O 4 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.

From Self-Assembly of Colloidal Crystals toward Ordered Porous Layer Interferometry

“…Ge et al have discussed the mechanism of magnetic-field-force-induced self-assembly and summarized the application of this method to fabricate CCs from 1D nanochains to 3D films [13]. Despite hard colloids, Wang et al proved that soft nanospheres like poly(N-isopropyl acrylamide) could also be assembled by magnetic field by adding Fe 3 O 4 into the nonmagnetic nanospheres [84].…”

Assembly and manipulation of responsive and flexible colloidal structures by magnetic and capillary interactions

“…Thus, one can fabricate permanent materials which can be macroscopically guided depending on the particle structure assembled within the composites. [210][211][212][213] For example, Shmauch et al used solvent casting method to embed magnetic field-aligned microparticles in an elastomer film. The resulting composite film was strongly and directionally responsive to an externally applied magnetic field.…”

Hydrogels for Chemical Sensing and Biosensing

“…The resulting colloidal crystals exhibit potential applications in sensing. [70] Overall, magnetoresponsive hydrogels offer exciting opportunities for sensing applications, and their development is rapidly advancing in the field.…”