Abstract
Superparamagnetism exists only in nanocrystals, and to endow micro/macro-materials with superparamagnetism, superparamagnetic nanoparticles have to be assembled into complex materials. Most techniques currently used to produce such assemblies are inefficient in terms of time and material. Herein, we used evaporation-guided assembly to produce superparamagnetic supraparticles by drying ferrofluid droplets on a superamphiphobic substrate in the presence of an external magnetic field. By tuning the concentration of ferrofluid droplets and controlling the magnetic field, barrel-like, cone-like, and two-tower-like supraparticles were obtained. These assembled supraparticles preserved the superparamagnetism of the original nanoparticles. Moreover, other colloids can easily be integrated into the ferrofluid suspension to produce, by co-assembly, anisotropic binary supraparticles with additional functions. Additionally, the magnetic and anisotropic nature of the resulting supraparticles was harnessed to prepare magnetically actuable microswimmers.
Highlights
Superparamagnetic materials display high magnetization only in the presence of a magnetic field, and they do not retain any magnetization once the magnetic field is removed.[1,2] This reversible magnetization allows the manipulation of the superparamagnetic materials by applying magnetic fields, resulting in numerous attractive applications such as microactuators,[3−5] magnetic separation,[6−8] and drug delivery.[9−11] superparamagnetism is strongly size dependent and only exists in nanocrystals
The final shape of supraparticles obtained by drying droplets of a magnetic colloidal suspension can be influenced by two main factors: the magnetization of the superparamagnetic NP dispersion and the surface and interfacial forces
The use of superamphiphobic substrates, which minimized the wetting of the droplet, was crucial to obtain 3D supraparticles
Summary
Superparamagnetic materials display high magnetization only in the presence of a magnetic field, and they do not retain any magnetization once the magnetic field is removed.[1,2] This reversible magnetization allows the manipulation of the superparamagnetic materials by applying magnetic fields, resulting in numerous attractive applications such as microactuators,[3−5] magnetic separation,[6−8] and drug delivery.[9−11] superparamagnetism is strongly size dependent and only exists in nanocrystals. We used evaporationguided assembly to produce superparamagnetic supraparticles by drying ferrofluid droplets on a superamphiphobic substrate in the presence of an external magnetic field. By tuning the concentration of ferrofluid droplets and controlling the magnetic field, barrel-like, cone-like, and two-tower-like supraparticles were obtained.
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