Shaping the Assembly of Superparamagnetic Nanoparticles

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.

S2 precipitated and rinsed with distilled water five times and dried in an oven at 65 °C overnight. In order to remove the excess oleate, the oleate-capped iron oxide nanoparticles were dispersed in ethanol (technical grade), separated from the solvent with a permanent magnet (NdFeB, 30 × 30 × 15 mm) and redispersed in fresh ethanol, this procedure was repeated three times.

Synthesis of hybrid Fe3O4/polystyrene nanoparticles (mgPS NPs)
The Fe3O4 NPs (below 20 nm) were immobilized inside polystyrene nanoparticles by emulsion polymerization. 3 Briefly, 1 g of Fe3O4 NPs was dispersed in 0.5 g of n-octane for 30 min in a sonication bath followed by the addition of 24 g of an aqueous solution containing 24 mg of sodium dodecyl sulfate (SDS). The two-phase suspension was sonicated with a tip (d (tip) = ½, Branson 450 Digital Sonifier, U.S.) for 3 min under ice cooling (70% amplitude, 5 s pulse, 5 s pause) and stirred using a mechanical stirrer (KPG) at room temperature. Separately, 1.2 g of styrene was mixed with 20 mg of n-hexadecane and 24 g of water containing 60 mg of SDS. An emulsion was prepared by the sonication of the water/styrene mixture with a tip (d (tip) = ½, Branson 450 Digital Sonifier, U.S.) for 1 min under ice cooling (10% amplitude, 5 s pulse, 5 s pause). The iron oxidein-water dispersion and the styrene-in-water dispersion were mixed and then nitrogen was bubbled in the combined dispersions for of 10 min. Then, 35 mg potassium persulfate (KPS) and 30 mg of sodium styrenesulfonate were added, and the reaction mixture was heated to 80°C under stirring for 14 h. Purification of the superparamagnetic polystyrene particles was carried out magnetically and by centrifugation. The final surface tension of the suspension was adjusted to 49 mN/m by controlling the SDS concentration.

S3
The CoFe2O4 NPs (20-30 nm) (Sigma Aldrich) were immobilized inside polyvinylpyridine nanoparticles by emulsion. Briefly, 0.5 g of CoFe2O4 NPs were dispersed under sonication in 5 g of a solution of 1% of PVP (1000 g/mol) in a 0.4 g/L solution of SDS. The clusters were purified from free PVP by 3 cycles of centrifugation followed by redispersion in a 0.4g/L solution of SDS.
The final surface tension of the suspension was adjusted to 50 mN/m by controlling the SDS concentration.

Fabrication of superparamagnetic supraparticles on superamphiphobic surfaces
First, soot-templated superamphiphobic surfaces were prepared following the previously described method. 4 Briefly, soot particles were deposited on the glass substrate by the candle-soot deposition method. Then 10-20 nm SiO2 layer was coated on those particles by chemical vapor deposition of tetraethoxysilane catalyzed with ammonia. The soot particles were removed by heating at 550 °C, resulting in a fractal-like structure of SiO2 with overhangs. The superamphiphobic surface was finally obtained by surface modification with trichloro(1H,1H,2H,2H-perfluorooctyl)silane which lowered the surface energy.
To fabricate the supraparticles, droplets (typically 5 µL) of an aqueous suspension of mgPS NPs (from 0.3wt% to 30wt%) were deposited using a micropipette on the superamphiphobic surface.
To generate the magnetic field, a permanent magnet (NdFeB, 30 × 30 × 15 mm) was placed under the superamphiphobic surface. By controlling the distance between the magnet and the surface, it was possible to tune the strength of the magnetic field. The magnetic field was measured at the surface of the superamphiphobic substrate. Unless noted otherwise, the supraparticles were formed using 5 uL of suspension dried at a temperature of 23 °C under a humidity of 25%.

S4
To obtain binary supraparticles, titanium dioxide nanoparticles (TiO2 NPs, 25 nm, Aldrich, Germany) and polystyrene nanoparticles (PS NPs, 270 nm) were dispersed in distilled water to reach different concentrations and then mixed with a concentrated suspension of mgPS NPs in order to obtain a final nanoparticles concentration of 6wt%.

Investigation of supraparticles magnetically-controlled movement
The supraparticles were put in a petri dish with distilled water. The movement of those supraparticles was controlled by displacing a permanent ring magnet (NdFeB, Ø = 19.1/9.5 mm, height 6.4 mm) at different positions. The trajectory, moving speed, and orientation of the supraparticle over time in distilled water were analyzed with ImageJ. 5

Characterizations
The drying process of droplets was monitored with a goniometer equipped with a side-camera (OCA 35, DataPhysics, Germany). The movie of drying droplets was recorded at 12 frames/min and for ca. 1 hour. The movie was extracted and analyzed to obtain the contact angle and contact line over time using MATLAB. The morphologies of the supraparticles were obtained with a stereomicroscope (M80, Leica, Germany) equipped with a digital camera (IC80 HD, Leica, Germany). To observed the inner structures of supraparticles, they were first embedded in epoxy resin and then sliced (slice thickness of 500 nm) with an ultramicrotome (EM UC7, Leica, Germany). The magnetic properties of mgPS NPs and supraparticles were obtained with a vibrating-sample magnetometer (Cryogenic Ltd. UK). The morphologies of the nanoparticles and supraparticles were observed by transmission electron microscopy (TEM, JEOL JEM1400, Japan) and scanning electron microscopy (SEM, Hitachi SU8000, Japan). For the chemical analysis of S5 the supraparticles´ surface and interior, the SU8000 with an energy dispersive x-ray spectrometer (EDX) was used to measure the elemental composition.   Figure S2 shows the morphology of the synthesized hybrid Fe3O4/polystyrene nanoparticles (mgPS NPs) obtained by TEM, the average diameter was 93 ± 16 nm. The hydrodynamic diameter S6 of the mgPS NPs measured by DLS (Nicomp, UK) was 110 ± 30 nm, and their Zeta potential (Zeta Nanosizer, Malvern Instruments, UK) was -67 ± 13 mV. The resulting hybrid mgPS NPs were supraparamagnetic has evidence by the variation of the magnetization in magnetic fields of different strength (Figure S2c).

Characterizations of CoFe2O4/polyvinylpyrrolidone ferromagnetic nanoparticles (mgPVP NPs)
CoFe2O4 nanoparticles (diameter below 50 nm, see Figure S3a) were used as building blocks to form the supraparticles. It is noted that the ferromagnetic nanoparticles have a magnetization even in the absence of a magnetic field. The hydrodynamic diameter of the resulting mgPVP colloidal suspension was 160 ± 16 nm ( Figure S3b).

Drying and assembly of ferromagnetic mgPVP NPs
With increasing the nanoparticle concentration, the final supraparticles can form the conical structure with high aspect ratio. A difference in magnetization and viscosity of the suspension, in comparison to mgPS, prevented the formation of more varied structures the low magnetic field used previously (16 kA/m) was not sufficient to induced the formation of anisotropic shapes.

Investigation of binary supraparticles
Triplicate of mgPS, PS/mgPS, and TiO2/mgPS supraparticles dried in a 160 kA/m magnetic field Figure S15. Reproducibility of the supraparticles structure obtained with the same drying conditions for 5µL droplets of a suspension containing 6wt% solid fraction. Scale bar = 0.5 mm. Figure S16 shows the cross-section of PS/mgPS and TiO2/mgPS supraparticles. In the case of coassembly between mgPS and PS NPs (Figure S16a), the PS NPs are clearly visible in the binary structure (white part). The gradient in the coloration of the supraparticle stemmed from the segregation of the mgPS and PS NPs. The same gradient of concentration was not obvious in the binary supraparticles made with TiO2. The density of the TiO2 NPs was much larger than the density of the PS NPs (respectively 4.23 and 1.04 g/cm 3 ) thus the volume fraction occupied by a constant loading of 9 wt% of non-magnetic nanoparticle was consequently 4 times smaller and more difficult to observe. However, using TiO2 NPs with a smaller diameter than the mgPS NPs allowed to clearly observe the distribution of the different nanoparticles using SEM. Figure S16d shows that in addition to the macroscopic segregation that is clearly observable in the PS/mgPS binary supraparticles, the TiO2 NPs also segregated into smaller microdomains dispersed in a mgPS-rich environment.