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.

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] However, superparamagnetism is strongly size dependent and only exists in nanocrystals. In the case of iron oxide nanoparticles (NPs), superparamagnetism is mostly observed in particles with a diameter smaller than ca. 30 nm.[12,13]

To build micro/macro-size superparamagnetic materials for further applications, superparamagnetic NPs have to be assembled into more complex hierarchical structures.[12,14] However, following aggregation and clustering, the magnetization and the coercitivity of the material could change,[15−20] and new magnetic response could be observed.[21,22] Therefore, the controlled assembly of superparamagnetic NPs is required. Such structures have been prepared by the direct formation of 1D arrays[23,24] or by the formation of controlled hybrid clustered beads.[12,25,26] Those clustered-beads can themselves undergo further assembly leading to the formation of more complex structures, that is, necklace-like chains.[18,27,28]

In comparison to those simples structures, three-dimensional (3D)-structured magneto-responsive materials, especially materials with an anisotropic magnetic response, would display an even larger array of potential applications such as flexible integrated sensors[29,30] or biomimetic soft-robots.[31,32] In order to obtain 3D-structured magneto-responsive materials, techniques such as self-assembly[33−35] and lithography[36−38] have been used. For example, the self-assembly of magnetite nanocubes into helical superstructure was realized by solvent evaporation at the liquid–air interface in the presence of an external magnetic field.[33] Similarly, lithography has been used to prepare complex 3D structures containing iron oxide NPs. For example, iron oxide NPs embedded in a monomer solution were used to produce superparamagnetic 3D-structured microrobots with a helical shape. These helical microrobots were able to mimic bacterial movements and swim under remote magnetic guidance in flows with low Reynolds numbers, for example, in blood.[39−41] Even though those processing methods have shown a range of potential applications for 3D-structured magneto-responsive materials, they still suffer from limited scalability and heavy use of non-ecofriendly conditions.

Recently, evaporation-guided colloidal assembly has been proposed as an efficient method for the preparation of 3D mesoscopic NP assemblies.[42,43] In this approach, droplets of NP suspension were dried on liquid-repellent surfaces to form supraparticles.[42−44] After evaporation-guided assembly, the fabricated supraparticles can be easily collected without a further processing step which prevents the use of toxic solvents.[42] Moreover, various sizes and shapes of supraparticles can be obtained. By tuning the concentration of the suspension and the volume of the drops, the size of the supraparticles can be varied from several microns to several millimeters. Supraparticles with various shapes, such as hemispherical,[45] doughnut-like,[46] and boat-like,[47,48] have been fabricated with the simple control of the wettability of the colloidal suspension on the substrates. Additionally, the composition of supraparticles can be varied by changing or mixing different types of colloids.[43] For example, suspensions of iron-nickel alloy particles were mixed into droplets of silica NPs and led to the formation of patchy supraparticles.[46]

An alternative method for the formation of large 3D superparamagnetic structures is to make use of the natural properties of ferrofluids. Colloidal suspensions of magnetic NPs, known as ferrofluid, can form a variety of transient structures when placed in a magnetic field. Ferrofluids produce reversible structures under the guidance of a magnetic field, such as Rosensweig pattern[49] or separated cone-like microdroplets when placed on non-wetting solid surfaces.[50] However, those patterns have never been used to purposely template solid materials.

Here, a suspension of hybrid Fe3O4/polystyrene nanoparticles (mgPS NPs) (SI, Figure S2) stabilized with sodium dodecyl sulfate (SDS) ([SDS] = 0.4 g/L, surface tension (γ) = 49 ± 1 mN/m) was dried in the presence of a magnetic field on a superamphiphobic surface (Figure a) to trap the transient shape of the suspension droplets during evaporation. By controlling the magnetic strength, magnetic orientation, and the initial concentration of NPs, supraparticles with distinct anisotropic shapes were obtained.

Figure 1

(a) Experimental system used for the production of supraparticles by evaporation-guided assembly of a magnetic NPs dispersion on a superamphiphobic surface. (b) Evolution of a 3 wt % droplet during drying without (upper panel) and with (bottom panel) magnetic field. Scale bars are 0.5 mm. Drying curve of the droplet (c) without and (d) with magnetic field. Insets represent the dimensions measured during drying.

Results and Discussion

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. When, instead, only hydrophobic substrates were used, film-like structures were observed due to the strong pinning effect caused by the more important wetting of the substrate by the colloidal suspension (SI, Figure S5).

To fully understand the effect of the presence of a magnetic field on the supraparticles formation, the drying process of superparamagnetic NPs suspensions with and without magnetic field was observed under the same drying conditions (Figure b–d). Figure b shows that in the absence of a magnetic field, the spherical droplet shrunk symmetrically as a consequence of water evaporation. After 35 min, the shape of the droplet changed through a buckling mechanism, that is, after reaching a critical composition, a sudden deformation of the droplet occurred. The resulting anisotropic structure, shaped like a deflating ball, subsequently shrunk further without notable alteration in its shape during the final part of the drying process. (Figure b and Video S1, SI). This buckling behavior was caused by the non-uniform distribution of mgPS NPs and surfactant molecules within the droplet. Energy dispersive X-ray spectroscopy was used to analyze the distribution of surfactant and mgPS NP across the volume of the supraparticle (SI, Figure S6). The local concentrations of sulfur and iron were used as respective indicators for the distribution of surfactant and mgPS NPs (SI, Table S1). The results show that the surface of the supraparticles was enriched in surfactant in comparison to the bulk which was richer in mgPS NPs. During the evaporation, the concentration of NPs and surfactants at the air/water interface increased, leading to the formation of a shell. As the amount of water in the droplet kept decreasing, a mechanical stress built up in the shell. To release this stress, the shell rapidly deformed, and the droplet buckled. As a result, the contact line remained constant, while the surface of the drop wrinkled (Figure c). This buckling behavior was similar to what has been previously observed for suspension of silica or polystyrene NPs dried on superhydrophobic surfaces,[46,47] and reminiscent of what has been observed during spray drying of microdroplets of colloidal suspensions.[51]

In the presence of a magnetic field of 16 kA/m, the initial contact line was larger than that observed without magnetic field since the magnetic attractive force is driving the NPs to partially segregate at the bottom of the droplet. Initially, the volume of the droplet decreased symmetrically by evaporation, preserving the original aspect ratio. However, after 30–40 min, the aspect ratio of the droplet started to change; the droplet became narrower and more elongated (SI, Figure S13). Over the following 2–10 min, as the concentration of mgPS NPs increased due to the evaporation of water, the height of the droplet quickly increased, and the droplet formed a cone-like structure. After this point, the shape and aspect ratio of the supraparticles were preserved until evaporation of the remaining water. Ultimately, the final shape of the supraparticle was reached and preserved after ca. 45 min (Figure d and Video S2, SI).

In ferrofluids suspensions, Rosensweig instabilities are formed only when the magnetization (M) of the suspension exceeds a critical magnetization (Mc) defined by the surface tension (γ) of the suspension as[49]where μ and μ0 are the magnetic permeability of the ferrofluid suspension and of the vacuum, respectively, and g and ρ are the gravitational constant and the density of the ferrofluid suspension. Initially, Figure b shows that there are no differences in the shape of the droplets dried at 0 and 16 kA/m. However, as the system dried, the magnetization of the droplet of mgPS suspension crossed the Mc threshold, and the droplet deformed leading to the formation of a conical supraparticle. The magnetization of the mgPS suspension was influenced by several factors. The net magnetization of mgPS NPs changes with the applied magnetic field (H) (SI, Figure S2c) and is described by[49]where χ is the volume magnetic susceptibility of the material. Furthermore, the magnetization (M) of the ferrofluid suspension depends on the NP concentration, as it is proportional to the volume fraction of superparamagnetic material in the suspension. According to the Rosensweig theory:[49,52]where VFe is the volume of iron oxide NPs and Vdroplet is the volume of the droplet. Thus, the magnetization of the suspension depends both on the initial concentration of mgPS in suspension and on the amount of water that has evaporated. As the suspension dried, the concentration increased, and at some point during the process, the magnetization of the droplet surpassed Mc, and the droplet started to elongate as evidenced by the increase in the aspect ratio of the droplet (SI, Figure S13).

The results obtained suggest that the shape adopted by the ferrofluid droplets, and ultimately by the supraparticles, was strongly influenced by the magnetization of the droplets. Consequently, the initial concentration of NPs (cNP) in the ferrofluid droplet and the applied magnetic field were varied to investigate the relationship between the final shape of the supraparticle and conditions under which drying was performed.

The initial cNP was varied from 0.3 wt % to 30 wt %, and the droplets were dried in a magnetic field of 16 kA/m. The initial contact angle decreased and the initial contact line increased with increasing cNP, since at higher concentration, a larger number of NPs were dragged to the interface between the droplet and the substrate. This led to an increase in the apparent wetting of the surface by the suspension, characterized by a lower contact angle and higher contact line (Figure a).

Figure 2

Drying kinetics of ferrofluid droplets in magnetic fields of (a–d) 16 kA/m and (e–f) 160 kA/m. (a and e) Contact lines of the initial droplets and of the resulting supraparticles as a function of initial concentration of NPs (cNP). The red regime indicates cone-like structures. The blue regime indicates barrel-like structures. The green regime indicates two-tower-like structures. (b and f) Optical photos of supraparticles prepared with different cNP. (c and g) Side-view pictures of the drying process for cNP = 21 and 30 wt %, respectively. (d and h) Evolution of contact angle and contact line during the drying of initial cNP = 21 and 30 wt %, respectively. Scale bar = 0.5 mm.

The drying of suspensions with different cNP resulted in the formation of supraparticles with distinct shapes. In a magnetic field of 16 kA/m, two populations of supraparticles were observed: cone-like structures at low cNP (≤6 wt %) and barrel-like structures at high cNP (Figure b). The cones were partially empty; a cavity was formed at the interface between the droplet and the superamphiphobic surface (SI, Figure S7). The formation of this cavity can be attributed to a buckling effect directed by the magnetic field and the presence of the superamphiphobic surface. Particle jamming created by the combined effect of solvent evaporation and magnetic field led to the formation of a stiff layer of particles at the droplet/surface interface, when this layer was unable to shrink further, buckling happened to create the cavity.

Conversely, suspensions with high cNP (≥12 wt %) resulted in supraparticles with barrel-like structures. The inner part was completely empty and was opened from top-to-bottom; the result of the combined effect of the buckling and dewetting (SI, Figure S8). Figure c shows that, initially, the ferrofluid droplet was spherical and shrunk symmetrically during the water evaporation. Then the aspect ratio of the droplet started to increase, which is similar to what was observed for lower cNP leading to the formation of a conical droplet (SI, Figure S13). Afterward, the height of the conical ferrofluid droplet increased until it reached a critical point where buckling happened, resulting in the formation of a barrel-like structure within minutes (Video S3, SI). This buckling effect was also observable in the variation of the contact line over time (Figure d). The buckling happened where a strong discontinuity was monitored toward the end of the drying process. For example, during the drying of a suspension with cNP= 21 wt % in a magnetic field of 16 kA/m, the contact line decreased during the first 25 min. Then, the contact line and the width of the droplet remained constant for ca. 5 min, and only the height of the droplet decreased until the droplet buckled. Following the buckling of the droplet, a rapid decrease in the contact line was observed, and the final barrel-like supraparticle was obtained. The barrel like-structure is the stretched version of toroids formed with non-magnetic suspension drops.[46]

To investigate further the influence of the strength of the magnetic field, we conducted a series of experiments at 80 and 160 kA/m. The results observed at 80 kA/m (SI, Figure S11) were similar to those observed at 16 kA/m. However, the behavior of the suspension drying in a magnetic field of 160kA/m showed marked differences due to the higher magnetization of the suspension. Similar to the behavior observed in H = 16 kA/m, the contact line of the droplets increased with increasing cNP, but, in every case, the contact line at 160 kA/m was larger than at 16 kA/m. In a field of 160 kA/m, especially at higher cNP, the effect of the strong field led to an increase of the contact line of the droplets on the surface (Figure e), since the magnetic attraction forces acting on the NPs were stronger and dragged the mgPS NPs to the substrate, increasing the apparent wetting of the superamphiphobic surface.

In a magnetic field of H = 160 kA/m, two different types of shapes, either cone-like structures or two-tower-like structures (Figure f), were observed. Similar to what was observed with H = 16 kA/m, lower cNP (≤12 wt %) led to the formation of empty cone-like structures (SI, Figure S7). However, at higher cNP (≥21 wt %), the NPs formed two-tower-like structures instead of barrel-like shapes. At high H and high cNP, the ferrofluid droplets did not buckle into a barrel-like structure, but rather split into two cone-like shapes during evaporation (Figure g and Video S4, SI). This splitting effect was observed in the variation of the contact line over time (SI, Figure S9). For example, during the drying of a suspension with cNP = 30 wt % in a magnetic field of 160 kA/m, the contact line first decreased for ca. 25 min. Then the contact line remained constant, and during that time, the top part of the droplet split into two separated fractions (SI, Video S4).

This behavior was similar to what has been observed for the splitting of ferrofluid droplets on superhydrophobic substrates under variable magnetic fields.[50] It is known that ferrofluid drops can spontaneously split into several smaller droplets when the drop size (contact line of the droplet) is larger than a critical wavelength (λc):[50]where H is the total magnetic field intensity, M is the magnetization of the drop, and d/dz is the change in z-direction (normal to the surface). In the present case, the splitting phenomenon was more complex, because during evaporation, both γ and M are changing since those parameters are concentration dependent. During the drying of the droplets, the change of γ was moderate and similar for all samples with different cNP because the initial concentration of surfactant molecules was the same in all samples. However, the change of magnetization over time, M(t), was not the same at every cNP. As the water evaporated, Vdroplet decreased over time resulting in an increase of the magnetization during drying, which finally led to a decrease in λc. Consequently, even if, initially, at high cNP the system was below the λc, droplet splitting could be observed during the drying process as the concentration and, consequently, the magnetization increased. The critical concentration where the magnetization was large enough to induce a splitting of the droplet was reached after ca. 30 min at 21 wt % and after 26 min at 30 wt % (SI, Figure S9) when the concentration of mgPS NPs in suspension reached ca. 50 wt %.

The relation between the size of the contact line and the splitting of the droplet was also corroborated by using droplets of different volume with cNP= 21 wt % (SI, Figure S10). During the drying kinetics, droplets of different volumes have the same magnetization and surface tension, but the contact line decreased with the initial volume of the droplet while λc stayed constant. Thus, smaller droplets, having smaller initial contact lines, did not experience splitting because the contact line remained constantly smaller than λc.

Figure a summarizes the diversity of supraparticle shapes obtained by mapping the structures obtained as a function of the initial NPs concentrations and applied magnetic fields. Four different shapes were obtained for the resulting supraparticles: deflated ball, cone, barrel, and two-tower. Other factors such as the drying speed or the total volume of droplet also influences the final shape of supraparticles. For example, by drying the 21 wt % droplets with different volumes, cone and two-tower-like shapes could be obtained (SI, Figure S10). The difference in the final structure adopted by the drying droplet depended on the complex balance of forces present in the system, where wetting, buckling, and magnetization influenced, through different mechanisms, the final structure observed. Gravitational forces can also have an effect, however, it is not expected to play an important role here since the gravitational forces were significantly smaller than magnetization.[53] As shown in the structure map, the overall shapes of supraparticles were mainly influenced by the magnetization. Although the wetting of the surface by the droplet was minimized by using superamphiphobic surfaces, wetting forces were responsible for the buckling and the formation of hollow structures. The shapes of the supraparticles can be adjusted by changing the direction of the applied magnetic field during the drying process. When the direction of the magnetic field was changed from 90° to 60°, the resulting supraparticles showed similar structures but have different orientation following the directions of the magnetic field (Figure b).

Figure 3

(a) Structure map of the supraparticles obtained with the variation of the magnetic strength and initial concentration of ferrofluid. The regime in gray, red, blue, and green indicates supraparticle with deflated ball, cone, barrel, two-tower shape, respectively. These supraparticles were obtained by drying of 5 μL NPs suspension. b) Two-tower shape supraparticles with different tower orientations by drying the 21 wt % of ferrofluid under different directions of the magnetic field (left one with the vertical direction and the right one with non-vertical direction, both were under a magnetic field of 160 kA/m). (c) Microscopic images of supraparticles obtained by drying CoFe2O4 NPs suspension on superamphiphobic surfaces with or without magnetic field. (d) Aspect ratio of binary supraparticles as a function of weight fraction of mgPS NPs. (e) Binary supraparticles fabricated by drying a cosuspension of mgPS NPs with titanium dioxide NPs (TiO2 /mgPS, upper panel) or polystyrene NPs (PS/mgPS, bottom panel). The weight fraction of mgPS NPS was varied from 0%, 50%, 83% to 91%. The total initial NPs concentration remained constant at 6 wt %, and the magnetic field was 160 kA/m. Scale bars are 0.5 mm.

Similarly, a variety of supraparticles with various structures can be obtained when using a suspension of ferromagnetic NPs (Figure c and SI, Figure S14) instead of a suspension of superparamagnetic mgPS NPs. As shown in Figure c, the supraparticles with a cone-like structure were obtained by drying the ferromagnetic suspension (from 1% to 20%) in the presence of a magnetic field (160 kA/m).

Furthermore, complex heterostructures can be prepared by drying a cosuspension of mgPS NPs and other NPs.[46,54] For example, cone-shape supraparticles were prepared by drying a mixture of TiO2 NPs and mgPS NPs (Figure d–e). Similarly, anisotropic supraparticles were also obtained when mgPS NPs were mixed with pure polystyrene NPs. In both cases, the mgPS NPs drove the droplet to form anisotropic supraparticles (aspect ratio >1) when the fraction of mgPS NPs in a cosuspension was above a threshold (75%) to actively act as a templating agent. Since the magnetization of the droplet is a function of the concentration of magnetic iron oxide in the droplet, as the ratio of non-magnetic particles increased, the shape selectivity was lost. When the concentration of mgPS in the NP cosuspension decreased below 50 wt % of the solid content, then the critical magnetization (Mc) needed to trigger the formation anisotropic structures cannot be reached, and spherical supraparticles were obtained. When the loading of mgPS was larger than this threshold composition, a magnetically induced deformation of the supraparticle was observed. However, segregation of the different NPs was observed in the final structures (SI, Figure S16). The mgPS NPs segregated in the regions where the magnetic field was the strongest. The formation of supraparticles by the evaporation of cosuspension droplets shows the flexibility of magnetic templating to form complex supraparticles with different architectures, chemical compositions, and functions.

The anisotropic supraparticles obtained by drying mgPS NPs preserved the superparamagnetism of the building blocks. The saturated magnetization of mgPS NPs and supraparticles was 55 Am2/kg and 52 Am2/kg, respectively (Figure a). Thus, their magneto-responsive behavior could be harnessed to magnetically control the supraparticles. Figure b shows the behavior of a cone-like supraparticle in a magnetic field acting as an active microswimmer. First, the movement of the supraparticle in suspension in water was controlled by applying selected magnetic fields. The trajectory of this supraparticle was tailored by the on/off triggering the magnetic field at different locations (Video S5, SI). The response speed of the supraparticle was fast, immediately when the magnetic field was turned on, the supraparticle moved and quickly stopped when the magnetic field was removed (Figure c). In an inhomogeneous magnetic field, the supraparticle was propelled in the direction of increasing gradient of magnetic field. The supraparticle not only followed the magnetic field but also reoriented itself in response to the application of the magnetic field (SI, Figure S17). The bottom part of the cone-like supraparticle was always facing the magnet due to the anisotropy of the supraparticle. Similar behavior was also observed in a rotational magnetic field (Video S6, SI).

Figure 4

Supraparticle in a remote magnetic field. (a) Magnetic property of the supraparticles characterized by vibrating-sample magnetometer. (b) Trajectory of a cone-like supraparticle in suspension in water with varied magnetic fields. Insets indicate the positions of magnets. The order of applied magnetic fields was top, left, right, and top. (c) Velocity (black curve) of the cone-like supraparticle shown in Figure b as the function of time. The blue curve shows the magnetic field experienced by the supraparticle. (d) Acceleration of supraparticle shown in Figure b as the function of the strength of the magnetic field (first and second displacement correspond to the application of the first and second gradient of magnetic field).

Conclusion

In summary, we prepared superparamagnetic supraparticles by the evaporation-guided assembly of ferrofluid droplets on a superamphiphobic surface in the presence of a magnetic field. These magneto-responsive supraparticles have well-defined structures such as deflated ball, cone, barrel, and two-tower shapes. The final 3D architecture of the supraparticles can be tuned by systematically varying the initial concentration of superparamagnetic NPs and the applied magnetic field. Moreover, the ferrofluid suspension can be used as the driving force to template the assembly of superparamagnetic NPs cosuspension and other functional NPs to fabricate functional anisotropic binary supraparticles. Thus, this flexible fabrication process offers the possibility to create anisotropic and magneto-responsive supraparticles with various materials. Since these anisotropic supraparticles are magneto-responsive, their orientation and motion can be spatially controlled by magnetic fields.

Methods

Superparamagnetic iron oxide NPs (Fe3O4 NPs) with a diameter of 13 ± 4 nm were synthesized by co-precipitation. The Fe3O4 NPs were then encapsulated in polystyrene by emulsion polymerization to produce hybrid Fe3O4/polystyrene nanoparticles (mgPS NPs) with a diameter of 110 ± 30 nm. The mgPS NPs were washed and finally dispersed in water and stabilized with sodium dodecyl sulfate ([SDS] = 0.4 g/L, surface tension of suspension was 49 mN/m). The mgPS NPs were used as building blocks for the fabrication of supraparticles. To fabricate the supraparticles, unless noted otherwise, 5 μL droplets of mgPS NPs suspension (with a concentration ranging from 0.3 wt % to 30 wt %) were dried on a superamphiphobic surface at a temperature of 23 °C and a humidity of 25%. The silica structured superamphiphobic surfaces were fabricated by the soot deposition method.[55] The magnetic field was generated by placing a permanent magnet (NdFeB, 30 × 30 × 15 mm) under the superamphiphobic surface. To obtain binary supraparticles, the titanium dioxide nanoparticles (TiO2 NPs, 25 nm, Aldrich, Germany) and polystyrene nanoparticles (PS NPs, 270 nm) were dispersed in distilled water and then mixed with a concentrated suspension of mgPS NPs in order to obtain a total NP concentration of 6 wt % with varying fraction of mgPS NPs.