Modular super-assembly of hierarchical superstructures from monomicelle building blocks.

Manipulating the super-assembly of polymeric building blocks still remains a great challenge due to their thermodynamic instability. Here, we report on a type of three-dimensional hierarchical core-satellite SiO2@monomicelle spherical superstructures via a previously unexplored monomicelle interfacial super-assembly route. Notably, in this superstructure, an ultrathin single layer of monomicelle subunits (~18 nm) appears in a typically hexagon-like regular discontinuous distribution (adjacent micelle distance of ~30 nm) on solid spherical interfaces (SiO2), which is difficult to achieve by conventional super-assembled methods. Besides, the number of the monomicelles on colloidal SiO2 interfaces can be quantitatively controlled (from 76 to 180). This quantitative control can be precisely manipulated by tuning the interparticle electrostatic interactions (the intermicellar electrostatic repulsion and electrostatic attractions between the monomicelle units and the SiO2 substrate). This monomicelle interfacial super-assembly strategy will enable a controllable way for building multiscale hierarchical regular micro- and/or macroscale materials and devices.

INTRODUCTION

Organizing colloidal nanoparticles into hierarchical superstructures is fascinating owing to their outstanding and synergistic properties (–), which have been explored in wide applications, involving photonics (), mechanics (), drug delivery (), catalysis (), sensing (), and energy storage (). Superstructuring is also of fundamental significance for deepening our understanding of the super-assembly sciences from molecular, nanoscopic, to macroscopic scales (, ). From a basic perspective, conventional super-assembly mainly focuses on inorganic colloidal nanoparticles (Au, Ag, Fe3O4, SiO2, etc.) because of their high thermodynamic stability (, –). However, in most cases, superstructures from these inorganic particles are typically superlattice configuration, coming from the normal one-by-one close packing of building blocks for minimizing the total surface energy (, ).

Because of this closely packing arrangement, inorganic nanoparticles are difficult to super-assemble on interfaces to generate the hierarchical superstructures, especially with a unique discontinuous interfacial distribution of building blocks (e.g., building superstructures of core-satellite configuration) (, ). To overcome these limitations, some structure-directed agents (e.g., DNA, protein, liposome, and polyphenol) (, –) or external directing forces (e.g., electric and magnetic fields) (, ) are needed to drive these inorganic nanoparticles for interfacial super-assembly. Nevertheless, these extra added agents could adversely restrain the supraparticle activity in practical applications (–). Meanwhile, the supraparticles obtained with the assistance of external fields (electric or magnetic) normally bear serious structural instability when transferred to new surroundings (without such fields) (, ). Thus, it is urgent to develop new building subunits and control their interfacial super-assembly to construct the hierarchical core-satellite superstructures.

The amphiphilic block copolymer micelle may be one of the most promising candidates to build the hierarchical core-satellite superstructures (–). Owing to the merits of the tunable copolymer block species, controlled molecular weight, etc., polymeric micelles can not only act as templates to fabricate the well-known conventional ordered mesoporous materials (–), monolayered mesoporous nanoparticles (), and ultrafine organic-inorganic nanohybrids (, ) but also perform as robust super-assembly building units to create a lot of fascinating hierarchical superstructures (–). In the past decade, some endeavors based on organic polymeric nanoparticles have been paid to obtain the desirable superstructures. For instance, vesicle-like or hemispherical hierarchical polymeric core-satellite supraparticles have been prepared on the basis of interfacial super-assembly, driven by directing agents or stabilized emulsions (–). However, such superstructures are normally irregular in configuration, and the interparticle distances of subunits on interfaces are difficult to manipulate, because such polymeric soft matters are easily aggregated and interwoven (–). To date, for interfacial super-assembly, it still remains a great challenge to accurately manipulate the adjacent distance of super-assembled building blocks on a substrate. In addition, an organic polymeric subunit–based hierarchical superstructure with ultrahigh stability has never been reported due to the inherently thermodynamic instability of polymeric micelles.

Here, we report an unprecedented type of hierarchical polymeric micelle subunit–based SiO2@monomicelle core-satellite superstructure via a novel monomicelle interfacial super-assembly route. In this superstructure, the polymeric polystyrene-block-poly(4-vinylpyridine)-block-poly(ethylene oxide) (PS-PVP-PEO) monomicelle (~18-nm) subunits typically appear in a hexagon-like regular discontinuous distribution on colloidal SiO2 interfaces, totally different from the conventional closely packing super-assembly. The number of regular monomicelles on colloidal SiO2 interfaces can be quantitatively controlled (ranging from 76 to 180). This quantitative control can be well obtained via precise manipulation of the interparticle electrostatic interactions (the intermicellar electrostatic repulsion and electrostatic attractions between the monomicelle units and the SiO2 substrate) without the need of any extra directing templates or external fields. As a universal approach, a vast number of unique superstructures have successfully been achieved via a modular super-assembly of monomicelles with tunable configurations (spherical and cylindrical), sizes (from 10 to 46 nm), and layers (one and two) on diverse interfaces with adjustable components (polymers, oxides, and semiconductors) and geometries [one-dimensional (1D), 2D, and 3D]. Besides, the obtained super-assemblies are also structurally very stable, which can be freely redispersed in various solvents (e.g., ethanol, methanol, water, and acetic acid) without any disassembly. This novel monomicelle interfacial super-assembly strategy paves a new way for building multiscale hierarchical superstructures with unique structural complexity and functional integration.

RESULTS

Materials synthesis and characterization

Modular super-assembly of superstructures from single-micelle building subunits on diverse solid interfaces could be achieved via a monomicelle interfacial super-assembly method (Fig. 1). Uniform and monodispersed PS-PVP-PEO monomicelles were first obtained by the thermokinetically mediated methodology (fig. S1) (). For this thermokinetically mediated methodology, the PS-PVP-PEO triblock copolymers undergo a process of heating (to the glass transition temperature of PS block, ~100°C) and cooling (back to room temperature) successively. It is worth noting that the PS-PVP-PEO monomicelles obtained from this thermokinetically mediated approach are structurally very stable. The reason is that during the cooling process, the long-chain PS core of PS-PVP-PEO monomicelles can progressively cross-link and solidify to a highly compact structure due to its “frozen” effect. This kind of structurally stable monomicelles can bear the changed solvent environments (e.g., ethanol, methanol, water, and acetic acid) (fig. S2) and strong ultrasonic treatments (fig. S3). Afterward, such ultrastable polymeric PS-PVP-PEO monomicelles performed as a stable building block to super-assemble on various solid interfaces with adjustable configurations and components for generating core-satellite superstructures (Fig. 1A) by controlling the electrostatic attraction. For such interfacial super-assembly, the electrostatic interaction between the monomicelles and the substrates could be highly controlled by tuning the concentration of the added ammonia (Fig. 1A). Meanwhile, followed by surface decoration, such core-satellite supraparticles can further be transformed into a new interface for the second-layer monomicelle super-assembly to create brand-new multiscale core-satellite-satellite superstructures (Fig. 1B). In addition, owing to the intermicellar electrostatic repulsion, these monomicelles on the interface are typically regular discontinuous, hardly achievable by the methods reported previously.

Fig. 1.

Modular super-assembly of monomicelles to build core-satellite superstructures via a monomicelle interfacial super-assembly route.

Schematic representation of the core-satellite SiO2@monomicelle (A) and core-satellite-satellite SiO2@monomicelle@monomicelle (B) superstructures.

Taking SiO2@monomicelle supraparticles as a demonstration, the PS-PVP-PEO monomicelles are well super-assembled on the colloidal spherical SiO2 (~300-nm) interfaces (fig. S4). Field-emission scanning electron microscopy (FESEM) images show that the hierarchical core-satellite SiO2@monomicelle supraparticles prepared by the monomicelle interfacial super-assembly approach are highly uniform in macroscopic domains with the spherical SiO2 cores and ultrasmall monomicelle shells (Fig. 2A and fig. S5A). High-resolution FESEM (HRSEM) image (Fig. 2B) further depicts that the monomicelle subunits appear in a unique discontinuous distribution on the spherical SiO2 interfaces by a very regular hexagonal non–close-packed pattern. The diameter of monomicelle subunits is ~18 nm, and the spatial distance of the monomicelles on an interface is almost identical (~30 nm; the micellar distance is defined as the center distance of two adjacent spherical monomicelles on an interface) (Fig. 2B). Furthermore, transmission electron microscopy (TEM) images (fig. S5B) clearly reveal that a monolayer of the monomicelles is homogeneously distributed on the colloidal SiO2 core surfaces. Dynamic light scattering (DLS) results show a narrow and sharp peak centered at ~340 nm, further demonstrating the macroscopical uniformity of this hierarchical SiO2@monomicelle superstructure (fig. S6). The high-resolution TEM (HRTEM) image (Fig. 2C) depicts a typical core-satellite structure with 18-nm monomicelles on the colloidal SiO2 nanospheres. The corresponding high-angle annular dark-field scanning TEM (HAADF-STEM; Fig. 2D) further demonstrates that merely single-layer discontinuous particles are uniformly anchored on the spherical interfaces (colloidal SiO2). In addition, energy-dispersive x-ray (EDX) elemental mapping images clearly show that C and N elements (ascribed to PS-PVP-PEO monomicelles) are homogeneously distributed out of Si and O elements (ascribed to SiO2), further proving the core-satellite architectures (Fig. 2, D to I). Such hierarchical SiO2@monomicelle superstructures can retain intact structure without any disassembly even when transferred to different solvent environments (e.g., ethanol, methanol, water, and acetic acid) (fig. S7) or under vigorous ultrasound treatment (fig. S8), indicating their high structural stability. In addition, such superstructures are also stable in a weak acid solution (pH 6.0; fig. S9A), in solution with high ionic strength (NaCl concentration of 2 M; fig. S9B), or under a high temperature of 100°C (fig. S9C) for 6 hours.

Fig. 2.

Characterization of the core-satellite SiO2@monomicelle superstructures.

Low-magnification (A) and high-magnification (B) FESEM, HRTEM (C), HAADF-STEM (D), and EDX elemental mapping images (E to I) of the core-satellite SiO2@monomicelle (PS-PVP-PEO) superstructures. Scale bars, 200 nm (A), 100 nm (B and C), and 50 nm (D to I).

Precise control of the monomicelle interfacial super-assembly

The monomicelle interfacial super-assembly strategy has high flexibility in regulating the architectures and constituents of the hierarchical superstructures by tuning the synthetic conditions (Fig. 3). For the SiO2@monomicelle superstructures, the diameter of the monomicelles on the colloidal SiO2 interface is highly adjustable (Fig. 3, A to D, and fig. S10, A to D). Low-magnification TEM images show that all the SiO2@monomicelle supraparticles are highly dispersed and uniform (fig. S10, A to D). The average sizes of the monomicelles on the core SiO2 interfaces are measured to be ~10, 18, 30, and 46 nm, respectively (Fig. 3, A to D). For these products, no overlay of monomicelles can be observed on each core SiO2 particle (Fig. 3, A to D), clearly indicating the high control of this monomicelle interfacial super-assembly strategy. In addition, two kinds of monomicelles with different sizes (i.e., 18 and 46 nm) can also be jointly super-assembled on a spherical SiO2 interface in a monolayer manner (fig. S11). This monomicelle interfacial super-assembly strategy is highly universal, and a host of the hierarchical core-satellite supraparticles can be successfully built by super-assembly of the monomicelles on diverse substrates, including the 3D spherical resorcinol-formaldehyde (RF), titanium oxide (TiO2) nanospheres, 2D graphene oxide (GO) nanosheets, and 1D ferric oxide (Fe2O3) nanorods (Fig. 3, E to H, and figs. S12 to S15).

Fig. 3.

Precise control of the monomicelle interfacial super-assembly strategy.

(A to D) TEM images of the core-satellite SiO2@monomicelle superstructures with different sizes of monomicelles, ranging from 10 (A), 18 (B), 30 (C), to 46 nm (D). (E to J) Numerous core-satellite superstructures obtained by the super-assembly of monomicelles with a size of 18 nm on RF (E), TiO2 (F), and GO (G) and 46-nm monomicelles on Fe2O3 (H). (I and J) CSS Fe2O3@RF@monomicelle (I) and upconversion nanoparticle@SiO2@monomicelle (J) superstructures. (K) TQD-decorated SiO2@monomicelle supraparticles. (L) Core-satellite-satellite SiO2@monomicelle@monomicelle superstructures by super-assembly of two-layer monomicelles, and the first and second layers of monomicelles are 18 and 46 nm, respectively. Scale bars, 100 nm in all TEM and SEM images.

In addition to the core-satellite supraparticles, some core-shell-satellite (CSS) functional supraparticles can also be obtained (Fig. 3, I and J, and figs. S16 and S17) by using this monomicelle interfacial super-assembly strategy. The TEM image shows that a shuttle-like CSS-type Fe2O3@RF@monomicelle superstructure is achieved (Fig. 3I) as the monomicelles assembled on the surfaces of the core-shell Fe2O3@RF nanoparticles. SEM images exhibit that the CSS-type superstructures are highly controlled with excellent uniformity (fig. S16, A and B). In addition, HRTEM images further prove that the monomicelles are intimately anchored on the RF surfaces of the shuttle-like core-shell Fe2O3@RF particles (fig. S16, C and D). Besides, a CSS-type upconversion nanoparticle@RF@monomicelle (denoted as UCNP@RF@monomicelle) superstructure can also be constructed (Fig. 3J and fig. S17, D to F) when a core-shell upconversion nanoparticle@RF nanosphere is selected as an assembly substrate.

In addition, the obtained core-satellite SiO2@monomicelle supraparticles above (size of monomicelles, ~8 nm) can also perform as hierarchical reservoirs for functional species to anchor on. TEM images show that the functional TiO2 quantum dots (TQDs) (a diameter size of about 2 to 3 nm) are homogeneously anchored on the surfaces of the SiO2@monomicelle supraparticles, resulting in the formation of a new multifunctional hierarchical SiO2@monomicelle@TQD supraparticle (Fig. 3K and fig. S18). Besides this superstructure with 18-nm monomicelles on the surface, the supraparticles with 46-nm subunits can also act as efficient reservoirs for TQDs to anchor on (fig. S19). The elemental mapping and HAADF-STEM images clearly reveal the homogeneous distribution of the TQDs on the surfaces of superstructures (fig. S19, C to H).

Moreover, a brand-new type of the core-satellite-satellite SiO2@monomicelle@ monomicelle supraparticles can also be prepared by successive interfacial super-assembly of monomicelle units with two different sizes. SEM and TEM images clearly disclose that a complex multilevel core-satellite-satellite SiO2@monomicelle@monomicelle supraparticle has successfully been fabricated (Fig. 3L and fig. S20, D and E) by using the RF-decorated (surface-coating) SiO2@monomicelle supraparticles as a substrate for interfacial super-assembly of the second-layer monomicelles. DLS result shows that the hydrodynamic diameter of this core-satellite-satellite superstructure is ~450 nm (fig. S21). The sizes of the monomicelles in the first and second layers are ~18 and 46 nm, respectively (Fig. 3L and fig. S20, D and E). As the size of the SiO2@monomicelle particles is ~340 nm, as mentioned above (fig. S6), we can thus calculate that the size of the core-satellite-satellite superstructure is about 110 nm larger than that of the core-satellite particles. Such an increase of 110 nm in diameter is almost two times of the size of the second-layer monomicelles. Such accurate increase in diameter can further demonstrate that the second-layer monomicelles are successfully anchored on the surface of the first-layer monomicelles. HRTEM and HRSEM images further demonstrate that the monomicelles in the second layer are intimately anchored on the surface of the RF-decorated first-layer monomicelles (fig. S20, E and F).

Besides the spherical monomicelles, the cylindrical PS-PVP-PEO ones (fig. S22) can also perform as building blocks for interfacial super-assembly. TEM images show that the length of these cylindrical monomicelle subunits is 200 to 500 nm, and the diameter is ~46 nm (fig. S22). The formed SiO2@cylindrical monomicelle (SiO2@CMM) superstructures are uniform (fig. S23), with a great number of cylindrical monomicelles anchoring on the spherical SiO2 surfaces. HRTEM images reflect that the cylindrical monomicelles prefer to vertically super-assemble on the substrates, with one end attached on the surfaces of the core colloidal SiO2 (fig. S23, C and D). This vertical assembly of the cylindrical monomicelles can be ascribed to the strong electrostatic interaction between the negatively charged core SiO2 and the positively charged ends of the cylindrical monomicelle (). This is due to the more exposed positively charged PVP on the arc-shaped ends (high curvature) of cylindrical configuration when compared with the long sides (low curvature) of the cylindrical monomicelles ().

Surface monomicelle number and wetting behaviors

The number of surface monomicelles for the hierarchical SiO2@monomicelle core-satellite supraparticles is well controlled (the total number of the monomicelles can be counted by using the method as follows: the number on the boundary adds two times of the number on the side of the sphere facing us). SEM images show that the hierarchical SiO2@monomicelle supraparticles with different numbers of monomicelles on the surface are all highly uniform, indicating the high control of this monomicelle interfacial super-assembly. HRSEM images further show that the monomicelle subunits exhibit homogeneously discontinuous distribution on the colloidal SiO2 interfaces for all these SiO2@monomicelle supraparticles (Fig. 4, G to K). In addition, the number of polymeric monomicelles on SiO2 interfaces can be precisely tuned, ranging from 0 (Fig. 4F), 76 (Fig. 4, A and G), 90 (Fig. 4, B and H), 110 (Fig. 4, C and I), 130 (Fig. 4, D and J), to 180 (Fig. 4, E and K). In addition, the number of monomicelles on SiO2 was also calculated by a mathematical model (fig. S24). The number from calculation is 0, 68, 93, 132, 156, and 203, respectively, well consistent with the number from the SEM observations mentioned above. This precisely manipulated micelle number on solid interfaces could be well controlled by changing the added ammonia concentrations [ranging from 0, 5, 10, 15, 20, to 25% (w/w)] during the monomicelle interfacial super-assembly (Fig. 4M). This is ascribed to the changed ammonia concentration that can gradually decrease the surface charge density of SiO2 (−0.40, −0.60, −1.1, −1.8, −2.8, and −4.0 mC/m2) (Fig. 4M) (, ), resulting in gradually increased mutual electrostatic interactions between the negatively charged SiO2 and positively charged monomicelles.

Fig. 4.

The nanoscale surface roughness and wetting behaviors of the core-satellite SiO2@monomicelle supraparticles.

Low-magnification (A to E) and high-magnification (F to K) SEM images, the corresponding structural models, and water contact angle (from 2°, 24°, 41°, 53°, 65°, to 78°) of the SiO2@monomicelle nanostructures with a tunable number of monomicelles on the surface, ranging from 0 (F), 76 (A and G), 90 (B and H), 110 (C and I), 130 (D and J), to 180 (E and K). (L) Relationship between the water contact angle and the number of monomicelles on each SiO2 nanosphere. (M) Relationship between the surface charge density of SiO2 (−0.40, −0.60, −1.1, −1.8, −2.8, and −4.0 mC/m2), the number of monomicelles on the surface of the colloidal SiO2 nanospheres, and the added ammonia amounts [0, 5, 10, 15, 20, and 25% (w/w)] in solution. Scale bars, 200 nm (A to E) and 100 nm (F to K).

This precisely controlled number of the surface monomicelles on substrates is greatly attributed to the widely adjustable surface wetting behaviors of the core-satellite SiO2@monomicelle supraparticles (Fig. 4, F to L). Before the monomicelle super-assembly, the static water contact angles appear to be small (2°), reflecting an intrinsic hydrophilic surface of pristine naked SiO2 nanospheres (Fig. 4F). However, when a large number (~76) of polymeric monomicelles are anchored on the colloidal SiO2 interfaces, the water contact angles of the hierarchical SiO2@monomicelle superstructures can markedly increase to 24° (Fig. 4, A and G), reflecting the distinctly decreased surface hydrophilicity. Moreover, when the number of monomicelles on the surface further rises up to 90 (Fig. 4, B and H), 110 (Fig. 4, C and I), 130 (Fig. 4, D and J), and 180 (Fig. 4, E and K), the water contact angles of supraparticles can highly reach up to 41°, 53°, 65°, and 78°, respectively (Fig. 4, H to K), resembling a positive linear correlation (Fig. 4L). These results firmly indicate that the surface wetting behaviors of these superstructures can be widely manipulated, ranging from ultrahigh (2°) to ultralow (78°) hydrophilicity (e.g., close to hydrophobicity) via a facile monomicelle interfacial super-assembly. Such highly adjustable surface wetting behaviors, combined with the precisely tuned structures, can make such hierarchical superstructure a promising candidate for wide applications, such as bionics, biomedicines, batteries, and catalysis.

DISCUSSION

Modular super-assembly mechanism of monomicelle-based superstructures

The polymeric PS-PVP-PEO monomicelles (PS, a core; PVP, a shell; and PEO, a corona) can endow unprecedented control of interfacial super-assembly to create core-satellite superstructures. First, because of a solidified hydrophobic PS core, the thermokinetically mediated polymeric monomicelles are structurally very stable (). This high stability can overcome the conventional micellar dissociation or aggregation during the super-assembly process, which is uncontrollable in other super-assembly systems. Second, the building blocks from the triblock copolymer PS-PVP-PEO monomicelles are monodispersed in the assembly solution due to the interparticle electrostatic repulsions (the protonated PVP blocks are positively charged) (). This great monodispersity is critical to precisely manipulate the arrangement of the monomicelle units on super-assembled interfaces in a regular manner. Third, the tunable configuration and molecular weight of the block copolymers provide high flexibility in controlling the morphology and size of the micellar subunits, making the assembly architecture more adjustable (, ). All the merits above attribute to great potential for preparing polymeric monomicelle–based hierarchical core-satellite superstructures.

Notably, ammonia is an important factor for driving this monomicelle interfacial super-assembly. To validate this assumption, concentration control experiments were carried out. The SEM image shows that only a few monomicelles can eventually assemble on the colloidal SiO2 interfaces in the absence of ammonia (fig. S25B). When ammonia is added, the superstructures are successfully built. In addition, as ammonia concentration rises [from 5, 10, 15, 20, to 25% (w/w)], numerous monomicelles, increasing from 76, 90, 110, 130, to 180, can anchor on interfaces (Fig. 4, G to K). This gradually increased number of monomicelles on interfaces may be ascribed to the conspicuously enhanced mutual interaction between monomicelles and SiO2 substrates. This enhanced interaction can be explained by the variation of the zeta potential and surface charge density of substrates in solution when increasing the concentration of ammonia (figs. S26 to S28). The zeta potential of the colloidal SiO2 (negatively charged) is −43.0 mV in neutral aqueous solution (pH 7.0) (). In addition, it can be increased to −8.8 mV when some acetic acid is added (pH 3.7; fig. S26A). However, when ammonia (25%, w/w) is added, the zeta potential is reversely decreased to −29.5 mV (pH 4.9) (fig. S26B). Furthermore, when the concentration of ammonia is controlled, ranging from 0, 5, 10, 15, 20, to 25% (w/w) (Fig. 4M), the zeta potential of SiO2 gradually decreases from ~−8.8, −12.6, −16.7, −20.4, −25.1, to −29.5 mV, respectively (figs. S26 and S27). In addition, the corresponding surface charge density of SiO2 ranges from −0.40, −0.60, −1.1, −1.8, −2.8, to −4.0 mC/m2 (, ). Such an obvious decrease in surface charge density and zeta potential can greatly boost the mutually electrostatic attraction between the monomicelle subunits and the SiO2 substrate (because the zeta potential of monomicelle is +6.9 mV, opposite to that of the colloidal SiO2) (fig. S26C). This improved electrostatic attraction is attributed to the well-controlled number of monomicelles on SiO2 interfaces. To further verify the key force of this electrostatic interaction, different organic amines including lauryl amine, ethylenediamine, triethanolamine, and phenylamine (which can decrease the zeta potential of SiO2) were also used (to replace ammonia) to boost their mutual interactions between the PS-PVP-PEO subunits and SiO2 substrates. With the assistance of these organic amines, the SiO2@monomicelle superstructures could be successfully constructed (fig. S29), firmly demonstrating the crucial role of the electrostatic interactions. However, when using the negatively charged poly(ethylene oxide)-b-poly(acrylic acid)-b-polystyrene (PEO-PAA-PS; zeta potential of −14.2 mV; fig. S30A) monomicelles as a super-assembly building block, the core-satellite superstructures failed to be constructed due to the strong electrostatic repulsions between PEO-PAA-PS and SiO2 (fig. S30, B and C).

Besides the electrostatic attraction between the monomicelles and SiO2 substrates, the electrostatic repulsion among different monomicelle units is also an important factor to regulate this discontinuous monomicelle interfacial super-assembly. Owing to these intermicellar repulsions, the monomicelle subunits on the SiO2 interface exhibit a unique regular discontinuous distribution, totally different from the conventional closely packing super-assembly pattern. Moreover, the excellent uniformity of the PS-PVP-PEO monomicelle subunits is also very pivotal, which can guarantee that the electrostatic repulsions among all adjacent monomicelles are totally identical. Hence, such identical monomicellar electrostatic repulsions can greatly endow a highly regular arrangement of the monomicelles on SiO2.

On the basis of these observations, we have reasons to suppose that the coexistence of electrostatic attractions (between monomicelle units and SiO2 substrates) and intermicellar electrostatic repulsion is the key to controlling the monomicelle interfacial super-assembly in a regular and discontinuous manner. Without the electrostatic attraction, the monomicelle units cannot be super-assembled on SiO2 surfaces, because under this condition the driving force is weak to bind the monomicelles on the SiO2 interface. In the absence of the intermicellar electrostatic repulsion, the monomicelle units cannot display a discontinuous distribution on the SiO2 interface. However, by combining such intermicellar electrostatic repulsion with the micelle-SiO2 electrostatic attraction mentioned above, the total driving force can efficiently direct the monomicelle subunits to regularly distribute on SiO2 interfaces in a unique discontinuous form. By finely adjusting the electrostatic attraction between monomicelles and the surface of the colloidal SiO2 (by changing the ammonia concentration), the number of the assembled micelles on each SiO2 nanosphere can be well manipulated, ranging from 76, 90, 110, 130, to 180 (Fig. 4), which is hardly achievable by other super-assembly strategies. In addition, the maximum number of discontinuous monomicelles on the spherical SiO2 interface with a diameter of 300 nm is ~180 (Fig. 4, E and K).

A theoretical simulation was also taken for this monomicelle interfacial super-assembly (Fig. 5) (–). With the assistance of ammonia (Fig. 5A), when the (n + 1)th monomicelle (Fig. 5C) is inclined to reach the SiO2 interface, all the n monomicelles (Fig. 5B) already preanchored on the interface have electrostatic repulsion on this (n + 1)th monomicelle. As the total electrostatic repulsions [from all these n interfacial monomicelles to the (n + 1)th monomicelle] are equal to the electrostatic attraction between the (n + 1)th monomicelle and the SiO2 substrate, the number of monomicelles on the spherical SiO2 interface reaches the maximum. In addition, under this condition, no more monomicelle can be further anchored on the SiO2 interface. The relationship between the total energy of micelles (Ecoul) and the micellar number (n) on the SiO2 interface presents a positive exponential correlation (Fig. 5D). The energy change curves of micelle-micelle and SiO2-micelle show that the theoretical maximum number of monomicelle units on SiO2 is ~162, which is well consistent with the number from the experimental observation (~180) (Fig. 5E).

Fig. 5.

The theoretical simulation of the monomicelle unit–based interfacial super-assembly.

(A) Super-assembly process of the superstructures. The mutual interaction model of monomicelle units on the SiO2 interface with different numbers of monomicelles, n (B) and n + 1 (C), respectively. (D) Relationship between the normalized total energy and monomicelle number on each solid SiO2. (E) Energy change curves of micelle-micelle and SiO2-micelle when different numbers of monomicelles are on a solid SiO2 interface.

Other conditions involving the surface functionalities, solvent types, and reaction time were also investigated. When using SiO2 with less surface hydroxyl group (-OH) as a substrate, the monomicelles cannot be super-assembled on interfaces (fig. S31). Moreover, when using PS nanospheres without -OH group on the surface as a substrate, no superstructure could be formed (fig. S32). These results obviously prove that the existence of surface -OH groups on substrates is also necessary for the interfacial super-assembly of the monomicelles, because the Si-OH groups can be changed to negatively charged Si-O− groups in the presence of basic ammonia. Such negatively charged Si-O− groups can boost the electrostatic attractions between the monomicelles and SiO2 substrates. In addition, when the super-assembly process was conducted in different solvents, including methanol, alcohol, propanol, and isopropanol (fig. S33), the core-satellite SiO2@monomicelle superstructures were successfully built. These results reveal that the solvent environments exert an ignorable role in driving this monomicelle interfacial super-assembly. Besides, at the very beginning of the reactions (short of 1 min), a large number of monomicelles have already assembled on the surfaces of the SiO2 nanospheres (fig. S34A). In addition, when the reaction time further increased to 2 hours (fig. S34D), no more additional monomicelles can be further anchored on the SiO2 interfaces. Hence, we can speculate that the monomicelle interfacial super-assembly process is highly efficient, which can totally finish in a very short time (less than 1 min).

Moreover, the hierarchical core-satellite SiO2@monomicelle supraparticles demonstrate a widely adjustable surface wetting behavior (from 2° to 78°) and apparently improved static water contact angles when compared with that of the naked core SiO2. Such greatly improved water contact angles for the superstructures can be ascribed to two reasons. First, the colloidal SiO2 nanospheres with abundant surface Si-OH groups are more hydrophilic than the “crew-cut” PS-PVP-PEO monomicelles with long hydrophobic PS chains (, ). Hence, when the SiO2 surface is occupied by more monomicelles, the total surface hydrophilic ability for whole supraparticles gradually decreases, resulting in an increased water contact angle. Second, as more monomicelles are anchored on the interface of the colloidal SiO2, a sharp increase in surface roughness for supraparticles is greatly achieved. For this improved surface roughness, the spatial distance of the adjacent monomicelles is decreased, leading to abundant cavities and air cushion on SiO2 surfaces (, ). This air cushion, as a spatial barrier, can effectively retard the water drops to contact and spread on SiO2 surfaces, resulting in an increased water contact angle (). Thus, by finely regulating the number of monomicelles on the colloidal SiO2 interfaces, the nanoscale surface roughness and surface wetting behaviors of this core-satellite SiO2@monomicelle superstructure can be precisely controlled. Such regularly multiscale hierarchical structure with highly rough surface for these supraparticles is similar to the biological complex configuration in nature, e.g., mosquito compound eyes, lotus leaves, butterfly wings, rice leaves, red rose petals, and water spider legs, which can exhibit outstanding and excellent physical, mechanical, optical, or electronic properties in practical applications (–).

In summary, we have created a new type of hierarchically regular core-satellite SiO2@monomicelle superstructure from the ultrastable polymeric building blocks via the monomicelle interfacial super-assembly route. In the superstructure, the PS-PVP-PEO monomicelle subunits on colloidal SiO2 interfaces typically appear in a hexagon-like regular discontinuous distribution. The number of such regular discontinuous monomicelles on colloidal SiO2 interfaces can be quantitatively controlled (ranging from 76 to 180). This quantitative control can be precisely manipulated by tuning the interparticle electrostatic interactions (the intermicellar electrostatic repulsion and electrostatic attractions between the monomicelle units and the SiO2 substrate). Moreover, the monomicelle interfacial super-assembly strategy is highly simple, versatile, and robust, which can greatly enable a modular super-assembly of monomicelle subunits with tunable configuration (spherical and cylindrical), sizes (from 10 to 46 nm), and layers (one and two) on various substrates with adjustable dimensions (e.g., 3D, 2D, and 1D) and components (e.g., SiO2, graphene, TiO2, and Fe2O3). This monomicelle interfacial super-assembly strategy could pave a new and highly controllable way for building multiscale hierarchical micro- and/or macroscale super-assembled materials and devices with desirable and promising properties.

MATERIALS AND METHODS

The core-satellite superstructures were fabricated via a monomicelle interfacial super-assembly method (see Supplementary Materials and Methods for detailed synthetic process). Briefly, ultrastable, uniform, and monodispersed PS-PVP-PEO monomicelles were first obtained by the thermokinetically mediated methodology. Then, such ultrastable polymeric PS-PVP-PEO monomicelles performed as a stable building block to super-assemble on various solid interfaces with adjustable configurations and components for generating various interesting core-satellite superstructures with the assistance of ammonia. In addition, the number of the monomicelle building units on assembly interfaces was accurately manipulated by tuning the amount of ammonia.