The self-assembly of micro/nanoparticles into suprastructures is a promising way to develop reconfigurable materials and to gain insights into the fundamental question of how matter organizes itself. The geometry of particles, especially those deviating from perfectly spherical shapes, is of significant importance in colloidal assembly because it influences the particle “recognition”, determines the particle packing, and ultimately dictates the formation of assembled suprastructures. In order to organize particles into desired structures, it is of vital importance to understand the relationship between the shape of the colloidal building blocks and the assembled suprastructures. This fundamental issue is an enduring topic in the assembly of molecular surfactants, but it remained elusive in colloidal assembly. To address this issue, we use snowman-shaped Janus nanoparticles (JNPs) as a model to systematically study the effect of colloidal geometries on their assembled suprastructures. Ten types of JNPs with identical chemical compositions but with different geometries were synthesized. Specifically, the synthesized JNPs differ in their lobe size ratios, phase separation degrees, and overall sizes. We show that by altering these parameters, both finite suprastructures, such as capsules with different curvatures, and nonfinite suprastructures, including free-standing single-layered or double-layered JNPs sheets, can be obtained via self-assembly. All these different types of suprastructures are constituted by highly oriented and hexagonally packed JNPs. These findings demonstrate the significance of geometries in colloidal assembly, such that slightly changing the building block geometries could result in a large variety of very different assembled structures, without altering the chemistry of the particles.

The ability to finely tune the amphiphilic balance of Janus nanoparticles (JNPs) could represent a step forward towards creating the next generation of solid-state amphiphiles with significant potential for applications. The inherent amphiphilicity of JNPs stemming from an intrinsic polarity contrast between two surface regions is well-acknowledged, but remained difficult to demonstrate experimentally in the absence of surfactants and stabilizers. We have designed two homologous series of surfactant-free polymeric JNPs starting from polystyrene (PS) seed nanoparticles (NPs) on which we grew Janus lobes of different sizes via seed polymerization and phase separation of the 3-(triethoxysilyl)propyl-methacrylate (3-TSPM) monomer. The two series differ only by the radical initiator used in the seed polymerization: polar ionic ammonium persulfate (APS) vs. non-polar oil-soluble 2,2′-azobis(2-methylpropionitrile) (AIBN). To compare the two series, we employed them in the emulsification of water with heptane or molten paraffin wax. A polarity reversal of the JNPs within AIBN-JNP series could be observed from the catastrophic and transitional emulsion phase inversions and occurred when the more polar lobe was larger than the non-polar seed PS lobe. Furthermore, the AIBN-JNPs appeared to be amphiphilic and adopt preferred orientation within the monolayer at the oil/water interface. We therefore demonstrated that in the absence of surfactants the amphiphilicity of the JNPs depends not only on the relative size of the lobes, but also on the surface polarity contrast, which can be tuned by changing the nature of radical initiator.

The ability of amphiphilic Janus nanoparticles (JNPs) to partition at the oil–water and air–water interfaces can be especially attractive for a plethora of new applications. Unlike molecular surfactants, the JNPs could act as “bulk-to-surface” carriers of different small molecules/actives or bulklike properties—magnetic, optic, or electric—without a dramatic effect on their ability to partition at interfaces. Here we report surfactant-free JNPs with pH-triggered switchable amphiphilicity that could be used in such interfacial applications. The polarity balance of the JNPs can be tuned by the pH such that the amphiphilicity of JNPs is switched on or off at low (pH 7.0) pH values, respectively. When the amphiphilicity of JNPs is switched off, the interfacial activity of JNPs is comparable to that of homogeneous nanoparticles (HNPs), and when switched on, the interfacial activity is enhanced, proving that the amphiphilicity of JNPs plays a role in enhancing the interfacial activity even at scales above 100 nm. In addition, we show that the ability to lower the interfacial tension (ΔIFT) is likely the most appropriate indicator to compare the interfacial activities of different particles because it does not scale with the size of the particles like the calculated activation energy E_a, or attachment energy ΔE. By comparing the mentioned parameters, we conclude that the enhancement in the interfacial activity is slightly larger than the earlier theoretical predictions that only compare the ΔE values. These pH-responsive JNPs were further applied in the generation of pH-responsive Pickering emulsions. Upon adsorption of these JNPs at the oil–water interfaces, the particle-laden interface acquires a pH-responsiveness functionality. For example, Pickering emulsions stabilized by the above pH-responsive JNPs undergo reversible water-in-oil (w/o) to oil-in-water (o/w) phase inversion when the aqueous phase changes from basic to acidic.