We report the novel use of the naturally occurring saponin, glycyrrhizic acid (GA) as a structuring material to transform liquid oil into a soft-solid structured emulsion system. The GA nanofibrils from the anisotropic self-assembly of GA molecules were first used as stabilizers to fabricate olive oil-in-water emulsions using a facile one-step emulsification at high temperature. Then, the obtained emulsions were further self-organized into the emulsion gel by applying a subsequent cooling to trigger the gel network formation, which is mostly due to the enhanced noncovalent interactions among GA fibrils in the continuous phase as well as at the droplet surface. The GA fibrils could adsorb at the interface in a multilayer form, leading to the formation of unique fibril shells with high electrostatic repulsive force, which could provide superior stability for the GA fibril-stabilized oil droplets and thus the whole emulsion gel during storage and heating. The thermoreversible gel-sol transitions of a self-assembled GA fibrillar network in the continuous phase endow the stable emulsion gels with a temperature-responsive switchable behavior. Moreover, the GA fibril-coated oil droplets embedded in the network were found to be closely packed together and connected with the gel matrix. As a consequence, the emulsion gels exhibited many interesting rheological behaviors, including a high gel strength, shear sensitivity, and good thixotropic recovery. These simple and inexpensive smart responsive oil structuring materials based on natural saponins could find novel applications in the fields of food, pharmaceuticals, or cosmetics.

Saponin nanofibrils assembled from natural glycyrrhizic acid (GA) have been recently shown to be an effective structurant for edible oil structuring. This work showed that the microstructure and mechanical properties of the novel emulsion gels formed by GA fibrils could be well tuned by oil phase polarity. For more polar oils (algal oil), the GA fibrils had a higher affinity to the oil-water interface, showing a faster adsorption kinetics, thus leading to the formation of fine multilayer emulsion droplets with smaller droplet size. Accordingly, the emulsion gels had a denser network microstructure and higher mechanical strength, which should be attributed to the fact that the smaller emulsion droplets could be packed more tightly within the continuous network, providing stronger interdroplet interactions, and thereby contribute to reinforcing the gel matrix. In addition, all emulsion gels had interesting thermoresponsive behavior, independent of oil phase, which is probably due to the thermoreversibility of the hydrogen-bond fibrillar network in the continuous phase.

Nanofibrils from the self-assembly of the naturally occurring saponin glycyrrhizic acid (GA) can be used to produce an oil-in-water emulsion foam with a long-term stability. Through homogenization and aeration followed by rapid cooling, stable emulsion foams can be produced from the mixtures of sunflower oil and saponin nanofibrils. At high temperatures, the GA fibrils form a multilayer assembly at the interface, creating an interfacial fibrillar network to stabilize the oil droplets and air bubbles generated during homogenization. A subsequent rapid cooling can trigger the self-assembly of free GA fibrils in the continuous phase, forming a fibrillar hydrogel and thus trapping the oil droplets and air bubbles. The viscoelastic bulk hydrogel showed a high yield stress and storage modulus, which lead to a complete arrest of the liquid drainage and a strong slowdown of the bubble coarsening in emulsion foams. The jamming of the emulsion droplets in the liquid channels as well as around the bubbles was also found to be able to enhance the foam stability. We show that such stable foam systems can be destroyed rapidly and on demand by heating because of the melting of the bulk hydrogel. The reversible gel-sol phase transition of the GA hydrogel leads to thermoresponsive emulsion foams, for which the foam stability can be switched from stable to unstable states by simply raising the temperature. The emulsion foams can be further developed to be photoresponsive by incorporating internal heat sources such as carbon black particles, which can absorb UV irradiation and convert the absorbed light energy into heat. This new class of smart responsive emulsion foams stabilized by the natural, sustainable saponin nanofibrils has potential applications in the food, pharmaceutical, and personal care industries.

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A novel, highly biocompatible bacterial cellulose (BC)-zein composite nanofiber with a controlled hydrophobic biosurface was successfully developed through a simple and green solution impregnation method, followed by evaporation-induced self-assembly (EISA) of adsorbed zein protein. The surface hydrophobicity of the zein-modified BC nanofibers could be controlled by simply changing the zein concentration, which is able to tune the morphology of self-assembled zein structures after EISA, thus affecting the surface roughness of composite membranes. Zein self-assembly at low concentrations (5 mg/mL) resulted in the formation of hierarchical zein structures (spheres and bicontinuous sponges) on the BC surface, thus increasing the surface roughness and leading to high hydrophobicity (the water contact angle reached 110.5°). However, at high zein concentrations, these large zein spheres assembled into a flat zein film, which decreased the surface roughness and hydrophobicity of membranes. The homogeneous incorporation of zein structures on the BC surface by hydrogen bonding did not significantly change the internal structure and mechanical performance of BC nanofibers. In comparison with pure BC, the BC-zein nanofibers had a better biocompatibility, showing a significantly increased adhesion and proliferation of fibroblast cells. This is probably due to the rough surface structure of BC-zein nanofibers as well as the high biocompatibility of natural zein protein. The novel BC-zein composite nanofibers with controlled surface roughness and hydrophobicity could be of particular interest for the design of BC-based biomaterials and biodevices that require specific surface properties and adhesion.

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N-doped mesoporous carbons, NMCs, have attracted intensive attention recently and have shown potential applications in various scientific fields including catalysis and energy conversion/storage. Via modification with foreign N elements and construction of mesoporous structures for NMCs, their electronic and spin structure, as well as their porosity can be greatly tailored. And the resultant electron-donor property, surface wettability, conductivity, ion/molecular transfer and reactivity are changed accordingly. In this review, we will summarize the recent research progress of these metal-free NMCs, with an emphasis on their synthesis and performance, especially for their synthetic strategy and catalytic properties toward oxygen and nitro compound reductions, as well as their electrochemical properties as electrode materials for lithium-ion/sulfur batteries and supercapacitors. We hope for future developments, such as controlling doping methods more precisely, generating more active sites by N-doping, and finding wider applications of NMCs in other fields.