The discovery of graphene has led to a rising interest in seeking quasi two-dimensional allotropes of several elements and inorganic compounds. Boron, carbon’s neighbour in the periodic table, presents a curious case in its ability to be structured as graphene. Although it cannot independently constitute a honeycomb planar structure, it forms a graphenic arrangement in association with electron-donor elements. This is exemplified in magnesium diboride (MgB2): an inorganic layered compound comprising boron honeycomb planes alternated by Mg atoms. Till date, MgB2 has been primarily researched for its superconducting properties; it hasn’t been explored for the possibility of its exfoliation. Here we show that ultrasonication of MgB2 in water results in its exfoliation to yield few-layer-thick Mg-deficient hydroxyl-functionalized nanosheets. The hydroxyl groups enable an electrostatically stabilized aqueous dispersion and create a heterogeneity leading to an excitation wavelength dependent photoluminescence. These chemically modified MgB2 nanosheets exhibit an extremely small absorption coefficient of 2.9 ml mg−1 cm−1 compared to graphene and its analogs. This ability to exfoliate MgB2 to yield nanosheets with a chemically modified lattice and properties distinct from the parent material presents a fundamentally new perspective to the science of MgB2 and forms a first foundational step towards exfoliating metal borides.

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with a large specific surface area featuring both hydrophilic and hydrophobic characters. [4] The hydrophobic nature of GO nanosheets originates from the basal plane, i.e., carbon rings, while hydrophilicity is imparted to GO by the surface and edge functional groups, e.g., hydroxyl, carboxylic, and epoxy groups. Due to this dual nature, GO nanosheets self-assemble at oil/water (O/W) interfaces, forming nanometer-thick barriers separating water and oil. Thus, by tuning the carbon to oxygen ratio, GO can be assembled at liquid-liquid interfaces to generate hierarchical structures with defined functionalities. [5,6] The promise of GO for numerous applications has increased interest in its interfacial behavior.The assembly of GO at liquid-liquid interfaces has been investigated primarily by dynamic interfacial tension (IFT). Previously, we [7] showed that GO assembles at the O/W interfaces, forming, predominantly a tessellated, nanosheet barrier reducing the interfacial surface energy between the liquids. The low bending modulus of GO enables the assemblies to conform to the curvature of the interface. Similarly, Kim et al. [1] investigated the activity of GO nanosheets at air-liquid, liquid-liquid, and liquid-solid interfaces, showing that, despite the stable dispersion of GO in water, GO segregates to the interfaces to reduce the interfacial tension. Imperiali et al. [8] reported on GO film formation at air/W interfaces, performing compression/expansion experiments in a Langmuir trough. They found that GO assembles at the surface and, upon compression, maitains a single layer thickness, resisting overlap due to attractive lateral forces. However, the influence of GO on the viscoelastic properties of the O/W interfaces has not been thoroughly investigated, which is critical for applications, including emulsification, enhanced oil recovery (EOR), and all-liquid 2D and 3D printing. We recently demonstrated, for example, the importance of the interfacial rheology of O/W on the stabilization of Pickering emulsions. [9] It was shown that the interfacial rheology plays a decisive role in emulsion formation, [9,10] controlling the emulsion morphology and stability. [11,12] In all-liquid 3D printing, reducing the interfacial tension to retard Plateau Rayleigh (PR) instabilities and stabilize the interface [13] is essential. Several nanomaterials have been recently proposed for sculpting liquids. [14] For instance, the printability Tailoring the oil/water (O/W) interface is a prerequisite for structuring these two immiscible liquids into prescribed architectures, i.e., liquid-in-liquid printing, which is an emerging area in material science. Here, assemblies of graphene oxide (GO) at O/W and air/W interfaces are characterized using a wide range of interfacial rheological techniques. It is shown that the GO nanosheets assemble at the interface, even at extremely low concentrations as low as 0.04 vol%, significantly increasing the elasticity at O/W or air/W interfaces. This is attributed to the combined hydrophobic and hydrop...

Monoclonal antibodies (mAbs) are proteins that uniquely identify targets within the body, making them well-suited for therapeutic applications. However, these amphiphilic molecules readily adsorb onto air-solution interfaces where they tend to aggregate. We investigated two mAbs with different propensities to aggregate at air-solution interfaces. The understanding of the interfacial rheological behavior of the two mAbs is crucial in determining their aggregation tendency. In this work, we performed interfacial stress relaxation studies under compressive step strain using a custom-built dilatational rheometer. The dilatational relaxation modulus was determined for these viscoelastic interfaces. The initial value and the equilibrated value of relaxation modulus were larger in magnitude for the mAb with a higher tendency to aggregate in response to interfacial stress. We also performed single-bubble coalescence experiments using a custom-built dynamic fluid-film interferometer (DFI). The bubble coalescence times also correlated to the mAbs aggregation propensity and interfacial viscoelasticity. To study the influence of surfactants in mAb formulations, polyethylene glycol (PEG) was chosen as a model surfactant. In the mixed mAb/PEG system, we observed that the higher aggregating mAb coadsorbed with PEG and formed domains at the interface. In contrast, for the other mAb, PEG entirely covered the interface at the concentrations studied. We studied the mobility of the interfaces, which was manifested by the presence or the lack of Marangoni stresses. These dynamics were strongly correlated with the interfacial viscoelasticity of the mAbs. The influence of competitive destabilization in affecting the bubble coalescence times for the mixed mAb/PEG systems was also studied.