The Origin of High Activity of Amorphous MoS2 in the Hydrogen Evolution Reaction

Molybdenum disulfide (MoS2) and related transition metal chalcogenides can replace expensive precious metal catalysts such as Pt for the hydrogen evolution reaction (HER). The relations between the nanoscale properties and HER activity of well‐controlled 2H and Li‐promoted 1T phases of MoS2, as well as an amorphous MoS2 phase, have been investigated and a detailed comparison is made on Mo−S and Mo−Mo bond analysis under operando HER conditions, which reveals a similar bond structure in 1T and amorphous MoS2 phases as a key feature in explaining their increased HER activity. Whereas the distinct bond structure in 1T phase MoS2 is caused by Li+ intercalation and disappears under harsh HER conditions, amorphous MoS2 maintains its intrinsic short Mo−Mo bond feature and, with that, its high HER activity. Quantum‐chemical calculations indicate similar electronic structures of small MoS2 clusters serving as models for amorphous MoS2 and the 1T phase MoS2, showing similar Gibbs free energies for hydrogen adsorption (ΔGH*) and metallic character

Nanoscopy of single antifreeze proteins reveals that reversible ice binding is sufficient for ice recrystallization inhibition but not thermal hysteresis

Antifreeze proteins (AFPs) bind ice to reduce freezing temperatures and arrest ice crystal ripening, making AFPs essential for the survival of many organisms in ice-laden environments and attractive as biocompatible antifreezes in many applications. While their activity was identified over 50 years ago, the physical mechanisms through which they function are still debated because experimental insights at the molecular scale remain elusive. Here, we introduce subzero nanoscopy by the design and incorporation of a freezing stage on a commercial super-resolution setup to resolve the interfacial dynamics of single AFPs with ice crystal surfaces. Using this method, we demonstrate irreversible binding and immobilization (i.e., pinning) of individual proteins to the ice/water interface. Surprisingly, pinning is lost and adsorption becomes reversible when freezing point depression activity, but not ice recrystallization inhibition, is eliminated by a single mutation in the ice-binding site of the AFP. Our results provide direct experimental evidence for the adsorption-inhibition paradigm, pivotal to all theoretical descriptions of freezing point depression activity, but also reveal that reversible binding to ice must be accounted for in an all-inclusive model for AFP activity. These mechanistic insights into the relation between interfacial interactions and activity further our understanding and may serve as leading principles in the future design of highly potent, biocompatible antifreezes with tunable affinity

Noncovalent Synthesis of Self-Assembled Nanotubes through Decoupled Hierarchical Cooperative Processes

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/jacs.9b07868Because of their wide number of biological functions and potential applications, self-assembled nanotubes constitute highly relevant targets in noncovalent synthesis. Herein, we introduce a novel approach to produce supramolecular nanotubes with defined inner and outer diameters from rigid rod-like monomers programmed with complementary nucleobases through two distinct, decoupled cooperative processes of different hierarchy and acting in orthogonal directions: chelate cooperativity, responsible for the formation of robust Watson-Crick H-bonded cyclic tetramers, and nucleation-growth cooperative polymerizationFunding from the European Research Council (ERC-Starting Grant 279548 PROGRAM-NANO) and MINECO (CTQ2014-57729-P and CTQ2017-84727-P) is gratefully acknowledged. V.V.-G. is grateful to MINECO for a FPI Grant. I.K.V. would like to acknowledge The Netherlands Organization for Scientific Research (NWO VIDI Grant 723.014.006) for financial suppor

Inhibition of Ice Recrystallization by Nanotube-Forming Cyclic Peptides

While most native ice-binding proteins are rigid, artificial (macro)molecular ice-binders are usually flexible. Realizing a regular array with precisely positioned ice-binding motifs on synthetic proteins, (macro)molecular ice-binders are thus challenging. Here, we exploit the predictable assembly of cyclic peptides into nanotubes as a starting point to prepare large, rigid ice-binders bearing an ice-binding site that is found in hyperactive ice-binding proteins in insects. First, we designed, synthesized, and purified cyclic octapeptide Lys2CP8 bearing a TaT motif to promote ice binding and investigated their solution assembly and activity using circular dichroism (CD) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, light scattering (LS), cryogenic transmission electron microscopy (cryo-TEM), and ice recrystallization inhibition (IRI) assays. The cyclic peptide Lys2CP8 was synthesized in good yield using Fmoc chemistry and purified by reversed-phase HPLC. Upon dissolution in aqueous solutions, Lys2CP8 was observed to assemble in a pH- and concentration-dependent manner into objects with nanoscopic dimensions. LS revealed the presence of small and large aggregates at pH 3 and 11, held together through a network of intermolecular antiparallel β-sheets as determined by FTIR and CD spectroscopy. Cryo-TEM revealed the presence of one-dimensional objects at pH 3 and 11. These are mostly well-dispersed at pH 3 but appear to bundle at pH 11. Interestingly, the pH-dependent self-assembly behavior translates into a marked pH dependence of IRI activity. Lys2CP8 is IRI-active at pH 3 while inactive at pH 11 hypothetically because the ice-binding sites are inaccessible at pH 11 due to bundling

Artificial Organic Skin Wets Its Surface by Field-Induced Liquid Secretion

Living organisms enhance their survival rate by excreting fluids at their surface, but man-made materials can also benefit from liquid secretion from a solid surface. Known approaches to secrete a liquid from solids are limited to passive release driven by diffusion, surface tension, or pressure. Remotely triggered release would give active control over surface properties but is still exceptional. Here, we report on an artificial skin that secretes functional fluids by means of radiofrequency electrical signals driven by dielectric liquid transport in a (sub-)microporous smectic liquid crystal network. The smectic order of the polymer network and its director determine the flow direction and enhance fluid transport toward the surface at pre-set positions. The released fluid can be reabsorbed by the skin using capillary filling. The fluid-active skins open avenues for robotic handling of chemicals and medicines, controlling tribology and fluid-supported surface cleaning

From a eutectic mixture to a deep eutectic system via anion selection: Glutaric acid + tetraethylammonium halides

In pursuit of understanding structure-property relationships for the melting point depression of binary eutectic mixtures, the influence of the anion on the solid-liquid (S-L) phase behavior was explored for mixtures of glutaric acid + tetraethylammonium chloride, bromide, and iodide. A detailed experimental evaluation of the S-L phase behavior revealed that the eutectic point is shifted toward lower temperatures and higher salt contents upon decreasing the ionic radius. The salt fusion properties were experimentally inaccessible owing to thermal decomposition. The data were inter- and extrapolated using various models for the Gibbs energy of mixing fitted to the glutaric-acid rich side only, which allowed for the assessment of the eutectic point. Fitting the experimental data to a two-parameter Redlich-Kister expansion with Flory entropy, the eutectic depth could be related to the ionic radius of the anion. The anion type, and in particular its size, can therefore be viewed as an important design parameter for the liquid window of other acid and salt-based deep eutectic solvents/systems

Solvent selectivity governs the emergence of temperature responsiveness in block copolymer self-assembly

In highly selective solvents, block copolymers (BCPs) form association colloids, while in solvents with poor selectivity, they exhibit a temperature-controlled (de)mixing behavior. Herein, it is shown that a temperature-responsive self-assembly behavior emerges in solvent mixtures of intermediate selectivity. A biocompatible poly-ethylene(oxide)-block-poly-ϵ-caprolactone (PEO-PCL) BCP is used as a model system. The polymer is dissolved in solvent mixtures containing water (a strongly selective solvent for PEO) and ethanol (a poorly selective solvent for PEO) to tune the solvency conditions. Using synchrotron X-ray scattering, cryogenic transmission electron microscopy, and scanning probe microscopy, it is shown that a rich temperature-responsive behavior can be achieved in certain solvent mixtures. Crystallization of the PCL block enriches the phase behavior of the BCP by promoting sphere-to-cylinder morphology transitions at low temperatures. Increasing the water fraction in the solvent causes a suppression of the sphere-to-cylinder morphology transition. These results open up the possibility to induce temperature-responsive properties on demand in a wide range of BCP systems

Effects of structural variation on the self-assembly of bis-urea based bolaamphiphiles

We report on the self-assembly in water of a set of bis-urea amphiphiles. A range of techniques, including dynamic light scattering, Cryo-TEM, SAXS, and MS are used to study the effect of structural variation on the morphology of the assemblies. The length, dispersity, and end-group of the ethylene glycol hydrophilic part of the molecule, as well as of the alkyl chain length are varied to tailor the morphology towards soluble wormlike micelles. Slight modification on molecular structures gave a large difference in self-assembly behavior in water, giving guidelines for the design of rodlike supramolecular fibers with novel functionalities, such as strain-stiffening and bioactivity

The Origin of High Activity of Amorphous MoS2 in the Hydrogen Evolution Reaction

Molybdenumdisulfide(MoS2)and related transition metal chal-cogenides can replace expensive preciousmetal catalysts suchas Pt for the hydrogen evolution reaction(HER). The relationsbetween the nanoscale properties and HER activity of well-controlled 2H and Li-promoted1Tphases of MoS2,aswell asan amorphous MoS2phase, have been investigated and ade-tailed comparison is made on Mo@Sand Mo@Mo bond analysisunder operando HER conditions, which reveals asimilar bondstructure in 1T and amorphous MoS2phases as akey feature inexplaining their increased HER activity.Whereasthe distinctbond structure in 1T phase MoS2is caused by Li+intercalationand disappears under harsh HER conditions, amorphous MoS2maintainsits intrinsic short Mo@Mo bond feature and, withthat, its high HER activity.Quantum-chemical calculations indi-cate similarelectronic structures of small MoS2clusters servingas modelsfor amorphous MoS2and the 1T phase MoS2,show-ing similarGibbs free energies for hydrogen adsorption (DGH*)and metalliccharacter

Weathering of a polyester-urethane clearcoat: lateral inhomogeneities

This paper is devoted to the surface analysis of a polyester-urethane coating during weathering under different conditions using artificial weathering machines. By means of atomic force microscopy (AFM), the evolution of the surface topology of the coatings is studied. Degradation is shown to be a laterally inhomogeneous process. The presence of water facilitates material removal and leads to an increase in the surface roughness and consequently a gloss loss. In addition, by comparing degradation under aerobic and anaerobic conditions, it is shown that oxidation reactions are the main cause of lateral inhomogeneous degradation of coatings