RESUMEN

Energy absorbing efficiency is a key determinant of a structure's ability to provide mechanical protection and is defined by the amount of energy that can be absorbed prior to stresses increasing to a level that damages the system to be protected. Here, we explore the energy absorbing efficiency of additively manufactured polymer structures by using a self-driving lab (SDL) to perform >25,000 physical experiments on generalized cylindrical shells. We use a human-SDL collaborative approach where experiments are selected from over trillions of candidates in an 11-dimensional parameter space using Bayesian optimization and then automatically performed while the human team monitors progress to periodically modify aspects of the system. The result of this human-SDL campaign is the discovery of a structure with a 75.2% energy absorbing efficiency and a library of experimental data that reveals transferable principles for designing tough structures.

RESUMEN

Tunable porous composite materials to control metal and metal oxide functionalization, conductivity, pore structure, electrolyte mass transport, mechanical strength, specific surface area, and magneto-responsiveness are critical for a broad range of energy storage, catalysis, and sensing applications. Biotemplated transition metal composite aerogels present a materials approach to address this need. To demonstrate a solution-based synthesis method to develop cobalt and cobalt oxide aerogels for high surface area multifunctional energy storage electrodes, carboxymethyl cellulose nanofibers (CNF) and alginate biopolymers were mixed to form hydrogels to serve as biotemplates for cobalt nanoparticle formation via the chemical reduction of cobalt salt solutions. The CNF-alginate mixture forms a physically entangled, interpenetrating hydrogel, combining the properties of both biopolymers for monolith shape and pore size control and abundant carboxyl groups that bind metal ions to facilitate biotemplating. The CNF-alginate hydrogels were equilibrated in CaCl2 and CoCl2 salt solutions for hydrogel ionic crosslinking and the prepositioning of transition metal ions, respectively. The salt equilibrated hydrogels were chemically reduced with NaBH4, rinsed, solvent exchanged in ethanol, and supercritically dried with CO2 to form aerogels with a specific surface area of 228 m2/g. The resulting aerogels were pyrolyzed in N2 gas and thermally annealed in air to form Co and Co3O4 porous composite electrodes, respectively. The multifunctional composite aerogel's mechanical, magnetic, and electrochemical functionality was characterized. The coercivity and specific magnetic saturation of the pyrolyzed aerogels were 312 Oe and 114 emu/gCo, respectively. The elastic moduli of the supercritically dried, pyrolyzed, and thermally oxidized aerogels were 0.58, 1.1, and 14.3 MPa, respectively. The electrochemical testing of the pyrolyzed and thermally oxidized aerogels in 1 M KOH resulted in specific capacitances of 650 F/g and 349 F/g, respectively. The rapidly synthesized, low-cost, hydrogel-based synthesis for tunable transition metal multifunctional composite aerogels is envisioned for a wide range of porous metal electrodes to address energy storage, catalysis, and sensing applications.

RESUMEN

While loop motifs frequently play a major role in protein function, our understanding of how to rationally engineer proteins with novel loop domains remains limited. In the absence of rational approaches, the incorporation of loop domains often destabilizes proteins, thereby requiring massive screening and selection to identify sites that can accommodate loop insertion. We developed a computational strategy for rapidly scanning the entire structure of a scaffold protein to determine the impact of loop insertion at all possible amino acid positions. This approach is based on the Rosetta kinematic loop modeling protocol and was demonstrated by identifying sites in lipase that were permissive to insertion of the LAP peptide. Interestingly, the identification of permissive sites was dependent on the contribution of the residues in the near-loop environment on the Rosetta score and did not correlate with conventional structural features (e.g., B-factors). As evidence of this, several insertion sites (e.g., following residues 17, 47-49, and 108), which were predicted and confirmed to be permissive, interrupted helices, while others (e.g., following residues 43, 67, 116, 119, and 121), which are situated in loop regions, were nonpermissive. This approach was further shown to be predictive for ß-glucosidase and human phosphatase and tensin homologue (PTEN), and to facilitate the engineering of insertion sites through in silico mutagenesis. By enabling the design of loop-containing protein libraries with high probabilities of soluble expression, this approach has broad implications in many areas of protein engineering, including antibody design, improving enzyme activity, and protein modification.

Asunto(s)

RESUMEN

The realization and optimization of multifunctional materials is difficult, especially when the functionalities are directly incompatible. For example, it is challenging to make surfaces both enzymatically active and water repellent, as these two properties are directly competitive because of the hydrophilic nature of the enzyme-laden surfaces. Patterning discrete domains of distinct functionalities can represent a path to multifunctionality, but the innumerable possible domain permutations present a major barrier to optimizing performance. Here, we develop a high-throughput approach for exploring patterned multifunctional surfaces that is inspired by the microtiter plate architecture. As a model system, patterned surfaces are realized with horseradish peroxidase-decorated domains amidst a background of hydrophobic fluorinated self-assembled monolayers. In experiments exploring effects of pattern geometry, the measured enzyme activity is dependent only on the surface coverage. In contrast, roll-off behavior strongly depends on the parameters of the pattern geometry. Importantly, this finding enables the precise tailoring of distinct wetting behavior of the surfaces in a manner that is independent of their enzymatic activity. The high-throughput nature of the platform facilitates multiobjective optimization of surface functionalities in a general and flexible manner.

Asunto(s)

RESUMEN

The delicate balance between hydrogen bonding and van der Waals interactions determines the stability, structure, and chirality of many molecular and supramolecular aggregates weakly adsorbed on solid surfaces. Yet the inherent complexity of these systems makes their experimental study at the molecular level very challenging. In this quest, small alcohols adsorbed on metal surfaces have become a useful model system to gain fundamental insight into the interplay of such molecule-surface and molecule-molecule interactions. Here, through a combination of scanning tunneling microscopy and density functional theory, we compare and contrast the adsorption and self-assembly of a range of small alcohols from methanol to butanol on Au(111). We find that longer chained alcohols prefer to form zigzag chains held together by extended hydrogen bonded networks between adjacent molecules. When alcohols bind to a metal surface datively via one of the two lone electron pairs of the oxygen atom, they become chiral. Therefore, the chain structures are formed by a hydrogen-bonded network between adjacent molecules with alternating adsorbed chirality. These chain structures accommodate longer alkyl tails through larger unit cells, while the position of the hydroxyl group within the alcohol molecule can produce denser unit cells that maximize intermolecular interactions. Interestingly, when intrinsic chirality is introduced into the molecule as in the case of 2-butanol, the assembly changes completely and square packing structures with chiral pockets are observed. This is rationalized by the fact that the intrinsic chirality of the molecule directs the chirality of the adsorbed hydroxyl group meaning that heterochiral chain structures cannot form. Overall this study provides a general framework for understanding the effect of simple alcohol molecular adstructures on hydrogen bonded aggregates and paves the way for rationalizing 2D chiral supramolecular assembly.

RESUMEN

The compatibility of multiple functions at a single interface is difficult to achieve, but is even more challenging when the functions directly counteract one another. This study provides insight into the creation of a simultaneously multifunctional surface formed by balancing two orthogonal functions; water repellency and enzyme catalysis. A partially fluorinated thiol is used to impart bulk hydrophobicity on the surface, and an N-hydroxysuccinimide ester-terminated thiol provides a specific anchoring sites for the covalent enzyme attachment. Different ratios of the two thiols are mixed together to form amphiphilic self-assembled monolayers, which are characterized with polarization-modulation infrared reflection-absorption spectroscopy and contact angle goniometry. The enzyme activity is measured by a fluorescence assay. With the results collected here, specific surface compositions are identified at which the orthogonal functions of water repellency and enzyme catalysis are balanced and exist simultaneously. An understanding of how to effectively balance orthogonal functions at surfaces can be extended to a number of higher-scale applications.

Asunto(s)

RESUMEN

The assembly of complex structures in nature is driven by an interplay between several intermolecular interactions, from strong covalent bonds to weaker dispersion forces. Understanding and ultimately controlling the self-assembly of materials requires extensive study of how these forces drive local nanoscale interactions and how larger structures evolve. Surface-based self-assembly is particularly amenable to modeling and measuring these interactions in well-defined systems. This study focuses on 2-butanol, the simplest aliphatic chiral alcohol. 2-butanol has recently been shown to have interesting properties as a chiral modifier of surface chemistry; however, its mode of action is not fully understood and a microscopic understanding of the role non-covalent interactions play in its adsorption and assembly on surfaces is lacking. In order to probe its surface properties, we employed high-resolution scanning tunneling microscopy and density functional theory (DFT) simulations. We found a surprisingly rich degree of enantiospecific adsorption, association, chiral cluster growth and ultimately long range, highly ordered chiral templating. Firstly, the chiral molecules acquire a second chiral center when adsorbed to the surface via dative bonding of one of the oxygen atom lone pairs. This interaction is controlled via the molecule's intrinsic chiral center leading to monomers of like chirality, at both chiral centers, adsorbed on the surface. The monomers then associate into tetramers via a cyclical network of hydrogen bonds with an opposite chirality at the oxygen atom. The evolution of these square units is surprising given that the underlying surface has a hexagonal symmetry. Our DFT calculations, however, reveal that the tetramers are stable entities that are able to associate with each other by weaker van der Waals interactions and tessellate in an extended square network. This network of homochiral square pores grows to cover the whole Au(111) surface. Our data reveal that the chirality of a simple alcohol can be transferred to its surface binding geometry, drive the directionality of hydrogen-bonded networks and ultimately extended structure. Furthermore, this study provides the first microscopic insight into the surface properties of this important chiral modifier and provides a well-defined system for studying the network's enantioselective interaction with other molecules.

RESUMEN

Methanol is a versatile chemical feedstock, fuel source, and energy storage material. Many reactions involving methanol are catalyzed by transition metal surfaces, on which hydrogen-bonded methanol overlayers form. As with water, the structure of these overlayers is expected to depend on a delicate balance of hydrogen bonding and adsorbate-substrate bonding. In contrast to water, however, relatively little is known about the structures methanol overlayers form and how these vary from one substrate to another. To address this issue, herein we analyze the hydrogen bonded networks that methanol forms as a function of coverage on three catalytically important surfaces, Au(111), Cu(111), and Pt(111), using a combination of scanning tunneling microscopy and density functional theory. We investigate the effect of intermolecular interactions, surface coverage, and adsorption energies on molecular assembly and compare the results to more widely studied water networks on the same surfaces. Two main factors are shown to direct the structure of methanol on the surfaces studied: the surface coverage and the competition between the methanol-methanol and methanol-surface interactions. Additionally, we report a new chiral form of buckled hexamer formed by surface bound methanol that maximizes the interactions between methanol monomers by sacrificing interactions with the surface. These results serve as a direct comparison of interaction strength, assembly, and chirality of methanol networks on Au(111), Cu(111), and Pt(111) which are catalytically relevant for methanol oxidation, steam reforming, and direct methanol fuel cells.

RESUMEN

We report a novel synthesis of nanoparticle Pd-Cu catalysts, containing only trace amounts of Pd, for selective hydrogenation reactions. Pd-Cu nanoparticles were designed based on model single atom alloy (SAA) surfaces, in which individual, isolated Pd atoms act as sites for hydrogen uptake, dissociation, and spillover onto the surrounding Cu surface. Pd-Cu nanoparticles were prepared by addition of trace amounts of Pd (0.18 atomic (at)%) to Cu nanoparticles supported on Al2O3 by galvanic replacement (GR). The catalytic performance of the resulting materials for the partial hydrogenation of phenylacetylene was investigated at ambient temperature in a batch reactor under a head pressure of hydrogen (6.9 bar). The bimetallic Pd-Cu nanoparticles have over an order of magnitude higher activity for phenylacetylene hydrogenation when compared to their monometallic Cu counterpart, while maintaining a high selectivity to styrene over many hours at high conversion. Greater than 94% selectivity to styrene is observed at all times, which is a marked improvement when compared to monometallic Pd catalysts with the same Pd loading, at the same total conversion. X-ray photoelectron spectroscopy and UV-visible spectroscopy measurements confirm the complete uptake and alloying of Pd with Cu by GR. Scanning tunneling microscopy and thermal desorption spectroscopy of model SAA surfaces confirmed the feasibility of hydrogen spillover onto an otherwise inert Cu surface. These model studies addressed a wide range of Pd concentrations related to the bimetallic nanoparticles.

RESUMEN

Investigation of methanol's surface chemistry on metals is a crucial step towards understanding the reactivity of this important chemical feedstock. Cu is a relevant metal for methanol synthesis and reforming, but due to the weak interaction of methanol with Cu, an atomic scale view of methanol's coverage-dependent ordering and self-assembly on Cu(111), the most abundant facet of most nanoparticles, has not yet been possible. Low and variable temperature scanning tunneling microscopy coupled with density functional theory reveal a coverage-dependent range of highly ordered structures stabilized by two hydrogen bonds per molecule. While extended chains that resemble the hydrogen-bonded zigzag structures reported for solid methanol are an efficient way to pack methanol at higher coverages, lower surface coverages yield isolated hexamer units. These hexamers form the same number of hydrogen bonds as the chains but appear to repel one another on the surface. Annealing treatments lead to the desorption of methanol with almost no decomposition. This data serves as a useful guide to both the preferred adsorption geometries and energies of a variety of methanol structures on Cu(111) surfaces as a function of surface coverage.

RESUMEN

Facile dissociation of reactants and weak binding of intermediates are key requirements for efficient and selective catalysis. However, these two variables are intimately linked in a way that does not generally allow the optimization of both properties simultaneously. By using desorption measurements in combination with high-resolution scanning tunneling microscopy, we show that individual, isolated Pd atoms in a Cu surface substantially lower the energy barrier to both hydrogen uptake on and subsequent desorption from the Cu metal surface. This facile hydrogen dissociation at Pd atom sites and weak binding to Cu allow for very selective hydrogenation of styrene and acetylene as compared with pure Cu or Pd metal alone.

RESUMEN

Using a combination of scanning tunneling microscopy (STM) and density functional theory the hydrogen bond directionality and associated chirality of enantiopure clusters is visualized and controlled. This is demonstrated with methanol hexamers adsorbed on Au(111), which depending on their chirality, adopt two distinct molecular footprints on the surface. Controlled STM tip manipulations were used to interconvert the chirality of entire clusters and to break up metastable chain structures into hexamers.