Graphene exfoliation using multidomain peptides.

Liquid-phase exfoliation using biomolecules in aqueous solution is a promising approach to obtain high quality 2D nanosheets. For example, the well-studied graphene-binding peptide, P1 (sequence HSSYWYAFNNKT), has been previously investigated and shown to have a good ability to exfoliate graphene sheets in aqueous conditions under sonication, maintaining colloidal stability. Building on this, the biomolecular exfoliant and assembly motif (BEAM) peptide, that features a graphene-binding domain at one end and a hexagonal boron nitride (h-BN) binding domain at the other, separated by a 10-carbon fatty acid chain in the centre, is shown to exfoliate graphene sheets from bulk graphite in aqueous media. An in-depth examination of the ability of the BEAM to both facilitate sheet exfoliation under sonication conditions and also maintain colliodal stability is provided through molecular dynamics simulations. These findings open new possibilities for designing multi-functional molecules that can both exfoliate and organise 2D materials into heterostructures under ambient conditions in aqueous media.

Accelerated lithium-ion diffusion via a ligand 'hopping' mechanism in lithium enriched solvate ionic liquids.

Solvate ionic liquids (SILs), equimolar amounts of lithium salts and polyether glymes, are well studied highly customisable "designer solvents". Herein the physical, thermal and ion mobility properties of SILs with increased LiTFSI (LiTFSA) concentration, with ligand 1 : >1 LiTFSI stoichiometric ratios, are presented. It was found that between 60-80 °C, the lithium cation diffuses up to 4 times faster than the corresponding anion or ligand (glyme). These systems varied from viscous liquids to self-supporting gels, though were found to thin exponentially when heated to mild temperatures (50-60 °C). They were also found to be thermally stable, up to 200 °C, well in excess of normal operating temperatures. Ion mobility, assessed under an electric potential via ionic conductivity, showed the benefit of SIL optimisation for attaining greater concentrations of Li+ cations to store charge during supercapacitor charging and discharging. Molecular dynamics simulations interrogate the mechanism of enhanced diffusion at high temperatures, revealing a lithium hopping mechanism that implicates the glyme in bridging two lithiums through changes in the denticity.

Using Nitroxides to Enhance Carbon Fiber Interfacial Adhesion and as an Anchor for "Graft to" Surface Modification Strategies.

Nitroxide groups covalently grafted to carbon fibers are used as anchoring sites for TEMPO-terminated polymers (poly-n-butylacrylate and polystyrene) in a "graft to" surface modification strategy. All surface-modified fibers are evaluated for their physical properties, showing that several treatments have enhanced the tensile strength and Young's modulus compared to the control fibers. Up to an 18% increase in tensile strength and 12% in Young's modulus are observed. Similarly, the evaluation of interfacial shear strength in an epoxy polymer shows improvements of up to 144% relative to the control sample. Interestingly, the polymer-grafted surfaces show smaller increases in interfacial shear strength compared to surfaces modified with a small molecule only. This counterintuitive result is attributed to the incompatibility, both chemical and physical, of the grafted polymers to the surrounding epoxy matrix. Molecular dynamics simulations of the interface suggest that the diminished increase in mechanical shear strength observed for the polymer grafted surfaces may be due to the lack of exposed chain ends, whereas the small molecule grafted interface exclusively presents chain ends to the resin interface, resulting in good improvements in mechanical properties.

Single Amino Acid Modifications for Controlling the Helicity of Peptide-Based Chiral Gold Nanoparticle Superstructures.

Assembling nanoparticles (NPs) into well-defined superstructures can lead to emergent collective properties that depend on their 3-D structural arrangement. Peptide conjugate molecules designed to both bind to NP surfaces and direct NP assembly have proven useful for constructing NP superstructures, and atomic- and molecular-level alterations to these conjugates have been shown to manifest in observable changes to nanoscale structure and properties. The divalent peptide conjugate, C16-(PEPAu)2 (PEPAu = AYSSGAPPMPPF), directs the formation of one-dimensional helical Au NP superstructures. This study examines how variation of the ninth amino acid residue (M), which is known to be a key Au anchoring residue, affects the structure of the helical assemblies. A series of conjugates of differential Au binding affinities based on variation of the ninth residue were designed, and Replica Exchange with Solute Tempering (REST) Molecular Dynamics simulations of the peptides on an Au(111) surface were performed to determine the approximate surface contact and to assign a binding score for each new peptide. A helical structure transition from double helices to single helices is observed as the peptide binding affinity to the Au(111) surface decreases. Accompanying this distinct structural transition is the emergence of a plasmonic chiroptical signal. REST-MD simulations were also used to predict new peptide conjugate molecules that would preferentially direct the formation of single-helical AuNP superstructures. Significantly, these findings demonstrate how small modifications to peptide precursors can be leveraged to precisely direct inorganic NP structure and assembly at the nano- and microscale, further expanding and enriching the peptide-based molecular toolkit for controlling NP superstructure assembly and properties.

Blocking Directional Lithium Diffusion in Solid-State Electrolytes at the Interface: First-Principles Insights into the Impact of the Space Charge Layer.

Understanding the degradation mechanisms in solid-state lithium-ion batteries at interfaces is fundamental for improving battery performance and for designing recycling methodologies for batteries. A key source of battery degradation is the presence of the space charge layer at the solid-state electrolyte-electrode interface and the impact that this layer has on the thermodynamics of the electrolyte structure. Currently, Li10GeP2S12 in its pristine form has one of the highest lithium conductivities and has been used as a template for designing even higher conductivity derived structures. However, being an ionic material with mostly linear diffusion, it is prone to path-blocker defects, which we show here to be especially prevalent in the space charge layer. We analyze the thermodynamic properties of a number of path-blocker defects using density functional theory and their potential crystal decomposition and find that the presence of an electrostatic potential in the space charge layer elevates the likelihood of existence of these defects, which otherwise would not be likely to form in the bulk of the electrolyte away from electrodes. We use ab initio molecular dynamics to assess the impact of these defects on the diffusivity of the crystal and find that they all reduce the lithium diffusivity. While our work focuses on Li10GeP2S12, it is relevant to any solid-state electrolyte with mainly linear diffusion.

Achieving regioselective materials binding using multidomain peptides.

The ability to integrate two disparate materials-binding domains into a single ligand to achieve regiospecific binding would be powerful to direct material assembly; however, this has proven challenging to achieve due to cross-materials binding. Accomplishing this goal might be achieved by harnessing the precision of biology to exploit the recognition between peptides and specific nanomaterials. Here, a designed bifunctional molecule termed Biomolecular Exfoliant and Assembly Motifs (BEAM) is introduced, featuring two different materials-binding peptide domains, one for graphene and one for hexagonal boron nitride (h-BN), at each end of the molecule, separated by a fatty acid spacer. The BEAM is demonstrated to bind strongly to both graphene and h-BN surfaces, and in each case the materials-binding peptide domain is shown to preferentially bind its target material. Critically, the two materials-binding domains exhibited limited cross-domain interaction. The BEAM design concept shows substantial potential to eventually guide self-organization of a range of materials in aqueous media.

Manipulation of peptide-fatty acid bioconjugates on graphene: effects of fatty acid chain length and attachment point.

The non-destructive functionalisation of graphene in aqueous media is a critical process with the potential to enhance the versatility of the 2D nanosheet material as a technological enabler. This could also unlock strategies for a wider uptake of graphene in bio-related applications. Graphene functionalisation can be achieved using peptides that specifically recognise the carbon-based material, resulting in persistent non-covalent adsorption without damaging the nanosheet. Bio-conjugation of non-natural moieties with these peptides can incorporate multifunctionality, further extending the applicability of these interfaces. Here, bio-conjugates comprising a graphene-binding peptide with a fatty acid chain of varying length are investigated for their binding affinity and adsorbed structures at the aqueous graphene interface. Through an integration of quartz crystal microbalance and atomic force microscopy data with advanced sampling molecular simulations, variations in the binding of these bio-conjugates is determined. Conjugation at either terminus led to good interfacial contact, and for a given attachment point, the changes in the fatty acid length did not substantially disrupt the conformations of the adsorbed peptide domain. These findings provide a solid foundation for designing multi-functional bio-interfaces for sensing and healthcare.

Predicting biomolecule adsorption on MoS2 nanosheets with high structural fidelity.

A new force field, MoSu-CHARMM, for the description of bio-interfacial structures at the aqueous MoS2 interface is developed, based on quantum chemical data. The force field describes non-covalent interactions between the MoS2 surface and a wide range of chemistries including hydrocarbon, alcohol, aldehyde, ketone, carboxylic acid, amine, thiol, and amino acid groups. Density functional theory (DFT), using the vdW-DF2 functional, is employed to create training and validation datasets, comprising 330 DFT binding energies for 21 organic compounds. Development of MoSu-CHARMM is guided by two criteria: (i) minimisation of energetic differences compared to target DFT data and (ii) preservation of the DFT energetic rankings of the different binding configurations. Force-field performance is validated against existing high-quality structural experimental data regarding adsorption of four 26-residue peptides at the aqueous MoS2 interface. Adsorption free energies for all twenty amino acids in liquid water are calculated to provide guidance for future peptide design, and interpret the properties of existing experimentally-identified MoS2-binding peptides. This force field will enable large-scale simulations of biological interactions with MoS2 surfaces in aqueous media where an emphasis on structural fidelity is prioritised.

ContactAngleCalculator: An Automated, Parametrized, and Flexible Code for Contact Angle Estimation in Visual Molecular Dynamics.

Accurate, fast, and flexible approaches for contact angle estimation in molecular dynamics simulations are of great importance for characterization of surface wettability, especially for machine learning approaches which would usually require thousands of computational contact angle evaluations for training and prediction purposes. However, evaluation of the contact angle from molecular simulations is typically a human-intensive process, which hinders the required fast throughput. To address this challenge, here a flexible and automated contact angle estimation tool, ContactAngleCalculator, is developed to meet these new requirements. In contrast to the current widely used computational approaches that are laborious and human intensive, this code is based on the concepts of the coarse-graining technique and equivalent contact area and volume of the droplet. Once the parameters are determined for a target liquid, it can automatically estimate the contact angle of different time points of one case or multiple cases by only one click. This tool is targeted for integration with machine learning methods, in which it can substantially streamline and reduce human labor and time in a computational contact angle estimation.

From graphene to graphene oxide: the importance of extended topological defects.

Graphene oxide (GO) represents a complex family of materials related to graphene: easy to produce in large quantities, easy to process, and convenient to use as a basis for further functionalization, with the potential for wide-ranging applications such as in nanocomposites, electronic inks, biosensors and more. Despite their importance, the key structural traits of GO, and the impact of these traits on properties, are still poorly understood due to the inherently berthollide character of GO which complicates the establishment of clear structure/property relationships. Widely accepted structural models of GO frequently neglect the presence of extended topological defects, structural changes to the graphene basal plane that are not removed by reduction methods. Here, a combination of experimental approaches and molecular simulations demonstrate that extended topological defects are a common feature across GO and that the presence of these defects strongly influences the properties of GO. We show that these extended topological defects are produced following even controlled 'gentle' functionalization by atomic oxygen and are comparable to those obtained by a conventional modified Hummers' method. The presence of the extended topological defects is shown to play an important role in the retention of oxygen functional groups after reduction. As an exemplar of their effect on the physical properties, we show that the GO sheets display a dramatic decrease in strength and stiffness relative to graphene and, due to the presence of extended structural defects, no improvement is seen in the mechanical properties after reduction. These findings indicate the importance of extended topological defects to the structure and properties of functionalized graphene, which merits their inclusion as a key trait in simple structural models of GO.

Peptide-Driven Exfoliation and Dispersion Mechanisms of Graphene in Aqueous Media.

Peptide-mediated exfoliation and suspension of graphene in aqueous media is a promising strategy for bioapplications such as drug delivery, tissue engineering, and biosensors. A few peptide sequences are known as graphene exfoliators/dispersants in water, but the mechanisms underpinning this process remain underexplored. Here, molecular simulations investigate two key steps: sheet exfoliation and subsequent sheet reunification, in aqueous media. Umbrella sampling simulations predict the energy required to separate a graphene sheet from a graphite stack in both the presence/absence of the graphene-exfoliant peptide, P1. The free-energy barrier for reunification of two P1-coated graphene sheets is similarly calculated. Under sonication, the benefit from the relatively lower free-energy barrier associated with exfoliation in the absence of the peptide is negated by its facile reunification postsonication. In contrast, although P1 slightly increases the energy barrier to exfoliation under sonication, the peptides confer high-energy barriers to sheet reunification, thus ensuring stable aqueous graphene dispersions.

Tuning Materials-Binding Peptide Sequences toward Gold- and Silver-Binding Selectivity with Bayesian Optimization.

Peptide sequence engineering can potentially deliver materials-selective binding capabilities, which would be highly attractive in numerous biotic and abiotic nanomaterials applications. However, the number of known materials-selective peptide sequences is small, and identification of new sequences is laborious and haphazard. Previous attempts have sought to use machine learning and other informatics approaches that rely on existing data sets to accelerate the discovery of materials-selective peptides, but too few materials-selective sequences are known to enable reliable prediction. Moreover, this knowledge base is expensive to expand. Here, we combine a comprehensive and integrated experimental and modeling effort and introduce a Bayesian Effective Search for Optimal Sequences (BESOS) approach to address this challenge. Through this combined approach, we significantly expand the data set of Au-selective peptide sequences and identify an additional Ag-selective peptide sequence. Analysis of the binding motifs for the Ag-binders offers a roadmap for future prediction with machine learning, which should guide identification of further Ag-selective sequences. These discoveries will enable wider and more versatile integration of Ag nanoparticles in biological platforms.

Controlling the Orientation and Viscoelasticity of Materials-Binding Peptides on Hexagonal Boron Nitride Using Fatty Acids.

The adsorption of materials-binding peptides to technologically relevant 2D nanosheets of h-BN could be transformative for both property modulation and materials applications. To enhance binding, integration of non-natural functionalities into the biomolecule could prove to be important. However, very little is understood regarding the impact of these biomolecular structural alterations on the binding, which could influence the affinity and surface-adsorbed structures. Here, the effect of fatty acid incorporation site and carbon chain length is investigated using the BP7 peptide, previously identified with affinity for h-BN. The peptide was modified at either the N- or C-terminus with a fatty acid chain length of 6-12 carbons long. The binding affinity and bio-overlayer viscoelasticity are quantified using quartz crystal microbalance analysis. While fatty acid conjugation did not substantially affect the affinity of the resultant biomolecules, it did alter the viscoelasticity of the biomolecular overlayer on the h-BN surface based upon the carbon chain length and incorporation site. Molecular dynamics simulations demonstrate interplay between enthalpic and entropic effects in modifying the overlayer viscoelasticity. The simulations predict that C-terminal conjugation promotes the enhancement of upright adsorbed states, compared with the N-terminal case, with this effect most pronounced for the 10-carbon chain.

Dynamical Water Ingress and Dissolution at the Amorphous-Crystalline Cellulose Interface.

The use of cellulose has considerable promise in a wide range of industrial applications but is hampered by degradation in mechanical properties due to ambient moisture uptake. Existing models of equilibrium moisture content can predict the impact of these effects, but at present, the dynamical, atomic-scale picture of water ingress into cellulose is lacking. The present work reports nonequilibrium molecular simulations of the interface between cellulose and water aimed at capturing the initial stages of two simultaneous dynamical processes, water ingress into cellulose and cellulose dissolution into water. These simulations demonstrate that the process depends on the temperature and chain length in the amorphous region, where high temperatures can induce more mass exchange and short chains can easily detach from amorphous cellulose. A cooperative mechanism that involves both chemical and physical aspects, namely, hydrogen bonding and chain intertwining, respectively, is proposed to interpret the incipient dual ingress/dissolution process. Outcomes of this work will provide a foundation for cellulose functionalization strategies to impede moisture uptake and preserve the mechanical properties of nanocellulose in applications.

Force fields for water-surface interaction: is reproduction of the experimental water contact angle enough?

A new protocol based on quantum chemical calculations and molecular dynamics simulations is proposed to revisit water-MoS2 interfacial force fields (FFs). The accurate reproduction of experimental water contact angles is suggested to be insufficient to ensure reliable FFs for recovering structural properties of the interfacial solvent. As an example, this protocol is used to develop a new set of FF parameters to both capture interfacial structural phenomena at the interface between water and MoS2 and recover experimental water contact angle data. This approach can be applied to any interface where contact angle data are available.

Selective manipulation of peptide orientation on hexagonal boron nitride nanosheets.

The bio-recognition capabilities of materials-specific peptides offer a promising route to obtaining and organizing 2D nanosheet materials in aqueous media. Although significant advances have been made for graphene, little is currently understood regarding how to apply this strategy to hexagonal boron nitride (h-BN) due to a lack of knowledge regarding peptide/h-BN interactions. Here, one of the few peptide sequences known with affinity for h-BN, BP7, is the focus of mutation studies and bio-conjugation. A combination of experimental methods and modeling reveals the importance of Tyrosine in peptide/h-BN interactions. This residue is identified as the key anchoring species, which is then leveraged via bio-conjugation of BP7 to a fatty acid to create new interfacial properties. Specific placement of the fatty acid in the bio-conjugate results in dramatic manipulation of the surface-bound biotic overlayer to generate a highly viscoelastic interface. This viscoelasticity is a consequence of the fatty acid binding, which also down-modulates Tyrosine contact to h-BN, resulting in presentation of the extended peptide to solution. In this orientation, the biomolecule is available for subsequent bioconjugation, providing new pathways to programmable organization and conjugation of h-BN nanosheets in liquid water.

Identification of Parameters Controlling Peptide-Driven Graphene Exfoliation in Aqueous Media.

Bio-inspired approaches represent potentially transformational methods to fabricate and activate non-natural materials for applications ranging from biomedical diagnostics to energy harvesting platforms. Recently, bio-based methods for the exfoliation of graphene in water have been developed, resulting in peptide-capped nanosheets; however, a clear understanding of the reaction system and peptide ligand structure remains unclear, limiting the advance of such approaches. Here the effects of reaction solution conditions and peptide ligand structure were systematically examined for graphene exfoliation, identifying key parameters to optimize material production. For this, the P1 peptide, identified with affinity for graphene, was exploited to drive exfoliation of bulk graphite to generate the final materials. The peptide was modified at both the N- and C-terminus with a 10-carbon chain fatty acid to explore the effects of a hydrophobic domain on the exfoliation process. The system was examined as a function of sonication time, pH, reagent concentration, and graphite source, where the final materials were fully characterized using a suite of approaches. Collectively, these results demonstrated that maximum graphene production was achieved using the parent P1 peptide after 12 h of sonication under basic conditions. While the exfoliation efficiency was slightly lower for the fatty acid modified peptides, the graphene produced using these biomolecules had fewer defects incorporated, potentially from the wrapping of the nanosheet edge by the aliphatic domain. Such results are important to provide key reaction designs to optimize the reproducibility of graphene exfoliation using biomimetic approaches.

Moisture Ingress at the Molecular Scale in Hygrothermal Aging of Fiber-Epoxy Interfaces.

Almost all applications of carbon fiber reinforced composites are susceptible to water aging, either via ambient humidity or through direct exposure to liquid water environments. Although the impacts of water aging in composites can be readily quantified via experimental efforts, details regarding the mechanisms of moisture ingress and aging, particularly at the incipient stages of aging under hygrothermal conditions, have proven challenging to resolve using experimental techniques alone. A deeper understanding of the factors that drive incipient moisture ingress during aging is required for more targeted approaches to combat water aging. Here, molecular dynamics simulations of a novel epoxy/carbon fiber interface exposed to liquid water under hygrothermal conditions are used to elucidate molecular details of the moisture ingress mechanisms at the incipient stages of the aging process. Remarkably, the simulations show that the fiber-matrix interface is not vulnerable to a moisture-wicking type of incipient water ingress and does not readily flood in these early stages of water aging. Instead, water is preferentially absorbed via the matrix-water interface, an ingress pathway that is facilitated by the dynamic mobility of polymer chains at this interface. These chains present electronegative sites that can capture water molecules and provide a conduit to transiently exposed pores and channels on the polymer surface, which creates a presoaked staging reservoir for subsequent deeper ingress into the composite. Characterization of the absorbed water is according to hydrogen bonding to the matrix, and the distributions and transport behavior of these waters are consistent with experimental observations. This work introduces new insights regarding the molecular-level details of moisture ingress and spatial distribution of water in these materials during hygrothermal aging, informing future design directions for extending both the service life and shelf life of next-generation composites.

Material composition and peptide sequence affects biomolecule affinity to and selectivity for h-boron nitride and graphene.

Nanosheet heterostructures offer emergent optical/electronic properties. These could be achieved using selective materials binding peptides, but lack of understanding of selectivity impedes advancement. Here we examine peptides with affinity for graphene or h-BN using quantitative experiments and molecular simulation to identify traits for design of 2D nanosheet selective peptides.

Side-chain effects on the co-existence of emergent nanopatterns in amino acid adlayers on graphene.

The spontaneous tendency of amino acid adlayers to self-assemble into ordered patterns on non-reactive surfaces is thought to be chiefly influenced by amino acid termination state. Experiments have shown that different side chains can produce different patterns, with a distinction drawn between side chains that can support hydrogen bonds or electrostatic interactions, and those that are hydrophobic. However, as is demonstrated in this work, this distinction is not clear cut, implying that there is currently no way to predict in advance what type of pattern will be formed. Here, we use molecular dynamics simulations of amino acid adlayers in neutral, zwitterion, and neutral-zwitterion states for two types of amino acids, either histidine or alanine, adsorbed at the in-vacuo graphene interface. In contrast to earlier studies on adlayers of tryptophan and methionine on graphene that reveal the presence of only a single type of pattern motif, the canonical dimer row, here we find that emergent patterns of histidine and alanine adlayers supported the co-existence of several different types of motifs, influenced by the different side-chain characteristics. For alanine, the compact side-chain does not support hydrogen bonding and engages weakly with the surface, leading to the emergence of a new dimer row configuration in addition to the canonical dimer row motif. On the contrary, for histidine, the side-chain supports hydrogen bonding, leading to the emergence of a dimer row motif different from the canonical dimer row, co-existing with several different monomer row motifs. On this basis, we propose that emergent canonical dimer row patterns are more likely for amino acids with side-chains that are non-compact and that also lack extensive hydrogen bonding capacity, and that engage strongly with the underlying substrate. These findings provide a fundamental basis to rationally guide the design of desired self-assembled nanostructures on planar surfaces.