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.

Biomimetic Control over Bimetallic Nanoparticle Structure and Activity via Peptide Capping Ligand Sequence.

The controlled design of bimetallic nanoparticles (BNPs) is a key goal in tailoring their catalytic properties. Recently, biomimetic pathways demonstrated potent control over the distribution of different metals within BNPs, but a direct understanding of the peptide effect on the compositional distribution at the interparticle and intraparticle levels remains lacking. We synthesized two sets of PtAu systems with two peptides and correlated their structure, composition, and distributions with the catalytic activity. Structural and compositional analyses were performed by a combined machine learning-assisted refinement of X-ray absorption spectra and Z-contrast measurements by scanning transmission electron microscopy. The difference in the catalytic activities between nanoparticles synthesized with different peptides was attributed to the details of interparticle distribution of Pt and Au across these markedly heterogeneous systems, comprising Pt-rich, Au-rich, and Au core/Pt shell nanoparticles. The total amount of Pt in the shells of the BNPs was proposed to be the key catalytic activity descriptor. This approach can be extended to other systems of metals and peptides to facilitate the targeted design of catalysts with the desired activity.

Peptide/Nanoparticle Biointerfaces for Multistep Tandem Catalysis.

The realization of multifunctional nanoparticle systems is essential to achieve highly efficient catalytic materials for specific applications; however, their production remains quite challenging. They are typically achieved through the incorporation of multiple inorganic components; however, incorporation of functionality could also be achieved at the organic ligand layer. In this work, we demonstrate the generation of multifunctional nanoparticle catalysts using peptide-based ligands for tandem catalytic functionality. To this end, chimeric peptides were designed that incorporated a Au binding sequence and a catalytic sequence that can drive ester hydrolysis. Using this chimera, Au nanoparticles were prepared, which sufficiently presented the catalytic domain of the peptide to drive tandem catalytic processes occurring at the peptide ligand layer and the Au nanoparticle surface. This work represents unique pathways to achieve multifunctionality from nanoparticle systems tuned by both the inorganic and bio/organic components, which could be highly important for applications beyond catalysis, including theranostics, sensing, and energy technologies.

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.

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.

Design of Pd-Decorated SrTiO3/BiOBr Heterojunction Materials for Enhanced Visible-Light-Based Photocatalytic Reactivity.

The development of photocatalytic materials that exploit visible light is imperative for their sustainable application in environmental remediation. While a variety of approaches have been attempted, facile routes to achieve such structures remain limited. In this contribution, a direct route for the production of a SrTiO3/BiOBr/Pd heterojunction is presented that employs a low temperature, sustainable production method. The materials were produced in a two-step process wherein BiOBr nanoplates are fabricated in the presence of the SrTiO3 nanospheres, generating a highly integrated composite material. Pd nanoparticle surface decoration was subsequently employed to facilitate and enhance charge separation lifetimes to optimize reactivity. The structures were fully characterized via a suite of approaches to confirm the final material composition and arrangement. Their reactivity was explored for the degradation of both colored and colorless model environmental pollutants, where the SrTiO3/BiOBr/Pd demonstrated significant reactivity using visible light, leading to substrate degradation in <10 min in some cases. The enhanced reactivity was attributed to the significant integration between materials, facilitating electron transfer. Such studies provide key information for the development of new materials with optimized visible-light-driven photocatalytic reactivity for sustainable environmental remediation.

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.

Atomically Resolved Characterization of Optically Driven Ligand Reconfiguration on Nanoparticle Catalyst Surfaces.

Dynamic ligand layers on nanoparticle surfaces could prove to be critically important to enhance the functionality of individual materials. Such capabilities could complement the properties of the inorganic component to provide multifunctionality or the ability to be remotely actuated. Peptide-based ligands have demonstrated the ability to be remotely responsive to structural changes when adsorbed to nanoparticle surfaces via incorporation of photoswitches into their molecular structure. In this contribution, direct spectroscopic evidence of the remote actuation of a photoswitchable peptide adsorbed onto Au nanoparticles is demonstrated using X-ray absorption fine structure spectroscopic methods. From this analysis, Au-X (X = C or N) coordination numbers confirm the changes before and after photoswitching in the surface ligand conformation, which was correlated directly to variations in the catalytic application of the materials for nitrophenol reduction processes. In addition, the catalytic application of the materials was demonstrated to be significantly sensitive to the structure of the nitrophenol substrate used in the reaction, suggesting that changes in the reactivity are likely based upon the peptide conformation and substrate structure. Such results confirm that surface ligands can be remotely reconfigured on nanoparticle surfaces, providing pathways to apply such capabilities to a variety of applications beyond catalysis ranging from drug delivery to sensing.

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.

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.

Identification of Toxicity Effects of Cu2O Materials on C. elegans as a Function of Environmental Ionic Composition.

Previous work has shown that spherical CuO nanomaterials show negative effects on cell and animal physiology. The biological effects of Cu2O materials, which posess unique chemical features compared to CuO nanomaterials and can be synthesized in a similarly large variety of shapes and sizes, are comparatively less studied. Here, we synthesized truncated octahedral Cu2O particles and characterized their structure, stability, and physiological effects in the nematode worm animal model, Caenorhabditis elegans. Cu2O particles were found to be generally stable in aqueous media, although the particles did show signs of oxidation and leaching of Cu2+ within hours in worm growth media. The particles were found to be especially sensitive to inorganic phosphate (PO4 3-) found in standard NGM nematode growth medium. Cu2O particles were observed being taken up into the nematode pharynx and detected in the lumen of the gut. Toxicity experiments revealed that treatment with Cu2O particles caused a significant reduction in animal size and lifespan. These toxic effects resembled treatment with Cu2+, but measurements of Cu leaching, worm size, and long-term behavior experiments show the particles are more toxic than expected from Cu ion leaching alone. These results suggest worm ingestion of intact Cu2O particles enhances their toxicity and behavior effects while particle exposure to environmental phosphate precipitates leached Cu2+ into biounavailable phosphate salts. Interestingly, the worms showed an acute avoidance of bacterial food with Cu2O particles, suggesting that animals can detect chemical features of the particles and/or their breakdown products and actively avoid areas with them. These results will help to understand how specific, chemically-defined particles proposed for use in polluted soil and wastewater remediation affect animal toxicity and behaviors in their natural environment.

Biomolecular Material Recognition in Two Dimensions: Peptide Binding to Graphene, h-BN, and MoS2 Nanosheets as Unique Bioconjugates.

Two-dimensional nanosheet-based materials such as graphene, hexagonal boron nitride, and MoS2 represent intriguing structures for a variety of biological applications ranging from biosensing to nanomedicine. Recent advances have demonstrated that peptides can be identified with affinity for these three materials, thus generating a highly unique bioconjugate interfacial system. This Review focuses on recent advances in the formation of bioconjugates of these types, paying particular attention to the structure/function relationship of the peptide overlayer. This is achieved through the amino acid composition of the nanosheet binding peptides, thus allowing for precise control over the properties of the final materials. Such bioconjugate systems offer rapid advances via direct property control that remain difficult to achieve for biological applications using nonbiological approaches.

Biomimetic strategies to produce catalytically reactive CuS nanodisks.

Copper sulfide materials have diverse applications from cancer therapy to environmental remediation due to their narrow bandgap and easily tuned plasmon. The synthesis of these materials often involves toxic reagents and harsh conditions where biomimetic methods may provide opportunities to produce these structures under sustainable conditions. To explore this capability, simple amino acids were exploited as biological ligands for the ambient synthesis of CuS materials. Using an aqueous-based approach, CuS nanodisks were prepared using acid-containing amino acid molecules that stabilize the materials against bulk aggregation. These structures were fully characterized by UV-vis analysis, transmission electron microscopy, dynamic light scattering, atomic force microscopy, selected area electron diffraction, and X-ray diffraction, which confirmed the formation of CuS. The materials possessed a vibrant plasmon band in the near IR region and demonstrated enhanced photocatalytic reactivity for the advanced oxidation of organic dyes in water. These results demonstrate a room temperature synthetic route to optically important materials, which could have important application in catalysis, optics, nanomedicine, etc.

Optical Control of Nanoparticle Catalysis Influenced by Photoswitch Positioning in Hybrid Peptide Capping Ligands.

Here, we present an in-depth analysis of structural factors that modulate peptide-capped nanoparticle catalytic activity via optically driven structural reconfiguration of the biointerface present at the particle surface. Six different sets of peptide-capped Au nanoparticles were prepared, in which an azobenzene photoswitch was incorporated into one of two well-studied peptide sequences with known affinity for Au, each at one of three different positions: the N- or C-terminus or mid-sequence. Changes in the photoswitch isomerization state induce a reversible structural change in the surface-bound peptide, which modulates the catalytic activity of the material. This control of reactivity is attributed to changes in the amount of accessible metallic surface area available to drive the reaction. This research specifically focuses on the effect of the peptide sequence and photoswitch position in the biomolecule, from which potential target systems for on/off reactivity have been identified. Additionally, trends associated with photoswitch position for a peptide sequence (Pd4) have been identified. Integrating the azobenzene at the N-terminus or central region results in nanocatalysts with greater reactivity in the trans and cis conformations, respectively, however, positioning the photoswitch at the C-terminus gives rise to a unique system that is reactive in the trans conformation and partially deactivated in the cis conformation. These results provide a fundamental basis for new directions in nanoparticle catalyst development to control activity in real time, which could have significant implications in the design of catalysts for multistep reactions using a single catalyst. Additionally, such a fine level of interfacial structural control could prove to be important for applications beyond catalysis, including biosensing, photonics, and energy technologies that are highly dependent on particle surface structures.

Tunable assembly of biomimetic peptoids as templates to control nanostructure catalytic activity.

Nanostructured materials present new opportunities to achieve sustainable catalytic reactivity. Fabrication and organization of these catalytic particles for enhanced reactivity remain challenging due to limited synthetic and organization strategies. Biomimetic approaches represent new avenues to address such challenges. Here we report the tunable assembly of sequence-defined peptoids as templates to control the formation of highly reactive Pd nanostructures of different arrangements. In this regard, peptoid 2D membranes and 1D fibers were assembled and used to template Pd nanoparticles in specific orientations. Catalytic analysis of the resulting materials demonstrated enhanced reactivity from the fiber-based system due to changes in inorganic material display. These results suggest that the morphology of peptoid-based templates plays an important role in controlling material properties, which could open a new direction of using peptoid assemblies for applications in optics, plasmonics, sensing, etc.

Solution Effects on Peptide-Mediated Reduction and Stabilization of Au Nanoparticles.

Biomimetic methods for the preparation and application of inorganic nanomaterials represent a unique avenue to sustainably generating functional materials with long-term activity. Typically, for the fabrication of these structures, the peptide is mixed with metal ions in solution prior to the addition of an exogenous reductant such as NaBH4, leading to nanoparticle nucleation and growth. In biological systems, strong reductants such as NaBH4 are not available, thus different metal ion reduction methods must be exploited. Recent work has shown that the AuBP1 peptide (WAGAKRLVLRRE), a Au binding peptide with an N-terminal tryptophan, can spontaneously reduce Au3+ without an exogenous reductant. Remarkably, this system initially demonstrated the formation of large Au aggregates that disassemble to form individual Au nanoparticles, stabilized by the peptide bound to the inorganic surface. In this contribution, we demonstrate the significant effects of aqueous solvent-processing conditions (pH, ionic strength, and ion composition) on the rate of particle evolution. Understanding how such effects alter the metal ion reduction process and subsequent nanoparticle fabrication is important in controlling the final structure/function relationship of the resultant peptide-capped materials. This work identifies conditions that may enhance nanoparticle synthesis using biomimetic approaches where the peptide has complete control over the complexation, reduction, nucleation, and growth of nanomaterials.

Biointerface Structural Effects on the Properties and Applications of Bioinspired Peptide-Based Nanomaterials.

Peptide sequences are known to recognize and bind different nanomaterial surfaces, which has resulted in the screening and identification of hundreds of peptides with the ability to bind to a wide range of metallic, metal oxide, mineral, and polymer substrates. These biomolecules are able to bind to materials with relatively high affinity, resulting in the generation of a complex biointerface between the biotic and abiotic components. While the number of material-binding sequences is large, at present, quantitative materials-binding characterization of these peptides has been accomplished only for a relatively small number of sequences. Moreover, it is currently very challenging to determine the molecular-level structure(s) of these peptides in the materials adsorbed state. Despite this lack of data related to the structure and function of this remarkable biointerface, several of these peptide sequences have found extensive use in creating functional nanostructured materials for assembly, catalysis, energy, and medicine, all of which are dependent on the structure of the individual peptides and collective biointerface at the material surface. In this Review, we provide a comprehensive overview of these applications and illustrate how the versatility of this peptide-mediated approach for the growth, organization, and activation of nanomaterials could be more widely expanded via the elucidation of biointerfacial structure/property relationships. Future directions and grand challenges to realize these goals are highlighted for both experimental characterization and molecular-simulation strategies.

Nature of peptide wrapping onto metal nanoparticle catalysts and driving forces for size control.

Colloidal metal nanocrystals find many applications in catalysis, energy conversion devices, and therapeutics. However, the nature of ligand interactions and implications on shape control have remained uncertain at the atomic scale. Large differences in peptide adsorption strength and facet specificity were found on flat palladium surfaces versus surfaces of nanoparticles of 2 to 3 nm size using accurate atomistic simulations with the Interface force field. Folding of longer peptides across many facets explains the formation of near-spherical particles with local surface disorder, in contrast to the possibility of nanostructures of higher symmetry with shorter ligands. The average particle size in TEM correlates inversely with the surface coverage with a given ligand and with the strength of ligand adsorption. The role of specific amino acids and sequence mutations on the nanoparticle size and facet composition is discussed, as well as the origin of local surface disorder that leads to large differences in catalytic reactivity.