Toward a modular multi-material nanoparticle synthesis and assembly strategy via bionanocombinatorics: bifunctional peptides for linking Au and Ag nanomaterials.

Materials-binding peptides represent a unique avenue towards controlling the shape and size of nanoparticles (NPs) grown under aqueous conditions. Here, employing a bionanocombinatorics approach, two such materials-binding peptides were linked at either end of a photoswitchable spacer, forming a multi-domain materials-binding molecule to control the in situ synthesis and organization of Ag and Au NPs under ambient conditions. These multi-domain molecules retained the peptides' ability to nucleate, grow, and stabilize Ag and Au NPs in aqueous media. Disordered co-assemblies of the two nanomaterials were observed by TEM imaging of dried samples after sequential growth of the two metals, and showed a clustering behavior that was not typically observed without both metals and the linker molecules. While TEM evidence suggested the formation of AuNP/AgNP assemblies upon drying, SAXS analysis indicated that no extended assemblies existed in solution, suggesting that sample drying plays an important role in facilitating NP clustering. Molecular simulations and experimental data revealed tunable materials-binding based upon the isomerization state of the photoswitchable unit and metal employed. This work is a first step in generating externally actuated biomolecules with specific material-binding properties that could be used as the building blocks to achieve multi-material switchable NP assemblies.

Sequence-Dependent Structure/Function Relationships of Catalytic Peptide-Enabled Gold Nanoparticles Generated under Ambient Synthetic Conditions.

Peptide-enabled nanoparticle (NP) synthesis routes can create and/or assemble functional nanomaterials under environmentally friendly conditions, with properties dictated by complex interactions at the biotic/abiotic interface. Manipulation of this interface through sequence modification can provide the capability for material properties to be tailored to create enhanced materials for energy, catalysis, and sensing applications. Fully realizing the potential of these materials requires a comprehensive understanding of sequence-dependent structure/function relationships that is presently lacking. In this work, the atomic-scale structures of a series of peptide-capped Au NPs are determined using a combination of atomic pair distribution function analysis of high-energy X-ray diffraction data and advanced molecular dynamics (MD) simulations. The Au NPs produced with different peptide sequences exhibit varying degrees of catalytic activity for the exemplar reaction 4-nitrophenol reduction. The experimentally derived atomic-scale NP configurations reveal sequence-dependent differences in structural order at the NP surface. Replica exchange with solute-tempering MD simulations are then used to predict the morphology of the peptide overlayer on these Au NPs and identify factors determining the structure/catalytic properties relationship. We show that the amount of exposed Au surface, the underlying surface structural disorder, and the interaction strength of the peptide with the Au surface all influence catalytic performance. A simplified computational prediction of catalytic performance is developed that can potentially serve as a screening tool for future studies. Our approach provides a platform for broadening the analysis of catalytic peptide-enabled metallic NP systems, potentially allowing for the development of rational design rules for property enhancement.

Elucidation of peptide-directed palladium surface structure for biologically tunable nanocatalysts.

Peptide-enabled synthesis of inorganic nanostructures represents an avenue to access catalytic materials with tunable and optimized properties. This is achieved via peptide complexity and programmability that is missing in traditional ligands for catalytic nanomaterials. Unfortunately, there is limited information available to correlate peptide sequence to particle structure and catalytic activity to date. As such, the application of peptide-enabled nanocatalysts remains limited to trial and error approaches. In this paper, a hybrid experimental and computational approach is introduced to systematically elucidate biomolecule-dependent structure/function relationships for peptide-capped Pd nanocatalysts. Synchrotron X-ray techniques were used to uncover substantial particle surface structural disorder, which was dependent upon the amino acid sequence of the peptide capping ligand. Nanocatalyst configurations were then determined directly from experimental data using reverse Monte Carlo methods and further refined using molecular dynamics simulation, obtaining thermodynamically stable peptide-Pd nanoparticle configurations. Sequence-dependent catalytic property differences for C-C coupling and olefin hydrogenation were then elucidated by identification of the catalytic active sites at the atomic level and quantitative prediction of relative reaction rates. This hybrid methodology provides a clear route to determine peptide-dependent structure/function relationships, enabling the generation of guidelines for catalyst design through rational tailoring of peptide sequences.

Reductant and sequence effects on the morphology and catalytic activity of peptide-capped Au nanoparticles.

The use of peptides as capping ligands for materials synthesis has been widely explored. The ambient conditions of bio-inspired syntheses using molecules such as peptides represent an attractive route for controlling the morphology and activity of nanomaterials. Although various reductants can be used in such syntheses, no comprehensive comparison of the same bio-based ligand with different reductants has been reported. In this contribution, peptides AuBP1, AuBP2, and Pd4 are used in the synthesis of Au nanoparticles. The reductant strength is varied by using three different reducing agents: NaBH4, hydrazine, and ascorbic acid. These changes in reductant produce significant morphological differences in the final particles. The weakest reductant, ascorbic acid, yields large, globular nanoparticles with rough surfaces, whereas the strongest reductant, NaBH4, yields small, spherical, smooth nanomaterials. Studies of 4-nitrophenol reduction using the Au nanoparticles as catalysts reveal a decrease in activation energy for the large, globular, rough materials relative to the small, spherical, smooth materials. These studies demonstrate that modifying the reductant is a simple way to control the activity of peptide-capped nanoparticles.

Atomic-scale identification of Pd leaching in nanoparticle catalyzed C-C coupling: effects of particle surface disorder.

C-C coupling reactions are of great importance in the synthesis of numerous organic compounds, where Pd nanoparticle catalyzed systems represent new materials to efficiently drive these reactions. Despite their pervasive utility, the catalytic mechanism of these particle-based reactions remains highly contested. Herein we present evidence of an atom leaching mechanism for Stille coupling under aqueous conditions using peptide-capped Pd nanoparticles. EXAFS analysis revealed Pd coordination changes in the nanoparticle consistent with Pd atom abstraction, where sizing analysis by SAXS confirmed particle size changes associated with a leaching process. It is likely that recently discovered highly disordered surface Pd atoms are the favored catalytic active sites and are leached during oxidative addition, resulting in smaller particles. Probing the mechanism of nanoparticle-driven C-C coupling reactions through structural analyses provides fundamental information concerning these active sites and their reactivity at the atomic-scale, which can be used to improve catalytic performance to meet important sustainability goals.

Determining peptide sequence effects that control the size, structure, and function of nanoparticles.

The ability to tune the size, shape, and composition of nanomaterials at length scales <10 nm remains a challenging task. Such capabilities are required to fully realize the application of nanotechnology for catalysis, energy storage, and biomedical technologies. Conversely, nature employs biomacromolecules such as proteins and peptides as highly specific nanoparticle ligands that demonstrate exacting precision over the particle morphology through controlling the biotic/abiotic interface. Here we demonstrate the ability to finely tune the size, surface structure, and functionality of single-crystal Pd nanoparticles between 2 and 3 nm using materials directing peptides. This was achieved by selectively altering the peptide sequence to change the binding motif, which in turn modifies the surface structure of the particles. The materials were fully characterized before and after reduction using atomically resolved spectroscopic and microscopic analyses, which indicated that the coordination environment prior to reduction significantly affects the structure of the final nanoparticles. Additionally, changes to the particle surface structure, as a function of peptide sequence, can allow for chloride ion coordination that alters the catalytic abilities of the materials for the C-C coupling Stille reaction. These results suggest that peptide-based approaches may be able to achieve control over the structure/function relationship of nanomaterials where the peptide sequence could be used to selectivity tune such capabilities.

Nanotechnology Meets Biology: Peptide-based Methods for the Fabrication of Functional Materials.

Nature exploits sustainable methods for the creation of inorganic materials on the nanoscale for a variety of applications. To achieve such capabilities, biomolecules such as peptides and proteins have been developed that recognize and bind the different compositions of materials. While a diverse set of materials binding sequences are present in the biosphere, biocombinatorial techniques have been used to rapidly identify peptides that facilitate the formation of new materials of technological importance. Interestingly, the binding motif of the peptides at the inorganic surface is likely to control the size, structure, composition, shape, and functionality of the final materials. In order to advance these intriguing new biomimetic approaches, a complete understanding of this biotic/abiotic interface is required. In this Perspective, we highlight recent advances in the biofunctionalization of nanoparticles with potential applications ranging from catalysis and energy storage to plasmonics and biosensing. We specifically focus on the physical characterization of the peptide-based surface from which specificity and activity are likely embedded.

Crystallographic recognition controls peptide binding for bio-based nanomaterials.

The ability to control the size, shape, composition, and activity of nanomaterials presents a formidable challenge. Peptide approaches represent new avenues to achieve such control at the synthetic level; however, the critical interactions at the bio/nano interface that direct such precision remain poorly understood. Here we present evidence to suggest that materials-directing peptides bind at specific time points during Pd nanoparticle (NP) growth, dictated by material crystallinity. As such surfaces are presented, rapid peptide binding occurs, resulting in the stabilization and size control of single-crystal NPs. Such specificity suggests that peptides could be engineered to direct the structure of nanomaterials at the atomic level, thus enhancing their activity.