• Comparative study of materials-binding peptide interactions with gold and silver surfaces and nanostructures: A thermodynamic basis for biological selectivity of inorganic materials

      Palafox-Hernandez, J.P.; Tang, Z.; Hughes, Zak E.; Li, Y.; Swihart, M.T.; Prasad, P.N.; Walsh, T.R.; Knecht, M.R. (2014-09)
      Controllable 3D assembly of multicomponent inorganic nanomaterials by precisely positioning two or more types of nanoparticles to modulate their interactions and achieve multifunctionality remains a major challenge. The diverse chemical and structural features of biomolecules can generate the compositionally specific organic/inorganic interactions needed to create such assemblies. Toward this aim, we studied the materials-specific binding of peptides selected based upon affinity for Ag (AgBP1 and AgBP2) and Au (AuBP1 and AuBP2) surfaces, combining experimental binding measurements, advanced molecular simulation, and nanomaterial synthesis. This reveals, for the first time, different modes of binding on the chemically similar Au and Ag surfaces. Molecular simulations showed flatter configurations on Au and a greater variety of 3D adsorbed conformations on Ag, reflecting primarily enthalpically driven binding on Au and entropically driven binding on Ag. This may arise from differences in the interfacial solvent structure. On Au, direct interaction of peptide residues with the metal surface is dominant, while on Ag, solvent-mediated interactions are more important. Experimentally, AgBP1 is found to be selective for Ag over Au, while the other sequences have strong and comparable affinities for both surfaces, despite differences in binding modes. Finally, we show for the first time the impact of these differences on peptide mediated synthesis of nanoparticles, leading to significant variation in particle morphology, size, and aggregation state. Because the degree of contact with the metal surface affects the peptide’s ability to cap the nanoparticles and thereby control growth and aggregation, the peptides with the least direct contact (AgBP1 and AgBP2 on Ag) produced relatively polydispersed and aggregated nanoparticles. Overall, we show that thermodynamically different binding modes at metallic interfaces can enable selective binding on very similar inorganic surfaces and can provide control over nanoparticle nucleation and growth. This supports the promise of bionanocombinatoric approaches that rely upon materials recognition.
    • Halo-substituted azobenzenes adsorbed at Ag(111) and Au(111) interfaces: Structures and optical properties

      Hughes, Zak E.; Baev, A.; Prasad, P.N.; Walsh, T.R. (2017-05-19)
      The adsorption of azobenzene (AB), ortho fluoro-azobenzene (FAB) and ortho chlor-azobenzol (ClAB), in both the cis and trans isomers, at the Au(111) and Ag(111) surfaces is investigated using plane-wave density functional calculations with the revPBE-vdW-DF functional. The resulting adsorption energies and internal structures of AB adsorbed to both metal surfaces are in broad agreement with available experimental data. In the gas phase, FAB and ClAB feature a significant reduction in the energy difference between the two isomeric states, compared with AB. This relative reduction in the energy difference is still significant for the adsorbed form of FAB but is only weakly apparent for ClAB. The absorption spectra of the molecules have also been calculated, with the halogen substituents generating significant changes in the gas phase, but only a modest difference for the adsorbed molecules.
    • Optical actuation of inorganic/organic interfaces: comparing peptide-azobenzene ligand reconfiguration on gold and silver nanoparticles

      Palafox-Hernandez, J.P.; Lim, C-K.; Tang, Z.; Drew, K.L.M.; Hughes, Zak E.; Li, Y.; Swihart, M.T.; Prasad, P.N.; Knecht, M.R.; Walsh, T.R. (2016-01-13)
      Photoresponsive molecules that incorporate peptides capable of material-specific recognition provide a basis for biomolecule-mediated control of the nucleation, growth, organization, and activation of hybrid inorganic/organic nanostructures. These hybrid molecules interact with the inorganic surface through multiple noncovalent interactions which allow reconfiguration in response to optical stimuli. Here, we quantify the binding of azobenzene-peptide conjugates that exhibit optically triggered cis-trans isomerization on Ag surfaces and compare to their behavior on Au. These results demonstrate differences in binding and switching behavior between the Au and Ag surfaces. These molecules can also produce and stabilize Au and Ag nanoparticles in aqueous media where the biointerface can be reproducibly and reversibly switched by optically triggered azobenzene isomerization. Comparisons of switching rates and reversibility on the nanoparticles reveal differences that depend upon whether the azobenzene is attached at the peptide N- or C-terminus, its isomerization state, and the nanoparticle composition. Our integrated experimental and computational investigation shows that the number of ligand anchor sites strongly influences the nanoparticle size. As predicted by our molecular simulations, weaker contact between the hybrid biomolecules and the Ag surface, with fewer anchor residues compared with Au, gives rise to differences in switching kinetics on Ag versus Au. Our findings provide a pathway toward achieving new remotely actuatable nanomaterials for multiple applications from a single system, which remains difficult to achieve using conventional approaches.
    • Optical control of nanoparticle catalysis influenced by photoswitch positioning in hybrid peptide capping ligands

      Lawrence, R.L.; Hughes, Zak E.; Cendan, V.J.; Liu, Y.; Lim, C.K.; Prasad, P.N.; Swihart, M.T.; Walsh, T.R.; Knecht, M.R. (2018-10)
      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.