Natural surfaces are often structured with nanometre-scale domains, yet a framework providing a quantitative understanding of how nanostructure affects interfacial energy, gamma(SL), is lacking. Conventional continuum thermodynamics treats gamma(SL) solely as a function of average composition, ignoring structure. Here we show that, when a surface has domains commensurate in size with solvent molecules, gamma(SL) is determined not only by its average composition but also by a structural component that causes gamma(SL) to deviate from the continuum prediction by a substantial amount, as much as 20% in our system. By contrasting surfaces coated with either molecular- (<2 nm) or larger-scale domains (>5 nm), we find that whereas the latter surfaces have the expected linear dependence of gamma(SL) on surface composition, the former show a markedly different non-monotonic trend. Molecular dynamics simulations show how the organization of the solvent molecules at the interface is controlled by the nanostructured surface, which in turn appreciably modifies gamma(SL).
Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Atomic force microscopy's 7 (AFM) ability of visualizing the topography and the property of surfaces and interfaces at a molecular level 8 has enabled a rapid development in the understanding of surface phenomena. Its versatility allows the exploration of hard and soft materials in vacuum 9-12 , in air 13 , but also in complex liquids 14,15 , often allowing imaging at sub-nanometre and sometimes atomic resolution 16 . In dynamic mode 17 (vibrating cantilever), AFM has proven sensitive to the interfacial compliance of viscous liquids and provided quantitative information about the structure of liquid layers between the AFM tip and the solid surface 18 , with, in some cases, atomic resolution 15 . However, specialized instruments were used and the nature of the tip-sample interaction remains an issue of debate. Dynamic AFM has the ability to probe the solid-liquid interface [18][19][20][21] , but an interpretation of experimental results remains difficult 22,23 . Traditionally, interfaces are characterized by an interfacial energy, IE, the sum of the two surface energies in vacuum minus the work of adhesion (W SL ) necessary to separate the surfaces (Dupré Equation 24 ). The latter is de facto the energy spent to restructure the interface due to the atomistic interaction between the two materials (Fig. 1c). In practice at a solid-liquid interface this is the energy that generates density variations and structuring of the liquid close to the interface. Hereafter we will call this layer of liquid which differs from the bulk
Monolayer-protected metal nanoparticles (MPMNs) are a newly discovered class of nanoparticles with an ordered, striped domain structure that can be readily manipulated by altering the ratio of the hydrophobic to hydrophilic ligands. This property makes them uniquely suited to systematic studies of the role of nanostructuring on biomolecule adsorption, a phenomenon of paramount importance in biomaterials design. In this work, we examine the interaction of the simple, globular protein cytochrome C (Cyt C) with MPMN surfaces using experimental protein assays and computational molecular dynamics simulations. Experimental assays revealed that adsorption of Cyt C generally increased with increasing surface polar ligand content, indicative of the dominance of hydrophilic interactions in Cyt C-MPMN binding. Protein-surface adsorption enthalpies calculated from computational simulations employing rigid-backbone coarse-grained Cyt C and MPMN models indicate a monotonic increase in adsorption enthalpy with respect to MPMN surface polarity. These results are in qualitative agreement with experimental results and suggest that Cyt C does not undergo significant structural disruption upon adsorption to MPMN surfaces. Coarse-grained and atomistic simulations furthermore elucidated the important role of lysine in facilitating Cyt C adsorption to MPMN surfaces. The amphipathic character of the lysine side chain enables it to form close contacts with both polar and nonpolar surface ligands simultaneously, rendering it especially important for interactions with surfaces composed of adjacent nanoscale chemical domains. The importance of these structural characteristics of lysine suggests that proteins may be engineered to specifically interact with nanomaterials by targeted incorporation of unnatural amino acids possessing dual affinity to differing chemical motifs.
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