Local water diffusivity as a molecular probe of surface hydrophilicity
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Introduction Understanding the interaction of water with biological and soft material surfaces is critical to rationalizing and predicting basic functions and properties of hydrated surfaces.1,2 Binding and adsorption events, the solubility of macromolecules, enzyme catalysis, the overall wettability of macroscopic surfaces, and ion or molecular transport through membrane materials can be modulated or influenced by the degree of attraction of water to—that is, the hydrophilicity of—the macromolecular or material surface. As demonstrated by the intuitive example of a protein whose function disappears upon dehydration and reappears upon rehydration,3 hydrophilicity is a functional property mediated through a proximate water network,4,5 the characteristics of which vary with the influence of this surface chemical property.6,7 If net surface wettability8 is characterized by a common contact angle measurement and extrapolated to net surface hydrophilicity, the lower spatial boundary of this ex situ technique will be the minimum span and volume of a water droplet. However, the interaction between water and a realistic surface varies not just from surface to surface but also from site to site, at length scales encompassing a few water layers from the surface and chemical residues spanning less than 1 nm laterally.9,10
Such variability is not limited to the biological domain: Analogous hydrophilic or hydrophobic regions making up a heterogeneous protein surface landscape11–13 or a soft material surface, such as those of cell membranes11,14,15 and sulfonylgroup-decorated polymer electrolyte fuel cell membranes,16 also display heterogeneous surface hydration dynamics with dramatically varying hydrophilicity characteristics across different chemical moieties, domains, water channels, and polymer backbones. Crucially, this variability can encode functionrelated properties of materials operating in aqueous environments, for example, mediating adsorption, chemical catalysis, or transport.12–15 Thus, to elucidate a potentially functional role of surface water at material and macromolecular interfaces, an experimental approach to probe local surface hydrophilicity at the (sub-) nanometer scale, with high sensitivity, is crucial. However, note that an effective surface hydrophilicity probe, even if site-specifically resolved, incorporates multiple forces into a single measure—the adhesion of water molecules to the surface and the cohesion of water molecules near the surface in particular—as these forces determine, in concert, behaviors such as whether a foreign molecule is likely to approach a binding site protected by a repulsive hydration shell.
Jinsuk Song, Department of Chemistry and Biochemistry, University of California, Santa Barbara, USA; [email protected] Brendan Allison, Department of Chemistry and Biochemistry, University of California, Santa Barbara, USA; [email protected] Songi Han, Department of Chemistry and Biochemistry, University of California, Santa Barbara, USA; [email protected] DOI: 10.1557/mrs.2014.254
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