Direct Mapping Of Surface-bound Liquid With Sub-nanometer Resolution

Abstract

At the solid-liquid interface molecules of the liquid adopt a particular arrangement which depends primarily on the interfacial energy and geometry. At the nanoscale this molecular arrangement is of central importance in a wide range of fields from molecular biology to surface physics, heterogeneous catalysis and electronics. In biology the role of interfacial liquid is further emphasized by the soft nature of most biomolecules whose conformation and dynamics depends on the surrounding medium. This is the case for protein function and folding (1), self-assembly processes and bio-electronics where a complex interplay between surface-bound liquid molecules and ions strongly affects any motion. Using amplitude modulation atomic force microscopy (AM-AFM) operated in liquid and in a particular regime, it is possible to simultaneously image the topography of the surface-bound liquid while measuring its adhesion energy to the solid investigated. We have used this method to map the binding energy of water to gold nanoparticles coated with mixed ligands, self-assembled in controlled patterns (2, 3). Such functionalized nanoparticles can mimic the typical surface of proteins (hydrophobicity, charge, surface domains) while allowing careful control of the domains’ size and properties (2). Our results show that the average binding energy of water to the surface of the nanoparticles strongly depends on the spatial arrangement of the ligands molecules. The geometry as well as the size of the ligand domains both affect the local adhesion energy of the solvent in a non-linear fashion. Our findings provide experimental and quantitative insight into the inter-play between solvent and surfaces in nanoscale biophysical processes. (1) H. Frauenfelder et al, (2006), Proc. Natl. Acad. Sci. 103, 15469-15472. (2) A. Verma et al, (2008), Nature Mat. 7, 588-595. (3) A. Centrone et al, (2008), Proc. Natl. Acad. Sci. 105, 9886-9891.