Directly Investigating Ions and their Dynamics at Biological Interfaces: an AFM Perspective

For three decades, atomic force microscopy (AFM) has pushed the limits of nanoscale imaging for researchers from all fields of condensed matter science. The benefits of being able to work on non-conducting samples at room temperature and immersed in fluid lend this technique to the biophysics community, especially with the ability to probe mechanical properties of the sample. Recently, however, advances in resolution have allowed the imaging of single ions adsorbed at solid-liquid interfaces. This allows for examination of molecular-level effects such as Hofmeister-like ion specificity that cannot be understood in the framework of continuum models.

I will broadly introduce the mechanisms behind this Ångström-level resolution as well as a few key examples from the literature, before looking in detail at the case study of alkali metal ions adsorbed to anionic lipid bilayers. The specific interaction between metal ions and cell membranes has long been known to affect membrane stiffness, permeability and the adsorption behaviour of macromolecules such as proteins. We show that the nature of the condensed layer of ions at the surface varies dramatically depending on the size of the cation. This can be explained by the competition between the metal and hydronium (H3O+) ions as they neutralise the negatively-charged surface. We demonstrate correlations between ions in the Stern layer as well as long time-scale evolution of ionic domains. The results have important implications for signal transduction, phospho-lipid synthesis and pH sensing in cells.