Mesoscopic Ordering of Water Dynamics in Self-Assembled Materials Revealed by Transient VSFG Microscopy

Abstract

<b>We report, for the first time, observations of mesoscopically homogeneous but macroscopically heterogenous water dynamics in self-assembled materials by a new, spatially resolved infrared (IR) pump vibrational sum frequency generation (VSFG) probe microscope. Using this new technique, we spatially resolved dynamics of water bounded by host-guest, self-assembled sheets comprised of sodium dodecyl sulfate (SDS) and β-cyclodextrin (β-CD). We found that the strong hydrogen-bond interactions between β-CD and nearby water not only template nearby water networks to adopt the chirality of β-CD, but also allow resonant energy transfer from β-CD to nearby water. More interestingly, the resonant energy transfer dynamics are heterogeneous among domains, while remaining uniform within domains. This surprising result indicates that the water near self-assembled materials can be templated uniformly across micron domains. Because SDS@2β-CD is a synthetic analogue that parallels the morphology, rigidity and crystallinity of protein assemblies, similar mesoscopic ordering of water structure and dynamics could also exist in biological soft materials. The advancement of adding spatial resolution to ultrafast molecular vibrational spectroscopy opens a new way to probe mesoscopic molecular structure ordering and relaxation dynamics in biological systems, and hydro-responsive self-assembly materials for micro-optics and electronics. </b>