Vibrational Sum Frequency Generation Spectroscopy Study of Nanoscale to Mesoscale Polarity and Orientation of Crystalline Biopolymers in Natural Materials.

As a nonlinear optical process, sum frequency generation (SFG) requires noncentrosymmetry across multiple length scales, ranging from individual molecular functional groups to their arrangements in space. This principle makes SFG not only intrinsically sensitive to molecular species at surfaces but also useful for studying 3D structures of crystalline biopolymers in natural materials. Examples of such biopolymers are cellulose, starch, and chitin in the polysaccharide family and collagen, silk, and keratin in the fibrous protein family. These biopolymers are noncentrosymmetric at multiple length scales, with chirality at the molecular scale, unit cell structure at the nanoscale, and crystallite orientation and polarity at the mesoscale; thus, they are SFG active. In this review, we describe how SFG can be used to determine nano- to mesoscale polarity and orientational orders of crystalline biopolymers interspersed in natural materials containing the same or similar biopolymers in amorphous states, which cannot be obtained with other characterization methods.