Understanding Mesoscale Structures of Nanocrystalline Domains in Silk using Sum Frequency Generation Vibrational Spectroscopy

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With support from the Chemical Measurement and Imaging (CMI) program in the Division of Chemistry (CHE), and co-funding from the Biomaterials Program (BMAT) in the Division of Materials Research (DMR), Dr. Seong Kim and his team at Penn State University are developing powerful probes of the way living organisms assemble water-soluble materials to produce insoluble semi-crystalline polymers to serve various mechanical functions such as structural support and energy storage. Their work aims to advance current understanding of how structural order is exploited by living organisms to tailor mechanical properties of crystalline biopolymers like those found in silk fibers. The approach will utilize sophisticated spectroscopic tools to quantitatively probe structural order and assembly inside natural silk. Students from underrepresented groups, including first-generation college students, will be involved in these modern spectroscopic studies, working in close collaboration with Dr. Kim and his graduate students. The research motivations and methodologies will be integrated into a graduate-level materials characterization course. Outreach activities associated with the Frost Entomological Museum at Penn State will help familiarize the general public with the science of materials produced by arthropods The main characterization method of this work is vibrational sum-frequency-generation (SFG) spectroscopy - a nonlinear optical process originating from noncentrosymmetry. While SFG is typically considered to be a surface-sensitive tool, it can also distinguish noncentrosymmetric arrangements of molecules from their disordered surroundings. While potentially useful as a probe of nano- to meso-scale order of noncentrosymmetric crystals dispersed in an amorphous polymer matrix (e.g., beta-sheet domains in silk fibers), the challenges encountered in the analysis of multi-scale three-dimensional structures are far beyond the complexity of two-dimensional surfaces. To address this obstacle, the Kim group is working to: (i) establish the SFG spectral interpretation of beta-sheet moieties prevalent in natural silk fibers; (ii) decipher mesoscale three-dimensional order of crystalline beta-sheets in silk; and (iii) conduct operando SFG analysis of silk fibers under stress-free and tensile strain states in controlled humidity conditions. The operando SFG methodology and knowledge established from this study will likely be applicable to studies of other natural biopolymers such as chitin, collagen and elastin as well as biomimetic materials, highlighting the potential for broad scientific impact of this work.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.