Imaging Orientation of a Single Molecular Hierarchical Self-Assembled Sheet: The Combined Power of a Vibrational Sum Frequency Generation Microscopy and Neural Network

Abstract

In this work, we determined the tilt angles of molecular units in hierarchical self-assembled materials on a single-sheet level, which were not available previously. This was achieved by developing a fast line-scanning vibrational sum frequency generation (VSFG) hyperspectral imaging technique in combination with neural network analysis. Rapid VSFG imaging enabled polarization resolved images on a single sheet level to be measured quickly, circumventing technical challenges due to long-term optical instability. The polarization resolved hyperspectral images were then used to extract the supramolecular tilt angle of a self-assembly through a set of spectra-tilt angle relationships which were solved through neural network analysis. This unique combination of both novel techniques offers a new pathway to resolve molecular level structural information on self-assembled materials. Understanding these properties can further drive self-assembly design from a bottom-up approach for applications in biomimetic and drug delivery research.

Multimodal Nonlinear Vibrational Hyperspectral Imaging

“…The inverted hyperspectral VSFG microscope builds on previous designs of a VSFG imaging setup 53 . A SolidWorks sketch of the inverted hyperspectral VSFG microscope can be found in Figure 1.…”

“…To show the proof-of-principle, we focus on FF sample. SDS@2β-CD has been analyzed by us previously 22,29,40,41,53 and the collagen hyperspectra are complicated which we will discuss in future publications. The spectral similarity images of FF for both the -CH and amide spectral regions are summarized in Figure 3.…”

“…Our group and others have developed the VSFG microscopy platform to accept biomimetic and biological materials. 22,29,30,40,41,43,44,46,47,52,53 Briefly, we have implemented a point-scanning VSFG microscope and used it to demonstrate superior phase stability for heterodyne detected VSFG images 41 , and the importance of visualizing local ordering in an overall disordered system. We further used it to resolve hydration levels of self-assembled biomimetic sheets at the single domain level 29 , and even conducted the first ultrafast vibrational dynamics of individual self-assembled sheets and reveal the heterogeneous hydration among sheets 22 .…”

“…We further used it to resolve hydration levels of self-assembled biomimetic sheets at the single domain level 29 , and even conducted the first ultrafast vibrational dynamics of individual self-assembled sheets and reveal the heterogeneous hydration among sheets 22 . Recently, we further developed the point scanning geometry to a fast scanning VSFG microscope that can acquire data 10 times faster than the original point-scanning version 53 . This development reduced optomechanical and electronic instability in the system over the course of data collection.…”

“…Furthermore, the original fast-scanning setup adopted horizontal geometry which limited samples fixed to a substrate. Building off our previous work 53 , we incorporate and report here, an inverted multimodal microscope. The new microscope enabled seamless collection of VSFG, SHG, and brightfield microscopy images.…”