Nanoscale Chemical Imaging of Amyloid Fibrils in Water Using Total-Internal-Reflection Tip-Enhanced Raman Spectroscopy.

Total-internal-reflection tip-enhanced Raman spectroscopy (TIR-TERS) imaging of amyloid-β (Aβ _1-42 -L34T) fibrils is performed with nanoscale spatial resolution in water, using TERS tips fabricated by bipolar electrodeposition. Ideal experimental parameters are corroborated by both theoretical simulations and TIR-TERS measurements. TIR-TERS imaging reveals the predominant parallel β-sheet secondary structure of Aβ _1-42 -L34T fibrils as well as the nanoscale spatial distribution of tyrosine, histidine, and phenylalanine aromatic amino acids. Their proportion in TERS spectra can be qualitatively explained by the combined effect of their localization in the Aβ _1-42 -L34T fibril structure and their molecular orientation with respect to the excitation laser light polarization. Conclusions drawn from the TERS experiments in water corroborate and significantly enrich our previous study in ambient air, thus confirming that hydration has only a marginal impact on the structure of such amyloid fibrils. This first TIR-TERS study in liquid opens fascinating perspectives for future applications in biology.