Probing Interfacial Charge Transfer between Amyloid-β and Graphene during Amyloid Fibrillization Using Raman Spectroscopy

Charge transfer plays a key role in the structural transformation of amyloid-β proteins (Aβs), as it fibrillizes from small monomers to intermediate oligomers and to ordered fibrils. While the protein fibrillization states have been identified using cryo-electron microscopy, X-ray diffraction, Raman, infrared, terahertz spectroscopies, etc., there is little known about the electronic states during the fibrilization of Aβ protein. Here, we probe the charge transfer of Aβ 42 proteins at different aggregation stages adsorbed on monolayer graphene (Gr) and molybdenum disulfide (MoS 2 ) using Raman spectroscopy. Monomers, oligomers, and fibrils prepared in buffer solutions were deposited and dried separately on Gr and MoS 2 where well-established characteristic Raman modes (G, 2D for Gr and E 2g, A 1g for MoS 2 ) were monitored. The shifts in Raman parameters showed that the small Aβ monomers withdraw electrons, whereas fibrils donate electrons to Gr and MoS 2 . Oligomers undergo transient charge states near the neutrality point. This is explained in terms of modulated carrier concentration in Gr and MoS 2 . This finding provides insight into the electronic properties of Aβs that could be essential to identifying the onset of toxic fibril forms and developing a straightforward, label-free diagnosis using Gr and MoS 2 .