Interfacial charge transfer of amyloid-β proteins with graphene examined using Raman spectroscopy

The abnormal aggregation of amyloid-β (Aβ) protein is associated with many human diseases, including Alzheimer’s disease. Therefore, for early diagnosis and treatment, methods to identify the Aβ aggregation stages – monomer, oligomer, fibril – has been rigorously investigated. Previously, Aβ states were determined based on their structural properties, namely their morphologies and vibrational modes. However, there is little known about the electronic properties of Aβ proteins.

In this work, we investigate the electronic states of Aβ proteins at different fibrillization stages. We achieve this by monitoring the interfacial charge transfer between Aβ proteins and monolayer graphene using Raman spectroscopy. Graphene, an atomically thin semimetal, reacts sensitively to charge-transfer-induced doping and the shifts in the carrier concentrations and in the Fermi levels can be distinguished from the characteristic Raman bands in graphene (G and 2D). In particular, we found that graphene was p-doped by monomers while n-doped by fibrils, indicating that the electronic energy levels of Aβs shift as they aggregate. This charge-transfer polarity between the monomer and the fibril states provides critical insights into the electronic properties of Aβs that are essential to identifying the onset of neurotoxic Aβ fibril formation and developing a rapid, label-free diagnosis protocol based on two-dimensional materials.