Graphene Nanodots as Substrates for SEIRAS and SERS Studies on Membrane Proteins

Graphene nanostructures are capable of supporting plasmonic resonances in the visible and infrared parts of the spectrum. Thus, they can be exploited as platforms for Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS) and Surface-Enhanced Raman Spectroscopy (SERS) studies. One application of SEIRAS and SERS is the study of proteins at very low concentrations, down to the picomolar range. Among the different forms of graphene, graphene nanodots are ideal nanostructures that can be produced on a large scale using established protocols relying on sonication-assisted exfoliation under specific experimental conditions. Their rich surface chemistry facilitates stable and nondenaturing adsorption of membrane proteins, ensuring preservation of their native secondary structure upon immobilization. In this study, we exploited graphene nanodots deposited by drop casting or spray coating onto a silicon wafer as a substrate to study the cytochrome <i>bd-</i>I oxidase from<i>E. coli</i>, a membrane protein that is present in the respiratory chains of bacteria. The amide I signal was examined to confirm the structural integrity of the protein once immobilized onto the graphene nanodots. SEIRAS and SERS experiments revealed reproducible enhancement of the protein signal, approximately by a factor of 2 and 6-10 compared to other substrates, respectively, enabling analyte detection with a sensitivity down to the nanomolar range. Furthermore, our tailored substrate exhibited high stability of the protein exceeding 6 days, thus underscoring its high potential for biosensing.