Single-protein dynamics revealed by meta-surface-enhanced Raman spectroscopy

Probing single-protein dynamics at the molecular level is crucial for understanding conformational changes and functional mechanisms. Surface-enhanced Raman spectroscopy (SERS) offers a promising label-free approach to study protein. However, traditional SERS substrates face challenges due to the large size of proteins that are too large to fit in conventional hotspots. Here, we demonstrate that coupled plasmonic nanocavities enhance the Raman scattering up to 108 in an unconfined area. We harness the enhanced fields to reveal the dynamics of a single protein by observing time-dependent changes in the Raman spectrum, which is a unique probe of the secondary structure of the protein. We study the effect of the pH and the surface charge of the substrate on the conformation of a single protein of bovine serum albumin (BSA). The dominant secondary structure of BSA is α-helix at pH 7, while at pH 3 and 10, more β sheet and random structure are observed. We use principal component analysis to classify the proteins on the basis of their secondary structure. This work establishes a new possibility of studying large biomolecules and proteins crucial for biomedical applications and understanding the biological functions of proteins and the origin of diseases.