Graphene-based flexible SERS substrates with Ag nanohole structures for biochemical sensing

Flexible substrates for surface-enhanced Raman spectroscopy (SERS) are essential for practical applications, such as point-of-care testing, wearable sensing, and environmental analysis that require high sensitivity, portability, and mechanical flexibility to fit curved surfaces. To meet these demands, we developed flexible SERS substrates consisting of graphene on thin mica with Ag nanohole structures. The biosensing capability of the substrates was demonstrated using a Raman-based enzyme-linked immunosorbent assay, in which bovine interleukin-6 was detected at concentrations as low as 50 pg/mL with a limit of detection of 6 pg/mL. The spectroscopic properties of the substrates were primarily characterized by using Raman measurements, along with scanning electron microscopy and atomic force microscopy. Raman analyses indicated that graphene contributed to the enhancement through fluorescence quenching and increased analyte adsorption associated with its hydrophobic surface. For chemical sensing of Rhodamine 6 G, we evaluated the geometrical enhancement effect of the Ag nanohole structures on mica under different Raman excitation configurations. Far-side illumination through the mica substrate increased the Raman intensity about twofold compared with near-side excitation. These results show the potential of graphene-based flexible SERS substrates for practical, high-sensitivity biochemical sensing.