Size-Controllable Gold Nanopores with High SERS Activity

Nanopore structures have been successfully employed in next-generation DNA sequencing. For more complicated protein which normally contains 20 different amino acids, identifying the fluctuation of ionic current caused by different amino acids appears inadequate for protein sequencing. Therefore, it is highly desirable to develop size-controllable nanopores with optical activity that can provide additional structural information. Herein, we discovered the novel nanopore properties of the self-assembled ultramicroelectrodes originally developed by Bard and co-workers. Using a slightly modified method, the self-assembly of 7 ± 1 nm gold nanoparticles (AuNPs) can be precisely controlled to form a gold nanoporous sphere (GPS) on the tip of a glass capillary. Different dithiol linker molecules (1,3-propanedithiol, C3; 1,6-hexanedithiol, C6; and 1,9-nonanedithiol, C9) reproducibly led to rather similar nanopore sizes (5.07 ± 0.02, 5.13 ± 0.02, and 5.25 ± 0.01 nm), respectively. The GPS nanostructures were found to exhibit high ionic current rectification as well as surface-enhanced Raman scattering (SERS) activity due to the presence of nanopores and numerous "hot spots" among the cross-linked AuNPs on the surface of GPS. The rectification effect of the small nanopores was observed even under high concentration of electrolyte (290 mM), along with SERS enhancement factors well above 1 × 10⁵. The GPS nanostructures were successfully applied for SERS-based detection of glutathione from a single HeLa cell.