DLC based substrate enabling single molecule detection by Surface Enhanced Raman Spectroscopy (SERS)

Abstract

This work shows a new and surprising application of Diamond-Like Carbon as Surface Enhanced Raman Spectroscopy (SERS) substrate. It was investigated with the well-known Rhodamine 6G (R6G) as SERS test molecule and extended the detection limit to the astonishing attomolar level, which means single molecule detection. The SERS substrate started by depositing excellent quality Diamond-Like Carbon (DLC) on aluminum, followed by laser modification of the DLC in a delimited area that defines the SERS substrate. The laser action gives electrical conductivity between the scratched surface and the aluminum. Silver was electrodeposited on this delimited area. The high Enhancement Factor was around 5 × 10¹² at an R6G concentration of 7 × 10⁻¹⁸ M, observed only at few (and difficult to hit upon) points since the surface density was smaller than 2 R6G molecules/mm². At each of the larger concentrations tested (7 × 10⁻¹⁵, 7 × 10⁻¹², 7 × 10⁻⁹ and 7 × 10⁻⁶ M), the Raman intensities were in the same order of magnitude along the whole substrate, indicating a pretty homogeneous sensitivity. The repeatability among 5 samples tested at 7 × 10⁻¹² M showed a standard deviation of only 18 %. The nano porous structure of the silver deposits, shown by Field Emission Gun Scanning Electron Microscopy (FEG-SEM) appears to be like many other studies with electroplated silver. However, the Raman spectra backgrounds show that amorphous carbon is interacting with the silver nanoparticles. A probable explanation for the superior EF is the synergistic contributions of plasmon enhancement from silver and chemical enhancement from amorphous carbon nanostructure.