Characterization of multilayer-enhanced surface-enhanced raman scattering (SERS) substrates and their potential for SERS nanoimaging

Multilayer gold surface-enhanced Raman scattering (SERS) substrates, which consist of continuous gold films that are separated by self-assembled monolayers (SAMs) and cast over 430-nm diameter silica nanospheres on a glass slide, have been evaluated as a means of further enhancing the SERS signals produced from conventional metal film over nanostructure substrates. Evaluation of the effect of various SAMs, with different terminal functional groups, on the SERS enhancement factor were measured and compared to conventional single-layer gold film over nanostructure substrates, revealing relative enhancements as great as 22.4-fold in the case of 2-mercapto-ethanol spacer layers. In addition to evaluation of the effect of different terminal functionalities, the effect of spacer length was also investigated, revealing that the shorter chain length alcohols provided the greatest signals. Employing the optimal SERS multilayer geometry, SERS nanoimaging probes were fabricated and the SERS enhancement factor and variability in enhancement factor were measured over the SERS active imaging area, providing absolute enhancements similar to previous silver-based SERS nanoimaging probes (i.e., 1.2×108). Varying the size of the multilayer gold islands that were deposited on the tip of the SERS active nanoimaging probe, it is possible to tune the optimal SERS excitation wavelength accurately and predictably over the range of approximately 450 to 600 nm, without coating the entire surface of the probe and significantly reducing the transmission and resulting signal-to-noise ratio of the images obtained.