A successful detection of inherently weak Raman signal from molecules is possible with giant enhancement of signal by the process of surface-enhanced Raman scattering (SERS). The SERS-induced enhancement is typically achieved when the molecules adsorbed onto the surface of a noble-metal substrate with nanometric roughness. Such SERS-substrate could be economically fabricated by convective assembly of polystyrene beads followed by metal deposition. The
characterization of mono-metallic substrate showed that the SERS enhancement factor increases with increasing
thickness of Ag or Au, with Ag-substrate giving the greatest SERS enhancement. However, the formation of silver oxide
layer could reduce the shelf-life of the Ag-substrate. Alternatively, Au is also used as the coating material owing to its
chemical inertness and biocompatibility. Despite the decent enhancement of the Au-substrate, Au-layer was found to be
unstable after prolonged incubation in crystal violet solution. The inherent deficiency in adhesiveness of Au to the glass
limits its use as a reliable and cost-effective substrate. In an attempt to improve the SERS-substrate, bimetallic substrate
was fabricated by depositing the Au-film, as a protective layer, on the Ag-substrate. In this case, the top layer of Au of
the bimetallic substrate remained intact after chemical treatment. Furthermore, the bimetallic substrate was shown to
give comparable level of enhancement as an Ag-substrate by choosing a proper thickness ratio of the bimetallic layers.
The result suggests that the design of bimetallic substrate could be optimized to maximize the SERS enhancement while
retaining a decent stability after laser illumination and chemical treatment. Our findings suggest that bimetallic substrates
are potentially useful for a reliable SERS-based biosensing.