Development of Au/Ag substrate with alternating nanosphere array for SERS-based biosensing

Abstract

Nanostructure substrates are effective biosensor to spectrally differentiate multiple compounds by Surface-enhanced Raman scattering (SERS). Metal film over nanosphere (MFON) has been demonstrated to exhibit reproducible and predictable Raman enhancement. MFON can be fabricated using an economical process in which polystyrene (PS) nanospheres are self-assembled on a planar solid supports and then followed by metal coating. In this work, we investigate the MFON substrates with bimetallic coating to combine the optical-enhancing and stability features from Ag and Au layers. The SERS responses are then quantified from the resultant bimetallic structures with 2-Naphthalenethiol. We show that the bimetallic substrate of optimal Au/Ag thickness ratio renders SERS enhancement and stability exceeding those of the Au-coated MFON. Compared to Au array, the bimetallic substrate exhibits quasi-bimetallic nanoparticles of surpassing SERS (2.5 times) with enhancement factor determined to be 2×10⁷. As a proof-of-concept for biosensing in microfluidics, SERS nanotag was prepared and tested on the optimized BMFON. In addition, we propose a fabrication scheme to construct MFON with alternating sizes (100nm and 400nm) of nanosphere. At optimal proportional amount, the 100nm-spheres were packed within the gaps between the 400nm-spheres. The resultant morphology renders additional nanogaps that could possibly lead to increment in SERS enhancement.