Photonic Crystal Enhanced SERS Detection of Analytes Separated by Ultrathin Layer Chromatography Using a Diatom Frustule Monolayer

Diatoms are single-celled algae that biologically fabricate nanostructured silica shells with ordered pore arrays called frustules that resemble a 2D photonic crystal. A monolayer of Pinnularia frustules isolated from cell culture is deposited on a glass substrate and then conformally coated with silver nanoparticles (AgNPs) to serve as a nanostructured thin film for ultrathin layer chromatography (UTLC). Malachite green and Nile red are resolved in toluene mobile phase and the separated analytes are profiled micro-Raman spectroscopy, where plasmonic AgNPs provide surface-enhanced Raman scattering (SERS). The AgNP-diatom frustule monolayer improves SERS detection of malachite green by an average factor of 1.8 +/- 0.1 over the plasmonic AgNP layer on glass. Analysis of hot spots on the AgNP-diatom frustule monolayer reveals that nearly 20% of the SERS active area intensifies the SERS signal at least tenfold over the SERS signal for AgNP on glass. Diatom-SERS enhancement is attributed to guided-mode resonances of the Raman laser source, which in turn further enhances the localized surface plasmonic resonance from AgNPs. Overall, the AgNP-diatom frustule monolayer thin film is a new functional material that uniquely enables separation of analytes by UTLC, quantitative SERS detection of separated analytes, and photonic enhancement of the SERS signals.