Reproducible single-molecule optofluidic-SERS analysis on nanostar-activated diatom biosilica capsules

Abstract

Single-molecule (SM) analysis with surface-enhanced Raman scattering (SERS) usually requires sophisticated engineering on plasmonic nanostructures to provide extremely strong hot-spots to enhance the intrinsically weak signal. In this work, we report a cell-sized biosilica capsule based on the diatom Pinnularia sp. frustule decorated with high-density bimetallic nanostars consisting of gold and silver, capable of achieving reproducible single-molecule SERS analysis. Such a plasmonic nanostar-activated biological photonic crystal nanostructure synthesized by a simple self-assembly method enhances hot-spots universally, which is proved by finite difference time domain simulation. The cell-sized biosilica capsule also concentrates trace-levels of target molecules from ultra-small fluidic droplets through a drop-on-demand inkjet-printing technique. We experimentally demonstrated reproducible SM optofluidic-SERS sensing from 120 nL of 10⁻¹⁵ M rhodamine 6G solution containing only 72 molecules through statistical analysis of mapping data over the cell-sized biosilica capsule, achieving a 3× higher signal-to-noise ratio and 9× better SM detection possibility compared to that on the glass substrate.