Raman spectroscopy with a microfluidic device embedded with plasmonic metasurface.

Metasurfaces offer a powerful tool to realize label-free and highly sensitive Raman spectroscopy. Embedding metasurfaces into microfluidic channels is promising to establish a new characterizing platform for microfluids. In this Letter, we present a highly stable method for improving the Raman scattering intensity of biological microfluids by using a microfluidic chip embedded with a plasmonic metasurface. The embedded metasurface consists of a nanosphere array coated with a silver layer, where the diameter of the nanosphere is ∼100 nm. The Langmuir-Blodgett method and a chemical spraying method were adopted to prepare the nanosphere-array metasurface. In the case of red blood cell measurement, a giant enhancement of Raman spectra intensity is achieved with a metasurface compared to that without a metasurface. Moreover, a two-time enhancement of Raman spectra intensity is obtained with a metasurface under radially polarized beam illumination compared to linearly polarized beam illumination. Furthermore, a microfluidic device embedded with a plasmonic metasurface was applied to monitor the environmental variation of rat red blood cells. Peaks in the range from 2143 cm^-1 to 2303 cm^-1 arise with the addition of glucose and are still obviously distinguishable when the additive concentration is down to 10^-3 M. This indicates high sensitivity to the concentration of glucose mixed with rat red blood cells, which could be further applied to monitor biological cell environments such as glucose concentration, pH, and sodium salt concentration.