Partial Leidenfrost Evaporation-Assisted Fast Solute Enrichment in a Water Droplet on Hierarchical Micro/Nanostructures

The conventional methods of creating superhydrophobic surface-enhanced Raman spectroscopy (SERS) devices are by conformally coating a nanolayer of hydrophobic materials on micro/nanostructured plasmonic substrates. However, the hydrophobic coating may partially block hot spots and therefore compromise Raman signals of analytes. Here, we report partial Leidenfrost evaporation-assisted fast enrichment of low-concentration analytes in water droplets on hierarchical plasmonic micro/nanostructures, which are fabricated by implanting nanoantennas on carbon nanotubes-decorated Si micropillar arrays. In comparison with natural evaporation, partial Leidenfrost-assisted evaporation provides a levitating force to maintain the analyte droplet in the Cassie-Wenzel hybrid state. In this process, the continuous shrinking circumferential rim of the droplet, which is in the Cassie state, towards the pinned central region of the droplet, which is in the Wenzel state, results in a fast concentration of dilute analyte molecules on a significantly reduced footprint within several minutes. Partial Leidenfrost droplet can reduce the final deposition footprint by 3-4 orders of magnitude and enable ultrasensitive detection of nanomolar analytes.