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    Partial Leidenfrost Evaporation-Assisted Ultrasensitive Surface-Enhanced Raman Spectroscopy in a Janus Water Droplet on Hierarchical Plasmonic Micro-/Nanostructures

    Year: 2020

    Journal: ACS Nano, Volume 14, AUG 25, page 9521–9531

    Authors: Song, Junyeob; Cheng, Weifeng; Nie, Meitong; He, Xukun; Nam, Wonil; Cheng, Jiangtao; Zhou, Wei

    Organizations: U.S. National Science Foundation (NSF)National Science Foundation (NSF) [ECCS 1808931, CBET 1550299]; Air Force Office of Scientific Research (AFOSR) Young Investigator AwardUnited States Department of DefenseAir Force Office of Scientific Research (AFOSR) [FA9550-18-1-0328]; National Institute of Standards and Technology (NIST)National Institute of Standards & Technology (NIST) - USA [70NANB18H201]

    Keywords: surface-enhanced Raman scattering; hierarchical plasmonic micro-/nanostructures; Leidenfrost evaporation; ultrasensitive biochemical detection; surface wettability control by heating

    The conventional methods of creating super-hydrophobic 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. In this paper, we report a partial Leidenfrost evaporation-assisted approach for ultrasensitive SERS detection of low-concentration analytes in water droplets on hierarchical plasmonic micro-/ nanostructures, which are fabricated by integrating nano-laminated metal nanoantennas on carbon nanotube (CNT)-decorated Si micropillar arrays. In comparison with natural evaporation, partial Leidenfrost-assisted evaporation on the hierarchical surfaces can provide a levitating force to maintain the water-based analyte droplet in the Cassie-Wenzel hybrid state, i.e., a Janus droplet. By overcoming the diffusion limit in SERS measurements, the continuous shrinking circumferential rim of the droplet, which is in the Cassie state, toward 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. Here, we demonstrate that a partial Leidenfrost droplet on the hierarchical plasmonic surfaces can reduce the final deposition footprint of analytes by 3-4 orders of magnitude and enable SERS detection of nanomolar analytes (10(-9) M) in an aqueous solution. In particular, this type of hierarchical plasmonic surface has densely packed plasmonic hot spots with SERS enhancement factors (EFs) exceeding 10(7). Partial Leidenfrost evaporation-assisted SERS sensing on hierarchical plasmonic micro-/nanostructures provides a fast and ultrasensitive biochemical detection strategy without the need for additional surface modifications and chemical treatments.
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