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. 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 nanolaminated 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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Metallic nano-optoelectrode arrays can simultaneously serve as nanoelectrodes to increase the electrochemical surface-to-volume ratio for high-performance electrical recording and optical nanoantennas to achieve nanoscale light concentrations for ultrasensitive optical sensing. However, it remains a challenge to integrate nano-optoelectrodes with a miniaturized multifunctional probing system for combined electrical recording and optical biosensing in vivo. Here, we report that flexible nano-optoelectrode-integrated multifunctional fiber probes can have hybrid optical–electrical sensing multimodalities, including optical refractive index sensing, surface-enhanced Raman spectroscopy, and electrophysiological recording. By physical vapor deposition of thin metal films through free-standing masks of nanohole arrays, we exploit a scalable nanofabrication process to create nano-optoelectrode arrays on the tips of flexible multifunctional fiber probes. We envision that the development of flexible nano-optoelectrode-integrated multifunctional fiber probes can open significant opportunities by allowing for multimodal monitoring of brain activities with combined capabilities for simultaneous electrical neural recording and optical biochemical sensing at the single-cell level.

Surface-enhanced Raman spectroscopy (SERS) has become a sensitive detection technique for biochemical analysis. Despite significant research efforts, most SERS substrates consisting of single-resonant plasmonic nanostructures on the planar surface suffer from limitations of narrowband SERS operation and unoptimized nano-bio interface with living cells. Here, it is reported that nanolaminate plasmonic nanocavities on 3D vertical nanopillar arrays can support a broadband SERS operation with large enhancement factors (>10 6 ) under laser excitations at 532, 633, and 785 nm. The multi-band Raman mapping measurements show that nanolaminate plasmonic nanocavities on vertical nanopillar arrays exhibit broadband uniform SERS performance with diffraction-limited resolution at a single nanopillar footprint. By selective exposure of embedded plasmonic hotspots in individual metal-insulatormetal (MIM) nanogaps, nanoscale broadband SERS operation at the single MIM nanocavity level with visible and near-infrared (vis-NIR) excitations is demonstrated. Numerical studies reveal that nanolaminate plasmonic nanocavities on vertical nanopillars can support multiple hybridized plasmonic modes to concentrate optical fields across a broadband wavelength range from 500 to 900 nm at the nanoscale.