Surface-enhanced Raman spectroscopy (SERS) has emerged as an ultrasensitive molecular-fingerprint-based technique for label-free biochemical analysis of biological systems. However, for conventional SERS substrates, SERS enhancement factors (EFs) strongly depend on background refractive index (RI), which prevents reliable spatiotemporal SERS analysis of living cells consisting of different extra-/intracellular organelles with a heterogeneous distribution of local RI values between 1.30 and 1.60. Here, we demonstrate that nanolaminated SERS substrates can support uniform arrays of vertically oriented nanogap hot spots with large SERS EFs (>107) insensitive to background RI variations. Experimental and numerical studies reveal that the observed RI-insensitive SERS response is due to the broadband multiresonant optical properties of nanolaminated plasmonic nanostructures. As a proof-of-concept demonstration, we use RI-insensitive nanolaminated SERS substrates to achieve label-free Raman profiling and classification of living cancer cells with a high prediction accuracy of 96%. We envision that RI-insensitive high-performance nanolaminated SERS substrates can potentially enable label-free spatiotemporal biochemical analysis of living biological systems.

Ultrasensitive surface-enhanced Raman spectroscopy (SERS) still faces difficulties in quantitative analysis because of its susceptibility to local optical field variations at plasmonic hotspots in metallo-dielectric nanostructures. Current SERS calibration approaches using Raman tags have inherent limitations due to spatial occupation competition with analyte molecules, spectral interference with analyte Raman peaks, and photodegradation. Herein, we report that plasmon-enhanced electronic Raman scattering (ERS) signals from metal can serve as an internal standard for spatial and temporal calibration of molecular Raman scattering (MRS) signals from analyte molecules at the same hotspots, enabling rigorous quantitative SERS analysis. We observe a linear dependence between ERS and MRS signal intensities upon spatial and temporal variations of excitation optical fields, manifesting the |E|4 enhancements for both ERS and MRS processes at the same hotspots in agreement with our theoretical prediction. Furthermore, we find that the ERS calibration’s performance limit can result from orientation variations of analyte molecules at hotspots.

can support gap plasmon modes with much tighter optical field confinement in nanogaps, [5] and thus can produce much higher SERS EFs theoretically up to 10 10 . [6] Experiments have demonstrated that plasmonic nanogaps based on metal nanoparticle aggregations possess single molecule SERS detection capabilities. [7] However, the hotspots in bottom-up assembled nanoparticle aggregations show a random and uncontrollable distribution because of the extremely sensitive dependence of gap plasmon resonances on nanogap dimensions. Calculations have illustrated that SERS enhancement factors for coupled metal nanostructures can change by five orders of magnitude with a nanogap variation less than 10 nm. [8] Toward controlled engineering of plasmonic nanogap hotspots, significant efforts have been made to create highperformance uniform SERS substrates using scalable nanofabrication strategies. Based on the orientation of plasmonic nanogaps, SERS substrates can be further classified into two types: 1) horizontally oriented nanogap structures where the dominant electric field is parallel to the substrate surface and 2) vertically oriented nanogap structures where the dominant electric field is perpendicular to the substrate surface. Since it is more straightforward to create horizontally oriented plasmonic nanogaps by modifying lithography defined in-plane nanopatterns, most top-down fabricated SERS substrates exploit horizontally oriented nanogap hotspots typically with SERS EFs on the order of 10 6 -10 7 . [9][10][11][12][13] However, for sub-10 nm control of horizontally oriented plasmonic nanogaps, most of these works require the use of nonstandard fabrication techniques to go beyond the nanolithography resolution limit, such as microcapillary forces driven nanogap generation by self-coalescence of nanopillar arrays, [9] ion-milling or adhesive-stripping assisted exposure of atomic layer deposition defined nanogaps, [10] self-aligned generation of nanogaps by off-angle shadow deposition through nanomasks, [11] and thermal annealing-assisted nanogap generation by forming metal nanoislands between nanopillar arrays. [12] Therefore, despite significant improvement, current scalable SERS substrates by engineering horizontally oriented plasmonic nanogaps still face challenges in relatively high fabrication costs and limited spectral Metallic nanogap structures can support gap surface plasmon modes and strongly concentrate optical fields to enable surface-enhanced Raman spectroscopy (SERS) for label-free biochemical analysis down to single molecule level. However, current scalable SERS substrates based on horizontally oriented plasmonic nanogaps still face challenges for accurate sub-10 nm control of in-plane nanostructures. Here, we report a new type of scalable high-performance SERS substrate based on multistack vertically oriented nanogap hotspots in metal-insulator-metal nanolaminated plasmonic crystals. In contrast to horizontally oriented nanogaps, vertically oriented plasmonic nanogaps can be controlled at subnanometer...

Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful tool for ultrasensitive fingerprint recognition of molecules with considerable potential in wearable biochemical sensing. However, previous efforts to fabricate wearable SERS devices by directly treating fabrics with plasmonic nanoparticles have generated a nonuniform assembly of nanoparticles, weakly adsorbed on fabrics via van der Waals forces. Here, we report the creation of washing reusable SERS membranes and textiles via template-assisted self-assembly and micro/nanoimprinting approaches. Uniquely, we employ the capillary force driven self-assembly process to generate micropatch arrays of Au nanoparticle (NP) aggregates within hydrophobic microstructured templates, which are then robustly bonded onto semipermeable transparent membranes and stretchable textiles using the UV-resist based micro/nanoimprinting technique. A mild reactive ion etching (RIE) treatment of SERS membranes and textiles can physically expose the SERS hotspots of Au NP-aggregates embedded within the polymer UV resist for further improvement of their SERS performance. Also, we demonstrate that the semipermeable transparent SERS membranes can keep the moisture content of meat from evaporating to enable stable in situ SERS monitoring of biochemical environments at the fresh meat surface. By contrast, stretchable SERS textiles can allow the spreading, soaking, and evaporation of solution analyte samples on the fabric matrix for continuous enrichment of analyte molecules at the hotspots in biochemical SERS detection. Due to the mechanical robustness of the UV-resist immobilized Au NP aggregates, simple detergentwater washing with ultrasound sonication or mechanical stirring can noninvasively clean contaminated hot spots to reuse SERS textiles. Therefore, we envision that washing reusable SERS membranes and textiles by template-assisted self-assembly and micro/ nanoimprinting fabrication are promising for wearable biochemical sensing applications, such as wound monitoring and body fluid monitoring.

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.