Large Format Surface-Enhanced Raman Spectroscopy Substrate Optimized for Enhancement and Uniformity

Evolutionary Optimized, Monocrystalline Gold Double Wire Gratings as a Novel SERS Sensing Platform

Achieving reliable and quantifiable performance in large-area surface-enhanced Raman spectroscopy (SERS) substrates poses a formidable challenge, demanding signal enhancement while ensuring response uniformity and reproducibility. Conventional SERS substrates often made of inhomogeneous materials with random resonator geometries, resulting in multiple or broadened plasmonic resonances, undesired absorptive losses, and uneven field enhancement. These limitations hamper reproducibility, making it difficult to conduct comparative studies with high sensitivity. This study introduces an innovative approach that addresses these challenges by utilizing monocrystalline gold flakes to fabricate well-defined plasmonic double-wire resonators through focused ion-beam lithography. Inspired by biological strategy, the double-wire grating substrate (DWGS) geometry is evolutionarily optimized to maximize the SERS signal by enhancing both excitation and emission processes. The use of monocrystalline material minimizes absorption losses and ensures shape fidelity during nanofabrication. DWGS demonstrates notable reproducibility (RSD = 6.6%), repeatability (RSD = 5.6%), and large-area homogeneity > 104 µm2. It provides a SERS enhancement for sub-monolayer coverage detection of 4-Aminothiophenol analyte. Furthermore, DWGS demonstrates reusability, long-term stability on the shelf, and sustained analyte signal stability over time. Validation with diverse analytes, across different states of matter, including biological macromolecules, confirms the sensitive and reproducible nature of DWGSs, thereby establishing them as a promising platform for future sensing applications.

Significantly increased Raman enhancement enabled by hot-electron-injection-induced synergistic resonances on anisotropic ReS2 films

Two-dimensional (2D) materials are an excellent platform for surface-enhanced Raman spectroscopy (SERS). However, a poor detection sensitivity hinders their practical application. Exciton resonance (μ_ex) can improve SERS significantly by lending intensity to nearby charge-transfer resonance. Coincidentally, for ReS₂, the enhanced μ_ex can be achieved through the injection of excited-state electrons which can adjust the energy band to the SERS detection range. Moreover, ReS₂ has strong anisotropic properties, which adds an additional dimension for SERS. Therefore, ReS₂ is an ideal candidate to realize highly sensitive anisotropic SERS. In this paper, the metallic T phase of ReS₂ is introduced to the semiconducting Td phase by phase engineering. The photoinduced electron tunneling from the T phase to the Td phase can tune exciton emissions to the visible region, which effectively facilitates the photoinduced charge transfer processes. With RhB as the probe molecule, the synergistic resonance effects improve the limit of detection to 10^-9 M with the enhancement factor up to about 10⁸. Meanwhile, the obtained ultrasensitive SERS substrates also show good uniformity, stability as well as unique anisotropy. Our results open a new perspective in the improvement of the SERS performance.

Large-area metal-dielectric heterostructures for surface-enhanced raman scattering

Metal-dielectric heterostructures have shown great application potentials in physics, chemistry and material science. In this work, we have designed and manufactured ordered metal-dielectric multiple heterostructures with tunable optical properties, which can be as large as the order of square centimeters in size. We experimentally realized that the surface-enhanced Raman scattering signal of the periodic multiple heterostructures increased 50 times compared with the silicon nanodisk-gold film arrays, which is attributed to the large-scale hotspots and high efficient coupling between the optical cavities and surface plasmon resonance modes. More importantly, the substrate also features a good uniformity and an excellent reproducible fabrication, which is very promising for practical applications.

Directly Convert Carbonaceous Microspheres to Three-Dimensional Porous Carbon Microspheres with a Robust Self-Supporting Structure as a Metal-Free SERS Substrate for Online High-Throughput Analysis

It is of great significance for practical applications to directly convert readily available biomass carbon into three-dimensional (3D) porous carbon microspheres with a self-supporting structure. Herein, we report the convenient conversion of biomass carbon microspheres to hierarchical porous carbon microspheres (HP-CMSs) with a robust self-supporting framework structure. A general SiO₂-induced etching mechanism is proposed for the formation of the HP-CMSs. Benefiting from this robust 3D self-supporting frame structure, these HP-CMSs have outstanding mechanical, chemical, and thermal stability. As a metal-free surface-enhanced Raman scattering (SERS) substrate with an ultrahigh specific surface area (4216 m² g^-1) and a high density of active sites, the HP-CMSs exhibit high sensitivity with a detection limit of 10^-10 M and a Raman enhancement factor of 3.5 × 10⁶. By integrating the enrichment and sensing functions of the HP-CMSs in a microfluidic channel, online high-throughput SERS detection of 20 samples within 5 min is achieved in a single channel, and the relative standard deviation of the signals between samples is only 5.1%. The current work develops a convenient preparation method that converts sustainable biomass carbon to 3D hierarchical porous carbon and provides a potential material for sensing, energy, catalysis, and other fields.

Fiber Optic Plasmonic Sensors Based on Nanodome Arrays with Nanogaps

In this study, a high-performance fiber optic surface plasmon resonance (FO-SPR) sensor using a dome array with nanogaps (DANG) is proposed for label-free real-time detection of biomolecules. A novel and simple method using polymer beads enables high sensitivity by allowing hotspots with nanometer spacing between the Au dome and the surrounding film. The nanodome structure, which comprises a polymer core and a Au shell, induces a localized surface plasmon, expands the sensing area, and extensively enhances the electromagnetic field. The refractive index sensitivity of the FO-SPR sensor with nanostructures, i.e., with nanogaps and nanodomes, was found to be 7.8 times higher than that of the FO-SPR sensor without nanostructures. The proposed sensor achieved a low detection limit of 38 fg/mL while quantifying thyroglobulin antibody-antigen interactions and exhibited excellent selectivity. In addition, it helped detect serum samples with a 103% recovery rate.

Surface-enhanced Raman Scattering of Au-Ag bimetallic nanopillars fabricated using surface-plasmon lithography

Arrays of gold-silver (Au-Ag) bimetallic nanopillars were fabricated by a newly developed surface-plasmon lithography (SPL) and their enhancement properties as surface-enhanced Raman scattering (SERS) substrates have been studied. We demonstrated that the SPL is a low-cost and high efficiency method for the fabrication of SERS substrates with both high sensitivity and reproducibility. The nanopillars showed a good response in the detection of methylene blue molecules at a low concentration of 1.0 × 10^-11mol· l^-1. The SERS enhancement factors (EFs) are on the orders of 10⁷and the relative standard deviation of SERS intensity is <8% over an area of 50μm × 50μm. The EFs increase fast with the height increasing from 200 to 530 nm, then increase slowly when further increase the height of the nanopillars to 1100 nm. In addition, the Au-Ag bimetallic coating has shown much higher SERS enhancement than the coatings of either the pure Au or Ag. The excellent SERS enhancement and reproducibility of the Au-Ag coated nanopillars indicated that the fabricated SERS substrates can be used for the detection of biochemical molecules at trace level and the SPL is a promising method for fabrication of SERS substrates.

Large-Area Nanosphere Self-Assembly Monolayers for Periodic Surface Nanostructures with Ultrasensitive and Spatially Uniform SERS Sensing

Colloidal lithography provides a rapid and low-cost approach to construct 2D periodic surface nanostructures. However, an impressive demonstration to prepare large-area colloidal template is still missing. Here, a high-efficient and flexible technique is proposed to fabricate self-assembly monolayers consisting of orderly-packed polystyrene spheres at air/water interface via ultrasonic spray. This "non-contact" technique exhibits great advantages in terms of scalability and adaptability due to its renitent interface dynamic balance. More importantly, this technique is not only competent for self-assembly of single-sized polystyrene spheres, but also for binary polystyrene spheres, completely reversing the current hard situation of preparing large-area self-assembly monolayers. As a representative application, hexagonal-packed silver-coated silicon nanorods array (Si-NRs@Ag) is developed as an ultrasensitive surface-enhanced Raman scattering (SERS) substrate with very low limit-of-detection for selective detection of explosive 2,4,6-trinitrotoluene down to femtomolar (10^-14 m) range. The periodicity and orderliness of the array allow hot spots to be designed and constructed in a homogeneous fashion, resulting in an incomparable uniformity and reproducibility of Raman signals. All these excellent properties come from the Si-NRs@Ag substrate based on the ordered structure, open surface, and wide-range electric field, providing a robust, consistent, and tunable platform for molecule trapping and SERS sensing for a wide range of organic molecules.

Novel Clarification of Surface Plasmon Coupling Reactions of Aromatic Alkynamine and Nitro Compounds

This work presents a theoretical and experimental approach for the coupling of 4-ethynylaniline (4-APA) and 4-ethynylnitrobenzene (4-NPA) in the theoretical application of density functional theory (DFT) and experimental monitoring of surface-enhanced Raman spectroscopy (SERS). The results support electromagnetic enhancement to drive the conversion of aromatic alkynamine and nitro compounds and regulation by the catalytic coupling reaction conditions. In addition, this work investigates the adsorption site effect of surface plasmon coupling reactions of 4-APA and 4-NPA molecules into alkynyl azo compounds. This study presents theoretical and experimental images used to analyze the plasmon-driven surface catalytic reaction system.

Metallic Plasmonic Array Structures: Principles, Fabrications, Properties, and Applications

The vast development of nanofabrication has spurred recent progress for the manipulation of light down to a region much smaller than the wavelength. Metallic plasmonic array structures are demonstrated to be the most powerful platform to realize controllable light-matter interactions and have found wide applications due to their rich and tunable optical performance through the morphology and parameter engineering. Here, various light-management mechanisms that may exist on metallic plasmonic array structures are described. Then, the typical techniques for fabrication of metallic plasmonic arrays are summarized. Next, some recent applications of plasmonic arrays are reviewed, including plasmonic sensing, surface-enhanced spectroscopies, plasmonic nanolasing, and perfect light absorption. Lastly, the existing challenges and perspectives for metallic plasmonic arrays are discussed. The aim is to provide guidance for future development of metallic plasmonic array structures.

Improved Sensitivity of Surface-Enhanced Raman Scattering with Gold Nanoparticles-Insulator-Metal Sandwich Layers on Flat Sapphire Substrate

Surface-enhanced Raman scattering (SERS) as a high sensitivity analytical method for molecule detection has attracted much attention in recent research. In this work, we demonstrated an improved SERS substrate, which has the gold nanoparticles randomly distributed on a SiO₂ interception layer over a gold thin film layer on the flat sapphire substrate (AuNP/SiO₂/Au/Sapphire), over the dispersed gold nanoparticles on a silicon substrate (AuNP/Si), for detection of R6G (1 × 10⁻⁶ M) in a Raman microscope. The fabrication of sandwich layers on top of the sapphire substrate involves evaporation of a gold mirror as thick as 100 nm, plasma enhanced chemical vapor deposition of the silica insulator layer 10 nm thick, and evaporation of a thin gold layer 10 nm thick for forming gold nanoparticles. For comparison, a gold thin film with a thickness of 5 nm and 10 nm was evaporated on a silicon substrate, respectively (AuNP/Si), as the reference SERS substrates in the experiment. The AuNP/SiO₂/Au/Sapphire substrate demonstrated improved sensitivity in detection of molecules in Raman microscopy, which can enable the molecules to be recognizable at a low laser power as 8.5 × 10⁻³ mW, 0.017 mW, 0.085 mW, and 0.17 mW for ultrashort exposure time. The simulation of AuNP/SiO₂/Au/Sapphire substrate and AuNP/Si substrate, based on the finite-difference time-domain (FDTD) method, explained the improved sensitivity for detection of R6G molecules from the view of classical electromagnetics, and it suggested the optimized size for the gold nanoparticles and the optimized laser wavelength for Raman microscopy for further research.

Nanofocusing of a metallized double periodic arranged nanocone array for surface-enhanced Raman spectroscopy

A plasmonic double periodic arranged nanocone array (DPANA) integrated by nanotips and nanogaps exhibit strong capability of light compression, and thus lead to extremely enhanced electric near-field intensity. The DPANA is fabricated by the self-assembled mask integrated with the inductively couple plasma (ICP) etching technology. Finite-difference time-domain (FDTD) simulations suggest that the metallized DPANA can generate a strong hotspot at the sharp tip apex and the nanogap between adjacent sharp tips. The electric-field enhancement characteristic is firstly verified with the help of the second-order surface nonlinear optical response of the metallized DPANA. The surface-enhanced Raman spectroscopy (SERS) examination of the metallized DPANA exhibits high sensitivity due to clearly presenting the Raman spectra of Rhodamine-6G (R6G) with concentrations down to 10 pM and has excellent uniformity, time stability, and recyclability, simultaneously. Furthermore, the principle demonstration of SERS practical application is also performed for thiram. This as-prepared SERS substrate has great potential application for trace amount detection.

Coupling enhancement mechanisms, materials, and strategies for surface-enhanced Raman scattering devices

Surface-enhanced Raman scattering (SERS) has become one of the most sensitive analytical techniques for identifying the chemical components, molecular structures, molecular conformations, and the interactions between molecules. However, great challenges still need to be addressed until it can be widely accepted by the absolute quantification of analytes. Recently, many efforts have been devoted to addressing these issues via various electromagnetic (EM), chemical (CM), and EM-CM hybrid coupling enhancement strategies. In comparison with uncoupled SERS devices, they offer key advantages in terms of sensitivity, reproducibility, uniformity, stability, controllability and reliability. This review provides an in-depth analysis of coupled SERS devices, including coupling enhancement mechanisms, materials and approaches. Finally, we also discuss the remaining bottlenecks and possible strategies for the development of coupling-enhanced SERS devices in the future.

Investigations of Shape, Material and Excitation Wavelength Effects on Field Enhancement in SERS Advanced Tips

This article, a part of the larger research project of Surface-Enhanced Raman Scattering (SERS), describes an advanced study focusing on the shapes and materials of Tip-Enhanced Raman Scattering (TERS) designated to serve as part of a novel imager device. The initial aim was to define the optimal shape of the "probe": tip or cavity, round or sharp. The investigations focused on the effect of shape (hemi-sphere, hemispheroid, ellipsoidal cavity, ellipsoidal rod, nano-cone), and the effect of material (Ag, Au, Al) on enhancement, as well as the effect of excitation wavelengths on the electric field. Complementary results were collected: numerical simulations consolidated with analytical models, based on solid assumptions. Preliminary experimental results of fabrication and structural characterization are also presented. Thorough analyses were performed around critical parameters, such as the plasmonic metal-Silver, Aluminium or Gold-using Rakic model, the tip geometry-sphere, spheroid, ellipsoid, nano-cone, nano-shell, rod, cavity-and the geometry of the plasmonic array: cross-talk in multiple nanostructures. These combined outcomes result in an optimized TERS design for a large number of applications.

Growth of Nanostructured Silver Flowers by Metal-Mediated Catalysis for Surface-Enhanced Raman Spectroscopy Application

Metallic flowers with nanoscale surface roughness can provide a platform for highly sensitive and reproductive surface-enhanced Raman spectroscopy (SERS). Here, we present a method to grow a nanostructured silver flower (NSF) at the apex of a plasmonic tip based on metal-mediated catalysis, where the NSF was rapidly generated in no more than 1 min. The NSF was used as the SERS substrate under linear polarization beam (LPB) excitation to achieve a 10^-9 M detection sensitivity for the malachite green analyte. The reproducibility for SERS is examined to have been guaranteed by comparing Raman intensity enhanced by different NSFs. Compared with the LPB, the azimuthal vector beam (AVB) excitation can further improve the SERS activity of the NSF, which is consistent with the simulation result that the gap mode can be effectively generated between two adjacent Ag nanoparticles (NPs) and between the NPs and the Ag pyramids on the surface of the NSF under AVB illumination. This work makes it promising for plasmonic tip-mediated catalysis to be applied in nanofabrication, the products of which can be further exploited in nanostructure-based ultrasensitive detection.

Optical and Photoacoustic Properties of Laser-Ablated Silver Nanoparticles in a Carbon Dots Solution

This study used the carbon dots solution for the laser ablation technique to fabricate silver nanoparticles. The ablation time range was from 5 min to 20 min. Analytical methods, including Fourier transform infrared spectroscopy (FTIR), UV-visible spectroscopy, transmission electron microscopy, and Raman spectroscopy were used to categorize the prepared samples. The UV-visible and z-scan techniques provided optical parameters such as linear and nonlinear refractive indices in the range of 1.56759 to 1.81288 and 7.3769 × 10^-10 cm² W^-1 to 9.5269 × 10^-10 cm² W^-1 and the nonlinear susceptibility was measured in the range of 5.46 × 10^-8 to 6.97 × 10^-8 esu. The thermal effusivity of prepared samples, which were measured using the photoacoustic technique, were in the range of 0.0941 W s^1/2 cm^-2 K^-1 to 0.8491 W s^1/2 cm^-2 K^-1. The interaction of the prepared sample with fluoride was investigated using a Raman spectrometer. Consequently, the intensity of the Raman signal decreased with the increasing concentration of fluoride, and the detection limit is about 0.1 ppm.

Synergistic plasmon resonance coupling and light capture in ordered nanoarrays as ultrasensitive and reproducible SERS substrates

An effective SERS substrate for on-field detection needs to satisfy high sensitivity to analyte and signal reproducibility even in the special case of tilting or bending of substrates. Herein, we transferred monolayer AuNPs into a nanocavity to construct a Au particle-in-hemispherical honeycomb nanoarray (PIHHN) as an ultrasensitive and spatially reproducible SERS substrate. The capacity of detection for R6G in an optimal PIHHN substrate is as low as a concentration of 10^-15 M, and the RSD of signal deviation is no more than 5.6%. FDTD simulations explain that placing AuNPs into a metallic nanocavity can capture and focus the light field to improve the interaction between the light and the substrate and provide the collective effect of multiple plasmon coupling, which can induce a stronger electromagnetic field. In addition, the system can generate more hot spots between AuNPs and between AuNPs and the metallic nanocavity. In particular, when the substrate is tilted or bent at an angle from 0° to 60°, the SERS performance remains stable due to the rotational symmetry of the PIHHN structure, which demonstrates the capability of on-field detection. Furthermore, the PIHHN substrate is employed as a highly sensitive multiplex sensor in on-field analysis for contaminated soil, achieving the detection of analytes down to 0.5 ppb.

Uniform Periodic Bowtie SERS Substrate with Narrow Nanogaps Obtained by Monitored Pulsed Electrodeposition

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive technique with molecular specificity, making it an ideal analytical tool in various fields. However, the breadth of practical applications of SERS has been severely limited because it is still a great challenge to achieve simultaneously a high sensitivity and a high reproducibility. Herein, we report a highly controllable method to fabricate periodic bowtie SERS substrates with a narrow nanogap, high SERS enhancement, and good uniformity over a large area. The periodic bowtie template is first fabricated over a gold film by holographic lithography (HL), followed by Au deposition to obtain a conductive plasmonic bowtie array. The gap size is then narrowed down by pulsed electrodeposition of Ag simultaneously monitored in situ by electrochemical dark field spectroscopy. Thus, we are able to observe the most sensitive change in the scattering spectra when the gap is just about to merge and obtain uniform SERS substrates with a gap size down to around 5 nm. The average enhancement factor of 5 × 10⁷ to 1 × 10⁸ is obtained, which is 50 times larger than that from Au nanoparticle-assembled substrates and 140 times larger than that from commercial Klarite chips. This substrate offers a promising opportunity for SERS practical applications.

Spatial-Tunable Au Nanoparticle Functionalized Si Nanorods Arrays for Surface Enhanced Raman Spectroscopy

In this study, hexagonal-packed Si nanorods (SiNRs) arrays were fabricated and conjugated with Au nanoparticles (AuNPs) in different spatial distributions for surface-enhanced Raman spectroscopy (SERS). The AuNPs were functionalized on the bottom of SiNRs (B-SiNRs@AuNPs), top of SiNRs (T-SiNRs@AuNPs) and sides of SiNRs (S-SiNRs@AuNPs), respectively. Our results demonstrated that the SiNRs conjugated with AuNPs on the sides achieved high reproducibility in detection of R6G molecules, while the AuNPs on the top of the SiNRs obtained the strongest Raman enhancement. In addition, the substrate with S-SiNRs@AuNPs obtained the highest spatial uniformity of enhancement. The finite-difference time-domain simulation gave further evidence that the incident light could be confined in the space of SiNRs arrays and yield a zero-gap enhancement coupled with the AuNPs. Our study provided a spatially tunable SiNRs@AuNPs substrate with high sensitivity and reproducibility in molecular detection.

Electrotunable Nanoplasmonics for Amplified Surface Enhanced Raman Spectroscopy

Tuning the properties of optical metamaterials in real time is one of the grand challenges of photonics. Being able to do so will enable a class of adaptive photonic materials for use in applications such as surface enhanced Raman spectroscopy and reflectors/absorbers. One strategy to achieving this goal is based on the electrovariable self-assembly and disassembly of two-dimensional nanoparticle arrays at a metal | liquid interface. As expected, the structure results in plasmonic coupling between NPs in the array but perhaps as importantly between the array and the metal surface. In such a system, the density of the nanoparticle array can be reversibly controlled by the variation of electrode potential. Theory suggests that due to a collective plasmon-coupling effect  less than 1 V variation of electrode potential can give rise to a dramatic simultaneous change in optical reflectivity from ∼93% to ∼1% and the amplification of the SERS signal by up to 5 orders of magnitude. This is experimentally demonstrated using a platform based on the voltage-controlled assembly of 40 nm Au-nanoparticle arrays at a TiN/Ag electrode in contact with an aqueous electrolyte. We show that all the physics underpinning the behavior of this platform works precisely as suggested by the proposed theory, setting the electrochemical nanoplasmonics as a promising direction in photonics research.

Three-dimensional assembly of silver nanoparticles spatially confined by cellular structure of Spirulina, from nanospheres to nanosheets

Three-dimensional (3D) ordered construction of nanoparticles (NPs) has attracted much attention in wide applications, however, techniques with respect to cost effective nanofabrication of well defined functional architectures is still lacking. To address this specific issue, a bio-interface confinement approach is proposed that precisely replicates the complex cellular structural features of microbes and integrates silver NP (SNP) building blocks into their 3D framework in a precise, low cost and mass production way. Herein, the SNPs with nanospheres and nanosheets structure were synthesized by way of electroless deposition using Spirulina as template. Results showed that SNPs were orderly assembled along the cellular structure, and the spatially confinement of cellular texture induced the transformation of SNPs from sphere to flake morphology during their continuous growth. The silver assembly not only shows good antibacterial activity, but also exhibits excellent surface enhanced Raman scattering (SERS) performance with the enhancement factor as high as 5.95 × 10⁸ and good recuperability towards Rhodamine 6G. The fascinating SERS performance can be ascribed to the combined action of nanosheets morphology of SNPs, hierarchical nanostructure of the cellular structure, and the small interparticle spacing. This strategy provides an effective strategy for controllable and ordered 3D assembly of NPs by using the cellular texture.

Engineering State-of-the-Art Plasmonic Nanomaterials for SERS-Based Clinical Liquid Biopsy Applications

Precision oncology, defined as the use of the molecular understanding of cancer to implement personalized patient treatment, is currently at the heart of revolutionizing oncology practice. Due to the need for repeated molecular tumor analyses in facilitating precision oncology, liquid biopsies, which involve the detection of noninvasive cancer biomarkers in circulation, may be a critical key. Yet, existing liquid biopsy analysis technologies are still undergoing an evolution to address the challenges of analyzing trace quantities of circulating tumor biomarkers reliably and cost effectively. Consequently, the recent emergence of cutting-edge plasmonic nanomaterials represents a paradigm shift in harnessing the unique merits of surface-enhanced Raman scattering (SERS) biosensing platforms for clinical liquid biopsy applications. Herein, an expansive review on the design/synthesis of a new generation of diverse plasmonic nanomaterials, and an updated evaluation of their demonstrated SERS-based uses in liquid biopsies, such as circulating tumor cells, tumor-derived extracellular vesicles, as well as circulating cancer proteins, and tumor nucleic acids is presented. Existing challenges impeding the clinical translation of plasmonic nanomaterials for SERS-based liquid biopsy applications are also identified, and outlooks and insights into advancing this rapidly growing field for practical patient use are provided.

Refractive-Index-Insensitive Nanolaminated SERS Substrates for Label-Free Raman Profiling and Classification of Living Cancer Cells

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.

Surface-Enhanced Raman Scattering-Based Odor Compass: Locating Multiple Chemical Sources and Pathogens

Olfaction is important for identifying and avoiding toxic substances in living systems. Many efforts have been made to realize artificial olfaction systems that reflect the capacity of biological systems. A sophisticated example of an artificial olfaction device is the odor compass which uses chemical sensor data to identify odor source direction. Successful odor compass designs often rely on plume-based detection and mobile robots, where active, mechanical motion of the sensor platform is employed. Passive, diffusion-based odor compasses remain elusive as detection of low analyte concentrations and quantification of small concentration gradients from within the sensor platform are necessary. Further, simultaneously identifying multiple odor sources using an odor compass remains an ongoing challenge, especially for similar analytes. Here, we show that surface-enhanced Raman scattering (SERS) sensors overcome these challenges, and we present the first SERS odor compass. Using a grid array of SERS sensors, machine learning analysis enables reliable identification of multiple odor sources arising from diffusion of analytes from one or two localized sources. Specifically, convolutional neural network and support vector machine classifier models achieve over 90% accuracy for a multiple odor source problem. This system is then used to identify the location of an Escherichia coli biofilm via its complex signature of volatile organic compounds. Thus, the fabricated SERS chemical sensors have the needed limit of detection and quantification for diffusion-based odor compasses. Solving the multiple odor source problem with a passive platform opens a path toward an Internet of things approach to monitor toxic gases and indoor pathogens.

Quasi-Metal for Highly Sensitive and Stable Surface-Enhanced Raman Scattering

Compared with the noble-metal surface-enhanced Raman scattering (SERS) substrates activated by the surface plasmon resonance (SPR)-induced electromagnetic mechanism (EM), the relative low sensitivity and stability of the chemical mechanism (CM)-based substrates are the biggest obstacles to their applications. Herein, we report that quasi-metallic VO₂ nanosheet arrays can be used as a sensitive and stable SERS substrate. The lowest detectable limit of analyte adsorbed on the VO₂ nanosheets achieves 10⁻¹⁰ M and the maximum Raman enhancement factor (EF) reaches 6.7 × 10⁷, which is comparable with that of the noble metals. The experimental and theoretical results demonstrate that the SERS performance of the VO₂ nanosheets comes from the strong interfacial interactions based on charge transfer and the vigorous SPR effects. Our research results demonstrate that quasi-metals are very promising SERS detection platforms and reveal that CM, like EM, contributes significantly to the SERS activity of quasi-metals.

•

Surface-enhanced Raman scattering (SERS) on quasi-metallic VO₂

•

High SERS enhancement factor and low limit of detection have been achieved

•

Synergistic effect of electromagnetic enhancement and chemical enhancement

Hybrid nanostructure of SiO2@Si with Au-nanoparticles for surface enhanced Raman spectroscopy

In this study, a structure of large-area orderly-arranged SiO₂@Si core-shell nanoparticles decorated with Au nanoparticles was fabricated for surface-enhanced Raman spectroscopy (SERS). This hybrid structure features light confinement in the Si shells and a uniform distribution of localized electric hot spots. FDTD simulations were carried out to examine the near-field enhancement response of this structure. Results indicate that the strongly enhanced local electric field is attributed to the WGM-LSPR coupling, that is, the coupling of the whispering gallery mode (WGM) of Si nanoshells with the localized surface plasmon resonance (LSPR) of Au nanoparticles. The excitation of WGM comes primarily from the magnetic response of the Si shell with a minor modification by its electric response. The WGM-LSPR coupling of the structure is tunable through the change of geometric parameters of SiO₂@Si particles. Raman scattering measurements were conducted on the samples fabricated, which agree well with the simulated results. The measured data gave a SERS G factor of ∼2 × 10⁸ and showed highly sensitive and reproducible SERS signals of R6G with a high spatial uniformity on a 2 × 2 cm² substrate consisting of an array of SiO₂@Si (D = ∼220 nm/290 nm) particles whose outer surfaces were scattered with d = ∼20 nm Au particles.

A Review on Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

Sources of variability in SERS spectra of bacteria: comprehensive analysis of interactions between selected bacteria and plasmonic nanostructures

The surface-enhanced Raman spectroscopy (SERS)-based analysis of bacteria suffers from the lack of a standard SERS detection protocol (type of substrates, excitation frequencies, and sampling methodologies) that could be employed throughout laboratories to produce repeatable and valuable spectral information. In this work, we have examined several factors influencing the spectrum and signal enhancement during SERS studies conducted on both Gram-negative and Gram-positive bacterial species: Escherichia coli and Bacillus subtilis, respectively. These factors can be grouped into those which are related to the structure and types of plasmonic systems used during SERS measurements and those that are associated with the culturing conditions, types of culture media, and method of biological sample preparation.

Nanopillar-Assisted SERS Chromatography

Practical implementation of surfaced enhanced Raman spectroscopy (SERS) sensing is hindered by complexity of real-life samples, which often requires long and costly pretreatment and purification. Here, we present a novel nanopillar-assisted SERS chromatography (NPC-SERS) method for simultaneous quantitation of target molecules and analysis of complex, multicomponent fluids, e.g., human urine spiked with a model drug paracetamol (PAR). Gold-coated silicon nanopillar (AuNP) SERS substrates and a centrifugal microfluidic platform are tactfully combined, which allows (i) a precise and fully automated sample manipulation and (ii) spatial separation of different molecular species on the AuNP substrate. The NPC-SERS technique provides a novel approach for wetting the stationary phase (AuNP) using the "wicking effect", and thus minimizes dilution of analytes. Separation of PAR and the main human urine components (urea, uric acid, and creatinine) has been demonstrated. Quantitative detection of PAR with ultrawide linear dynamic range (0-500 ppm) is achieved by analyzing the spreading profiles of PAR on the AuNP surface. NPC-SERS transforms SERS into a sensing technique with general applicability, facilitating rapid and quantitative detection of analytes in complex biofluids, such as saliva, blood, and urine.

Ultralarge Area Sub-10 nm Plasmonic Nanogap Array by Block Copolymer Self-Assembly for Reliable High-Sensitivity SERS

Effective surface enhancement of Raman scattering (SERS) requires strong near-field enhancement as well as effective light collection of plasmonic structures. To this end, plasmonic nanoparticle (NP) arrays with narrow gaps or sharp tips have been suggested as desirable structures. We present a highly dense and uniform Au nanoscale gap array enabled by the customized design of NP shape and arrangement employing block copolymer self-assembly. Block copolymer self-assembly in thin films offers uniform hexagonally packed nanopost template arrays over the entire surface of a 2 in. wafer. Conventional evaporative metal deposition over the nanotemplate surface allows precise geometric control and positional arrangement of metal NPs, constituting tunable, strong plasmonic near-field enhancement particularly at the "hot spots" near interparticular nanoscale gaps. Underlying field distribution has been investigated by a finite-difference time-domain simulation. In the detection of thiophenol, our Au nanogap array shows a remarkable enhancement of Raman intensity greater than ∼10⁴, a standard deviation as small as 12.3% compared to that of the planar Au thin film. In addition, adenine biomolecules can be detected with a detection limit as low as 100 nM. Our approach proposes highly sensitive and reliable SERS on the basis of a scalable, low-cost bottom-up strategy.

One-Pot Synthesis of Multi-Branch Gold Nanoparticles and Investigation of Their SERS Performance

Gold nanoparticles with multiple branches have attracted intensive studies for their application in sensing of low trace molecules. A large number of the merits found on the gold nanoparticles for the above applications are attributed to the strong localized surface plasmon resonance excited by the incident radiation. However, a facile and flexible way of synthesizing the multi-branch gold nanoparticles with tunable localized surface plasmon resonance frequency is still a challenge for the plasmonic research field. Herein, we report an efficient one-pot synthesis of multi-branch gold nanoparticles method that resembles a seed-medicated approach while using no further chemicals except chloroauric acid, ascorbic acid and 4-(2-Hydroxyethyl)-1-piperazinyl]-ethanesulfonic acid. By controlling the amounts of ascorbic acid volumes in the reaction mixture, the morphology and the localized surface plasmon resonance frequency of the synthesized multi-branch gold nanoparticles can be manipulated conveniently. Moreover, using the 4-Mercaptobenzoic acid as the Raman reporter, the multi-branch gold nanoparticles show superior surface-enhanced Raman spectroscopy characteristics that can be potentially used in chemical and biological sensing.

Label-Free SERS Quantum Semiconductor Probe for Molecular-Level and in Vitro Cellular Detection: A Noble-Metal-Free Methodology

Accurate in vitro molecular-level analysis is an essential step prior to in vivo and clinical application for early diagnosis and cancer treatment. Among the diagnostic techniques, surface-enhanced Raman scattering (SERS) biosensing has shown growing potential due to its noninvasive and real-time characterization of the biomolecules. However, the application of SERS biosensing is mostly limited to the plasmonic noble metals, in the form of either nanoparticles or tips and substrates (fixed probe), on which surface plasmon resonance (SPR) is the prominent enhancement principle. The semiconductor quantum particles have been explored in several optoelectronics applications, but have never been reported to be exploited as a means of surface-enhanced Raman scattering (SERS) for molecular-level and intracellular sensing. Here, we report on the new generation of noble-metal-free SERS probe; Si@SiO₂ quantum probe (Si@SiO₂ Q-probe) whose affinity to functional groups not only imitates a self-driven labeling attribution that enables charge transfer (CT) as an augmented enhancement principle but also its mobile nature in miniaturized scale facilitates endocytosis for in situ live cell biosensing. Moreover, a significant enhancement factor of 10⁶ of rhodamine 6G (R6G) and 10⁷ of glutathione (GSH) at ∼5 × 10^-12 pM concentration has been achieved that is comparable to inherently plasmonic noble metals. Our results showed a capability of the Si@SiO₂ Q-probe to unveil the "biochemical fingerprint" of substantial components of mammalian and cancerous cervical cells, which leads to diagnosis of cervical cancer. These unique attributions of the Si@SiO₂ Q-probe can provide better insight into cell mutation and malignancy.

CO2 -Assisted Conversion of Crystal Two-Dimensional Molybdenum Oxide to Amorphism with Plasmon Resonances

Localized surface plasmon resonances (LSPRs) of ultra-thin two-dimensional (2D) nanomaterials have opened up a new regime in plasmonics in the last several years. 2D plasmonic materials are currently concentrated on the crystal structure, with amorphous materials hardly being reported because of their limited preparation methods rather than undesired plasmonic properties. Taking molybdenum oxides as an example, herein, we elaborate the 2D amorphous plasmons prepared with the assistance of supercritical CO₂ . In brief, we examine the reported characteristic plasmonic properties of molybdenum oxides, and applications of supercritical CO₂ in formations of 2D layer materials as well as introduced phase and disorder engineering based on our research. Furthermore, we propose our perspective on the development of 2D plasmons, especially for amorphous layer materials in the future.

Quantitative SERS by electromagnetic enhancement normalization with carbon nanotube as an internal standard

Quantitative surface-enhanced Raman scattering (SERS) in practical applications still remain unresolved, mainly due to low reproducibility relying on the quality of the SERS substrates. In this paper, a carbon nanotube and Ag nanoparticles composite (CNT/AgNPs) is reported, and the carbon nanotube is as an internal standard for the calibration of SERS intensity of analyte molecules. The quantification of analyte molecules rhodamine 6G (R6G) is demonstrated in an aqueous solution with the concentration of 10-9 to 10-7 M. Raman mapping is used to investigate the stability of SERS spectra in a large scanning area, and the corresponding relative standard deviation (RSD) is calculated. SERS mapping reveals that the uniformity of Raman enhancement is improved through the build-in calibration with 2D Raman peak of CNT. Meanwhile, CNT/AgNPs samples are used to detect N2 in natural air, indicating that such self-calibration method can improve the reliability of the SERS analysis.

Ultrathin Molybdenum Dioxide Nanosheets as Uniform and Reusable Surface-Enhanced Raman Spectroscopy Substrates with High Sensitivity

Metal oxides have advantages over the traditional noble metals to be used as substrate materials for surface-enhanced Raman spectroscopy (SERS) with low cost, versatility, and biocompatibility, but their enhancement factors are generally quite low with a poor limit of detection. Here, ultrathin molybdenum dioxide (MoO₂ ) nanosheets synthesized by chemical vapor deposition demonstrated in large area are used as SERS substrates with superior signal uniformity in the whole area with a limit of detectable concentration down to 4 × 10^-8 m and enhancement factor up to 2.1 × 10⁵ , exceeding that of 2D materials and comparable to that of noble metal films. More practically important, the planar MoO₂ substrate is more robust than noble metals and shows excellent reusability and uniformity, which is usually prohibited for nanostructured or nanoparticle-based metal oxide substrates. The enhancement is mainly attributed to the surface plasmon resonance effect as evidenced by the first principle calculations and UV-vis absorption spectroscopy characterization, which can be further increased by decreasing the thickness of the MoO₂ nanosheets. The overall superior performance makes the MoO₂ nanosheets an ideal substrate for practical SERS applications.

Flexible, transparent and highly sensitive SERS substrates with cross-nanoporous structures for fast on-site detection

A flexible and transparent film assembled from the cross-nanoporous structures of Au on PET (CNS of Au@PET) is developed as a versatile and effective SERS substrate for rapid, on-site trace analysis with high sensitivity. The fabrication of the CNS of Au can be achieved on a large scale at low cost by employing an etching process with super-aligned carbon nanotubes as a mask, followed by metal deposition. A strongly enhanced Raman signal with good uniformity can be obtained, which is attributed to the excitation of "hot spots" around the metal nanogaps and sharp edges. Using the CNS of Au@PET film as a SERS platform, real-time and on-site SERS detection of the food contaminant crystal violet (CV) is achieved, with a detection limit of CV solution on a tomato skin of 10-7 M. Owing to its ability to efficiently extract trace analytes, the resulting substrate also achieves detection of 4-ATP contaminants and thiram pesticides by swabbing the skin of an apple. A SERS detection signal for 4-ATP has a relative standard deviation of less than 10%, revealing the excellent reproducibility of the substrate. The flexible, transparent and highly sensitive substrates fabricated using this simple and cost-effective strategy are promising for practical application in rapid, on-site SERS-based detection.

Facing Challenges in Real-Life Application of Surface-Enhanced Raman Scattering: Design and Nanofabrication of Surface-Enhanced Raman Scattering Substrates for Rapid Field Test of Food Contaminants

Surface-enhanced Raman scattering (SERS) is capable of detecting a single molecule with high specificity and has become a promising technique for rapid chemical analysis of agricultural products and foods. With a deeper understanding of the SERS effect and advances in nanofabrication technology, SERS is now on the edge of going out of the laboratory and becoming a sophisticated analytical tool to fulfill various real-world tasks. This review focuses on the challenges that SERS has met in this progress, such as how to obtain a reliable SERS signal, improve the sensitivity and specificity in a complex sample matrix, develop simple and user-friendly practical sensing approach, reduce the running cost, etc. This review highlights the new thoughts on design and nanofabrication of SERS-active substrates for solving these challenges and introduces the recent advances of SERS applications in this area. We hope that our discussion will encourage more researches to address these challenges and eventually help to bring SERS technology out of the laboratory.

Plasmonic Nanowires for Wide Wavelength Range Molecular Sensing

In this paper, we propose the use of a standing nanowires array, constituted by plasmonic active gold wires grown on iron disks, and partially immersed in a supporting alumina matrix, for surface-enhanced Raman spectroscopy applications. The galvanic process was used to fabricate nanowires in pores of anodized alumina template, making this device cost-effective. This fabrication method allows for the selection of size, diameter, and spatial arrangement of nanowires. The proposed device, thanks to a detailed design analysis, demonstrates a broadband plasmonic enhancement effect useful for many standard excitation wavelengths in the visible and NIR. The trigonal pores arrangement gives an efficiency weakly dependent on polarization. The devices, tested with 633 and 830 nm laser lines, show a significant Raman enhancement factor, up to around 6 × 10⁴, with respect to the flat gold surface, used as a reference for the measurements of the investigated molecules.

Self-assembled vertically aligned Au nanorod arrays for surface-enhanced Raman scattering (SERS) detection of Cannabinol

Self-assembled multi-layered vertically aligned gold nanorod (AuNR) arrays have been fabricated by a simple preparation process that requires a balance between the particle concentration and the ionic strength of the solvent. An experimentally determined critical AuNR concentration of 2.0nM and 50mM NaCl produces well-ordered vertically aligned hexagonally close-packed AuNR arrays. We demonstrate surface treatment via UV Ozone cleaning of such samples to allow introduction of analyte molecules (benzenethiol and cannabinol) for effective surface enhanced Raman scattering detection. This is the first demonstration of the SERS analysis of cannabinol. This approach demonstrates a cost-effective, high-yield and simple fabrication route to SERS sensors with application in the screening for the cannabinoids.

SERS polarization-dependent effects for an ordered 3D plasmonic tilted silver nanorod array

Hexagonal close-packed tilted Ag nanorod arrays that exhibit excellent uniformity and reproducibility were prepared. The tilt angle was easily controlled by regulating the sputtering angle, accompanied by a reduction and constancy in the gap size of adjacent nanorods, which is 30° and 90° relative to the sputtering direction. The surface enhanced Raman spectroscopy (SERS) technique was used to characterize the interaction of tilted Ag nanorod arrays with polarized laser excitation. Interestingly, the SERS polarization-dependence increased with increasing tilt angle of the Ag nanorods. To elucidate the essential factors responsible for this SERS result, three-dimensional (3D) electromagnetic enhancement distribution for the proposed system was numerically simulated based on p- and s-polarization excitation. Most importantly, the fundamental reasons for the polarization dependence of SERS were obtained by a quantitative 3D numerical simulation of hotspot distribution for adjacent nanorods.

Highly robust, uniform and ultra-sensitive surface-enhanced Raman scattering substrates for microRNA detection fabricated by using silver nanostructures grown in gold nanobowls

Highly sensitive and reproducible surface enhanced Raman spectroscopy (SERS) requires not only a nanometer-level structural control, but also superb uniformity across the SERS substrate for practical imaging and sensing applications. However, in the past, increased reproducibility of the SERS signal was incompatible with increased SERS sensitivity. This work presents multiple silver nanocrystals inside periodically arrayed gold nanobowls (SGBs) via an electrochemical reaction at an overpotential of -3.0 V (vs. Ag/AgCl). The gaps between the silver nanocrystals serve as hot spots for SERS enhancement, and the evenly distributed gold nanobowls lead to a high device-to-device signal uniformity. The SGBs on the large sample surface exhibit an excellent SERS enhancement factor of up to 4.80 × 10⁹, with excellent signal uniformity (RSD < 8.0 ± 2.5%). Furthermore, the SGBs can detect specific microRNA (miR-34a), which plays a widely acknowledged role as biomarkers in diagnosis and treatment of diseases. Although the small size and low abundance of miR-34a in total RNA samples hinder their detection, by utilizing the advantages of SGBs in SERS sensing, reliable and direct detection of human gastric cancer cells has been successfully accomplished.

Surface-Enhanced Raman Spectroscopy-Based Label-Free Insulin Detection at Physiological Concentrations for Analysis of Islet Performance

Label-free optical detection of insulin would allow in vitro assessment of pancreatic cell functions in their natural state and expedite diabetes-related clinical research and treatment; however, no existing method has met these criteria at physiological concentrations. Using spatially uniform 3D gold-nanoparticle sensors, we have demonstrated surface-enhanced Raman sensing of insulin in the secretions from human pancreatic islets under low and high glucose environments without the use of labels such as antibodies or aptamers. Label-free measurements of the islet secretions showed excellent correlation among the ambient glucose levels, secreted insulin concentrations, and measured Raman-emission intensities. When excited at 785 nm, plasmonic hotspots of the densely arranged 3D gold-nanoparticle pillars as well as strong interaction between sulfide linkages of the insulin molecules and the gold nanoparticles produced highly sensitive and reliable insulin measurements down to 100 pM. The sensors exhibited a dynamic range of 100 pM to 50 nM with an estimated detection limit of 35 pM, which covers the reported concentration range of insulin observed in pancreatic cell secretions. The sensitivity of this approach is approximately 4 orders of magnitude greater than previously reported results using label-free optical approaches, and it is much more cost-effective than immunoassay-based insulin detection widely used in clinics and laboratories. These promising results may open up new opportunities for insulin sensing in research and clinical applications.

Driving Chemical Reactions in Plasmonic Nanogaps with Electrohydrodynamic Flow

Nanoparticles from colloidal solution-with controlled composition, size, and shape-serve as excellent building blocks for plasmonic devices and metasurfaces. However, understanding hierarchical driving forces affecting the geometry of oligomers and interparticle gap spacings is still needed to fabricate high-density architectures over large areas. Here, electrohydrodynamic (EHD) flow is used as a long-range driving force to enable carbodiimide cross-linking between nanospheres and produces oligomers exhibiting sub-nanometer gap spacing over mm² areas. Anhydride linkers between nanospheres are observed via surface-enhanced Raman scattering (SERS) spectroscopy. The anhydride linkers are cleavable via nucleophilic substitution and enable placement of nucleophilic molecules in electromagnetic hotspots. Atomistic simulations elucidate that the transient attractive force provided by EHD flow is needed to provide a sufficient residence time for anhydride cross-linking to overcome slow reaction kinetics. This synergistic analysis shows assembly involves an interplay between long-range driving forces increasing nanoparticle-nanoparticle interactions and probability that ligands are in proximity to overcome activation energy barriers associated with short-range chemical reactions. Absorption spectroscopy and electromagnetic full-wave simulations show that variations in nanogap spacing have a greater influence on optical response than variations in close-packed oligomer geometry. The EHD flow-anhydride cross-linking assembly method enables close-packed oligomers with uniform gap spacings that produce uniform SERS enhancement factors. These results demonstrate the efficacy of colloidal driving forces to selectively enable chemical reactions leading to future assembly platforms for large-area nanodevices.

Highly Sensitive, Uniform, and Reproducible Surface-Enhanced Raman Spectroscopy Substrate with Nanometer-Scale Quasi-periodic Nanostructures

We introduce a simple and cost-effective approach for fabrication of effective surface-enhanced Raman spectroscopy (SERS) substrates. It is shown that the as-fabricated substrates show excellent SERS effects in various probe molecules with high sensitivity, that is, picomolar level detection, and also good reliability. With a SERS enhancement factor beyond 10⁸ and excellent reproducibility (deviation less than 5%) of signal intensity, the fabrication of the SERS substrate is realized on a four-inch wafer and proven to be effective in pesticide residue detection. The SERS substrate is realized first through the fabrication of quasi-periodic nanostructured silicon with dimension features in tens of nanometers using superaligned carbon nanotubes networks as an etching mask, after which a large amount of hot spots with nanometer gaps are formed through deposition of a gold film. With rigorous nanostructure design, the enhanced performance of electromagnetic field distribution for nanostructures is optimized. With the advantage of cost-effective large-area preparation, it is believed that the as-fabricated SERS substrate could be used in a wide variety of actual applications where detection of trace amounts is necessary.