Volume-Enhanced Raman Scattering Detection of Viruses

SERS-based microdevices for use as in vitro diagnostic biosensors

Advances in surface-enhanced Raman scattering (SERS) detection have helped to overcome the limitations of traditional in vitro diagnostic methods, such as fluorescence and chemiluminescence, owing to its high sensitivity and multiplex detection capability. However, for the implementation of SERS detection technology in disease diagnosis, a SERS-based assay platform capable of analyzing clinical samples is essential. Moreover, infectious diseases like COVID-19 require the development of point-of-care (POC) diagnostic technologies that can rapidly and accurately determine infection status. As an effective assay platform, SERS-based bioassays utilize SERS nanotags labeled with protein or DNA receptors on Au or Ag nanoparticles, serving as highly sensitive optical probes. Additionally, a microdevice is necessary as an interface between the target biomolecules and SERS nanotags. This review aims to introduce various microdevices developed for SERS detection, available for POC diagnostics, including LFA strips, microfluidic chips, and microarray chips. Furthermore, the article presents research findings reported in the last 20 years for the SERS-based bioassay of various diseases, such as cancer, cardiovascular diseases, and infectious diseases. Finally, the prospects of SERS bioassays are discussed concerning the integration of SERS-based microdevices and portable Raman readers into POC systems, along with the utilization of artificial intelligence technology.

Deciphering biomolecular complexities: the indispensable role of surface-enhanced Raman spectroscopy in modern bioanalytical research

Surface-enhanced Raman spectroscopy (SERS) has emerged as an indispensable analytical tool in biomolecular research, providing unmatched sensitivity critical for the elucidation of biomolecular structures. This review presents a thorough examination of SERS, outlining its fundamental principles, cataloging its varied applications within the biomolecular sphere, and contemplating its future developmental trajectories. We begin with a detailed analysis of SERS's mechanistic principles, emphasizing both the phenomena of surface enhancement and the complexities inherent in Raman scattering spectroscopy. Subsequently, we delve into the pivotal role of SERS in the structural analysis of diverse biomolecules, including proteins, nucleic acids, lipids, carbohydrates, and biochromes. The remarkable capabilities of SERS extend beyond mere detection, offering profound insights into biomolecular configurations and interactions, thereby enriching our comprehension of intricate biological processes. This review also sheds light on the application of SERS in real-time monitoring of various bio-relevant compounds, from enzymes and coenzymes to metal ion-chelate complexes and cellular organelles, thereby providing a holistic view and empowering researchers to unravel the complexities of biological systems. We also address the current challenges faced by SERS, such as enhancing sensitivity and resolution, developing stable and reproducible substrates, and conducting thorough analyses in complex biological matrices. Nonetheless, the continual advancements in nanotechnology and spectroscopy solidify the standing of SERS as a formidable force in biomolecular research. In conclusion, the versatility and robustness of SERS not only deepen our understanding of biomolecular intricacies but also pave the way for significant developments in medical research, therapeutic innovation, and diagnostic approaches.

Ag Nanoparticles@Au Nanograting Array as a 3D Flexible and Effective Surface-Enhanced Raman Scattering Substrate

Surface-enhanced Raman scattering (SERS) is a powerful analytical technique for chemical identification, but it remains a great challenge to realize the large-scale and well-controlled fabrication of sensitive and repeatable SERS substrates. Here, we report a facile strategy to fabricate centimeter-sized periodic Au nanograting (Au-NG) decorated with well-arranged Ag nanoparticles (Ag-NPs) (denoted as Ag-NPs@Au-NG) as a three-dimensional (3D) flexible hybrid SERS substrate with high sensitivity and good reproducibility. The Au-NG patterns with periodic ridges and grooves are fabricated through nanoimprint lithography by employing a low-cost digital versatile disc (DVD) as a master mold, and the Ag-NPs are assembled by a well-controlled interface self-assembly method without any coupling agents. Multiple coupling electromagnetic field effects are created at the nanogaps between the Ag-NPs and Au-NG patterns, leading to high-density and uniform hot spots throughout the substrate. As a result, the Ag-NPs@Au-NG arrays demonstrate an ultrahigh SERS sensitivity as low as 10-13 M for rhodamine 6G with a high average enhancement factor (EF) of 1.85 × 108 and good signal reproducibility. For practical applications, toxic organic pollutants including crystal violet, thiram, and melamine have been successfully detected with high sensitivity at a low detection limit, showing a good perspective in the rapid detection of toxic organic pollutants.

Transferable G/Au Film for Constructing a Variety of SERS Substrates

Surface-enhanced Raman scattering (SERS), as one of the most powerful analytical methods, undertakes important inspection tasks in various fields. Generally, the performance of an SERS-active substrate relies heavily on its structure, which makes it difficult to integrate multiple-functional detectability on the same substrate. To address this problem, here we designed and constructed a film of graphene/Au nanoparticles (G/Au film) through a simple method, which can be conveniently transferred to different substrates to form various composite SERS substrates subsequently. By means of the combination of the electromagnetic enhancement mechanism (EM) and the chemical enhancement mechanism (CM) of this structure, the film realized good SERS performance experimentally, with the enhancement factor (EF) approaching ca. 1.40 × 105. In addition, the G/Au film had high mechanical strength and had large specific surface area and good biocompatibility that is beneficial for Raman detection. By further transferring the film to an Ag/Si composite substrate and PDMS flexible film, it showed enhanced sensitivity and in situ detectability, respectively, indicating high compatibility and promising prospect in Raman detection.

Cascade Bowl Multicavity Structure for In Situ Surface-Enhanced Raman Scattering Detection of Organic Gas Molecules

With the increasing emphasis on atmospheric environmental protection, it is crucial to find an efficient, direct, and accurate method to identify pollutant species in the atmosphere. To solve this problem, we designed and prepared the cascade multicavity (CMC) structure composed with silver nanoparticles (Ag NPs) as a surface-enhanced Raman scattering (SERS) substrate with favorable light transmittance and flexibility. The multicavity structure distributed on the surface introducing the homogeneous connecting holes endows the structure to more fully utilize the incident light while slowing the gas movement rate. Theoretical and experimental results have demonstrated that the Ag NPs/cascade multicavity (Ag-CMC) SERS substrate is a highly sensitive SERS substrate that can be used for in situ detection of gases under non-perpendicularly incident laser irradiation or bending of the substrate. We believe that the SERS substrate can provide a more efficient and feasible way for in situ detection of gaseous pollutants.

SERS detection of volatile gas in spoiled pork with the Ag/MoS2 nano-flower cavity/PVDF micron-bowl cavity (FIB) substrate

Putrescine and cadaverine are significant volatile indicators used to assess the degree of food spoilage. Herein, we propose a micro-nano multi cavity structure for surface-enhanced Raman spectroscopy (SERS) to analyze the volatile gas putrescine and cadaverine in decomposing food. The MoS2 nano-flowers are inserted into a PVDF micro-cavity through in-situ growth, followed by vacuum evaporation technology of Ag nanoparticles to form an Ag/MoS2 nano-flower cavity/PVDF micron-bowl cavity (FIB) substrate. The micro-nano multi cavity structure can improve the capture capacity of both light and gas, thereby exhibiting high sensitivity (EF = 7.71 × 107) and excellent capability for gas detection of 2-naphthalenethiol. The SERS detections of the putrescine and cadaverine are achieved in the spoiled pork samples with the FIB substrate. Therefore, this substrate can provide an efficient, accurate, and feasible method for the specific and quantitative detection in the food safety field.

Fabrication of multifunctional g-C3N4-modified Au/Ag NRs arrays for ultrasensitive and recyclable SERS detection of bisphenol A residues

Monolayer g-C3N4-modified Au/Ag nanorods (g-C3N4/Au/Ag NRs) array is fabricated as a dual-function platform with high surface-enhanced Raman scattering (SERS) response and excellent photocatalytic degradation ability for bisphenol A (BPA) residues. FDTD simulation results of Au/Ag NRs proves that the electromagnetic field intensity is significantly enhanced at the gap of Ag NRs and Au NPs and the protrusion of Au NPs, which endows the arrays with excellent SERS activity. The arrays exhibit high sensitivity for rhodamine 6G (R6G) (LOD = 1.1 × 10-11 mol/L) and high SERS enhancement (EF = 9.2 × 107). In addition, the g-C3N4/Au/Ag NRs could degrade ˃90% of BPA adsorbed on the substrate surface within 140 min under visible light irradiation, and maintains its SERS activity after repeated use for 4 times. The dual-function platform with high SERS response and excellent recycling capability is proved to be reliable and is very promising for monitoring of BPA residues in food.

Self-assembly of Au nanocubes for ultrasensitive detection of Alzheimer's disease biomarkers by SERS

Since presently Alzheimer's disease (AD) is incurable, early diagnosis of AD is crucial. Aβ 1-42 and tau-441 proteins are promising core biomarkers for early diagnosis and early therapeutic intervention in AD. Here we constructed a surface-enhanced Raman spectroscopy (SERS) biosensor for highly sensitive quantitative detection of Aβ 1-42 and tau proteins by preparing gold nanocube (AuNC) superlattices through evaporation self-assembly. The results showed that the method has a wide response range (0.1-10 000 ng mL-1 and 0.01-1000 ng mL-1, respectively) and high sensitivity. The detection limits of Aβ1-42 and tau protein were 0.0416 ng mL-1 and 0.0087 ng mL-1, respectively. In addition, the method was able to rapidly and simultaneously detect the two biomarkers in serum, which showed the feasibility of the method in complex biological environments. The detection of Aβ 1-42 and tau protein has great potential for the accurate prediction and early diagnosis of Alzheimer's disease.

Manipulating Coupled Field Enhancement in Slot-under-Groove Nanoarrays for Universal Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) is an ultrasensitive spectroscopic technique that can identify materials and chemicals based on their inelastic light-scattering properties. In general, SERS relies on sub-10 nm nanogaps to amplify the Raman signals and achieve ultralow-concentration identification of analytes. However, large-sized analytes, such as proteins and viruses, usually cannot enter these tiny nanogaps, limiting the practical applications of SERS. Herein, we demonstrate a universal SERS platform for the reliable and sensitive identification of a wide range of analytes. The key to this success is the prepared "slot-under-groove" nanoarchitecture arrays, which could realize a strongly coupled field enhancement with a large spatial mode distribution via the hybridization of gap-surface plasmons in the upper V-groove and localized surface plasmon resonance in the lower slot. Therefore, our slot-under-groove platform can simultaneously deliver high sensitivity for small-sized analytes and the identification of large-sized analytes with a large Raman gain.

Highly Uniform Self-Assembly of Gold Nanoparticles by Butanol-Induced Dehydration and Its SERS Applications in SARS-CoV-2 Detection

We report the development of a reproducible and highly sensitive surface-enhanced Raman scattering (SERS) substrate using a butanol-induced self-assembly of gold nanoparticles (AuNPs) and its application as a rapid diagnostic platform for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The butanol-induced self-assembly process was used to generate a uniform assembly of AuNPs, with multiple hotspots, to achieve high reproducibility. When an aqueous droplet containing AuNPs and target DNAs was dropped onto a butanol droplet, butanol-induced dehydration occurred, enriching the target DNAs around the AuNPs and increasing the loading density of the DNAs on the AuNP surface. The SERS substrate was evaluated by using Raman spectroscopy, which showed strong electromagnetic enhancement of the Raman signals. The substrate was then tested for the detection of SARS-CoV-2 using SERS, and a very low limit of detection (LoD) of 3.1 × 10-15 M was obtained. This provides sufficient sensitivity for the SARS-CoV-2 screening assay, and the diagnostic time is significantly reduced as no thermocycling steps are required. This study demonstrates a method for the butanol-induced self-assembly of AuNPs and its application as a highly sensitive and reproducible SERS substrate for the rapid detection of SARS-CoV-2. The results suggest the potential of this approach for developing rapid diagnostic platforms for other biomolecules and infectious diseases.

Rapid Detection of the Monkeypox Virus Genome and Antigen Proteins Based on Surface-Enhanced Raman Spectroscopy

The conventional detection methods cannot satisfy the need for early and rapid detection of monkeypox virus (MPXV) infection. This is due to complicated pretreatment, time consumption, and complex operation of the diagnostic tests. Based on surface-enhanced Raman spectroscopy (SERS), this study attempted to capture the characteristic fingerprints of the MPXV genome and multiple antigenic proteins without the need to design specific probes. The minimum detection limit of this method is 100 copies/mL, with good reproducibility and signal-to-noise ratio. Therefore, the relationship between characteristic peak intensity and the protein and nucleic acid concentration can be used to construct a concentration-dependent spectral line with a good linear relationship. Additionally, principal component analysis (PCA) could identify the SERS spectra of four different MPXV proteins in serum. Therefore, this rapid detection method in the current outbreak of monkeypox control and the future response to possible new outbreaks has broad application prospects.

Detection of live SARS-CoV-2 virus and its variants by specially designed SERS-active substrates and spectroscopic analyses

A method using label-free surface enhanced Raman spectroscopy (SERS) based on substrate design is provided for an early detection and differentiation of spike glycoprotein mutation sites in live SARS-CoV-2 variants. Two SERS-active substrates, Au nanocavities (Au NCs) and Au NPs on porous ZrO₂ (Au NPs/pZrO₂), were used to identify specific peaks of A.3, Alpha, and Delta variants at different concentrations and demonstrated the ability to provide their SERS spectra with detection limits of 0.1–1.0% (or 10⁴⁻⁵ copies/mL). Variant identification can be achieved by cross-examining reference spectra and analyzing the substrate-analyte relationship between the suitability of the analyte upon the hotspot(s) formed at high concentrations and the effective detection distance at low concentrations. Mutation sites on the S1 chain of the spike glycoprotein for each variant may be related and distinguishable. This method does not require sample preprocessing and therefore allows for fast screening, which is of high value for more comprehensive and specific studies to distinguish upcoming variants.

LSP-SPP Coupling Structure Based on Three-Dimensional Patterned Sapphire Substrate for Surface Enhanced Raman Scattering Sensing

Although the fabrication of controllable three-dimensional (3D) microstructures on substrates has been proposed as an effective solution for SERS, there remains a gap in the detection and manufacturability of 3D substrates with high performance. In this study, photolithography is adopted to obtain a pyramid-like array on a patterned sapphire substrate (PSS), with Al₂O₃ as the dielectric layer. In addition, silver nanoparticles (AgNPs) are used to decorate Au films to obtain mass-producible 3D SRES substrates. In the case of low fluorescence, the substrate realizes the coupling of localized surface plasmon polaritons (LSPs) and surface plasmon polaritons (SPPs), which is consistent with the simulation results obtained using the finite element method. The performance of the SERS substrate is evaluated using rhodamine 6G (R6G) and toluidine blue (TB) as probe molecules with detection limits of 10^-11 M and 10^-9 M, respectively. The substrate exhibits high hydrophobicity and excellent light-capturing capability. Moreover, it shows self-cleaning ability and long-term stability in practical applications. Allowing for the consistency of the composite substrate in the preparation process and the high reproducibility of the test results, it is considered to be promising for mass production.

A robust SERS calibration using a pseudo-internal intensity reference

Surface-enhanced Raman scattering (SERS) with high molecular sensitivity and specificity is a powerful nondestructive analytical tool. Since its discovery, SERS measurements have suffered from the vulnerability of calibration curve, which makes quantification analysis a great challenge. In this work, we report a robust calibration method by introducing a referenced measurement as the intensity standard. This intensity reference not only has the advantages of the internal standard method such as reflecting the SERS substrate enhancement, but also avoids the introduction of competing adsorption between target molecules and the internal standard. Based on the normalized calibration curve, the magnitude of the R6G concentration can be well evaluated from 10^-7 M to 10^-12 M. Furthermore, we demonstrate that this pseudo-internal standard method can also work well using a different type of molecule as the reference. This SERS calibration method would be beneficial for the development of quantitative SERS analysis.

Raman spectroscopy for viral diagnostics

Raman spectroscopy offers the potential for fingerprinting biological molecules at ultra-low concentration and therefore has potential for the detection of viruses. Here we review various Raman techniques employed for the investigation of viruses. Different Raman techniques are discussed including conventional Raman spectroscopy, surface-enhanced Raman spectroscopy, Raman tweezer, tip-enhanced Raman Spectroscopy, and coherent anti-Stokes Raman scattering. Surface-enhanced Raman scattering can play an essential role in viral detection by multiplexing nanotechnology, microfluidics, and machine learning for ensuring spectral reproducibility and efficient workflow in sample processing and detection. The application of these techniques to diagnose the SARS-CoV-2 virus is also reviewed.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-023-01059-4.

Capillary-force-assisted self-assembly of gold nanoparticles into highly ordered plasmonic thin films for ultrasensitive SERS

In this study, a capillary device based on the surface plasmon-enhanced Raman scattering effect was prepared by a simple and easy method. First, the capillary was treated with APTES solution. Due to the electrostatic effect, gold nanoparticles could be easily and tightly assembled in the capillary inner wall. On this basis, the effects of changing the concentration of APTES, the concentration of colloids and the soaking time of the capillary in the colloids on the assembly of gold nanoparticles on the inner wall of the capillary were studied, and the SERS enhancement effect under different conditions was analyzed, and the optimal solution was successfully found. At the same time, the reason why the capillary substrate shows better SERS performance than the traditional planar substrate is deeply discussed. Since the nanoparticles can be attached to the upper and lower surfaces of the inner wall of the capillary, the utilization rate of nanoparticles and laser is improved, thereby achieving higher enhancement. For the detection of the probe molecule rhodamine 6G, it was proved that the substrate has good uniformity and the lowest detection limit can reach 10^-10 M. Finally, the real-life pesticide thiram and the food additive aspartame were tested, and the detection limits could reach 10^-6 M and 0.25 g L^-1. It is confirmed that the prepared capillary shows excellent SERS performance and can be used for rapid detection in various fields.

Double-sided plasmonic metasurface for simultaneous biomolecular separation and SERS detection

Porous membrane-based nanofiltration separation of small biomolecules is a widely used biotechnology for which size-based selectivity is a critical parameter of technological relevance. Efficient determination of size selectivity calls for an advanced detection method capable of performing sensitive, rapid, and on-membrane examination. Surface-enhanced Raman spectroscopy (SERS) is such a detection method that has been widely recognized as an ultrasensitive technique for trace-level detection with sensitivity down to the single-molecule level. In this work, we for the first time develop a double-sided hierarchical porous membrane-like plasmonic metasurface to realize high-selectivity bimolecular separation and simultaneous ultrasensitive SERS detection. This highly flexible device, consisting of subwavelength nanocone pairs surrounded by randomly orientated sub-5 nm nanogrooves, was prepared by combining customized "top-down" fabrication of conical nanopores in an ion-track registered polycarbonate membrane and self-assembly of nanogrooves on the membrane surface through physical vapor deposition. The unique tip-to-tip oriented conical nanopores in the device enables excellent size-based molecular selectivity; the hierarchical groove-pore structure supports a peculiar cascaded electromagnetic near-field enhancement mechanism, endowing the device with SERS-based molecular detection of ultrahigh sensitivity, uniformity, repeatability, and polarization independence. With such dual structural merits and performance enhancement, we demonstrate effective nanofiltration separation of small-sized adenine from big-sized ss-DNA and synergistic SERS determination of their species. We experimentally demonstrate an ultrasensitive detection of 4-mercaptopyridine down to 10 pM. Together with its unparalleled mechanical flexibility, this double-side-responsive plasmonic metasurface membrane can find great potential in real-world molecular filtration and detection under extremely complex working conditions.

Aptamer-coated track-etched membranes with a nanostructured silver layer for single virus detection in biological fluids

Aptasensors based on surface-enhanced Raman spectroscopy (SERS) are of high interest due to the superior specificity and low limit of detection. It is possible to produce stable and cheap SERS-active substrates and portable equipment meeting the requirements of point-of-care devices. Here we combine the membrane filtration and SERS-active substrate in the one pot. This approach allows efficient adsorption of the viruses from the solution onto aptamer-covered silver nanoparticles. Specific determination of the viruses was provided by the aptamer to influenza A virus labeled with the Raman-active label. The SERS-signal from the label was decreased with a descending concentration of the target virus. Even several virus particles in the sample provided an increase in SERS-spectra intensity, requiring only a few minutes for the interaction between the aptamer and the virus. The limit of detection of the aptasensor was as low as 10 viral particles per mL (VP/mL) of influenza A virus or 2 VP/mL per probe. This value overcomes the limit of detection of PCR techniques (∼10³ VP/mL). The proposed biosensor is very convenient for point-of-care applications.

Plasmonic Biosensors with Nanostructure for Healthcare Monitoring and Diseases Diagnosis

Nanophotonics has been widely utilized in enhanced molecularspectroscopy or mediated chemical reaction, which has major applications in the field of enhancing sensing and enables opportunities in developing healthcare monitoring. This review presents an updated overview of the recent exciting advances of plasmonic biosensors in the healthcare area. Manufacturing, enhancements and applications of plasmonic biosensors are discussed, with particular focus on nanolisted main preparation methods of various nanostructures, such as chemical synthesis, lithography, nanosphere lithography, nanoimprint lithography, etc., and describing their respective advances and challenges from practical applications of plasmon biosensors. Based on these sensing structures, different types of plasmonic biosensors are summarized regarding detecting cancer biomarkers, body fluid, temperature, gas and COVID-19. Last, the existing challenges and prospects of plasmonic biosensors combined with machine learning, mega data analysis and prediction are surveyed.

Engineered Au@CuO Nanoparticles for Wide-Range Quantitation of Sulfur Ions by Surface-Enhanced Raman Spectroscopy

Efficient detection of sulfide ions (S^2-), especially in a wide quantitative range, is of significance but faces challenges. This work strategizes and fabricates Au@CuO nanoparticles for quantitative surface-enhanced Raman spectroscopy (SERS) detection of the S^2- ions based on the S^2- concentration-dependent ion-solid interactions. We have achieved fast and quantitative S^2- detection in a wide range from 5 ppb to 64,000 ppm (saturation concentration of the S^2- source). We also demonstrated that the optimal CuO shell thickness for the detection is about 7 nm and that the detection can be further improved by prolonging the soaking duration. Moreover, this detection method has also shown the merits of reusable substrates (especially for low S^2- concentrations) and good anti-interference ability to many common anions (Cl^-, NO₃^-, OH^-, HCOO^-, CO₃^2-, and SO₄^2-). Finally, the high feasibility of this detection in actual water (tap water and pond water) has also been demonstrated. This work provides efficient S^2- detection with great potential in practical use and also inspires the design of quantifiable SERS substrates for detecting more small inorganic molecules and ions.

3D Flexible SERS Substrates Integrated with a Portable Raman Analyzer and Wireless Communication for Point-of-Care Application

With the development of flexible surface-enhanced Raman spectroscopy (SERS) substrates that can realize rapid in situ detection, the SERS technique accompanied by miniaturized Raman spectrometers holds great promise for point-of-care testing (POCT). For an in situ detection strategy, constructing high-performance flexible and transparent SERS substrates through a facile and cost-effective fabrication method is critically important. Herein, we present a simple method for fabricating a large-area flexible and transparent SERS substrate consisting of a silver-nanoparticle-grafted wrinkled polydimethylsiloxane (Ag NPs@W-PDMS) film, using a surface-wrinkling technique and magnetron sputtering technology. By characterizing rhodamine 6G as a probe molecule with a portable Raman spectrometer, the flexible SERS substrate shows a low detection limit (10^-7 M), a high enhancement factor (6.11 × 10⁶), and excellent spot-spot and batch-batch reproducibilities (9.0% and 4.2%, respectively). Moreover, the Ag NPs@W-PDMS substrate maintains high SERS activity under bending and twisting mechanical deformations of over 100 cycles, as well as storage in air for 30 days. To evaluate its practical feasibility, in situ detection of malachite green on apple and tomato peels is performed with a detection limit of 10^-6 M. In addition, for point-of-care analysis, we develop a wireless transmission system to transmit the collected SERS spectral data to a computer in real time for signal processing and analysis. Therefore, the proposed Ag NPs@W-PDMS SERS substrate fabricated through a simple and mass-producible method, combined with the utilization of a portable Raman spectrometer and wireless communication, offers a promising opportunity to extend the SERS technique from the laboratory to POCT applications.

Recent Progress on Solid Substrates for Surface-Enhanced Raman Spectroscopy Analysis

Surface-enhanced Raman spectroscopy (SERS) is a powerful vibrational spectroscopy technique with distinguished features of non-destructivity, ultra-sensitivity, rapidity, and fingerprint characteristics for analysis and sensors. The SERS signals are mainly dependent on the engineering of high-quality substrates. Recently, solid SERS substrates with diverse forms have been attracting increasing attention due to their promising features, including dense hot spot, high stability, controllable morphology, and convenient portability. Here, we comprehensively review the recent advances made in the field of solid SERS substrates, including their common fabrication methods, basic categories, main features, and representative applications, respectively. Firstly, the main categories of solid SERS substrates, mainly including membrane substrate, self-assembled substrate, chip substrate, magnetic solid substrate, and other solid substrate, are introduced in detail, as well as corresponding construction strategies and main features. Secondly, the typical applications of solid SERS substrates in bio-analysis, food safety analysis, environment analysis, and other analyses are briefly reviewed. Finally, the challenges and perspectives of solid SERS substrates, including analytical performance improvement and largescale production level enhancement, are proposed.

Iron Oxide-Au Magneto-Plasmonic Heterostructures: Advances in Their Eco-Friendly Synthesis

In recent years, nanotechnologies have attracted considerable interest, especially in the biomedical field. Among the most investigated particles, magnetic based on iron oxides and Au nanoparticles gained huge interest for their magnetic and plasmonic properties, respectively. These nanoparticles are usually produced starting from processes and reagents that can be the cause of potential human health and environmental concerns. For this reason, there is a need to develop simple, green, low-cost, and non-toxic synthesis methods and reagents. This review aims at providing an overview of the most recently developed processes to produce iron oxide magnetic nanoparticles, Au nanoparticles, and their magneto-plasmonic heterostructures using eco-friendly approaches, focusing the attention on the microorganisms and plant-assisted syntheses and showing the first results of the development of magneto-plasmonic heterostructures.

Nanoscale mapping of shifts in dark plasmon modes in sub 10 nm aluminum nanoantennas

In this work, we report the fabrication and spectroscopic characterization of subwavelength aluminum nanocavities-consisting of hexamer or tetramer clusters of sub 10 nm width Al nanorods-with tunable localized surface plasmon resonance (LSPR) energies on suspended SiN_xmembranes. Here the volume plasmon (VP) and LSPR modes of lithographically-fabricated Al nanocavities are revealed by low-loss electron energy-loss spectroscopy (EELS) in an aberration corrected scanning transmission electron microscope (STEM). We show that the existence of grain boundaries (GBs) in these nanocavities results in shifts in the VP energy and a reduction in the VP lifetime. We map the VP energy and lifetime across GBs and we observe a decrease in VP energy and lifetime at GBs that is consistent with a reduction in free carrier density and increased plasmon scattering at these locations. Dipolar LSPR modes resonant in the UV and blue regions of the electromagnetic spectrum as well as higher-energy optically dark quadrupolar and hexapolar LSPR modes are also observed and mapped by STEM and EELS. All LSPR modes are confirmed via electromagnetic simulations based on the boundary element method. Both tetramer and hexamer structures support the excitation of dipolar bright and dipolar dark modes. Finally, we find that asymmetries in fabricated nanorod hexamer and tetramer nanocavities result in a mode mixing leading to a shift in dipolar dark LSPR modes.

Kerr nonlinear medium assisted double-face absorbers for differential manipulation via an all-optical operation

Recently, light absorbers have attracted great attentions due to their promising in applications in functional optoelectronic devices. Herein, we theoretically propose and numerically demonstrate a new absorber platform, which consists of a 280-nm-thick photonic nonlinear waveguide film covering on the metal grating structure. Strong reflection inhibition and absorption enhancement is achieved in both the forward and backward directions, which indicates potential novel performances since the previous reports only achieved absorption in one side due to the using of opaque metal film substrate or the reflective mirror. The anti-reflection bands or the absorption peaks at the shorter and longer wavelength ranges are related to the excitation of the propagating surface plasmon resonance by the slit-assisted grating and the cavity mode by the slit in the metal film. Strong differential manipulation is realized for the double-face absorbers via the all-optical operation. Moreover, the operation wavelengths for the double-face light absorber can be modified strongly via using an asymmetric dielectric medium for the coating films. These new findings pave approaches for subtractive lightwave modulation technology, selective filtering, multiplex sensing and detection, etc.

Particle-in-Molybdenum Disulfide-Coated Cavity Structure with a Raman Internal Standard for Sensitive Raman Detection of Water Contaminants from Ions to <300 nm Nanoplastics

To develop a universal and precise detection strategy that can be applied to water contaminants of various sizes, we designed a particle-in-MoS₂ coated cavity structure of AAO/MoS₂/Ag with a Raman internal standard. This modified particle-in-cavity structure not only successfully integrates both "surface hot spots" and "volume hot spots" via dressing and manipulating the cascaded optical-field mode inside the cavity but also introduces the chemical enhancement and internal standard attribute of MoS₂. Because of its unique three-dimensional structure, AAO/MoS₂/Ag accurately detects water contaminants of various sizes from ions to nanoplastics (<300 nm) for the first time. This work proposes a novel and universal surface-enhanced Raman scattering strategy for detecting multiple-size water contaminants and demonstrates the potential to build a security line in early warning systems for the prevention of water pollution.

Rapid detection of viruses: Based on silver nanoparticles modified with bromine ions and acetonitrile

Nearly 200 million people have been diagnosed with COVID-19 since the outbreak in 2019, and this disease has claimed more than 5 million lives worldwide. Currently, researchers are focusing on vaccine development and the search for an effective strategy to control the infection source. This work designed a detection platform based on Surface-Enhanced Raman Spectroscopy (SERS) by introducing acetonitrile and calcium ions into the silver nanoparticle reinforced substrate system to realize the rapid detection of novel coronavirus. Acetonitrile may amplify the calcium-induced hot spots of silver nanoparticles and significantly enhanced the stability of silver nanoparticles. It also elicited highly sensitive SERS signals of the virus. This approach allowed us to capture the characteristic SERS signals of SARS-CoV-2, Human Adenovirus 3, and H1N1 influenza virus molecules at a concentration of 100 copies/test (PFU/test) with upstanding reproduction and signal-to-noise ratio. Machine learning recognition technology was employed to qualitatively distinguish the three virus molecules with 1000 groups of spectra of each virus. Acetonitrile is a potent internal marker in regulating the signal intensity of virus molecules in saliva and serum. Thus, we used the SERS peak intensity to quantify the virus content in saliva and serum. The results demonstrated a satisfactory linear relationship between peak intensity and protein concentration. Collectively, this rapid detection method has a broad application prospect in clinical diagnosis of viruses, management of emergent viral infectious diseases, and exploration of the interaction between viruses and host cells.

High-Throughput Surface-Enhanced Raman Scattering for Screening Chemical Sensor Candidates Enabled by Bipolar Electrochemistry

A variety of hydrothermal or electrochemical methods have been explored to prepare noble metal nanostructures as surface-enhanced Raman scattering (SERS) substrates. However, most of those metallic nanoarrays are structurally homogeneous, which makes it laborious to select the high-performance substrates for particular Raman sensing purposes. Here, a high-throughput SERS imaging strategy is demonstrated for the first time for screening chemical sensors with sub-nanomolar sensitivities. Bipolar electrochemistry was applied to generate Au or Au-Ag gradient nanoarrays with diverse chemical compositions, morphologies, and particle dimensions ranging from several nanometers to micrometers. The selected "hot-spots" on the Au-Ag alloy nanoarray exhibited a 660-fold enhancement in SERS intensity compared to those on the pure Au gradient nanoarray. The SERS screening of 4-aminothiophenol, 4-nitrothiophenol, and 4-mercaptobenzoic acid was carried out that provided a limit of detection (LOD) between 1 and 5 pM. The distinctive LODs among three thiophenolic Raman probes are ascribed to the differences in the affinity of the probe to the alloy, orientation of the metal-ligand monolayer, or plasmonic environment of the nanoarray surface. As a continuous, rapid, and cost-effective manner to fabricate transitional nanostructures and screen out SERS responsive sites, this method not only facilitates controllable synthesis of noble metal nanoarrays but has the potential to provide an alternative tool for ultrasensitive chemical sensing on a wide range of bimetallic substrates.

Label-Free Spectroscopic SARS-CoV-2 Detection on Versatile Nanoimprinted Substrates

Widespread testing and isolation of infected patients is a cornerstone of viral outbreak management, as underscored during the ongoing COVID-19 pandemic. Here, we report a large-area and label-free testing platform that combines surface-enhanced Raman spectroscopy and machine learning for the rapid and accurate detection of SARS-CoV-2. Spectroscopic signatures acquired from virus samples on metal-insulator-metal nanostructures, fabricated using nanoimprint lithography and transfer printing, can provide test results within 25 min. Not only can our technique accurately distinguish between different respiratory and nonrespiratory viruses, but it can also detect virus signatures in physiologically relevant matrices such as human saliva without any additional sample preparation. Furthermore, our large area nanopatterning approach allows sensors to be fabricated on flexible surfaces allowing them to be mounted on any surface or used as wearables. We envision that our versatile and portable label-free spectroscopic platform will offer an important tool for virus detection and future outbreak preparedness.

Applications of surface-enhanced Raman spectroscopy in environmental detection

As the human population grows, the anthropogenic impacts from various agricultural and industrial processes produce unwanted contaminants in the environment. The accurate, sensitive and rapid detection of such contaminants is vital for human health and safety. Surface-enhanced Raman spectroscopy (SERS) is a valuable analytical tool with wide applications in environmental contaminant monitoring. The aim of this review is to summarize recent advancements within SERS research as it applies to environmental detection, with a focus on research published or accessible from January 2021 through December 2021 including early-access publications. Our goal is to provide a wide breadth of information that can be used to provide background knowledge of the field, as well as inform and encourage further development of SERS techniques in protecting environmental quality and safety. Specifically, we highlight the characteristics of effective SERS nanosubstrates, and explore methods for the SERS detection of inorganic, organic, and biological contaminants including heavy metals, pharmaceuticals, plastic particles, synthetic dyes, pesticides, viruses, bacteria and mycotoxins. We also discuss the current limitations of SERS technologies in environmental detection and propose several avenues for future investigation. We encourage researchers to fill in the identified gaps so that SERS can be implemented in a real-world environment more effectively and efficiently, ultimately providing reliable and timely data to help and make science-based strategies and policies to protect environmental safety and public health.

Atomically Thin TaSe2 Film as a High-Performance Substrate for Surface-Enhanced Raman Scattering

An atomically thin TaSe₂ sample, approximately containing two to three layers of TaSe₂ nanosheets with a diameter of 2.5 cm is prepared here for the first time and applied on the detection of various Raman-active molecules. It achieves a limit of detection of 10^-10  m for rhodamine 6G molecules. The excellent surface-enhanced Raman scattering (SERS) performance and underlying mechanism of TaSe₂ are revealed using spectrum analysis and density functional theory. The large adsorption energy and the abundance of filled electrons close to the Fermi level are found to play important roles in the chemical enhancement mechanism. Moreover, the TaSe₂ film enables highly sensitive detection of bilirubin in serum and urine samples, highlighting the potential of using 2D SERS substrates for applications in clinical diagnosis, for example, in the diagnosis of jaundice caused by excess bilirubin in newborn children.

Abnormally Weak Surface-Enhanced Raman Scattering Activity of Tip-Rich Au Nanostars: The Role of Interfacial Defects

Designing and regulating the geometry of a given plasmonic metal (Au, Ag, etc.) has become one of the most efficient approaches to achieve highly active surface-enhanced Raman spectroscopy (SERS) substrates, but this work demonstrates that plain efforts on this may not be enough. Here, we report that the often-neglected inner crystal defects also have huge impacts on the SERS activity, through a case of Au nanostars (NSs) with good SERS geometry but rich in defects. The results suggest that the interfacial defects (twin boundaries and superlattices) in the NSs aggravate the electronic oscillation damping via reducing the free path of electron scattering. This eventually results in weak local electromagnetic fields near the NS surfaces (or weak SERS activity of the NSs). This study has demonstrated the huge impact of interfacial defects on SERS activity and thus has a significant guideline for the design and fabrication of efficient SERS substrates.

Simple self-organization-based synthesis of gold nanoparticle-implanted ZnO aerogels with good sensing performance to gaseous ethanol

Simple fabrication of metal-modified oxide aerogels is expected but remains challenging. This work presents a sample one-pot synthesis method for gold nanoparticle (NP) implanted ZnO (Au-ZnO) aerogels just by sequentially adding (CH₃COO)₂Zn and NaBH₄solutions into a pre-prepared Au colloidal solution. The typically fabricated Au-ZnO aerogels are constituted by ZnO networks implanted with uniform Au NPs. The Au NPs had a size of about 100 nm, and the ZnO nanochains in the networks were about 10 nm in thickness. Further, the proportion of the Au NPs in the final aerogels could be tuned by using different amounts of the Zn precursors. Furthermore, a mechanism based on metal oxidation and oriented connection growth (a self-organization process) has been presented for describing the formation of such Au-ZnO aerogels. In the typical formation, the Zn²⁺ions first convert into ZnO beads, and then are self-organized to form networks wrapping the colloidal Au NPs under the effect of linker molecules, and this matches well with the observed experimental results. Most importantly, these Au-ZnO aerogels show great structurally enhanced gas sensing properties to gaseous ethanol compared with a pure ZnO film. They have a fast response (about 30 s), a high selectivity, and quantitative sensing to the target gas. This work has provided a simple preparation method for Au-ZnO aerogels, and also shows their great potential in gas sensing applications.

Three-dimensional surface-enhanced Raman scattering substrates constructed by integrating template-assisted electrodeposition and post-growth of silver nanoparticles

Three-dimensional (3D) plasmonic nano-arrays can provide high surface-enhanced Raman scattering (SERS) sensitivity, good spectral uniformity and excellent reproducibility. However, it is still a challenge to develop a simple and efficient method for fabrication of 3D plasmonic nano-arrays with high SERS performance. Here we report a facile approach to construct ordered arrays of silver (Ag) nanoparticles-assembled spherical micro-cavities using polystyrene (PS) sphere template-assisted electrodeposition and post-growth. The electrodeposited small Ag nanoparticles grow into bigger stable nanoparticles during the post-growth process, which could significantly improve the SERS sensitivity. The Ag nanoparticles-assembled 3D micro-cavity array provides much more hotspots in the excitation laser beam-covered volume than the two-dimensional counterpart. The relative standard deviation (RSD) of 612 cm^-1 peak of rhodamine 6G (R6G) was calculated to be 8%, and the RSD of the characteristic peak taken from substrates of different batches was less than 10%. The detectable lower concentration as low as 1 fM was achieved for an aqueous solution of R6G. Such SERS substrate also showed high sensitivity to thiram (fungicide) and paraquat (herbicide) in water with limits of detection of 0.067 nM and 2.5 nM respectively. Furthermore, it also demonstrated that SERS detection of pesticide residues on fruits can be realized, showing a potential application in rapid monitoring food safety.

Identifying infectiousness of SARS-CoV-2 by ultra-sensitive SnS2 SERS biosensors with capillary effect

The current COVID-19 pandemic urges us to develop ultra-sensitive surface-enhanced Raman scattering (SERS) substrates to identify the infectiousness of SARS-CoV-2 virions in actual environments. Here, a micrometer-sized spherical SnS₂ structure with the hierarchical nanostructure of "nano-canyon" morphology was developed as semiconductor-based SERS substrate, and it exhibited an extremely low limit of detection of 10^-13 M for methylene blue, which is one of the highest sensitivities among the reported pure semiconductor-based SERS substrates. Such ultra-high SERS sensitivity originated from the synergistic enhancements of the molecular enrichment caused by capillary effect and the charge transfer chemical enhancement boosted by the lattice strain and sulfur vacancies. The novel two-step SERS diagnostic route based on the ultra-sensitive SnS₂ substrate was presented to diagnose the infectiousness of SARS-CoV-2 through the identification standard of SERS signals for SARS-CoV-2 S protein and RNA, which could accurately identify non-infectious lysed SARS-CoV-2 virions in actual environments, whereas the current PCR methods cannot.

Two-Dimensional Self-Assembly of Au@Ag Core-Shell Nanocubes with Different Permutations for Ultrasensitive SERS Measurements

Different self-assembly methods not only directly change the arrangement of noble metal particles on the substrate but also indirectly affect the local electromagnetic field distribution and intensity of the substrate under specific optical excitation conditions, which leads to distinguished different enhancement effects of the structure on molecular Raman signals. In this paper, first, the gold species growth method was used to prepare the silver-coated gold nanocubes (Au@Ag NCs) with regular morphology and uniform size, and then the two-phase and three-phase liquid-liquid self-assembly and evaporation-induced self-assembly methods were used to obtain the substrate structure with different NC arrangement patterns. The optimal arrangement of NCs was found by transverse comparison of Raman signal detection of probe molecules with the same concentration. Subsequently, surface-enhanced Raman scattering (SERS) measurements of Rhodamine (Rh6G) and aspartame (APM) were carried out. Furthermore, the finite element method (FEM) was employed to calculate the local electromagnetic fields of the substrates with different Au@Ag NC arrangements, and the calculated results were in agreement with the experimental results. The experimental results show that the SERS-active substrate was largely associated with the different arrangements of Au@Ag NCs, and the island membrane Au@Ag NCs array substrate obtained by evaporation-induced self-assembly can generate a strong local electromagnetic field due to the edge and corner bonding gap between the tightly arranged NCs; this endows the substrate with benign sensitivity and reproducibility and has great potential in molecular detection, biosensing, and food safety monitoring.

Advances in the Application of Exosomes Identification Using Surface-Enhanced Raman Spectroscopy for the Early Detection of Cancers

Exosomes are small nanoscale vesicles with a double-layered lipid membrane structure secreted by cells, and almost all types of cells can secrete exosomes. Exosomes carry a variety of biologically active contents such as nucleic acids and proteins, and play an important role not only in intercellular information exchange and signal transduction, but also in various pathophysiological processes in the human body. Surface-enhanced Raman Spectroscopy (SERS) uses light to interact with nanostructured materials such as gold and silver to produce a strong surface plasmon resonance effect, which can significantly enhance the Raman signal of molecules adsorbed on the surface of nanostructures to obtain a rich fingerprint of the sample itself or Raman probe molecules with ultra-sensitivity. The unique advantages of SERS, such as non-invasive and high sensitivity, good selectivity, fast analysis speed, and low water interference, make it a promising technology for life science and clinical testing applications. In this paper, we briefly introduce exosomes and the current main detection methods. We also describe the basic principles of SERS and the progress of the application of unlabeled and labeled SERS in exosome detection. This paper also summarizes the value of SERS-based exosome assays for early tumor diagnosis.

Advanced Point-of-Care Testing Technologies for Human Acute Respiratory Virus Detection

The ever-growing global threats to human life caused by the human acute respiratory virus (RV) infections have cost billions of lives, created a significant economic burden, and shaped society for centuries. The timely response to emerging RVs could save human lives and reduce the medical care burden. The development of RV detection technologies is essential for potentially preventing RV pandemic and epidemics. However, commonly used detection technologies lack sensitivity, specificity, and speed, thus often failing to provide the rapid turnaround times. To address this problem, new technologies are devised to address the performance inadequacies of the traditional methods. These emerging technologies offer improvements in convenience, speed, flexibility, and portability of point-of-care test (POCT). Herein, recent developments in POCT are comprehensively reviewed for eight typical acute respiratory viruses. This review discusses the challenges and opportunities of various recognition and detection strategies and discusses these according to their detection principles, including nucleic acid amplification, optical POCT, electrochemistry, lateral flow assays, microfluidics, enzyme-linked immunosorbent assays, and microarrays. The importance of limits of detection, throughput, portability, and specificity when testing clinical samples in resource-limited settings is emphasized. Finally, the evaluation of commercial POCT kits for both essential RV diagnosis and clinical-oriented practices is included.

Recent advances of vibrational spectroscopy and chemometrics for forensic biological analysis

Biological materials found at a crime scene are crucially important evidence for forensic investigation because they provide contextual information about a crime and can be linked to the donor-individuals through combination with DNA analysis. Applications of vibrational spectroscopy to forensic biological analysis have been emerging because of its advantageous characteristics such as the non-destructivity, rapid measurement, and quantitative evaluation, compared to most current methods based on histological observation or biochemical techniques. This review presents an overview of recent developments in vibrational spectroscopy for forensic biological analysis. We also emphasize chemometric techniques, which can elicit reliable and advanced analytical outputs from highly complex spectral data from forensic biological materials. The analytical subjects addressed herein include body fluids, hair, soft tissue, bones, and bioagents. Promising applications for various analytical purposes in forensic biology are presented. Simultaneously, future avenues of study requiring further investigation are discussed.

Human ACE2-Functionalized Gold "Virus-Trap" Nanostructures for Accurate Capture of SARS-CoV-2 and Single-Virus SERS Detection

The current COVID-19 pandemic urges the extremely sensitive and prompt detection of SARS-CoV-2 virus. Here, we present a Human Angiotensin-converting-enzyme 2 (ACE2)-functionalized gold "virus traps" nanostructure as an extremely sensitive SERS biosensor, to selectively capture and rapidly detect S-protein expressed coronavirus, such as the current SARS-CoV-2 in the contaminated water, down to the single-virus level. Such a SERS sensor features extraordinary 10⁶-fold virus enrichment originating from high-affinity of ACE2 with S protein as well as "virus-traps" composed of oblique gold nanoneedles, and 10⁹-fold enhancement of Raman signals originating from multi-component SERS effects. Furthermore, the identification standard of virus signals is established by machine-learning and identification techniques, resulting in an especially low detection limit of 80 copies mL^-1 for the simulated contaminated water by SARS-CoV-2 virus with complex circumstance as short as 5 min, which is of great significance for achieving real-time monitoring and early warning of coronavirus. Moreover, here-developed method can be used to establish the identification standard for future unknown coronavirus, and immediately enable extremely sensitive and rapid detection of novel virus.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-021-00620-8.

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.

Plasmonic alloy nanochains assembled via dielectrophoresis for ultrasensitive SERS

It is great challenge and interesting for researchers to fabricate substrates for enhanced Raman and sensor, and assemble some easy-to-synthesize metallic nanomaterials into controllable nanostructures with special morphologies and arrangements, via alternating current (AC) electric field. The Au-Ag alloy nanoparticles (Au-Ag alloy NPs) colloidal suspension with excellent dispersibility synthesized by wet chemical method, and the morphology of the assembly can be well controlled by regulating the frequency of the AC electric field. Au-Ag alloy nanochains array (Au-Ag ANCs) with dense plasmonic "hot spots" is formed when the AC electric field of 4V_pp-30kHz is applied, which is supported by the result of finite element method (FEM) numerical simulation. Experimental results demonstrate that Au-Ag ANCs show excellent SERS activity: Au-Ag ANCs can detect both Rhodamine 6G (Rh6G) and crystal violet (CV) in the magnitude order of 10^-10 M, and the Raman peaks intensity and analyte concentration has a strong linear correlation (R² is 0.99339 and 0.95916, respectively). Besides, the introduction of Au-Ag ANCs makes the Raman spectra intensity of thiram (a pesticide) with a concentration of 30 ppm on the surface of the blank ITO glass significantly enhanced, and it can detect thiram with a concentration as low as 0.03 ppm. In addition, Au-Ag ANCs substrate exhibits great uniformity and stability, so they have considerable application potential in the field of quantitative detection of trace substances.

A New Look into Cancer-A Review on the Contribution of Vibrational Spectroscopy on Early Diagnosis and Surgery Guidance

Simple Summary

Cancer is a leading cause of death worldwide, with the detection of the disease in its early stages, as well as a correct assessment of the tumour margins, being paramount for a successful recovery. While breast cancer is one of most common types of cancer, head and neck cancer is one of the types of cancer with a lower prognosis and poor aesthetic results. Vibrational spectroscopy detects molecular vibrations, being sensitive to different sample compositions, even when the difference was slight. The use of spectroscopy in biomedicine has been extensively explored, since it allows a broader assessment of the biochemical fingerprint of several diseases. This literature review covers the most recent advances in breast and head and neck cancer early diagnosis and intraoperative margin assessment, through Raman and Fourier transform infrared spectroscopies. The rising field of spectral histopathology was also approached. The authors aimed at expounding in a more concise and simple way the challenges faced by clinicians and how vibrational spectroscopy has evolved to respond to those needs for the two types of cancer with the highest potential for improvement regarding an early diagnosis, surgical margin assessment and histopathology.

Abstract

In 2020, approximately 10 million people died of cancer, rendering this disease the second leading cause of death worldwide. Detecting cancer in its early stages is paramount for patients’ prognosis and survival. Hence, the scientific and medical communities are engaged in improving both therapeutic strategies and diagnostic methodologies, beyond prevention. Optical vibrational spectroscopy has been shown to be an ideal diagnostic method for early cancer diagnosis and surgical margins assessment, as a complement to histopathological analysis. Being highly sensitive, non-invasive and capable of real-time molecular imaging, Raman and Fourier transform infrared (FTIR) spectroscopies give information on the biochemical profile of the tissue under analysis, detecting the metabolic differences between healthy and cancerous portions of the same sample. This constitutes tremendous progress in the field, since the cancer-prompted morphological alterations often occur after the biochemical imbalances in the oncogenic process. Therefore, the early cancer-associated metabolic changes are unnoticed by the histopathologist. Additionally, Raman and FTIR spectroscopies significantly reduce the subjectivity linked to cancer diagnosis. This review focuses on breast and head and neck cancers, their clinical needs and the progress made to date using vibrational spectroscopy as a diagnostic technique prior to surgical intervention and intraoperative margin assessment.

Noble metal modified ReS2 nanocavity for surface-enhanced Raman spectroscopy (SERS) analysis

The rhenium disulphide (ReS₂) nanocavity-based surface enhanced Raman scattering (SERS) substrates ware fabricated on the gold-modified silicon pyramid (PSi) by thermal evaporation technology and hydrothermal method. In this work, the ReS₂ nanocavity was firstly combined with metal nanostructures in order to improve the SERS properties of ReS₂ materials, and the SERS response of the composite structure exhibits excellent performance in sensitivity, uniformity and repeatability. Numerical simulation reveals the synergistic effect of the ReS₂ nanocavity and the plasmon resonance generated by the metal nanostructures. And the charge transfer between the metal, ReS₂ and the analytes was also verified and plays an non-ignorable role. Besides, the plasmon-driven reaction for p-nitrothiophenol (PNTP) to p,p'-dimercaptobenzene (DMAB) conversion was successfully in-situ monitored. Most importantly, it is found for the first time that the SERS properties of ReS₂ nanocavity-based substrates are strongly temperature dependent, and the SERS effect achieves the best performance at 45 °C. In addition, the low concentration detection of malachite green (MG) and crystal violet (CV) molecules in lake water shows its development potential in practical application.

Material-assisted mass spectrometric analysis of low molecular weight compounds for biomedical applications

Low molecular weight compounds play an important role in encoding the current physiological state of an individual. Laser desorption/ionization mass spectrometry (LDI MS) offers high sensitivity with low cost for molecular detection, but it is not able to cover small molecules due to the drawbacks of the conventional matrix. Advanced materials are better alternatives, showing little background interference and high LDI efficiency. Herein, we first classify the current materials with a summary of compositions and structures. Matrix preparation protocols are then reviewed, to enhance the selectivity and reproducibility of MS data better. Finally, we highlight the biomedical applications of material-assisted LDI MS, at the tissue, bio-fluid, and cellular levels. We foresee that the advanced materials will bring far-reaching implications in LDI MS towards real-case applications, especially in clinical settings.

Three-Dimensional Au/Ag Nanoparticle/Crossed Carbon Nanotube SERS Substrate for the Detection of Mixed Toxic Molecules

Research on engineering "hotspots" in the field of surface-enhanced Raman scattering (SERS) is at the forefront of contributing to the best sensing indicators. Currently, there is still an urgent need to design a high-strength and large-scale electric field distribution method in order to obtain an ideal SERS sensor. Here, we designed a three-dimensional (3D) Au/Ag nanoparticle (NP)/crossed carbon nanotube film SERS substrate. The proposed structure formed by the simple preparation process can perfectly coordinate the interaction between the SERS substrates, lasers, and molecules. The denser "hotspots" can be induced and then distributed in holes enclosed by Au/AgNPs and the gaps between them. This process was verified by numerical simulations. The experimental results show that the proposed SERS substrate possesses an excellent sensitivity of 10^-12 M (rhodamine 6G (R6G)), an enhancement factor of 1.60 × 10⁹, and a good signal reproducibility (the relative standard deviation is ~6.03%). We further use a Au/AgNP/crossed CNT substrate to detect complex solutions composed of toxic molecules, which shows that our proposed SERS substrate has a wide range of application potentials, especially in food safety.

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.

SERS substrate with wettability difference for molecular self-concentrating detection

The surface-enhanced Raman spectroscopy (SERS) has attracted much attention due to the powerful capability of quantificational analysis. Nowadays, most of the enhancement effect by SERS substrate is provided by the 'hot spots' occupying relatively small space. When the amount of analyte is too low, it is difficult to ensure that all the probe molecules can be placed into the 'hot spots', which is a headache in SERS quatification. In order to solve this problem, we have developed a structure of CuO nanowires/Ag nanoparticles with wettability capacity difference, which can aggregate molecules in water and oil simultaneously under two different mechanisms. The limit of detection and enhancement factor of this structure are estimated as 10^-15M and 1.55 × 10¹¹respectively (for rhodamine 6G, R6G). In a proof-in-principle experiment of sewage detection, it successfully achieved the aggregation and additional enhancement of both the R6G molecules in aqueous solution and thiuram molecules in toluene, realizing efficient and accurate Raman detection.