Semiconductor SERS enhancement enabled by oxygen incorporation

Ultrafast Laser-Induced 1T'/2H-MoTe2 Nanopattern with Au-Nanoclusters for Raman Monitoring of Cellular Drug Metabolism

The development of surface-enhanced Raman spectroscopy (SERS) as an ultrasensitive fingerprint analysis technique in precision medicine requires high-performance SERS substrates with controllable nanostructure (hot-spot) distribution, simple fabrication, superior stability, biocompatibility, and extraordinary optical responses. Unfortunately, fabrication of arbitrary nanostructures with high homogeneity on a large scale for SERS is still challenging. Herein, we report an ultrafast laser parallel fabrication protocol for Au/2D-transition-metal dichalcogenide hybrid SERS biosensors. The leveraged photonic nanojets (PNJs) are generated by a micron-sized microsphere monolayer to simultaneously trigger localized phase transition in 2H-MoTe2, achieving a 1T'-MoTe2 nanopattern array with a density of 1 million per mm2 by a single laser shot. The Au nanoparticle clusters (AuNCs) are subsequently grown in situ from the 1T' regions, creating a AuNCs on 1T'/2H-MoTe2 (AuNCs@1T'/2H-MoTe2) hybrid SERS substrate. The fabricated feature diameter and overlay accuracy of the patterned AuNCs are 210.1 ± 3.4 and 9.2 ± 1.7 nm, respectively. To eliminate background noise, we designed dimer-AuNCs@1T'/2H-MoTe2 (dAuNCs@1T'/2H-MoTe2), achieving a detection limit of 10-13 M with an enhancement factor of 4.9 × 108 for the methylene blue (MB) analyte. The strong localized surface plasmon resonances in the dAuNCs as well as efficient charge transfers between Au, 2H-MoTe2, and MB contribute to the majority of Raman enhancement. The multiscale dAuNCs@1T'/2H-MoTe2 array provides a powerful SERSome (comprising multiple SERS spectra) platform for therapeutic drug monitoring, by which we successfully identified the metabolic behaviors of living gastric adenocarcinoma cells administered with two drugs, i.e., capecitabine, oxaliplatin, and their combination. The present work establishes opportunities for creating a highly ordered nanopattern array for ultrasensitive SERSome analysis of cell metabolism in cancer therapy.

Energy-level rich nanorings hybridizing Ag, Au and AgCl as high-performance SERS substrate for numerous molecules

The current surface-enhanced Raman scattering (SERS) substrates typically feature a single energy level, posing challenges in coordinating electromagnetic enhancement (EM) and chemical enhancement (CM), thereby limiting the sensitive detection of numerous crucial target molecules. In this study, novel aggregated nanorings (a-NRs) hybridizing Ag, Au and AgCl are constructed as SERS substrates. On one hand, the obtained a-NRs exhibit robust localized surface plasmon resonance absorption, whose wavelength can be tuned to match three commonly used laser wavelengths (532, 633 and 785 nm) to gain strong EM effect. On the other hand, these materials possess the Fermi levels of Au nanoparticles and Au/Ag alloy, in addition to the valence band and conduction band of AgCl. The abundant energy levels of the obtained a-NRs facilitate increased charge transfer opportunities for molecules, leading to a strong CM effect. Therefore, the obtained a-NRs show ultra-high SERS sensitivity towards numerous molecules. Moreover, the unique chemical composition makes the obtained a-NRs have good long-term stability in terms of SERS activity. Besides providing high-performance SERS substrates, the valuable experience for coordinating EM and CM to construct highly active SERS substrate demonstrated in this work are expected to significantly advance the application of SERS.

Recent advances in semiconductor nanostructure-based surface-enhanced Raman scattering sensors

Surface-enhanced Raman scattering (SERS) has become a transformative analytical tool, attracting growing interest for its wide-ranging applications. The development of SERS-active materials is now a central research area, spurring innovation in various types of SERS substrates. While noble metal-based substrates remain extensively studied, semiconductor-based, non-metal substrates are garnering attention due to their unique advantages: excellent chemical stability, high carrier mobility, biocompatibility, and precise fabrication control. However, their generally weaker enhancement effects limit their utility, underscoring the need for strategies to boost their SERS activity. Understanding the complex enhancement mechanisms in semiconductor-based SERS substrates is critical for designing next-generation materials with metal-like enhancement factors (EFs). The interplay of charge transfer, localized surface plasmon resonance, and photonic effects makes the enhancement process inherently challenging to unravel. Therefore, the search for new materials with exciting optoelectronic properties, as well as more innovative solutions to increase their SERS sensitivity, continues to grow. In this review, we explore the latest advancements in semiconductor-based SERS substrates, dissecting the complex enhancement mechanisms and various modification strategies aimed at achieving metal-like high EFs. We present a comprehensive analysis of the methods used to improve the SERS performance of semiconductor substrates and conclude with potential future directions for advancing this dynamic field.

Lead-free halide double perovskite nanoflakes as high-performance SERS substrates for detection of trace organic pollutants: chemical enhancement versus electromagnetic enhancement

Surface-enhanced Raman scattering (SERS) is a powerful spectroscopic method known for its ultrasensitive characterization capabilities. Recently, lead-based halide perovskites incorporated with noble metals have gained much attention as SERS substrates. However, their practical applications are often hindered by the toxicity of the lead content and low stability. Herein, for the first time, we have synthesized lead-free halide double perovskite (DP) Cs2AgBiBr6 to overcome the toxicity, stability, and uniformity issues and studied the performance of lead-based perovskite SERS substrates. The self-trapped exciton (STE) defects were controlled by post-growth annealing of the sample under an argon (Ar) atmosphere, minimizing the AgBi and BiAg anti-site disorder. The sample with the highest STE defects demonstrates the highest SERS performance owing to the defect-assisted charge transfer process. We successfully identified methylene blue (MB) and rhodamine 6G (R6G) at concentrations as low as ∼10-10 M, achieving a remarkable SERS enhancement factor (EF) of 5.04 × 107 and 1.37 × 107, respectively, which is highly significant for a semiconductor-based SERS substrate. Additionally, notable amplification was observed for other cationic dyes, including crystal violet (CV), rhodamine B (RhB), and malachite green (MG). By varying the annealing temperature and the deconvolution of the photoluminescence spectra, we demonstrate a direct correlation between the defect density and the SERS signal intensity. To further understand the underlying enhancement mechanisms, we analyzed the individual contributions of chemical and electromagnetic enhancements to the overall SERS amplification. This analysis was conducted using finite element method (FEM) simulations and density functional theory (DFT) computations. These insights provide a foundational basis for designing highly efficient metal-free SERS substrates, opening new possibilities for advanced detection technologies. Additionally, the Cs2AgBiBr6 substrate exhibited excellent stability, retaining performance after four months of storage under ambient conditions, highlighting its potential for environmental monitoring.

Development of Defect-Rich WO3-x/TiO2 Heterojunction Toward Dual-Functional Enhancement: Boosting SERS and Photocatalytic Performance

Semiconductors have emerged as promising candidates for surface-enhanced Raman scattering (SERS) applications due to their inexpensiveness and good chemical stability. Nevertheless, their low enhancement ability compared to noble metals makes it desirable to explore strategies for improving SERS performance. Since charge transfer (CT) between semiconductors and analytes plays a crucial role on the chemical enhancement mechanism of SERS, heterojunction engineering, a powerful method to boost optoelectronic performance via tailoring interfacial charge transfer, provides a promising approach. Here, we prepared defect-rich WO3-x/TiO2 nanocomposites via a facile solvothermal method to achieve dual-functional enhancement in SERS and photocatalytic activity. Due to suppressed recombination of charge carriers in WO3-x/TiO2 heterojunction with type II band alignment, more photogenerated carriers are available for CT, consequently increasing molecular polarizability. The SERS intensity of WO3-x/TiO2 is at least three times that of its component semiconductors, with a detection limit of 10-10 M for methyl orange (MO). Meanwhile, the suppressed recombination of charge carriers also results in higher degradation efficiency of WO3-x/TiO2 heterojunction (93%) than WO3-x (47%) and TiO2 (54%) under visible-light irradiation for 120 min. This work provides insightful information on the development of dual-functional semiconductor systems through band structure engineering for ultrasensitive sensing and efficient remediation of environmental pollutants.

Enhanced Detection of Organic Pollutants Using ReS2/ZnO/Au Ternary Surface-Enhanced Raman Spectroscopy Substrate with Multiple Charge Transfer Channels

Persistent organic pollutants pose significant environmental and health risks, highlighting the urgent need for highly sensitive sensing technologies capable of detecting trace concentrations. In this study, we synthesized a ReS2/ZnO/Au ternary surface-enhanced Raman spectroscopy (SERS) substrate via a simple hydrothermal and reduction method based on a novel strategy that leverages multiple charge transfer channels to significantly enhance SERS performance. The unique bandgap and energy level alignment of ReS2 facilitate both excitonic resonance and charge transfer transitions. Additionally, the semiconductor ZnO acts as an efficient charge transfer mediator by borrowing energy from molecular transitions. As a result of the synergistic combination of electromagnetic and chemical enhancements, the ReS2/ZnO/Au ternary substrate demonstrates excellent versatility and high sensitivity for detecting various pollutants, including rhodamine 6G (R6G), crystal violet, malachite green, and tetracycline (TC). Notably, the detection limit for TC can reach 10-10 M, with an enhancement factor as high as 2.78 × 108. Our strategy provides comprehensive insight into SERS enhancement, offering a pathway for designing sensitive and versatile SERS systems, with significant potential for monitoring and quantitative analysis of organic pollutants.

Chemically Enhanced Raman Scattering Enabled by Organic Semiconductor Molecules with Deep Lowest Unoccupied Molecular Orbitals

Organic semiconductor materials (OSMs) have emerged as innovative platforms for surface-enhanced Raman scattering (SERS). For now, SERS activity has been established in only a few materials like thiophene-based derivatives, and the potential of the broader OSM library is largely untapped. Systematic exploration of energy level alignment between analytes and the OSM substrates is highly desirable for further material screening and optimization. We introduce a strategy utilizing OSMs with the deep lowest unoccupied molecular orbital (LUMO) levels, exemplified by TCNQ and HATCN, as novel SERS active platforms realizing efficient detection of multiple organic dyes otherwise undetectable under low-energy incident laser irradiation at 785 nm. Our study showcases selective SERS enhancement for analytes with diverse highest occupied molecular orbital levels, highlighting the pivotal role of LUMO levels in both SERS activity and molecular sensitivity. This work elucidates the molecular structure-SERS activity correlation, facilitating the development of novel SERS substrates via the strategy of LUMO level tuning.

High-Stability Printable Perovskite SERS Substrates in an Aqueous Environment via Plasmon-Induced Resonance Energy Transfer

The excellent photoelectric conversion efficiency and tunable bandgap of metal halide perovskites make them highly suitable for SERS applications. However, the low stability of perovskites in water and oxygen greatly hinders their use in SERS detection, particularly in biomolecule detection applications, which often require water-based test solutions. Herein, we report a gold (Au)/perovskite-polyvinylidene difluoride (PVDF) nanocomposite/ZnO nanoflower (GPPZ) SERS substrate capable of functioning in aqueous solutions. Its enhancement ability is attributed to plasmon-induced resonance energy transfer (PIRET) and an electromagnetic mechanism. The surface plasmon resonance created by ultrathin Au and ZnO nanoflowers induces resonance energy transfers to the perovskite via PIRET, facilitating a quasi-matched charge transfer between the perovskite and the probe molecule. The PVDF coating protects the perovskite from water and oxygen without affecting the resonance energy-transfer process. As a result, an enhancement factor (EF) approaching 1 × 106 was achieved for the crystal violet molecule. Additionally, we fabricated a flexible GPPZ substrate using silk screen printing, enabling mass production of an SERS array substrate. The printed flexible GPPZ substrates demonstrated micromole-level cysteine detection with an EF of 6.8 × 105, showing potential for application in hyperhomocysteinemia diagnosis.

Decoration of Silver Nanoparticles on WS2-WO3 Nanosheets: Implications for Surface-Enhanced Resonance Raman Spectroscopy Detection and Material Characteristics

This study investigates the chemical and structural modifications of vertically aligned tungsten disulfide-tungsten trioxide (WS2-WO3) nanosheets decorated with silver nanoparticles (Ag(NPs)) under nitrogen plasma conditions. The synthesized vertically aligned WS2-WO3 nanosheets were functionalized through direct-current (DC) magnetron sputtering, forming silver-decorated samples. Structural changes, as well as the size and distribution of Ag(NPs), were characterized using scanning electron microscopy (SEM). Chemical state analysis was conducted via X-ray photoelectron spectroscopy (XPS), while Raman spectroscopy was employed to investigate vibrational modes. The findings confirmed the successful decoration of Ag(NPs) and identified unexpected compound transformations that were dependent on the duration of functionalization. The synthesized and functionalized samples were evaluated for their sensing capabilities towards Rhodamine B (RhB) through surface-enhanced resonance Raman scattering (SERRS). This study discusses the impact of substrate morphology and the shape and size of nanoparticles on the enhancement of SERRS mechanisms, achieving an enhancement factor (EF) of approximately 1.6 × 106 and a limit of detection (LOD) of 10-9 M.

Activating SERS Signals of Inactive Analytes: Creating an Energy Bridge between Metal/Molecule Energy Alignment via Metal/Semiconductor Transitions

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical technique, yet it faces challenges with certain probe molecules exhibiting weak or inactive signals, limiting their applicability. In a recent study, we investigated this phenomenon using a set of four probe molecules─chloramphenicol (CAP), 4-nitrophenol (4-NP), amoxicillin (AMX), and furazolidone (FZD)─deposited on Ag-based nanostructured SERS substrates. Despite being measured under identical conditions, CAP and 4-NP exhibited SERS activity, while AMX and FZD did not. We also demonstrated that the alignment of the target molecule's lowest unoccupied molecular orbital (LUMO) energy level with the substrate's Fermi level plays a critical role in influencing the SERS signal. When the LUMO level diverges from the Fermi level, hindrance of the charge transfer process occurs due to a high potential barrier, leading to weak or absent SERS signals. To overcome this challenge, in this study, we introduce an approach inspired by metal-semiconductor interfacial charge transfer dynamics. By employing TiO2/Ag nanostructures, we not only enhance SERS signals for CAP and 4-NP but also activate signals for inactive molecules AMX and FZD. Importantly, we demonstrate that controlling the crystalline phase composition of the TiO2 semiconductor allows for tailored conduction band minimum energy level (ECBM) positions, significantly impacting the overall SERS efficiency of the TiO2/Ag substrate. Our findings highlight the pivotal role of the semiconductor's ECBM position in the energy alignment of the metal-semiconductor-analyte three-body interaction for an optimal SERS sensing platform. These findings also offer a novel strategy to enhance and activate the SERS phenomenon of important yet underexplored analytes.

Ultrasensitive detection of contaminants in milk using a novel NMS-Ag modified water-resistant paper substrate

Rapid and precise detection of harmful substances in food products is essential for ensuring public health and safety. This study introduces a novel surface-enhanced Raman spectroscopy (SERS) substrate, composed of a molybdenum disulfide‑silver nanocomposite, applied to flexible, water-resistant filter paper for detecting melamine and bisphenol A (BPA) in milk. Optimized molybdenum disulfide (NMS) nanoflowers (NFs) were synthesized through hydrothermal methods and high-temperature annealing, then modified with silver (Ag) nanoparticles to form the NMS-Ag nanocomposite (NMSA6). This substrate greatly enhances the Raman signal, achieving an enhancement factor of approximately 1.49 × 107 and a detection limit as low as 10-11 M for simultaneous multi-component analysis. Finite-difference time-domain (FDTD) simulations confirm the enhancement mechanism. The NMSA6 substrate demonstrates remarkably low detection limits for BPA and melamine, facilitating the analysis of various hazardous substances. These findings highlight the substrate's potential for highly sensitive, label-free detection, presenting a viable tool for food safety monitoring.

Nanorod structure tuning and defect engineering of MoOx for high-performance SERS substrates

In recent years, surface-enhanced Raman scattering (SERS) based on metal oxide semiconductors has been an active area of research and development, attracting significant scientific interest. These SERS substrates are known as plasmon-free SERS substrates because they are not based on noble metal nanoparticles but mainly on the defects, structure, and surface morphology of semiconductors to enhance the Raman signal. In this study, we fabricated a SERS substrate based on molybdenum oxide, using reactive DC magnetron sputtering and then used different simple and effective strategies to enhance the Raman signal. The results show that nanorod structure, oxygen deficiency engineering, phase engineering, and optical properties can be easily controlled by varying sputtering time and annealing time of MoOx SERS substrates. The analysis methods XRD, PL, and Raman show that with the optimal fabricated conditions. The presence of oxygen defects and a mixed MoO3, Mo9O26 phase structure in as well as the nanorod structure of MoOx SERS substrates could likely enhance Raman signals via a chemical mechanism (CM) and electromagnetic mechanism (EM). The MoOx SERS substrates were also used to detect R6G at low concentrations, with an EF of 1.14 × 106 (at 0.01 ppm), LOD of 0.01 ppm, and good temporal stability and reproducibility.

Synergistic enhancement of chemical and electromagnetic effects in a Ti3C2Tx/AgNPs two-dimensional SERS substrate for ultra-sensitive detection

BACKGROUND

It is well established that surface-enhanced Raman scattering (SERS) is one of the most commonly used spectral analysis techniques in real-world applications, including chemical and biological sensing, analytical detection, and even forensics. It offers high sensitivity, high resistance to solvents, photobleaching, and limited spectrum bands. In general, SERS is caused by two mechanisms, the electromagnetic enhancement mechanism (EM) and the chemical enhancement mechanism (CM), although the exact mechanism is not yet known. For increased sensitivity, a SERS substrate based on EM coupled with CM is essential.

RESULTS

Using electrostatic self-assembly, we fabricated many homogeneous hot spots for the substrate by evenly mixing positive charge Ag nanoparticles (AgNPs) with negative charge Ti3C2Tx. In addition, there is a clearly enhanced effect due to the high affinity between the tested molecule and Ti3C2Tx, which facilitates molecule-to-molecule charge transfer. After successfully preparing the Ti3C2Tx/AgNPs substrate, the R6G dye molecule was used to investigate its SERS activity. According to the results, the substrate can reach an enhancement factor of 3.8 × 108. Furthermore, it has been demonstrated that the coupling effect between EM and CM is the main reason for the excellent performance of the Ti3C2Tx/AgNPs composite substrate. Based upon the results of detecting the two biomarkers, adenosine triphosphate and folic acid, the detection limits were determined to be 4.27 × 10-9 M and 7.26 × 10-13 M, respectively.

SIGNIFICANCE AND NOVELTY

Two-dimensional metal carbide Ti3C2Tx material can be used to obtain CM in surface Raman scattering. It has been demonstrated that the combination of CM and EM with nano-precious metals can produce an extremely sensitive SERS substrate that is dependable and stable. Additionally, the Ti3C2Tx/AgNPs study offers a novel perspective for the advancement of the SERS coupling mechanism in addition to providing direction for realistic detection.

Cascade internal electric field dominated carbon nitride decorated with gold nanoparticles as SERS substrate for thiram assay

The development of valid chemical enhancement strategy with charge transfer (CT) for semiconductors has great scientific significance in surface-enhanced Raman scattering (SERS) technology. Herein, a phosphorus doped crystalline/amorphous polymeric carbon nitride (PCPCN) is fabricated by a facile molten salt method, and is employed as a SERS substrate for the first time. Upon the synergies of phosphatization and molten salt etching, PCPCN owns a cascaded internal electric field (IEF) due to the formation of p-n homojunction (interface-IEF) and crystalline/amorphous homojunction (bulk-IEF). The interface-IEF and bulk-IEF could effectively suppress the recombination of charge carriers and promote electron transfer between PCPCN and target methylene blue (MB), respectively. The strong CT interaction endows PCPCN substrate with superior SERS activity with an enhancement factor (EF) of 5.53 × 105. Au nanoparticles (Au NPs) are subsequently decorated on PCPCN to introduce electromagnetic enhancement for a better SERS response. The Au/PCPCN substrate allows to reliably detect trace crystal violet, as well as the thiram residue on cherry tomato. This work offers an integrated solution to enhance CT efficiency based on collaborative homojunction and internal electric field, and may inspire the design of novel semiconductor-based SERS substrates.

Advances in Surface-Enhanced Raman Spectroscopy for Urinary Metabolite Analysis: Exploiting Noble Metal Nanohybrids

This review examines recent advances in surface-enhanced Raman spectroscopy (SERS) for urinary metabolite analysis, focusing on the development and application of noble metal nanohybrids. We explore the diverse range of hybrid materials, including carbon-based, metal-organic-framework (MOF), silicon-based, semiconductor, and polymer-based systems, which have significantly improved SERS performance for detecting key urinary biomarkers. The principles underlying SERS enhancement in these nanohybrids are discussed, elucidating both electromagnetic and chemical enhancement mechanisms. We analyze various fabrication methods that enable precise control over nanostructure morphology, composition, and surface chemistry. The review critically evaluates the analytical performance of different hybrid systems for detecting specific urinary metabolites, considering factors such as sensitivity, selectivity, and stability. We address the analytical challenges associated with SERS-based urinary metabolite analysis, including sample preparation, matrix effects, and data interpretation. Innovative solutions, such as the integration of SERS with microfluidic devices and the application of machine learning algorithms for spectral analysis, are highlighted. The potential of these advanced SERS platforms for point-of-care diagnostics and personalized medicine is discussed, along with future perspectives on wearable SERS sensors and multi-modal analysis techniques. This comprehensive overview provides insights into the current state and future directions of SERS technology for urinary metabolite detection, emphasizing its potential to revolutionize non-invasive health monitoring and disease diagnosis.

Phase-Engineered Transition Metal Dichalcogenides for Highly Efficient Surface-Enhanced Raman Scattering

Phase engineering of two-dimensional (2D) transition metal dichalcogenides (TMDs) is an attractive avenue to construct new surface-enhanced Raman scattering (SERS) substrates. Herein, 2D WS2 and MoS2 monolayers with high-purity distorted octahedral phase (1T') are prepared for highly sensitive SERS detection of analytes (e.g., rhodamine 6G, rhodamine B and crystal violet). 1T'-WS2 and 1T'-MoS2 monolayers show the detection limits of 8.28 × 10-12 and 8.57 × 10-11 M for rhodamine 6G, with the enhancement factors of 4.6 × 108 and 3.9 × 107, respectively, which are comparable to noble-metal substrates, outperforming semiconducting 2H-W(Mo)S2 monolayers and most of the reported non-noble-metal substrates. First-principles density functional theory calculations show that their Raman enhancement effect is mainly ascribed to highly efficient interfacial charge transfer between the 1T'-W(Mo)S2 monolayers and analytes. Our study reveals that 2D TMDs with semimetallic 1T' phase are promising as next-generation SERS substrates.

Defect-Engineered Coordination Compound Nanoparticles Based on Prussian Blue Analogues for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for label-free chemical analysis. The emergence of nonmetallic materials as SERS substrates, offering chemical signal enhancements, presents an exciting direction for achieving reproducible and biocompatible SERS, a challenge with traditional metallic substrates. Despite the potential, the realm of nonmetallic SERS substrates, particularly nanoparticles, remains largely untapped. Here, we present defect-engineered coordination compounds (DECCs) based on Prussian blue analogues (PBAs) as a class of nonmetallic nanoparticle-based SERS substrates. We demonstrate the utility and flexibility of the DECC template by incorporating various metal (M) elements into PBAs to synthesize nanoparticles that deliver substantial chemical mechanism (CM)-based enhancements to the Raman signal with a ∼ 108-fold increase. The introduction of the M-PBA-based DECC nanoparticles as a class of SERS substrates represents a pioneering stride, enabling the straightforward and systematic exploration of a library of compounds for SERS-based analysis of a wide range of target molecules, especially biomolecules.

Impact of Surface Enhanced Raman Spectroscopy in Catalysis

Catalysis stands as an indispensable cornerstone of modern society, underpinning the production of over 80% of manufactured goods and driving over 90% of industrial chemical processes. As the demand for more efficient and sustainable processes grows, better catalysts are needed. Understanding the working principles of catalysts is key, and over the last 50 years, surface-enhanced Raman Spectroscopy (SERS) has become essential. Discovered in 1974, SERS has evolved into a mature and powerful analytical tool, transforming the way in which we detect molecules across disciplines. In catalysis, SERS has enabled insights into dynamic surface phenomena, facilitating the monitoring of the catalyst structure, adsorbate interactions, and reaction kinetics at very high spatial and temporal resolutions. This review explores the achievements as well as the future potential of SERS in the field of catalysis and energy conversion, thereby highlighting its role in advancing these critical areas of research.

Selective SERS Sensing of R6G Molecules Using MoS2 Nanoflowers under Pressure

Pressure-induced surface-enhanced Raman spectroscopy (PI-SERS) has garnered significant attention as a subfield of SERS detection due to its capacity to regulate the band gap between molecules and substrates through pressure modulation. Currently, SERS detection primarily focuses on single molecules at atmospheric pressure with limited investigations conducted under high pressure conditions. Herein, we employed rose-shaped MoS2 nanoflowers as the SERS substrate and realized selective PI-SERS enhancement of R6G molecules in the binary (MV+R6G) and ternary (MV+R6G+RhB) systems. The MoS2 demonstrated an exceptionally low SERS detection limit of 5 × 10-6 M in binary and ternary systems with equimolar amounts of molecules. High-pressure experimental results indicate that MoS2 displays selective enhancement for R6G molecules, as evidenced by the comparison of the PI-SERS peak intensity ratio between MoS2 and the probe molecules. The proposed enhancement mechanism in binary and ternary SERS systems under high pressure involves pressure-induced changes in both the band structures of the MoS2 substrate and molecules, thereby influencing their charge transfer dynamics. Consequently, this approach holds great promise for practical applications in complex SERS systems operating under extreme conditions.

Tuning Nanographene-Enhanced Raman Scattering for Rapid Label-Free Detection of Amino Acids

The rapid and sensitive detection of amino acids is important not only for fundamental studies but also for the establishment of a healthy society. However, conventional detection methods have been hampered by the difficulties of low sensitivity, long sampling and detection times, and expensive operation and instruments. Here, we report the plasma engineering of bioresource-derived graphene quantum dots (GQDs) as surface-enhanced Raman scattering (SERS)-active materials for the rapid and sensitive detection of amino acids. Surface-functionalized GQDs with tuned structures and band gaps were synthesized from earth-abundant bioresources by using reactive microplasmas under ambient conditions. Detailed microscopy and spectroscopy studies indicate that the SERS properties of the synthesized GQDs can be tuned by controlling the band gaps of synthesized GQDs. The plasma-synthesized metal-free GQDs with surface functionalities showed improved SERS properties for rapid amino acid detection with low detection limits of 10-5 M for tyrosine and phenylalanine. Theoretical calculations suggest that charge transfer between GQDs and amino acids can enhance the SERS response of the GQDs. Our work provides insights into the controlled engineering of SERS-active nanographene-based materials using the plasma-enhanced method.

2D Material-Based Surface-Enhanced Raman Spectroscopy Platforms (Either Alone or in Nanocomposite Form)-From a Chemical Enhancement Perspective

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopic technique with molecular fingerprinting capability and high sensitivity, even down to the single-molecule level. As it is 50 years since the observation of the phenomenon, it has now become an important task to discuss the challenges in this field and determine the areas of development. Electromagnetic enhancement has a mature theoretical explanation, while a chemical mechanism which involves more complex interactions has been difficult to elucidate until recently. This article focuses on the 2D material-based platforms where chemical enhancement (CE) is a significant contributor to SERS. In the context of a diverse range (transition metal dichalcogenides, MXenes, etc.) and categories (insulating, semiconducting, semimetallic, and metallic) of 2D materials, the review aims to realize the influence of various factors on SERS response such as substrates (layer thickness, structural phase, etc.), analytes (energy levels, molecular orientation, etc.), excitation wavelengths, molecular resonances, charge-transfer transitions, dipole interactions, etc. Some examples of special treatments or approaches have been outlined for overcoming well-known limitations of SERS and include how CE benefits from the defect-induced physicochemical changes to 2D materials mostly via the charge-transport ability or surface interaction efficiency. The review may help readers understand different phenomena involved in CE and broaden the substrate-designing approaches based on a diverse set of 2D materials.

Localized surface plasmon resonance (LSPR) excitation on single silver nanoring with nanoscale surface roughness

With the expansion of the application of high-sensitivity Surface-enhanced Raman scattering (SERS) technique, micro SERS-active substrates with rich optical properties and high-level functions are desired. In this study, silver nanorings with nanoscale surface roughness were fabricated as a new type of enclosed quasi-2D micro-SERS-active substrate. Highly-crystalline spherical and hemispherical silver nanoprotrusions were densely and uniformly distributed over the entire surface of the nanorings. The SERS signals were significantly enhanced on the roughened silver nanorings which were mainly derived from the maximal localized surface plasmon resonance (LSPR) points at the junctions between adjacent coupled nanoprotrusions on the roughened nanorings. The mapping image shows a uniform and intense LSPR enhancement over the nanorings, owing to the uniform and dense distribution of silver nanoprotrusions and the resulting uniform distribution of maximal LSPR points on the roughened nanorings. The dark-field spectra further indicated that the single roughened silver nanoring had significant LSPR enhancement, a wide LSPR frequency-range response, and adaptability for SERS enhancement. Notably, both the measured and simulated results demonstrate that the maximal LSPR enhancement at the junctions between the nanoprotrusions, which are distributed on the inner surface of the silver nanoring, is higher than that on the outer surface because of the plasmon-focusing effect of the enclosed silver nanoring, which leads to the lateral asymmetrical distribution of LSPR intensity, indicating more LSPR and SERS features. These results indicate that single roughened silver nanorings exhibit excellent performance as a new type of enclosed quasi-2D silver nanoring micro-SERS-active substrate, microzone LSPR catalysis, and micro/nanodevices.

SERS Detection of Trace Carcinogenic Aromatic Amines Based on Amorphous MoO3 Monolayers

Aromatic amines are important commercial chemicals, but their carcinogenicity poses a threat to humans and other organisms, making their rapid quantitative detection increasingly urgent. Here, amorphous MoO3 (a-MoO3) monolayers with localized surface plasmon resonance (LSPR) effect in the visible region are designed for the trace detection of carcinogenic aromatic amine molecules. The hot-electron fast decay component of a-MoO3 decreases from 301 fs to 150 fs after absorption with methyl orange (MO) molecules, indicating the plasmon-induced hot-electron transfer (PIHET) process from a-MoO3 to MO. Therefore, a-MoO3 monolayers present high SERS performance due to the synergistic effect of electromagnetic enhancement (EM) and PIHET, proposing the EM-PIHET synergistic mechanism in a-MoO3. In addition, a-MoO3 possesses higher electron delocalization and electronic state density than crystal MoO3 (c-MoO3), which is conducive to the PIHET. The limit of detection (LOD) for o-aminoazotoluene (o-AAT) is 10-9 M with good uniformity, acid resistance, and thermal stability. In this work, trace detection and identification of various carcinogenic aromatic amines based on a-MoO3 monolayers is realized, which is of great significance for reducing cancer infection rates.

Phase-Tunable Molybdenum Boride Ceramics as an Emerging Sensitive and Reliable SERS Platform in Harsh Environments

Traditional surface-enhanced Raman scattering (SERS) sensors rely heavily on the use of plasmonic noble metals, which have limitations due to their high cost and lack of physical and chemical stability. Hence, it is imperative to explore new materials as SERS platforms that can withstand high temperatures and harsh conditions. In this study, the SERS effect of molybdenum boride ceramic powders is presented with an enhancement factor of 5 orders, which is comparable to conventional noble metal substrates. The molybdenum boride powders synthesized through liquid-phase precursor and carbothermal reduction have β-MoB, MoB2, and Mo2B5 phases. Among these phases, β-MoB demonstrates the most significant SERS activity, with a detection limit for rhodamine 6G (R6G) molecules of 10-9 m. The impressive SERS enhancement can be attributed to strong molecule interactions and prominent charge interactions between R6G and the various phases of molybdenum boride, as supported by theoretical calculations. Additionally, Raman measurements show that the SERS activity remains intact after exposure to high temperature, strong acids, and alkalis. This research introduces a novel molybdenum boride all-ceramic SERS platform capable of functioning in harsh conditions, thereby showing the promising of boride ultrahigh-temperature ceramics for detection applications in extreme environments.

Toward Modeling the Complexity of the Chemical Mechanism in SERS

Surface-enhanced Raman scattering (SERS) provides detailed information about the binding of molecules at interfaces and their interactions with the local environment due to the large enhancement of Raman scattering. This enhancement arises from a combination of the electromagnetic mechanism (EM) and chemical mechanism (CM). While it is commonly accepted that EM gives rise to most of the enhancement, large spectral changes originate from CM. To elucidate the rich information contained in SERS spectra about molecules at interfaces, a comprehensive understanding of the enhancement mechanisms is necessary. In this Perspective, we discuss the current understanding of the enhancement mechanisms and highlight their interplay in complex local environments. We will also discuss emerging areas where the development of computational and theoretical models is needed with specific attention given to how the CM contributes to the spectral changes. Future efforts in modeling should focus on overcoming the challenges presented in this review in order to capture the complexity of CM in SERS.

Mechanism Switch in Surface-Enhanced Raman Scattering: The Role of Nanoparticle Dimensions

Surface-enhanced Raman scattering (SERS) is renowned for amplifying Raman signals, with electromagnetic mechanism (EM) enhancement arising from localized surface plasmon resonances and chemical mechanism (CM) enhancement as a result of charge transfer interactions. Despite the conventional emphasis on EM as a result of plasmonic effects, recent findings highlight the significance of CM when noble metals appear as smaller entities. However, the threshold size of the noble metal clusters/particles corresponding to the switch in SERS mechanisms is not clear at present. In this work, the VSe2-xOx/Au composites with different Au sizes are employed, in which a clear view of the SERS mechanism switch is observed at the Au size range of 16-21 nm. Our findings not only provide insight into the impact of noble metal size on SERS efficiency but also offer quantitative data to assist researchers in making informed judgments when analyzing SERS mechanisms.

Facet junction engineering for enhanced SERS activity of Ag/Cu2O composite substrates

Featuring ultra-high sensitivity and molecule-specific detection ability, surface-enhanced Raman scattering (SERS) is suitable for the rapid sensing of trace-level chemicals in biological, environmental, and agricultural samples. Although crystal facet junction engineering is a powerful tool to manipulate the optoelectronic properties of semiconducting materials, its correlation with the SERS sensing activity of noble metal/semiconductor composites has still not been clarified. In this work, Ag was deposited on Cu2O nanocrystals enclosed by different facets, including {100} (cube), {111} (octahedron), and {100}/{111} (truncated octahedron), and a detailed study of their SERS performance was carried out. It was found that Ag/truncated-octahedral Cu2O (Ag/Cu2O(J3)) exhibited superior SERS activity to Ag/cubic Cu2O (Ag/Cu2O(C)) and Ag/octahedral Cu2O (Ag/Cu2O(O)). The {100}/{111} facet junction in Cu2O can promote the separation and transfer of photogenerated charge carriers, which is beneficial for enhancing SERS sensing performance. Moreover, Ag/Cu2O(J3) has a higher content of oxygen vacancies, providing extra interfacial charge-transfer pathways to the analyte, which also contribute to improving the SERS activity. The low detection limit of Ag/Cu2O(J3) was 1 × 10-11 M for 4-nitrobenzenethiol, two orders of magnitude lower than that of Ag/Cu2O(C) and Ag/Cu2O(O). In addition, Ag/Cu2O(J3) could detect CV and R6G at concentrations down to 1 × 10-10 M and 1 × 10-8 M, respectively. The findings of this work can provide insightful information for designing metal/semiconductor substrates toward SERS sensing application by regulating the crystal facet junction.

Plasmon AgNPs/MoS2/ZnO nanorods array ternary heterojunctions enabling high-efficiency solar-light energy utilization for photocatalysis and recyclable SERS detection

BACKGROUND

Surface-enhanced Raman scattering (SERS) has gained widespread use in molecule-level detection benefiting from its high sensitivity, nondestructive data acquisition, and capacity for providing molecular fingerprint information. However, the strong adhesion of target molecules to the substrate (known as the "memory effect") inherently hinders the reusability of SERS substrates. Research has shown that self-cleaning SERS substrates based on versatile semiconductor materials with SERS enhancement capabilities and solar photocatalytic properties offer an effective platform for the sensitive detection and degradation of harmful molecules.

RESULTS

In this research, a resuable SERS-active substrate was facilely fabricated by anchoring silver nanoparticles (AgNPs) to the edges of MoS2 nanosheet decorated on ZnO nanorod arrays (NRAs). This innovative design exhibited a remarkable SERS enhancement factor (EF) of 4.6 × 107 and demonstrated significant solar photocatalytic efficiency. Such superior characteristics of ternary plasma heterojunction were ascribable to the synergistic effect of the "Schottky barrier" and "hot spots" between MoS2 and AgNPs, the inherent chemical enhancement proficiency of the MoS2/ZnO NRAs heterojunction, as well as the ultrafast electron transfer within the ternary heterojunction.

SIGNIFICANCE

The developed ternary heterojunction substrate enabled highly sensitive SERS detection of trace amounts of organic molecules. Moreover, this SERS substrate exhibited self-cleaning and recyclability via solar-light-driven photocatalysis. This bifunctional recyclable SERS substrate proved capable of meeting various requirements for routine monitoring of environmental organic pollutants and provided a robust avenue for advancing energy utilization materials that serve as high-performance SERS sensors and catalysts.

Beyond the Charge Transfer Mechanism for 2D Materials-Assisted Surface Enhanced Raman Scattering

Two-dimensional (2D) materials have been extensively implemented as surface-enhanced Raman scattering (SERS) substrates, enabling trace-molecule detection for broad applications. However, the accurate understanding of the mechanism remains elusive because most theoretical explanations are still phenomenological or qualitative based on simplified models and rough assumptions. To advance the development of 2D material-assisted SERS, it is vital to attain a comprehensive understanding of the enhancement mechanism and a quantitative assessment of the enhancement performance. Here, the microscopic chemical mechanism of 2D material-assisted SERS is quantitatively investigated. The frequency-dependent Raman scattering cross sections suggest that the 2D materials' SERS performance is strongly dependent on the excitation wavelengths and the molecule types. By analysis of the microscopic Raman scattering processes, the comprehensive contributions of SERS can be revealed. Beyond the widely postulated charge transfer mechanisms, the quantitative results conclusively demonstrate that the resonant transitions within 2D materials alone are also capable of enhancing the molecular Raman scattering through the diffusive scattering of phonons. Furthermore, all of these scattering routines will interfere with each other and determine the final SERS performance. Our results not only provide a complete picture of the SERS mechanisms but also demonstrate a systematic and quantitative approach to theoretically understand, predict, and promote the 2D materials SERS toward analytical applications.

Highly ordered nanocavity as photonic-plasmonic-polaritonic resonator for single molecule miRNA SERS detection

Strong light-matter coupling between molecules and electromagnetic field lead to the formation of hybrid polaritonic states for surface enhanced Raman scattering (SERS) detection. However, owing to the inefficient interaction between zero-point fluctuations of photons/plasmons and molecular electronic transitions, the Raman enhancement is limited in relative low levels. Here, we propose and fabricate a TiOx/Cu2-xSe/R6G nanocavity based photonic-plasmonic-polaritonic resonator for single molecular SERS detection. Through precisely matching the energy levels of illuminated photon, generated plasmon, and molecular polariton, an extremely high Raman enhancement factor of 2.6 × 109 is implemented. The rationally designed SERS substrate allows sensitive detection of miRNA-21 in single molecular level with a detection limit of 1.58 aM. The hybrid SERS mechanism both from electromagnetic and chemical perspectives in this photonic-plasmonic-polaritonic resonance strategy provides insight into polaritonic semiconductor systems, thus paving the way for new experimental possibilities in light-matter hybrids.

Surface-Enhanced Raman Scattering Using 2D Materials

The use of surface-enhanced Raman scattering (SERS) as a technique for detecting small amounts of (bio)chemical analytes has become increasingly popular in various fields. While gold and silver nanostructures have been extensively studied as SERS substrates, the availability of other types of substrates is currently expanding the applications of this spectroscopic method. Recently, researchers have begun exploring two-dimensional (2D) materials (e. g., graphene-like nanostructures) as substrates for SERS analysis. These materials offer unique optical properties, a well-defined structure, and the ability to modify their surface chemistry. As a contribution to advance this field, this concept article highlights the significance of understanding the chemical mechanism that underlies the experimental Raman spectra of chemisorbed molecules onto 2D materials' surfaces. Therefore, the article discusses recent advancements in fabricating substrates using 2D layered materials and the synergic effects of using their metallic composites for SERS applications. Additionally, it provides a new perspective on using Raman imaging in developing 2D materials as analytical platforms for Raman spectroscopy, an exciting emerging research area with significant potential.

Interfacial Bonding Induced Charge Transfer in Two-Dimensional Amorphous MoO3-x/Graphdiyne Oxide Non-Van der Waals Heterostructures for Dominant SERS Enhancement

Two-dimensional semiconductor-based nanomaterials have shown to be an effective substrate for Surface-enhanced Raman Scattering (SERS) spectroscopy. However, the enhancement factor (EF) tends to be relatively weak compared to that of noble metals and does not allow for trace detection of molecules. In this work, we report the successful preparation of two-dimensional (2D) amorphous non-van der Waals heterostructures MoO3-x/GDYO nanomaterials using supercritical CO2. Due to the synergistic effect of the localized surface plasmon resonance (LSPR) effect and the charge transfer effect, it exhibits excellent SERS performance in the detection of methylene blue (MB) molecules, with a detection limit as low as 10-14 M while the enhancement factor (EF) can reach an impressive 2.55×1011. More importantly, the chemical bond bridging at the MoO3-x/GDYO heterostructures interface can accelerate the electron transfer between the interfaces, and the large number of defective surface structures on the heterostructures surface facilitates the chemisorption of MB molecules. And the charge recombination lifetime can be proved by a ~1.7-fold increase during their interfacial electron-transfer process for MoO3-x/GDYO@MB mixture, achieving highly sensitive SERS detection.

Exploring and Engineering 2D Transition Metal Dichalcogenides toward Ultimate SERS Performance

Surface-enhanced Raman spectroscopy (SERS) is an ultrasensitive surface analysis technique that is widely used in chemical sensing, bioanalysis, and environmental monitoring. The design of the SERS substrates is crucial for obtaining high-quality SERS signals. Recently, 2D transition metal dichalcogenides (2D TMDs) have emerged as high-performance SERS substrates due to their superior stability, ease of fabrication, biocompatibility, controllable doping, and tunable bandgaps and excitons. In this review, a systematic overview of the latest advancements in 2D TMDs SERS substrates is provided. This review comprehensively summarizes the candidate 2D TMDs SERS materials, elucidates their working principles for SERS, explores the strategies to optimize their SERS performance, and highlights their practical applications. Particularly delved into are the material engineering strategies, including defect engineering, alloy engineering, thickness engineering, and heterojunction engineering. Additionally, the challenges and future prospects associated with the development of 2D TMDs SERS substrates are discussed, outlining potential directions that may lead to significant breakthroughs in practical applications.

Design superhydrophobic no-noble metal substrates for highly sensitive and signal stable SERS sensing

Surface-enhanced Raman spectroscopy (SERS) is an analytical technique with a broad range of potential applications in fields such as biomedicine, electrochemistry, and hazardous chemicals. However, it is challenging to develop SERS substrates that are both good sensitive and signal stable. Here we designed a superhydrophobic Nd doped MoS2 uniformly assembled on the activated carbon fiber cloth (CFC) to avoid the coffee ring effect and enrich the analyte, improving the enhancement factor (EF) to 3.9 × 109 and pesticide endosulfan (<10-10) analyte detection. We demonstrate our strategy by density-functional theory (DFT) calculations confirming that both adsorption energy and density of states are enhanced after doping Nd leading to SERS enhancement. Beside DFT calculations, our experiments also provide an effective means to demonstrate that the high SERS sensitivity is based on multiple charge transfer processes combined with the activated carbon cloth. Our results address the limitations of low sensitivity and limit of detection (LOD) of semiconductor SERS substrates for trace analysis and are a step towards practical applications.

Unique gap-related SERS behaviors ofp-aminothiophenol molecules absorbed on TiO2surface in periodic TiO2/Ni nanopillar arrays

We observed a unique interpillar gap-related surface-enhanced Raman scattering (SERS) behavior ofp-aminothiophenol (PATP) molecules from periodic TiO2nanopillar arrays with three gap sizes of 191, 297 and 401 nm, which is completely different from that on Ag and Ni nanopillar arrays. Especially, the gap-size-dependent charge-transfer (CT) resonance enhancement from TiO2/Ni has been indicated through comparisons of variation trend of SERS intensities with inter-pillar gap size between TiO2/Ni and Ag/TiO2/Ni as well as Ni nanoarrays, and been confirmed by spectra of ultraviolet-visible absorption and photoluminescence. Results demonstrate that the CT resonance enhancement is more susceptible to the change of the gap size compared with the surface plasmon resonance (SPR) enhancement in TiO2/Ni nanoarrays. Hence, SPR and CT enhancement showing different variation trend and rate with the gap size that leads to a different relative contribution of CT resonance to the overall SERS enhancement as gap size changes, and consequently results in a unique gap-related SERS behavior for TiO2/Ni nanoarrays. The present study is not only helpful for investigating SERS mechanism for semiconductors but also providing a method to design and optimize periodic metal/semiconductor SERS substrates in a controllable way.

Recent progress in SERS monitoring of photocatalytic reactions

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical technique renowned for its ultra-high sensitivity. Extensive research in SERS has led to the development of a wide range of SERS substrates, including plasmonic metals, semiconductors, metal organic frameworks, and their assemblies. Some of these materials are also excellent photocatalysts, and by taking advantage of their bifunctional characteristics, the photocatalytic processes that occur on their surface can be monitored in situ via SERS. This provides us with unique opportunities to gain valuable insights into the intricate details of the photocatalytic processes that are challenging to access using other techniques. In this review, we highlight key development in in situ and/or real-time SERS-tracking of photocatalytic reactions. We begin by providing a brief account of recent developments in SERS substrates, followed by discussions on how SERS can be used to elucidate crucial aspects of photocatalytic processes, including: (1) the influence of the surrounding media on charge carrier extraction; (2) the direction of charge carrier transfer; (3) the pathway of photocatalytic activation; and (4) differentiation between the effects of photo-thermal and energetic electrons. Additionally, we discuss the benefits of tip-enhanced Raman spectroscopy (TERS) due to the ability to achieve high-spatial-resolution measurements. Finally, we address major challenges and propose potential directions for the future of SERS monitoring of photocatalytic reactions. By leveraging the capabilities of SERS, we can uncover new insights into photocatalytic processes, paving the way for advancements in sustainable energy and environmental remediation.

A new semiconductor heterojunction SERS substrate for ultra-sensitive detection of antibiotic residues in egg

Antibiotic residues in animal-derived food (egg) are threatening human health. Semiconductor heterojunction surface-enhanced Raman scattering (SERS) substrates can be used for ultra-sensitive detection of antibiotic residues in egg. Here, a TiO2/ZnO heterojunction was developed as a new SERS substrate based on an interface engineering strategy. Due to strong interfacial coupling and efficient carrier separating in heterostructure, utilization rate of photo-induced electrons in substrate was improved greatly, which realized the efficient charge transfer in substrate-molecule system, resulting in a prominent SERS enhancement. Taking the detection of enrofloxacin residue in egg as an example, the limit of detection (LOD) is only 13.1 μg/kg, which is far below the European Union standard, and lower than LODs of other conventional analytical methods and existing noble metal-based SERS methods. More importantly, benefiting from high sensitivity and selectivity of heterojunction and fingerprint characteristics of SERS, multiple antibiotic residues in egg can be identified simultaneously.

Resonance-Assisted Surface-Enhanced Raman Spectroscopy Amplification on Hierarchical Rose-Shaped MoS2/Au Nanocomposites

Surface-enhanced Raman spectroscopy (SERS) has emerged as a highly sensitive trace detection technique in recent decades, yet its exceptional performance remains elusive in semiconductor materials due to the intricate and ambiguous nature of the SERS mechanism. Herein, we have synthesized MoS2 nanoflowers (NFs) decorated with Au nanoparticles (NPs) by hydrothermal and redox methods to explore the size-dependence SERS effect. This strategy enhances the interactions between the substrate and molecules, resulting in exceptional uniformity and reproducibility. Compared to the unadorned Au nanoparticles (NPs), the decoration of Au NPs induces an n-type effect on MoS2, resulting in a significant enhancement of the SERS effect. This augmentation empowers MoS2 to achieve a low limit of detection concentration of 2.1 × 10-9 M for crystal violet (CV) molecules and the enhancement factor (EF) is about 8.52 × 106. The time-stability for a duration of 20 days was carried out, revealing that the Raman intensity of CV on the MoS2/Au-6 substrate only exhibited a reduction of 24.36% after undergoing aging for 20 days. The proposed mechanism for SERS primarily stems from the synergistic interplay among the resonance of CV molecules, local surface plasma resonance (LSPR) of Au NPs, and the dual-step charge transfer enhancement. This research offers comprehensive insights into SERS enhancement and provides guidance for the molecular design of highly sensitive SERS systems.

Near-Neighbor Electron Orbital Coupling Effect of Single-Atomic-Layer Au Cluster Intercalated Bilayer 2H-TaS2 for Surface Enhanced Raman Scattering Sensing

It is difficult to perfectly analyze the enhancement mechanism of two-dimensional (2D) materials and their combination with precious metals as surface enhanced Raman scattering (SERS) substrates using chemical enhancement mechanisms. Here, we propose a new mentality based on the coupling effect of neighboring electron orbitals to elucidate the electromagnetic field enhancement mechanism of single-atom-layer Au clusters embedded in double-layer 2H-TaS2 for SRES sensing. The insertion of Au atoms into the 2H-TaS2 interlayer was verified by XRD, AFM, and HRTEM, and a SERS signal enhancement of 2 orders of magnitude was obtained compared to the pure 2H-TaS2. XPS and micro-UV/vis-NIR spectra indicate that the outer electrons of neighboring Au and 2H-TaS2 overlap and migrate from Au to 2H-TaS2. First-principles calculations suggest strong electronic coupling between Au and 2H-TaS2. This study offers insights into SERS enhancement in nonprecious metal compounds and guides the development of new SERS substrates.

Plasmon-Induced Charge Transfer-Enhanced Raman Scattering on a Semiconductor: Toward Amplification-Free Quantification of SARS-CoV-2

Semiconductors demonstrate great potentials as chemical mechanism-based surface-enhanced Raman scattering (SERS) substrates in determination of biological species in complex living systems with high selectivity. However, low sensitivity is the bottleneck for their practical applications, compared with that of noble metal-based Raman enhancement ascribed to electromagnetic mechanism. Herein, a novel Cu2 O nanoarray with free carrier density of 1.78×1021  cm-3 comparable to that of noble metals was self-assembled, creating a record in enhancement factor (EF) of 3.19×1010 among semiconductor substrates. The significant EF was mainly attributed to plasmon-induced hot electron transfer (PIHET) in semiconductor which was never reported before. This Cu2 O nanoarray was subsequently developed as a highly sensitive and selective SERS chip for non-enzyme and amplification-free SARS-CoV-2 RNA quantification with a detection limit down to 60 copies/mL within 5 min. This unique Cu2 O nanoarray demonstrated the significant Raman enhancement through PIHET process, enabling rapid and sensitive point-of-care testing of emerging virus variants.

Insights into the Semiconductor SERS Activity: The Impact of the Defect-Induced Energy Band Offset and Electron Lifetime Change

The significant boost in surface-enhanced Raman scattering (SERS) by the chemical enhancement of semiconducting oxides is a pivotal finding. It offers a prospective path toward high uniformity and low-cost SERS substrates. However, a detailed understanding of factors that influence the charge transfer process is still insufficient. Herein, we reveal the important role of defect-induced band offset and electron lifetime change in SERS evolution observed in a MoO3 oxide semiconductor. By modulating the density of oxygen vacancy defects using ultraviolet (UV) light irradiation, SERS is found to be improved with irradiation time in the first place, but such improvement later deteriorates for prolonged irradiation even if more defects are generated. Insights into the observed SERS evolution are provided by ultraviolet photoelectron spectroscopy and femtosecond time-resolved transient absorption spectroscopy measurements. Results reveal that (1) a suitable offset between the energy band of the substrate and the orbitals of molecules is facilitated by a certain defect density and (2) defect states with relatively long electron lifetime are essential to achieve optimal SERS performance.

Alloy Engineering Allows On-Demand Design of Ultrasensitive Monolayer Semiconductor SERS Substrates

The chemical mechanism (CM) of surface-enhanced Raman scattering (SERS) has been recognized as a decent approach to mildly amplify Raman scattering. However, the insufficient charge transfer (CT) between the SERS substrate and molecules always results in unsatisfying Raman enhancement, exerting a substantial restriction for CM-based SERS. In principle, CT is dominated by the coupling between the energy levels of a semiconductor-molecule system and the laser wavelength, whereas precise tuning of the energy levels is intrinsically difficult. Herein, two-dimensional transition-metal dichalcogenide alloys, whose energy levels can be precisely and continuously tuned over a wide range by simply adjusting their compositions, are investigated. The alloys enable on-demand construction of the CT resonance channels to cater to the requirements of a specific target molecule in SERS. The SERS signals are highly reproducible, and a clear view of the SERS dependences on the energy levels is revealed for different CT resonance terms.

Au Nanoparticles Decorated CoP Nanowire Array: A Highly Sensitive, Anticorrosive, and Recyclable Surface-Enhanced Raman Scattering Substrate

Metal-semiconductor composites are promising candidates for surface-enhanced Raman scattering (SERS) substrates, but their inert basal plane, poor active sites, and limited stability hamper their commercial prospects. Herein, we report a three-dimensional CoP nanowire array decorated with Au nanoparticles on carbon cloth (Au@CoP/CC) as a self-supporting flexible SERS substrate. The Au nanoparticles spontaneously grew on the surface of the CoP nanowire array to form efficient SERS hot spots by a redox reaction with HAuCl₄ without any additional reducing agents. Such Au@CoP/CC substrate exhibited a limit of detection of 10^-11 M using rhodamine 6G as a model dye with outstanding corrosion resistance ability even under extreme acid and alkali conditions, which is better than many recently reported Au-based SERS substrates. Finite-difference time-domain simulation results demonstrated that Au@CoP/CC can provide a high density of regions with intense local electric field enhancement. Moreover, Au@CoP/CC can degrade target organic dyes for the self-cleaning and reproduction of SERS-active substrates under visible light irradiation. This work provides a novel means of using the plasmonic metal-transition metal phosphide composites for high-performance SERS sensing and photodegradation.

Heteropolyacids: An Ultrasensitive Ionic Volume-Enhanced Raman Scattering Platform

Surface-enhanced Raman scattering (SERS) is regarded as the most direct and powerful tool to identify chemical fingerprints. However, current SERS substrate materials still face some critical challenges, including low molecular utilization efficiency and low selectivity. Herein, a novel oxygen vacancy heteropolyacid─H₁₀Fe₃Mo₂₁O₅₁ (HFMO)─is developed as a high-performance volume-enhanced Raman scattering (VERS)-active platform. Due to its merit of water solubility, HFMO forms a special coordination bond with the probe molecule at the molecular level, which allows its enhancing ability to be comparable to that of noble metals. An enhancement factor of 1.26 × 10⁹ and a very low detection limit of 10^-13 M for rhodamine 6G were obtained. A robust O-N coordination bond was formed between the anion of HFMO and the probe molecule, resulting in a special electron transfer path (Mo-O-N) with high selectivity, which is verified using X-ray photoelectron spectroscopy analysis and density functional theory calculations. That is to say, the proposed HFMO platform has excellent VERS enhancing effect, specifically for the molecules containing the imino group (e.g., methyl blue, detection limit: 10^-11 M), offering the merits of high reproducibility and uniformity, high-temperature resistance, long-time laser irradiation, and strong acid resistance. Such an initial effort on the ionic type VERS platform may enable the further development of highly sensitive, highly selective, and water-soluble VERS technology.

Development of a MoS2/Ag NP Nanopocket to Trap Target Molecules for Surface-Enhanced Raman Scattering Detection with Long-Term Stability and High Sensitivity

Surface-enhanced Raman scattering (SERS) substrates mostly achieve highly sensitive detection by designing various hot spots; however, how to guide molecules to hot spots and prevent them from leaving has not been thoroughly considered and studied. Here, a composite MoS₂/Ag NP nanopocket detector composed of MoS₂ covered with a Ag NP film was fabricated to develop a general SERS method for actively capturing target molecules into hotspots. A finite element method (FEM) simulation of the multiphysics model was used to analyze the distributions of electric field enhancements and hydrodynamic processes in solution and air of the MoS₂/Ag NP nanopocket. The results revealed that covering MoS₂ slowed the evaporation of the solution, extended the window period for SERS detection, and enhanced the electric field in comparison with the monolayer Ag NP film. Therefore, in the process of dynamic detection, the MoS₂/Ag NP nanopocket can provide an efficient and stable signal within 8 min, increasing the high sensitivity and long-term stability of the SERS method. Furthermore, a MoS₂/Ag NP nanopocket detector was applied to detect antitumor drugs and monitor hypoxanthine structural changes in serum, which demonstrated long-term stability and high sensitivity for SERS analysis. This MoS₂/Ag NP nanopocket detector paves the way for developing the SERS method in various fields.

Polyhedral Au Nanoparticle/MoOx Heterojunction-Enhanced Ultrasensitive Dual-Mode Biosensor for miRNA Detection Combined with a Nonenzymatic Cascade DNA Amplification Circuit

A novel homologous surface-enhanced Raman scattering (SERS)-electrochemical (EC) dual-mode biosensor based on a 3D/2D polyhedral Au nanoparticle/MoO_x nanosheet heterojunction (PAMS HJ) and target-triggered nonenzyme cascade autocatalytic DNA amplification (CADA) circuit was constructed for highly sensitive detection of microRNA (miRNA). Mixed-dimensional heterostructures were prepared by in situ growth of polyhedral Au nanoparticles (PANPs) on the surface of MoO_x nanosheets (MoO_x NSs) via a seed-mediated growth method. As a detection substrate, the resulting PAMS HJ shows the synergistic effects of both electromagnetic and chemical enhancements, efficient charge transfer, and robust stability, thus achieving a high SERS enhancement factor (EF) of 4.2 × 10⁹ and strong EC sensing performance. Furthermore, the highly efficient molecular recognition between the target and smart lock probe and the gradually accelerated cascade amplification reaction further improved the selectivity and sensitivity of our sensing platform. The detection limits of miRNA-21 in SERS mode and EC mode were 0.22 and 2.69 aM, respectively. More importantly, the proposed dual-mode detection platform displayed excellent anti-interference and accuracy in the analysis of miRNA-21 in human serum and cell lysates, indicating its potential as a reliable tool in the field of biosensing and clinical analysis.

Ultrafast charge transfer in mixed-dimensional WO3-x nanowire/WSe2 heterostructures for attomolar-level molecular sensing

Developing efficient noble-metal-free surface-enhanced Raman scattering (SERS) substrates and unveiling the underlying mechanism is crucial for ultrasensitive molecular sensing. Herein, we report a facile synthesis of mixed-dimensional heterostructures via oxygen plasma treatments of two-dimensional (2D) materials. As a proof-of-concept, 1D/2D WO3-x/WSe2 heterostructures with good controllability and reproducibility are synthesized, in which 1D WO3-x nanowire patterns are laterally arranged along the three-fold symmetric directions of 2D WSe2. The WO3-x/WSe2 heterostructures exhibited high molecular sensitivity, with a limit of detection of 5 × 10-18 M and an enhancement factor of 5.0 × 1011 for methylene blue molecules, even in mixed solutions. We associate the ultrasensitive performance to the efficient charge transfer induced by the unique structures of 1D WO3-x nanowires and the effective interlayer coupling of the heterostructures. We observed a charge transfer timescale of around 1.0 picosecond via ultrafast transient spectroscopy. Our work provides an alternative strategy for the synthesis of 1D nanostructures from 2D materials and offers insights on the role of ultrafast charge transfer mechanisms in plasmon-free SERS-based molecular sensing.

The Convenience of Polydopamine in Designing SERS Biosensors with a Sustainable Prospect for Medical Application

Polydopamine (PDA) is a multifunctional biomimetic material that is friendly to biological organisms and the environment, and surface-enhanced Raman scattering (SERS) sensors have the potential to be reused. Inspired by these two factors, this review summarizes examples of PDA-modified materials at the micron or nanoscale to provide suggestions for designing intelligent and sustainable SERS biosensors that can quickly and accurately monitor disease progression. Undoubtedly, PDA is a kind of double-sided adhesive, introducing various desired metals, Raman signal molecules, recognition components, and diverse sensing platforms to enhance the sensitivity, specificity, repeatability, and practicality of SERS sensors. Particularly, core-shell and chain-like structures could be constructed by PDA facilely, and then combined with microfluidic chips, microarrays, and lateral flow assays to provide excellent references. In addition, PDA membranes with special patterns, and hydrophobic and strong mechanical properties can be used as independent platforms to carry SERS substances. As an organic semiconductor material capable of facilitating charge transfer, PDA may possess the potential for chemical enhancement in SERS. In-depth research on the properties of PDA will be helpful for the development of multi-mode sensing and the integration of diagnosis and treatment.

Recent Progress in the Application of Metal Organic Frameworks in Surface-Enhanced Raman Scattering Detection

Metal-organic framework (MOF) compounds are centered on metal ions or metal ion clusters, forming lattices with a highly ordered periodic porous network structure by connecting organic ligands. As MOFs have the advantages of high porosity, large specific surface area, controllable pore size, etc., they are widely used in gas storage, catalysis, adsorption, separation and other fields. SERS substrate based on MOFs can not only improve the sensitivity of SERS analysis but also solve the problem of easy aggregation of substrate nanoparticles. By combining MOFs with SERS, SERS performance is further improved, and tremendous research progress has been made in recent years. In this review, three methods of preparing MOF-based SERS substrates are introduced, and the latest applications of MOF-based SERS substrates in biosensors, the environment, gases and medical treatments are discussed. Finally, the current status and prospects of MOF-based SERS analysis are summarized.

ZnO nanorods decorated with Ag nanoflowers as a recyclable SERS substrate for rapid detection of pesticide residue in multiple-scenes

Pesticide residues threaten the ecological environment and human health. Therefore, developing high performance SERS substrate to achieve highly sensitive detection of pesticide residues is meaningful. In this study, based on the strategy of combining "hot spots" engineering and material hybridization, we construct a novel hybrid SERS substrate by depositing Ag nanoflowers (NFs) on ZnO nanorods (NRs). Benefiting from the synergistic effect of electromagnetic enhancement and charge transfer effect, the Ag NFs@ZnO NRs substrate exhibits a low detection limit (10^-13 M) for crystal violet molecules. This SERS substrate has good uniformity with a relative standard deviation of 7.463 %. Besides, owning to the photocatalytic property of ZnO NRs, the hybrid substrate can degrade probe molecules after SERS detection and realize recyclability. As a demonstration, we employed our SERS substrate for the trace detection of pesticide residues on apple surface and in river water. This study provides a new idea for improving the SERS performance of hybrid substrates.