Two-Dimensional TiVC Solid-Solution MXene as Surface-Enhanced Raman Scattering Substrate

Tailoring SERS sensitivity in AgNWs-ZIF-67 substrates: Effects of MOF thickness and probe-pore size matching

In this study, we develop a filter paper-supported Ag nanowire (AgNWs) substrate coated with a tunable layer of zeolitic imidazolate framework-67 (ZIF-67) metal-organic framework (MOF) for surface-enhanced Raman scattering (SERS) applications. The thickness of the ZIF-67 layer is precisely controlled by adjusting the soaking time, allowing us to investigate the influence of MOF thickness on SERS performance using probes with different sizes. Sensitivity for large, impenetrable molecules such as thiram and rhodamine 6G (R6G) are primarily governed by localized surface plasmon resonance (LSPR), while smaller, penetrable molecules like 4-mercaptobenzoic acid (4-MBA) exhibit enhanced sensitivity driven by charge transfer (CT) effects due to their ability to penetrate the MOF layer. A finite-difference time-domain (FDTD) simulation reveals that the porous structure of ZIF-67 facilitates electric field propagation to the probe, maintaining significant LSPR effects even beyond 200 nm from the AgNWs. Practical applicability is demonstrated using a wiping mode to detect thiram spiked on apple surfaces, with reliable linear detection achieved across a wide range of concentrations. This study underscores the potential of MOF-based hybrid SERS substrates for real-world applications, leveraging the complementary roles of CT and electromagnetic (EM) mechanisms to enhance sensitivity.

Curvature-Induced Electron Delocalization Activates the Bifunctional Catalytic Activity of COF/MXene for High-Performance Lithium-Sulfur Batteries

Covalent organic frameworks (COFs) have shown promise as bifunctional catalysts to simultaneously mitigate shuttle effects and Li dendrite issues of lithium-sulfur (Li-S) batteries. However, the inherent low conductivity of the COFs has significantly limited their catalytic activity and stability. Herein, bifunctional catalytic activity and durability of the COF/MXene heterostructure are activated by tuning the surface curvatures of COFs interfaced with MXene. The increased curvature of COFs could induce enhanced electron delocalization and alter heterostructure geometry, which in turn strengthens lithium polysulfide adsorption, lowers energy barriers, and stabilizes catalytic sites to promote sulfur redox reactions. Concurrently, the hierarchical COF/MXene structure improves electrolyte penetration and wettability, facilitates rapid ion transport, and homogenizes the Li-ion flux distribution, thus achieving uniform lithium deposition. Consequently, the 1D-COF/MXene Li-S batteries demonstrate a high-rate capacity of 926 mA h g-1 at 4C, a stable cycling performance with a reversible capacity of 589 mA h g-1 at 3C after 500 cycles, and a high reversible capacity of 604 mA h cm-2 with a sulfur loading of 3.5 mg cm-2 under a low electrolyte-to-sulfur ratio of 10 μL mg-1. This work offers an efficacious approach to regulate catalytic activity and stability of catalysts.

Anisotropically Shaped Plasmonic WO3-x Nanostructure-Driven Ultrasensitive SERS Detection and Machine Learning-Based Differentiation of Nitro-Explosives

Discovery of new surface-enhanced Raman spectroscopy (SERS) substrates consisting of inexpensive and earth-abundant elements is an unmet need for the advancement of future analysis techniques for the ultrasensitive detection and quantification of chemical and biological analytes. Nanostructures (NSs) of noble metals such as Au, Ag, and Cu are the benchmarks for the preparation of highly efficient SERS substrates because of their unique localized surface plasmon resonance (LSPR) properties. Non-noble-metal SERS substrates, e.g., metal chalcogenide semiconductors and transition metal oxides, have been prepared to mitigate the cost; however, their low sensitivity restricts widespread applications. In this article, we report for the first time that the structure of oxygen-deficient, LSPR-active, nonstoichiometric tungsten oxide (i.e., WO3-x) NSs can control the SERS enhancement factor (EF). SERS substrates prepared with colloidally synthesized WO3-x nanowires, nanorods, and nanoplatelets display SERS EF of 2.5 × 106, 3.1 × 107, and 5.5 × 107, respectively, using rhodamine 6G (R6G) molecules as Raman probes. Our experimentally acquired SERS data and spectroscopically determined electronic band structure of LSPR-active WO3-x NSs, and time-domain density functional theory (TDDFT)-based calculations support a dual enhancement scheme involving a strong plasmonic effect controlled electromagnetic field and their oxygen vacancy-induced chemical enhancement mechanisms, respectively. To demonstrate the practical utility of our WO3-x NCs, we are able to detect aromatic nitro-explosives (tetryl, TNT, and DNT) with a very low limit of detection (LOD, 10-9 M). Importantly, machine learning-driven chemometric analysis for SERS-based detection shows excellent classification between these three explosives. Finally, three nonaromatic, nitro-explosives, HMX, RDX, and PETN are also successfully detected utilizing our LSPR-active, WO3-x-based SERS substrates. To the best of our knowledge, this is the first example where LSPR-active, non-noble-metal NSs are used for the detection of both aromatic and aliphatic nitro-explosives. Taken together, our work represents the advancement of the fabrication of non-noble-metal-based SERS substrates, which can be widely employed for the low-cost detection of analytes across forensic science, chemistry, and biomedical fields.

Layer-by-layer thin films of Ti3C2 MXene and gold nanoparticles as an ideal SERS platform

The combination of plasmonic metals and MXene, as a new and interesting member of the 2D material class, may provide unique advantages in terms of low cost, versatility, flexibility, and improved activity as an ideal surface-enhanced Raman spectroscopy (SERS) platform. Despite the recent progress, the present studies on the utilization of plasmonic metal/MXene-based SERS systems are quite limited and thereby benefits of the extraordinary properties of this combination cannot be realized. In this study, for the first time, we propose layer-by-layer (LbL) thin films of Ti3C2 MXene and gold nanoparticles (AuNPs) as a robust SERS platform (Ti3C2/AuNPs). For this, Ti3C2 MXene was synthesized from the Ti3AlC2 MAX phase, and the Ti3C2/AuNP LbL film was fabricated via a vacuum-assisted filtration method to create consecutive layers of each material. This procedure produced densely distributed AuNPs in the LbL film in a well-controlled manner. The SERS activity tests for methylene blue and DTNB as Raman reporter molecules showed that they exhibited enhancement factors of 1.5 × 106 and 1.2 × 106 and limits of detection of 1 × 10-8 M, and 2.5 × 10-8 M, respectively. Various mechanisms, including the formation of hotspots due to AuNPs on the interlayer of Ti3C2, improved surface roughness and resultant optical activity, as well as the synergistic effect between Ti3C2 and AuNPs, contributed to the resultant SERS activity to some extent. This study has proven the feasibility of the Ti3C2/AuNP LbL system as a robust SERS-based sensor platform, paving the way for its use in various biological and chemical applications.

Structurally-Modulated Substrate of MXene for Surface-Enhanced Raman Scattering Sensing

The distinctive structure of MXene offers exceptional electron transport properties, abundant surface chemistry, and robust mechanical attributes, thereby bestowing it with remarkable advantages and promising prospects in the realm of surface-enhanced Raman scattering (SERS). This review comprehensively outlines the evolution, synthesis methodologies, and characterization techniques employed for MXene-based SERS substrates. It delves into the intricacies of its SERS enhancement mechanism, substrate variants, and performance metrics, alongside showcasing its diverse applications spanning molecular detection, biosensing, and environmental monitoring. Furthermore, it endeavors to pinpoint the research bottlenecks and chart the future research trajectories for MXene-based SERS substrates.

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.

Solid-Solution MXenes: Synthesis, Properties, and Applications

ConspectusMXenes, among other two-dimensional (2D) materials such as graphene, hexagonal BN, transition metal dichalcogenides (TMDs), 2D metal-organic frameworks (MOFs), and covalent organic frameworks (COFs), are the fastest growing class discovered thus far. The general formula of MXenes is Mn+1XnTx, where M, X, and Tx represent an early transition metal (Ti, V, Nb, Mo, etc.), C and/or N, and the surface functional groups (typically, O, OH, F, Cl), respectively, and n can be between 1 and 4. MXenes as a class of materials have extraordinary properties, such as high electrical conductivity, nonlinear optical properties, solution processability, scalability and ease of synthesis, redox capability, and tunable surface properties, among others; the specific properties, however, depend on their chemistry. Since the initial report of the first MXene in 2011, the research community has primarily focused on Ti3C2Tx, and the amount of research work to investigate its synthesis and properties has increased exponentially over the years. In materials science, alloying is a useful way of synthesizing new materials to improve the properties of a class of materials. Advancement of steel and synthesis of inorganic semiconductors can be regarded as some of the major historical advancements in the concept of alloying. Thus, just one year after the initial report of MXenes, the first solid-solution MXene, (TiNb)2CTx, was reported, which demonstrates the inherent chemical tunability of this class of materials.MXenes have two sites for compositional variation: elemental substitution on both the metal (M) and carbon/nitrogen (X) sites, presenting promising routes for tailoring their properties. X-site solid-solutions include carbonitride MXenes and are the least studied class of MXenes to date. Comparatively, multi-M MXenes have acquired significant attention, leading to the extreme example of high-entropy solid-solution MXenes. By using multiple M elements, a significant expansion of the structural and chemical diversity is possible, giving rise to novel chemical, magnetic, electronic, and optical properties that cannot be accessed by single-M MXenes. Solid-solution MXenes represent the largest and most tunable class of MXenes; solid-solution MXenes are those that have multiple metals that are randomly distributed on their M sites with no distinct chemical ordering. Using multiple M elements in MXenes, it is possible to synthesize novel MXene structures that cannot be produced otherwise, such as M5X4Tx MXenes. Based on their chemistry, it is possible to rationally control the electronic, optical, mechanical, and chemical properties in a way that no other class of MXenes can. In some cases, the resultant property is linearly related to the chemistry, such as the electrical conductivity, while in other cases the properties are nonlinear or emergent: optical properties, enabling these MXenes to fulfill roles that no other MXene, or 2D material, can.In this Account, we discuss the recent progress in the synthesis, properties, applications, and outlook of solid-solution MXenes. Importantly, we demonstrate how multi-M solid-solutions can be used to tailor properties for specific applications easily.

Development and Biomedical Application of Non-Noble Metal Nanomaterials in SERS

Surface-enhanced Raman scattering (SERS) is vital in many fields because of its high sensitivity, fast response, and fingerprint effect. The surface-enhanced Raman mechanisms are generally electromagnetic enhancement (EM), which is mainly based on noble metals (Au, Ag, etc.), and chemical enhancement (CM). With more and more studies on CM mechanism in recent years, non-noble metal nanomaterial SERS substrates gradually became widely researched and applied due to their superior economy, stability, selectivity, and biocompatibility compared to noble metal. In addition, non-noble metal substrates also provide an ideal new platform for SERS technology to probe the mechanism of biomolecules. In this paper, we review the applications of non-noble metal nanomaterials in SERS detection for biomedical engineering in recent years. Firstly, we introduce the development of some more common non-noble metal SERS substrates and discuss their properties and enhancement mechanisms. Subsequently, we focus on the progress of the application of SERS detection of non-noble metal nanomaterials, such as analysis of biomarkers and the detection of some contaminants. Finally, we look forward to the future research process of non-noble metal substrate nanomaterials for biomedicine, which may draw more attention to the biosensor applications of non-noble metal nanomaterial-based SERS substrates.

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.

Sensitive detection of kanamycin based on ECL resonance energy transfer between iridium complex doped SiO2 nanospheres and Au nanoparticles decorated TiVC MXene

Herein, a novel sandwich electrochemiluminescence (ECL) aptasensor was developed based on the resonance energy transfer (RET) with iridium complex doped silicate nanoparticles (SiO2@Ir) as energy donor and gold nanoparticles modified TiVC MXene (AuNPs@TiVC) as energy acceptor. Strong anodic ECL signal of SiO2@Ir was obtained through both co-reactant pathway and annihilation pathway. Electrochemical results showed that SiO2@Ir has good electron transfer rate and large specific surface area to immobilize more aptamers. AuNPs@TiVC apparently quenched the ECL signal of SiO2@Ir due to the ECL resonance energy transfer between them. In the presence of kanamycin (KAN), a sandwich type sensor was formed with the aptamer probes as connecters between the donor and the acceptor, resulting in the decrease of ECL intensity. Under the optimal condition, KAN could be sensitively detected in the range of 0.1 pg/mL to 10 ng/mL with a low detection limit of 24.5 fg/mL. The proposed ECL system exhibited satisfactory analytical performance, which can realize the detection of various biological molecules by adopting suitable aptamer.

Ti3C2TxMXene as surface-enhanced Raman scattering substrate

The electromagnetic field enhancement mechanisms leading to surface-enhanced Raman scattering (SERS) of R6G molecules near Ti3C2TxMXene flakes of different shapes and sizes are analyzed theoretically in this paper. In COMSOL simulations for the enhancement factor (EF) of SERS, the dye molecule is modeled as a small sphere with polarizability spectrum based on experimental data. It is demonstrated, for the first time, that in the wavelength range of500 nm-1000 nm, the enhancement of Raman signals is largely conditioned by quadrupole surface plasmon (QSP) oscillations that induce a strong polarization of the MXene substrate. We show that the vis-NIR spectral range quadrupole SP resonances are strengthened due to interband transitions (IBTs), which provide EF values of the order of 105-107in agreement with experimental data. The weak sensitivity of the EF to the shape and size of MXene nanoparticles (NPs) is interpreted as a consequence of the low dependence of the absorption cross-section of QSP oscillations and IBT on the geometry of the flakes. This reveals a new feature: the independence of EF on the geometry of MXene substrates, which allows to avoid the monitoring of the shape and size of flakes during their synthesis. Thus, MXene flakes can be advantageous for the easy manufacturing of universal substrates for SERS applications. The electromagnetic SERS enhancement is determined by the 'lightning rod' and 'hot-spot' effects due to the partial overlapping of the absorption spectrum of the R6G molecule with these MXene resonances.

Phase Transformation on Multilayer 2M-WS2 for Improved Surface-enhanced Raman Scattering

In recent years, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been widely recognized as an ideal platform for surface-enhanced Raman scattering (SERS). Given their rich structural phases, phase transformation in 2D TMDCs is an efficient strategy to tailor their SERS performance. In this paper, we present the great SERS performance of multilayer 2M-WS2 and then investigate the effect of its phase transformation on SERS performance. It is observed that multilayer 2M-WS2 nanosheets undergo a thermally induced single-crystal phase transition from 2M-WS2 to 2H-WS2 upon thermal annealing or laser treatment. Distinguishing from the commercially available pure 2H-WS2 (P-2H-WS2), 2H-WS2 obtained by annealing and laser treatment still retain SERS properties comparable to those of 2M-WS2, among which the detection limits for CV molecules (10-8 M) are 3 orders of magnitude lower than that of P-2H-WS2 and the Raman intensity enhancements are ∼10-37 times higher. In contrast to the charge transfer (CT) mechanism governed by the Fermi level in metallic-phase 2M-WS2, 2H-WS2 obtained by phase transition exhibits accelerated CT facilitated by the bandgap reduction and reorganization resulting from the abundance of vacancies. This study introduces an interesting perspective and potential avenue for enhancing SERS through metal-to-semiconductor phase transitions in 2D TMDCs materials.

Tailoring Interfacial Electric Field by Gold Nanoparticles Enable Electrocatalytic Lithium Polysulfides Conversion for Lithium-Sulfur Batteries

Although lithium-sulfur batteries (LSBs) are considered as the promising next rechargeable storage system ascribing to their decent specific capacity of inorganic sulfur, the development is partially impeded by inferior electronic conductivity, severe shuttle effect, and large volume variation. To tackle the issues above, a great deal of effort is made on sulfur-containing polymer (SCP) that shows better electrochemical performance. Nevertheless, sluggish conversion of lithium polysulfides (LiPSs) obstructs battery performance yet. Herein, electrocatalytic LiPSs with full conversion by tailoring the interfacial electric field are discovered based on gold nanoparticles (AuNPs) anchored on sulfurized polyaniline (SPANI). A downhill path of Gibbs free energy from organosulfur polymer to intermediate product means more spontaneously and favorable for full conversion, as the significant enhancement of electron density of state in the vicinity of the HOMO level for the AuNPs increase the electron transition probability rate. This composite delivers satisfactory electrochemical performance, especially increased rate capacity of >300 mAh g-1. Furthermore, catalyst mechanism on molecule level is proposed that AuNPsdominate chemical enhancement and higher electron delocalizablility betweenAuNPs and LiPSs molecules. These results can erect a promising strategy for enhancing lithium polysulfides full conversion.

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.

Improved Surface-Enhanced Raman Scattering Performance of 2D Ti3C2Tx MXene Embedded in PVDF Film Enabled by Photoinduction and Electric Field Modulation

In this study, we introduce a synergistic approach to enhance the surface-enhanced Raman scattering (SERS) signal in two-dimensional (2D) MXene through photo-irradiation and electric field modulation. Our methodology involves the integration of 2D Ti3C2Tx MXene with piezoelectric polyvinylidene fluoride (PVDF) polymer, resulting in the creation of a free-standing, flexible composite film. On this composite film, a thin layer of Au was deposited. Our flexible substrate was able to sense methylene blue (MB), crystal violet (CV), 4-aminothiophenol (ATP), and melamine. The SERS substrate exhibits low detection limit of 10-8 M MB with a 6.7 × 106 enhancement factor (EF). The SERS substrate enables picomolar (pM) detection sensitivity for CV molecules with an EF of 9.2 × 109. Furthermore, the introduction of photo-irradiation leads to an additional ∼3.5-fold enhancement in the SERS signal, which is attributed to the altered work function and defects. The application of mechanical force to the piezoelectric PVDF/Ti3C2Tx film results in a ∼4.5-fold boost in SERS signal due to mechanical force-induced electrical energy. The fabrication strategy employed here for producing a flexible piezoelectric PVDF/Ti3C2Tx film holds significant promise for expanding the potential application of 2D MXene in rapid, on-site sensing scenarios.

Carboxyl-functionalized two-dimensional MXene-Au nanocomposites were prepared as SERS substrates for the detection of melamine in dairy products

In the present study, we address the limitations of conventional surface-enhanced Raman scattering (SERS) techniques for sensitive and stable detection of melamine in food products, especially dairy. To overcome these challenges, we developed a novel SERS-active substrate by incorporating gold nanoparticles (AuNPs) onto carboxyl-functionalized two-dimensional (2D) MXene material doped with nitrides, specifically Au-Ti2N-COOH. Our strategy leverages the unique physicochemical properties of MXene, a class of atomically thin, 2D transition metal carbides/nitrides, with tunable surface functionalities. By modifying the MXene surface with AuNPs and introducing carboxyl groups (-COOH), we successfully enhanced the interaction between the substrate and melamine molecules. The carboxyl groups form hydrogen bonds with the amino groups on the melamine's triazine ring, facilitating the adsorption of melamine molecules within the 'hotspot' regions responsible for SERS signal amplification. A series of characterization methods were used to confirm the successful synthesis of Au-Ti2N-COOH composites.Using Au-Ti2N-COOH as the SERS substrate, we detected melamine in spiked dairy product samples with significantly enhanced sensitivity and stability compared to nitride-doped MXene alone. The detection limit in liquid milk stands at 3.7008 μg kg-1, with spike recovery rates ranging from 99.84% to 107.55% and an approximate RSD of 5%. This work demonstrates the effectiveness of our approach in designing a label-free, rapid, and robust SERS platform for the accurate quantitation of melamine contamination in food, thereby mitigating health risks associated with melamine adulteration.

Promoting effect of TiVC MXene on cathodic electrogenerated chemiluminescence of Ru(bpy)32+ and its application in the sensitive detection of sulfite

The enhanced cathodic ECL of Ru(bpy)32+ at a bimetallic element MXenes (TiVC MXene) modified electrode in neutral aqueous condition is reported. TiVC MXene significantly catalyzed the oxygen reduction reaction (ORR) as well as the electrochemical reduction of Ru(bpy)32+ to produce reactive oxygen species and Ru(bpy)3+. The obtained hydroxyl radical (OH∙) not only oxidized Ru(bpy)3+ to generate Ru(bpy)32+* and emit light through coreactant pathway, but also oxidized Ru(bpy)32+ to Ru(bpy)33+, which caused an annihilation ECL reaction. As a result, two pathways occurred simultaneously to generate strong cathodic ECL signal. Sulfite removes the dissolved oxygen in water and reduces the occurrence of ORR, which prohibits the generation of OH∙ to decrease the ECL signal. The decrement of ECL intensity varied linearly with the concentration of sulfite in the range 2 nM to 50 μM with a detection limit of 0.14 nM (3σ). The proposed sensor exhibited good analytical performance, and could be used in the detection of sulfite in real samples. The results revealed that the electrocatalytic behavior of TiVC MXene is the key factor for strong cathodic Ru(bpy)32+ ECL, which provides new application in ECL sensing field.

Therapeutic drug monitoring mediated by the cooperative chemical and electromagnetic effects of Ti3C2TX modified with Ag nanocubes

It is pivotal for the credible utilization of surface-enhanced Raman scattering (SERS) technique in clinical drug monitoring to exploit versatile substrates with dependable quantitative detection and robust recognition abilities. Herein, a commendable electromagnetic-chemical dual-enhancement SERS substrate dependent on Ti3C2TX and Ag nanocubes (Ag NCs) was fabricated for the precise quantification of ritonavir and ibrutinib in serum. Specifically, it was revealed that numerous electromagnetic "hotspots" emerged nearby the extremely tiny nanogaps among the intimately clustered Ag NCs, which also acted as optimal channels to facilitate effective photo-induced charge transfer (PICT) between the two-dimensional Ti3C2TX matrix and target molecules. The cooperation between electromagnetic and chemical effects yielded a satisfactory enhancement factor (EF) of 4.77 × 107 for the composite substrate. Benefiting from the remarkable sensitivity of the Ti3C2TX/Ag NCs composite substrate, the low limit of detection (LOD) at 10-6 mg/mL was successfully attained, along with exceptional recoveries of exceeding 90% for ritonavir and ibrutinib in serum. Considering its reliability and simplicity, our strategy holds immense promise for its utilization in efficient monitoring and identification of clinical blood drug concentration.

Bimetallic MXene with tailored vanadium d-band as highly efficient electrocatalyst for reversible lithium-oxygen battery

Lithium-oxygen (Li-O2) battery possesses high theoretical energy density of ∼ 3500 Wh kg-1, yet the sluggish kinetics of oxygen redox reactions hinder its practical application. Herein, TiVC bimetallic MXene solid solution is prepared as the efficient electrocatalyst for Li-O2 battery. The results of experiment and theoretical calculations reveal that through the formation of Ti-C-V bond in TiVC, electrons transfer from V site to Ti site enhances electron delocalization of V sites, which causes the upshift of d band center of V site and strengthens the adsorption of intermediate products (LiO2) on TiVC electrode surface. Due to the strong adsorption of intermediates, the film-like Li2O2 can be formed on TiVC electrode via the surface-adsorbed pathway, which ensures the full contact between the electrode and discharged product and thus facilitates the charge transfer between TiVC electrode and oxygen species during charge process. As a consequence, the TiVC based Li-O2 battery exhibits superior electrochemical performance including large discharge capacity (12780 mAh/g) and extended cycling stability (422 cycles) at the current density of 300 mA g-1.

Modulation of the emission spectrum of rare-earth ions using inverse-designed photonic crystals cavities

Rare-earth elements play an indispensable role in the optical communication and laser industries, due to their superior luminescent properties. Nevertheless, the selective enhancement and suppression of different emission bands during energy level transitions for multi-band emitting rare-earth ions presents a significant research challenge, which we aim to address. This study explores the potential of leveraging an inverse-designed dual-cavity photonic crystals structure to manipulate the emission spectrum, thereby facilitating the augmentation or suppression of distinct emission bands. We utilized a convolutional neural network model to establish the relationship between geometric parameters and the local density of states, forecasting the optimal cavity geometry parameters for achieving the desired modulation outcomes. This paper delineates the neural network's generalization capabilities, along with the modulation efficacy of the dual-cavity configuration, both confirmed through numerical validation. Our findings highlight the modulatory capacity of Dy3+ ions, which exhibit three emission spectrum in the visible range, to achieve pure color light emission within the devised cavity structure. Notably, our approach yielded enhancements of up to 2.79-fold and 2.81-fold in pure yellow and red light emissions respectively, compared to free space emissions. The single-sided emission enhancement reaches 16.28-fold for yellow light and 30.79-fold for red light. This emphasizes the transformative potential of this methodology in crafting rare-earth-based luminescent materials with meticulously engineered emission attributes.

Wearable Plasmonic Sensors Engineered via Active-Site Maximization of TiVC MXene for Universal Physiological Monitoring at the Molecular Level

Two-dimensional transition metal carbon/nitrides (MXenes) are promising candidates to revolutionize next-generation wearable sensors as high-performance surface-enhanced Raman scattering (SERS) substrates. However, low sensitivity of pure MXene nanosheets and weak binding force or uncontrolled in situ growth of plasmonic nanoparticles on hybrid MXene composites limit their progress toward universal and reliable sensors. Herein, we designed and manufactured a highly sensitive, structurally stable wearable SERS sensor by in situ fabrication of plasmonic nanostructures on the flexible TiVC membranes via the maximization of chemically reducing sites using alkaline treatment. DFT calculations and experimental characterization demonstrated that the hydroxyl functional groups on the surface of MXenes can facilitate the reduction of metal precursors and the nucleation of gold nanoparticles (AuNPs) and can be covalently attached to AuNPs. Thus, the fabricated flexible TiVC-OH-Au sensor satisfied the rigorous mechanical requirements for wearable sensors. In addition, combining the electromagnetic (EM) enhancement from dense AuNPs formed by the activation of nucleation sites and charge transfer (CT) between target molecule and substrate induced by the abundant DOS near the Fermi level of TiVC, the fabricated sensor exhibits ultrasensitivity, long-term stability, good signal repeatability, and excellent mechanical durability. Moreover, the proof-of-concept application of the wearable SERS sensor in sweat sensing was demonstrated to monitor the content of nicotine, methotrexate, nikethamide, and 6-acetylmorphine in sweat at the molecular level, which was an important step toward the universality and practicality of the wearable sensing technology.

Molecularly Confined Topochemical Transformation of MXene Enables Ultrathin Amorphous Metal-Oxide Nanosheets

Two-dimensional (2D) amorphous nanosheets with ultrathin thicknesses have properties that differ from their crystalline counterparts. However, conventional methods for growing 2D materials often produce either crystalline flakes or amorphous nanosheets with an uncontrollable thickness. Here, we report that ultrathin amorphous metal-oxide nanosheets featuring superior flatness can be realized through the molecularly confined topochemical transformation of MXene. Using MXene Ti2CTx as an example, we show that surface modification of Ti2CTx nanosheets with molecular ligands, such as oleylamine (OAm) and oleic acid (OA), not only imparts notable colloidal dispersity to Ti2CTx nanosheets in nonpolar organic solvents but also confines their subsequent oxidation to in-plane configurations. We demonstrate that unlike the drastic oxidation conventionally observed for pristine MXene, hydrophobizing MXene with OAm and OA ligands enables individual Ti2CTx nanosheets to undergo independent oxidation in a nondestructive manner, resulting in amorphous titanium oxide (am-TiO2) nanosheets that faithfully retain the dimension and flatness of pristine MXene. These am-TiO2 nanosheets exhibit exceptional activity as substrates for surface-enhanced Raman scattering. Importantly, this molecular confinement strategy can be extended to other MXene materials, providing a versatile approach for synthesizing ultrathin amorphous metal-oxide nanosheets with tailored compositions and functionalities.

SERS-active immunoassay kit for SARS-CoV‑2 mediated by the cooperative chemical and electromagnetic effects of MXene modified with gold nanowires

Surface-enhanced Raman scattering (SERS) technique with high sensitivity, reliable specificity, and rapid recognition ability exhibits attractive promise for the effective fast-monitoring of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Herein, a novel SERS-active immunoassay kit for SARS-CoV-2 nucleocapsid (N) protein was prepared by in-situ growing gold (Au) nanowire forests (NFs) onto Ti3C2Tx, which was then modified onto polymethyl methacrylate (PMMA) matrix and encapsulated into kit. It was noted that the Au nanowires with fibrous structures which vertically anchored on Ti3C2Tx served as perfect channels to promote photo-induced charge transfer. The synergistic action of electromagnetic and chemical effects resulted in an enhancement factor (EF) of 1.27 × 107. Furthermore, the unreliable fluctuation of the enhanced signal was eliminated by using the intrinsic Raman signal of the flexible PMMA platform, achieving an improved correlation coefficient (R2) value from 0.950 to 0.990. Moreover, the as-designed immunoassay kit with both high sensitivity and remedied quantitative ability rendered by the Ti3C2Tx@Au NFs-PMMA composite exhibited a powerful performance in the practical detection of N-protein with concentration low to 5.0 × 10-8 mg/mL.

MXene-based electrochemical devices applied for healthcare applications

The initial part of the review provides an extensive overview about MXenes as novel and exciting 2D nanomaterials describing their basic physico-chemical features, methods of their synthesis, and possible interfacial modifications and techniques, which could be applied to the characterization of MXenes. Unique physico-chemical parameters of MXenes make them attractive for many practical applications, which are shortly discussed. Use of MXenes for healthcare applications is a hot scientific discipline which is discussed in detail. The article focuses on determination of low molecular weight analytes (metabolites), high molecular weight analytes (DNA/RNA and proteins), or even cells, exosomes, and viruses detected using electrochemical sensors and biosensors. Separate chapters are provided to show the potential of MXene-based devices for determination of cancer biomarkers and as wearable sensors and biosensors for monitoring of a wide range of human activities.

Surface-Enhanced Raman Spectroscopy for Boosting Electrochemical CO2 Reduction on Amorphous-Surfaced Tin Oxide Supported by MXene

Amorphous materials disrupt the intrinsic linear scalar dependence seen in their crystalline counterparts, typically exhibiting enhanced catalytic characteristics. Nevertheless, substantial obstacles remain in terms of boosting their stability, enhancing their conductivity, and elucidating distinct catalytic mechanisms. Herein, a core-shell catalyst, comprising a crystalline SnO2 core and an amorphous SnOx shell supported on MXene (denoted as SnO2@SnOx/MXene), was prepared utilizing hydrothermal and solution reduction methods. The SnO2@SnOx/MXene catalyst excels in the electrocatalytic conversion of CO2 to formate, yielding a Faradaic efficiency (FE) as high as 93% for formate production at -1.17 V vs RHE and demonstrating exceptional durability. Both density functional theory (DFT) calculations and experimental results indicate that the SnOx shell bolsters formate formation by fine-tuning the adsorption energy of the *OCHO intermediate. In SnO2@SnOx/MXene, MXene plays a vital role in enhancing the conductivity and stability of the amorphous shell and especially amplifying Raman signals of catalyst components. The ex/in situ surface-enhanced Raman scattering (SERS) application further confirms the formation of amorphous SnOx and further enables the direct detection of the formation of the intermediate species. This work provides the basis for the application of amorphous materials in practical electrocatalytic reduction of CO2 reduction.

Noble metal-free SERS: mechanisms and applications

Surface-enhanced Raman scattering (SERS) is a very important tool in vibrational spectroscopy. The coupling of nanomaterials induces local surface plasmon resonance (LSPR), which contributes greatly to SERS. Due to its remarkable sensitivity in trace detection, SERS has gained prominence in the fields of catalysis, biosensors, drug tracking, and optoelectronic devices. SERS activity is believed to be closely related to the LSPR and charge transfer (CT) of the material. Noble metal nanostructures have been commonly used as SERS-active substrates due to their strong local electric fields and relatively mature preparation, application, and enhancement mechanisms. In recent years, SERS research based on semiconductor materials has attracted significant attention because semiconductor materials have advantages such as repeatable preparation, simple pretreatment, stable SERS spectra and superior biocompatibility, stability, and reproducibility. Semiconductor-based SERS has the potential to enrich SERS theory and applications. Thus, the development of semiconductor materials will introduce a new epoch for SERS-based research. In this review, we outline the two main kinds of semiconductor SERS-active substrates: inorganic and organic semiconductor SERS-active substrates. We also provide an overview of the SERS mechanism for different kinds of materials and SERS-based applications.

Three-Dimensional MXene-AgNP Hollow Spheres for In Situ Surface-Enhanced Raman Scattering Detection of Catalysis Reactions

MXenes are attractive candidates in the fields of surface-enhanced Raman scattering (SERS) and catalysis. However, most of the current studies on MXenes are based on blocks and nanosheets, limiting their SERS and catalytic properties. Herein, we have prepared 3D MXene hollow spheres wrapped with silver nanoparticles (Ti3C2-AgNP HSs) using a sacrificial template method, which exhibits excellent sensitivity with a low detection limit due to good light-trapping capability of the hollow sphere and strong localized surface plasmon resonance (LSPR) effect of AgNPs. Furthermore, it shows outstanding photocatalytic performance and realizes in situ SERS monitoring of the 4-nitrobenzenethiol (4-NTP) to 4-aminothiophenol (4-ATP) catalysis reaction. The finite-difference time-domain (FDTD) simulations confirm that 3D Ti3C2-AgNP hollow structures have stronger hot spots than 3D solid structures and higher SERS sensitivity for molecule detection. Therefore, it promises to be an excellent bifunctional material for highly sensitive SERS detection and the in situ monitoring of catalytic reactions.

Superhydrophobic Surface Modification of Polymer Microneedles Enables Fabrication of Multimodal Surface-Enhanced Raman Spectroscopy and Mass Spectrometry Substrates for Synthetic Drug Detection in Blood Plasma

Microneedles are widely used substrates for various chemical and biological sensing applications utilizing surface-enhanced Raman spectroscopy (SERS), which is indeed a highly sensitive and specific analytical approach. This article reports the fabrication of a nanoparticle (NP)-decorated microneedle substrate that is both a SERS substrate and a substrate-supported electrospray ionization (ssESI) mass spectrometry (MS) sample ionization platform. Polymeric ligand-functionalized gold nanorods (Au NRs) are adsorbed onto superhydrophobic surface-modified polydimethylsiloxane (PDMS) microneedles through the control of various interfacial interactions. We show that the chain length of the polymer ligands dictates the NR adsorption process. Importantly, assembling Au NRs onto the micrometer-diameter needle tips allows the formation of highly concentrated electromagnetic hot spots, which provide the SERS enhancement factor as high as 1.0 × 106. The micrometer-sized area of the microneedle top and high electromagnetic field enhancement of our system can be loosely compared with tip-enhanced Raman spectroscopy, where the apex of a plasmonic NP-functionalized sharp probe produces high-intensity plasmonic hot spots. Utilizing our NR-decorated microneedle substrates, the synthetic drugs fentanyl and alprazolam are analyzed with a subpicomolar limit of detection. Further analysis of drug-molecule interactions on the NR surface utilizing the Langmuir adsorption model suggests that the higher polarizability of fentanyl allows for a stronger interaction with hydrophilic polymer layers on the NR surface. We further demonstrate the translational aspect of the microneedle substrate for both SERS- and ssESI-MS-based detection of these two potent drugs in 10 drug-of-abuse (DOA) patient plasma samples with minimal preanalysis sample preparation steps. Chemometric analysis for the SERS-based detection shows a very good classification between fentanyl, alprazolam, or a mixture thereof in our selected 10 samples. Most importantly, ssESI-MS analysis also successfully identifies fentanyl or alprazolam in these same 10 DOA plasma samples. We believe that our multimodal detection approach presented herein is a highly versatile detection technology that can be applicable to the detection of any analyte type without performing any complicated sample preparation.

Triple-enhanced Raman scattering sensors from flexible MXene/Au nanocubes platform via attenuating the coffee ring effect

Developing substrates that combine sensitivity and signal stability is a major challenge in surface enhanced Raman scattering (SERS) research. Herein, we present a flexible triple-enhanced Raman Scattering MXene/Au nanocubes (AuNCs) sensor fabricated by selective filtration of Ti3C2Tx MXene/AuNCs hybrid on the Ti3C2Tx MXene membrane and subsequent treatment with 1H,1H,2H,2H-perfluoro-octyltriethoxysilane (FOTS). The resultant superhydrophobic MXene/AuNCs-FOTS membrane not only provides the SERS substrate with environmental stability, but also imparts analyte enrichment to enhance the sensitivity (LOD = 1 × 10-14 M) and reliability (RSD = 6.41%) for Rhodamine 6G (R6G) molecules owing to the attenuation of the coffee ring effect. Moreover, the triple enhancement mechanism of combining plasmonic coupling enhancement from plasmonic coupling (EM) of nearby AuNCs at lateral and longitudinal direction of MXene/AuNCs-FOTS membrane, charge transfer (CT) from Ti3C2Tx MXene and target molecules and analyte enrichment function provides the substrate with excellent SERS performance (EF = 3.19 × 109), and allows efficient quantification of biomarkers in urine. This work could provide new insights into MXenes as building blocks for high-performance substrates and fill existing gaps in SERS techniques.

Optically Active MXenes in Van der Waals Heterostructures

The vertical integration of distinct 2D materials in van der Waals (vdW) heterostructures provides the opportunity for interface engineering and modulation of electronic as well as optical properties. However, scarce experimental studies reveal many challenges for vdW heterostructures, hampering the fine-tuning of their electronic and optical functionalities. Optically active MXenes, the most recent member of the 2D family, with excellent hydrophilicity, rich surface chemistry, and intriguing optical properties, are a novel 2D platform for optoelectronics applications. Coupling MXenes with various 2D materials into vdW heterostructures can open new avenues for the exploration of physical phenomena of novel quantum-confined nanostructures and devices. Therefore, the fundamental basis and recent findings in vertical vdW heterostructures composed of MXenes as a primary component and other 2D materials as secondary components are examined. Their robust designs and synthesis approaches that can push the boundaries of light-harvesting, transition, and utilization are discussed, since MXenes provide a unique playground for pursuing an extraordinary optical response or unusual light conversion features/functionalities. The recent findings are finally summarized, and a perspective for the future development of next-generation vdW multifunctional materials enriched by MXenes is provided.

PEC-SERS Dual-Mode Detection of Foodborne Pathogens Based on Binding-Induced DNA Walker and C3N4/MXene-Au NPs Accelerator

In this paper, a photoelectrochemical (PEC)-surface-enhanced Raman scattering (SERS) dual-mode biosensor is constructed coupled with a dual-recognition binding-induced DNA walker with a carbon nitride nanosheet (C3N4)/MXene-gold nanoparticles (C/M-Au NPs) accelerator, which is reliable and capable for sensitive and accurate detection of Staphylococcus aureus (S. aureus). Initially, a photoactive heterostructure is formed by combining C3N4 and MXene via a simple electrostatic self-assembly as they possess well-matched band-edge energy levels. Subsequently, in situ growth of gold nanoparticles on the formed surface results in better PEC performance and SERS activity, because of the synergistic effects of surface plasmon resonance and Schottky barrier. Furthermore, a three-dimensional, bipedal, and dual-recognition binding-induced DNA walker is introduced with the formation of Pb2+-dependent DNAzyme. In the presence of S. aureus, a significant quantity of intermediate DNA (I-DNA) is generated, which can open the hairpin structure of Methylene Blue-tagged hairpin DNA (H-MB) on the electrode surface, thereby enabling the switch of signals for the quantitative determination of S. aureus. The constructed PEC-SERS dual-mode biosensor that can be mutually verified under one reaction effectively addresses the problem of the low detection accuracy of traditional sensors. Experimental results revealed that the effective combination of PEC and SERS is achieved for amplification detection of S. aureus with a detection range of 5-108 CFU/mL (PEC) and 10-108 CFU/mL (SERS), and a detection of limit of 0.70 CFU/mL (PEC) and 1.35 CFU/mL (SERS), respectively. Therefore, this study offers a novel and effective dual-mode sensing strategy, which has important implications for bioanalysis and health monitoring.

Two-Dimensional MXenes Derived from Medium/High-Entropy MAX Phases M2 GaC (M = Ti/V/Nb/Ta/Mo) and their Electrochemical Performance

Two-dimensional (2D) transition metal carbides and/or nitrides, MXenes, are prepared by selective etching of the A-site atomically thin metal layers from their MAX phase precursors. High entropy MXenes, the most recent subfamily of MXenes, are in their infancy and have attracted great interest recently. They are currently synthesized mainly through wet chemical etching of Al-containing MAX phases, while various MAX phases with A-sites elements other than Al have not been explored. It is important to embody non-Al MAX phases as precursors for the high entropy MXenes synthesis to allow for new compositions. In this work, it is reported on the design and synthesis of Ga-containing medium/high entropy MAX phases and then their corresponding medium/high entropy MXenes. Gallium atomic layer etching is carried out using a Lewis acid molten salt (CuCl2). The as-prepared (Ti1/4 V1/4 Nb1/4 Ta1/4 )2 CTx exhibits a Li+ specific capacity of ≈400 mAh g-1 . For (Ti1/5 V1/5 Nb1/5 Ta1/5 Mo1/5 )2 CTx a specific capacity of 302 mAh g-1 is achieved after 300 cycles, and high cycling stability is observed at high current densities. This work is of great significance for expanding the family members of MXenes with tunable chemistries and structures.

MXene-Embedded Electrospun Polymeric Nanofibers for Biomedical Applications: Recent Advances

Recently MXenes has gained immense attention as a new and exciting class of two-dimensional material. Due to their unique layered microstructure, the presence of various functional groups at the surface, earth abundance, and attractive electrical, optical, and thermal properties, MXenes are considered promising candidates for various applications such as energy, environmental, and biomedical. The ease of dispersibility and metallic conductivity of MXene render them promising candidates for use as fillers in polymer nanocomposites. MXene-polymer nanocomposites simultaneously benefit from the attractive properties of MXenes and the flexibility and facile processability of polymers. However, the potentiality of MXene to modify the electrospun nanofibers has been less studied. Understanding the interactions between polymeric nanofibers and MXenes is important to widen their role in biomedical applications. This review explores diverse methods of MXene synthesis, discusses our current knowledge of the various biological characteristics of MXene, and the synthesis of MXene incorporated polymeric nanofibers and their utilization in biomedical applications. The information discussed in this review serves to guide the future development and application of MXene-polymer nanofibers in biomedical fields.

Spotting the driving forces for SERS of two-dimensional nanomaterials

Recently, two-dimensional (2D) layered nanomaterials have become promising candidates for surface-enhanced Raman scattering (SERS) substrates due to their unique characteristics of ultrathin layer structure, outstanding optical properties and good biocompatibility, significantly contributing to remarkable SERS sensitivity, stability, and compatibility. Unlike traditional SERS substrates, 2D nanomaterials possess unparalleled layer-dependent, phase transition induced and anisotropic optical properties, which as driving forces significantly promote the SERS performance and development, as well as greatly enrich the SERS substrates and provide versatile resources for SERS research. For a profound understanding of the SERS effect of 2D nanomaterials, a review concentrating on these driving forces for SERS enhancement on 2D nanomaterials is written here for the first time, which strongly emphasizes the importance and influence of these driving forces on the SERS effect of 2D nanomaterials, including their intrinsic physical and chemical properties and external influencing factors. Moreover, the essential mechanisms of these driving forces for the SERS effect are also elaborated systematically. Finally, the challenges and future perspectives of SERS substrates based on 2D nanomaterials are concluded. This review will provide guiding principles and strategies for designing highly sensitive 2D nanomaterial SERS substrates and extending their potential applications based on SERS.

Structural Engineering Enabled Bimetallic (Ti1- y Nby )2 AlC Solid Solution Structure for Efficient Electromagnetic Wave Absorption in Gigahertz

Microstructures play a critical role to influence the polarization behavior of dielectric materials, which determines the electromagnetic response ability in gigahertz. However, the relationship between them, especially in the solid-solution structures is still absent. Herein, a series of (Ti_{1- y} Nb_y )₂ AlC MAX phase solid solutions with nano-laminated structures have been employed to illuminate the aforementioned problem. The relationship has been investigated by the lattice distortion constructed via tuning the composition from Ti to Nb in the M-site atomic layer. Experimental characterizations indicated that the dielectric response behaviors between declined conduction loss and boosted polarization loss can be well balanced by niobium atom manipulative solid-solution engineering, which is conducive to impedance matching and electromagnetic absorption performance. Theoretical calculation further proved that the origin of electric dipoles is ascribed to the charge density differences resulting from the altered microscopic atomic distribution. As a result, the Ti_1.2 Nb_0.8 AlC exhibits the mostly optimized microwave absorption property, in which a minimum reflection loss of -42 dB and an effective absorption bandwidth of 4.3 GHz under an ultra-thin thickness of 1.4 mm can be obtained. This work provides insight into the structural engineering in modifying electromagnetic response performance at gigahertz and which can be expanded to other solid-solution materials.

Plasmonic Coupling of Au Nanoclusters on a Flexible MXene/Graphene Oxide Fiber for Ultrasensitive SERS Sensing

High sensitivity, good signal repeatability, and facile fabrication of flexible surface enhanced Raman scattering (SERS) substrates are common pursuits of researchers for the detection of probe molecules in a complex environment. However, fragile adhesion between the noble-metal nanoparticles and substrate material, low selectivity, and complex fabrication process on a large scale limit SERS technology for wide-ranging applications. Herein, we propose a scalable and cost-effective strategy to a fabricate sensitive and mechanically stable flexible Ti₃C₂T_x MXene@graphene oxide/Au nanoclusters (MG/AuNCs) fiber SERS substrate from wet spinning and subsequent in situ reduction processes. The use of MG fiber provides good flexibility (114 MPa) and charge transfer enhancement (chemical mechanism, CM) for a SERS sensor and allows further in situ growth of AuNCs on its surface to build highly sensitive hot spots (electromagnetic mechanism, EM), promoting the durability and SERS performance of the substrate in complex environments. Therefore, the formed flexible MG/AuNCs-1 fiber exhibits a low detection limit of 1 × 10^-11 M with a 2.01 × 10⁹ enhancement factor (EF_exp), signal repeatability (RSD = 9.80%), and time retention (remains 75% after 90 days of storage) for R6G molecules. Furthermore, the l-cysteine-modified MG/AuNCs-1 fiber realized the trace and selective detection of trinitrotoluene (TNT) molecules (0.1 μM) via Meisenheimer complex formation, even by sampling the TNT molecules at a fingerprint or sample bag. These findings fill the gap in the large-scale fabrication of high-performance 2D materials/precious-metal particle composite SERS substrates, with the expectation of pushing flexible SERS sensors toward wider applications.

Application of SERS in In-Vitro Biomedical Detection

Surface-enhanced Raman scattering (SERS), as a rapid and nondestructive biological detection method, holds great promise for clinical on spot and early diagnosis. In order to address the challenging demands of on spot detection of biomedical samples, a variety of strategies has been developed. These strategies include substrate structural and component engineering, data processing techniques, as well as combination with other analytical methods. This report summarizes the recent SERS developments for biomedical detection, and their promising applications in cancer detection, virus or bacterial infection detection, miscarriage spotting, neurological disease screening et al. The first part discusses the frequently used SERS substrate component and structures, the second part reports on the detection strategies for nucleic acids, proteins, bacteria, and virus, the third part summarizes their promising applications in clinical detection in a variety of illnesses, and the forth part reports on recent development of SERS in combination with other analytical techniques. The special merits, challenges, and perspectives are discussed in both introduction and conclusion sections.

Monolayer Iron Oxychloride with a Resonant Band Structure for Ultrasensitive Molecular Sensing

Two-dimensional layered materials (2DLMs) are expected to be next-generation commercial sensors for surface-enhanced Raman scattering (SERS) sensing owing to their unique structural features and physicochemical properties. The low sensitivity and poor universality of 2DLMs are the dominant barriers toward their practical applications. Herein, we report that monolayer iron oxychloride (FeOCl) with a naturally suitable band structure is a promising candidate for ultrasensitive SERS sensing. The generally boosted Raman scattering cross section of different analyte-FeOCl systems benefits from the resonant photoinduced charge transfer processes and strong ground-state interactions. In addition, the strong adsorption ability of monolayer FeOCl is crucial for rapid detection in practical applications, which is proven to be much better than those of conventional SERS sensors. Consequently, monolayer FeOCl enables diverse SERS applications, including multicomponent analysis, chemical reaction monitoring, and indirect ion sensing.

Engineered Two-Dimensional Nanostructures as SERS Substrates for Biomolecule Sensing: A Review

Two-dimensional nanostructures (2DNS) attract tremendous interest and have emerged as potential materials for a variety of applications, including biomolecule sensing, due to their high surface-to-volume ratio, tuneable optical and electronic properties. Advancements in the engineering of 2DNS and associated technologies have opened up new opportunities. Surface-enhanced Raman scattering (SERS) is a rapid, highly sensitive, non-destructive analytical technique with exceptional signal amplification potential. Several structurally and chemically engineered 2DNS with added advantages (e.g., π-π* interaction), over plasmonic SERS substrates, have been developed specifically towards biomolecule sensing in a complex matrix, such as biological fluids. This review focuses on the recent developments of 2DNS-SERS substrates for biomolecule sensor applications. The recent advancements in engineered 2DNS, particularly for SERS substrates, have been systematically surveyed. In SERS substrates, 2DNS are used as either a standalone signal enhancer or as support for the dispersion of plasmonic nanostructures. The current challenges and future opportunities in this synergetic combination have also been discussed. Given the prospects in the design and preparation of newer 2DNS, this review can give a critical view on the current status, challenges and opportunities to extrapolate their applications in biomolecule detection.

Facet-Dependent SERS Activity of Co3O4

Surface-enhanced Raman spectroscopy (SERS) is an ultra-sensitive and rapid technique that is able to significantly enhance the Raman signals of analytes absorbed on functional substrates by orders of magnitude. Recently, semiconductor-based SERS substrates have shown rapid progress due to their great cost-effectiveness, stability and biocompatibility. In this work, three types of faceted Co₃O₄ microcrystals with dominantly exposed {100} facets, {111} facets and co-exposed {100}-{111} facets (denoted as C-100, C-111 and C-both, respectively) are utilized as SERS substrates to detect the rhodamine 6G (R6G) molecule and nucleic acids (adenine and cytosine). C-100 exhibited the highest SERS sensitivity among these samples, and the lowest detection limits (LODs) to R6G and adenine can reach 10^-7 M. First-principles density functional theory (DFT) simulations further unveiled a stronger photoinduced charge transfer (PICT) in C-100 than in C-111. This work provides new insights into the facet-dependent SERS for semiconductor materials.

Band Structure Engineering within Two-Dimensional Borocarbonitride Nanosheets for Surface-Enhanced Raman Scattering

Herein, with two-dimensional (2D) borocarbonitride (BCN) as a metal- and plasmon-free surface-enhanced Raman scattering (SERS) platform, we demonstrate a band structure engineering strategy to facilitate the charge transfer process for an enhanced SERS response. Especially, when the conduction band of the BCN substrate is tuned to align with the LUMO of the target molecule, remarkable SERS performance is achieved, ascribed to the borrowing effect from the vibronic coupling of resonances through the Herzberg-Teller coupling term. Meanwhile, fluorescence quenching is achieved due to the efficient charge transfer between the BCN substrate and target molecule. Consequently, BCN can accurately detect 20 kinds of trace chemical and bioactive analytes. Moreover, BCN exhibits excellent thermal and chemical stability, which can not only withstand high-temperature (300 °C) heating in the air but also resist long-term corrosion in harsh acid (pH = 0, HCl) and base (pH = 14, NaOH). This work provides new insight into band structure engineering in promoting the SERS performance of plasmon- and metal-free semiconductor substrates.

Mxenes–Au NP Hybrid Plasmonic 2D Microplates in Microfluidics for SERS Detection

Combined with microfluidics, surface-enhanced Raman spectroscopy (SERS) exhibits huge application prospective in sensitive online detection. In current studies, the design and optimization of plasmonic enhanced structures in microfluidics for SERS detection could be an interesting challenge. In this work, hybrid plasmonic 2D microplates composed of Mxenes (Ti3C2Tx) microplates and in-situ synthesized Au nanoparticles (Au NPs) are fabricated in a microchannel for enhanced structures in SERS microfluidics. Benefiting from the 2D Mxenes microplates with complex distributions, the enhanced areas generated by Au NPs are quite enlarged in a microchannel, which exhibits high sensitivity in SERS detection at 10−10 M for Nile blue (NB) molecules in microfluidics. The mechanism of electromagnetic enhancement (EM) and chemical enhancement (CM) is analyzed. The experimental data indicate the ultrasonic times of Mxenes and the concentration of Au3+ play important roles in the sensitivity of SERS detection, which is confirmed by the simulated electric field distributions. Furthermore, a typical pesticide (thiram) at 100 ppm in water is detected on these SERS microfluidics with hybrid plasmonic enhanced structures, which demonstrates that our work not only strengthens the knowledge of plasmonics but also enlarges the application of SERS.

2D TiVCTx layered nanosheets grown on nickel foam as highly efficient electrocatalysts for the hydrogen evolution reaction

Exploring highly efficient and durable catalysts for the hydrogen evolution reaction (HER) is crucial for the hydrogen economy and environmental protection issues. Numerous studies have now found that transition metal carbide MXenes are ideal candidates as catalysts for the hydrogen evolution reaction. However, MXenes are inclined to easily undergo lamellar structure agglomeration and stacking, which impedes their further applications. Besides, most of the extant research has focused on single transition metal carbides, and the investigation of double transition metal carbide MXenes is rather rare. In this research work, a three-dimensional (3D) TiVCT_x-based conductive electrode was constructed by depositing 2D TiVCT_x nanosheets on 3D network structured nickel foam (NF) to synthesize a hybrid electrode material (abbreviated as TiVCT_x@NF). TiVCT_x@NF exhibits efficient electrochemical properties with a low overpotential of 151 mV at 10 mA cm⁻² and a small Tafel slope of 116 mV dec⁻¹. Benefitting from the open layer structure and strong interfacial coupling effect, compared to the pristine structure, the resulting TiVCT_x@NF has greatly increased active sites for the hydrogen evolution reaction (HER) and encounters less resistance for charge transfer. In addition, TiVCT_x@NF exhibits better stability in long-term acidic electrolytes. This work provides a tactic to prepare three-dimensional network electrode materials and broadens the application of single transition metal carbide MXenes as water splitting electrodes in the HER, which is beneficial to the application of noble metal-free electrocatalysts.

The TiVCT_x MXene was obtained by etching and peeling methods, and the TiVCT_x@NF hybrid electrode material was obtained by the deposition method. The electrochemical performance was evaluated using a variety of characterization methods.