Surface-Modified Two-Dimensional Titanium Carbide Sheets for Intrinsic Vibrational Signal-Retained Surface-Enhanced Raman Scattering with Ultrahigh Uniformity

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.

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.

Surfactant-free interfacial growth of graphdiyne hollow microspheres and the mechanistic origin of their SERS activity

As a two-dimensional carbon allotrope, graphdiyne possesses a direct band gap, excellent charge carrier mobility, and uniformly distributed pores. Here, a surfactant-free growth method is developed to efficiently synthesize graphdiyne hollow microspheres at liquid‒liquid interfaces with a self-supporting structure, which avoids the influence of surfactants on product properties. We demonstrate that pristine graphdiyne hollow microspheres, without any additional functionalization, show a strong surface-enhanced Raman scattering effect with an enhancement factor of 3.7 × 107 and a detection limit of 1 × 10-12 M for rhodamine 6 G, which is approximately 1000 times that of graphene. Experimental measurements and first-principles density functional theory simulations confirm the hypothesis that the surface-enhanced Raman scattering activity can be attributed to an efficiency interfacial charge transfer within the graphdiyne-molecule system.

Electrostatic Interfacial Cross-Linking and Structurally Oriented Fiber Constructed by Surface-Modified 2D MXene for High-Performance Flexible Pseudocapacitive Storage

Fiber supercapacitors are promising power supplies suitable for wearable electronics, but the internally insufficient cross-linking and random structure of fiber electrodes restrict their performance. This study describes how interfacial cross-linking and oriented structure can fabricate an MXene fiber with high flexibility and electrochemical performance. The continuous and highly oriented macroscopic fibers were constructed by 2D MXene sheets via a liquid-crystalline wet-spinning assembly. The oxyanion-enriched terminations of surface-modified MXene in situ could reinforce the interfacial cross-linking by electrostatic interactions while mediating the sheet-to-sheet lamellar structure within the fiber. The resultant MXene fiber exhibits high electrical conductivity (3545 S cm^-1) and mechanical strength (205.5 MPa) and high pseudocapacitance charge storage capability up to 1570.5 F cm^-3. Notably, the assembled fiber supercapacitor delivers an energy density of 77.6 mWh cm^-3 at 401.9 mW cm^-3, exceptional flexibility and stability exhibiting ∼99.5% capacitance retention under mechanical deformation, and can be integrated into commercial textiles to power microelectronic devices. This work provides insight into the fabrication of an advanced MXene fiber and the development of high-performance flexible fiber supercapacitors.

Deposition of hydrophilic Ti3C2Tx on a superhydrophobic ZnO nanorod array for improved surface-enhanced raman scattering performance

BACKGROUND

Superhydrophobic substrate modifications are an effective way to improve SERS sensitivity by concentrating analyte molecules into a small surface area. However, it is difficult to manipulate low-volume liquid droplets on superhydrophobic substrates.

RESULTS

To overcome this limitation, we deposited a hydrophilic Ti₃C₂T_x film on a superhydrophobic ZnO nanorod array to create a SERS substrate with improved analyte affinity. Combined with its interfacial charge transfer properties, this enabled a rhodamine 6G detection limit of 10^-11 M to be achieved. In addition, the new SERS substrate showed potential for detection of biological macromolecules, such as microRNA.

CONCLUSION

Combined with its facile preparation, the SERS activity of ZnO/Ti₃C₂T_x suggests it may provide an ultrasensitive environmental pollutant-monitoring and effective substrate for biological analyte detection.

Flexible Two-Dimensional Vanadium Carbide MXene-Based Membranes with Ultra-Rapid Molecular Enrichment for Surface-Enhanced Raman Scattering

Two-dimensional (2D) MXene materials have attracted broad interest in surface-enhanced Raman scattering (SERS) applications by virtue of their abundant surface terminations and excellent photoelectric properties. Herein, we propose to design highly sensitive MXene-based SERS membranes by integrating a 2D downsizing strategy with molecular enrichment approaches. Two types of 2D vanadium carbide (V₄C₃ and V₂C) MXenes are demonstrated for ultrasensitive SERS sensing, and corresponding SERS mechanisms including the effect of 2D vanadium carbide thickness on their electron density states and interfacial photoinduced charge transfer resonance were discussed. A 2D downsizing strategy authorizes nonplasmonic SERS detection with a sensitivity of 1 × 10^-7 M. Moreover, the performance can be further upgraded by vacuum-assisted filtration, which enables an ultrarapid molecular enrichment (within 2 min), ultrahigh molecular removal rate (over 95%), and improved sensitivity (5 × 10^-9 M). This work may shed light on the MXene-based materials as an innovative platform for nonplasmonic SERS detection.

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.

Raman spectroscopy studies of black phosphorus

Black phosphorus has important applications in many fields such as optics, optoelectronics and thermals. Many of its excellent properties are related to its special anisotropy. In this work, we adopted Raman spectroscopy, which can obtain fast response optical signals without destroying the structure of the sample, to identify its crystal orientation and explore its thermal and SERS properties. We successfully distinguished the armchair and zigzag directions of black phosphorus by angle-resolved polarized Raman spectroscopy of A_g mode in a less studied orthogonal polarization configuration. Then we used temperature dependent Raman spectroscopy to study its thermal properties. It is found that the first order temperature coefficients of its three Raman vibration modes A_g¹, B_2g, and A_g² are -0.0133 cm^-1 K^-1, -0.0232 cm^-1 K^-1 and -0.0229 cm^-1 K^-1, respectively for the 3.2 nm sample. Furthermore, we studied the surface enhancement effect of black phosphorus with different thicknesses as SERS substrates. We found that few-layer black phosphorus has better enhancement effects and its limit of detection for MB and CV are both 10^-6M. The analytical enhancement factor of black phosphorus substrates on CV can achieve 1.2 × 10³ by calculation. These methods can be extended to other similar two-dimensional materials.

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

Two-dimensional (2D) MXenes are attractive candidates as surface-enhanced Raman scattering (SERS) substrates because of their metallic conductivity and abundant surface terminations. Herein, we report the facile synthesis of bimetallic solid-solution TiVC (MXene) and its application in SERS. The few-layered MXene nanosheets with high crystallinity were successfully prepared using a one-step chemical etching method without ultrasonic and organic solvent intercalation steps. SERS activity of the as-prepared MXene was investigated by fabricating free-standing TiVC film as the substrate. A SERS enhancement factor of 10¹² and femtomolar-level detection limit were confirmed using rhodamine 6G as a model dye with 532 nm excitation. The fluorescent signal of the rhodamine 6G dye was effectively quenched, making the SERS spectrum clearly distinguishable. Furthermore, we demonstrate that the TiVC-analyte system with ultrahigh sensitivity is dominated by the chemical mechanism (CM) based on the experimental and simulation results. The abundant density of states near the Fermi level of the TiVC and the strong interaction between the TiVC and analyte promote the intermolecular charge transfer resonance in the TiVC-analyte complex, resulting in significant Raman enhancement. Additionally, several other probe molecules were used for SERS detection to further verify CM-based selectivity enhancement on the TiVC substrates. This work provides guidance for the facile synthesis of 2D MXene and its application in ultrasensitive SERS detection.

A photochemical approach to anchor Au NPs on MXene as a prominent SERS substrate for ultrasensitive detection of chlorpromazine

As a novel two-dimensional (2D) material, metal carbide (MXene) has been identified as a hotspot research topic in the field of surface-enhanced Raman spectroscopy (SERS). Herein, we report the increment of SERS activity of titanium carbide (TiC) by incorporation of gold nanoparticles (Au NPs) by a facile photoreduction process for the detection of antipsychotic drug. TiC anchored with Au NPs produce a remarkable SERS enhancement by the synergistic action of chemical and electromagnetic mechanisms. The hotspots are formed in the nanometer-scale gaps between Au NPs on the TiC surface for the effective interaction with probe molecules. The proposed TiC/Au-NPs SERS substrate was employed for the detection of chlorpromazine (CPZ) with the wide linear range of 10^-1-10^-10 M and the ultra-low limit of detection of 3.92 × 10^-11 M. Besides, the SERS effect of the optimized TiC/Au-NPs for the 532 nm excitation exhibits the enhancement factor in the order of 10⁹ with the relative standard deviation of < 13% for the uniformity and < 8.80% for the reproducibility. To ensure the practical feasibility of the proposed TiC/Au-NPs SERS substrate, the spike and recovery method was used for the detection of CPZ in human biological fluids like urine and saliva. This work can open up a new approach to improve the SERS activity of MXene-based SERS substrate for practical applications, especially the determination of antipsychotic drugs in environmental pollution management.

The charge transfer effect on SERS in a gold-decorated surface defect anatase nanosheet/methylene blue (MB) system

The increase in the transition route of the CT process enhances the Raman scattering of MB.

Surface Enhanced Raman Scattering Revealed by Interfacial Charge-Transfer Transitions

Surface enhanced Raman scattering (SERS) is a fingerprint spectral technique whose performance is highly dependent on the physicochemical properties of the substrate materials. In addition to the traditional plasmonic metal substrates that feature prominent electromagnetic enhancements, boosted SERS activities have been reported recently for various categories of non-metal materials, including graphene, MXenes, transition-metal chalcogens/oxides, and conjugated organic molecules. Although the structural compositions of these semiconducting substrates vary, chemical enhancements induced by interfacial charge transfer are often the major contributors to the overall SERS behavior, which is distinct from that of the traditional SERS based on plasmonic metals. Regarding charge-transfer-induced SERS enhancements, this short review introduces the basic concepts underlying the SERS enhancements, the most recent semiconducting substrates that use novel manipulation strategies, and the extended applications of these versatile substrates.