Charge-Transfer Resonance and Electromagnetic Enhancement Synergistically Enabling MXenes with Excellent SERS Sensitivity for SARS-CoV-2 S Protein Detection

Photoinduced Charge Transfer Empowers Ta4C3 and Nb4C3 MXenes with High SERS Performance

This study introduces two-dimensional (2D) Ta4C3 and Nb4C3 MXenes as outstanding materials for surface-enhanced Raman scattering (SERS) sensing, marking a significant departure from traditional noble-metal substrates. These MXenes exhibit exceptional SERS capabilities, achieving enhancement factors around 105 and detection limits as low as 10-7 M for various analytes, including environmental pollutants and drugs. The core of their SERS functionality is attributed to the robust interfacial photoinduced charge-transfer interactions between the MXenes and the adsorbed molecules. This deep insight not only advances our understanding of MXene materials in SERS applications but also opens new avenues for developing highly sensitive and selective SERS sensors. The potential of Ta4C3 and Nb4C3 MXenes to revolutionize SERS technology underscores their importance in environmental monitoring, food safety, and beyond.

CRISPR/Cas12a coupled with loop-mediated isothermal amplification and lateral flow assay for SARS-CoV-2 detection

Point-of-care testing (POCT) is rapid, exhibits highly sensitive performance, can facilitate home self-testing and avoids cross-contamination. Herein, we developed a biosensor that combines Si-OH magnetic bead (MB)-based fast RNA extraction, reverse transcription-loop-mediated isothermal amplification (RT-LAMP), CRISPR-Cas12a, and lateral flow assay (LFA) for rapid detection of SARS-CoV-2 RNA within 1.5 h. In the presence of the SARS-CoV-2 LAMP amplicon, the trans-cleavage activity of Cas12a was activated to cleave the probe, separating streptavidin from the AuNPs-digoxin (Dig) antibody, resulting in the inability of the test line to capture the AuNPs-Dig antibody. The method can distinguish SARS-CoV-2 from other RNA viruses, with a limit-of-detection (LOD) of 6.2 × 102 copies per mL. Therefore, LAMP-CRISPR-LFA has high specificity and sensitivity and is convenient to develop into commercial assay kits, which could have a broad prospect for practical application.

On-mask detection of SARS-CoV-2 related substances by surface enhanced Raman scattering

We have developed a convenient surface-enhanced Raman scattering (SERS) platform based on vertical standing gold nanowires (v-AuNWs) which enabled the on-mask detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) related substances such as the Spike-1 protein and the corresponding pseudo-virus. The Spike-1 protein was clearly distinguished from BSA protein with an accuracy above 99 %, and the detection limit could be achieved down to 0.01 μg/mL. Notably, a similar accuracy was achieved for the pseudo-SARS-CoV-2 (pSARS-2) virus as compared to the pseudo-influenza H7N9 (pH7N9) virus. The sensing strategy and setups could be easily adapted to the real SARS-CoV-2 virus and other highly contagious viruses. It provided a promising way to screen the virus carriers by a fast evaluation of their wearing v-AuNWs integrated face-mask which was mandatory during the pandemic.

Mo-doped Co LDHs as Raman enhanced substrate for detection of roxarsine

BACKGROUND

Roxarsone (ROX) is widely used as a feed additive, which is indigestible after ingestion by poultry, and most of it can only be excreted into the natural environment and degraded into highly toxic and carcinogenic inorganic arsenic compounds, which pose a hazard to the ecosystem and human health. However, for roxarsone, traditional detection methods require complex and time-consuming procedures, so it is urgent to find a new fast detection method for detection of ROX.

RESULTS

In this work, we developed a novel Raman enhancement material and utilized the Surface-enhanced Raman scattering (SERS) technique to achieve rapid and sensitive detection of roxarsone. Specifically, Mo-doped cobalt layered double hydroxides (Co-LDHs) semiconductor material (abbreviated as CMM-100) was prepared by a simple method of using ion-assisted MOF etching. Under laser excitation at a wavelength of 532 nm, the CMM-100 showed excellent SERS property to various organic dye molecules such as methylene blue (MB), Toluidine Blue(TB), and Crystal Violet (CV). Especially, an enhancement factor (EF) of 1.4 × 106 was achieved for MB. Compared with the traditional method, this work utilized the fast and non-destructive SERS technology, which achieved a rapid detection of ROX. The detection limit was as low as 9.73 × 10-10 M, and the detection range was from 10-9 M to 10-3 M.

SIGNIFICANCE

In this work, SERS technology was adopted for the rapid and sensitive detection of ROX. This study provides a Raman-enhanced substrate named CMMs for detection of food additives, pesticides, biomolecules, etc., which also broadens the application areas of SERS materials.

Cellulose Surface Nanoengineering for Visualizing Food Safety

Food safety is vital to human health, necessitating the development of nondestructive, convenient, and highly sensitive methods for detecting harmful substances. This study integrates cellulose dissolution, aligned regeneration, in situ nanoparticle synthesis, and structural reconstitution to create flexible, transparent, customizable, and nanowrinkled cellulose/Ag nanoparticle membranes (NWCM-Ag). These three-dimensional nanowrinkled structures considerably improve the spatial-electromagnetic-coupling effect of metal nanoparticles on the membrane surface, providing a 2.3 × 108 enhancement factor for the surface-enhanced Raman scattering (SERS) effect for trace detection of pesticides in foods. Notably, the distribution of pesticides in the apple peel and pulp layers is visualized through Raman imaging, confirming that the pesticides penetrate the peel layer into the pulp layer (∼30 μm depth). Thus, the risk of pesticide ingestion from fruits cannot be avoided by simple washing other than peeling. This study provides a new idea for designing nanowrinkled structures and broadening cellulose utilization in food safety.

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.

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.

Quantitative detecting low concentration polystyrene nanoplastics in aquatic environments via an Ag/Nb2CTx (MXene) SERS substrate

In this study, the plasmonic Ag nanoparticles (Ag NPs) were uniformly anchored on the high conductivity Nb2CTx (MXene) nanosheets to construct an Ag/Nb2CTx substrate for surface-enhanced Raman spectroscopy (SERS) detection of polystyrene (PS) nanoplastics. The KI addition (0.15 mol/L), the volume ratio between substrate colloid and nanoplastic suspension (2:1), and the mass ratio of Nb2CTx in substrate (14%) on SERS performance were optimized. The EM hot spots of Ag/Nb2CTx are significantly enlarged and enhanced, elucidated by FDFD simulation. Then, the linear relationship between the PS nanoplastics concentration with three different sizes (50, 300, and 500 nm) and the SERS intensity was obtained (R2 > 0.976), wherein, the detection limit was as low as 10-4 mg/mL for PS nanoplastic. Owing to the fingerprint feature, the Ag/Nb2CTx-14% substrate successfully discerns the mixtures from two-component nanoplastics. Meanwhile, it exhibits excellent stability of PS nanoplastics on different detection sites. The recovery rates of PS nanoplastics with different sizes in lake water ranged from 94.74% to 107.29%, with the relative standard deviation (RSD) ranging from 2.88% to 8.30%. Based on this method, the expanded polystyrene (EPS) decomposition behavior was evaluated, and the PS concentrations in four water environments were analyzed. This work will pave the way for the accurate quantitative detection of low concentration of nanoplastics in aquatic environments.

Open-Nanogap-Induced Strong Electromagnetic Enhancement in Au/AgAu Monolayer as a Stable and Uniform SERS Substrate for Ultrasensitive Detection

Nanogap-based plasmonic metal nanocrystals have been applied in surface-enhanced Raman scattering detection, while the closed and insufficient electromagnetic fields as well as the nonreproducible Raman signal of the substrate greatly restrict the actual application. Herein, a highly uniform Au/AgAu monolayer with abundant nanogaps and huge electromagnetic enhancement is prepared, which shows ultrasensitive and reproducible SERS detection. Au/AgAu with an inner nanogap is first prepared based on Au nanotriangles, and the nanogap is opened from the three tips via a subsequent etching process. The open-gap Au/AgAu displays much higher SERS efficiency than Au and Au/AgAu with an inner nanogap on detecting crystal violet due to the open-gap induced electromagnetic enhancement and improved molecular absorption. Furthermore, the open-gap Au/AgAu monolayer is prepared via interfacial self-assembly, which shows further improved SERS due to the dense and strong hotspots in the nanocavities induced by the electromagnetic coupling between adjacent open gaps. The monolayer possesses excellent signal stability, uniformity, and reproducibility. The analytic enhancement factor and relative standard deviation reach to 2.12 × 108 and 4.65% on detecting crystal violet, respectively. Moreover, the monolayer achieves efficient detection of thiram in apple juice, biphenyl-4-thiol, 4-mercaptobenzoic, melamine, and a mixed solution of four different molecules, showing great promise in practical detection.

Charge-transfer-driven ultrasensitive SERS sensing in a two-dimensional titanium carbonitride MXene

Two-dimensional (2D) MXenes stand out as promising platforms for surface-enhanced Raman scattering (SERS) sensing owing to their metallic feature, various compositions, high surface area, compatibility with functionalization, and ease of fabrication. In this work, we report a high-performance 2D titanium carbonitride (Ti3CN) MXene SERS substrate. We reveal that the abundant electronic density of states near the Fermi level of Ti3CN MXene boosts the efficiency of photo-induced charge transfer at the interface of Ti3CN/molecule, resulting in significant Raman enhancement. The SERS sensitivity of Ti3CN MXene is further promoted through a 2D morphology regulation and molecular enrichment strategies. Moreover, prohibited drugs are detectable on this substrate, presenting the potential of trace-amount analysis on Ti3CN MXene. This work provides a deep insight of the SERS mechanisms of Ti3CN MXene and broadens the practical application of transition metal carbonitride MXene SERS substrates.

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.

Detection of SARS-CoV-2 N protein using AgNPs-modified aligned silicon nanowires BioSERS chip

The SARS-CoV-2 (COVID-19) pandemic had a strong impact on societies and economies worldwide and tests for high-performance detection of SARS-CoV-2 biomarkers are still needed for potential future outbreaks of the disease. In this paper, we present the different steps for the design of an aptamer-based surface-enhanced Raman scattering (BioSERS) sensing chip capable of detecting the coronavirus nucleocapsid protein (N protein) in spiked phosphate-buffered solutions and real samples of human blood serum. Optimization of the preparation steps in terms of the aptamer concentration used for the functionalization of the silver nanoparticles, time for affixing the aptamer, incubation time with target protein, and insulation of the silver active surface with cysteamine, led to a sensitive BioSERS chip, which was able to detect the N protein in the range from 1 to 75 ng mL-1 in spiked phosphate-buffered solutions with a detection limit of 1 ng mL-1 within 30 min. Furthermore, the BioSERS chip was used to detect the target protein in scarcely spiked human serum. This study demonstrates the possibility of a clinical application that can improve the detection limit and accuracy of the currently commercialized SARS-CoV-2 immunodiagnostic kit. Additionally, the system is modular and can be applied to detect other proteins by only changing the aptamer.

Reversible Thermoelectric Regulation of Electromagnetic and Chemical Enhancement for Rapid SERS Detection

Actively controlling surface-enhanced Raman scattering (SERS) performance plays a vital role in highly sensitive detection or in situ monitoring. Nevertheless, it is still challenging to achieve further modulation of electromagnetic enhancement and chemical enhancement simultaneously in SERS detection. In this study, a silver nanocavity structure with graphene as a spacer layer is coupled with thermoelectric semiconductor P-type gallium nitride (GaN) to form an electric-field-induced SERS (E-SERS) for dual enhancement. After applying the electric field, the intensity of SERS signals is further enhanced by over 10 times. The thermoelectric field enables fast and reproducible doping of graphene, thereby modulating its Fermi level over a wide range. The thermoelectric field also regulates the position of the plasmon resonance peak of the silver nanocavity structure, rendering synchronous dual electromagnetic and chemical regulation. Additionally, the method enables the trace detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A detailed theoretical analysis is performed based on the experimental results and finite-element calculations, paving the way for the fabrication of high-efficient E-SERS substrates.

COVID-19 Virus Structural Details: Optical and Electrochemical Detection

The increasing viral species have ruined people's health and the world's economy. Therefore, it is urgent to design bio-responsive materials to provide a vast platform for detecting a different family's passive or active virus. One can design a reactive functional unit for that moiety based on the particular bio-active moieties in viruses. Nanomaterials as optical and electrochemical biosensors have enabled better tools and devices to develop rapid virus detection. Various material science platforms are available for real-time monitoring and detecting COVID-19 and other viral loads. In this review, we discuss the recent advances of nanomaterials in developing the tools for optical and electrochemical sensing COVID-19. In addition, nanomaterials used to detect other human viruses have been studied, providing insights for developing COVID-19 sensing materials. The basic strategies for nanomaterials develop as virus sensors, fabrications, and detection performances are studied. Moreover, the new methods to enhance the virus sensing properties are discussed to provide a gateway for virus detection in variant forms. The study will provide systematic information and working of virus sensors. In addition, the deep discussion of structural properties and signal changes will offer a new gate for researchers to develop new virus sensors for clinical applications.

Two-Dimensional MXene as a Promising Adsorbent for Trihalomethanes Removal: A Density-Functional Theory Study

This groundbreaking research delves into the intricate molecular interactions between MXene and trihalomethanes (THs) through a comprehensive theoretical study employing density-functional theory (DFT). Trihalomethanes are common carcinogenic chlorination byproducts found in water sanitation systems. This study focuses on a pristine MXene [Mn+1·Xn] monolayer and its various terminal [Tx] functional groups [Mn+1·XnTx], strategically placed on the surface for enhanced performance. Our investigation involves a detailed analysis of the adsorption energies of THs on different MXene types, with the MXene-Cl layer emerging as the most compatible variant. This specific MXene-Cl layer exhibits remarkable properties, including a total dipole moment (TDM) of 12.443 Debye and a bandgap of 0.570 eV, achieved through meticulous geometry optimization and computational techniques. Notably, THs such as trichloromethane (CHCl3), bromide-chloromethane (CHBrCl2), and dibromochloromethane (CHBr2Cl) demonstrate the highest TDM values, indicating substantial changes in electronic and optical parameters, with TDM values of 16.363, 15.998, and 16.017 Debye, respectively. These findings highlight the potential of the MXene-Cl layer as an effective adsorbent and detector for CHF3, CHClF2, CHCl3, CHBrCl2, and CHBr2Cl. Additionally, we observe a proportional increase in the TDM and bandgap energy, indicative of conductivity, for various termination atom combinations, such as Mxene-O-OH, Mxene-O-F, Mxene-O-Cl, Mxene-OH-F, Mxene-F-Cl, and Mxene-OH-Cl, with bandgap energies measured at 0.734, 0.940, 1.120, 0.835, and 0.927 eV, respectively. Utilizing DFT, we elucidate the adsorption energies of THs on different MXene surfaces. Our results conclusively demonstrate the significant influence of the termination atom nature and quantity on MXene's primitive TDM value. This research contributes to our understanding of MXene-THs interactions, offering promising avenues for the development of efficient adsorbents and detectors for THs. Ultimately, these advancements hold the potential to revolutionize water sanitation practices and enhance environmental safety.

Design of MXene-Based Multiporous Nanosheet Stacking Structures Integrating Multiple Synergistic SERS Enhancements for Ultrasensitive Detection of Chloramphenicol

Motivated by the desire for more sensitivity and stable surface-enhanced Raman scattering (SERS) substrates to trace detect chloramphenicol due to its high toxicity and ubiquity, MXene has attracted increasing attention and is encountering the high-priority task of further observably improving detection sensitivity. Herein, a universal SERS optimization strategy that incorporates NH4VO3 to induce few-layer MXenes assembling into multiporous nanosheet stacking structures was innovatively proposed. The synthesized Nb2C-based multiporous nanosheet stacking structure can achieve a low limit of detection of 10-10 M and a high enhancement factor of 2.6 × 109 for MeB molecules, whose detection sensitivity is improved by 3 orders of magnitude relative to few-layer Nb2C MXenes. Such remarkably enhanced SERS sensitivity mainly originates from the multiple synergistic contributions of the developed physical adsorption, the chemical enhancement, and the conspicuously improved electromagnetic enhancement arising from the intersecting MXenes. Furthermore, the improved SERS sensitivity endows Nb2C-based multiporous structures with the capability to achieve ultrasensitive detection of chloramphenicol with a wide linear range from 100 μg/mL to 1 ng/mL. We believe it is of great significance in conspicuously developing the SERS sensitivity of other MXenes with surficial negative charges and has a great promising perspective for the trace detection of other antibiotics in microsystems.

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.

Construction of dense film inside capillary wall and SERS application research

The high-density particle distribution in capillary was a crucial factor for enhancing SERS properties and a difficult point in the preparation process. The direct high-temperature method was used to fuse the particles and form a uniform and dense particle distribution on the capillary's inner wall, providing a foundation for enhancing Raman signals. The prepared capillary SERS substrate strongly enhances the rhodamine 6G (R6G) signal, and the RSD values of several characteristic peaks of R6G are about 10 %, demonstrating high sensitivity, uniformity, and stability. Using capillary SERS substrate for detecting goat serum. Embedding precious metal particles into capillary SERS substrate can effectively encapsulate the tested liquid and avoid contamination, which improves the disadvantage of traditional substrates exposing the liquid to air. The prepared capillary SERS substrate could be used for field and biomedical sensitivity detection, providing a theoretical and experimental basis for developing the capillary SERS substrate.

Thermoelectrically Driven Dual-Mechanism Regulation on SERS and Application Potential for Rapid Detection of SARS-CoV-2 Viruses and Microplastics

Electric-induced surface-enhanced Raman scattering (E-SERS) has been widely studied for its flexible regulation of SERS after the substrate is prepared. However, the enhancement effect is not sufficiently high in the E-SERS technology reported thus far, and no suitable field of application exists. In this study, a highly sensitive thermoelectrically induced SERS substrate, Ag/graphene/ZnO (AGZ), was fabricated using ZnO nanoarrays (NRs), graphene, and Ag nanoparticles (NPs). Applying a temperature gradient to the ZnO NRs enhanced the SERS signals of the probe molecules by a factor of approximately 20. Theoretical and experimental results showed that the thermoelectric potential enables the simultaneous modulation of the Fermi energy level of graphene and the plasma resonance peak of Ag NPs, resulting in a double enhancement in terms of physical and chemical mechanisms. The AGZ substrate was then combined with a mask to create a wearable thermoelectrically enhanced SERS mask for collecting SARS-CoV-2 viruses and microplastics. Its SERS signal can be enhanced by the temperature gradient created between a body heat source (∼37 °C) and a cold environment. The suitability of this method for virus detection was also examined using a reverse transcription-polymerase chain reaction and SARS-CoV-2 virus antigen detection. This work offers new horizons for research of E-SERS, and its application potential for rapid detection of the SARS-CoV-2 virus and microplastics was also studied.

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.

Machine Learning for COVID-19 Determination Using Surface-Enhanced Raman Spectroscopy

The rapid, low cost, and efficient detection of SARS-CoV-2 virus infection, especially in clinical samples, remains a major challenge. A promising solution to this problem is the combination of a spectroscopic technique: surface-enhanced Raman spectroscopy (SERS) with advanced chemometrics based on machine learning (ML) algorithms. In the present study, we conducted SERS investigations of saliva and nasopharyngeal swabs taken from a cohort of patients (saliva: 175; nasopharyngeal swabs: 114). Obtained SERS spectra were analyzed using a range of classifiers in which random forest (RF) achieved the best results, e.g., for saliva, the precision and recall equals 94.0% and 88.9%, respectively. The results demonstrate that even with a relatively small number of clinical samples, the combination of SERS and shallow machine learning can be used to identify SARS-CoV-2 virus in clinical practice.

Rapid assays of SARS-CoV-2 virus and noble biosensors by nanomaterials

The COVID-19 outbreak caused by SARS-CoV-2 in late 2019 has spread rapidly across the world to form a global epidemic of respiratory infectious diseases. Increased investigations on diagnostic tools are currently implemented to assist rapid identification of the virus because mass and rapid diagnosis might be the best way to prevent the outbreak of the virus. This critical review discusses the detection principles, fabrication techniques, and applications on the rapid detection of SARS-CoV-2 with three categories: rapid nuclear acid augmentation test, rapid immunoassay test and biosensors. Special efforts were put on enhancement of nanomaterials on biosensors for rapid, sensitive, and low-cost diagnostics of SARS-CoV-2 virus. Future developments are suggested regarding potential candidates in hospitals, clinics and laboratories for control and prevention of large-scale epidemic.

SERS immunoassay analysis of Escherichia coli and Staphylococcus aureus based on sandwich-structured complex probe and target-induced strand displacement

A direct and ultra-sensitive surface-enhanced Raman scattering (SERS) immunoassay method is introduced for the detection of Escherichia coli and Staphylococcus aureus. This methodology is based on a sandwich-structured complex probe (SCP) mechanism, combined with target-induced strand displacement. Moreover, by leveraging the amplified SERS signal from gold nanoparticles (AuNPs) corresponding to an increase in bacterial count, we achieve quantitative determination. The SCP demonstrates remarkable specificity, sensitivity, and anti-interference capability in bacterial detection. The detection limits for both bacterial strains are as low as 10 CFU/mL. In our selectivity tests, all peak intensities had standard deviations (n = 3) below 6%. Recoveries in normal human serum were 101-110% for E. coli and 96-101% for S. aureus. In milk, the recoveries were 102-105% for E. coli and 100-105% for S. aureus, respectively, demonstrating a high level of accuracy and resistance to interference. In addition, the SCP offers a dual-detection capability, enabling simultaneous diagnosis of multiple targets, which greatly simplifies the testing procedure. The findings underscore that this immunoassay platform fulfills the demand for rapid and precise pathogenic bacterial diagnosis, holding substantial potential for practical applications.

Atomic Scale Design of MXenes and Their Parent Materials─From Theoretical and Experimental Perspectives

More than a decade after the discovery of MXene, there has been a remarkable increase in research on synthesis, characterization, and applications of this growing family of two-dimensional (2D) carbides and nitrides. Today, these materials include one, two, or more transition metals arranged in chemically ordered or disordered structures of three, five, seven, or nine atomic layers, with a surface chemistry characterized by surface terminations. By combining M, X, and various surface terminations, it appears that a virtually endless number of MXenes is possible. However, for the design and discovery of structures and compositions beyond current MXenes, one needs suitable (stable) precursors, an assessment of viable pathways for 3D to 2D conversion, and utilization or development of corresponding synthesis techniques. Here, we present a critical and forward-looking review of the field of atomic scale design and synthesis of MXenes and their parent materials. We discuss theoretical methods for predicting MXene precursors and for assessing whether they are chemically exfoliable. We also summarize current experimental methods for realizing the predicted materials, listing all verified MXenes to date, and outline research directions that will improve the fundamental understanding of MXene processing, enabling atomic scale design of future 2D materials, for emerging technologies.

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.

Antibody- and aptamer-free SERS substrate for ultrasensitive and anti-interference detection of SARS-CoV-2 spike protein in untreated saliva

Sensitive and anti-interference detection of targeted signal(s) in body fluids is one of the paramount tasks in biosensing. Overcoming the complication and high cost of antibody/aptamer-modification, surface-enhanced Raman spectroscopy (SERS) based on antibody/aptamer-free (AAF) substrates has shown great promise, yet with rather limited detection sensitivity. Herein, we report ultrasensitive and anti-interference detection of SARS-CoV-2 spike protein in untreated saliva by an AAF SERS substrate, applying the evanescent field induced by the high-order waveguide modes of well-defined nanorods for SERS for the first time. A detection limit of 3.6 × 10-17 M and 1.6 × 10-16 M are obtained in phosphate buffered saline and untreated saliva, respectively; the detection limits are three orders of magnitude improved than the best records from AAF substrates. This work unlocks an exciting path to design AAF SERS substrates for ultrasensitive biosensing, not limited to detection of viral antigens.

Ultra-sensitive and specific detection of pathogenic nucleic acids using composite-excited hyperfine plasma spectroscopy combs sensitized by Au nanoarrays functionalized with 2D Ta2C-MXene

Accurate and rapid screening techniques on a population scale are crucial for preventing and managing epidemics like COVID-19. The standard gold test for nucleic acids in pathogenic infections is primarily the reverse transcription polymerase chain reaction (RT-PCR). However, this method is not suitable for widespread screening due to its reliance on large-scale equipment and time-consuming extraction and amplification processes. Here, we developed a collaborative system that combines high-load hybridization probes targeting N and OFR1a with Au NPs@Ta2C-M modified gold-coated tilted fiber Bragg grating (TFBG) sensors to enable direct nucleic acid detection. Multiple activation sites of SARS-CoV-2 were saturable modified on the surface of a homogeneous arrayed AuNPs@Ta2C-M/Au structure based on a segmental modification approach. The combination of hybrid probe synergy and composite polarisation response in the excitation structure results in highly specific hybridization analysis and excellent signal transduction of trace target sequences. The system demonstrates excellent trace specificity, with a limit of detection of 0.2 pg/mL, and achieves a rapid response time of 1.5 min for clinical samples without amplification. The results showed high agreement with the RT-PCR test (Kappa index = 1). And the gradient-based detection of 10-in-1 mixed samples exhibits high-intensity interference immunity and excellent trace identification. Therefore, the proposed synergistic detection platform has a good tendency to curb the global spread of epidemics such as COVID-19.

Heterogeneous-Nucleation Biosensor for Long-Term Collection and Mask-Based Self-Detection of SARS-CoV-2

The effective control of infectious diseases, including Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection, depends on the availability of rapid and accurate monitoring techniques. However, conventional SARS-CoV-2 detection technologies do not support continuous self-detection and may lead to cross-infection when utilized in medical institutions. In this study, we introduce a prototype of a mask biosensor designed for the long-term collection and self-detection of SARS-CoV-2. The biosensor utilizes the average resonance Rayleigh scattering intensity of Au nanocluster-aptamers. The inter-mask surface serves as a medium for the long-term collection and concentration enhancement of SARS-CoV-2, while the heterogeneous-nucleation nanoclusters (NCs) contribute to the exceptional stability of Au NCs for up to 48 h, facilitated by the adhesion of Ti NCs. Additionally, the biosensors based on Au NC-aptamers exhibited high sensitivity for up to 1 h. Moreover, through the implementation of a support vector machine classifier, a significant number of point signals can be collected and differentiated, leading to improved biosensor accuracy. These biosensors offer a complementary wearable device-based method for diagnosing SARS-CoV-2, with a limit of detection of 103 copies. Given their flexibility, the proposed biosensors possess tremendous potential for the continuous collection and sensitive self-detection of SARS-CoV-2 variants and other infectious pathogens.

Nanosphere Lithography-Enabled Hybrid Ag-Cu Surface-Enhanced Raman Spectroscopy Substrates with Enhanced Absorption of Excitation Light

We demonstrated a low-cost, highly sensitive hybrid Ag-Cu substrate with enhanced absorption for the excitation laser beam via the nanosphere lithography technique. The hybrid Ag-Cu surface-enhanced Raman spectroscopy (SERS) substrate consists of a Cu nanoarray covered with Ag nanoparticles. The geometry of the deposited Cu nanoarray is precisely determined through a self-assembly nanosphere etching process, resulting in optimized absorption for the excitation laser beam. Further Raman enhancement is achieved by incorporating plasmonic hotspots formed by dense Ag nanoparticles, grown by immersing the prepared Cu nanoarray in a silver nitrate solution. The structural design enables analytical enhancement factor of hybrid Ag-Cu SERS substrates of 1.13 × 105. The Ag-Cu SERS substrates exhibit a highly sensitive and reproducible SERS activity, with a low detection limit of 10-13 M for Rhodamine 6G detection and 10-9 M for 4,4'-Bipyridine. Our strategy could pave an effective and promising approach for SERS-based rapid detection in biosensors, environmental monitoring and food safety.

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.

Spontaneous Redox-Reaction-Driven Growth of Ag Nanoparticles on Co(OH)2 Nanoflower Arrays for Surface-Enhanced Raman Scattering

A simple and reliable method is developed to fabricate Ag-nanoparticle-decorated Co(OH)₂ nanoflowers grafted on polyacrylonitrile (PAN) nanopillar arrays as uniform and sensitive surface-enhanced Raman scattering (SERS) substrates. First, Co(OH)₂-nanosheet-assembled nanoflowers are achieved on the highly uniform PAN nanopillar arrays via electrochemical deposition. Then, Ag nanoparticles (Ag-NPs) are decorated onto the Au-nanoparticle-precoated Co(OH)₂ nanoflowers based on a spontaneous redox reaction (SRR) between the silver ions and Co(OH)₂ nanosheets at room temperature. Ag-NPs can be successfully in situ synthesized on the Co(OH)₂ nanoflowers, and Au nanoparticles precoated on the surface of the Co(OH)₂ nanosheets can ensure that the Co(OH)₂ nanoflower structure does not collapse. Because of the highly uniform PAN nanopillar arrays and the high-density sub-10 nm gaps between the neighboring Ag-NPs on the surface of the Co(OH)₂ nanoflowers, the hierarchical three-dimensional Ag@Co(OH)_x grown on PAN nanopillar arrays can produce a reproducible and sensitive SERS effect. To verify the SERS performance of the substrate, 4-aminothiophenol (4-ATP) is used as the probe molecule, and the Ag@Co(OH)_x grown on PAN nanopillar arrays is employed as the SERS substrate. As a result, 4-ATP concentrations as low as 10^-10 M can still be identified, exhibiting high SERS activity. Additionally, the relative standard deviation value of the main characteristic peak of 10^-5 M 4-ATP is 9.43%, indicating good uniformity of the SERS signal of the substrate. The SRR between silver ions and Co(OH)₂ can provide a simple route to prepare heterostructures as SERS substrates, which has great potential for application in the field of analysis.

Ultra-sensitive specific detection of nucleic acids in pathogenic infections by Ta2C-MXene sensitization-based ultrafine plasmon spectroscopy combs

Accurate and rapid population-scale screening techniques based on SARS-CoV-2 RNA are essential in preventing and controlling the COVID-19 epidemic. However, the sensitivity and specificity of the assay signal are challenged by the problems of target dilution and sample contamination inherent in high-volume pooled testing. Here, we reported a collaborative system of high-loaded hybrid probes targeting N and OFR1a coupling with the novel Ta₂C-M/Au/TFBG biosensor, providing high-intensity vector signals for detecting SARS-CoV-2. The method relies on a segmental modification approach to saturable modify multiple activation sites of SARS-CoV-2 on the high-performance Ta₂C-M surface. The coupling of multi-site synergy with composite excited TFBG results in excellent signal transduction, detection limits (0.2 pg/mL), and hybridization efficiency. Without relying on amplification, the collaborative system achieved specific differentiation of 30 clinical samples in an average diagnostic time of 1.8 min. In addition, for the first time, a kinetic determination of dilution mixed samples was achieved and showed a high-intensity carrier signal and fantastic stability. Therefore, it can be used as a collaborative, integrated tool to play a massive role in the screening, prevention, and control of COVID-19 and other epidemics.

Biomedical equipments, vaccine and drug in the prevention, diagnosis and treatment of COVID-19

SARS-CoV-2 virus caused an infectious disease, named COVID-19. Biomedical equipments, vaccine and drug have played a crucial role in the prevention, diagnosis and treatment. Nevertheless, up to now, there still has been no literature summarizing the diagnosis, prevention and treatment of this infectious disease from the perspective of biomedical equipments. Thus, this review wants to give an overview on the biomedical equipments, vaccine and drug in the prevention, diagnosis and treatment of this disease, and avoids the overlap with previous research, more emphasis on biomedical equipments, and less emphasis on biomaterials. The existing problems in the current research and application were summarized, and the future research direction was proposed, so as to provide reference to deal with similar viral infections in the future.

Nano-biosensor for SARS-CoV-2/COVID-19 detection: methods, mechanism and interface design

The epidemic of coronavirus disease 2019 (COVID-19) was a huge disaster to human society. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which led to COVID-19, has resulted in a large number of deaths. Even though the reverse transcription-polymerase chain reaction (RT-PCR) is the most efficient method for the detection of SARS-CoV-2, the disadvantages (such as long detection time, professional operators, expensive instruments, and laboratory equipment) limit its application. In this review, the different kinds of nano-biosensors based on surface-enhanced Raman scattering (SERS), surface plasmon resonance (SPR), field-effect transistor (FET), fluorescence methods, and electrochemical methods are summarized, starting with a concise description of their sensing mechanism. The different bioprobes (such as ACE2, S protein-antibody, IgG antibody, IgM antibody, and SARS-CoV-2 DNA probes) with different bio-principles are introduced. The key structural components of the biosensors are briefly introduced to give readers an understanding of the principles behind the testing methods. In particular, SARS-CoV-2-related RNA mutation detection and its challenges are also briefly described. We hope that this review will encourage readers with different research backgrounds to design SARS-CoV-2 nano-biosensors with high selectivity and sensitivity.

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.

Recent development of surface-enhanced Raman scattering for biosensing

Surface-Enhanced Raman Scattering (SERS) technology, as a powerful tool to identify molecular species by collecting molecular spectral signals at the single-molecule level, has achieved substantial progresses in the fields of environmental science, medical diagnosis, food safety, and biological analysis. As deepening research is delved into SERS sensing, more and more high-performance or multifunctional SERS substrate materials emerge, which are expected to push Raman sensing into more application fields. Especially in the field of biological analysis, intrinsic and extrinsic SERS sensing schemes have been widely used and explored due to their fast, sensitive and reliable advantages. Herein, recent developments of SERS substrates and their applications in biomolecular detection (SARS-CoV-2 virus, tumor etc.), biological imaging and pesticide detection are summarized. The SERS concepts (including its basic theory and sensing mechanism) and the important strategies (extending from nanomaterials with tunable shapes and nanostructures to surface bio-functionalization by modifying affinity groups or specific biomolecules) for improving SERS biosensing performance are comprehensively discussed. For data analysis and identification, the applications of machine learning methods and software acquisition sources in SERS biosensing and diagnosing are discussed in detail. In conclusion, the challenges and perspectives of SERS biosensing in the future are presented.

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.

Raman spectroscopy for viral diagnostics

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

Graphical abstract

Supplementary Information

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

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.

Revolutionary approaches for cancer diagnosis by terahertz-based spectroscopy and imaging

Most tumors are easily missed and misdiagnosed due to the lack of specific clinical signs and symptoms in the early stage. Thus, an accurate, rapid and reliable early tumor detection method is highly desirable. The application of terahertz (THz) spectroscopy and imaging in biomedicine has made remarkable progress in the past two decades, which addresses the shortcomings of existing technologies and provides an alternative for early tumor diagnosis. Although issues such as size mismatch and strong absorption of THz waves by water have set hurdles for cancer diagnosis by THz technology, innovative materials and biosensors in recent years have led to possibilities for new THz biosensing and imaging methods. In this article, we reviewed the issues that need to be solved before THz technology is used for tumor-related biological sample detection and clinical auxiliary diagnosis. We focused on the recent research progress of THz technology, with an emphasis on biosensing and imaging. Finally, the application of THz spectroscopy and imaging for tumor diagnosis in clinical practice and the main challenges in this process were also mentioned. Collectively, THz-based spectroscopy and imaging reviewed here is envisioned as a cutting-edge approach for cancer diagnosis.

Recent Trends in SERS-Based Plasmonic Sensors for Disease Diagnostics, Biomolecules Detection, and Machine Learning Techniques

Surface-enhanced Raman spectroscopy/scattering (SERS) has evolved into a popular tool for applications in biology and medicine owing to its ease-of-use, non-destructive, and label-free approach. Advances in plasmonics and instrumentation have enabled the realization of SERS's full potential for the trace detection of biomolecules, disease diagnostics, and monitoring. We provide a brief review on the recent developments in the SERS technique for biosensing applications, with a particular focus on machine learning techniques used for the same. Initially, the article discusses the need for plasmonic sensors in biology and the advantage of SERS over existing techniques. In the later sections, the applications are organized as SERS-based biosensing for disease diagnosis focusing on cancer identification and respiratory diseases, including the recent SARS-CoV-2 detection. We then discuss progress in sensing microorganisms, such as bacteria, with a particular focus on plasmonic sensors for detecting biohazardous materials in view of homeland security. At the end of the article, we focus on machine learning techniques for the (a) identification, (b) classification, and (c) quantification in SERS for biology applications. The review covers the work from 2010 onwards, and the language is simplified to suit the needs of the interdisciplinary audience.

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) have opened a variety of exciting research fields. However, although a vast number of applications have been proposed since the two techniques were first reported, none has been applied to real practical use. This calls for an update in the recent fundamental and application studies of SERS and TERS. Thus, the goals and scope of this review are to report new directions and perspectives of SERS and TERS, mainly from the viewpoint of combining their mechanism and application studies. Regarding the recent progress in SERS and TERS, this review discusses four main topics: (1) nanometer to subnanometer plasmonic hotspots for SERS; (2) Ångström resolved TERS; (3) chemical mechanisms, i.e., charge-transfer mechanism of SERS and semiconductor-enhanced Raman scattering; and (4) the creation of a strong bridge between the mechanism studies and applications.

Regulating the microenvironment with nanomaterials: Potential strategies to ameliorate COVID-19

COVID-19, caused by SARS-CoV-2, has resulted in serious economic and health burdens. Current treatments remain inadequate to extinguish the epidemic, and efficient therapeutic approaches for COVID-19 are urgently being sought. Interestingly, accumulating evidence suggests that microenvironmental disorder plays an important role in the progression of COVID-19 in patients. In addition, recent advances in nanomaterial technologies provide promising opportunities for alleviating the altered homeostasis induced by a viral infection, providing new insight into COVID-19 treatment. Most literature reviews focus only on certain aspects of microenvironment alterations and fail to provide a comprehensive overview of the changes in homeostasis in COVID-19 patients. To fill this gap, this review systematically discusses alterations of homeostasis in COVID-19 patients and potential mechanisms. Next, advances in nanotechnology-based strategies for promoting homeostasis restoration are summarized. Finally, we discuss the challenges and prospects of using nanomaterials for COVID-19 management. This review provides a new strategy and insights into treating COVID-19 and other diseases associated with microenvironment disorders.

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.

Highly sensitive AuNSs@AgNR SERS substrates for rapid determination of aromatic amines

The leakage of aromatic amines will pose a great threat to human health and the ecological environment. Therefore, there is an urgent need to achieve rapid and high-sensitivity detection of such substances. In this study, a simple surface-enhanced Raman scattering (SERS) method based on gold nanostars-modified silver nanorods (AuNSs@AgNRs) was established for the detection of benzidine and 4-aminobiphenyl (4-ABP). The enhancement factors of the substrate towards rhodamine 6G (R6G) and crystal violet (CV) were 4.67 × 10⁸ and 1.11 × 10⁸, respectively. Combined with density functional theory (DFT), the AuNSs@AgNR substrate achieved the rapid detection of benzidine and 4-ABP and obtained low detection limits (LOD_benzidine = 7.09 × 10^-9 M; LOD_4-ABP = 1.20 × 10^-9 M). Furthermore, the AuNSs@AgNR substrate can realize the high-sensitivity detection of benzidine and 4-ABP in the spiked river water samples within 3 min, which means that the AuNSs@AgNR-based SERS method can be used as a portable platform to realize the on-site rapid detection of environmental pollutants.

Two-dimensional MBenes with ordered metal vacancies for surface-enhanced Raman scattering

As an emerging class of two-dimensional (2D) materials, MBenes show enormous potential for optoelectronic applications. However, their use in molecular sensing as surface-enhanced Raman scattering (SERS)-active material is unknown. Herein, for the first time, we develop a brand-new high-performance MBene SERS platform. Ordered vacancy-triggered highly sensitive SERS platform with outstanding signal uniformity based on a 2D Mo_4/3B₂ MBene material was designed. The 2D Mo_4/3B₂ MBene presented superior SERS activity to most of the semiconductor SERS substrates, showing a remarkable Raman enhancement factor of 3.88 × 10⁶ and an ultralow detection limit of 1 × 10^-9 M. The underlying SERS mechanism is revealed from systematic experiments and density functional theory calculations that the ultrahigh SERS sensitivity of 2D Mo_4/3B₂ MBene is derived from the efficient photoinduced charge transfer process between MBene substrates and adsorbed molecules. The abundant electronic density of states near the Fermi level of 2D Mo_4/3B₂ MBene enables its Raman enhancement by a factor of 100 000 times higher than that of the bulk MoB. Consequently, the 2D Mo_4/3B₂ MBene could accurately detect various trace chemical analytes. Moreover, with ordered metal vacancies in the 2D Mo_4/3B₂ MBene, uniform charge transfer sites are formed, resulting in an outstanding signal uniformity with a relative standard deviation down to 6.0%. This work opens up a new horizon for the high-performance SERS platform based on MBene materials, which holds great promise in the field of chemical sensing.

SARS-CoV-2 proteins monitored by long-range surface plasmon field-enhanced Raman scattering with hybrid bowtie nanoaperture arrays and nanocavities

The identification of biomacromolecules by using surface-enhanced Raman scattering (SERS) remains a challenge because of the near-field effect of traditional substrates. Long-range surface plasmon resonance (LRSPR) is a special type of surface optical phenomenon that provides higher electromagnetic field enhancement and longer penetration depth than conventional surface plasmon resonance. To break the limit of SERS detection distance and obtain a SERS substrate with increased enhancement ability, a bowtie nanoaperture array was sandwiched between two symmetric dielectric environments. Then, an Au mirror was inserted to form a metal-insulator-metal configuration. Finite-difference time-domain simulations revealed that numerous hybrid modes can be provided by this novel configuration (denoted as long-range SERS [LR-SERS] substrate). In particular, the LRSPR mode can be excited and reach the maximum value through the regulation of the polarizations of the incident light and the geometrical parameters of the LR-SERS substrate. The optimized LR-SERS substrate was then applied to detect SARS-CoV-2 spike (S) and nucleocapsid (N) proteins. This substrate displayed ultralow detection limits of ∼9.2 and ∼11.3 pg mL^-1 for the S and N proteins, respectively. Moreover, with the help of principal component analysis and receiver operating characteristic methods, our fabricated sensors exhibited excellent selectivity and hold great potential for the diagnosis of SARS-CoV-2 proteins in real samples.

Rapid Detection of SARS-CoV-2 Spike RBD Protein in Body Fluid: Based on Special Calcium Ion-Mediated Gold Nanoparticles Modified by Bromide Ions

The receptor-binding domain of the SARS-CoV-2 spike mediates the key to binding the virus to the host receptor, but capturing the molecular signal of this spike RBD remains a formidable challenge. Here, we report a new surface-enhanced Raman spectroscopy (SERS) approach, which used gold nanoparticles prepared by low-speed constant-temperature centrifugation by bromine and calcium ions in two cleaning steps as the enhanced substrate to rapidly and accurately detect spike RBD large protein molecules in body fluids. The detection signal was extremely stable, and the orientation of the spike RBD on the enhanced substrate surface was also determined. This approach was specific in distinguishing different SARS-CoV-2 variants of spike RBD, including Delta, Beta, Gamma, and Omicron. Additionally, the enhanced substrate can identify biologically active or inactive spike RBD. This two-step cleaning enhanced substrate opens up opportunities not only for early diagnostics of SARS-CoV-2 virus but also for developing targeted drugs against viruses.

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.

Plasmon Coupling and Efficient Charge Transfer in Rough-Surfaced Au Nanotriangles/MXene Hybrids as an Ultrasensitive Surface-Enhanced Raman Scattering Platform

The rational design of Raman substrate materials with prominent electromagnetic enhancement and charge transfer is quite important for surface-enhanced Raman scattering (SERS). Herein, an efficient SERS substrate based on two-dimensional ultrathin Ti₃C₂T _x MXene and rough-surfaced Au nanotriangles (NTs) was successfully prepared for efficient detection of organic molecules due to the synthetic effect of an optimized electromagnetic field and charge transfer. Uniform Au NTs with tunable surface roughness were controllably prepared by selectively depositing of Au on the smooth Au NTs. Due to the large surface area, tunable plasmon resonance, and abundant hotspots on the planar surface, the modified Au NTs showed much better SERS performance than initial Au NTs. By combination of the rough-surfaced Au NTs with MXene, the Ti₃C₂T _x /Au NT hybrids exhibited much better SERS performance than initial Au NTs and Au NTs with a rough surface. The detection limit is down to 10^-12 M, and the analytical enhancement factors reach 3.6 × 10⁹ (at 1174 cm^-1) on detecting crystal violet excited at 785 nm. This is because the strong plasmon coupling between the in-plane resonance of Au NTs and transversal plasmon resonance of Ti₃C₂T _x MXene around 785 nm can generate an intense interfacial electromagnetic field for amplifying SERS signals. Additionally, the efficient charge transfer between Au NTs, MXene, and molecules also plays an important role in enhancing the SERS performance. This work presents a new insight to develop high-performance SERS substrates based on plasmon.