Highly Sensitive and Reproducible SERS Substrates Based on Ordered Micropyramid Array and Silver Nanoparticles

Semiconductor-Bimetallic Plasmonic Heterojunction ZnO-Ag-Cu as Reusable SERS Substrate with Attomolar Detection Limit

Semiconductor-bimetallic ZnO-Ag-Cu (ZAC) heterojunction of different compositions were fabricated as highly sensitive SERS substrates. ZnO nanorods were synthesized using a facile hydrothermal route. ZAC composites were synthesized via impregnation method by keeping ZnO content same and varying the mole fractions of Ag and Cu. The ZnO matrix, known for its stability and photocatalytic properties, was decorated with Ag and Cu nanoparticles to enhance plasmonic activity and boost SERS. Introducing semiconductor oxide as SERS substrate reduces the substrate cost due to its self-cleaning property upon exposure to UV light. When SERS activity of ZAC composites were compared with either ZnO-Ag (ZA) or ZnO-Cu (ZC) composites, the best SERS performance was recorded with ZAC55, where the Ag and Cu content are same. ZAC55 produced a SERS enhancement factor of 6.2×10⁶ and a limit of detection of 10-18 M and 10-15 M for the analyte molecules Rhodamine 6 G (R6G) and Methylene Blue (MB), respectively, using 532 nm laser excitation. The enhanced SERS performance is attributed to the synergistic effects of ZnO, Ag, and Cu, unveiling ZAC55 as a promising next-generation SERS substrate. Along with remarkable sensitivity, ZAC55 showed promising reusability and reproducibility, indicating its potential for practical applications in chemical sensing.

Protocol for preparing cellulose-based aerogels in a deep eutectic solvent as surface-enhanced Raman scattering substrates

There is an increasing need to detect and identify analytes of diverse origins in low molar concentrations and from complex matrices. Here, we introduce a protocol for preparing surface-enhanced Raman scattering (SERS) substrates that enable the detection of aqueous analytes in ultralow concentrations. We describe the procedure for synthesizing plasmonic aerogels using L-ascorbic acid as a reductor and eutectic solvents as reaction media. We then detail procedures for measurements using these substrates. For complete details on the use and execution of this protocol, please refer to Panikar et al.1.

SERS-based simplified analysis of paraquat in poisoning cases: Bypassing complicated pretreatment with antioxidant sensor

Applying antioxidant coating materials to prepare surface-enhanced Raman spectroscopy (SERS) sensing substrates can effectively enhance the sensitivity and stability for the analysis of molecules. In this study, we have leveraged SERS to develop an innovative sensor for the swift identification of Paraquat (PQ), enabling on-site detection of this herbicide. The newly devised sensor distinguishes itself through its exceptional oxidation resistance. This resistance is attributed to the physical properties of the nanoparticles, specifically the silver shell coating and loading on the molybdenum disulfide (MoS2). By the creation of "hot spots" of the composite nanoparticles (Ag@AuBPs on flower-like MoS2), the kit achieves a remarkably low detection limit as low as 1.0 × 10-10 M for Paraquat in lake water, soil, and clothing samples, allowing for rapid and direct identification of PQ in complex environments.

Surface-Enhanced Raman Scattering (SERS)-based biosensors for advanced extracellular vesicle detection: A review

BACKGROUND

Extracellular Vesicles (EVs), as nano-scale vesicles rich in biological information, hold an indispensable status in the biomedical field. However, due to the intrinsic small size and low abundance of EVs, their effective detection presents significant challenges. Although various EV detection techniques exist, their sensitivity and ease of operation still need enhancement.

RESULTS

Surface-Enhanced Raman Scattering (SERS) is known for its high sensitivity and specificity. It stands out in tackling the challenges that traditional EV detection methods face. In this review, we focus on the application of SERS-based biosensors in EV detection. It provides a detailed introduction to the recognition and capture of EVs, strategies for mediating signal amplification, and detection of EV biomarkers. Finally, the challenges and prospects of SERS-based biosensors are discussed.

SIGNIFICANCE

SERS-based biosensor enhances the Raman signal, allowing for the detection of biomarkers at low concentrations. This capability reveals its substantial potential in identifying EVs and analyzing molecular data. It paves the path for advanced EV detection.

Anti-counterfeiting labels with controllable and anti-interference coding information based on core-shell Ag@SiO2 nanomaterials for ink printing

The core-shell structured Ag@SiO2 nanomaterial integrated with surface-enhanced Raman scattering (SERS) spectroscopy promises a critical application in anti-counterfeiting. Security labels have been fabricated based on Ag@SiO2 embedded with Raman reporters. The Ag@SiO2 nanomaterial shows good stability and excellent anti-interference property for anti-counterfeiting. Multiple kinds of Raman probe molecules have been anchored in the Ag@SiO2 labels to provide specific and abundant encoding information. The flexible encoding security information could be controlled conveniently by adjusting probe molecules, which not only enrich the SERS information but also improve the level of anti-counterfeiting. Furthermore, the Ag@SiO2 shown excellent stability in organic solvent, and successfully used in ink for the anti-counterfeiting application.

Label-free and liquid state SERS detection of multi-scaled bioanalytes via light-induced pinpoint colloidal assembly

Surface-enhanced Raman scattering (SERS) has been extensively applied to detect complex analytes due to its ability to enhance the fingerprint signals of molecules around nanostructured metallic surfaces. Thus, it is essential to design SERS-active nanostructures with abundant electromagnetic hotspots in a probed volume according to the dimensions of the analytes, as the analytes must be located in their hotspots for maximum signal enhancement. Herein, we demonstrate a simple method for detecting robust SERS signals from multi-scaled bioanalytes, regardless of their dimensions in the liquid state, through a photothermally driven co-assembly with colloidal plasmonic nanoparticles as signal enhancers. Under resonant light illumination, plasmonic nanoparticles and analytes in the solution quickly assemble at the focused surface area by convective movements induced by the photothermal heating of the plasmonic nanoparticles without any surface modification. Such collective assemblies of plasmonic nanoparticles and analytes were optimized by varying the optical density and surface charge of the nanoparticles, the viscosity of the solvent, and the light illumination time to maximize the SERS signals. Using these light-induced co-assemblies, the intrinsic SERS signals of small biomolecules can be detected down to nanomolar concentrations based on their fingerprint spectra. Furthermore, large-sized biomarkers, such as viruses and exosomes, were successfully detected without labels, and the complexity of the collected spectra was statistically analyzed using t-distributed stochastic neighbor embedding combined with support vector machine (t-SNE + SVM). The proposed method is expected to provide a robust and convenient method to sensitively detect biologically and environmentally relevant analytes at multiple scales in liquid samples.

Non-linear responses via agglomeration and aggregation of gold nanoparticles for surface-enhanced Raman spectroscopy (SERS) coupled with chemometric analysis for chlorpyrifos detection

This study investigates the behaviour of gold nanoparticles (AuNPs) when exposed to chlorpyrifos, an agricultural pesticide, and its application in detecting the pesticide via surface-enhanced Raman spectroscopy (SERS). Under synergistic addition of NaCl, AuNPs undergo agglomeration at lower chlorpyrifos concentrations but aggregation at higher concentrations, resulting in a distinctive nonlinear SERS response. A linear relationship is obtained between 0.001 and 1 ppm with detection limit (LOD) of 0.009 ppm, while an inverse response is observed at higher concentrations (1-1000 ppm) with a LOD of 1 ppm. Combining the colorimetric response of AuNP solutions, their absorbance spectra, and principal component analysis can improve detection reliability. The assay, coupled with a simple recovery method using acetonitrile swabbing, achieves high reproducibility in detecting chlorpyrifos in cucumber, even at concentrations as low as 0.11 ppm. This approach can be tailored for various chlorpyrifos concentrations not only in cucumbers but also in different food matrices.

Surface-enhanced Raman scattering based on noble metal nanoassemblies for detecting harmful substances in food

Residues of harmful substances in food can severely damage human health. The content of these substances in food is generally low, making detection difficult. Surface-enhanced Raman scattering (SERS), based on noble metal nanomaterials, mainly gold (Au) and silver (Ag), has exhibited excellent capabilities for trace detection of various substances. Noble metal nanoassemblies, in particular, have extraordinary flexibility and tunable optical properties, which cannot be offered by single nanoparticles (NPs). These nanoassemblies, with their various morphologies synthesized using NPs through artificially induced self-assembly or template-driven preparation, can significantly enhance the local electric field and create "hot spots" due to the gaps between adjacent NPs. Consequently, the SERS properties of NPs become more prominent, leading to improved performance in the trace detection of various substances and detection limits that are considerably lower than the current relevant standards. Noble metal nanoassemblies show promising potential in ensuring food safety. This review discusses the synthesis methods and SERS properties of noble metal nanoassemblies and then concentrates on their application in detecting biotoxins, drug residues, illegal additives, and heavy metals. The study provides valuable references for further research into the application of nanoassemblies in food safety detection.

SERS detection of thiram using a 3D sea cucumber-like composite flexible porous substrate

Nowadays, trace detection of thiram is in urgent demand due to its widespread application in agriculture and significant harmful effects on public health. In this work, a three-dimensional (3D) sea cucumber-like flexible porous surface-enhanced Raman scattering (SERS) substrate composed of a poly(vinylidene fluoride) (PVDF) membrane, ZnO nanorods, gold films, and Ag nanoparticles (Ag/Au/ZnO/P) has been established for the highly sensitive detection of thiram. The substrate takes advantage of the 3D morphology of the Ag/Au/ZnO system on a flexible porous PVDF membrane to produce abundant plasmonic hot spots. Meanwhile, the employment of an AgNPs/Au shell system combined the benefits of both gold and silver metals, thus guaranteeing stable and sensitive detection. With 4-mercaptobenzoic acid (4-MBA) as a probe molecule, the Ag/Au/ZnO/P substrate exhibited excellent linear detection in the range of 10-11-10-5 M, with a correlation coefficient (R2) of 0.99 and an enhancement cofactor of 7.09 × 107. The substrate exhibited excellent uniformity with a related standard deviation (RSD) value of 3.82% and demonstrated high stability during a 15 d-storage test. In addition, the substrate could detect thiram in an aqueous solution at concentrations as low as 10-10 M with excellent selectivity. Meanwhile, thiram on the surface of apple peel could be easily detected by the Ag/Au/ZnO/P substrate with the "paste-and-peel" method in less than 10 s, and the detection limit could be as low as 0.48 ng cm-2. Overall, the remarkable performance of the Ag/Au/ZnO/P SERS substrate demonstrated its great potential for the environmental monitoring of thiram.

Optimisation of a Microwave Synthesis of Silver Nanoparticles by a Quality by Design Approach to Improve SERS Analytical Performances

A major limitation preventing the use of surface-enhanced Raman scattering (SERS) in routine analyses is the signal variability due to the heterogeneity of metallic nanoparticles used as SERS substrates. This study aimed to robustly optimise a synthesis process of silver nanoparticles to improve the measured SERS signal repeatability and the protocol synthesis repeatability. The process is inspired by a chemical reduction method associated with microwave irradiation to guarantee better controlled and uniform heating. The innovative Quality by Design strategy was implemented to optimise the different parameters of the process. A preliminary investigation design was firstly carried out to evaluate the influence of four parameters selected by means of an Ishikawa diagram. The critical quality attributes were to maximise the intensity of the SERS response and minimise its variance. The reaction time, temperature and stirring speed are critical process parameters. These were optimised using an I-optimal design. A robust operating zone covering the optimal reaction conditions (3.36 min-130 °C-600 rpm) associated with a probability of success was modelled. Validation of this point confirmed the prediction with intra- and inter-batch variabilities of less than 15%. In conclusion, this study successfully optimised silver nanoparticles by a rapid, low cost and simple technique enhancing the quantitative perspectives of SERS.

A novel SERS-lateral flow assay (LFA) tray for monitoring of miR-155-5p during pyroptosis in breast cancer cells

In the study, a novel surface-enhanced Raman scattering (SERS)-lateral flow assay (LFA) tray for the real-time detection of pyroptosis-associated miR-155-5p in breast cancer cells was established and validated. The SERS probe modified with monoclonal antibodies and functionalized HP1@5-FAM was first synthesized. When miR-155-5p was present, HP1@5-FAM on the SERS probe specifically recognized target miRNAs and hybridized with them, resulting in HP2 on the T line only capturing some SERS probes that were not bound to miR-155-5p. The T line appeared as a light orange band or there was no color change, and the corresponding Raman detection result showed a weak or insignificant Raman signal. The SERS probe showed high selectivity, satisfactory stability, and excellent reproducibility, and the limit of detection (LOD) for miR-155-5p was 7.26 aM. Finally, the proposed SERS-LFA tray was applied to detect miR-155-5p in MBA-MD-468 cells that underwent varying degrees of pyroptosis, and the detection results of SERS were consistent with those of the conventional real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) assay. The study demonstrated that the SERS-LFA tray was a convenient and ultrasensitive method for miR-155-5p real-time detection, which could provide more detailed information for pyroptosis and be of potential value in guiding the treatment of breast cancer.

Cellulose Nanofiber Films with Gold Nanoparticles Electrostatically Adsorbed for Facile Surface-Enhanced Raman Scattering Detection

Cellulose nanofiber (CNF) holds great promise in applications such as surface-enhanced Raman scattering (SERS), catalysis, esthesia, and detection. This study aimed to build novel CNF-based SERS substrates through a facile synthetic method. Citrate-reduced gold nanoparticles (AuNPs) were adsorbed on the cationized CNF surface due to electrostatic interactions, and uniform AuNPs@(2,3-epoxypropyl trimethylammonium chloride)EPTMAC@CNF flexible SERS substrates were prepared by a simple vacuum-assisted filtration method. The probe molecule methylene blue was chosen to assess the performance of the CNF-based SERS substrate with a sensitivity up to 10-9 M, superior signal reproducibility (relative standard deviation (RSD) = 4.67%), and storage stability (more than 30 days). Tensile strength tests indicated that the CNF-based films had good mechanical properties. In addition, CNF-based substrates can easily capture and visually identify microplastics in water. These results demonstrate the potential application of the flexible, self-assembled AuNPs@EPTMAC@CNF flexible SERS substrate for prompt and sensitive detection of trace substances.

Colloidal surface-enhanced Raman spectroscopic study of grouper epidermal mucus using acidified sodium sulphate as the aggregating agent

Fish epidermal mucus is an important reservoir of antipathogenic compounds which serves as the first line of the immune defence. Despite its significant role in the physiology and health of fish, detailed profiling of fish epidermal mucus has yet to be explored. Therefore, this study investigates a label-free colloidal surface-enhanced Raman spectroscopic (SERS) method for profiling grouper mucus. Gold nanoparticles were first synthesised using the standard citrate reduction and characterised using ultraviolet-visible spectroscopy, transmission electron microscopy and dynamic light scattering. The influence of acidified sodium sulphate (Na2SO4) at pH 3 as the aggregating agent on the enhancement of the SERS spectrum of different analyte samples including rhodamine 6G (R6G) dye, lysozyme solution and hybrid grouper (Epinephelus fuscoguttatus × Epinephelus lanceolatus) mucus was observed. Based on the results, an optimal Na2SO4 concentration of 1 M was recorded to achieve the highest enhancement of the SERS signal for R6G and grouper mucus, while the optimal concentration for lysozyme was 0.1 M. The results indicated a higher degree of aggregation induced by lysozyme than R6G and grouper mucus. A few overlapping peaks of the SERS spectra of lysozyme and grouper mucus made it possible to confirm the presence of lysozyme as potential biomarkers.

Planting gold nanoflower for harvesting reproducible SERS substrate

Surface-enhanced Raman scattering (SERS) is an ultrasensitive analytical method which has been applied in many fields, and the reproducibility of the substrate is important for reliable SERS analysis. In present work, an innovative method inspired by the flower planting process is put forward to acquire gold nanoflower (AuNF) SERS substrate. Three steps (digging holes, sowing the gold nanoseeds and seeds grow into gold nanoflowers) are included in the substrate fabrication process, and the influence of preparing conditions (like reacting time and Na3Au(SO3)2 concentration) on the substrate morphology and SERS performance are investigated. The acquired AuNF substrate not only exhibits good SERS performance but also possesses excellent reproducibility while being used to detect the rhodamine 6G (R6G) molecular. The relative standard deviation (RSD) of Raman signals among substrates acquired in distinct batches (substrate-to-substrate) is as low as 6.67 %. Since the AuNF substrate is prepared by the wet chemistry route based on seed-mediated growth and there are no expensive reagents or complicated process used, the new process to obtain AuNF substrate is cost-effective and easy to scale up.

Plasmonic substrates for biochemical applications of surface-enhanced Raman spectroscopy

Due to its great practical importance, the detection and determination of many biomolecules in body fluids and other samples is carried out in a large number of laboratories around the world. One of the most promising analytical techniques now being widely introduced into medical analysis is surface-enhanced Raman scattering (SERS) spectroscopy. SERS is one of the most sensitive analytical methods, and in some cases, a good quality SERS spectrum dominated by the contribution of even a single molecule can be obtained. Highly sensitive SERS measurements can only be carried out on substrates generating a very high SERS enhancement factor and a low Raman spectral background, and so using of right nanomaterials is a key element in the success of SERS biochemical analysis. In this review article, we present progress that has been made in the preparation of nanomaterials used in SERS spectroscopy for detecting various kinds of biomolecules. We describe four groups of nanomaterials used in such measurements: nanoparticles of plasmonic metals and deposits of plasmonic nanoparticles on macroscopic substrates, nanocomposites containing plasmonic and non-plasmonic parts, nanostructured macroscopic plasmonic metals, and nanostructured macroscopic non-plasmonic materials covered by plasmonic films. We also describe selected SERS biochemical analyses that utilize the nanomaterials presented. We hope that this review will be useful for researchers starting work in this fascinating field of ​​science and technology.

Aramid nanofiber membrane decorated with monodispersed silver nanoparticles as robust and flexible SERS chips for trace detection of multiple toxic substances

Aramid nanofibers (ANFs) as an innovative nanoscale building block exhibit great potential for novel high-performance multifunctional membranes attributed to their extraordinary performance. However, the application of aramid nanofibers in the field of surface enhanced Raman scattering (SERS) sensing has been rarely reported. In this work, aramid nanofibers derived from commercial Kevlar fibers were synthesized by a facile dimethyl sulfoxide/potassium hydroxide (DMSO/KOH) solution treatment. The monodispersed silver nanoparticle-decorated aramid nanofiber (m-Ag@ANF) membranes were constructed by an efficient vacuum filtration technique. Taking advantages of unique intrinsic properties of ANF, the m-Ag@ANF substrates exhibit good flexibility, excellent mechanical properties and prominent thermal stability. Besides, due to the abundance of positively charged amino-group on the ANF substrates, the negatively charged m-AgNPs were uniformly and firmly deposited on the surface of ANF substrate through electrostatic interactions. As a result, the optimal flexible m-Ag-9@ANF SERS substrate exhibits high sensitivity of 10-9 M for methylene blue (MB) and excellent signal reproducibility (RSD = 6.37 %), as well as outstanding signal stability (up to 15 days). Besides, the 2D Raman mapping and FDTD simulations further reveal prominent signal homogeneity and strong electric field distribution for flexible m-Ag-9@ANF SERS substrate. Finally, it is demonstrated that the flexible m-Ag-9@ANF SERS substrate can also be used for detection of toxic molecules on irregular surfaces by a feasible paste-and-read process. The m-Ag@ANF paper exhibits potential applications as a flexible, low-cost, robust and stable SERS sensing platform for trace detection of toxic materials.

Preparation of cellulose-based flexible SERS and its application for rapid and ultra-sensitive detection of thiram on fruits and vegetables

In response to the prevalent issue of thiram as a common pesticide residue on the surface of fruits and vegetables, our research team employed an acidic hydrated metal salt low co-fusion solvent to dissolve cellulose lysis slurry. Subsequently, a regenerated cellulose membrane (RCM) was successfully prepared via sol-gel method. Uniformly sized Ag nanoparticles (NPs) were deposited on RCM utilizing the continuous ion layer adsorption and reaction (SILAR) technique. The resulting Ag NPs/RCM flexible surface-enhanced Raman spectroscopy (SERS) substrates exhibited a minimum detection limit of 5 × 10-9 M for Rhodamine 6G (R6G), demonstrating good uniformity (RSD = 4.86 %) and reproducibility (RSD = 3.07 %). Moreover, the substrate displayed a remarkable sensitivity of 10-10 M toward thiram standard solution. Given its inherent flexibility, the substrate proves advantageous for the detection of three-dimensional environments such as fruit and vegetable surfaces, and its practicality has been confirmed in the detection of thiram residue on apples, tomatoes, pears, and other fruits and vegetables.

Gold-Deposited Graphene Nanosheets for Self-Cleaning Graphene Surface-Enhanced Raman Spectroscopy with Superior Charge-Transfer Contribution

The interaction of graphene with metals initiates charge-transfer interaction-induced chemical enhancements, which critically depend on the doping effect from deposited metallic configurations. In this paper, we have explored the gold nanoparticle-decorated monolayer graphene nanosheets for the large graphene-induced Raman enhancement of adsorbed analytes, indicating the surface-enhanced Raman spectroscopy (SERS) capabilities of metal-doped graphene (G-SERS). Here, the systematically sputtered Au thickness optimization procedure revealed noticeable modifications in the graphene Raman spectra and photoluminescence (PL) background quenching, which indicated favorable charge transfer through n-type doping of chemical vapor deposition-grown graphene nanosheets. The highly consistent, individually distributed morphology of the gold nanoislands over graphene nanosheets depicted a reproducibly uniform G-SERS signal with excellent relative standard deviation values (<5%), resulting in the strongest Raman intensity enhancement factors of ∼108 (MB) (methylene blue) and 107 (DPA) (2,6-pyridinedicarboxylic acid) composed of the weakest PL background. The combined charge-transfer-induced chemical enhancement and electromagnetic enhancement from individual Au nanoislands result in a lowering of detectability down to 10-16 M (MB) and 10-11 M (DPA) concentrations with efficient time-dependent signal stability. Additionally, the GAu demonstrated its effective (∼94.4%) photocatalytic degradation capabilities by decomposing MB dye molecules from a concentration of 1 μM to 2.52 fM within 60 min. Therefore, the prominent charge-transfer contribution through controlled Au decoration over graphene nanosheets provides a potential strategy for fabricating superior SERS sensors and photocatalysts exhibiting adequate signal consistency, stability, and photodegradation efficiency through overcoming the limitations of the traditional sensing platforms.

Photochemical decoration of gold nanoparticles on MoS2 nanoflowers grafted onto the flexible carbon cloth as a recyclable SERS sensor for the detection of antibiotic residues on curved surfaces

Surface-enhanced Raman spectroscopy (SERS)-based flexible substrate has recently been demonstrated to be effective in detecting molecules on curved surfaces, however a suitable method for fabricating the flexible SERS substrate still remains a hurdle. In this paper, we fabricated a flexible SERS substrate by anchoring the plasmonic gold nanoparticles (Au-NPs) onto the hydrothermally grown flower-like molybdenum disulfide (MoS2) grafted onto carbon cloth (CC) via a facile photoreduction route. Benefitting from the abundant hotspots generation of the Au-NPs and photo-induced charge-transfer ability of MoS2, the constructed Au-NPs/MoS2/CC substrate exhibit a superior SERS sensing ability, excellent SERS enhancement factor, high flexibility and mechanical stability towards the nitrofurantoin (NFT) with an ultra-low detection limit of 10-11 M. As a trial for practical applications, the flexible substrate was used to detect NFT (10-4 M) in the curved surfaces of meat samples via swab technique. The ability of the flexible Au-NPs/MoS2/CC substrate to sustain the robust Raman signals of NFT even after recycling up to 4 cycles validated its reusability. The proposed flexible SERS substrate with reusable capability indicates its great potential in practical applications for the detection of target molecules on the curved surfaces.

Ultrahighly Sensitive Surface-Enhanced Raman Spectroscopy Film of Silver Nanoparticles Dispersed in Three Dimensions on a Thin Alumina Nanowire Framework

To develop highly sensitive surface-enhanced Raman spectroscopy (SERS) films, various types of aggregated Ag nanowire (NW) and nanoparticle (NP) complex structures were fabricated using anodic aluminum oxide (AAO) templates and thermal evaporation. Aggregated AgNW structures with numerous tapered nanogaps were fabricated via Ag deposition on aggregated thin alumina nanowires of different lengths. AgNP complex structures were obtained by collapsing vertically aligned thin alumina nanowires 1 μm in length and depositing AgNPs on their tops and sides using surface tension during ethanol drying after functionalization. The Raman signal enhancement factors (EFs) of the samples were evaluated by comparing the SERS signal of the thiophenol (TP) self-assembled monolayer (SAM) on the nanostructures with the Raman signal of neat TP. EFs as high as ~2.3 × 107 were obtained for the optimized aggregated AgNW structure (NW length of 1 μm) and ~3.5 × 107 for the optimized AgNP complex structure. The large EF of the AgNP complex film is attributed mainly to the AgNPs dispersed in three dimensions on the sides of the thin alumina nanowires, strongly implying some important, relevant physics yet to be discovered and also a very promising nanostructure scheme for developing ultrahighly sensitive SERS films with EF > 108.

Light-induced synthesis of silver nanoprisms as a surface-enhanced Raman scattering substrate for N-acetyl procainamide drug quantification

Triangle-shaped silver nanoprisms (AgNPMs) were prepared by a photo-induced method through a seed-mediated growth process and were successfully employed as an ultra-sensitive surface-enhanced Raman scattering (SERS) substrate for the detection of the chemotherapeutic N-acetyl procainamide (NAPA) compound. The transformation of the morphology of the nanoprisms substrate could be noted with a remarkable change in color, possessing an average size of 95 nm. The shape-modified AgNPMs exhibited interesting optical characteristics owing to the truncated dual edges, which led to a pronounced longitudinal localized surface plasmonic resonance (LLSPR) behavior. The nanoprisms-based SERS substrate demonstrated an outstanding sensitivity for NAPA in aqueous solutions with the lowest ever reported detection limit of 0.5 × 10-13 M corresponding to excellent recovery and stability. A steady linear response with a broad dynamic range (10-4-10-12 M) and an R2 of 0.945 was also achieved. The results proved that the NPMs demonstrated excellent efficiency, reproducibility (97%), and stability (30 days) with a superior Raman signal enhancement reaching an ultralow detection limit of 0.5 × 10-13 M compared to the nanosphere particles which could show an LOD of 0.5 × 10-9 M.

Diblock copolymer pattern protection by silver cluster reinforcement

Pattern fabrication by self-assembly of diblock copolymers is of significant interest due to the simplicity in fabricating complex structures. In particular, polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP) is a fascinating base material as it forms an ordered micellar structure on silicon surfaces. In this work, silver (Ag) is applied using direct current magnetron sputter deposition and high-power impulse magnetron sputter deposition on an ordered micellar PS-b-P4VP layer. The fabricated hybrid materials are structurally analyzed by field emission scanning electron microscopy, atomic force microscopy, and grazing incidence small angle X-ray scattering. When applying simple aqueous posttreatment, the pattern is stable and reinforced by Ag clusters, making micellar PS-b-P4VP ordered layers ideal candidates for lithography.

Surface-Enhanced Raman Spectroscopy (SERS) Activity of Gold Nanoparticles Prepared Using an Automated Loop Flow Reactor

This study used automatic control methods to prepare gold nanoparticles (AuNPs) as the substrate and rhodamine 6G molecule as the probe to investigate the enhancement effect, stability, and consistency of surface-enhanced Raman spectroscopy (SERS). The gold nanosols were prepared via automatic control using loop flow-reactor technology, and the synthesis of nanoparticles with different sizes was precisely controlled by optimizing the ratio of the solution required for the reaction between sodium citrate and chloroauric acid during the preparation process. The morphology, structure, and optical properties of the prepared AuNPs were investigated using field-emission scanning electron microscopy, transmission electron microscopy, and ultraviolet visible spectroscopy. Using the proposed method, AuNPs with average particle sizes of 72, 85, 93, and 103 nm were synthesized in a precisely controlled manner. The 93 nm particles exhibited good SERS activity for rhodamine 6G under 785 nm excitation with a detection limit of 2.5 × 10-10 M. The relative standard deviation of the SERS spectra synthesized multiple times was <3.5%, indicating their good sensitivity and reproducibility. The results showed that the AuNPs prepared by the automatic control of the loop-flow method have high sensitivity, stability, and reproducibility. Moreover, they exhibited notable potential for in situ measurement and quantitative analysis using SERS.

Honeycomb-like Ag Nanocavity Array for SERS Observations Using Plasmon-Mediated Chemical Reactions

Organized two-dimensional polystyrene bead arrays perform ion etching, and protruding nanostructures are created on polystyrene beads due to the shadow effects from the ring beads, leading to nucleus selection and growth in Au nanostructure deposition. Ag nanostructures are prepared via plasmon-mediated chemical reactions (PMCRs), leading to the Ag nanocavity geometry of the honeycomb pattern when the etching time and Ag growth time are tuned. Due to the strong electromagnetic coupling, the Ag honeycomb-shaped nanocavity array works as the SERS substrate with high sensitivity and good repeatability, which is used to detect thiram pesticide residues with a concentration down to 10-9 M.

Casting liquid PDMS on self-assembled bilayer polystyrene nanospheres to prepare a SERS substrate with two layers of nanopits for detection of p-nitrophenol

p-Nitrophenol (PNP) is widely used in pesticides, pharmaceuticals, and dyestuffs. It is vital to detect trace PNP in the environment, because it poses significant environmental hazards due to its high toxicity. In this paper, a new method was reported for preparing a SERS substrate with excellent SERS activity by combining self-assembly techniques and flexible materials. First, the three-dimensional (3D) polystyrene (PS) photonic crystal (PC) structural master was fabricated by stacking two layers of self-assembled PS nanospheres with different diameters. Polydimethylsiloxane (PDMS) with a complementary structure to the master was obtained by casting, curing and peeling off. Finally, the PDMS-Ag substrate was fabricated by sputtering a thin Ag layer on the PDMS structure. The enhancement factor (EF) of the PDMS-Ag substrate was calculated to be 2.90 × 109 by using 4-amino thiophenol (ATP) as the probe molecule, and the limit of detection (LOD) for ATP can reach 10-11 M. And the RSD of the SERS intensity for the peak at 1078 cm-1 on the PDMS-Ag substrates from batch to batch was within 2%, indicating the high reproducibility of the as-prepared substrate. The quantitative analysis of PNP was achieved with a LOD of 10-8 M. Therefore, the PDMS-Ag substrate exhibits high sensitivity and reproducibility, and it can detect PNP in trace amounts, with great potential for detecting other contaminants.

Simple and Ultrasensitive Detection of Glioma-Related ctDNAs in Mice Serum by SERS-Based Catalytic Hairpin Assembly Signal Amplification Coupled with Magnetic Aggregation

Purpose

Circulating tumor DNA (ctDNA) is more representative and accurate than biopsy and is also conducive to dynamic monitoring, facilitating accurate diagnosis and prognosis of glioma. Therefore, the present study aimed to establish and validate a novel amplified method for the detection of IDH1 R132H and BRAF V600E, which were associated with the genetic diagnosis of glioma.

Patients and Methods

A dual-signal amplification method based on magnetic aggregation and catalytic hairpin assembly (CHA) was constructed for the simultaneous detection of ctDNAs. When target ctDNAs are present, the CHA reaction is initiated and leads to the assembly of Au-Ag nanoshuttles (Au-Ag NSs) onto magnetic beads (MBs). Further enrichment of MBs under an external magnetic field facilitated the dual-signal amplification of SERS.

Results

The limit of detection (LOD) for IDH1 R132H and BRAF V600E in serum was as low as 6.01 aM and 5.48 aM. The reproducibility and selectivity of the proposed SERS analysis platform was satisfactory. Finally, the platform was applied to quantify IDH1 R132H and BRAF V600E in the serum of subcutaneous-tumor‑bearing nude mice, and the results obtained by SERS were consistent with those from quantitative real-time polymerase chain reaction (qRT-PCR).

Conclusion

The present study showed that the dual-signal amplification method is a simple and ultrasensitive strategy for gliomas-associated ctDNAs detection, which is crucial for early diagnosis and dynamic monitoring.

Flexible 3D Substrate of Ag Nanoparticle-Loaded Carbon Aerogels with Outstanding Surface-Enhanced Raman Scattering Performance

Surface-enhanced Raman scattering (SERS), an ultra-sensitive and non-destructive analytic technique, has attracted wide attention from the scientific community. Despite its rapid development, limited hotspots on the SERS substrates have restricted their potential in practical applications. Herein, we developed a facile method to fabricate a flexible three-dimensional (3D) SERS substrate composed of silver nanoparticles (Ag NPs)-loaded carbon aerogels (CAs). Such a flexible Ag NPs/CAs substrate exhibited numerous hotspots, which can facilely be adjusted not only by tuning the density of Ag NPs but also by controlling the bending degree of the flexible substrate. In addition, the influence of hotspots on the local electric field enhancement was investigated by theoretical calculations. Moreover, the 3D network structure of the CAs with a large specific surface area and strong adsorption ability can improve the capture of target molecules. Consequently, the optimal Ag NPs/CAs substrate has a low detection limit of 10^-12 M for rhodamine 6G molecules as well as good repeatability. Furthermore, based on the good performance of SERS detection of the Ag NPs/CAs substrate, it can also be practically used for the detection of thiram molecules on the surface of cherry tomatoes. Such a flexible 3D Ag NPs/CAs substrate has great potential for practical environmental monitoring applications.

Self-Assembly of Silver Nanowire Films for Surface-Enhanced Raman Scattering Applications

The development of SERS detection technology is challenged by the difficulty in obtaining SERS active substrates that are easily prepared, highly sensitive, and reliable. Many high-quality hotspot structures exist in aligned Ag nanowires (NWs) arrays. This study used a simple self-assembly method with a liquid surface to prepare a highly aligned AgNW array film to form a sensitive and reliable SERS substrate. To estimate the signal reproducibility of the AgNW substrate, the RSD of SERS intensity of 1.0 × 10^-10 M Rhodamine 6G (R6G) in an aqueous solution at 1364 cm^-1 was calculated to be as low as 4.7%. The detection ability of the AgNW substrate was close to the single molecule level, and even the R6G signal of 1.0 × 10^-16 M R6G could be detected with a resonance enhancement factor (EF) as high as 6.12 × 10¹¹ under 532 nm laser excitation. The EF without the resonance effect was 2.35 × 10⁶ using 633 nm laser excitation. FDTD simulations have confirmed that the uniform distribution of hot spots inside the aligned AgNW substrate amplifies the SERS signal.

Dual-Functional Solar-to-Steam Generation and SERS Detection Substrate Based on Plasmonic Nanostructure

Solar-to-steam (STS) generation based on plasmonic materials has attracted significant attention as a green method for producing fresh water. Herein, a simple in situ method is introduced to fabricate Au nanoparticles (AuNPs) on cellulose filter papers as dual-functional substrates for STS generation and surface-enhanced Raman spectroscopy (SERS) sensing. The substrates exhibit 90% of broadband solar absorption between 350 and 1800 nm and achieve an evaporation rate of 0.96 kg·m^-2·h^-1 under 1-sun illumination, room temperature of 20 °C, and relative humidity of 40%. The STS generation of the substrate is stable during 30 h continuous operation. Enriched SERS hotspots between AuNPs endow the substrates with the ability to detect chemical contamination in water with ppb limits of detection for rhodamine 6G dye and melamine. To demonstrate dual-functional properties, the contaminated water was analyzed with SERS and purified by STS. The purified water was then analyzed with SERS to confirm its purity. The developed substrate can be an improved and suitable candidate for fresh water production and qualification.

Au@SiO2 SERS nanotags based lateral flow immunoassay for simultaneous detection of aflatoxin B1 and ochratoxin A

Agricultural products are frequently contaminated by mycotoxins. Multiplex, ultrasensitive, and rapid determination of mycotoxins is still a challenging problem, which is of great significance to food safety and public health. Herein, a surface-enhanced Raman scattering (SERS) based lateral flow immunoassay (LFA) for the simultaneous on-site determination of aflatoxin B₁ (AFB₁) and ochratoxin A (OTA) on the same test line (T line) was developed, in this study. In practice, two kinds of Raman reporters 4-mercaptobenzoic acid (4-MBA), and 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) encoded silica-encapsulated gold nanotags (Au^4-MBA@SiO₂ and Au^DNTB@SiO₂) were used as detection markers to identify the two different mycotoxins. Through systematic optimization of the experimental conditions, this biosensor has high sensitivity and multiplexing with the limits of detection (LODs) at 0.24 pg mL^-1 for AFB₁ and 0.37 pg mL^-1 for OTA. These are far below the regulatory limits set by the European Commission, in which the minimum LODs for AFB₁ and OTA are 2.0 and 3.0 μg kg^-1. In the spiked experiment, the food matrix are corn, rice, and wheat, and the mean recoveries of the two mycotoxins ranged from 91.0% ± 6.3%-104.8% ± 5.6% for AFB₁ and 87.0% ± 4.2%-112.0% ± 3.3% for OTA. These results demonstrate that the developed immunoassay has good stability, selectivity, and reliability, which can be used for routine monitoring of mycotoxin contamination.

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.

Core-shell SERS nanotags-based western blot

Western blot (WB) is the most commonly used scheme for protein identification in life science, but it still faces great challenges in the accurate quantitative detection of low-abundance proteins. Here, we proposed a novel surface-enhanced Raman scattering-based Western blot (SERS-WB) to solve this challenge. SERS nanotags were used as quantitative labels of proteins, which were composed of gold-silver core-shell nanoparticles, and Nile blue A (NBA) molecules were anchored on the interface of the core and shell. The results show that the SERS-WB possessed excellent sensitivity with detection limit of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) protein of 0.15 pg, as well as wide linear dynamic range (LDR) of 382 fg to 382 ng. In addition, the target protein on nitrocellulose (NC) membrane could be directly identified by colorimetric signal due to the aggregation effect of nanoparticles, which greatly simplifies the procedure. This as-proposed strategy will bring new thoughts to technological innovation of WB.

Ameliorating SERS Sensitivity for Pesticide Malathion Detection with Synergistic Boosting Effect by Hydrogen Cations and Chloride Anions

Although SERS has been widely recognized as one of the highly sensitive analytical methods that can be deployed in the field with high sensitivity and short analysis time, reports regarding the fast determination of malathion at low concentrations are still scarce. Here, in this work, the solution pH and various halogen co-adsorbates were explored to promote the SERS signal of malathion using the citrate-reduced Ag NPs. It was found that chloride anions were the most efficient signal booster among the three halogen ions screened. Further examination of the SERS profile of the malathion in the presence of different halogen species found that the stretching mode of the P-S bond shifted to a lower frequency with Cl^-, which may imply closer (and stronger) binding of malathion to the Ag NPs. This concurs with literature reports that halogen ions could facilitate the adsorption of a certain analyte onto the SERS substrate. In addition, hydrogen ions showed a synergistic effect on SERS signal enhancement when combined with chloride anions. At optimum conditions, the malathion could be detected with a limit of detection (LOD) of 3 ppb. Malathion-spiked cherry tomatoes and oranges were analyzed, and the recovery rates were found to be within 85-100%.

DEEP Surveillance of Brain Cancer Using Self-Functionalized 3D Nanoprobes for Noninvasive Liquid Biopsy

Brain cancers, one of the most fatal malignancies, require accurate diagnosis for guided therapeutic intervention. However, conventional methods for brain cancer prognosis (imaging and tissue biopsy) face challenges due to the complex nature and inaccessible anatomy of the brain. Therefore, deep analysis of brain cancer is necessary to (i) detect the presence of a malignant tumor, (ii) identify primary or secondary origin, and (iii) find where the tumor is housed. In order to provide a diagnostic technique with such exhaustive information here, we attempted a liquid biopsy-based deep surveillance of brain cancer using a very minimal amount of blood serum (5 μL) in real time. We hypothesize that holistic analysis of serum can act as a reliable source for deep brain cancer surveillance. To identify minute amounts of tumor-derived material in circulation, we synthesized an ultrasensitive 3D nanosensor, adopted SERS as a diagnostic methodology, and undertook a DEEP neural network-based brain cancer surveillance. Detection of primary and secondary tumor achieved 100% accuracy. Prediction of intracranial tumor location achieved 96% accuracy. This modality of using patient sera for deep surveillance is a promising noninvasive liquid biopsy tool with the potential to complement current brain cancer diagnostic methodologies.

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

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

LSPR Tunable Ag@PDMS SERS Substrate for High Sensitivity and Uniformity Detection of Dye Molecules

At present, the use of efficient and cost-effective methods to construct plasmonic surface-enhanced Raman scattering (SERS) substrates of high sensitivity, uniformity and reproducibility is still crucial to satisfy the practical application of SERS technology. In this paper, a localized surface plasmonic resonance (LSPR) tunable flexible Ag@PDMS substrate was successfully constructed by the low-cost bio-template-stripping method and magnetron sputtering technology. The theory proves that the local electromagnetic field enhancement and "hot spot" distribution is adjustable by modifying the size of the optical cavity unit in the periodicity nanocavity array structure. Experimentally, using rhodamine 6G (R6G) as the target analyte, the SERS performance of optimal Ag@PDMS substrate (Ag film thickness for 315 nm) was researched in detail, which the minimum detection limit was 10^-11 M and the enhancement factor was calculated as 8.03 × 10⁸, indicating its high sensitivity. The relative standard deviation (RSD) was calculated as 10.38%, showing that the prepared substrate had excellent electromagnetic field enhancement uniformity. At last, the trace detection of Crystal violet (CV, LOD = 10^-9 M) and the simultaneous detection of three common dyes (R6G, CV and Methylene blue (MB) mixture) were also realized. This result suggests that the SERS substrate has a good application prospect in the quantitative and qualitative detection of dye molecules.

Ultra-sensitive detection of streptomycin in foods using a novel SERS switch sensor fabricated by AuNRs array and DNA hydrogel embedded with DNAzyme

Detrimental health effects caused by the intake of food contaminated with streptomycin have drawn concerns on effective monitoring using sensitive and selective methods. In this work, a DNA hydrogel surface enhanced Raman spectroscopy (SERS) sensor was successfully developed for the ultrasensitive determination of streptomycin residues in foods. The sensor used a DNA hydrogel containing DNAzyme (Pb-DNAzyme), triggering release of the Raman reporter 4-mercaptobenzonitrile, which was detected using a gold nanorods (AuNRs) array. The linear range of the sensor was 0.01-150 nM and the limit of detection was 4.85 × 10^-3 nM. Tests conducted with four streptomycin structural analogues confirmed the sensor was specific. Milk and honey samples spiked with streptomycin were analysed, resulting in standard recoveries in the range 98.2-117.3%. These findings demonstrated that such a sensor can be used for ultrasensitive detection of streptomycin in foods.

Highly Sensitive, Robust, and Recyclable TiO2/AgNP Substrate for SERS Detection

Label-free biosensors provide an important platform for detecting chemical and biological substances without needing extra labeling agents. Unlike surface-based techniques such as surface plasmon resonance (SPR), interference, and ellipsometry, surface-enhanced Raman spectroscopy (SERS) possesses the advantage of monitoring analytes both on surfaces and in solutions. Increasing the SERS enhancement is crucial to preparing high-quality substrates without quickly losing their stability, sensitivity, and repeatability. However, fabrication methods based on wet chemistry, nanoimprint lithography, spark discharge, and laser ablation have drawbacks of waste of time, complicated processes, or nonreproducibility in surface topography. This study reports the preparation of recyclable TiO₂/Ag nanoparticle (AgNP) substrates by using simple arc ion plating and direct-current (dc) magnetron sputtering technologies. The deposited anatase-phased TiO₂ ensured the photocatalytic degradation of analytes. By measuring the Raman spectra of rhodamine 6G (R6G) in titrated concentrations, a limit of detection (LOD) of 10⁻⁸ M and a SERS enhancement factor (EF) of 1.01 × 10⁹ were attained. Self-cleaning was performed via UV irradiation, and recyclability was achieved after at least five cycles of detection and degradation. The proposed TiO₂/AgNP substrates have the potential to serve as eco-friendly SERS enhancers for label-free detection of various chemical and biological substances.

Bimetallic Gold Nanostars Having High Aspect Ratio Spikes for Sensitive Surface-Enhanced Raman Scattering Sensing

There has been increasing interest in evolution of plasmonic nanoplatforms based on noble metal nanoparticles to achieve ultrasensitive detection of trace analyte molecules through solution-based surface-enhanced Raman spectroscopy (SERS). This work presents a surfactant-free synthesis method of bimetallic gold nanostars coated with silver (BGNS-Ag) having sharp, high aspect-ratio spikes for achieving ultrahigh detection sensitivity and high reproducibility. Specifically, the unique BGNS-Ag platform combines both the strong SERS enhancement effects of gold nanostar sharp spikes and the high scattering feature of the silver-gold bimetallic structure. To achieve SERS reproducibility, this solution-based SERS measurement requires minimal sample preparation without addition of any external reagents, which can cause irregular aggregation of nanoparticles and reduce the reproducibility of SERS measurements. Moreover, we have streamlined our SERS sensing procedure by using standard well-plates and a portable Raman device for SERS measurements, which could be utilized for rapid on-site detection. This solution-based SERS performance was studied using methylene blue (MB) as a model analyte system. The detection limit of MB was as low as 42 pM, indicating high sensitivity of detection using BGNS-Ag. To illustrate the usefulness for environmental sensing, we showed that the SERS sensor can detect a pesticide, thiram, at a concentration as low as 0.8 nM. This study demonstrated that the BGNS-Ag system could serve as an effective and versatile plasmonic-active platform for reproducible, fast, and in-field detection of small organic analytes at trace levels.

Plasmonic enhanced piezoelectric photoresponse with flexible PVDF@Ag-ZnO/Au composite nanofiber membranes

The coordination of piezoelectric and plasmonic effects to regulate the separation and migration of photo-generated carriers is still a significant method to improve the performance of visible-light photoresponse. Herein, we propose the PVDF@Ag-ZnO/Au composite nanofiber membranes utilizing the piezoelectric and plasmonic effects to promote the photocatalytic degradation of organic dyes. Here, ZnO nanorods can generate a built-in electric field under vibration to separate electron-hole pairs. The Schottky junction formed by noble metal/semiconductor can not only inhibit the recombination of photo-generated carriers and accelerate the migration of carriers, but also enhance the utilization of visible light. In addition, the structure has excellent flexibility and easy recycling characteristics. We demonstrate that the plasmonic effect of noble metal can enhance the light response of membranes and broaden light absorption from ultraviolet to visible light region. With the help of the surface-enhanced Raman scattering (SERS), modulation effects of the piezoelectric effect on light response is proved. For catalytic processes, rhodamine B (98.8%) can be almost completely degraded using PVDF@Ag-ZnO/Au within 120 minutes in the piezoelectric photocatalysis process, which is 2.2 and 2.8 times higher than photocatalysis and piezoelectric catalysis, respectively. This work provides a promising strategy for harnessing solar and mechanical energy.

Flexible surface-enhanced Raman scatting substrates: recent advances in their principles, design strategies, diversified material selections and applications

Surface-enhanced Raman scattering (SERS) is widely used as a powerful analytical technology in cutting-edge areas such as food safety, biology, chemistry, and medical diagnosis, providing ultra-fast, ultra-sensitive, nondestructive characterization and achieving ultra-high detection sensitivity even down to the single-molecule level. Development of Raman spectroscopy is strongly dependent on high-performance SERS substrates, which have long evolved from the early days of rough metal electrodes to periodic nanopatterned arrays building on solid supporting substrates. For rigid SERS substrates, however, their applications are restricted by sophisticated pretreatments for detecting solid samples with non-planar surfaces. It is therefore essential to reassert the principles in constructing flexible SERS substrates. Herein, we comprehensively review the state-of-the-art in understanding, preparing and using flexible SERS. The basic mechanisms behind the flexible SERS are briefly outlined, typical design strategies are highlighted and diversified selection of materials in preparing flexible SERS substrates are reviewed. Then the recent achievements of various interdisciplinary applications based on flexible SERS substrates are summarized. Finally, the challenges and perspectives for future evolution of flexible SERS and their applications are demonstrated. We propose new research directions focused on stimulating the real potential of SERS as an advanced analytical technique for commercialization.

Highly Sensitive Exosome Detection for Early Diagnosis of Pancreatic Cancer Using Immunoassay Based on Hierarchical Surface-Enhanced Raman Scattering Substrate

Exosomes have emerged as potential biomarkers for pancreatic cancer (PaC). However, it is still challenging to get quantitative detection of exosomes with the specific surface receptors. In this study, a highly sensitive detection system is first constructed for the direct quantitation of specific exosomes in real samples using hierarchical surface-enhanced Raman scattering substrate (H-SERS substrate) and rapid enrichment strategy magnetic beads @ exosomes @ SERS detection probes (MEDP). It is found that the detection system (MEDP @ H-SERS substrate) could provide a 3.5 times higher SERS intensity compared with MEDP sandwich immunocomplex only. Moreover, LRG1-positive exosomes (LRG1-Exos) and GPC1-positive exosomes (GPC1-Exos) are chosen to distinguish PaC through exosome proteomics and database screening. The lower limit of detection (LOD) is 15 particles µL-1 using the MEDP @ H-SERS substrate. Significantly, the detection in clinical samples shows that the innovative combination of LRG1-Exos and GPC1-Exos could improve the diagnostic efficiency of PaC, with an area under the operating characteristic curve (AUC) of 0.95. Even for the early-stage PaC, the diagnostic accuracy is still high (AUC = 0.95). Collectively, the findings indicate that the MEDP @ H-SERS substrate has great potential for the early diagnosis of PaC.

High Sensitivity and a Wide Sensing Range Flexible Strain Sensor Based on the V-Groove/Wrinkles Hierarchical Array

Flexible strain sensors occupying a large part of human body detection and wearable electronics, which have a wide sensing range and high sensitivity, are crucial in fully monitoring human motion signals. This study proposed a strategy to construct flexible strain sensors based on the V-groove/wrinkles hierarchical array. The V-groove array was prepared on a polydimethylsiloxane (PDMS) substrate through mold transfer printing. The gold film was sputtered on the prestretching PDMS substrate, and the V-groove/wrinkles hierarchical array was formed after strain release. Compared with the sensors based on single-scale wrinkle structures and a V-groove array, the fabricated strain sensor with the hierarchical array showed high sensitivity (maximum gauge factor up to 2,557.71) and a wide sensing range (up to 45%). In addition, the dynamic characteristics of the sensor were investigated in detail, indicating that the sensor had a fast response (less than 130 ms), a low detection limit (0.1% strain), and good stability (almost no performance loss after 10,000 cycles). In practical applications, the sensor was used to detect sizable physical motion and weak physiological signals, demonstrating great potential application value in human motion detection. This study could provide new ideas for preparing high-performance flexible strain sensors.

Hybrid Surface-Enhanced Raman Scattering Substrates for the Trace Detection of Ammonium Nitrate, Thiram, and Nile Blue

We report the fabrication and performance evaluation of hybrid surface-enhanced Raman scattering (SERS) substrates involving laser ablation and chemical routes for the trace-level detection of various analyte molecules. Initially, picosecond laser ablation experiments under ambient conditions were performed on pure silver (Ag) and gold (Au) substrates to achieve distinct nanosized features on the surface. The properties of the generated surface features on laser-processed portions of Ag/Au targets were systematically analyzed using UV-visible reflection and field emission scanning electron microscopy studies. Later, hybrid-SERS substrates were achieved by grafting the chemically synthesized Au nanostars on the plain and laser-processed plasmonic targets. Subsequently, we employed these as SERS platforms for the detection of a pesticide (thiram), a molecule used in explosive compositions [ammonium nitrate (AN)], and a dye molecule [Nile blue (NB)]. A comparative SERS study between the Au nanostar-decorated bare glass, silicon, Ag, Au, and laser-processed Ag and Au targets has been established. Our studies and the obtained data have unambiguously determined that laser-processed Ag structures have demonstrated reasonably good enhancements in the Raman signal intensities for distinct analytes among other substrates. Importantly, the fabricated hybrid SERS substrate of "Au nanostar-decorated laser-processed Ag" exhibited up to eight times enhancement in the SERS intensity compared to laser-processed Ag (without nanostars), as well as up to three times enhancement than the Au nanostar-loaded plain Ag substrates. Additionally, the achieved detection limits from the Au nanostar-decorated laser-processed Ag SERS substrate were ∼50 pM, ∼5 nM, and ∼5 μM for NB, thiram, and AN, respectively. The estimated enhancement factors accomplished from the Au nanostar-decorated laser-processed Ag substrate were ∼106, ∼106, and ∼104 for NB, thiram, and AN, respectively.

Exosome detection via surface-enhanced Raman spectroscopy for cancer diagnosis

As nanoscale extracellular vesicles, exosomes are secreted by various cell types, and they are widely distributed in multiple biological fluids. Studies have shown that tumor-derived exosomes can carry a variety of primary tumor-specific molecules, which may represent a novel tool for the early detection of cancer. However, the clinical translation of exosomes remains a challenge due to the requirement of large quantities of samples when enriching the cancer-related exosomes in biological fluids, the insufficiency of traditional techniques for exosome subpopulations, and the complex exosome isolation of the current commercially available exosome phenotype profiling approaches. The evolving surface-enhanced Raman scattering (SERS) technology, with properties of unique optoelectronics, easy functionalization, and the particular interaction between light and nanoscale metallic materials, can achieve sensitive detection of exosomes without large quantities of samples and multiplexed phenotype profiling, providing a new mode of real-time and noninvasive analysis for cancer patients. In the present review, we mainly discussed exosome detection based on SERS, especially SERS immunoassay. The basic structure and function of exosomes were firstly introduced. Then, recent studies using the SERS technique for cancer detection were critically reviewed, which mainly included various SERS substrates, biological modification of SERS substrates, SERS-based exosome detection, and the combination of SERS and other technologies for cancer diagnosis. This review systematically discussed the essential aspects, limitations, and considerations of applying SERS technology in the detection and analysis of cancer-derived exosomes, which could provide a valuable reference for the early diagnosis of cancer through SERS technology. STATEMENT OF SIGNIFICANCE: Surface-enhanced Raman scattering (SERS) has been applied to exosomes detection to obtain better diagnostic results. In past three years, several reviews have been published in exosome detection, which were narrowly focus on methods of exosome detection. Selection and surface functionalization of the substrate and the combination detection with different methods based on SERS will provide new strategies for the detection of exosomes. This review will focus on the above aspects. This emerging detection method is constantly evolving and contributing to the early discovery of diseases in the future.

Face-to-Face Assembly of Ag Nanoplates on Filter Papers for Pesticide Detection by Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) technology has been regarded as a most efficient and sensitive strategy for the detection of pollutants at ultra-low concentrations. Fabrication of SERS substrates is of key importance in obtaining the homogeneous and sensitive SERS signals. Cellulose filter papers loaded with plasmonic metal NPs are well known as cost-effective and efficient paper-based SERS substrates. In this manuscript, face-to-face assembly of silver nanoplates via solvent-evaporation strategies on the cellulose filter papers has been developed for the SERS substrates. Furthermore, these developed paper-based SERS substrates are utilized for the ultra-sensitive detection of the rhodamine 6G dye and thiram pesticides. Our theoretical studies reveal the creation of high density hotspots, with a huge localized and enhanced electromagnetic field, near the corners of the assembled structures, which justifies the ultrasensitive SERS signal in the fabricated paper-based SERS platform. This work provides an excellent paper-based SERS substrate for practical applications, and one which can also be beneficial to human health and environmental safety.

Influence of Material Microstructure on Machining Characteristics of OFHC Copper C102 in Orthogonal Micro-Turning

Micro-cutting is different from conventional cutting in its mechanics. The workpiece material is not considered to be homogeneous in the micro-cutting process. As a result, it is critical to comprehend how microstructure affects surface integrity, cutting forces, and chip formation. In this paper, we experimented with micro-turning on oxygen-free high-conductivity (OFHC) copper with different microstructures after annealing. Feed rate parameters were smaller than, larger than, and equal to the grain size, respectively. Experimental results show that when the feed rates are equivalent to the grain size, the surface roughness of the machined surface is low and the width of the flake structure on the free surface of chips is minimal, and the explanations for these occurrences are connected to dislocation slip.

Applications of surface-enhanced Raman spectroscopy in environmental detection

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