Efficient Enrichment and Self-Assembly of Hybrid Nanoparticles into Removable and Magnetic SERS Substrates for Sensitive Detection of Environmental Pollutants

A SERS-based point-of-care testing approach for efficient determination of diquat and paraquat residues using a flexible silver flower-coated melamine sponge

Diquat (DQ) and paraquat (PQ) residues in food are potential hazards to consumers' health. Point-of-care testing (POCT) of them remains challenging. Based on surface-enhanced Raman spectroscopy (SERS) technology, we developed a POCT strategy for DQ and PQ on apple surface and in apple juice. A point-of-use composite was fabricated using a piece of porous melamine sponge (MS) modified with silver nanoflowers (AgNFs), combining the specificity of the SERS fingerprint and the excellent adsorption capacity of MS. Using this dual-functional AgNFs@MS, the on-site determination of the DQ and PQ residues was completed within 3 min without pretreatment. Clear trends were observed between SERS intensity and logarithmic concentrations, with r values from 0.962 to 0.984. The limit of detection of DQ and PQ were 0.14-0.70 ppb in apple juice and on apple surface. This study provides a new point-of-use alternative for rapidly detecting DQ and PQ residues in nonlaboratory settings.

Controllable preparation of Ti3C2Tx/Ag composite as SERS substrate for ultrasensitive detection of 4-nitrobenzenethiol

Currently, metal carbonitride (MXene) has been identified as a hot research topic in the research area of surface-enhanced Raman scattering (SERS). In this study, Ti3C2Tx/Ag composite was fabricated as SERS substrate with different Ag contents. The fabricated Ti3C2Tx/Ag composites show good SERS behavior by detecting 4-Nitrobenzenethiol (4-NBT) probe molecules. Through calculation, the SERS enhancement factor (EF) of the Ti3C2Tx/Ag substrate was as high as 4.15 × 106. It is worth noting that the detection limit of 4-NBT probe molecules can be achieved ultralow concentration of 10-11 M. In this system, electromagnetic enhancement mechanism and chemical enhancement mechanism have synergistic effects on SERS phenomenon. Meanwhile, the Ti3C2Tx/Ag composite substrate exhibited good SERS reproducibility. In addition, the SERS detection signal hardly changed after 6 months of natural standing, and the substrate showed good stability. This work suggests that the Ti3C2Tx/Ag substrate could be used as a sensitivity SERS sensor for practical application, and could be applied in the field of environmental monitoring.

High-Yield Gold Nanohydrangeas on Three-Dimensional Carbon Nanotube Foams for Surface-Enhanced Raman Scattering Sensors

Recently, carbon nanomaterial-supported plasmonic nanocrystals used as high-performance surface-enhanced Raman scattering (SERS) substrates have attracted increasing attention due to their ultra-high sensitivity of detection. However, most of the work focuses on the design of 2-D planar substrates with traditional plasmonic structures, such as nanoparticles, nanorods, nanowires, and so forth. Here, we report a novel strategy for the preparation of high-yield Au nanohydrangeas on three-dimensional porous polydopamine (PDA)/polyvinyl alcohol (PVA)/carbon nanotube (CNT) foams. The structures and growth mechanisms of these specific Au nanocrystals are systematically investigated. PDA plays the role of both a reducing agent as well as an anchoring site for Au nanohydrangeas' growth. We also show that the ratio of surfactant KBr to the gold precursor (HAuCl4) is key to obtain these structures in a manner of high production. Moreover, the substrate of the CNT foam-Au nanohydrangea hybrid can be employed as SERS sensors and can detect the analytes down to 10-9 M.

Single versus Double Coffee-Ring Effect Patterns in Thin-Layer Chromatography Coupled with Surface-Enhanced Raman Spectroscopic Analysis of Anti-Diabetic Drugs Adulterated in Herbal Products

In thin-layer chromatography coupled with surface-enhanced Raman spectroscopy (TLC-SERS), the coffee ring effect (CRE) describes the formation of a ring-shape spot (blank in the middle and darker on the edge) caused by the aggregation of silver nanoparticles (Ag NPs), alone (single CRE) or with the analytes (double CRE). In this work, the SCRE and DCRE were investigated in two anti-diabetic drugs, hydrophobic glibenclamide (GLB) and more hydrophilic metformin (MET). The SCRE occurred in GLB analysis, as opposed to the DCRE that occurred in MET. It was proven that for optimization of the TLC-SERS analytical procedure, it is necessary to distinguish the CRE patterns of analytes. Additionally, MET and GLB were analyzed with the developed TLC-SERS method and confirmed by another validated method using high-performance liquid chromatography. Four herbal products collected on the market were found to be adulterated with GLB or/and MET; among those, one product was adulterated with both MET and GLB, and two products were adulterated with GLB at a higher concentration than the usual GLB prescription dose. The TLC-SERS method provided a useful tool for the simultaneous detection of adulterated anti-diabetic herbal products, and the comparison of the SCRE and DCRE provided more evidence to predict CRE patterns in TLC-SERS.

Nanoparticle Self-Assembly: From Design Principles to Complex Matter to Functional Materials

The creation of matter with varying degrees of complexities and desired functions is one of the ultimate targets of self-assembly. The ability to regulate the complex interactions between the individual components is essential in achieving this target. In this direction, the initial success of controlling the pathways and final thermodynamic states of a self-assembly process is promising. Despite the progress made in the field, there has been a growing interest in pushing the limits of self-assembly processes. The main inception of this interest is that the intended self-assembled state, with varying complexities, may not be "at equilibrium (or at global minimum)", rendering free energy minimization unsuitable to form the desired product. Thus, we believe that a thorough understanding of the design principles as well as the ability to predict the outcome of a self-assembly process is essential to form a collection of the next generation of complex matter. The present review highlights the potent role of finely tuned interparticle interactions in nanomaterials to achieve the preferred self-assembled structures with the desired properties. We believe that bringing the design and prediction to nanoparticle self-assembly processes will have a similar effect as retrosynthesis had on the logic of chemical synthesis. Along with the guiding principles, the review gives a summary of the different types of products created from nanoparticle assemblies and the functional properties emerging from them. Finally, we highlight the reasonable expectations from the field and the challenges lying ahead in the creation of complex and evolvable matter.

A Rapid Therapeutic Drug Monitoring Strategy of Carbamazepine in Serum by Using Coffee-Ring Effect Assisted Surface-Enhanced Raman Spectroscopy

Carbamazepine (CBZ) has a narrow therapeutic concentration range, and therapeutic drug monitoring (TDM) is necessary for its safe and effective individualized medication. This study aims to develop a procedure for CBZ detection in serum using coffee-ring effect assisted surface-enhanced Raman spectroscopy (SERS). Silver nanoparticles deposited onto silicon wafers were used as the SERS-active material. Surface treatment optimization of the silicon wafers and the liquid-liquid extraction method were conducted to eliminate the influence of impurities on the silicon wafer surface and the protein matrix. The proposed detection procedure allows for the fast determination of CBZ in artificially spiked serum samples within a concentration range of 2.5-40 μg·mL^-1, which matches the range of the drug concentrations in the serum after oral medication. The limit of detection for CBZ was found to be 0.01 μg·mL^-1. The developed method allowed CBZ and its metabolites to be ultimately distinguished from real serum samples. The developed method is anticipated to be a potential tool for monitoring other drug concentrations.

Recent Progress on Solid Substrates for Surface-Enhanced Raman Spectroscopy Analysis

Surface-enhanced Raman spectroscopy (SERS) is a powerful vibrational spectroscopy technique with distinguished features of non-destructivity, ultra-sensitivity, rapidity, and fingerprint characteristics for analysis and sensors. The SERS signals are mainly dependent on the engineering of high-quality substrates. Recently, solid SERS substrates with diverse forms have been attracting increasing attention due to their promising features, including dense hot spot, high stability, controllable morphology, and convenient portability. Here, we comprehensively review the recent advances made in the field of solid SERS substrates, including their common fabrication methods, basic categories, main features, and representative applications, respectively. Firstly, the main categories of solid SERS substrates, mainly including membrane substrate, self-assembled substrate, chip substrate, magnetic solid substrate, and other solid substrate, are introduced in detail, as well as corresponding construction strategies and main features. Secondly, the typical applications of solid SERS substrates in bio-analysis, food safety analysis, environment analysis, and other analyses are briefly reviewed. Finally, the challenges and perspectives of solid SERS substrates, including analytical performance improvement and largescale production level enhancement, are proposed.

RhB-encapsulated metal-organic cage as a dual-emission fluorescence sensor for detection of malachite green and glycine

RhB@ZrT-1-OH composite was constructed by introduction of Rhodamine B (RhB) into the cages of zirconium-based metal-organic cage that had two fluorescence emission peaks at 466 and 612 nm upon excitation at 327 nm. The dual-emission fluorescence sensor exhibits ultra-high sensitive detection for malachite green (MG) and glycine (Gly) in phosphate buffer solution (pH = 6.86). RhB@ZrT-1-OH as a ratiometric fluorescence probe was applied to detect MG with a low LOD of 0.2879 μM and presented obvious fluorescence visual changes from orange to purple to blue under 254 nm UV-vis lamp. Moreover, RhB@ZrT-1-OH also can be utilized as a "turn-on" fluorescence sensor to recognize Gly with a low LOD of 0.3747 μM and exhibits fluorescence color changes from orange to pink to purple. Notably, the corresponding test papers for sensing MG and Gly were designed for recognize the concentration of MG and Gly. Furthermore, the dual-emission fluorescence sensor can be used to detect MG and Gly in fish and human serum with high sensitivity and reliable. The possible detecting mechanisms of RhB@ZrT-1-OH for sensing MG and Gly were detailedly explored.

Hybridizing Silver Nanoparticles in Hydrogel for High-Performance Flexible SERS Chips

An ideal surface-enhanced Raman scattering (SERS) substrate should have high sensitivity, long-term stability, excellent repeatability, and strong anti-interference. In the present work, single-layer carbon-based dot (CD)-capped Ag nanoparticle aggregates (a-AgNPs/CDs) with high SERS activity are synthesized and hybridized with a hydrogel to prepare novel hydrogel SERS chips. Benefiting from the unique properties of a-AgNPs/CDs and the hydrogel, the constructed hydrogel SERS chips show excellent performances. Taking crystal violet detection as an example, the hydrogel SERS chips show a detection limit of around 1 × 10^-16 mol/L (high sensitivity), maintain above 96.40% of SERS activity even after 14 weeks of storage (long-term stability), and display point-to-point relative standard deviation (RSD) in one chip as low as 1.43% (outstanding repeatability) and RSD in different chips as low as 2.75% (excellent reproducibility). Furthermore, the self-extraction effect of the hydrogel enables the flexible hydrogel SERS chips to be used for analyzing various real samples including soybean milk, juices, and fruits without any complex pretreatment. For instance, the hydrogel SERS chips are able to detect trace thiram and 2-(4-thiazolyl)benzimidazole with the detection limits of 1 and 5 ppb in liquid samples, respectively, and of 1 and 2.5 ng/cm² on the peel of fruits, respectively. The self-extraction functional flexible SERS chips offer a reliable and convenient platform for the quick detection and on-site monitoring of chemical contaminants.

Iridescent Colloidal Crystals Composed of SiO2 Porous Hollow Sphere for SERS Application

In recent years, the application of low-refractive-index materials in the optical field has attracted considerable attention due to it high transmittance and high optical sensitivity. In this study, we synthesized SiO₂ porous hollow spheres (SPHS) with an ultralow refractive index (n = 1.05) by using a templating method. Their refractive indices could be easily controlled from 1.05 to 1.08 by tuning the thickness of shell. In addition, a droplet coatings method is proposed for SPHS colloidal crystal (CC) by controlling the temperature and humidity. The SPHS CCs displayed distinct structural colors when the incident angle was adjusted and demonstrated high angular resolution. Moreover, the iridescent color changes could be observed with the naked eye. For surface-enhanced Raman scattering application, more analyte could be absorbed by the porous shells, and metal nanoparticles were coated on the SPHSs surface to increase the hot spot density for improving the SERS intensity.

Facile and robust fabrication of hierarchical Au nanorods/Ag nanowire SERS substrates for the sensitive detection of dyes and pesticides

Surface enhanced Raman spectroscopy (SERS) has emerged as a promising tool for the rapid and ultrasensitive recognition of trace amounts of environmental pollutants. Hierarchical SERS substrates usually show superior performance to single-component substrates but require complicated preparation protocols. Herein, a facile, robust and low-cost route for the fabrication of hierarchical SERS substrates has been reported, in which no complicated laborious protocols or sophisticated equipment is needed. In the hierarchical SERS substrate, Au nanorods were distributed onto the network of Ag nanowires through evaporation induced self-assembly. The density of the Au nanorods and Ag nanowires could be easily tailored by tuning the number of droplets of gold nanorod solution and the concentration of silver nanowire solution. The nanogaps formed between Au nanorods and Ag nanowires were able to induce a rich enhanced electromagnetic field area via localized surface plasmon resonances and surface plasmon polaritons to achieve amplification of the Raman signal. The as-prepared substrate showed high uniformity and was capable of identifying 10^-12 M rhodamine 6G, 10^-10 M thiram and 10^-10 M crystal violet, with correlation coefficients (R²) all higher than 0.98. This approach can be employed for the detection of trace dyes, pesticides or other environmental pollutants with high sensitivity and uniformity.

A wearable screen-printed SERS array sensor on fire-retardant fibre gloves for on-site environmental emergency monitoring

Glove-based wearable sensors can offer the potential ability to a fast and on-site environmental threat assessment, which is crucial for timely and informed incident management. In this study, an on-demand surface-enhanced Raman scattering (SERS) array sensor has been patterned on fire-retardant fibre gloves via the screen-printing technique in large batches. The screen-printed ink contains one-pot synthesized silver nanoparticle and molybdenum disulfide nanocomposite (Ag/MoS₂), and polyanionic cellulose (PAC) as a new adhesive agent. Rhodamine 6G (R6G) was employed as an initial probe molecule to systematically evaluate the performance of the resulting sensor. The results suggest that the fabricated fire-retardant screen-printed SERS array sensor displays high reproducibility and stability at 250 °C, with the lower detection limit of 10^-13 M for R6G. The spot-to-spot SERS signals show that the intensity variation was less than 10%. Besides, the SERS signals can be maintained over 7 weeks. Further investigation was then successfully carried out to detect polycyclic aromatic hydrocarbons (PAHs), which are commonly used as flammable chemicals. In our perception, this wearable fire-retardant screen-printed SERS array sensor would be an ideal candidate for practical on-site environmental emergency monitoring due to its fire-retardant capability and timely measurement on a portable carrier.

Layered filter paper-silver nanoparticle-ZIF-8 composite for efficient multi-mode enrichment and sensitive SERS detection of thiram

A SERS substrate FP/Ag/ZIF-8 composed of filter paper (FP), silver nanoparticles (AgNPs) and zeolitic imidazolate framework (ZIF-8) film arranged in a layered structure was developed for sensitive detection of pesticide thiram in various samples. Roles of these components in analyte adsorption and Raman signal enhancement were studied using a pesticide intermediate 4-Aminothiophenol (4-ATP) as the probe. The substrate showed high adsorption and optimized SERS response with thick metal organic framework (MOF) coating (125 nm), which is different from previous reported plasmonic particle-MOF composite substrate, where thinnest MOF coating produced the strongest SERS signal. Detection limit for 4-ATP improved 1000-fold on FP/Ag/ZIF-8 (3 pM) compared with that on FP/Ag (3 nM). Importantly, the FP/Ag/ZIF-8 with porous and flexible property can efficiently capture pesticide thiram in different real samples using soaking, filtration or swabbing operation. The subsequent SERS detection of thiram showed advantages of low detection limit (soaking, LOD: 0.04 nM in lake water), fast detection (filtration, within 1 min in peach juice) and suitable for curve surface analysis (swabbing, LOD: 0.1 ng/cm² on apple peel), respectively. The substrate also displayed good reproducibility, high stability and size-selective response for thiram detection. Such a layered plasmonic particle/MOF hybrid may hold great promise for toxicant analysis in environment and food.

Interface engineering with self-assembling Au@Ag@β-cyclodextrin bimetal nanoparticles to fabricate a ring-like arrayed SERS substrate for sensitive recognition of phthalate esters based on a host-guest interaction and the coffee ring effect

Herein, Au@Ag@β-cyclodextrin (β-CD) nanoparticles with a relatively uniform shape and size of ∼13 nm in diameter have been successfully synthesized, and the surfaces of the synthesized nanoparticles are successful modified by β-CD. A highly efficient synthetic approach was developed for the preparation of a surface-enhanced Raman spectroscopy (SERS) substrate, which self-assembles Au@Ag@β-CD nanoparticles and analytes into a coffee ring pattern via the coffee ring effect. The coffee ring effect can make phthalates (PAEs) aggregate to the edge together with the Au@Ag@β-CD nanoparticles and concentration enrichment can be achieved. In addition, the surface of the core-shell Au@Ag@β-CD is modified with β-CD with a cavity structure, which can enrich analyte concentration by adsorbing the analytes into the hydrophobic cavity using host-guest recognition. This enrichment process not only improves the concentration of the surface of the analyte but also effectively distinguishes it from other substances in the analyte solution. The mechanism of enrichment and host-guest recognition is verified by subsequent molecular docking simulation. Thus, a ring-like arrayed SERS substrate with dual-strategy enrichment is used to detect PAEs. The experiments using the ring-like arrayed SERS substrate, gave a limit of detection of 0.2 nM for DOP detection, the recovery rate of the spiked samples ranged from 92.3% to 106.6%, and an RSD of less than 6% for PAE detection is obtained. This work provided a simple, rapid, low-cost, highly sensitive and stable method for PAE detection in life and the environment.

Ultra-rapid and highly efficient enrichment of organic pollutants via magnetic mesoporous nanosponge for ultrasensitive nanosensors

Currently, owing to the single-molecule-level sensitivity and highly informative spectroscopic characteristics, surface-enhanced Raman scattering (SERS) is regarded as the most direct and effective detection technique. However, SERS still faces several challenges in its practical applications, such as the complex matrix interferences, and low sensitivity to the molecules of intrinsic small cross-sections or weak affinity to the surface of metals. Here, we show an enrichment-typed sensing strategy with both excellent selectivity and ultrahigh detection sensitivity based on a powerful porous composite material, called mesoporous nanosponge. The nanosponge consists of porous β-cyclodextrin polymers immobilized with magnetic NPs, demonstrating remarkable capability of effective and fast removal of organic micropollutants, e.g., ~90% removal efficiency within ~1 min, and an enrichment factor up to ~10³. By means of this current enrichment strategy, the limit of detection for typical organic pollutants can be significantly improved by 2~3 orders of magnitude. Consequently, the current enrichment strategy is proved to be applicable in a variety of fields for portable and fast detection, such as Raman and fluorescent sensing.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Combination of bacitracin-based flocculant and surface enhanced Raman scattering labels for flocculation, identification and sterilization of multiple bacteria in water treatment

Bacteria, especially antibiotic-resistant bacteria, in water threaten public health in countries. Simultaneous flocculation, sterilization and identification of bacteria are great challenge in water treatment. Herein we presented a three-in-one compound through combining a novel Bacitracin-based flocculant (B-g-PAMDAC) and surface enhanced Raman scattering (SERS) labels, the modified Au@AgNPs using graphene oxide (GO) and 4-mercaptophenylboronic acid (4-MPBA). B-g-PAMDAC with bactericidal groups and microblock structure was synthesized via copolymerization and self-assembly. Its functional groups and microblock structure contributed to the excellent performance in flocculation of bacteria. 4-MPBA as bacterial capture bound to the bacterial cell membrane and contributed to recognition of bacteria in flocculation. Bacteria aggregating around Au@AgNPs resulted in abundant "hot spots" and strong Raman signals. SERS labels obviously improved the sensitivity, accuracy and stability of bacteria identification even at low bacterial concentration of 1 × 10³ CFU mL^-1. They presented distinct fingerprints of bacteria, Escherichia coli, Pseudomonas aeruginosa, Bacillus cereus and Enterococcus faecalis, in Raman mappings. Bacitracin improved sterilization efficiency of B-g-PAMDAC in four bacteria treatment in terms of sterilization rate and time. β-galactosidase and respiratory activity of bacteria revealed sterilization mechanism of B-g-PAMDAC that changed permeability of cell membrane before it reduced the respiration activity of bacteria and ruptured cell wall.

Simultaneous SERS detection of illegal food additives rhodamine B and basic orange II based on Au nanorod-incorporated melamine foam

In food safety assessment, surface-enhanced Raman spectroscopy (SERS) is a novel detection method with the advantages of being fast, easy, and of high sensitivity. However, many SERS substrate synthesis methods are complex, and there are only a few studies on the simultaneous detection of multiple substances. In this study, a new, simple, low-cost SERS substrate was synthesised for the first time to simultaneously detect illegal food additives rhodamine B and basic orange II in chilli products. A lightweight, porous, and low-cost material of melamine foam (MF) was employed as the SERS synthesis template. The substrate's SERS effect on, and sensitivity to, rhodamine B and basic orange II were demonstrated. The molecular vibration and SERS enhancement mechanisms of the two target molecules were analysed by density functional theory (DFT) calculations. The results reveal that this fabricated substrate has great application potential for the supervision and testing industry.

Development of affinity between target analytes and substrates in surface enhanced Raman spectroscopy for environmental pollutant detection

Environmental pollution has long been a social concern due to the variety of pollutants and their wide distribution, persistence and being detrimental to health. It is therefore necessary to develop rapid and sensitive strategies to trace and detect these compounds. Among various detection methodologies, surface enhanced Raman spectroscopy (SERS) has become an attractive option as it enables accurate analyte identification, simple sample preparation, rapid detection and ultra-high sensitivity without any interference from water. For SERS detection, an essential yet challenging step is the effective capture of target analytes onto the surface of metal nanostructures with a high intensity of enhanced electromagnetic field. This review has systematically summarized recent advances in developing affinity between targets and the surface of SERS substrates via direct adsorption, hydrophobic functional groups, boronate affinity, metal organic frameworks (MOFs), DNA aptamers and molecularly imprinted polymers (MIPs). At the end of this review, technical limitations and outlook have been provided, with suggestions on optimizing SERS techniques for real-world applications in environmental pollutant detection.

Ratiometric Sensing of Polycyclic Aromatic Hydrocarbons Using Capturing Ligand Functionalized Mesoporous Au Nanoparticles as a Surface-Enhanced Raman Scattering Substrate

The absorption behavior between plasmonic nanostructures and a target molecule plays key roles in effective surface-enhanced Raman scattering (SERS) detection. However, for analytes with low surface affinity to the metallic surface, e.g., polycyclic aromatic hydrocarbons (PAHs), it remains challenging to observe the enhanced Raman signal. In this work, we reported a ratiometric SERS strategy for sensitive PAH detection through the surface functionalization of 3D ordered mesoporous Au nanoparticles (meso-Au NPs). By employing mono-6-thio-β-cyclodextrin (HS-β-CD) as capture ligands, the hydrophobic molecules, e.g., anthracene, could be effectively absorbed on the meso-Au NP surface via a host-guest interaction. Besides, a hydrophobic slippery surface is used as a concentrator to deliver and enrich the Au/analyte droplets into a small area. Consequently, the detection limits of anthracene and naphthalene are down to 1 and 10 ppb. The improved SERS enhancement is mainly ascribed to the host-guest effect of HS-β-CD ligands, large surface area and high-density of sub-10 nm mesopores of Au networks, as well as the enrichment effect of hydrophobic slippery surface. Moreover, the HS-β-CD (480 cm^-1 band) could serve as an internal standard, leading to the ratiometric determination of anthracene ranging from 1 ppm to 1 ppb. The proposed surface modification strategy in combination with the hydrophobic slippery surface shows great potential for active capture and trace detection of persistent organic pollutants in real-world SERS applications.

A Versatile SERS Sensor for Multiple Determinations of Polycyclic Aromatic Hydrocarbons and Its Application Potential in Analysis of Fried Foods

Polycyclic aromatic hydrocarbons (PAHs), due to their high hydrophobicity, have low affinity for metallic SERS-active surfaces, which leads to their low SERS detection sensitivity. Various functional groups have been used to improve the affinity of metallic substrates towards the target PAHs. However, a large portion of the signals generated from the “first-layer effect” of the functionalized substrates may complicate the spectrum, leading to a distortion in the assignment of the intrinsic SERS fingerprints of PAHs. In this study, a SERS sensor composed of Au nanoparticles (AuNPs) and reoxidized graphene oxide (rGO) was developed for the simultaneous determination of 16 EPA priority PAHs. The synthesis of the rGO/AuNP substrate can be realized without a complicated modification process. All the 16 PAHs could be identified based on their characteristic peaks in the presence of the composited substrate, with estimated LOD as low as 0.2–2 ng·mL⁻¹. The binary linear regression was optimized as the fitting model for all PAHs except for benzo(k)fluoranthene, with the linear correlation coefficient ranging from 0.9889 to 0.9997. Based on the developed SERS substrates and sample pretreatment, the characteristic SERS peaks of four PAHs in Chinese traditional fried food (youtiao) were identified without any background interference. The whole detection process only takes approximately 15 minutes. The results demonstrate the potential of the multicomponent on-field detection of PAHs.

Hotspots on the Move: Active Molecular Enrichment by Hierarchically Structured Micromotors for Ultrasensitive SERS Sensing

Surface-enhanced Raman scattering (SERS) is recognized as one of the most sensitive spectroscopic tools for chemical and biological detections. Hotspots engineering has expedited promotion of SERS performance over the past few decades. Recently, molecular enrichment has proven to be another effective approach to improve the SERS performance. In this work, we propose a concept of "motile hotspots" to realize ultrasensitive SERS sensing by combining hotspots engineering and active molecular enrichment. High-density plasmonic nanostructure-supporting hotspots are assembled on the tubular outer wall of micromotors via nanoimprint and rolling origami techniques. The dense hotspots carried on these hierarchically structured micromotors (HSMs) can be magnet-powered to actively enrich molecules in fluid. The active enrichment manner of HSMs is revealed to be effective in accelerating the process of molecular adsorption. Consequently, SERS intensity increases significantly because of more molecules being adjacent to the hotspots after active molecular enrichment. This "motile hotspots" concept provides a synergistical approach in constructing a SERS platform with high performance. Moreover, the newly developed construction method of HSMs manifests the possibility of tailoring tubular length and diameter as well as surface patterns on the outer wall of HSMs, demonstrating good flexibility in constructing customized micromotors for various applications.

A Europium-based MOF Fluorescent Probe for Efficiently Detecting Malachite Green and Uric Acid

Lanthanide (such as Tb and Eu) metal-organic frameworks (MOFs) have been widely used in fluorescent probes because of their multiple coordination modes and brilliant fluorescence characteristic. Many lanthanide MOFs were applied in detecting metal ions, inorganic anions, and small molecules. However, it's rarely reported that Ln-MOF was devoted to detecting malachite green (MG) and uric acid (UA). We prepared a europium-based metal-organic framework (Eu-TDA) (TDA = 2,5-thiophenedicarboxylic acid group). Luminescence studies demonstrated that Eu-TDA can rapidly detect MG and UA with excellent selectivity and sensitivity, where individual quenching efficiency K_sv (MG: 5.8 × 10⁵ M^-1; UA: 4.15 × 10⁴ M^-1) and detection limit (MG: 0.0221 μM; UA: 0.689 μM) were regarded as the excellent MOF sensors for detecting MG and UA. The quenching of Eu-TDA's fluorescence emission by MG and UA was likely due to the spectral overlap, energy transfer, and competition. Among 11 metal cations and 14 anions, Eu-TDA can quickly and effectively recognize MG and UA with highly selective and sensitive properties. Our method possesses potential application in detecting UA in human blood and MG in the fishpond.

Self-assembled "bridge" substance for organochlorine pesticides detection in solution based on Surface Enhanced Raman Scattering

Pesticide residues pose a great threat to human health, and it is an urgent matter to realize fast and accurate detection of pesticide. SERS (Surface Enhanced Raman Scattering), as a nondestructive detection technology, performs a prominent role in fast detection field due to the strong surface plasmon resonance from short range effect between analyte and nanoparticle. Therefore, in order to solve the incompatibility between organochlorine pesticides molecules and noble metal nanoparticles, this paper proposed a concept of "bridge" substances acting as an interconnect function role to achieve a binding model (object-binder-metal (OBM)) and developed a droplet concentration method to enhance Raman signals. Both combination mode of pesticide molecules to bridge molecules and energy transfer of SERS experiment may relate to the compound ring according to the changes of peaks based on surface plasmon resonance. The selectivity and stability of different bridge substances interacting with pesticides molecules were illumined via binding energy of these two substances obtained by DFT calculations. A droplet can capture nanoparticles and analytes, which is conducive to SERS performance. Chloride ions in the solution contribute to rearrangement of nanoparticles and can validly promote surface activation of Ag nanoparticles to improve energy transfer efficiency of plasma resonance, resulting in superior SERS effect.

Controlling the Morphologies of Silver Aggregates by Laser-Induced Synthesis for Optimal SERS Detection

Controlling the synthesis of metallic nanostructures for high quality surface-enhanced Raman scattering (SERS) materials has long been a central task of nanoscience and nanotechnology. In this work, silver aggregates with different surface morphologies were controllably synthesized on a glass-solution interface via a facile laser-induced reduction method. By correlating the surface morphologies with their SERS abilities, optimal parameters (laser power and irradiation time) for SERS aggregates synthesis were obtained. Importantly, the characteristics for largest near-field enhancement were identified, which are closely packed nanorice and flake structures with abundant surface roughness. These can generate numerous hot spots with huge enhancement in nanogaps and rough surface. These results provide an understanding of the correlation between morphologies and SERS performance, and could be helpful for developing optimal and applicable SERS materials.

3D aluminum/silver hierarchical nanostructure with large areas of dense hot spots for surface-enhanced raman scattering

Plasmonic nanomaterials possessing large-volume, high-density hot spots with high field enhancement are highly desirable for ultrasensitive surface-enhanced Raman scattering (SERS) sensing. However, many as-prepared plasmonic nanomaterials are limited in available dense hot spots and in sample size, which greatly hinder their wide applications in SERS devices. Here, we develop a two-step physical deposition protocol and successfully fabricate 3D hierarchical nanostructures with highly dense hot spots across a large scale (6 × 6 cm² ). The nanopatterned aluminum film was first prepared by thermal evaporation process, which can provide 3D quasi-periodic cloud-like nanostructure arrays suitable for noble metal deposition; then a large number of silver nanoparticles with controllable shape and size were decorated onto the alumina layer surfaces by laser molecular beam epitaxy, which can realize large-area accessible dense hot spots. The optimized 3D-structured SERS substrate exhibits high-quality detection performance with excellent reproducibility (13.1 and 17.1%), whose LOD of rhodamine 6G molecules was 10^-9 M. Furthermore, the as-prepared 3D aluminum/silver SERS substrate was applied in detection of melamine with the concentration down to 10^-7 M and direct detection of melamine in infant formula solution with the concentration as low 10 mg/L. Such method to realize large-area hierarchical nanostructures can greatly simplify the fabrication procedure for 3D SERS platforms, and should be of technological significance in mass production of SERS-based sensors.

Electrostatic Self-Assembly of Ti3C2Tx MXene and Gold Nanorods as an Efficient Surface-Enhanced Raman Scattering Platform for Reliable and High-Sensitivity Determination of Organic Pollutants

A reliable surface-enhanced Raman scattering (SERS) substrate composed of two-dimensional (2D) MXene (Ti₃C₂T_x) nanosheets and gold nanorods (AuNRs) is designed and fabricated for sensitive detection of organic pollutants. The AuNRs are uniformly distributed on the surface of the 2D MXene nanosheets because of the strong electrostatic interactions, forming abundant SERS hot spots. The MXene/AuNR SERS substrate exhibits high sensitivity and excellent reproducibility in the determination of common organic dyes such as rhodamine 6G, crystal violet, and malachite green. The detection limits are 1 × 10^-12, 1 × 10^-12, and 1 × 10^-10 M, and relative standard deviations determined from 13 areas on each sample are 18.1, 10.1, and 15.6%, respectively. In the determination of more complex organic pesticides and pollutants, the substrate also shows excellent sensitivity and quantitative detection, and the detection limits for thiram and diquat of 1 × 10^-10 and 1 × 10^-8 M, respectively, are much lower than the contaminant levels stipulated by the US Environmental Protection Agency. The MXene/AuNR composite constitutes an efficient SERS platform for reliable and high-sensitivity environmental analysis and food safety monitoring.

Template growth of Au/Ag nanocomposites on phosphorene for sensitive SERS detection of pesticides

We report template growth and dense packing of noble metallic nanoparticles (NPs) on few-layer phosphorene for sensitive surface-enhanced Raman scattering (SERS) detection. Phosphorene obtained by electrochemical exfoliation serves as both the template and reductant in the fabrication of noble metallic NPs which are dispersed on phosphorene without aggregation or pile-up. The BP/Ag/Au and BP/Au/Ag nanocomposites with a nanogap structure exhibit excellent SERS sensitivity and reproducibility with respect to Rhodamine 6G. The BP/Au/Ag nanocomposite is further utilized as the SERS substrate for the detection of two fungicides, thiabendazole and thiram, and detection limits of 10^-7 and 10^-8 M are achieved. Template growth of Au/Ag nanocomposites is demonstrated to be a green strategy for the fabrication of SERS substrates and has large potential in applications such as pesticide detection.

Applications of magnetic nanoparticles in surface-enhanced Raman scattering (SERS) detection of environmental pollutants

Environmental pollution, a major problem worldwide, poses considerable threat to human health and ecological environment. Efficient and reliable detection technologies, which focus on the appearance of emerging environmental and trace pollutants, are urgently needed. Surface-enhanced Raman scattering (SERS) has become an attractive analytical tool for sensing trace targets in environmental field because of its inherent molecular fingerprint specificity and high sensitivity. In this review, we focused on the recent developments in the integration of magnetic nanoparticles (MNPs) with SERS for facilitating sensitive detection of environmental pollutants. An overview and classification of different types of MNPs for SERS detection were initially provided, enabling us to categorize the huge amount of literature that was available in the interdisciplinary research field of MNPs based SERS technology. Then, the basic working principles and applications of MNPs in SERS detection were presented. Subsequently, the detection technologies integrating MNPs with SERS that eventually were used for the detection of various environmental pollutions were reviewed. Finally, the advantages of MNP-basedSERS detection technology for environmental pollutants were concluded, and the current challenges and future outlook of this technology in practical applications were highlighted. The application of the MNPs-basedSERS techniques for environmental analysis will be significantly advanced with the great progresses of the nanotechnologies, optics, and materials.

Ratiometric SERS detection of polycyclic aromatic hydrocarbons assisted by β-cyclodextrin-modified gold nanoparticles

A surface-enhanced Raman scattering (SERS) method is described for the determination of trace polycyclic aromatic hydrocarbons (PAHs) in the environment efficiently and economically. Detection sensitivity is improved by modifying gold nanoparticles (AuNPs) with 4-mercaptophenylboronic acid (4-MPBA) conjugated to β-cyclodextrin (β-CD) as a new method for ratiometric determination of PAHs in solution. Pyrene (with a Raman band at 580 cm^-1) and anthracene (750 cm^-1) were used as the model analytes, while 4-MPBA (1570 cm^-1) was used as the internal reference to normalize the SERS signals. The intensity ratios of pyrene/4-MPBA increase linearly in the 2 to 10 nM pyrene concentration range, and the intensity ratios of anthracene/4-MPBA increase linearly in the 10 to 100 nM anthracene concentration range. The detection limits are 0.4 nM for pyrene and 4.4 nM for anthracene. This method was applied to the determination of the two analytes in soil sample extracts and the recoveries of pyrene (at levels of 236 ng∙g^-1 and 170 ng∙g^-1) and anthracene (334 ng∙g^-1 and 510 ng∙g^-1) agreed well with the results from GC-MS analyses. The good recovery rates (101.8% and 102.5% for pyrene and 106.4% and 101.7% for anthracene) confirmed the reliability of the method. Graphical abstract Schematic illustration of SERS signal enhancement of pyrene, as an example of polycyclic aromatic hydrocarbons, by β-cyclodextrin modified gold nanoparticles.

Magnetite-Supported Gold Nanostars for the Uptake and SERS Detection of Tetracycline

Magnetite nanoparticles (MNPs) decorated with gold nanostars (AuNSs) have been prepared by using a seed growth method without the addition of surfactants or colloidal stabilizers. The hybrid nanomaterials were investigated as adsorbents for the uptake of tetracycline (TC) from aqueous solutions and subsequent detection using surface-enhanced Raman scattering (SERS). Several parameters were investigated in order to optimize the performance of these hybrid platforms on the uptake and SERS detection of TC, including variable pH values and the effect of contact time on the removal of TC. The spatial distribution of TC and AuNS on the hybrid composites was accomplished by coupling SERS analysis with Raman imaging studies, allowing also for the determination of the detection limit for TC when dissolved in ultrapure water (10 nM) and in more complex aqueous matrices (1 μM). Attempts were also made to investigate the adsorption modes of the TC molecules at the surface of the metal NPs by taking into account the enhancement of the Raman bands in these different matrices.

Grating-like SERS substrate with tunable gaps based on nanorough Ag nanoislands/moth wing scale arrays for quantitative detection of cypermethrin

Considering the complexity and high-consumption of the existing approaches to fabricate three-dimensional (3D) regular substrate templates, the scales of the moth wings with evenly-distributed nanoarrays were discovered to provide an ideal bioscaffold for metal silver (Ag) to decorate on to fabricate a flexible, highly-ordered, low-cost and large-scale Ag nanoislands/moth wing (Ag/MW) SERS-active substrate. The grating-like substrate with the optimal morphology of rough and hierarchical Ag nanoislands exhibited high enhancement factor (EF, ~4.16 × 10⁵), low detection limit (10^-10 M) to 4-aminothiophenol (4-ATP), outstanding signal uniformity (the relative standard deviations were less than 15%) and superior identification performance in the quantitative detection of pesticide cypermethrin. The three-dimensional finite-difference time-domain (3D-FDTD) method simulated the spatial distribution of the electric field intensity in the substrates with different morphologies, showing a potential strong enhancement of Raman signals in sub-10 nm gaps between two adjacent Ag nanoislands of different layers. These prominent SERS properties of novel Ag/MW SERS-active substrates suggest their potential value in rapid on-side biological and chemical sensing. Meanwhile, the highly-ordered nanoarrays of moth wings provide a new idea for the preparation of regular biomimetic nanomaterials.

Surface-Enhanced Raman Scattering-Active Substrate Prepared with New Plasmon-Activated Water

Conventionally, reactions in aqueous solutions are prepared using deionized (DI) water, the properties of which are related to inert "bulk water" comprising a tetrahedral hydrogen-bonded network. In this work, we demonstrate the distinguished benefits of using in situ plasmon-activated water (PAW) with reduced hydrogen bonds instead of DI water in electrochemical reactions, which generally are governed by diffusion and kinetic controls. Compared with DI water-based systems, the diffusion coefficient and the electron-transfer rate constant of K₃Fe(CN)₆ in PAW in situ can be increased by ca. 35 and 15%, respectively. These advantages are responsible for the improved performance of surface-enhanced Raman scattering (SERS). On the basis of PAW in situ, the SERS enhancement of twofold higher intensity of rhodamine 6G and the corresponding low relative standard deviation of 5%, which is comparable to and even better than those based on complicated processes shown in the literature, are encouraging.

Antipsychotic drug poisoning monitoring of clozapine in urine by using coffee ring effect based surface-enhanced Raman spectroscopy

Antipsychotics are the drugs most often involved in drug poisoning cases, and therefore, therapeutic drug monitoring (TDM) is necessary for safe and effective medication administration of these drugs. In this study, a coffee ring effect-based surface-enhanced Raman spectroscopy (CRE-SERS) method was developed and successfully used to monitor antipsychotic poisoning by using urine samples for the first time. The established method exhibited excellent SERS performance since more hot spots were obtained in the "coffee ring". Using the optimized CRE-SERS method, the sensitivity was improved one order more than that of the conventional method with reasonable reproducibility. The antipsychotic drug clozapine (CLO) spiked into urine samples at 0.5-50 μg mL^-1 was quantitatively detected, at concentrations above the thresholds for toxicity. The CRE-SERS method allowed CLO and its metabolites to be ultimately distinguished from real poisoning urine samples. The coffee-ring effect would provide more opportunities for practical applications of the SERS-based method. The frequent occurrence of drug poisoning may have created a new area for the application of the CRE-SERS method. It is anticipated that the developed method will also have great potential for other drug poisoning monitoring.

Highly Sensitive, Uniform, and Reproducible Surface-Enhanced Raman Spectroscopy Substrate with Nanometer-Scale Quasi-periodic Nanostructures

We introduce a simple and cost-effective approach for fabrication of effective surface-enhanced Raman spectroscopy (SERS) substrates. It is shown that the as-fabricated substrates show excellent SERS effects in various probe molecules with high sensitivity, that is, picomolar level detection, and also good reliability. With a SERS enhancement factor beyond 10⁸ and excellent reproducibility (deviation less than 5%) of signal intensity, the fabrication of the SERS substrate is realized on a four-inch wafer and proven to be effective in pesticide residue detection. The SERS substrate is realized first through the fabrication of quasi-periodic nanostructured silicon with dimension features in tens of nanometers using superaligned carbon nanotubes networks as an etching mask, after which a large amount of hot spots with nanometer gaps are formed through deposition of a gold film. With rigorous nanostructure design, the enhanced performance of electromagnetic field distribution for nanostructures is optimized. With the advantage of cost-effective large-area preparation, it is believed that the as-fabricated SERS substrate could be used in a wide variety of actual applications where detection of trace amounts is necessary.