Facile on-site detection of substituted aromatic pollutants in water using thin layer chromatography combined with surface-enhanced Raman spectroscopy

Multi-channel surface-enhanced Raman spectroscopy (SERS) platform for pollutant detection in water fabricated on polydimethylsiloxane

A miniature multi-channel surface-enhanced Raman scattering (SERS) sensor based on polydimethylsiloxane (PDMS) is constructed to achieve rapid delivery of polluted water and specific identification of multiple components. Hg2+, organic pollutants, and sodium nitrite are successfully identified by the multi-channel SERS sensor using Cy5, cyclodextrin, and urea in the corresponding detection area. This multi-channel sensor exhibits excellent sensitivity and specificity, with detection limits of 3.2 × 10-10 M for Hg2+, 1.0 × 10-8 M for aniline, 6.9 × 10-9 M for diphenylamine, 9.1 × 10-8 M for PCB-77, and 7.5 × 10-9 M for pyrene, and 5.0 × 10-7 M for sodium nitrite. Compared with traditional analysis techniques, this method exhibited excellent recovery for the water pollutants ranging from 82.1 to 115.8%. The PDMS-based microchannel allows for simultaneous and rapid identification of multiple environmental pollutants, offering a portable detection method for emergency testing of environmental pollutants and routine determination of water pollutants.

Decoration of Ag nanoparticle on MXene sheets for SERS studies

MXene sheets with the unique electrical and optical properties show the excellent potential for surface-enhanced Raman spectroscopy (SERS) applications. In this study, we chose Ti3C2Tx, a type of MXene, to decorate silver nanoparticles (Ag NPs) on the ultrathin two-dimensional (2D) MXene sheets. The designed Ag-MXene substrates with SERS activity showed high sensitivity, high stability, and reproducibility. The SERS signal was enhanced by the synergistic contribution of both charge-transfer (CT) and surface plasmon resonance (SPR) involving the Ag NPs and the MXene sheets. Due to the strong interaction between the probe molecules and Ag NPs which provided the nanoscale gap, the substrate exhibited remarkable SERS performance. A novel experimental strategy was developed to facilitate the controlled synthesis of noble metal NPs and MXene sheets and provide insights for further improving the practical applications of these materials in SERS detection.

Biosensing of multiple aromatic xenobiotics in water by in-house fabricated prototype device

Portable, low-cost, and accurate monitoring of hazardous mono-aromatic pollutants, such as phenol or benzene group of compounds in water is a challenging task due to the lack of suitable detectable functional groups and complex matrix of environmental samples. Here, we use a series of protein-based biosensing recognition scaffolds to enable specific detection of several mono-aromatic classes of xenobiotics. The biosensor is tuned to perform in intricate environmental conditions and is interfaced with an in-house manufactured, multi-channel device (AroTrack) capable of direct and sensitive detection of several of these aromatic contaminants, such as phenol, benzene, and 2,3-dimethylphenol (2,3-DMP) in the low ppb range (10-200 ppb). The efficiency of the prototype device was benchmarked in both simulated wastewater and real environmental samples comprising 10 times higher isostructural aromatic pollutants or ions. It was established that AroTrack is reliable for environmental sample testing with a high degree of reproducibility and efficiency comparable to that of modern spectrophotometers (<5 % error). The battery-operated device costs less than $50 to fabricate and this low cost makes it effective to be implemented in rural and low-income settings which suggests immense field deployable potential.

A recyclable SERS-DGT device for in-situ sensing of sulfamethazine by Au@g-C3N4NS in water

Although diffusion gradient in thin-film technique (DGT) has realized the in-situ sampling Sulfamethazine (SMT), the traditional DGT devices cannot be served as sensing devices but in-situ sampling devices. Here we report a recyclable surface enhanced Raman scattering (SERS) responsive DGT sensing device (recyclable SERS-DGT Sensing Device) capable of in-situ sensing of SMT in water. This is achieved by innovatively utilizing a recyclable SERS responsive liquid suspension of Au nanoparticles supported on g-C3N4 (Au@g-C3N4NS) as DGT binding phase. Au@g-C3N4NS is synthesized via in-situ growth method and embed in DGT binding phase, which exhibits good SERS activity, aqueous stability recyclable and adsorption performance. The SERS-DGT Sensing Device is valid for measuring SMT under a wide range of conditions (i.e., deployment time 24∼180 h, concentrations range of 1.031∼761.9 ng mL-1, pH 5∼9, ionic strength 0.0001∼0.05 mol L-1 NaCl, DOM concentrations 0∼100 mg L-1, four recycles). Furthermore, substrate combined with DGT binding phase, can integrate the sampling, pretreatment and SERS detection of SMT, which can be recycled, improving the reliability and efficiency of environmental monitoring. In this article, recyclable SERS-DGT Sensing Device, a platform for recyclable in-situ sensing of antibiotics, holds great potential for environmental monitoring.

Thin layer chromatography coupled with surface enhanced Raman scattering for rapid separation and on-site detection of multi-components

The separation and detection of multi-component mixtures has always been a challenging task. Traditional detection methods often suffer from complex operation, high cost, and low sensitivity. Surface enhanced Raman scattering (SERS) technique is a high sensitivity, powerful and rapid detection tool, which can realize the specific detection of single substance components, but it must solve the problem that multi-component mixtures cannot be accurately determined. Thin layer chromatography (TLC) technology, as a high-throughput separation technology, uses chromatographic plate as the stationary phase, and could select different developing phases for separation experiments. The advantages of TLC technology in short distance and rapid separation are widely used in protein, dye and biomedical fields. However, TLC technology has limitations in detection ability and difficulty in obtaining ideal signal intensity. The combination of TLC technology and SERS technology made the operation procedure simple and the sample size small, which can achieve rapid on-site separation and quantitative detection of mixtures. Due to the rapid development of TLC-SERS technology, it has been widely used in the investigation of various complex systems. This paper reviews the application of TLC-SERS technology in food science, environmental pollution and biomedicine.

Optically trapped SiO2@Au particle-dye hybrid-based SERS detection of Hg2+ ions

The selective ultra-sensitive detection of a very low concentration of analyte in a liquid environment using surface-enhanced Raman spectroscopy (SERS) is a challenging task owing to the poor reproducibility of the Raman signals arising from the nonstationary nature of the substrate. However, plasmonic metal particle-incorporated microparticles can be effectively 3-D arrested in a liquid environment that can serve as a stable SERS substrate by employing an optical trapping force. Herein, we demonstrate a 3-D optically trapped Au-attached SiO₂ microparticle as an efficient SERS substrate that can detect 512 pM for Rhodamine6G and 6.8 pM for crystal violet. Further, the substrate allows the simultaneous detection of multiple analytes. By utilizing the Raman signal from Rhodamine 6G as the probe beam, the selective detection of Hg²⁺ ions as low as 100 pM is demonstrated.

Rapid Detection of Malathion, Phoxim and Thiram on Orange Surfaces Using Ag Nanoparticle Modified PDMS as Surface-Enhanced Raman Spectroscopy Substrate

Malathion, phoxim, and thiram are organophosphates and organosulfur pesticides widely used in agricultural products. The residues of these pesticides present a direct threat to human health. Rapid and on-site detection is critical for minimizing such risks. In this work, a simple approach was introduced using a flexible surface-enhanced Raman spectroscopy (SERS) substrate. The prepared Ag nanoparticles-polydimethylsiloxane (AgNPs-PDMS) substrate showed high SERS activity, good precision (relative standard deviation = 5.33%), and stability (30 days) after optimization. For target pesticides, the linear relationship between characteristic SERS bands and concentrations were achieved in the range of 10~1000, 100~5000, and 50~5000 μg L-1 with LODs down to 3.62, 41.46, and 15.69 μg L-1 for thiram, malathion, and phoxim, respectively. Moreover, SERS spectra of mixed samples indicated that three pesticides can be identified simultaneously, with recovery rates between 96.5 ± 3.3% and 118.9 ± 2.4%, thus providing an ideal platform for detecting more than one target. Pesticide residues on orange surfaces can be simply determined through swabbing with the flexible substrate before acquiring the SERS signal. This study demonstrated that the prepared substrate can be used for the rapid detection of pesticides on real samples. Overall, this method greatly simplified the pre-treatment procedure, thus serving as a promising analytical tool for rapid and nondestructive screening of malathion, phoxim, and thiram on various agricultural products.

Functional cotton fabric-based TLC-SERS matrix for rapid and sensitive detection of mixed dyes

A facile cotton fabric with a built-in TLC-SERS structure was fabricated to demonstrate an integrated TLC separation and SERS identification of mixed dyes. The soft and flexible SERS fabric was firstly fabricated using a simple method in which gold nanoparticles were in-situ synthesized on cotton fabrics by heating. β-CD was then grafted onto cotton fabric through crosslinking with citric acid in presence of sodium hypophosphite monohydrate via esterification reaction. The adsorption and TLC development performance of β-CD grafted fabrics were comprehensively investigated with two organic dyes, one anionic dye and one nonionic dye. Besides, the recyclable adsorption and separation performance were tested to evaluate its sustainable application prospects. It displayed less adsorption capacity loss and reusable separation performance after several cycles than the pristine cotton fabrics. Finally, two sets of mixed dyes were successfully separated on the TLC fabrics and then identified via on-site SERS according to their different migration distance. The developed TLC-SERS fabric shows the advantage of quick, easy to handle, low-cost, sensitive, and could be exploited in on-site study of synthetic dyes in art objects, textile and packaging products or forensic applications.

Review of Thin-Layer Chromatography Tandem with Surface-Enhanced Raman Spectroscopy for Detection of Analytes in Mixture Samples

In the real world, analytes usually exist in complex systems, and this makes direct detection by surface-enhanced Raman spectroscopy (SERS) difficult. Thin layer chromatography tandem with SERS (TLC-SERS) has many advantages in analysis such as separation effect, instant speed, simple process, and low cost. Therefore, the TLC-SERS has great potential for detecting analytes in mixtures without sample pretreatment. The review demonstrates TLC-SERS applications in diverse analytical relevant topics such as environmental pollutants, illegal additives, pesticide residues, toxic ingredients, biological molecules, and chemical substances. Important properties such as stationary phase, separation efficiency, and sensitivity are discussed. In addition, future perspectives for improving the efficiency of TLC-SERS in real sample detecting are outlined.

Dynamic SPME-SERS Induced by Electric Field: Toward In Situ Monitoring of Pharmaceuticals and Personal Care Products

The core of the surface-enhanced Raman spectroscopy (SERS)-based techniques for dynamic monitoring is to realize rapid and reversible adsorption. Herein, the integration technology of electro-enhanced adsorption, solid-phase microextraction, and surface-enhanced Raman spectroscopy (EE-SPME-SERS) was developed to obtain sensitive, ultrafast, and reversible SERS response toward in situ monitoring of pharmaceuticals and personal care products (PPCPs). In the EE-SPME-SERS method, a roughened Ag fiber with Au modification (r-Ag/Au fiber) was used as the SERS substrate, SPME sorbent, and working electrode. The r-Ag/Au fiber displayed good SERS sensitivity, ultrahigh photostability, and adsorption properties. The adsorption efficiency of benzidine was 76 times accelerated in EE-SPME-SERS compared to that in static adsorption. The whole process of "sampling and detection" in EE-SPME-SERS can be finished within 1 s. Reversible adsorption and desorption can be achieved in situ by switching the direction of electric field, and the regeneration process takes only a few minutes. Simulated release of benzidine from household wastewater was in situ and dynamically monitored using this strategy. EE-SPME-SERS was proved universal for ionized PPCPs and can detect multicomponents simultaneously. In addition, EE-SPME-SERS showed very good analytical properties. Great potential of EE-SPME-SERS can be expected in environmental monitoring.

Recent Advances in the Use of Surface-Enhanced Raman Scattering for Illicit Drug Detection

The rapid increase in illicit drug use and its adverse health effects and socio-economic consequences have reached alarming proportions in recent years. Surface-enhanced Raman scattering (SERS) has emerged as a highly sensitive analytical tool for the detection of low dosages of drugs in liquid and solid samples. In the present article, we review the state-of-the-art use of SERS for chemical analysis of illicit drugs in aqueous and complex biological samples, including saliva, urine, and blood. We also include a review of the types of SERS substrates used for this purpose, pointing out recent advancements in substrate fabrication towards quantitative and qualitative detection of illicit drugs. Finally, we conclude by providing our perspective on the field of SERS-based drug detection, including presently faced challenges. Overall, our review provides evidence of the strong potential of SERS to establish itself as both a laboratory and in situ analytical method for fast and sensitive drug detection and identification.

Controlled Citrate Oxidation on Gold Nanoparticle Surfaces for Improved Surface-Enhanced Raman Spectroscopic Analysis of Low-Affinity Organic Micropollutants

Surface-enhanced Raman spectroscopy (SERS) provides an ultrasensitive, fast, and inexpensive method for organic micropollutant analysis, but its applications are limited by the low affinity of most organic micropollutants toward plasmonic nanoparticle surfaces. Particularly, the citrate layer on gold nanoparticle (AuNP) surfaces exerts strong resistance to ligand exchange and prevents carboxylic and phenolic pollutants from entering SERS "hot spots". In this study, we aim to extend the application of SERS to low-affinity organic micropollutants by oxidative decomposition of the citrate layer on AuNP surfaces. The kinetics of citrate oxidation were carefully controlled using sulfate radicals that were slowly released from peroxydisulfate photolysis, which guarantees both the stability of AuNP colloid and generation of a high density of SERS hot spots for pollutant analysis. In situ Raman spectroscopic monitoring demonstrates that citrate is first oxidized to di- and monocarboxylate acids and subsequently displaced by guest ligands. This oxidation-induced ligand exchange has been applied for SERS analysis of various low-affinity organic micropollutants, including monochloro-substituted carboxylates and phenols, as well as a widely used herbicide 2.4-dichlorophenoxyacetic acid. This study substantially broadens the library of organic micropollutants for label-free SERS analysis and advances SERS toward a holistic analytical tool for water quality monitoring.

Multiplex Sensing of Complex Mixtures by Machine Vision Analysis of TLC-SERS Images

Thin layer chromatography in tandem with surface-enhanced Raman scattering (TLC-SERS) has demonstrated tremendous potentials as a new analytical chemistry tool to detect a wide range of substances from real-world samples. However, it still faces significant challenges of multiplex sensing from complex mixtures due to the imperfect separation by TLC and the resulting interference of SERS detection. In this article, we propose a multiplex sensing method of complex mixtures by machine vision analysis of the scanning image of the TLC-SERS results. Briefly, various pure substances in solution and the complex mixture solution are separated by TLC followed by one-dimensional SERS scanning of the entire TLC plate, which generates TLC-SERS images of all target substances along the chromatography path. After that, a machine vision method is employed to extract the template images from the TLC-SERS images of pure substance solutions. Finally, we apply a feature point matching strategy based on the Winner-take-all principle, which matches the template image of each pure substance with the mixture image to confirm the existence and derive the position of each target substance in the TLC plate, respectively. Our experimental results based on the mixture solution of five different substances show that the proposed machine vision analysis is highly selective, sensitive and does not require artificial analysis of the SERS spectra. Therefore, we envision that the proposed machine vision analysis of the TLC-SERS imaging is an objective, accurate, and efficient method for multiplex sensing of trace level of target substances from complex mixtures.

Graphene-embedded oblique V-shaped silver nanoarrays for hydrophobic pollutants pre-concentration and high-sensitivity SERS detection

A surface enhanced Raman scattering (SERS) substrate of silver nanorod modified with graphene and silver nanorod (AgNR@Graphene@AgNR) has been fabricated to improve the sensitivity of SERS detection of hydrophobic pollutants, in which, graphene is an interlayer and AgNR is arranged on both sides of the graphene. The embedded graphene could help the oblique V-shaped AgNR structure to improve the sensitivity of SERS detection with a significant electric field enhancement effect. The annealing treatment of the substrate, shortening the nanometer gap between the graphene and AgNR, is benefit for producing a large number of "hot spots" at the fold, which has been verified by the finite difference time domain (FDTD) simulation. The enhancement factor (EF) of AgNR@Graphene@AgNR could reach up to 1.6 × 10⁸ with a good reproducibility. The substrate could achieve high-sensitivity detection of 4-chlorobiphenyl (PCB-3) and 3, 3', 4, 4'-tetrachlorobiphenyl (PCB-77) with the limit of detections (LODs) of 1.72 × 10^-10 M and 2.11 × 10^-8 M, and the effective identification of PCBs mixture has been realized through principal component analysis (PCA), which means that the AgNR@Graphene@AgNR substrate has a potential significance for the detection and analysis of hydrophobic pollutant mixtures in the environment.

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.

Highly sensitive SERS substrates with multi-hot spots for on-site detection of pesticide residues

Pesticide residues will be a huge threat to food security and ecological environment; therefore, there is an urgent need to achieve rapid and on-site detection of pesticide residues. Herein, a plasmonic substrate with multiple "hot spots" was fabricated by transferring three-dimensional (3D) Au nanoparticles (NPs) onto the polydimethylsiloxane (PDMS) membrane for highly sensitive surface-enhanced Raman scattering (SERS) detection of pesticide residues. In combination with 3D-FDTD simulations, high SERS enhancement (EF = 1.2 × 10⁸) and high detection sensitivity (LOD = 6.3 × 10^-10 M) were achieved, mainly due to the enhanced electromagnetic fields around the "hot spots". Additionally, the PDMS-based SERS substrate held good transparency and flexibility, enabling conformal contact with non-planar surfaces and allowing the laser to penetrate the back of the analytes. Combined with a portable Raman spectrometer, the substrates holds great potential for rapid, high-sensitive, and on-site detection of contaminants in food, especially for the analyte on the nonplanar surfaces.

Efficient fabrication of highly sensitive AgNPs-drawing paper SERS substrates by robotic writing approach

Inspired by hand writing approach for preparing surface-enhanced Raman scattering (SERS) substrates, silver nanoparticles (AgNPs) decorated drawing paper substrates were prepared by robotic writing technique. The wettabilities and surface morphologies of the drawing paper before and after the deposition of AgNPs were characterized by contact angle analyzer and scanning electron microscope, respectively. Malachite green was employed as a probe molecule to evaluate the SERS activities of the AgNPs-drawing paper substrates. The AgNPs-drawing paper substrates exhibited extremely high sensitivity that the detection limit for malachite green was down to 10^-18 mol/L and the Raman enhancement factor was calculated to be 10¹⁵. The relative standard deviation (RSD) values of the Raman peaks intensities collected from twelve points on a single substrate and fifteen substrates were used to evaluate the uniformity and reproducibility of the AgNPs-drawing paper substrates. It was found that the substrates had good reproducibility and uniformity with RSD values of 7.29% and 9.70%, respectively. Furthermore, the prepared AgNPs-drawing paper substrates exhibited long-term stability among six months.

Combined Paper Centrifugal Chromatographic Separation and SERS Detection for Multicomponent Substances

The separation and chemical analysis of mixtures in an emergency situation represent major challenges, especially in remote or poverty-stricken areas. A novel method was developed for the rapid separation and detection of multiple components via paper centrifugal chromatography, which costs as little as $2.26 US. The method was realized based on the combination of portable paper centrifugal chromatography and surface-enhanced Raman scattering (SERS) detection. This coupled technique was successfully implemented for the separation and qualitative analysis of a rhodamine 6G-crystal violet mixture and a colorless aniline-pyrocatechol-benzidine mixture. A chromatographic mobile phase was collected using absorbent cotton, which was demonstrated to have no effect on the SERS results. The optimized device achieved rapid and effective separation of the colorless aniline-pyrocatechol-benzidine mixture with a high centrifugal force (0.3303π² N). The newly developed method involving multicomponent paper centrifugal chromatography-SERS detection will be of great value for emergency-related substance separation and analysis in remote and poor areas.

High-Sensitivity Raman Gas Probe for In Situ Multi-Component Gas Detection

Multiple reflection has been proven to be an effective method to enhance the gas detection sensitivity of Raman spectroscopy, while Raman gas probes based on the multiple reflection principle have been rarely reported on. In this paper, a multi-reflection, cavity enhanced Raman spectroscopy (CERS) probe was developed and used for in situ multi-component gas detection. Owing to signal transmission through optical fibers and the miniaturization of multi-reflection cavity, the CERS probe exhibited the advantages of in situ detection and higher detection sensitivity. Compared with the conventional, backscattering Raman layout, the CERS probe showed a better performance for the detection of weak signals with a relatively lower background. According to the 3σ criteria, the detection limits of this CERS probe for methane, hydrogen, carbon dioxide and water vapor are calculated to be 44.5 ppm, 192.9 ppm, 317.5 ppm and 0.67%, respectively. The results presented the development of this CERS probe as having great potential to provide a new method for industrial, multi-component online gas detection.

Recent progress in mycotoxins detection based on surface-enhanced Raman spectroscopy

Mycotoxins are toxic compounds naturally produced by certain types of fungi. The contamination of mycotoxins can occur on numerous foodstuffs, including cereals, nuts, fruits, and spices, and pose a major threat to humans and animals by causing acute and chronic toxic effects. In this regard, reliable techniques for accurate and sensitive detection of mycotoxins in agricultural products and food samples are urgently needed. As an advanced analytical tool, surface-enhanced Raman spectroscopy (SERS), presents several major advantages, such as ultrahigh sensitivity, rapid detection, fingerprint-type information, and miniaturized equipment. Benefiting from these merits, rapid growth has been observed under the topic of SERS-based mycotoxin detection. This review provides a comprehensive overview of the recent achievements in this area. The progress of SERS-based label-free detection, aptasensor, and immunosensor, as well as SERS combined with other techniques, has been summarized, and in-depth discussion of the remaining challenges has been provided, in order to inspire future development of translating the techniques invented in scientific laboratories into easy-to-operate analytic platforms for rapid detection of mycotoxins.

Plasmonic Microgels for Raman-Based Molecular Detection Created by Simultaneous Photoreduction and Photocross-linking

Molecular detection in complex mixtures is of great importance in biomedical diagnosis, food safety, and environmental monitoring. Although surface-enhanced Raman scattering serves as one of the most promising detection methods, metal surfaces are prone to contamination, making the direct detection of small molecules in mixtures elusive. Metal nanoparticle-loaded hydrogels have been used for the exclusion of large adhesive molecules and direct detection of small molecules. Here, we design microgels containing highly concentrated gold nanoparticles through the simultaneous formation of hydrogel and gold nanoparticles in emulsion droplets. Monodisperse water-in-oil droplets are microfluidically prepared to contain a gold precursor, hydrogel precursor, and photoinitiator. Upon ultraviolet irradiation, a hydrogel is gradually formed in the drop by photocross-linking at which gold nanoparticles are synthesized and grown by photo and thermal reduction. The in situ synthesis provides the uniform distribution of gold nanoparticles at very high concentrations without aggregation, which is otherwise very difficult to achieve. Using the microgels, small molecules in albumin solutions can be detected by Raman measurement with high signal sensitivity and reproducibility in the absence of interruption from albumin. As a proof of concept, we demonstrate the direct detection of pyocyanin, a biomarker for Pseudomonas infection spiked in unpurified saliva.

Surface Enhanced Raman Spectroscopy in environmental analysis, monitoring and assessment

Environmental pollution is usually monitored via mass spectrometry-based approaches. Such techniques are extremely sensitive but have several disadvantages. The instruments themselves are expensive, require specialized training to use and usually cannot be taken into the field. Samples also usually require extensive pre-treatment prior to analysis which can affect the final result. The development of analytical methods that matched the sensitively of mass spectrometry but that could be deployed in the field and require minimal sample processing would be highly advantageous for environmental monitoring. One method that may meet these criteria is Surface Enhanced Raman Spectroscopy (SERS). This is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on rough nanostructure surfaces such as gold or silver nanoparticles. SERS gives selective spectral enhancement such that increases in sensitivity of 10¹⁰ to 10¹⁴ have been reported. While this means SERS is, theoretically at least, capable of single molecule detection such a signal enhancement is hard to achieve in practice. In this review the background of SERS is introduced for the environmental scientist and the recent literature on the detection of several classes of environmental pollutants using this technique is discussed. For heavy metals the lowest limit of detection reported was 0.45 μg/L for Mercury; for pharmaceuticals, 2.4 μg/L for propranolol; for endocrine disruptors, 0.35 μg/L for 17β-estradiol; for perfluorinated compounds, 500 μg/L for perfluorooctanoic acid and for inorganic pollutants, 37g/L for general pesticide markers. The signal enhancements achieved in each case show great promise for the detection of pollutants at environmentally relevant concentrations and, although it does not yet routinely match the sensitivity of mass spectrometry. Further work to develop SERS methods and apply them for the detection of contaminants could be of wide benefit for environmental science.

Rapid capture and SERS detection of triclosan using a silver nanoparticle core - protein satellite substrate

Triclosan (TCS) is a synthetic antimicrobial compound that has been widely used in consumer products. However, increasing evidence suggests adverse effects of TCS to human health and environment, raising great public concerns. The existing methods for detecting TCS are limited to time-consuming and complicated procedure. Here, we developed a rapid method for capture and detection of TCS using surface-enhanced Raman spectroscopy (SERS) based on a silver nanoparticle (Ag NP) core - protein satellite nanostructure. Bovine serum albumin (BSA) assembled on Ag NPs as satellites configuration could anchor a large number of TCS molecules close to the surface of Ag NPs, producing amplified SERS signals. As low as 50 nM TCS standard was successfully detected within 30 min. We also demonstrated its capability for TCS detection in pond water. The developed SERS method holds a great promise for rapid screening of TCS in environmental and food samples.

Factors of Importance for Arsenic Migration/Separation under Coffee-Ring Effect on Silver Nanofilms

Surface-enhanced Raman spectroscopy (SERS) has been recognized as a promising analytical technique owing to its merit of nondestructive and fast detection capabilities. However, SERS usually suffers signal interferences from different analytes or a complicated matrix. Separation is an effective approach to solve the signal interference in the application of SERS. It was proposed that two concentric coffee rings could serve as a simple separation platform; however, there are still many questions to be answered for in-depth understanding. In this study, critical parameters during the formation of two concentric coffee rings are characterized for a better understanding of this phenomenon, including surface tension, surface morphology, and surface energy. Two arsenicals, including arsenate (As^V) and cacodylic acid (DMA^V), are chosen to study the arsenicals' separation/migration mechanism due to their significant difference in chemical properties. In the typical coffee ring, these two arsenicals have signal interference and only DMA^V is detected via SERS; however, they are detected along the radius of the two concentric coffee rings. The distribution of arsenicals on the two concentric coffee rings is further verified by the chromatographic method. Under this simple platform, interactions between the arsenicals and the surface of the silver nanofilm are pivotal to their migration/separation. By surface modification of silver nanofilm with small molecules, the surface polarity and surface ζ potential are manipulated. The signal dynamics of these two arsenicals are studied on these modified silver nanofilms. It is clear that the electrostatic interaction plays a more important role than the polarity in the arsenicals' migration. This study reveals the mechanism of small molecule migration/separation in the two concentric coffee rings and provides insights for future study of employing this simple platform.

Two-dimensional MXene modified AgNRs as a surface-enhanced Raman scattering substrate for sensitive determination of polychlorinated biphenyls

A MXene/AgNR substrate was prepared through a facile modification strategy. The substrate can perform sensitive SERS detection of polychlorinated biphenyls, which may have potential in environmental monitoring at the point of need.

Quaternion-based Parallel Feature Extraction: Extending the Horizon of Quantitative Analysis using TLC-SERS Sensing

Quantitative analysis using thin-layer chromatography coupled in tandem with surface-enhanced Raman scattering (TLC-SERS) still remains a grand challenge due to many uncontrollable variations during the TLC developing process and the random nature of the SERS substrates. Traditional chemometric methods solve this problem by sampling multiple SERS spectra in the sensing spot and then conducting statistical analysis of the SERS signals to mitigate the variation of quantitative analysis, while still ignoring the spatial distribution of the target species and the correlation among the multiple sampling points. In this paper, we proposed for the first time a parallel feature extraction and fusion method based on quaternion signal processing techniques, which can enable quantitative analysis using recently established TLC-SERS techniques. By marking three deterministic sampling points, we recorded spatially correlated SERS spectra to constitute an integral representation model of triple-spectra by a pure quaternion matrix. Quaternion principal component analysis (QPCA) was utilized for features extraction and followed by feature crossing among the quaternion principal components to obtain final fusion spectral feature vectors. Support vector regression (SVR) was then used to establish the quantitative model of melamine-contaminated milk samples with seven concentrations (1ppm to 250ppm). Compared with traditional TLC-SERS analysis methods, QPCA method significantly improved the accuracy of quantification by reaching only 7% and 2% quantization errors at 20 and 105 ppm concentration. Validation testing based on reasonable amount of statistic measurement results showed consistently smaller measurement errors and variance, which proved the effectiveness of QPCA method for TLC-SERS based quantitative sensing applications.

Quantitative TLC-SERS detection of histamine in seafood with support vector machine analysis

Scombroid fish poisoning caused by histamine intoxication is one of the most prevalent allergies associated with seafood consumption in the United States. Typical symptoms range from mild itching up to fatal cardiovascular collapse seen in anaphylaxis. In this paper, we demonstrate rapid, sensitive, and quantitative detection of histamine in both artificially spoiled tuna solution and real spoiled tuna samples using thin layer chromatography in tandem with surface-enhanced Raman scattering (TLC-SERS) sensing methods, enabled by machine learning analysis based on support vector regression (SVR) after feature extraction with principal component analysis (PCA). The TLC plates used herein, which were made from commercial food-grade diatomaceous earth, served simultaneously as the stationary phase to separate histamine from the blended tuna meat and as ultra-sensitive SERS substrates to enhance the detection limit. Using a simple drop cast method to dispense gold colloidal nanoparticles onto the diatomaceous earth plate, we were able to directly detect histamine concentration in artificially spoiled tuna solution down to 10 ppm. Based on the TLC-SERS spectral data of real tuna samples spoiled at room temperature for 0 to 48 hours, we used the PCA-SVR quantitative model to achieve superior predictive performance exceling traditional partial least squares regression (PLSR) method. This work proves that diatomaceous earth based TLC-SERS technique combined with machine-learning analysis is a cost-effective, reliable, and accurate approach for on-site detection and quantification of seafood allergen to enhance food safety.

Arsenic Speciation on Silver Nanofilms by Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS), as a nondestructive and fast detection technique, is a promising alternative approach for arsenic detection, particularly for in situ applications. SERS-based speciation analysis according to the fingerprint SERS signals of different arsenicals has the potential to provide a superior technique in species preservation over the conventional chromatographic separation methods, albeit with some difficulties due to the similarity in SERS patterns. In this study, we explored a novel SERS method for arsenic speciation by using the separation potential of the coffee ring effect on negatively charged silver nanofilms (AgNFs). Four arsenic species, including arsenite (As^III), arsenate (As^V), monomethylarsonic acid (MMA^V), and dimethylarsinic acid (DMA^V), were measured for fingerprint SERS signals in solution and on the films. Significant enhancement of SERS signals on the dried coffee ring stains by the AgNFs were observed except for As^III, and more importantly, arsenicals migrated varying distances during coffee ring development, promoting better speciation. Sodium dodecyl sulfate was then introduced into the droplet to reduce the droplet surface tension, facilitating the migration of solution into the peripheral region. Under the combined interactions of arsenicals with the AgNFs, solvent, and surfactant, enhanced separation between arsenicals was observed as a result of the formation of two concentric rings. Combining the SERS fingerprint signals and physical separation of arsenicals on the surface, arsenic speciation was achieved using the AgNFs substrate-based SERS technology, demonstrating the potential of the coffee ring effect for rapid separation and analysis of small molecules by SERS.

On-site visualized classification of transparent hazards and noxious substances on a water surface by multispectral techniques

This paper investigated the use of spectra and multispectral images for on-site visualized classification of transparent hazards and noxious substances (HNS), such as benzene, xylene, and palm oil, floating on a water surface with the potential use for rapid classification of multiple HNS during a leak accident. Partial least-squares discrimination analysis (PLS-DA) and least-squares support vector machine (LS-SVM) models achieved a classification accuracy of 100% for spectral reflectance (325-900 nm) and multispectral image at nine wavelengths. Wavelength division and selection were applied for spectra and spectral images, respectively, to reduce the difficulty in data collection and to simplify the redundant bands. This was followed by PLS-DA and LS-SVM modeling. The LS-SVM model based on the least wavelengths (365, 410, 450, and 850 nm) of multispectral images was suggested as the most effective method for on-site visualized classification of transparent HNS on a water surface.

Novel Surface-Enhanced Raman Spectroscopy Techniques for DNA, Protein and Drug Detection

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopic technique in which the Raman scattering signal strength of molecules, absorbed by rough metals or the surface of nanoparticles, experiences an exponential growth (10³-10⁶ times and even 10¹⁴-10¹⁵ times) because of electromagnetic or chemical enhancements. Nowadays, SERS has attracted tremendous attention in the field of analytical chemistry due to its specific advantages, including high selectivity, rich informative spectral properties, nondestructive testing, and the prominent multiplexing capabilities of Raman spectroscopy. In this review, we present the applications of state-of-the-art SERS for the detection of DNA, proteins and drugs. Moreover, we focus on highlighting the merits and mechanisms of achieving enhanced SERS signals for food safety and clinical treatment. The machine learning techniques, combined with SERS detection, are also indicated herein. This review concludes with recommendations for future studies on the development of SERS.

Influence of smear matrix types on detection behaviors and efficiencies of polycyclic aromatic hydrocarbons using ion mobility spectrometry

Influence of smear matrix types on detection behaviors and efficiencies of polycyclic aromatic hydrocarbons (PAHs) with different molecular weights in ion mobility spectrometry (IMS) were investigated. Various smear matrices of stainless steel mesh (SM), cellulose paper (CP), and cotton fabric (CF) were employed. Anisole was used as the solvent and IMS analysis was performed without evaporation step of the solvent to apply charge transfer reactions between PAH molecules and the molecular ions of solvent. Shapes of reactant ion peaks (RIPs) were varied according to the smear matrix types. At the beginning of the sample inlet, intensity of RIPs of air and moisture notably decreased due to the lots of solvent vapor. The SM with good gas permeability showed relatively strong RIPs of air and moisture, whereas the CP with no gas permeability showed weak ones. Detection times and efficiencies of PAH ions were varied according to the smear matrix types as well as the kinds of PAHs. PAHs were on the whole detected well in 1-3 s after the sample inlet. Detection limits of PAHs measured using the SM were slightly better than those measured using the CP, while those measured using the CP were much better than those measured using the CF. The experimental results could be explained by structures of the smear matrices and evaporation behaviors of the PAH solutions.

Highly sensitive detection of an antidiabetic drug as illegal additives in health products using solvent microextraction combined with surface-enhanced Raman spectroscopy

The combined SME-SERS approach realized the effective separation and sensitive detection of illegal drug additives spiked in different healthy products.

A fast response TLC-SERS substrate for on-site detection of hydrophilic and hydrophobic adulterants in botanical dietary supplements

Schematic illustration of TLC-SERS for detection of hydrophilic and hydrophobic adulterants in botanical dietary supplements.

Galvanic displacement-induced codeposition of reduced-graphene-oxide/silver on alloy fibers for non-destructive SPME@SERS analysis of antibiotics

This work describes the integration of solid-phase microextraction (SPME) and surface-enhanced Raman spectroscopy (SERS) by codeposition of a hybrid consisting of reduced graphene oxide and silver on silver-copper alloy fibers. The morphology and structure of the coating were characterized by a variety of microscopic and spectroscopic techniques that confirmed the hybrid structure of the material. A galvanic-displacement-induced process is assumed to be involved during the codeposition of the hybrid coating on the alloy. In this process, Ag(I) is reduced to Ag(0) by Cu(0), and the presence of conjugated domains in GO facilitates the long-range transfer of electrons from Cu to Ag+. Simultaneously, GO accepts electrons and is converted into RGO. The hybrid coating exhibits a high SERS enhancement factor and good spatial uniformity. The needle-like coated alloy fibers are shown to be a viable tool for non-destructive sampling and SERS-based determination of trace levels of the antibiotics sulfadiazine and sulfamethoxazole in a spiked tissue mimic. The SERS peaks at 1149 cm^-1 for sulfadiazine and 1144 cm^-1 for sulfamethoxazole are selected as the reference peaks in the quantitative analysis. The linear range is from 0.01 to 100 μg·cm^-3. The detection limits are 1.9 ng·cm^-3 for sulfadiazine and 4.4 ng·cm^-3 for sulfamethoxazole. Graphical abstract Schematic presentation of I: Galvanic-displacement-induced reduction of graphene oxide (brown films) and Ag⁺ (purple dots) on silver-copper alloy; II: Codeposition of reduced-graphene-oxide (grey films)/Ag (blue stars) on alloy fiber; III: Non-destructive SPME of antibiotics from spiked tissue mimic; IV: SERS detection using Raman spectroscope.

Cu2O@β-cyclodextrin as a synergistic catalyst for hydroxyl radical generation and molecular recognitive destruction of aromatic pollutants at neutral pH

Wastewater systems contain a large number of compounds, such as anthropogenic aromatic pollutants and natural organic matter (NOM), and usually have pH higher than 4. Fenton-like reaction is the most widespread method for removal of organic pollutants, but their reactivity with H₂O₂ may be inhibited by NOM due to the competition of hydroxyl radicals and chelating agents. In this work, Cu₂O@β-cyclodextrin was developed to achieve the collaboration between molecular recognition and Fenton-like catalysis to destruct aromatic pollutants at neutral pH. In Cu₂O@β-CD, covalent CuOC bond was topotaxially converted from CuCl assisted by β-CD at room temperature. Covalently linked β-CD could keep humic acid from interfering catalytic performance of Cu₂O surfaces and inhibit the leaching of copper. A higher catalytic ability was observed for Cu₂O@β-CD with rate constant 0.0331 min^-1 than Cu₂O (0.0064 min^-1) at neutral pH. A mechanism of synergistic catalysis was proposed on the basis of Cu⁺, β-CD and phenoxo-Cu²⁺ complexes in the Cu₂O@β-CD/BPA/H₂O₂ system. The strategy of coupling molecular recognition into Fenton-like reaction provides an efficient and promising approach to the destruction of aromatic pollutants at neutral pH.

Flexible, transparent and highly sensitive SERS substrates with cross-nanoporous structures for fast on-site detection

A flexible and transparent film assembled from the cross-nanoporous structures of Au on PET (CNS of Au@PET) is developed as a versatile and effective SERS substrate for rapid, on-site trace analysis with high sensitivity. The fabrication of the CNS of Au can be achieved on a large scale at low cost by employing an etching process with super-aligned carbon nanotubes as a mask, followed by metal deposition. A strongly enhanced Raman signal with good uniformity can be obtained, which is attributed to the excitation of "hot spots" around the metal nanogaps and sharp edges. Using the CNS of Au@PET film as a SERS platform, real-time and on-site SERS detection of the food contaminant crystal violet (CV) is achieved, with a detection limit of CV solution on a tomato skin of 10-7 M. Owing to its ability to efficiently extract trace analytes, the resulting substrate also achieves detection of 4-ATP contaminants and thiram pesticides by swabbing the skin of an apple. A SERS detection signal for 4-ATP has a relative standard deviation of less than 10%, revealing the excellent reproducibility of the substrate. The flexible, transparent and highly sensitive substrates fabricated using this simple and cost-effective strategy are promising for practical application in rapid, on-site SERS-based detection.

Development of surface-enhanced Raman spectroscopy application for determination of illicit drugs: Towards a practical sensor

Surface-enhanced Raman spectroscopy (SERS) has been widely applied to identify or detect illicit drugs, because of the ability for highly specific molecular fingerprint and independence of aqueous solutions impact. We summarize the progress in determination of illicit drugs using SERS, including trace illicit drugs, suspicious objects and drugs or their metabolites in real biological system (urine, saliva and so on). Even though SERS detection of illicit drugs in real samples still remains a huge challenge because of the complex unknown environment, the efficient sample separation and the improved hand-held Raman analyzer will provide the possibility to make SERS a practically analytical technique. Moreover, we put forward a prospective overview for future perspectives of SERS as a practical sensor for illicit drugs determination. Perhaps the review is not exhaustive, we expect to help researchers to understand the evolution and challenges in this field and further interest in promoting Raman and SERS as a practical analyzer for convenient and automated illicit drugs identification.

SERS Sensors: Recent Developments and a Generalized Classification Scheme Based on the Signal Origin

Owing to its extreme sensitivity and easy execution, surface-enhanced Raman spectroscopy (SERS) now finds application for a wide variety of problems requiring sensitive and targeted analyte detection. This widespread application has prompted a proliferation of different SERS-based sensors, suggesting the need for a framework to classify existing methods and guide the development of new techniques. After a brief discussion of the general SERS modalities, we classify SERS-based sensors according the origin of the signal. Three major categories emerge from this analysis: surface-affinity strategy, SERS-tag strategy, and probe-mediated strategy. For each case, we describe the mechanism of action, give selected examples, and point out general misconceptions to aid the construction of new devices. We hope this review serves as a useful tutorial guide and helps readers to better classify and design practical and effective SERS-based sensors.

Diazotization-Coupling Reaction-Based Determination of Tyrosine in Urine Using Ag Nanocubes by Surface-Enhanced Raman Spectroscopy

A novel, simple, and highly sensitive method was developed to detect the concentration of tyrosine-derived azo dye indirectly using silver nanocubes (AgNCs) as a substrate on a super-hydrophobic silver film by surface-enhanced Raman spectroscopy (SERS). Diazotization-coupling reaction occurred between diazonium ions and the phenolic tyrosine, resulting in three new typical peaks in the SERS spectrum of the azo dye that was formed on the AgNCs, indicating strong SERS activity. Subsequently, the limit of detection of this approach was as low as 10^-12 M for tyrosine. Moreover, the SERS intensities of the three typical SERS signals of the analyte were linearly correlated with the logarithm of concentration of the Tyrosine. The proposed method shows great potential for tyrosine detection in the urine samples of normal humans.