Based on time and spatial-resolved SERS mapping strategies for detection of pesticides

Real-time and in situ monitoring of organosilicon-induced thiram penetration into cabbage leaves by surface-enhanced Raman scattering mapping

BACKGROUND

Understanding pesticide penetration behavior is important for effective application of pesticides. However, there is a lack of an effective method to monitor pesticide penetration behavior and its changing process. In the present study, a novel surface-enhanced Raman scattering (SERS) mapping method was used for real-time and in situ tracking of the penetration behaviors of thiram and thiram-organosilicon mixture on cabbage leaves.

RESULTS

The results suggest that thiram has very weak ability to penetrate into cabbage leaves. However, when the thiram-organosilicon mixture was placed on leaf surfaces, a clear thiram signal was detected inside the leaf after 2 h of exposure, a strong signal was observed after 12 h, and the penetration depth of thiram was approximately 200 μm after 48 h.

CONCLUSION

SERS mapping was demonstrated to be a reliable method for in situ monitoring of organosilicon-induced thiram penetration into cabbage leaf over time. The present study provides a new reference for rationally selecting adjuvants, effectively applying pesticides, and reducing pesticides residue in food. © 2022 Society of Chemical Industry.

Multi-scale convolution neural network with residual modules for determination of drugs in human hair using surface-enhanced Raman spectroscopy with a gold nanorod film self-assembled by inverted evaporation

Detection of illegal drug users is crucial in controlling drug-related crimes, reducing drug prevalence, and protecting human lives to ensure social stability. In this study, surface-enhanced Raman spectroscopy (SERS) and deep learning networks were employed to determine methamphetamine, ketamine, and morphine in human hair. Drugs were obtained from hair through alkaline hydrolysis and liquid-liquid extraction, and gold nanorods were employed to prepare the self-assembled film as the SERS substrate by inverted evaporation. The film showed good uniformity and excellent sensitivity, with a relative standard deviation of 15.6% and a detection limit of at least 10^-10 M in the SERS detection of crystal violet. The spectra of methamphetamine, ketamine, and morphine at 0.05-1.0, 0.1-2.0, and 0.1-2.0 ng/mg were obtained, and the three drugs could be detected. Inception, a multi-scale feature extraction network, was combined with residual modules (Inception-ResNet) to develop the identification models of drugs, and the effect of spectral input form as a vector or matrix was explored. Inception-ResNet with input form of matrix outweighed other methods with 100.00%, 100.00%, and 99.23% accuracies in the training, validation, and prediction sets, respectively. In brief, SERS and Inception-ResNet with the spectra in matrix form provide an efficient and accurate determination of drugs in human hair, enabling the retrospective evaluation of drug use, and the method will be anticipated to detect excitant, poison, and toxic chemicals in human hair.

A super-hydrophobic perfluoropolyether coated polytetrafluoroethylene sheets substrate for detection of acetamiprid surface-enhanced Raman spectroscopy

In this paper, a hydrophobic substrate as concentrators including an inner layer of polytetrafluoroethylene (PTFE) and an outer layer covered a thin layer of perfluoropolyether (PFPE) was constructed to achieve a higher sensitivity for acetamiprid (AC) SERS detection. The condensation effect of the PTFE-PFPE hydrophobic substrate-induced aggregation of gold nanoparticles (Au NPs) result ''hot spots'' for SERS. The hydrophobic substrate is better reproducibility (RSD < 5%) compared with that on a conventional silicon wafer. A further application of the hydrophobic substrate was demonstrated by the detection of AC in tea samples within a detection range of 0.03 mg/L to 3 mg/L. The hydrophobic substrate eliminates the problem of solution diffusion to avoid the "coffee ring" effect (When a droplet adheres to a solid surface, the suspended molecular particles usually deposit on the edge of the droplet to form a ring).

Dimensional Design for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that enables specific identification of target analytes with sensitivity down to the single-molecule level by harnessing metal nanoparticles and nanostructures. Excitation of localized surface plasmon resonance of a nanostructured surface and the associated huge local electric field enhancement lie at the heart of SERS, and things will become better if strong chemical enhancement is also available simultaneously. Thus, the precise control of surface characteristics of enhancing substrates plays a key role in broadening the scope of SERS for scientific purposes and developing SERS into a routine analytical tool. In this review, the development of SERS substrates is outlined with some milestones in the nearly half-century history of SERS. In particular, these substrates are classified into zero-dimensional, one-dimensional, two-dimensional, and three-dimensional substrates according to their geometric dimension. We show that, in each category of SERS substrates, design upon the geometric and composite configuration can be made to achieve an optimized enhancement factor for the Raman signal. We also show that the temporal dimension can be incorporated into SERS by applying femtosecond pulse laser technology, so that the SERS technique can be used not only to identify the chemical structure of molecules but also to uncover the ultrafast dynamics of molecular structural changes. By adopting SERS substrates with the power of four-dimensional spatiotemporal control and design, the ultimate goal of probing the single-molecule chemical structural changes in the femtosecond time scale, watching the chemical reactions in four dimensions, and visualizing the elementary reaction steps in chemistry might be realized in the near future.

Transparent, Flexible Plasmonic Ag NP/PMMA Substrates Using Chemically Patterned Ferroelectric Crystals for Detecting Pesticides on Curved Surfaces

Transparent and flexible surface-enhanced Raman scattering (SERS) substrates have attracted much interest for the detection of probe molecules on the curved surfaces of real samples, but a facile route to fabricate such substrates is still lacking. Herein, we present a rationally designed, high-performance flexible SERS substrate fabricated using a simple drop and peel-off technique for the ultrasensitive detection of pesticides. The proposed SERS substrate consists of a polymethyl methacrylate (PMMA) film anchored with plasmonic silver nanoparticles (Ag NPs), which are photoreduced using chemically patterned ferroelectric templates. The photoreduced Ag NPs extracted onto the PMMA film offer strong electromagnetic enhancement and produce intensive hotspots for the effective enhancement of the Raman signal. They provide superior SERS performance for the detection of parathion (PT) and fenitrothion (FNT) at trace-level concentrations of 10^-9 M and 10^-10 M with excellent enhancement factors in the order of 10⁸ and 10⁹, respectively. Moreover, the Ag NP/PMMA SERS substrate has good spot-to-spot uniformity and batch-to-batch reproducibility with the reservation of high detection sensitivity even after the mechanical deformation of bending and torsion up to 50 cycles. The multiplex detection ability is also investigated for the simultaneous detection of PT and FNT. To ensure the practical feasibility, the in-situ, real-time detection of PT and FNT on the curved surfaces of tomato and lemon using a fiber-coupled Raman probe is performed with limits of detection of 4.24 × 10^-8 M and 2.74 × 10^-9 M. The proposed Ag NP/PMMA flexible SERS substrate possesses unique features, such as easy fabrication through a simple, economical, rapid process, and facilitates straightforward implementation of in-situ SERS detection on curved fruit/vegetable surfaces.

Surface-enhanced Raman spectroscopy for bioanalysis and diagnosis

In recent years, bioanalytical surface-enhanced Raman spectroscopy (SERS) has blossomed into a fast-growing research area. Owing to its high sensitivity and outstanding multiplexing ability, SERS is an effective analytical technique that has excellent potential in bioanalysis and diagnosis, as demonstrated by its increasing applications in vivo. SERS allows the rapid detection of molecular species based on direct and indirect strategies. Because it benefits from the tunable surface properties of nanostructures, it finds a broad range of applications with clinical relevance, such as biological sensing, drug delivery and live cell imaging assays. Of particular interest are early-stage-cancer detection and the fast detection of pathogens. Here, we present a comprehensive survey of SERS-based assays, from basic considerations to bioanalytical applications. Our main focus is on SERS-based pathogen detection methods as point-of-care solutions for early bacterial infection detection and chronic disease diagnosis. Additionally, various promising in vivo applications of SERS are surveyed. Furthermore, we provide a brief outlook of recent endeavours and we discuss future prospects and limitations for SERS, as a reliable approach for rapid and sensitive bioanalysis and diagnosis.

Detection of Mycotoxins in Food Using Surface-Enhanced Raman Spectroscopy: A Review

Mycotoxins are toxic metabolites produced by fungi that contaminate many important crops worldwide. Humans are commonly exposed to mycotoxins through the consumption of contaminated food products. Mycotoxin contamination is unpredictable and unavoidable; it occurs at any point in the food production system under favorable conditions, and they cannot be destroyed by common heat treatments, because of their high thermal stability. Early and fast detection plays an essential role in this unique challenge to monitor the presence of these compounds in the food chain. Surface-enhanced Raman spectroscopy (SERS) is an advanced spectroscopic technique that integrates Raman spectroscopic molecular fingerprinting and enhanced sensitivity based on nanotechnology to meet the requirement of sensitivity and selectivity, but that can also be performed in a cost-effective and straightforward manner. This Review focuses on the SERS methodologies applied to date for qualitative and quantitative analysis of mycotoxins based on a variety of SERS substrates, as well as our perspectives on current limitations and future trends for applying this technique to mycotoxin analyses.

The sensitive detection of methylene blue using silver nanodecahedra prepared through a photochemical route

In this work, we have carried out systematic studies on the critical role of polyvinyl pyrrolidone (PVP) and citrate in the well-known chemical reduction route to synthesize silver nanodecahedra (AgND). Silver nitrate (AgNO₃) was used as silver source, which can be directly converted to metallic silver after being reduced by sodium borohydride (NaBH₄) under blue light-emitting diode (LED) irradiation (λ _max = 465 nm), and polyvinyl pyrrolidone (PVP) as a capping agent to assist the growth of AgND. The obtained products were silver nanodecahedra of excellent uniformity and stability with high efficiency and yield. The results showed that PVP acted as a capping agent to stabilize the silver nanoparticles, prolonging the initiation time required for nanodecahedra nucleation, thus inducing anisotropic growth, allowing the size and morphology of the AgND to be controlled successfully. This improved understanding allows a consistent process for the synthesis of AgND with significantly enhanced reproducibility to be developed and the formation mechanism of these nanostructures to be elucidated. This is a simple, cost-effective and easily reproducible method for creating AgND. The typical absorption maxima in the UV-vis spectroscopy of Ag seeds was λ _max ∼400 nm and that of AgND was λ _max ∼480 nm. The size of the prepared AgND was in the range of 60-80 nm. SEM images confirmed the uniform and high density of AgND when the concentration of PVP was 0.5 mM. The XRD pattern showed that the final product of AgND was highly crystallized. In addition, the prepared AgND can be used to detect methylene blue (MB) in a sensitive manner with good reproducibility and stability using Surface-Enhanced Raman Scattering (SERS) phenomenon. Out of the obtained products, the AgND prepared with 50 min blue LED light irradiation (AgND-50) displayed the strongest SERS signal. Interestingly, MB in diluted solution can be detected with a concentration as low as 10^-7 M (the limit of detection, LOD) and the linear dependence between SERS intensity and the MB concentration occurred in the range from 10^-7 to 10^-6 M. The enhancement factor (EF) of the SERS effect was about 1.602 × 10⁶ with a MB concentration of 10^-7 M using 532 nm laser excitation.

Attomolar SERS detection of organophosphorous pesticides using silver mirror-like micro-pyramids as active substrate

Surface-enhanced Raman spectroscopy (SERS) is gaining importance as an ultrasensitive analytical tool for routine high-throughput analysis of a variety of molecular compounds. One of the main challenges is the development of robust, reproducible and cost-effective SERS substrates. In this work, we study the SERS activity of 3D silver mirror-like micro-pyramid structures extended in the z-direction up to 3.7 μm (G0 type substrate) or 7.7 μm (G1 type substrate), prepared by Si-based microfabrication technologies, for trace detection of organophosphorous pesticides, using paraoxon-methyl as probe molecule. The average relative standard deviation (RSD) for the SERS intensity of the peak displayed at 1338 cm^-1 recorded over a centimetre scale area of the substrate is below 13% for pesticide concentrations in the range 10^-6 to 10^-15 mol L^-1. This data underlies the spatial uniformity of the SERS response provided by the microfabrication approach. According to finite-difference time-domain (FDTD) simulations, such remarkable feature is mainly due to the contribution on electromagnetic field enhancement of edge plasmon polaritons (EPPs), propagating along the pyramid edges where the pesticide molecules are preferentially adsorbed. Graphical abstract.

Mapping of Pesticide Transmission on Biological Tissues by Surface Enhanced Raman Microscopy with a Gold Nanoparticle Mirror

We presented an improved surface-enhanced Raman scattering (SERS) mapping technique for the imaging of pesticides on biological samples including tomato leaves, fruits, and mouse skin using a gold nanoparticle mirror as the SERS substrate. The gold nanoparticle mirror was fabricated using 50 nm commercial citrate-capped gold nanoparticles upon the interface of water and a mediating solvent that was prepared using acetonitrile and hexane. The properties of the gold nanoparticle mirror were compared with gold nanoparticles, and the mirror displayed higher sensitivity with a limit of detection of 0.07 μg/cm² and better reproducibility with a relative standard deviation of 5.48% for the SERS mapping of pesticide (ferbam) on biological samples. The gold mirror-based SERS mapping technique was also used to investigate pesticide transmission from tomato fruit surfaces to mouse skin after 1 mg/cm² of pesticides was administered upon the fruit, and the results showed that about 23% of the pesticide was transmitted from the fruit to the mouse skin. We also found that pesticides on the contaminated hand could not be completely removed by routine rinsing with tap water for 2 min. This study provides an effective approach for the imaging of pesticides on biological tissues that would facilitate research on pesticide behaviors both on and in biological systems.

The synthesis of Ag-coated tetrapod gold nanostars and the improvement of surface-enhanced Raman scattering

In this work, the novel tetrapod gold nanostars with four specific numbers of sharp tips were synthesized with good uniformity, high yield and chemical stability. The size of tetrapod gold nanostars has been increased effectively by increasing the amount of HAuCl₄ or DEA added to the reaction solution, and the corresponding in-plane localized surface plasmon resonance (LSPR) band can shift from 700 to 1100 nm. In order to combine the advantages of different metals, the Ag-coated tetrapod gold nanostars were prepared with different Ag coating thicknesses by increasing the volume of AgNO₃. With the increase of Ag coating thickness, the morphology of the nanoparticles has changed from gold nanostars with four sharp tips to bimetallic cuboids with four sharp tips and finally to crore-shell cuboids completely. The surface-enhanced Raman scattering (SERS) activity of Ag-coated tetrapod gold nanostars has also been improved with the different Ag coating thicknesses. Combined the tip effect and the enhancement effect of Ag, the SERS activity has the strongest enhancement when the four sharp tips of tetrapod gold nanostars are not completely coated by the Ag layer but still exposed to the outsides. For tetrapod gold nanostars with larger size to grow Ag-coated tetrapod gold nanostars, the bimetallic cuboids with four sharp tips nanostructure have the biggest SERS enhancement factor (EF) of 1.73 × 10⁶ for Rhodamine 6G (R6G), making it an ideal candidate in SERS-based sensing application.

Surface-Enhanced Raman Spectroscopy on Self-Assembled Au Nanoparticles Arrays for Pesticides Residues Multiplex Detection under Complex Environment

The high reproducibility of trace detection in complex systems is very hard but crucial to analytical technology and science. Here, we present a surface-enhanced Raman scattering (SERS) platform made by large-scale self-assembly of Au nanoparticle (NP) arrays at the cyclohexane/water interface and its use for pesticides residues trace detection. The analyte molecules spontaneously localize into the Au NPs’ nanogaps during the self-assembly process, yielding excellent Raman signal enhancement by surface effects, and possibly both by the concentration of the analytes into the array and by plasmonic hot-spot formation. Transmission electron microscopy (TEM) images demonstrate a good uniformity of interparticle distances (2–3 nm) in the Au NP arrays. SERS experiments on crystal violet (CV) molecules demonstrated that the relative standard deviations (RSD) of the band intensities at 1173, 1376, and 1618 cm⁻¹ were 6.3%, 6.4%, and 6.9%, respectively, indicating high reproducibility of the substrate. Furthermore, we demonstrate that two pesticides dissolved in organic and aqueous phases could be simultaneously detected, suggesting an excellent selectivity and universality of this method for multiplex detection. Our SERS platform opens vast possibilities for repeatability and sensitivity detection of targets in various complex fields.

A CO2-tunable plasmonic nanosensor based on the interfacial assembly of gold nanoparticles on diblock copolymers grafted from gold surfaces

A general stepwise strategy for the fabrication of CO₂-tunable plasmonic nanosensors was described for the first time, based on gold surface functionalization by CO₂-responsive poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA) brushes via a surface-initiated atom transfer radical polymerization (SI-ATRP) method, then the extremity of PDEAEMA was functionalized by linking the polyacrylamide (PAAm) brushes via ATRP, where they were assembled with gold nanoparticles (AuNPs) efficiently by altering the deposition time. The swelling–shrinking states of the PDEAEMA brushes can be tuned just by passing CO₂ and N₂ through a solution alternately. The unique plasmonic surface-enhanced Raman scattering (SERS) sensing properties of these stimulable substrates were investigated using 4-mercaptophenol (4MPh) as a molecular probe. When alternating CO₂ and N₂ bubbling in the water solution, the reversible switching of the SERS signals was complete. By in situ contact-mode atomic force microscopy, the thickness of the polymer layer was observed to be 26 nm in CO₂ saturated water, and after N₂ bubbling to remove CO₂ it decreased to 15 nm, causing the AuNPs to move near to the gold surface. Meanwhile, the distance between the nearby AuNPs becomes smaller, and the surface coverage (φ) of the AuNPs increased from 27% to 35%. The reported CO₂-responsive plasmonic nanosensor provided a dynamic SERS platform, with reversible regulation for electromagnetic coupling between the AuNPs and the gold surface, and between nearby AuNPs.

A CO₂-tunable plasmonic nanosensor: just by passing CO₂ and N₂ through a water solution alternately, the swelling–shrinking states of PDEAEMA brushes can be tuned, and reversible switching of SERS signals can be obtained.

Aptasensors for quantitative detection of Salmonella Typhimurium

Salmonella is one of the most frequent causes of food borne infectious disease. Among nearly 2500 documented serotypes are reported, Salmonella Typhimurium is the number one serotype associated with salmonellosis worldwide. Many different methods have been developed for the detection and quantification of S. typhimurium. Most of these assays are usually expensive, time consuming and require difficult sample preparation steps. Therefore, it is necessary to develop rapid, robust, cost-effective and sensitive alternative detection methods. In the last years, aptasensors, used for detection of S. typhimurium in different samples. In this review, recent advances and applications of aptasensors for the detection and quantification of S. typhimurium in details have been summarized.

Ultrasensitive SERS aptasensor for the detection of oxytetracycline based on a gold-enhanced nano-assembly

This paper investigated a new detection method of oxytetracycline (OTC) in aquatic products with ultrasensitive detection limit. The method was constructed on the basis of raman hot spot between gold nanoparticles (AuNPs) (13nm and 80nm diameter respectively) linked by an DNA sequence. The DNA sequence combined with the OTC aptamer including its complementary sequence as well as a stem-loop structure. The raman signal molecule (4-MBA) was modified at the surface of 13nm AuNPs. After the exposure of OTC, the aptamer sequence was preferentially combined with OTC and partially dehybridized with its complementary sequence which led the 13nm AuNPs to get more closer to the 80nm AuNPs. The raman intensity was thus increased for the more enhanced hot spot generated. Under the optimal experimental conditions, the SERS signal was positively related to the OTC concentration with a wide working range of 4.60×10^-2-4.60×10²fg/mL and the limit of detection (LOD) was as low as 4.35×10^-3fg/mL. The recovery rates of fishmeal ranged from 91.29-110.98%. The specificity of this method was further examined, and the results showed that the AuNPs based aptasensor was highly selective. This developed ultrasensitive aptamer-based SERS detection platform suggested that it may be a promising strategy for a variety of sensing applications.

Nanoshell-Enhanced Raman Spectroscopy on a Microplate for Staphylococcal Enterotoxin B Sensing

A sensitive surface-enhanced Raman scattering (SERS) immunosensor based on the Au nanoparticle (Au NP) shell structure was developed to detect staphylococcal enterotoxin B (SEB) on a microplate. Au NPs modified with 4-nitrothiophenol (4-NTP) and coated with Ag shell of controlled thickness at 6.6 nm exhibited excellent SERS intensity and were used as signal reporters in the detection of SEB. The engaged 4-NTP allowed the significant electromagnetic enhancement between Au NPs and the Ag shell and prevented the dissociation of the Raman reporter. More importantly, 4-NTP-differentiated SERS signals between the sample and microplate. The SERS-based immunosensor had a limit of detection of 1.3 pg/mL SEB. Analysis of SEB-spiked milk samples revealed that the developed method had high accuracy. Therefore, the SERS-encoded Au@Ag core-shell structure-based immunosensor is promising for the detection of biotoxins, pathogens, and environmental pollutants.

Surface-enhanced Raman scattering activities of carbon nanotubes decorated with silver nanoparticles

We synthesized a carbon nanotube (CNT) loaded with silver nanoparticles (AgNPs) composite as an effective surface enhanced Raman scattering (SERS) substrate via a low-cost, simple citrate reduction method.

Salmonella typhimurium detection using a surface-enhanced Raman scattering-based aptasensor

Surface-enhanced Raman spectroscopy (SERS) has been used in a variety of biological applications due to its high sensitivity and specificity. Here, we report a SERS-based aptasensor approach for quantitative detection of pathogenic bacteria. A SERS substrate bearing Au@Ag core/shell nanoparticles (NPs) is functionalized with aptamer 1 (apt 1) for the capture of target molecules. X-rhodamine (ROX)-modified aptamer 2 (apt 2) is used as recognition element and Raman reporter. Salmonella typhimurium specifically interacted with the aptamers to form Au@Ag-apt 1-target-apt 2-ROX sandwich-like complexes. As a result, the concentration of S. typhimurium was determined using this developed aptasensor structure, and a calibration curve is obtained in the range of 15 to 1.5 × 10(6) cfu/mL with a limit of detection of 15 cfu/mL. Our method was successfully applied to real food samples, and the results are consistent with the results obtained using plate counting methods. We believe that the developed method shows potential for the rapid and sensitive detection of pathogenic bacteria in food safety assurance.