Using Standing Gold Nanorod Arrays as Surface-Enhanced Raman Spectroscopy (SERS) Substrates for Detection of Carbaryl Residues in Fruit Juice and Milk

Carbon Nanodots-Based Sensors: A Promising Tool for Detecting and Monitoring Toxic Compounds

The increasing prevalence of toxic compounds in food, agriculture, and the environment presents a critical challenge to public health and ecological sustainability. Carbon nanodots (CNDs), with their excellent photoluminescence, biocompatibility, and ease of functionalization, have emerged as highly promising materials for developing advanced sensors that target hazardous substances. This review provides a comprehensive overview of the synthesis, functionalization, and sensing mechanisms of CND-based sensors, highlighting their versatile application in detecting toxic compounds such as heavy metals, pesticides, mycotoxins, and emerging contaminants. The article outlines recent advancements in fluorescence, electrochemical, and colorimetric detection strategies and presents key case studies that illustrate the successful application of CNDs in real-world monitoring scenarios. Furthermore, it addresses the challenges associated with reproducibility, scalability, selectivity, and sensor stability and explores future directions for integrating CNDs with smart and sustainable technologies. This review emphasizes the transformative potential of CNDs in achieving rapid, cost-effective, and environmentally friendly toxin detection solutions across multiple domains.

Ag@CDS SERS substrate coupled with lineshape correction algorithm and BP neural network to detect thiram in beverages

Surface enhanced Raman scattering (SERS) has been proved an effective analytical technique due to its high sensitivity, however, how to identify and extract useful information from raw SERS spectra is still a problem that needs to be resolved. In this work, a composite SERS substrate was prepared by encapsulating Ag nanoparticles within dialdehyde starch (Ag@CDS) to obtain dense "hot spot", and then a novel spectral preprocessing algorithm namely lineshape correction algorithm (LCA) was developed to separate the characteristic peaks of analytes from the original SERS spectra. Based on Ag@CDS and LCA, thiram residues in different beverages were quantitatively detected using back propagation (BP) neural network regression model. It was found that LCA provided an easy-to-use method for improving prediction ability of BP model. The Rp2 of BP model was improved from 0.2384, 0.3647 and 0.5581 to 0.9327, 0.9127 and 0.9251 for the quantitative detection of thiram residue in apple juice, grape juice and milk, respectively, while LCA was used for SERS spectra preprocessing. The optimal model can accurately detect thiram residue with a low limit of detection at 1.0 × 10-7 M, which is far below the maximum residue limit of thiram (2.9 × 10-5 M) regulated by the US Environmental Protection Agency. This study demonstrated that the proposed LCA can be used as a simple and valid spectra-preprocessing method in SERS quantitative detection.

Fabrication of high performance 2D flexible SERS substrate based on cellulose nanofibrils and its application for pesticide residue detection

Cellulose nanofibrils (CNFs) can serve as an efficient surface enhanced Raman scattering (SERS) platform for in situ detection of trace targets. In this study, a highly reproducible SERS platform based on TEMPO-oxidized CNFs (T-CNFs) was fabricated by the ion-exchange. Self-assembly of silver nanoparticles (AgNPs) was accomplished in only 120 s. The abundant carboxylate groups and good hydrophilicity of T-CNFs facilitated uniform and dense loading of AgNPs over the surface area. The obtained SERS substrate greatly enhanced the Raman signal of different pesticides, and the detection limits of thiram and thiabendazole were 5.81 × 10-8 M and 9.63 × 10-8 M, respectively. SERS substrate could produce homogeneous Raman-enhanced signals (relative standard deviation (RSD) = 6.59 %). In addition, due to the good flexibility, SERS substrate could collect and detect pesticide residues from the surface of apples. The intensities of Raman characteristic peak at 1384 cm-1 showed a good linear relationship with the analyte concentrations (0.96 ng/cm2-9600 ng/cm2). The constructed SERS substrate provided a theoretical basis for the preliminary rapid screening of hazardous chemical residues in food, which was of great value for the SERS technique to become a routine on-site analysis method for pesticide residues.

Rapid and ultrasensitive detection of thiram and carbaryl pesticide residues in fruit juices using SERS coupled with the chemometrics technique

A gold nanogap substrate was used to measure the thiram and carbaryl residues in various fruit juices using surface-enhanced Raman scattering (SERS). The gold nanogap substrates can detect carbaryl and thiram with limits of detection of 0.13 ppb (0.13 μgkg-1) and 0.22 ppb (0.22 μgkg-1). Raw SERS data were first preprocessed to reduce noise and undesirable effects and, were later used for model creation, implementing classification, and regression analysis techniques. The partial least-squares regression models achieved the highest prediction correlation coefficient (R2) of 0.99 and the lowest root mean square of prediction value below 0.62 ppb for both pesticide-infected juice samples. Furthermore, to differentiate between juice samples contaminated by both pesticides and control (pesticide-free), logistic-regression classification models were produced and achieved the highest classification accuracies of 100% and 99% for contaminated juice containing thiram and 100% accurate results for contaminated juice containing carbaryl. This indicates that the gold nanogap surface has significant potential for achieving high sensitivity in detecting trace contaminants in food samples.

Mechanistic understanding of nanoparticle interactions to achieve highly-ordered arrays through self-assembly for sensitive surface-enhanced Raman scattering detection of trace thiram

Over the last few decades, there is increasing worldwide concern over human health risks associated with extensive use of pesticides in agriculture. Developing excellent SERS substrate materials to achieve highly sensitive detection of pesticide residues in the food is very necessary owing to their serious threat to human health through food chains. Self-assembled metallic nanoparticles have been demonstrated to be excellent SERS substrate materials. Hence, alkanethiols-protected gold nanoparticles have been successfully prepared for forming larger-scale two-dimensional monolayer films. These films can be disassembled into a fluid state and re-assembled back to crystallized structure by controlling surface pressure. Further investigations reveal that their self-assembled structures are mainly dependent on the diameter of gold nanoparticles and ligand length. These results suggest that the size ratio of nanoparticle diameter/ligand length within the range of 4.45-2.35 facilitates the formation of highly ordered 2D arrays. Furthermore, these arrays present excellent Surface-Enhanced Raman Scattering performances in the detection of trace thiram, which can cause environmental toxicity to the soil, water, animals and result in severe damage to human health. Therefore, the current study provides an effective way for preparing monodispersed hydrophobic gold nanoparticles and forming highly ordered 2D close-packed SERS substrate materials via self-assembly to detect pesticide residues in food. We believe that, our research provides not only advanced SERS substrate materials for excellent detection performance of thiram in food, but also novel fundamental understandings of self-assembly, manipulation of nanoparticle interactions, and controllable synthesis.

Self-Assembled Bimetallic Au-Ag Nanorod Vertical Array for Single Molecule Plasmonic Sensing

Ordered plasmonic nanoparticle arrays are highly desirable for optical sensing, as they provide uniformly distributed plasmonic hotspots due to their periodic order and near-field coupling. Anisotropic-shaped bimetallic nanoparticles are of particular interest as their hybridized plasmonic modes enable precise tuning of plasmonic resonance and optical responses. However, the controlled assembly of large-scale arrays of bimetallic nanoparticles with uniformly distributed hotspots remains a challenge. In this study, we present a highly robust and reproducible method for creating large-area vertically aligned arrays of bimetallic Au-Ag nanorods by epitaxially growing Ag over preassembled Au nanorods. Structural characterization using electron microscopy and X-ray photoelectron spectroscopy confirms the formation of a uniform thin layer of Ag, creating a bimetallic Au-Ag nanorod array. We also demonstrate the efficacy of the designed nanoarrays for Surface-Enhanced Raman Scattering (SERS) spectroscopy. Our experimental and computational studies show considerably enhanced optical responses of bimetallic Au-Ag nanorods compared to their monometallic counterparts. The scalability, cost-effectiveness, and reproducibility of this method make it a versatile platform for creating various structures by varying guest nanoparticles in suspensions, with broad applications in biomedical research, food safety surveillance, and environmental monitoring.

Mesoporous Silica-Layered Gold Nanorod Core@Silver Shell Nanostructures for Intracellular SERS Imaging and Phototherapy

Precision diagnosis-guided efficient treatment is crucial to extending the lives of cancer patients. The integration of surface-enhanced Raman scattering (SERS) imaging and phototherapy into a single nanoplatform has been considered a more accurate diagnosis and treatment strategy for cancer nanotheranostics. Herein, we constructed a new type of mesoporous silica-layered gold nanorod core@silver shell nanostructures loaded with methylene blue (GNR@Ag@mSiO2-MB) as a multifunctional nanotheranostic agent for intracellular SERS imaging and phototherapy. The synthesized GNR@Ag@mSiO2-MB nanostructures possessed a uniform core-shell structure, strong near-infrared (NIR) absorbance, photothermal conversion efficiency (65%), dye loading ability, SERS signal, and Raman stability under phototherapy conditions. Under single 785 nm NIR laser irradiation, the intracellular GNR@Ag@mSiO2-MB nanostructures were dramatically decreased to <9%, which showed excellent photothermal and photodynamic effects toward cancer cell killing, indicating that the combination of photothermal therapy (PTT) and photodynamic therapy (PDT) of the GNR@Ag@mSiO2-MB nanostructures could greatly enhance the therapeutic efficacy of cancer cell death. GNR@Ag@mSiO2-MB nanostructures demonstrated a strong Raman signal at 450 and 502 cm-1, corresponding to the δ(C-N-C) mode, suggesting that the Raman bands of GNR@Ag@mSiO2-MB nanostructures were more efficient to detect CT-26 cell SERS imaging with high specificity. Our results indicate that GNR@Ag@mSiO2-MB nanostructures offer an excellent multifunctional nanotheranostic platform for SERS imaging and synergistic anticancer phototherapy in the future.

Construction of Graphene@Ag-MLF composite structure SERS platform and its differentiating performance for different foodborne bacterial spores

A new surface-enhanced Raman spectroscopy (SERS) biosensor of Graphene@Ag-MLF composite structure has been fabricated by loading AgNPs on graphene films. The response of the biosensor is  based on plasmonic sensing. The results showed that the enhancement factor of three different spores reached 107 based on the Graphene@Ag-MLF substrate. In addition, the SERS performance was stable, with good reproducibility (RSD<3%). Multivariate statistical analysis and chemometrics were used to distinguish different spores. The accumulated variance contribution rate was up to 96.35% for the top three PCs, while HCA results revealed that the spectra were differentiated completely. Based on optimal principal components, chemometrics of KNN and LS-SVM were applied to construct a model for rapid qualitative identification of different spores, of which the prediction set and training set of LS-SVM achieved 100%. Finally, based on the Graphene@Ag-MLF substrate, the LOD of three different spores was lower than 102 CFU/mL. Hence, this novel Graphene@Ag-MLF SERS substrate sensor was rapid, sensitive, and stable in detecting spores, providing strong technical support for the application of SERS technology in food safety.

A ratiometric SERS sensor with one signal probe for ultrasensitive and quantitative monitoring of serum xanthine

Xanthine can be converted into uric acid, and a high concentration of xanthine in the human body can cause many diseases. Therefore, it is important to develop a sensitive, simple, and reliable approach for measuring xanthine in biological liquids. Hence, a ratiometric surface-enhanced Raman spectroscopy (SERS) sensing strategy with one signal probe was exploited for reliable, sensitive, and quantitative monitoring of serum xanthine. 3-Mercaptophenylboronic acid (3-MPBA) was used as a typical reference with a Raman peak at 996 cm-1. First, 3-MPBA was bound to gold nanoflowers@silica (GNFs@Si) through Au-S bonds. Xanthine oxidase (XOD) catalyzed the oxidation of xanthine into H2O2 on GNFs@Si. Afterward, the obtained H2O2 further reduced 3-MPBA to 3-hydroxythiophenol (3-HTP) accompanied by the emergence of a new Raman peak at 883 cm-1. Meanwhile, the Raman intensity at 996 cm-1 remained constant. Therefore, the ratio of I883/I996 increased with the increasing of xanthine concentration, thus realizing quantitative detection of xanthine. As a result, a ratiometric SERS sensor for the detection of xanthine was proposed with a detection limit of 5.7 nM for xanthine. The novel ratiometric SERS sensor provides a new direction for analyzing other biomolecules with high sensitivity and reliability.

Recent development of surface-enhanced Raman scattering for biosensing

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

Au nanogap SERS substrate for the carbaryl pesticide determination in juice and milk using chemomterics

Conventional spectroscopic methods like IR, and Raman are not very effective at detecting low levels of pesticides or harmful chemicals in food matrices. A quick, highly accurate approach that can identify pesticides present in different food products at lower levels must be developed in order to address this problem and ensure food safety. In this study, a highly sensitive and uniform wafer-scale Au nanogap surface-enhanced Raman spectroscopy (SERS) substrate was used for the quantitative analysis of carbaryl pesticide levels in standard solution, mango juice, and milk samples using chemometrics. Carbaryl was detected up to 3 ppb concentration levels for all three group of samples. However, due to the higher sensitivity, uniformity, and enhancement factors of the SERS substrate used in this investigation, the limit of detection (LOD) values for the standard solution, mango juice, and milk were 0.37 ppb, 0.57 ppb, and 0.15 ppb at 1380 cm^-1, 1380 cm^-1, and 1364 cm^-1 wavenumber ranges. In order to predict different carbaryl concentrations (1, 2, 3, 4, and 5 ppb), the variable importance in projection (VIP) method combined with partial least squares regression (PLSR) and attained the coefficient of determination (R²) values of 0.994, 0.989, and 0.978 along with minimum root mean square error (RMSE) values of 0.112, 0.190, and 0.278 ppb for the prediction datasets. Furthermore, PLS-DA was able to distinguish between pure and adulterated samples with the highest classification accuracy of 100 % for a standard solution, and mango juice and 94.4 % for milk samples. Considering this, we can conclude that the SERS Au Nanogap substrate can rapidly and effectively detect carbaryl pesticides quantitatively and qualitatively in mango juice and milk.

An accuracy improved ratiometric SERS sensor for rhodamine 6G in chili powder using a metal-organic framework support

Internal standard molecule 4-mercaptobenzoic acid (4-MBA) embedded Au core-Ag shell nanorods (Au-MBA@Ag NRs) were prepared by a seed-mediated growth method, then loaded on octahedral MIL-88B-NH₂ to obtain a novel ratiometric SERS substrate of Au-MBA@Ag NRs/PSS/MIL-88B-NH₂ (AMAPM) for detecting rhodamine 6G (R6G) in chili powder. The porous structure and excellent adsorption ability of MIL-88B-NH₂, allowed for increased loading of Au-MBA@Ag NRs, thereby shortening the distance between adsorbed R6G and the "hot spot" resulting from local surface plasmon resonance (LSPR) of Au-MBA@Ag NRs. Based on the SERS characteristic peak ratio of R6G to 4-MBA, the ratiometric SERS substrate displayed improved accuracy and excellent performance for R6G detection, with a wide linear range of 5-320 nM and a low detection limit of 2.29 nM as well as fine stability, reproducibility and specificity. The proposed ratiometric SERS substrate offered a simple, fast and sensitive sensing strategy for R6G detection in chili powder, which demonstrated potential applications in food safety and the analysis of trace analytes in complex matrices.

SERS-based error calibration of a TMB-H2O2 colorimetric system

3,3',5,5'-tetramethylbenzidine (TMB)-H₂O₂ is widely used as an effective colorimetric system, in which the color reaction is implemented with peroxidase-catalyzed TMB oxidation by H₂O₂ that usually measured UV-vis absorption spectra or Raman spectra. However, its low accuracy significantly limits its application. Blue charge transfer complex (CTC), which is the product of TMB and H₂O₂ reaction and is used as the basis for partial colorimetric methods, usually causes colorimetric error owing to changes in the UV-vis absorption and Raman spectra during TMB oxidation under various environmental conditions (catalyst type, temperature, H₂O₂ concentration). Herein, we propose a surface-enhanced Raman spectrum (SERS)-based error calibration method to improve the accuracy of the TMB-H₂O₂ colorimetric system. It is found that under 633 nm laser excitation, TMB has three Raman peaks at 1189, 1335 and 1609 cm^-1 in the single-electron oxidation phase, and these peaks disappear completely in the two-electron oxidation phase. By comparing these Raman peaks, we can conveniently obtain the actual process information during TMB oxidation. Using the proposed method, the accuracy of the TMB-H₂O₂ colorimetric system improved by more than 15%. Importantly, this SERS-based TMB-H₂O₂ error calibration method will open a new horizon for enzyme-linked immunosorbent assay (ELISA) and other biomedical applications.

Novel Microneedle Patch-Based Surface-Enhanced Raman Spectroscopy Sensor for the Detection of Pesticide Residues

Pesticide residues are a global threat to human health, and conventional sensors fail to simultaneously detect pesticide residues on the surface and inside agricultural products. In this work, we present a new microneedle (MN) patch-based surface-enhanced Raman spectroscopy (SERS) sensor. The needles and the basement of MNs can simultaneously detect pesticide residues on the surface and inside agricultural products. The Ag nanoparticles and sodium hyaluronate/poly(vinyl alcohol) (HA/PVA) hydrogel used in this MN patch-based sensor efficiently amplify the Raman signals of the pesticide residues. In addition, the HA/PVA hydrogel can effectively and quickly collect the residues, allowing this sensor to detect pesticide residues more conveniently. Furthermore, the stepped structure of the MNs increases the sensor's surface area. Experimental results show that the sensor can detect thiram and thiabendazole (TBZ) pesticide residues with detection limits of 10^-7 and 10^-8 M, respectively. The detection process is minimally invasive and not harmful to agricultural products. The application of this MN patch-based SERS sensor can be extended to the safety and health monitoring of other plants and animals.

Engineering an Ag/Au bimetallic nanoparticle-based acetylcholinesterase SERS biosensor for in situ sensitive detection of organophosphorus pesticide residues in food

Developing simple, efficient, and inexpensive method for trace amount organophosphorus pesticides' (OPs) detection with high sensitivity and specificity is of significant importance for guaranteeing food safety. Herein, an Ag/Au bimetallic nanoparticle-based acetylcholinesterase (AChE) surface-enhanced Raman scattering (SERS) biosensor was constructed for in situ simple and sensitive detection of pesticide residues in food. The principle of this biosensor exploited 4-mercaptophenylboronic acid (4-MPBA)-modified Ag/Au bimetallic nanoprobes as SERS signal probe to improve sensitivity and stability. The combination of AChE and choline oxidase (CHO) can hydrolyze acetylcholine (ATCh) to generate H₂O₂. The product of H₂O₂ selectively oxidizes the boronate ester of 4-MPBA, decreasing the Raman intensity of the B-O symmetric stretching. In the presence of OPs, it could inhibit the production of H₂O₂ by destroying the AChE activity, so the reduction of the SERS signal was also alleviated. Based on the principle, an Ag/Au bimetallic nanoparticle-based AChE SERS sensor was established without any complicated pretreatments. Benefiting from the synergistic effects of Ag/Au bimetallic hybrids, a linear detection range from 5×10^-9 to 5×10^-4 M was achieved with a limit of detection down to 1.7×10^-9 M using parathion-methyl (PM) as the representative model of OPs. Moreover, the SERS biosensor uses readily available reagents and is simple to implement. Importantly, the proposed SERS biosensor was used to quantitatively analyze OP residues in apple peels. The levels of OPs detected in real samples by this method were consistent with those obtained using gas chromatography-mass spectrometry (GC-MS), suggesting the proposed assay has great potential applications for OPs in situ detection in food safety fields.

Fabrication and evaluation of chitosan modified filter paper for chlorpyrifos detection in wheat by surface-enhanced Raman spectroscopy

BACKGROUND

Chlorpyrifos is a commonly used organophosphorus pesticide in agriculture. However, its neurotoxicity poses a huge threat to human health. In the present study, a chitosan-modified filter paper-based surface enhanced Raman scattering active substrate (Ch/AgNPs/paper) was fabricated and used to detect trace amounts of chlorpyrifos in 120 treated wheat samples.

RESULTS

Results showed that the Ch/AgNPs/paper substrate could be used to enhance the chlorpyrifos spectral fingerprint only up to a concentration of 0.000558 mg L^-1 . Following Raman spectra acquisition, three pre-processing methods, including Savitzky-Golay (Savitsky-Golay filter with a second order polynomial) smoothing with first derivative and second derivative and normalization, were used to reduce baseline variation and increase resolutions of spectral peak features of the original spectra dataset. Then, prediction models based on partial least squares were established for detecting chlorpyrifos pesticide residue in wheat. The partial least squares model with normalization yielded optimal result, with a correlation coefficient of 0.9764, root mean square error of prediction of 1.22 mg L^-1 in the prediction, and relative analysis deviation of 4.12. Five unknown samples were prepared to verify the accuracy of the prediction model. The predicted recoveries were calculated to be between 97.25% and 119.38% with an absolute t value of 0.598. The value of a t-test shows that the prediction model is accurate and reliable.

CONCLUSION

The present study demonstrates that the proposed method can achieve rapid detection of chlorpyrifos in wheat. © 2022 Society of Chemical Industry.

Au Nanorods on Carbon-Based Nanomaterials as Nanohybrid Substrates for High-Efficiency Dynamic Surface-Enhanced Raman Scattering

Gold nanorods (AuNRs) with different aspect ratios were prepared by the seed-mediated growth method and combined with three carbon-based nanomaterials of multiple dimensions (i.e., zero-dimensional (0D) carbon black (CB), one-dimensional (1D) carbon nanotubes (CNTs), and two-dimensional (2D) graphene oxide (GO)). The AuNR/carbon-based nanomaterial hybrids were utilized in dynamic surface-enhanced Raman scattering (D-SERS). First, cetyltrimethylammonium bromide (CTAB) was used to stabilize and coat the AuNRs, enabling them to be dispersed in water and conferring a positive charge to the surface. AuNR/carbon-based nanomaterial hybrids were then formed via electrostatic attraction with the negatively charged carbon-based nanomaterials. Subsequently, the AuNR/carbon-based nanomaterial hybrids were utilized as large-area and highly sensitive Raman spectroscopy substrates. The AuNR/GO hybrids afforded the best signal enhancement because the thickness of GO was less than 5 nm, which enabled the AuNRs adsorbed on GO to produce a good three-dimensional hotspot effect. The enhancement factor (EF) of the AuNR/GO hybrids for the dye molecule Rhodamine 6G (R6G) reached 1 × 10⁷, where the limit of detection (LOD) was 10^-8 M. The hybrids were further applied in D-SERS (detecting samples transitioning from the wet state to the dry state). During solvent evaporation, the system spontaneously formed many hotspots, which greatly enhanced the SERS signal. The final experimental results demonstrated that the AuNR/GO hybrids afforded the best D-SERS signal enhancement. The EF value for R6G reached 1.1 × 10⁸ after 27 min, with a limit of detection of 10^-9 M at 27 min. Therefore, the AuNR/GO nanohybrids have extremely high sensitivity as molecular sensing elements for SERS and are also very suitable for the rapid detection of single molecules in water quality and environmental management.

Sensitive and handy detection of pesticide residue on fruit surface based on single microsphere surface-enhanced Raman spectroscopy technique

HYPOTHESIS

Surface-enhanced Raman spectroscopy (SERS) has become an emerging and reliable tool for detecting pesticide residues due to its high sensitivity, fast testing speed and easy sample handling. SERS active substrates are the key to achieve efficient and sensitive detection. However, for the most widely used noble metal nanoparticles, there are problems of high noble metal nanoparticle usage and random aggregation. The micron-scale Raman spot is focused on multiple randomly aggregated nanoparticles during the test, resulting in poor reproducibility. Therefore, the development of micron-scale cost-effective SERS substrates with good reproducibility and simple detecting method is of great significance in practical detection.

EXPERIMENTS

Through deposition of silver nanoparticles (Ag-NPs) by chemical reduction on the surface of monodisperse sulfonated polystyrene (SPS) microspheres, micron-sized PS@Ag-NPs core-shell microspheres were prepared with excellent SERS activity. After that, two simple protocols (Method I and Method II) were explored for the determination of thiram on apple epidermis.

FINDINGS

Based on our developed strategy of the single microsphere SERS technique, we successfully fabricated uniform PS@Ag-NPs substrate with high SERS activity and excellent detection sensitivity. The single microsphere SERS technique possesses the capability of anti-dilutability and the utilization of ultra-low PS@Ag-NPs microsphere dosage, realizing qualitative and quantitative detection of thiram on apple with detection limits far below the standard stipulated by China and the European Union.

Identification of milk quality and adulteration by surface-enhanced infrared absorption spectroscopy coupled to artificial neural networks using citrate-capped silver nanoislands

Milk is one of the most important multicomponent superfoods owing to its rich macronutrient composition. It requires quality control at all the production stages from the farm to the finished products. A localized surface plasmon resonance optical sensor based on a citrate-capped silver nanoparticle (Cit-AgNP)–coated glass substrate was developed. The fabrication of such sensors involved a single-step synthesis of Cit-AgNPs followed by surface modification of glass slides to be coated with the nanoparticles. The scanning electron microscope micrographs demonstrated that the nanoparticles formed monolayer islands on glass slides. The developed surface-enhanced infrared absorption spectroscopy (SEIRA) sensor was coupled to artificial neural networking (ANN) for the qualitative differentiation between cow, camel, goat, buffalo, and infants’ formula powdered milk types. Moreover, it can be used for the quantitative determination of the main milk components such as fat, casein, urea, and lactose in each milk type. The qualitative results showed that the obtained FTIR spectra of cow and buffalo milk have high similarity, whereas camel milk resembled infant formula powdered milk. The most difference in FTIR characteristics was evidenced in the case of goat milk. The developed sensor adds several advantages over the traditional techniques of milk analysis using MilkoScan™ such as less generated waste, elimination of pre-treatment steps, minimal sample volume, low operation time, and on-site analysis.

Digital image colorimetry in combination with chemometrics for the detection of carbaryl based on the peroxidase-like activity of nanoporphyrins and the etching process of gold nanoparticles

Carbaryl is a typical carbamate pesticide that plays an essential role in agricultural production, but its residues cause serious harm to the environment and human health. Here, we developed a polychromatic colorimetric sensor based on ZnTPyP-DTAB peroxidase activity and gold nano-bipyramids (Au NBPs) etching to detect carbaryl. ZnTPyP-DTAB catalyzes the decomposition of H₂O₂ to hydroxyl radicals, and Au NBPs are etched. The coordination of zinc and nitrogen in nanometer porphyrins was affected by the steric effects of carbaryl, which resulted in decreased activity of ZnTPyP-DTAB peroxidase. The detection limit of carbaryl was 0.26 mg/kg. The recoveries of carbaryl in reaal sample ranged from 91 % to 107% (RSD ≤ 0.7%). The sensor platform displayed a series of high-resolution multicolor variations of rainbow colors within the above concentration range. The rich color variation facilitates the acquisition of digital images. RGB value transformation combined with partial least squares regression model can accurately and quantitatively detect carbaryl in vegetables, fruits and Chinese medicinal materials.

Activated Porous Carbon Supported Pd and ZnO Nanocatalysts for Trace Sensing of Carbaryl Pesticide in Water and Food Products

Nanomaterials-based sensors are a dire need for credible and accurate determination of pesticides in water and food samples as a monitoring tool. Herein, electrocatalysts of Pd and ZnO NPs supported...

Nanotheranostic Probe Built on Methylene Blue Loaded Cucurbituril [8] and Gold Nanorod: Targeted Phototherapy in Combination with SERS Imaging on Breast Cancer Cells

Recent advancements in a nanoarchitecture platform for safe and effective targeted phototherapy in a synergistic fashion is an absolute necessity in localized cancer therapy. Photothermal and photodynamic therapies (PTT and PDT) are considered as the most promising localized therapeutic intervention for cancer management as they have no long-term side effects and are minimally invasive and affordable. Herein, we have demonstrated a tailor-made nanotheranostic probe in which macrocyclic host cucurbituril [8] (CB[8]) is placed as a glue between two gold nanorods (GNRs) within ∼3 nm gaps in linear nanoassemblies with exquisitely sensitive plasmonics that exert combined phototherapy to investigate the therapeutic progression on human breast cancer cells. Photosensitizer methylene blue was positioned on CB[8] to impart the PDT effect, whereas GNR was responsible for PTT on a single laser trigger ensuring the synchronized phototherapy. Furthermore, the nanoconstruct was tagged with targeting anti-Her2 monoclonal antibody (MB-CB[8]@GNR-anti-Her2) for localized PTT and PDT on Her2 positive SKBR3 cells, subsequent cellular recognition by surface-enhanced Raman spectroscopy (SERS) platform, and further assessment of the combined intracellular phototherapy. Hence, the current strategy is definitely marked as a proof-of-concept straightforward approach that implies the perfect nature of the combined phototherapy to achieve an efficient cancer treatment.

Recent Advancements in the Technologies Detecting Food Spoiling Agents

To match the current life-style, there is a huge demand and market for the processed food whose manufacturing requires multiple steps. The mounting demand increases the pressure on the producers and the regulatory bodies to provide sensitive, facile, and cost-effective methods to safeguard consumers' health. In the multistep process of food processing, there are several chances that the food-spoiling microbes or contaminants could enter the supply chain. In this contest, there is a dire necessity to comprehend, implement, and monitor the levels of contaminants by utilizing various available methods, such as single-cell droplet microfluidic system, DNA biosensor, nanobiosensor, smartphone-based biosensor, aptasensor, and DNA microarray-based methods. The current review focuses on the advancements in these methods for the detection of food-borne contaminants and pathogens.

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.

Symmetric and asymmetric overgrowth of a Ag shell onto gold nanorods assisted by Pt pre-deposition

The plasmonic properties of noble metallic nanoparticles could be tuned by morphology and composition, enabling opportunities for applications in sensors, photocatalysis, biomedicine, and energy conversion. Here, we report a method of the symmetric and asymmetric overgrowth of a Ag shell onto gold nanorods assisted by Pt pre-deposition via a 2-step approach. Firstly, gold nanorods (AuNRs), synthesized via a seed-mediated method, were used as seeds to form a AuNR–Pt structure, by using K₂PtCl₄ as the precursor. In this step, most of the Pt material was selectively deposited on the tips of the AuNR. Secondly, by using AgNO₃ as the precursor, a Ag shell was overgrown on the surface of the AuNRs–Pt nanoparticles, resulting in a (AuNR–Pt)–Ag core–shell tri-metallic nanostructure. Due to the surface energy and lattice matching between Au and Ag, the Ag shell preferred to be epitaxially overgrown on the side of AuNR. The Ag shell thickness and symmetry of the (AuNR–Pt)–Ag could be tuned by changing the amounts of AgNO₃ precursor. With the increase of the Ag shell thickness, the (AuNR–Pt)–Ag nanostructures changed from symmetric to asymmetric. The obtained (AuNR–Pt)–Ag nanostructures were studied using UV-vis-NIR spectroscopy, transmission electron microscopy, EDS mapping, DLS, and ICP-MS. The growth mechanism was discussed.

Demonstrating asymmetric (AuNR–Pt)–Ag tri-metallic nanostructures by a two-step seed-mediated method. The shell thickness was controlled by the amount of AgNO₃.

Nanostructure-Based Surface-Enhanced Raman Spectroscopy Techniques for Pesticide and Veterinary Drug Residues Screening

Pesticide and veterinary drug residues in food and environment pose a threat to human health, and a rapid, super-sensitive, accurate and cost-effective analysis technique is therefore highly required to overcome the disadvantages of conventional techniques based on mass spectrometry. Recently, the surface-enhanced Raman spectroscopy (SERS) technique emerges as a potential promising analytical tool for rapid, sensitive and selective detections of environmental pollutants, mostly owing to its possible simplified sample pretreatment, gigantic detectable signal amplification and quick target analyte identification via finger-printing SERS spectra. So theoretically the SERS detection technology has inherent advantages over other competitors especially in complex environmental matrices. The progress in nanostructure SERS substrates and portable Raman appliances will promote this novel detection technology to play an important role in future rapid on-site assay. This paper reviews the advances in nanostructure-based SERS substrates, sensors and relevant portable integrated systems for environmental analysis, highlights the potential applications in the detections of synthetic chemicals such as pesticide and veterinary drug residues, and also discusses the challenges of SERS detection technique for actual environmental monitoring in the future.

Point-of-Care Detection of Salivary Nitrite Based on the Surface Plasmon-Assisted Catalytic Coupling Reaction of Aromatic Amines

Rapid quantification of nitrite (NO₂^-) in food, drink and body fluids is of significant importance for both food safety and point-of-care (POA) applications. However, conventional nitrite analytical methods are complicated, constrained to sample content, and time-consuming. Inspired by a nitrite-triggered surface plasmon-assisted catalysis (SPAC) reaction, a rapid point-of-care detection salivary nitrate was developed in this work. NO₂^- ions can trigger the rapid conversion of p-aminothiophenol (PATP) to p,p'-dimercaptozaobenzene (DMAB) on gold nanoparticles (GNPs) under light illumination, and the emerged new bands at ca. 1140, 1390, 1432 cm^-1 originating from DMAB can be used to the quantification of nitrite. Meanwhile, to make the method entirely suitable for on-site fast screen or point-of-care application, the technique is needed to be further optimized. The calibration graph for nitrates was linear in the range of 1-100 µM with a correlation coefficient of 0.9579. The limit of detection was 1 µM. The facile method could lead to a further understanding of the progression and treatment of periodontitis and to guide professionals in planning on-site campaigns to effectively control periodontal diseases.

Quantitative detection of dithiocarbamate pesticides by surface-enhanced Raman spectroscopy combined with an exhaustive peak-seeking method

Surface-enhanced Raman spectroscopy (SERS) based on nanosilver colloid substrates has great potential for rapid detection of pesticide residues because of its advantages of sensitivity, rapidity, simplicity, low cost, etc. However, its poor repeatability and narrow linear quantitative range limit its practical application. In this paper, a silver colloid SERS analysis method combined with an exhaustive peak-seeking method was introduced for quantitative determination of thiram and ziram. This method can establish a linear quantitative relationship in a wide range by use of an own characteristic peak of analysis as an internal standard (IS) which is found via judging the linear correlation between the intensity ratios of two SERS peaks of analytes and the concentrations. Combined with improving the preparation method of silver colloids, adding suitable activators and optimizing the detection process, a liquid detection system with good repeatability and a wide linear quantitation range was obtained. The relative standard deviation (RSD) of the strongest SERS peak is no more than 8.98%, which is better than the general case of the silver colloid SERS substrate. The ratio of I₁₃₈₄/I₁₁₄₈ has a good linear relationship with the concentration of thiram solution, and the 1148 cm^-1 characteristic peak was utilized as the IS to establish the standard curve equation for the determination of thiram concentration. The equation is I₁₃₈₄/I₁₁₄₈ = -1.7930 × lg[c_thiram (ppm)] + 6.0078 with a linear range of 10^-2 to 10² ppm (4.16 × 10^-8 to 4.16 × 10^-4 mol L^-1) and a limit of detection (LOD) of 10^-2 ppm. The peak of IS for the determination of ziram concentration is at 938 cm^-1, and the equation is I₁₃₈₄/I₉₃₈ = 4.5531 × lg[c_ziram (ppm)] + 6.4792 with a linear range of 10^-1 to 10² ppm (3.27 × 10^-7 to 3.27 × 10^-4 mol L^-1) and a LOD of 10^-4 ppm. Thiram or ziram in apple juice was successfully detected by using this liquid detection system. This analysis system effectively solves the problem of poor repeatability and a narrow linear quantification range in SERS analysis based on silver colloid substrates, and the linear quantification range meets the requirements of the national standard (GB-2763-2019).

Optimization of gold nanorod arrays for surface enhanced Raman spectroscopy (SERS) detection of atrazine

Recently, there has been increasing concern over the widespread use of the herbicide atrazine which has been reported to have problematic side effects on local ecosystems. This has highlighted the need for rapid and accurate point-of-need assessment tools for analytical determination of herbicides in ground and surface waters. Surface enhanced Raman spectroscopy (SERS) is a sensitive vibrational spectroscopy technique which has recently been employed for the analysis of a variety of analytes in water, ranging from pharmaceuticals to pesticides. In this work, SERS sensors constructed using gold nanorod (AuNR) arrays are optimized and then utilized for the rapid and sensitive detection of atrazine. In this study, the effect of relative humidity on the self-assembly of gold nanorods into arrays was explored, and the SERS performance was assessed using para-aminothiophenol as a SERS probe. Once the SERS performance of the substrates was deemed optimal, the detection of atrazine was highlighted. This work represents the first time that relative humidity has been explored as an optimization strategy for controlled alignment of gold nanorods for SERS analysis of atrazine.

A copper nanoparticle-based electrochemical immunosensor for carbaryl detection

A hapten-protein conjugate with copper nanoparticles (Hap-Car-BSA@CuNPs) was first synthesized in the present work for the determination of carbaryl. The copper nanoparticles (CuNPs) of the conjugate were used as electrochemical labels in the direct solid-phase competitive determination of carbaryl residues in flour from different crops. The signal was read by linear sweep anodic stripping voltammetry (LSASV) of copper (through the electrochemical stripping of accumulated elemental copper) on a gold-graphite electrode (GGE). To form a recognition receptor layer of monoclonal antibodies against the carbaryl on the surface of the GGE, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) and 1-hydroxy-2,5-pyrrolidinedione (NHS) were used as the best covalent cross-linkers. The concentrations of the antibodies and the Hap-Car-BSA@CuNPs conjugate were optimized for carbaryl detection by the electrochemical immunosensor. The electrochemical immunosensor can be used for highly sensitive determination of carbaryl residues in flour samples in the concentration range 0.8-32.3 μg·kg^-1, with a limit of detection 0.08 μg·kg^-1. The present work paves the path for a novel method for monitoring carbaryl in other food products, drinks, and soil samples.

Towards a point-of-care SERS sensor for biomedical and agri-food analysis applications: a review of recent advancements

The growing demand for reliable and robust methodology in bio-chemical sensing calls for the continuous advancement of sensor technologies. Over the last two decades, surface-enhanced Raman spectroscopy (SERS) has emerged as one of the most promising analytical techniques for sensitive and trace analysis or detection in biomedical and agri-food applications. SERS overcomes the inherent sensitivity limitation associated with Raman spectroscopy, which provides vibrational "fingerprint" spectra of molecules that makes it unique and versatile among other spectroscopy techniques. This paper comprehensively reviews the recent advancements of SERS for biomedical, food and agricultural applications over the last 6 years, and we envision that, in the near future, some of these platforms have the potential to be translated as a point-of-care and rapid sensor for real-life end-user applications. The merits and limitations of various SERS sensor designs are analysed and discussed based on critical features such as sensitivity, specificity, usability, repeatability and reproducibility. We conclude by highlighting the opportunities and challenges in the field while stressing the technological gaps to be addressed in realizing commercially viable point-of-care SERS sensors for practical biomedical and agri-food technological applications.

Core-shell magnetic Ag-molecularly imprinted composite for surface enhanced Raman scattering detection of carbaryl

Surface-enhanced Raman scattering (SERS) is a promising technique for rapid detection of pesticide residues. However, conventional SERS substrates require extraction processes which are time consuming and they also lack selectivity, stability and reproducibility. Herein, we present a multifunctional stable zero-valent iron based core-shell substrate. It combines magnetic separation, selective adsorption by molecular imprinting technique and sensitive detection of carbaryl by SERS. The core-shell substrate was successfully prepared by immobilizing silver on the surface of zero-valent iron microspheres. Subsequent molecular imprinting on the bimetallic magnetic silver microspheres ensured selective removal and detection. The substrate exhibited magnetization saturation of 8.89 emu/g providing efficient analyte separation. It showed high sensitivity and selectivity toward carbaryl detection to nanomolar concentration level. Linear regression models for peaks at Raman shift 1599 cm^-1 and 2233 cm^-1 demonstrated a good linear fit with R²=0.9738 and R²=0.8952 respectively. The composite was successfully applied on spiked water samples resulting in average recovery rate of 89%. The findings of this study demonstrate great substrate potential for application in separation and detection of trace quantities of chemical contaminants for environment safety and protection.

A 3D Plasmonic Crossed-Wire Nanostructure for Surface-Enhanced Raman Scattering and Plasmon-Enhanced Fluorescence Detection

In this manuscript, silver nanowire 3D random crossed-wire woodpile (3D-RCW) nanostructures were designed and prepared. The 3D-RCW provides rich "antenna" and "hot spot" effects that are responsive for surface-enhanced Raman scattering (SERS) effects and plasmon-enhanced fluorescence (PEF). The optimal construction mode for the 3D-RCW, based on the ratio of silver nanowire and control compound R6G, was explored and established for use in PEF and SERS analyses. We found that the RCW nanochip capable of emission and Raman-enhanced detections uses micro levels of analysis volumes. Consequently, and SERS and PEF of pesticides (thiram, carbaryl, paraquat, fipronil) were successfully measured and characterized, and their detection limits were within 5 μM~0.05 µM in 20 µL. We found that the designed 3D plasmon-enhanced platform cannot only collect the SERS of pesticides, but also enhance the fluorescence of a weak emitter (pesticides) by more than 1000-fold via excitation of the surface plasmon resonance, which can be used to extend the range of a fluorescence biosensor. More importantly, solid-state measurement using a 3D-RCW nanoplatform shows promising potential based on its dual applications in creating large SERS and PEF enhancements.

Tetragonal Superlattice of Elongated Rhombic Dodecahedra for Sensitive SERS Determination of Pesticide Residues in Fruit

The self-assembly of plasmonic nanoparticles into highly ordered superlattices could pave the way toward novel nanomaterials for surface-enhanced Raman scattering (SERS). Here, we propose the formation of large-area superlattices of elongated rhombic dodecahedra in a vertical orientation via a controlled droplet evaporation process. Expectedly, the constant humidity of the experimental condition could control the evaporation speed of droplets and this procedure promotes the balance between driven depletion attraction and electrostatic repulsion in the system, leading to the generation of well-organized three-dimensional (3D) superlattices. The unique geometry of elongated rhombic dodecahedra could establish the tetragonal superlattices, which breaks the conventional hexagonal symmetry of gold nanorods. Specifically, the influence of the type and concentration of surfactants, the concentration of nanoparticles, and the amount of droplets on the preparation results were systematically investigated to find the optimal assembly parameters. Remarkably, such close-packed tetragonal arrays of vertically aligned elongated rhombic dodecahedra exhibit more excellent SERS performance compared with the traditional hexagonal superstructure of gold nanorods. Benefiting from the high sensitivity and reproducibility of elongated rhombic dodecahedron superlattices, their applications in the determination of pesticide residues in apple and grape peels were successfully demonstrated. As a result, this study may advance the production of innovative plasmonic nanomaterials for a broad range of fields.

Applications of surface-enhanced Raman spectroscopy in detection fields

Surface-enhanced Raman spectroscopy (SERS) is a Raman spectroscopy technique that has been widely used in food safety, environmental monitoring, medical diagnosis and treatment and drug monitoring because of its high selectivity, sensitivity, rapidness, simplicity and specificity in identifying molecular structures. This review introduces the detection mechanism of SERS and summarizes the most recent progress concerning the use of SERS for the detection and characterization of molecules, providing references for the later research of SERS in detection fields.

Fabrication of nano/microstructures for SERS substrates using an electrochemical method

Based on an electrochemical method, three-dimensional arrayed nanopore structures are machined onto a Mg surface. The structured Mg surface is coated with a thin gold (Au) film, which is used as a surface-enhanced Raman scattering (SERS) substrate. A rhodamine 6G (R6G) probe molecule is used as the detection agent for the SERS measurement. Different sizes of arrayed micro/nanostructures are fabricated by different treatment time using the electrochemical process. The topographies of these micro/nanostructures and the thickness of the Au film have an influence on the Raman intensity of the Mg substrate. Furthermore, when the thickness of Au film coating is held constant, the Raman intensity on the structured Mg substrates is about five times higher after a treatment time of 1 min when compared with other treatment times. The SERS enhancement factor ranges from 10⁶ to 1.75 × 10⁷ under these experimental conditions. Additionally, a 10^-6 mol·L^-1 solution of lysozyme was successfully detected using the Mg-Au nanopore substrates. Our low-cost method is reproducible, homogeneous, and suitable for the fabrication of SERS substrates.

Gold Nanorods as an Efficient Substrate for the Detection and Degradation of Pesticides

The rapid, ultralow detection, degradation, and complete removal of pesticides demand the design of potential substrates. Herein, we discussed gold nanorods (Au NRs) as the potential substrate for the naked eye detection and degradation of two common and broad-spectrum pesticides, chlorpyrifos (CPF) and malathion (MLT), up to 0.15 ppt concentration within 2 min. Under certain environmental conditions, both the pesticides degraded and adsorbed on the surface of Au NRs. The degraded moieties of CPF and MLT on the surface of Au NRs formed side-to-side and end-to-end interactions, respectively, leading to a long-range assembly. This shows that no external agent is required, and only CPF and MLT analytes are quite enough for the formation of assembly of Au NRs. Assembly of Au NRs is confirmed by transmission electron microscopy (TEM) analysis, and degradation is supported by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and gas chromatography-mass spectrometry (GC-MS) analyses. Au NRs were recovered and reused for four consecutive cycles. The fast and ultralow detection of pesticides demonstrates that Au NRs are a potential substrate for the detection and degradation of pesticides.