Electrokinetic Preseparation and Molecularly Imprinted Trapping for Highly Selective SERS Detection of Charged Phthalate Plasticizers

Low-cost and sensitive chemiluminescence detection of phthalates in environment by signal sensing of carbon-based materials from PVC/coal gangue dechlorination

BACKGROUND

The detection of plasticizers in the environment is important to prevent environmental risks and people's health hazards. Improving recycling efficiency of waste PVC still faced challenges.

RESULTS

In this work, it was found that solid products from waste PVC/coal gangue dechlorination in subcritical water (dPVC) had strong catalysis activity for luminol-H2O2 chemiluminescence (CL) reaction. Phthalates, common plasticizers, could bond and adsorb on dPVC, which greatly inhibited the luminol-H2O2-dPVC CL reaction. Based on this, a low-cost CL analysis was constructed for the detection of phthalates combinations (PACs) and di-(2-ethylhexyl) phthalate (DEHP) in the environment. The detection limit for PACs and DEHP was 0.048 ng/L and 0.13 ng/L, respectively. Compared with HPLC standard method, the dPVC CL analysis had accuracy and reliability for the detection of phthalates in actual environmental samples. Besides, the results of life cycle assessment (LCA) revealed that dPVC for CL sensing materials had significantly small global warming potential (GWP).

SIGNIFICANCE

The use of dPVC for CL sensing not only improved the recycling efficiency of PVC, but also reduced carbon emissions of obtaining CL sensing materials.

Raman and Surface-Enhanced Raman Scattering Detection in Flowing Solutions for Complex Mixture Analysis

Raman scattering provides a chemical-specific and label-free method for identifying and quantifying molecules in flowing solutions. This review provides a comprehensive examination of the application of Raman spectroscopy and surface-enhanced Raman scattering (SERS) to flowing liquid samples. We summarize developments in online and at-line detection using Raman and SERS analysis, including the design of microfluidic devices, the development of unique SERS substrates, novel sampling interfaces, and coupling these approaches to fluid-based chemical separations (e.g., chromatography and electrophoresis). The article highlights the challenges and limitations associated with these techniques and provides examples of their applications in a variety of fields, including chemistry, biology, and environmental science. Overall, this review demonstrates the utility of Raman and SERS for analysis of complex mixtures and highlights the potential for further development and optimization of these techniques.

All-in-One Electrokinetic Strategy Coupled with a Miniaturized Chip for SERS Detection of Multipesticides

Complexed and tiresome pretreatment processes have significantly impeded in-field analysis of environmental specimens. Herein, an all-in-one sample separation and enrichment strategy based on a compact charge-selective capture/nanoconfined enrichment (CSC/NCE) device is exploited for marker-free surface-enhanced Raman spectroscopy (SERS) detection of charged pesticides in matrix specimens. This tactic incorporating in situ separations, seizing, and nanoconfined enhancement can greatly elevate the effectiveness of sample pretreatment. Importantly, CSC/NCE with excellent adsorption performances and excellent plasmonic features facilitates concentration and signal amplification of electrically charged pesticides. With the introduction of an electric field on this integrated CSC/NCE, the matrix effect in samples could be significantly eradicated, and a distinct SERS response is witnessed for targeted analytes. Accurate quantification of multipesticides is achieved by synergizing the CSC/NCE chip and chemometrics, and the contents found by the CSC/NCE-based sensing strategy agree with those obtained from chromatography assays with relative deviations lower than 10%. The facile and versatile all-in-one tactic infused in a compact chip exhibits enormous potential for field-test application in chemical measurement and food safety.

Rapid Sample Pretreatment Facilitating SERS Detection of Trace Weak Organic Acids/Bases in Complex Matrices

Fast and efficient sample pretreatment is the prerequisite for realizing surface-enhanced Raman spectroscopy (SERS) detection of trace targets in complex matrices, which is still a big issue for the practical application of SERS. Recently, we have proposed a highly performed liquid-liquid extraction (LLE)-back extraction (BE) for weak acids/bases extraction in drinking water and beverage samples. However, the performance efficiency decreased drastically on facing matrices like food and biological blood. Based on the total interaction energies among target, interferent, and extractant molecules, solid-phase extraction (SPE) with a higher selectivity was introduced in advance of LLE-BE, which enabled the sensitive (μg L-1 level) and rapid (within 10 min) SERS detection of both koumine (a weak base) and celastrol (a weak acid) in different food and biological samples. Further, the high SERS sensitivity was determined unmanned by Vis-CAD (a machine learning algorithm), instead of the highly demanded expert recognition. The generality of SPE-LLE-BE for various weak acids/bases (2 < pKa < 12), accompanied by the high efficiency, easy operation, and low cost, offers SERS as a powerful on-site and efficient inspection tool in food safety and forensics.

Highly sensitive SERS sensors for glucose detection based on enzyme@MOFs and ratiometric Raman

Diabetes is a common chronic metabolic disease. The frequent fluctuation of glucose is the main cause of most diabetes complications, which in turn causes harm to the health of patients. Surface-enhanced Raman scattering (SERS) spectroscopy has attracted much attention in the rapid detection of glucose due to its unique molecular fingerprinting ability, ultra-high sensitivity and fast response. However, due to the low affinity between glucose and SERS substrate, poor signal, susceptibility to complex environmental interference, and poor stability of SERS detection, it is still a challenge for SERS to accurately and sensitively determine glucose in complex environments. In this work, we encapsulated 4-mercaptobutyronitrile (4-MBN) as an internal standard (IS) in Au@Ag NRs inside and then Au@4-MBN@Ag NRs, Leucomalachite Green (LMG), glucose oxidase (GOx) and horseradish peroxidase (HPR) were encapsulated in ZIF-8 to prepare a tandem enzyme catalytic ratiometric SERS sensor Au@4-MBN@Ag@LMG@ZIF-8(GOx, HPR) for the detection of glucose in saliva. Because ZIF-8 enhanced the catalytic activity of the enzyme, the ability of glucose enrichment, and weakens the aggregation of Ag NRs. The internal standard signal molecule improves the accuracy and sensitivity of detection. The ratiometric Raman signal I412/I2233 of glucose has a good linear relationship with the concentration in the range of 0.1-100 μM, and the limit of detection (LOD) could be down to 0.03 μM. At the same time, it has excellent selectivity, repeatability and accuracy. The recovery rate of glucose in saliva is 96.50%-105.56 %, which proves the feasibility of the method. The Au@4-MBN@Ag@LMG@ZIF-8(GOx, HPR) sensor prepared in this study showed excellent SERS performance, which was able to detect glucose quickly, sensitively and accurately. This work provides a new strategy for the design of enzyme-catalyzed SERS sensors.

Lanthanide metal-organic framework-based surface molecularly imprinted polymers ratiometric fluorescence probe for visual detection of perfluorooctanoic acid with a smartphone-assisted portable device

Perfluorooctanoic acid (PFOA) poses a threat to the environment and human health due to its persistence, bioaccumulation, and reproductive toxicity. Herein, a lanthanide metal-organic framework (Ln-MOF)-based surface molecularly imprinted polymers (SMIPs) ratiometric fluorescence probe (Eu/Tb-MOF@MIPs) and a smartphone-assisted portable device were developed for the detection of PFOA with high selectivity in real water samples. The integration of Eu/Tb MOFs as carriers not only had highly stable multiple emission signals but also prevented deformation of the imprinting cavity of MIPs. Meanwhile, the MIPs layer preserved the fluorescence of Ln-MOF and provided selective cavities for improved specificity. Molecular dynamics (MD) was employed to simulate the polymerization process of MIPs, revealing that the formation of multiple recognition sites was attributed to the establishment of hydrogen bonds between functional monomers and templates. The probe showed a good linear relationship with PFOA concentration in the range of 0.02-2.8 μM, by giving the limit of detection (LOD) of 0.98 nM. Additionally, The red-green-blue (RGB) values analysis based on the smartphone-assisted portable device demonstrated a linear relationship of 0.1-2.8 μM PFOA with the LOD of 3.26 nM. The developed probe and portable device sensing platform exhibit substantial potential for on-site detecting PFOA in practical applications and provide a reliable strategy for the intelligent identification of important targets in water environmental samples.

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.

Point-of-Use SERS Approach for Efficient Determination and Removal of Phthalic Acid Esters Based on a Metal-Organic Framework-Coated Melamine Sponge

Phthalic acid esters (PAEs) are ubiquitous environmental contaminants, and their real-time monitoring and removal remain challenging. Herein, a point-of-use (POU) device integrating adsorption, surface-enhanced Raman spectroscopy (SERS), and removal strategy was developed and realized ultrafast on-site determination of PAEs and cleanup of them from water. A piece of flexible melamine sponge (MS) was coated with gold nanostars (AuNSs) and metal-organic frameworks (MOFs), thus obtaining SERS activity and adsorption capacity. Based on this multifunctional AuNSs@MOFs/MS composite, clear trends were observed between SERS signal intensity and concentration of di(2-ethylhexyl)phthalate (DEHP) and dibutyl phthalate (DBP). The method detection limits of DEHP and DBP were calculated to be 0.75 × 10-7 and 0.67 × 10-7 M in water, respectively, proving good sensitivity. Furthermore, this POU device exhibited satisfactory adsorption capacity (∼82.3 g/g for DBP and ∼90.0 g/g for DEHP), high adsorption efficiency (equilibrium in 100 s), and good regeneration capability (removal efficiency >70% after 5 cycles). The applicability of this device was verified by its good determination and removal performance in real environmental water matrices. The whole process could be completed within 5 min. This approach provides a new POU alternative for real-time monitoring and removal of PAEs in water.

Surface-Enhanced Raman Scattering in Silver-Coated Suspended-Core Fiber

In this paper, the silver-coated large-core suspended-core fiber (LSCF) probe was fabricated by the dynamic chemical liquid phase deposition method for surface-enhanced Raman scattering (SERS) sensing. The 4-mercaptophenylboronic acid (4-MPBA) monolayer was assembled in the LSCF as the recognition monolayer. Taking advantage of the appropriate core size of the LSCF, a custom-made Y-type optical fiber patch cable was utilized to connect the semiconductor laser, Raman spectrometer, and the proposed fiber SERS probe. The SERS signal is propagated in the silver-coated air channels, which can effectively reduce the Raman and fluorescence background of the silica core. Experiments were performed to measure the Raman scattering spectra of the 4-MPBA in the silver-coated LSCF in a non-enhanced and enhanced case. The experiment results showed that the Raman signal strength was enhanced more than 6 times by the surface plasmon resonance compared with the non-enhanced case. The proposed LSCF for SERS sensing technology provides huge research value for the fiber SERS probes in biomedicine and environmental science. The combination of SERS and microstructured optical fibers offers a potential approach for SERS detection.

Trace analysis of food by surface-enhanced Raman spectroscopy combined with molecular imprinting technology: Principle, application, challenges, and prospects

Surface-enhanced Raman spectroscopy (SERS) is a rapid detection method with high sensitivity and simple pretreatment, but can be affected by interference from matrix components. By incorporating molecularly imprinted polymers (MIPs) that recognize specific targets, MIP-SERS sensors effectively overcome the interference of complex matrices and offer improved stability and sensitivity. This review provides a comprehensive understanding of the applications of MIP-SERS sensors for the detection of trace toxic substances in food. The underlying mechanism and development of SERS technology and the principle and classification of MIPs technology are discussed. Furthermore, the types of MIP-SERS sensors are introduced, with their advantages and disadvantages systematically illustrated. Recent advances in MIP-SERS technology for the detection of mycotoxins, additives, prohibited dyes, pesticides, veterinary drug residues, and other hazardous substances in food are highlighted. Finally, this review discusses the challenges associated with MIP-SERS technology and proposes future development prospects.

Design and fabrication of Zn@ZnO@Ag chip for Raman scattering analysis of norfloxacin in liquid milk, fish and animal feeds

A novel and easy-to-prepare plasmonic nanoparticles doped semiconductor substrate-Zn@ZnO@Ag chip with ultra-high surface-enhanced Raman scattering (SERS) activity was fabricated for label-free, rapid and sensitive analysis of norfloxacin. The Zn@ZnO array was synthesized by surface oxidation at low temperature, followed by in-situ reduction to form leaf-like AgNPs on Zn@ZnO array without extra reducing agent, thus fabricating a Zn@ZnO@Ag chip. The ultra-high SERS activity is attributed to the synergistic effect of semiconductor characteristics of ZnO and surface plasmon resonance properties of leaf-like AgNPs. The possible enhancement mechanism was verified by density functional theory simulations. The proposed SERS method showed a wide linear range (3.0-500.0 μg/L) and low limit of detection (0.8 μg/L) for norfloxacin analysis. High sensitivity, good selectivity and acceptable recoveries (82.7-113.6%) in real sample analysis were obtained. This study offers a promising SERS chip-based platform for norfloxacin detection in the field.

A review on solution- and vapor-responsive sensors for the detection of phthalates

Phthalic acid esters, largely referred to as phthalates, are today acknowledged as important pollutants used in the manufacture of polyvinyl chloride (PVC)-based plastics, whose use extends to almost every aspect of modern life. The risk of exposure to phthalates is particularly relevant as high concentrations are regularly found in drinking water, food-contact materials and medical devices, motivating an immense body of research devoted to methods for their detection in liquid samples. Conversely, phthalate vapors have only recently been acknowledged as potentially important atmospheric pollutants and as early fire indicators; additionally, deposition of these vapors can pose significant problems to the proper functioning of spacecraft and diverse on-board devices, leading to major space agencies recognizing the need of developing vapor-responsive phthalate sensors. In this manuscript we present a literature survey on solution- and vapor-responsive sensors and analytical assays for the detection of phthalates, providing a detailed analysis of a vast array of analytical data to offer a clear idea on the analytical performance (limits of detection and quantification, linear range) and advantages provided by each class of sensor covered in this review (electrochemical, optical and vapor-responsive) in the context of their potential real-life applications; the manuscript also gives detailed fundamental information on the various physicochemical responses exploited by these sensors and assays that could potentially be harnessed by new researchers entering the field.

Efficient photoelectrochemical aptasensing of di-2-ethylhexyl phthalate in environmental samples based on N, S co-doped graphene quantum dots/TiO2 nanorods

An efficient photoelectrochemical (PEC) sensing platform was developed for detection of di-2-ethylhexyl phthalate (DEHP) based on nitrogen and sulfur co-doped graphene quantum dots/TiO₂ nanorods (N, S-GQDs/TiO₂ NRs) coupling with exonuclease I (Exo I)-assisted target recycling for remarkable signal amplification. N, S-GQDs uniformly grown on TiO₂ NRs by simple hydrothermal method showed high electron-hole separation efficiency and superior photoelectric performance, which was explored as the photoactive substrate for anchoring anti-DEHP aptamer and its complementary DNA (cDNA). With the addition of DEHP, aptamer molecules fell from the electrode surface owing to the specific recognition of aptamer to DEHP, resulting in the increment of photocurrent signal. At this moment, Exo I could stimulate aptamer hydrolysis in the aptamer-DEHP complexes, so that DEHP was released from the complexes to take part in the next reaction cycling, which remarkably increasing the photocurrent response and achieving signal amplification. The designed PEC sensing platform exhibited excellent analytical performance for DEHP with a low detection limit of 0.1 pg L^-1. Also, its applications in real samples were further investigated in detail. Thus, the established method would provide a simple and efficient tool for DEHP or other pollutants monitoring in the environment.

Fabrication of molecularly-imprinted gold nanoparticle-embedded Fe-MOFs for highly selective SERS detection of 17β-estradiol in milk

17β-Estradiol (17β-E2) could accumulate in humans through milk, thus causing diseases by interfering with the function of the endocrine system. However, its detection at a trace level in milk is still a challenge because of matrix interferences. In this work, a core-shell structured polydopamine molecular-imprinted gold nanoparticles (AuNP@MIP-PDA) were embedded into Fe metal-organic framework materials to form a well-defined hexagonal microspindle structure of AuNP@MIP-PDA@MIL-101(Fe). AuNP@MIP-PDA were successfully encapsulated within the MIL-101 crystals through the hydrophobic interaction between organic ligands and the aromatic groups of PDA, the chelating power of catechol groups, as well as the introduction of acetic acid. Combined with the SERS activity of AuNPs, the specific recognition sites from MIPs, and the adsorption and enrichment capability of MIL-101, the fabricated nanohybrids could be designed as highly selective SERS sensors for the detection. By effectively preventing the macromolecule adsorption and the preconcentration of 17β-E2 near the SERS-active surface, the SERS sensor could be directly applied in the selective detection of 17β-E2 in milk without tedious pretreatment. The method demonstrated an outstanding detection limit of 1.95 × 10^-16 mol L^-1, without the interference mainly originating from the two analogues, estrone and estriol. These promising results foresee the potential application of this novel MIP-based SERS sensor in food and environmental sensing.

The Convenience of Polydopamine in Designing SERS Biosensors with a Sustainable Prospect for Medical Application

Polydopamine (PDA) is a multifunctional biomimetic material that is friendly to biological organisms and the environment, and surface-enhanced Raman scattering (SERS) sensors have the potential to be reused. Inspired by these two factors, this review summarizes examples of PDA-modified materials at the micron or nanoscale to provide suggestions for designing intelligent and sustainable SERS biosensors that can quickly and accurately monitor disease progression. Undoubtedly, PDA is a kind of double-sided adhesive, introducing various desired metals, Raman signal molecules, recognition components, and diverse sensing platforms to enhance the sensitivity, specificity, repeatability, and practicality of SERS sensors. Particularly, core-shell and chain-like structures could be constructed by PDA facilely, and then combined with microfluidic chips, microarrays, and lateral flow assays to provide excellent references. In addition, PDA membranes with special patterns, and hydrophobic and strong mechanical properties can be used as independent platforms to carry SERS substances. As an organic semiconductor material capable of facilitating charge transfer, PDA may possess the potential for chemical enhancement in SERS. In-depth research on the properties of PDA will be helpful for the development of multi-mode sensing and the integration of diagnosis and treatment.

Self-corrected dual-optical immunosensors using carbon dots@SiO2@MnO2 improving diethyl phthalate detection accuracy

The traditional immunoassay is widely used for pollutant detection and bioanalysis, but there are still some challenges in ensuring its sensitivity and reliable accuracy. Dual-optical measurement can prove mutual evidence to effectively improve the accuracy of the method by self-correction, which will overcome this problem. In this study, we developed a "visualization and sensing" dual-modal immunoassay based on blue carbon dots@SiO₂@MnO₂ (B-CDs@SiO₂@MnO₂) as "color and fluorescence" immunosensors. Here, MnO₂ nanosheets have the activity of simulating oxidase. 3,3', 5,5'-Tetramethylbenzidine (TMB) can be oxidized to TMB²⁺ under acidic conditions and the color of the solution from colorless to yellow. On the other hand, the MnO₂ nanosheets can quench the fluorescence of B-CDs@SiO₂. After adding ascorbic acid (AA), MnO₂ nanosheets were reduced to Mn²⁺, thereby the fluorescence of B-CDs@SiO₂ was restored. Under the optimum conditions, as the concentration of target substance (diethyl phthalate) increased from 0.05 to 100 ng/mL, the method showed a good linear relationship. The fluorescence measurement signal and the color change signal of the solution visualization support each other and give the information of the corresponding material content. The results of the dual-optical immunoassay maintain good consistency, which proves the accuracy of the developed dual-optical immunoassay for detection of diethyl phthalate is reliable. Additionally, it is demonstrated that the dual-modal method exhibits high accuracy and stability in the assays, pointing to a broad range of application prospects in pollutant analysis.

Surface molecularly imprinted-based matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for highly selective and sensitive direct analysis of paraquat in complicated samples

Herein, a novel surface molecularly imprinted-based matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (SMI-MALDI-TOF MS) method for direct target paraquat (PQ) analysis in complicated samples is reported. Notably, a captured analyte-imprinted material can be directly detected via MALDI-TOF MS by using imprinted material as nanomatrix. Using this strategy, the molecular specific affinity performance of surface molecularly imprinted polymers (SMIPs) and the high-sensitivity detection capability of MALDI-TOF MS was integrated. The introduction of SMI endowed the nanomatrix with the capacity for rebinding the target analyte and ensuring specificity, prevented the interfering organic matrix, and enhanced the analyzing sensitivity. By using paraquat (PQ) as a template, dopamine as a monomer, and covalent organic frameworks with a carboxyl group (C-COFs) as a substrate, polydopamine (PDA) was decorated on C-COFs via a simple self-assembly procedure to generate an analyte-based surface molecularly imprinted polymer (C-COF@PDA-SMIP), which served the dual function of SMIP capturing the target analytes and high-efficiency ionization. Thus, a reliable MALDI-TOF MS detection PQ with high selectivity and sensitivity as well as an interference-free background was achieved. The synthesis and enrichment conditions of C-COF@PDA-SMIPs were optimized, and its structure and property were characterized. Under optimal experimental conditions, the proposed method achieved highly selective and ultrasensitive detection of PQ from 5 to 500 pg mL^-1, and the limit of detection was as low as 0.8 pg mL^-1, which is at least three orders of magnitude lower than that achieved without enrichment. In addition, the specificity of the proposed method was superior to that of C-COFs and nonimprinted polymers. Moreover, this method exhibited reproducibility, stability, and high salt tolerance. Lastly, the practical applicability of the method was successfully verified by analyzing complicated samples, such as grass and orange.

Nickel hexacyanoferrate nanoparticle-decorated 3D rGO composites-based electrochemical sensing platform for detection of di-2-ethylhexyl phthalate

A label-free and efficient electrochemical (EC) sensing platform for di-2-ethylhexyl phthalate (DEHP) was developed based on in situ probe nickel hexacyanoferrate nanoparticle (NiHCF NP)-decorated three-dimensional reduced graphene oxide (3D rGO) composites. NiHCF NPs in the composites as an in situ probe show a pair of well-defined peaks with good reversibility and stability. Coupling 3D rGO with NiHCF NPs not only improved the electron transfer capability of NiHCF NPs but also provided more sites for aptamer immobilization. The synthesized NiHCF NP-decorated 3D rGO composites were used to act as a substrate for the immobilization of anti-DEHP aptamer by the covalent bonding method. The designed EC sensing platform displays excellent sensing performance for DEHP with a low detection limit of 3.64 pg/L, and a linear working range of 0.01 - 1000 ng/L. The application of the sensing platform to actual environmental samples was studied and satisfactory results were obtained. Thus, the proposed EC sensing platform would provide a potential tool for efficient detection of pollutants in the environment.

Fabrication of hierarchical β-Bi2O3/AuAg microspheres for sensitive, selective and rapid detection of environment pollutants by surface-enhanced Raman spectroscopy

In this paper, we report a novel surface-enhanced Raman spectroscopy (SERS) substrate based on hierarchical β-Bi2O3/Au2Ag2 microspheres for rapid, sensitive and selective detection of environment pollutants including o-dianisidine (o-diASD) and Hg2+ in environmental samples. The sheet-like β-Bi2O3 not only provides large specific surface areas for adsorption of molecules and AuAg, but also emerges as semiconductor matrix with chemical enhancement combined with AuAg with electromagnetic enhancement, making promising SERS activity. Particularly, the β-Bi2O3/Au2Ag2 shows high SERS performance for 4-mercaptobenzoic acid and TMB with minimum detectable concentration of 0.1 μg/L with enhancement factor of 3.1 × 107 and 6.3 × 107, respectively. The density functional theory simulations were further adopted to explain the high SERS activity and selectivity for o-diASD and TMB. Finally, the β-Bi2O3/Au2Ag2 was applied to direct detection of o-diASD, and indirect detection of Hg2+ by TMB marking in environmental samples. The linearity range of 0.5-200.0 and 0.2-500.0 μg/L with limit of detection of 0.2 and 0.07 μg/L for o-diASD and Hg2+ ions can be achieved, respectively. This method provides a novel strategy in designing and fabricating SERS substrates with high performance for rapid, sensitive and accurate analysis of environmental pollutants.

Molecularly imprinted 3D SERS sensor with inorganic frameworks for specific and recyclable SERS sensing application

Poor selectivity and reusability of Au/Ag nanostructures are the main challenges for surface-enhanced Raman spectroscopy (SERS) in real sample detection. Herein, a novel specific and reusable three-dimensional (3D) SERS sensor with dual functions of selective trapping and photocatalytic degradation was designed. Firstly, Au-Ag bimetallic nanoparticles decorated silicon nanowires array (SiNWs-AuAg) were prepared as 3D SERS substrate. Then, silicon-based inorganic-framework molecularly imprinted TiO₂ (TiO₂@SiMIP) was synthesized and immobilized on SiNWs-AuAg by using rhodamine 6G (R6G) as template molecule. Owing to the excellent SERS performance of SiNWs-AuAg and the specific affinity of TiO₂@SiMIP to template molecule, the prepared SERS sensor enables sensitive and selective detection of R6G in food samples with a limit of detection (LOD) of 0.27 nM. In addition, due to the photocatalysis of TiO₂ and the stability of silicon-based inorganic framework, the residual templates in TiO₂@SiMIP can be completely removed by UV irradiation, and the imprinted cavity of regenerated sensors still maintained good selectivity after regeneration by UV irradiation.

A high-performance SERS imprinted membrane based on Ag/CNTs for selective detection of spiramycin

In this test, the eggshell membrane (ESM) is selected as the support membrane for the biocompatibility and anchors CNTs on the surface to increase the mechanical properties. Then Ag NPs are decorated on CNTs-ESM substrate as SERS substrate by twice in-situ reduction. Finally, a layer of imprinted polymers is coated on the surface of the substrate to synthesize the imprinted membrane for selective detection of spiramycin. It is exhibited from the characteristic results that the CNTs significantly increase the mechanical properties and the detection sensitivity, simultaneously. When the concentration of SP changes between 10^-6 ∼ 10^-11 M, there is a linear relationship between SERS intensity and SP concentration. The detection limit is 10^-11 M, and the correlation coefficient R² is 0.9864. The SERS imprinted membrane can be applied into the detection of antibiotics in practical sample, which broadens the research field of antibiotics detection.

Adsorption of rhodamine 6G and choline on gold electrodes: a molecular dynamics study

The adsorption of analyte molecules on nano-optoelectrodes (e.g. a combined nanoantenna and nanoelectrode device) significantly affects the signal characteristics in surface-enhanced Raman scattering (SERS) measurements. Understanding how different molecules adsorb on electrodes and their electrical potential modulation helps interpret SERS measurements better. We use molecular dynamics simulations to investigate the adsorption of prototypical analyte molecules (rhodamine 6G and choline) on gold electrodes with negative, neutral, and positive surface charges. We show that both molecules can readily adsorb on gold surfaces at all surface charge densities studied. Nevertheless, the configurations of the adsorbed molecules can differ for different surface charge densities, and adsorption can also change a molecule's conformation. Rhodamine 6G molecules adsorb more strongly than choline molecules, and the adsorption of both molecules is affected by electrode charge in 0.25 M NaCl solutions. The mechanisms of these observations are elucidated, and their implications for voltage-modulated SERS measurements are discussed.

Facile Regulation of Shell Thickness of the Au@MOF Core-Shell Composites for Highly Sensitive Surface-Enhanced Raman Scattering Sensing

Metal-organic frameworks (MOFs)-based core-shell composites have advanced the development of surface-enhanced Raman scattering (SERS) analysis, which originates from the promising structural characteristics of the outer framework material as well as the inherent plasmonic properties of the novel metal structure core (for example, nanoparticle, MNP). However, the SERS effect only exists directly in the surface of MNP or restricted around the plasmonic MNP surface. Consequently, the nanoscale control of the thickness of MOF shell in hybrid core-shell substrates is highly desirable. Despite the great effects which have been made to integrate various MOF matrices with MNP for the purpose of improving the SERS activity, the nanoscale thickness control of MOF shell remains a significant challenge. Here, we report a facile regulation method that enables the Au NP to be encapsulated by a zirconium-based MOF (BUT-17) with different thickness through the controlling of synthesis parameters. This method provides a promising strategy for optimizing the activity of core-shell SERS substrates for potential trace detection.

Fabrication of branched gold copper nanoalloy doped mesoporous graphitic carbon nitride hybrid membrane for surface-enhanced Raman spectroscopy analysis of carcinogens

Carcinogens in food samples show great potential threat to human health due to their wide distribution and high carcinogenicity. In this work, branched AuCu nanoalloy doped mesoporous graphitic carbon nitride hybrid membrane (mpg-C₃N₄/AuCu) was fabricated for SERS analysis of carcinogens including benzidine and zearalenone in food. The AuCu was in-situ grown on mpg-C₃N₄ to form mpg-C₃N₄/AuCu composites. The as-fabricated mpg-C₃N₄/AuCu membrane can effectively combined synergistic effect of localized surface plasmon resonance properties of branched AuCu nanoalloy and semiconductor characteristics of mpg-C₃N₄. The limit of detection for crystal violet is 1.0 ng/L with enhancement factor of 3.7 × 10⁸. The mechanism of high SERS activity of mpg-C₃N₄/AuCu membrane was investigated by density functional theory simulations. The mpg-C₃N₄/AuCu membrane was used for direct determination of benzidine, and indirect determination of zearalenone with 3,3',5,5'-tetramethylbenzidine as markers in food. The limits of detection of SERS method were 0.14 and 0.03 μg/L for benzidine and zearalenone, respectively. It provides a new strategy for design and fabrication of high-quality SERS substrates for carcinogens analysis.

New Trends in Nanoarchitectured SERS Substrates: Nanospaces, 2D Materials, and Organic Heterostructures

This article reviews recent fabrication methods for surface-enhanced Raman spectroscopy (SERS) substrates with a focus on advanced nanoarchitecture based on noble metals with special nanospaces (round tips, gaps, and porous spaces), nanolayered 2D materials, including hybridization with metallic nanostructures (NSs), and the contemporary repertoire of nanoarchitecturing with organic molecules. The use of SERS for multidisciplinary applications has been extensively investigated because the considerably enhanced signal intensity enables the detection of a very small number of molecules with molecular fingerprints. Nanoarchitecture strategies for the design of new NSs play a vital role in developing SERS substrates. In this review, recent achievements with respect to the special morphology of metallic NSs are discussed, and future directions are outlined for the development of available NSs with reproducible preparation and well-controlled nanoarchitecture. Nanolayered 2D materials are proposed for SERS applications as an alternative to the noble metals. The modern solutions to existing limitations for their applications are described together with the state-of-the-art in bio/environmental SERS sensing using 2D materials-based composites. To complement the existing toolbox of plasmonic inorganic NSs, hybridization with organic molecules is proposed to improve the stability of NSs and selectivity of SERS sensing by hybridizing with small or large organic molecules.

Organic Anion Detection with Functionalized SERS Substrates via Coupled Electrokinetic Preconcentration, Analyte Capture, and Charge Transfer

Detecting ultralow concentrations of anionic analytes in solution by surface-enhanced Raman spectroscopy (SERS) remains challenging due to their low affinity for SERS substrates. Two strategies were examined to enable in situ, liquid phase detection using 5(6)-carboxyfluorescein (5(6)-FAM) as a model analyte: functionalization of a gold nanopillar substrate with cationic cysteamine self-assembled monolayer (CA-SAM) and electrokinetic preconcentration (EP-SERS) with potentials ranging from 0 to +500 mV. The CA-SAM did not enable detection without an applied field, likely due to insufficient accumulation of 5(6)-FAM on the substrate surface limited by passive diffusion. 5(6)-FAM could only be reliably detected with an applied electric field with the charged molecules driven by electroconvection to the substrate surface and the SERS intensity following the Langmuir adsorption model. The obtained limits of detection (LODs) with an applied field were 97.5 and 6.4 nM on bare and CA-SAM substrates, respectively. For the CA-SAM substrates, both the ligand and analyte displayed an ∼15-fold signal enhancement with an applied field, revealing an additional enhancement due to charge-transfer resonance taking place between the metal and 5(6)-FAM that improved the LOD by an order of magnitude.

Highly sensitive and selective photoelectrochemical aptasensing of di-2-ethylhexyl phthalate based on graphene quantum dots decorated TiO2 nanotube arrays

A photoelectrochemical (PEC) sensing platform for di-2-ethylhexyl phthalate (DEHP) was constructed using graphene quantum dots decorated TiO₂ nanotube arrays (GQDs-decorated TiO₂ NTs) as the transducer species and the anti-DEHP aptamer as the biological recognition element. GQDs were synthesized using the alkali-mediated hydrothermal method, and then anchored onto the TiO₂ NTs uniformly and intimately via pronounced electrostatic interaction. Coupling GQDs with TiO₂ NTs not only enhanced visible-light absorption, but promoted charge separation and transportation, exhibiting excellent photocurrent response, and PEC activity. Various means were conducted to explore morphologies, optical, structural and PEC properties of the materials. As an identification unit, the anti-DEHP aptamer molecules were immobilized on GQDs-decorated TiO₂ NTs using a cross-linking coupling method. The developed PEC sensing platform exhibits excellent sensing behavior for DEHP, and provides a low detection limit of 0.1 ng/L, high selectivity and stability. Meanwhile, its application in real environmental samples was evaluated and satisfying results were achieved. Thus, the established sensing platform provides a promising tool to detect DEHP in the environment.

Nanotechnology in food and water security: on-site detection of agricultural pollutants through surface-enhanced Raman spectroscopy

Agricultural pollutants are harmful components threatening human health, wildlife, the environment, and the ecosystem. To avoid their exposure, developing prevention and detection systems with high sensitivity and selectivity is required. Most conventional methods, including molecular and chromatographic techniques, cannot be adopted for outdoor on-site detection even though they can provide sensitive and selective detection. Thus, detection platforms that can provide on-site detection via miniaturized and high throughput systems should be developed. As an alternative method, surface-enhanced Raman scattering (SERS) provides unique information about the substances in the presence of plasmonic nanostructures, and it can be portable with the use of portable detection systems and spectrometers. In this study, on-site detection of agricultural pollutants through SERS is reviewed. Three different types of agricultural pollutants were pointed out. On-site detection of biological pollutants, including bacteria and viruses, is reviewed as the first type of pollutant. As a second type, the detection of pesticides, antibiotics, and additives are focused on as chemical pollutants. The third group includes the detection of microplastics and also nanoparticles from the environment.