Plasmonic nanoparticles on metal-organic framework: A versatile SERS platform for adsorptive detection of new coccine and orange II dyes in food

Recent and emerging trends of metal-organic frameworks (MOFs)-based sensors for detecting food contaminants: A critical and comprehensive review

Interest in the use of sensors based on metal-organic frameworks (MOFs) to detect food pollutants has been growing recently due to the desirable characteristics of MOFs, including uniform structures, large surface area, ultrahigh porosity and easy-to-functionalize surface. Fundamentally, this review offers an excellent solution using MOFs-based sensors (e.g., fluorescent, electrochemical, electrochemiluminescence, surface-enhanced Raman spectroscopy, and colorimetric sensors) to detect food contaminants such as pesticide residues, mycotoxins, antibiotics, food additives, and other hazardous candidates. More importantly, their application scenarios and advantages in food detection are also introduced in more detail. Therefore, this systematic review analyzes detection limits, linear ranges, the role of functionalities, and immobilized nanoparticles utilized in preparing MOFs-based sensors. Additionally, the main limitations of each sensing type, along with the enhancement mechanisms of MOFs in addressing efficient sensing are discussed. Finally, the limitations and potential trends of MOFs-based materials in food contaminant detection are also highlighted.

Developing a magnetic SERS nanosensor utilizing aminated Fe-Based MOF for ultrasensitive trace detection of organophosphorus pesticides in apple juice

The unreasonable use of organophosphorus pesticides leads to excessive pesticide residues in food, seriously threatening public health, and the potential of surface-enhanced Raman spectroscopy (SERS) technology, incorporating a metal-organic framework, is substantial for the rapid detection of trace pesticide residues. Here, a novel Fe3O4@NH2-MIL-101(Fe)@Ag (FNMA) SERS nanosensor was developed. Results indicated that the FNMA had a high enhancement factor of 1.53 × 108, a low limit of detection (LOD) of 4.55 × 10-12 M, and a relative standard deviation of 7.73 % for 4-nitrothiophenol, demonstrating its good SERS sensitivity and uniformity, and also possessed good storage stability for one month. In quantifying fenthion and methyl parathion in standard solutions and apple juice in the range of 0.05/0.02-20 mg/L, it showed LODs of 3.02 × 10-3 mg/L and 1.43 × 10-3 mg/L, and 0.0407 and 0.0075 mg/L, respectively, demonstrating potentials in ultrasensitive trace detection of pesticides in food.

Fluorescent composite based on peptide nanotubes activating coumarin 6 for sensitive detection of new coccine in food samples

New coccine (NC), as a kind of common colorant, has been frequently used in our daily life. Herein, the fluorescent composite (PNTs@C6) prepared by the hydrophobic non-covalent interaction between peptide nanotubes and coumarin 6 (C6) was designed for the determination of NC. Due to the activation of C6 by peptide nanotubes, the composite exhibits strong green fluorescence emission, which can be selectively quenched by NC through the inner filter effect. Therefore, a new fluorescent method based on the PNTs@C6 composite for NC detection was constructed. Under optimal conditions, the fluorescence quenching of the sensor exhibits a good linear relationship with the concentration of NC in the range of 0.01-10 μM and the limit of detection is 3.6 nM. Furthermore, the strategy shows simplicity, rapid response and high selectivity and has been successfully applied to the detection of NC in food samples.

Dual-functionalized two-dimensional metal-organic framework composite with highly hydrophilicity for effective enrichment of glycopeptides

Protein glycosylation research is currently focused on the development of various functionalized materials that can effectively enrich the levels of glycopeptides in samples. However, most of these materials possess limited glycopeptide-specific recognition sites because of large steric hindrance, unsuitable mass transfer kinetics, and relatively low surface areas. Herein, a highly hydrophilic two-dimensional (2-D) metal-organic framework (MOF) nanosheet modified with glutathione (GSH) and l-cysteine (l-Cys) (denoted as Zr-Fc MOF@Au@GC) has been synthesized for efficient glycopeptide enrichment. Using this composite material, 39 and 44 glycopeptides from horseradish peroxidase (HRP) and human serum immunoglobulin G (IgG) digests were detected, respectively, which represents a higher efficiency for glycopeptide enrichment from model glycoprotein digests than has been previously reported. The material Zr-Fc MOF@Au@GC exhibited ultra-high sensitivity (0.1 fmol/µL), excellent selectivity (weight ratio of HRP tryptic digest to bovine serum albumin (BSA) tryptic digest = 1:2000), good binding capacity (200 mg/g), satisfactory reusability, and long-term storage capacity. In addition, 655 glycopeptides corresponding to 366 glycoproteins were identified from human serum samples. To the best of our knowledge, this is the largest number of glycoproteins detected in human serum samples to date. These results indicated that Zr-Fc MOF@Au@GC has the potential to be used for the enrichment of glycopeptides in biological samples and the analysis of protein glycosylation.

Developments in food neonicotinoids detection: novel recognition strategies, advanced chemical sensing techniques, and recent applications

Neonicotinoid insecticides (NEOs) are a new class of neurotoxic pesticides primarily used for pest control on fruits and vegetables, cereals, and other crops after organophosphorus pesticides (OPPs), carbamate pesticides (CBPs), and pyrethroid pesticides. However, chronic abuse and illegal use have led to the contamination of food and water sources as well as damage to ecological and environmental systems. Long-term exposure to NEOs may pose potential risks to animals (especially bees) and even human health. Consequently, it is necessary to develop effective, robust, and rapid methods for NEOs detection. Specific recognition-based chemical sensing has been regarded as one of the most promising detection tools for NEOs due to their excellent selectivity, sensitivity, and robust interference resistance. In this review, we introduce the novel recognition strategies-enabled chemical sensing in food neonicotinoids detection in the past years (2017-2023). The properties and advantages of molecular imprinting recognition (MIR), host-guest recognition (HGR), electron-catalyzed recognition (ECR), immune recognition (IR), aptamer recognition (AR), and enzyme inhibition recognition (EIR) in the development of NEOs sensing platforms are discussed in detail. Recent applications of chemical sensing platforms in various food products, including fruits and vegetables, cereals, teas, honey, aquatic products, and others are highlighted. In addition, the future trends of applying chemical sensing with specific recognition strategies for NEOs analysis are discussed.

Solid chip-based detection of trace morphine in solutions via portable surface-enhanced Raman spectroscopy

The accurate, sensitive and portable detection of morphine is important to handle judicial cases, but remains to be a great challenge. In this work, a flexible route is presented for the accurate identification and efficient detection of trace morphine in solutions based on surface-enhanced Raman spectroscopy (SERS) and a solid substrate/chip. A gold-coated jagged silicon nanoarray (Au-JSiNA) is designed and prepared via Si-based polystyrene colloidal template-reactive ion etching and sputtering deposition of Au. Such Au-JSiNA has three-dimensional nanostructure with good structural uniformity, high SERS activity and hydrophobic surface. Adopting this Au-JSiNA as SERS chip, trace morphine in solutions could be detected and identified in both dropping and soaking ways, and the limit of detection is below 10-4 mg/mL. Importantly, such chip is especially suitable for the detection of trace morphine in aqueous solutions and even domestic sewage. The good SERS performance is attributed to the high-density nanotips and nanogaps on this chip as well as its hydrophobic surface. Additionally, the appropriate surface modification of this Au-JSiNA chip with 3-mercapto-1-propanol or 3-mercaptopropionic acid/1-(3-dimethylaminopropyl)-3-ethylcarbodiimide can further increase its SERS performances to morphine. This work provides a facile route and practical solid chip for SERS detection of trace morphine in solutions, which is significant to develop the portable and reliable instruments for on-site analysis of drugs in solutions.

Recent advancements in nanomaterial based optical detection of food additives: a review

Food additives have become a critical component in the food industry. They are employed as preservatives to decelerate the negative effects of environmental and microbial factors on food quality. Currently, food additives are used for a variety of purposes, including colorants, flavor enhancers, nutritional supplements, etc., owing to improvements in the food industry. Since the usage of food additives has increased dramatically, the efficient monitoring of their acceptable levels in food products is quite necessary to mitigate the problems associated with their inappropriate use. The traditional methods used for detecting food additives are generally based on standard spectroscopic and chromatographic techniques. However, these analytical techniques are limited by their high instrumentation cost and time-consuming procedures. The emerging field of nanotechnology has enabled the development of highly sensitive and specific sensors to analyze food additives in a rapid manner. The current article emphasizes the need to detect various food additives owing to their potential negative effects on humans, animals, and the environment. In this article, the role of nanomaterials in the optical sensing of food additives has been discussed owing to their high accuracy, ease-of-use, and excellent sensitivity. The applications of nanosensors for the detection of various food additives have been elaborated with examples. The current article will assist policymakers in developing new rules and regulations to mitigate the adverse effects of toxic food additives on humans and the environment. In addition, the prospects of nanosensors for the optical detection of food additives at a commercial scale have been discussed to combat their irrational use in the food industry.

High-Performance Au@Ag Nanorods Substrate for SERS Detection of Malachite Green in Aquatic Products

In order to improve the detection performance of surface-enhanced Raman scattering (SERS), a low-cost Au@Ag nanorods (Au@Ag NRs) substrate with a good SERS enhancement effect was developed and applied to the detection of malachite green (MG) in aquaculture water and crayfish. By comparing the SERS signal enhancement effect of five kinds of Au@Ag NRs substrates with different silver layer thickness on 4-mercaptobenzoic acid (4-MBA) solution, it was found that the substrate prepared with 100 µL AgNO3 had the smallest aspect ratio (3.27) and the thickest Ag layer (4.1 nm). However, it showed a good signal enhancement effect, and achieved a detection of 4-MBA as low as 1 × 10-11 M, which was 8.7 times higher than that of the AuNRs substrate. In addition, the Au@Ag NRs substrate developed in this study was used for SRES detection of MG in crayfish; its detection limit was 1.58 × 10-9 M. The developed Au@Ag NRs sensor had the advantages of stable SERS signal, uniform size and low cost, which provided a new tool for SERS signal enhancement and highly sensitive SERS detection method development.

Metal-organic frameworks (MOFs) based luminescent and electrochemical sensors for food contaminant detection

With the increasing population, food toxicity has become a prevalent concern due to the growing contaminants of food products. Therefore, the need for new materials for toxicant detection and food quality monitoring will always be in demand. Metal-organic frameworks (MOFs) based on luminescence and electrochemical sensors with tunable porosity and active surface area are promising materials for food contaminants monitoring. This review summarizes and studies the most recent progress on MOF sensors for detecting food contaminants such as pesticides, antibiotics, toxins, biomolecules, and ionic species. First, with the introduction of MOFs, food contaminants and materials for toxicants detection are discussed. Then the insights into the MOFs as emerging materials for sensing applications with luminescent and electrochemical properties, signal changes, and sensing mechanisms are discussed. Next, recent advances in luminescent and electrochemical MOFs food sensors and their sensitivity, selectivity, and capacities for common food toxicants are summarized. Further, the challenges and outlooks are discussed for providing a new pathway for MOF food contaminant detection tools. Overall, a timely source of information on advanced MOF materials provides materials for next-generation food sensors.

Material design, development, and trend for surface-enhanced Raman scattering substrates

Surface-enhanced Raman scattering (SERS) is a powerful and non-invasive spectroscopic technique that can provide rich and specific chemical fingerprint information for various target molecules through effective SERS substrates. In view of the strong dependence of the SERS signals on the properties of the SERS substrates, design, exploration, and construction of novel SERS-active nanomaterials with low cost and excellent performance as the SERS substrates have always been the foundation and the top priority for the development and application of the SERS technology. This review specifically focuses on the extensive progress made in the SERS-active nanomaterials and their enhancement mechanism since the first discovery of SERS on the nanostructured plasmonic metal substrates. The design principles, unique functions, and influencing factors on the SERS signals of different types of SERS-active nanomaterials are highlighted, and insight into their future challenge and development trends is also suggested. It is highly expected that this review could benefit a complete understanding of the research status of the SERS-active nanomaterials and arouse the research enthusiasm for them, leading to further development and wider application of the SERS technology.

A highly sensitive surface-enhanced Raman scattering sensor with MIL-100(Fe)/Au composites for detection of malachite green in fish pond water

Concerns about food safety have been arisen due to the improper use of chemicals in aquaculture. Malachite green (MG) has attracted attention because of its illegal usage and its potential negative impacts on the environment and public health. Surface-enhanced Raman scattering (SERS) platforms coupled with different SERS substrates have been employed for rapid analysis of MG residues in food. However, the most commonly used SERS substrates were non-reusable and showed limited detection sensitivity. In this study, a novel SERS substrate with a good recyclability and a high sensitivity was prepared by electrostatically assembling together a metal-organic framework material called materials of institute lavoisie-100(Fe) (MIL-100(Fe)) and Au NPs. The lowest detectable concentration of MG was 10^-13 M based on the optimal substrate. The SERS sensor was applied for the detection of the trace MG in fish pond water, which was accomplished with the correlation coefficients R² = 0.991-0.996 in a concentration range of 10^-6-10^-13 M. Moreover, MIL-100(Fe)/Au was recycled at least five times, realizing a "detection to degradation", showing great potential for food contamination monitoring due to its distinguished performance.

Recent Progress in the Application of Metal Organic Frameworks in Surface-Enhanced Raman Scattering Detection

Metal-organic framework (MOF) compounds are centered on metal ions or metal ion clusters, forming lattices with a highly ordered periodic porous network structure by connecting organic ligands. As MOFs have the advantages of high porosity, large specific surface area, controllable pore size, etc., they are widely used in gas storage, catalysis, adsorption, separation and other fields. SERS substrate based on MOFs can not only improve the sensitivity of SERS analysis but also solve the problem of easy aggregation of substrate nanoparticles. By combining MOFs with SERS, SERS performance is further improved, and tremendous research progress has been made in recent years. In this review, three methods of preparing MOF-based SERS substrates are introduced, and the latest applications of MOF-based SERS substrates in biosensors, the environment, gases and medical treatments are discussed. Finally, the current status and prospects of MOF-based SERS analysis are summarized.

Recent Progress of Surface-Enhanced Raman Spectroscopy for Bacteria Detection

There are various pathogenic bacteria in the surrounding living environment, which not only pose a great threat to human health but also bring huge losses to economic development. Conventional methods for bacteria detection are usually time-consuming, complicated and labor-intensive, and cannot meet the growing demands for on-site and rapid analyses. Sensitive, rapid and effective methods for pathogenic bacteria detection are necessary for environmental monitoring, food safety and infectious bacteria diagnosis. Recently, benefiting from its advantages of rapidity and high sensitivity, surface-enhanced Raman spectroscopy (SERS) has attracted significant attention in the field of bacteria detection and identification as well as drug susceptibility testing. Here, we comprehensively reviewed the latest advances in SERS technology in the field of bacteria analysis. Firstly, the mechanism of SERS detection and the fabrication of the SERS substrate were briefly introduced. Secondly, the label-free SERS applied for the identification of bacteria species was summarized in detail. Thirdly, various SERS tags for the high-sensitivity detection of bacteria were also discussed. Moreover, we emphasized the application prospects of microfluidic SERS chips in antimicrobial susceptibility testing (AST). In the end, we gave an outlook on the future development and trends of SERS in point-of-care diagnoses of bacterial infections.

A fluorescence and surface-enhanced Raman scattering dual-mode aptasensor for sensitive detection of deoxynivalenol based on gold nanoclusters and silver nanoparticles modified metal-polydopamine framework

Deoxynivalenol (DON), a common mycotoxin produced by Fusarium species, poses a great threat to human and animal body. Hence, it is of significance to develop an ultrasensitive and reliable method for DON detection. Herein, a fluorescence and surface-enhanced Raman scattering (FL-SERS) dual-mode aptasensor was designed for the detection of DON based on gold nanoclusters (Au NCs) and silver nanoparticles modified metal-polydopamine framework (Ag NPs/MPDA). In this aptasensor, complementary DNA modified Au NCs (cDNA-Au NCs) was selected as fluorescence probe, and 6-carboxytetramethylrhodamine (TAMRA)-labeled aptamer modified Ag NPs/MPDA (Ag NPs/MPDA-Apt-TAMRA) was employed as SERS probe, in which Ag NPs/MPDA acted as SERS substance and fluorescence quencher, and TAMRA acted as Raman label. The superior binding affinity of the aptamer with DON to cDNA can regulate the fluorescence and Raman signal intensities and realize the quantitative determination of DON. Under the optimal conditions, the aptasensor exhibited a low detection limit of 0.08 ng mL^-1 (0.1-100 ng mL^-1) in FL mode and 0.06 ng mL^-1 (0.1-100 ng mL^-1) in SERS mode. In addition, it was successfully applied for DON detection in wheat flour. We believe that the proposed FL-SERS strategy has a promising application in the detection of mycotoxins.

Catalytic metal-organic framework-melamine foam composite as an efficient material for the elimination of organic pollutants

Water-insoluble organic pollutants in environment, such as sea oil spill, industrial reagents, and the abused organic pesticides, bring great risks to global water systems, which thus requires effective approaches for organic pollutant elimination. In this study, we report a catalytic metal-organic framework (MOF)-melamine foam (MF) composite material (DDT-UiO-66-NH₂@MF) showing excellent oil-water separation performance and enzyme-like degradation ability toward organophosphorus pesticides. The fabrication of DDT-UiO-66-NH₂@MF is based on the immobilization of a MOF-derived nanozyme (UiO-66-NH₂) on MF sponge, and followed by the hydrophobic modification of UiO-66-NH₂ by 1-dodecanethiol (DDT). The obtained DDT-UiO-66-NH₂@MF thus displayed superhydrophobic/superhydrophilic property with a high water contact angle (WCA = 144.6°) and specific adsorption capacity toward various oils/organic solvents (62.2-119.8 g/g), which leads to a continuous oil-water separation on a simple device. In the meanwhile, owing to the enzyme-like property of UiO-66-NH₂, DDT-UiO-66-NH₂@MF also displayed good ability to hydrolyze paraoxon under mild conditions, which facilitates the elimination of toxic pesticide residuals in water systems. This work provides a simple, efficient, and green approach for the separation and treatment of water-insoluble organic pollutants, as well as expands the use of MOFs-MF sponge composite materials in environmental sustainability.

Electrochemical synthesis of Co/Ni bimetal-organic frameworks: A high-performance SERS platform for detection of tetracycline

Surface-enhanced Raman scattering (SERS) enables food contaminants monitoring become facile and efficient. Herein, a facile strategy of integrating three-dimensional Ni form with Co/Ni bimetal-organic frameworks combining Ag nanoparticles via electrochemical synthesis method was proposed to develop a high-performance SERS substrate (CoNi-ZIFs@Ag@NF) for efficient detection of tetracycline. The flexible Ni foam (NF) acted as scaffold which can contribute to dramatically enhancing intrinsic electrical conductivity and endowing prepared substrate with high stability and uniform distribution of Ag nanoparticles. Furthermore, the pre-concentration effect of CoNi-ZIFs@Ag@NF for target molecules enhanced SERS performance dramatically. Besides, tetracycline was sensitively detected using CoNi-ZIFs@Ag@NF with low limit of detection (1.0 × 10^-11 M) and wide linear detection range (10^-10 - 10^-5 M) in aqueous solution. Also, the satisfactory recovery (94.45 - 114.25 %) was realized with less than 6.78 % of RSD in real samples. This method would provide a potential and high-performance substrate for SERS monitoring of tetracycline in food and environment.

Rapid and sensitive SERS detection of opioids in solutions based on the solid chip Au-coated Si nano-cone array

Rapid and flexible detection or accurate recognition of trace drugs is of great importance in cracking down on drug crimes, but it remains to be expected. Here, a solid chip is presented for the efficient detection and recognition of trace opioids (typically morphine) in aqueous solutions based on surface-enhanced Raman spectroscopy (SERS). Firstly, a Au-coated Si nano-cone array (Au-SNCA) is designed and fabricated via Si-based organic colloidal template etching and Au deposition. This Au-SNCA shows three-dimensional nanostructure with high densities of nanotips and deep nanogaps as well as high structural consistency, which exhibits strong SERS activity to morphine and outstanding stability. Then, such Au-SNCA is used as solid SERS chip to detect morphine in aqueous solutions. It has been demonstrated that using such solid chip, trace morphine in solutions could be recognized and detected within 1 min, and the detection limit is 10^-5 mg/mL (∼10 ppb), showing rapid and sensitive detection, which is much better than the previous reports. Meanwhile, the Au-SNCA chip also can be utilized to detect trace morphine in tap water and reservoir water, the recoveries range from 90.4% to 102.4%. Such excellent SERS performance of this Au-SNCA chip is attributed to its special structure which enhances not only local electromagnetic field but also molecular adsorption. The experimental results about the effects of immersion time and concentration show that the adsorption behavior of morphine molecules on such Au-SNCA chip can be explained by the pseudo-second-order kinetic model and Freundlich adsorption mode. Moreover, the Au-SNCA chip is also suitable for the identification of morphine homologues and the broad-spectrum detection of various common drugs. This study presents a practical solid chip and a simple approach for the efficient SERS detection and recognition of trace drugs in solutions. This is of significance to on-site detect drugs in forensic science.

Surface-enhanced Raman spectroscopy combined with stable isotope probing to assess the metabolic activity of Escherichia coli cells in chicken carcass wash water

Rapid evaluation of the metabolic activity of microorganisms is crucial in the assessment of the disinfection ability of various antimicrobial agents in the food industry. In this study, surface-enhanced Raman spectroscopy combined with isotope probing was employed for the analysis of the disinfection of single bacterial cells in the chicken carcass wash water. The Raman signals from single Escherichia coli O157:H7 cells were enhanced by in situ synthesis of silver nanoparticles. The ΔCD of the cells grown in presence of 0.5% hydrogen peroxide and 50 ppm chlorine was 5.86 ± 1.86% and 5.1 ± 2.3%, respectively, which showed significant reduction compared with cells grown in the absence of disinfecting agents (19.86 ± 2.51%) after 2 h of incubation. The study proved that the proposed method had the potential to assess the metabolic activity of microorganisms in other food products and optimize the disinfection process.

Electrochemical Sensing Platform Based on Functionalized Multi-Walled Carbon Nanotubes and Metal Oxide for the Detection and Degradation Studies of Orange II Dye

Textile industry effluents are heavily contaminated with dyes. The discharge of these toxic dyes into waterbodies poses a serious threat to aquatic flora and fauna. The ultimate entrance of these toxins from thereon into the food chain affects the primary and secondary consumers. Therefore, the adoption of a sustainable solution for protection against the detrimental effects associated with adulterated water is an immediate need of the hour. To address the severity of the issue, the present work aims to design an electrochemical sensing platform by modifying the glassy carbon electrode (GCE) with zinc oxide nanoparticles and amino group-functionalized multi-walled carbon nanotubes (NH₂-fMWCNTs) for the detection of Orange II, which is a toxic azo dye. Zinc oxide nanoparticles facilitate electron transfer between the transducer and the analyte. While, the positively charged NH₂-fMWCNTs in acidic medium help in preconcentration of negatively charged analyte molecules at the electrode/electrolyte interface. The modification of the GCE catalyzed the oxidation of Orange II, as evidenced by the negative shift of the oxidation potential and enhancement in peak current intensity. Square wave voltammetry was used to optimize various experimental conditions, such as the supporting electrolyte, pH of the electrolyte, deposition potential, and deposition time for the best performance of the designed sensor. Under the optimized conditions, the detection limit and quantification of the designed sensor were found to be 0.57 and 1.92 nM, respectively. The catalytic degradation studies of Orange II was shown to be facilitated by titanium dioxide, which acted as a photocatalyst. The addition of hydrogen peroxide further promoted the extent and rate of degradation of dye. The breakdown of Orange II was probed by the designed sensing platform electrochemically and also by UV-visible spectroscopy. The dye degraded up to 92% by following pseudo-first-order kinetics.

Metal-organic frameworks based surface-enhanced Raman spectroscopy technique for ultra-sensitive biomedical trace detection

Metal-organic frameworks (MOFs) have attracted widespread interest due to their unique and unprecedented advantages in microstructures and properties. Besides, surface-enhanced Raman scattering (SERS) technology has also rapidly developed into a powerful fingerprint spectroscopic technique that can provide rapid, non-invasive, non-destructive, and ultra-sensitive detection, even down to single molecular level. Consequently, a considerable amount of researchers combined MOFs with the SERS technique to further improve the sensing performance and broaden the applications of SERS substrates. Herein, representative synthesis strategies of MOFs to fabricate SERS-active substrates are summarized and their applications in ultra-sensitive biomedical trace detection are also reviewed. Besides, relative barriers, advantages, disadvantages, future trends, and prospects are particularly discussed to give guidance to relevant researchers.

Visualization and quantification of content and hydrogen bonding state of water in apple and potato cells by confocal Raman microscopy: A comparison study

Water is the most abundant component in fresh fruit and vegetables and its distribution and hydrogen bonding state in cells has a significant influence on food processing. In the current study, an improved method based on our earlier studies was developed to directly visualize the spatial distribution of content and hydrogen bonding state of water in apple and potato cells for the first time and the difference in water distribution in these cells was compared. Additionally, based on the distribution images of content and hydrogen bonding state of water in different regions in apple and potato tissues, the total water and free water contents, and the hydrogen bonding state of free water were quantified and compared with those obtained by nuclear magnetic resonance and Marinchik methods, demonstrating that the method could be successfully used for quantifying the content and hydrogen bonding state of water in fruit and vegetable cells.

Flexible surface-enhanced Raman scatting substrates: recent advances in their principles, design strategies, diversified material selections and applications

Surface-enhanced Raman scattering (SERS) is widely used as a powerful analytical technology in cutting-edge areas such as food safety, biology, chemistry, and medical diagnosis, providing ultra-fast, ultra-sensitive, nondestructive characterization and achieving ultra-high detection sensitivity even down to the single-molecule level. Development of Raman spectroscopy is strongly dependent on high-performance SERS substrates, which have long evolved from the early days of rough metal electrodes to periodic nanopatterned arrays building on solid supporting substrates. For rigid SERS substrates, however, their applications are restricted by sophisticated pretreatments for detecting solid samples with non-planar surfaces. It is therefore essential to reassert the principles in constructing flexible SERS substrates. Herein, we comprehensively review the state-of-the-art in understanding, preparing and using flexible SERS. The basic mechanisms behind the flexible SERS are briefly outlined, typical design strategies are highlighted and diversified selection of materials in preparing flexible SERS substrates are reviewed. Then the recent achievements of various interdisciplinary applications based on flexible SERS substrates are summarized. Finally, the challenges and perspectives for future evolution of flexible SERS and their applications are demonstrated. We propose new research directions focused on stimulating the real potential of SERS as an advanced analytical technique for commercialization.

Analyzing macromolecular composition of E. Coli O157:H7 using Raman-stable isotope probing

Metabolic dynamics of bacterial cells is needed for understanding the correlation between changes in environmental conditions and cell metabolic activity. In this study, Raman spectroscopy combined with deuterium labelling was used to analyze the metabolic activity of a single Escherichia coli O157:H7 cell. The incorporation of deuterium from heavy water into cellular biomolecules resulted in the formation of carbon-deuterium (CD) peaks in the Raman spectra, indicating the cell metabolic activity. The broad vibrational peaks corresponding to CD and CH peaks encompassed different specific shifts of macromolecules such as protein, lipids, and nucleic acid. The utilization of tryptophan and oleic acid by the cell as the sole carbon source led to changes in cell lipid composition, as indicated by new peaks in the second derivative spectra. Thus, the proposed method could semi-quantitatively determine total metabolic activity, macromolecule specific identification, and lipid and protein metabolism in a single cell.

Colorimetric and ratiometric fluorescent dual-mode sensitive detection of Hg2+ based on UiO-66-NH2@Au composite

A colorimetric and ratiometric fluorescent dual-mode assay is constructed for sensitive and specific Hg²⁺ sensing based on UiO-66-NH₂ and Au composite (UiO-66-NH₂@Au). The addition of Hg²⁺ stimulates the peroxidase-like activity of UiO-66-NH₂@Au by the formation of Au-Hg amalgam, promoting the oxidizing of chromogenic substrate OPD to DAP with the aid of H₂O₂, which lead to the change of colorimetric and fluorescent signals. The absorbance of the sensing system at 450 nm is linear positive correlation with Hg²⁺ concentration of 30-1400 nM and the color of the solution under visible light shaded from light yellow to dark yellow. With the increase of Hg²⁺ concentration, the fluorescence signal at 570 nm (DAP) increased whereas that at 455 nm (intrinsic fluorescence of UiO-66-NH₂) decreased due to inner filter effect (IFE), the fluorescence intensity ratio (F₄₅₅/F₅₇₀) decreasing linearly with Log [Hg²⁺] over the range 60-1700 nM; the fluorescence emission of sensing system under UV excitation changed from blue to yellow, which can easily be discerned visually. This assay was successfully applied to the determination of Hg²⁺ in tap water and river water. The results indicate that the colorimetric and ratiometric fluorescent dual-mode assay based on UiO-66-NH₂@Au realized visual determination of Hg²⁺ rapidly and reliably, revealed application prospect in Hg²⁺ monitoring.

Detection of Bioactive Metabolites in Escherichia Coli Cultures Using Surface-Enhanced Raman Spectroscopy

Detection of bioactive metabolites produced by bacteria is important for identifying biomarkers for infectious diseases. In this study, a surface-enhanced Raman spectroscopy (SERS)-based technique was developed for the detection of bioactive metabolite indole produced by Escherichia coli (E. coli) in biological media. The use of highly sensitive Au@Ag core-shell nanoparticles resulted in the detection of indole concentration as low as 0.0886 mM in standard solution. The supplementation of growth media with 5 mM of exogenous tryptophan resulted in the production of a maximum yield of indole of 3.139 mM by E. coli O157:H7 at 37 °C. The growth of bacterial cells was reduced from 47.73 × 10⁸ to 1.033 × 10⁶ CFU/mL when the cells were grown in 0 and 10 mM exogenous tryptophan, respectively. The amount of indole in the Luria-Bertani (LB) media had an inverse correlation with the growth of cells, which resulted in a three-log reduction in the colony-forming unit when the indole concentration in the media was 20 times higher than normal. This work demonstrates that SERS is an effective and highly sensitive method for rapid detection of bioactive metabolites in biological matrix.

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.

Highly catalysis amplification of MOFNd-loaded nanogold combined with specific aptamer SERS/RRS assay of trace glyphosate

A neodymium metal-organic framework (MOFNd) was prepared using 1H-pyrazole-3,5-dicarboxylic acid (H3pdc) and 2-pyrazinecarboxylic acid as ligands. Through the addition of HAuCl4 as a precursor and NaBH4 as a reducing agent, a new MOFNd-loaded nanogold (AuNPs) (Au@MOFNd) nanosol with good stability and high catalytic activity was conveniently prepared via a solvothermal-reduction method and characterized. It was found that the indicator reaction of reducing HAuCl4 by Na2SO3 to generate AuNPs was slow. Au@MOFNd strongly catalyzes this nanoreaction, and the produced AuNPs exhibit a strong resonance Rayleigh scattering (RRS) peak at 370 nm, and a strong surface-enhanced Raman scattering (SERS) peak at 1617 cm-1 with the addition of the molecular probe Victoria blue 4R (VB4r). A novel SERS/RRS di-mode quantitative analysis method for glyphosate (GLY) was established by coupling this new Au@MOFNd catalytic indicator reaction with the aptamer (Apt) reaction of GLY, with SERS and RRS detection limits of 0.02 nM and 0.3 nM, respectively. It has been applied to the analysis of soil samples with a recovery rate of 93.0%-106.5% and precision of 2.2%-4.1%, and the results were satisfactory.

SERS determination of dopamine using metal-organic frameworks decorated with Ag/Au noble metal nanoparticle composite after azo derivatization with p-aminothiophenol

A specific surface-enhanced Raman scattering (SERS) assay for dopamine (DA) based on an azo derivatization reaction is proposed for the first time by preparation of p-aminothiophenol (PATP)-modified composite SERS substrate, composed of metal-organic framework (MIL-101) decorated with Au and Ag nanoparticles. As the result, the SERS method for detection of the azo reaction between PATP and DA exhibits superior sensitivity, selectivity, and stability. A reasonable linearity in the range 10^-6 to 10^-10 mol∙L^-1 is achieved, and the limit of detection is 1.2 × 10^-12 mol∙L^-1. The reactive SERS assay is free from interference in complex physiological fluid. The feasibility of the proposed SERS method for the detection of DA levels in fetal bovine serum (FBS) samples and human serum samples is validated by HPLC-MS methods, displaying promising application potential in early disease diagnosis.

Photosensitized Peroxidase Mimicry at the Hierarchical 0D/2D Heterojunction-Like Quasi Metal-Organic Framework Interface for Boosting Biocatalytic Disinfection

Metal-organic frameworks (MOFs) are a versatile toolbox for the bioinspired design of nanozymes for antibacterial applications and beyond, however, designing a nanozyme by the hierarchical quasi-MOF scheme remains largely unpracticed. This work exemplifies the preferential structure-activity correlation of a bimetallic quasi-MOF (Q-MOF_Ce0.5 ) among three series of MOF-derived peroxidase (POD) mimics. The biomimetic quasi-MOF_Ce0.5 nanosheets accommodate both oxygen vacancy-coupled multivalent redox cycles and photosensitive energy band layout, benefiting from the hierarchical heterojunction-like 0D/2D interface featuring isolated nodes-derived CeOCu sites upon the 2D decarboxylated MOF scaffold. These integrated unique merits enable the POD-like Q-MOF_Ce0.5 to generate sustained reactive oxygen species to effectively eradicate the surface-adhered bacteria under visible light, resulting in significant inactivation of Escherichia coli (99.74 %) and Staphylococcus aureus (99.35%) in vitro, and potent disinfection of skin wounds in vivo in safe and on-demand manners. It is hoped that this work can intensify the interventions of MOF nanozymes against the microbial world.

A fluorescence aptasensor based on carbon quantum dots and magnetic Fe3O4 nanoparticles for highly sensitive detection of 17β-estradiol

Trace amounts of 17β-estradiol (E2) in food and the environment poses a threat to human health, which has created the demand for sensitive analytical methods to detect E2. In this study, a novel fluorescent aptasensor was developed for sensitive detection of E2 based on double-chain hybridization between carbon quantum dots-labelled with E2 aptamer (CQDs-aptamer) and Fe₃O₄ nanoparticles modified by complementary DNA (Fe₃O₄-cDNA). Under the optimal conditions, the aptasensor displayed a good linear range of 10^-11-10^-6 M for E2 with the coefficient of determination (R²) of 0.996, and a low detection limit of 3.48 × 10^-12 M was obtained. Besides, the aptasensor showed high selectivity and good reproducibility for E2 detection, which was successfully applied to the sensitive detection of E2 in milk as compared with tap water and lake water with satisfactory recoveries from 85.21% to 114.80%, suggesting the great significance of this aptasensor for detecting food contaminants in the food industry.

Novel graphene oxide/polymer composite membranes for the food industry: structures, mechanisms and recent applications

The membrane can not only be used as food packaging, but also for the separation, fractionation and recovery of food ingredients. Graphene oxide (GO) sheets are a two-dimensional (2 D) material with a unique structure that exhibit excellent mechanical properties, biocompatibility, and flexibility. The corporation of polymer matrix membrane with GO can significantly improve the permeability, selectivity, and antibacterial activity. In this review, the chemical structures of GO, GO membranes and GO/polymer composite membranes are introduced, the permeation mechanisms of molecules through the membranes are discussed and key factors affecting the permeability are presented in detail. In addition, recent applications in the food industry for filtration, bioreactions and active food packaging are analyzed, and limitations and future trends of GO membranes development are also highlighted. GO/polymer composite membranes exhibit excellent permeability, selectivity and strong barrier properties against bacterial and gas permeation. However, current food material filtration and packaging applications of GO/polymer composite membranes are still in the laboratory stage. Future work can focus on the development of large scale uniformly sized GO production, the homogeneous distribution and tight combination of GO in polymer matrixes, the sensing function of GO in packaging, and the verification method of GO toxicology.

Semiconductor-based surface enhanced Raman scattering (SERS): from active materials to performance improvement

Surface enhanced Raman scattering (SERS) is a powerful spectral analysis technique and has exhibited remarkable application prospects in various fields. The design and fabrication of high-performance SERS substrates is key to promoting the development of SERS technology. Apart from noble metal substrates, non-metal substrates based on semiconductor materials have received increasing attention in recent years owing to their unique physical, chemical, and optical properties. However, compared with noble metal substrates, most semiconductor substrates show weak Raman enhancement ability. Therefore, exploring effective strategies to improve the SERS sensitivity is an urgent task. Numerous reviews have outlined the research progress of semiconductor SERS substrates, which mainly focused on summarizing the material category of semiconductor substrates. However, reviews that systematically summarize the strategies for improving the SERS performance of semiconductor substrates are lacking. In this review, we comprehensively discuss the research on semiconductor SERS from the aspects of mechanism, materials, and modification. Firstly, the Raman enhancement mechanism of semiconductor substrates and the SERS-active materials are discussed. Then, we summarize several effective approaches to boost the SERS performance of semiconductor substrates. In conclusion, we propose some prospects for this field.

Interfacing metal-polyphenolic networks upon photothermal gold nanorods for triplex-evolved biocompatible bactericidal activity

Gold nanorods (GNRs) outstand in photothermal disinfection but are faced with severe surface chemistry and dose relevant biotoxicity. Herein, a naturally green building block, metal-phenolic networks (MPNs), was employed to functionalize GNRs via coordination reaction, yielding a tunable and biocompatible core-shell photothermal nano-bactericide (GNRs@MPNs). The bioactive GNRs@MPNs built with iron and polyphenols (tannic acid, epigallocatechin gallate, and procyanidins) exhibited superior light-to-heat conversion efficiencies with η = 29.29-44.00%, remarkably preceding that of GNRs (η = 12.24%), which could rapidly ablate 99.8% of Escherichia coli O157: H7 and 98.6% of Staphylococcus aureus bacteria in relatively low efficacy doses (10 ppm of Au). Moreover, local heat triggered by GNRs@MPNs accelerated the healing of the cutaneous wound of a mice model infected by methicillin-resistant S. aureus. The facile synthesis, photothermal synergy, polyphenolic bioactivity, and significantly low efficacy dose of GNRs@MPNs empower them satisfactory efficiency and biosafety in the future broad-spectrum photothermal sterilization applications.

Ti3C2Tx MXenes loaded with Au nanoparticle dimers as a surface-enhanced Raman scattering aptasensor for AFB1 detection

Mycotoxin B1 (AFB1) contamination in agricultural products pose a deadlydangertoanimal and human health and its rapid and reliable detection is thus very important. Herein, a ratiometric surface-enhanced Raman scattering (SERS) aptasensor for AFB1 detection was developed, in which 1,2-bis(4-pyridyl) ethylene (BPE) was used to trigger the assembly of Au nanoparticle dimers (AuNP dimers) and form intensive SERS "hot spots", and MXenes nanosheets could load aptamer-modified AuNP dimers due to the hydrogen bonding and the chelation between the phosphate groups of aptamers and the Ti ion of MXenes. With the presence of AFB1 preferentially binding to AFB1 aptamer, AuNP dimers were separated from MXenes nanosheets, leading to a decrease in SERS intensity. Regression analysis in the range from 0.001 to 100 ng·mL^-1 showed the limit of detection (LOD) being 0.6 pg·mL^-1 in standard solution, indicating that the great prospects of the AuNP dimers/MXenes SERS substrate for detecting AFB1.

A label-free fluorescence nanosensor based on nitrogen and phosphorus co-doped carbon quantum dots for ultra-sensitive detection of new coccine in food samples

In this paper, an innovative method for the sensitive detection of new coccine using N, P-doped carbon quantum dots (N,P-CQDs) as fluorescent nanosensor is reported for the first time. The sensing mechanism is based on the fluorescence quenching of N,P-CQDs by new coccine through inner filter effect (IFE). N,P-CQDs were prepared by simple hydrothermal treatment of citric acid, phosphoric acid and ethylenediamine. Under the optimal conditions, the new coccine has two good linear responses in the concentration range of 0.2-100 and 100-200 μM, and the detection limits are as low as 24.8 and 9.4 nM, respectively. Our developed nanosensor has been successfully used for the determination of new coccine in food samples with good precision and high accuracy. This work highlights the economic, rapid, simple, selective and ultra-sensitive for new coccine detection, and opens up a new way for the monitoring of new coccine in actual food samples.

A novel SERS substrate of MIL-100(Fe)/AgNFs for sensitive detection of ascorbic acid in cellular media

A novel SERS substrate of MIL-100(Fe)/AgNFs was firstly prepared for sensitive and selective detection of ascorbic acid (L-AA), with a LOD of 10⁻¹¹ M. A spectral decrease of MIL-100(Fe)/AgNFs towards L-AA solution thanks to the efficient capture and reduction of Fe³⁺ in MIL-100(Fe) constituted the assay, which was demonstrated to function well in food samples and in cellular media for L-AA sensing.

A novel SERS substrate of MIL-100(Fe)/AgNFs was firstly prepared for sensitive and selective detection of ascorbic acid (L-AA), with a LOD of 10⁻¹¹ M.

Precision release systems of food bioactive compounds based on metal-organic frameworks: synthesis, mechanisms and recent applications

Controlled release (CR) systems have become a powerful platform for accurate and effective delivery of bioactive compounds (BCs). Metal-organic frameworks (MOFs) are one of the best BCs-loaded carriers for CR systems. In the review, the principles and methods of the design and synthesis of MOFs-CR systems are summarized in detail, the encapsulation of BCs by MOFs and CR mechanisms are explored, and their biological toxicity and biocompatibility are highlighted and applications in the food industry are discussed. In addition, current challenges in this field and possible future development directions are also presented. MOFs have been proven to encapsulate BCs effectively, including gaseous and solid molecules, and control the release of BCs through spontaneous diffusion or stimulus-response. The solubility, stability and biocompatibility of BCs encapsulated by MOFs are greatly improved, which expands their applications in foods. The effective CR of BCs by MOFs-CR systems is beneficial to assist in maintaining or even improving the quality and safety of food, reduce the BCs usage while increasing the bioavailability. Low- or non-biotoxic MOFs, especially bio-MOFs, show greater application prospects in the food industry.

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

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

In-Situ Synthesis of Methyl Cellulose Film Decorated with Silver Nanoparticles as a Flexible Surface-Enhanced Raman Substrate for the Rapid Detection of Pesticide Residues in Fruits and Vegetables

The purpose of this study was to develop a flexible substrate methylcellulose-decorated silver nanoparticles (MC/Ag NPs) film and explore its application in fruits and vegetables by surface enhanced Raman spectroscopy (SERS) technology for rapid detection of pesticides. The performance of the MC/Ag NPs film substrate was characterized by Nile blue A (NBA), and the detection limit was as low as 10⁻⁸ M. The substrate also exhibited satisfactory Raman signal strength after two months of storage. The impressive sensitivity and stability were due to the excellent homogeneity of the silver nanoparticles that were grown in situ in the methylcellulose matrix, which generated “hot spots” between the silver nanoparticles without a large amount of aggregation, and resulted in the ultra-high sensitivity and excellent stability of the MC/Ag NPs film substrate. The MC/Ag NPs film substrate was used to detect thiram pesticides on tomato and cucumber peels, and the minimum detectable level of thiram was 2.4 ng/cm², which was much lower than the maximum residue level. These results indicate that the MC/Ag NPs film is sensitive to rapid detection of multiple pesticides in food.