Detection of melamine in milk using molecularly imprinted polymers–surface enhanced Raman spectroscopy

Potentiality of using front face fluorescence spectroscopy for quantitative analysis of cow milk adulteration in buffalo milk

In current study, synchronous front-face fluorescence spectroscopy together with partial least squares regression (PLSR) is used to predict the adulteration of cow and buffalo milk quantitatively. Fresh (unprocessed milk) samples of cow and buffalo were collected from local dairy farms. Fluorescence emission from milk samples mixed in different concentrations, show intensity variations at wavelengths 370-380 nm, 410 nm, 442 nm and 520-560 nm. Among them, the emissions at band position of 442 nm and 525 nm are highly selective between the two species and could help in finding adulteration of cow milk in buffalo milk and vice versa. The emissions at these wavelength positions correspond to fat-soluble vitamin-A as well as β-carotene. PLS regression is used as a statistical prediction model, which is developed by training with the emission spectra of milk samples having known level of adulterations. The developed model predicts the unknown level of adulterations by means of their spectral data. The goodness of the model is determined by the correlation coefficient R-square (r²) value, which in our case is 0.99. Furthermore, the model root mean square error in cross validation (RMSECV) and in prediction (RMSECP) remains 1.16 and 6.24 respectively. This approach can effectively be applied to determine milk adulterations among other species as well as in detecting external agents (fraudulent) added into milk and other dairy products by further studies.

Soft and flexible material-based affinity sensors

Recent advances in biosensors and point-of-care (PoC) devices are poised to change and expand the delivery of diagnostics from conventional lateral-flow assays and test strips that dominate the market currently, to newly emerging wearable and implantable devices that can provide continuous monitoring. Soft and flexible materials are playing a key role in propelling these trends towards real-time and remote health monitoring. Affinity biosensors have the capability to provide for diagnosis and monitoring of cancerous, cardiovascular, infectious and genetic diseases by the detection of biomarkers using affinity interactions. This review tracks the evolution of affinity sensors from conventional lateral-flow test strips to wearable/implantable devices enabled by soft and flexible materials. Initially, we highlight conventional affinity sensors exploiting membrane and paper materials which have been so successfully applied in point-of-care tests, such as lateral-flow immunoassay strips and emerging microfluidic paper-based devices. We then turn our attention to the multifarious polymer designs that provide both the base materials for sensor designs, such as PDMS, and more advanced functionalised materials that are capable of both recognition and transduction, such as conducting and molecularly imprinted polymers. The subsequent content discusses wearable soft and flexible material-based affinity sensors, classified as flexible and skin-mountable, textile materials-based and contact lens-based affinity sensors. In the final sections, we explore the possibilities for implantable/injectable soft and flexible material-based affinity sensors, including hydrogels, microencapsulated sensors and optical fibers. This area is truly a work in progress and we trust that this review will help pull together the many technological streams that are contributing to the field.

A Simple SERS-Based Trace Sensing Platform Enabled by AuNPs-Analyte/AuNPs Double-Decker Structure on Wax-Coated Hydrophobic Surface

In this work, a simple and versatile SERS sensing platform enabled by AuNPs-analyte/AuNPs double-decker structure on wax-coated hydrophobic surface was developed using a portable Raman spectrometer. Wax-coated silicon wafer served as a hydrophobic surface to induce both aggregation and concentration of aqueous phase AuNPs mixed with analyte of interest. After drying, another layer of AuNPs was drop-cast onto the layer of AuNPs-analyte on the substrate to form double-decker structure, thus introducing more “hot spots” to further enhance the Raman signal. To validate the sensing platform, methyl parathion (pesticide), and melamine (a nitrogen-enrich compound illegally added to food products to increase their apparent protein content) were employed as two model compounds for trace sensing demonstration. The as-fabricated sensor showed high reproducibility and sensitivity toward both methyl parathion and melamine detection with the limit of detection at the nanomolar and sub-nanomolar concentration level, respectively. In addition, remarkable recoveries for methyl parathion spiked into lake water samples were obtained, while reasonably good recoveries for melamine spiked into milk samples were achieved. These results demonstrate that the as-developed SERS sensing platform holds great promise in detecting trace amount of hazardous chemicals for food safety and environment protection.

Rapid determination of atrazine in apple juice using molecularly imprinted polymers coupled with gold nanoparticles-colorimetric/SERS dual chemosensor

Rapid and reliable determination of atrazine, a common chemical contaminant, in agri-foods is highly necessary. We reported a novel dual-chemosensor coupling, a separation [molecularly imprinted polymers (MIPs)], an instrumental-free detection [gold nanoparticles (AuNPs)-based colorimetric assay] and an instrument-based quantification [surface enhanced Raman spectroscopy (SERS)] method for high-throughput and sensitive determination of atrazine in apple juice. Used as the selective sorbent for the solid phase extraction, MIPs effectively extracted atrazine from apple juice with high recoveries (∼93%). AuNPs of different sizes (large; medium; and small) performed differently in the two analytical methods. Large-AuNPs provided the highest sensitivity in colorimetric analysis (<0.01 mg L^-1), while medium-AuNPs achieved the lowest limit of detection (0.0012 mg L^-1) and quantification (0.0040 mg L^-1) in SERS analysis. With minor modifications, protocols for both analytical methods can rapidly detect and/or quantify atrazine in different food products complying with the Health Canada regulation (0.005 mg L^-1).

Melamine-Induced Decomposition and Anti-FRET Effect from a Self-Assembled Complex of Rhodamine 6G and DNA-Stabilized Silver Nanoclusters Used for Dual-Emitting Ratiometric and Naked-Eye-Visible Fluorescence Detection

In this work, blue-emitting silver nanoclusters (AgNCs) were prepared in a matrix of single-stranded deoxyribonucleic acid (DNA) on the basis of ambient hydrothermal reactions. DNA acted as the stabilizer or coating agent, and NaBH₄ was used as the reducing agent. Through the interactions between rhodamine 6G (Rh6G) and the synthesized DNA-AgNCs, the self-assembled complex of DNA-AgNC-Rh6G was generated. Meanwhile, fluorescence emission of AgNCs was weakened as a result of fluorescence-resonance-energy transfer (FRET) from AgNCs (donor) to Rh6G (acceptor). In the DNA-AgNC-Rh6G complex aqueous suspension, the addition of melamine induced obvious emission recovery of AgNCs and fluorescence decrease of Rh6G, attributable to melamine-induced decomposition of the self-assembled complex and anti-FRET effects. There was a well-plotted linear relationship of ratiometric fluorescence intensities ( I_AgNCs/ I_Rh6G) versus melamine concentration in the range of 0.1-10 μM, with a low detection limit of 25 nM. Responses of I_AgNCs/ I_Rh6G to melamine were highly selective and sensitive over potential interferents. A novel dual-emitting ratiometric fluorescence sensor of melamine was facilely constructed on the basis of the DNA-AgNC-Rh6G complex. In particular, the sensor enabled visual fluorescence detection of melamine both in aqueous solution and on wetted filter paper. Superior detection results of the sensor were experimentally obtained and confirmed its high feasibility for melamine detection in practical samples.

Advancements of molecularly imprinted polymers in the food safety field

Molecularly imprinted technology (MIT) has been widely employed to produce stable, robust and cheap molecularly imprinted polymer (MIP) materials that possess selective binding sites for recognition of target analytes in food, such as pesticides, veterinary drugs, mycotoxins, illegal drugs and so on. Because of high selectivity and specificity, MIPs have drawn great attention in the food safety field. In this review, the recent developments of MIPs in various applications for food safety, including sample preparation, chromatographic separation, sensing, immunoassay etc., have been summarized. We particularly discuss the advancements and limitations in these applications, as well as attempts carried out for their improvement.

Rapid, On-Site, Ultrasensitive Melamine Quantitation Method for Protein Beverages Using Time-Resolved Fluorescence Detection Paper

To ensure protein beverage safety and prevent illegal melamine use to artificially increase protein content, a rapid, on-site, ultrasensitive detection method for melamine must be developed because melamine is detrimental to human health. Herein, an ultrasensitive time-resolved fluorescence detection paper (TFDP) was developed to detect melamine in protein beverages within 15 min using a one-step sample preparation. The lower limits of detection were 0.89, 0.94, and 1.05 ng/mL, and the linear ranges were 2.67-150, 2.82-150, and 3.15-150 ng/mL (R² > 0.982) for peanut, walnut, and coconut beverages, respectively. The recovery rates were 85.86-110.60% with a coefficient of variation <7.80% in the spiking experiment. A high specificity was observed in the interferent experiment. When detecting real protein beverage samples, the TFDP and ultraperformance liquid chromatography-tandem mass spectrometer (UPLC-MS/MS) results were consistent. This method is a promising alternative for rapid, on-site detection of melamine in beverages.

Rapid detection and quantification of 2,4-dichlorophenoxyacetic acid in milk using molecularly imprinted polymers-surface-enhanced Raman spectroscopy

We report the development of a molecularly imprinted polymers-surface-enhanced Raman spectroscopy (MIPs-SERS) method for rapid detection and quantification of a herbicide residue 2,4-dichlorophenoxyacetic acid (2,4-D) in milk. MIPs were synthesized via bulk polymerization and utilized as solid phase extraction sorbent to selectively extract and enrich 2,4-D from milk. Silver nanoparticles were synthesized to facilitate the collection of SERS spectra of the extracts. Based on the characteristic band intensity of 2,4-D (391 cm^-1), the limit of detection was 0.006 ppm and the limit of quantification was 0.008 ppm. A simple logarithmic working range (0.01-1 ppm) was established, satisfying the sensitivity requirement referring to the maximum residue level of 2,4-D in milk in both Europe and North America. The overall test of 2,4-D for each milk sample required only 20 min including sample preparation. This MIPs-SERS method has potential for practical applications in detecting 2,4-D in agri-foods.

A Surface Plasmon Resonance-Based Optical Fiber Probe Fabricated with Electropolymerized Molecular Imprinting Film for Melamine Detection

Molecularly imprinted polymer (MIP) films prepared by bulk polymerization suffer from numerous deficiencies, including poor mass transfer ability and difficulty in controlling reaction rate and film thickness, which usually result in poor repeatability. However, polymer film synthesized by electropolymerization methods benefit from high reproducibility, simplicity and rapidity of preparation. In the present study, an Au film served as the refractive index-sensitive metal film to couple with the light leaked out from optical fiber core and the electrode for electropolymerizing MIP film simultaneously. The manufactured probe exhibited satisfactory sensitivity and specificity. Furthermore, the surface morphology and functional groups of the synthesized MIP film were characterized by Atomic Force Microscopy (AFM) and Fourier transform infrared microspectroscopy (FTIR) for further insights into the adsorption and desorption processes. Given the low cost, label-free test, simple preparation process and fast response, this method has a potential application to monitor substances in complicated real samples for out-of-lab test in the future.

Detection and quantification of offal content in ground beef meat using vibrational spectroscopic-based chemometric analysis

As less consumed animal by-product, beef and pork offal have chances to sneak into the authentic ground beef meat products, and thus a rapid and accurate detection and quantification technique is highly required. In this study, Fourier transformed-infrared (FT-IR) spectroscopy was investigated to develop an optimized protocol for analyzing ground beef meat potentially adulterated with six types of beef and pork offal. Various chemometric models for classification and quantification were constructed for the collected FT-IR spectra. Applying optimized chemometric models, FT-IR spectroscopy could differentiate authentic beef meat from adulterated samples with >99% accuracy, to identify the type of offal in the sample with >80% confidence, and to quantify five types of offal in an accurate manner (R² > 0.81). An optimized protocol was developed to authenticate ground beef meat as well as identify and quantify the offal adulterants using FT-IR spectroscopy coupled with chemometric models. This protocol offers a limit of detection <10% w/w of offal in ground beef meat and can be applied by governmental laboratories and food industry to rapidly monitor the integrity of ground beef meat products.

Integrated preservation and sample clean up procedures for studying water ingestion by recreational swimmers via urinary biomarker determination

The use of cyanuric acid as a biomarker for ingestion of swimming pool water may lead to quantitative knowledge of the volume of water ingested during swimming, contributing to a better understanding of disease resulting from ingestion of environmental contaminants. When swimming pool water containing chlorinated cyanurates is inadvertently ingested, cyanuric acid is excreted quantitatively within 24 h as a urinary biomarker of ingestion. Because the volume of water ingested can be quantitatively estimated by calculation from the concentration of cyanuric acid in 24 h urine samples, a procedure for preservation, cleanup, and analysis of cyanuric acid was developed to meet the logistical demands of large scale studies. From a practical stand point, urine collected from swimmers cannot be analyzed immediately, given requirements of sample collection, shipping, handling, etc. Thus, to maintain quality control to allow confidence in the results, it is necessary to preserve the samples in a manner that ensures as quantitative analysis as possible. The preservation and clean-up of cyanuric acid in urine is complicated because typical approaches often are incompatible with the keto-enol tautomerization of cyanuric acid, interfering with cyanuric acid sample preparation, chromatography, and detection. Therefore, this paper presents a novel integration of sample preservation, clean-up, chromatography, and detection to determine cyanuric acid in 24 h urine samples. Fortification of urine with cyanuric acid (0.3-3.0 mg/L) demonstrated accuracy (86-93% recovery) and high reproducibility (RSD < 7%). Holding time studies in unpreserved urine suggested sufficient cyanuric acid stability for sample collection procedures, while longer holding times suggested instability of the unpreserved urine. Preserved urine exhibited a loss of around 0.5% after 22 days at refrigerated storage conditions of 4 °C.

Determination of thiabendazole in orange juice using an MISPE-SERS chemosensor

Thiabendazole, a systemic fungicide used to treat vegetables and fruits during postharvest process, persists as detrimental residue to consumers. We combine a molecularly imprinted polymers (MIPs) with surface enhanced Raman spectroscopy (SERS) to form a novel MISPE-SERS chemosensor and determined thiabendazole in orange juice. Kinetic and static adsorption tests validated the efficient and selective adsorption of thiabendazole using synthesized MIPs via precipitation polymerization. Synthesized MIPs were packed into solid phase extraction (SPE) cartridge to serve as tailor-made sorbents for the separation of thiabendazole in orange juice. Silver colloids synthesized by reduction of AgNO₃ by trisodium citrate were used as SERS-active substrate to quantify the eluted thiabendazole from MISPE. The overall process including sample preparation and detection took 23min and the limit of detection of this chemosensor was 4ppm for thiabendazole in orange juice. This chemosensor can be used for rapid and sensitive detection of thiabendazole in agri-foods.

Application of nuclear magnetic resonance spectroscopy in food adulteration determination: the example of Sudan dye I in paprika powder

Carcinogenic Sudan I has been added illegally into spices for an apparent freshness. ¹H solution and solid-state (SS) nuclear magnetic resonance (NMR) spectroscopies were applied and compared for determination of Sudan I in paprika powders (PPs). For solution NMR, PPs spiked with Sudan I were extracted with acetonitrile, centrifuged, rotor-evaporated, and re-dissolved in DMSO-d6 for spectral collection. For SSNMR, Sudan I contaminated PPs were mixed with DMSO-d6 solution and used for spectral collection. Linear regression models constructed for quantitative analyses resulted in the average accuracies for unknown samples as 98% and 105%, respectively. Limits of detection for the solution NMR and SSNMR spectrometers were 6.7 and 128.6 mg kg^-1, while the limits of quantification were 22.5 and 313.7 mg kg^-1. The overall analysis time required by both methods was similar (35 and 32 min). Both NMR techniques are feasible for rapid and accurate determination of Sudan I adulteration in PPs.

Advantage of Eu3+ -Doped Polystyrene Microspheres Compared with Colloidal Gold Used in Immunochromatographic Assays for the Detection of Melamine in Milk

Colloidal gold and Eu³⁺ -doped fluorescent microspheres were applied as labels to develop the immunochromatographic strips for detecting melamine in milk. Under the optimized condition, the visual detection limit of colloidal gold-immunochromatographic test strip (ICTS) was 150 μg/L of melamine in phosphate-buffered saline (PBS), although the visual detection limit of fluorescent nanoparticles (FN)-ICTS was 75 μg/L in PBS. As thermal acceleration test, FN-ICTS could be stored at 37 °C for at least 11 d before sample testing, but the color of the lines on colloidal gold-ICTS faded away after 7-d storage. The visual result of FN-ICTS was more stable than that of colloidal gold-ICTS, and the fluorescence intensity of the line on FN-ICTS could be maintained up to 30 d at 22 °C after sample testing. Once the immunochromatographic strips were used to detect melamine in milk, no negative effect of milk components on the performance of FN-ICTS was identified, whereas the performance of colloidal gold-ICTS was significantly influenced by milk matrix.

Molecularly imprinted polymers for sample preparation and biosensing in food analysis: Progress and perspectives

Molecularly imprinted polymers (MIPs) are biomimetics which can selectively bind to analytes of interest. One of the most interesting areas where MIPs have shown the biggest potential is food analysis. MIPs have found use as sorbents in sample preparation attributed to the high selectivity and high loading capacity. MIPs have been intensively employed in classical solid-phase extraction and solid-phase microextraction. More recently, MIPs have been combined with magnetic bead extraction, which greatly simplifies sample handling procedures. Studies have consistently shown that MIPs can effectively minimize complex food matrix effects, and improve recoveries and detection limits. In addition to sample preparation, MIPs have also been viewed as promising alternatives to bio-receptors due to the inherent molecular recognition abilities and the high stability in harsh chemical and physical conditions. MIPs have been utilized as receptors in biosensing platforms such as electrochemical, optical and mass biosensors to detect various analytes in food. In this review, we will discuss the current state-of-the-art of MIP synthesis and applications in the context of food analysis. We will highlight the imprinting methods which are applicable for imprinting food templates, summarize the recent progress in using MIPs for preparing and analysing food samples, and discuss the current limitations in the commercialisation of MIPs technology. Finally, future perspectives will be given.

Bimodal counterpropagating-responsive sensing material for the detection of histamine

A dual-mode system for simultaneous fluorescence and SERS sensing of histamine.

A high-performance SERS-imprinted sensor doped with silver particles of different surface morphologies for selective detection of pyrethroids in rivers

Ag-MIPs were prepared through a multistep procedure, in which MPS and LC were selected as the template molecules. These materials could selectively rebind the templates and could be detected using Raman spectroscopy.

Highly sensitive determination of ethyl carbamate in alcoholic beverages by surface-enhanced Raman spectroscopy combined with a molecular imprinting polymer

A simple and reliable method for fast extraction and sensitive detection of ethyl carbamate in alcoholic beverages was developed.