Melamine In Milk Products Research Articles

Development of a new phosphorescence sensor based on surface molecularly imprinted Mn-doped ZnS quantum dots for detection of melamine in milk products

This paper presents a novel room temperature phosphorescence sensor (IMIPs-ZnS QDs RTP sensor) based on inorganic surface molecularly imprinted polymers and Mn-doped ZnS quantum dots (QDs) for the rapid detection of trace melamine (MEL) in commercial milk products. The surface of Mn-ZnS QDs was modified with 3-(mercaptopropyl) trimethoxy silane (MPTS). Then, MEL, 3-aminopropyltriethoxysilane (APTES) and tetraethoxysilane (TEOS) were used as a template/target molecule, functional monomer, and cross-linker, respectively. IMIPs-ZnS QDs RTP sensor was characterized using spectrofluorimeter, UV–Vis spectrophotometer, FT-IR, transmission electron microscope (TEM), and X-ray photoelectron spectrometer (XPS).Detection time and linear range for IMIPs-ZnS QDs RTP sensor were 30 min and 4.0–79.2 µM with a correlation coefficient value of 0.9946, respectively. Furthermore, LOD and LOQ values were calculated using Stern-Volmer equation as 0.29 and 0.97 µM, respectively. Thus, IMIPs-ZnS QDs RTP sensor was successfully applied for the detection of MEL residue in milk samples. Recovery values were in the range of 88.62–90.22 % with relatively high precision values (0.57–0.92 % RSD). Our findings indicate that the developed IMIPs-ZnS QDs RTP sensor exhibits high sensitivity and selectivity towards the MEL in milk sample containing potentially relatively high number of interfering compounds.

Determination of Melamine by Label-Free Ratiometric Fluorescence Using the Inner-Filter Effect (IFE) between Cadmium Tellurium/Cadmium Sulfide Quantum Dots (QDs) and Gold Nanoparticles (Au NPs)

A simple and sensitive ratiometric fluorescent assay has been developed for the determination of melamine based on the inner-filter effect (IFE) between gold nanoparticles (AuNPs) and fluorescent quantum dots (QDs). In the IFE-based fluorescent assay, a complementary overlap was established between the absorption spectrum of AuNPs and emission spectrum of cadmium tellurium/cadmium sulfide quantum dots (CdTe/CdS QDs). Consequently, AuNPs in the dispersion state quench the emission of green CdTe/CdS QDs at 520 nm (QDs520), while the aggregated AuNPs quench emission of red CdTe/CdS QDs at 680 nm (QDs680). Along with the addition of melamine, the fluorescence of the QDs at 680 nm was quenched while the fluorescence of at 520 nm was recovered by the aggregation of AuNPs so the ratiometric fluorescent detection of melamine was complete. The system determined melamine from 30 to 1030 nM and was employed to determine melamine in milk products with satisfactory recoveries. The simple proposed sensing approach may improve the selectivity and sensitivity of IFE-based ratiometric fluorescent assays.

Electrochemical sensor based on EDTA-functionalized polyorthophenylene diamine g-C3N4 nanocomposite for determination of melamine in milk

Melamine has little acute oral toxicity, but excessive consumption of melamine can be harmful to human health, and the development of a rapid and accurate detection method for melamine is very important. Since the electrochemical activity of melamine is very low, potassium ferrocyanide was used as a redox probe to help detect melamine using a modified carbon paste electrode. In this study, we present a novel, simple, sensitive, and low-cost electrochemical sensor for the rapid detection of melamine. The sensor was based on electrochemically synthesized polyorthophenylene diamine and graphitic-carbon nitride (g-C3N4) nanocomposite (g-C3N4/POPD), which was additionally functionalized by ethylenediaminetetraacetic acid (EDTA) in order to get structure (g-C3N4/POPD /EDTA) with advanced selectivity towards melamine. The synthesized g-C3N4/POPD/EDTA nanocomposite was characterized by FE-SEM, EDS, Mapping, ATR-IR, Raman, AFM, BET, and EIS techniques. Cyclic voltammetry and differential pulse voltammetry were used for the investigation of the electrochemical behavior of the modified electrode in the presence of melamine. The oxidation peak currents of ferrocyanide were linear with the concentration of melamine in the range from 0.01 µM to 0.1 µM with a linear coefficiency of 0.996. The detection limit was 9.0 nM. The sensor was successfully applied to the determination of melamine in milk products and showed high selectivity, sensitivity, and reproducibility.

A Highly Sensitive Determination for the Melamine in Milk on MIL-101/AuNPs/CTS-PVP-rGO/GCE Electrochemical Sensor

A highly sensitive melamine electrochemical sensor was successfully constructed by self-assembling based on the composite of chitosan with polyvinyl pyrrolidone-dispersed reduced graphene oxide (CTS-PVP-rGO), gold nanoparticles (AuNPs) and metal-organic framework MIL-101. The characterizations of modified materials and electrodes were investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron dispersive X-ray (EDX) spectroscopy, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV). The results indicated that the sensor of MIL-101/AuNPs/CTS-PVP-rGO/GCE exhibited a high sensitivity and selectivity as well as a good stability and reproducibility for the determination of melamine since CTS-PVP-rGO or AuNPs could enhance the conductivity of the sensor greatly and MIL-101 could promote the adsorption of melamine on the surface of the modified electrode remarkably. At pH 7.0, the scan rate of 100 mV/s and the frequency of 50 Hz, the determination limit of melamine was as low as 5.00 × 10–11 mol/L with the linear range from 5.00 × 10–11 to 1.00 × 10–8 mol/L and the correlation coefficient (R) of 0.996. Based on the electrochemical behavior of melamine on MIL-101/AuNPs/CTS-PVP-rGO/GCE, the possible redox procedure of melamine was put forward. Furthermore, the sensor of MIL-101/AuNPs/CTS-PVP-rGO/GCE was applied to the determination of melamine in milk products and a satisfying result was obtained.

Ionic liquid based ion-pairing microextraction combined with spectrophotometry for preconcentration and quantitation of melamine in milk and milk-based products

A simple, rapid and economical ultrasound-assisted microextraction (UA-ME) procedure was developed for extraction and determination of trace amounts of melamine in milk products, followed at 310 nm by UV-Visible spectrophotometry. In the study, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C6mim][Tf2N] as ionic liquid (IL) was used in presence of sodium dodecyl sulfate (SDS) as an oppositely charged auxiliary ligand at pH 4.0 for extraction of melamine, and methanol was selected as a disperser solvent. Under the optimal conditions, the analytical features of method were established by calibration curves prepared from both solvent based calibration and matrix-matched calibration solutions. The method exhibits a linear relationship (from 5 to 250 μg L−1), low detection limit (1.6 μg L−1), good recovery (93.5–97.8%), and high sensitivity enhancement factor (125) by solvent based calibration curve while it allows a detection limit of 2.1 μg L−1 in range of 5–210 μg L−1 by the matrix-matched calibration curve. The method accuracy (trueness and precision) was checked by applying matrix-matched calibration strategy to all the spiked samples including quality control samples as well as comparison with an independent micellar LC method, and then it was successfully applied to the determination of melamine in milk based products.

Colorimetric detection of melamine based on p-chlorobenzenesulfonic acid-modified AuNPs

A highly selective and sensitive method is developed for colorimetric detection of melamine using gold nanoparticles (AuNPs) functionalized with p-chlorobenzenesulfonic acid. The addition of melamine induced the aggregation of AuNPs, as evidenced from the morphological characterizations and the color changed from red wine to blue, which could also be monitored by the UV–visible spectrometer and even naked eyes. This process caused a significant increase in the absorbance ratio (A650nm/A520nm) of p-chlorobenzenesulfonic acid–AuNPs. Under optimized conditions, the system exhibited a linear response to melamine in the range of 6.0 × 10−7–1.5 × 10−6 mol L−1 with a correlation coefficient of 0.997, and the limit of detection can even be 2.3 nM, which was much lower than some other methods and the safe limits (20 μM in both the USA and EU, 8.0 μM for infant formula in China, 1.2 μM in the CAC (Codex Alimentarius Commission) review for melamine in liquid infant formula). More importantly, the developed method presented excellent tolerance to coexisting common metal ions such as Ca2+, Zn2+, whose concentration is 1000 times of melamine, so that it had been applied to the analysis of melamine in liquid milk and milk powder with the recovery of 97.0–101 % and 100–103 %, respectively, indicating that the proposed method is quite a highly effective means to determine melamine in milk products.

Identification and Quantitation of Melamine in Milk by Near-Infrared Spectroscopy and Chemometrics

Melamine is a nitrogen-rich substance and has been illegally used to increase the apparent protein content in food products such as milk. Therefore, it is imperative to develop sensitive and reliable analytical methods to determine melamine in human foods. Current analytical methods for melamine are mainly chromatography-based methods, which are time-consuming and expensive and require complex pretreatment and well-trained technicians. The present paper investigated the feasibility of using near-infrared (NIR) spectroscopy and chemometrics for identifying and quantifying melamine in liquor milk. A total of 75 samples were prepared. Uninformative variable elimination-partial least square (UVE-PLS) and partial least squares-discriminant analysis (PLS-DA) were used to construct quantitative and qualitative models, respectively. Based on the ratio of performance to standard deviate (RPD), UVE-PLS model with 3 components resulted in a better solution. The PLS-DA model achieved an accuracy of 100% and outperformed the optimal reference model of soft independent modeling of class analogy (SIMCA). Such a method can serve as a potential tool for rapid screening of melamine in milk products.

Polydopamine-assisted partial hydrolyzed poly(2-methyl-2-oxazolinze) as coating for determination of melamine in milk by capillary electrophoresis

Nitrogen-rich melamine (66% by mass) had been found to be illegally adulterated to milk products and animal feed in order to increase the apparent protein content, and ingestion of melamine may lead to the formation of kidney stones. Hence, the development of reliable analytical methods for the monitoring of melamine in milk products is necessary. The common methods for melamine analysis in milk products are performed via GC–MS or HPLC, however, these procedures require a large of organic solvent and several pretreatment steps. In this study, a new and facile approach was developed to modify a fused-silica capillary inner surface based on poly(dopamine) (PDA) and partial hydrolyzed poly(2-methyl-2-oxazoline) (PMOXA-EI) mixed coating, which was used to determine the melamine in milk powder quantitatively by capillary electrophoresis with ultraviolet (CE-UV) detection with simple analytes pretreatment. Firstly, the influences of the molecular mass of poly(2-methyl-2-oxazoline) (PMOXA), the hydrolysis degree of PMOXA, and the concentration ratio of DA to PMOXA-EI on the separation properties of a mixture of four basic proteins (cytochrome c, lysozyme, ribonuclease A, and α-chymotrypsinogen A) were studied in detail. As a consequence, fast and efficient separation of four proteins was obtained using PDA/PMOXA1k-EI0.4 coating (once coating). Then, a modified facile coating workflow was carried out to acquire a novel twice coating (i.e. PDA/PMOXA1k-EI0.4+PMOXA1k-EI0.4 coated capillary) for highly sensitive and stable analysis of melamine. Under the optimal conditions, the linear response of melamine concentration ranged from 6.25 to 100µg/mL with high correlation coefficient of 0.9910, and the limit of detection (LOD) for melamine was 0.097µg/mL. The minimum amount of melamine determined in milk powder with our proposed method was 4mg/kg. The recoveries and relative standard deviations (RSDs) were 92.13–102.47%, 92.22–100.30%, 2.07–4.98%, and 2.43–3.77%, respectively for the intraday and interday analysis. After being used continually for 45 days, the PDA/PMOXA1k-EI0.4+PMOXA1k-EI0.4 coated capillary could be still used to detect the melamine from milk powder, with the recovery from 91.85% to 106.14%. The technique developed in this study provides a simple and relatively sensitive method for determination of melamine in milk powder.

Melamine contamination in nutritional supplements--Is it an alarm bell for the general consumer, athletes, and 'Weekend Warriors'?

BackgroundNutritional supplements are used or experimented with by consumers, notably these are; competitive and recreational athletes of all ages, and ‘weekend warriors’. As a consequence the supplement industry has grown to meet the increasing demand. A Global Industry Analysts Inc. report indicates that the herbal supplement market has not declined during the worldwide recession, but in fact exhibited steady growth over the period 2008 to 2009. It is anticipated that the market will reach US$93.15 billion by the year 2015. These supplements may contain adulterated substances that may potentially have harmful short - and long-term health consequences to the consumer. “Scrap Melamine” is such an example, which has been implicated in the kidney failure and death of several cats, dogs and pigs. In China in 2008, reports described very severe health effects in infants and young children. At the time over 294 000 infants were screened and diagnosed with urinary tract stones and sand-like calculi associated with melamine in milk products, of which 50 000 infants were hospitalised, and at least six associated deaths, recorded. The extent that melamine contamination occurs in nutritional supplements is not known. Therefore, the aim of this study was to determine whether commercially available nutritional and traditional supplement products contain melamine, even though they are not declared by the manufacturer on the product label.MethodsA total of 138 nutritional supplements products were obtained from (i) direct purchases from shops, pharmacies and outlets, (ii) directly from consumers, and (iii) from suppliers, manufacturers and distributors. The products were laboratory analysed for melamine, using Tandem Liquid Chromatography Mass Spectrometry.ResultsForty-seven % of all the products (n = 138) tested positive for melamine. Eight-two % of the South African produced products (n = 27) tested positive and 58 % of the products imported into South Africa (n = 50) tested positive. The median concentration estimate for melamine in the products tested were, 6.0 μg/g for the 138 supplements tested, 8.9 μg/g for South African produced products, and 6.9 μg/g for products imported into South Africa.ConclusionThe melamine (undeclared on product label) levels detected in the nutritional supplements products investigated were within the Tolerable Daily intake (TDI) limit guidelines of 200 μg/g as set by WHO and others. Melamine over exposure within the context of the nutritional supplements consumption in the products investigated should not be of concern to the consumer provided the recommended guidelines of daily product use are adhered to. Further investigation is warranted to determine, (i) the link of melamine as (part) substitute for the perceived total declared protein content on the product label, (ii) cyanuric and uric acid presence in the supplement products that could form chemical-complex formation with melamine and/or analogues that could cause adverse health effects.

Simple Pretreatment and Portable UV‐Vis Spectrum Instrument for the Rapid Detection of Melamine in Milk Products

AbstractIn this paper, we report a simple and reliable method for detection of melamine (MEL) in the field of milk products safety screening. This method is developed based on gold nanoparticle (AuNP) probe as the presence of MEL can induce the AuNPs to aggregate and make its color change from wine red to purple. This color change can be quantitatively recorded by a portable UV‐vis spectrophotometer so as to calculate the amount of MEL simultaneously. Herein, alcohol is used as MEL extraction solvent without other complex sample processing method because alcohol can easily denature the casein in milk. Furthermore, under the optimized condition, only about 15 min for whole detection process is required to determine the amount of MEL in milk, and the detection limit of MEL can be as low as 2 ppm.Practical ApplicationsAt present, analysis of melamine (MEL) in these milk products is urgent and required. However, most of the published assays require expensive instruments and complicated pretreatment. Herein, a reliable and simple method for MEL detection for field screening of milk was developed, which is based on gold nanoparticle probe with a portable UV‐vis spectrophotometer. Herein, ethanol was used as an environmentally friendly solvent to extract MEL from milk, and the whole detection process only required about 15 min.

Molecularly imprinted electrochemical sensor for the highly selective and sensitive determination of melamine.

A selective and sensitive molecularly imprinted electrochemical sensor was developed for the determination of melamine. Carbon nanotube-ionic liquid composite was used as a carrier to enhance the number of imprinted cavities and then improve the selectivity and sensitivity of the sensor. The proposed sensor showed a linear relationship with the concentration of melamine in the range of 0.4 to 9.2 μM, with a detection limit of 0.11 μM. The sensor was successfully applied to the determination of melamine in milk products with good recovery.

Determination of melamine in powdered milk by molecularly imprinted stir bar sorptive extraction coupled with HPLC

A novel molecularly imprinted stir bar (MIP-SB) was developed with melamine (MA) as the template molecule in this study. The sorptive capacity of MIP-SB was nearly three times over that of non-imprinted stir bar (NIP-SB). The MIP-SB presented much better selectivity than NIP-SB when used to absorb MA and its analogues. An analytical method to determine MA in powdered milk was established by combining MIP-SB sorptive extraction with HPLC. The liner range was 0.0631–12.6ng/mL with good correlation coefficient of 0.9983, and the limit of detection was 0.0127ng/mL based on three times ratio of signal to noise. This method was successfully applied to the determination of MA in powdered milk with satisfactory results. The method built is simple and suitable for the determination of MA in milk products which is of great significance for quality control of milk products.

Highly sensitive and selective determination of melamine in milk using glassy carbon electrode modified with molecularly imprinted copolymer

A novel electrochemical sensor was constructed for highly sensitive and selective determination of melamine by depositing a molecularly imprinted copolymer film, prepared by electrocopolymerization of gallic acid and o-phenylenediamine from aqueous electrolyte solution, onto glassy carbon electrode surface. The morphologic feature of the modified electrode was characterized via cyclic voltammetry, electrochemical impedance spectrometry, and scanning electron microscopy. The interactions between the molecularly imprinted copolymer and melamine cause the decrease in the peak currents of hexacyanoferrate, which could be used for electrochemical sensing of melamine. In this study, square wave voltammetry was selected as transducing mechanism. Several important parameters controlling the performance of the molecularly imprinted polymer (MIP) electrode were investigated and optimized. In the optimal conditions, the sensor response was sensitive to melamine from 5.0 to 100.0 nmol L−1 with good selectivity. The limit of detection based on a signal to noise ratio of 3 was low up to 1.4 nmol L−1. The method was applied successfully to determine melamine in milk products with satisfactory precision (2.11~3.38 % of relative standard deviation (RSD)) and acceptable recoveries (96.6~106.4 %), demonstrating a promising feature for applying the MIP sensor to trace detection of melamine in milk and other related products.

Development of melamine sensor based on ionic liquid/nanoparticles/chitosan with modified gold electrode for determination of melamine in milk product

The illegal adulteration of infant milk powder with melamine in China has resulted in chronic kidney and urinary tract failure in September 2008. To date, several analytical techniques have been used for the determination of melamine, although these techniques are complicated, time consuming and costly. In this study, we developed a novel electrochemical biosensor method based on the modification of gold electrode with chitosan, calcium oxide nanoparticles and an ionic liquid for the determination in the presence of melamine in milk products. The electrochemical behaviour of the modified gold electrode was studied by using cyclic voltammetry and differential pulse voltammetry in the presence of methylene blue used as a redox indicator. The morphological characteristics of nanomaterials were observed under scanning electron microscope. Under optimal conditions, differential pulse voltammetry was detected at different concentrations of melamine from 9.6×10−3 to 9.6×10−15M, with a detection limit of 9.6×10−16M. The developed melamine sensor method is a very simple and fast procedure for analysing melamine level in milk and milk products.

Melamine detection in dairy products by using a reusable evanescent wave fiber-optic biosensor

A sensitive, simple and rapid detection of trace amounts of melamine in milk products was achieved in a reusable, portable optical sensor system based on the principle of immunoassay and evanescent wave-excited fluorescence. The evanescent wave fiber-optic immunosensor (EWFI) employed a single-multi mode fiber optic coupler for light excitation and collection of fluorescence generated from the fiber optic probe. A reusable immunosurface of fiber probe was established to allow the performance of more than 300 assay cycles. Each assay cycle was less than 15min. The typical calibration curves of the EWFI obtained for melamine (MA) had a detection limit of 5.14μg/L with the detectable working range from 12.62 to 284.18μg/L when the concentration of Cy5.5-labeled antibody was 0.2μg/mL. The cross-reactivity against the organic compounds structurally similar to MA was negligible. The recoveries of MA in all sorts of dairy products ranged from 86.1% to 106.5% with relative standard deviation values of less than 4%, confirming the application potential in the measurement of MA in reality. The immunoassay performance of the EWFI was also validated with respect to conventional LC–MS/MS and the correlation between methods was in good agreement with the coefficient variation of less than 20% for majority samples.

A Fluoroimmunoassay Method Using Quantum Dot Labels for Detection of Melamine

An indirect competitive immunoassay using quantum dots (QDs) as the fluorescent label has been developed for detection of melamine in milk products. The method exhibited a response for melamine with IC50 value of 11.3 ng mL-1 and a detection limit of 0.27 ng mL-1. Milk and milk powder samples spiked with melamine have been analyzed, and the method provided recoveries ranging from 84.2% to 108.0%, and a CV of 12.8%. The implementation of QDs provides for at least 30 fold improvement of detection limit when compared with optically-based ELISA screening methods that have been reported. The analytical performance in combination with simplicity of sample and antibody preparation provides the basis for a practical method for melamine detection.

Lateral flow test strip based on colloidal selenium immunoassay for rapid detection of melamine in milk, milk powder, and animal feed.

Although high melamine (MEL) intake has been proven to cause serious health problems, MEL is sometimes illegally added to milk products and animal feed, arousing serious food safety concerns. A satisfactory method of detecting MEL in onsite or in-home testing is in urgent need of development. This work aimed to explore a rapid, convenient, and cost-effective method of identifying MEL in milk products or other food by colloidal selenium-based lateral flow immunoassay. Colloidal selenium was synthesized by L-ascorbic acid to reduce seleninic acid at room temperature. After conjugation with a monoclonal antibody anti-MEL, a test strip was successfully prepared. The detection limit of the test strip reached 150 μg/kg, 1,000 μg/kg, and 800 μg/kg in liquid milk, milk powder, and animal feed, respectively. No cross-reactions with homologues cyanuric acid, cyanurodiamide, or ammelide were found. Moreover, the MEL test strip can remain stable after storage for 1 year at room temperature. Our results demonstrate that the colloidal selenium MEL test strip can detect MEL in adulterated milk products or animal feed conveniently, rapidly, and sensitively. In contrast with a colloidal gold MEL test strip, the colloidal selenium MEL test strip was easy to prepare and more cost-efficient.

Electrospray ionization tandem mass spectrometry for rapid, sensitive and direct detection of melamine in dairy products

A simple, rapid, and sensitive method for detection of melamine in dairy products by electrospray ionization mass spectrometry (ESI–MS) without any sample preparation is demonstrated. The ESI–MS/MS on the protonated melamine at m/z 127 produced the characteristic fragment ions at m/z 110, 85, 68, 60. This method has a good linearity in the range of 0.5–10.0μg/mL, with detection limit 0.1μg/mL and RSD 8.9% (n=5). Together with existing recent methods on mass spectrometry and NP-based sensing methodologies for melamine detection, the present method provides an efficient platform for detection of melamine in milk products without any sample preparation steps or post-separation procedures.

Assay of melamine in milk products with a pH‐mediated stacking technique in capillary electrophoresis

A pH-mediated stacking method in capillary electrophoresis as an assay for low concentrations of melamine in milk products was established. Real samples were treated with acetone and sodium acetate and injected directly after centrifugation and filtration. Several experimental factors, such as buffer pH, buffer concentration, sample matrix, injection/sweeping ratio, sweeping time/voltages, separation voltages, as well as sample pretreatment, which affected stacking and separation, were investigated and optimized. Under the selected condition, a low LOD of 0.01 μmol/L (S/N = 5) and a wide range of linearity of 0.01∼1.0 μmol/L could be easily achieved with a good reproducibility (RSDs < 5.8% for both migration time and peak area) and an acceptable recovery of 94.0∼103.2% (for milk, infant formula, yogurt, and milk products). The proposed method was suitable for routine assay of melamine in real milk samples.

Stacking and analysis of melamine in milk products with acetonitrile-salt stacking technique in capillary electrophoresis.

Melamine was measured in real milk products with capillary electrophoresis (CE) based on acetonitrile-salt stacking (ASS) method. Real milk samples were deproteinized with acetonitrile at a final concentration of 60% (v/v) and then injected hydrodynamically at 50 mBar for 40.0 s. The optimized buffer contains 80.0 mmol/L pH 2.8 phosphates. Melamine could be detected within 20.0 min at +10 kV with a low limit of detection (LOD) of 0.03 μmol/L. Satisfactory reproducibility (inter- and intraday RSD% both for migration time and peak area was lower than 5.0%) and a wide linearity range of 0.05 μmol/L ~ 10.0 μmol/L were achieved. The proposed method was suitable for routine assay of MEL in real milk samples that was subjected to a simple treatment step.