Melamine Detection in Milk and Dairy Products: Traditional Analytical Methods and Recent Developments

Electrochemical Analysis of Curcumin in Real Samples Using Intelligent Materials

Curcumin is a compound of great importance in the food industry due to its biological and pharmacological properties, which include being an antioxidant, anti-inflammatory, antibacterial, antiviral, and anticarcinogenic. This paper proposes the synthesis of an electrochemical sensor based on molecularly imprinted polymers (MIPs) and MWCNT by drop casting deposited on a glassy carbon electrode (GCE) for the selective quantification of curcumin in food samples. The synthesized compounds are characterized by Fourier transform infrared (IR), Brunauer-Emmett-Teller (BET), and electrochemical techniques such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The optimal conditions for further experiments were determined by selecting these parameters. We examined three food products, commercial capsules, turmeric rhizomes, and commercial turmeric powder, employing both electrochemical and HPLC methods for the analysis. The electrochemical method revealed a limit of detection (LOD) value of 0.1365 µmol L-1, compared with the HPLC analysis, which gave a value of 3.55 µmol L-1. Furthermore, the MIP material demonstrated superior selectivity for the analyte compared to potential interferents. The recovery percentage, determined using the HPLC method, fell within the range of 87.5% to 102.6.

Melamine in Iranian foodstuffs: A systematic review, meta-analysis, and health risk assessment of infant formula

The presence of melamine in food is one of the most significant threats to consumer health and food safety now confronting the communities. The goal of this systematic review and meta-analysis was to determine the melamine content of different food products available on the Iranian market. The pooled melamine concentration (95% confidence interval) on 484 samples of animal-based foodstuffs was as follows: 0.22 (0.08, 0.36 mg kg^-1) for milk, 0.39 (0.25, 0.53 mg kg^-1) for coffee mate, 1.45 (1.36, 1.54 mg kg^-1) for dairy cream, 0.90 (0.50, 1.29 mg kg^-1) for yoghurt, 1.25 (1.20, 1.29 mg kg^-1) for cheese, 0.81 (-0.16, 1.78 mg kg^-1) for hen eggs, 1.28 (1.25, 1.31 mg kg^-1) for poultry meat, 0.58 (0.35, 0.80 mg kg^-1) for chocolates, and 0.98 (0.18, 1.78 mg kg^-1) for infant formula. Based on the results of health risk assessment study on toddlers under 2 years old who ingested infant formula (as a melamine-sensitive group), all groups of toddlers are at an acceptable level of non-carcinogenic risk (THQ ≤ 1). Toddlers were classified according to their ILCR (carcinogenic risk) levels due to infant formula consumption as follows: under 6 months (0.0000056), 6-12 months (0.0000077), 12-18 months (0.0000102), and 18-24 months (0.0000117). The melamine carcinogenicity in infant formula for children had an ILCR value of 0.000001-0.0001 in the investigation, which was considerable risk. According to the findings, Iranian food products (notably infant formula) should be analyzed for melamine contamination on a regular basis.

Primary amide-functionalized cyclotricatechylene covalent organic frameworks membrane for efficient enrichment of melamine and its derivatives in migration solution of food contact materials

A highly chemically stable primary amide-functionalized cyclotricatechylene covalent organic framework was synthesized by an irreversible reaction and a post-synthetic modification. It possessed a rod-like morphology and exhibited strong solvent stability owing to the polyether bonds. The material showed good adsorption performance for melamine and its derivatives and adsorption mechanism was investigated by molecular simulations. The adsorbent was coated on the nylon-66 membrane to prepare the enrichment membrane. Under optimized conditions, an in-syringe membrane-based extraction method, combined with ultra-high performance liquid chromatography-tandem mass spectrometry was developed for the analysis of melamine and six melamine derivatives in the migration solution. A good linearity was obtained with correlation coefficients ranging from 0.9924 to 0.9995. The limits of detection were 1-200 ng/L and the limits of quantification were 3-500 ng/L. This method was successfully applied to the migration solution of sushi bamboo rolling mats with spiked recoveries of 73.2%-115% and relative standard deviations of 0.9%-9.9%. This work shows a practical and perspective approach for the efficient enrichment of food contact material hazards.

A Molecularly Imprinted Polymer-Based Thermal Sensor for the Selective Detection of Melamine in Milk Samples

In recent years, melamine-sensing technologies have increasingly gained attention, mainly due to the misuse of the molecule as an adulterant in milk and other foods. Molecularly imprinted polymers (MIPs) are ideal candidates for the recognition of melamine in real-life samples. The prepared MIP particles were incorporated into a thermally conductive layer via micro-contact deposition and its response towards melamine was analyzed using the heat-transfer method (HTM). The sensor displayed an excellent selectivity when analyzing the thermal response to other chemicals commonly found in foods, and its applicability in food safety was demonstrated after evaluation in untreated milk samples, demonstrating a limit of detection of 6.02 μM. As the EU/US melamine legal limit in milk of 2.5 mg/kg falls within the linear range of the sensor, it can offer an innovative solution for routine screening of milk samples in order to detect adulteration with melamine. The results shown in this work thus demonstrate the great potential of a low-cost thermal platform for the detection of food adulteration in complex matrices.

Photonic crystal sensor for melamine based on magnetic molecularly imprinted nanoparticles self-assembled with an amphiphilic random copolymer

Magnetic molecularly imprinted nanoparticles (MMINPs) were obtained with a one-step process through miniemulsion self-assembly using an amphiphilic random copolymer as both an emulsifier and MMINP coating, oleic acid-modified magnetite nanoparticles as magnetic cores, and melamine (MEL) as the template molecule. MMINPs were assembled under an external magnetic field to construct photonic crystal (PC) sensor for naked-eye detection of MEL. The MMINPs were characterized by FT-IR, TEM, TGA, and VSM. The analytical performances of the magnetic molecularly imprinted PC sensor for MEL (MEL-MMIPCs) were investigated with respect to sensitivity, response time, selectivity, and stability. As the MEL concentration increases from 1.0 to 1.0 × 10⁶ μg/l, the reflection wavelength of MEL-MMIPCs shifted from 497 to 709 nm, and was linear with the logarithm of MEL concentration in this range. The detection limit was 0.21 μg/l (S/N = 3) and response time was within 30 s. The MEL-MMIPC sensor had an imprinting factor of 5.09, and selectivity factors for the analogs cyanuric acid and atrazine were 8.76 and 5.75, respectively, indicating the high sensitivity and selectivity. After 10 cycles of elution/response, MEL-MMIPCs still had a good ability to recognize MEL.

Influence of brands on a discrimination model for adulterated milk based on asynchronous two-dimensional correlation spectroscopy slice spectra

To improve the robustness of near infrared (NIR) identification models for the milk adulteration, a novel approach was explored based on asynchronous two-dimensional correlation spectroscopy (2D-COS) slice spectra obtained at characteristic wavebands for pure milk and adulterant combined with an N-way partial least squares discriminant analysis (NPLS-DA). NIR diffuse reflectance spectra from four different brands, Guangming (GM), Mengniu (MN), Sanyuan (SY), and Wandashan (WDS), were collected in range of 11,000 to 4000 cm^-1. Influence of brands on discrimination models for adulterated milk was analyzed. The asynchronous 2D-COS slice spectra at 10 characteristics wavebands, including 4 wavebands for pure milk and 6 wavebands for urea, were input into NPLS-DA to construct discriminant models. External validations using five independent calibration sets from intrabrand or interbrand were established. The same prediction set of 26 SY samples was used to assess the prediction ability of different calibration sets and compared with traditional one-dimensional (1D) NIR spectra based on a partial least squares discriminant analysis (PLS-DA). It showed that for intrabrand model, the correct rates for the calibration and predication sets were 100% and 96.15%, respectively. For the interbrand model, the correct rates by the NPLS-DA for the calibration set of GM, MN, and WDS milk were both 100%. The corresponding rates for the prediction set were 73%, 88.46% and 69.23%, respectively, which were much higher than those of PLS-DA (only 50%, 53.83% and 50%, respectively). It was proven that model robustness was sensitive to changes in the milk brands. The proposed method can effectively reduce the influence of brands on the discrimination models.

A signal-on fluorescent aptasensor by sensitized Tb3+ luminescence for detection of melamine in milk

A fluorescent aptasensor based on sensitized terbium(III) luminescence was constructed to detect melamine in milk. Tb³⁺ as the fluorescence probe can be sensitized by a guanine-rich single-stranded DNA sequence, so the complementary sequence of the polythymidine aptamer (cDNA) was modified with six consecutive guanine bases (G6). In the absence of melamine, melamine aptamer combined with cDNA to form a double helix structure, and G6 hybridized with the extended cytosine bases in the aptamer, resulting in low fluorescence intensity of Tb³⁺. In the presence of melamine, cDNA was released due to the specific recognition of melamine to the aptamer, resulting in stronger sensitized fluorescence intensity of Tb³⁺. Under the optimum conditions, the linear concentration of melamine in the milk ranged from 1.0 μg/mL to 10.0 μg/mL. This aptasensor can be used for the accurate and rapid detection of melamine in milk with a detection limit of 0.02 μg/mL, and has the advantages of high sensitivity, high efficiency, simple operation and low cost.

A Fluorescence-Based Chemical Sensor for Detection of Melamine in Aqueous Solutions

Melamine, an industrial chemical, receives wide attention nowadays because of its unethical usage as a nitrogen enhancer in protein-rich foods and dairy products. Since most of the existing melamine detection methods are highly expensive and time-consuming, high sensitivity biosensor-based detection methods have arisen in the scientific literature as promising alternatives. This study reports the design, synthesis, and fluorescent investigations of a carbazole-based sensor (CB) for the detection of melamine in aqueous solutions. The titration studies and microplate experiments on a CB-cyanuric acid mixture (CB-CA) with melamine suggested that the novel sensor could detect melamine even at very low concentrations in both aqueous solutions and dairy samples.

An isotope dilution liquid chromatography-mass spectrometry method for detection of melamine in milk powder

Adulteration/unintentional contamination of milk with melamine could have negative health and economic implications especially in the developing countries due to insufficient laboratory support and surveillance. This paper presents an Isotope Dilution Liquid Chromatography-Mass Spectrometry (ID LC-MS) method developed for detection of melamine in powdered milk. The rapid sample preparation involved dissolution of 1g of milk powder in 2.5% formic acid, precipitation of protein with acetonitrile, spiking of samples with melamine (triamine-¹⁵N₃) at 200 µg L^-1 and detection of intrinsic ¹⁴N-melamine molecular ratio to the spike. The isotope dilution calibration procedure was free from matrix effects, unlike other methods where the detector sensitivity can fluctuate up to several orders of magnitude. Limit of detection of the method was 13 µg kg^-1, and the recovery of melamine at 50, 100, and 250 µg kg^-1 was 78.7-126.3%. The method was used to determine melamine levels in 22 milk powder products (local and imported) available in Sri Lanka. Melamine was detected in all the samples (range = 0.33-0.96 mg kg^-1). Full cream milk powders (both local and imported) contained melamine in the range of 0.39-0.84 mg kg^-1, and various health and pregnancy formulas contained <0.5 mg kg^-1 of melamine. Two imported infant formula samples contained the highest levels of melamine (0.96 and 0.94 mg kg^-1). Although these melamine levels are below the regulatory limit in Sri Lanka (1 mg kg^-1), a monitoring programme would ensure consumer safety.

Determination of melamine in milk by fluorescence spectroscopy and second-order calibration

Melamine is a compound commonly used in the manufacturing of plastic and flame retardant products, but due to its solubility on water and high nitrogen content, it is also used to adulterate milk to mask adulteration by dilution in protein content tests. This work proposes a quick method using excitation-emission matrix (EEM) fluorescence spectroscopy and second-order calibration methods (PARAFAC and U-PLS/RBL) for the identification and quantification of melamine in milk. The proposed method uses a single clean-up step with acetic acid, resulting in a quick, low-cost, and environmentally friendly procedure, in agreement with green chemistry principles. Both PARAFAC and U-PLS/RBL were capable of detecting melamine in milk above 120.6 and 146.5 ppm respectively, adequate for adulterations above 2% in volume, with RMSEPs of 68.6 and 81.9 ppm, respectively.

Rapid and sensitive SERS detection of melamine in milk using Ag nanocube array substrate coupled with multivariate analysis

In this study, a facile Ag nanocube (NC) array substrate was fabricated for rapid SERS detection of melamine in milk. This easily-prepared substrate exhibited high Raman enhancement factor (~1.02 × 105) and good reproducibility with ~10.75% spot-to-spot variation in Raman intensity. Our proposed method can detect melamine as low as 0.01 ppm in standard solutions and 0.5 ppm in real milk samples after a simple one-step solvent extraction. Two multivariate analysis tools including partial least squares and support vector machines (SVM) were explored to develop reliable regression models for quantitative SERS analysis of melamine. By comparison, SVM regression models exhibited better predictive performance, especially in liquid milk, with root mean square error (RMSE) of calibration = 5.5783, coefficient of determination (R2) of calibration = 0.9807, RMSE of prediction = 1.9636, and R2 of prediction = 0.9736. Hence, this study offers a rapid and sensitive detection of adulterant melamine in milk samples.

Rapid Determination of Total Tryptophan in Yoghurt by Ultra High Performance Liquid Chromatography with Fluorescence Detection

Tryptophan (TRP) is an essential amino acid which cannot be synthesized by humans and animals, but has to be supplied by exogenous sources, notably through the diet. The bulk of dietary TRP flows into the synthesis of body’s proteins, but the TRP metabolism also involves several biochemical reactions (i.e., serotonin and kynurenine pathways). Defects in the TRP transport mechanism or catabolism are related to a large number of clinical abnormalities. Therefore, dietary TRP intake is necessary not only for the body’s growth but also for most of the body’s metabolic functions. Among protein-based foods, milk proteins provide a relatively high amount of TRP. In this paper, a rapid chromatographic method for TRP determination in yoghurt, by ultra high performance liquid chromatography on a reversed-phase column with fluorescence detection (280 nm Ex; 360 nm Em), is provided. A linear gradient elution of acetonitrile in water allowed TRP analysis in 8.0 min. The limit of detection and limit of quantification of the method were 0.011 ng/µL and 0.029 ng/µL, respectively, using 5-methyl-l-tryptophan as the internal standard. The analytical method was successfully applied to commercial yoghurts from different animal species, and the TRP values ranged between 35.19 and 121.97 mg/100 g (goat and cow Greek type yoghurt, respectively).

Exposure to melamine and its derivatives in childcare facilities

Melamine (MEL) and its derivatives are widely used in many consumer products, including furniture, kitchenware, and plastics. However, very limited knowledge exists on human exposure to MEL and its derivatives, especially in the indoor environment. Here, we determined the occurrence and distribution of 11 MEL derivatives in childcare facilities and estimated children's exposure through dust ingestion and dermal absorption. We analyzed dust and samples of nap mats, a commonly used item in many childcares, from eight facilities located in the United States. Eight MEL-based compounds were detected in dust, and total MEL concentrations ranged from 429 to 117,000 ng/g. The most abundant compounds found in the dust samples were MEL, cyanuric acid (CYA), ammeline (AMN), and ammelide (AMD), with median concentrations of 1620, 585, 1060, and 299 ng/g, respectively. MEL, CYA, AMN and 2,4,6-tris[bis(methoxymethyl)amino]-1,3,5-triazine (TBMMAT) were also detected in nap mats with median concentrations of 45.6, 19.8, 1510 and 2.5 ng/g, respectively. ΣMEL concentrations in mat covers (median 709 ng/g) were significantly higher than those in mat foam (median 15.1 ng/g). Estimated daily intakes (EDIs) of MEL and its derivatives via dust ingestion were two orders of magnitude higher than the EDIs through dermal absorption, but both were below the established tolerable daily intake levels. This is the first report on exposure to MEL and its derivatives in the childcare environment.

A simple magnetic solid-phase extraction method based on magnetite/graphene oxide nanocomposite for pre-concentration and determination of melamine by high-performance liquid chromatography

In this study, a clean and simple magnetic solid-phase extraction (MSPE) procedure using magnetite/graphene oxide nanocomposite as an adsorbent was developed for melamine separation and preconcentration from water and dairy products. After synthesis and characterization of the adsorbent, adsorption isotherms and kinetic studies of the adsorption were carried out. The analyte quantification was performed by reversed phase high-performance liquid chromatography after elution of the preconcentrated analytes from the adsorbent surface. Several factors affecting the extraction/preconcentration procedure such as pH, adsorbent amount, extraction time, sample volume, type, and volume of eluent were investigated. The optimizing of some important parameters was assessed by employing a response surface method. The constructed calibration curve in the optimized conditions is linear in the working range of 0.10-100 μg L^-1 with a correlation coefficient of 0.9999. The detection limit, limit of quantification, and enrichment factor are 0.03 μg L^-1, 0.10 μg L^-1, and 500, respectively. The melamine relative recoveries from different real samples are between 97.20 and 103.10% with relative standard deviations of 1.07-4.98%.

Occurrence and distribution of melamine and its derivatives in surface water, drinking water, precipitation, wastewater, and swimming pool water

The extensive use of melamine and its three derivatives (i.e., ammeline, ammelide, and cyanuric acid) resulted in their widespread occurrence in the environment. Nevertheless, limited information is available on their distribution in the aquatic environment. In this study, concentrations and profiles of melamine and its derivatives were determined in 223 water samples, comprising river water, lake water, seawater, tap water, bottled water, rain water, wastewater, and swimming pool water, collected from New York State, USA. The sum concentrations of melamine and its derivatives (∑₄MELs) decreased in the following order: swimming pool water (median: 1.5 × 10⁷ ng/L) ≫ wastewater (1240) > precipitation (739) > tap water (512) > river water (370) > lake water (347) > seawater (186) > bottled water (98). Cyanuric acid was the major compound, accounting for 60-100% of ∑₄MELs concentrations in swimming pool water, wastewater, precipitation, tap water, seawater, and bottled water, whereas melamine dominated in river and lake water (54-64% of ∑₄MELs). Significant positive correlations (0.499 < R < 0.703, p < 0.002) were found between the concentrations of melamine and atrazine (a triazine herbicide) in surface waters. The geographic distribution in the concentrations of ∑₄MELs in river, lake, and tap water corresponded with the degree of urbanization, suggesting that human activities contribute to the sources melamine and cyanuric acid in the aquatic environments. A preliminary hazard assessment of melamine and cyanuric acid in waters suggested that their ecological or human health risks were minimal. This is the first study to document the occurrence and spatial distribution of melamine and its derivatives in waters from the United States.

Presence of melamine in human milk and the evaluation of the effects on mother–infant pairs in a cohort study

Melamine contamination is a recent public health problem emerging globally. Present study aimed to detect the rate of melamine presence in human milk in a cohort study and to evaluate any possible differences in maternal–infant pair characteristics such as breastfeeding status, crying and sleep problems of infants, maternal postpartum depression, maternal–infant bonding, infant and maternal anthropometry, and maternal complete blood count caused by the melamine exposure. Mothers of infants born in Şanlıurfa were invited to participate in ‘Urfa Child Cohort Survey’. Overall, two breast milk samples were taken between 5 days and 15 days postpartum and between 4 weeks and 10 weeks. Randox Food’s InfiniPlex array was used to analyse the presence of melamine. Melamine was detected in 32.2% and 24.4% of the first and the second milk samples; 16.7% of mothers had two positive samples. z Scores for birth weight and z scores for height for age were detected to be significantly lower in cases with two positive samples compared to cases with negative samples. Mean maternal white blood cells counts were found to be lower in cases with repeated melamine contact. Melamine might have a detrimental effect on birth weight, infant height and maternal blood count. Further studies should be done to detect environmental contamination in different regions and countries.

Designing a sustainable mobile phase composition for melamine monitoring in milk samples based on micellar liquid chromatography and natural deep eutectic solvent

Modified micellar liquid chromatography (MLC) with a natural deep eutectic solvent (NADES), produced from choline chloride (ChCl) and ethylene glycol (EG), was employed for melamine (MEL) monitoring in milk matrix. This sustainable mobile phase was attained through chemometrical optimization of crucial variables including concentration of sodium dodecyl sulphate ([SDS]) along with volume percentages of both NADES and glacial acetic acid (GAC). The desirability function and central composite design were utilized as chemometrical tools. Retention time (t_R-MEL), and chromatographic peak width of MEL at 50% of its height (W_50%-MEL) were considered for finding the best possible arrangement of the influential factors in the configuration of the mobile phase. Under the optimal experimental conditions of 0.10 mol L^-1 SDS, 4% (v/v) NADES, and 4% (v/v) GAC, the results showed that both t_R-MEL and W_50%-MEL drastically decreased when NADES was a part of the mobile phase composition. This indicated that ChCl-EG-based NADES had a significant impact on improving the chromatographic behaviour of an ionizable polar compound, MEL. At the optimal point, MEL was eluted in approximately 10 min without being interfered by coexisting proteins and endogenous species in milk. The practical performance of the mobile phase was established through direct injection of milk samples into the MLC system. The eligibility criteria of the United State-Food and Drug Administration (US-FDA) were considered for validation of the introduced methodology.

Spatial and temporal trends of melamine and its derivatives in sediment from Lake Shihwa, South Korea

Occurrence, spatial distribution, and temporal trends of melamine and its derivatives (ammeline, ammelide, and cyanuric acid) were investigated in surface sediment and two sediment cores collected from Lake Shihwa, South Korea. ∑Melamine (sum of melamine and its three derivatives) was found in all surface sediment samples at concentrations that ranged from 16.6 to 4390 ng/g dry weight (dw), with an average value of 202 ng/g dw. ∑Melamine concentrations exhibited a clear spatial gradient, in decreasing order, as: creeks (mean: 577 ng/g dw) > inshore locations (41.9 ng/g dw) > offshore locations (24.3 ng/g dw). ∑Melamine concentrations were notably high in sediment collected near wastewater treatment plant (WWTP) outfalls. Melamine was the most prevalent compound in sediment collected from creeks (87%) and WWTP outfall locations (48%), whereas cyanuric acid was the dominant compound in sediment from inshore (51%) and offshore (63%) locations. The historical profiles of ∑Melamine in sediment cores corresponded with coastal development and environmental changes in this region. This is the first study to document the occurrence and temporal distribution of melamine in sediment cores, and this information is useful in understanding the fate of these relatively less-studied chemicals in the environment.

Occurrence and Profiles of Melamine and Cyanuric Acid in Bovine Feed and Urine from China, India, and the United States

Melamine and cyanuric acid have been reported to occur in animal products. Nevertheless, information that pertains to the occurrence of melamine and cyanuric acid in cattle feed and urine is lacking. In this study, the occurrence of melamine and its three derivatives (i.e., cyanuric acid, ammeline, and ammelide) was determined in 183 bovine urine and 29 matched feed samples collected from China, India, and the United States. ∑Melamine (sum of four target compounds) was found in all urine samples at concentrations that ranged from 4.2 to 5280 ng/mL (median: 370 ng/mL); cyanuric acid was the major derivative, accounting for 97% of the total concentrations, followed by melamine (2.2%). The ubiquitous occurrence of ∑Melamine in feed (21-6230 ng/g) suggests that it is the major source of melamine and its derivatives in bovines. Urinary concentrations of melamine and cyanuric acid varied significantly among the three countries, with samples from China as having the highest concentrations, followed by the United States and India. The calculated cumulative daily intakes of melamine and cyanuric acid were at least 10-fold below the current tolerable daily intake recommended for humans. Our study provides evidence-based data on exposure patterns and sources of melamine and cyanuric acid in cattle.

Spectral interval combination optimization (ICO) on rapid quality assessment of Solanaceae plant: a validation study

A novel spectral variable selection method, named as interval combination optimization (ICO), was proposed in the previous study of us. In the present study, ICO coupled with near infrared (NIR) spectroscopy was applied to the rapid determination of four primary constituents including total sugar, reducing sugar, total nitrogen and nicotine in Nicotiana plant. Partial least squares regressions was performed after ICO algorithm. The full spectrum was divided into forty equal-width intervals, and the interval with lower root mean squared error of cross-validation was selected for further analysis. As a result, only 155 variables were retained from 1555 variables for each constituent. Particularly, as a variables selection method, ICO improved the prediction accuracy of calibration model and obtained a satisfactory result compared with full-spectrum data. Results revealed that NIR combined with ICO could be efficiently used for rapid analysis of quality associated constituents of Nicotiana plant. Moreover, this study provided a supplementary verification of the proposed variable selection method for the further applications.

Facile detection of melamine by a FAM-aptamer-G-quadruplex construct

The development of a novel method for melamine detection that uses a FAM-aptamer-G-quadruplex construct due to the efficient quenching ability of an aptamer-linked G-quadruplex is reported herein. The construct, which is labeled with the fluorescent dye 6-carboxyfluorescein (FAM), consists of two parts: a melamine-binding aptamer and a G-rich sequence that can form a G-quadruplex structure. Because of the specific recognition of melamine by the T-rich aptamer, this aptamer folds into a hairpin structure in the presence of melamine, which draws the G-quadruplex closer to the FAM fluorophore, leading to the quenching of the fluorescence of FAM. Thus, a highly sensitive and selective fluorescence strategy for assaying melamine was established. Under optimal conditions, the fluorescence quenching is proportional to the concentration of melamine within the range 10-90 nM, and the method has a detection limit of 6.32 nM. Further application of the method to plastic cup samples suggested that it permitted recoveries of between 97.15% ± 1.02 and 101.92% ± 2.07. The detected amounts of melamine spiked into the plastic cup samples and the corresponding amounts measured by HPLC were in good accordance, indicating that this fluorescent method is reliable and practical. Owing to its high sensitivity, excellent selectivity, and convenient procedure, this strategy represents a promising alternative method of melamine screening. Graphical abstract.

Inter-day and inter-individual variability in urinary concentrations of melamine and cyanuric acid

Melamine is used extensively in household products, such as furniture, dinnerware, and food utensils. Several studies have shown that melamine adversely affects kidney function. Nevertheless, little is known about urinary melamine concentrations, and its temporal variability. In this study, 213 first-morning-void urine samples were collected from 19 volunteers for over a month to assess longitudinal variability in concentrations of melamine and its three structural analogues, i.e., cyanuric acid, ammeline, and ammelide. Target analytes were found in all urine samples at mean concentrations of 3.3, 16, 0.99, and 0.62 ng/mL, for melamine, cyanuric acid, ammelide, and ammeline, respectively. Cyanuric acid was the major compound found in all urine samples, accounting for 74-80% of the total concentrations, followed by melamine (12-20%), ammelide (4-6%), and ammeline (2-4%). Gender- and age-related differences in melamine concentrations were observed, although no such pattern was found for cyanuric acid. After adjusting for creatinine, melamine and cyanuric acid concentrations were moderately predictable with inter-day intraclass correlation coefficients (ICCs) in the range of 0.541-0.763. Nevertheless, substantial inter-individual variation in melamine levels existed even after creatinine adjustment, as evidenced by low ICCs (0.008-0.108). Cumulative daily intake of melamine and cyanuric acid was calculated on the basis of urinary concentrations and the mean values were found to be at least 10-fold below the current tolerable daily intake.

Distribution Profiles of Melamine and Its Derivatives in Indoor Dust from 12 Countries and the Implications for Human Exposure

Melamine and its derivatives (i.e., ammeline, ammelide, and cyanuric acid) are extensively used in household products, such as furniture, dinnerware, and food utensils. Nevertheless, limited information is available on human exposures, other than dietary sources, to melamine and its derivatives. In this study, the occurrence of melamine and its derivatives was determined, for the first time, in 341 samples of indoor dust, collected from 12 countries. All of the dust samples contained target analytes, with global median concentrations of 1800, 1100, 48, and 45 ng/g for melamine, cyanuric acid, ammelide, and ammeline, respectively. The total concentrations of melamine and its derivatives varied among countries in the following decreasing order: United States (median: 17 000 ng/g) > Japan (8400) > China (8000) > South Korea (7300) > Saudi Arabia (4400) > Romania (4300) > Greece (3400) > Kuwait (2400) > Vietnam (2300) > Colombia (1300) > Pakistan (820) > India (430). Correlation analysis indicated the existence of multiple and varied sources for melamine and cyanuric acid. Estimated daily intakes of melamine via indoor dust ingestion were at least 2 orders of magnitude below the current tolerable daily intake.

A versatile signal-enhanced ECL sensing platform based on molecular imprinting technique via PET-RAFT cross-linking polymerization using bifunctional ruthenium complex as both catalyst and sensing probes

Molecularly imprinted technique (MIT) has proven to be a significant tool in the analyzing area in virtue of its obvious advantages such as specific recognition, favorable stability to high temperature and higher sensitivity. Electrochemiluminescence (ECL) technology has also been receiving enormous attention as a powerful tool in sensing fields. However, sensors based on the combination of MIT and ECL technologies have seldom been reported yet. Herein, we find that Ru(bpy)₃²⁺ cannot only work as an efficient catalyst for photo-induced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization, but also as a sensing probe for ECL sensor. Based on this, we successfully construct ECL sensors via the combination of MIT and ECL techniques. In details, poly(methacrylic acid) (PMAA) and cross-linked PMAA were synthesized first via a well-controlled PET-RAFT polymerization using Ru(bpy)₃²⁺ as catalyst under illumination of visible light with a wavelength of 460 nm, as confirmed by ¹H NMR and gel permeation chromatography (GPC). Then, negatively-charged Au nanoparticles (AuNPs) with average sizes of 20 nm were prepared and modified with Ru(bpy)₃²⁺ via electrostatic incorporation. MIPs were prepared on the surface of AuNPs using melamine (MEL) as the template via PET-RAFT controlled cross-linking polymerization. The MIPs modified AuNPs (AuNPs-MIPs) were then fixed on the surface of working electrode with Nafion to achieve a solid-state ECL sensing platform employing Ru(bpy)₃²⁺ as the ECL probes. The as-prepared sensor showed a wide detection range of 5.0 × 10^-13 - 5.0 × 10^-6 mol/L and a low detection limit of 1.0 × 10^-13 mol/L (S/N ≥ 3) was reached in the detection of MEL. Moreover, further tests for analyzing MEL structural analogues proved that the constructed ECL sensing platform could be utilized to detect various substances via specific recognitions.

Direct Detection of Toxic Contaminants in Minimally Processed Food Products Using Dendritic Surface-Enhanced Raman Scattering Substrates

We present a method for the surface-enhanced Raman scattering (SERS)-based detection of toxic contaminants in minimally processed liquid food products, through the use of a dendritic silver nanostructure, produced through electrokinetic assembly of nanoparticles from solution. The dendritic nanostructure is produced on the surface of a microelectrode chip, connected to an AC field with an imposed DC bias. We apply this chip for the detection of thiram, a toxic fruit pesticide, in apple juice, to a limit of detection of 115 ppb, with no sample preprocessing. We also apply the chip for the detection of melamine, a toxic contaminant/food additive, to a limit of detection of 1.5 ppm in milk and 105 ppb in infant formula. All the reported limits of detection are below the recommended safe limits in food products, rendering this technique useful as a screening method to identify liquid food with hazardous amounts of toxic contaminants.