Melamine Concentration Research Articles

Deep Learning-Based Quantitative Assessment of Melamine and Cyanuric Acid in Pet Food Using Fourier Transform Infrared Spectroscopy.

Melamine and its derivative, cyanuric acid, are occasionally added to pet meals because of their nitrogen-rich qualities, leading to the development of several health-related issues. A nondestructive sensing technique that offers effective detection must be developed to address this problem. In conjunction with machine learning and deep learning technique, Fourier transform infrared (FT-IR) spectroscopy was employed in this investigation for the nondestructive quantitative measurement of eight different concentrations of melamine and cyanuric acid added to pet food. The effectiveness of the one-dimensional convolutional neural network (1D CNN) technique was compared with that of partial least squares regression (PLSR), principal component regression (PCR), and a net analyte signal (NAS)-based methodology, called hybrid linear analysis (HLA/GO). The 1D CNN model developed for the FT-IR spectra attained correlation coefficients of 0.995 and 0.994 and root mean square error of prediction values of 0.090% and 0.110% for the prediction datasets on the melamine- and cyanuric acid-contaminated pet food samples, respectively, which were superior to those of the PLSR and PCR models. Therefore, when FT-IR spectroscopy is employed in conjunction with a 1D CNN model, it serves as a potentially rapid and nondestructive method for identifying toxic chemicals added to pet food.

Centimeter level high surface roughness copper / silver nanoheterostructures for highly sensitive SERS detection

A novel method for centimeter level high surface roughness copper nanoparticles (CuNPs)/silver nanowires (AgNWs) heterostructures prepared by vacuum thermal evaporation method and solid-state ionics method under 6 ​μA direct current electric field (DCEF) using fast ionic conductor KAg4I5 was developed. The fractal dimension of the CuNPs/AgNWs heterostructures was calculated by grid-coverage method and the surface-enhanced Raman scattering (SERS) performance was detected by melamine aqueous solution as probe molecules. The results indicate that the macroscopic dendritic CuNPs/AgNWs heterostructures arranged neatly near cathode with the longest length of 6.3 ​cm were prepared, the diameters of CuNPs/AgNWs heterostructures ranged from 40 to 100 ​nm and many regularly arranged CuNPs with the diameter from 10 to 30 ​nm lie in the prepared nanoheterostructures, which lead to the CuNPs/AgNWs heterostructures have high surface roughness. The molar ratio of Ag:Cu is 11.87:1 and the fractal dimension of CuNPs/AgNWs heterostructures was 1.64 which means that there were more dentritic nanoheterostructures. The limiting concentration of melamine for the prepared high surface roughness CuNPs/AgNWs heterostructures SERS substrates is 10−16 ​mol/L, which is far below the strictest safety limits required of melamine in US and China. The EF for R6G/CuNPs/AgNWs heterostructures is estimated to be 4.82 ​× ​1012. These results indicate that the prepared CuNPs/AgNWs heterostructures with facile material synthesis, high surface roughness, low detection concentration and simple detection procedure has promising and strong application prospect for SERS detection in food safety.

“Lab-on-pol” colormatic sensor platforms: Melamine detection with color change on melamine imprinted membranes

There is an urgent need to improve a specific, equipment-free and user friendly technique for detecting melamine in food samples. Here, melamine imprinted polymeric membranes were synthesized, and tested for selectivity in aqueous solutions against cyromazine used as competitor agent, and real milk samples spiked with melamine. Scanning electron microscope, atomic force microscopy, energy-dispersive X-ray and optical profilometer devices were used for characterization. Fe+3 ions were used as marker to obtain color change based on melamine concentration. The color intensity on MIP membranes was monitored by means of a color analysis application (Image J software) via a smartphone. Some obtained results as fallows: Melamine binding to 3D cavities of membrane came true in about 20 min. The linear regression plot showed a correlation coefficient (R2) of 0.995 in the range of 10 μM-50 μM according to Beer Lambert's law. A low concentration of melamine, (e.g., 10 µM) was determined in raw milk. Melamine imprinted polymeric membranes showed 7.575 times more sensitivity for melamine than cyromazine. Limit of detection (LOD) and limit of quantification (LOQ) were computed as 9.9 µM and 30.2 µM, respectively. Here, a new kind of smart polymer was designed, and successfully applied for the easy, portable and on-site colorimetric analysis via imprinted polymers, and named as “lab-on-pol”system. HPLC analysis method was also used to validate this new analysis method.

Layer-by-Layer Assembly of Two-Dimensional Monolayer Films of Gold Nanoparticles for Electrochemical Determination of Melamine

L-Cysteine (L-Cys) and two-dimensional monolayer films of gold nanoparticles (2D MFGS) were employed to modify gold electrodes to build uniform and stable multilayer films by layer-by-layer (LbL) self-assembly, denoted by Au/(L-Cys/2D MFGS)n (n = number of layers). Electrochemical measurements showed that the specific surface area and current amplification capacity of the modified electrode increased with the value of n. In addition, considering the orderliness inside the multilayer 2D MFGS assembly and experimental accessibility, Au/(L-Cys/2D MFGS)n=4 was selected to be the working electrode and dopamine (DA) as the signal quantification molecule for the indirect detection of melamine (MA). The results showed that the oxidation current of DA was inhibited due to the competitive adsorption of melamine upon the working electrode. Under the optimal conditions, the decrease of the dopamine peak anode current is proportional to the melamine concentration from 10−12 to 10−3 M with a detection limit of 0.13 pM. The results demonstrate the potential of 2D MFGS for applications in food and biological analysis. The scheme for fabricating the multilayer electrodes is convenient and environment-friendly, without expensive instrumentation or complex modifications.

Fraud monitoring using a new disposable photoluminescence sensor in milk

A new, sensitive, and room-temperature On/Off photoluminescence sensor was developed for rapid, low-cost and convenient on-site determination of melamine (MEL) in dairy products using dithizone (Dz)-capped Eu3+ doped lithopone (LIT) particles (Dz/Eu3+-LIT) with the help of Pb2+ ions as the fluorescent probe. The interaction of Pb2+ ions with Dz led to form a rigid structure on the surface of Dz/Eu3+-LIT and turned on the luminescence intensity of the particles. The sensing mechanism is based on a simple complex formation strategy that quenches the luminescence signal of Pb2+-Dz/Eu3+-LIT complex by interaction with MEL. Polyvinyl alcohol (PVA) was used as the supporting material for the sensing particles and glutaraldehyde (GA) modifier was used to give the support insolubility property and make it suitable for water-based applications. The potential interferences including amino acids, ions, and other components had no significant effect on the luminescence quenching of the sensor. Under optimum conditions, there was a good linear relationship between the signal intensity of the probe and MEL concentration in the range of 10 to 1000 μg L-1 with a limit of detection (LOD) of 2.95 μg L-1. The relative standard deviation (RSD%) for 5 replicate measurements of MEL solution (50 μg L-1) was obtained 2.23 %. The developed method was applied to the determination of MEL concentrations in raw milk and milk infant powder and recovery values ranging from 98.9 to 102.1 % were obtained.

Comparison of melamine exposure by feeding practices in babies aged 0–6 months

This study was conducted to estimate the daily dietary intakes of melamine for human milk-fed (HMF) babies and mixed-fed (MF) babies. It was carried out in 70 mother-baby pairs (40 babies in the HMF group and 30 babies in the MF group). Human milk, formula milk, and baby urine samples were collected to assess the dietary exposure of babies. Melamine concentrations were analyzed by using a competitive enzyme-linked immunosorbent assay. Melamine was determined in 82.5 % of the human milk samples in the HMF group (median: 0.75 µg/L) while it was present in 96.7 % of human milk samples (median: 1.25 µg/L) and 96.7 % in formula milk samples (median: 0.95 µg/kg) in the MF group. The mean urinary melamine concentration of HMF babies (1.20 ± 0.21 µg/L) was not significantly different than MF babies (1.35 ± 0.49 µg/L). Melamine exposure was calculated as 0.12 µg/kg bw/day and 0.24 µg/kg bw/day in HMF and MF babies, respectively. Melamine exposure in both groups was below the tolerable daily intake. There were no significant associations between melamine exposure and various features of babies and mothers. As a result, it can be suggested that Turkish babies (aged 0–6 months) are not at risk for high melamine exposure through the diet.

Identification of adulterated milk based on auto-correlation spectra

A qualitative analysis of melamine-adulterated milk was proposed based on two-trace two-dimensional (2T2D) auto-correlation spectra. The concentration of melamine was used as external perturbation, and 40 adulterated samples of each brand with different concentrations of melamine (0.01 g/L to 1 g/L) were configured. Four brands of milk were used to configure experimental samples, including Guangming brand, Mengniu brand, Sanyuan brand and Wandashan brand. Spectroscopic data of pure milk and melamine-adulterated milk were measured by infrared (IR) (80–4000 cm−1) spectrophotometer. 2T2D auto-correlation spectral technology combined with least squares support vector machine (LS-SVM) method was used for qualitative analysis. The two strongest auto-correlation peaks in the auto-correlation spectra were selected for modeling. For Guangming brand, the intensities of auto-correlation at two wave numbers 2898 cm−1 and 2972 cm−1 were selected as independent variables. For Mengniu brand, the intensities of auto-correlation at two wave numbers 2852 cm−1 and 2920 cm−1 were selected. For Sanyuan brand, the intensities of auto-correlation at two wave numbers 2900 cm−1 and 2974 cm−1 were selected. For Wandashan brand, the intensities of auto-correlation at two wave numbers 2900 cm−1 and 2974 cm−1 were selected. For four brands fused together, the intensities of auto-correlation at two wave numbers 2900 cm−1 and 2974 cm−1 were selected. For each brand, the accuracy of qualitative analysis was 100 %. For four brands fused together, the accuracy of qualitative analysis was 99.05 %. In this way, it greatly reduced the amount of data to be processed. This study showed that 2T2D auto-correlation spectral technology combined with LS-SVM method was perfect for the discrimination of melamine-adulterated milk.

Melamine Adsorbed Catalyst for Mitigating Phosphoric Acid Poisoning in High Temperature Polymer Electrolyte Membrane Fuel Cell

High-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) is operate at over 100 °C, at which the water in the fuel cell cannot exist in the liquid phase. That is why the PBI-based membrane with phosphoric acid is used in HT-PEMFC and phosphoric acid as an electrolyte to carry hydrogen ions in the form of phosphate ions. However, phosphate ions are attached to the surface of the platinum catalyst and inhibit the catalytic activity [1]. In particular, phosphate ions attached to the cathode prevent oxygen from reacting to platinum and become a major resistance factor [2,3]In this study, melamine was introduced to the Pt/C under 7 different concentrations in a water/ethanol-based solution at ambient temperature and pressure conditions. In the prepared catalyst, the melamine layer formed on the Pt nanoparticles protects phosphate ions while allowing oxygen to pass through.When the catalyst surface was analyzed using XPS, a clear N 1s peak was confirmed in common in samples containing 10 mM or more melamine. In addition, when the surface element ratio was analyzed, samples of 20 mM or more showed a ratio of harmful nitrogen atoms from melamine of 4 wt% or more in common.Elemental analysis using EDS mapping and shape change through transmission electron microscopy (TEM) as a result showed no significant difference depending on the melamine concentration, but showed a nitrogen content of about 7 wt% in common.In a 0.1 M HClO4 electrolyte, the binding of melamine can be confirmed through the shape change of the hydrogen oxidation and oxygen reduction peaks on cyclic voltammetry. Also, the 5 mM melamine sample showed the greatest half-wave potential in the ORR polarization curve at 1600 rpm. When the CV was measured while increasing the phosphoric acid concentration, the commercial platinum under the high phosphoric acid concentration had a very reduced ECSA, but the melamine sample showed an increased ECSA. In addition, the electrochemical performance of commercial Pt/C through the ORR polarization curve decreased significantly as the phosphoric acid concentration increased. On the other hand, it was confirmed that the reduction amount was greatly reduced in the sample in which 10 mM or more of melamine was synthesized. As a result, the performance under 1 M phosphoric acid solution was the best in the 20 mM melamine sample, in which the voltage at 0.2 mA/cm2 is 0.817 V at melamine 10 mM versus 0.715 V at Pt/C.

Application of 2D-COS-FTIR spectroscopic analysis to milk powder adulteration: Detection of melamine

Milk and its products are important and widely traded food commodities, which made them the target for adulteration. Fourier Transform Infrared spectroscopy (FTIR) is one of the most widely used methods to identify chemical contaminations and determine their concentrations. In the present work, two-dimensional correlation spectroscopy (2D-COS) was applied to analyse FTIR spectra of milk powder having different concentration of melamine. With the help of sequential order analysis, the FTIR band at 815 cm −1 was determined as the most qualified FTIR band for carrying quantitative analysis. It is recommended to perform 2D-COS and determine the sequential order of intensity change among all peaks of the contamination. This is to establish the appropriate bands for quantitative analysis, and help to improve the values of analytical parameters. • FTIR spectrum of milk powder was examined for melamine adulteration. • 2D-COS-FTIR and the sequential analysis were used to determine a quantitative band. • The band at 815 cm −1 was the most qualified band for quantitative analysis. • The proposed method is particularly important when dealing with new adulterations. • FTIR combined with 2D-COS are powerful tools in milk powder adulteration.

Melamine Adsorbed Catalyst for Mitigating Phosphoric Acid Poisoning in High Temperature Polymer Electrolyte Membrane Fuel Cell

Phosphoric acid poisoning of catalysts in the cathode is a major performance degradation factor of high-temperature polymer electrolyte membrane fuel cells. Phosphoric acid is attached to the platinum surface and inhibits the ORR reaction. Thus, the surface treatment of Pt can block phosphoric acid. In this study, melamine is attached to the surface of commercial Pt/C at room temperature and pressure to form a melamine layer that can prevent phosphoric acid poisoning. The catalysts of various melamine concentrations were characterized by the optical method. In addition, an electrochemical evaluation was performed at a 1 M phosphoric acid concentration of the electrolyte. Catalysts with 20 mM or more of melamine attached to it form a sufficient melamine layer. These catalysts exhibited reduced performance loss in high concentrations of phosphoric acid electrolytes. In addition, an excessive amount of melamine layer at over 20 mM sample was beneficial to ORR activity in the high concentration of phosphoric acid.

Avatar-like body imaging of dermal exposure to melamine in factory workers analyzed by ambient mass spectrometry.

Ambient mass spectrometry thermal desorption-electrospray ionization/mass spectrometry (TD-ESI/MS) can rapidly identify chemicals without pretreatment of biological samples. This study used a rapid semi-quantitative TD-ESI/MS screening technique for the probe skin sampling of melamine workers occupationally exposed to different ambient melamine concentrations to create avatar-like body images, which were then used to study temporal and dynamic changes in nephrotoxic melamine exposure. We enrolled four voluntary melamine workers from one factory, each from one of four worksites. Melamine exposure was highest in manufacturing and molding, followed by grinding and polishing, packing, and administration, the lowest. Skin samples were collected Friday (end-of-shift) and Monday (pre-shift). Early morning one-spot urine samples were also collected right after skin sampling. 2198 probe skin samples were collected and subjected to semi-quantitative TD-ESI/MS analyses of melamine chemical within 40h. After normalization, converted body image scores revealed exposure to be highest in the manufacturing worker on Friday and lowest in the administrative worker on Monday. The absolute differences (Friday minus Monday) of normalized body image scores were all significantly positive in each individual worker and across all four workers (permutation test, all p-values < 0.002). The slope estimates of the linear regression line between body image scores and urinary melamine levels were 0.81 (p-value=0.008). We concluded that this fast and non-invasive technique can potentially be used to study temporal and dynamic changes in exposure to occupational hazards. A future study of developing an automatic and reproducible TD-ESI/MS sampling platform is needed.

A new colorimetric method for determination of formaldehyde in sea food based on anti-aggregation of gold nanoparticles

A new colorimetric method based on anti-aggregation of gold nanoparticles (AuNPs) in the presence of melamine for sensitive and selective determination of formaldehyde was developed. The citrate capped AuNPs was synthesized by Turkevich method. A certain concentration of melamine is responsible for color change of AuNPs from red to purple. The surface plasmon resonance peak was shifted from 520 to 640 nm. In the presence of formaldehyde, the reaction between melamine and formaldehyde to form methylol melamine was occurred, which resulted in the decreasing of aggregation of AuNPs. Based on the anti-aggregation mechanism, formaldehyde can be detected by observing the color change of AuNPs solution containing melamine, which was monitored by UV–visible spectrophotometer. Calibration curve plotted between absorbance ratio (A640/A520) and formaldehyde concentration was made with linearity of 0 – 2000 µg L−1. The limit of detection and the limit of quantitation were found at 26.0 µg L−1 and 88.0 µg L−1, respectively. The method also provided high accuracy (%recovery in range 97.3 – 104%) and high precision (%RSD < 5.6).

Exposure to melamine and its derivatives in Chinese adults: The cumulative risk assessment and the effect on routine blood parameters

Melamine (MEL) and its derivatives, ammeline (AMN), ammelide (AMD), cyanuric acid (CYA) are widely existed in environmental media. Animal studies have reported the cumulative risk assessment (CRA) of simultaneous exposure to MEL and its derivatives and explored the associations between exposure and routine blood parameters. Such information is largely unknown in human studies. In this study, we detected the urinary concentrations of MEL and its derivatives in 239 Chinese adults to conduct the CRA by evaluating their hazard quotients (HQ) and hazard Index (HI), and also explored the possible associations between exposure and measured routine blood parameters in study population. The detectable frequencies of MEL, AMN, AMD and CYA were 96.65%, 41.00%, 97.91% and 97.07%, respectively. The median values of creatinine (Cr)-adjusted MEL, AMN, AMD, CYA and the total concentrations of MEL and its derivatives (∑MEL) were 11.41 μg/g Cr, not detected (ND), 2.64 μg/g Cr, 15.30 μg/g Cr, 35.02 μg/g Cr, respectively. There were 9 (3.77%) participants with estimated daily intakes (EDIs) of CYA exceeding the tolerable daily intake (TDI) of 2500 ng/kg bw/day, and 12 (5.02%) participants with HI of ∑MEL exposure exceeding 1 based on the strictest TDI value. Urinary concentrations of MEL and its derivatives were positively associated with specific routine blood parameters, including hematocrit, hemoglobin, mean corpuscular volume, mean corpuscular hemoglobin concentration, mean corpuscular hemoglobin, white blood cell, neutrophil count (P < 0.05). Meanwhile, exposure to MEL and its derivatives increased the risk of red blood cell abnormality (P < 0.05). Our study is the first study to provide evidence-based data on the CRA of exposure to MEL and its derivatives in Chinese adults, and to propose a possible association between such exposure and routine blood parameters in human.

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 themagnetic 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 × 106μg/l, the reflection wavelength of MEL-MMIPCs shifted from 497 to 709nm, and was linear with the logarithm of MEL concentration in this range. The detection limit was 0.21μg/l (S/N = 3) and response time was within 30s. The MEL-MMIPC sensor had an imprinting factor of 5.09, and selectivity factors for theanalogs 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.

Associations of Dietary Intake with Urinary Melamine and Derivative Concentrations among Children in the GAPPS Cohort.

Melamine is a nephrotoxic industrial chemical. Diet is one source of melamine exposure, yet little work has examined the main dietary contributors, particularly among children. We evaluated associations of diet with urinary melamine and derivative concentrations among 123 children aged 4–6 years in the Global Alliance to Prevent Prematurity and Stillbirth cohort. Children’s diets on the day preceding urine collection were assessed using 24-h dietary recalls. Associations of meat, fruit, and grain intakes with melamine exposure were examined using multiple linear regression. Remaining food groups were examined in secondary analyses. Mean (SD) melamine, ammelide, and cyanuric acid concentrations were 6.1 (12.4), 1.9 (2.1), and 60.6 (221.2) ng/mL, respectively. The second tertile of red meat consumers had 98% (95% CI: 15%, 241%) greater melamine exposure than non-consumers, yet the highest consumers did not have increased exposure. Greater consumption of certain fruits was associated with lower urinary ammelide. The top yogurt consumers had 112% (95% CI: 29%, 247%) greater melamine exposure than non-consumers. Consumption of starchy vegetables excluding potatoes was associated with 139% (95% CI: 6%, 437%) greater urinary ammelide. These observed associations should be confirmed in future studies using larger samples and increased monitoring of non-dietary routes of exposure.

Single- and Multiple-Adulterants Determinations of Goat Milk Powder by NIR Spectroscopy Combined with Chemometric Algorithms

In this work, we quantified goat milk powder adulteration by adding urea, melamine, and starch individually and simultaneously, with the utilization of near infrared (NIR) spectroscopy coupled with chemometrics. For single-adulterant samples, the successive projections algorithm (SPA) selected three, three, and four optimal wavelengths for urea, melamine, and starch, respectively. Models were built based on partial least squares regression (PLS) and the selected wavelengths, exhibiting good predictive ability with an Rp2 above 0.987 and an RMSEP below 0.403%. For multiple-adulterants samples, PLS2 and multivariate curve resolution alternating least squares (MCR-ALS) were adopted to build the models to quantify the three adulterants simultaneously. The PLS2 results showed adequate precision and results better than those of MCR-ALS. Except for urea, MCR-ALS models presented good predictive results for milk, melamine, and starch concentrations. MCR-ALS allowed detection of adulteration with new and unknown substitutes as well as the development of models without the need for the usage of a large data set.

Elevated emissions of melamine and its derivatives in the indoor environments of typical e-waste recycling facilities and adjacent communities and implications for human exposure

Melamine and its derivatives are used as additives in flame retardants. Moreover, melamine-containing consumables such as laminates, adhesives, glues, and plastics are extensively used in electronic products. Nevertheless, there is no information concerning the identification of melamine and its derivatives during e-waste recycling. In the present study, the occurrence of melamine and three of its derivatives (cyanuric acid, ammelide, and ammeline) was identified for the first time in indoor dust from typical e-waste recycling areas and adjacent rural communities. Urban communities situated about 80 kilometers away were used as a control. The target analytes were detected in almost all the dust samples, which were dominated by melamine and cyanuric acid. The total concentrations of melamine and its derivatives varied among sampling locations in the following order: e-waste workshops (geometric mean: 15,018 ng/g) > urban houses (9060 ng/g) > local rural houses (6204 ng/g) > local streets (201 ng/g). This suggested that e-waste dismantling/recycling activities mainly contributed to the abundant emission of melamine and its derivatives in e-waste dust. Correlation analysis indicated that melamine and its derivatives were similarly applied in electronic products and e-waste recycling resulted in common emissions. The combined toxicity of melamine and its derivatives on human bladder cancer cells was observed. Importantly, Monte Carlo analysis was used to determine that the estimated daily intakes of these chemicals via dust ingestion for occupational e-waste recycling workers were 2.5–9 times higher than the estimated daily intakes for adult residents in both adjacent local communities and urban areas. These results are the first to demonstrate that e-waste dismantling workers are more vulnerable to the exposure risk posed by the presence of melamine and its derivatives in e-waste dust, which deserves more research attention.

High and stable surface-enhanced Raman spectroscopy activity of h-BN nanosheet/Au1Ag3 nanoalloy hybrid membrane for melamine determination

In this work, a hexagonal boron nitride (h-BN)/AuAg nanoalloy hybrids (NAHs) was synthesized to fabricate h-BN/Au1Ag3 membrane as a solid surface-enhanced Raman spectroscopy (SERS) substrate for sensitive SERS detection of melamine. The AuAg nanoalloys were in situ grown on h-BN by chemical reduction method, and the Au/Ag molar ratio was tuned to achieve optimal SERS performance. After the SERS performance of h-BN/AuAg NAHs with different Au/Ag ratios was analyzed, h-BN/Au1Ag3 NAHs were chosen for SERS analysis. The h-BN/Au1Ag3 membrane can be obtained through simple filtration of h-BN/Au1Ag3 NAHs on chromatographic paper. As expected, the solid SERS substrates of h-BN/Au1Ag3 membrane were uniform and demonstrated good selectivity, repeatability and reproducibility for SERS detection of melamine. The results demonstrate that h-BN/Au1Ag3 membrane exhibited high SERS activity for 4-mercaptobenzoic acid (4-MBA) with limit of detection (LOD) at 1.0 ng L−1, and its analytical enhancement factor (AEF) reached 3.6 × 108. The possible enhancement mechanism, including electromagnetic mechanisms (EM) and chemical mechanisms (CM) were illustrated by finite-difference time-domain (FDTD) and density functional theory (DFT) simulations in detail, respectively. The concentration of melamine in the 0.05–5.0 mg L−1 range showed good linear relationship (R2 = 0.9940) with SERS intensity with LOD of 0.01 mg L−1. Finally, the recoveries of melamine in liquid milk samples are 87.7–105.7% with relative standard deviations (RSDs) in range of 0.6–2.6%, providing precise safety evaluation of melamine in milk samples.

Chemical Detection by Analyte-Induced Change in Electrophoretic Deposition of Gold Nanoparticles

The electrophoretic deposition (EPD) of citrate-stabilized Au nanoparticles (cit-Au NPs) occurs on indium tin oxide (ITO)-coated glass electrodes upon electrochemical oxidation of hydroquinone (HQ) due to the release of hydronium ions. Anodic stripping voltammetry (ASV) for Au oxidation allows the determination of the amount of Au NP deposition under a specific EPD potential and time. The binding of Cr3+ to the cit-Au NPs inhibits the EPD by inducing aggregation and/or reducing the negative charge, which could lower the effective NP concentration of the cit-Au NPs and/or lower the electrophoretic mobility. This lowers the Au oxidation charge in the ASV, which acts as an indirect signal for Cr3+. The binding of melamine to cit-Au NPs similarly leads to aggregation and/or lowers the negative charge, also resulting in reduction of the ASV Au oxidation peak. The decrease in Au oxidation charge measured by ASV increases linearly with increasing Cr3+ and melamine concentration. The limit of detection (LOD) for Cr3+ is 21.1 ppb and 16.0 ppb for 15.1 and 4.1 nm diameter cit-Au NPs, respectively. Improving the sensing conditions allows for as low as 1 ppb detection of Cr3+. The LOD for melamine is 45.7 ppb for 4.1 nm Au NPs.

Determination of four melamine and its derivatives in urine by ultra-performance liquid chromatography-tandem mass spectrometry

To establish a method for simultaneous detection of melamine(MEL), ammeline(AMN), ammelide(AMD) and cyanuric acid(CYA) in urine by ultra-performance liquid chromatography-tandem mass spectrometry(UPLC-MS/MS). After the urine sample was added to the internal standard, they were mixed by acetonitrile, ultrasonic extraction and centrifugation, enriched by MCX or MAX solid phase extraction column, eluted with 5% ammoniated methanol or 2% formic acid methanol, dried under nitrogen at 40 ℃, separated by Amide chromatography column(2.1 mm×100 mm, 1.7 μm), qualitatively and quantitatively analyzed by mass spectrometer in the simultaneous scanning mode of positive and negative ions. Melamine and its derivatives were completely separated with great chromatography peak. The limit of detection(LOD) for melamine, ammeline, ammelide and cyanuric acid were 0.03, 0.04, 0.04 and 0.05 ng/mL, and the limit of quantification(LOQ) were 0.11, 0.12, 0.14 and 0.15 ng/mL, respectively. The recovery rate of standard addition samples of 0.2, 5, 50 and 200 ng/mL ranged from 94.6% to 106.5%, the intra-day precision was between 0.73% and 8.34%, and the inter-day precision was less than 6.80%(n=6). At low concentrations, AMD and CYA mainly showed matrix enhancing effect, while AMN showed matrix inhibitory effect with increasing concentration. The overall detection rate of melamine and its derivatives was 75.0% in 60 parcels of random urine. The average concentration of urinary melamine and its derivatives were higher in men. UPLC-MS/MS can simultaneously determine four melamine and its derivatives in urine. The LOD and LOQ of the current method were low with great accuracy and precision.