Melamine In Raw Milk Research Articles

Highly sensitive facile plasmonic scheme for assessment of melamine in raw milk

This work presents two novel devices with microcontroller and two different light sensors, namely, Light Dependent Resistor (LDR) and Ambient Light Sensor (ALS) which can provide a quantitative output from...

One step construction of an electrochemical sensor for melamine detection in milk towards an integrated portable system

Excessive intake of melamine (MEL) can be harmful to human health, and it is important to establish a rapid and accurate MEL detection method. As the electrochemical activity of MEL is very low, ferrocenylglutathione (Fc-ECG) was used as an electron transfer mediator to assist with the detection of MEL using screen-printed carbon electrode (SPCE). This modified electrode (MEL/Fc-ECG/SPCE) was prepared by stepwise drop-casting and was fully characterized. Results showed that MEL significantly enhanced signal of Fc-ECG/SPCE sensor due to the three p-π conjugated double bonds that facilitated electron transfer. Under optimal conditions, the sensor exhibits two linearities in the range of 0.20–2.00 μM and 8.00–800 μM, with a sensitivity of 15.03 μA·μM−1·cm−2. The selectivity, stability, and reproducibility were investigated and successfully used to detect MEL in raw milk and confirms safety verification of foods. Moreover, a portable testing platform was designed for MEL detection based on a CH32 chip.

Colorimetric recognition of melamine in milk using novel pincer zinc complex stabilized gold nanoparticles

A convenient colorimetric approach for visual detection of melamine in raw milk was realized by using gold nanoparticles (AuNPs) stabilized by an unsymmetrical terpyridyl zinc complex with a thymine fragment at one terminal and a quaternary ammonium salt at the other. Even without pre-addition of melamine or relative additives, obvious color change from red to blue was observed by naked eye in the presence of trace amount of melamine, which was attributed to the alternation of aggregation state of AuNPs caused by the selective binding between the thymine fragment and melamine via triple hydrogen-bonding interactions. Remarkably, the detection limit for melamine was as low as 2.4 ppb, providing a highly sensitive and efficient approach for the visual detection of melamine.

Detecting melamine‐adulterated raw milk by using near‐infrared transmission spectroscopy

AbstractChinese national standards regulate that the melamine in infant food should not exceed 1 mg/kg. To quickly and accurately identify whether the melamine in raw milk exceeds the limit, near‐infrared transmission (NIRT) spectra of 220 raw milk samples and 380 melamine‐adulterated raw milk samples were collected over the spectral range of 900–1,700 nm. Different spectral preprocessing and dimension reduction methods were used to preprocess the data of full spectra with 425 wavelengths. The qualitative prediction models of partial least squares discriminate analysis (PLS‐DA) and support vector machine (SVM) were built based on full spectra and reduced data. The results showed that the SVM was better than PLS‐DA in identifying the melamine‐adulterated raw milk whose melamine contents exceeded the limit of 1 mg/kg. The established SVM model using full spectra had the optimal prediction performance with the accuracy rate of 92.67%, sensitivity of 89.09%, and specificity of 94.74%. The study indicates that the raw milk whose melamine contents exceed the limit of 1 mg/kg could be identified by using NIRT spectroscopy.Practical ApplicationsThis study demonstrated that the raw milk whose melamine contents exceed the limit of 1 mg/kg could be identified by using near‐infrared transmission spectroscopy. The results provide technical support and effective information for developing portable instrument to detect melamine‐adulterated raw milk in situ or in field.

Highly sensitive detection of melamine in milk samples based on N-methylmesoporphyrin IX/G-quadruplex structure

Melamine has a high nitrogen content thus is often illegally added to milk powder to phonily boost the protein percentage composition. Milk products containing melamine are harmful to babies and children. In the present work, a simple, label-free and turn-on fluorescence method to detect melamine with a high sensitivity and selectivity has been developed. The N-methylmesoporphyrin IX (NMM)/G-quadruplex structure is used as a signal reporter in this method to realize label-free detection of melamine. The detection limit of this method is 12.5 nM, which is much less than the maximum level permitted for infant formulas. The linear range is 0.05–1 μM (R2 = 0.98). The selectivity of this method is also very satisfactory. Furthermore, this method can be successfully applied for detection melamine in raw milk and milk powder.

Sensitive colorimetric detection of melamine in processed raw milk using asymmetrically PEGylated gold nanoparticles

In this study, the asymmetrically modified gold nanoparticles (AuNPs) with relatively large size are proposed for highly sensitive detection of melamine in raw milk. At first, we found that the sensitivity of colorimetric detection of melamine based on the conventional citrate-stabilized AuNPs could be significantly increased (limit of detection by visual observation: from 1 μmol L−1 to 1 nmol L−1) via enlarging the size of AuNPs (from 13 to 42 nm). However, the large-sized AuNPs are unstable in the solution, and the accuracy of relevant aggregation-based colorimetric detection method is dramatically decreased due to the rapid evanishment of the color of solution caused by the precipitation of large-scaled AuNPs aggregates after introducing melamine. In view of this limitation, the stabilizing agent polyethylene glycol (PEG) is introduced onto the selected area at the surface of AuNPs to stabilize the large-sized AuNPs sensor and further adjust the aggregation behavior in an oriented and controllable manner. As a result, the addition of melamine into asymmetrically PEGylated AuNPs with large size could still induce a color change from purple to blue, but the color change could be maintained in aqueous solution for a long time. This improvement is mainly caused by forming stable and oriented oligomers of AuNPs taking advantage of the asymmetrical modification of AuNPs with PEG chain in this system. Such a way of aggregation leads to remarkably enhanced long-term stability and an at least 2 orders of magnitude wider dynamic detection range (1.05 nmol L−1 to 1 mmol L−1) compared with conventional AuNPs sensors. Finally, such a novel asymmetrically modified AuNPs sensor is successfully applied to detect melamine in the raw milk with the satisfactory spiked recovery from 99.6% to 103.7% and RSD of about 3.0%. This work provides a sensitive and reliable method to detect melamine in pretreated milk, indicating a new avenue to make innovation on colorimetric sensors for real sample detection.

An Aptamer-Based Biosensor for Direct, Label-Free Detection of Melamine in Raw Milk.

Melamine, a nitrogen-rich compound, has been used as a food and milk additive to falsely increase the protein content. However, melamine is toxic, and high melamine levels in food or in milk can cause kidney and urinary problems, or even death. Hence, the detection of melamine in food and milk is desirable, for which numerous detection methods have been developed. Several methods have successfully detected melamine in raw milk; however, they require a sample preparation before the analyses. This study aimed to develop an aptamer-DNAzyme conjugated biosensor for label-free detection of melamine, in raw milk, without any sample preparation. An aptamer-DNAzyme conjugated biosensor was developed via screening using microarray analysis to identify the candidate aptamers followed by an optimization, to reduce the background noise and improve the aptamer properties, thereby, enhancing the signal-to-noise (S/N) ratio of the screened biosensor. The developed biosensor was evaluated via colorimetric detection and tested with raw milk without any sample preparation, using N-methylmesoporphyrin IX for fluorescence detection. The biosensor displayed significantly higher signal intensity at 2 mM melamine (S/N ratio, 20.2), which was sufficient to detect melamine at high concentrations, in raw milk.

Visible and fluorescent detection of melamine in raw milk with one-step synthesized silver nanoparticles using carbon dots as the reductant and stabilizer

Using carbon dots as the reductant and stabilizer, a visible and fluorescent method was developed for melamine detection in raw milk with one-step synthesized silver nanoparticles. In this work, carbon dots (C-dots) were applied to reduce silver ions and stabilize the nanoparticles, resulting in the formation of silver nanoparticles (AgNPs). As a result, the inherent fluorescence emission of C-dots was significantly reduced after the formation of AgNPs. However, in the presence of melamine, silver ions could interact with the nitrogen atoms in amine and triazine groups of melamine. With 0–2μM melamine, aggregated AgNPs were found after the reduction by C-dots, resulting in color and absorbance changes. With further increase of melamine (2–20μM), both formation and aggregation of AgNPs were inhibited, and the fluorescence was gradually increased. This optical platform was optimized for melamine detection and then was applied to detecting melamine in raw milk samples. The results for melamine assay based on visible and fluorescent method showed the requisite sensitivity with a low detection limit of 30nM, as well as high selectivity.

Highly Sensitive Aptamer-Based Colorimetric Detection of Melamine in Raw Milk with Cysteamine-Stabilized Gold Nanoparticles.

In this paper, a rapid and facile visual method for detecting melamine in raw milk was developed by using T-rich single-stranded DNA (ssDNA) aptamers as the recognition element and cysteamine-stabilized gold nanoparticles (CS-AuNPs) as the color indicator. The electrostatic interaction between the negatively-charged melamine-binding aptamers and the positively-charged CS-AuNPs induced the aggregation of the CS-AuNPs and a remarkable change in color from wine red to blue. The specific recognition between melamine and T-rich aptamers could cause the formation of melamine-aptamer complex through hydrogen bonding. Compared with the electrostatic interaction between CS-AuNPs and aptamers, the strongly specific recognition between melamine and aptamers played a dominant role, thus the presence of melamine could prevent the aptamers-induced aggregation and color change of CS-AuNPs. As a result of this, using CS-AuNPs as the colorimetric probe, a simple and sensitive aptasensor for melamine determination is developed. Under the optimum conditions, the absorbance change (ΔA 526) of CS-AuNPs was linearly proportional to melamine concentration in the range of 1–24 nM, with the detection limit of 0.389 nM. The application of the proposed method for the determination of melamine in raw milk samples spiked with different amounts of melamine suggested satisfactory recoveries between 92.8% and 112.2%, which are in full accordance with the results from HPLC. Due to the advantages of low cost, high sensitivity, simple operation procedure, reduced detection time, and practical application, this method would be expected for on-site and real-time screening of melamine in raw milk.

Fluorescence Based Study for Melamine Detection Using Gold Colloidal Solutions.

A comparative study on detection of melamine with different sized citrate capped AuNPs namely 15nm (AuNPs-I), 30nm (AuNPs-II), and 40nm (AuNPs-III) was carried out by fluorescence spectroscopy. The AuNPs emitted strong fluorescence at 421nm with different intensity at 116.122, 220.511 and 253.665 for AuNPs-I, AuNPs-II and AuNPs-III respectively on excitation with 308nm. On interaction with melamine, the AuNPs aggregated resulting in the enhancement of the fluorescent intensity of AuNPs. The sensitivity of melamine detection was studied for three different sizes of AuNPs by drawing a calibration curve between the concentration of melamine and fluorescence intensity. A good sensitivity was observed for AuNPs-II having the detection limit as low as 0.66nM (3σ) which was lower in comparison to the detection limit of AuNPs-I (2.78nM) and AuNPs-III (7.74nM). The cost of synthesis was low as the usage of HAuCl4 in the synthesis of AuNPs-II was lower compared to the other sizes of AuNPs resulting in low cost of chemicals. The AuNPs-II was further chosen for carrying out selectivity study and for detecting the concentration of melamine. The recovery percentage of melamine in raw milk, liquid milk and milk powder after pre-treatment was found to be 100%, 97% to 100% and 94% to 99% respectively.

One-pot synthesis of mesoporous structured ratiometric fluorescence molecularly imprinted sensor for highly sensitive detection of melamine from milk samples.

A facile strategy was developed to prepare mesoporous structured ratiometric fluorescence molecularly imprinted sensor for highly sensitive and selective determination of melamine using CdTe QDs as target sensitive dye and hematoporphyrin as reference dyes. One-pot synthesis method was employed because it could simplify the imprinting process and shorten the experimental period. The as-prepared fluorescence MIPs sensor, which combined ratiometric fluorescence technique with mesoporous silica materials into one system, exhibited excellent selectivity and sensitivity. Under optimum conditions, these mesoporous structured ratiometric fluorescence MIP@QDs sensors showed detection limit as low as 38 nM, which was much lower than those non-mesoporous one. The recycling process was sustainable at least 10 times without obvious efficiency decrease. The feasibility of the developed method in real samples was successfully evaluated through the analysis of melamine in raw milk and milk powder samples with satisfactory recoveries of 92-101%. The developed method proposed in this work proved to be a convenient, rapid, reliable and practical way to prepared high sensitive and selective fluorescence sensors with potentially applicable for trace pollutants analysis in complicated samples.

An upconversion fluorescence resonance energy transfer nanosensor for one step detection of melamine in raw milk

Here we report a nanosensor based on fluorescence resonance energy transfer (FRET) between upconversion nanoparticles (UCNPs) and gold nanoparticles (AuNPs) for melamine detection. The positively charged UCNPs as donor and the negatively charged AuNPs as acceptor bound together through electrostatic interaction, which caused the fluorescence quenching of UCNPs. Upon addition of melamine, AuNPs were released from the surface of UCNPs and aggregation due to the N-Au interaction between melamine and AuNPs, which results in the fluorescence of UCNPs gradually recovered. Under the optimal conditions including media pH (7.0), the concentration of AuNPs (1.23nM) and incubation time (12min), the fluorescence enhanced efficiency shows a linear response to the melamine concentration ranging from 32.0 to 500nM with a detection limit of 18.0nM. Compared with other fluorescence methods, the fluorimetric nanosensor shows high sensitivity of 0.968, ease of operation and can be used for the determination of melamine in raw milk samples.

Label-free fluorescence turn-on sensing for melamine based on fluorescence resonance energy transfer between CdTe/CdS quantum dots and gold nanoparticles

A label-free fluorescence turn-on sensing system based on fluorescence resonance energy transfer (FRET) between CdTe/CdS quantum dots (QDs) and gold nanoparticles (AuNPs) is developed for the detection of melamine. This FRET process originated from a fluorescent donor–acceptor pair, in which CdTe/CdS quantum dots act as donors and AuNPs as acceptors, owing to the overlap of the fluorescence spectrum of the quantum dots and absorption spectrum of the AuNPs, leading to a decrease in the fluorescence signal. However, the presence of melamine resulted in the aggregation of AuNPs via multiple combinations between melamine and AuNPs, which reduced the spectral overlap and hampered the FRET process by restoring the fluorescence signal. The fluorescence enhancement exhibits a linear relationship to the melamine concentration from 5.0 × 10−8 M to 1.0 × 10−6 M with a detection limit of 3.0 × 10−8 M. Good detection of melamine in raw milk and milk products was also achieved. This sensing system is facile, rapid, easy to implement and may provide a new avenue for the determination of melamine in real samples.

Optical Detection of Melamine in Raw Milk

Optical Detection of Melamine in Raw Milk

Visual detection of melamine based on the peroxidase-like activity enhancement of bare gold nanoparticles

In this study, a facile method to sensitively detect melamine and highly improve the peroxidase-like activity of bare gold nanoparticles (Au NPs) at the same time is proposed for the first time. It is interesting to find that the addition of melamine could improve the peroxidase-like activity of Au NPs. By coupling with 3,3′,5,5′-tetramethlybenzidine (TMB)–H2O2 chormogenic reaction, a novel method for colorimetic detection of melamine is developed. The detection limit of this method is as low as 0.2nM with the help of UV–vis spectroscopy and 0.5µM by naked-eye observation, both which are far below the US food and Drug Administration estimated melamine safety limit of 20µM. In addition, the present method is successfully applied for the detection of melamine in raw milk and milk powder. More importantly, the proposed method could also improve the peroxidase-like activity of Au NPs, which may not only provide a new approach to develop effective nanomaterials-based mimetic enzyme, but also irradiative to develop new applications for Au NPs in varieties of cost-effective and simple sensors in medicine, biotechnology and environmental chemistry.

Detection of melamine on fractals of unmodified gold nanoparticles by surface-enhanced Raman scattering

A simple way of detecting melamine in raw milk is demonstrated via surface-enhanced Raman spectroscopy (SERS) using fractals of bare and nonfunctionalized ~30 nm gold nanoparticles (AuNP) distributed on a solid support. The technique demonstrates the formation of AuNP fractals, from a random distribution, upon exposure to melamine, that enhance the Raman scattering cross-section to enable detection by SERS. The agglomeration, which is pronounced at higher melamine concentrations, is demonstrated directly through imaging, and the red-shift of the plasmon absorption peak of the AuNP fractal away from 530 nm by finite difference time domain (FDTD) calculations. The agglomeration results in a strong plasmon field, shown by FDTD, over the interparticle sites that enhances the Raman scattering cross-section of melamine and ensures unambiguous detection. Limit of detection of 100 ppb could be achieved reproducibly.

Efficient inner filter effect of gold nanoparticles on the fluorescence of CdS quantum dots for sensitive detection of melamine in raw milk

A simple, rapid and ultra-sensitive analytical method for the determination of melamine in raw milk based on inner filter effect (IFE) of Au nanoparticles (AuNPs) on the fluorescence of CdS quantum dots (QDs) is described in this study. With the presence of citrate-stabilized AuNPs, the fluorescence of l-cysteine (l-Cys)-capped CdS QDs was remarkably quenched by AuNPs via IFE. The fluorescence of the AuNPs-CdS QDs system was recovered upon addition of melamine, since melamine could induce the aggregation of AuNPs accompanying the reduction of their characteristic plasmon absorption at ∼522 nm, which resulted in “turn-on” of the IFE-decreased fluorescence of CdS QDs. Under the optimum conditions, a good linear correlation for detection of melamine in raw milk was exhibited from 0.05 mg·L−1 to 0.35 mg·L−1, and the detection limit was 0.017 mg·L−1, which was much lower than the detection limit of HPLC coupled with UV detector and the safety limit required by USA, UK and China. This method was successfully carried out for the assessment of melamine in raw milk samples, which revealed many advantages such as high sensitivity, low cost and non-time-consuming compared with traditional methods.

A Rapid Colorimetric Detection of Melamine in Raw Milk by Unmodified Gold Nanoparticles

We report a rapid, visual, facile colorimetric detection of melamine in raw milk by unmodified gold nanoparticles (AuNPs). AuNPs can be induced by melamine from dispersion to aggregation rapidly, along with the color change from red to blue (or purple). After the optimization of reaction conditions, the whole analysis only needs about 20 min including the simple pretreatment we designed. The analysis can be observed by the naked eye and simple to operate without sophisticated instruments. In the raw milk, the present limit of detection for melamine is down to ∼70 ppb. Furthermore, we explore the interaction principle between melamine and citrate-stabilized AuNPs in this work and establish the foundation for modifying the AuNPs and optimizing the system of AuNPs–melamine in the future.

CuInS2 quantum dots-based fluorescence turn off/on probe for detection of melamine

In this paper, a sensitive and simple method for the determination of melamine (MA) was developed based on the fluorescence changes of the water-soluble CuInS2 quantum dots (QDs). The water-soluble CuInS2 QDs capped by mercaptopropionic acid (MPA) was directly synthesized by hydrothermal method based on our previous report. The fluorescence emission of CuInS2 QDs was quenched by the oxidation of the surface of the QDs with H2O2, and the quenched fluorescence of CuInS2 QDs could be recovered upon the addition of small amounts of MA, which might be due to the surface passivation of the CuInS2 QDs by MA. The other amino acids such as glycine and lysine had no effect on the quenched fluorescence of CuInS2 QDs. Under optimum conditions, there was a good linear relationship between the fluorescence intensity of CuInS2 QDs and the concentration range of MA from 1.0×10−8 to 1.0×10−5mol/L with a detection limit as low as 5nM. The proposed method was successfully applied to detect trace MA in raw milk with satisfactory results. Compared with previous reports, the proposed method manifested several advantages such as high sensitivity, short analysis time, low cost and ease of operation.

Sensitive fluorescent detection of melamine in raw milk based on the inner filter effect of Au nanoparticles on the fluorescence of CdTe quantum dots

A simple, rapid and sensitive fluorescent assay for determination of melamine has been developed based on inner filter effect (IFE) of gold nanoparticles (AuNPs) on the fluorescence of CdTe quantum dots (QDs). When thioglycolic acid-capped CdTe QDs was mixed with citrate-stabilized AuNPs, the fluorescence of CdTe QDs was significantly quenched via the IFE of AuNPs. With the presence of melamine, melamine could induce the aggregation and corresponding absorbance change of AuNPs, which then resulted in the recovery of IFE-decreased emission of CdTe QDs. Under the optimum conditions, the detection limit for melamine in raw milk was 0.02mgL−1. The application of this method in samples of melamine-spiked raw milk suggested a recovery between 103% and 104%. Therefore, the obvious merits provided by the present assay, such as simplicity, rapidity, low cost, and high sensitivity, would make it promising for on-site screening of melamine adulterant in raw milk.