Recognition Of Melamine Research Articles

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.

A unique supramolecular assembly between sulfated cyclodextrin, silver and melamine: Towards a fluorescence based dual wavelength detection approach for melamine

Nitrogen-rich melamine has been often used by fraudulent merchants as an adulterant to food and dairy products which exhibit toxic effects, leading to life-threatening diseases. This has triggered enormous activity on developing methods for the recognition of melamine in various food items for protecting public health and food safety. In recent times, fluorescence methods have emerged as powerful analytical tools for sensor development. The previously reported fluorescence-based methods for melamine detection, either require tedious synthesis procedures or operate through single wavelength-based signal output, making their accuracy susceptible to minute changes in the experimental environment. Herein, to overcome these limitations, we report the recognition of melamine which is achieved through the formation of a serendipitously discovered unique supramolecular assembly. The assembly consists of sulfated cyclodextrin, silver ion and melamine which, in turn, encapsulates a molecular rotor dye, Thioflavin-T, to yield a fluorescence turn-on response towards melamine. Importantly, the supramolecular assembly is responsive at dual wavelengths and is builtup with all commercially available economical molecules. Overall, the present system offers a simple, sensitive, label-free and highly selective detection for melamine. Importantly, the melamine containing supramolecular ensemble is also found to perform in commercial milk sample promising its future application prospect.

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.

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.

Study of the interaction mechanism between hydrophilic thiol capped gold nanoparticles and melamine in aqueous medium

In the last years, intense efforts have been made in order to obtain colloidal-based systems capable of pointing out the presence of melamine in food samples. In this work, we reported about the recognition of melamine in aqueous solution, using gold nanoparticles stabilized with 3-mercapto-1-propanesulfonate (AuNPs-3MPS), with the aim of deepening how the recognition process works. AuNPs were synthesized using a wet chemical reduction method. The synthesized AuNPs-3MPS probe was fully characterized, before and after the recognition process, by both physicochemical (UV–vis, FT-IR, 1H-NMR, DLS and ζ-potential) and morphostructural techniques (AFM, HR-TEM). The chemical and electronic structure was also investigated by SR-XPS. The sensing method is based on the melamine-induced aggregation of AuNPs; the presence of melamine was successfully detected in the range of 2.5−500 ppm. The results achieved also demonstrate that negatively charged AuNPs-3MPS are potentially useful for determining melamine contents in aqueous solution. SR-XPS measurements allowed to understand interaction mechanism between the probe and the analyte. The presence of sulfonate groups allows a mutual interaction mediated by electrostatic bonds between nanoparticles surface thiols and positively charged amino groups of melamine molecules.

Melamine Recognition: Molecularly Imprinted Polymer for Selective and Sensitive Determination of Melamine in Food Samples.

In this study, a sensitive and selective sensor is constructed to measure the melamine (MEL) using molecular imprinting polymer (MIP) technique. Chemical and electrochemical techniques are used to construct the MIP and quantitative measurements. The constructed sensor was modified with GO-Fe3O4@SiO2 nanocomposite. Screening and optimization of factors are done using statistical methods, including Plackett–Burman design (PBD) and central composite design (CCD). Under the optimized conditions, an MIP sensor showed a linear range from 5.0 × 10−7 to 1.0 × 10−5 M MEL concentration with a correlation coefficient (R2) of 0.9997. The limit of detection was obtained (0.028 µM) with a highly reproducible response (RSD 2.15%, n = 4). The electrochemical sensor showed good results for the determination of MEL in food samples.

Visual recognition of melamine in milk via selective metallo-hydrogel formation

A series of novel six-coordinated terpyridine zinc complexes, containing ammonium salts and thymine fragment at the two terminals, have been designed and synthesized, which can function as highly sensitive visualized sensors for melamine detection via selective metallo-hydrogel formation. After fully characterization by various techniques, the complementary triple-hydrogen-bonding between the thymine fragment and melamine, as well as π-π stacking interactions may be responsible for the selective metallo-hydrogel formation. In light of the possible interference aroused by milk ingredients (proteins, peptides and amino acids) and legal/illegal additives (urine, sugars and vitamins), a series of control experiments are therefore involved. To our delight, this visual recognition is highly selective, no gelation was observed with the selected milk ingredients or additives. Remarkably, this new developed protocol enables convenient and highly selective visual recognition of melamine at a concentration as low as 10 ppm in raw milk without any tedious pretreatment.

Molecular recognition of melamine and cyanuric acid by C2-symmetric phenylalanine based supramolecular hydrogels

Molecular recognition of achiral structural analogues, a remaining challenge for materials scientists and chemists, is the key to manipulating the applications and properties of supramolecular chiral materials. Herein, we report the molecular recognition of two harmful structural analogues i.e. melamine (MA) and cyanuric acid (CA) by using C2-symmetric phenylalanine chiral hydrogelators. Two distinct molecular recognition events between hydrogelator fibrils and triazine based achiral molecules are demonstrated: phenylalanine-co-melamine (PF-co-MA) gels showed chirality amplification while chirality inversion was observed for phenylalanine-co-cyanuric acid (PF-co-CA) gels. Moreover, viscoelastic properties and twist pitch were also regulated under the influence of the achiral molecules. Such phenomenon is mainly mediated by H-bonding among the two components, which may subtly change the arrangement of the molecules, in turn, the self-assembled nanostructures. This method of using C2-symmetric phenylalanine based gelators in the recognition of the achiral molecules may be broadly applicable for a wide variety of molecular recognition partners such as sugars.

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 10-90nM, and the method has a detection limit of 6.32nM. 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.

Column packing elimination in matrix solid phase dispersion by using water compatible magnetic molecularly imprinted polymer for recognition of melamine from milk samples

This paper reports a new variant of sample clean-up procedure based on miniaturized matrix solid phase dispersion (MSPD) combining with water compatible magnetic molecularly imprinted polymers (MMIP) for preconcentration of melamine from milk samples prior high-performance liquid chromatography with UV detection. By employing MMIP as dispersant sorbent matrix interferences could be eliminated completely and the column packing step was interestingly omitted that decreasing in glassware expenditure, labor effort and extraction time which cause improvement of the method performance. In order to investigate the functionality, morphology and magnetic properties of the prepared MMIP, it was fully characterized by FT-IR, VSM, SEM, and TEM. Multivariate optimization was applied as a versatile statistical approach for evaluating the influence of the main variables and their interactions on extraction efficiency. Under optimized conditions, the proposed method exhibited a wide linear response in the concentration range of 250.0–5000.0 μg L−1, satisfactory recoveries for all samples (86.6% ≤), and supreme repeatability (RSD less than 6.3%) were achieved as well. The low limit of detection close to 67.0 μg L−1 implied high ability of proposed method for isolation of melamine from complicate milk sample. These befitting results demonstrated that the developed method is unique in terms of the facility, rapidly and least solvent consumption as well as accurate analysis of melamine at trace levels.

Recognition of Melamine by Chromium Tricarbonyl (Thio)barbituric Acid Derivatives: Theoretical Insight into Multiple Hydrogen‐Bond Modes

AbstractMelamine‐contaminated milk products continually emerge in recent years and cause disease. Therefore, how to swiftly and efficiently differentiate melamine from protein becomes an urgent issue. In this work, three neutral receptors p‐dimethylaminobenzaldehyde‐barbituric acid chromium tricarbonyl (PBC), p‐dimethylaminobenzaldehyde‐2‐thiobarbituric acid chromium tricarbonyl (2‐PTC), and p‐dimethylaminobenzaldehyde‐4‐thiobarbituric acid chromium tricarbonyl (4‐PTC) were constructed for recognition of melamine, and their non‐covalent interactions were systematically studied using density functional theory and several theoretical analysis methods. Our results suggest that melamine can be theoretically recognized by receptors PBC and 2‐/4‐PTC due to the formation of multiple hydrogen bonds upon complexation, indicating the good detecting capability of the Cr(CO)3‐based (thio)barbituric acid moiety. Nevertheless, the IR analysis results show the receptors 2‐/4‐PTC cannot distinctly detect melamine by evident frequency shift of metal carbonyl. Therefore, we further proposed a structural modification strategy by introducing the cyano group into the (thio)barbituric acid moiety, and the corresponding IR analysis exhibits large frequency shift of metal carbonyl, indicating the potential of modified receptors in the infrared neutral molecular recognition of melamine. The results reported herein will be very useful in the design of potent receptors distinguishing melamine from protein.

Succinic acid functionalized silver nanoparticles (Suc-Ag NPs) for colorimetric sensing of melamine

In this study, a quantitative colorimetric sensing strategy is developed for the rapid, sensitive and selective determination of melamine. The sensing system relies on the application of succinic acid as a selective recognition probe functionalized over Ag NPs. The synthesized Ag NPs were modified with cysteamine to induce positively charged atmosphere which allowed easy and favorable functionalization of succinic acid. The di-carboxyl nature of succinic acid enabled its binding to both cysteamine and melamine. The strong and favorable linkage between succinic acids carbonyl and amine moieties of melamine triggered aggregation of silver NPs producing a significant shift in the measured absorption excitation. This change in the excitation along with the colorimetric response was found linearly proportional to the melamine concentration in the range of 0.1–1.2 μM. The developed sensor system is simple and unlike electrostatic attraction based sensor system utilize selective linkage for the recognition of melamine. In addition to this, the developed optical probe can efficiently be used for the determination of melamine in milk samples.

Discrete Molecular Recognition Induced Higher-Order Structures: Fibrous Formation Triggered by Melamine Recognition with a Cationic Ethynylpyridine Macrocyclic Host.

A tricationic shape-persistent macrocycle was obtained by methylation on the nitrogen atoms of the three 3,5-pyridylene groups of an alternating 2,6-/3,5-substituted ethynylpyridine macrocycle. The tricationic macrocycle recognized melamine in polar solvents such as DMSO and water, and the host-guest association in water induced a higher-order aggregate confirmed by UV-vis titration and dynamic light scattering experiments. Scanning electron microscopy, transmission electron microscopy, and atomic force microscopy indicated that fibrous network structures resulted from the stacking of the macrocycle and melamine complex.

In Situ Colorimetric Recognition of Melamine Based on Thymine Derivative-Functionalized Gold Nanoparticle

A simple, fast, and convenient colorimetric sensing system is presented for melamine recognition in milk samples by thymine-derivative- (NT-) decorated gold nanoparticles (AuNPs), based on the complementary hydrogen bond between thymine and melamine, wherein the introduction of melamine causes a rapid and obvious red-to-blue color change. Therefore, the melamine content in milk can be qualitatively recognized by the naked eye and quantitatively measured by measuring absorbance. NT-AuNPs show excellent selectivity to melamine against any other tested molecules, anions, and metal ions, as well as very good sensitivity that can distinguish melamine-contaminated milk from safe milk with a limit of detection of 3.5 nM.

Comparison of multi-recognition molecularly imprinted polymers for recognition of melamine, cyromazine, triamterene, and trimethoprim.

Three fragmental templates, including 2,4-diamino-6-methyl-1,3,5-triazine (DMT), cyromazine (CYR), and trimethoprim (TME), were used to prepare the fragment molecularly imprinted polymers (FMIPs), respectively, in polar ternary porogen which was composed of ionic liquid ([BMIM]BF4), methanol, and water. The morphology, specific surface areas, and selectivity of the obtained FMIPs for fragmental analogues were systematically characterized. The experimental results showed that the FMIPs possessed the best specific recognition ability to the relative template and the greatest imprinting factor (IF) was 5.25, 6.69, and 7.11 of DMT on DMT-MIPs, CYR on CYR-MIPs, and TME on TME-MIPs, respectively. In addition, DMT-MIPs also showed excellent recognition capability for fragmental analogues including CYR, melamine (MEL), triamterene (TAT), and TME, and the IFs were 2.08, 3.89, 2.18, and 2.60, respectively. The effects of pH and temperature on the retention of the fragmental and structural analogues were studied in detail. Van't Hoff analysis indicated that the retention and selectivity on FMIPs were an entropy-driven process, i.e., steric interaction. The resulting DMT-MIPs were used as a solid-phase extraction material to enrich CYR, MEL, TAT, and TME in different bio-matrix samples for high-performance liquid chromatography analysis. The developed method had acceptable recoveries (86.8-98.6%, n = 3) and precision (2.7-4.6%) at three spiked levels (0.05-0.5 μg g(-1)).

Activation of Mg(2+)-dependent DNAzymes based on AP site-containing triplex for specific melamine recognition.

Based on melamine binding-triggered triplex formation and subsequent activation of Mg(2+)-dependent DNAzymes, a novel strategy for DNAzyme regulation was proposed and developed for melamine recognition.

Triplex molecular beacons for sensitive recognition of melamine based on abasic-site-containing DNA and fluorescent silver nanoclusters.

A melamine aptamer derived from an abasic-site-containing triplex molecular beacon (tMB) was designed and developed for sensitive recognition of melamine by integrating tMBs and fluorescent silver nanoclusters (Ag NCs).

Establishment of a novel surface-imprinting system for melamine recognition and mechanism of template–matrix interactions

A surface molecular-imprinting system was developed on polypropylene (PP) fiber for melamine (Mel) as an N-containing template. In this article, acrylic acid was introduced onto the surface of PP for template binding. Subsequently, binding sites on PP were stabilized by crosslinking with ethylene glycol diglycidyl ether in the presence of Mel. The imprinted fiber (MIF-Mel) prepared with the optimal 15 % crosslinking density showed best-imprinting effect, with an imprinting factor of 2.18 respect to nonimprinted fiber, and a relative selectivity coefficient k′ of 10.40 for Mel with respect to its structural analog 2,4-dinitroaniline. MIF-Mel showed higher affinity to Mel with the maximum adsorption capacity of 15.5 mg g−1, while that on nonimprinted fiber was only 6.9 mg g−1. Its adsorption isotherm was well described using Langmuir model. Kinetic studies showed a rapid-binding interaction and high affinity of the MIF-Mel for its template, with a 2.5 times higher in binding amount and 4.7 times faster in binding speed than those of granular molecular-imprinting polymer with the same chemical structure. High degree of fitness with pseudo-second-order model revealed chemisorption was the rate-controlling step in the template-binding process. Basic theory of matrix–template interaction in this imprinting system was clarified to be dominated by electrostatic force synergized by hydrogen bonding between deprotonated carboxyl groups and protonated N atom in the template. It suggests that extension of this novel approach or theory to other imprinting system involving nitrogenous templates is very likely.

Micelles formation of polystyrene-co-poly (N-acryloylthymine) and its aggregation behavior induced by triple hydrogen bonding

Micelles of a new amphiphilic copolymer bearing pendant thymine groups, polystyrene-co-poly (N-acryloylthymine) (PS-co-PAT), were prepared in aqueous solutions. Transmission electron microscopy and laser light-scattering studies showed that the resultant spherical micelles were nanoscale with narrow size distribution. Moreover, ratio of hydrophilic and hydrophobic contents strongly influenced the hydrodynamic radius of the spherical micelles (113–227 nm). Upon exposure of the PS-co-PAT micelles to melamine, triple hydrogen bonding recognition between melamine and thymine resulted in the aggregation of polymer micelles, and the blue phase transparent solution of the polymer micelles was accordingly changed to turbid emulsion. The aggregation of micelles and hydrogen-bonding interaction between melamine and the PS-co-PAT shell was further confirmed by 1H NMR spectra and zeta-potential measurements. These results suggested that PS-co-PAT micelles could be a potential new vehicle for melamine recognition.

Chaotrope-Assisted Color Visualization Mechanism and Thermodynamics Involved in Molecular Recognition of Melamine by Bolaamphiphiles Embedded in Polydiacetylene Vesicles

Chaotrope-Assisted Color Visualization Mechanism and Thermodynamics Involved in Molecular Recognition of Melamine by Bolaamphiphiles Embedded in Polydiacetylene Vesicles