Melamine In Solution Research Articles

Synthesis of Molecularly Imprinted Polymer for the Removal of Melamine

In this article, molecularly imprinted polymer (MIP) of melamine was synthesized by non-covalent approach using microemulsion medium. In this study, melamine is used as a template molecule, acrylic acid as a functional monomer, N,N′-methylenebis(acrylamide) (MBAm) as a cross-linker and 2,2′-azobisisobutyronitrile (AIBN) as an initiator and microemulsion as a solvent. The produced polymers were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and thermogravimetric analysis (TGA). The batch binding analysis was conducted for the evaluation of MIP and NIP rebinding efficiency with the melamine template. The adsorption studies including the adsorbent dosage of polymer, initial concentrations and pH of melamine solution were successfully carried out on both MIP and NIP. For selectivity test, 2,4,6-trichlorophenol was chosen as the competitive molecule against melamine template. A very good relative selectivity coefficient was achieved. The removal of melamine from different samples were conducted successfully by achieving a good removal efficiency in milk, river water and human blood serum 84.30, 94.26 and 93.32 %, respectively.

Synthesis, characterization, and application of molecular imprinting polymer for extraction of melamine from spiked milk, water, and blood serum

Molecularly imprinted polymers (MIPs) of melamine (MEL) were synthesized by non-covalent approach with precipitation polymerization. In this research, MEL was used as the template molecule, acrylamide (AAm) as the functional monomer, divinylbenzene (DVB) as the cross-linker, 2,2′-azobisisobutyronitrile (AIBN) as an initiator, and the mixture of dimethyl sulfoxide (DMSO) and acetonitrile (ACN) was used as a reaction medium. The imprinted and non-imprinted polymer particles were characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), and thermogravimetric analysis (TGA). The rebinding efficiency was evaluated for all the synthesized polymers by batch binding analysis. The adsorption studies such as adsorbent dosage of polymer, initial concentrations, and pH of MEL solution were successfully conducted on the selected polymer. For selectivity test, 2,4,6-trichlorophenol (2,4,6-TCP) was selected as competitive molecule against the MEL. A good removal efficiency of MEL from different samples such as milk (81.97%), river water (91.52%), and human blood serum (90.66%) was achieved.

Bowl-like Pore array made of hollow Au/Ag alloy nanoparticles for SERS detection of melamine in solid milk powder

This article presented a facile and productive method to prepare a bowl-like pore array composed of hollow Au/Ag alloy nanoparticles (Au/Ag BPHAN array). It was found that Au/Ag BPHAN array showed the great SERS activity in the detection of melamine molecule. The experimental results for 10−5 M melamine solution indicated the SERS intensity was about five times larger than that of Ag or Au samples when the ratio of Au to Ag was close to 1:1. Especially, the detection limit was 10-9 M which was one order of magnitude lower than the reported value in references. SERS enhancement mechanism was the surface plasmon resonance in the alloy nanoparticles as presented in the simulation results by FDTD. To test the performance of the BPHAN arrays in real food samples, the melamine intentionally added to the milk powder was detected. The results showed that the SERS peaks assigned to melamine were very clear and the detection limit was already low to 0.1 ppm, 10 times lower than the tolerance level in infant milk products imposed by FDA of U.S. and 1.5 times lower than the strictest standards in the world, proposed by the Codex Alimentarius Commission in 2012. As far as we know, this was the highest sensitivity record of melamine in solid milk powder by SERS detection.

Mass-Sensitive Sensing of Melamine in Dairy Products with Molecularly Imprinted Polymers: Matrix Challenges.

Food standards and quality control are important means to ensure public health. In the last decade, melamine has become a rather notorious example of food adulteration: Spiking products with low-cost melamine in order to feign high amino acid content exploits the lack in specificity of the established Kjeldahl method for determining organic nitrogen. This work discusses the responses of a sensor based on quartz crystal microbalances (QCM) coated with molecularly imprinted polymers (MIP) to detect melamine in real life matrices both in a selective and a sensitive manner. Experiments in pure milk revealed no significant sensor responses. However, sensor response increased to a frequency change of −30Hz after diluting the matrix ten times. Systematic evaluation of this effect by experiments in melamine solutions containing bovine serum albumin (BSA) and casein revealed that proteins noticeably influence sensor results. The signal of melamine in water (1600 mg/L) decreases to half of its initial value, if either 1% BSA or casein are present. Higher protein concentrations decrease sensor responses even further. This suggests significant interaction between the analyte and proteins in general. Follow-up experiments revealed that centrifugation of tagged serum samples results in a significant loss of sensor response, thereby further confirming the suspected interaction between protein and melamine.

Synthesis of novel g-C3N4 microrods: A metal-free visible-light-driven photocatalyst

Novel graphitic carbon nitride (g-C3N4) microrods were prepared by chemical method via pre-treatment of melamine and molar solution of HNO3 at a lower temperature. The phase and morphological structure was examined by XRD and SEM, while elemental composition was found by performing EDX spectroscopy. SEM results revealed that the as-prepared g-C3N4 materials has rods like morphology with average length 3–5 µm. Furthermore, the vibrational modes were studied by FTIR, and optical properties were analyzed by UV–Vis and PL spectroscopy respectively. The optical measurements showed that the band gap of g-C3N4 microrods was 2.70 eV which make them useful for photocatalytic process under visible light. The blue PL emission at 455 nm was ascribed to band-to-band transition which also confirms the activation of g-C3N4 microrods in visible spectrum range. Finally, g-C3N4 microrods were utilized as photocatalyst for the decomposition of hazardous Methyl Orange (MO) dye, and the results showed that they exhibited excellent performance (k = 0.026 min−1) visible light irradiation.

Degradation of melamine in aqueous systems by vacuum UV-(VUV-) photolysis. An alternative to photocatalysis

Abstract VUV-irradiation experiments with aqueous solutions of melamine and related triazine derivatives were carried out in the presence or absence of molecular oxygen. Substrate degradation, total organic carbon and evolution profiles of intermediates were monitored. Relevant mineralization of the substrates was observed, in particular for melamine, its extent depending on the concentration of dissolved O2 and on the number of amino substituents at the s-triazine moiety of the substrate. In all experiments, cyanuric acid was found to be the only persistent final product. Experimental results indicate that the route of reactions leading to mineralization is independent of that leading to amino group substitution. Mechanistic hypotheses are proposed and compared with findings coming from TiO2-based photocatalysis.

Constructing 3D and Flexible Plasmonic Structure for High‐Performance SERS Application

AbstractSubstrate design has attracted much interest in development of an effective surface enhanced Raman scattering (SERS) sensor. A flexible SERS substrate with excellent performance needs to be sensitive to details of the preparation process; this sensitivity represents a significant challenge for practical applications as opposed to laboratory research applications. Here, a 3D flexible plasmonic structure, AgNPs@MoS2/pyramidal polymer (polymethyl methacrylate), is fabricated using a simple and low‐cost method. Using experiments and theoretical simulations, the SERS performance of the proposed substrate is assessed in terms of its high enhancement factor, ultrasensitivity, quantifiability, reproducibility, stability, and reusability. Finally, the ultralow in situ detection limits of the standard melamine solution and melamine in milk are found to be 10−10 and 10−6m, respectively, which meet the demand for real applications in the food security field.

A Comparative Study for Detection of Melamine in Solution by Interference Sensing Using Functionalized Silver Nanoparticles

In this research an attempt has been made for instantaneous sensing of melamine, a potential milk adulterant, at room temperature with the help of bio-functionalized silver nanoparticles. The bio-functionalized nanoparticles were made using leaf extracts of Parthenium hysterophorous and Calotropis procera. Also these methods were compared with the nanoparticles that were functionalized with sulfanillic acid. Colorimetric change was the basis of sensing the melamine using these bio-functionalized silver nanoparticles. Melamine upto a limit of 0.1 ppm was detected by the particles prepared using Parthenium hysterophorous leaf extract at room temperature. UV-visible spectroscopy (200–800 nm range) was used to detect the changes in the absorbance of silver nanoparticles with the addition of melamine in different concentrations. DLS studies were carried out to confirm the presence of nanoparticles and to study their surface morphology during sensing. Comparative study showed that Parthenium hysterophorous (noxious hysterophorous) leaf extract was far better and accurate in detecting melamine and was as accurate as the silver nanoparticles functionalized with sulfanillic acid. This can be attributed to the fact that Parthenium hysterophorous leaf extract contains caffeic acid which worked as a reducing agent and also as the bio functionalized chemical. Thus, this method can be further used as a rapid detection of melamine. In addition, this method can be converted into prototype for detection of melamine in industries.

Template-free large-scale synthesis of g-C3N4 microtubes for enhanced visible light-driven photocatalytic H2 production

A template-free hydrothermal-assisted thermal polymerization method has been developed for the large-scale synthesis of one-dimensional (1D) graphitic carbonnitride (g-C3N4) microtubes. The g-C3N4 microtubes were obtained by simple thermal polymerization of melamine-cyanuric acid complex microrods under N2 atmosphere, which were synthesized by hydrothermal treatment of melamine solution at 180 °C for 24 h. The as-obtained g-C3N4microtubes exhibited a large surface area and a unique one-dimensional tubular structure, which provided abundant active sites for proton reduction and also facilitated the electron transfer processes. As such, the g-C3N4 microtubes showed enhanced photocatalytic H2 productionactivity in lactic acid aqueous solutions under visible light irradiation (λ ≥ 420 nm), which was ∼ 3.1 times higher than that of bulk g-C3N4 prepared by direct thermal polymerization of the melamine precursor under the same calcination conditions.

A New Synthesis Approach for Carbon Nitrides: Poly(triazine imide) and Its Photocatalytic Properties.

Poly(triazine imide) (PTI) is a material belonging to the group of carbon nitrides and has shown to have competitive properties compared to melon or g-C3N4, especially in photocatalysis. As most of the carbon nitrides, PTI is usually synthesized by thermal or hydrothermal approaches. We present and discuss an alternative synthesis for PTI which exhibits a pH-dependent solubility in aqueous solutions. This synthesis is based on the formation of radicals during electrolysis of an aqueous melamine solution, coupling of resulting melamine radicals and the final formation of PTI. We applied different characterization techniques to identify PTI as the product of this reaction and report the first liquid state NMR experiments on a triazine-based carbon nitride. We show that PTI has a relatively high specific surface area and a pH-dependent adsorption of charged molecules. This tunable adsorption has a significant influence on the photocatalytic properties of PTI, which we investigated in dye degradation experiments.

One-step electrochemical strategy for in-situ synthesis of S,N-codoped graphene as metal-free catalyst for oxygen reduction reaction

A simple and economic method was developed for synthesizing efficient S,N-codoped graphene based catalyst for oxygen reduction reaction via a one-step electrochemical exfoliation of graphite rod in melamine and (NH4)2SO4 mixture solution. On one hand, melamine and (NH4)2SO4 co-worked as electrolyte to enhance the exfoliation efficiency of graphite into graphene nanosheets. On the other hand, they also provided nitrogen and sulphur sources for in-situ codoping of the as-prepared graphene. As evidenced, the N and S elements are distributed uniformly throughout the graphene nanosheets which possessed larger interlayer spacing, more lattice disorder and higher defect density. Accordingly, the obtained SNG shows efficient ORR catalytic activity with an onset potential of 0.88 V and a dominant four-electron transfer mechanism (n = 3.98 at 0.5 V), long-time durability and excellent methanol tolerance, which are comparable or superior to those of commercial Pt/C catalysts.

Plywood made from plasma-treated veneers: Shear strength after shrinkage-swelling stress

Thermally modified and unmodified beech veneers in untreated and plasma-treated state were immersed in melamine solution at different concentrations. The plasma pre-treated veneers exhibited significantly higher melamine loads than the untreated veneers at equal impregnation duration. Subsequently the veneers were manufactured into 5-layer plywood boards; the plywood samples then underwent an extreme testing procedure based on DIN-EN 314-1/2 in order to proof the bonding quality by means of shear strength. The plywood boards made of plasma pre-treated veneers exhibited up to 2.7-fold improvement in shear strength compared to plywood made of untreated veneers. Results differed markedly based on material (unmodified/thermally modified veneer) and concentration of melamine solution.

Facile synthesis and luminescence property of core–shell structured NaYF4: Yb, Er/g-C3N4 nanocomposites

A facile two-step route is used to synthesize core–shell structured NaYF4: Yb, Er/g-C3N4 nanocomposites (NaYF4/CN). A hot cyanuric acid solution and a melamine solution are put into a NaYF4: Yb, Er colloid to form ternary NaYF4: Yb, Er-cyanuric acid-melamine (NaYF4/CM) adducts. And then, the prismatic NaYF4/CM composites are annealed at 550°C, which results in a structure change from NaYF4/CM to NaYF4/CN. The g-C3N4 shell of NaYF4/CN weakens the surface luminescence quenching of the NaYF4: Yb, Er core. So the NaYF4/CN shows stronger upconversion (UC) luminescence than pure cubic NaYF4: Yb, Er nanoparticles. Besides, under excitation of 335nm, NaYF4/CN shows weaker downshifting luminescence than bulk g-C3N4.

Sensing trace levels of molecular species in solution via zinc oxide nanoprobe Raman spectroscopy

Surface‐enhanced Raman spectroscopy (SERS) has potential for unique clinical, environmental, and military applications, among many others, but it is limited by a rapid decrease in signal with distance from the sensing surface. For this reason, much study of SERS‐based biosensing involves chemical or physical adsorption of analytes to an active surface. Adsorption, however, limits the types of analytes that can be detected and detection sensitivity. The three‐dimensional closely packed architecture of zinc oxide (ZnO) nanowires decorated with silver nanoparticles increases SERS intensity, allowing for adsorption‐free biosensing. This approach greatly expands the potential applications of Raman spectroscopy as a biosensing technique. This work demonstrates a significant SERS enhancement from silver nanoparticle‐decorated ZnO nanoprobes to the Raman spectra of crystal violet, melamine, and adenine solutions. These enhancements were quantified by comparing the intensity of Raman peaks from each of the three solutions through ZnO nanowires decorated with silver nanoparticles with that through bare ZnO nanowires. Estimated enhancement of the Raman signal accounted for the volume difference between solution affected by SERS and solution sensed by the Raman system. More importantly, the detected SERS signal is from molecules in solution and unadsorbed to the sensing surfaces. This lack of adsorption was confirmed by tracking the SERS enhancement of a crystal violet Raman peak over time. This greatly enhances the value and flexibility of Raman spectroscopy as a detection technique for a wide variety of applications. Copyright © 2017 John Wiley & Sons, Ltd.

The Polyurethane Foams Based on Melamine-formaldehyde-cyclohexanone Resins

The conditions and methods for obtaining polyurethane foams based on melamine-formaldehyde-cyclohexanone resins are presented These foams were obtained from melamine solution in reactive solvents based on cyclohexanone and 4,4′-diphenylmethane diisocyanate. Some properties of obtained polyurethane foams were examined, e.g. apparent density, water absorbing capacity, dimensional stability, thermal conductivity, thermal stability, flammability and compression strength. Also a thermal analysis using the differential thermal analysis and differential scanning calorimetry methods was carried out. Polyurethane foams with increased thermal stability were obtained.

Oligoetherols based on melamine\u2013formaldehyde\u2013butanone resins

New oligoetherols with 1,3,5-triazine ring were obtained from melamine–formaldehyde–butanone resins using 12 molar excess propylene oxide (with respect to the amount of introduced melamine). The resins were obtained from melamine solutions in reactive solvent obtained from simple chemical compound—methyl ethyl ketone (butan-2-one) in its reaction with 3 molar excess formaldehyde. Physical and chemical properties of obtained oligoetherols were determined and their structure was confirmed on the basis of proton magnetic resonance, infrared spectral data and MALDI-ToF mass analysis. The amount of by-products was specified on the basis of chromatographic analysis. Initial tests proved that obtained oligoetherols can constitute a potential substrate for obtaining polyurethane foams of increased thermal resistance.

Determination of melamine in milk using surface plasma effect of aggregated Au@SiO2 nanoparticles by SERS technique

A simple and rapid method to detect melamine in liquid milk by Surface-enhanced Raman Scattering (SERS) technique was presented. The pretreatment procedure of milk samples only contains hydrochloric acid treatment and twice centrifugation. In order to reduce the distortion about the Raman signals originating from charge transfer and electronic tunnelling effect, SiO2 shell-isolated gold nanoparticles (Au@SiO2 NPs) instead of Au NPs were employed to enhance signal intensity. Aggregation occurs when the Au@SiO2 NPs colloid is mixed with the melamine solution or the treated milk containing melamine. Different from aggregated Au NPs, these aggregated Au@SiO2 NPs on Cu substrate can undistortedly enhance the Raman signals of melamine using surface plasma effect. Quantitative analysis was tried and the results showed a good linearity (R2 ≈ 0.99) when the melamine concentration was between 0.5 mgL−1 and 5 mgL−1. The detection sensitivity satisfies the requirement of the Codex Alimentarius Commission and this method can be practically used for melamine detection in milk.

Surface plasmon resonance coupling effect of micro-patterned gold film

Surface-enhanced Raman scattering has a high sensitivity in the detections of complex biological systems, and it has a lot of potential applications in food inspection, biological imaging and biosensors in biochemistry, etc. Here, we investigate the surface Raman enhancements on gold films of different morphologies and further simulate the enhancements by using the finite difference time domain. To prepare the substrates with different morphologies, polymethyl methacrylate (PMMA) is spin coated 2000 rpm in one minute on a silicon wafer, followed by annealing at 180℃ for 5 min. Then, PMMA is etched by a 20 kV electron beam lithography. With the PMMA used as a soft imprint template, polydimethylsiloxane (PDMS) is dropped on the template then removed gently from the template after drying at 60℃ for 4 h. Finally, a gold thin film is prepared on the PDMS by magnetron sputtering with a current of 10 mA for 15 min. We design two kinds of morphologies:a four-way grid and a square morphology. The dimension of the four-way grids is 40 m and the grid width is 4 upm. The dimension of the square is also 4 upm. The cystine and melamine solutions with concentrations of 50, 100, 200 and 400 ppm are deposited on the surfaces of the gold thin film, respectively. The Raman spectra of cystine and melamine solutions are measured on the substrates with four-way grids and dot arrays. The Raman spectra of cystine on two kinds of substrates show no obvious difference. Due to the relatively small enhancement of melamine, the Raman peaks of melamine solutions of concentrations 50 and 100 ppm on the substrate of square morphologies are not easy to detect. On the contrary, all of the Raman spectra of melamine on the substrate of four-way grid morphologies are clear. The result indicates that the substrate with four-way grids has better sensitivity and enhancement performance. To verify the influence of the morphologies of the substrates on surface Raman enhancement and understand the mechanism of the enhancement, we simulate the scattering spectra and field distributions of different morphologies on gold thin films by using the finite difference time domain method. It is indicated that more complex the structure, the more obvious the enhanced Raman spectra will be. The calculations show that the enhancements of four-way grid morphologies are better than those of square morphologies. The predicted results of the surface enhanced Raman scattering are consistent with the measurements. These results will provide guidance and theoretical basis for further applications of surface enhanced.

A thermo-sensitive supramolecular hydrogel derived from an onium salt with solution–gel–crystal transition properties

A self-assembled supramolecular hydrogel, which not only exhibits thermo-sensitive characteristics but has the property of solution–gel–crystal transition, is obtained by mixing the aqueous solutions of benzoic acid and melamine.

Melamine detection using phase-shift fiber-loop ring-down spectroscopy

Melamine is an illegal adulteration in milk, infant formula and pet food. Its misuse is toxic to humans and animals, so rapid and reliable screening methods for melamine detection are essential. In this paper, a phase-shift fiber-loop ring-down spectroscopy (PS-FLRDS)-based system is developed and evaluated for the detection and quantification of melamine. In the PS-FLRDS technique, an intensity-modulated continuous-wave (CW) laser is used as the light source, and a phase shift between the incident and emitted light of the fiber loop is introduced due to losses caused by the fiber loop and absorbing sample. By measuring this phase shift, one can readily obtain the concentration information of the measured species. Phase shifts of melamine solutions with different concentrations are measured and the experimental results show that there is a good linear relationship between the phase shift and the melamine concentration in low concentration range and a detection limit of 0.03mg/mL has been achieved.