Sensitive Detection Of Melamine Research Articles

DNA hydrogels combined with microfluidic chips for melamine detection

The illegal addition of melamine (MEL) into pet foods and infant milk powder has caused great concern among people. In this study, a point-of-care testing (POCT) method was developed by combining stimuli-responsive deoxyribonucleic acid (DNA) hydrogels with microfluidic chips to achieve portable and sensitive detection of MEL. With the MEL aptamer (MA) acting as a cross-linker, DNA hydrogel-coated gold nanoparticles (AuNPs) served as a basis for colorimetric detection and quantitative analysis. In the presence of MEL, it competitively binds to the aptamer, causing disintegration of the DNA hydrogels and a release of the coated AuNPs, making it possible to visually detect and quantitatively measure the MEL. Under optimal conditions, the detection range of MEL using DNA hydrogels was 0.1–100 μM and the limit of detection (LOD) was 42 nM. This portable, sensitive, and user-friendly field test equipment was developed to avoid the use of non-portable laboratory instruments. Furthermore, we combined microfluidic chips with the properties of DNA hydrogels, making it possible to quantitatively detect MEL by taking photos and analyzing the gray value using software in accordance with the different colors of copolymerization solutions after the reaction. The detection range of MEL using the microfluidic chip-based method was 0.2–50 μM, and the LOD was 37 nM. Neither trained operators nor large-scale instruments are needed for using this method to conduct portable and sensitive field detection of the targets, which highlights the methods excellent potential in food security, clinical tests, environmental monitoring, and other aspects.

Aquamarine blue emitting silver nanoparticles as fluorescent sensor for melamine detection.

Designing an exceptional probe to detect minute quantities of melamine is of huge importance for the safety and health of the human race. In this study, an aquamarine blue and rapid fluorescent sensor was devised by chemical reduction synthesis method for highly selective and sensitive detection of melamine in milk using polyethyleneimine protected silver nanoparticles (PEI-AgNPs). The optical properties of PEI-AgNPs were studied as a function of reactive material concentration, incubation time, temperature and pH. The formation of PEI-AgNPs was verified by using transmission electron microscopy (TEM) and found that the PEI-AgNPs were spherical in shape with an average diameter of 15 nm. The prepared PEI-AgNPs were aquamarine blue luminescent under UV light and show a maximum emission at 470 nm, when excited at 348 nm. The fluorescence of the PEI-AgNPs can be effectively quenched by the addition of melamine due to its ability to interact with citric acid (CA) through hydrogen bonding and form silver-melamine complexes with Ag+ via coordination bonds, while the addition of other nitro compounds induced feeble quenching of PEI-AgNPs, suggesting the high selectivity of PEI-AgNPs. The present sensor system exhibits a good linear response to melamine ranging from 0.16 to 56 μM, and the limit of detection was found to be 132 nM (Sb/K = 3). Significantly, the fluorescence sensor exhibited good selectivity and acted as a potential probe for detection of melamine in milk samples.

Preparation of Quasi-Three-Dimensional Porous Ag and Ag-NiO Nanofibrous Mats for SERS Application.

In this study, two new quasi-three-dimensional Surface Enhanced Raman Scattering (SERS) substrates, namely porous Ag and Ag-NiO nanofibrous mats, were prepared using a simple, electrospinning-calcination, two-step synthetic process. AgNO3/polyvinyl pyrrolidone (PVP) and AgNO3/Ni(NO3)2/PVP composites serving as precursors were electrospun to form corresponding precursory nanofibers. Porous Ag and Ag-NiO nanofibers were successfully obtained after a 3-h calcination at 500 °C under air atmosphere, and analyzed using various material characterization techniques. Synthesized, quasi-three-dimensional porous Ag and Ag-NiO nanofibrous mats were applied as SERS substrates, to measure the model compound Rhodamine 6G (R6G), and investigate the corresponding signal enhancement. Furthermore, porous Ag and Ag-NiO nanofibrous mats were employed as SERS substrates for melamine and methyl parathion respectively. Sensitive detection of melamine and methyl parathion was achieved, indicating their feasibility as an active SERS sensing platform, and potential for food safety and environmental monitoring. All the results suggest that the electrospinning-calcination, two-step method offers a new, low cost, high performance solution in the preparation of SERS substrates.

On-site detection of sub-mg/kg melamine mixed in powdered infant formula and chocolate using sharp-edged gold nanostar substrates

ABSTRACTWe report a facile method for sample preparation and sensitive on-site detection of melamine in powdered infant formula and chocolate using Raman spectroscopy on sharp-edged gold nanostars (AuNSs). The aggregation of AuNSs by sodium chloride (1.2 M) facilitates the more sensitive detection of melamine in comparison with spherical gold nanoparticles (AuNPs). Density functional theory quantum mechanical calculations were performed to determine the energetic stability on gold cluster atoms. Our spectroscopic data indicated that AuNSs are an efficient platform for detecting melamine in food mixtures. The detection limits of melamine in powdered infant formula and chocolate were found to be ~0.1 mg/kg and ~1 mg/kg, respectively, on AuNPs, whereas they were observed to be ~0.01 mg/kg and ~0.1 mg/kg, respectively, on AuNSs. Using a handheld Raman spectrometer, a sub-mg/kg detection of melamine in both powdered infant formula and chocolate could be achieved within a few minutes.

Sensitive detection of melamine by an electrochemiluminescence sensor based on tris(bipyridine)ruthenium(II)-functionalized metal-organic frameworks

Metal-organic frameworks (MOFs) are microporous materials that consist of metal ions or clusters and organic ligands. Their luminescence activities can be improved by functionalization, and the present work describes the synthesis of a luminescence-functionalized MOF in which tris(2,2′-bipyridyl) dichlororuthenium(II) hexahydrate (Ru(bpy)32+) was introduced into the MOF. The synthesized RuMOF was then fabricated as an ECL sensor that can be used for the rapid and sensitive detection of melamine in dairy products. The optimum detection conditions were established by single-factor analysis and surface response multivariate optimization. Under optimum conditions, the ECL intensity was proportional to log(melamine) concentration in the range 10−10–10−4 M with a detection limit of about 3.8 × 10−11 M (signal/noise = 3). In an application to the detection of melamine in milk and infant formula powder, recoveries in the range 98–104% and 95–103%, respectively were obtained from spiked samples. Thus this novel ECL sensor represents a potentially valuable product for the analysis of melamine food samples.

Three-Dimensional SERS Substrates Formed with Plasmonic Core-Satellite Nanostructures

We demonstrate three-dimensional surface-enhanced Raman spectroscopy (SERS) substrates formed by accumulating plasmonic nanostructures that are synthesized using a DNA-assisted assembly method. We densely immobilize Au nanoparticles (AuNPs) on polymer beads to form core-satellite nanostructures for detecting molecules by SERS. The experimental parameters affecting the AuNP immobilization, including salt concentration and the number ratio of the AuNPs to the polymer beads, are tested to achieve a high density of the immobilized AuNPs. To create electromagnetic hot spots for sensitive SERS sensing, we add a Ag shell to the AuNPs to reduce the interparticle distance further, and we carefully adjust the thickness of the shell to optimize the SERS effects. In addition, to obtain sensitive and reproducible SERS results, instead of using the core-satellite nanostructures dispersed in solution directly, we prepare SERS substrates consisting of closely packed nanostructures by drying nanostructure-containing droplets on hydrophobic surfaces. The densely distributed small and well-controlled nanogaps on the accumulated nanostructures function as three-dimensional SERS hot spots. Our results show that the SERS spectra obtained using the substrates are much stronger and more reproducible than that obtained using the nanostructures dispersed in solution. Sensitive detection of melamine and sodium thiocyanate (NaSCN) are achieved using the SERS substrates.

Sensitive and simple sonoluminescence detection of melamine via aggregation of Au nanoparticles

A novel nano-material-sonoluminescence design is proposed by converting the melamine responsive aggregation of Au nanoparticles into sonoluminescence signals for sensitive detection of melamine.

Dual hairpin-like molecular beacon based on coralyne-adenosine interaction for sensing melamine in dairy products

This study presents a novel dual hairpin-like molecular beacon (MB) for the selective and sensitive detection of melamine (MA) based on the conjugation of MA and thymine. In this protocol, the coordination between coralyne and adenosine (A) leaded a dual hairpin-like MB and the fluorophore–quencher pair is close proximity resulting in the fluorescence quenching. With the addition of MA, it conjugated with thymine in the loop part of dual hairpin-like MB by triple H-bonds, triggering the dissociation of the dual hairpin-like MB. The resulting spatial separation of the fluorophore from quencher induced the enhancement in fluorescence emission. Under the optimized conditions, the sensor exhibited a wide linear range of 8×10−9–1.6×10−5M (R2=0.9969) towards MA, with a low detection limit of 5nM, approximately 4000 times lower than the Drug Administration and the US Food estimated MA safety limit. The real milk samples were also investigated with a satisfying result.

Rapid and sensitive detection of melamine in milk with gold nanoparticles by Surface Enhanced Raman Scattering

A rapid and sensitive method to detect melamine in liquid milk based on Surface Enhanced Raman Scattering (SERS) spectroscopy is presented, exploiting the selective binding of gold nanoparticles (AuNPs) with this analyte. This interaction promotes the aggregation of the AuNPs inducing a huge enhancement of the melamine signals in the Raman spectrum due to the formation of SERS “hot spots”. An external standard calibration method was employed for quantitative analysis and the method was validated for linearity, sensitivity, repeatability and recovery. A good linearity (R2=0.99) was found in the concentration range of 0.31–5.0mgl−1 in milk with a limit of detection of 0.17mgl−1. This method does not require a long extraction procedure (total analysis time can be lower than 30min) and can be reliably used for melamine detection in milk matrix in accordance with the European law limits.

Rapid detection of melamine with 4-mercaptopyridine-modified gold nanoparticles by surface-enhanced Raman scattering

A surface-enhanced Raman scattering (SERS) strategy based on 4-mercaptopyridine (MPY)-modified gold nanoparticles (AuNPs) was developed for the rapid and sensitive detection of melamine in milk powder. The SERS measurement of melamine strongly relied on the "hotspot" effect, in which AuNPs immediately aggregated upon the addition of melamine, leading to significantly enhanced Raman intensity of the reporter molecule MPY and a color change for the solution from red to blue-gray. The limit of detection based on a signal to noise of 3 (S/N = 3) was found to be as low as 0.1 ppb of melamine, with an excellent linearity of 0.5-100 ppb, demonstrating a higher sensitivity and a wider quantitation range than direct SERS sensing methods based on enhanced substrate. An impressive specificity for melamine detection over various common metal ions and excipients in dairy products, even at concentrations of 100-fold higher than melamine, was achieved. Good recoveries of 88.5% and 111.7% were obtained from milk samples spiked to 20 and 100 ppb levels, respectively. The proposed method is potentially applicable for the rapid in situ determination of melamine in complex matrices.

Selective and sensitive detection of melamine by intra/inter liposomal interaction of polydiacetylene liposomes

We report a convenient melamine detection system based on polydiacetylene (PDA) liposomes having rapid, selective, and sensitive detection, and dual signal capabilities. The detection limit of the sensory PDA liposome is 1 and 0.5 ppm in the colorimetric and the fluorescence detection schemes, respectively, satisfying the world regulation level.

Highly sensitive detection of melamine and its derivatives by capillary electrophoresis coupled with online preconcentration techniques

Two kinds of online preconcentration techniques including large-volume sample stacking-sweeping and selective-exhaustive injection-sweeping (SEI-S) were employed in CE to develop a rapid, simple and sensitive method for determination of melamine (MM) and its derivatives such as ammeline (AMN), ammelide (AMD) and cyanuric acid (CA) in liquid milk products. Though the sensitivity of large-volume sample stacking-sweeping is not particularly high, it can be used to detect all the four compounds simultaneously. However, it is very difficult to improve the sensitivity of these four compounds by using single SEI-S method directly, owing to their completely different extents of protonation or deprotonation. Grouping can solve this problem perfectly. After the four compounds are divided into two groups (MM/AMN and AMD/CA), cation-SEI-S or anion-SEI-S can be applied for their online stacking. In cation-SEI-S, the detection limits for MM and AMN were both 0.01 ng/mL based on the signal-to-noise ratio of 3. In anion-SEI-S, the detection limits were 0.05 and 0.02 ng/mL for CA and AMD, respectively. Results of this study show a great potential for the SEI-S method to be a useful tool for the rapid and sensitive determination of MM and its derivatives in liquid milk products.

Detection of Melamine in Powdered Milk Using Surface-Enhanced Raman Scattering with No Pretreatment

We detected a trace amount of melamine in powdered milk using surface enhanced Raman scattering (SERS) on gold surfaces at an excitation wavelength of 632.8 nm. A detection limit of ∼100 ppm (μg/g) melamine in milk was attained within a few minutes by the gold nanoparticle substrate from chemical reduction; whereas, better sensitivity, as low as ∼200 ppb (ng/g), was achieved by the roughened gold substrate. Our method has the advantage of fast and sensitive detection of melamine in powdered milk without pre-treating the samples.

Analysis of Melamine Cyanurate in Urine Using Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) was applied to the direct analysis of melamine cyanurate (MC). The three commonly used MALDI matrixes, namely, alpha-cyano-4-hydroxycinnamic acid (CHCA), sinapinic acid (SA), and 2,5-dihydroxybenzoic acid (DHB), were able to desorb/ionize melamine from MC upon N(2) laser irradiation, with CHCA showing the highest detection sensitivity in the positive mode. Only DHB and SA were able to desorb/ionize cyanuric acid from MC in the negative mode but with remarkably lower sensitivity. The method is able to detect melamine unambiguously from a small amount of MC (down to 12.5 microg) spiked into urine and was successfully applied for the rapid and sensitive detection of melamine in urine stones/residues of the samples collected from patients clinically confirmed of having kidney stones associated with the consumption of melamine-tainted food products. The urine matrix resulted in interfering ion peaks and suppressed the ion intensity of melamine, while a cleanup process consisting of simply washing with water eliminated such interference and enhanced the ion intensity. The merit of the method is simplicity in sample preparation. The analytical time of the method for high-throughput analysis from the time of sample treatment to analysis is less than 7 minutes per sample, with sensitive detection of the presence of melamine in the urine stones/residues of the patient samples.