Simultaneous determination of urea and melamine in milk powder by nonlinear chemical fingerprint technique

Development of a novel and affordable point-of-care kit for rapid detection of urea and glucose adulteration in cow milk

The increasing global population has raised the demand for cow milk, leading to its adulteration with harmful substances, including urea and glucose, that cause damage to humans when consumed regularly. Hence, this study started with predicting urea and glucose toxicity using ProTox-III software, wherein the results revealed that urea belongs to class IV with an LD50 value of 6350 mg kg-1 and glucose belongs to class VI with an LD50 value of 23 000 mg kg-1. Then, a qualitative colorimetric kit and Fourier-transform infrared (FTIR) spectroscopy were used for the preliminary detection of urea and glucose in cow milk. The colorimetric kit confirmed the presence of urea and glucose by changing the sample colour. Based on these results, a point-of-care (PoC) kit was developed for urea and glucose detection in cow milk. The enzyme immobilization technique was used to coat urease and glucose oxidase/peroxidase on polystyrene strips to make PoC strips. The biochemical methods of the Berthelot assay and glucose oxidase/peroxidase (GOD/POD) assay were used to detect urea and glucose, respectively. The lowest detection limits of the developed microassay kit for urea and glucose were 1.5 and 3 μg from 300 μg of cow milk. The shelf life of the urease immobilized strip was ∼30 days, with 15 times the reusability of a single well, and for the GOD/POD immobilized strip it was ∼15 days, with 7 times the reusability, each with a detection efficiency of 85-90%. The strips provided results in ten minutes and were easily portable for on-site adulteration detection.

Detection of urea in milk by urease-inorganic hybrid nanoflowers combined with portable colorimetric microliter tube

A simple one-pot green synthesis method was used to prepare urease-inorganic hybrid nanoflowers (UE-HNFs), which had a high surface-to-volume ratio to improve enzyme catalytic efficiency and make urease reusable. A portable colorimetric microliter tube based on urease-inorganic hybrid nanoflowers (UE-HNFs-PCMT), as an urea colorimetric biosensor, was developed for determining urea concentration in milk. The combination of urea colorimetric biosensor and a smartphone is used for capturing the colour change of milk after reaction. There was a good linear relationship between colour intensity of the image (Δ intensity) and urea concentration (43-600 mg L-1), with a detection limit of 12.81 mg L-1. UE-HNFs-PCMT has the advantages of no need for complex equipment, easy operation, reusability, low detection cost, good portability, and environmental friendliness and can achieve urea detection in milk.

Detecting Low Concentrations of Nitrogen-Based Adulterants in Whey Protein Powder Using Benchtop and Handheld NIR Spectrometers and the Feasibility of Scanning through Plastic Bag

Nitrogen-rich adulterants in protein powders present sensitivity challenges to conventional combustion methods of protein determination which can be overcome by near Infrared spectroscopy (NIRS). NIRS is a rapid analytical method with high sensitivity and non-invasive advantages. This study developed robust models using benchtop and handheld spectrometers to predict low concentrations of urea, glycine, taurine, and melamine in whey protein powder (WPP). Effectiveness of scanning samples through optical glass and polyethylene bags was also tested for the handheld NIRS. WPP was adulterated up to six concentration levels from 0.5% to 3% w/w. The two spectrometers were used to obtain three datasets of 819 diffuse reflectance spectra each that were pretreated before linear discriminant analysis (LDA) and regression (PLSR). Pretreatment was effective and revealed important absorption bands that could be correlated with the chemical properties of the mixtures. Benchtop NIR spectrometer showed the best results in LDA and PLSR but handheld NIR spectrometers showed comparatively good results. There were high prediction accuracies and low errors attesting to the robustness of the developed PLSR models using independent test set validation. Both the plastic bag and optical glass gave good results with accuracies depending on the adulterant of interest and can be used for field applications.

One-step wet-chemical synthesis of ternary ZnO/CuO/Co3O4 nanoparticles for sensitive and selective melamine sensor development

Using one-step wet-chemically synthesized ternary ZnO/CuO/Co₃O₄ nanoparticles (NPs) fabricated GCE sensor probe, a selective and sensitive melamine chemical sensor was developed by electrochemical approach, which exhibited the highest sensitivity, better repeatability, broad linear dynamic range, good linearity, fast response time, and lowest detection limit.

Removal of a melamine contaminant with Ag-doped ZnO nanocomposite materials

In this approach, Ag-doped ZnO nanosheets (Ag/ZnO NSs) were prepared via a facile wet-chemical method using reducing agents in an alkaline medium at a low temperature.

A simple and sensitive SERS quantitative analysis method for urea using the dimethylglyoxime product as molecular probes in nanosilver sol substrate

Under the light wave irradiation, the stable and highly surface enhanced Raman scattering (SERS) active silver nanosol (AgNP) was prepared by reduction of AgNO₃, using sodium citrate as reducer. Urea reacted with dimethylglyoxime to produce 4,5-dimethyl-2-imidazole ketone that exhibited a strong SERS peak at 1320 cm^-1 in the as-prepared AgNPs substrate. Under the selected conditions, the increased SERS intensity ΔI was linear to urea concentration in the range of 8.25-825 nM, with a detection limit of 4.92 nM. Accordingly, a new, simple SERS quantitative analysis method for trace urea in foods was established, with relative standard deviation of 0.92-4.8% and recovery of 97.4-101%.