Cockle shell-derived nanoparticles for optical urea biosensor development based on reflectance transduction

An optical biosensor for urea based on urease enzyme immobilised on functionalised calcium carbonate nanoparticles (CaCO₃-NPs) was successfully developed in this study. CaCO₃-NPs were synthesised from discarded cockle shells via a simple and eco-friendly approach, followed by surface functionalisation with succinimide ester groups. The fabricated biosensor is comprised of two layers. The first (bottom layer) contained functionalised NPs covalently immobilised to urease, and the second (uppermost layer) was alginate hydrogel physically immobilised to the pH indicator phenolphthalein. The biosensor provided a colorimetric indication of increasing urea concentrations by changing from colourless to pink. Quantitative urea analysis was performed by measuring the reflectance intensity of the colour change at a wavelength of 633.16 nm. The determination of urea concentration using this biosensor yielded a linear response range of 30–1000 mM (R² = 0.9901) with a detection limit of 17.74 mM at pH 7.5. The relative standard deviation of reproducibility was 1.14%, with no signs of interference by major cations, such as K⁺, Na⁺, NH?⁺, and Mg²⁺. The fabricated biosensor showed no significant difference with the standard method for the determination of urea in urine samples.     

Dehydrated halloysite intercalated mechanochemically with urea: Thermal behavior and structural aspects

Urea has been intercalated mechanochemically into dehydrated halloysite and analyzed by X-ray powder diffraction (XRPD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance ultraviolet/visible spectroscopy (DRUV–VIS), thermal analysis (TGA/DTA), transmission electron microscopy (TEM), and electron paramagnetic resonance (EPR). The basal distance expands from 7.4 to 10.7 Å and the interaction of urea to adjacent layers of halloysite through hydrogen bonds increases the structural order of the matrix. After heat treatment in air at different temperatures, decomposition products begin to appear starting from 100 °C. Although the basal distance remains constant up to 160 °C and collapses to the original value at 200 °C, urea and the decomposition products are still present in the sample. Starting from 125 °C, urea decomposition products reduce halloysite structural Fe³⁺ centers to Fe²⁺, as indicated by DRUV–VIS and EPR spectroscopy.     

Purification,Properties, and Application of a Novel Acid Urease from <Emphasis Type="Italic">Enterobacter</Emphasis> sp.

It has been demonstrated that acid urease is capable of decomposing urea in fermented beverage and foods. As urea is a precursor of ethylcarbamate, a potential carcinogenic compound, measures must be taken to control the level of urea. We herein describe the purification and characterization of a novel acid urease from Enterobacter sp. R-SYB082 and its application to the removal of urea in Chinese rice wine. The enzyme was purified to electrophoretic homogeneity using ethanol precipitation, Superdex 200 and Mono Q with a fold purification of 21.1 and a recovery of 49%. The molecular weight of the enzyme was 430,000 Da by gel filtration and 72,000 Da by sodium dodecyl sulfate polyacrylamide gel electrophoresis, suggesting that it was a hexamer. The activity of this purified enzyme was optimal at pH 4.5 and 35 °C. The temperature stability was under 55 °C, and the pH stability was 4.0~5.0. The enzyme exhibited an apparent K _m of 19.5 μmol/l and a V _max of 109 μmol urea/mg·min at 35 °C and pH 4.5. When incubating two different kinds of Chinese rice wine with the enzyme (0.08 U/ml) at 35 °C for 7 days, over 85% of urea was decomposed, and at 20 °C, above 78% was removed. The result showed that the enzyme is applicable to elimination of urea in Chinese rice wine.     

Dual potentiometric and UV/Vis spectrophotometric disposable sensors with dispersion cast polyaniline

Simple conducting polymer–polyaniline-based sensors/biosensors, working either in potentiometric or UV/Vis spectrophotometric mode, are proposed. Disposable sensors were produced by coating polyaniline layers, cast from aqueous dispersion of the polymer nanoparticles, on a transparent plastic polyacetate foil. In the potentiometric mode, polyaniline layers are sensitive to a number of metal cations, while in UV/Vis mode, changes of absorbance were recorded only in case of a chemical reaction of cations with the polymer. Pronounced sensitivity of tested sensors to ammonia was used for potentiometric/spectrophotometric biosensing purposes studied on a model example of urease-based sensors of urea. The highest sensitivity and reproducibility of such sensors were observed in urea concentration ranging from 1 to 10 mM.     

Study of spin coated multilayer zirconia and silica films for non-quarterwavelength optical design based antireflection effect at 1,054 nm

A non-quarterwavelength optical design (design wavelength, λ_o = 1,054 nm) based antireflection (AR) coating was prepared by sol–gel spin coating technique. Two materials, zirconia and silica were chosen for the deposition of AR layers on borosilicate crown glass, refractive index (R. I. = 1.51). For this design, the bottom and middle layers were of zirconia with the R. I. range 1.941–1.958 while the top layer was of silica with R. I. 1.455. To understand the surface feature after each deposition, refractive index and physical thickness of the layers were measured ellipsometrically (λ = 632.8 nm) at different points over the area, 10 mm × 10 mm with an interval of 0.5 mm along the centre based perpendicular projection made on an imaginary chord. The surface feature was examined by plotting the measured values of the optical parameters against the displacement. The surface roughness decreased with increasing layers. This was verified by the study of AFM images of the layers. Specular reflection of the antireflection coated product at λ₀ was comparable to that of the theoretically simulated curve.     

A compact miniaturized continuous flow system for the determination of urea content in milk

A multicommutation-based flow system with photometric detection was developed, employing an analytical microsystem constructed with low temperature co-fired ceramics (LTCC) technology, a solid-phase reactor containing particles of Canavalia ensiformis DC (urease source) immobilized with glutaraldehyde, and a mini-photometer coupled directly to the microsystem which monolithically integrates a continuous flow cell. The determination of urea in milk was based on the hydrolysis of urea in the solid-phase reactor and the ammonium ions produced were monitored using the Berthelot reaction. The analytical curve was linear in the urea concentration range from 1.0 × 10⁻⁴ to 5.0 × 10⁻³ mol L⁻¹ with a limit of detection of 8.0 × 10⁻⁶ mol L⁻¹. The relative standard deviation (RSD) for a 2.0 × 10⁻³ mol L⁻¹ urea solution was lower than 0.4% (n = 10) and the sample throughput was 13 h⁻¹. To check the reproducibility of the flow system, calibration curves were obtained with freshly prepared solutions on different days and the RSD obtained was 4.7% (n = 6). Accuracy was assessed by comparing the results of the proposed method with those from the official procedure and the data are in close agreement, at a 95% confidence level.     

Metal nitrate/fuel mixture reactivity and its influence on the solution combustion synthesis of γ-LiAlO<Subscript>2</Subscript>

The reactivity of LiNO₃ and Al(NO₃)₃ with respect to urea and β-alanine was investigated. Experimental results proved that β-alanine is a more suitable fuel for LiNO₃, whereas urea seems to be more adequate for Al(NO₃)₃. Based on the different metal nitrate/fuel mixture reactivity, nanocrystalline γ-LiAlO₂ powders were prepared by solution combustion synthesis using a fuel mixture of urea and β-alanine. This fuel mixture yielded single-phase nanocrystalline γ-LiAlO₂ (32.6 nm) directly from the combustion reaction. The resulted powder had a specific surface area of 3.2 m²/g and no supplementary annealing was required. On the other hand, pure γ-LiAlO₂ could not be obtained by using a single fuel (urea, β-alanine) unless annealing at 900 °C for 1 h was performed.     

Deep eutectic silsesquioxane hybrids with quaternary ammonium/urea derivatives: synthesis and physicochemical and ion-conductive properties

We report the synthesis of deep eutectic silsesquioxane hybrids (DE-SQs) by simple mixing of quaternary-ammonium-containing SQ and urea derivatives. Cationic SQ, which was prepared by the hydrolytic condensation of a triethoxysilane precursor derived from 2-(dimethylamino)ethyl acrylate, followed by a quaternization reaction with methyl iodide, was used as a quaternary-ammonium-containing SQ component. Cationic SQ reacted with urea at a 1:2 M ratio at 80 °C for 48 h to yield a viscous DE-SQ (2Urea) liquid with a low glass transition temperature (T_g = ?11 °C). Urea derivatives—1,3-dimethylurea (DMU) and 1,3-dimethylthiourea (DMTU)—were additionally used as hydrogen bond donors to form low-T_g DE-SQs. The thermal, physical, and ion-conductive properties of the DE-SQ family of organic–inorganic hybrids were investigated and characterized, and the influences of the nature of the urea derivative and their feed ratios on DE-SQ formation were evaluated. Among the DE-SQs developed in this study, DE-SQ (2Urea) and DE-SQ (2DMTU) achieved the highest ionic conductivity, with DE-SQ (2Urea) exhibiting 2.35 × 10^?6 and 6.63 × 10^?4 S cm^?1 at 25 and 75 °C, respectively, under anhydrous conditions. This is the first report on the synthesis of DE-SQs by simple mixing of two solids, wherein the resulting compounds exhibit low T_g, thermal stability, and characteristic ionic conductivity. The ability to incorporate unique DE units into the SQ structure facilitates the development of advanced organic–inorganic hybrid materials possessing a wide range of functions and applications.     

Properties of cellulose films prepared from NaOH/urea/zincate aqueous solution at low temperature

Cellulose films were successfully prepared from NaOH/urea/zincate aqueous solution pre-cooled to −13 °C by coagulating with 5% H₂SO₄. The cellulose solution and regenerated cellulose films were characterized with dynamic rheology, ultraviolet–visible spectroscope, scanning electron microscopy, wide angle X-ray diffraction, Fourier transform infrared (FT-IR) spectrometer, thermogravimetry and tensile testing. The results indicated that at higher temperature (above 65 °C) or lower temperature (below −10 °C) or for longer storage time, gels could form in the cellulose dope. However, the cellulose solution remained a liquid state for a long time at 0–10 °C. Moreover, there was an irreversible gelation in the cellulose solution system. The films with cellulose II exhibited better optical transmittance, high thermal stability and tensile strength than that prepared by NaOH/urea aqueous solution without zincate. Therefore, the addition of zincate in the NaOH/urea aqueous system could enhance the cellulose solubility and improve the structure and properties of the regenerated cellulose films.     

Fabrication of flame-retardant and smoke-suppressant isocyanate-based polyimide foam modified by silica aerogel thermal insulation and flame protection layers

Because of containing urea groups, flame resistance and smoke releasing behaviors of isocyanate-based polyimide foam (IBPIF) produced using free foaming technology require further improvement. In this work, silica aerogel layers were incorporated into cells of IBPIF through an in situ growth process of silica sol (SS). Compared with silica aerogel particles directly mixed into the foaming slurry, the silica aerogel layers that firmly attached to the pores and surfaces of cells not only provided exceptional thermal insulation and flame protection, but also kept original cellular structure. With increase in ratio of SS mass to IBPIF volume, silica aerogel incorporation dosage was gradually increased. Accompanied by flame resistance was obviously improved and smoke releasing behavior was effectively suppressed. Those were indicated by the improved limiting oxygen index (LOI), decreased heat release rate (HRR), peak of HRR, and specific optical density of smoke (Ds) in trials with pilot flames. Compared with pure IBPIF, when the ratio reached to 5/15 g/cm³, it resulted in LOI increasing from 22.0% to 33.0%, peak of HRR, total smoke production (TSP), and maximum value of Ds decreasing from 174 to 72 kW/m², 1.11 to 0.37 m²/m², 45.90 to 17.45, respectively.     

The role of urea in Cu–Zn–Al catalysts for methanol steam reforming

Abstract  

Impregnated Cu–Zn over Al₂O₃ exhibits high activity with the use of a lower amount of active metal relative to conventional co-precipitation catalysts. The activity of the catalyst could be enhanced by addition of urea to the metal salt solution during impregnation. The H₂ yield from Cu–Zn catalysts with urea is 42%, while the H₂ yield from catalyst without urea is only 28% in a continuous system at 250 °C and 1.2 atm. The H₂ yield of the catalyst with urea in this study could compete with that of commercial catalysts. The role of urea in the Cu–Zn catalysts was investigated. X-ray diffraction (XRD) analysis of the catalysts shows that the crystal size of CuO could be reduced by the addition of urea. The XRD diffractogram of the catalyst prior to calcination also shows the formation of NH₄NO₃, which could aid in dissociation of metal clusters. Scanning electron microscopy (SEM) images of catalysts show the size of Cu–Zn compound clusters and also their dispersion over the Al₂O₃ surface on the impregnated catalysts. The addition of urea could also yield smaller Cu–Zn compound clusters and better dispersion compared with the impregnated catalyst without urea. Such impregnated Cu–Zn catalysts with urea could be alternative novel catalysts for methanol steam reforming.     

Mechanisms of amphipathic helical peptide denaturation by guanidinium chloride and urea: a molecular dynamics simulation study

Urea and GdmCl are widely used to denature proteins at high concentrations. Here, we used MD simulations to study the denaturation mechanisms of helical peptide in different concentrations of GdmCl and urea. It was found that the helical structure of the peptide in water simulation is disappeared after 5 ns while the helicity of the peptide is disappeared after 70 ns in 2 M urea and 25 ns in 1 M GdmCl. Surprisingly, this result shows that the helical structure in low concentration of denaturants is remained more with respect to that solvated in water. The present work strongly suggests that urea interact more preferentially to non-polar and aromatic side chains in 2 M urea; therefore, hydrophobic residues are in more favorable environment in 2 M urea. Our results also reveal that the hydrogen bonds between urea and the backbone is the dominant mechanism by which the peptide is destabilized in high concentration of urea. In 1 M and 2 M GdmCl, GdmCl molecules tend to engage in transient stacking interactions with aromatics and hydrophobic planar side chains that lead to displacement of water from the hydration surface, providing more favorable environment for them. This shows that accumulation of GdmCl around hydrophobic surfaces in 1 M and 2 M GdmCl solutions prevents proper solvation of the peptide at the beginning. In high GdmCl concentrations, water solvate the peptide better than 1 M and 2 M GdmCl. Therefore, our results strongly suggest that hydrogen bonds between water and the peptide are important factors in the destabilization of peptide in GdmCl solutions.     

Determination of human urinary kanamycin in one step using urea-enhanced surface plasmon resonance light-scattering of gold nanoparticles

The purpose of this study was to establish a simple, sensitive analytical method for kanamycin (KANA) in human urine. Enhancement of the plasmon resonance light-scattering (PRLS) of gold nanoparticles (AuNPs) by KANA provided the basis for this analytical method. At pH 6.7, KANA induced AuNPs aggregation with enhanced PRLS. The PRLS of the AuNPs–KANA system was further enhanced by addition of urea. The linear range and detection limit for KANA were from 20–800 nmol L⁻¹ and 2 nmol L⁻¹, respectively. Potential interfering substances present in urine had a negligible effect on the determination, thus preliminary sample separations were not necessary. Recovery of KANA from spiked human urine was 94–104%. This simple, sensitive method, using urea to enhance the PRLS of the AuNPs–KANA system, may provide a new approach for determination of compounds rich in OH groups.     

Ultra high performance liquid chromatography with tandem mass spectrometry method for determining dinotefuran and its main metabolites in samples of plants,animal‐derived foods,soil, and water

An ultra high‐performance liquid chromatography with tandem triple quadrupole mass spectrometry residue method was developed and validated for the quantification and identification of dinotefuran and its main metabolites 1‐methyl‐3‐(tetrahydro‐3‐furylmethyl) urea and 1‐methyl‐3‐(tetrahydro‐3‐furylmethyl) guanidine in fruit (watermelon), vegetable (cucumber), cereal (rice), animal‐derived foods (milk, egg, and pork), soil, and water. The samples were extracted with acetonitrile containing 15% v/v acetic acid and purified with dispersive solid‐phase extraction with octadecylsilane, primary secondary amine, graphitized carbon black, or zirconia‐coated silica prior to analysis. The method had an excellent linearity (R² ≥ 0.9942, 1–500 μg/L) and satisfactory recoveries (73–102%) at five spiked levels (0.001, 0.01, 0.05, 0.5, and 2 mg/kg) with intra‐ or interday precision in the range of 0.8–9.5% and 3.0–12.8% for the three compounds in the eight matrices. The limits of quantification were 10 μg/kg for 1‐methyl‐3‐(tetrahydro‐3‐furylmethyl) guanidine and 1 μg/kg for 1‐methyl‐3‐(tetrahydro‐3‐furylmethyl) urea and dinotefuran. The applicability of the developed method was demonstrated by determining the occurrence of dinotefuran, 1‐methyl‐3‐(tetrahydro‐3‐furylmethyl) guanidine, and 1‐methyl‐3‐(tetrahydro‐3‐furylmethyl) urea in various samples from plants, animal‐derived foods, and the environment. From 80 samples, 70 contained dinotefuran (0.8–11.7 μg/kg), among which six also contained 1‐methyl‐3‐(tetrahydro‐3‐furylmethyl) urea (water and rice, 0.5–0.9 μg/kg).     

Ultrasensitive polyaniline-nickel oxide cladding modified with urease immobilized intrinsic optical fiber urea biosensor

Herein, we report a polyaniline-nickel oxide (PANI-NiO) nanocomposite as an efficient immobilization matrix for development the optical fiber urea biosensor. Optical fiber sensing probe was developed by removing some portion of optical fiber at middle and modified with PANI-NiO matrix. After the modification of cladding removed portion, it was immobilized with enzyme urease via glutaraldehyde as a bi-functional cross-linking agent. The physicochemical and optical properties of the PANI-NiO matrix were explored by X-ray diffraction, scanning electron microscopy, ultraviolet–visible, and Fourier transform infrared spectroscopic techniques. The characteristic features and performance of the developed sensor were evaluated via recording the output power and modal power distribution by means of a charge-coupled device camera. The developed urea biosensor exhibits a selective response towards urea concentrations in the linear range 1 nM–100 mM with a lower detection limit of 1 nM. Sensor recorded as a 40 days stability and response time ~1 min. Thus, the obtained experimental results of the developed sensor promote its applicability with practical prospects in diverse field.     

Screening and Selection of Marine Isolate for l-Glutaminase Production and Media Optimization Using Response Surface Methodology

The current work details the screening of about 400 marine isolates from various marine niches, from which one isolate was finally selected based on the productivity of glutaminase (71.23 U/l). Further, biochemical identification tests and 16S rRNA sequencing identified this isolate to be Providencia sp. This isolate was taken up for further media optimization studies by using one-factor-at-a-time approach and subsequently by response surface methodology. A face centered central composite design was employed to investigate the interactive effects of four variables, viz., concentrations of glucose, methionine, urea, and succinic acid on glutaminase production. A significant influence of urea on glutaminase production was noted. Response surface methodology showed that a medium containing (g/l) glucose 10.0, urea 5.15, methionine 3.5, succinic acid 6.0, ammonium sulfate 2.5, and yeast extract 6.0 to be optimum for the production of glutaminase. The applied methodology was validated using this optimized media and enzyme activity 119 ± 0.12 U/l and specific activity of 0.63 U/mg protein after 28 h of incubation at 25 °C was obtained.     

Thermal properties and interpartical interactions of <Emphasis Type="SmallCaps">l</Emphasis>-proline,glycine, and <Emphasis Type="SmallCaps">l</Emphasis>-alanine in aqueous urea solutions at 288–318 K

The enthalpies of solution of l-proline have been measured in aqueous urea solutions at 0–6 mol urea kg⁻¹ water at 288.15, 298.15, 308.15, and 318.15 K by the calorimetric method. The two-parameter relation connecting the values of solution enthalpies of proline with urea concentration and temperature has been obtained. The enthalpy and heat capacity parameters of pair interaction of l-proline with urea in water have been computed. Using the thermodynamic relations, the temperature changes of reduced enthalpy, and also the change of entropy and reduced Gibbs energy of solution of l-proline in aqueous solutions of urea at the temperature rise from 288 to 318 K have been determined. Their comparison with the data for glycine and l-alanine has been carried out. It has been shown that the entropy–enthalpy compensation (Barclay–Butler rule) takes place for dissolution and transfer processes.     

Simultaneous quantification of N‐butylthiophosphoric triamide and dicyandiamide in urea formulation by liquid chromatography with tandem mass spectrometry

A new liquid chromatography with tandem mass spectrometry method employing a mixed‐mode zwitterionic stationary phase was developed for simultaneous determination of urease inhibitor (N‐butylthiophosphoric triamide) and nitrification inhibitor (dicyandiamide) in urea fertilizer. Molecular modeling based on density functional theory calculations was employed to provide an insight into the interaction mechanism of urea, dicyandiamide, and N‐butylthiophosphoric triamide with zwitterionic stationary phase in chromatographic separation system. The detection of analytes was performed on a triple quadrupole tandem mass spectrometer in multiple reaction monitoring mode using positive electrospray ionization. The ion transitions monitored were m/z 85→68 for dicyandiamide and m/z 168.2→74 for N‐butylthiophosphoric triamide, respectively. The standard calibration curves of dicyandiamide and N‐butylthiophosphoric triamide were linear over the range of 1.0 ? 15 ppm (coefficient of determination = 0.9984), 0.05 ? 1 ppm (coefficient of determination = 0.9995), with limit of detection of 25 and 5 ppb, respectively. The recoveries of low, middle, and high concentrations were from 96.7 to 105.8% for N‐butylthiophosphoric triamide and 94.4 to 105.8% for dicyandiamide with accuracy (relative error %) of ≤5.8% and ≤5.8%, the precision (coefficients of variation) was ≤2.0% and ≤2.9%, respectively. The validated method was successfully applied on real urea samples to determine N‐butylthiophosphoric triamide and dicyandiamide simultaneously.     

An amperometric urea biosensor based on covalent immobilization of urease on copolymer of glycidyl methacrylate and vinylferrocene

A unique urea biosensor construction based on the direct covalent attachment of urease onto a polymeric electron transfer mediator, poly(glycidyl methacrylate-co-vinylferrocene)-coated electrode is described. Amperometric response was measured as a function of urea concentration, at a fixed potential of +0.35 V vs. Ag/AgCl in phosphate-buffered saline (pH 7.0). Covalent immobilization of the urease directly to the functionalized ferrocene copolymer surface produced biosensors with a short response time (about 3 s) and provided low detection limits. The stability, reusability, pH, and temperature response of the biosensor, besides its kinetic parameter, were also studied.     

Biofouling of dextran-derivative layers investigated by quartz crystal microbalance

This study reports the fouling of carboxymethyl dextran (CMD) layers in cell culture medium, fibronectin, and serum solutions. CMD layers were covalently immobilized onto amine groups available either on an n-heptylamine plasma polymer (HApp) layer or onto a polyethylenimine (PEI) coating grafted to an acetaldehyde plasma polymer (AApp) layer. The successful immobilization of the graft layers was demonstrated by X-ray photoelectron spectroscopy (XPS). Primary amines on HApp and AApp + PEI surfaces were quantified using a colorimetric assay. Quartz crystal microbalance (QCM) was used to investigate in real-time the fouling of the graft layers upon incubation in cell culture medium (RPMI), fibronectin, and foetal bovine serum (FBS) solutions. HApp, AApp and AApp + PEI layers exhibited large fouling in fibronectin and FBS solutions, while fouling in RPMI cell culture medium was not significant. Protein repellent properties of CMD layers in FBS and fibronectin have been demonstrated compared to the other tested surfaces. QCM has shown that both CMD layers were fouled to a small extent in RPMI medium.