Chungu Xia

Colorimetric detection of melamine based on methanobactin-mediated synthesis of gold nanoparticles

A simple and rapid field-portable colorimetric method for the detection of melamine in liquid milk was reported. Methanobactin (Mb) could reduce Au (III) to Au (0) and mediate the synthesis of gold nanoparticles (Au-NPs). Upon the addition of melamine, melamine interacted with oxazolone ring of Mb, which interrupted the formation of Au-NPs. Melamine could also stimulate the aggregation of formed Au-NPs. In this paper, these characteristics have been used to detect melamine in liquid milk by naked eyes observation with a detection limit of 5.56 × 10(-6)M (0.7 mg/kg). Further, the plasmon absorbance of the formed Au-NPs allowed the quantitative detection of melamine by UV-vis spectrometer. A linear correlation was existed between the absorbance and the melamine concentration ranging from 3.90 × 10(-7)M to 3.97 × 10(-6)M with a correlation coefficient of 0.9685. The detection limit (3σ) obtained by UV-vis spectrum was as low as 2.38 × 10(-7)M (i.e., 0.03 mg/kg).

pH-Responsive N-heterocyclic carbene copper(I) complexes: syntheses and recoverable applications in the carboxylation of arylboronic esters and benzoxazole with carbon dioxide

A pH-controlled monophasic/biphasic switchable system has been developed as a green and novel strategy for homogeneous catalyst recycling, which has been successfully applied to the Cu-NHC-catalyzed carboxylation of organoboronic esters and benzoxazole with carbon dioxide. Additionally, the present strategy could also be extended to the Ag-NHC-catalyzed carboxylation of terminal alkyne. The tertiary amine-functionalized catalysts could be used for at least four times with a slight loss of activity.

Facile preparation of highly-dispersed cobalt- silicon mixed oxide nanosphere and its catalytic application in cyclohexane selective oxidation

Highly dispersed cobalt-silicon mixed oxide [Co-SiO2] nanosphere was successfully prepared with a modified reverse-phase microemulsion method. This material was characterized in detail by X-ray diffraction, transmission electron microscopy, Fourier transform infrared, ultraviolet-visible diffuse reflectance spectra, X-ray absorption spectroscopy near-edge structure, and N2 adsorption-desorption measurements. High valence state cobalt could be easily obtained without calcination, which is fascinating for the catalytic application for its strong oxidation ability. In the selective oxidation of cyclohexane, Co-SiO2 acted as an efficient catalyst, and good activity could be obtained under mild conditions.

The Methane Monooxygenase Intrinsic Activity of Kinds of Methanotrophs

Methanotrophs have promising applications in the epoxidation of some alkenes and some chlorinated hydrocarbons and in the production of a biopolymer, poly-β-hydroxybutyrate (poly-3-hydroxybutyrate; PHB). In contrast with methane monooxygenase (MMO) activity and ability of PHB synthesis of four kinds of methanotrophic bacteria Methylosinus trichosporium OB3b, M. trichosporium IMV3011, Methylococcus capsulatus HD6T, Methylomonas sp. GYJ3, and the mixture of the four kinds of strains, M. trichosporium OB3b is the highest of the four in the activity of propene epoxidation (10.72xa0nmol/min mg dry weight of cell [dwc]), the activity of naphthalene oxidation (22.7xa0mmol/mg dwc), and ability in synthesis of PHB(11% PHB content in per gram dry weight of cell in 84xa0h). It could be feasible to improve the MMO activity by mixing four kinds of methanotrophs. The MMO activity dramatically decreased when the cellular PHB accumulated in the second stage. The reason for this may be the dilution of the MMO system in the cells with increasing PHB contents. It has been found that the PHB contents at the level of 1–5% are beneficial to the cells for maintenance of MMO epoxidation activity when enough PHB have been accumulated. Moreover, it was also found that high particulate methane monooxygenase activity may contribute to the synthesis of PHB in the cell, which could be used to improve the yield of PHB in methanotrophs.

DFT studies of the substituent effects of dimethylamino on non-heme active oxidizing species: iron(V)-oxo species or iron(IV)-oxo acetate aminopyridine cation radical species?

Through the introduction of dimethylamino (Me2N) substituent at the pyridine ring of 2-((R)-2-[(R)-1-(pyridine-2-ylmethyl)pyrrolidin-2-yl]pyrrolidin-1-ylmethyl)pyridine (PDP) ligand, the non-heme FeII(Me2NPDP)/H2O2/AcOH catalyst system was found to exhibit significant higher catalytic activity and enantioselectivity than the non-substituent one in the asymmetric epoxidation experiments. The mechanistic origin of the remarkable substituent effects in these oxidation reactions has not been well established. To ascertain the potent oxidant and the related reaction mechanism, a detailed DFT calculation was performed. Interestingly, a novel Fe(IV)-oxo Me2NPDP cation radical species, [(Me2NPDP)+·FeIV(O)(OAc)]2+ (Me2N5), with about one spin spreading over the non-heme Me2NPDP ligand was formed via a carboxylic-acid-assisted O–O bond heterolysis, which is reminiscent of Compound I (an Fe(IV)(O)(porphyrin cation radical) species) in cytochrome P450 chemistry. Me2N5 is energetically comparable with the cyclic ferric peracetate species Me2N6, while in the pristine Fe(PDP) catalyst system, H6 is more stable than H5. Comparison of the activation energy for the ethylene epoxidation promoted by Me2N5 and Me2N6, Me2N5 is supposed as the true oxidant triggering the epoxidation of olefins. In addition, a systematic research on the substituent effects varied from the electron-donating substituent (dMM, the substituents at sites 3, 4, and 5 of the pyridine ring: methyl, methoxyl, and methyl) to the electron-withdrawing one (CF3, 2,6-bis(trifluoromethyl)phenyl) on the electronic structure of the reaction intermediates has also been investigated. An alternative cyclic ferric peracetate complex is obtained, indicating that the substituents at the pyridine ring of PDP ligands have significant impacts on the electronic structure of the oxidants.

Truncated concave octahedral Cu2O nanocrystals with {hkk} high-index facets for enhanced activity and stability in heterogeneous catalytic azide–alkyne cycloaddition

We report a facile synthesis of Cu2O truncated concave octahedral nanocrystals mainly with {511} high-index facets in a system of oil in water emulsion. In azide–alkyne cycloaddition with water as a green solvent, Cu2O truncated concave octahedra showed greatly enhanced activity with a turnover frequency (TOF) as high as 2413 h−1, which was about two and four times, respectively, that of Cu2O rhombic dodecahedra with {110} facets and cubes with {100} facets. After 10 rounds of recycling experiments, the Cu2O truncated concave octahedra still exhibited good catalytic and structural stability.

Lipase-Catalyzed Synthesis of α-Tocopheryl Ferulate

α-Tocopheryl ferulate (α-TF) can be used as an antioxidant in both lipid containing food and cosmetics. The ability of lipases to catalyze the transesterification between α-tocopherol and ethyl ferulate (EF) was investigated. Only the lipase B from Candida antarctica and the lipase from Candida rugosa catalyzed the transesterification. The influences of reaction condition on the yield of α-TF were investigated. A yield of 25.2% of α-TF was obtained using 5:1 mole ratio of α-tocopherol to EF in solvent-free medium at 72h. The yield of α-TF was further enhanced by applying vacuum-rotary evaporation procedure to remove the co-product ethanol.

Improved biological synthesis of gold nanoparticles using methanobactin

The present study reported a novel approach for the green synthesis of gold nanoparticles (AuNPs) by using methanobactin (Mb) from the spent medium of Methylosinus trichosporium 3011. This paper explored the synthesis of AuNPs under different Mb concentrations, temperatures, and reaction times. The biosynthesized AuNPs were characterized by using UV–Visible spectroscopy, fluorescence spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). Comparing with the previous research using hydroquinone as an extra electron donor to synthesis of AuNPs, this research was the first to show that rising temperature improved the formation rate of AuNPs in a higher concentration of Mb itself. The formation of AuNPs was confirmed by surface plasmon resonance using UV–Visible spectrophotometry. Fluorescence spectroscopy experiments revealed that Mb sample showed three prominent peaks. In addition, the fluorescence exhibited by Mb was rapidly quenched upon addition of HAuCl4. According to the image analysis performed on a TEM micrograph, the obtained AuNPs were predominantly spherical with an average size of 19.3xa0±xa05.5xa0nm. XPS analysis demonstrated that Mb molecules reduced Au(III) to Au(0). FTIR spectrum confirmed the involvement of biomass groups in capping and reduction of AuNPs, which increased the stability of the synthesized AuNPs.

Direct Electrochemistry of Methanobactin Functionalized Gold Nanoparticles on Au Electrode

Mathanobatins (Mb, Mbtins) were immobilized successfully on nanometer-sized gold colloid particles associated with β-mercaptoethylamine. The structures of Mb functionalized gold nanoparticles were characterized and confirmed by UV-vis spectroscopy (UV-vis), FTIR spectra and electrochemical analyses. Direct electron transfer between Mb or copper-loading Mbtins and the modified electrode was investigated without the aid of any electron mediator. The copper-loading Mbtins act as a better electrocatalyst for the reduction of H2O2 than Mb. The copper-loading Mb, with which gold nanoparticles were functionalized, as a model enzyme, was immobilized on gold electrode to construct a novel H2O2 biosensor. In pH 6.4 phosphate buffer solution, the reduction and oxidation peak potentials of Mb functionalized gold nanoparticles modified Au electrode (copper-loading Mbtins) were 0.115 and 0.222 V. On the surface, capacitance per unite area (Cd) of Mb functionalized gold nanoparticles modified electrode were 38 μF cm-2. The immobilized Mb displayed the features of a peroxidase and gave an excellent electrocatalytic response to the reduction of H2O2. The detection limit of Mb functionalized gold nanoparticles (copper-loading) were 09 × 10-5 mA/M (S/N = 3). The Michaelis-Menten constant (Km) was 0.787 mM. Good stability and sensitivity were assessed for the biosensor.

Propylene Carbonate as a Green Solvent for Kinetic Resolution of Secondary Alcohols Catalyzed by Candida antarctica Lipase

The kinetic resolution of secondary alcohols catalyzed by Candida antarctica lipase was studied using vinyl acetate as acyl donor in propylene carbonate. The effects of the reaction conditions included solvent, temperature, acyl donor and organic base. In the reactions, the enantioselectivity of Candida antarctica lipase increased with increasing the reaction time and the addition of triethylamine. Furthermore, it was shown that the addition of non-reactive base triethylamine to the reaction mixture enhanced the rate of the reaction. Propylene carbonate offers an environmentally friendly alternative in contrast to conventional solvents.