Comparison of membrane- and cloth-cathode assembly for scalable microbial fuel cells: Construction,performance and cost

The aim of this study is to compare the performance of different membrane cathode assembly (MCA) and cloth-cathode assembly (CCA) in air-chamber microbial fuel cells (MFCs) and provide an optimum cathode configuration for MFC scaling up. Two MCAs were prepared by hot-pressing carbon cloth containing cathodic catalyst to anion exchange membrane (AEM) and cation exchange membrane (CEM), respectively. A CCA was built by coating GORE-TEX^® cloth with a mixture of nickel-based conductive paint and cathodic catalyst. Under the fed-batch mode using brewery wastewater, the MFCs were compared with respect to power production, coulombic efficiency, COD removal, internal resistance and material cost. The experimental results show that CCA is a more favorable alternative than MCAs due to its easier preparation, higher maximum power density and COD removal, and lower internal resistance and cost. The optimum cathode assembly of CCA is cost-effective and mechanically robust enough to meet the important requirements for MFC scalability.     

Improving phosphate buffer-free cathode performance of microbial fuel cell based on biological nitrification

To reduce the amount of phosphate buffer currently used in Microbial Fuel Cell's (MFC's), we investigated the role of biological nitrification at the cathode in the absence of phosphate buffer. The addition of a nitrifying mixed consortia (NMC) to the cathode compartment and increasing ammonium concentration in the catholyte resulted in an increase of cell voltage from 0.3 V to 0.567 V (external resistance of 100 Ω) and a decrease of catholyte pH from 8.8 to 7.05. A large fraction of ammonium was oxidized to nitrite, as indicated by an increase of nitrate-nitrogen (NO₃⁻–N). An MFC inoculated with an NMC and supplied with 94.2 mgN/l ammonium to the catholyte could generate a maximum power of 2.1 ± 0.14 mW (10.94 ± 0.73 W/m³). This compared favorably to an MFC supplied with either buffered or non-buffered solution. The buffer-free NMC inoculated cathodic chamber showed the smallest polarization resistance, suggesting that nitrification resulted in improved cathode performance. The improved performances of the phosphate buffer-free cathode and cell are positively related to biological nitrification, in which we suggest additional protons produced from ammonium oxidation facilitated electrochemical reduction of oxygen at cathode.     

Quantification of hydrogen peroxide and glucose using 3-methyl-2-benzothiazolinonehydrazone hydrochloride with 10,11-dihydro-5H-benz(b,f)azepine as chromogenic probe

A sensitive, selective, and rapid enzymatic method is proposed for the quantification of hydrogen peroxide (H₂O₂) using 3-methyl-2-benzothiazolinonehydrazone hydrochloride (MBTH) and 10,11-dihydro-5H-benz(b,f)azepine (DBZ) as chromogenic cosubstrates catalyzed by horseradish peroxidase (HRP) enzyme. MBTH traps free radical released during oxidation of H₂O₂ by HRP and gets oxidized to electrophilic cation, which couples with DBZ to give an intense blue-colored product with maximum absorbance at 620 nm. The linear response for H₂O₂ is found between 5 × 10⁻⁶ and 45 × 10⁻⁶ mol L⁻¹ at pH 4.0 and a temperature of 25 °C. Catalytic efficiency and catalytic power of the commercial peroxidase were found to be 0.415 × 10⁶ M⁻¹ min⁻¹ and 9.81 × 10⁻⁴ min⁻¹, respectively. The catalytic constant (k_cat) and specificity constant (k_cat/K_m) at saturated concentration of the cosubstrates were 163.2 min⁻¹ and 4.156 × 10⁶ L mol⁻¹ min⁻¹, respectively. This method can be incorporated into biochemical analysis where H₂O₂ undergoes catalytic oxidation by oxidase. Its applicability in the biological samples was tested for glucose quantification in human serum.     

Neuropeptide glutamic-isoleucine (NEI) specifically stimulates the secretory activity of gonadotrophs in primary cultures of female rat pituitary cells

The neuropeptide EI (NEI) is derived from proMCH. It activates GnRH neurons, and has been shown to stimulate the LH release following intracerebroventricular administration in several experimental models. The aim of the present paper was to evaluate NEI actions on pituitary hormone secretion and cell morphology in vitro. Pituitary cells from female rats were treated with NEI for a wide range of concentrations (1–400 × 10⁻⁸ M) and time periods (1–5 h). The media were collected and LH, FSH, PRL, and GH measured by RIA. The interaction between NEI (1, 10 and 100 × 10⁻⁸ M) and GnRH (0.1 and 1 × 10⁻⁹ M) was also tested. Pituitary cells were harvested for electron microscopy, and the immunogold immunocytochemistry of LH was assayed after 2 and 4 h of NEI incubation. NEI (100 × 10⁻⁸ M) induced a significant LH secretion after 2 h of stimulus, reaching a maximum response 4 h later. A rapid and remarkable LH release was induced by NEI (400 × 10⁻⁸ M) 1 h after stimulus, attaining its highest level at 2 h. However, PRL, GH and FSH were not affected. NEI provoked ultrastructural changes in the gonadotrophs, which showed accumulations of LH-immunoreactive granules near the plasma membrane and exocytotic images, while the other populations exhibited no changes. Although NEI (10 × 10⁻⁸ M), caused no action when used alone, its co-incubation with GnRH (1 × 10⁻⁹ M), promoted a slight but significant increase in LH. These results demonstrate that NEI acts at the pituitary level through a direct action on gonadotrophs, as well as through interaction with GnRH.     

Graft copolymerization of N-vinylformamide onto sodium carboxymethylcellulose and study of its swelling, metal ion sorption and flocculation behaviour

The present paper reports the graft copolymerization of N-vinylformamide onto sodium carboxymethylcellulose by free radical polymerization using potassium peroxymonosulphate/thiourea redox system in an inert atmosphere. The reaction conditions for maximum grafting have been optimized by varying the reaction variables, including the concentration of N-vinylformamide (12.0 × 10⁻²–28.0 × 10⁻² mol dm⁻³), potassium peroxymonosulphate (4.0 × 10⁻³–12.0 × 10⁻³ mol dm⁻³), thiourea (1.2 × 10⁻³–4.4 × 10⁻³ mol dm⁻³), sulphuric acid (2.0 × 10⁻³–10.0 × 10⁻³ mol dm⁻³), sodium carboxymethylcellulose (0.2–1.8 g dm⁻³) along with time duration (60–180 min) and temperature (25–45° C). Water swelling capacity, metal ion sorption and flocculation studies of synthesized graft copolymer have been performed with respect to the parent polymer. The graft copolymer has been characterized by FTIR spectroscopy and thermogravimetric analysis.     

Laccase-catalyzed oxidation of phenolic compounds in organic media

Rhus vernificera laccase-catalyzed oxidation of phenolic compounds, i.e., (+)-catechin, (−)-epicatechin and catechol, was carried out in selected organic solvents to search for the favorable reaction medium. The investigation on reaction parameters showed that optimal laccase activity was obtained in hexane at 30 °C, pH 7.75 for the oxidation of (+)-catechin as well as for (−)-epicatechin, and in toluene at 35 °C, pH 7.25 for the oxidation of catechol. E_a and Q₁₀ values of the biocatalysis in the reaction media of the larger log p solvents like isooctane and hexane were relatively higher than those in the reaction media of lower log p solvents like toluene and dichloromethane. Maximum laccase activity in the organic media was found with 6.5% of buffer as co-solvent. A wider range of 0–28 μg protein/ml in hexane than that of 0–16.7 μg protein/ml in aqueous medium was observed for the linear increasing conversion of (+)-catechin. The kinetic studies revealed that in the presence of isooctane, hexane, toluene and dichloromethane, the K_m values were 0.77, 0.97, 0.53 and 2.9 mmol/L for the substrate of (+)-catechin; 0.43, 0.34, 0.14 and 3.4 mmol/L for (−)-epicatechin; 2.9, 1.8, 0.61 and 1.1 mmol/L for catechol, respectively, while the corresponding V_max values were 2.1 × 10⁻², 2.3 × 10⁻², 0.65 × 10⁻² and 0.71 × 10⁻² δA/μg protein min); 1.8 × 10⁻², 0.88 × 10⁻², 0.19 × 10⁻² and 1.0 × 10⁻² δA/μg protein min); 0.48 × 10⁻², 0.59 × 10⁻², 0.67 × 10⁻² and 0.54 × 10⁻² δA/μg protein min), respectively. FT-IR indicated the formation of probable dimer from (+)-catechin in organic solvent. These results suggest that this laccase has higher catalytic oxidation capacity of phenolic compounds in suitable organic media and favorite oligomers could be obtained.     

Polycrystalline bismuth oxide films for development of amperometric biosensor for phenolic compounds

An attractive biocomposite based on polycrystalline bismuth oxide (BiO_x) film and polyphenol oxidase (PPO) was proposed for the construction of a mediator-free amperometric biosensor for phenolic compounds in environmental water samples. The phenolic biosensor could be easily achieved by casting the biocomposite on the surface of glassy carbon electrode (GCE) via the cross-linking step by glutaraldehyde. The laboratory-prepared bismuth oxide semiconductor was polymorphism. Its hydrophilicity provided a favorable microenvironment for retaining the biological activity of the immobilized protein. The parameters of the fabrication process and the various experimental variables for the enzyme electrode were optimized. The proposed PPO/BiO_x biosensor provided a linear response to catechol over a concentration range of 4 × 10⁻⁹ M to 1.5 × 10⁻⁵ M with a dramatically developed sensitivity of 11.3 A M⁻¹ cm⁻² and a detection limit of 1 × 10⁻⁹ M based on S/N = 3. In addition, the PPO/BiO_x biocomposite was characterized by scanning electron microscope (SEM), Fourier transform infrared spectra (FTIR) and rotating disk electrode voltammetry.     

Granular activated carbon single-chamber microbial fuel cells (GAC-SCMFCs): A design suitable for large-scale wastewater treatment processes

As an emerging biotechnology capable of removing contaminants and producing electricity, microbial fuel cells (MFCs) hold a promising future in wastewater treatment. However, several main problems, including the high internal resistance (R_in), low power output, expensive material, and complicated configuration have severely hindered the large-scale application of MFCs. The study targeted these challenges by developing a novel MFC system, granular activated carbon single-chamber MFC, termed as GAC-SCMFC. The batch tests showed that GAC was a good substitute for carbon cloth and GAC-SCMFCs generated high and stable power outputs compared with the traditional two-chamber MFCs (2CMFCs). Critical operational parameters (i.e. wastewater substrate concentrations, GAC amount, electrode distance) affecting the performance of GAC-SCMFCs were examined at different levels. The results showed that the R_in gradually decreased from 60 Ω to 45 Ω and the power output increased from 0.2 W/m³ to 1.2 W/m³ when the substrate concentrations increased from 100 mg/L to 850 mg/L. However, at high concentrations of 1000–1500 mg/L, the power output leveled off. The R_in of MFCs decreased 50% when the electrode distance was reduced from 7.5 cm to 1 cm. The highest power was achieved at the electrode distance of 2 cm. The power generation increased with more GAC being added in MFCs due to the higher amount of biomass attached. Finally, the multi-anode GAC-SCMFCs were developed to effectively collect the electrons generated in the GAC bed. The results showed that the current was split among the multiple anodes, and the cathode was the limiting factor in the power production of GAC-SCMFCs.     

Inhibition of prostagland in E2-induced cyclic AMP accumulation in the rat anterior pituitary by II-deoxyprostaglandin E analogs (9-ketoprostynoic acids)

The effects of various II-deoxyprostaglandin E analogs on the basal and prostaglandin E₂ (PGE₂)-induced cyclic AMP accumulation in the rat anterior pituitary were studied in vitro. 13-Hydroxy-9-oxoprost-14-ynoic acid at 5 × 10⁻⁴M, but not 5 × 10⁻⁵M, decreased (45%) the induced accumulation and did not alter the basal accumulation; 15-hydroxy-9-oxoprost-13-ynoic acid at 5 × 10⁻⁴M caused less of a decrease (29%) in the induced and also did not alter the basal accumulation. (14Z)-13-Hydroxy-9-oxoprost-14-enoic acid at 5 × 10⁻⁴M did not alter the induced and caused a slight increase (5 fold) in the basal accumulation. 7-Oxa-13-prostynoic acid increased slightly the basal accumulation at 5 × 10⁻⁵M (2 fold) and 2.33 × 10⁻⁴M (6 fold) and did not antagonize the induced accumulation. Thus, the 9-ketoprostynoic acids are effective PGE₂ antagonists in this system.     

Inhibition of prostaglandin E2-induced cyclic AMP accumulation in the rat anterior pituitary by 11-deoxyprostaglandin E analogs (9-ketoprostynoic acids)

The effects of various 11-deoxyprostaglandin E analogs on the basal and prostaglandin E₂ (PGE₂)-induced cyclic AMP accumulation in the rat anterior pitutiary were studied . 13-Hydroxy-9-oxoprost-14-ynoic acid at 5 × 10⁻⁴M, but not 5 × 10⁻⁵M, decreased (45%) the induced accumulation and did not alter the basal accumulation; 15-hydroxy-9-oxoprost-13-ynoic acid at 5 × 10⁻⁴M caused less of a decrease (29%) in the induced and also did not alter the basal accumulation. (14Z)-13-Hydroxy-9-oxoprost-14-enoic acid at 5 × 10⁻⁴M did not alter the induced and caused a slight increase (5 fold) in the basal accumulation. 7-Oxa-13-prostynoic acid increased slightly the basal accumulation at 5 × 10⁻⁵M (2 fold) and 2.33 × 10⁻⁴M (6 fold) and did not antagonize the induced accumulation. Thus, the 9-ketoprostynoic acids are effective PGE₂ antagonists in this system.     

Hybridization biosensor using 2-nitroacridone as electrochemical indicator for detection of short DNA species of Chronic Myelogenous Leukemia

A new acridone derivative 2-nitroacridone (NAD) was synthesized in this paper, and it was found that NAD had excellent electrochemical activity on the glassy carbon electrode (GCE) with a couple reversible redox peaks at 0.051 V and 0.103 V, respectively. Voltammetry was used to investigate the electrochemical behavior of NAD and the interaction between NAD and salmon sperm DNA. In pH 4.0 phosphate buffer solution, the binding ratio between NAD and salmon sperm DNA was calculated to be 2:1 and the binding constant was 3.19 × 10⁵ L/mol. A Chronic Myelogenous Leukemia (CML, Type b₃a₂) DNA biosensor was developed by immobilizing covalently single-stranded CML DNA fragments to a modified GCE. The surface hybridization of the immobilized single-stranded CML DNA fragment with its complementary DNA fragment was evidenced by electrochemical methods using NAD as a novel electrochemical indicator, with a detection limit of 6.7 × 10⁻⁹ M and a linear response range of 1.8 × 10⁻⁸ M to 9.1 × 10⁻⁸ M for CML DNA. Selective determination of complementary ssDNA was achieved using differential pulse voltammetry (DPV).     

A comparison between freezing methods for the cryopreservation of stallion spermatozoa

The effects of sperm freezing concentration (40 × 10⁶ mL⁻¹ vs. 400 × 10⁶ mL⁻¹), straw size (0.25 mL vs. 0.5 mL) and freezing method (liquid nitrogen vapour in a Styrofoam^® box vs. programmable freezing machine) were evaluated in a 2 × 2 × 2 factorial experimental design using 3 split ejaculates from each of 4 stallions. Immediately after thawing, the total motility and forward progressive motility of spermatozoa frozen at a concentration of 40 × 10⁶ mL⁻¹ was higher than for spermatozoa frozen at 400 × 10⁶ mL⁻¹. No significant differences were observed in the semen parameters assessed after cryopreservation in either 0.25 or 0.5 mL straws. However, the programmable freezer provided a more consistent and reliable freezing rate than liquid nitrogen vapour. We conclude that an effective protocol for the cryopreservation of stallion spermatozoa at low concentrations would include concentrations of 40 × 10⁶ mL⁻¹ in 0.25 mL straws using a programmable freezer. This freezing protocol would be suitable for emerging sperm technologies such as sex-preselection of stallion spermatozoa as the sorting process yields only low numbers of spermatozoa in a small volume available for either immediate insemination or cryopreservation.     

Peroxidase catalyzed phenolic compounds oxidation in presence of surfactant Dynol 604: A kinetic investigation

The kinetics of fungal peroxidase-catalyzed phenolic compounds (PCs) oxidation was investigated in presence of acetylenic-based surfactant Dynol 604 at pH 5.5 and 25 °C. It was shown that the presence of ppm concentrations of surfactant did not influence initial rate of PCs oxidation. The calculated apparent bimolecular rate constants were (1.8 ± 0.2) × 10⁵ M⁻¹ s⁻¹, (1.4 ± 0.4) × 10⁷ M⁻¹ s⁻¹, (1.30 ± 0.06) × 10⁷ M⁻¹ s⁻¹ and 1.1 × 10⁸ M⁻¹ s⁻¹ for phenol, 1-naphthol, 2-naphthol and 1-hydroxypyrene, respectively.During an extensive substrates conversion Dynol 604 showed diverse action for different PCs. The oxidation of phenol practically did not change, whereas the surfactant enhanced the conversion of 1- and 2-naphthol and 1-hydroxypyrene in dose response manner. The results accounted by a scheme, which contains a stadium of enzyme inhibition by oligomeric PC oxidation products. The action of the surfactant was explained by avoidance the enzyme active center clothing with the oligomers. The results acquired demonstrate a remarkable increase of substrates conversion in the presence of Dynol 604.     

Air-cathode structure optimization in separator-coupled microbial fuel cells

Microbial fuel cells (MFC) with 30% wet-proofed air cathodes have previously been optimized to have 4 diffusion layers (DLs) in order to limit oxygen transfer into the anode chamber and optimize performance. Newer MFC designs that allow close electrode spacing have a separator that can also reduce oxygen transfer into the anode chamber, and there are many types of carbon wet-proofed materials available. Additional analysis of conditions that optimize performance is therefore needed for separator-coupled MFCs in terms of the number of DLs and the percent of wet proofing used for the cathode. The number of DLs on a 50% wet-proofed carbon cloth cathode significantly affected MFC performance, with the maximum power density decreasing from 1427 to 855 mW/m² for 1–4 DLs. A commonly used cathode (30% wet-proofed, 4 DLs) produced a maximum power density (988 mW/m²) that was 31% less than that produced by the 50% wet-proofed cathode (1 DL). It was shown that the cathode performance with different materials and numbers of DLs was directly related to conditions that increased oxygen transfer. The coulombic efficiency (CE) was more affected by the current density than the oxygen transfer coefficient for the cathode. MFCs with the 50% wet-proofed cathode (2 DLs) had a CE of >84% (6.8 A/m²), which was substantially larger than that previously obtained using carbon cloth air-cathodes lacking separators. These results demonstrate that MFCs constructed with separators should have the minimum number of DLs that prevent water leakage and maximize oxygen transfer to the cathode.     

Using porphyritic andesite as a new additive for improving hydrolysis and acidogenesis of solid organic wastes

The effects of porphyritic andesite on the hydrolysis and acidogenesis of solid organic wastes were investigated by batch and continuous experiments using a rotational drum fermentation system. The results of the batch experiment show that if porphyritic andesite (1%, 3%, and 5% reactants) is added initially, the pH level increases and hydrolysis and acidogenesis are accelerated. The highest surface based hydrolysis constant (26.4 × 10⁻³ kg m⁻² d⁻¹) and volatile solid degradation ratio (43.3%) were obtained at a 1% porphyritic andesite addition. In the continuous experiment, porphyritic andesite elevated the first order hydrolysis constant from 13.10 × 10⁻³ d⁻¹ to 18.82 × 10⁻³ d⁻¹. A particle mean diameter reduction rate of 33.05 μm/d and a volatile solid degradation rate of 3.53 g/L d⁻¹ were obtained under the hydraulic retention time of 4, 8, 12 and 16 d.     

Fatty acid transport and activation and the expression patterns of genes involved in fatty acid trafficking

These studies defined the expression patterns of genes involved in fatty acid transport, activation and trafficking using quantitative PCR (qPCR) and established the kinetic constants of fatty acid transport in an effort to define whether vectorial acylation represents a common mechanism in different cell types (3T3-L1 fibroblasts and adipocytes, Caco-2 and HepG2 cells and three endothelial cell lines (b-END3, HAEC, and HMEC)). As expected, fatty acid transport protein (FATP)1 and long-chain acyl CoA synthetase (Acsl)1 were the predominant isoforms expressed in adipocytes consistent with their roles in the transport and activation of exogenous fatty acids destined for storage in the form of triglycerides. In cells involved in fatty acid processing including Caco-2 (intestinal-like) and HepG2 (liver-like), FATP2 was the predominant isoform. The patterns of Acsl expression were distinct between these two cell types with Acsl3 and Acsl5 being predominant in Caco-2 cells and Acsl4 in HepG2 cells. In the endothelial lines, FATP1 and FATP4 were the most highly expressed isoforms; the expression patterns for the different Acsl isoforms were highly variable between the different endothelial cell lines. The transport of the fluorescent long-chain fatty acid C₁-BODIPY-C₁₂ in 3T3-L1 fibroblasts and 3T3-L1 adipocytes followed typical Michaelis–Menten kinetics; the apparent efficiency (k_cat/K_T) of this process increases over 2-fold (2.1 × 10⁶–4.5 × 10⁶ s⁻¹ M⁻¹) upon adipocyte differentiation. The V_max values for fatty acid transport in Caco-2 and HepG2 cells were essentially the same, yet the efficiency was 55% higher in Caco-2 cells (2.3 × 10⁶ s⁻¹ M⁻¹ versus 1.5 × 10⁶ s⁻¹ M⁻¹). The kinetic parameters for fatty acid transport in three endothelial cell types demonstrated they were the least efficient cell types for this process giving V_max values that were nearly 4-fold lower than those defined form 3T3-L1 adipocytes, Caco-2 cells and HepG2 cells. The same cells had reduced efficiency for fatty acid transport (ranging from 0.82 × 10⁶ s⁻¹ M⁻¹ to 1.35 × 10⁶ s⁻¹ M⁻¹).     

A nano-Ni based ultrasensitive nonenzymatic electrochemical sensor for glucose: Enhancing sensitivity through a nanowire array strategy

Highly ordered Ni nanowire arrays (NiNWAs) were synthesized for the first time using a template-directed electropolymerization strategy with a nanopore polycarbonate (PC) membrane template, and their morphological characterization were examined by scanning electron microscopy (SEM) and transmission electron microscope (TEM). A NiNWAs based electrode shows very high electrochemical activity for electrocatalytic oxidation of glucose in alkaline medium, which has been utilized as the basis of the fabrication of a nonenzymatic biosensor for electrochemical detection of glucose. The biosensor can be applied to the quantification of glucose with a linear range covering from 5.0 × 10⁻⁷ to 7.0 × 10⁻³ M, a high sensitivity of 1043 μA mM⁻¹ cm⁻², and a low detection limit of 1 × 10⁻⁷ M. The experiment results also showed that the sensor exhibits good reproducibility and long-term stability, as well as high selectivity with no interference from other oxidable species.     

Synthesis, characterization and applications of graft copolymer (Chitosan-g-N,N-dimethylacrylamide)

An unreported graft copolymer of N,N-dimethylacrylamide (DMA) with chitosan has been synthesized under nitrogen atmosphere using peroxymonosulphate/mandelic acid redox pair. The effect of reaction conditions on grafting parameters i.e. grafting ratio, efficiency, conversion, add on and homopolymer has been studied. Experimental results show that maximum grafting has been obtained at 1.0 g dm⁻³ concentration of chitosan, 30 × 10⁻² mol dm⁻³ concentration of N,N-dimethylacrylamide and 7.0 × 10⁻³ mol dm⁻³ concentration of hydrogen ion. It has also been observed that grafting ratio, add on, conversion and efficiency increase upto 3.2 × 10⁻³ mol dm⁻³ of mandelic acid, 12.0 × 10⁻³ mol dm⁻³ of potassium peroxymonosulphate, 150 min of time and 40 °C of temperature. Grafted polymer has been characterized by FTIR spectroscopy and thermogravimetric analysis. Water swelling capacity of chitosan-g-N,N-dimethylacrylamide has been determined. It has been observed that the graft copolymer is thermally more stable than parent backbone.     

Xanthine oxidase/laponite nanoparticles immobilized on glassy carbon electrode: Direct electron transfer and multielectrocatalysis

In this work, colloidal laponite nanoparticles were further expanded into the design of the third-generation biosensor. Direct electrochemistry of the complex molybdoenzyme xanthine oxidase (XnOx) immobilized on glassy carbon electrode (GCE) by laponite nanoparticles was investigated for the first time. XnOx/laponite thin film modified electrode showed only one pair of well defined and reversible cyclic voltammetric peaks attributed to XnOx–FAD cofactor at about −0.370 V vs. SCE (pH 5). The formal potential of XnOx–FAD/FADH₂ couple varied linearly with the increase of pH in the range of 4.0–8.0 with a slope of −54.3 mV pH⁻¹, which indicated that two-proton transfer was accompanied with two-electron transfer in the electrochemical reaction. More interestingly, the immobilized XnOx retained its biological activity well and displayed an excellent electrocatalytic performance to both the oxidation of xanthine and the reduction of nitrate. The electrocatalytic response showed a linear dependence on the xanthine concentration ranging from 3.9 × 10⁻⁸ to 2.1 × 10⁻⁵ M with a detection limit of 1.0 × 10⁻⁸ M based on S/N = 3.     

A sugar-aza-crown ether-based fluorescent sensor for Hg and Cu

A fluorescent sensor, 5, based upon the sugar-aza-crown ether structure with two anthracenetriazolymethyl groups was prepared and its fluoroionophoric properties toward transition metal ions were investigated. In methanol, the sensor exhibits highly selective recognition of Cu²⁺ and Hg²⁺ ions among a series of tested metal ions. The association constant for 5·Cu²⁺ and 5·Hg²⁺ in methanol was calculated to be 4.0 × 10⁵ M⁻¹ and 1.1 × 10⁵ M⁻¹, respectively. The detection limits for the sensing of Cu²⁺ and Hg²⁺ ions were 1.39 × 10⁻⁶ M and 1.39 × 10⁻⁵ M, respectively.