Selective determination of chloramphenicol at trace level in milk samples by the electrode modified with molecularly imprinted polymer

A novel voltammetric sensor for determination of chloramphenicol (CAP) in milk samples was introduced. The CAP selective molecularly imprinted polymer (MIP) and non imprinted polymer (NIP) were synthesized and then added to the carbon paste (CP) electrode composition in order to prepare MIP-CP electrode. The MIP, embedded in the carbon paste electrode, acted as the selective recognition element and pre-concentrator agent for CAP. CAP was extracted in the electrode for a definite time and then it was analyzed by differential pulse voltametry, found to be an effective determination method. The MIP-CP showed very high CAP recognition ability, compared to NIP-CP. The electrode washing, after CAP extraction, led to an enhanced selectivity. Various factors, known to affect the response behavior of the electrode, were investigated and optimized. This sensor showed a linear response range of 8.0 × 10⁻⁹−1.0 × 10⁻⁶ M and lower detection limit of 2.0 × 10⁻⁹ M (S/N = 3). The sensor was successfully applied to the determination of CAP in milk samples.     

A novel nanosensor based on Pt:Co nanoalloy ionic liquid carbon paste electrode for voltammetric determination of vitamin B9 in food samples

In this work we describe a novel nanoalloy (Pt:Co) room temperature ionic liquid (RTIL) modified carbon paste electrode as a high sensitive sensor for voltammetric determination of vitamin B₉ in food samples. The sensor exhibits an enhanced effectiveness for the electro-oxidation of vitamin B₉ in aqueous solution. The oxidation peak potential for this matter at a surface of the ionic liquid (n-hexyl-3-methylimidazolium hexafluoro phosphate) Pt:Co carbon paste electrode (Pt:Co/IL/CPE) appeared at 685 mV that was about 110 mV lower than the oxidation peak potential at the surface of the traditional carbon paste electrode (CPE) under similar condition. The mechanism of the electro-oxidation process on the surface of the modified electrode was analyzed. Square wave voltammetry (SWV) was applied as a very sensitive electrochemical method for the determination of sub-micro-molar amounts of vitamin B₉. The linear response range and detection limit were found to be 1.0 × 10⁻⁷ to 5.0 × 10⁻⁴ M and 4.0 × 10⁻⁸ M, respectively. The prepared modified electrode shows several advantages such as simple preparation method, high stability, high sensitivity, and excellent catalytic activity, long-term stability and remarkable voltammetric reproducibility for eletrooxidation of vitamin B₉. The proposed sensor was successfully applied for the determination of vitamin B₉ in food samples.     

Sensitive and Selective Determination of Riboflavin in Food and Pharmaceutical Samples Using Manganese (III) Tetraphenylporphyrin Modified Carbon Paste Electrode

A chemically modified electrode was constructed by incorporating manganese (III) tetraphenyl porphyrine into a carbon paste matrix. The modified electrode was used as a sensitive electrochemical sensor for measuring of riboflavin. The constructed electrode exhibited catalytic properties for the electro-oxidation of riboflavin and lowered the over potential for the oxidation of this compound; consequently, the corresponding peak currents of riboflavin increased significantly. The modified electrode showed a near-Nernstian behavior for electro-oxidation of riboflavin hence, it could be a suitable voltammetric sensor for the fast and easy determination of riboflavin. A linear response in concentration range 1.0 × 10^?8 – 1.0 × 10^?5 M was obtained with a detection limit of 8.0 × 10^?9 M (S /n = 3) for the determination of riboflavin. The electrode showed long-term stability and the standard deviation of the slope obtained after repeated calibration during a period of 3 months was 3.5% (n = 10). The modified electrode was used for differential pulse voltammetric determination of riboflavin in pharmaceutical and food samples.     

Potentiometric Determination of Aluminum in Foods, Pharmaceuticals, and Alloys by AlMCM-41-Modified Carbon Paste Electrode

A new chemically modified electrode is constructed by incorporating AlMCM-41 into carbon paste matrix (AlMCM-41-MCPE) and used as a sensitive sensor for detection of aluminum in aqueous and nonaqueous solutions. The rapid exchange kinetics in the membrane results in a near-Nernstian behavior of the modified electrode and makes it a suitable potentiometric sensor for detection of aluminum. A linear response in concentration range from 1.0?×?10^?6 to 1.0?×?10^?1 mol/L (0.027 μg/mL–2.7 mg/mL) was obtained with a detection limit of 4.6?×?10^?7 mol/L for the potentiometric detection of aluminum. Selectivity coefficients of a number of interfering cations have been estimated. The interference from many of the investigated ions is negligible. The AlMCM-41-MCPE is suitable for use in aqueous solution of pH 2–6 and in partially nonaqueous medium. The modified electrode exhibited a fast response time (~8 s), good stability, and an extended lifetime. The developed sensor was used successfully for the determination of Al³⁺ in some alloys, drugs, and food products.     

Application of ZnO/CNTs Nanocomposite Ionic Liquid Paste Electrode as a Sensitive Voltammetric Sensor for Determination of Ascorbic Acid in Food Samples

In this study, a simple and rapid analytical method development for ascorbic acid (AA) determination in food samples by using differential pulse voltammetry (DPV) method on ZnO/CNTs nanocomposite ionic liquid modified carbon paste electrode. For this, several parameters, such as ZnO/CNTs nanocomposite, ionic liquid ratio, and pH, have been studied. The cyclic voltammogram showed an irreversible oxidation peak at 0.61 V (vs. Ag/AgCl_sat), which corresponded to the oxidation of AA. Compared to common carbon paste electrode, the electrochemical response was greatly improved. Under the optimized conditions, the oxidation peak current of AA showed linear dynamic range 0.1–450 μmol l^?1 with a detection limit of 0.07 μmol l^?1, using the DPV method. The proposed sensor was successfully applied to the determination of AA in fresh vegetable juice, fruit juices and food supplement samples without previous preparation and was compared with a published electrochemical method.     

SWCNT-modified carbon paste electrode as an electrochemical sensor for histamine determination in alcoholic beverages

In this work, a simple and rapid electrochemical method is presented for the voltammetric determination of histamine based on carbon paste electrodes bulk-modified with single-walled carbon nanotubes. As monitored in cyclic voltammetry histamine undergoes an irreversible electrochemical oxidation with a peak potential of ca. +1.25 V (vs. Ag/AgCl, 3 mol L^?1 KCl) in phosphate buffer solution (PBS, 0.1 mol L^?1, pH 6.0). At optimized differential pulse voltammetric parameters, the current response of histamine was linearly proportional to its concentration in the range from 4.5 to 720 μmol L^?1. A low limit of detection of 1.26 μmol L^?1 and a limit of quantification of 3.78 μmol L^?1 of histamine, as well as good reproducibility (RSD?=?0.48–3.40 %) were obtained using the carbon paste electrode modified with single-walled carbon nanotubes. The proposed sensor was successfully applied to the determination of histamine in commercial beer and wine samples.     

Quantitation of NAD+ biosynthesis from the salvage pathway in Saccharomyces cerevisiae

Nicotinamide adenine dinucleotide (NAD⁺) is synthesized via two major pathways in prokaryotic and eukaryotic systems: the de novo biosynthesis pathway from tryptophan precursors, or the salvage biosynthesis pathway from either extracellular nicotinic acid or various intracellular NAD⁺ decomposition products. NAD⁺ biosynthesis via the salvage pathway has been linked to an increase in yeast replicative lifespan under calorie restriction (CR). However, the relative contribution of each pathway to NAD⁺ biosynthesis under both normal and CR conditions is not known. Here, we have performed lifespan, NAD⁺ and NADH (the reduced form of NAD⁺) analyses on BY4742 wild‐type, NAD⁺ salvage pathway knockout (npt1Δ) and NAD⁺ de novo pathway knockout (qpt1Δ) yeast strains cultured in media containing either 2% glucose (normal growth) or 0.5% glucose (CR). We have utilized ¹⁴C labelled nicotinic acid in the culture media combined with HPLC speciation and both UV and ¹⁴C detection to quantitate the total amounts of NAD⁺ and NADH and the amounts derived from the salvage pathway. We observed that wild‐type and qpt1Δ yeast exclusively utilized extracellular nicotinic acid for NAD⁺ and NADH biosynthesis under both the 2% and 0.5% glucose growth conditions, suggesting that the de novo pathway plays little role if a functional salvage pathway is present. We also observed that NAD⁺ concentrations decreased in all three strains under CR. However, unlike the wild‐type strain, NADH concentrations did not decrease and NAD⁺: NADH ratios did not increase under CR for either knockout strain. Lifespan analyses revealed that CR resulted in a lifespan increase of approximately 25% for the wild‐type and qpt1Δ strains, while no increase in lifespan was observed for the npt1Δ strain. In combination, these data suggest that having a functional salvage pathway is required for lifespan extension under CR. Copyright © 2009 John Wiley & Sons, Ltd.     

Determination of Ascorbic Acid in Commercial Juices,on a Modified Carbon Paste Electrode,by Using a Taguchi Experimental Design

In this paper, a simple and rapid analytical method development for ascorbic acid determination by using differential pulse voltammetry method on copper(II)-phthalocyanine carbon paste electrode is shown. For this, several parameters, such as copper(II)-phthalocyanine (gram)/carbon (gram)/Nujol (millilitre) ratio, supporting electrolyte, pH and some others have been studied. In order to know the copper(II)-phthalocyanine influence on voltammetry peak height, a comparison between the analytical behaviour of a chemically modified carbon paste electrode and a conventional carbon paste electrode was carried out. The experimental conditions have been optimised by means of the experimental design using the Taguchi methodology. The catalytic oxidation peak current showed a linear depending on the ascorbic acid concentration in the range between 0.88 and 10.03 mg L⁻¹, with the detection limit of 0.55 mg L⁻¹. The proposed procedure was successfully applied to the determination of ascorbic acid in fruit juices without previous preparation and was compared with HPLC method applied at the same samples previously filtered.     

Gene cloning and characterization of dihydrolipoamide dehydrogenase from Microbacterium luteolum: A useful enzymatic regeneration system of NAD from NADH

Dihydrolipoamide dehydrogenase (LPD), a useful biocatalyst for regenerating NAD⁺, was purified from Microbacterium luteolum JCM 9174, and the gene encoding LPD was cloned from the genomic DNA. The gene contained an opening reading frame consisting of 1395 nucleotides encoding 465 amino acid residues with a predicted molecular weight of 49912.1 Da, which displayed 36–78% homology to known LPDs. Moreover, the FAD- and NAD⁺-binding sites and the two catalytic residues in the LPDs were conserved. The enzyme was expressed in recombinant Escherichia coli cells and purified to homogeneity by column chromatography. LPD of M. luteolum (MluLPD) accepted not only lipoamide but also some artificial electron acceptors such as dichlorophenolindophenol (DCIP) and nitrotetrazolium blue (NTB), that is, it functions as a diaphorase. NAD⁺ demonstrated a strong activating effect on MluLPD, and the activity was 5.2 times higher than that without NAD⁺. The enzyme was suitable for regenerating NAD⁺ in biocatalytic reactions because of its high affinity for NADH (6.1 μM). An NAD⁺-regenerating system with MluLPD and laccase using 2,5-dimethoxy-1,4-benzoquinone as a hydrogen acceptor was demonstrated.     

Lactate-mediated enzymatic reduction of metmyoglobin in vitro

This study examined the lactate-mediated enzymatic reduction of equine metmyoglobin (MMb) by using various lactate salts (sodium, potassium and calcium) and bovine m. triceps brachi extracts in vitro. The equine MMb was reduced by the assay mixture containing lactate, NAD⁺, and the muscle extract at pH 5.7. In the absence of the bovine muscle extract, NAD⁺ or l-lactate in the assay mixture, MMb reduction was not observed. Replacing l-lactate with d-lactate or adding oxalate (lactate dehydrogenase inhibitor) to the assay mixture resulted in no MMb reduction, suggesting that lactate dehydrogenase is involved in the MMb reduction, interacting with l-lactate and NAD⁺. An increase (P < 0.05) in the rate of MMb reduction was also observed with an increase of the amount of bovine muscle extract, NAD⁺ or lactate concentration in the assay mixture. There was no difference (P > 0.05) in the rate of MMb reduction when potassium lactate was added to the assay mixture instead of sodium lactate. The MMb reduction through the lactate–NAD⁺-muscle extract system was increased (P < 0.05) with an elevation in pH from 5.1 to 6.1. The results of this research confirm that the inclusion of l-lactate can induce the MMb reduction through the lactate– NAD⁺-enzyme coupling interaction.     

ZnO Nanoparticle Ionic Liquids Carbon Paste Electrode as a Voltammetric Sensor for Determination of Sudan I in the Presence of Vitamin B<Subscript>6</Subscript> in Food Samples

Room-temperature ionic liquid n-hexyl-3-methylimidazolium hexafluoro phosphate as a binder and ZnO nanoparticle (ZnO/NPs) as a sensor were used to construct a new ZnO/NPs carbon ionic liquid paste electrode (ZnO/NPs/IL/CPE), which exhibited enhanced electrochemical behavior as compared with the traditional carbon paste electrode with paraffin for electrooxidation of Sudan I. This modified electrode exhibited a potent and persistent electron mediating behavior followed by well separated oxidation peaks of Sudan I and vitamin B₆. The peaks current of square wave voltammograms (SWV) of Sudan I and vitamin B₆ increased linearly with their concentration in the ranges of 0.01–400 μM Sudan I and 0.5–800 μM vitamin B₆. The detection limits for Sudan I and vitamin B₆ were 0.008–0.2 μM, respectively. The modified electrode has been successfully applied for the assay of Sudan I and vitamin B₆ in food samples.     

A Biosensor Utilizing l-Glutamate Dehydrogenase and Diaphorase Immobilized on Nanocomposite Electrode for Determination of l-Glutamate in Food Samples

Amperometric biosensor utilizing bienzymatic composition consisting of l-glutamate dehydrogenase and diaphorase for the determination of l-glutamate has been developed. Enzymes were immobilized between chitosan layers onto the surface of planar nanocomposite electrodes consisting of multi-walled carbon nanotubes (diameter?=?60–100 nm; length?=?5–15 μm, 95+?% purity). Linear response was obtained from 10 to 3,495 μM in phosphate buffer solution of pH 9.0 and in the presence of enzyme cofactor NAD⁺ (2 mM) and mediator ferricyanide (5 mM). The limit of detection was 5.4 μM, and sensitivity was found to be 28 nA μM^?1 cm^?2. The biosensor showed a short response time (within 60 s), good storage (no loss of activity for at least 3 months), and operational (response ability above 90 % after 7 days since its first use) stability. Finally, the results obtained from measurements of the food samples were compared with those obtained with an enzymatic–spectrophotometric method and correlated well. Analytical performance of the biosensor indicated that the bienzyme system utilizing diaphorase as a secondary enzyme could be a general basis for other biosensors based on NAD⁺-dependent dehydrogenases.     

Electrochemical Determination of Tert-Butyl Hydroquinone in Edible Oil Samples at Poly (Crystal Violet) Modified Glassy Carbon Electrode

The synthetic phenolic antioxidant tert-butyl hydroquinone (TBHQ) is frequently associated to adverse health effects. A polymerized film of crystal violet (CV) was prepared on the surface of a glass carbon electrode (GCE) by electropolymerization in alkaline solution, and then the modified electrode was successfully used to determine TBHQ. This electrode was characterized by scanning electron microscopy and electrochemical impedance spectra. The voltammetric behavior of TBHQ over an extended pH range using cyclic voltammetry at poly (crystal violet) modified glassy carbon electrode (PCV/GCE) was also studied. The resulting electrode exhibited excellent electrocatalytic activity towards the oxidation of TBHQ, and this was confirmed by the observed increased redox peak currents and shifted potentials. The electrochemical sensor can be applied to the quantification of TBHQ with a linear range covering 5?×?10^?7–1?×?10^?4 mol?·?L^?1 (with a correlation coefficient of 0.9969) and the limit detection was 3?×?10^?8 mol?·?L^?1(S/N?=?3). The recovery was between 97.1 and 102 % in edible oil samples. The electrochemical sensor method was also compared with a HPLC method, which proves its capability in commercial market surveillance.     

A novel potentiometric pH electrode based on sulfated natural Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> and analytical application in food samples

A novel solid state potentiometric pH electrode based on sulfated natural Fe₃O₄ silicone was fabricated. The optimum potentiometric performances such as Nernstian response, response time, selectivity, life-time and reproducibility of pH electrodes were investigated by using a computer-controlled potentiometric device. Moreover, the potentiometric performance of the solid state pH electrodes was studied with different mixtures sulfated natural Fe₃O₄, silicone and graphite powder. The best potentiometric behavior of proposed pH electrode was obtained with a composition of 20% (w/w) sulfated natural Fe₃O₄, 40% (w/w) graphite powder and 40% (w/w) silicone. The sub-Nernstian response for pH electrode was exhibited with a slope of 30.8?±?1.4 mV/pH (r?=?0.9963) from pH 2 to pH 12. In addition, the dynamic response time was found as 10 s in acidic medium and further the proposed pH electrode can be used for at least 1 year without any significant slope of the pH–potential curve. The selectivity coefficient of pH electrode was interpreted according to fixed interference method in the presences of Na⁺, Li⁺, K⁺, NH₄⁺, SO₄^2?, CH₃COO^? and NO₃^? ions. The reproducibility of pH electrode was calculated in pH 4 and pH 6 phosphate buffers and it was found as 0.24 RSD (%) and 0.27 RSD (%) respectively. The proposed pH electrode was used to determine of pH in acid?bases titration compared with glass pH electrode and is highly stable in corrosive systems including HF solution. Terminally, the pH value was successfully determined in some soft drinks and milk samples by proposed solid state pH electrode at 95% confidence level with satisfactory agreement compared with glass pH electrode.     

Development of novel potentiometric sensors for determination of tartrazine dye concentration in foodstuff products

Tartrazine dye Na₃TZ in foodstuff products was determined by a new modified carbon paste electrode, encoded sensor A, and a coated silver wire electrode, encoded sensor B, based on tartrazine TZ- cetryltrimethyl ammoniumbromide CTAB as a chemical modifier TZ-CTA. The electrodes exhibit the following characteristics listed respectively: a Nernstian slope of 17.9 ± 0.5 and 19.4 ± 0.2 mV/decade for tartrazine ion over a wide concentration range from 4.3 × 10⁻⁷ to 1.0 × 10⁻² and 1.1 × 10⁻⁷ to 1.0 × 10⁻² M. The lower detection limits: 3.2 × 10⁻⁷ and 5.5 × 10⁻⁸ M. Short response time (5–8 s) over the pH range 3.8–7.7 and 4.2–8.1. The proposed sensors display significantly high selectivity for TZ ion over a wide variety of sugars, some anions, common organic, inorganic compounds and additives. The developed electrodes were applied to the potentiometric determination of tartrazine ion in different kinds of foodstuffs: solid jelly (strawberry and custard) powder samples and soft drink (orange) samples with satisfactory results.     

Novel Sensor Fabrication for the Determination of Nanomolar Concentrations of Hg2+ in Some Foods and Water Samples Based on Multi-walled Carbon Nanotubes/Ionic Liquid and a New Schiff Base

The air and water stable room temperature ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMP]Tf₂N, multi-walled carbon nanotubes (MWCNTs), graphite powder and also a new synthesized Schiff base have been used to fabricate a new carbon composite electrode with very attractive sensing behavior. The new Schiff base was synthesized to apply as a selective agent in electrochemical responses of the voltammetric sensor for the determination of mercury (II) ions in aqueous solutions. The proposed sensor allowed the determination of mercury (II) in the wide linear dynamic range of 0.2–260.0 nmol l^?1. The limit of detection (LOD) was noticed to be 0.05 nmol l^?1. The prepared electrode was used for mercury (II) determination in the real samples.     

KINETIC STUDIES AND MECHANISM OF CATALYSIS OF MALIC DEHYDROGENASE FROM DIOSCOREA ROTUNDATA TUBER

ABSTRACT The kinetic mechanism of catalysis by malic dehydrogenase (EC 1.1.1.37) isolated from yam (Dioscorea rotundata) tuber has been delineated. Initial velocity studies with the enzyme in the presence and absence of products of the reaction revealed an ordered sequential mechanism. The K_m values obtained from secondary plots were 0.05, 0.08, 0.48 and 2.56 mM for NADH, OAA, and NAD⁺ and L-malic acid, respectively. Product inhibition studies in both the forward and backward reactions support an ordered Bi-Bi sequential mechanism. This begins with an obligatory binding of NAD⁺ to form the first binary complex and a final release of NADH.     

Electrochemical Determination of Bisphenol A at Multi-walled Carbon Nanotubes/Poly (Crystal Violet) Modified Glassy Carbon Electrode

Bisphenol A (BPA) can disrupt endocrine system. In this study, a novel and sensitive electrochemical senor based on multi-walled carbon nanotubes (MWCNTs) and poly crystal violet (PCV) modified glass carbon electrode (GCE) was developed for determination of BPA. The morphologies and properties of modified electrode were characterized by scanning electron microscopy and electrochemical impedance spectra. Compared with bare GCE and other modified electrodes, this MWCNTs/PCV/GCE exhibited an excellent electrocatalytic role for the oxidation of BPA by significantly enhancing the current response and decreasing the BPA oxidation overpotential. Under optimum conditions, the electrochemical sensor can be applied to the quantification of BPA by linear sweep voltammetry (LSV) with a linear range covering 5 × 10^?8–1 × 10^?4 mol L^?1 (with a correlation coefficient of 0.9969), and the limit detection was 1 × 10^?8 mol L^?1 (S/N = 3). The recovery was between 98.38 and 103.51% in real plastic samples. This strategy might enable more opportunities for the electrochemical determination of BPA in practical applications.     

Application of reduced graphene oxide and carbon nanotube modified electrodes for measuring the enzymatic activity of alcohol dehydrogenase

An electrochemical method was developed to measure the enzymatic activity of alcohol dehydrogenase (ADH) by monitoring the amount of reduced nicotinamide adenine dinucleotide (NADH) generated in the catalysed oxidation of ethanol by ADH. The concentration of NADH was determined by amperometric measurements, which recorded the oxidation current of NADH versus time on reduced graphene oxide and functionalised multi-walled carbon nanotube modified electrodes. The initial reaction rates and the apparent Michaelis constants of the enzymatic reaction were obtained in the absence and presence of Al³⁺ and nanometre-sized tridecameric aluminium polycationic (nano-Al₁₃) species. The results showed that Al³⁺ and nano-Al₁₃ exhibited inhibitory effect on the enzymatic activity of ADH. Fluorescence and circular dichroism spectra indicated the inhibitory effect was likely caused by the conformational changes of ADH and/or NADH induced by Al³⁺ and nano-Al₁₃.     

Determination of nitrite in food samples by anodic voltammetry using a modified electrode

A glassy carbon (GC) electrode modified with alternated layers of iron(III) tetra-(N-methyl-4-pyridyl)-porphyrin (FeT4MPyP) and copper tetrasulfonated phthalocyanine (CuTSPc) was employed for nitrite determination by differential pulse voltammetry (DPV). This modified electrode showed excellent catalytic activity for the nitrite oxidation. After optimizing the operational conditions, a linear response range from 0.5 to 7.5 μmol l⁻¹ with a low detection limit of 0.1 μmol l⁻¹ was obtained. The proposed sensor was stable with a sensitivity of 20.0 μA, 1 μmol⁻¹ and good repeatability, evaluated in terms of relative standard deviation (R.S.D. = 1.3%) for n = 10. Possible interferences from several common ions were evaluated. This sensor was applied for the voltammetric determination of nitrite in some food samples. The results were consistent with those obtained with the standard spectrophotometric procedure.