Abstract

A nitrogen-doped nanocrystalline electrochemical graphite sensor for the sensitive determination of oleuropein (OL) from extra virgin olive oils (EVOOs) is presented. The sensor was developed by the deposition of nanocrystalline graphite (NCG) using plasma-enhanced chemical vapour deposition (PECVD) on silicon wafers. Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD) were used to characterise the microstructure and morphology of the developed materials. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and square wave voltammetry (SWV) were used to investigate the electrochemical properties of the material and the performance of the sensor. The developed sensor showed good analytical performance against OL over a concentration range of 5.00-500.00 mu M, with a good detection limit of 3.93 mu M and a good sensitivity of 0.057 mu A mu M-1. The reproducibility of the electrochemical sensor was excellent, with a relative standard deviation (RSD) of 8.56% for seven measurements.

Abstract

In recent years, various studies have been carried out to increase the concentration of antioxidant active principles in red wines as a consequence of the effects of winemaking techniques on the polyphenols content. In this study, in order to obtain the most optimal wine in terms of content and efficiency of antioxidant activity, various winemaking technologies (punching-down and pumping-over maceration) were tried with diverse gradations (Feteasca Neagra and Cabernet Sauvignon wines) and the addition of different concentrations of melatonin in must. Suitable HPLC and spectrophotometric methods were used to follow the evolution of the antioxidant compounds from wines during aging (for 12 months). After comparing the acquired results, an increase was observed in the antioxidant compound concentrations, particularly in resveratrol (85%), peonidin-3-glucoside (100%) or cyanidin-3-glucoside (100%), and antioxidant activity (10-40%). The most enriched wine was obtained in the case of Feteasca Neagra by the addition of 0.5 mg of melatonin per 1 kg of must using the punch-down technology and, in the case of Cabernet Sauvignon, by the addition of 0.05 mg of melatonin per 1 kg of must using the pumping-over technique. This study can provide winemakers with an approach to enhance red wines with antioxidant compounds.

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Lately, there has been increased interest in the development of phytochemical alternatives for the prevention and treatment of type 2 diabetes, the alternatives that are able to reduce or prevent glucose absorption by inhibiting digestive enzymes. In this context, this study aims to analyze the inhibitory alpha-amylase and alpha-glucosidase activities of Artemisia abrotanum and Symphytum officinale polyphenolic compound-rich extracts obtained by membrane technologies (micro- and ultrafiltration). Polyphenols and flavones content, HPLC-MS polyphenolic compounds profiling, antioxidant activity, and cytotoxic potential of these herbs were determined. Major phenolic acid compounds were chlorogenic acid, ellagic acid, caffeic acid, and rosmarinic acid. The flavone content was higher in the case of A. abrotanum extracts, and the major compounds were rutin and umbelliferone. The polyphenolic-rich extract of A. abrotanum had the highest quantities of polyphenols, 977.75 mu g/mL, and flavones, 552.85 mu g/mL, as well as a pronounced alpha-amylase inhibitory activity (IC50 1881.21 +/- 1.8 mg/mL), a value close to acarbose inhibitory activity (IC50 1110.25 +/- 8.82 mg/mL) that was used as the control for both enzymes. The alpha-glucosidase inhibitory activity was higher for both herb extracts, more pronounced for S. officinale polyphenolic-rich extract (IC50 291.56 +/- 2.1 mg/mL), a value higher than that of acarbose (IC50 372.35 +/- 3.2 mg/mL). These plants show potential as a complementary therapy for type 2 diabetes management.

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In recent years, the wine industry has shown a considerable degree of interest in the occurrence of melatonin in wines. Sample pretreatment may be the most important step in trace analysis. Since wine is a complex matrix and melatonin is present in low amounts (ppb), an adequate extraction technique is required. In this study, the effect of several extraction methods, such as solid phase extraction (SPE), Quick, Easy, Cheap, Effective, Rugged, and Safe extraction (QuEChERS), and dispersive liquid-liquid micro-extraction (DLLME) was studied and the variable parameters that can arise throughout the extraction process were optimized to obtain the best results. A high-performance liquid chromatography with fluorescence detector (HPLC-FL) method was adapted and validated, including measurement uncertainty, for the analysis of melatonin in wines and to assess the efficiency of the extraction yield. After comparing the acquired results, the DLLME method was optimized. Extraction recoveries values ranging from 95 to 104% demonstrated that the approach may be successfully applied for the extraction and concentration (enrichment factor of almost eight) of melatonin in wine samples prior to HPLC-FL analysis. The first report of melatonin levels in Feteasca Neagra wines has been made. The data obtained for Cabernet Sauvignon revealed that the final levels of melatonin in the wines are dependent on the winemaking process.

Abstract

Until recently, the main antioxidant role among wine constituents was attributed to polyphenolic compounds, but once the presence of melatonin in wines was confirmed, an interesting new field of research opened up due to its possible synergistic effects with other antioxidants in the winemaking process, which may lead to a change in the profile of polyphenolic compounds and antioxidant activity. In order to investigate the evolution of active principles from the phenylpropanoid metabolism associated with the synergistic effects of melatonin, for the first time, a melatonin treatment was performed in the pre-stage of the different winemaking processes of Feteasca Neagra and Cabernet Sauvignon wines with different melatonin concentrations. After comparing the acquired results for the evolution of the polyphenolic compound profile and antioxidant activity of treated wines, we ascertained an increase in the antioxidant compound concentrations, especially in resveratrol, quercetin, and cyanidin-3-glucoside, directly proportional to the used melatonin concentration; an intensification in activity of PAL and C4H enzymes; and the modification in the expression of specific anthocyanin biosynthesis genes, especially UDP-D-glucose-flavonoid-3-O-glycosyltransferase. It was also shown that the application of melatonin in the pre-stage of the winemaking process can be successfully used to obtain red wines with increased antioxidant activity (almost 14%).

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A thick nano-crystalline graphite film (NCG, similar to 350 nm) was grown on dielectric substrate (SiO2, similar to 110 nm) using plasma-enhanced chemical vapour deposition (PECVD) aiming the development of an electrochemical sensor for caffeic acid. The obtained sensor was characterised using several investigation techniques such as atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The data evidenced the coexistence in the investigated sample of the nano-crystalline graphite/graphene domains, while the NCG sheet resistance (similar to 340 U/sq.) is matching well the value reported for few-layers graphene film, revealing for the nano-crystalline graphite film a better ordered graphene structure and an improved electrical conduction in the interconnected graphene domains. The sensor for caffeic acid, operating at an applied potential of +0.4 V vs. Hg/Hg2Cl2 reference electrode showed a linear working range comprised between 5.0 X 10(-5) and 1.0 X 10(-3) M, and very good capabilities against real samples analysis. The reproducibility of the developed sensor was assessed in the presence of 0.1mM caffeic acid and the relative standard deviation was 5.84% (n = 11). (c) 2019 Elsevier Ltd. All rights reserved.

Abstract

A Si wafer coated with a low temperature oxide (LTO) was used as substrate (Si/SiO2) during the deposition of a thick nano-crystalline graphite (NCG) film by means of plasma-enhanced chemical vapour deposition (PECVD) procedure. The process parameters, the atomic force (AFM) and scanning electron (SEM) micrographs, Raman spectrum and X-ray diffraction (XRD) pattern are herein illustrated. The as deposited NCG film was electrochemically pretreated (3 mA applied current, during 240 s, in 10 mM phosphate buffer saline (PBS) solution containing 0.1 M KCl, pH 7) and thereafter used as electrode for sensing the caffeic acid content in lyophilised berries and dried chokeberries in Nano-crystalline graphite film on SiO2: Electrochemistry and electro-analytical application [1]. (C) 2019 The Author(s). Published by Elsevier Inc.

Abstract

An interconnected graphene network (IGN) structure with excellent photoluminescence (PL) properties was synthesized using a one-pot microwave-assisted hydrothermal carbonization route. The material exhibited intense and excitation-wavelength dependent PL emission located mainly in the UV-blue light range (300-450 nm). The result demonstrates that graphene networks could also be included in the emerging class of tunable PL carbon nanomaterials. Furthermore, we have taken a first step towards their incorporation into solar cell devices by fabricating IGN/p-SiNWs radial heterojunctions using the versatile potentiostatic electrodeposition technique. The IGN modified p-SiNW solar cell showed the best performance with an overall enhancement of power conversion efficiency of 7.5 times higher than the reference cell. We emphasize that the structural and electronic characteristics of the as-prepared IGN combined with tapering effects are directly responsible for the tripled short circuit current density and 9% improvement of open circuit voltage with respect to the reference cell. Finally, we have demonstrated that the IGN successfully passivated the Si nanowires' surface using intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS). These promising findings indicate that further IGN exploitation may help to gain efficiency in future energy conversion applications.

Abstract

A thick nano-crystalline graphite film (NCG, ∼350 nm) was grown on dielectric substrate (SiO 2 , ∼110 nm) using plasma-enhanced chemical vapour deposition (PECVD) aiming the development of an electrochemical sensor for caffeic acid. The obtained sensor was characterised using several investigation techniques such as atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The data evidenced the coexistence in the investigated sample of the nano-crystalline graphite/graphene domains, while the NCG sheet resistance (∼340 Ω/sq.) is matching well the value reported for few-layers graphene film, revealing for the nano-crystalline graphite film a better ordered graphene structure and an improved electrical conduction in the interconnected graphene domains. The sensor for caffeic acid, operating at an applied potential of +0.4 V vs. Hg/Hg 2 Cl 2 reference electrode showed a linear working range comprised between 5.0 × 10 −5 and 1.0 × 10 −3 M, and very good capabilities against real samples analysis. The reproducibility of the developed sensor was assessed in the presence of 0.1 mM caffeic acid and the relative standard deviation was 5.84% (n = 11). © 2019 Elsevier Ltd

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The challenge for conformal modification of the ultra-high internal surface of nanoporous silicon was tackled by electrochemical polymerisation of 2,6-dihydroxynaphtha lene using cyclic voltammetry or potentiometry and, notably, after the thermal treatment (800 degrees C, N-2, 4 h) an assembly of interconnected networks of graphene strongly adhering to nanoporous silicon matrix resulted. Herein we demonstrate the achievement of an easy scalable technology for solid state supercapacitors on silicon, with excellent electrochemical properties. Accordingly, our symmetric supercapacitors (SSC) showed remarkable performance characteristics, comparable to many of the best high-power and/ or high-energy carbon-based supercapacitors, their figures of merit matching under battery-like supercapacitor behaviour. Furthermore, the devices displayed high specific capacity values along with enhanced capacity retention even at ultra-high rates for voltage sweep, 5V/s, or discharge current density, 100 A/g, respectively. The cycling stability tests performed at relatively high discharge current density of 10 A/g indicated good capacity retention, with a superior performance demonstrated for the electrodes obtained under cyclic voltammetry approach, which may be ascribed on the one hand to a better coverage of the porous silicon substrate and, on the other hand, to an improved resilience of the hybrid electrode to pore clogging.

Abstract

The goal of the present work was to study the influence of winemaking procedures and the effect of bottle-aging on the antioxidant activity and on the variation of low molecular weight phenolics of Feteasca Neagra and Negru Aromat red wine from the Valea Calugareasca vineyard, Romania. The free radical-scavenging activity was measured using the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radical from bottles sampled every 3 months up to the 9th month. The results indicated that Negru Aromat obtained by classical maceration had the highest activity at 6th month. The highperformance liquid chromatography results showed that the most abundant low molecular weight phenolic compounds were catechin, (-)-epicatechin, gallic acid, malvidin, and peonidin 3-O-glucoside. For the phenolics, Feteasca Neagra prepared by classical maceration and Negru Aromat by run-off juice were the richest. For anthocyanidins, the Feteasca Neagra and Negru Aromat wine by classical maceration had the highest concentrations. These results suggest that classical maceration is the winemaking procedure of choice to obtain the highest quality wine from the Valea Calugareasca vineyard.

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The quality of red wines is given by phenolic compounds and anthocyanins and is associated with colour, taste and therapeutic effects on human health. This work aims to provide a detailed profile of phenolic compounds and anthocyanins that are found in five red wine samples from Dealu Mare-Valea Calugareasca region. The phenolic and anthocyanin profiles of the red wine samples were determined by high-performance liquid chromatography-photodiode array-mass spectrometry and matrix-assisted laser desorption/ionisation-time of flight, respectively. The results obtained showed that Feteasca Neagra had the highest content of phenolic compounds followed by Pinot Noir while the preponderant compound was gallic acid. Amongst anthocyanins, malvidin was found to be the major compound and the highest anthocyanin content was found also for Feteasca Neagra wine. A simple high-performance liquid chromatography-photodiode array-mass spectrometry method was developed, optimised and applied for the quantification of phenolic compounds in red wine samples from Dealu Mare-Valea Calugareasca region. In the same time, a rapid matrix-assisted laser desorption/ionisation-time of flight method that does not need sample preparation was applied for the identification of anthocyanins. Moreover, the phenolic and anthocyanin composition of red wines from Dealu Mare-Valea Calugareasca region is reported for the first time. The phenolic and anthocyanin profile determination will be beneficial for the Romanian winemakers to produce high-quality red wines.

Abstract

A nanocomposite formed from molybdenum disulphide (MoS2) and graphene quantum dots (GQDs) was proposed as a novel and suitable support for enzyme immobilisation displaying interesting electrochemical properties. The conductivity of the carbon based screen-printed electrodes was highly improved after modification with MoS2 nanoflakes and GQDs, the nanocomposite also providing compatible matrix for laccase immobilisation. The influence of different modification steps on the final electroanalytical performances of the modified electrode were evaluated by UV-vis absorption and fluorescence spectroscopy, scanning electron microscopy, transmission electron microscopy, X ray diffraction, electrochemical impedance spectroscopy and cyclic voltammetry. The developed laccase biosensor has responded efficiently to caffeic acid over a concentration range of 038-100 mu M, had a detection limit of 0.32 mu M and a sensitivity of 17.92 nA mu M-1. The proposed analytical tool was successfully applied for the determination of total polyphenolic content from red wine samples. (C) 2015 Elsevier B.V. All rights reserved.

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Abstract

In this work a novel sensor array platform based on a dual carbon screen-printed electrode was developed for the simultaneous determination of chlorogenic acid and caffeine. One of the carbon working electrodes was modified with platinum nanoparticles, reduced graphene oxide and laccase (C-SPE/Pt-NPs/RGO/lacc-biosensor) for chlorogenic acid determination and the second carbon working electrodes was modified with reduced graphene oxide and Nafion (C-SPE/RGO/Nafion-sensor) for caffeine determination. Cyclic voltammetry was used to characterise and optimise the dual sensor array while chronoamperometry was used to investigate the bioelectrocatalytic response. The C-SPE/Pt-NPs/RGO/lacc for biosensing chlorogenic acid exhibited a sensitivity of 0.02 mu A mu M (1) and a detection limit of 2.67 mu M whereas the C-SPE/RGO/Nafion used for sensing caffeine has showed a sensitivity of 1.38 mu A mu M-1 and a detection limit of 0.22 mu M. The developed sensor array was used to determine these two major coffee bean compounds from real coffee samples. Due to its simplicity, feasibility and accessibility, the developed dual sensor array could represent the basis of a valuable analytical tool able to screen both chlorogenic acid and caffeine content from coffee samples offering important information about the phytochemical composition of the samples.

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The present work describes the development of an electrochemical method based on the use of 2,2'-diphenyl-1-picrylhidrazyl free radical (DPPH.) for the determination of the antiradical properties of several olive oils. Differential pulse voltammetry was used as measuring technique while the electrochemical process was recorded at a platinum screen-printed working electrode. The decrease in 2,2'-diphenyl-1-picrylhidrazyl peak current intensity was measured at a specific potential value of +160 mV vs. screen-printed pseudo-reference electrode, in the presence of alpha-, delta- and gamma-tocopherol and olive oil samples, respectively. The obtained results using differential pulse voltammetry, as detection technique for real samples analysis, showed a satisfactory agreement with those obtained by high-performance liquid chromatography coupled with fluorescence detection. The reported electrochemical method is rapid and easy to use, feasible and accessible to be used as an alternative to 2,2'-diphenyl-1-picrylhidrazyl spectrophotometric based method. (C) 2014 Elsevier Ltd. All rights reserved.

Abstract

The main goal of our study was to evaluate the effect of the individual administration of five lyophilized lactic acid bacteria strains (Lactobacillus fermentum 428ST, Lactobacillus rhamnosus E4.2, Lactobacillus plantarum FCA3, Lactobacillus sp. 34.1, Weissella paramesenteroides FT1a) against the in vitro simulated microbiota of the human colon using the GIS1 system. The influence on the metabolic activity was also assessed by quantitative determination of proteins and polysaccharides at each segment of human colon. The obtained results indicated that the lactic acid bacteria L. rhamnosus E4.2 and W. paramesenteroides FTa1 had better efficiency in synthesising exopolysaccharides and also a better probiotic potential and therefore could be recommended for use in probiotics products or food industry.

Abstract

An optimized electrochemical sensor was developed to assess the antioxidant capacity of carotenoids, accumulating during the life cycle of Haematococcus pluvialis cell cultures. The sensor was improved with a composite renewable surface made of immobilised phosphatidylcholine (PC) on magnetic nanobeads of iron oxide (Fe3O4) and PC/Fe3O4, and it was used to monitor the antioxidant properties of the ketocarotenoid astaxanthin against in situ generated phosphatidylcholine lipoperoxides. The surface configuration was able to mimic the natural position and orientation of astaxanthin in the cellular membrane, conferring to the whole experimental set-up good sensitivity for reactive oxygen species (limit of detection for peroxyl radicals 9.1 x 10(-10) mol L-1) with a linear response ranging between 10(-8) and 1.6 x 10(-6) mol L-1. The sensor has been proved suitable for both batch and flow measurements. The accuracy of the flow measurements was unaffected by the magnetic field intensity. Electrochemical measurements confirmed that natural astaxanthin is a more effective antioxidant than synthetic astaxanthin, vitamin E and lutein and the protective effect of astaxanthin correlates with its concentration inside the cell. The newly developed sensor is hence useable for in-line monitoring of whole-cell based industrial bioprocesses for the production of astaxanthin.

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The present work describes the development of a nanocomposite system and its application in construction of a new amperometric biosensor applied in the determination of total polyphenolic content from propolis extracts. The nanocomposite system was based on covalent immobilization of laccase on functionalized indium tin oxide nanoparticles and it was morphologically and structural characterized. The casting of the developed nanocomposite system on the surface of a screen-printed electrode was used for biosensor fabrication. The analytical performance characteristics of the settled biosensor were determined for rosmarinic acid, caffeic acid and catechol (as laccase specific substrate). The linearity was obtained in the range of 1.06x10(-6) - 1.50x10(-5) mol L-1 for rosmarinic acid, 1.90x10(-7) - 2.80x10(-6) mol L-1 for caffeic acid and 1.66x10(-6) - 7.00x10(-6) mol L-1 for catechol. A good sensitivity of amperometric biosensor 141.15 nA A mu mol(-1) L-1 and fair detection limit 7.08x10(-8) mol L-1 were obtained for caffeic acid. The results obtained for polyphenolic content of propolis extracts were compared with the chromatographic data obtained by liquid-chromatography with diode array detection.

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The main products generated by lipid peroxidation are the lipid hydroperoxides, compounds that are involved in the pathology of several diseases and in the process of food rancidification. In this context, simple, fast, and reliable analytical tools for the determination of lipid hydroperoxides are highly required. The present work describes the development of a single use ferrocenemethanol modified carbon screen-printed electrode for the determination of lipid hydroperoxides. The modified electrode is equally applicable to evaluation of peroxidation inhibition degree of two well-known antioxidants. The performance characteristics of the proposed non-enzymatic sensor are: linear working range 6.0 x 10(-6) to 2.7 x 10(-4) mol L-1, limit of detection 1.7 x 10(-6) mol L-1, upper control limit 3 x 10(-4) mol L-1 and sensitivity 826.53 nA mM(-1); the required time to reach the steady state was 60 s. The sensor was then applied for the determination of lipid hydroperoxide formation from four real samples of vegetal oils, walnut, sunflower, linseed and soybean oils, the results being expressed as linoleic acid equivalents. Furthermore, the efficacy of Trolox and butylated hydroxyanisole in inhibiting lipid hydroperoxide formation was tested.

Abstract

A disposable amperometric biosensor was developed for the detection of total polyphenolic compounds from tea infusions. The biosensor was designed by modifying the surface of a carbon screen-printed electrode with platinum nanoparticles and reduced graphene oxide, followed by the laccase drop-casting and stabilization in neutralised 1% Nafion solution. The obtained biosensor was investigated by scanning electron microscopy and electrochemical techniques. It was observed that platinum nanoparticles-reduced graphene oxide composite had synergistic effects on the electron transfer and increased the electroactive surface area of the carbon screen-printed electrode. The constructed analytical tool showed a good linearity in the range 0.2-2 mu M for caffeic acid and a limit of detection of 0.091 mu M. The value of Michaelis-Menten apparent constant was calculated from the electrochemical version of Lineweaver-Burk equation to be 2.75 mu M. This disposable laccase biosensor could be a valuable tool for the estimation of total polyphenolic content from tea infusions. (C) 2013 Elsevier B.V. All rights reserved.

Abstract

Pristine graphene platelets and graphene oxide were used as electrode modifiers, aiming the investigation of their electrochemical efficacy towards beta-nicotinamide adenine dinucleotide (NADH). The electrochemical detection of NADH is one of the most studied areas of bioelectroanalysis because of the ubiquity of NAD(P)H-based enzymatic reactions in nature. Commercially available graphene and laboratory prepared graphene oxide were used to modify glassy carbon electrodes and the behaviour of such modified electrodes against potassium ferricyanide (III) and NADH was reported. Relying on the graphene-modified transducer, l-lactic dehydrogenase (l-LDH) was successfully immobilised in a 1 % Nafion(A (R)) membrane. The developed biosensor, working at +250 mV versus Ag/AgCl reference electrode, was used to assess l-lactic acid in four different types of yogurts, revealing an l-lactic acid concentration ranging between 0.3 and 0.6 %.

Abstract

A fluorimetric method for the determination of cysteine (CSH) and cystine (CSSC) at nanomolar concentrations is reported. The detection limit was 2.46x10(-11) M for CSH when derivatisation is performed by 5-iodoacetamidofluorescein (5-IAF) and respectively 7.2x10(-10) M when monobromobimane was used as fluorescent reagent. The dynamic range of analytical response was 3x10(-10) - 3x10(-8) M ( in the case of CSH-5-IAF). The method was further applied to determine the [CSH]/[CSSC] ratio at nanomolar concentration levels as potential marker of aminoacids oxidative status. An original algorithm to calculate the exact concentrations for each form was established, being provided even the experimental data to support it. Zinc influence on avoiding CSH oxidation was studied. Data were confirmed by high-performance liquid chromatography with fluorescence detection.

Abstract

Introduction A recently developed laccase based biosensor is used for polyphenols determination from in vitro Salvia cultures, the results being expressed as rosmarinic acid equivalent content. Objective The aim of this work was to use a previously developed laccase biosensor for the determination of total phenolic content from in vitro cultivated Salvia, and to support the biosensors further application for the assessment of polyphenols metabolites. Methodology The biosensor was constructed by drop casting 3 mu L of laccase solution and stabilisation with 0.1 % Nafion solution onto a DropSens carbon screen-printed electrode. Electrochemical measurements were carried out in a 0.1 mol/L phosphate buffer (pH 4.50), the applied working potential being -30 mV versus reference electrode. Results The response of the biosensor developed was characterised in terms of repeatability, accuracy and precision; the limit of detection was 7.5?x?10-7 mol/L, the limit of determination was 9.5?x?10-7 mol/L, and linear response range for rosmarinic acid was 1 x 10-610-5 mol/L. Conclusion A stable, sensitive and simple biosensor based on laccasenafion was used for monitoring the total polyphenolic content from two in vitro cultivated plants. The biosensor response was free of electrochemical interferences and of possible interferences from growth media constituents, demonstrating a high sensitivity for rosmarinic acid determination in cell culture suspensions. Copyright (c) 2012 John Wiley & Sons, Ltd.

Abstract

A simple, fast and sensitive high-performance liquid chromatography/electrospray ionization-mass spectrometry method is described and validated in terms of linearity, accuracy/recovery and reproducibility, as well as limit of detection for the determination of ascorbic acid from grape (Vitis vinifera L.) samples from Murfatlar vineyard. Good linearity (correlation factor > 0.9970) was achieved in the concentration range 0.5-15 mu g mL(-1) for AA in acetic acid 0.1% solution. The obtained RSD values (below 5%) indicated excellent repeatability of the proposed method. The limit of detection was 0.32 mu g mL(-1) while the recovery ranged between 96.86 and 102.41%. The advantages of the method are: small amounts of sample and solvents, short analysis time and minimum steps for sample preparation.

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Multiwalled carbon nanotubes with nanosized sputtered gold were used to modify a glassy carbon electrode (GCE). The substrate was characterized by scanning electron microscopy (SEM), X-ray diffraction, cyclic voltammetry and amperometry. SEM micrographs indicated an uniform coverage of the carbon nanotubes with nanosized (poly)crystalline gold. Cyclic voltammetry reveals that peak separation of the unmodified GCE in the presence of 1 mM ferricyanide is 131 mV, but 60 mV only for the modified GCE. In addition, the oxidation of NADH (1 mmol L-1 solution) begins at negative potentials (around -100 mV vs. Ag/AgCl), and the anodic peak potential (corresponding to the irreversible oxidation of NADH) is found at +94 mV. The effect of pH on the electrocatalytic activity was studied in the range from 5.4 to 8.0. The relationship between the anodic peak potential and the pH indicated a variation of -33.5 mV/pH which is in agreement with a two-electron and one-proton reaction mechanism. Amperometry, performed at either -50 or +50 mV vs. an Ag/AgCl reference electrode, indicates that the modified electrode is a viable amperometric sensor for NADH. At a working potential of +50 mV, the response to NADH is linear in the concentration range from 1 to 100 mu mol L-1, with an RSD of 6% (n = 4).

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