Analytical determination of the reducing and stabilization agents present in different Zostera noltii extracts used for the biosynthesis of gold nanoparticles.

The objective of this work was to ascertain the nature of the components responsible for the reducing and stabilizing properties of Zostera noltii extracts that lead to gold nanoparticle formation using chemical techniques of analysis. In order to achieve this aim, we try the synthesis of AuNPs with three different extracts from plants collected in the Bay of Cádiz (Spain). The n-butanol extract produced the best results. Taking this into account, four fractions were isolated by Sephadex LH-20 column chromatography from this extract and we studied their activity. The chemical study of these fractions led to the isolation of several flavone sulfates and these were identified as the species' responsible for the formation and stabilization of the AuNPs. Flavone sulfates were purified by high performance liquid chromatography and the structures were established by means of spectroscopic methods nuclear magnetic resonance and mass spectroscopy. AuNPs have an average lifetime of about 16weeks. Additionally, the morphology and crystalline phase of the gold nanoparticles were characterized by transmission electron microscopy. The composition of the nanoparticles was evaluated by electron diffraction and energy dispersive X-ray spectroscopy. An 88% of the gold nanoparticles has a diameter in the range 20-35nm, with an average size of 26±2nm.

Amperometric inhibition biosensors based on horseradish peroxidase and gold sononanoparticles immobilized onto different electrodes for cyanide measurements.

New biosensors based on inhibition for the detection of cyanide and the comparison of the analytical performances of nine enzyme biosensor designs by using three different electrodes: Sonogel-Carbon, glassy carbon and gold electrodes were discussed. Three different horseradish peroxidase immobilization procedures with and without gold sononanoparticles were studied. The amperometric measurements were performed at an applied potential of -0.15V vs. Ag/AgCl in 50mM sodium acetate buffer solution pH=5.0. The apparent kinetic parameters (Kmapp, Vmaxapp) of immobilized HRP were calculated in the absence of inhibitor (cyanide) by using caffeic acid, hydroquinone, and catechol as substrates. The presence of gold sononanoparticles enhanced the electron transfer reaction and improved the analytical performance of the biosensors. The HRP kinetic interactions reveal non-competitive binding of cyanide with an apparent inhibition constant (Ki) of 2.7µM and I50 of 1.3µM. The determination of cyanide can be achieved in a dynamic range of 0.1-58.6µM with a detection limit of 0.03µM which is lower than those reported by previous studies. Hence this biosensing methodology can be used as a new promising approach for detecting cyanide.

Study of the electrocatalytic activity of cerium oxide and gold-studded cerium oxide nanoparticles using a Sonogel-Carbon material as supporting electrode: electroanalytical study in apple juice for babies.

The present work reports a study of the electrocatalytic activity of CeO2 nanoparticles and gold sononanoparticles (AuSNPs)/CeO2 nanocomposite, deposited on the surface of a Sonogel-Carbon (SNGC) matrix used as supporting electrode and the application of the sensing devices built with them to the determination of ascorbic acid (AA) used as a benchmark analyte. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to investigate the electrocatalytic behavior of CeO2- and AuSNPs/CeO2-modified SNGC electrodes, utilizing different concentrations of CeO2 nanoparticles and different AuSNPs:CeO2 w/w ratios. The best detection and quantification limits, obtained for CeO2 (10.0 mg·mL(-1))- and AuSNPs/CeO2 (3.25% w/w)-modified SNGC electrodes, were 1.59 × 10(-6) and 5.32 × 10(-6) M, and 2.93 × 10(-6) and 9.77 × 10(-6) M, respectively, with reproducibility values of 5.78% and 6.24%, respectively, for a linear concentration range from 1.5 µM to 4.0 mM of AA. The electrochemical devices were tested for the determination of AA in commercial apple juice for babies. The results were compared with those obtained by applying high performance liquid chromatography (HPLC) as a reference method. Recovery errors below 5% were obtained in most cases, with standard deviations lower than 3% for all the modified SNGC electrodes. Bare, CeO2- and AuSNPs/CeO2-modified SNGC electrodes were structurally characterized using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). AuSNPs and AuSNPs/CeO2 nanocomposite were characterized by UV-vis spectroscopy and X-ray diffraction (XRD), and information about their size distribution and shape was obtained by transmission electron microscopy (TEM). The advantages of employing CeO2 nanoparticles and AuSNPs/CeO2 nanocomposite in SNGC supporting material are also described. This research suggests that the modified electrode can be a very promising voltammetric sensor for the determination of electroactive species of interest in real samples.

Recent advances in graphite powder-based electrodes.

Graphite powder-based electrodes have the electrochemical performance of quasi-noble metal electrodes with intrinsic advantages related to the possibility of modification to enhance selectivity and their easily renewable surface, with no need for hazardous acids or bases for their cleaning. In contrast with commercial electrodes, for example screen-printed or sputtered-chip electrodes, graphite powder-based electrodes can also be fabricated in any laboratory with the form and characteristics desired. They are also readily modified with advanced materials, with relatively high reproducibility. All these characteristics make them a very interesting option for obtaining a large variety of electrodes to resolve different kinds of analytical problems. This review summarizes the state-of-the-art, advantages, and disadvantages of graphite powder-based electrodes in electrochemical analysis in the 21st century. It includes recent trends in carbon paste electrodes, devoting special attention to the use of emergent materials as new binders and to the development of other composite electrodes. The most recent advances in the use of graphite powder-modified sol-gel electrodes are also described. The development of sonogel-carbon electrodes and their use in electrochemical sensors and biosensors is included. These materials extend the possibilities of applications, especially for industrial technology-transfer purposes, and their development could affect not only electroanalytical green chemistry but other interesting areas also, for example catalysis and energy conversion and storage.

Use of a Sonogel-Carbon electrode modified with bentonite for the determination of diazepam and chlordiazepoxide hydrochloride in tablets and their metabolite oxazepam in urine.

Sonogel-Carbon electrode (SngCE) modified with bentonite (BENT) shows an interesting alternative electrode to be used in the determination of 1,4-benzodiazepines by square wave adsorptive cathodic stripping voltammetry (SWAdCSV). Diazepam (DZ) and chlordiazepoxide hydrochloride (CPZ), were determined using SngCE modified by 5% BENT. An electrochemical study of different parameters (such as pH, buffer type, ionic strength, accumulation potential, scan rate, and accumulation time) which affect the determination of DZ and CPZ is reported. Linear concentration ranges of 0.028-0.256 µg mL(-1) DZ (r=0.9997) and 0.034-0.302 µg mL(-1) CPZ (r=0.9997) are successfully obtained after an accumulation time of 60s. The quantification and detection limits were calculated to be 14.0 and 4.0 ng mL(-1) for DZ, and 16.0 and 5.0 ng mL(-1) for CPZ, respectively. The surface of the proposed electrode was characterized by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX). The developed method was applied to the analysis of commercially available tablets and human urine real samples. Analysis was performed with better precision, very low detection limits, and faster than previously reported voltammetric techniques.

Graphite grains studded with silver nanoparticles: description and application in promoting direct biocatalysis between heme protein and the resulting carbon paste electrode.

The impregnation of graphite grains with silver nanoparticles (AgNPs) is proposed for making a novel carbon paste electrode (CPE). The resulting material promotes direct electron transfer and direct biocatalysis of embedded heme protein. It is demonstrated that the impregnation of graphite grains with AgNPs of 16-25 nm, incorporated in a CPE, can promote measurable bio-electrochemical phenomena involving hemoglobin and myoglobin. Unlike other biosensors prepared with simple carbon, those based on carbon grains studded with AgNPs show well-defined and quasi-reversible voltammetric peak with heterogeneous electron transfer rate k(s) of approximately 0.037±0.007 and 0.013±0.005s(-1) for hemoglobin and myoglobin, respectively. The embedded proteins also retain their bio-catalytical activity for hydrogen peroxide and nitrite reduction with linear ranges of 0.5-3000 µM and 30-150 µM, sensitivities of 73.6±0.6nA µM(-1) and 5.72±0.11 nA µM(-1), and detection limits close to 0.08 µM and 5.80 µM, for these two analytes respectively. These results support the viability of this preliminary approach for the development of advanced third-generation biosensors.

1-Furoylthiourea-Sonogel-Carbon electrodes: structural and electrochemical characterization.

A new type of sensor based on Sonogel-Carbon materials modified with 1-(2-furoyl)-3-(1-naphthyl)thiourea is presented to be used as an amperometric sensor for metals. Different percentages of modifier were tested in order to optimise the structural and mechanical properties, as well as the electrochemical behaviour. Scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), Fourier transform-infrared spectroscopy (FTIR), single crystal X-ray diffraction (XRD) and powder XRD were used for the structural characterization of these electrodes. The 1-(2-furoyl)-3-(1-naphthyl)thiourea did not modify significantly the structural and mechanical properties of the Sonogel-Carbon electrodes, being similar to other modifications carried out previously in these materials. For the study of the electrochemical response, anodic stripping differential pulse voltammetry (ASDPV) was employed, optimising other parameters of measurement such as pH of the buffer, potential and accumulation time and pulse amplitude. The electrochemical response of the modified electrodes improved significantly with respect to the non-modified electrode, giving good signals and acceptable detection limit.

Sonogel-carbon electrode based on hemin for detection of superoxide.

The Sonogel electrode was immersed in hemin solution to obtain the hemin-Sonogel-modified electrode by cyclic voltammetry (CV). It was applied to detect superoxide radicals produced by xanthine oxidase catalyzed hypoxanthine oxidation. The characterization of hemin-Sonogel was modified by impedance and CV.

beta-Sonogel-carbon electrodes: a new alternative for the electrochemical determination of catecholamines.

In this work, a new alternative for the electrochemical determination of catecholamines based on beta-cyclodextrin-Sonogel-Carbon electrodes is reported. The incorporation of beta-CD and graphite in the preparation of the Sonogel-Carbon material leads to a modification of the electrode surface properties which causes a significant increase in the oxidation peak current of biomolecules such as dopamine, L-epinephrine, D,L-norepinephrine and catechol. This phenomenon might be attributed to the formation of an inclusion complex between beta-CD and the catecholamines. The amount of beta-CD necessary to form the Sonogel electrode was studied and optimization of electrochemical parameters, perm selectivity and mechanical stability of the sensor are discussed. Scanning electron microscopy and electrochemical impedance spectroscopy measurements were employed to characterize the electrical parameters and the structural properties of the new electrode surface, respectively. Cyclic voltammetry (CV) and Adsorptive differential pulse voltammetry (AdDPV) measurements were also used to explore the electrochemical behaviour of the electrode versus the quoted catecholamines. The beta-CD-Sonogel-Carbon electrode offers fast and linear responses towards dopamine, norepinephrine, epinephrine and catechol, with good and low detection limits: 0.164, 0.294, 0.699 and 0.059 micromol L(-1), respectively.

Alumina sol-gel/sonogel-carbon electrode based on acetylcholinesterase for detection of organophosphorus pesticides.

Two new amperometric biosensors based on immobilization of acetylcholinesterase on a sonogel-carbon electrode for detection of organophosphorous compounds are proposed. The electrodes were prepared applying high-energy ultrasounds directly to the precursors. The first biosensor was obtained by simple entrapping acetylcholinesterase in Al(2)O(3) sol-gel matrix on the sonogel-carbon. The second biosensor was produced in a sandwich configuration. Its preparation involved adsorption of the enzyme and modification via a polymeric membrane such as polyethylene glycol and the ion-exchanger Nafion. The optimal enzyme loading was found to be 0.7 mIU. Both biosensors showed optimal activity in 0.2 M phosphate buffer, pH 7.0, at an operating potential of 210 mV. The detection limit achieved for chlorpyriphos-ethyl-oxon was 2.5x10(-10)M at a 10-min incubation time.

Investigation of biosensor signal bioamplification: comparison of direct electrochemistry phenomena of individual Laccase, and dual Laccase-Tyrosinase copper enzymes, at a Sonogel-Carbon electrode.

Direct electrochemistry of Trametes versicolor Laccase (LAC) was found at a Sonogel-Carbon electrode. The bioamplification, performed by dual immobilization of this enzyme and Mushroom Tyrosinase (TYR), of the bio-electrocatalytic reduction of O(2) was investigated. The calculated alpha transfer coefficients were 0.64 and 0.67, and the heterogeneous electron-transfer rate constants were 6.19 and 8.52 s(-1), respectively, for the individual LAC and dual LAC-TYR-based Nafion/Sonogel-Carbon bio-electrodes. The responses of the dual enzymes electrode to polyphenols were stronger than those of the individual LAC or TYR biosensors. Hypotheses are offered about the mechanism of bioamplification. The surfaces of the biosensors were also characterized by AFM.

A comparison of three amperometric phenoloxidase-Sonogel-Carbon based biosensors for determination of polyphenols in beers.

Three phenoloxidases based biosensors were successfully developed using as electrochemical transducer a new type of electrode recently developed by our group: the "Sonogel-Carbon electrode". The employed enzymes were Trametes versicolor laccase (Lac), Mushroom tyrosinase (Tyr), and Horseradish peroxidase (HRP). Immobilization step was accomplished by doping the electrode surface with a mixture of the individual enzyme and Nafion ion exchanger as additive-protective. The biosensor responses, optimized in beer real samples, were evaluated for five individual polyphenols. It was found that the developed biosensors were sensitive to nanomolar concentrations of the tested polyphenols. As example, the limit of detection, sensitivity, and response linear range for caffeic acid for Nafion-Lac/Sonogel-Carbon biosensor were 0.06µmolL(-1), 99.6nAµmol(-1)L, and 0.04-2µmolL(-1), respectively. The stability and reproducibility of the biosensors were evaluated by applying them directly to beer real samples. It has been demonstrated that the Nafion-Lac/Sonogel-Carbon system is the more stable with a relative standard deviation of 3.3% (n=10), maintaining 84% of its stable response for at least three weeks. Estimation of polyphenol index in eight lager beers and a comparison of the results with those obtained by a classical method was carried out.

Dual laccase-tyrosinase based Sonogel-Carbon biosensor for monitoring polyphenols in beers.

A biosensor based on the bi-immobilization of laccase and tyrosinase phenoloxidase enzymes has been successfully developed. This biosensor employs as the electrochemical transducer the Sonogel-Carbon, a novel type of electrode developed by our group. The immobilization step was accomplished by doping the electrode surface with a mixture of the enzymes, glutaricdialdehyde, and Nafion-ion exchanger, as protective additive. The response of this biosensor, denoted the dual Trametes versicolor laccase (La) and Mushroom tyrosinase (Ty) based Sonogel-Carbon, was optimized directly in beer real samples and its analytical performance with respect to five individual polyphenols was evaluated. The Lac-Ty/Sonogel-Carbon electrode responds to nanomolar concentrations of flavan-3-ols, hydroxycinnamic acids, and hydroxybenzoic acids. The limit of detection, sensitivity, and linear range for caffeic acid, taken as an example, were 26 nM, 167.53 nA M (-1), and 0.01-2 microM, respectively. In addition, the stability and reproducibility of the biosensor were also evaluated in beer samples. The Lac-Ty/sonogel-carbon electrode was verified as very stable in this matrix, maintaining 80% of its stable response for at least three weeks, with a RSD of 3.6% ( n = 10). The biosensor was applied to estimate the total polyphenol index in ten beer samples, and a correlation of 0.99 was obtained when the results were compared with those obtained using the Folin-Ciocalteau reagent.

The Sonogel-Carbon materials as basis for development of enzyme biosensors for phenols and polyphenols monitoring: a detailed comparative study of three immobilization matrixes.

Three amperometric biosensors based on immobilization of tyrosinase on a new Sonogel-Carbon electrode for detection of phenols and polyphenols are described. The electrode was prepared using high energy ultrasounds (HEU) directly applied to the precursors. The first biosensor was obtained by simple adsorption of the enzyme on the Sonogel-Carbon electrode. The second and the third ones, presenting sandwich configurations, were initially prepared by adsorption of the enzyme and then modification by mean of polymeric membrane such as polyethylene glycol for the second one, and the ion-exchanger Nafion in the case of the third biosensor. The optimal enzyme loading and polymer concentration, in the second layer, were found to be 285 U and 0.5%, respectively. All biosensors showed optimal activity at the following conditions: pH 7, -200 mV, and 0.02 mol l(-1) phosphate buffer. The response of the biosensors toward five simple phenols derivatives and two polyphenols were investigated. It was found that the three developed tyrosinase Sonogel-Carbon based biosensors are in satisfactory competitiveness for phenolic compounds determination with other tyrosinase based biosensors reported in the literature. The detection limit, sensitivity, and the apparent Michaelis-Menten constant K(m)(app) for the Nafion modified biosensor were, respectively, 0.064, 0.096, and 0.03 micromol, 82.5, 63.4, and 194 nA micromol(-1)l(-1), and 67.1, 54.6, and 12.1 micromol l(-1) for catechol, phenol, and 4-chloro-3-methylphenol. Hill coefficient values (around 1 for all cases), demonstrated that the immobilization method does not affect the nature of the enzyme and confirms the biocompatibility of the Sonogel-Carbon with the bioprobe. An exploratory application to real samples such as beers, river waters and tannery wastewaters showed the ability of the developed Nafion/tyrosinase/Sonogel-Carbon biosensor to retain its stable and reproducible response.

Electrochemical analysis of endosulfan using a C18-modified carbon-paste electrode.

Successful applications of different analytical procedures to determine quantitatively endosulfan and its metabolites in aqueous media can be found in recent literature. Fundamentally, they have made use of solid-phase extraction (SPE) and gas (GC) or liquid chromatography (LC), sometimes coupled to mass spectrometry (MS). In this paper, a new and alternative methodology to determine quantitatively endosulfan in aqueous media is reported. A C18-modified carbon-paste electrode has been used to determine voltammetrically endosulfan, despite its unfavourable electrochemical properties and behaviour. The methodology proposed is based on the decrease experienced by the peak intensity corresponding to voltammetric signals of Cu(II) when successive and constant additions of endosulfan are carried out. This decrease is directly proportional to the concentration of endosulfan what allows to perform an indirect quantification of the pesticide. The detection limit obtained is 40 ng l(-1), this value being under the limits specified by European norms and EPA reports.

The Sonogel-Carbon electrode as a sol-gel graphite-based electrode.

In this work, a novel sol-gel-based procedure to obtain solid carbon composite electrodes is proposed. The procedure is based on the use of sonocatalysis to apply high-energy ultrasound directly to the precursors; this way, ultrasonic cavitation is achieved so that hydrolysis with acidic water is promoted in the absence of any additional solvent and the time needed to get a unique phase is reduced drastically. The mix of sonogel with spectroscopic grade graphite leads to a new type of sol-gel electrode: the Sonogel-Carbon electrode. The amount of water, necessary for hydrolysis to occur, has been studied, as well as the sol pH value and the electrochemical behavior of the Sonogel-Carbon electrode in Britton-Robinson buffer. Mechanical renewal of the electrodes surfaces and background signal stability for at least 42 days have been also tested. We have carried out some tests to check the ability of the synthesized material to give electrochemical response when redox species are present in the background electrolyte. Mechanical and electrochemical studies indicate that the Sonogel-Carbon electrode shows good properties for use as an electrochemical sensor.