Gel polymer electrolyte-based dual screen-printed electrodes for the headspace quantification of 4-ethylphenol and ethanethiol simultaneously in wines.

The identification and correction of negative factors, such as 4-ethylphenol and ethanethiol, is important to comply with food safety regulations and avoid economic losses to wineries. A simple amperometric measurement procedure that facilitates the simultaneous quantification of both compounds in the gas phase has been developed using fullerene and cobalt (II) phthalocyanine-modified dual screen-printed electrodes coated with a room temperature ionic liquid-based gel polymer electrolyte. The replacement of the typical aqueous supporting electrolyte by low-volatility ones improves both operational and storage lifetime. Under the optimum conditions of the experimental variables, Britton Robinson buffer pH 5 and applied potentials of + 0.86 V and + 0.40 V for each working electrode (vs. Ag ref. electrode), reproducibility values of 7.6% (n = 3) for 4-ethylphenol and 6.6% (n = 3) for ethanethiol were obtained, as well as capability of detection values of 23.8 µg/L and decision limits of 1.3 µg/L and 9.2 µg/L (α = ß = 0.05), respectively. These dual electrochemical devices have successfully been applied to the headspace detection of both compounds in white and red wines, showing their potential to be routinely used for rapid analysis control in wineries.

Simultaneous determination of 4-ethylphenol and 4-ethylguaicol on C60 modified dual screen-printed electrochemical sensors.

4-ethylphenol and 4-ethylguaicol levels in wine are associated to organoleptic defects that cause consumer rejection accompanied by significant economic losses for producers. Thus, electrochemical sensors based on screen-printed carbon electrodes (SPCEs) modified with activated fullerene C60 (AC60) have been developed for the analysis of both phenols by direct headspace amperometric measurements. Upon optimization of the experimental variables affecting the sensors performance, the AC60/SPCE sensors presented linearity ranges from 9.9 to 65.4 µg/L and from 19.6 to 107.1 µg/L for 4-ethylphenol and 4-ethylguaicol, respectively. The achieved detection capacities were 10.3 µg/L (4-ethylphenol) and 19.6 µg/L (4-ethylguaicol), with a reproducibility of 6.3 % and 9.1 % (n = 3), respectively. In addition, dual-working AC60/SPCE devices were developed for the simultaneous analysis of both phenols using different working potentials for each electrode. The dual systems were successfully applied in the analysis of different spiked wine samples, obtaining good recoveries ranging from 94 to 108 %.

Headspace detection of ethanethiol in wine by cobalt phthalocyanine modified screen-printed carbon electrodes.

The formation of thiols has a notable and detrimental sensory impact, especially in the aroma of bottled wines. Their detection in wine is of great interest to avoid important economic and image losses for wineries. This work reports the study of different cobalt phthalocyanine/nanomaterials-based sensors for the headspace detection of volatile thiols. The amperometric procedure based on the use of carbon sensors simply modified with cobalt phthalocyanine showed the best performance. Under the optimum conditions of applied potential, +0.8 V, and pH of the supporting electrolyte, 2.6, this procedure shows a reproducibility of 7% (n = 5) in terms of relative standard deviation of the slopes of calibration curves built in the concentration range from 9.9 to 82.6 µg/L, a capability of detection of 12.5 µg/L and a decision limit of 6.5 µg/L (α = ß = 0.05). The use of this electrocatalytic material and the headspace measurements reduce interferents, increasing the selectivity of the procedure, which allows the easy and successful quantification of ethanethiol in white and red wines.

Electrochemical sensors for the determination of 4-ethylguaiacol in wine.

The development of an electrochemical procedure for the determination of 4-ethylguaiacol and its application to wine analysis is described. Modified screen-printed carbon electrodes (SPCEs) with fullerene C60 (C60) have been shown to be efficient in this kind of analysis. The developed activated C60/SPCEs (AC60/SPCEs) were adequate for the determination of 4-ethylguaicol, showing a linear range from 200 to 1000 µg/L, a reproducibility of 7.6% and a capability of detection (CCß) value of 200 µg/L, under optimized conditions. The selectivity of the AC60/SPCE sensors was evaluated in the presence of possibly interfering compounds, and their practical applicability was demonstrated  in the analysis of different wine samples obtaining recoveries ranging from 96 to 106%.

Electrochemical devices for cholesterol detection.

Cholesterol can be considered as a biomarker of illnesses such as heart and coronary artery diseases or arteriosclerosis. Therefore, the fast determination of its concentration in blood is interesting as a means of achieving an early diagnosis of these unhealthy conditions. Electrochemical sensors and biosensors have become a potential tool for selective and sensitive detection of this biomolecule, combining the analytical advantages of electrochemical techniques with the selective recognition features of modified electrodes. This review covers the different approaches carried out in the development of electrochemical sensors for cholesterol, differentiating between enzymatic biosensors and non-enzymatic systems, highlighting lab-on-a-chip devices. A description of the different modification procedures of the working electrode has been included and the role of the different functional materials used has been discussed.

Molecularly imprinted polypyrrole based electrochemical sensor for selective determination of ethanethiol.

This work describes a molecularly imprinted (MIP) sensor, based on the electropolymerization of pyrrole on a glassy carbon electrode (GCE), for the determination of ethanethiol. Ethanethiol was used as a template molecule for the formation of cavities in the imprinted polymer. The effect of molar ratios template molecules/functional monomers and time needed to remove the template were optimized. The developed MIP/GCE sensor presented a linear range from 6.1 to 32.4 mg L-1 with capability detection and reproducibility values of 7.2 mg L-1 and 10.4%, respectively. The sensitivity of the developed sensor was enhanced by the incorporation of gold nanoparticles (AuNPs). The AuNPs/MIP/GCE showed a capability of detection and reproducibility values of 0.4 mg L-1 and 4.1%, respectively (calibration range from 0.3 to 3.1 mg L-1). The sensor was successfully applied to the determination of ethanethiol in spiked wine samples with recoveries ranging from 99% to 107%.

Molecularly imprinted polypyrrole based electrochemical sensor for selective determination of 4-ethylphenol.

This work describes the development of an electrochemical sensor based on a molecularly imprinted polymer (MIP) for sensitive and selective determination of 4-ethylphenol in wine. The sensor has been built by means of the electrosynthesis of the MIP on a glassy carbon electrode surface using cyclic voltammetry. The electropolymerization has been performed in the presence of 4-ethylphenol and pyrrole as template molecule and functional monomer, respectively. The influence of the molar ratios of template molecules to functional pyrrole monomers and the time needed to remove the template have been optimized taking into account the differential pulse voltammetric response of 4-ethylphenol. Under the optimal experimental conditions the developed MIP/GCE sensor shows good capability of detection (0.2 µM, α = ß = 0.05) and reproducibility (3.0%) in the concentration range from 0.2 to 34.8 µM. The influence of possible interfering species in the analytical response has been studied and the sensor has successfully been applied to the determination of 4-ethylphenol in different wine samples.

Determination of ascorbic acid in serum samples by screen-printed carbon electrodes modified with gold nanoparticles.

This paper shows the ability of simple modified screen-printed devices for the electrochemical detection of ascorbic acid in different samples, including serum samples. Even if the presence of uric acid does not greatly affect the analysis of ascorbic acid using screen-printed carbon electrodes modified by gold nanoparticles, dopamine must be considered an interferent. Thus, the joint determination of the three compounds has been set out using methods of multivariate calibration. The validated models, built by partial least squares regressions, have been used to determine the concentration of ascorbic acid, dopamine and uric acid of spiked serum samples, with successful results.

Cytochrome P450 2D6 based electrochemical sensor for the determination of codeine.

Considering the enzymatic activity of the cytochrome P450 2D6 on substrates such as codeine, the current paper includes the development of an enzymatic biosensor for detection of this drug. Home-made screen-printed electrodes were used as electrochemical transducers of the biosensor, in which the enzyme was covalently attached to the carbon surface of the working electrode, this type of modification being the most suitable for the immobilization of the biological element. Chronoamperometric measurements were carried out under optimum conditions of pH and working potential, pH 7 and +200 mV vs. screen-printed Ag/AgCl electrode, giving a reduction signal related to the concentration of codeine in solution. Consecutive additions of a solution of codeine were performed to obtain calibration curves in order to validate the electrochemical method in terms of precision and calculate its capability of detection. These biosensors were used for the determination of codeine in urine and commercial pharmaceutical samples.

Speciation of chromium using chronoamperometric biosensors based on screen-printed electrodes.

Chronoamperometric assays based on tyrosinase and glucose oxidase (GOx) inactivation have been developed for the monitoring of Cr(III) and Cr(VI). Tyrosinase was immobilized by crosslinking on screen-printed carbon electrodes (SPCEs) containing tetrathiafulvalene (TTF) as electron transfer mediator. The tyrosinase/SPC(TTF)E response to pyrocatechol is inhibited by Cr(III). This process, that is not affected by Cr(VI), allows the determination of Cr(III) with a capability of detection of 2.0±0.2 µM and a reproducibility of 5.5%. GOx modified screen-printed carbon platinised electrodes (SPCPtEs) were developed for the selective determination of Cr(VI) using ferricyanide as redox mediator. The biosensor was able to discriminate two different oxidation states of chromium being able to reject Cr(III) and to detect the toxic species Cr(VI). Chronoamperometric response of the biosensor towards glucose decreases with the presence of Cr(VI), with a capability of detection of 90.5±7.6 nM and a reproducibility of 6.2%. A bipotentiostatic chronoamperometric biosensor was finally developed using a tyrosinase/SPC(TTF)E and a GOx/SPC(Pt)E connected in array mode for the simultaneous determination of Cr(III) and Cr(VI) in spiked tap water and in waste water from a tannery factory samples.

Sensitive and selective cocaine electrochemical detection using disposable sensors.

This paper describes the voltammetric determination of cocaine in presence of three different interferences that could be found in street samples using disposable sensors. The electrochemical analysis of this alkaloid can be affected by the presence of codeine, paracetamol or caffeine, whose oxidation peaks may overlap and lead to false positives. This work describes two different solutions to this problem. On one hand, the modification of disposable carbon sensors with carbon nanotubes allows the voltammetric quantification of cocaine by using ordinary least squares regressions in the concentration range from 10 to 155 µmol L(-1), with a reproducibility of 5.6% (RSD, n = 7. On the other hand, partial least squares regressions are used for the resolution of the overlapped voltammetric signals when using screen-printed carbon electrodes without any modification. Both procedures have been successfully applied to the evaluation of the purity of cocaine street samples.

Sulfite oxidase biosensors based on tetrathiafulvalene modified screen-printed carbon electrodes for sulfite determination in wine.

Screen-printed carbon electrodes have been modified with tetrathiafulvalene and sulfite oxidase enzyme for the sensitive and selective detection of sulfite. Amperometric experimental conditions were optimized taking into account the importance of quantifying sulfite in wine samples and the inherent complexity of these samples, particularly red wine. The biosensor responds to sulfite giving a cathodic current (at +200 mV vs screen-printed Ag/AgCl electrode and pH 6) in a wide concentration range, with a capability of detection of 6 µM (α=ß=0.05) at 60°C. The method has been applied to the determination of sulfite in white and red samples, with averages recoveries of 101.5% to 101.8%, respectively.

Screen-printed biosensor based on the inhibition of the acetylcholinesterase activity for the determination of codeine.

The current paper presents the chronoamperometric determination of codeine using screen-printed carbon electrodes that incorporate tetrathiafulvalene in the matrix of the working electrode, as mediator, and cross-linked acetylcholinesterase. Applying a potential of +250 mV, a 1mM solution of acetylthiocholine in electrolyte solution pH 7 gives an oxidation signal due to the dimerization of its metabolite after the reaction with the enzyme. This electrochemical signal is decreased by consecutive additions of a solution of codeine, which allows the performance of curves of calibration for the validation of this electrochemical method, giving a reproducibility of 3.31% (n=6) and a capability of detection of 20 µM. This type of inhibition has been studied by means of a Lineweaver-Burk plot. Additionally, the developed biosensor was used for the determination of the quantity of codeine in pharmaceutical commercial tablets and urine samples.

Electrochemical determination of cocaine using screen-printed cytochrome P450 2B4 based biosensors.

This paper describes the development of screen-printed cytochrome P450 2B4 based biosensors, from the fabrication of the device using a single screen-printing technology to its characterization and application in the determination of cocaine street samples. Voltammetric measurements were first performed in order to study the electrochemical behavior of the enzyme trapped in the structure of the working electrode, as well as its interaction with one of its substrates, cocaine. Then, chronoamperometric measurements were carried out for the characterization of the biosensors. Calibration curves under optimum conditions of the experimental variables have been used for establishing the reproducibility (5.91%, n=5), the repeatability (10.45%, n=3) and the capability of detection (0.2mM, α=ß=0.05) of the described method in the calibration range from 0.2 to 1.2mM. Finally, the performance of the developed biosensors has been successfully probed by the analysis of the purity of cocaine street samples.

Screen-printed acetylcholinesterase-based biosensors for inhibitive determination of permethrin.

An amperometric assay based on acetylcholinesterase (AChE) inactivation has been developed for the monitoring of permethrin using a screen-printed three-electrode system. The enzyme AChE catalyzes the hydrolysis of acetylthiocholine to thiocholine, which can be electrochemically oxidized. The presence of permethrin inhibits the AChE activity, resulting in a lower thiocholine production and thus, a decrease in the amperometric oxidation current. Immobilization of AChE was performed by cross-linking giving a capability of detection of 8.1±0.4 µM. Repeatability and reproducibility of the developed AChE biosensor were also calculated, yielding values of 9.6% (n=4) and 5.4% (n=5), respectively related to the slopes of the calibration curves performed in the range from 6.2 up to 41 µM. The method was successfully applied to the determination of permethrin content in a commercial lice gel.

Fabrication and characterization of disposable sensors and biosensors for detection of formaldehyde.

Screen-printed carbon electrodes (SPCEs) electrochemically platinised (Pt-SPCEs) and screen-printed platinised carbon electrodes (SPC(Pt)Es) have been chronoamperometrically characterized for the determination of formaldehyde (FA). The oxidation current registered at 600 mV in the FA concentration range from 0.99 to 9.09 mmol L(-1), led to higher precision values in terms of repeatability and reproducibility when using SPC(Pt)Es. SPC(Pt)Es have been also used for alcohol oxidase (AOX) cross-linked immobilization in the development of enzymatic biosensors for FA. The enzymatic reaction produces hydrogen peroxide, which has been chronoamperometrically monitored and related to FA concentration in different kinds of samples. Experimental design methodology has been performed to optimize the pH and the applied potential. This method has shown a repeatability and reproducibility of 3% (n=4) and 6% (n=4) respectively, related to the slopes of the calibration curves performed in the range from 60 to 460 µmol L(-1). The use of this kind of biosensor, which has a detection capability of 60 µmol L(-1) for a given probability of false positive and negative equal to 0.05, has been probed in the determination of FA in commercial samples for histology.

Horseradish peroxidase-screen printed biosensors for determination of Ochratoxin A.

This work summarizes the manufacturing procedure of Horseradish peroxidase (HRP) based biosensors for the determination of the mycotoxin Ochratoxin A (OTA). The biosensors have been fabricated using the single technology of screen-printing. That is to say, an HRP containing ink has been directly screen-printed onto carbon electrodes, which offers a higher rapidity and simplicity in the manufacturing process of biosensors for OTA determination. The formal redox potential of the Fe(III/II) moiety of HRP has been used to demonstrate the effective loading of enzyme into the ink. The chronoamperometric oxidation current registered has been successfully related to the concentration of OTA in solution from different samples, including beer ones. Under the optimum conditions of the experimental variables, precision in terms of reproducibility and repeatability has been calculated in the concentration range from 23.85 to 203.28 nM. A relative standard deviation for the slopes of 10% (n = 4) was obtained for reproducibility. In the case of repeatability, the biosensor retained a 30% of the initial sensitivity after the third calibration. The average capability of detection for 0.05% probabilities of false positive and negative was 26.77 ± 3.61 nM (α = 0.05 and ß=0.05, n = 3).

CYP450 biosensors based on screen-printed carbon electrodes for the determination of cocaine.

A new electrochemical method has been described and characterized for the determination of cocaine using screen-printed biosensors. The enzyme cytochrome P450 was covalently attached to screen-printed carbon electrodes. Experimental design methodology has been performed to optimize the pH and the applied potential, both variables that have an influence on the chronoamperometric determination of the drug. This method showed a reproducibility of 3.56% (n=4) related to the slopes of the calibration curves performed in the range from 19 up to 166nM. It has been probed the used of this kind of biosensors in the determination of cocaine in street samples, with an average capability of detection of 23.05±3.53nM (n=3, α=ß=0.05).

Electrochemical sensors in the development of selective methods for antiepileptic drugs determination.

The measurement of antiepileptic drugs (AEDs) in different samples has received considerable attention due to the directly correlation between clinical effects and plasma concentration. Numerous methods have been extensively applied to the analysis of AEDs since many years ago providing reliable and accurate results. This paper provides an overview of electrochemical techniques used for the analysis of different AEDs. More than sixty papers from refereed analytical chemistry journals on the analysis of AEDs in pharmaceutical formulations and biological samples are included. The present review shows the development of novel measurement electrochemical based on the use of different types of electrodes including mercury electrodes, screen-printed electrodes (SPEs) and electrodes modified with metal films and nanoparticles. Electrochemical biosensors and immunosensors developed for the analysis of AEDs have been also reviewed.

Disposable biosensors for determination of biogenic amines.

This work reports monoamine oxidase (MAO)/horseradish peroxidase (HRP) and diamine oxidase (DAO)/horseradish peroxidase (HRP) based biosensors using screen-printed carbon electrodes for the determination of biogenic amines (BA). The enzymes have been covalently immobilized onto the carbon working electrode, previously modified by an aryl diazonium salt, using hydroxysuccinimide and carbodiimide. The detection has been performed by measuring the cathodic current due to the reduction of the mediator hydroxymethylferrocene at a low potential, 250 mV vs screen-printed Ag/AgCl reference electrode. The experimental conditions for the enzymes immobilization, as well as for the main variables that can influence the chronoamperometric current have been optimized by the experimental design methodology. Under these optimum conditions, the disposable biosensors have been characterized. A linear response range from 0.2 up to 1.6 microM and from 0.4 to 2.4 microM of histamine was obtained for DAO/HRP and MAO/HRP based biosensors, respectively. The biosensor construction was highly reproducible, yielding relative standard deviations of 10% and 11% in terms of sensitivity for DAO/HRP and MAO/HRP based biosensors, respectively. The capability of detection, 0.18+/-0.01 microM in the case of DAO/HRP and 0.40+/-0.04 microM (alpha=0.05 and beta=0.005) for MAO/HRP based biosensors, and the biosensor sensitivity towards different BA has also been analyzed. Finally, the developed biosensors have been applied to the determination of the total amine content in fish samples.