Voltammetric determination of taurine in energy drinks after o-phthalaldehyde-ethanethiol derivatization.

A completely new voltammetric method has been developed for quantitative determination of food additive Taurine (Tau) in energy drinks. This electroanalytical method is based on voltammetric oxidation of o-phthalaldehyde-ethanthiol derivative of Tau at glassy carbon electrode in 95% methanol containing 0.1 mol L-1 lithium perchlorate. Working conditions necessary for quantitative Tau derivatization reaction and electrochemical detection using square wave voltammetry were optimized. Linear range from 1.0 × 10-5 to 1.0 × 10-4 mol L-1 characterized by coefficient of determination 0.9998, limits of quantification 6.8 × 10-6 mol L-1 and detection 2.1 × 10-6 mol L-1 were obtained at pulse amplitude 50 mV and frequency 80 Hz. Analytical method of calibration curve was used for evaluation of Tau content in several commercially available energy drinks. The procedure was validated using standard reference high performance liquid chromatography (HPLC) method. Both methods showed nearly identical Tau content, around 0.35% (w/w). Besides its reliability to the Tau determination, that is totally comparable to reference method used, present voltammetric approach is more advantageous on the economic and simplicity basis. Finally, developed voltammetric method could find employment in food quality control.

Recent Applications of Silver Amalgam Electrodes for Analysis of Pharmaceuticals and Vitamins.

The features and applications of silver amalgam electrodes in electroanalysis of pharmaceuticals and vitamins are summarized in this review. The state-of-the-art in the preparation and construction of solid silver amalgam electrodes for prolonged and userfriendly use is presented. Although not as widely spread as other electrode materials, silver amalgam possesses a unique and viable combination of favorable electrochemical properties, which are close to ideal mercury electrodes, non-toxic character, high durability and mechanical stability, if properly prepared. Its capability of conducting the redox processes at highly negative potentials is essential for the analysis of large number of bioactive organic compounds. The review features also overview information for each application in drug or vitamin analysis divided to the sections according to the construction variants of silver amalgam electrodes.

Immunosensors in Early Cancer Diagnostics: From Individual to Multiple Biomarker Assays.

Cancer is a widespread disease characterized by high mortality. To improve the survival rate or facilitate effective therapy, accurate and reliable diagnosis at an early stage is needed. For this reason, there is a continuous push to develop sensitive methods which can be used in cancer diagnosis. Current diagnosis relies on the quantification of cancer biomarkers defined as molecules that are measurable in body fluids or tissues and indicate a change in physiological processes with subsequent pathological manifestations. This contribution reviews recent developments in the area of electrochemical immunosensors applicable for the detection of cancer biomarkers that occur in a wide concentration range including extremely low levels, which are typical for the early stage of the disease. A summary of various antibody labels used for biomarker analysis and combined with electrochemical detection is presented. The potential of multiple biomarker analysis, with its indisputable clinical impact for accurate diagnosis, is also highlighted.

Tyrosinase Electrochemical Biosensors Monitoring Medicinally Significant Substances.

Nowadays in biosensing, development of analytical techniques is focused on improvement for monitoring biologically active species in living organisms, especially in real-time analysis. This article provides an overview of applications of electrochemical tyrosinase biosensors in the analysis of medicinally significant substances, also known as biomarkers. At the beginning, special attention is paid to characterisation of the tyrosinase enzyme, explanation of reaction mechanisms of tyrosinase with various types of electrochemical transducers and techniques needed for stable immobilization of this biocatalyst on the transducer surface used. In this case, amperometric transducers represent the most frequently used type of electrochemical sensing because they usually provide sensitive current response to the presence of analyte in the sample. Many scientific works suggest that these very selective bioanalytical devices could find application in the clinical diagnosis of various serious diseases because they represent the effective analytical tools for diagnosis of neurodegenerative disorders, detection of microbial pathogens responsible for foodborne illness and diagnosis of cytopathology, especially melanoma cancer. Additionally, various kinds of electrochemical tyrosinase biosensors are presented which were developed to determine the catalytic activity of other enzymes significant for human metabolism, medicaments and their metabolic products, several hormones, amino acids and proteins. Despite all of their advantages, it is necessary to state that tyrosinase biosensors are still not used in the routine laboratory practice due to their relatively short service life which is not longer than one month unfortunately. Never-ending development of catalytic polymers imitating the tyrosinase active site could be one of ways to solve this serious drawback.

Trace level voltammetric determination of lead and cadmium in sediment pore water by a bismuth-oxychloride particle-multiwalled carbon nanotube composite modified glassy carbon electrode.

Two multiwalled carbon nanotubes-based composites modified with bismuth and bismuth-oxychloride particles were synthesized and attached to the glassy carbon electrode substrate. The resultant configurations, Bi/MWCNT-GCE and BiOCl/MWNT-GCE, were then characterized with respect to their physicochemical properties and electroanalytical performance in combination with square-wave anodic stripping voltammetry (SWASV). Further, some key experimental conditions and instrumental parameters were optimized; namely: the supporting electrolyte composition, accumulation potential and time, together with the parameters of the SWV-ramp. The respective method with both electrode configurations has then been examined for the trace level determination of Pb(2+) and Cd(2+) ions and the results compared to those obtained with classical bismuth-film modified GCE. The different intensities of analytical signals obtained at the three electrodes for Pb(2+) and Cd(2+) vs. the saturated calomel reference electrode had indicated that the nature of the modifiers and the choice of the supporting electrolyte influenced significantly the corresponding stripping signals. The most promising procedure involved the BiOCl/MWCNT-GCE and the acetate buffer (pH 4.0) offering limits of determination of 4.0 µg L(-1) Cd(2+) and 1.9 µg L(-1) Pb(2+) when accumulating for 120 s at a potential of -1.20 V vs. ref. The BiOCl/MWCNT electrode was tested for the determination of target ions in the pore water of a selected sediment sample and the results agreed well with those obtained by graphite furnace atomic absorption spectrometry.

Combined macro-/mesoporous microelectrode arrays for low-noise extracellular recording of neural networks.

Microelectrode arrays (MEAs) are appealing tools to probe large neural ensembles and build neural prostheses. Microelectronics microfabrication technologies now allow building high-density MEAs containing several hundreds of microelectrodes. However, several major problems become limiting factors when the size of the microelectrodes decreases. In particular, regarding recording of neural activity, the intrinsic noise level of a microelectrode dramatically increases when the size becomes small (typically below 20-µm diameter). Here, we propose to overcome this limitation using a template-based, single-scale meso- or two-scale macro-/mesoporous modification of the microelectrodes, combining the advantages of an overall small geometric surface and an active surface increased by several orders of magnitude. For this purpose, standard platinum MEAs were covered with a highly porous platinum overlayer obtained by lyotropic liquid crystal templating possibly in combination with a microsphere templating approach. These porous coatings were mechanically more robust than Pt-black coating and avoid potential toxicity issues. They had a highly increased active surface, resulting in a noise level ∼3 times smaller than that of conventional flat electrodes. This approach can thus be used to build highly dense arrays of small-size microelectrodes for sensitive neural signal detection.

Voltammetric determination of the herbicide Linuron using a tricresyl phosphate-based carbon paste electrode.

This paper summarises the results of voltammetric studies on the herbicide 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (Linuron), using a carbon paste electrode containing tricresyl phosphate (TCP-CPE) as liquid binder. The principal experimental conditions, such as the pH effect, investigated in Britton-Robinson buffer solutions (pH 2.0-7.0), the peak characteristics for the analyte of interest, or instrumental parameters for the differential pulse voltammetric mode were optimized for the method. As found out, the best electroanalytical performance of the TCP-CPE was achieved at pH 2.0, whereby the oxidation peak of Linuron appeared at ca. +1.3 V vs. SCE. The analytical procedure developed offers good linearity in the concentration range of 1.25-44.20 µg mL(-1) (1.77 × 10(-4)-5.05 × 10(-6) mol L(-1)), showing-for the first time-the applicability of the TCP-CPE for anodic oxidations in direct voltammetry (without accumulation). The method was then verified by determining Linuron in a spiked river water sample and a commercial formulation and the results obtained agreed well with those obtained by the reference HPLC/UV determination.

Stripping voltammetric determination of mercury(II) at antimony-coated carbon paste electrode.

A new procedure was elaborated to determine mercury(II) using an anodic stripping square-wave voltammetry at the antimony film carbon paste electrode (SbF-CPE). In highly acidic medium of 1M hydrochloric acid, voltammetric measurements can be realized in a wide potential window. Presence of cadmium(II) allows to separate peaks of Hg(II) and Sb(III) and apparently catalyses reoxidation of electrolytically accumulated mercury, thus allowing its determination at ppb levels. Calibration dependence was linear up to 100 ppb Hg with a detection limit of 1.3 ppb. Applicability of the method was tested on the real river water sample.

Screen-printed carbon electrodes modified by rhodium dioxide and glucose dehydrogenase.

The described glucose biosensor is based on a screen-printed carbon electrode (SPCE) modified by rhodium dioxide, which functions as a mediator. The electrode is further modified by the enzyme glucose dehydrogenase, which is immobilized on the electrode's surface through electropolymerization with m-phenylenediamine. The enzyme biosensor was optimized and tested in model glucose samples. The biosensor showed a linear range of 500-5000 mg L(-1) of glucose with a detection limit of 210 mg L(-1) (established as 3σ) and response time of 39 s. When compared with similar glucose biosensors based on glucose oxidase, the main advantage is that neither ascorbic and uric acids nor paracetamol interfere measurements with this biosensor at selected potentials.

New approaches to the characterization of carbon paste electrodes using the ohmic resistance effect and qualitative carbon paste indexes.

In this article, some new approaches to characterize the carbon paste mixtures and the respective carbon paste electrodes (CPEs) are presented, discussed, and critically evaluated. Particular attention has been paid to the changes of the ohmic resistance, relative to the dependence on composition of the CPE, the materials used, the time, and the position of storage. Four types of carbon pastes were examined, and for the interpretation of experimental data, a new simple model of "close-packing of spheres" has been applied. This model resembles the percolation theory for solid matter. In our case, however, it is possible to explain not only the "bent" or "broken" shape of the dependence of the electrode resistance upon the binder:carbon ratio and the corresponding electrochemical current response, but also differences caused by various material used and three various effects observed during the electrode aging. Furthermore, the report presents the significance of practical utilization of the recently introduced carbon paste index (denoted as chi(CPE)), which is a qualitative hitherto unused factor based on the evaluation of cyclic voltammograms for standard redox systems (e.g., [Fe(CN)(6)](3-/4-)) and specifying the electrochemical properties of a CPE. Some problems connected with homogeneity and stability of carbon pastes, their handling, storage, or eventual aging effects are also discussed.

A spectroelectrochemical approach to the electrodeposition of bismuth film electrodes and their use in stripping analysis.

UV-vis reflection spectroelectrochemistry has proven to be a very useful multiresponse technique to evaluate the quality of bismuth films obtained by electrochemical deposition on glassy carbon electrodes (GCEs). Bismuth films have recently emerged as a promising and environmental friendly alternative to mercury electrodes for stripping analysis. Spectroelectrochemical measurements, carried out in a flow cell, allowed us to follow in situ the electrodeposition and stripping of bismuth and cadmium. Electrochemical and spectroscopic responses individually have led to successfully quantify the amount of cadmium in test solutions.

Enzyme-Linked Electrochemical Detection of PCR-Amplified Nucleotide Sequences Using Disposable Screen-Printed Sensors. Applications in Gene Expression Monitoring.

Electrochemical enzyme-linked techniques for sequence-specific DNA sensingare presented. These techniques are based on attachment of streptavidin-alkalinephosphatase conjugate to biotin tags tethered to DNA immobilized at the surface ofdisposable screen-printed carbon electrodes (SPCE), followed by production andelectrochemical determination of an electroactive indicator, 1-naphthol. Via hybridizationof SPCE surface-confined target DNAs with end-biotinylated probes, highly specificdiscrimination between complementary and non-complementary nucleotide sequences wasachieved. The enzyme-linked DNA hybridization assay has been successfully applied inanalysis of PCR-amplified real genomic DNA sequences, as well as in monitoring of planttissue-specific gene expression. In addition, we present an alternative approach involvingsequence-specific incorporation of biotin-labeled nucleotides into DNA by primerextension. Introduction of multiple biotin tags per probe primer resulted in considerableenhancement of the signal intensity and improvement of the specificity of detection.

Antimony film electrode for electrochemical stripping analysis.

In this work, an antimony film electrode (SbFE) is reported for the first time as a possible alternative for electrochemical stripping analysis of trace heavy metals. The SbFE was prepared in situ on a glassy carbon substrate electrode and employed in combination with either anodic stripping voltammetry or stripping chronopotentiometry in nondeaerated solutions of 0.01 M hydrochloric acid (pH 2). Several key operational parameters influencing the electroanalytical response of SbFE were examined and optimized, such as deposition potential, deposition time, and composition of the measurement solution. The SbFE exhibited well-defined and separated stripping signals for both model metal ions, Cd(II) and Pb(II), surrounded with low background contribution and a relatively large negative potential range. The electrode revealed good linear behavior in the examined concentration range from 20 to 140 microg L-1 for both test metal ions, with a limit of detection (3sigma) of 0.7 microg L-1 for Cd(II) and 0.9 microg L-1 for Pb(II) obtained after a 120 s deposition step, and good reproducibility, with a relative standard deviation (RSD) of +/-3.6% for Cd(II) and +/-6.2% for Pb(II) (60 microg L-1, n = 12). When comparing the SbFE with the commonly used mercury film electrode and recently introduced bismuth film electrode, the newly proposed electrode offers a remarkable performance in more acidic solutions (pH < or = 2), which can be advantageous in electrochemical analysis of trace heavy metals, hence contributing to the wider applicability of electrochemical stripping techniques in connection with "mercury-free" electrodes.

Catalytic activity of iron hexacyanoosmate(II) towards hydrogen peroxide and nicotinamide adenine dinucleotide and its use in amperometric biosensors.

Hydrogen peroxide and nicotinamide adenine dinucleotide (NADH) may be determined amperometrically using screen-printed electrodes chemically modified with iron(III) hexacyanoosmate(II) (Osmium purple) in flow injection analysis (FIA). The determination is based on the exploitation of catalytic currents resulting from the oxidation/reduction of the modifier. The performance of the sensor was characterized and optimized by controlling several operational parameters (applied potential, pH and flow rate of the phosphate buffer). Comparison has been made with analogous complexes of ruthenium (Ruthenium purple) and iron (Prussian blue). Taking into account the sensitivity and stability of corresponding sensors, the best results were obtained with the use of Osmium purple. The sensor exhibited a linear increase of the amperometric signal with the concentration of hydrogen peroxide in the range of 0.1-100 mg L(-1) with a detection limit (evaluated as 3sigma) of 0.024 mg L(-1) with a R.S.D. 1.5% for 10 mg L(-1) H2O2 under optimized flow rate of 0.4 mL min(-1) in 0.1M phosphate buffer carrier (pH 6) and a working potential of +0.15 V versus Ag/AgCl. Afterwards, a biological recognition element--either glucose oxidase or ethanol dehydrogenase--was incorporated to achieve a sensor facilitating the determination of glucose or ethanol, respectively. The glucose sensor gave linearity between current and concentration in the range from 1 to 250 mg L(-1) with a R.S.D. 2.4% for 100 mg L(-1) glucose, detection limit 0.02 mg L(-1) (3sigma) and retained its original activity after 3 weeks when stored at 6 degrees C. Optimal parameters in the determination of ethanol were selected as: applied potential +0.45 V versus Ag/AgCl, flow rate 0.2 mL min(-1) in 0.1 M phosphate buffer carrier (pH 7). Different structural designs of the ethanol sensor were tested and linearity obtained was up to 1000 mg L(-1) with a maximum R.S.D. of 5.1%. Applications in food analysis were also examined.

Stripping voltammetric determination of platinum metals at a carbon paste electrode modified with cationic surfactants.

In this contribution, a novel method is described for the determination of platinum metals. The procedure developed employs a carbon paste electrode modified in situ with cationic surfactants of the quaternary ammonium salt type. The pre-concentration step is based on a specific accumulation mechanism involving ion-pair formation; the detection being performed by cathodic scanning in the differential pulse voltammetric mode. Regarding the individual forms of platinum metals, the method has been found convenient for the determination of three heavy platinum metals in the form of Pt(IV), Ir(III) and Os(IV), whereas for the remaining elements (Ru, Rh, and Pd) was almost inapplicable. Platinum metals of the former group can be pre-concentrated in chloride-containing supporting media via PtCl(6)(2-), IrCl(6)(3-) and OsCl(6)(2-) complex anions, the central atom of each species being fairly reducible during the voltammetric scan. Stripping signals for both platinum and iridium were proportional to the concentration in a range of 1-10x10(-6)M Pt(IV) and Ir(III); the response for osmium being linear within 0.1-6x10(-7)M Os(IV) with a detection limit of about 5x10(-9)moll(-1). During optimisation, special attention was paid to the accumulation mechanism, choice of key experimental conditions, and to interference effects from foreign ions with potentially ion-pairing capabilities (AuCl(4)(-), TlCl(4)(-), CrO(4)(2-), MnO(4)(-), SCN(-), and I(-)). The method elaborated has been tested on both model solutions and real samples of industrial waste water, showing in both cases satisfactory analytical performance.

Isotachophoretic determination of hydrosulfite and metabisulfite in technical samples.

A method for determination of metabisulfite and hydrosulfite in poultice and decolorant by isotachophoresis was developed. Metabisulfite and hydrosulfite are ionizable oxoanions of sulfur of similar character that can easily be oxidized to sulfates. To protect the analytes from oxidation the solid samples were dissolved in a 1% (w/v) solution of formaldehyde. Hydrosulfite and metabisulfite present in the samples were transformed by the reaction with formaldehyde to stable compounds, hydroxymethanesulfinate and hydroxymethanesulfonate that were determined isotachophoretically without any pretreatment except for sample filtering and degassing. A capillary of 0.4mm i.d. and 100mm effective length made of fluorinated ethylene-propylene copolymer was filled with an electrolyte system consisting of 10 mmol L(-1) HCl+11 mmol L(-1) imidazole, 0.15% (w/v) hydroxyethylcellulose, pH 6.0 (leading electrolyte) and 5 mmol L(-1) benzoic acid+6 mmol L(-1) imidazole, pH 6.5 (terminating electrolyte). Separation was performed at a driving current of 80 microA and for detection current was decreased to 30 microA. Using contactless conductivity detection, the calibration curves in the tested concentration range up to 2.5 mmol L(-1) were linear for both metabisulfite and hydrosulfite complexes. The concentration detection limits for metabisulfite and hydrosulfite were 2.9 and 3.4 micromol L(-1), respectively. For 1 mmol L(-1) concentration, values of R.S.D. (n=6) were 2.6% for hydrosulfite and 0.8% for metabisulfite. Isotachophoretic determination took about 20 min. The elaborated isotachophoretic procedure is simple to perform, sufficiently sensitive and accurate. In addition to this, low cost of analyses makes the method an alternative procedure to methods used so far for the determination of oxoanions of sulfur.

Direct simultaneous determination of eight sweeteners in foods by capillary isotachophoresis.

A method for isotachophoretic determination of sweeteners of different character in candies and chewing gums was developed. A capillary of 0.8 mm ID and 90 mm effective length made of fluorinated ethylene-propylene copolymer is filled with an electrolyte system consisting of 10 mM HCl + 14 mM Tris, pH 7.7 (leading electrolyte) and 5 mM L-histidine + 5 mM Tris, pH 8.3 (terminating electrolyte). The analysis is performed at a driving current of 200 microA and for detection current is decreased to 100 microA. Boric acid is added to the aqueous sample solution to form borate complexes with substances of polyhydroxyl nature and make them migrate isotachophoretically. Using conductivity detection, the calibration curves in the tested concentration range up to 2.5 mM were linear for all components of interest: acesulfame K, saccharine, aspartame, cyclamate, sorbitol, mannitol, lactitol, and xylitol. The concentration detection limits ranged between 0.024 and 0.081 mM. Good precision of the ITP method is evidenced by favorable RSD values ranging from 0.8 to 2.8% obtained at the analyte concentration of 1.0 mM (n = 6). The analysis time was about 20 min. Simplicity, accuracy, and low cost of analyses make ITP an alternative procedure to methods used so far for the determination of ionizable sweeteners.

A study on the determination of chromium as chromate at a carbon paste electrode modified with surfactants.

A procedure for the determination of chromium is described based on synergistic pre-concentration of the chromate anion at a carbon paste electrode modified in situ with quarternary ammonium salts such as 1-ethoxycarbonylpentadecyltrimethylammonium bromide (Septonex((R))), cetyltrimethylammonium bromide (CTAB) or cetylpyridinium bromide (CPB). The proper electrochemical detection utilises the reduction Cr(VI) --> Cr(III) performed in the differential pulse cathodic voltammetric mode. In discussion, considerable attention has been paid to the accumulation mechanism at the carbon paste electrode in the presence of surfactants. Furthermore, after optimising the corresponding experimental conditions (0.1-0.3M HCl + 0.1M NaCl as the supporting electrolyte, 2.5-25muM as the total concentration of modifier, pre-concentration at +0.7V versus Ag/AgCl and the stripping from +0.7 to -0.4V), the analytical performance of the method has been evaluated. The signal of interest was reproducible within +/-8% and proportional to the concentration in a range of 0.5-50muM CrO(4)(2-), with a limit of detection (S/N = 3:1) of about 5x10(-8)M CrO(4)(2-) (with accumulation for 300s). Interference studies were focused mainly on the species capable of forming ion-pairs with the modifier; i.e., TlCl(4)(-), AuCl(4)(-), PdCl(4)(2-), PtCl(6)(2-), VO(4)(3-), MnO(4)(-) and I(-). Practical applicability of the method was tested on model solutions via the recovery rates (typically 90-110%) or using selected certified reference materials (tea, bush leaves, clover) and two samples of black tea when the respective results were compared to those obtained by the reference determinations with ICP-AES.

(Bio)sensors based on manganese dioxide-modified carbon substrates: retrospections, further improvements and applications.

An overview is presented which summarizes our accomplishment in the development of sensors and biosensors based on heterogenous carbon electrodes modified with manganese dioxide. Brief account of each sensor and biosensor has been given and example of real sample applications provided where appropriate.

Carbon paste electrodes modified with Bi(2)O(3) as sensors for the determination of Cd and Pb.

Carbon paste electrodes bulk-modified with Bi(2)O(3)were used for the determination of Cd(II) and Pb(II). The best composition was 1% (wt%) Bi(2)O(3) in the paste. The measurements were made by differential pulse voltammetry in the potential range from -1.2 V to -0.3 V. The peak potential of the reoxidation of Cd is -0.85 V, and of Pb -0.60 V vs. SCE. The lowest concentration that could be determined was 5 microg L(-1) of both metals (preconcentration time 240 s), the relative standard deviation was 3.5%-5.0% (four determinations). The correlation coefficient ( r(2)) of the calibration curves was 0.9966 (for Cd) and 0.9971 (for Pb). The Bi(2)O(3)-modified electrode could be used for the analysis of drinking water, mineral water and urine.