Simple and sensitive electrochemical determination of higenamine in dietary supplements using a disposable pencil graphite electrode

Monatshefte für Chemie - Chemical Monthly - In this study, a cheap, rapid, simple, and modification-free electroanalytical methodology based on the disposable pencil graphite electrode is...     

Electrodeposition of Ag nanoparticles on graphenized pencil lead electrode as a sensitive and low-cost sensor for iodate determination

In this study, a simple, sensitive and low-cost iodate electrochemical sensor based on graphenized pencil lead electrode (GPLE) modified with Ag nanoparticles (AgNPs) was presented. The GPLE was simply prepared via electrochemical exfoliation of pencil lead electrode (PLE) by applying an optimized potential in acidic media. Afterward, silver nanoparticles were electrochemically deposited on the surface of GPLE using chronoamperometry technique. The fabricated electrode was carefully characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) techniques. Electrochemical behavior and also the electrocatalytic performance of the modified electrode toward the reduction of iodate were studied in details using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. The fabricated sensor responds efficiently to iodate over the concentration range of 0.05 to 75 mM with a detection limit of 0.017 mM and sensitivity of 0.26 µA µM^?1 cm^?2. Remarkably enhanced electrocatalytic performance of the modified electrode was ascribed to the synergistic effect of graphene-like nanostructures with high surface to volume ratio, excellent conductivity and also the excessive electrocatalytic behavior of silver nanoparticles. The modified electrode was successfully employed for the determination of iodate in table and sea salt samples.     

Investigation of organic phase biosensor for measuring glucose in flow injection analysis system

The aim of our present work was to develop a flow-through measuring apparatus for the determination of glucose content as model system in organic media and to compare the properties of the biosensor in organic and in aqueous solutions. Glucose oxidase (GOx) enzyme was immobilized on a natural protein membrane in a thin-layer enzyme cell, made of Teflon. The enzyme cell was connected into a flow injection analyzer (FIA) system with an amperometric detector. After optimizing the system the optimal flow rate was found at 0.8 ml min⁻¹. In this case 50-60 samples were measured per hour. Adding ferrocene monocarboxylic acid (FMCA) to acetonitrile and to 2-propanol the optimal concentration was 5 mg l⁻¹, while in the case of tetrabutylammonium-p-toluenesulfonate (TBATS) the optima were 2.7 and 8.0 mg l⁻¹, respectively. With 6% buffer in acetonitrile containing FMCA more than 100 samples could be measured with the enzyme cell without any loss of activity. Measuring the hydrogen peroxide content produced in 2-propanol, the optimal concentration of buffer solution was at about 20%. The linear measuring range was 0-0.5 mM glucose in acetonitrile and 0-1.0 mM in 2-propanol.Glucose concentration of oily food samples was measured and compared with results obtained by the reference UV-photometric method. The correlation between the results measured by the two methods was very good with correlation coefficient (r) as high as 0.976.     

Voltammetric behavior of acebutolol on pencil graphite electrode: highly sensitive determination in real samples by square-wave anodic stripping voltammetry

In this work, an electrochemical investigation of acebutolol (ACE), a beta-blocker drug, was carried out in alkaline medium using pencil graphite (PG) electrode. In cyclic voltammetry, the compound displayed a reversible and adsorption-controlled oxidation peak. By using square-wave anodic stripping voltammetry, the oxidation peak current observed at +0.78 V showed a linear relationship with concentration at 0.4–7 nM interval in Britton–Robinson buffer (pH 10.0) and a detection limit of 0.09 nM. The relative standard deviation of 4.72% for the concentration level of 2.0 nM (n = 11) was also calculated. The PG electrode that is used for the first time in this method was successfully applied to determine the ACE in pharmaceutical formulations and urine.     

Shape-controlled electrodeposition of silver using chitosan as structure-directing agent on disposable pencil graphite electrodes: low-cost electrocatalysts for the detection of hydrogen peroxide and hydrazine hydrate

A facile synthetic protocol for the electrodeposition of diverse morphologies of silver on disposable pencil graphite electrodes (Ag/PGE) in the presence of chitosan as structure-directing agent (SDA) is reported. The influence of various electrodeposition parameters on the morphology of Ag deposited has been studied and interpreted using electron microscopic techniques. Several impressive morphologies such as hexahedron, leaf and dendrites have been observed for Ag/PGE with respect to change in experimental conditions. Furthermore, the crucial role of chitosan in determining the morphology of Ag/PGE has been elucidated with the help of three-dimensional Scharifker-Hills nucleation and growth model. The electrocatalytic activities of various Ag/PGEs towards the reduction of hydrogen peroxide (HP) and oxidation of hydrazine hydrate (HH) have been studied in detail with the help of diverse electrochemical techniques. In comparison with PGE, the Ag hexahedron- (Ag-Hex/PGE) and Ag dendrite- (Ag-Dend/PGE) modified PGEs exhibited excellent electrocatalytic activity towards HP and HH, respectively. The Ag-Hex/PGE displayed a wide linear range of 0.1–20,000 μM with a limit of detection (LOD, 3σ/m) of 0.06 μM for HP reduction. On the other hand, a linear range of 25–20,000 μM with LOD of 1.8 μM for HH oxidation has been observed for Ag-Dend/PGE. Furthermore, the modified Ag/PGEs revealed remarkable reproducibility and long-term storage stability. The practical applicability of the Ag-Hex/PGE and Ag-Dend/PGE was demonstrated through the electrocatalytic detection of HP in milk and HH in tap water samples with satisfactory recovery results.

     

Flow injection analysis of L-lactate in milk and yoghurt by on-line microdialysis and amperometric detection at a disposable biosensor.

A disposable lactate biosensor able to operate in flow injection analysis is described and characterized. The biosensing layer, obtained by glutaraldehyde co-crosslinking of lactate oxidase with bovine serum albumin, was cast on an underlying electropolymerized layer of overoxidized polypyrrole. The resulting biosensor was interference-free and showed a K'M value of 2.4 mmol l-1 and a maximum current density of 440 microA cm-2. When integrated in a flow injection analysis system, a sensitivity of 300 +/- 10 nA mmol-1 l, a linear response up to 1 mmol l-1 and detection limits in the low micromolar range were obtained. The introduction of a microdialysis membrane-based sampler reduced the sensitivity to 7.9 +/- 0.2 nA mmol-1 l and extended the linear range up to 50 mmol l-1 lactate. The anti-interference characteristics of the biosensor permitted lactate determination in untreated milk and diluted yoghurt samples.     

Flow injection analyses: Part I. A new concept of fast continuous flow analysis

The concept of a new continuous flow analyser system is described. Based on instant discrete sampling by injection into a carrier stream, the system allows continuous flow analysis to be performed in a fast, much simplified way. As the continuous flowing stream is characterized by a turbulent rather than a laminar flow, the discrete instant sampling creates geometrically well-defined segments of sample solution within the flowing stream. Because of the absence of lag phase, an unprecedented sampling rate for continuous flow analysis of well over 200 samples per hour can be achieved; and even manual injection of the samples allows a very high degree of accuracy and precision to be obtained ( ? ± 1%). Uses of the system in various analytical procedures are described and discussed. A potentiometric sensor (the air-gap electrode used in a flow-through unit) and a spectrophotometric arrangement with a flow-through cell have been used as detector units.     

Amperometric lactate biosensor for flow injection analysis based on a screen-printed carbon electrode containing Meldola's Blue-Reinecke salt, coated with lactate dehydrogenase and NAD

A biosensor for the measurement of lactate in serum has been developed, which is based on a screen-printed carbon electrode, modified with Meldola's Blue-Reinecke Salt (MBRS-SPCE), coated with the enzyme lactate dehydrogenase NAD⁺ dependent (from Porcine heart), and NAD⁺. A cellulose acetate layer was deposited on the top of the device to act as a permselective membrane. The biosensor was incorporated into a commercially available, thin-layer, amperometric flow cell operated at a potential of only +0.05 V vs. Ag/AgCl. The mobile phase consisted of 0.2 M phosphate buffer pH 10 containing 0.1 M potassium chloride solution; a flow rate of 0.8 ml min⁻¹ was used throughout the investigation. The biosensor response was linear over the range 0.55-10 mM lactate; the former represents the detection limit. The precision of the system was determined by carrying out 10 repeat injections of 10 mM l(+)lactic acid standard; the calculated coefficient of variation was 4.28%. It was demonstrated that this biosensor system could be applied to the direct measurement of lactate in serum without pre-treatment; therefore, this would allow high throughput-analysis, at low cost, for this clinically important analyte.     

A flow injection biosensor system for highly sensitive detection of 2,4,6-trichlorophenol based on preoxidation by ceric sulfate.

A flow injection biosensor system was proposed for the highly sensitive detection of 2,4,6-trichlorophenol (2,4,6-TCP). The system is based on the preoxidation by ceric sulfate to the corresponding benzoquinone (2,6-dichloro-1,4-benzoquinone: 2,6-DC-1,4-BQ), which was characterized using cyclic voltammetry, hydrodynamic voltammetry, and UV-vis spectrophotometry. The laccase-based biosensor used in this analytical system responded sensitively to 2,4,6-TCP after the preoxidation by ceric sulfate. The response could be based on the bioelectrocatalytic recycling of oxidation product (2,6-DC-1,4-BQ) between laccase membrane and the electrode, because the oxidation product (2,6-DC-1,4-BQ) of 2,4,6-TCP was an electrochemically reversible redox species. The signal current was linearly related to the 2,4,6-TCP concentrations in a dynamic range of 2 nM - 2 microM; the slope and the y-intercept of the straight line were 1150 nA microM(-1) and 0.88 nA, respectively. The detection limit was 1.2 nM (S/N = 3) for a 20 microl injection. Among a variety of chlorophenols and some phenolic compounds, the only interferent was 2,4-dichlorophenol.     

Digital versatile disc bipolar electrode: A fast and low-cost approach for visual sensing of analytes and electrocatalysts screening

This work represents a new, extremely low cost and easy method for fabrication of bipolar electrode (BPE) for rapid and simultaneous screening of potential candidates for electrocatalytic reactions and sensing applications. Our method takes advantage of the silver reflective layer deposited on already available recordable digital versatile disc (DVD-R) polycarbonate substrate which acts as BPE. Oxidation of the reflective layer of the DVD-R in anodic pole of the BPE results in a permanent and visually measurable dissolute length. Therefore, one could correlate the electrocatalytic activity of the catalyst at the cathodic pole of the BPE, as well as the concentration of analyte in the solution, to the dissolution length of the BPE. To illustrate the promising applications of this new substrate as BPE, p-benzoquinone (BQ) and hydrogen peroxide were tested as model targets for the sensing application. Moreover, in order to show the feasibility of using DVD BPEs for screening applications, the electrocatalytic activity of Pt, Pd, Au, and pristine DVD substrate toward hydrogen evolution reaction (HER) were compared using an array of BPEs prepared on DVD substrate.     

A fast and sensitive continuous flow nanobiodetector based on polyaniline nanofibrils

Polyaniline nanofibrils, fabricated as a freestanding nanonetwork in a micro gap between two gold electrodes, have been applied in a fast and sensitive analytical device for detecting microorganism cells in a flowing liquid, including the “on-line” regime. The electrical response of the device (in this case an increase in the electrical conductivity of the nanonetwork) depends on the number of cells in the sample analyzed: it is linear within the range of 0–5000 and 0–15 × 10⁶ cells injected for yeast and bacteria examined, respectively. The detection limit depends on the type of microorganism: it is about 300 and 1.9 × 10⁵ (per mL) for yeast (Saccharomyces cerevisiae) and bacteria cells (Lactobacillus rhamnosus), respectively. The results are registered within few seconds (typically 5–15 s). The device is designed to detect a low level of bio-contamination and it is expected to be useful in biomedical applications, environmental protection and anti bioterrorism systems, including “on-line” monitoring.     

Polarographic study of acrolein and its determination by flow injection with amperometric detection at a mercury electrode

A study of the electrochemical behavior of acrolein at a dropping mercury electrode using different polarographic techniques is described. Theoretical studies of the reversibility of the wave of acrolein were carried out using two different polarographic techniques: direct current tast and differential pulse. Differential pulse polarography may be used to determine acrolein concentration in a Britton-Robinson buffer solution of pH 10 in the ranges 2 x 10(-7)10(-8) and 5 x 10(-8)-10(-4) mol dm(-3) and a coefficient of variation of 1.7% for a concentration of 10(-5)mol dm(-3). A flow injection method with amperometric detection at a potential of -1.4V using a mercury electrode is also described. Before each injection, any drop hanging from the tip of the capillary needs to be dislodged and a new electrode drop dispensed; three different drop sizes were tested. A linear relationship between peak intensity and acrolein concentration was obtained in the range 10(-5)-10(-7) mol dm(-3), with a detection limit of 9.8 x 10(-8) mol dm(-) 3 and a coefficient of variation of 2.9% for a 2 x 10(-7) mol dm(-3) concentration. Several organic and inorganic species were tested in order to ascertain whether they interfered with the signal for acrolein. The proposed methods were applied to the determination of acrolein in seawater samples.     

Amperometric assay of glucose and lactic acid by flow injection analysis

A computer-controlled flow-injection system is described for the assay of D-glucose and L-lactic acid in undiluted plasma. Glucose or lactate is quantified by coupling an immobilized glucose oxidase or lactate oxidase membrane with an amperometric sensor; the hydrogen peroxide generated is directly related to the concentration of glucose or lactate. The linear range is 0–40 mM and 0–10 mM for glucose and lactic acid, respectively. The sample frequency is 60 h^?1 with a standard deviation of less than 1.5%. Correlation with the results for blood plasma obtained by routine clinical analyzers was good for both glucose and lactic acid.     

Fluoride ion-selective electrode in flow injection analysis : Part 2. Interference Studies

The influence of the sample composition on the response characteristics of the fluoride ion-selective electrode in flow injection analysis is described. Sample parameters such as ionic strength, viscosity and pH affect the response time of the electrode and cause transient signals when limiting values are exceeded. The respective limiting values depend on the total ionic-strength adjustment buffer (TISAB) used and these interferences can be minimized by proper choice of the TISAB. The complex formation of fluoride by several elements in the presence of TISAB containing CDTA is discussed. Aluminium and magnesium were found to interfere when present at levels above 1 and 100 mg l^?1, respectively. The signal decrease in the presence of iron, calcium and silicon can be attributed to ionic strength effects rather than complexation. Provided that the ionic strength is taken into account and corrected for, no influence occurs even in the presence of 0.5, 2 and 5% of iron, calcium and silicon, respectively.     

Amperometric sensor for detection of bisphenol A using a pencil graphite electrode modified with polyaniline nanorods and multiwalled carbon nanotubes

We report on a simple and highly sensitive amperometric method for the determination of bisphenol A (BPA) using pencil graphite electrodes modified with polyaniline nanorods and multiwalled carbon nanotubes. The modified electrodes display enhanced electroactivity for the oxidation of BPA compared to the unmodified pencil graphite electrode. Under optimized conditions, the sensor has a linear response to BPA in the 1.0 and 400?μM concentration range, with a limit of detection of 10?nM (at S/N?=?3). The modified electrode also has a remarkably stable response, and up to 95 injections are possible with a relative standard deviation of 4.2% at 100?μM of BPA. Recoveries range from 86 to 102% for boiling water spiked with BPA from four brands of baby bottles.

Determination of glucose in blood by flow injection analysis and an immobilized glucose oxidase column

A flow-injection system for glucose determination is described. Glucose oxidase is immobilized on controlled porosity glass (CPG) and used in a glass column (2.5 mm diameter × 2.5 cm). The hydrogen peroxide produced by the enzymatic reaction (? 1 × 10^?6 M) is detected by the current produced in a flow-through cell, with two platinum electrodes having a potential difference of 0.6 V. Glucose (0–20 mmol l^?1) can be determined in blood plasma either with a dialyser in the system or, better, by incorporating a column of copper(II) diethyldithiocarbamate on CPG before the enzyme column. The results compared well with those obtained by a conventional analyser system. The glucose oxidase column showed little change in activity over a 10-month period.     

Highly sensitive and wide-range nonenzymatic disposable glucose sensor based on a screen printed carbon electrode modified with reduced graphene oxide and Pd-CuO nanoparticles

A nanocomposite consisting of reduced graphene oxide decorated with palladium-copper oxide nanoparticles (Pd-CuO/rGO) was synthesized by single-step chemical reduction. The morphology and crystal structure of the nanocomposite were characterized by field-emission scanning electron microscopy, high resolution transmission electron microscopy and X-ray diffraction analysis. A 3-electrode system was fabricated by screen printing technology and the Pd-CuO/rGO nanocomposite was dropcast on the carbon working electrode. The catalytic activity towards glucose in 0.2 M NaOH solutions was analyzed by linear sweep voltammetry and amperometry. The steady state current obtained at a constant potential of +0.6 V (vs. Ag/AgCl) showed the modified electrode to possess a wide analytical range (6 μM to 22 mM), a rather low limit of detection (30 nM), excellent sensitivity (3355 μA∙mM⁻¹∙cm⁻²) and good selectivity over commonly interfering species and other sugars including fructose, sucrose and lactose. The sensor was successfully employed to the determination of glucose in blood serum.

A highly sensitive nonenzymatic electrochemical sensor was fabricated using a Pd-CuO composite with reduced graphene oxide. The sensor has a wide detection range and was used to sense glucose in blood serum

     

Continuous flow and flow injection stripping voltammetric determination of silver(I), mercury(II), and bismuth(III) at a bulk modified graphite tube electrode

The epoxy-impregnated graphite tube electrode bulk modified with 2-mercaptobenzoxazole, employed in a wall-jet configuration, was found to be useful for the continuous flow and flow injection stripping voltammetric determinations of Ag^I, Hg^II and Bi^III. For continuous flow, detection limits for Ag^I, Hg^II and Bi^III were 1.8 × 10⁻¹⁰ M, 1.9 × 10⁻⁹ M and 9.5 × 10⁻⁹ M, respectively (10 min accumulation, S/N = 3). Precisions for 5.00 × 10⁻⁹ M Ag^I, 1.00 × 10⁻⁸ M Hg^II and 1.00 × 10⁻⁷ M Bi^III were 10.5%, 5.77 % and 7.90% (relative standard deviations, n = 6), respectively. In the case of flow injection stripping, with a 500 μL injection loop, detection limits of 0.59 ng, 2.0 ng and 120 ng were obtained for Ag^I, Hg^II and Bi^III, respectively (S/N = 3). Selected metal ions, inorganic and organic substances were investigated for interferences. The electrode was tested with a certified sample and then applied to the determinations of the metal ions in a urine and a sea-water sample.