mA Mol Research Articles

Evaluation of a Multi-Walled Carbon Nanotube-Hemin Composite for the Voltammetric Determination of Hydrogen Peroxide in Dental Products

A simple, low cost sensor was developed for the voltammetric determination of hydrogen peroxide in mouthwash and dental whitening gel based on multi-walled carbon nanotubes incorporated with hemin. The sensor showed electrocatalytic activity toward the reduction of hydrogen peroxide in 0.05 mol L−1 Tris-HCl buffer solution (pH 7.0) using cyclic voltammetry. The optimum composition of paste was 20:10:70% (m/m/m) (multi-walled carbon nanotubes:hemin:mineral oil). A linear plot of the square root of scan rate vs. cathodic peak current showed that reduction of hydrogen peroxide is diffusion controlled. Using linear sweep voltammetry, the analytical curve ranged from 0.2 up to 1.4 mmol L−1 (r = 0.9996) with a sensitivity of 3.62 × 10−2 mA mol−1 L. The limits of detection and quantification were found to be 12.5 µmol L−1 and 41.7 µmol L−1, respectively. The developed method was applied for hydrogen peroxide determination in dental formulations. The results were compared with a volumetric method as a reference technique. No significant differences at the 95% level (paired student t test) were observed, thus demonstrating the accuracy of the sensor for the analysis of real samples.

A flower-like nickel oxide nanostructure: Synthesis and application for choline sensing

Flower-like nickel oxide nanostructure was synthesized by a simple desolvation method. The nanostructure was then employed as the modifier of a carbon paste electrode to fabricate a choline sensor. The mechanism and kinetics of the electrocatalytic oxidation of choline on the modified electrode surface were studied by cyclic voltammetry, steady-state polarization curve, and chronoamperometry. The catalytic rate constant and the charge transfer coefficient of the choline electrooxidation process by an active nickel species, and the diffusion coefficient of choline were reported. An amperometric method was developed for determination of choline with a sensitivity of 60.5 mA mol(-1)Lcm(-2) and a limit of detection of 25.4 μmol L(-1). The sensor had the advantages of high electrocatalytic activity and sensitivity, and long-term stability toward choline, with a simple fabrication method without complications of immobilization steps and using any enzyme or reagent.

Voltammetric analysis of cocaine using platinum and glassy carbon electrodes chemically modified with Uranyl Schiff base films

Platinum and glassy carbon electrodes chemically modified with films of Schiff bases of [UO2(3-MeOSalen)(H2O)] · H2O and [UO2(5-MeOSalen)(H2O)] · H2O, to determine cocaine were developed. The stability of these films in aqueous solution containing 1.0 mol L− 1 KCl and HCl 1.0 mM in pH 3 as supporting electrolyte and conducted cyclic voltammetric analysis of cocaine in the same supporting electrolyte were investigated. A reversible interaction between cocaine and the working electrodes increased the original peak current of the surface-modifier, according to the employed species. The determination of cocaine using the developed electrodes depended linearly on cocaine concentration in the range 0.54–9.10 μmol L− 1, with amperometric sensitivities of 5.21 × 107 and 1.66 × 105 μA mol− 1 L, limits of quantification of 0.29 and 0.50 μmol L− 1, and limits of detection of 0.07 and 0.15 μmol L− 1, respectively.

FIA-automated system used to electrochemically measure nitrite and its interfering chemicals through a 1-2 DAB / Au electrode: gain of sensitivity at upper potentials

The measurement of nitrite and its interfering-chemicals (paracetamol, ascorbic acid and uric acid) was performed employing a Flow-injection Analysis (FIA) system, which was automated using solenoid valves and air-pump. It is very important to quantify nitrite from river water, food and biologic fluids due to its antibacterial capacity in moderated concentrations, or its toxicity for human health even at low concentrations (> 20 μmol L−1 in blood fluids). Electrodes of the electrochemical planar sensor were defined by silk-screen technology. The measuring electrode was made from gold paste covered with 1-2 cis Diaminobenzene (DAB), which allowed good selectivity, linearity, repeatability, stability and optimized gain of sensitivity at 0.5 VAg/AgCl Nafion®117 (6.93 μA mol−1 L mm−2) compared to 0.3 VAg/AgCl Nafion® 117. The reference electrode was obtained from silver/palladium paste modified with chloride and covered with Nafion® 117. The auxiliary electrode was made from platinum paste. It was noteworthy that nitrite response adds to the response of the studied interfering-chemicals and it is predominant for concentrations lower than 175 μmol L−1.

Supramolecular immobilization of glucose oxidase on gold coated with cyclodextrin-modified cysteamine core PAMAM G-4 dendron/Pt nanoparticles for mediatorless biosensor design

Cysteamine core polyamidoamine G-4 dendron branched with β-cyclodextrins was chemisorbed on the surface of Au electrodes and further coated with Pt nanoparticles. Adamantane-modified glucose oxidase was subsequently immobilized on the nanostructured electrode surface by supramolecular association. This enzyme electrode was used to construct a reagentless amperometric biosensor for glucose, making use of the electrochemical oxidation of H2O2 generated in the enzyme reaction. The amperometric response of the biosensor was rapid (6 s) and a linear function of glucose concentration between 5 and 705 μmol L(-1). The biosensor had a low detection limit of 2.0 μmol L(-1), sensitivity of 197 mA mol(-1) L cm(-2), and retained 94% of its initial response after storage for nine days at 4 °C.

A highly sensitive method for determination of β-blocker drugs using a Nafion-coated glassy carbon electrode

The electrochemical response of bisoprolol fumarate (BF) and propranolol hydrochloride (PR) is significantly enhanced by immobilization of a Nafion film on the surface of the glassy carbon electrode. The positively charged drug molecules were accumulated in 0.1 mol L −1 sulfuric acid solution at a potential of +0.6 V for 120 s, and then determined by square-wave anodic voltammetry. The enhancement mechanism and effect factors such as pH value, mass of Nafion, accumulation potential and pre-concentration time, were explored. The peak current is proportional to the concentration in the range from 5 × 10 −8 to 5 × 10 −7 mol L −1 for BF and from 8 × 10 −8 to 6 × 10 −7 mol L −1 for PR, with the sensitivities of 1.49 × 10 8 and 2.47 × 10 7 μA mol −1 L, respectively. The modified electrode was used to determine both β-blockers in commercial tablets. The quantification of BF in urine samples with no prior extraction was carried out after selective accumulation at the electrode surface under stirring. Together with low cost and ease preparation, the modified electrode is a promising sensor in electroanalysis, not only for the detection of BF and PR, but also for other β-blocker drugs.

Hg 2+ detection by measuring thiol groups with a highly sensitive screen-printed electrode modified with a nanostructured carbon black film

A sensor based on a screen-printed electrode (SPE) modified with a stable dispersion of commercially available carbon black (CB) N220 was optimised and challenged with several thiol-containing compounds. This probe showed a significantly enhanced electrochemical activity respect to a bare SPE when tested with thiocholine, cysteine, glutathione and cysteamine. When challenged in amperometric batch mode, the response was stable and showed a linear dependence up to 1 × 10 −5 mol l −1 for thiocholine and cysteine. The very high sensitivity towards these thiols (299 mA mol −1 l cm −2 for thiocholine and 441 mA mol −1 l cm −2 for cysteine) was then used as the basis for developing an analytical method for mercury ion detection since a non electroactive complex (thiol–Hg) is formed in the presence of the metal. By selecting an appropriate concentration of thiocholine, a concentration of mercury as low as 5 × 10 −9 mol l −1 (1 ppb) was detected. Satisfactory recovery was obtained when the system was tested on drinking water samples.

Electro-catalysis by immobilised human flavin-containing monooxygenase isoform 3 (hFMO3)

Human flavin-containing monooxygenases are the second most important class of drug-metabolizing enzymes after cytochromes P450. Here we report a simple but functional and stable enzyme-electrode system based on a glassy carbon (GC) electrode with human flavin-containing monooxygenase isoform 3 (hFMO3) entrapped in a gel cross-linked with bovine serum albumin (BSA) by glutaraldehyde. The enzymatic electrochemical responsiveness is characterised by using well-known substrates: trimethylamine (TMA), ammonia (NH(3)), triethylamine (TEA), and benzydamine (BZD). The apparent Michaelis-Menten constant (K'(M)) and apparent maximum current (I'(max)) are calculated by fitting the current signal to the Michaelis-Menten equation for each substrate. The enzyme-electrode has good characteristics: the calculated sensitivity was 40.9 +/- 0.5 mA mol(-1) L cm(-2) for TMA, 43.3 +/- 0.1 mA mol(-1) L cm(-2) for NH(3), 45.2 +/- 2.2 mA mol(-1) L cm(-2) for TEA, and 39.3 +/- 0.6 mA mol(-1) L cm(-2) for BZD. The stability was constant for 3 days and the inter-electrode reproducibility was 12.5%. This is a novel electrochemical tool that can be used to investigate new potential drugs against the catalytic activity of hFMO3.

Sensitive lactate determination based on acclimated mixed bacteria and palygorskite co-modified oxygen electrode

A sensitive bacteria biosensor was prepared for the detection of trace lactate. The sensitive bioelement, Lactobacillus bulgaricus and Streptococcus thermophilus mixed cultrue, and palygorskite, a perfect matrix for bacteria, was co-immobilized on the surface of oxygen electrode. The biosensor possesses fine selective specificity, good sensitivity and longer operational life time, which were due to the mutual help relationship of symbiotic bacteria and 240 days acclimation with lactate as the carbon source. Hydrodynamic amperometry, an advanced electrochemical method, is suitable for on-line monitoring the concentration change of dissolved oxygen that is closely accompanied with the metabolism of lactate. Electrochemical data show that the current is very sensitive to the changes of the concentration of lactate. The response current was linear with lactic acid concentration in the range from 0 to 300 μmol L − 1 , where the response time is no more than 240 s (R = 0.9952), and the sensitivity was 1.87 mA mol − 1 L. Experiments show the biosensor is also very useful for long time on-line monitoring of lactate, such as fermentation progress.

Immobilization of biotinylated biomolecules onto electropolymerized poly(pyrrole-nitrilotriacetic acid)–Cu 2+ film

A novel and simple immobilization strategy for biotinylated biological macromolecules onto electropolymerized poly(pyrrole-nitrilotriacetic acid)(NTA)–Cu 2+ films without avidin as connecting bridge is reported. After complexation of Cu 2+ by the polymerized NTA chelator, biotinylated biomolecules were immobilized by coordination of the biotin groups on the NTA–Cu 2+ complex. The anchoring of biotinylated glucose oxidase was demonstrated by fluorescent characterization via FITC-labeled avidin and amperometric measurement of glucose. The resulting calibration curve led to a sensitivity and maximum current density values of 0.6 mA mol − 1 L cm − 2 and 13.2 μA cm − 2 , respectively. Thus, biotinylated polyphenol oxidase was fixed leading to a catechol sensor with a sensitivity of 656 mA mol − 1 L cm − 2 and maximum current density of 25.4 μA cm − 2 . This system was also applied to the efficient immobilization of biotinylated DNA, illustrated by impedimetric detection of the formation of the DNA duplex.

Carbon and diamond paste microelectrodes based on Mn(III) porphyrins for the determination of dopamine

Three Mn(III) porphyrins were used for the design of carbon paste and diamond paste based microelectrodes, which were employed for the determination of dopamine in pharmaceutical and biological samples using differential pulse voltammetry (DPV). The limits of detection lie between 1.6 × 10 −13 and 2.0 × 10 −6 mol L −1 while the sensitivities were between 230 pA μmol L −1 and 3.24 μA mol L −1. Dopamine was recovered reliable from pharmaceutical and biological samples in percentages higher than 91.00% and 92.00%, respectively. The surface of the microelectrodes can easily be renewed by simple polishing, obtaining a fresh surface ready for use in a new assay.

Characterization of carbon paste electrodes modified with manganese based perovskites-type oxides from the amperometric determination of hydrogen peroxide

This work proposes the amperometric determination of hydrogen peroxide reduction and oxidation as a tool for the characterization of La 1− x A x MnO 3 perovskites dispersed in a graphite composite electrode (carbon paste electrode, CPE). The catalytic activity of perovskites towards the oxidation and reduction of hydrogen peroxide is highly dependent on the nature of the A cation and on the temperature and time of calcination employed during the synthesis. Therefore, the selection of the optimal synthesis conditions to obtain the best catalytic activity towards hydrogen peroxide can be performed from amperometric determinations. We also report the analytical application of the perovskite modified CPE through the quantification of hydrogen peroxide in two real samples. Some preliminary results about the usefulness of La 0.66Sr 0.33MnO 3–CPE to develop a glucose biosensor by incorporation of the enzyme glucose oxidase (GOx) within the electrode are also reported. The difference in sensitivity to glucose at CPE–GOx and CPE–La 0.66Sr 0.33MnO 3–GOx (11.9 μA mol −1 L and 158.1 μA mol −1 L, respectively), clearly demonstrate the advantages of the association of the biocatalytic activity of GOx and the catalytic activity of perovskites towards hydrogen peroxide oxidation/reduction, and opens the doors to the development of new sensors for other important bioanalytes.

A Chloro‐Bridged Linear Chain Imine‐Copper(II) Complex and Its Application as an Enzyme‐Free Amperometric Biosensor for Hydrogen Peroxide

AbstractThe synthesis, structural characterisation and magnetic properties of a new chloro‐bridged linear chain imine‐copper(II) compound are reported. The results indicate that uniform chains are formed by stacking of parallel pyramidal units formed from the copper centre coordinated to the imine ligands and bridged by chloride ions, with the uncoordinated perchlorate ions located between the layers to balance the charge. Magnetic measurements carried out on a powder sample at temperatures from 1.8 to 290 K and a field of 500 Oe, indicated an intramolecular magnetic coupling between the copper centres [J/k = (–12.3 ± 0.2) K]. Moreover, an antiferromagnetic interaction can also be observed between the polynuclear arrays [J′ z/k = –(10.4 ± 1.7) K]. An analysis of the sample at high temperature indicated that the overall interaction would be antiferromagnetic (ΘCW = –14 K). Furthermore, this new inorganic‐organic zigzag polymer was linked covalently to the poly(N‐vinylimidazole) polymer, PVI, on a vitreous carbon electrode surface and used as a biomimetic material for the efficient analytical determination of hydrogen peroxide. Raman spectroscopy and electrochemical techniques were used to give a complete characterisation of the hybrid film obtained. The proposed enzyme‐free biosensor for catalase‐like activity exhibited good sensitivity (268 mA mol–1 L cm–2) at –0.4 V relative to the. SCE in a phosphate buffer solution (pH 7.0) and a wide linear range from 1 to 30 mmol L–1 for hydrogen peroxide detection.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Fabrication of a multichannel PDMS/glass analytical microsystem with integrated electrodes for amperometric detection

The fabrication process of novel multichannel microfluidic devices with integrated electrodes for amperometric detection is described. Soft-lithography, lift-off and O(2) plasma surface activation sealing techniques were employed for rapid prototyping of cost effective PDMS/glass microchips. The capabilities of the proposed microdevices were demonstrated by the electrooxidation of hydroquinone and N-acetyl-p-aminophenol (APAP) on a Au working electrode at +800 mV and +700 mV, respectively, against a Au pseudo reference electrode, and of thiocyanate on a Cu working electrode at +700 mV against a Ag/AgCl (KCl saturated) reference electrode. Linear response over the range up to 1.0 mmol L(-1) for APAP and up to 4.0 mmol L(-1) for hydroquinone and thiocyanate were verified through calibration curves with correlation coefficients greater than 0.97 (minimum of five data points). The sensitivities for hydroquinone, thiocyanate, and APAP were 28, 19, and 78 microA mol(-1) L, respectively. Under the experimental conditions used, the estimated limits of detection were 0.21, 0.95, and 0.12 mmol L(-1) for hydroquinone, thiocyanate and APAP, respectively. The geometries of the devices were designed to allow fast calibration procedures and reliable results for in-field applications. Exerting a strong influence over the device performance, the sealing process was greatly enhanced by depositing auxiliary TiSiO(2) thin-films. The general performance of the system was verified by amperometric assays of N-acetyl-p-aminophenol standard solutions, and the influences exerted by the present fabrication methods regarding reproducibility and reliability are addressed. The proposed device was successfully applied in the determination of the concentration of APAP in two commercial formulations.

Electrocatalytic oxidation of thiocholine at chemically modified cobalt hexacyanoferrate screen-printed electrodes

In this work, cobalt hexacyanoferrate (CoHCF) modified screen-printed electrodes were developed and characterised electrochemically. The effect of different cations was investigated showing a more reversible system with a well-defined peak around +0.35 V vs. Ag/AgCl when Na + was used as cation in the supporting electrolyte. The transfer coefficient ( α) and the apparent charge transfer rate constant ( k s) for electron transfer between the electrode and the CoHCF layer was calculated. The electrocatalytic behaviour of CoHCF towards oxidation of enzymatically generated thiocholine was thoroughly investigated. Cyclic voltammetry experiments performed in thiocholine solution showed the classical shape of a mediated redox system. The diffusion constant of thiocholine was estimated using chronoamperometry. Also, the catalytic rate constant k of thiocholine at CoHCF–SPE was determined. The current due to thiocholine mediated oxidation was then evaluated in batch amperometric mode at +0.5 V vs. Ag/AgCl obtaining a linear range (5 × 10 −7–1 × 10 −5 mol L −1) with a low detection limit and a high sensitivity equal to 5 × 10 −7 mol L −1 and 435 mA mol −1 L cm −2, respectively.

Titania sol–gel-derived tyrosinase-based amperometric biosensor for determination of phenolic compounds in water samples. Examination of interference effects

For detection of phenolic compounds in environmental water samples we propose an amperometric biosensor based on tyrosinase immobilized in titania sol-gel. The analytical characteristics toward catechol, p-cresol, phenol, p-chlorophenol, and p-methylcatechol were determined. The linear range for catechol determination was 2.2 x 10(-7)-1.3 x 10(-5) mol L(-1) with a limit of detection of 9 x 10(-8) mol L(-1) and sensitivity 2.0 x 10(3) mA mol(-1) L. The influence of sample matrix components on the electrode response was studied according to Plackett-Burman experimental design. The potential interferents Mg(2+), Ca(2+), HCO3(-), SO4(2-), and Cl(-), which are usually encountered in waters, were taken into account in the examination. Cu(2+) was also taken into account, because CuSO(4) is sometimes added to a water sample, as a preservative, before determination of phenolic compounds. It was found that among the ions tested only Mg(2+) and Ca(2+) did not directly affect the electrode response. The developed biosensor was used for determination of catechol in spring and surface water samples using the standard addition method.

Determination of Nickel in Fuel Ethanol Using a Carbon Paste Modified Electrode Containing Dimethylglyoxime

A mercury-free electrode chemically modified with carbon paste containing dimethylglyoxime was used for determination of nickel in fuel ethanol. The instrumental parameters and composition of the modified paste were optimized. The analytical curve for nickel determination from 5.0 × 10−9 to 5.0 × 10−7 mol L−1 was obtained using 25 min of accumulation time. The detection limit and amperometric sensitivity obtained for this method were 2.7 × 10−9 mol L−1 and 5.2 × 108 µA mol−1 L, respectively. The values for nickel concentration in four commercial samples of fuel ethanol were obtained in the range of 1.1 × 10−8 to 6.9 × 10−8 mol L−1. A comparison to graphite furnace atomic absorption spectrometry (GFAAS) was performed for nickel determination in commercial samples of ethanol.

Voltammetric detection of the interactions between RNO 2[rad]− and electron acceptors in aqueous medium at highly boron doped diamond electrode (HBDDE)

This paper describes the electrochemical behavior of the nitrofurazone (NFZ), in predominantly aqueous medium, in the absence and presence of glutathione (reduced form) (GSH), l-cysteine (Cys) and O 2 using a highly boron doped diamond electrode (HBDDE). In presence of [Thiol] ≥ 3.7 × 10 −2 mol L −1 NFZ is directly reduced to RNO–Thiol adducts in an electrochemical process involving two electrons and two protons. On the other side, O 2 acts as a RNO 2 •− scavenger and the velocity constant for the reaction, k O 2 , is 60 L mol −1 s −1. The process is catalytic and can be used to the analytical determination of NFZ in the range of 9.9 × 10 −7 ≤ [NFZ] ≤ 1.1 × 10 −5 mol L −1 at pH 8.0, with sensitivity of 2.2 × 10 6 μA mol −1 cm −2 and detection limit of 3.4 × 10 −7 mol L −1. The analytical parameters were similar to those obtained at pH 4.0 using the direct reduction of NFZ to the respective amine derivative in a process involving six electrons and six protons. The characterization of NFZ global reduction process in aqueous medium and at relative low scan rate, 100 mV s −1, was only possible due the intrinsic superficial characteristics of the HBDDE, which stabilize the RNO 2 − free radical, allowing to work in a large potential window, without losing the RNO 2 − oxidation signal.

A probe for NADH and H2O2 amperometric detection at low applied potential for oxidase and dehydrogenase based biosensor applications

Modified screen-printed electrodes for amperometric detection of H(2)O(2) and nicotinamide adenine dinucleotide (NADH) at low applied potential are presented in this paper. The sensors are obtained by modifying the working electrode surface with Prussian Blue, a well known electrochemical mediator for H(2)O(2) reduction. The coupling of this sensor with phenazine methosulfate (PMS) in the working solution gives the possibility of measuring both NAD(P)H and H(2)O(2). PMS reacts with NADH producing PMSH, which in the presence of oxygen, gives an equimolar amount of H(2)O(2). This allows the measurement of both analytes with similar sensitivity (357 mA mol(-1)L cm(-2) for H(2)O(2) and 336 mA mol(-1)L cm(-2) for NADH) and LOD (5x10(-7)mol L(-1) for H(2)O(2) and NADH) and opens the possibility of a whole series of biosensor applications. In this paper, results obtained with a variety of dehydrogenase enzymes (alcohol, malic, lactate, glucose, glycerol and glutamate) for the detection of enzymatic substrates or enzymatic activity are presented demonstrating the suitability of the proposed method for future biosensor applications.

The electroanalytical detection of hydrazine: A comparison of the use of palladium nanoparticles supported on boron-doped diamond and palladium plated BDD microdisc array

We show that both a random distribution of palladium nanoparticles supported on a BDD electrode or a palladium plated BDD microelectrode array can each provide a sensing platform for the electrocatalytic detection of hydrazine. The palladium nanoparticle modified electrode displays a sensitivity and limit of detection of 60 mA mol(-1) L and 2.6 microM respectively while the array has a sensitivity of 8 mA mol(-1) L with a detection limit of 1.8 microM. The beneficial cost implications of using palladium nano- or micro-particles in sensors compared to a palladium macroelectrode are evident. Interestingly the array of the nanoparticles shows similar sensitivity and limit of detection to the microelectrode array which probably indicates that the random distribution of the former leads to 'clumps' of nanoparticles that effectively act as microelectrodes.