[show abstract][hide abstract] ABSTRACT: In voltammetric analysis the signal—in most cases—depends on transport processes. When getting concentration values of analytes from voltammetric calibrating curves, it is expected that the mass transport characteristics in sample solutions and in calibrating standards are identical. Standard addition methods are used in analytical practice when making calibrating standard with transport properties matching that of the samples would be difficult. Voltammetric measurements can also be carried out in soil—in sediment—or in gel samples. The mass transport conditions in these media, however, can considerably differ from those existing in aqueous solutions. The application of standard addition technique, however, is not an option there. In this work, a glassy carbon electrode was applied with a built-in diffusion layer on its measuring surface and chronoamperometric measurements were carried out. The current–time transients taken in aqueous solution and in tortuous, diffusion-hindered media were compared. Ascorbic acid and iodine as analytes as well as silica sand sediments and green pepper pulps as tortuous matrices were used. It was proved that if the modified electrode is used for analysis, then short time chronoamperometric transients taken in tortuous media can be evaluated by calibration data taken in aqueous standard solutions.
Journal of Solid State Electrochemistry 12/2013; · 2.28 Impact Factor
[show abstract][hide abstract] ABSTRACT: Banana tissue biosensor Dynamic voltammetry Scanning electrochemical microscopy Periodically interrupted amperometry Dopamine a b s t r a c t A model biosensor based on a reaction layer containing banana pulp was constructed and dopamine (DA) has been selected as analyte for the investigations. On the basis of dynamic voltammograms it was found that the optimal potential for DA detection should be −0.2 V vs. SCE, potential where the oxygen interference was avoided. Based on vertical scanning electrochemical microscopy (SECM) the optimal width of the reaction layer for the model biosensor was determined to be in between 130 and 160 m. Optimal resting potential in periodically interrupted amperometry (PIA) detection mode was determined to be in the range from −0.02 V to −0.03 V vs. SCE. The resting time used in PIA experiments was 0.5 s. A detection limit of 2 × 10 −6 M was obtained for DA by using PIA detection mode.
Sensors and Actuators B Chemical 08/2013; 190:149-156. · 3.54 Impact Factor
[show abstract][hide abstract] ABSTRACT: The fabrication of a solid-contact, micropipette-based magnesium ion-selective micro-tipped electrode (ISME) suitable for scanning electrochemical microscopy is reported and compared against a conventional micro-tipped ISME having a conventional aqueous internal reference electrode. Measurements showed that the solid-contact ISME had a lower internal resistance and a faster response time than the one with a liquid-contact. These advantages increased the spatial distribution and improved 2D images depicting concentration distributions of Mg2+. The ability of the microelectrode to image local ionic concentration has been tested over magnesium surfaces freely corroding or galvanically coupled to iron in aqueous chloride-containing solution. Scans of magnesium ion distribution, in the absence of corrosion currents, were also made over a micro-pipette source containing a concentrated magnesium chloride gel as a source of Mg2+ and over a current source in the absence of Mg2+. From these measurements it was concluded that the potentiometric measurements over corroding surfaces were dominated by the changes in Mg2+ distributions with small electric potential contributions due to corrosion current.
Journal of The Electrochemical Society 08/2013; 160(9):C451-C459. · 2.59 Impact Factor
[show abstract][hide abstract] ABSTRACT: The applicability of scanning electrochemical microscopy for the local detection and quantification of relevant species participating in the corrosion of magnesium-based materials is presented. The iron-magnesium galvanic pair exposed to aqueous NaCl solution was adopted as model system for this purpose. Mg2+ ion concentration and pH profiles were investigated using ion selective electrodes, containing a liquid membrane and Sb/Sb2O3 as sensor elements, respectively. Oxygen consumption at the substrate related to the cathodic reaction was also monitored with the antimony-based electrode though operated amperometrically. Data show a major production of hydroxyl anions at the cathodic sites as result of the oxygen reduction half-reaction, whereas in the vicinity of the magnesium surface pH is greatly affected by the anodic dissolution process instead. The later produces the release of metal cations accompanied by hydrogen evolution.
[show abstract][hide abstract] ABSTRACT: The spatial resolution of Mg2+ release from magnesium and its alloys during exposure to aqueous environments has been imaged using a new, solid contact, micropipette-based magnesium-ion selective electrode employed as potentiometric tip in SECM. The detection of metal dissolution is crucial factor to detect the local microelectrodes established on the surface of the metal, and distinguish the processes related to anodic and cathodic half-cell reactions. Concentration distribution images have been obtained for the magnesium-based alloy AZ63 when galvanically coupled to pure iron during exposure to 1 mM NaCl solution.
[show abstract][hide abstract] ABSTRACT: A new approach for studying the local distribution of anodic and cathodic sites in a corroding system using the scanning vibrating electrode technique (SVET) and scanning electrochemical microscopy (SECM) is presented. When zinc is coupled to iron, dissolution of the active metal occurs in a localized manner (e.g. pitting corrosion), allowing for cathodic sites to be developed on zinc. Local alkalization and oxygen consumption related to cathodic activity are detected above portions of the zinc surface using SECM. Anionic fluxes related to the generation of OH- ions could be measured on the zinc surface outside the corroding pit by rastering the SVET probe closer to the substrate.
[show abstract][hide abstract] ABSTRACT: Scanning electrochemical microscopy (SECM) with coupled potentiometric and amperometric measurement modes is used to study corrosion reactions in aqueous environments. Whereas conventional electrochemical techniques lack spatial resolution and provide little information on behaviour at sites of corrosion initiation or at defects, the advent of scanning electrochemical microscopy is contributing to overcome these limitations when applied to the investigation of corrosion processes in situ. Ion-selective microelectrodes (ISME) and dual amperometric/potentiometric probes have been developed that can be employed as SECM tips as they exhibit sufficiently low resistances and response times. The applicability of this experimental approach is demonstrated by chemically imaging the behaviour of an iron-magnesium galvanic couple immersed in a diluted aqueous electrolyte. The dissolution of metal ions from anodic sites, the consumption of oxygen at the cathodic sites, and the local pH changes associated with both half cell reactions are thus effectively monitored.
[show abstract][hide abstract] ABSTRACT: Micron-size ion selective micropipettes can be used in scanning electrochemical microscopy (SECM). They can provide excellent spatial resolution. Unfortunately the resistance of these small sensors is high. Their application needs special shielding and slow scanning rates. Usually their lifetime hardly exceeds a few days.
Zinc layer or dispersed zinc particles containing films are often used for providing cathodic protection against corrosion in case of metal surfaces. Therefore, in corrosion studies, measurements of local zinc ion concentration can give important information about the nature of the process. For corrosion studies we needed SECM measuring tips for imaging concentration profiles of Zn2+ ions involved in surface processes. Based on our earlier experience, solid contact micropipettes for selective measurements of Zn2+ ion concentration were prepared with a tip size of a few micrometers. The properties of the micropipettes were investigated. They were also used in SECM imaging. In this paper, details of Zn2+ ion selective microelectrode preparation are described. Data about their properties, lifetime, resistance, and ion activity response are shown. Preliminary findings in SECM imaging of zinc ion concentration profiles are shown. The improvement of the scanning rate achieved by lowering tip resistance is a main advantage in potentiometric SECM.
[show abstract][hide abstract] ABSTRACT: Ion-selective microelectrodes can be employed as tips in scanning electrochemical microscopy (SECM) for chemical imaging of corrosion processes. They present higher chemical selectivity than conventional amperometric microdisks, and may be the only effective option to visualize the dissolution of metals with negative redox potentials in aqueous environments when the use of Pt microelectrodes is limited by the onset of oxygen reduction and hydrogen evolution reactions. A robust micro-sized ion selective electrode has been developed which allows the spatial distribution of Zn2+ during galvanic corrosion of a model Fe/Zn couple to be investigated using SECM. Owing to the low internal contact potential achieved with the novel design, the resistance of the micropipette electrodes is only fractions of the resistance of conventional micropipette electrodes of the same size. As a result, no special shielding of the microelectrodes is required and higher scanning rate can be used for scanning in the potentiometric modes using these micropipette tips. Concentration profiles over corroding surfaces measured with this technique will be presented.
[show abstract][hide abstract] ABSTRACT: A combined scanning microelectrochemical procedure is proposed to obtain information on the action of corrosion inhibitors on metals. The method uses the scanning vibrating electrode technique (SVET) and the scanning electrochemical microscopy (SECM). Antimony tips are employed as the sensing probe in SECM, allowing this technique to be operated in both potentiometric and amperometric modes. This novel approach allows the spatial distributions of pH, concentration of redox active species, and ionic currents associated to corrosion processes to be monitored. The potential of the proposed experimental strategy is tested on a model system, the inhibitor action of benzotriazole (BTAH) on the galvanic corrosion of an iron/copper couple immersed in sodium chloride solution. In this way, the effect of BTAH on the cathodic half-cell reaction occurring on the copper specimen was studied in situ. The results reveal that the inhibitor films formed on copper render this metal electrochemically inactive, though electrically connected to the iron specimen, and cathodic sites to be developed on the less noble iron surface in addition to the anodic sites.
[show abstract][hide abstract] ABSTRACT: Iron(II) phthalocyanine (FePc) modified multi-wall carbon nanotubes paste electrodes (MWCNTPEs) were used as voltammetric sensors to selectively detect dopamine (DA) in the presence of serotonin (5-HT). The electrochemical behavior of DA at the new modified electrode was investigated using CV. The enhanced current response of DA indicates that FePc modification of the MWCNTPE surface results in a high catalytic activity for the redox reaction of DA. Differential pulse voltammetry was applied in detection of DA and 5-HT at FePc–MWCNTPE. The method parameters were optimized. Detection limit of 2.05 × 10−7 M was obtained for DA by using the electrocatalytic oxidation signal corresponding to the FeII/FeIII redox process. The separation between the peak potentials of DA and 5-HT is 170 mV which is large enough for the simultaneously, selective determination of the two chemical species in their mixtures. There was no electrochemical response for ascorbic acid (AA) added in the sample. The monoamine neurotransmitter measuring method has been tested in analyzing deproteinized serum samples.
Sensors and Actuators B Chemical 08/2011; 156(2):731–736. · 3.54 Impact Factor
[show abstract][hide abstract] ABSTRACT: A new method for the spatially resolved measurement of pH during corrosion processes with the scanning electrochemical microscope (SECM) is presented. Antimony tips are employed because the dual-function characteristics of this material allow the combined amperometric/potentiometric operation of the SECM. The applicability of this technique is illustrated by considering the galvanic corrosion of a model zinc–iron pair immersed in 0.1M NaCl aqueous solution. Spatially resolved images of pH and oxygen concentration above the metal specimens could be obtained in the same experiment.
[show abstract][hide abstract] ABSTRACT: The stability and catalytic action of glucose oxidase has been investigated in different water-butyl-methyl imidazolium tetrafluoroborate (BMIM(BF4)) mixtures. A flat amperometric measurement cell was fabricated embedding silver and platinum wires in dental cement. The rate of the enzyme catalyzed reaction was observed amperometrically detecting the hydrogen peroxide produced in the enzyme catalyzed reaction. The arrangement used allowed to carry out one measurement in reaction media volume as small as 12 μl. The rate of the enzyme catalyzed reaction decreased by increasing the BMIM(BF4) content of the solvent. The half life of the enzyme, however, was found longer in BMIM(BF4) containing media measured at different temperatures than in aqueous buffer.
[show abstract][hide abstract] ABSTRACT: A method for quantification of the main sugar components of honey and nectars are worked out and investigated. The method employs IC separation on CarboPac PA1 column with sodium hydroxide eluent and home made, electrochemical detector cells that contain copper working electrode. The detector cells operate in amperometric mode. The electrochemical oxidation of the sugar components was studied, estimating the number of electrons exchanged, the rate constant of the electrode reaction. The electrode processes showed electrocatalytic character with formate ions as main product of oxidation. Real samples of honey and nectar taken from different plants were analyzed with the method proposed.
[show abstract][hide abstract] ABSTRACT: The electrochemical determinations of 4-chlorophenol (4-CP) and 4-nitrophenol (4-NP) by chronoamperometry (CA) and multiple pulsed amperometry (MPA) using expanded graphite-epoxy composite (EG-Epoxy) and rotating spectral graphite disc (SG) electrodes are reported. The electrochemical behaviours of both electrodes in the presence of organics informed about oxidation peak potential and the electrode fouling with organics concentration increasing. Setting up the oxidation peak potential as detection potential, only SG gave good electroanalytical performance using CA. However, by MPA applying both electrodes exhibited the capability to assess electrochemically and quantitatively the pollutants from aqueous solutions. UV spectrometric method detecting 4-CP and 4-NP at lambda=280nm and lambda=398nm wavelength, respectively was used for validation and parallel determinations.
[show abstract][hide abstract] ABSTRACT: Micropipette ion selective electrodes are very small, but fragile, short-life time sensors with very high resistance. Their high resistance is a draw back considering application in scanning electrochemical microscopy (SECM) and in life sciences. New, low resistance potassium micropipette electrodes were prepared, and applied. The electrode contains solid internal contact made of a carbon fiber lowered down all the way close to the orifice of the micropipette. The internal contact potential was kept constant by applying a doped, electrochemically prepared PEDOT coating on the fiber surface. The electrode performed well in in vivo experiments both in plant and animal tissue without capacitance neutralization and in SECM.
[show abstract][hide abstract] ABSTRACT: The diffusion coefficient of glucose in different media is an important parameter in life sciences, as well as in biotechnology and microbiology. In this work a simple, fast method is proposed that is based on the electrochemical time of flight principle. In most of the earlier time of flight experiments performed, a constant flight distance was applied. In the present work a scanning electrochemical microscope (SECM) was applied as a measuring tool. With use of the SECM, the flying distance could be changed with high precision, making measurements with several flight distances more accurate and reliable values could be obtained for solutions as well as for gels. The conventional voltammetric methods are not applicable for glucose detection. In our work electrocatalytic copper oxide coated copper microelectrodes and micro-sized amperometric enzyme sensors were used as detectors, while microdroplet-ejecting pneumatically driven micropipettes were used as a source.
Analytical and Bioanalytical Chemistry 07/2009; 394(7):1955-63. · 3.66 Impact Factor
[show abstract][hide abstract] ABSTRACT: Amadori peptides were enriched using boronate affinity tips and measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/MS). As demonstrated by electrochemical measurements, the tips show the highest binding efficiency for glucose at pH 8.2 employing ammonium chloride/ammonia buffer with ionic strength of 150 mM, exceeding taurine buffer at the same concentration. The bound constituents were released by sorbitol and formic acid. It was also demonstrated that elution with sorbitol at 1.2 M is superior to acidic media. Comparison of results was based on the numbers of detected peptides and their glycated sites. Using sorbitol for elution requires desalting prior to analysis. Therefore, three different sorbents were tested: fullerene-derivatized silica, ZipTip (C18), and C18 silica. Fullerene-derivatized silica and ZipTip showed the same performance regarding the numbers of glycated peptides, and sites were better than C18 silica. The elaborated off-line method was compared with liquid chromatography-tandem mass spectrometry (LC-MS/MS) measurements, by which considerable less modified peptides were detected. Affinity tips used under optimized conditions were tested for the analysis of human serum albumin (HSA) from sera of healthy and diabetic individuals. A peptide with a mass of 1783.9 Da could be detected only in samples of diabetic patients and, therefore, could be a very interesting biomarker candidate.
[show abstract][hide abstract] ABSTRACT: The principle of a simple time of flight (TOF) diffusion coefficient measuring method is presented. It is based on the special capability of Scanning Electrochemical Microscopy (SECM). Its applicability has been investigated experimentally in comparison with conventional electrochemical methods. Complex irreversible electrode processes usually bring in uncertainty into electrochemical diffusion coefficient measurements. One of these, the electrochemical oxidation of saccharides in basic media at a copper electrode, was selected for the studies. Microsize copper electrodes used also as SECM tip, and conventional size copper electrodes were used. Diffusion coefficients of D-glucose, D-arabinose, D-ribose, and D-galactose were measured. The results prove the advantages of the SECM-TOF method in the case investigated.