Showing papers in "Electroanalysis in 2004"

Electroanalytical and Bioelectroanalytical Systems Based on Metal and Semiconductor Nanoparticles

Abstract: Metal, semiconductor and magnetic particles act as functional units for electroanalytical applications. Metal nanoparticles provide three important functions for electroanalysis. These include the roughening of the conductive sensing interface, the catalytic properties of the nanoparticles permiting their enlargement with metals and the amplified electrochemical detection of the metal deposits and the conductivity properties of nanoparticles at nanoscale dimensions that allow the electrical contact of redox-centers in proteins with electrode surfaces. Also, metal and semiconductor nanoparticles provide versatile labels for amplified electroanalysis. Dissolution of the nanoparticle labels and the electrochemical collection of the dissolved ions on the electrode followed by the stripping-off of the deposited metals represents a general electroanalytical procedure. These unique functions of nanoparticles were employed for developing electrochemical gas sensors, electrochemical sensors based on molecular- or polymer-functionalized nanoparticle sensing interfaces, and for the construction of different biosensors including enzyme-based electrodes, immunosensors and DNA sensors. Semiconductor nanoparticles enable the photoelectrochemical detection of analytes. Several studies have revealed the photocurrent generation by enzyme-mediated processes and as a result of DNA hybridization. Magnetic particles act as functional components for the separation of biorecognition complexes and for the amplified electrochemical sensing of DNA or antigen/antibody complexes. Also, electrocatalytic and bioelectrocatalytic processes at electrode surfaces are switched by means of functionalized magnetic particles and in the presence of an external magnet.

Disposable Carbon Nanotube Modified Screen‐Printed Biosensor for Amperometric Detection of Organophosphorus Pesticides and Nerve Agents

Abstract: A disposable carbon nanotube-based biosensor was successfully developed and applied to the detection of organophosphorus (OP) pesticides and nerve agents The biosensors using acetycholinesterase (AChE)/choline oxidase (CHO) enzymes provided a high sensitivity, large linear range, and low detection limits for the analysis of OP compounds Such characteristics may be attributed to the catalytic activity of carbon nanotube to promote the redox reaction of hydrogen peroxide produced during AChE/CHO enzymatic reactions with their substrate, as well as the large surface area of carbon nanotube materials

An Extended Method for the Practical Evaluation of the Standard Rate Constant from Cyclic Voltammetric Data

Abstract: The Nicholson's treatment for the evaluation of the heterogeneous standard rate constant k0 of electron transfer from cyclic voltammetric (CV) data is combined with the Klingler and Kochi method suitable to totally irreversible systems in order to use very large ΔEpx n values, where n is the number of electrons involved in a simple electron-transfer process O+n e−⇌R. An empirical relationship between the dimensionless parameter Ψ=k0 [πDnvF/(RT)]−1/2 and ΔEpx n which practically replaces the Nicholson's working curve up to about 200 mV is reported.

FET‐Based Biosensors for The Direct Detection of Organophosphate Neurotoxins

Abstract: Recent world-wide terrorist events associated with the threat of hazardous chemical agent proliferation, and outbreaks of chemical contamination in the food supply has demonstrated an urgent need for sensors that can directly detect the presence of dangerous chemical toxins. Such sensors must enable real-time detection and accurate identification of different classes of pesticides (e.g., carbamates and organophosphates) but must especially discriminate between widely used organophosphate (OP) pesticides and G- and V-type organophosphate chemical warfare nerve agents. Present field analytic sensors are bulky with limited specificity, require specially-trained personnel, and, in some cases, depend upon lengthy analysis time and specialized facilities. Most bioanalytical based systems are biomimetic. These sensors utilize sensitive enzyme recognition elements that are the in-vivo target of the neurotoxic agents which the sensor is attempting to detect. The strategy is well founded; if you want to detect cholinesterase toxins use cholinesterase receptors. However, this approach has multiple limitations. Cholinesterase receptors are sensitive to a wide range of non-related compounds and require lengthy incubation time. Cholinesterase sensors are inherently inhibition mode and therefore require baseline testing followed by sample exposure, retest and comparison to baseline. Finally, due to the irreversible nature of enzyme-ligand interactions, inhibition-mode sensors cannot be reused without regeneration of enzyme activity, which in many cases is inefficient and time-consuming. In 1996, we pioneered a new “kinetic” approach for the direct detection of OP neurotoxins based on agent hydrolysis by the enzyme organophosphate hydrolase (OPH; EC 3.1.8.2; phosphotriesterase) and further identified a novel multi-enzyme strategy for discrimination between different classes of neurotoxins. The major advantage of this sensor strategy is it allows direct and continuous measurement of OP agents using a reversible biorecognition element. We also investigated incorporation of enzymes with variations in substrate specificity (e.g., native OPH, site-directed mutants of OPH, and OPAA (EC 3.1.8.1), based upon preferential hydrolysis of PO, PF and PS bonds to enable discrimination among chemically diverse OP compounds. Organophosphate hydrolase enzymes were integrated with several different transduction platforms including conventional pH electrodes, fluoride ion-sensitive electrodes, and pH-responsive fluorescent dyes. Detection limit for most systems was in the low ppm concentration range. This article reviews our integration of organophosphate hydrolase enzymes with pH sensitive field effect transistors (FETs) for OP detection.

Solubilization of Multiwall Carbon Nanotubes by 3‐Aminopropyltriethoxysilane Towards the Fabrication of Electrochemical Biosensors with Promoted Electron Transfer

Abstract: A simple procedure has been described for the fabrication of multi-wall carbon nanotube (CNT) based electrochemical sensors. 3-Aminopropyltriethoxysilane (APTES) induced solubilization of CNTs allowed for the modification of electrode surfaces. With glucose oxidase (GOx), a flavin (FAD) containing enzyme as a model system, APTES was used as a solubilizing agent for CNTs as well as an immobilization matrix for GOx to construct a mediatorless biosensor. Our biosensor was able to efficiently monitor direct electroactivity of GOx at the electrode surface. A well-defined glucose response was observed at −0.45 V (vs. Ag/AgCl) whereas relevant physiological levels (0.1 mM) of three common interfering species, uric acid, ascorbic acid, and acetaminophen, resulted in no response. Although CNTs modified by APTES acted as semiconductors to reduce the exposed sensing surface, we reasoned nanoscale “dendrites” of CNTs modified by APTES formed a network and projected outwards from the electrode surface and acted like bundled ultra-microelectrodes that allowed access to the active FAD site and facilitated direct electron transfer to the immobilized enzyme. The glucose biosensor prepared using a carbon fiber (11 μm) exhibited picoamperometric current response within 5 s with detection limits of 5–10 μM.

Direct electron transfer between ligninolytic redox enzymes and electrodes

Abstract: The electrochemistry of the ligninolytic redox enzymes, which include lignin peroxidase, manganese peroxidase and laccase and possibly also cellobiose dehydrogenase, is reviewed and discussed in conjunction with their basic biochemical characteristics. It is shown that long-range electron transfer between these enzymes and electrodes can be established and their ability to degrade lignin through a direct electron transfer mechanism is discussed.

Batch Injection Analysis: An Almost Unexplored Powerful Tool

Abstract: The main purpose of this review is to report the development and potentialities of batch injection analysis (BIA) and call the attention of researchers about the power of this (until now) almost unexplored tool. The text focuses on the concepts and potentialities of BIA combined with other techniques for analytical purposes. The association of BIA with amperometry, potentiometry, voltammetry, calorimetry, fluorescence and spectrophotometry is presented and important aspects related with each technique are discussed. Some aspects comparing the similarities and differences between BIA, FIA and wall-jet are presented along the text.

Contactless Conductivity Detection in Capillary Electrophoresis: A Review

Abstract: The popularity of contactless conductivity detection in capillary electrophoresis has been growing steadily over the last few years. Improvements have been made in the design of the detector in order to facilitate its handling, to allow easy incorporation into available instruments or to achieve higher sensitivity. The understanding of its fundamental working principles has been advanced and the detection approach has also been transferred to lab-on-chip devices. The range of applications has been extended greatly from the initial work on small inorganic ions to include organic species and biomolecules. Concurrent determination of cations and anions by dual injection from opposite ends has been demonstrated as well as sample introduction by using flow-injection systems for easy automation of the process.

Characterization and Applications of a Bismuth Bulk Electrode

Abstract: A bismuth bulk electrode (BiBE), a new solid-state electrode, is presented. The polycrystalline metal bismuth disk-shaped electrode was examined for its anodic stripping voltammetry performance, which was found to be well comparable to that achieved with the bismuth or mercury film electrodes. Useful potential windows of the BiBE in aqueous solutions of pH 1 to 13 were found to range from approximately −1.7 to −0.1 V, depending on pH, where either hydrogen evolution or anodic dissolution of metallic bismuth limit the electrochemical inertness of the BiBE. Employing cyclic voltammetry (CV), the cathodic behavior of the BiBE was examined by testing inorganic (cadmium(II) ions) and organic (2-nitrophenol) model compounds; a CV quasi-reversible behavior was recorded in the case of the Cd(II)-Cd(0) couple. The characteristics of the BiBE under anodic conditions, i.e., at bismuth surface coated with a thin conductive Bi2O3 film, was examined by testing two well-established redox systems, potassium hexacyanoferate(III) and ruthenium(III) hexaaminechloride; a nearly reversible behavior was recorded in the latter case. Based on the presented preliminary results, BiBE can be considered as an interesting alternative to common solid and (toxic) mercury electrodes for possible use in electrochemical studies and electroanalytical applications.

Direct Electrochemistry of Catalase at a Gold Electrode Modified with Single-Wall Carbon Nanotubes

Abstract: The direct electrochemistry of catalase (Ct) was accomplished at a gold electrode modified with single-wall carbon nanotubes (SWNTs). A pair of well-defined redox peaks was obtained for Ct with the reduction peak potential at 0.414 V and a peak potential separation of 32 mV at pH 5.9. Both reflectance FT-IR spectra and the dependence of the reduction peak current on the scan rate revealed that Ct adsorbed onto the SWNT surfaces. The redox wave corresponds to the Fe(III)/Fe(II) redox center of the heme group of the Ct adsorbate. Compared to other types of carbonaceous electrode materials (e.g., graphite and carbon soot), the electron transfer rate of Ct redox reaction was greatly enhanced at the SWNT-modified electrode. The peak current was found to increase linearly with the Ct concentration in the range of 8 10 6 ±8 10 5 M used for the electrode preparation and the peak potential was shown to be pH dependent. The catalytic activity of Ct adsorbates at the SWNTs appears to be retained, as the addition of H2O2 produced a characteristic catalytic redox wave. This work demonstrates that direct electrochemistry of redox-active biomacromolecules such as metalloenzymes can be improved through the use of carbon nanotubes.

Characterization and Optimization of Interdigitated Ultramicroelectrode Arrays as Electrochemical Biosensor Transducers

Abstract: Interdigitated ultramicroelectrode arrays (IDUAs) were fabricated on glass wafers and investigated to obtain optimal oxidation and reduction reactions of potassium ferro/ferrihexacyanide, Fe2+/3+(CN)6, when using a 2-electrode set up. These electrodes will be used as transducers in portable microfluidic-based biosensors in the future for the detection in an aqueous, biocompatible matrix. IDUAs were designed to maximize the signal-to-noise ratio (S/N) investigating electrode height, gap size, finger width, and material. Interesting differences in the electrode materials gold and platinum were found, which were due to the oxidization of platinum and gold during the IDUA fabrication process. It resulted in gold IDUAs being by far superior in respect to signal-to-noise ratio and overall signal magnitude to those made of platinum. The effects of gap size, electrode width and number of electrode fingers were as expected. Optimal electrode heights were in the range of 70 nm–140 nm, much larger and smaller electrodes had lower signal-to-noise ratios due to overall reduced signal or increased background. The optimized IDUA was made out of gold, had 400 fingers with a finger width of 2.7 μm, a finger height between 70 nm and 140 nm and a gap size of 0.9–1 μm. A detection limit of as low as 0.1 μM ferro/ferrihexacyanide measured in a simple 2-electrode set up was obtained with a signal-to-noise ratio of 9.7.

Direct Peroxidase Bioelectrocatalysis on a Variety of Electrode Materials

Abstract: The role of the electrode material in the efficiency of direct (non-mediated) bioelectrocatalytic reduction of H2O2, catalyzed by horseradish peroxidase (HRP) is studied and discussed. The variations in direct peroxidase bioelectrocatalysis when coming from carbon/graphite to metal electrodes and oxides, as well as modified electrodes, are analyzed regarding the variations in adsorption/orientation of peroxidase at the electrodes, interfacial electron transfer rates and mechanism of catalysis.

Amplified Electrical Transduction of DNA Hybridization Based on Polymeric Beads Loaded with Multiple Gold Nanoparticle Tags

Abstract: A new strategy for amplifying particle-based electrical DNA detection based on oligonucleotides functionalized with polymeric beads carrying numerous gold nanoparticle tags is described. The gold-tagged beads were prepared by binding biotinylated metal nanoparticles to streptavidin-coated polystyrene spheres. Such use of carrier-sphere amplification platforms is combined with catalytic enlargement of the multiple gold tags and an ultrasensitive electrochemical stripping detection of the dissolved gold tags. The gold-nanoparticle loaded beads and the resulting DNA-linked assembly were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Factors affecting the performance have been optimized. Such amplified electrical transduction allows detection of DNA targets down to the 300amol level, and offers great promise for ultrasensitive detection of other biorecogntion events.

Electrochemistry of DNA at Single‐Wall Carbon Nanotubes

Abstract: Single-wall carbon nanotubes (SWNTs) can be used as an electrochemical probe to study the electrochemical oxidation of DNA, two voltammetric oxidation peaks corresponding to electrochemical oxidation of guanine and adenine residues in DNA could be observed, demonstrating a new potential application of SWNTs in DNA analysis. Compared to other types of electrode materials, the electrochemical response of natural DNA was greatly enhanced at the SWNT-modified electrode. This method can be used to investigate the interaction of SWNTs with DNA.

Nanotube Membrane Based Biosensors

Abstract: We review highly sensitive detection based on electrochemical methods. These methods are based on monodisperse gold and alumina nanotubule membranes with inside diameter approaching molecular dimensions. The analyte species can be detected by measuring a change in trans-membrane current when the analyte is added to the nanotubule-based cell. The second method entails the use of a concentration change based on the nanotubule membrane. Biomemtic ion-gated channels micropore and nanotubule membrane sensors are also reviewed. These synthetic ion channels can be switched from an “off” state to an “on” state in response to an external chemical stimulus. Using these methods, we have achieved detection limits as low as 10 pM. Potential applications for these biosensors are in fields such as bioanalytical, biomedical, pharmaceutical and drug discovery.

Single-wall carbon nanotube paste electrodes: A comparison with carbon paste, platinum and glassy carbon electrodes via cyclic voltammetric data

Abstract: A new carbon electrode material, obtained by mixing single wall carbon nanotubes (SWNTs) with a mineral oil binder is studied Carbon nanotube pastes show the special properties of carbon nanotubes combined with the various advantages of composite electrodes such as a very low capacitance (background current) and the possibility of an easy preparation, modification and renewal A better knowledge of the characteristics of electrode reactions at carbon nanotube paste (CNTP) electrodes was obtained studying the electron transfer rates of various redox couples under different pretreatment conditions A critical comparison with carbon paste (CP), platinum (Pt) and glassy carbon (GC) electrodes was also carried out Capacitance and resistance values were also calculated for all electrodes investigated Both untreated and treated CNTP electrodes showed a low resistance while the capacitance was markedly reduced with CNTP electrodes previously treated with concentrated nitric acid An electrochemical pretreatment on CNTP electrodes was developed which showed an excellent result towards two-electron quinonic structure species After this treatment the heterogeneous standard rate constants for p-methylaminophenol sulfate (MAP) and dopamine resulted to be significantly higher (21×10−2 cm/s and 20×10−2 cm/s, respectively) than those obtained with the other electrodes studied Reproducibility, stability and storage characteristics of CNTP electrodes were also reported

Carbon Nanotubes-Based Amperometric Cholesterol Biosensor Fabricated Through Layer-by-Layer Technique

Abstract: A carbon nanotubes-based amperometric cholesterol biosensor has been fabricated through layer-by-layer (LBL) deposition of a cationic polyelectrolyte (PDDA, poly(diallyldimethylammonium chloride)) and cholesterol oxidase (ChOx) on multi-walled carbon nanotubes (MWNTs)-modified gold electrode, followed by electrochemical generation of a nonconducting poly(o-phenylenediamine) (PPD) film as the protective coating. Electrochemical impedance measurements have shown that PDDA/ChOx multilayer film could be formed uniformly on MWNTsmodified gold electrode. Due to the strong electrocatalytic properties of MWNTs toward H2O2 and the low permeability of PPD film for electroacitve species, such as ascorbic acid, uric acid and acetaminophen, the biosensor has shown high sensitivity and good anti-interferent ability in the detection of cholesterol. The effect of the pH value of the detection solution on the response of the biosensor was also investigated. A linear range up to 6.0 mM has been observed for the biosensor with a detection limit of 0.2 mM. The apparent Michaelis-Menten constant and the maximum response current density were calculated to be 7.17 mM and 7.32 m Ac m � 2 , respectively.

Analysis and Speciation of Traces of Arsenic in Environmental, Food and Industrial Samples by Voltammetry: a Review

Abstract: Voltammetric approaches for the determination of arsenic and speciation at trace levels are critically appraised in a review covering the literature from 1970 to 2002. Special attention is devoted to stripping modes and to issues related to the choice of working material and supporting electrolyte. A section is dedicated to the management of real samples and aspects of sample preparation. An extensive compilation, organized by real sample type, gathers essential experimental conditions. Potentiometric stripping analysis is introduced for sake of comparison. The coupling of voltammetric detection or preaccumulation with FIA, chromatography, capillary electrophoresis and ICP techniques is also addressed.

Probing DNA Hybridization by Impedance Measurement Based on CdS‐Oligonucleotide Nanoconjugates

Abstract: A novel, sensitive DNA hybridization detection protocol, based on DNA-quantum dots nanoconjugates coupled with electrochemical impedance spectroscopy (EIS) detection, is described. The amino-linked ss-DNA probe was covalently immobilized onto a self-assembled mercaptoacetic acid monolayer modified gold electrode; after hybridization with the target ssDNA-CdS nanoconjugates, EIS was used to detect the change of interfacial electron-transfer resistance (Ret) of the redox marker, [Fe(CN)6]4−/3−, from solution to transducer surface. The results showed that when target ssDNA-CdS nanoconjugates hybridized with probe oligonucleotide, a double helix film formed on the electrode, a remarkably increased Ret value was observed. Only complementary DNA sequence had an obvious signal compared with three-base mismatched or non-completely matched sequences under the optimized experimental conditions. Due to having more negative charges, space resistance and the semiconductor property, CdS nanoparticle labels on target DNA could improve the sensitivity to two orders of magnitude when compared with non-CdS tagged DNA sequences.

Cathodic Electrochemical Detection of Nitrophenols at a Bismuth Film Electrode for Use in Flow Analysis

Abstract: The bismuth film electrode (BiFE) is presented for use in both batch voltammetric and flow injection (FI) amperometric detection of some nitrophenols (2-nitrophenol, 2-NP; 4-nitrophenol, 4-NP; 2,4-dinitrophenol, 2,4-DNP). The bismuth film was deposited ex situ (batch measurements) and in-line (FI) onto a glassy carbon substrate electrode. Batch analysis of the nitrophenols was carried out in 0.04 M Britton Robinson (BR) buffer pH 4, while for FI measurements, a carrier/electrolyte solution composed of 0.1 M BR buffer pH 4 mixed with methanol (20+80, v/v%) was employed to resemble media used in preconcentration/clean-up and flow separation sample pretreatment procedures. Under batch conditions, the voltammetric behavior of the nitrophenols was examined for dependence on medium pH in the range of 2 to 10. Employing the square-wave voltammetry mode, the limits of detection were 0.4 μg L−1, 1.4 μg L−1, and 3.3 μg L−1 for 2-NP, 4-NP, and 2,4-DNP, respectively. Under flow conditions, a simple in-line electrochemical bismuth film renewal procedure was tested and shown to provide very good inter- and intra-electrode reproducibility of the current signals at low μg L−1 analyte concentrations. The limits of detection for 2-NP, 4-NP and 2,4-DNP obtained using FI and amperometric detection at −1.0 V (vs. Ag/AgCl) were 0.3 μg L−1, 0.6 μg L−1 and 0.7 μg L−1, respectively, with linear ranges extending up to 20 μg L−1. The attractive performance of the BiFE under flow analysis conditions offers great promise with respect to its detection capability and to its use for a prolonged period of time with no need for inconvenient removal of the electrode from the system for mechanical surface treatment.

Bioelectrocatalysis of Oxygen Reduction Reaction by Laccase on Gold Electrodes

Abstract: Direct electron transfer (DET) reaction of oxygen electroreduction catalyzed by enzyme laccase on monolayer modified gold electrodes was studied. Three different monolayers were investigated, from which 4-aminothiophenol was found to be optimal for the direct electron transfer to take place. The electrocatalytic reduction of the oxygen at the electrode surface was found to depend significantly on the method of immobilization. Fungal laccase from Coriolus hirsitus modified with sodium-periodate demonstrated more anodic onset potential for oxygen reduction than the tree laccase from Rhus vernicifera. Physical immobilization of enzyme did not result in any manifestation of bioelectrocatalytic activity. A maximum anodic shift in reduction potential of 300 mV was observed for fungal laccase covalently coupled on the electrode surface.

Constant‐Distance Mode Scanning Electrochemical Microscopy. Part II: High‐Resolution SECM Imaging Employing Pt Nanoelectrodes as Miniaturized Scanning Probes

Abstract: The potential of needle-type Pt disk nanoelectrodes as extremely miniaturized scanning probes for high resolution scanning electrochemical microscopy (SECM) was investigated. The accuracy of a piezoelectric shear-force based distance control allowed a precise positioning of the Pt nanoelectrodes in close proximity to the surface of interest not only in tip approach experiments but also throughout scanning and SECM imaging. As proof of the advanced quality of SECM imaging, high-resolution current and topography images of a three-dimensional LIGA microstructure will be presented both simultaneously acquired by operating Pt nanoelectrodes in the constant-distance mode of SECM.

Sensitive Electrochemical Detection of Native and Aggregated α-Synuclein Protein Involved in Parkinson's Disease

Abstract: The aggregation of α-synuclein, a 14 kDa protein, is involved in several human neurodegenerative disorders, including Parkinson's disease. We studied native and in vitro aggregated α-synuclein by circular dichroism (CD), atomic force microscopy (AFM) and electrochemical methods. We used constant current chronopotentiometric stripping analysis (CPSA) to measure hydrogen evolution catalyzed by α-synuclein (peak H) at hanging mercury drop electrodes (HMDE) and square-wave stripping voltammetry (SWSV) to monitor tyrosine oxidation at carbon paste electrodes (CPE). To decrease the volume of the analyte, most of the electrochemical measurements were performed by adsorptive transfer (medium exchange) from 3-6 L drops of α-synuclein samples. With both CPE and HMDE we observed changes in electrochemical responses of α-synuclein corresponding to protein fibrillization detectable by CD, fluorescence and AFM. Aggregation-induced changes in peak H at HMDE were relatively large in strongly aggregated samples, suggesting that this electrochemical signal may find use in the analysis of early stages of α-synuclein aggregation. This assumption was documented by marked changes in the peak H potential and height in samples withdrawn at the end of the lag and the beginning of the elongation phase. Native α-synuclein can be detected down to subnanomolar concentrations by CPSA.

High-temperature electrochemistry: a review

Abstract: A review. High-temp. electrochem. remains a relatively unexplored field of research, although in recent years significant developments were made. This report details the main exptl. methods and approaches to heating an electrochem. system under both isothermal and nonisothermal conditions and gives an insight into the exptl. and electroanal. results obtainable under such conditions. The promotion of mass transport at high-temps., through diffusion or convection, often results in increased current signals. This increase benefits electroanal. measurements by lowering detection limits. High temps. also usefully enhance the sensitivity of systems with sluggish kinetics. [on SciFinder(R)]

A Novel Amperometric O2 Gas Sensor Based on Supported Room‐Temperature Ionic Liquid Porous Polyethylene Membrane‐Coated Electrodes

Abstract: A novel solid-state amperometric O2 gas sensor based on the supported 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) porous polyethylene membrane-coated electrodes has been proposed with its characterization. By electrochemical impedance technique, the ionic conductivity of the supported EMIBF4 membrane was estimated to be ca. 0.6 S m−1, indicating that the supported EMIBF4 membrane (the thickness: 50 μm) can be used as a solid state ionic conductor at room temperature. The cyclic voltammograms obtained for the one-electron redox reaction of O2/O2−.(O2−.: superoxide ion) couple at high scan rates (>100 mV s−1) showed a couple of usual redox peaks, while at low scan rates (<30 mV s−1) S-shaped steady-state voltammograms similar to those obtained by rotating disk voltammetry were obtained. These results were explained on the basis of the mass transport of O2 at the supported EMIBF4 membrane-coated electrode system. The transient and steady-state reduction currents for the reduction of O2 to O2−.as well as the transient oxidation current for the reoxidation of O2−.to O2, which were obtained by potential-step chronoamperometry, could be used to measure the change of O2 concentration in O2-N2 mixed gas stream. The present O2 gas sensor demonstrated a wide detection range, a high sensitivity and an excellent reproducibility.

Detecting Both Physical and (Bio‐)Chemical Parameters by Means of ISFET Devices

Abstract: A multi-parameter sensor system for the detection of eight (bio-)chemical and physical parameters (pH, potassium concentration, penicillin concentration, diffusion coefficient of H þ - and OH – -ions, temperature, flow velocity, flow direction and liquid level) is realized by using the same transducer principle. A Ta2O5-gate ISFET (ion-sensitive fieldeffect transistor) is applied as basic transducer for all kinds of sensors. The multi-parameter detection is achieved by means of sequentially or simultaneously scheduling of the hybride sensor modules consisting of four ISFET structures and an ion generator in different sensor arrangements and/or different operation modes. Thus, more parameters (eight) can be detected than the number of sensors (four) in the system.

Trace Determination of Chromium by Square-Wave Adsorptive Stripping Voltammetry on Bismuth Film Electrodes

Abstract: This works reports the use of adsorptive stripping voltammetry (AdSV) for the trace determination of chromium on a rotating-disk bismuth-film electrode (BFE). During the reductive accumulation step, all the chromium species in the sample were reduced to Cr(III) which was complexed with cupferron and the complex was accumulated by adsorption on the surface of a preplated BFE. The stripping step was carried out by using a square-wave (SW) potential-time voltammetric signal. Electrochemical cleaning of the bismuth film was employed, enabling the same bismuth film to be used for a series of measurements in the presence of dissolved oxygen. The experimental variables as well as potential interferences were investigated and the figures of merit of the method were established. Using the selected conditions, the 3σ limit of detection for chromium was 100 ng L−1 (for 120 s of preconcentration) and the relative standard deviation was 3.6% at the 2 μg L−1 level (n=8). Finally, the method was applied to the determination of chromium in real samples with satisfactory results.

Electrocatalytic Properties of Heme Proteins in Layer‐by‐Layer Films Assembled with SiO2 Nanoparticles

Abstract: Electrochemical and electrocatalytic properties of heme proteins, including myoglobin (Mb), hemoglobin (Hb), and horseradish peroxidase (HRP), in layer-by-layer films assembled with SiO2 nanoparticles were investigated at pyrolytic graphite (PG) electrodes. The stable layer-by-layer {SiO2/protein}n films showed a pair of well-defined, nearly reversible cyclic voltammetric peaks at about −0.25 V (vs. SCE) at pH 5.5, characteristic of the protein heme FeIII/FeII redox couples. The proteins in the films displayed good electrocatalytic activities toward various substrates such as oxygen, hydrogen peroxide, trichloroacetic acid, and nitrite, and showed a potential applicability in fabricating the new kind of biosensors without using mediators. Electrochemical parameters of the protein films such as apparent heterogeneous electron transfer rate constant (ks) and formal potential (E°′) were estimated by square-wave voltammetry and nonlinear regression analysis. The influencing factors like the number of bilayer and the pH of external solution were also investigated.

Lead Sulfide Nanoparticle as Oligonucleotides Labels for Electrochemical Stripping Detection of DNA Hybridization

Abstract: We report a method for the detection of DNA hybridization in connection to lead sulfide (PbS) nanoparticle tags and electrochemical stripping measurement of the lead. A kind of lead sulfide nanoparticle with free carboxyl groups on its surface was synthesized in aqueous solution. The nanoparticle was used as a marker to label a sequence-known oligonucleotide, which was then employed as a DNA probe for identifying a target ssDNA immobilized on a PPy modified electrode based on a specific hybridization reaction. The hybridization events were monitored by the oxidation dissolution of the lead sulfide anchored on the hybrids and the indirect determination of the lead ions by anodic stripping voltammetry (ASV). The detection limit is 0.3 pmol L−1 of target oligonucleotides. The PbS nanoparticle combining its easy conjugation to the DNA molecule with the highly sensitive stripping voltammetry detection of lead shows its promising application in the electrochemical DNA hybridization analysis assay.

Voltammetric Oxidation of Drugs of Abuse I. Morphine and Metabolites

Abstract: A detailed study of the electrochemical oxidative behavior of morphine in aqueous solution is reported. Through the synthesis of several metabolites and derivatives, pseudomorphine, morphine N-oxide, normorphine, dihydromorphine and 2-(N,N-dimethylaminomethyl)morphine, and their voltammetric study it was possible to identify the oxidation peaks for morphine. The anodic waves are related with the oxidation of phenolic and tertiary amine groups. It is also possible to verify that a poorly defined peak observable during morphine oxidation is not a consequence of further oxidation of pseudomorphine but due to formation of a dimer during phenolic group oxidation. The results obtained and especially those regarding the formation of a new polymer based on a CO coupling could be useful for clarifying the discoloration phenomenon occurring during storage of morphine solutions as well as leading to a better understanding of its oxidative metabolic pathways.