Redox buffering in an electrospray ion source using a copper capillary emitter.

An electrospray ion source used in electrospray mass spectrometry is a two-electrode, controlled-current electrochemical flow cell. Electrochemical reactions at the emitter electrode (oxidation and reduction in positive and negative ion modes respectively) provide the excess charge necessary for the quasi-continuous production of charged droplets and ultimately gas-phase ions with this device. We demonstrate here that a copper capillary emitter, in place of the more commonly used stainless-steel capillary emitter, can be utilized as a redox buffer in positive ion mode. Anodic corrosion of the copper capillary during normal operation liberates copper ions to solution and in so doing maintains the interfacial potential at this electrode near the equilibrium potential for the copper corrosion process [E degrees = 0.34 V versus standard hydrogen electrode (SHE)]. Fixing the interfacial potential at the emitter electrode provides control over the electrochemical reactions that take place at this electrode. It is shown that the oxidation of N-phenyl-1,4-phenylenediamine to N-phenyl-1,4-phenylenediimine (E(p/2) = 0.48 V versus SHE) can be completely avoided using the copper emitter, whereas this analyte is completely oxidized with a stainless-steel capillary emitter under the same conditions. Moreover, using N-phenyl-1,4-phenylenediimine, we demonstrate that reduction reactions can occur at the copper emitter electrode in positive ion mode. Emitter corrosion, in addition to redox buffering, provides a convenient means to introduce metal ions into solution for analytical use in electrospray mass spectrometry.     

Ascorbic acid for homogenous redox buffering in electrospray ionization–mass spectrometry

Electrospray ionization (ESI) involves the dispersion of a liquid containing analytes of interest into a fine aerosol by applying a high potential difference to the sample solution with respect to a counter electrode. Thus, from the electrochemical point of view, the ESI source represents a two-electrode controlled-current electrochemical flow cell. The electroactive compounds part of the solvent sprayed may be altered by occurring electrolysis (oxidation in positive ion mode and reduction in negative ion mode). These reactions can be troublesome in the context of unknown identification and quantification. In the search for a simple, inexpensive, and efficient way to suppress electrochemical oxidation in positive ESI, the usability of ascorbic acid, hydroquinone, and glutathione for homogenous redox buffering was tested. Performance of the antioxidants was assessed by analyzing pharmaceutical compounds covering a broad range of functional groups prone to oxidation. Different emitter setups were applied for continuous infusion, flow injection, and liquid chromatography/mass spectrometry experiments. Best performance was obtained with ascorbic acid. In comparison to hydroquinone and glutathione, ascorbic acid offered superior antioxidant activity, a relatively inert oxidation product, and hardly any negative effect on the ionization efficiency of analytes. Furthermore, ascorbic acid suppressed the formation of sodiated forms and was able to induce charge state reduction. Only in the very special case of analyzing a compound isobaric to ascorbic acid, interference with the low-abundant [ascorbic acid+H](+) signal may become a point of attention.     

Expanded use of a battery-powered two-electrode emitter cell for electrospray mass spectrometry

A battery-powered, controlled-current, two-electrode electrochemical cell containing a porous flow-through working electrode with high surface area and multiple auxiliary electrodes with small total surface area was incorporated into the electrospray emitter circuit to control the electrochemical reactions of analytes in the electrospray emitter. This cell system provided the ability to control the extent of analyte oxidation in positive ion mode in the electrospray emitter by simply setting the magnitude and polarity of the current at the working electrode. In addition, this cell provided the ability to effectively reduce analytes in positive ion mode and oxidize analytes in negative ion mode. The small size, economics, and ease of use of such a battery-powered controlled-current emitter cell was demonstrated by powering a single resistor and switch circuit with a small-size, 3 V watch battery, all of which might be incorporated on the emitter cell.     

蒽醌/聚吡咯复合膜修饰电极的电化学行为和电催化活性

在水溶液中制备了掺杂蒽醌磺酸盐(AQS)的聚吡咯(PPy)/玻碳复合膜修饰电极,采用循环伏安法和旋转圆盘电极技术研究了该修饰电极在不同pH值溶液中的电化学行为以及在pH=5.5的磷酸盐缓冲溶液中对氧还原反应的电催化性能和动力学.结果表明,与裸玻碳电极相比,PPy膜的存在不仅降低了AQS的反应电位和峰电位差,而且增大了其氧化还原反应的峰电流,H2AQ/HAQ-氧化还原对的电离常数为9.5.AQS/PPy膜修饰电极上氧的还原主要是两电子还原为H2O2的不可逆过程,H2AQ对氧还原反应起主要催化作用,还原过程符合异相氧化还原催化机理.该修饰电极具有良好的电化学重现性.     

Electrochemical Polymerization of 3,4‐Ethylenedioxythiophene from Aqueous Solution Containing Hydroxypropyl‐β‐cyclodextrin and the Electrocatalytic Behavior of Modified Electrode Towards Oxidation of Sulfur Oxoanions and Nitrite

Poly(3,4‐ethylenedioxythiophene) (PEDOT) film was prepared on glassy carbon electrode from 0.1 M LiClO₄ aqueous solution containing 3,4‐ethylenedioxythiophene (EDOT) monomer and hydroxypropyl‐β‐cyclodextrin (HP‐β‐CD), by multiple scan cyclic voltammetry. The effect of oxidation potentials on electropolymerization of EDOT was examined by chronoamperometry and cyclic voltammetric techniques. The results of potentiostatic experiments show that optimum potential range to obtain compact stable film was 0.9 to 1.05 V (vs. Ag/AgCl). At higher positive potential, i.e. above 1.05 V, polymer growth was hindered by passivation effect. The PEDOT film exhibited a strong absorption at 550 nm in the UV‐vis region and also a multicolor electrochromism in different buffer solutions (sky blue‐purple red). Cyclic voltammetric features of PEDOT‐coated electrode in pure supporting electrolyte suggested that charge transfer of the film resembles that of surface‐confined redox species. Finally, the electrocatalytic behavior of PEDOT‐modified electrode was tested towards oxidation of sulfur oxoanions and nitrite using cyclic voltammetry.     

Insights into analyte electrolysis in an electrospray emitter from chronopotentiometry experiments and mass transport calculations

Insights into the electrolysis of analytes at the electrode surface of an electrospray (ES) emitter capillary are realized through an examination of the results from off-line chronopotentiometry experiments and from mass transport calculations for flow through tubular electrodes. The expected magnitudes and trends in the interfacial potential in an ES emitter under different solution conditions and current densities, using different metal electrodes, are revealed by the chronopotentiometry data. The mass transport calculations reveal the electrode area required for complete analyte electrolysis at a given volumetric flow rate. On the basis of these two pieces of information, the design of ES emitters that may maximize and those that may minimize analyte electrolysis during ES mass spectrometry are discussed.     

导电高聚物修饰纳米尺度TiO~2多孔膜电极的光电化学研究

用光电化学方法研究了用导电高聚物修饰的纳米晶TiO~2多孔膜电极在不含氧化还原对和含不同氧化还原对体系电解质溶液中的光电转换过程。TiO~2/导电高聚物多孔膜电极为双层n型半导体结构,内层TiO~2多孔膜的禁带宽度为3.26eV,外层聚吡咯(PPy)膜的禁带宽度为2.23eV,而聚苯胺(PAn)膜的禁带宽度为2.88eV。用导电高聚物修饰半导体电极能使其在可见光区的光吸收增加,光电流增强,且起始波长红移至>600nm,使宽禁带半导体电极的光电转换效率得到明显改善。     

Electrochemically Controlled Ion‐exchange Property of Carbon Nanotubes/Polypyrrole Nanocomposite in Various Electrolyte Solutions

The electrochemically controlled ion‐exchange properties of multi‐wall carbon nanotube (MWNT)/electronically conductive polypyrrole (PPy) polymer composite in the various electrolyte solutions have been investigated. The ion‐exchange behavior, rate and capacity of the electrochemically deposited polypyrrole with and without carbon nanotube (CNT) were compared and characterized using cyclic voltammetry (CV), chronoamperometry (CA), electrochemical quartz crystal microbalance (EQCM), X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). It has been found that the presence of carbon nanotube backbone resulted in improvement in ion‐exchange rate, stability of polypyrrole, and higher anion loading capacity per PPy due to higher surface area, electronic conductivity, porous structure of thin film, and thinner film thickness providing shorter diffusion path. Chronoamperometric studies show that electrically switched anion exchange could be completed more than 10 times faster than pure PPy thin film. The anion selectivity of CNT/PPy film is demonstrated using X‐ray photoelectron spectroscopy (XPS).     

PREPARATION, CHARACTERIZATION AND ELECTROCHEMICAL PROPERTIES OF POLYPYRROLE-POLYSTYRENE SULFONIC ACID COMPOSITE FILM

Polypyrrole-polystyrene sulfonic acid (PPy-PSSA) composite films have been electrosynthesized in an aqueous solution of PSSA. The electro-active films exhibit cation exchange during the redox process. Infrared, Raman and energy-dispersive spectroscopic results demonstrated that the polyanion of PSS^- is co-deposited into the PPy matrix and couldn‘t be stripped from the film extensively by dedoping. The doping level together with dipolaron content of the PPy-PSSA composite film increases during electrochemical polymerization process. SEM images revealed that the composite film has smooth and compact morphology and AFM pictures suggested that PPy chains are possibly grown perpendicular to the electrode surface. TGA tests indicated that the composite films has much better thermal stability than that of pure PPy.Furthermore, electrochemical studies showed that the relaxation process at certain holding potential has great effect on the shape of the cyclic voltammetric curves of PPy-PSSA composite film. The composite film exhibits cation and anion exchange during the redox process after undergoing the relaxation step. It is more difficult for divalent anion to enter the polymer matrix than a univalent ion, and a large cation such as (CH3CH2CH2CH2)4N^ cannot be involved in the ion exchange process.     

Triboelectric spray ionization

Triboelectric spray ionization (TESI) is a variation of electrospray ionization (ESI) using common instrumental components, including gas flow, solvent flow rate and heat, the only difference being the use of a high‐voltage power supply for ESI or a static charge for TESI. The ionization of solvent or analyte is due to the electrostatic potential difference formed between the spray electrode and counter electrode. The ion source contains a pneumatic spray operated over a range of flow rates (0.15–1.5 µl/min) and gas pressures (0–100). This new design contains a standalone spray assembly and an optional metal mesh in front of the spray. There are several parameters that affect the performance during ionization of molecules including the flow rate of solvent, gas pressure, temperature, solvent acidity, distance and potential difference between emitter and counter electrode. A variable electrostatic potential can be applied for higher ionization efficiency. The new ionization method was successfully applied to solutions of various proteins under different conditions. The same charge‐state distributions compared to other ESI techniques are observed for all the protein samples. The unique feature of TESI is very efficient spraying by using a natural electrostatic potential even at the potential that a human body can produce. This provides very gentle ionization efficiency of peptides and proteins in different solvents. Copyright © 2013 John Wiley & Sons, Ltd.     

A corona discharge initiated electrochemical electrospray ionization technique

We report here the development of a corona discharge (CD) initiated electrochemical (EC) electrospray ionization (ESI) technique using a standard electrospray ion source. This is a new ionization technique distinct from ESI, electrochemistry inherent to ESI, APCI, and techniques using hydroxyl radicals produced under atmospheric pressure conditions. By maximizing the observable CD at the tip of a stainless steel ESI capillary, efficient electrochemical oxidation of electrochemically active compounds is observed. For electrochemical oxidation to be observed, the ionization potential of the analyte must be lower than Fe. Ferrocene labeled compounds were chosen as the electrochemically active moiety. The electrochemical cell in the ESI source was robust, and generated ions with selectivity according to the ionization potential of the analytes and up to zeptomolar sensitivity. Our results indicate that CD initiated electrochemical ionization has the potential to become a powerful technique to increase the dynamic range, sensitivity, and selectivity of ESI experiments.     

Analytical Characteristics of Electrochemical Biosensor Using Pt‐Dispersed Graphene on Boron Doped Diamond Electrode

An electrochemical biosensor was developed using Pt‐nanoparticles (Pt‐NPs) dispersed graphene based on a boron‐doped diamond thin film electrode. To compare its performances with those of other biosensors, glucose was used as a target analyte. This biosensor exhibited a wide linear range, a low detection limit and a higher sensitivity compared to other amperometric biosensors using graphene‐based electrodes. In addition, the biosensor promotes a direct electron transfer between the redox enzymes and the electrode surface and detects low concentration analytes. The excellent performance of the biosensor is attributed to the synergistic effect of the Pt‐NPs, graphene sheet and the BDD thin film. Therefore, it can be a promising application for electrochemical detection of analytes.     

Direct Electron Communication Between Glucose Oxidase and Carbon Electrode Covered with Polypyrrole

IntroductionDirectelectrochemistryofenzymeshasarousedincreasinginterestofmanyresearchersasitisapreferablewayforproducingarealreagent1essbiosensor.Glucoseoxidase(GOD),beingaflavoprotein,iswell-knownduetoitswidespreaduseinthebiosensors'Recently,agreatnumberofpeoplehaveat-temptedtoachievedirectelectrontransferbetweenGODandvariouselec-trodes[l-19].Szucselal.determinedtheshapeandsizeoftheGODmoleculead-sorbedonagoldelectrodebyel1ipsometry['jandexamineddirectelectrontrans-ferbetweentheadsorbedenz…     

Coupling of a Reagentless Electrochemical DNA Biosensor with Conducting Polymer Film and Nanocomposite as Matrices for the Detection of the HIV DNA Sequences

Abstract

This paper describes a reagentless electrochemical DNA biosensor applied to the detection of human immunodeficiency virus (HIV) sequences based on electrochemical impedance spectroscopy (EIS). The novel DNA biosensor has been elaborated by means of an opposite‐charged adsorption Au‐Ag nanocomposite to a conductive polymer polypyrrole (PPy) modified platinum electrode (Pt) and self‐assembly the mercapto oligonucleotide probes onto the surface of modified electrode via the nanocomposite. The duplex formation was detected by measuring the electrochemical impedance signal of nucleic acids in phosphate buffer solution (PBS). Such response is based on the concomitant conductivity changes of the PPy film and nanocomposite. The reagentless scheme has been characterised using 21‐mer synthetic oligonucleotides as models: parameters affecting the hybridization assay were explored and optimized. The detection limit is 5.0×10^?10 M of target oligonucleotides at 3σ. The potential for development of reagentless DNA hybridization analysis in the clinical diagnosis is being pursued.     

新型导电聚合物-聚吡咯溴离子化学传感器

本文用电化学方法把导电聚合物聚吡咯(PPy)修饰在玻碳(GC)电极上, 研究了Br^-离子的掺杂效应和薄膜电极的电化学行为, 研制一种新型Br^-离子选择电极, 电极的响应机制是基于导电聚合物中阴离子的掺杂效应, 详细研究了聚合条件对电极电位响应性能的影响, 电极具有内阻小、响应快、抗毒化能力强、制备简便等优点, 电极对1x10^-^1-x10^-^4MBr^-呈能斯特响应, 检测下限7x10^-^5M, 斜率61mV/PBr^-。本文结果是化学修饰电极技术在化学传感器方面应用的有意义的尝试, 薄膜的良好导电性质使之更易于制备离子敏感电子学器件和生物电子学器件。     

Electropreparation of Poly(benzophenone‐4) Film Modified Electrode and Its Electrocatalytic Behavior Towards Dopamine,Ascorbic Acid and Nitrite

The oxidation of benzophenone‐4 (2‐hydroxy‐4‐methoxybenzophenone‐5‐sulfonic acid) at glassy carbon electrode gives rise to stable redox active electropolymerized film during repetitive potential cycling between 0 to 1.3 V (Ag/AgCl). Cyclic voltammogram of poly(benzophenone‐4) film shows a redox couple with well‐defined peaks. The redox response of the modified electrode was found to be depending on the pH of the contacting solution. The peak potentials were shifted to a less positive region with increasing pH and the dependence of the peak potential was found to be 51 mV/pH. The electrocatalytic behavior of poly(benzophenone‐4) film modified electrode towards oxidation of dopamine, ascorbic acid and reduction of nitrite was investigated. The oxidation of dopamine and ascorbic acid occurred at less positive potential on poly(benzophenone‐4) film compared to bare glassy carbon electrode. For dopamine, the overpotential was reduced about 180 mV. Feasibility of utilizing poly(benzophenone‐4) film coated electrode in analytical estimation of dopamine, ascorbic acid and nitrite was also demonstrated.     

Nanocomposite with Polypyrrole Encapsulated within SBA‐15 Mesoporous Silica: Preparation and Its Electrochemical Application

The nanocomposite with polypyrrole (PPy) confined in ordered mesoporous silica SBA‐15 channels was synthesized by in situ electropolymerization. X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, N₂ adsorption/desorption, and FT‐IR studies indicated that the nanocomposite has the well‐ordered hexagonal structures and PPy was in situ polymerized into the channels instead of the outer surface of SBA‐15. Furthermore, the PPy/SBA‐15 nanocomposite was used as an electrode modifier. We found that the nanocomposite‐modified electrode exhibited good electrocatalytic activities for hydroquinone oxidation where PPy chains could facilitate the electron transfer between molecular sieves and electrode surface. Three dihydroxybenzene isomers (hydroquinone, catechol and resorcinol) have been successfully detected at PPy/SBA‐15 modified electrode by preconcentration of the analyte.     

Preparation of an Electrode That Immobilizes NAD‐Dependent Alcohol Dehydrogenase by Using a Redox Polymer Containing 1,10‐Phenanthroline‐5,6‐dione

A redox polymer (PAHA‐Ru), modified electrode exhibited excellent catalytic activity for the electrochemical oxidation of β‐nicotinamide adenine dinucleotide (NADH). PAHA‐Ru was composed of carboxyl groups and ruthenium complexes containing 1,10‐phenanthroline‐5,6‐dione (phen‐dione). The stability of the PAHA‐Ru film was increased by incorporating poly(diallyldimethylammonium chloride) (PDDA) owing to the formation of a polyelectrolyte complex between the PAHA‐Ru and PDDA. The catalytic efficiency of the oxidation of NADH using the PAHA‐Ru/PDDA‐modified electrode was also greater than that using a PAHA‐Ru‐modified electrode. NAD‐dependent alcohol dehydrogenase (ADH) was entrapped in the PAHA‐Ru/PDDA film on the surface of the glassy carbon electrode. Electrochemical oxidation of ethanol using the ADH‐entrapped electrode was also observed.     

Systematic investigation of ion suppression and enhancement effects of fourteen stable‐isotope‐labeled internal standards by their native analogues using atmospheric‐pressure chemical ionization and electrospray ionization and the relevance for multi‐analyte liquid chromatographic/mass spectrometric procedures

In clinical and forensic toxicology, multi‐analyte procedures are very useful to quantify drugs and poisons of different classes in one run. For liquid chromatographic/tandem mass spectrometric (LC/MS/MS) multi‐analyte procedures, often only a limited number of stable‐isotope‐labeled internal standards (SIL‐ISs) are available. If an SIL‐IS is used for quantification of other analytes, it must be excluded that the co‐eluting native analyte influences its ionization. Therefore, the effect of ion suppression and enhancement of fourteen SIL‐ISs caused by their native analogues has been studied. It could be shown that the native analyte concentration influenced the extent of ion suppression and enhancement effects leading to more suppression with increasing analyte concentration especially when electrospray ionization (ESI) was used. Using atmospheric‐pressure chemical ionization (APCI), methanolic solution showed mainly enhancement effects, whereas no ion suppression and enhancement effect, with one exception, occurred when plasma extracts were used under these conditions. Such differences were not observed using ESI. With ESI, eleven SIL‐ISs showed relevant suppression effects, but only one analyte showed suppression effects when APCI was used. The presented study showed that ion suppression and enhancement tests using matrix‐based samples of different sources are essential for the selection of ISs, particularly if used for several analytes to avoid incorrect quantification. In conclusion, only SIL‐ISs should be selected for which no suppression and enhancement effects can be observed. If not enough ISs are free of ionization interferences, a different ionization technique should be considered. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrochemical processes in a wire-in-a-capillary bulk-loaded, nano-electrospray emitter

Experiments are described that illustrate solvent oxidation, emitter electrode corrosion, and analyte oxidation in positive ion mode nano-electrospray mass spectrometry using a wire-in-a-capillary, bulk-loaded nano-electrospray emitter geometry. Time-lapsed color photography of pH and metal specific indicator solutions within operating nano-electrospray emitters, as well as temporal changes in the ions observed in the nano-electrospray mass spectra, are used to probe these reactions, judge their magnitude, and study the time dependent changes in solution composition and gas-phase ion signal brought about as a result of these electrochemical reactions. The significance of these observations for analytical applications of nano-electrospray mass spectrometry are discussed.