Conductance maps by electrochemical tunneling spectroscopy to fingerprint the electrode electronic structure

We describe a methodology to perform reliable tunneling spectroscopy in electrochemical media. Sequential in situ tunneling spectra are recorded while the electrochemical potential of the electrode is scanned. Spectroscopic data are presented as conductance maps or conductograms that show the in situ electronic structure of an electrode surface while it undergoes an electrochemical reaction. The conductance map or conductogram represents the redox fingerprint of an electrode/liquid interface in a specific medium and can serve to predict its electrochemical behavior in a quantitative energy scale. The methodology is validated studying the reversible oxidation and passivity of an iron electrode in borate buffer, and we describe the main quantitative information that can be extracted concerning the semiconducting properties of the Fe passive film. This methodology is useful to study heterogeneous catalysis, electrochemical sensing and bioelectronic systems.     

全钒液流电池用碳纳米管-石墨复合电极的研究

将石墨(GP)和多壁碳纳米管(MWNT)按不同比例压片制成电极,用于全钒氧化还原液流电池电极材料,通过循环伏安、交流阻抗、充放电测试、SEM手段对MWNT-GP复合电极进行表征和分析.研究结果表明,MWNT含量为15wt%的MWNT-GP复合电极性能最佳,对组装成的静态电池在电流密度20~80mA/cm2进行充放电性能比较,电流效率均在93%以上,电压效率随着电流密度的增加而有所下降.     

Scanning electrochemical cell microscopy: theory and experiment for quantitative high resolution spatially-resolved voltammetry and simultaneous ion-conductance measurements

Scanning electrochemical cell microscopy (SECCM) is a high resolution electrochemical scanning probe technique that employs a dual-barrel theta pipet probe containing electrolyte solution and quasi-reference counter electrodes (QRCE) in each barrel. A thin layer of electrolyte protruding from the tip of the pipet ensures that a gentle meniscus contact is made with a substrate surface, which defines the active surface area of an electrochemical cell. The substrate can be an electrical conductor, semiconductor, or insulator. The main focus here is on the general case where the substrate is a working electrode, and both ion-conductance measurements between the QRCEs in the two barrels and voltammetric/amperometric measurements at the substrate can be made simultaneously. In usual practice, a small perpendicular oscillation of the probe with respect to the substrate is employed, so that an alternating conductance current (ac) develops, due to the change in the dimensions of the electrolyte contact (and hence resistance), as well as the direct conductance current (dc). It is shown that the dc current can be predicted for a fixed probe by solving the Nernst-Planck equation and that the ac response can also be derived from this response. Both responses are shown to agree well with experiment. It is found that the pipet geometry plays an important role in controlling the dc conductance current and that this is easily measured by microscopy. A key feature of SECCM is that mass transport to the substrate surface is by diffusion and, for charged analytes, ion migration which can be controlled and varied quantifiably via the bias between the two QRCEs. For a working electrode substrate this means that charged redox-active analytes can be transported to the electrode/solution interface in a well-defined and controllable manner and that relatively fast heterogeneous electron transfer kinetics can be studied. The factors controlling the voltammetric response are determined by both simulation and experiment. Experiments demonstrate the realization of simultaneous quantitative voltammetric and ion conductance measurements and also identify a general rule of thumb that the surface contacted by electrolyte is of the order of the pipet probe dimensions.     

On-line electrochemistry/thermospray/tandem mass spectrometry as a new approach to the study of redox reactions: the oxidation of uric acid

The electrochemical oxidation pathway of uric acid was determined by on-line electrochemistry/thermospray/tandem mass spectrometry. Intermediates and products formed as a result of electrooxidation were monitored as the electrode potential was varied. Several reaction intermediates have been identified and characterized by tandem mass spectrometry. The tandem mass spectrometric results provide convincing evidence that the primary intermediate produced during the electrooxidation of uric acid has a quinonoid diimine structure. The results indicate that once formed via electrooxidation, the primary intermediate can follow three distinct reaction pathways to produce the identified final products. The final electrochemical oxidation products observed in these studies were urea, CO2, alloxan, alloxan monohydrate, allantoin, 5-hydroxyhydantoin-5-carboxamide, and parabanic acid. The solution reactions that follow the initial electron transfer at the electrode are affected by the vaporizer tip temperature of the thermospray probe. In particular, it was found that at different tip temperatures either hydrolysis or ammonolysis reactions of the initial electrochemical oxidation products can occur. Most importantly, the results show that the on-line combination of electrochemistry with thermospray/tandem mass spectrometry provides otherwise difficult to obtain information about redox and associated chemical reactions of biological molecules such as the structure of reaction intermediates and products, as well as providing insight into reaction pathways.     

NiO微球的微波辅助合成及电荷传导能力优化

为了优化电荷传导特性, 提高电极的电化学性能, 本工作采用微波辅助合成了分级多孔结构的氧化镍微球。通过XRD、SEM和TEM对产物的形貌进行了表征。研究结果表明, 开放多孔结构的氧化镍微球是由极薄纳米片自组装而成, 以硫酸镍为镍源, 得到的氧化镍微球的粒径约为2 µm。作为超级电容器电极材料, 在电流密度为0.5 A/g时, 电极的比容量达到455 F/g, 由于NiO微球独特的多孔特性, 使电极表现出良好的阻抗特性, 为法拉第反应过程提供了较多的活性反应点, 从而提高了电极的电容性能。     

Electrochemical properties of ferrocene adsorbed on multi-walled carbon nanotubes electrode

The adsorbed process of ferrocene on a glassy carbon (GC) electrode modified by multi-walled carbon nanotubes (MWNTs) and electrochemical properties of the adsorbed layers are investigated. It is found that the redox process of ferrocene in solution is controlled by diffusion and surface electrochemical steps on the MWNT/GC electrode in contrast to the diffusion-controlled process of ferrocene on the GC electrode. The adsorbed ferrocene exhibits a pair of well-defined redox waves in the potential range from − 0.2 V to 0.6 V. Interestingly, two pairs of obvious redox waves for the adsorbed ferrocene are observed at the switching potential over 0.8 V and the peak current values of redox waves in more positive potential increase with the enlarging switching potential. The electrochemical reaction model of ferrocene adsorbed on the MWNT/GC electrode is proposed.     

Electrochemical immunosensor using p-aminophenol redox cycling by hydrazine combined with a low background current

Signal amplification and noise reduction are crucial for obtaining low detection limits in biosensors. Here, we present an electrochemical immunosensor in which the signal amplification is achieved using p-aminophenol (AP) redox cycling by hydrazine, and the noise level is reduced by implementing a low background current. The redox cycling is obtained in a simple one-electrode, one-enzyme format. In a sandwich-type heterogeneous immunosensor for mouse IgG, an alkaline phosphatase label converts p-aminophenyl phosphate into AP for 10 min. This generated AP is electrooxidized at an indium tin oxide (ITO) electrode modified with a partially ferrocenyl-tethered dendrimer (Fc-D). The oxidized product, p-quinone imine (QI), is reduced back to AP by hydrazine, and then AP is electrooxidized again to QI, resulting in redox cycling. Moreover, hydrazine protects AP from oxidation by air, enabling long incubation times. The small amount of ferrocene in a 0.5% Fc-D-modified ITO electrode, where 0.5% represents the ratio of ferrocene groups to dendrimer amines, results in a low background current, and this electrode exhibits high electron-mediating activity for AP oxidation. Moreover, there is insignificant hydrazine electrooxidation on this electrode, which also results in a low background current. The detection limit of the immunosensor using a 0.5% Fc-D-modified electrode is 2 orders of magnitude lower than that of a 20% Fc-D-modified electrode (10 pg/mL vs 1 ng/mL). Furthermore, the presence of hydrazine reduces the detection limit by an additional 2 orders of magnitude (100 fg/mL vs 10 pg/mL). These results indicate that the occurrence of redox cycling combined with a low background current yields an electrochemical immunosensor with a very low detection limit (100 fg/mL). Mouse IgG could be detected at concentrations ranging from 100 fg/mL to 100 microg/mL (i.e., 9 orders of magnitude) in a single assay.     

Highly Stable Vanadium Redox‐Flow Battery Assisted by Redox‐Mediated Catalysis

With good operation flexibility and scalability, vanadium redox‐flow batteries (VRBs) stand out from various electrochemical energy storage (EES) technologies. However, traditional electrodes in VRBs, such as carbon and graphite felt with low electrochemical activities, impede the interfacial charge transfer processes and generate considerable overpotential loss, which significantly decrease the energy and voltage efficiencies of VRBs. Herein, by using a facile electrodeposition technique, Prussian blue/carbon felt (PB/CF) composite electrodes with high electrochemical activity for VRBs are successfully fabricated. The PB/CF electrode exhibits excellent electrochemical activity toward VO²⁺/VO₂⁺ redox couple in VRB with an average cell voltage efficiency (VE) of 90% and an energy efficiency (EE) of 88% at 100 mA cm^?2. In addition, due to the uniformly distributed PB particles that are strongly bound to the surface of carbon fibers in CF, VRBs with the PB/CF electrodes show much better long‐term stabilities compared with the pristine CF‐based battery due to the redox‐mediated catalysis. A VRB stack consisting of three single cells (16 cm²) is also constructed to assess the reliability of the redox‐mediated PB/CF electrodes for large‐scale application. The facile technique for the high‐performance electrode with redox‐mediated reaction is expected to shed new light on commercial electrode design for VRBs.     

Localized high resolution electrochemistry and multifunctional imaging: scanning electrochemical cell microscopy

We describe highly localized electrochemical measurements and imaging using a simple, mobile theta pipet cell. Each channel (diameter <500 nm) of a tapered theta pipet is filled with electrolyte solution and a Ag/AgCl electrode, between which a bias is applied, resulting in a conductance current across a thin meniscus of solution at the end of the pipet, which is typically deployed in air or a controlled gaseous environment. When the position of the pipet normal to a surface of interest is oscillated, an oscillating component in the conductance current is generated when the meniscus at the end of the probe comes into contact with the surface and undergoes periodic (reversible) deformation, so as to modulate the solution resistance. This oscillating current component can be used to maintain gentle contact of the solution from the pipet cell with the surface and as a set point for high resolution topographical imaging with the pipet. Simultaneously, the mean conductance current that flows between the pipet channels can be measured and is sensitive to the local nature of the interface, informing one, for example, on wettability and ion flow into or out of the surface investigated. Furthermore, conductor or semiconductor surfaces can be connected as a working electrode, with one of the electrodes in the pipet serving as a quasi-reference electrode. This pipet cell then constitutes part of a dynamic electrochemical cell, with which direct voltammetric-amperometric imaging can be carried out simultaneously with conductance and topographical imaging. This provides multifunctional electrochemical maps of surfaces and interfaces at high spatial resolution. The prospects for the use of this new methodology widely are highlighted through exemplar studies and a brief discussion of future applications.     

Protein-modified nanocrystalline diamond thin films for biosensor applications

Diamond exhibits several special properties, for example good biocompatibility and a large electrochemical potential window, that make it particularly suitable for biofunctionalization and biosensing. Here we show that proteins can be attached covalently to nanocrystalline diamond thin films. Moreover, we show that, although the biomolecules are immobilized at the surface, they are still fully functional and active. Hydrogen-terminated nanocrystalline diamond films were modified by using a photochemical process to generate a surface layer of amino groups, to which proteins were covalently attached. We used green fluorescent protein to reveal the successful coupling directly. After functionalization of nanocrystalline diamond electrodes with the enzyme catalase, a direct electron transfer between the enzyme's redox centre and the diamond electrode was detected. Moreover, the modified electrode was found to be sensitive to hydrogen peroxide. Because of its dual role as a substrate for biofunctionalization and as an electrode, nanocrystalline diamond is a very promising candidate for future biosensor applications.     

The effect of an electrical double layer on the voltammetric performance of nanoscale interdigitated electrodes: a simulation study

Finite-element based computational simulation is performed to investigate the effect of an electrical double layer (EDL) on the electrochemical processes of nanometer-scale interdigitated electrodes (nano-IDEs). Results show that the EDL structure will alter the voltammetric current response of nano-IDEs due to the expansion of the diffuse layer into the diffusion layer at the electrode surfaces and the overlap of the electrical fields of the neighboring electrodes. The EDL induced change in the voltammetric current response is more severe for nano-IDEs with a smaller electrode size and gap spacing, and the EDL effect is influenced by the compact layer thickness, the charge valence of the redox species, the electron transfer rate, and the absence of the supporting electrolyte.     

Electrochemical and morphological studies of an electroactive material derived from 3-hydroxyphenylacetic acid: a new matrix for oligonucleotide hybridization

This paper describes the formation of polymeric films derived from 3-hydroxyphenylacetic acid electropolymerized onto graphite electrodes through cyclic voltammetry. We observed the formation of an electroactive material over the electrode surface. The modified electrode showed significant blocking behavior to electron transfer reaction of the pair redox ferricyanide/ferrocyanide, indicating repulsion electrostatic with the negatively charged carboxylate groups of the polymer. The quasi-reversible behavior to Ru(NH₃)₆Cl₂ suggests electrostatic attraction, facilitating the charge transfer. The modified electrode was studied through electrochemical quartz crystal microbalance, electrochemical impedance spectroscopy, and atomic force microscopy. These analyses indicate modification of the graphite electrode. Surface analysis by AFM showed that the morphology of the modified electrode surface presents globular form, randomly distributed, and formed by lower globules with diameter near 100 nm. Immobilization and hybridization of oligonucleotide onto the modified electrode were successfully carried out by using both direct electrochemical oxidation of nitrogenated bases and the redox electroactive indicator methylene blue.     

SiC-C fiber electrode for biological sensing

Microelectrodes implanted in the brain can act as transducers for the neuronal impulses. In this work a composite fiber (SiC-C) electrode was studied for neuronal activity sensing and for biochemical detection of electroactive neurotransmitters. An electrolytic etching technique was also developed for fabricating electrode tips from SiC-C composite fiber. SEM was used to study different tip geometries and shapes. Cyclic voltammetry and electrochemical impedance spectroscopy were used for the electrochemical characterization and modeling. Almost square voltammograms showed that the current generated was due to the charging and discharging of the capacitive double layer without any significant redox reaction. The electrodes showed capacitive interaction with the surrounding electrolyte solution which is highly desirable for safe charge transfer. Biochemical sensing of neurotransmitters including dopamine hydrochloride and vitamin C was done with the SiC-C composite electrodes and oxidation currents were found to vary linearly with concentration. In vivo action potential recordings from anesthetized rat's brain with very high signal to noise ratio were obtained.     

Electrochemical sensing using quantum-sized gold nanoparticles

This paper describes the electrocatalytic activity of quantum-sized thiolate protected Au(25) nanoparticles and their use in electrochemical sensing. The Au(25) film modified electrode exhibited excellent mediated electrocatalytic activity that was utilized for amperometric sensing of biologically relevant analytes, namely, ascorbic acid and uric acid. The electron transfer dynamics in the Au(25) film was examined as a function of Au(25) concentration, which manifested the dual role of Au(25) as an electronic conductor as well as a redox mediator. The electron transfer study has further revealed the correlation between the electronic conductivity of the Au(25) film and the sensing sensitivity.     

Electrochemically reduced graphene modified carbon ionic liquid electrode for the sensitive sensing of rutin

In this paper a graphene (GR) modified carbon ionic liquid electrode that was obtained by one-step potentiostatic electroreduction of a graphene oxide solution was described. The resulting electrode displayed excellent electrochemical performance due to the formation of highly conductive GR film on the electrode surface. Electrochemistry of rutin was carefully studied with a pair of well-defined redox peaks appeared in pH 2.5 buffer solution. Rutin exhibited a diffusion-controlled two-electron and two-proton transfer reaction on the modified electrode with the electrochemical parameters calculated. The reduction peak currents are linearly related to rutin concentration in the concentration range from 0.070 to 100.0 μmol/L with a detection limit as low as 24.0 nmol/L (3σ). The modified electrode displayed excellent selectivity with good stability, and was applied to the determination of rutin content in tablet, human serum and urine samples with satisfactory results.     

High-resolution mapping of redox-immunomarked proteins using electrochemical-atomic force microscopy in molecule touching mode

We explore the possibility of using molecule touching atomic force electrochemical microcopy (Mt/AFM-SECM) for high-resolution mapping of proteins on conducting surfaces. The proposed imaging strategy relies on making surface-immobilized proteins electrochemically "visible" via redox-immunomarking by specific antibodies conjugated to poly(ethylene glycol) (PEG) chains terminated by redox ferrocene (Fc) heads. The flexibility and length of the PEG chains are such that, upon approaching a combined AFM-SECM microelectrode tip toward the surface, the Fc moieties can efficiently shuttle electrons from the surface to the tip. The so-generated SECM positive feedback tip current allows the specific localized detection of the sought protein molecules on the surface. This new electrochemical imaging scheme is validated experimentally on the basis of a model system consisting of mouse IgGs adsorbed onto electrode surfaces and recognized by Fc-PEG-labeled antimouse antibodies. In order to estimate the resolution of Mt/AFM-SECM for protein imaging, regular arrays of submicrometer-sized spots of mouse IgGs are fabricated onto gold electrode surfaces using particle lithography. The Fc-PEG-immunomarked mouse IgG spots are imaged by Mt/AFM-SECM operated in tapping mode. Both an electrochemical image, reflecting the surface distribution of the redox-labeled IgGs, and a topography image are then simultaneously and independently acquired, with a demonstrated resolution in the ~100 nm range. The strength of Mt/AFM-SECM imaging is to combine the nanometric resolution of AFM with the selectivity of the electrochemical detection, potentially allowing individual target proteins to be identified amidst similarly sized "nano objects" present on a conducting surface.     

A lactate electrochemical biosensor with a titanate nanotube as direct electron transfer promoter

Hydrogen titanate (H(2)Ti(3)O(7)) nanotubes (TNTs) have been synthesized by a one-step hydrothermal processing. Lactate oxidase (LOx) enzyme has been immobilized on the three-dimensional porous TNT network to make an electrochemical biosensor for lactate detection. Cyclic voltammetry and amperometry tests reveal that the LOx enzyme, which is supported on TNTs, maintains their substrate-specific catalytic activity. The nanotubes offer the pathway for direct electron transfer between the electrode surface and the active redox centers of LOx, which enables the biosensor to operate at a low working potential and to avoid the influence of the presence of O(2) on the amperometric current response. The biosensor exhibits a sensitivity of 0.24?μA?cm(-2)?mM(-1), a 90% response time of 5?s, and a linear response in the range from 0.5 to 14?mM and the redox center of enzyme obviates the need of redox mediators for electrochemical enzymatic sensors, which is attractive for the development of reagentless biosensors.     

"Off-On" Electrochemical Hairpin-DNA-Based Genosensor for Cancer Diagnostics

A simple and robust "off-on" signaling genosensor platform with improved selectivity for single-nucleotide polymorphism (SNP) detection based on the electronic DNA hairpin molecular beacons has been developed. The DNA beacons were immobilized onto gold electrodes in their folded states through the alkanethiol linker at the 3'-end, while the 5'-end was labeled with a methylene blue (MB) redox probe. A typical "on-off" change of the electrochemical signal was observed upon hybridization of the 27-33 nucleotide (nt) long hairpin DNA to the target DNA, in agreement with all the hitherto published data. Truncation of the DNA hairpin beacons down to 20 nts provided improved genosensor selectivity for SNP and allowed switching of the electrochemical genosensor response from the on-off to the off-on mode. Switching was consistent with the variation in the mechanism of the electron transfer reaction between the electrode and the MB redox label, for the folded beacon being characteristic of the electrochemistry of adsorbed species, while for the "open" duplex structure being formally controlled by the diffusion of the redox label within the adsorbate layer. The relative current intensities of both processes were governed by the length of the formed DNA duplex, potential scan rate, and apparent diffusion coefficient of the redox species. The off-on genosensor design used for detection of a cancer biomarker TP53 gene sequence favored discrimination between the healthy and SNP-containing DNA sequences, which was particularly pronounced at short hybridization times.     

Additional Lithium Storage on Dynamic Electrode Surface by Charge Redistribution in Inactive Ru Metal

Beyond a traditional view that metal nanoparticles formed upon electrochemical reaction are inactive against lithium, recently their electrochemical participations are manifested and elucidated as catalytic and interfacial effects. Here, ruthenium metal composed of ≈5 nm nanoparticles is prepared and the pure ruthenium as a lithium‐ion battery anode for complete understanding on anomalous lithium storage reaction mechanism is designed. In particular, the pure metal electrode is intended for eliminating the electrochemical reaction‐derived Li₂O phase accompanied by catalytic Li₂O decomposition and the interfacial lithium storage at Ru/Li₂O phase boundary, and thereby focusing on the ruthenium itself in exploring its electrochemical reactivity. Intriguingly, unusual lithium storage not involving redox reactions with electron transfer but leading to lattice expansion is identified in the ruthenium electrode. Size‐dependent charge redistribution at surface enables additional lithium adsorption to occur on the inactive but more environmentally sensitive nanoparticles, providing innovative insight into dynamic electrode environments in rechargeable lithium chemistry.