On the mechanism of H2O2 reduction at Prussian Blue modified electrodes

Prussian Blue deposited on the electrode surface under certain conditions is known to be a selective electrocatalyst of hydrogen peroxide (H₂O₂) reduction in the presence of O₂. The electrocatalyst was stabilized at cathodic potentials preventing its loss from the electrode surface. Hydrodynamic voltammograms of H₂O₂ reduction indicated the transfer of two electrons per catalytic cycle. The operational stability of Prussian Blue in H₂O₂ reduction was highly dependent on the buffer capacity of the supporting electrolyte. Since Prussian Blue is known to be dissolved in alkaline solution, it was confirmed that in neutral aqueous solutions the product of H₂O₂ electrocatalytic reduction is OH⁻.     

Local deposition and characterisation of K2Co[Fe(CN)6] and K2Ni[Fe(CN)6] by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) is used to form local deposits of different Prussian blue analogs on macroscopic surfaces of gold and glassy carbon. Dissolution of Co and Ni sacrificial ultramicroelectrodes (UMEs) generates divalent cations in the gap between the UME and the macroscopic specimen electrode. Co²⁺ or Ni²⁺ precipitate with [Fe(CN)₆]^4– formed by reduction of [Fe(CN)₆]^3– at the macroelectrode. By moving the UME while generating Co²⁺ or Ni²⁺, lines can be "drawn" with a width of 130 μm. The line width can be adjusted by reagent concentration and translation speed of the UME. Different pulse programs allow the formation of ring-shaped structures. The deposited hexacyanoferrate microstructures show catalytic activity for the reduction of Fe³⁺ which was imaged in the feedback and generation-collection modes of the SECM. Electronic Publication     

Fabrication of Prussian Blue modified ultramicroelectrode for GOD imaging using scanning electrochemical microscopy

A Prussian Blue (PB) film modified disk ultramicroelectrode (UME) was fabricated by electrochemical deposition technique on a Pt-disk UME. The electrocatalytical reductions of hydrogen peroxide derived from glucose oxidase (GOD) on this modified UME were investigated. The enzymatic biochemical reactivity was imaged by scanning electrochemical microscopy (SECM) utilizing the PB film modified UME. It is evident that sensitivity and spatial resolution for hydrogen peroxide measurement were improved obviously. SECM images obtained clearly revealed the concentration profile of the reaction products around the enzymes. The PB film modified microelectrode is in the nature of simple preparation, high catalytic activity on hydrogen peroxide and substrate selectivity for SECM etc.     

Local Activities of Hydroxide and Water Determine the Operation of Silver‐Based Oxygen Depolarized Cathodes

Local ion activity changes in close proximity to the surface of an oxygen depolarized cathode (ODC) were measured by scanning electrochemical microscopy (SECM). While the operating ODC produces OH^? ions and consumes O₂ and H₂O through the electrocatalytic oxygen reduction reaction (ORR), local changes in the activity of OH^? ions and H₂O are detected by means of a positioned Pt microelectrode serving as an SECM tip. Sensing at the Pt tip is based on the pH‐dependent reduction of PtO and obviates the need for prior electrode modification steps. It can be used to evaluate the coordination numbers of OH^? ions and H₂O, and the method was exploited as a novel approach of catalyst activity assessment. We show that the electrochemical reaction on highly active catalysts can have a drastic influence on the reaction environment.     

The local electrochemical behavior of the AA2098-T351 and surface preparation effects investigated by scanning electrochemical microscopy

In this work, scanning electrochemical microscopy (SECM) measurements were employed to characterize the electrochemical activities on polished and as-received surfaces of the 2098-T351 aluminum alloy (AA2098-T351). The effects of the near surface deformed layer (NSDL) and its removal by polishing on the electrochemical activities of the alloy surface were evaluated and compared by the use of different modes of SECM. Confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) were also employed to characterize the morphology of the surfaces. The surface chemistry was analyzed by X-ray photoelectron spectroscopy (XPS). The surface generation/tip collection (SG/TC) and competition modes of the SECM were used to study hydrogen gas (H₂) evolution and oxygen reduction reactions, respectively. H₂ evolution and oxygen reduction were more pronounced on the polished surfaces. The feedback mode of SECM was adopted to characterize the electrochemical activity of the polished surface that was previously corroded by immersion in a chloride-containing solution, in order to investigate the influence of the products formed on the active/passive domains. The precorroded surface and as-received surfaces revealed lower electrochemical activities compared with the polished surface showing that either the NSDL or corrosion products largely decreased the local electrochemical activities at the AA2098-T351 surfaces.     

Study on Electrocatalytic Activity of Platinum for Hydrogen Oxidation in Acidic Solution by Scanning Electrochemical Microscopy

The electrocatalytic activity of platinum for hydrogen oxidation in 0.01 M H₂SO₄ + 0.1 MNa₂SO₄ solution has been investigated by scanning electrochemical microscopy (SECM) technique. The cyclic voltammogram (CV), approach curve, area scan imaging and chronoamperometric methods have been used. The results indicate that the imaging capability of the SECM feedback mode can be used more efficiently to visually identify materials' electrocatalytic activity, compared with the approach curve method for identification of the conductive or insulating nature of a surface. The SECM imaging method has demonstrated the effects of Pt substrate potential on the electrocatalytic oxidation of hydrogen under a constant tip potential. It is found that the more positive the Pt substrate potential, the lower the electrocatalytic activity of the Pt. Furthermore, the chronoamperometric results support the variation of the electrocatalytic activity with the Pt substrate potential as well.     

Short-term influence of interfering ion activity change on ion-selective micropipette electrode potential; another factor that can affect the time needed for imaging in potentiometric SECM

The applicability of ion-selective microelectrodes (ISME) in scanning electrochemical microscopy (SECM) in the presence of potentially interfering ions is investigated. Decreasing the time needed for potentiometric SECM imaging is a key aim. It is generally accepted that the long response time of potentiometric cells with high impedance microelectrodes limits the imaging speed that can be obtained without distortion. In this study we show that a change in the activity of interfering ions can result in a short-term electrode potential transient that should be taken into consideration in parameter setting for SECM measurements. The activity step method was employed, using NH₄⁺-ion-measuring micropipettes. Solutions containing equal concentrations of NH₄⁺ ions with or without K⁺ ions were rapidly introduced and the short-term change in electrode potential was recorded. Rapid transient potential signals appeared following the K⁺ activity step in the range where the interfering K⁺ did not affect the long-term electrode potential. The possible influence of this on SECM applications is discussed.     

Stability Improvement of Prussian Blue by a Protective Cellulose Acetate Membrane for Hydrogen Peroxide Sensing in Neutral Media

The cellulose acetate covered Prussian blue modified glassy carbon electrode (GCE/PB/CA) was fabricated as a novel hydrogen peroxide sensor. It was shown by scanning electron microscope (SEM) and atomic force microscope (AFM) that Prussian blue was covered and protected by cellulose acetate perfectly. The modified electrode showed a good electrocatalytic activity for H₂O₂ reduction in neutral aqueous solution. H₂O₂ was detected amperometrically in 0.05 mol/L phosphate buffer solutions (pH 7.0, containing 0.1 mol/L KCl as supporting electrolyte) at an applied potential of ?0.2 V (vs. SCE). The response current was proportional to the concentration of H₂O₂ in the range of 1.0×10^?5 mol/L to 2.5×10^?4 mol/L with the detection limit of 2.2×10^?6 mol/L at a signal to noise ratio 3.     

Evaluation of Redox Activity of Human Myocardium‐derived Mesenchymal Stem Cells by Scanning Electrochemical Microscopy

In this study the redox activity of human myocardium‐derived mesenchymal stem cells (hmMSC) were investigated by redox‐competition (RC‐SECM) and generation‐collection (GC‐SECM) modes of scanning electrochemical microscopy (SECM), using 2‐methylnaphthalene‐1,4‐dione (menadione, MD) as a redox mediator. The redox activity of human healthy and dilated hmMSCs was evaluated by measuring reduction of MD. Measurements were performed by approaching and retracting the UME from the surface of growing hmMSC cells. The current study shows that the RC‐SECM mode can be applied to investigate integrity of cell membranes, whereas the most promising results were observed by using the GC‐SECM mode and applying the Hill's equation for the calculation/fitting of dependencies of electrical current vs menadione concentration. The calculated apparent Michaelis constant (K_M) for the production of menadiol (MDH₂) in the pathological hmMSC cells was 14.4 folds higher compared to that of the healthy hmMSC revealing the lover redox activity of pathological cells. Moreover, the calculated Hill's coefficient n shows a negative cooperative binding between MD and healthy hmMSC and positive cooperative binding between MD and pathological hmMSC. It means that healthy hmMSC is of lower affinity to MD, which is also related to the better membrane integrity of healthy cells. Data of this study demonstrate that SECM can be applied to investigate intracellular redox and membrane changes ongoing in human dilated myocardium‐derived hmMSC in order to improve their functioning and further regenerative potential.     

Electrochemically deposited thiophene-based polymers as protective agents for Prussian Blue thin films

A composite material based on overlapped layers of electrochemically synthesized Prussian Blue (PB) and terthiophene-derived polymer is described, aiming at enhancing the stability of the hexacyanoferrate thanks to the protective action of the polymer. Two bilayer configurations and deposition methods (for the polymer component) were tested. The morphology and electrochemical behavior in organic solvent and in aqueous solutions containing different supporting electrolytes were carried out. The best performances of electrodes modified with films of the composite material as to increased stability of PB were achieved with the potentiostatically deposited polymer covering the PB layer, in acetate buffer at pH 5.5. As for potential cycling stress, the anodic and cathodic peak currents due to PB were not decreased after 20 cycles. Conversely, PB alone displayed the anodic peak currents relevant to PB/Prussian White (PW) and PB/Berlin Green (BG) systems decreased by about 30 %. The stability to local pH increase was assessed by cyclic voltammetry after electrochemical reduction of H₂O₂. For example, the anodic peak currents were decreasing by 15 % and 5 % for the two PB redox systems, while for PB alone the same currents decreased by 35 % and 10 %. The response sensitivity to hydrogen peroxide was improved by 54 %, with respect to PB alone, as evaluated by chronoamperometry.     

Prussian Blue-coated interdigitated array electrodes for possible analytical application

Thin films of iron(III) hexacyanoferrate(II) (Prussian Blue) were electrochemically deposited on interdigitated array (IDA) electrodes, yielding systems which can be considered as chemiresistors in sensing alkali metal ion concentrations in an adjacent electrolyte. This is due to the fact that the conductivity of the film being measured by a steady-state current on application of a voltage to the two-fingered electrodes of the IDA depends on both the redox stare of the film and the cation concentration in the electrolyte. From the dependence of the steady-state current on the electrode (bias) potential at variable cation concentrations for different alkali metal ions and for mixtures of alkali metal ions, the possibilities of analytical application were elucidated. In addition, by using the methods of staircase coulometry and scanning conductivity, the electron diffusion coefficient D_e was determined as a function of the redox state of Prussian Blue. It is concluded that Prussian Blue-coated IDA electrodes are, in principle, suitable as chemiresistors for the determination of alkali metal ion concentrations with increasing selectivity in the series Li < Na < K < Rb < Cs.     

ITO上AuPd合金和Au阵列图案的制备及电催化活性的SECM表征

利用电化学湿法印章技术在氧化铟锡(ITO)导电玻璃上制备AuPd合金和Au的双组分阵列图案. 采用具有微浮雕图案的琼脂糖印章存储足够多的溶液,并通过控制电沉积的时间来控制图案厚度. 应用场发射扫描电子显微镜(FE-SEM),X射线能谱分析(EDX)和原子力显微镜(AFM)分别对ITO表面上的AuPd合金和Au的形貌和组分进行表征,并通过循环伏安(CV)技术和扫描电化学显微镜(SECM)研究比较了Au和AuPd合金的催化活性. 利用扫描电化学显微镜(SECM)的针尖产生-基底收集(TG-SC)模式和氧化还原竞争(RC)模式,发现Au电极对二茂铁甲醇氧化物(FcMeOH⁺)电催化还原能力高于AuPd合金电极,而在AuPd合金上催化还原H₂O₂的能力显著高于Au.     

Probing the Activity of Iron Peroxo Porphyrin Intermediates in the Reaction Layer during the Electrochemical Reductive Activation of O2

Herein we report the first example of using scanning electrochemical microscopy (SECM) to quantitatively analyze O₂ reductive activation in organic media catalyzed by three different Fe porphyrins. For each porphyrin, SECM can provide in one single experiment the redox potential of various intermediates, the association constant of Fe^II with O₂, and the pK_a of the Fe^III(OOH^?)/ Fe^III(OO^2?) couple. The results obtained can contribute to a further understanding of the parameters controlling the catalytic efficiency of the Fe porphyrin towards O₂ activation and reduction.     

High Sensitive Microsensor Based on Organic‐Inorganic Composite for Two‐Dimensional Mapping of H2O2 by SECM

The present work describes the development of a selective, sensitive and stable sensing microsensor for scanning electrochemical microscopy (SECM) to measure H₂O₂ during electrochemical reduction of oxygen. The microsensor is based on graphene and Poly(3,4‐ethylenedioxythiophene) composite as support to iron (III) hexacyanoferrate (II) (PEDOT/graphene/Fe^III₄[Fe^II(CN)₆]₃ microsensor). The electrochemical properties of the PEDOT/graphene/Fe^III₄[Fe^II(CN)₆]₃ microsensor were investigated by cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM). The PEDOT/graphene/Fe^III₄[Fe^II(CN)₆]₃ microsensor showed an excellent electrocatalytic activity toward hydrogen peroxide (H₂O₂) reduction with a diminution of the overpotential of about 500 mV in comparison to the process at a bare gold microelectrode. The microsensor presented excellent performance for two dimensional mapping of H₂O₂ by SECM in 0.1 mol L^?1 phosphate buffer solution (pH 7.0). Under optimized conditions, a linear response range from 1 up to 1000 µmol L^?1 was obtained with a sensitivity of 0.08 nA L µmol^?1 and limit of detection of 0.5 µmol L^?1.     

Probing the Local Reaction Environment During High Turnover Carbon Dioxide Reduction with Ag-Based Gas Diffusion Electrodes

Discerning the influence of electrochemical reactions on the electrode microenvironment is an unavoidable topic for electrochemical reactions that involve the production of OH⁻ and the consumption of water. That is particularly true for the carbon dioxide reduction reaction (CO₂RR), which together with the competing hydrogen evolution reaction (HER) exert changes in the local OH⁻ and H₂O activity that in turn can possibly affect activity, stability, and selectivity of the CO₂RR. We determine the local OH⁻ and H₂O activity in close proximity to a CO₂-converting Ag-based gas diffusion electrode (GDE) with product analysis using gas chromatography. A Pt nanosensor is positioned in the vicinity of the working GDE using shear-force-based scanning electrochemical microscopy (SECM) approach curves, which allows monitoring changes invoked by reactions proceeding within an otherwise inaccessible porous GDE by potentiodynamic measurements at the Pt-tip nanosensor. We show that high turnover HER/CO₂RR at a GDE lead to modulations of the alkalinity of the local electrolyte, that resemble a 16 m KOH solution, variations that are in turn linked to the reaction selectivity.     

Detection and Quantification of Sulfonamide Antibiotic Residues in Milk Using Scanning Electrochemical Microscopy

The use of scanning electrochemical microscopy (SECM) for the qualitative and quantitative determination of sulfapyridine (SPY) in milk is described. A direct competitive immunoassay was performed involving an antibiotic horseradish peroxidase (HRP)‐labeled analog and using selective capture antibodies immobilized on the surface of Protein G‐modified glassy carbon plates. SECM detection was accomplished by means of the sample generator/tip collector (GC) mode involving the reduction of benzoquinone (BQ) generated upon the HRP‐catalyzed oxidation of hydroquinone (HQ) at the modified substrate surface in the presence of H₂O₂. The detection limit for SPY in milk samples was as low as 0.13 ng mL^?1.     

Fabrication of Active Horseradish Peroxidase Micropatterns with a High Resolution by Scanning Electrochemical Microscopy

We used a new reactive species OH^? to fabricate active horseradish peroxidase (HRP) micropatterns with a high resolution by scanning electrochemical microscopy (SECM) coupled with a carbon fiber disk electrode as the SECM tip. In this method, except for active HRP micropatterns predesigned other regions on a HRP‐immobilized substrate were deactivated by OH^? generated at the tip held at ?1.7 V in 1.0 mol/L KCl containing 2.0×10^?3 mol/L benzoquinone (BQ) (pH 8.0). The feedback mode of SECM with a tip potential of ?0.2 V was used to characterize the active HRP micropatterns in 1.0 mol/L KCl containing 2.0×10^?3 mol/L BQ and 2.0×10^?3 mol/L H₂O₂.     

Synthesis,Characterization and Applications of a New Prussian Blue Type Material

This work reports on a material synthesized via 1‐butyl‐3‐methylimidazolium tetrachloroferrate and K₃[Fe(CN)₆]. Its structure and properties were characterized by IR, XPS, AFM and CV, which was inferred as Prussian Blue Type (PBT) material with particle size distribution between 80–120 nm and spike‐like aggregation from the characterization results. PBT is insoluble in acetone, water and ethanol uniformly, which gives magnetic properties. It can be reacted with 30 % H₂O₂, and produce gas bubbles when a voltage is applied. Modified on the glassy carbon electrode, the material showed obviously electrocatalytic activity to sodium nitrite, which has potential application prospect for sensors.     

Prussian Blue acts as a mediator in a reagentless cytokinin biosensor

An electrochemical biosensor for detection of the plant hormone cytokinin is introduced. Cytokinin homeostasis in tissues of many lower and higher plants is controlled largely by the activity of cytokinin dehydrogenase (CKX, EC 1.5.99.12) that catalyzes an irreversible cleavage of N⁶-side chain of cytokinins. Expression of Arabidopsis thaliana CKX2 from Pichia pastoris was used to prepare purified AtCKX2 as the basis of the cytokinin biosensor. Prussian Blue (PrB) was electrodeposited on Pt microelectrodes prior to deposition of the enzyme in a sol–gel matrix. The biosensor gave amperometric responses to several cytokinins. These responses depended on the presence of both the enzyme and the Prussian Blue. Thus Prussian Blue must act as an electron mediator between the FAD centre in CKX2 and the Pt surface.     

Localization of proteins in paint cross-sections by scanning electrochemical microscopy as an alternative immunochemical detection technique

The qualitative identification of proteinaceous substances, as well as their location within a complex paint stratigraphy, is one of the most challenging issues in the characterization of painting materials. Nevertheless, information on paint components represent a crucial task for studies concerning both the ancient painting techniques adopted and the state of conservation, being fundamental investigations for the selection of appropriate conservation actions. The present research was aimed at developing a new detection approach for the immunochemical localization of ovalbumin in paint cross-sections based on the use of scanning electrochemical microscopy (SECM). The immunochemical analyses were performed using an anti-ovalbumin primary antibody and a secondary antibody labelled with horseradish peroxidase (HRP). SECM measurements were performed in feedback mode using benzoquinone (BQ)/hydroquinone (H₂Q) redox couple. In presence of hydrogen peroxide (H₂O₂), HRP catalyzes the re-oxidation of H₂Q to BQ and the increment of BQ concentration in correspondence of the target protein was detected by SECM through the electrochemical reduction of the regenerated BQ at the microelectrode. Indeed, the localization of ovalbumin was possible thanks to a clear discrimination of SECM currents, achieved by the comparison of the measurements recorded before and after H₂O₂ administration, based on the HRP on/off approach. The method was evaluated both on samples from standard mocks-up and on a historical sample, collected from a Renaissance wood painting. The obtained results were promising, foreseeing a wider application of SECM on cultural heritage researches.