Mode Scanning Electrochemical Microscopy Research Articles

Electrochemical reduction of Bicarbonate to Formate with Silver Nanoparticles and Silver Nanoclusters supported on Multiwalled Carbon Nanotubes

In this work, we report heterogeneous catalysis of bicarbonate reduction to formate (FA) using silver nanoparticles (Ag NP) and silver nanoclusters (Ag NC) supported on multi-wall carbon nanotubes (MWCNT). FA detection was performed by substrate generation/tip collection (SG/TC) mode of scanning electrochemical microscopy (SECM) using Pt UME in 100mM HCO3− at pH 8. The Ag NP and Ag NC showed high carbonate reduction current, and similar activity as massive gold electrode targets them as alternative catalysts for FA production. Prior to bicarbonate reduction, the electrochemical activity of Ag NP and Ag NC was evaluated using carbon fiber ultra microelectrodes (UME), these carbon UME were fabricated and characterized for use in SCEM mapping; carbon UME provided an excellent feedback response and are dispensable electrodes with renewable surfaces. Also, a carbon UME enables the fabrication of an enzymatic sensor which was designed for NAD+ (Nicotine amide dinucleotide) dependent formate dehydrogenase immobilization. The enzymatic reaction coupled with the redox mediator 2,2V-Azino- bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) allowed a sensitive FA detection. The production of FA on silver particles and the FA detection methods presented in this work will be the target of further investigation for energy storage molecules.

Soft Probe Scanning Electrochemical Microscopy with Spider Array for Visualizing Biomarkers and Redox Active Proteins in Animal Tissues

We present soft spider array for gentle contact mode scanning electrochemical microscopy (SECM) imaging of tissues. The spider array contains up to eight completely individual, parallel, and flexible carbon microelectrodes designed to scan square centimeter sized areas on reasonable time scales while avoiding sample aging and electrode fouling. The contact mode enables the scanning of largely curved samples at a constant distance. The spider array is applied for the SECM imaging of the distribution of certain biomarkers, such as tyrosinase, in thin tumor tissues as well as thick tumor blocks. Furthermore, the spider array can be applied for mapping the antioxidant distribution in an apple peel.

Evaluation of Enzymatic Kinetics of GOx‐based Electrodes by Scanning Electrochemical Microscopy at Redox Competition Mode

AbstractGlucose oxidase (GOx) is an enzyme, which is used for the development of enzymatic biofuel cells. Therefore in this research redox competition mode of scanning electrochemical microscopy (RC‐SECM) was applied for the investigation of glucose oxidase (GOx) catalyzed reaction kinetics. The GOx was immobilized by glutaraldehyde on substrates of different electrical conductivity: (i) gold covered glass was used as conducting substrate and (ii) plastic poly(methyl methacrylate) was used as non‐conducting substrate. Current vs distance dependencies were registered by SECM at different concentrations of glucose in the absence of redox mediator. The potential of −750 mV vs Ag/AgCl(3 M KCl) was applied to the microelectrode (ME), which was used as a probe in SECM, in order to register oxygen reduction current. Consumption of oxygen by the GOx based layer was evaluated according to principles determined by Michaelis‐Menten kinetics. Apparent Michaelis constants KM(app.) were calculated from the dependencies of current vs glucose concentration. In both these cases the KM(app.) value increased when the distance between ME and enzyme modified surface was increasing from 10 to 30 μm, while the KM(app.) value decreased by increasing the distance from 30 to 60 μm.

Theory and Simulations for the Electron‐Transfer/Ion‐Transfer Mode of Scanning Electrochemical Microscopy in the Presence or Absence of Homogenous Kinetics

AbstractThe front cover artwork is provided by the groups of Amatore (Paris, France) and Mirkin (New York, USA). The image illustrates the delivered and captured flux gradients inside the nanopipette as well as those generated inside the nanocleft delimited by the nanopipette tip and the electrode surface. It is a single‐barrel nanopipette, so the two gradients are artificially separated for graphical purposes by their individual projection on each face of the crosscut volume. Read the full text of the Article at 10.1002/celc.201600583.

Redox titration of gold and platinum surface oxides at porous microelectrodes.

The extension of the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) is demonstrated for porous electrode materials. These materials are often high surface area powders which are very important electrocatalysts for instance in fuel cells or water electrolyzers. The powdered electrocatalyst material is filled into a cavity-microelectrode which is then operated as the sample electrode in SECM. After a surface oxide generation step, the oxides on the porous sample are reduced by [Ru(NH3)6]2+ formed at the microelectrode probe of the SECM while the sample is at open circuit potential. Such porous electrodes pose the difficulty to cope with unavoidable variations in the filling of the cavity and to access the entire surface by the mediator. The electrochemically active surface area is used to compensate the variation in filling. It can also be used for calculating coverages of surface oxides for a better comparison between different electrodes. We found a complete and fast accessibility for all investigated porous electrodes which is based on electron transfer. Therefore, we propose a "vertical feedback" mechanism analogous to SECM feedback experiments on extended flat samples at open circuit potential. Moreover, the current transients indicate that distinctive oxide species with different kinetics are present. Taken together, these measures ensure consistent determination of oxide coverages for nanoporous gold and carbon-supported platinum nanoparticles.

Scanning electrochemical microscopy: Visualization of local electrocatalytic activity of transition metals hexacyanoferrates

The redox competition mode of scanning electrochemical microscopy (SECM) was used to visualize differences in local electrocatalytic activity of Fe and Ni hexacyanoferrates (HCFs) in hydrogen peroxide reduction. The uniform round-shaped spots of electrocatalysts for the SECM measurements were electrochemically deposited using a scanning droplet cell. A negligible activity of NiHCF towards H2O2 reduction compared to Prussian Blue (PB) was observed. The dependence of local Prussian Blue activity on the applied potential was investigated. The proposed strategy explores the potential application of SECM as a rapid screening tool for HCF film activity within a single experiment.

Electrochemical analysis of nanostructured iron oxides using cyclic voltammetry and scanning electrochemical microscopy

Iron oxides in general and especially hematite, α-Fe2O3, have become promising materials for the alkaline water electrolysis and photoelectrochemical water splitting, respectively. In the present study electrocatalytic electrodes with a thin film of α-Fe2O3 and with vertically aligned α-Fe2O3 nanowires were prepared. Cyclic voltammograms of the α-Fe2O3 nanowires revealed differences including a series of three unreported cathodic signals when compared to previously published voltammograms for polycrystalline iron oxides. The generation-collection mode of scanning electrochemical microscopy (SECM) using nanostructured Pt microdisc probes was exploited to detect soluble reaction products formed at the voltammetric peaks of the α-Fe2O3 electrode. SECM tip-substrate voltammetry unexpectedly showed that the reduction of FeVI to FeIII on the cathodic sweep is accompanied by significant O2 evolution.

Theory and Simulations for the Electron‐Transfer/Ion‐Transfer Mode of Scanning Electrochemical Microscopy in the Presence or Absence of Homogenous Kinetics

AbstractThe electron‐transfer/ion‐transfer (ET/IT) mode of scanning electrochemical microscopy (SECM) was developed recently and applied to studies of heterogeneous reactions at the substrate surface. The charged products or intermediates are detected by measuring the ion‐transfer current of the species across the liquid/liquid interface supported at the tip of a nanopipette. In this article, we develop the theory for this technique and explore its potential advantages and limitations. By using the COMSOL Multiphysics package, the approach curves were simulated for three commonly encountered experimental situations, that is, the surface‐generated ionic species is either chemically stable or participates in a first‐ or second‐order homogeneous reaction. The simulation results are generalized in the form of analytical approximations derived under limiting conditions.

Spatially Resolved Analysis of Screen Printed Photoanodes of Dye-Sensitized Solar Cells by Scanning Electrochemical Microscopy

Different approaches are compared for imaging local differences in the performance of nanostructured dye-sensitized solar cells (DSCs) using scanning electrochemical microscopy (SECM). The DSCs were fabricated from TiO2 and the triphenylamine dye (E)-3-(5-(4-(bis(2',4'-dibutoxy-[1,1'-biphenyl]-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid, called D35. The components of the redox electrolytes cobalt trisbipyridine ([Co(bpy)3]3+/2+) and iodide/triiodide (I‐/I3−) were used as SECM mediators. Imaging was performed by the feedback (FB) mode and the substrate-generation/tip collection (SG/TC) mode of SECM with additional options of local and temporal illumination. In FB mode, the SECM microelectrode (ME) reduces the mediator which is re-oxidized at the illuminated photoanode. In the SG/TC mode, the reduced form of the mediator is oxidized at the photoanode and the oxidized form is detected at the ME. It is expected that the SG/TC is more sensitive than the FB mode but provides lower lateral resolution. However, imaging is complicated by the strong light scattering in the nanoporous photoanode and the long residence time of charge carriers under the conditions of SECM imaging with low mediator concentrations. This prevents approaches based on local illumination or temporal illumination. Using shear force SECM (SF-SECM) in the FB mode, local differences in the morphology and performance of screen-printed photoanodes could be resolved that resulted from screen printing process. The morphological variations are also corroborated by scanning force microscopy and optical phase contrast microscopy. Furthermore, isolated irregularities were detected in which morphology and local performance were not correlated.

Mixed Transition Metal Oxide Supported on Nitrogen Doped Carbon Nanotubes – a Simple Bifunctional Electrocatalyst Studied with Scanning Electrochemical Microscopy

AbstractA facile way to prepare a bifunctional electrocatalyst, which is active for both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline media, is presented. A Ni0.9Co0.1Fe2O4 mixed oxide, which is synthesized using a polyol mediated synthesis route, is combined with nitrogen doped carbon nanotubes (NCNT) known as electrically conducting material with high activity for the ORR. The combined bifunctional catalyst was investigated towards its structural properties using TEM, XRD, XPS and Raman spectroscopy and studied with relevant electrochemical characterization techniques, namely rotating disk electrode measurements for ORR and linear sweep voltammetry for OER as well as with scanning electrochemical microscopy (SECM). The difference between onset potentials for OER and for ORR was similar to that of other published bifunctional catalysts, indicating that the simple synthetic approach yields materials well suited for bifunctional catalysis applications. For ORR investigations the redox competition mode of SECM indicated high selectivity of the materials towards 4‐electron reduction of oxygen to water. The mixed material showed an enhanced activity for both reactions in comparison to the untreated and pristine materials. An additional heat treatment of the bifunctional catalyst led to a material with elemental Ni, Co and Fe exhibiting a decreased activity for both reactions.

A novel nonenzymatic biosensor for evaluation of oxidative stress based on nanocomposites of graphene blended with CuI

A high-sensitive nonenzymatic hydrogen peroxide (H2O2) biosensor based on cuprous iodide and graphene (CuI/Gr) composites has been explored for the detection of H2O2 released by living cells and monitoring the oxidative stress of cells under excellular stimulation. The biosensor properties were evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), amperometric i-t curve, and the redox-competition mode of scanning electrochemical microscopy (SECM). Our observations demonstrate that the CuI/Gr nanocomposites modified glassy carbon electrode (GCE) exhibits excellent catalytic activity for H2O2 with relatively low detection limit and a wide linear range from 0.5 μM to 3 mM. Moreover, the redox-competition mode of SECM imaging study further illustrates the improved electrochemical catalytic capability for H2O2 reduction with CuI/Gr nanocomposites deposited on graphite electrode. Hence, the as-prepared nonenzymatic H2O2 biosensor could be used to detect H2O2 release from different kinds of living cells under stimulation while eliminating the interference of ascorbic acid.

Mesoporous nitrogen containing carbon materials for the simultaneous detection of ascorbic acid, dopamine and uric acid

Mesoporous nitrogen rich carbonaceous (MNC) materials have been synthesized by pyrolyzing the polymerized ethylenediamine nanocasted into a SBA-15 hard template at 600 and 800°C and explored for simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The electrocatalytic activity of these materials for the oxidation of analyte molecules was examined by means of redox-competition mode of scanning electrochemical microscopy (SECM), voltammetric, chronoamperometric and rotating disc electrode (RDE) measurements. MNC material exhibits a superior sensitivity towards the oxidation of AA, DA, and UA with a lowest detection limit of 0.01μM, 0.001μM and 0.01μM respectively without any substantial interferences including glucose at physiologically relevant concentrations.

Investigation of the Electron Transfer at Si Electrodes: Impact and Removal of the Native SiO2 Layer

Silicon is considered as one of the promising alternatives to graphite as negative electrode material in lithium-ion batteries. The electron transfer at uncharged microstructured and planar Si was characterized using the feedback mode of scanning electrochemical microscopy (SECM) and 2,5-di-tert-butyl-1,4-dimethoxybenzene as redox mediator. Approach curves and images demonstrate that the electron transfer rate constants at pristine Si are relatively small due to the native SiO2 surface layer. In addition, the electron transfer rate constants show local variations because of the heterogeneous coverage of SiO2. The SiO2 layer is at least partially removed by mechanical contact and abrasion with the microelectrode probe. After SiO2 removal by the microelectrode or by a hydrofluoric acid dip, the electron transfer rate constants increase strongly and remain heterogeneous. Moreover, the surface of the Si electrodes is at least stable over hours after SiO2 removal. The consequences for investigating the formation of the solid electrolyte interphase (SEI) on Si are discussed.

Impact of the Native SiO2 Surface Layer on the Electron Transfer at Amorphous Si Electrodes

The electron transfer at uncharged microstructured and planar amorphous (a-)Si was characterized using the feedback mode of scanning electrochemical microscopy (SECM) and 2,5-di-tert-butyl-1,4-dimethoxybenzene as redox mediator in carbonate electrolytes. Approach curves and images demonstrate that the electron transfer rate at pristine a-Si is relatively small due to the native SiO2 surface layer. In addition, the electron transfer rates show local variations because of the heterogeneous coverage of SiO2. After removal of the SiO2 layer, the effect of the solid electrolyte interphase (SEI) on electron transport rate can be studied. The SiO2 layer is at least partially removed by approach curve contact and scratching with the microelectrode probe. After SiO2 removal, the electron transfer rates increase strongly and remain heterogeneous.

Electro‐oxidation and reduction of H2 on platinum studied by scanning electrochemical microscopy for the purpose of local detection of H2 evolution

Electrochemical detection of H2 using scanning electrochemical microscopy (SECM) has shown to hold great promise as a sensitive characterization method with high spatial resolution for active surfaces generating H2. Herein, the factors contributing to the current that is measured by SECM in generation/collection mode for H2 detection are studied. In particular, the concentration gradient of H2 at the substrate, the H2/H+ recycling between the SECM tip and substrate and hemispherical profile of H2 diffusion has been discussed. It was postulated that H2/H+ recycling plays a dominant role in the oxidative current measured in generation/collection mode of SECM when the microelectrode is positioned in close vicinity of substrate. Copyright © 2015 John Wiley & Sons, Ltd.

Non-destructive Patterning of Carbon Electrodes by Using the Direct Mode of Scanning Electrochemical Microscopy.

Patterning of glassy carbon surfaces grafted with a layer of nitrophenyl moieties was achieved by using the direct mode of scanning electrochemical microscopy (SECM) to locally reduce the nitro groups to hydroxylamine and amino functionalities. SECM and atomic force microscopy (AFM) revealed that potentiostatic pulses applied to the working electrode lead to local destruction of the glassy carbon surface, most likely caused by etchants generated at the positioned SECM tip used as the counter electrode. By applying galvanostatic pulses, and thus, limiting the current during structuring, corrosion of the carbon surface was substantially suppressed. After galvanostatic patterning, unambiguous proof of the formation of the anticipated amino moieties was possible by modulation of the pH value during the feedback mode of SECM imaging. This patterning strategy is suitable for the further bio-modification of microstructured surfaces. Alkaline phosphatase, as a model enzyme, was locally bound to the modified areas, thus showing that the technique can be used for the development of protein microarrays.

Investigation of Electron Transfer Properties of the SEI on Metallic Lithium Electrodes by SECM at Open Circuit

The electron transfer at metallic lithium was characterized using the feedback mode of scanning electrochemical microscopy (SECM) and 2,5-di-tert-butyl-1,4-dimethoxybenzene (DBDMB) as redox mediator at open circuit. As a result the solid electrolyte interphase (SEI) at Li is not passivating towards DBDMB as redox mediator. In comparison to charged graphite composite electrodes, the electron transfer rate is smaller at Li and the ranges for the electron transfer rates at both electrodes overlap. Both, graphite composite and Li electrodes show a local variation of electron transfer rates and temporal changes within a time span of hours. In addition, significant short term changes of SEI passivity occur at both electrodes. However, the frequency of such events is smaller for metallic Li then on graphite.

Comparison of Spatiotemporal Changes of SEI Properties on Different Anodes By SECM

Li ion batteries are widely used in our society. Mainly during the first charging process the solid electrolyte interphase (SEI) between the electrode and electrolyte is formed by the decomposition of electrolyte components at the lithiated graphite. This layer is critical for the performance and safety of the Li ion batteries.[1] The aim of this study is to characterize the electron transport across the SEI, which determines the ongoing reduction of electrolyte components. Thus, understanding the electron transport across the SEI helps to improve Li ion batteries. Characterization of the SEI is challenging, because of the variety of chemically similar components and enclosed electrolyte species. Furthermore, ex situ analysis of the SEI requires separation and isolation of the SEI, which may change the content and the structure of the SEI.[2] Recently we used the feedback mode of scanning electrochemical microscopy (SECM) to investigate in situ the electron transport at the lithiated graphite (Figure).[3] 2,5-di-tert-butyl-1,4-dimethoxy benzene was identified as an useful SECM mediator providing sufficient stability and sensitivity to study the passivation properties of the SEI. Our results by SECM show gradual and significant short-term spatiotemporal changes of the SEI properties and demonstrate the dynamic and spontaneous behavior of SEI formation, damage and reformation under open circuit conditions above lithiated graphite anodes. The results emphasize that spatiotemporal changes of the passivating SEI properties are highly localized and occur preferentially in between the gaps of graphite particles. A HOPG model electrode shows significant differences to a graphite composite electrode, indicating the impact of stressed graphite particles on the SEI stability. In addition, imaging experiments show the formation, detachment and reformation of gas bubbles. The outcome of significant short-term spatiotemporal changes of the SEI properties clarifies that electrolyte reduction still occurs after SEI formation at localized spots. All measurements were conducted under open circuit conditions and thus the charging state remained constant. Consequently, the SEI is unstable even under constant charging state. The detection of gas bubbles is in line with the electrolyte reduction events nearby, since electrolyte reduction is a requirement for gas formation. Further research using this methodology focuses on Li and Si electrodes. H.B. thanks the Lower Saxony Ministry of Science and Art for funding within the Graduate Program GEENI - Graduate school of electromobility and energy storage in Lower Saxony. M.S. and M. W. gratefully acknowledge the provision of anode substrates by Infineon Technologies Austria AG.

In Situ Reactive Imaging of Photoassisted Water Splitting Reaction Intermediates on n-Doped Strontium Titanate Using Surface Interrogation Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) gives spatially resolved information about electrochemical reactions occurring on surfaces, and easily probes reaction mechanisms. The in situ surface study of photoelectrochemical processes, such as water splitting at semiconductor electrodes, is challenging due to its fast kinetics and the multiple reactive intermediates formed. By combining the surface interrogation mode (SI-) of SECM [1] with transport simulations, the reactivity of multiple adsorbed intermediates resulting from water splitting on gently n-doped strontium titanate (STO) was investigated. Using this technique, we addressed the in situ detection of long lived holes and photogenerated hydroxyl radical on STO. Combined with the measurements on the local production of oxygen using the collection mode of SECM, our methodology provides an integrated view of reactivity at photocatalytic surfaces. In our SI-SECM measurements, a micro- or nano-electrode used as SECM tip was positioned close to the STO surface and activated to generate a redox titrant that provided information about the surface coverage and reaction kinetics of adsorbed intermediates. By addressing different parts on the electrode surface, it is possible to obtain information about different surface condition son the STO electrode. This combined methodology brings spatial resolution to SI-SECM, and measured different adsorbate coverage on pristine and defective STO (100) surfaces. For the first time, SI-SECM was adapted to the reactive imaging of adsorbates to show how these affect photocatalytic performance on a heterogeneous surface. [1] Rodríguez-López, J. The Surface Interrogation Mode of Scanning Electrochemical Microscopy (SI-SECM): an approach to the study of adsorption and (electro)catalysis at electrodes. In Electroanalytical Chemistry, a series of advances. Vol. 24. Bard, A.J. and Zoski, C.G., Eds. 2012, CRC Press, pp. 287-352.

(Invted) The Use of Redox Nano-Titrations for Elucidating Reactive Heterogeneity on Electrodes for Energy Conversion and Charge Storage

We present on the use of the Surface Interrogation mode of Scanning Electrochemical Microscopy (SI-SECM)[1] for the quantification of reactive intermediates at heterogeneous reactive electrode structures. In this mode, an SECM probe is approached to the electrode under study and a pulse of reactive redox mediator is directed to the substrate while it rests at open circuit. Since the tip operates in the feedback mode, if this pulse induces a chemical reaction on the surface species, an increased flux of the redox mediator sensed by the tip can be used to obtain the local titrated charge and the kinetics of reaction between the mediator and the surface species. Here, we will present how SI-SECM can be used to quantify by titration the reactive species present on an extended surface of a relatively inert electrode. This technique was used for interfaces of interest for energy conversion and storage: 1) for the detection of photogenerated hydroxyl radical on a semiconducting surface, and 2) for the detection of redox active polymer particles [2] on single layer graphene (SLG). Both semiconducting and SLG interfaces permit only the reactivity of reactive species on their surface to be detected. Studies using finite element simulations indicate that the efficiency and extent of titration of a surface are strongly dependent on the SECM tip electrode size and on the tip-substrate distance. As the SECM tip size is made smaller, the apparent titrated charge increases. Likewise, the distance between the SECM tip and substrate determines the titrated charge – very small tip-substrate gaps lead to a constrained area, while large tip-substrate gaps increase the area titrated at the expense of a decreased signal-to-background. An optimum distance can be obtained. We will present on a system that validates these findings, as well as examples of interest to our laboratory, which include the titration of photogenerated intermediates on semiconductors for water splitting, and the titration of redox active polymer particles for charge storage in redox flow batteries. In the first case, we were able to decouple the reactivity of hydroxyl radicals and obtained their coverage and reaction kinetics. In the second case, the relaxed tip-substrate geometry suggested by our simulated findings allowed to evaluate the charge stored in polydisperse polymer nanoparticles for flow batteries. SI-SECM redox nano-titrations will allow to evaluate the role and importance of reactive intermediates in charge storage and energy conversion. [1] Rodríguez-López, J. The Surface Interrogation Mode of Scanning Electrochemical Microscopy (SI-SECM): an approach to the study of adsorption and (electro)catalysis at electrodes. In Electroanalytical Chemistry, a series of advances. Vol. 24. Bard, A.J. and Zoski, C.G., Eds. 2012, CRC Press, pp. 287-352[2] Gavvalapalli, N.; Hui, J.; Cheng, K.; Lichtenstein, T.; Shen, M.; Moore, J.S.; Rodríguez-López, J. J. Am. Chem. Soc. 2014, 136, 16309-16316