Feedback Mode Of Scanning Electrochemical Microscopy Research Articles

Investigation of the Electrocatalytic Reduction of Peroxydisulfate Using Scanning Electrochemical Microscopy.

The elementary steps of the electrocatalytic reduction of S2O82- using the Ru(NH3)63+/2+ redox couple were investigated using scanning electrochemical microscopy (SECM) and steady-state voltammetry (SSV). SECM investigations were carried out in a 0.1 M KCl solution using a 3.5 μm radius carbon ultramicroelectrode (UME) as the SECM tip and a 25 μm radius platinum UME as the substrate electrode. Approach curves were recorded in the positive feedback mode of SECM by reducing Ru(NH3)63+ at the tip electrode and oxidizing Ru(NH3)62+ at the substrate electrode, as a function of the tip-substrate separation and S2O82- concentration. The one-electron reaction between electrogenerated Ru(NH3)62+ and S2O82- yields the unstable S2O83•-, which rapidly dissociates to produce highly oxidizing SO4•-. Because SO4•- is such a strongly oxidizing species, it can be further reduced at both the tip and the substrate, or it can react with Ru(NH3)62+ to regenerate Ru(NH3)63+. SECM approach curves display a complex dependence on the tip-substrate distance, d, due to redox mediation reactions at both the tip and the substrate. Finite element method (FEM) simulations of both SECM approach curves and SSV confirm a previously proposed mechanism for the mediated reduction of S2O82- using the Ru(NH3)63+/2+ redox couple. Our results provide a lower limit for dissociation rate constant of S2O83•- (∼1 × 106 s-1), as well as the rate constants for electron transfer between SO4•- and Ru(NH3)62+ (∼1 × 109 M-1 s-1) and between S2O82- and Ru(NH3)62+ (∼7 × 105 M-1 s-1).

Combined SECM and spectroscopy investigation of the interfacial chemistry of chalcopyrite during anodic oxidation

Hydrometallurgical copper extraction from chalcopyrite (CuFeS2), the most abundant copper-containing resource in the world, is always challenged by its passivation during industrial application, whose mechanisms are still not fully understood. In this study, the interfacial chemistry of chalcopyrite during the anodic oxidation was studied by combined Scanning Electrochemical Microscopy (SECM) and spectroscopic analyzes of Raman spectroscopy and X-ray absorption near-edge spectroscopy (XANES). Ex-situ SECM feedback mode was firstly used to detect and image the formation of passivating films. Then, the in-situ SECM substrate generation-tip collection (SG-TC) experiment by positioning the micro-platinum probe just above the chalcopyrite surface was conducted so that electrochemically active dissolution products can be deposited and monitored. Subsequently, Raman spectroscopy was performed on the SECM tip and the chalcopyrite substrate electrode respectively to identify the products that were released into the solution and then collected on the tip, as well as the ones left behind on the mineral surface. The SECM feedback mode mapping and approach curves demonstrated that under the passive and trans-passive potential regions, low conductive passivation films formed on the chalcopyrite surface, which contributed to the hindered dissolution of chalcopyrite due to the decreased electron transfer rate. In-situ SECM SG-TC results from the anodically oxidized chalcopyrite suggested that soluble sulfur species, most likely thiosulfate ions (S2O32−) were dissolved from the surface during the passivation films formation. The released S2O32− was evidenced by comparing with the pyrite anodic oxidation experiment and sulfur K-edge XANES analysis of the electrolytes collected after chalcopyrite anodic dissolution. In-situ Raman spectra of the oxidized chalcopyrite surface confirmed metal deficient layers were the passivation layers.

Transparent Ultramicroelectrodes for Studying Interfacial Charge-Transfer Kinetics of Photoelectrochemical Water Oxidation at TiO2 Nanorods with Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) has been extensively applied to the electrochemical analysis of the surfaces and interfaces of a photoelectrochemical (PEC) system. A semiconductor photoelectrode with a well-defined geometry and active surface area comparable to SECM's tip is highly desired for accurately quantifying interfacial charge-transfer activities and photoelectrochemically generated redox species, where the broadening effects due to the mass transfer gradient and nonlocal electron transfer at a planar semiconductor surface can be minimized. Here, we present a newly developed platform as a SECM substrate for investigating semiconductor PEC activities, which is based on a transparent ultramicroelectrode (UME) fabricated by using two-step photolithographic patterning and ion milling methods. This transparent UME with a 25 μm recessed disk shape is fully characterized with SECM for quantifying the interfacial charge-transfer rates of IrCl62-/IrCl63- by comparing with theoretical results from finite element simulations in COMSOL Multiphysics. When coated with TiO2 nanorods as a model semiconductor material, the transparent UME can be used to quantify the catalytic PEC water oxidation in a feedback mode of SECM by sampling tip and substrate current signals simultaneously. This transparent UME-SECM study provides insights into the potential-dependent PEC water oxidation reaction mechanism and the quantitative analysis of photocurrent contributions from water oxidation and the SECM tip-generated redox mediator. The transparent UME-SECM method can be potentially expanded to other SECM operation modes such as surface interrogation for understanding the dynamics of the electrode surfaces and interfaces of a PEC system.

Transparent Ultramicroelectrodes for Studying Interfacial Charge Transfer Kinetics of Photoelectrochemical Water Oxidation at TiO2 Nanorods with Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) has been extensively applied to high-throughput screening and quantification of the surface and interfaces of a photoelectrochemical (PEC) system. It is highly desirable to have a semiconductor photocatalytic electrode with well-defined geometry and active area comparable to the SECM’s tip electrode for accurately quantifying interfacial charge transfer activities and photoelectrochemically generated redox species of a photoelectrode. This is because the broadening effects due to mass transfer gradient and non-local electron transfer at a planar semiconductor surface can be minimized by utilizing the well-defined semiconductor substrate. Here, we present a newly developed platform as SECM substrate for investigating semiconductor PEC activities, which is based on a transparent ultramicroelectrode (UME) fabricated by using two-step photolithographic patterning and ion milling methods. This transparent UME with a 25 µm recessed disk shape is fully characterized with SECM for quantitatively understanding the interfacial electron transfer kinetics of IrCl6 2-/IrCl6 3- by comparing with theoretical results from finite element simulations in COMSOL Multiphysics. When coated with TiO2 nanorods as a model semiconductor material, the transparent UME can be used to quantify the catalytic PEC water oxidation in a feedback mode of SECM by sampling tip and substrate current signals simultaneously. This transparent UME-SECM study provides insights into the potential-dependent interfacial photogenerated charge transfer kinetics and quantitative analysis of photocurrent contributions from water oxidation and SECM tip-generated redox mediators. The transparent UME-SECM method can be potentially expanded to other SECM operation modes for understanding the dynamics of the surfaces and interfaces of a PEC catalyst.

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.

Visualization of Inhomogeneuous Reactivity on Battery Material Using Scanning Electrochemical Cell Microscopy

To understand the metal oxide coating effect on battery performance, the following two techniques are required: 1) constructing a flat thin-film electrode surface to realize a well-defined interface and 2) analyzing the electrode/electrolyte interface reaction with nanoscale resolution. We previously studied flat LiCoO2 thin-film electrodes using in situ surface-sensitive X-ray absorption spectroscopy (XAS) and reported that Co reduction at the LiCoO2 surface resulting from electrolyte contact caused the initial degradation. We also showed that the ZrO2 layer successfully prevented physical contact between LiCoO2 and the electrolyte. And it confirmed that a thicker ZrO2 layer (above 2 nm) increased the diffusion resistance of the lithium ions in the ZrO2 layer. However, since XAS lacks in-plane resolution and provides only averaged information, it is impossible to analyze the ZrO2 morphology in detail. Recently, Taguchi et al. investigated a thin Li-Zr-layer (ca. 2 nm) on a LiCoO2 composite electrode by transmission electron microscopy (TEM). They suggested that this thin layer could improve the durability. However, it is difficult to analyze the electrochemical properties using TEM. To evaluate the intrinsic mechanism of the metal oxide coating effect, it is necessary to develop a novel in-situ method that can analyze the surface morphology with high spatial resolution and simultaneously determine the local electrochemical properties. Scanning electrochemical microscopy (SECM) is a powerful technique for linking the surface morphology of a sample to its electrochemical properties. For the battery materials, the SECM feedback mode is effective in monitoring solid electrolyte interphase formation. To directly and quantitatively investigate spatially resolved ionic processes, mercury-capped platinum ultramicroelectrodes were developed and employed for Li+ imaging based on Li stripping. However, it is difficult to visualize the Li+ flux in battery materials at the sub-micrometer scale by SECM. Scanning electrochemical cell microscopy (SECCM), which uses a nanopipette as a probe and forms a local electrochemical cell, is effective in characterizing surface reactivity. We recently applied SECCM for visualization of electrochemical activities on a lithium-ion battery cathode material at sub-micrometer resolution. The SECCM was applied to collect or provide Li in specified area confined by the nanopipette. Further, it collection visualized the electrochemical properties by scanning the nanopipette as an image. There are some strong advantages in SECCM for battery material research such as its high spatial resolution, small capacitive current, and isolated electrochemical cell. In this report, we applied SECCM to characterize a ZrO2-coated LiCoO2 thin-film electrode prepared by pulsed laser deposition. Local cyclic voltammetry (CV) and galvanostatic charge/discharge were performed to characterize the cycle durability and rate performance of ZrO2-coated LiCoO2 thin-film electrodes and to reveal the relationship between the ZrO2 morphology and thickness.

Mapping the antioxidant activity of apple peels with soft probe scanning electrochemical microscopy

We present a non-invasive electrochemical strategy for mapping the antioxidant (AO) activity of apple peels, which counterbalances oxidative stress caused by various external effectors. Soft carbon microelectrodes were used for soft probe scanning electrochemical microscopy (SECM) enabling the gentle and scratch-free contact mode scanning of rough and delicate apple peels in an electrolyte solution. The SECM feedback mode was applied using ferrocene methanol (FcMeOH) as redox mediator that gets electrochemically oxidized at the soft probe and diffuses towards the apple peel where it gets regenerated by certain AOs leading to a redox mediator recycling and increased current signal. The global AO activity in the apple peel including lenticels and regions with artificially degraded AOs were mapped using the soft microelectrodes. Finally, in an apple cross-section the higher and homogeneous AO concentration in the peel with a heterogeneously decaying AO gradient towards the apple inward was visualized, demonstrating the adequate micrometer resolution of the SECM probe and the possibility to get information of the interior AO activity of the apple.

Fixation and Permeabilization Approaches for Scanning Electrochemical Microscopy of Living Cells.

Scanning electrochemical microscopy (SECM) has been widely used for the electrochemical imaging of dynamic topographical and metabolic changes in alive adherent mammalian cells. However, extracting intracellular information by SECM is challenging, since it requires redox species to travel in and out the lipid cell membrane. Herein, we present cell fixation and permeabilization approaches as an alternative tool for visualizing cell properties by SECM. With this aim, adherent cells were analyzed in the SECM feedback mode in three different conditions: (i) alive; (ii) fixed, and (iii) fixed and permeabilized. The fixation was carried out with formaldehyde and does not damage lipid membranes. Therefore, this strategy can be used for the SECM investigation of cell topography or the passive transport of the redox mediator into the cells. Additional permeabilization of the cell membrane after fixation enables the analysis of the intracellular content through the coupling of SECM with immunoassay strategies for the detection of specific biomarkers. The latter was successfully applied as an easy and fast screening approach to detect the expression of the melanoma-associated marker tyrosinase in adherent melanoma cell lines corresponding to different cancer progression stages using the SECM substrate generation-tip collection mode. The present approach is simple, fast, and reliable and can open new ways to analyze cell cultures with electrochemically based scanning probe techniques.

Kinetic differentiation of bulk/interfacial oxygen reduction mechanisms at/near liquid/liquid interfaces using scanning electrochemical microscopy

The present work describes the application of scanning electrochemical microscopy (SECM) in the feedback mode to determine the kinetics of oxygen reduction at or near the liquid/liquid interface – between water and 1,2-dichloroethane (w/DCE). The system contained decamethylferrocene (DMFc) in DCE as the electron donor and acids in water as a proton source. In this approach, decamethylferrocenium (DMFc+) is reduced at the tip of a microelectrode in DCE and the electrogenerated DMFc reacts with protons and oxygen to be re-oxidized in a following chemical reaction (catalytic EC’ mechanism). When a high Galvani potential difference was applied across the liquid/liquid interface, protons would transfer rapidly to the organic phase. Under this condition, SECM approach curves toward the liquid/liquid interface showed dramatic current increases at distances far from the interface. This indicates that oxygen reduction takes place mainly in the bulk DCE; however, at lower Galvani potential differences, where the proton transfer is slow, oxygen reduction was also observed at the interface. Finally, SECM feedback mode measurements with the tip approaching a conductive substrate were used to determine the kinetics of the homogeneous reaction, with an obtained apparent rate constant of 0.2–0.5m3mol−1s−1.

Kinetic Investigation on the Confined Etching System of n-Type Gallium Arsenide by Scanning Electrochemical Microscopy

Confined etchant layer technique (CELT) has been proved an effective electrochemical microfabrication method for both 3D microstructures and a supersmooth surface. From a physical chemistry viewpoint, the confined etching system of n-GaAs includes an etchant generation reaction from Br– to Br2 (E) followed by two parallel reactions: the confining reaction between Br2 and l-cystine (C1), and the etching reaction between Br2 and n-GaAs (C2). In this paper, the homogeneous EC1 process is investigated first through the tip generation/substrate collection (TG/SC) mode of scanning electrochemical microscopy (SECM), and the reaction rate of C1 is determined as (8.0 ± 1.0) × 103 dm3 mol–1 s–1; second, the heterogeneous EC2 process is investigated through the feedback mode of SECM, and the reaction rate of C2 is determined as (3.2 ± 0.5) × 10–2 cm s–1; third, a deformed geometry finite element model is established to simulate the etching topography coupling E(C1∥C2) processes by using the obtained data. The theoretical profiles of pits etched at different concentrations of scavenger, l-cystine, are analyzed and compared with experimental results. This model allows the prediction of spatial resolution of CELT as a function of reaction rates of C1 and C2 but also of the concentration of scavenger.

A Leveling Method Based on Current Feedback Mode of Scanning Electrochemical Microscopy

Substrate leveling is an essential but neglected instrumental technique of scanning electrochemical microscopy (SECM). In this technical note, we provide an effective substrate leveling method based on the current feedback mode of SECM. By using an air-bearing rotary stage as the supporter of an electrolytic cell, the current feedback presents a periodic waveform signal, which can be used to characterize the levelness of the substrate. Tuning the adjusting screws of the tilt stage, substrate leveling can be completed in minutes by observing the decreased current amplitude. The obtained high-quality SECM feedback curves and images prove that this leveling technique is valuable in not only SECM studies but also electrochemical machining.

Scanning Electrochemical Microscopy Study of Ion Annihilation Electrogenerated Chemiluminescence of Rubrene and [Ru(bpy)3]2+

Scanning electrochemical microscopy (SECM) was used for the study of electrogenerated chemiluminescence (ECL) in the radical annihilation mode. The concurrent steady-state generation of radical ions in the microgap formed between a SECM probe and a transparent microsubstrate provides a distance-dependent ECL signal that can provide information about the kinetics, stability, and mechanism of the light emission process. In the present study, the ECL emission from rubrene and [Ru(bpy)(3)](2+) was used to model the system by carrying out experiments with the SECM and light-detecting apparatus inside an inert atmosphere box. We studied the influence of the distance between the two electrodes, d, and the annihilation kinetics on the ECL light emission profiles under steady-state conditions, as well as the ECL profiles when carrying out cyclic voltammetry (CV) at a fixed d. Experimental results are compared to simulated results obtained through commercial finite element method software. The light produced by annihilation of the ions was a function of d; stronger light was observed at smaller d. The distance dependence of the ECL emission allows the construction of light approach curves in a similar fashion as with the tip currents in the feedback mode of SECM. These ECL approach curves provide an additional channel to describe the reaction kinetics that lead to ECL; good agreement was found between the ECL approach curve emission profile and the simulated results for a fast, diffusion-limited second-order annihilation process (k(ann) > 10(7) M(-1) s(-1)). In the CV mode at fixed distance, the ECL emission of rubrene showed two distinct signals at different potentials when fixing the substrate to generate the radical cation and scanning the tip to generate the radical anion. The first signal (pre-emission) corresponded to an emission well before reaching the generation of the radical anion and was more intense on Au than on Pt. The second ECL signal showed the expected steady-state behavior from the second-order annihilation reaction and agreed well with the simulation. A comparison of the emission obtained with rubrene and [Ru(bpy)(3)](2+) to test the direct formation of lower energy triplets directly at the electrode showed that triplets are not the cause of the pre-emission observed. Wavelength selection experiments for the rubrene system showed that the pre-emission ECL signal also appeared slightly red-shifted with respect to the main luminophore emission; a possible explanation for this phenomenon is inverse photoemission, where the injection of highly energetic holes by the oxidized species into the negatively biased tip electrode causes emission of states in the metal that appear at a different wavelength than the singlet emission from the ECL luminophore.

Effect of Cation on Dye Regeneration Kinetics of N719-Sensitized TiO2 Films in Acetonitrile-Based and Ionic-Liquid-Based Electrolytes Investigated by Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) in the feedback mode has been used to investigate the kinetics of dye regeneration on cis-di(thiocyanato)-bis(2,2′-bipyridyl-4,4″-dicarboxylate) ruthenium(II)-sensitized TiO2 photoelectrochemical electrodes in contact with room temperature ionic liquids electrolytes. Acetonitrile based electrolytes were used for comparison. The data are analyzed with the help of a revised model for relating steady state approach curves in the SECM feedback mode to an effective rate constant for the dye regeneration process. This effective rate constant summarizes the effect of electron transfer kinetics, mass transport of the mediator inside the porous electrode, and recombination processes. We found a remarkable dependence of the dye regeneration on the nature and concentration of deliberately added cations.

Spontaneous adsorbed layers of 4-nitrobenzenediazonium salt on gold and glassy carbon: Local characterization by SECM and electron-transfer kinetics evaluation

The adsorption of 4-nitrobenzenediazonium tetrafluoroborate (NBD) in acidic aqueous solution was investigated by scanning electrochemical microscopy (SECM) on polycrystalline gold and glassy carbon (GC) substrates. The feedback mode of SECM was used to study the local properties of each substrate before and after immersion in the aryldiazonium solution, by performing approach curves with two redox mediators, Fe ( CN ) 6 4 - and Ru ( NH 3 ) 6 3 + . The fitting of the approach curves with analytical approximate equation was used to evaluate the apparent heterogeneous rate constant k for Fe ( CN ) 6 3 - / 4 - redox system at NBD-adsorbed GC ( k = 2.6 × 10 −4 cm s −1).

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

AbstractThe 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.

Local Modulation of the Redox State of p‐Nitrothiophenol Self‐Assembled Monolayers Using the Direct Mode of Scanning Electrochemical Microscopy

Local reduction of the terminating nitro groups of a p-nitrothiophenol self-assembled monolayer (SAM) under formation of either hydroxylamine or amino groups is invoked using the direct mode of scanning electrochemical microscopy (SECM). By choosing the appropriate potential and a potential pulse sequence, the reduction of the SAM end groups to the desired oxidation state can be achieved, locally restricted to the area of the sample surface directly underneath the positioned SECM tip. Following the "writing" of redox microstructures within the SAM end groups, the local modification of the redox states is visualized ("reading") by using the feedback mode of SECM. The current at the Pt tip electrode is determined by the electron-transfer rate for reoxidation of the redox mediator at the sample surface. Thus, heterogeneities in the SAM surface, which are caused by local differences in the redox state of the end groups, are distinguishable due to the different electron-transfer rates governed by the redox state of the SAM end groups. To further unequivocally prove the successful local modification of the redox state of the SAM end groups during the writing process, the micropatterned surface is selectively modified with biotin at areas with reduced SAM end groups for further complementary binding of an avidin-enzyme conjugate. Selective post-functionalization with an avidin-alkaline phosphatase conjugate allows visualization of the microstructure using the generator-collector mode of SECM.

Local Imaging of an Electrochemical Active/Inactive Region on a Conductive Carbon Surface by Using Scanning Electrochemical Microscopy

We demonstrated the imaging of local electron transfer-rate differences on a flat conductive carbon substrate, attributed to only surface functional groups, by using a scanning electrochemical microscopy (SECM) technique. These differences were clearly imaged by using a redox mediator with surface state sensitive electron transfer rates, even if the conductivity of each imaging area were almost identical. The carbon electrode surface was masked with a patterned photoresist, and selectively introduced oxygen functional groups using an oxygen plasma treatment. This patterned surface exhibited hardly any topographical features when observed by scanning electron microscopy (SEM), and a height difference of only 1.0 nm was observed with atomic force microscopy (AFM). However, the SECM feedback mode and substrate generation-tip collection (SG-TC) mode are able to distinguish these interfaces with an almost micrometer order resolution by utilizing the difference in the electron transfer rate for the Fe(2+/3+) mediator, and the current ratios for regions rich and poor in oxygen containing groups were 1.5 and 2.0, respectively. This technique could be employed for imaging and monitoring the electron transfer rates on various electrode surfaces, including fluorine and nitrogen terminated surfaces and a monolayer film patterned with micro or nano contact printing techniques.

Micropatterning of active enzyme with a high-resolution by scanning electrochemical microscopy coupled with a nanometer-sized carbon fiber disk tip

We developed a new method for fabrication of nanometer-sized carbon fiber disk electrodes and applied them to micropattern active horseradish peroxidase (HRP) with a high-resolution by scanning electrochemical microscopy (SECM). In order to pattern active HRP, except for active HRP micropatterns predesigned other regions on a HRP-immobilized substrate was deactivated by a reactive species generated at the electrode as the tip of SECM held at 1.7 V through oxidation of Br − in 0.20 mol/L phosphate buffer (PB) containing 2.5 × 10 −2 mol/L KBr and 2.0 × 10 −3 mol/L BQ (pH 7.0). The micropatterns of active HRP were characterized using the feedback mode of SECM in PB containing 2.0 × 10 −3 mol/L BQ and 2.0 × 10 −3 mol/L H 2O 2, when the tip potential was held at −0.2 V.

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

AbstractWe 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.

Imaging of DNA hybridization on microscopic polypyrrole patterns using scanning electrochemical microscopy (SECM): the HRP bio-catalyzed oxidation of 4-chloro-1-naphthol

We illustrate in this paper the successful combination of the direct and feedback mode of scanning electrochemical microscopy (SECM) for the writing of oligonucleotide patterns on thin gold films alongside the imaging of DNA hybridization. The patterning process was achieved using the direct mode of SECM, where the electrical field established between the SECM tip and the gold interface was used to drive the local deposition of micrometre sized polypyrrole spots to which a 15(mer) oligonucleotide (ODN) strand was linked covalently. Imaging of the deposited polypyrrole-ODNs was achieved by means of the feedback mode of SECM using Ru(NH(3))(6)(3+) as the mediator. The detection of the hybridization reaction of the ODN probes with their biotinylated complementary strands using SECM was possible after subsequent reactions with streptavidin and biotinylated horseradish peroxidase (HRP). The HRP-biocatalyzed oxidation of 4-chloro-1-naphthol (1) in the presence of H(2)O(2), and the precipitation of the insoluble product 4-chloro-1-naphthon (2) on the hybridized areas on the gold film caused a local alteration of conductivity. Such a change in conductivity was sensitively detected by the SECM tip and allowed imaging of DNA arrays in a fast and straightforward way.