Fabrication of nanometer-sized electrodes and tips for scanning electrochemical microscopy

A novel method for fabrication of nanometer-sized electrodes and tips suitable for scanning electrochemical microscopy (SECM) is reported. A fine etched Pt wire is coated with polyimide, which was produced by polymerization on the Pt surface initiated by heat. This method can prepare electrodes with effective radii varying from a few to hundreds of nanometers. Scanning electron microscopy, cyclic voltammetry, and SECM were used to characterize these electrodes. Well-defined steady-state voltammograms could be obtained in aqueous or in 1,2-dichloroethane solutions. Ibis method produced the nanoelectrodes with exposed Pit on the apex, and they can also be employed as the nanotips for SECM investigations. Different sizes of Pt nanotips made by this method were employed to evaluate the kinetics of the redox reaction of Ru(NH3)6(3+) on the surface of a large Pt electrode by SECM, and the standard rate constant kappa (o) of this system was calculated from the best fit of the SECM approach curve. This result is similar to the values obtained by analysis of the obtained voltammetric data.

Investigation of the model compounds at 4-aminobenzoic acid modified glassy carbon electrode by scanning electrochemical microscopy

In this paper, we studied the reactions of both potassium ferricyanide and hexaammineruthenium(III) chloride at a 4-aminobenzoic acid (4-ABA) modified glassy carbon electrode (GCE) by scanning electrochemical microscopy (SECM) in different pH solutions. The surface of the modified electrode has carboxyl groups, the dissociation of which are strongly dependent upon the solution pH values. The rate constant kb of the oxidation of ferrocyanide on the modified electrode can be obtained by fitting the experimental tip current-distance (I-T-d) curves with the theoretical values. The surface pK(a) of the 4-ABA modified GCE was estimated from the plot of standard rate constant k(o) versus the solution pH and is equal to 3.2, which is in good agreement with the reported result. The SECM approach curves for Ru(NH3)(6)(3+) both on the bare and the modified electrodes show similar diffusion control processes. These results can be explained by the electrostatic interactions between the modified electrode surface and the model compounds with different charges. (C) 2001 Elsevier Science BN. All rights reserved.

Scanning electrochemical microscopy study of electron transfer across the water/1,2-dichloroethane interface induced by common ions

In this work, we report the reverse electron transfer reaction between TCNQ in 1, 2-dichloroethane (DCE) and ferrocyanide in water. This process is a thermodynamic unfavorable reaction and the reverse electron transfer reaction can only be obtained by scanning electrochemical microscopy(SECM) in the presence of suitable potential-determining ions, which govern the interfacial potential difference. In our case, the potential determining ions are tetrabutylammonium ion(TBA(+)) and tetraphenylarsonium ion (TPAs+). The effects of the concentrations of TBA(+) and TPAs+ in two phases and other parameters have been studied in detail. The apparent heterogeneous rate constants(k(i)) were obtained under different values of K-p(K-p=c(i)(w)/c(i)(o)) for both cases by fitting the SECM approach curves with theoretical ones and the results showed that they were controlled by the interfacial potential differences. The relationship between apparent heterogeneous rate constants and the interfacial potential differences obeys Butler-Volmer theory.

Recent advances in scanning electrochemical microscopy

The basic principle, instrumentation and recent advances of scanning electrochemical microscopy are briefly reviewed with 55 references.

Some unusual features of highly oriented pyrolytic graphite observed by electrochemical scanning tunneling microscopy

Highly oriented pyrolytic graphite (HOPG) is the substrate often used in scanning tunneling microscopy (STM). It is well known that STM images of the basal plane of HOPG show some unusual structural patterns. In this letter, we present in situ STM images of some unusual features on HOPG in solutions, including normal or abnormal chain-like features and hexagonal or oblique superperiodic structures. These features emerge both next to and apart from the step of HOPG.

Molecular characterization of beef liver catalase by scanning tunneling microscopy

Beef liver catalase molecules can stick tenaciously to the highly oriented pyrolytic graphite (HOPG) surface which has been activated by electrochemical anodization. The immobilized sample is stable enough for high resolution scanning tunneling microscope (STM) imaging. When the anodized conditions are controlled properly, the HOPG surface will be covered with a very thin oxide layer which can bind the protein molecules. Individual molecules of native beef liver catalase are directly observed in detail by STM, which shows an oval-shape structure with a waist. The dimensions of one catalase molecule in this study are estimated as 9.0 x 6.0x 2.0 nm(3), which are in good agreement with the known data obtained from X-ray analysis, except the height can not be exactly determined from STM. Electrochemical results confirm that the freshly adsorbed catalase molecules maintain their native structures with biological activities. However, the partly unfolding structure of catalase molecules is observed after the sample is stored for 15 days, this may be caused by the long-term interaction between catalase molecules and the anodized HOPG surface.

Electrochemical scanning tunneling microscopy and electrochemical quartz crystal microbalance study of the adsorption of phenanthraquinone accompanied by an electrochemical redox reaction on the Au electrode

In situ electrochemical scanning tunneling microscopy (ECSTM) and an electrochemical quartz crystal microbalance (EQCM) have been employed to follow the adsorption/desorption processes of phenanthraquinone (PQ sat. in 0.1 mol l(-1) HClO4, solution) accompanied with an electrochemical redox reaction on the Au electrode. The result shows that: (1) the reduced form PQH(2) adsorbed at the Au electrode and the desorption occurred when PQH(2) was oxidized to PQ; (2) the adsorption process initiates at steps or kinks which provide high active sites on the electrode surface for adsorption, and as the potential shifts to negative, a multilayer of PQH(2) may be formed at the Au electrode; (3) the reduced PQH(2) adsorbed preferentially in the area where the tip had been scanned continually; this result suggests that the tip induction may accelerate the adsorption of PQH(2) on the Au(111) electrode. Two kinds of possible reason have been discussed; (4) high resolution STM images show the strong substrate lattice information and the weak monolayer adsorbate lattice information simultaneously. The PQH(2) molecules pack into a not perfectly ordered condensed physisorbed layer at potentials of 0.1 and 0.2 V with an average lattice constant a = 11.5 +/- 0.4 Angstrom, b = 11.5 +/- 0.4 Angstrom, and gamma = 120 +/- 2 degrees; the molecular lattice is rotated with respect to the substrate lattice by about 23 +/- 2 degrees. (C) 1997 Elsevier Science S.A.

Scanning tunnelling microscopy observation of cytochrome-c denaturation induced by bromopyrogal red on highly oriented pyrolytic graphite

The denaturation of cytochrome-e (cyt-c) induced by bromopyrogal red (BPR) was studied by scanning tunnelling microscopy (STM) on the electrochemically pretreated highly oriented pyrolytic graphite (HOPG) surface. STM images reveal that denatured cyt-c molecules exist in variable states including aggregates, globular compact, partially unfolded and combined with BPR molecule. The apparently low image contrast of denatured cyt-c observed in this experiment comparing to that of native cyt-c molecules, and the relative low image contrast of the unfolded part comparing with the compact globular part, are ascribed to the unfavourable tunnelling paths for the conformational variations of denatured cyt-c molecules. (C) 1997 Elsevier Science B.V.

In situ scanning tunneling microscopy studies of nanometer size palladium particles on highly oriented pyrolytic graphite

A special electrodeposition process of palladium was studied by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). A kind of palladium(IV) complex was attached to the highly oriented pyrolytic graphite (HOPG) electrode surface by electro-oxidation of palladium(II) complex first, and was then reduced to palladium particles. The surface complexes and particles of palladium were both characterized by in situ STM and XPS. The Pd particles are in the nanometer range of size and exhibit electrocatalytic activity towards the oxidation of hydrazine and hydroxylamine.

Tip-induced dynamic characterization of multilayers and variable morphologies of cytochrome c molecules on highly oriented pyrolytic graphite revealed by in-situ scanning tunnelling microscopy

The dynamic states of cytochrome c multilayers on electrochemically pretreated highly oriented pyrolytic graphite (HOPG) have been studied by in-situ scanning tunnelling microscopy (STM) under potential control of both the tip and the substrate in cytochrome c and phosphate buffer solution. The dynamic characterization of cytochrome c multilayers and relatively stable adsorbed single cytochrome c molecules scattered on HOPG imply that physically adsorbed multilayers were more easily influenced by the STM tip than those of chemically adsorbed single molecules. In-situ STM images of chemically adsorbed cytochrome c molecules with discernible internal structures on HOPG revealed that morphologies of cytochrome c molecules also suffered tip influence; possible tip-sample-substrate interactions have been discussed.

In situ electrochemical scanning tunneling microscopy investigation of renewing graphite surface accompanied by electrochemical reaction

In situ electrochemical scanning tunneling microscopy (ECSTM) has been employed to follow the renewal process of a graphite electrode accompanied by flavin adenine dinucleotide (FAD) electrochemical reaction which involves adsorption of the reduced form (FADH(2)) and desorption of the oxidized form (FAD). The renewal process initiates from steps or kinks on the electrode surface, which provide high active sites for adsorption. This renewal depends on the working electrode potential, especially in the range near the FAD redox potential. Our experiment suggests that delamination of the graphite surface is caused by interaction between the substrate and adsorbed molecules. A simple model is proposed to explain this phenomenon.

In situ electrochemical scanning tunnelling microscopy investigation of structure for horseradish peroxidase and its electrocatalytic property

Immobilization of protein molecules is a fundamental problem for scanning tunnelling microscopy (STM) measurements with high resolution. In this paper, an electrochemical method has been proved to be an effective way to fix native horseradish peroxidase (HRP) as well as inactivated HRP from electrolyte onto a highly oriented pyrolytic graphite (HOPG) surface. This preparation is suitable for both ex situ and in situ electrochemical STM (ECSTM) measurements. In situ STM has been successfully employed to observe totally different structures of HRP in three typical cases: (1) in situ ECSTM reveals an oval-shaped pattern for a single molecule in neutral buffer solution, which is in good agreement with the dimension determined as 6.2 x 4.3 x 1.2. nm(3) by ex situ STM for native HRP; (2) in situ ECSTM shows that the adsorbed HRP molecules on HOPG in a denatured environment exhibit swelling globes at the beginning and then change into a V-shaped pattern after 30 min; (3) in situ ECSTM reveals a black hole in every ellipsoidal sphere for inactivated HRP in strong alkali solution. The cyclic voltammetry results indicate that the adsorbed native HRP can directly catalyse the reduction of hydrogen peroxide, demonstrating that a direct electron transfer reduction occurred between the enzyme and HOPG electrode, whereas the corresponding cyclic voltammograms for denatured HRP and inactivated HRP adsorbed on HOPG electrodes indicate a lack of ability to catalyse H2O2 reduction, which confirms that the HRP molecules lost their biological activity. Obviously, electrochemical results powerfully support in situ STM observations.

Scanning tunneling microscopy characterization of electrode materials in electrochemistry

Ex situ and in situ STM characterization of the electrode materials, including HOPG, GC, Au, Pt and other electrodes, is briefly surveyed and critically evaluated. The relationship between the electrode activity and surface microtopography is discussed.

EFFECTS OF ANODIC-OXIDATION ON THE SURFACE-STRUCTURE OF HIGHLY ORIENTED PYROLYTIC-GRAPHITE REVEALED BY IN-SITU ELECTROCHEMICAL SCANNING-TUNNELING-MICROSCOPY IN H2SO4 SOLUTION

Effects of the potential of anodic oxidation and of potential cycling on the surface structure of a highly oriented pyrolytic graphite (HOPG) electrode were observed by in situ electrochemical scanning tunnelling microscopy (ECSTM) in dilute H2SO4 solution with atomic resolution. With potential cycling between -0.1 V and 1.8 V vs. Ag/AgCl (sat. KCI), some atoms on the top layer of HOPG protrude out of the base plane, and the graphite lattice of these protrusions is still intact but is strained and expanded. With further potential cycling, some protrusions coalesced and some grew larger, and an anomalous superperiodic feature was observed (spacing 90 Angstrom with a rotation 30 degrees relative to atomic corrugations) which superimposed on the atomic corrugation of HOPG. On the topmost of these protrusions, some atoms form oxides and others are still resolved by the ECSTM image. With potential cycling between -0.1 V and + 2.0 V vs. Ag/AgCl (sat. KCl), damage to freshly cleaved HOPG surface is more serious and fast, some ridges are observed, the atomic structure of the HOPG surface is partially and then completely damaged due to the formation of oxide. We also found that anodic oxidation occurred nonuniformly on the surface of HOPG near defects during potential cycling.