Tip position modulation and lock-in detection in scanning electrochemical microscopy

Small amplitude tip-position modulation (TPM) in combination with lock-in detection of the modulated current signal is applied to a scanning electrochemical microscope (SECM) to improve signal-to-noise ratio and to enhance image resolution. Phase shift information from the alternating current TPM signal and the bipolar response of TPM over insulators and conductors make TPM-SECM superior to conventional SECM imaging methods as well as useful for measuring substrate conductivity.Board of Regents, University of Texas Syste

Electrochemistry and electrogenerated chemiluminescence with a single faradaic electrode

Described herein is an apparatus comprising an electrochemical cell that employs a capacitive counter electrode and a faradaic working electrode. The capacitive counter electrode reduces the amount of redox products generated at the counter electrode while enabling the working electrode to generate redox products. The electrochemical cell is useful for controlling the redox products generated and/or the timing of the redox product generation. The electrochemical cell is useful in assay methods, including those using electrochemiluminescence. The electrochemical cell can be combined with additional hardware to form instrumentation for assay methods.Board of Regents, University of Texas Syste

Dynamic Potential-Ph Diagrams Application to Electrocatalysts for Water Oxidation

The construction and use of "dynamic potential-pH diagrams" (DPPDs), that are intended to extend the usefulness of thermodynamic Pourbaix diagrams to include kinetic considerations is described. As an example, DPPDs are presented for the comparison of electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), an important electrochemical reaction because of its key role in energy conversion devices and biological systems (water electrolyses, photoelectrochemical water splitting, plant photosynthesis). The criteria for obtaining kinetic data are discussed and a 3-D diagram, which shows the heterogeneous electron transfer kinetics of an electrochemical system as a function of pH and applied potential is presented. DPPDs are given for four catalysts: IrO(2), Co(3)O(4), Co(3)O(4) electrodeposited in a phosphate medium (Co-Pi) and Pt, allowing a direct comparison of the activity of different electrode materials over a broad range of experimental conditions (pH, potential, current density). In addition, the experimental setup and the factors affecting the accurate collection and presentation of data (e. g., reference electrode system, correction of ohmic drops, bubble formation) are discussed.Ministry of Education, University and Research PRIN 2008PF9TWZ, 2008N7CYL5Universita degli Studi di MilanoNational Science Foundation CHE-0808927Robert A. Welch Foundation F-0021Center for Electrochemistr

Application of scanning electrochemical microscopy to biological samples

The scanning electrochemical microscope can be used in the feedback mode in two-dimensional scans over biological substrates to obtain topographic information at the micrometer level. In this mode, the effect of distance between a substrate (either conductive or insulating) and a scanning ultramicroelectrode tip on the electrolytic current flowing at the tip is recorded as a function of the tip x-y position. Scans of the upper surface of a grass leaf and the lower surface of a Ligustrum sinensis leaf (which show open stomata structures) immersed in aqueous solution are shown. Scans of the upper surface of an elodea leaf in the dark and under irradiation, where the tip reaction is the reduction of oxygen produced by photosynthesis, demonstrate the possibility of obtaining information about the distribution of reaction sites on the substrate surface

Electrochemical reduction of allyl halides in nonaqueous solvents - a reinvestigation

Abstract: The electrochemical reduction of allyl iodide (la), allyl bromide (lb), (E)-3-bromo-l-phenyl-l-propene (IC), and (E)-5-bromo-2,2,6,6-tetramethyl-3-heptene (Id) was studied in dry acetonitrile with TBAP as supporting electrolyte by means of cyclic voltammetry and coulometry at mercury, platinum, and vitreous carbon electrodes. Compounds la-c showed multiple waves on platinum and mercury because of halide surface effects. However on vitreous carbon la-d gave single reduction waves, with half-peak potentials of -1.38, -1.64, -1.1 I , and -1.89 V (vs. SCE), respectively. Apparent coulometric n values of 1 for la-c were shown to arise from rapid nucleophilic substitution of the allyl anion intermediates with starting halide to give electroinactive dimers. Sterically hindered Id showed an napp of 2 and did not exhibit surface interactions with Hg and Pt. Thus allyl halides, contrary to previous statements in the literature, are reduced via a two-electron electrode reaction and the reduction of the allyl radical to the allyl anion cannot be seen as a separate step. Evidence was also obtained for the formation of the allyl anion by reduction of l a and I b with solvated electrons in liquid ammonia; the electrochemical oxidation of this species in this medium occurred at ca. -1.2 V. Introduction The electrochemical reduction of organic halogen compounds has been widely investigated and several reviews are available.* As was first shown by von Stackelberg and S t r a~k e ,~ the overall reaction involves cleavage of the carbon-halogen bond in a single two-electron polarographic wave to give a carbanion which is subsequently protonated (Schem

Photocatalytic methods for preparing metallized powders

Photocatalytic methods are disclosed for the preparation of metallized powders. Specifically, such methods include the photodeposition of platinum, copper and other metals on TiO.sub.2 powder and other semiconductor powders. The powders thus prepared are particularly useful as catalysts.Board of Regents, University of Texas Syste

Method for metal nanoparticle electrocatalytic amplification

The present invention includes methods, compositions and kits for analyzing a chemical analyte having an electrochemical cell connected to a measuring apparatus. The electrochemical cell contains a solution having one or more nanoparticles, one or more chemical analytes, an indicator. In addition, the electrochemical cell contains one or more electrodes in communication with the solution. One or more electrocatalytic properties are generated by the interaction of the one or more nanoparticles and the liquid sample and measured at the one or more electrodes.Board of Regents, University of Texas Syste

Method and apparatus for nanoparticle electrogenerated chemiluminescence amplification

Methods, compositions and kits for analyzing a chemical analyte using an electrochemical cell connected to a measuring apparatus are provided. The electrochemical cell contains a solution having one or more conductive or redox active NPs (nanoparticles), one or more chemical analytes, and an indicator. In addition, the electrochemical cell contains one or more electrodes in communication with the solution. One or more catalytic ECL properties are generated by the interaction of the one or more conductive or redox active NPs and the liquid sample and measured at the one or more electrodes or with an optical detection system.Board of Regents, University of Texas Syste

Ellipsometric, electrochemical, and elemental characterization of the surface phase produced on glassy carbon electrodes by electrochemical activation

Ellipsometry was used to monitor the In situ growth of a fllm on a glassy carbon electrode, whlch was electrochemlcally anodized at 1.8 V In 0.1 M H,SO,. The layer grew contlnuously In an uninhlblted fashion wlth constant optical constants to a thlckness of at least 925 nm. I t was nearly transparent at wavelengths of 545 and 632.8 nm. X-ray and elemental analysls of bulk quantltles of the phase Indicate that It Is an amorphous form of graphlte oxlde. Comblned elllpsometrlc and electrochemical measurements show that the phase activates the surface and that deactivation occurs upon extennslve reductlon of the layer. The electrode actlvlty, as monitored by the voltammetrlc response In dilute solutlons of catechol, hydroquhne, and 2,3-dlcyanohydroqulnone, varled wlth the extent of reductlon of the layer. Surface treatments have been used extensively to improve the electrochemical performance of several types of carbon electrodes: vitreous (glassy) carbon (1-7), pyrolytic graphite @-IO), carbon paste (11,12), carbon fibers Evidence for increased surface oxygen concentration upon activation There is some evidence, however, that does not support the theory of mediation by oxygen functionalities, especially for GC. Heat treatment has been shown to remove oxygen functionalities but yet improve the voltammetry of ascorbic acid and ferricyanide (7). Dioxygen reduction was shown to be independent of pH a t an activated GC electrode, but a predictable function of pH at a quinone-coated electrode (with no reduction a t a pH C pK, of the semiquinone (25)). The charge under the reductive stripping peak associated with the "functional groups" formed by anodization was estimated to be the equivalent of 11 groups/A2 (assuming n = 2) (4); such a density of groups would require a multilayer structure, rather than simple functionalization of the surface. The results presented here show that a dominant process during electrochemical activation of the GC surface is the formation of a nearly transparent homogeneous graphitic oxide phase. Ellipsometry has been used extensively to study electrode surfaces (26-31). The technique can detect monolayer coverages and (microscopic) surface roughness on an optically flat surface. The glasslike structure (32) and hardness of vitreous carbon allow polishing to give a very flat, reflective, and isotropic surface. These properties make GC an attractive material for specular reflectance techniques (18). Ellipsometry was used to monitor the optical constants and thickness of a nearly transparent film that developed during electrochemical activation of GC. EXPERIMENTAL SECTION Chemicals. Reagent grade sulfuric acid was used as received and diluted with deionized water. The electrolyte solution was 0.1 M sulfuric acid in all experiments. Hydroquinone (HQ) was used without purification. Catechol was sublimed twice under vacuum. Day old solutions of catechol produced extraneous voltammetric waves superimposed on the catechol waves. Therefore, solutions containing catechol were prepared immediately before use. 2,3-Dicyanohydroquinone (DCHQ) was recrystallized several times from EtOH/water solution. Stock solutions of the redox couples were about 1.0 mM. Electrodes. The polished end of a 5 mm diameter glassy carbon (GC) rod (Tokai Carbon Co., Ltd., grad GC-20) served as the working electrode. A 10 mm length of the rod was attached coaxially to a brass rod of equal diameter with graphite epoxy (Dylon Industries, Inc., Cleveland, OH, grade PX), which had been doped with platinum black. A gold-plated pin connector was soldered to the brass rod, giving an assembly with an internal resistance of about 10 R. The rod assembly was sealed in heatshrinkable FEP Teflon tubing and then pressed into an undersized (by about 0.25 mm) cylinder of TFE Teflon. An optically flat working surface was prepared by facing-off the Teflon shroud on a lathe and sanding the GC rod with 6W-grit carborundum paper. By application of only gentle pressure during sanding, coplanarity was achieved without removing much of the shroud or destroying its planarity. The surface was polished successively with 3.0-, LO-, and 0.25-fim diamond paste on microcloth-felt with an 8-in. polishing wheel (all polishing suppliers from Buehler, Ltd.). The electrode was sonicated in deionized water twice for 15 min between polishings. It was repolished with 0.25-pm diamond paste between experiments. A silver wire served as a quasi-reference electrode (Ag-QRE) and a 5-cm2 Pt screen as the counter electrode. The Ag-QRE potential was about 0.24 (10.02) V positive of a sodium-saturated calomel electrode (0.48 (k0.02) V positive of NHE (normal hydrogen electrode)) and varied slightly from week to week, as determined by comparing either the E I j z of HQ or the onset of hydrogen evolution from Pt relative to each reference electrode. Activation Procedure. The electrode was activated in blank solution and in solutions of HQ and DCHQ by sweeping th

Electrostatic Electrochemistry at Insulators

The identity of charges generated by contact electrification on dielectrics has remained unknown for centuries and the precise determination of the charge density is also a long-standing challenge. Here, electrostatic charges on Teflon (polytetrafluoroethylene) produced by rubbing with Lucite (polymethylmethacrylate) were directly identified as electrons rather than ions by electrochemical (analogous to electrogenerated chemiluminescence). Moreover, copper deposition could be amplified by depositing Pd first in a predetermined pattern, followed by electroless deposition to produce Cu lines. This process could be potentially important for microelectronic and other applications because Teflon has desirable properties including a low dielectric constant and good thermal stability. Charge density was determined using Faraday's law and the significance of electron transfer processes on charged polymers and potentially other insulators have been demonstrated. Although both contact electrification of insulating materials (dielectrics), such as Teflon and glass 1 , and electrochemistry at electronic conductors, such as metals and semiconductors 2 , deal with charged interfaces, they have largely remained distinct fields. The possible chemical effects of electrostatic charge have not been widely studied. Despite its long history 3 , the charge identity (electron or ion) on rubbed insulators is still poorly understood. Whereas Harper recognized the role of an electron transfer mechanism for metals and semiconductors 4,5 , on the basis of their relative Fermi level energies, he favoured an ion transfer mechanism for insulators 11 . Experiments designed to test if ion transfer occurred during contact electrification were not successful Experiments were carried out by immersion of charged Teflon into an acidic solution to note any change in pH and formation of hydrogen gas. After 37 pieces of Teflon septa were rubbed with Lucite discs and then briefly immersed in 3 ml of a 0.1 mM HCl solution one after another, the solution pH increased from 4 to 6.2. In another experiment, the pH of 3 ml of an HCl solution changed from 3.1 to 4.1, 5.2 and 7.3 after consecutive contact with charged Teflon tapes. However, this result alone does not prove that the negative charges on Teflon were electrons instead of ions, because H + could also adsorb on charged Teflon or an adsorbed anion, such as hydroxide, transferred to the surface during charging 15 could leach into the solution and cause a pH change. However, if hydrogen gas was produced, the charge carriers on Teflon must be electrons because there is no known way for adsorbed ions to generate hydrogen. Indeed, hydrogen was detected by ultrahigh vacuum (UHV) mass spectrometry. In this case, D 2 O was used and samples were prepared inside a glove box. Charged Teflon tape was introduced through a Teflon tube into a glass reactor with 50 ml D 2 O solution containing 1.5 ml DCl (35%). The reactor, which was equipped with a metal joint, was then connected to a stainlesssteel tube sealed with a valve. Note that some tape stayed above the DCl solution; careful shaking and tilting of the reactor were necessary for them to fully contact the solution. The reactor was then taken out and connected to a UHV system (1.5 × 10 −9 torr). Liquid nitrogen was used to freeze the reactor solution and the gas was first introduced into a sample transfer chamber before it reached the main UHV chamber. A clear D 2 peak appeared in the mass spectrum, whereas a control experiment carried out under the same conditions without contact to charged Teflon showed only a flat baseline. Hydrogen generation clearly shows that energetic electrons were present on the Teflon surface and caused a reduction process that should be faradaic as in conventional electrochemistry (2H + + 2e → H 2 ). In this process, as opposed to that of a typical two-electrode electrochemical cell, the solution becomes negatively charged with an excess of anions. If all of the pH change can be ascribed to the proton reduction, the observed pH change could be used as an accurate way to measure the electrostatic charge density on an insulator. Indeed, when the total number of H + ions removed from the solution is divided by the geometric nature materials ADVANCE ONLINE PUBLICATION www.nature.com/naturematerials