In Situ Scanning Tunneling Microscopy of Polycrystalline Platinum Electrodes under Potential Control: Copper Electrodeposition and Pyrrole Electropolymerization

Proc. Natl. Acad. Sci. U.S.A.

Bard

1999

Self Cite

111

Imaging of DNA, keyhole limpet hemocyanin, mouse monoclonal IgG, and glucose oxidase on a mica substrate has been accomplished by scanning electrochemical microscopy with a tungsten tip. The technique requires the use of a high relative humidity to form a thin film of water on the mica surface that allows electrochemical reactions to take place at the tip and produce a faradaic current (Ϸ1 pA) that can be used to control tip position. The effect of relative humidity and surface pretreatment with buffer solutions on the ionic conductivity of a mica surface was investigated to find appropriate conditions for imaging. Resolution of the order of 1 nm was obtained.H igh-resolution imaging of nanostructured materials, especially soft and sensitive biological samples, poses a great challenge in material science and biology. The structures of individual biological molecules at nanometer-scale resolution have been traditionally studied by high-resolution electron microscopy and x-ray crystallography. However, these techniques often require difficult sample-preparation procedures and potentially damaging experimental conditions or depend on the availability of obtaining crystalline samples. The atomic-force microscope has been extensively used to image biological samples and is especially convenient because the molecules can be observed under ambient or physiological conditions. However, atomic-force microscopy does not provide chemical information, and special precautions are required to prevent the tip from damaging or altering the sample. In the family of scanning probe microscopes (1), the scanning electrochemical (EC) microscope (SECM) allows for both structural and chemical information of the surface (so-called chemical imaging). However, the application of SECM has largely focused on analytical investigations, e.g., in studies of interfacial kinetics, rather than as a highresolution imaging tool (2). In the amperometric mode, the SECM is similar to the better-known scanning tunneling microscope (STM), in that it measures the current flow through a conductive tip. However, it differs from the STM in that the current response is an EC one, and the sample is interrogated by solution species generated or reacting at the tip rather than by a strong interaction with the tip itself.The resolution attainable with a SECM is largely governed by the tip size and the distance between tip and sample. Most SECM measurements are carried out with the sample under a thick liquid layer; under these conditions, the tip must be sheathed in an insulator so that only the very end is exposed to solution to achieve high resolution. SECM measurements can also be carried out in ambient or humid air. When a substrate, like mica, with a hydrophilic surface is exposed to air, a thin layer of water forms on the surface. As shown by Guckenberger et al. (3), imaging is possible within this thin liquid layer. As we have discussed previously (4, 5), the current that passes between tip and contact is a faradaic one, based on EC reactions that take ...

Section: Methodsmentioning

confidence: 99%

Imaging of biological macromolecules on mica in humid air by scanning electrochemical microscopy

Proc. Natl. Acad. Sci. U.S.A.

Bard

1999

Self Cite

111

“…The SECM instrument consists of a combination of electrochemical components (cell and potentiostat) and those used in a scanning tunneling microscope for manipulating a tip at high resolution (piezoelectric drivers) and acquiring the data (computer/interface). The SECM instrument has been described in detail previously . The ECL intensity was measured with a photomultiplier tube (PMT) installed under the electrochemical cell.…”

Section: Methodsmentioning

confidence: 99%

Inverted Region Electron Transfer Demonstrated by Electrogenerated Chemiluminescence at the Liquid/Liquid Interface

Bard

1999

J. Phys. Chem. B

Self Cite

We describe several transinterface electron-transfer processes occurring at the benzonitrile (PhCN)/water interface that produce excited states. When an electrode was moved very close to the interface or even through the interface to produce an electrode/thin layer organic phase/aqueous solution configuration, the electrogenerated radical species formed in the PhCN phase (C12−Ru(bpy)3 3+, DPA+, or TH+) diffused to the liquid/liquid interface to react with (oxidize) a coreactant (C2O4 2-) soluble only in the aqueous solution, leading to the formation of a strongly reducing radical species (CO2 •-). The electron transfer from CO2 •- in the aqueous solution to the oxidized species in the organic phase across the liquid/liquid interface produced an electronically excited state which then emitted light. The appearance of an ECL signal in these systems supports the suggestion of Marcus inverted region behavior in very exothermic heterogeneous electron-transfer reactions at the liquid/liquid interface.

“…The Macor support and gel slab were then immersed in a solution containing a suitable redox couple for EC and SECM measurements. Details of the construction and operation of the SECM 1a,b have been described elsewhere.…”

Section: Methodsmentioning

confidence: 99%

Electrochemical Studies on Ion Transport in Gels with Scanning Electrochemical Microscopy

1998

J. Phys. Chem. B

Ion transport properties of two gels made from polyacrylate and polyacrylamide (PAAM) were studied with scanning electrochemical microscopy (SECM) by using three ferrocene derivatives, which carry different charges, as the probe molecules. Ion diffusion in these gels could be nearly as fast as in aqueous electrolytes and did not adhere to the Stokes−Einstein equation. Passivation of the electrode surface in polyacrylate gels reported previously was mainly caused by the electrophoretic deposition of gel particles on the surface of the electrode. The deposited gel film can act as a cation-exchange membrane and shows permselectivity, which affects both the electrochemical and SECM behavior. No detectable film formation was found in the PAAM gel perhaps because of its neutral charge and rigid three-dimensional network structure. The small change in diffusion coefficients as a function of PAAM gel concentration and charges of the probe molecules suggest much of the transport in the gel occurs via a water-filled domain. The transient (chronoamperometric) SECM response could be used to determine diffusion coefficients from the critical transient time as a function of tip displacement without knowing the concentration of the redox mediator, the tip radius, and the number of electrons transferred in the electrode reaction.