Scanning electrochemical microscopy for direct imaging of reaction rates

Microfluidic push-pull probe for scanning electrochemical microscopy

This paper presents a microfluidic push-pull probe for scanning electrochemical microscopy (SECM) consisting of a working microelectrode, an integrated counter/reference electrode and two microchannels for pushing and pulling an electrolyte solution to and away from a substrate. With such a configuration, a droplet of a permanently renewed redox mediator solution is maintained just at the probe tip to carry out SECM measurements on initially dry substrates or in microenvironments. For SECM imaging purposes, the probe fabricated in a soft polymer material is used in a contact regime. SECM images of various gold-on-glass samples demonstrate the proof-of-concept of a push-pull probe for local surface activity characterization with high spatial resolution even on vertically oriented substrates. Finite element computations were performed to guide the improvement of the probe sensitivity.

In situ electrochemical imaging of membrane glycan expression on micropatterned adherent single cells

A scanning electrochemical microscopic (SECM) method for in situ imaging of four types of membrane glycan motifs on single adherent cells was proposed using BGC-823 human gastric carcinoma (BGC) cells as the model. These adherent cells were first micropatterned in the microwell of poly(dimethylsiloxane) membrane for precisely controlling the localized surface interaction, and the membrane glycans were then specifically recognized with corresponding lectins labeled with horseradish peroxidase (HRP). On the basis of the enzymatic oxidization of ferrocenylmethanol (FMA) by H(2)O(2) to yield FMA(+), the glycan expression level was detected by the reduction current of FMA(+) at the SECM tip. The cell-surface glycans could, thus, be in situ imaged by SECM at a single-cell level without peeling the cells from culture dish. Under the optimized conditions, four types of membrane glycan motifs showed statistically distinguishable expression levels. The SECM results for different glycan motifs on adherent single cells were consistent with those estimated by flow cytometric assay. This work provides a reliable approach for in situ evaluation of the characteristic glycopattern of single living cells and can be applied in cell biologic study based on cell surface carbohydrate expression.

Synthesis, Physical Properties, Structural, and Electrochemical Characterization of Methimidazolium and Imidazolium-based Tetracyanoquinodimethane Anion Radical Salts

Two methimazolium and two imidazolium-based salts derived from combination with the tetracyanoquinodimethane (TCNQ) radical anion have been synthesized (1–4). The 1:1 (cation:anion) stoichiometry of the chemically synthesized materials is fully supported by steady-state voltammetric measurements at a microdisc electrode in acetonitrile. The methimazolium TCNQ salts (1 and 2), which contain an acidic proton on the cation, exhibit a protonation step coupled to the TCNQ1–/2– charge-transfer process. Solid–solid transformations at a TCNQ-modified electrode also lead to electrochemical synthesis of 1–4, but also indicate that other cation:anion stoichiometries are accessible. Atomic force microscopy for electrochemically synthesized samples exhibit rod-like morphology. Conductivity measurements on chemically and electrochemically prepared salts are in the semiconducting range. Scanning electrochemical microscopy approach curve data support the substantial conductivity of these solids. Extensive physicochemical characterization of these materials is in complete accordance with the X-ray crystal structure of 1-acetonitrile-3-methylimidazolium tetracyanoquinodimethane, [AMim+][TCNQ1–], 4.

Intermittent contact-scanning electrochemical microscopy (IC-SECM): a new approach for tip positioning and simultaneous imaging of interfacial topography and activity

A new scanning electrochemical microscopy (SECM) tip positioning method that allows surface topography and activity to be resolved simultaneously and independently is presented. The tip, controlled by a piezoelectric positioner operated in closed loop, is oscillated normal to the substrate surface. Changes in the oscillation amplitude, caused by the intermittent contact (IC) of the tip with the substrate surface, are used as a feedback signal to control the tip height. The method is illustrated with amperometric feedback approach curve measurements to inert (insulating) and active (conducting) substrates using 12.5 and 1 microm radii Pt disk electrodes. Imaging of gold bands on a glass substrate demonstrates the capabilities for simultaneous topography and activity mapping. The prospect for using IC methodology more widely with other types of tips is highlighted briefly.

Touching surface-attached molecules with a microelectrode: mapping the distribution of redox-labeled macromolecules by electrochemical-atomic force microscopy

We report on the development of a mediator-free electrochemical-atomic force microscopy (AFM-SECM) technique designed for high-resolution imaging of molecular layers of nanometer-sized redox-labeled (macro)molecules immobilized onto electrode surfaces. This new AFM-SECM imaging technique, we call molecule touching atomic force electrochemical microscopy (Mt/AFM-SECM), is based on the direct contact between surface-anchored molecules and an incoming microelectrode (tip). To validate the working-principle of this microscopy, we consider a model system consisting of a monolayer of nanometer long, flexible, polyethylene glycol (PEG) chains covalently attached by one extremity to a gold surface and bearing at their free end a ferrocene (Fc) redox tag. Using Mt/AFM-SECM in tapping mode, i.e., by oscillating the tip so that it comes in intermittent contact with the grafted chains, we show that the substrate topography and the distribution of the redox-tagged PEG chains immobilized on the gold surface can be simultaneously and independently imaged at the sub-100 nm scale. This novel type of SECM imaging may be found useful for characterizing the surface of advanced biosensors which use electrode-grafted, redox-tagged, linear biochains, such as peptides or DNA chains, as sensing elements. In principle, Mt/AFM-SECM should also permit in situ imaging of the distribution of any kind of macromolecules immobilized on electrode surfaces or simply conducting surfaces, provided they are labeled by a suitable redox tag.

SECM investigations of immobilized porphyrins films

Electronic properties of electrogenerated Zn-porphyrin layers linked by an electroactive linker and immobilized on a semitransparent ITO electrode were investigated by steady-state SECM in unbiased conditions in view of the numerous possible applications of such surface. This SECM strategy took advantage of the variations of the charge transfer kinetics of the organic redox couple (the mediator used in SECM) on ITO surface with the standard potential of the mediator. After preliminary characterization of nonmodified ITO, analysis of the SECM approach curves recorded with a series of redox mediators allows the characterizations of both film permeability and charge transport inside the organic film in conditions close to a "real optoelectronic device". Two types of porphyrin films were considered. In the first one, the film was produced by electropolymerization of a modified zinc-β-octaethylporphyrin in which the bipyridinium pendant substituent is first introduced. The second type of film was prepared directly from an in situ electropolymerization method in which the Zn porphyrin is simply oxidized in the presence of 4,4'-bipyridine. Experiments show the occurrence of efficient charge transport inside both films after initial reduction of the electroactive linker. However, the first preparation method leads to films with stronger blocking character versus organic molecules and higher charge injection rates.

Real-time monitoring of cell viability by its nanoscale height change with oxygen as endogenous indicator

A method for real-time evaluation of cell viability was developed by using oxygen as an endogenous indicator in scanning electrochemical microscopy to monitor the nanoscale height change of a single cell in a physiological environment with a novel Pt nanodisk electrode and a newly designed step-approaching strategy.

Combining scanning electrochemical microscopy with infrared attenuated total reflection spectroscopy for in situ studies of electrochemically induced processes

The combination of scanning electrochemical microscopy (SECM) with single-bounce attenuated total reflection Fourier-transformed infrared spectroscopy (FT-IR-ATR) has been developed for in situ studies on electrochemically induced processes at IR waveguide surfaces via evanescent field absorption spectroscopy. The feasibility of the combined microelectrochemical FT-IR setup was demonstrated by spectroscopically monitoring microstructured polymer depositions induced via feedback mode SECM using a 25 mum Pt disk ultramicroelectrode (UME). The surface of a ZnSe ATR crystal was initially coated with 2,5-di-(2-thienyl)-pyrrole (SNS) layer, which was then locally polymerized during Ru(bpy)(3)(2+) mediated feedback mode SECM experiments. The polymerization reaction was simultaneously monitored by recording absorption intensity changes of SNS specific IR bands, thereby providing information on the polymerization mechanism and on the percentage of surface modification.

Monitoring scanning electrochemical microscopy approach curves with mid-infrared spectroscopy: toward a novel current-independent positioning mode

Single-bounce attenuated total reflection infrared spectroscopy in the 3-20 mum range (mid-infrared, MIR) has been combined with scanning electrochemical microscopy (SECM) for in situ spectroscopic detection of electrochemically induced localized surface modifications using an ultramicroelectrode (UME). In this study, a novel current-independent approach for positioning the UME in aqueous electrolyte solution is presented using either changes of infrared (IR) absorption intensity associated with borosilicate glass (BSG), which is used as shielding material of the UME wire, or by monitoring IR changes of the water spectrum within the penetration depth of the evanescent field due to displacement of water molecules in the volume between the sample surface and the UME within the evanescent field. The experimental results show that the UME penetrates into the exponentially decaying evanescent field in close vicinity (a few micrometer) to the attenuated total reflection (ATR) crystal surface. Hence, the resulting intensity changes of the IR absorption spectra for borosilicate glass (increase) and for water (decrease) can be used to determine the position of the UME relative to the ATR crystal surface independent of the current measured at the UME.

Microfabrication of patterns of adherent marine bacterium Phaeobacter inhibens using soft lithography and scanning probe lithography

Two lithographic approaches have been explored for the microfabrication of cellular patterns based on the attachment of marine bacterium Phaeobacter inhibens strain T5. Strain T5 produces a new antibiotic that makes this bacterium potentially interesting for the pharmaceutical market and as a probiotic organism in aquacultures and in controlling biofouling. The microcontact printing (microCP) method is based on the micropatterning of self-assembled monolayers (SAMs) terminated with adhesive end groups such as CH(3) and COOH and nonadhesive groups (e.g., short oligomers of ethylene glycol (OEG)) to form micropatterned substrates for the adhesion of strain T5. The scanning probe lithographic method is based on the surface modification of OEG SAM by using a microelectrode, the probe of a scanning electrochemical microscope (SECM). Oxidizing agents (e.g., Br(2)) were electrogenerated in situ at the microelectrodes from Br(-) in aqueous solution to remove OEG SAMs locally, which allows the subsequent adsorption of bacteria. Various micropatterns of bacteria could be formed in situ on the substrate without a prefabricated template. The fabricated cellular patterns may be applied to a variety of marine biological studies that require the analysis of biofilm formation, cell-cell and cell-surface interactions, and cell-based biosensors and bioelectronics.

Synthesis and immobilization of Ag(0) nanoparticles on diazonium modified electrodes: SECM and cyclic voltammetry studies of the modified interfaces

A versatile method was used to prepare modified surfaces on which metallic silver nanoparticles are immobilized on an organic layer. The preparation method takes advantage, on one hand, of the activated reactivity of some alkyl halides with Ag-Pd alloys to produce metallic silver nanoparticles and, on the other hand, of the facile production of an anchoring polyphenyl acetate layer by the electrografting of substituted diazonium salts on carbon surfaces. Transport properties inside such modified layers were investigated by cyclic voltammetry, scanning electrochemical microscopy (SECM) in feedback mode, and conducting AFM imaging for characterizing the presence and nature of the conducting pathways. The modification of the blocking properties of the surface (or its conductivity) was found to vary to a large extent on the solvents used for surface examination (H(2)O, CH(2)Cl(2), and DMF).

Analytical expressions for quantitative scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM), is a recent analytical technique in electrochemistry, which was developed in the 1990s and uses microelectrodes to probe various surfaces. Even with the well-known disc microelectrodes, the system geometry is not as simple as in regular electrochemistry. As a consequence even the simplest experiments, the so-called positive and negative feedback approach curves, cannot be described with exact analytical expressions. This review gathers all the analytical expressions available in the SECM literature in steady-state feedback experiments. Some of them are claimed as general expressions, other are presented as approximate. Their validity is discussed in the light of the current understanding and computer facilities.

Influence of electrode size and geometry on electrochemical experiments with combined SECM-SFM probes

Gold electrodes integrated into silicon scanning force microscopy (SFM) probes allow the acquisition of spatially correlated data for sample morphology (via SFM) and local electrochemical reactivity via scanning electrochemical microscopy (SECM). The lateral resolution of both techniques is controlled by different properties of the integrated probes. The topographic tracking provided by the SFM mechanism allows the realization of very small working distances for the SECM measurements. Microfabrication technology was used in order to reduce the size of the active electrode area of the tip into the sub-100 nm regime. The functionality of the probes was tested using electrochemical methods. Experiments revealed that the response could be quantitatively compared to numerical simulation. The low working distance, in combination with the small size of the active electrode area, allows for high lateral resolution in the SECM images. This is illustrated with different model substrates that cover a range of different rate constants and illustrate the dependence of the SECM contrast on the local kinetics of the sample in the sub-micrometre size range.

Scanning electrochemical microscopy in neuroscience

This article reviews recent work involving the application of scanning electrochemical microscopy (SECM) to the study of individual cultured living cells, with an emphasis on topographical and functional imaging of neuronal and secretory cells of the nervous and endocrine system. The basic principles of biological SECM and associated negative amperometric-feedback and generator/collector-mode SECM imaging are discussed, and successful use of the methodology for screening soft and fragile membranous objects is outlined. The drawbacks of the constant-height mode of probe movement and the benefits of the constant-distance mode of SECM operation are described. Finally, representative examples of constant-height and constant-distance mode SECM on a variety of live cells are highlighted to demonstrate the current status of single-cell SECM in general and of SECM in neuroscience in particular.

Electrochemical atomic force microscopy using a tip-attached redox mediator for topographic and functional imaging of nanosystems

We describe the development of a new type of high-resolution atomic force electrochemical microscopy (AFM-SECM), labeled Tarm (for tip-attached redox mediator)/AFM-SECM, where the redox mediator, a ferrocene (Fc), is tethered to the AFM-SECM probe via nanometer long, flexible polyethylene glycol (PEG) chains. It is demonstrated that the tip-attached ferrocene-labeled PEG chains effectively shuttle electrons between the tip and substrate, thus acting as molecular sensors probing the local electrochemical reactivity of a planar substrate. Moreover the Fc-PEGylated AFM-SECM probes can be used for tapping mode imaging, allowing simultaneous recording of electrochemical feedback current and of topography, with a vertical and a lateral resolution in the nanometer range. By imaging the naturally nanostructured surface of HOPG, we demonstrate that Tarm/AFM-SECM microscopy can be used to probe the reactivity of nanometer-sized active sites on surfaces. This new type of SECM microscopy, being, by design, free of the diffusional constraints of classical SECM, is expected to, in principle, enable functional imaging of redox nanosystems such as individual redox enzyme molecules.