Contactless surface conductivity mapping of graphene oxide thin films deposited on glass with scanning electrochemical microscopy.

The present article introduces a rapid, very sensitive, contactless method to measure the local surface conductivity with Scanning Electrochemical Microscopy (SECM) and obtain conductivity maps of heterogeneous substrates. It is demonstrated through the study of Graphene Oxide (GO) thin films deposited on glass. The adopted substrate preparation method leads to conductivity disparities randomly distributed over approximately 100 µm large zones. Data interpretation is based on an equation system with the dimensionless conductivity as the only unknown parameter. A detailed prospection provides a consistent theoretical framework for the reliable quantification of the conductivity of GO with SECM. Finally, an analytical approximation of the conductivity as a function of the feedback current is proposed, making any further interpretation procedure straightforward, as it does not require iterative numerical simulations any more. The present work thus provides not only valuable information on the kinetics of GO reduction in mild conditions but also a general and simplified interpretation framework that can be extended to the quantitative conductivity mapping of other types of substrates.

Accurate and simplified consideration of the probe geometrical defaults in scanning electrochemical microscopy: theoretical and experimental investigations.

Fabrication of scanning electrochemical microscopy (SECM) tips cannot always guarantee a perfect disk geometry. In the present work, the impact of these defaults is investigated both theoretically and experimentally. The situations where these defaults can accurately be taken into account by considering that the probe behaves like a microdisk with effective geometric parameters are determined. In these situations, the quantitative analysis of the experimental results is greatly simplified. The study also proposes expressions to evaluate the apparent microdisk parameters from a picture of the probe.

Surface reactivity from electrochemical lithography: illustration in the steady-state reductive etching of perfluorinated surfaces.

The scanning electrochemical microscope (SECM) in the lithographic mode is used to assess quantitatively, from both theoretical and experimental points of view, the kinetics of irreversible transformation of electroactive molecular moieties immobilized on a surface as self-assembled monolayers (SAMs). The SECM tip allows the generation of an etchant that transforms the surface locally and irreversibly. The resulting surface patterning is detectable by different surface analyses. The quantification of the surface transformation kinetics is deduced from the evolution of the pattern dimensions with the etching time. The special case of slow etching kinetics is presented; it is predicted that the pattern evolution follows the expansion of the etchant at the substrate surface. The case of a chemically unstable etchant is considered. The model is then tested by inspecting the slow reductive patterning of a perfluorinated SAM. Good agreement is found with different independent SECM interrogation modes, depending on the insulating or conducting nature of the covered substrate. The surface transformation measurements are also compared to the reduction of solutions of perfluoroalkanes. The three-orders-of-magnitude-slower electron transfer observed at the immobilized molecules likely describes the large reorganization associated with the generation of a perfluoroalkyl-centered radical anion.

Synchronization of Dictyostelium discoideum adhesion and spreading using electrostatic forces.

Synchronization of cell spreading is valuable for the study of molecular events involved in the formation of adhesive contacts with the substrate. At a low ionic concentration (0.17 mM) Dictyostelium discoideum cells levitate over negatively charged surfaces due to electrostatic repulsion. First, a two-chamber device, divided by a porous membrane, allows to quickly increase the ionic concentration around the levitating cells. In this way, a good synchronization was obtained, the onsets of cell spreading being separated by less than 5 s. Secondly applying a high potential pulse (2.5 V/Ref, 0.1s) to an Indium Tin Oxide surface attracts the cells toward the surface where they synchronously spread as monitored by LimE(Deltacoil)-GFP. During spreading, actin polymerizes in series of active spots. On average, the first spot appears 8-11s after the electric pulse and the next ones appear regularly, separated by about 10s. Synchronized actin-polymerization activity continues for 40s. Using an electric pulse to control the exact time point at which cells contact the surface has allowed for the first time to quantify the cellular response time for actin polymerization. Electrochemical synchronization is therefore a valuable tool to study intracellular responses to contact.

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.