Unraveling the Link between Catalytic Activity and Agglomeration State with Scanning Electrochemical Microscopy and Atomic Force Microscopy.

In this article, we set up a methodology to investigate the relationship between the catalytic activity and the agglomeration state of platinum group metal-free ORR catalysts. To this end, we have developed a statistical approach based on scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM). Two catalysts are investigated at very low loadings in order to access their intrinsic activity. Differences in terms of dispersion, stability of the inks, and adherence on the substrate are observed, highlighting the importance of measuring the exact amount and agglomeration state of the materials under study. The agglomeration state of the deposits measured by AFM explains the differences in activity measured by SECM. The performances of the catalysts are compared, and the contributions of the intrinsic activity and the agglomeration state are identified. This work paves the way toward various applications ranging from the benchmarking of new catalysts to the optimization of an ink formulation, for ORR and beyond.

Electronic Transport of MoS2 Monolayered Flakes Investigated by Scanning Electrochemical Microscopy.

The amazing properties of 2D materials are envisioned to revolutionize several domains such as flexible electronics, electrocatalysis, or biosensing. Herein we introduce scanning electrochemical microscopy (SECM) as a tool to investigate molybdenum disulfide in a straightforward fashion, providing localized information regarding the electronic transport within chemical vapor deposition (CVD)-grown crystalline MoS2 single layers having micrometric sizes. Our investigations show that within flakes assemblies some flakes are well electrically interconnected, with no detectable contact resistance, whereas others are not electrically connected at all, independent of the size of the physical contact between them. Overall, the work shows how the complex electronic behavior of MoS2 flake assemblies (semiconducting nature, contact quality between flakes) can be investigated with SECM.

Carbon nanotube-templated synthesis of covalent porphyrin network for oxygen reduction reaction.

The development of innovative techniques for the functionalization of carbon nanotubes that preserve their exceptional quality, while robustly enriching their properties, is a central issue for their integration in applications. In this work, we describe the formation of a covalent network of porphyrins around MWNT surfaces. The approach is based on the adsorption of cobalt(II) meso-tetraethynylporphyrins on the nanotube sidewalls followed by the dimerization of the triple bonds via Hay-coupling; during the reaction, the nanotube acts as a template for the formation of the polymeric layer. The material shows an increased stability resulting from the cooperative effect of the multiple π-stacking interactions between the porphyrins and the nanotube and by the covalent links between the porphyrins. The nanotube hybrids were fully characterized and tested as the supported catalyst for the oxygen reduction reaction (ORR) in a series of electrochemical measurements under acidic conditions. Compared to similar systems in which monomeric porphyrins are simply physisorbed, MWNT-CoP hybrids showed a higher ORR activity associated with a number of exchanged electrons close to four, corresponding to the complete reduction of oxygen into water.

Localized reduction of graphene oxide by electrogenerated naphthalene radical anions and subsequent diazonium electrografting.

Herein, we describe a new localized functionalization method of graphene oxide (GO) deposited on a silicon oxide surface. The functionalization starts with the reduction of GO by electrogenerated naphthalene radical anions. The source of reducers is a microelectrode moving close to the substrate in a typical scanning electrochemical microscopy (SECM) configuration. Then, the recovery of electronic conductivity upon reduction enables the selective electrochemical functionalization of the patterns. The illustrative example is the electrografting of reduced-GO with a diazonium salt bearing a protonated amino group that can further immobilize gold nanoparticles by simple immersion. This study opens new routes for the construction of multifunctional patterned surfaces.

Investigating catalase activity through hydrogen peroxide decomposition by bacteria biofilms in real time using scanning electrochemical microscopy.

Catalase activity through hydrogen peroxide decomposition in a 1 mM bulk solution above Vibrio fischeri (γ-Protebacteria-Vibrionaceae) bacterial biofilms of either symbiotic or free-living strains was studied in real time by scanning electrochemical microscopy (SECM). The catalase activity, in units of micromoles hydrogen peroxide decomposed per minute over a period of 348 s, was found to vary with incubation time of each biofilm in correlation with the corresponding growth curve of bacteria in liquid culture. Average catalase activity for the same incubation times ranging from 1 to 12 h was found to be 0.28 ± 0.07 µmol H2O2/min for the symbiotic biofilms and 0.31 ± 0.07 µmol H2O2/min for the free-living biofilms, suggesting similar catalase activity. Calculations based on Comsol Multiphysics simulations in fitting experimental biofilm data indicated that approximately (3 ± 1) × 10(6) molecules of hydrogen peroxide were decomposed by a single bacterium per second, signifying the presence of a highly active catalase. A 2-fold enhancement in catalase activity was found for both free-living and symbiotic biofilms in response to external hydrogen peroxide concentrations as low as 1 nM in the growth media, implying a similar mechanism in responding to oxidative stress.

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.

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.

Detection of hydrogen peroxide produced during the oxygen reduction reaction at self-assembled thiol-porphyrin monolayers on gold using SECM and nanoelectrodes.

Porphyrin molecules were immobilized on polycrystalline gold and glassy carbon by coordinating cobalt(II) 5,10,15,20-tetraphenyl-21H,23H-porphine to a 4-aminothiophenol self-assembled monolayer. The resulting electrocatalytic activity of the metalloporphyrin-modified substrates with regard to the oxygen reduction reaction was characterized by means of cyclic voltammetry and scanning electrochemical microscopy (SECM) using nanoelectrodes of well-defined geometry. From substrate generation tip collection (SG-TC) mode SECM measurements performed under steady-state conditions and at different applied substrate potentials, it is possible to extract kinetic information relevant to electrocatalyst substrates such as metalloporphyrin-modified gold and glassy-carbon electrodes. Such an approach allows for the isolation of the unique contribution of the electrocatalyst to the oxygen reduction reaction and peroxide formation.

Ideal optical contrast for 2D material observation using bi-layer antireflection absorbing substrates.

The capability to observe 2D materials with optical microscopy techniques is of central importance in the development of the field and is a driving force for the assembly and study of 2D material van der Waals heterostructures. Such an observation of ultrathin materials usually benefits from antireflection conditions associated with the choice of a particular substrate geometry. The most common configuration uses a transparent oxide layer with a thickness minimizing light reflection at the air/substrate interface when light travels from air to the substrate. Backside Absorbing Layer Microscopy (BALM) is a newly proposed configuration in which light travels from glass to air (or another medium such as water or a solvent) and the antireflection layer is a light-absorbing material (typically a metal). We recently showed that this technique produces images of 2D materials with unprecedented contrast and can be ideally coupled to chemical and electrochemical experiments. Here, we show that contrast can be optimal using double-layer antireflection coatings. By following in situ and with sub-nm precision the controlled deposition of molecules, we notably establish precisely the ideal observation conditions for graphene oxide monolayers which represent one of the most challenging 2D material cases in terms of transparency and thickness. We also provide guidelines for the selection of antireflection coatings applicable to a large variety of nanomaterials. This work strengthens the potential of BALM as a generic, powerful and versatile technique for the study of molecular-scale materials and phenomena.

Bifunctional coatings: coupling an organic adhesion promoter with an anticorrosion inorganic layer.

In this work, a multifunctional non-toxic chromium free treatment is proposed. Hexavalent chromium, largely used for anticorrosion surface treatments of aluminum alloys in aeronautics, will soon be completely banned due to its high toxicity (European REACH regulation) and new solutions are required. Here, in a first step, a polymeric film was grafted at the aluminum surface by the surface induced reduction of a diazonium salt. In a second step, the grafted surface was submitted to an anodization treatment, forming a thick aluminum oxide layer protecting the underlying metal against corrosion. No change in the organic coating was detected after the second step of the process. This leads to a multilayer coating, which provides competitive results regarding both the adhesion of paint and corrosion protection.

Backside absorbing layer microscopy: Watching graphene chemistry.

The rapid rise of two-dimensional nanomaterials implies the development of new versatile, high-resolution visualization and placement techniques. For example, a single graphene layer becomes observable on Si/SiO2 substrates by reflected light under optical microscopy because of interference effects when the thickness of silicon oxide is optimized. However, differentiating monolayers from bilayers remains challenging, and advanced techniques, such as Raman mapping, atomic force microscopy (AFM), or scanning electron microscopy (SEM) are more suitable to observe graphene monolayers. The first two techniques are slow, and the third is operated in vacuum; hence, in all cases, real-time experiments including notably chemical modifications are not accessible. The development of optical microscopy techniques that combine the speed, large area, and high contrast of SEM with the topological information of AFM is therefore highly desirable. We introduce a new widefield optical microscopy technique based on the use of previously unknown antireflection and absorbing (ARA) layers that yield ultrahigh contrast reflection imaging of monolayers. The BALM (backside absorbing layer microscopy) technique can achieve the subnanometer-scale vertical resolution, large area, and real-time imaging. Moreover, the inverted optical microscope geometry allows its easy implementation and combination with other techniques. We notably demonstrate the potentiality of BALM by in operando imaging chemical modifications of graphene oxide. The technique can be applied to the deposition, observation, and modification of any nanometer-thick materials.

Enhancing the Performances of P3HT:PCBM-MoS3-Based H2-Evolving Photocathodes with Interfacial Layers.

Organic semiconductors have great potential for producing hydrogen in a durable and economically viable manner because they rely on readily available materials and can be solution-processed over large areas. With the objective of building efficient hybrid organic-inorganic photoelectrochemical cells, we combined a noble-metal-free and solution-processable catalyst for proton reduction, MoS3, and a poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ). Different interfacial layers were investigated to improve the charge transfer between P3HT:PCBM and MoS3. Metallic Al/Ti interfacial layers led to an increase of the photocurrent by up to 8 mA cm(-2) at reversible hydrogen electrode (RHE) potential with a 0.6 V anodic shift of the H2 evolution reaction onset potential, a value close to the open-circuit potential of the P3HT:PCBM solar cell. A 50-nm-thick C60 layer also works as an interfacial layer, with a current density reaching 1 mA cm(-2) at the RHE potential. Moreover, two recently highlighted1 figures-of-merit, measuring the ratio of power saved, Φsaved,ideal and Φsaved,NPAC, were evaluated and discussed to compare the performances of various photocathodes assessed in a three-electrode configuration. Φsaved,ideal and Φsaved,NPAC use the RHE and a nonphotoactive electrode with an identical catalyst as the dark electrode, respectively. They provide different information especially for differentiation of the roles of the photogenerating layer and catalyst. The best results were obtained with the Al/Ti metallic interlayer, with Φsaved,ideal and Φsaved,NPAC reaching 0.64% and 2.05%, respectively.

Versatile Wafer-Scale Technique for the Formation of Ultrasmooth and Thickness-Controlled Graphene Oxide Films Based on Very Large Flakes.

We present a new strategy to form thickness-adjusted and ultrasmooth films of very large and unwrinkled graphene oxide (GO) flakes through the transfer of both hemispherical and vertical water films stabilized by surfactants. With its versatility in terms of substrate type (including flexible organic substrates) and in terms of flake density (from isolated flakes to continuous and multilayer films), this wafer-scale assembly technique is adapted to a broad range of experiments involving GO and rGO (reduced graphene oxide). We illustrate its use through the evaluation of transparent rGO electrodes.

New Insights into the Electronic Transport of Reduced Graphene Oxide Using Scanning Electrochemical Microscopy.

The present work investigates the electronic conduction of reduced graphene oxide flakes and the coupling between flakes through a combined SECM (scanning electrochemical microscopy), AFM, and SEM analysis. Images of individual and interconnected flakes directly reveal the signature of the contact resistance between flakes in a noncontact and substrate-independent way. Quantitative evaluation of the parameters is achieved with the support of numerical simulations to interpret the experimental results. The interflakes contact resistance importantly impacts the transport of electrons, which can be anticipated as a key parameter in r-GO-based materials used in fuel cells, lithium batteries, supercapacitors, and organic electronic devices.

Electrochemical nanoprobes for single-cell analysis.

The measurement of key molecules in individual cells with minimal disruption to the biological milieu is the next frontier in single-cell analyses. Nanoscale devices are ideal analytical tools because of their small size and their potential for high spatial and temporal resolution recordings. Here, we report the fabrication of disk-shaped carbon nanoelectrodes whose radius can be precisely tuned within the range 5-200 nm. The functionalization of the nanoelectrode with platinum allowed the monitoring of oxygen consumption outside and inside a brain slice. Furthermore, we show that nanoelectrodes of this type can be used to impale individual cells to perform electrochemical measurements within the cell with minimal disruption to cell function. These nanoelectrodes can be fabricated combined with scanning ion conductance microscopy probes, which should allow high resolution electrochemical mapping of species on or in living cells.

Assessing multidrug resistance protein 1-mediated function in cancer cell multidrug resistance by scanning electrochemical microscopy and flow cytometry.

Cancer cell multidrug resistance is a molecular signature that highly influences the outcome of chemotherapy treatment and for which there is currently no robust method to monitor in vitro its activity. Herein, we demonstrate that ferrocenemethanol (FcCH(2)OH) and its oxidized form ([FcCH(2)OH](+)) affect the redox state of cancer cells. Specifically, the interaction of FcCH(2)OH with the glutathione couple (GSH/GSSG) is shown in human adenocarcinoma cervical cancer cells HeLa and a multidrug resistant variant overexpressing the multidrug resistant associated protein 1 (MRP1) using bioanalytical techniques, such as flow cytometry and fluorescence microscopy. It is further demonstrated that the differential response to FcCH(2)OH in multidrug-resistant cells is in part due to MRP1's unspecific efflux. Scanning electrochemical microscopy confirmed the interaction between FcCH(2)OH and the cells, and the differential response was observed to depend on MRP1 expression. This newly established relation between FcCH(2)OH/[FcCH(2)OH](+), GSH/GSSG and multidrug resistance in human cancer cells enables than the acquisition of scanning electrochemical microscopy images.