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

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 highlighted figures-of-merit, measuring the ratio of power saved, Φ_saved,ideal and Φ_saved,NPAC, were evaluated and discussed to compare the performances of various photocathodes assessed in a three-electrode configuration. Φ_saved,ideal and Φ_saved,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 Φ_saved,ideal and Φ_saved,NPAC reaching 0.64% and 2.05%, respectively

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 <i>Vibrio fischeri</i> (γ-<i>Protebacteria-Vibrionaceae</i>) 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) × 10⁶ 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

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) <i>meso</i>-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

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