Spontaneous adsorbed layers of 4-nitrobenzenediazonium salt on gold and glassy carbon: Local characterization by SECM and electron-transfer kinetics evaluation

Quantitative Feedback Referencing for Improved Kinetic Fitting of Scanning Electrochemical Microscopy Measurements

Scanning electrochemical microscopy (SECM) has matured as a technique for studying local electrochemical processes. The feedback mode is most commonly used for extracting quantitative kinetic information. However, approaching individual regions of interest, as is commonly done, does not take full advantage of the spatial resolution that SECM has to offer. Moreover, fitting of experimental approach curves remains highly subjective due to the manner of estimating the tip-to-substrate distance. We address these issues using negative or positive feedback currents as a reference to calculate the tip-to-substrate distance directly for quantitative kinetic fitting of approach curves and line profiles. The method was first evaluated by fitting simulated data and then tested experimentally by resolving negative feedback and intermediate kinetics behavior in a spatially controlled fashion using (i) a flat, binary substrate composed of Au and SiO₂ segments and (ii) a dual-mediator system for live-cell measurements. The methodology developed herein, named quantitative feedback referencing (QFR), improves fitting accuracy, removes fitting subjectivity, and avoids substrate-microelectrode contact.

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.

Scanning Electrochemical Microscopy Study of Permeability of a Thiolated Aryl Multilayer and Imaging of Single Nanocubes Anchored to It

Electroreduction of diazonium salts is a widely used technique for grafting organic films on various surfaces. In this paper, scanning electrochemical microscopy (SECM) was used for high-resolution characterization of a thiolated aryl multilayer film obtained by electrografting of thiophenol diazonium on highly ordered pyrolytic graphite (HOPG). The blocking properties of the film were evaluated, and the origins of incomplete surface passivation were elucidated by comparing current-distance curves and surface reactivity maps obtained with nanometer- and micrometer-sized tips. In this way, one can distinguish between different pathways of charge transport in the film, e.g., pinhole defects versus rate-limiting charge transfer through the film. Pd nanocubes were anchored to the film by thiol groups and imaged by SECM. The applicability of SECM to in situ visualization of the geometry of non-spherical nanoparticles has been demonstrated.

Local surface modification via confined electrochemical deposition with FluidFM

Hollow AFM cantilevers enable local electroplating and grafting followed by the in situ imaging of the created surface patterns.

X-RAY PHOTOELECTRON SPECTROSCOPY AND SCANNING ELECTROCHEMICAL MICROSCOPY STUDIES OF BRANCHED MULTIWALLED CARBON NANOTUBE PAPER MODIFIED BY ELECTROCHEMICAL GRAFTING AND CLICK CHEMISTRY

Modification of nanomaterials through electrochemical grafting is a useful approach to introduce linking groups on to the surface of these structures. This work shows the possibility of applying electrochemical grafting to branched multiwalled carbon nanotube paper with an electrical resistance of 0.1 ohm-cm, and subsequent reaction of the grafted 4-azidobenzenediazonium with ethynylferrocene through the Sharpless click chemistry reaction. A comparison is made between this paper electrode and adsorbed single-walled carbon nanotubes on a glassy carbon electrode, with electrochemistry, X-ray photoelectron spectroscopy and scanning electrochemical microscopy used for characterization.

Elucidation of the mechanism of redox grafting of diazotated anthraquinone

Redox grafting of aryldiazonium salts containing redox units may be used to form exceptionally thick covalently attached conducting films, even in the micrometers range, in a controlled manner on glassy carbon and gold substrates. With the objective to investigate the mechanism of this process in detail, 1-anthraquinone (AQ) redox units were immobilized on these substrates by electroreduction of 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate. Electrochemical quartz crystal microbalance was employed to follow the grafting process during a cyclic voltammetric sweep by recording the frequency change. The redox grafting is shown to have two mass gain regions/phases: an irreversible one due to the addition of AQ units to the substrate/film and a reversible one due to the association of cations from the supporting electrolyte with the AQ radical anions formed during the sweeping process. Scanning electrochemical microscopy was used to study the relationship between the conductivity of the film and the charging level of the AQ redox units in the grafted film. For that purpose, approach curves were recorded at a platinum ultramicroelectrode for AQ-containing films on gold and glassy carbon surfaces using the ferro/ferricyanide redox system as redox probe. It is concluded that the film growth has its origin in electron transfer processes occurring through the layer mediated by the redox moieties embedded in the organic film.