Electron-transfer mediation on poly-aryl dendrimer-modified electrodes

Electrochemical properties of a dendrimer-modified electrode that was prepared by immobilization of ferrocenyl-terminated dendrimers on a poly-phenyl acetate anchoring layer were investigated in CH₂Cl₂. The anchoring layer was made by electro-grafting of the corresponding diazonium salt on a glassy carbon surface. The method allows the fabrication of a robust interface where the properties of the dendrimers are well-preserved. Moreover, the control of the layer properties as the permeation of molecules from the solution to the surface could be tuned up from only limited to totally blocked through the electrochemical conditions used during the electro-grafting of the anchoring layer. Detailed investigations performed with cyclic voltammetry and on different types of layers show that the modified electrode catalyses the oxidation of redox substrates. The process depends on the standard potential of the redox couple compared to that of the adsorbed dendrimer molecules. Experiments indicate that the electron exchange with molecules in solution takes place mainly at the dendrimer film–solution interface as the dendrimers inside the film permit the charge-transfer through the modified film to the carbon substrate. The interest of using robust electrode dendrimer relies on the possibility of large structural variations allowing the careful introduction of specific properties in the layer.     

Optimized preparation and scanning electrochemical microscopy analysis in feedback mode of glucose oxidase layers grafted onto conducting carbon surfaces

An optimized immobilization procedure based on the electroreduction of aryldiazonium salt followed by covalent attachment of a cross-linked hydrogel was used to graft glucose oxidase on a carbon surface. Scanning electrochemical microscopy (SECM) and cyclic voltammetry were used to follow the construction steps of the modified electrode. By adjusting the compactness of the layer through the electrografting reaction, the penetration of the mediator through the layer can be controlled to allow the monitoring of the enzymatic activity by both cyclic voltammetry and SECM in feedback mode. The enzymatic activity of the film is finally characterized by SECM.     

Quantification of photoelectrogenerated hydroxyl radical on TiO(2) by surface interrogation scanning electrochemical microscopy

The surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was used for the detection and quantification of adsorbed hydroxyl radical ˙OH((ads)) generated photoelectrochemically at the surface of a nanostructured TiO(2) substrate electrode. In this transient technique, a SECM tip is used to generate in situ a titrant from a reversible redox pair that reacts with the adsorbed species at the substrate. This reaction produces an SECM feedback response from which the amount of adsorbate and its decay kinetics can be obtained. The redox pair IrCl(6)(2-/3-) offered a reactive, selective and stable surface interrogation agent under the strongly oxidizing conditions of the photoelectrochemical cell. A typical ˙OH((ads)) saturation coverage of 338 μC cm(-2) was found in our nanostructured samples by its reduction with the electrogenerated IrCl(6)(3-). The decay kinetics of ˙OH((ads)) by dimerization to produce H(2)O(2) were studied through the time dependence of the SI-SECM signal and the surface dimerization rate constant was found to be ～k(OH) = 2.2 × 10(3) mol(-1) m(2) s(-1). A radical scavenger, such as methanol, competitively consumes ˙OH((ads)) and yields a shorter SI-SECM transient, where a pseudo-first order rate analysis at 2 M methanol yields a decay constant of k'(MeOH) ～ 1 s(-1).     

Atomic contacts via electrochemistry in water/cyclodextrin media: a step toward protected atomic contacts

Atomic contacts are nanoscience devices proposed for applications such as single-atom switches in nanoelectronic circuits or one-molecule sensing devices. The conductance of such contacts varies in a stepwise fashion with a tendency to quantize near integer multiples of the conductance quantum (G0) but can also deviate significantly from integer values upon molecular adsorption. However, for sensing applications it is first necessary to coat the contact permanently to avoid nonspecific adsorption. Here, we show that marked differences are observed between atomic contacts generated in water, and in water/beta-CD. In this latter medium, atomic contacts with unusual properties can be generated. They have below 1 G0 conductance, low conductance fluctuation with time, and appear to be protected or partially protected from salicylate external molecular probes. Such contacts are not obtained in water, in water/glucose, or when beta-CD is added after 1 G0 contacts have been generated in water. These results indicate specific adsorption of beta-cyclodextrin on the atomic contacts and are compatible with the formation of encapsulated atomic contacts. However, direct independent structural evidence is still needed to confirm or infirm this interpretation.     

Covalent assembly and micropatterning of functionalized multiwalled carbon nanotubes to monolayer-modified Si(111) surfaces

Multiwalled carbon nanotubes (MWNTs) covalently bound to monocrystalline p-type Si(111) surfaces have been prepared by attaching soluble amine-functionalized MWNTs onto a preassembled undecanoic acid monolayer using carbodiimide coupling. SEM analysis of these functionalized surfaces shows that the bound MWNTs are parallel to the surface rather than perpendicular. The voltammetric and electrochemical impedance spectroscopy measurements reveal that the electron transfer at the MWNT-modified surface is faster than that observed at a MWNT-free alkyl monolayer. We have also demonstrated that it is possible to prepare MWNT micropatterns using this surface amidation reaction and a "reagentless" UV photolithography technique. Following this approach, MWNT patterns surrounded by n-dodecyl areas have been produced and the local electrochemical properties of these micropatterned surfaces have been examined by scanning electrochemical microscopy. In particular, it is demonstrated that the MWNT patterns allow a faster charge transfer which is consistent with the results obtained for the uniformly modified surfaces.     

Variations of diffusion coefficients of redox active molecules in room temperature ionic liquids upon electron transfer

In ionic liquids, the diffusion coefficients of a redox couple vary considerably between the neutral and radical ion forms of the molecule. For a reduction, the inequality of the diffusion coefficients is characterized by the ratio gamma = D(red)/D(ox), where D(red) and D(ox) are the diffusion coefficients of the electrogenerated radical anion and of the corresponding neutral molecule, respectively. In this work, measurements of gamma have been performed by scanning electrochemical microscopy (SECM) in transient feedback mode, in three different room temperature ionic liquids (RTILs) sharing the same anion and with a series of nitro-derivative compounds taken as a test family. The smallest gamma ratios were determined in an imidazolium-based RTIL and with the charge of the radical anion localized on the nitro group. Conversely, gamma tends to unity when the radical anion is fully delocalized or when the nitro group is sterically protected by bulky substituents. The gamma ratios, standard potentials of the redox couple measured in RTILs, and those observed in a classical organic solvent were compared for the investigated family of compounds. The stabilization energies approximately follow the gamma ratios in a given RTIL but change considerably between ionic liquids with the nature of the cation.