Scanning electrochemical microscopy (SECM) is used for studying the intrinsic photo-electrochemical properties of CdSe/CdS quantum rods. They are deposited on a transparent and non-conductive glass plate and investigated by SECM in feedback and generator–collector modes using a series of redox mediators. The method allows the interrogation of the quantum rods under illumination without the interference of the substrate, notably that due to the electron photo-ejection from the substrate, a process that is inherent to any polarized electrode material. Beside the methodological demonstration that could easily be extended to the investigations of the photo-redox properties of nanoparticles, studies highlight the strong reductive properties of quantum rods under illumination.

Charge transport through an insulating layer was probed using ferrocenyl-terminated dendrimers and scanning electrochemical microscopy. Experiments show that the passage through the layer is considerably enhanced when the transferred charges are brought globally to the surface by the ferrocenyl dendrimer instead of a single ferrocene molecule. This result shows that charge tunneling through an insulator could be promoted by a purely molecular nano-object.

Localized “electroclick” was achieved on azido-terminated self-assembled monolayers using Scanning Electrochemical Microscopy (SECM) in feedback mode, in which the substrate is not electrically connected (unbiased conditions). The method allows both the local immobilization of diverse functional moieties and the monitoring of each modification step at a micrometer scale. Conditions of the “click” coupling reaction were optimized especially to avoid the deposit of metallic copper by the choice of a specific ligand to stabilize the Cu(I) species. The catalytic efficiency in localized “electroclick” reaction of Cu(II)TMPA (TMPA: tris(2-pyridylmethyl)amine) as the “click” catalyst was compared with a derivative containing an alkyne group Cu(II)6eTMPA, the same molecule playing the role of the catalyst and the substrate. Evidences for surface self-catalysis propagation are demonstrated through SECM imaging showing a random 2D progression of the catalytic modification.

The electrogeneration of aryl radicals from protected diazonium salts combined with protection–deprotection steps was evaluated to design functional monolayers on carbon substrates with a well-controlled organization at the nanometric scale. The structure of the obtained monolayer is adjusted by varying the size of the protecting group that is introduced on the precursors (trimethylsilyl, triethylsilyl, and tri(isopropyl)silyl were tested in the present study). After deprotection, a robust ethynylaryl monolayer is obtained whatever the substituent that serves as a platform to attach other functional groups by a specific “click chemistry” coupling step. Electrochemical and structural analyses show that the organization of the attached monolayer is totally governed by the size of the protecting group that leaves a footprint after removal but maintains a total availability of the immobilized functional groups. Properties of the monolayer (charge transfer, permeation of molecules through the layer, density of functional groups) were examined in combination with the performances for postfunctionalization taken with an alkyl-ferrocene derivative as an example of the immobilized species.Keywords: carbon materials surfaces; diazonium redox chemistry; modified organized surfaces;

The use of a chemically irreversible redox probe in scanning electrochemical microscopy (SECM) was evaluated for the determination of the absolute tip–substrate distance. This data is required for a quantitative use of the method in the analysis of functional surfaces with an unknown redox response. Associated with the relevant model curves, the electrochemical response allows an easy positioning of the tip versus the substrate that is independent of the nature of the materials under investigation. The irreversible oxidation of polyaromatic compounds was found to be well adapted for such investigations in organic media. Anthracene oxidation in acetonitrile was chosen as a demonstrative example for evaluating the errors and limits of the procedure. Interest in the procedure was exemplified for the local investigations of surfaces modified by redox entities. This permits discrimination between the different processes occurring at the sample surface as the permeability of the probe through the layer or the charge transfer pathways. It was possible to observe small differences with simple kinetic models (irreversible charge transfer) that are related to permeation: charge transport steps through a permeable redox layer.

•Electrochemical preparation of azobenzene monolayers on carbon materials.•Electrografting of sylil-protected aryldiazonium salts.•Reversible variation of permeation on a modified carbon surface.•Electrochemical responses of redox probes on a blocked electrode.Nanostructured monolayer of azobenzene derivatives was prepared on carbon materials. The method is based on the electrochemical reduction of a specific silyl-protected aryl diazonium salt followed by a deprotection step and a click chemistry coupling reaction. This procedure leads to a robust modification of the surface where molecular entities are placed in a “frozen” structuration thanks to the covalent nature of the modification. The photo-switchable properties of the surface were examined by electrochemistry using different redox probes. UV–visible light irradiation provides an easy way to reversibly modulate the permeability in a monolayer film grafted onto carbon materials.

The influences of the size, position, nature of different silyl protecting groups on resulting covalently bound films obtained by electroreduction of ethynyl-aryldiazonium salts were investigated in view of preparing robust and functional interfaces with well-controlled structure and properties. Three different protecting groups where the active function was introduced on one meta position of the aryl ring were considered: trimethylsilyl (TMS), triethylsilyl (TES) and tri(isopropyl)silyl (TIPS). As a general tendency, ultra thin and robust active functional layers were obtained on carbon substrates after deprotection (loss of the silyl group) by treatment with nBu4BF4. Multilayer formation occurs when the protective group is just a TMS but becomes negligible when TIPS is used. Interestingly, the structure of the final layer keeps a memory of the silyl group used during the electrografting that permits a tweaking of the blocking properties and of the layer thicknesses. As a remarkable feature, all layers present large density of active alcyne terminations that remain totally available for further “click chemistry” coupling. These possibilities were tested with a redox label (azidomethylferrocene).Highlights► Electroreduction of silyl meta protected ethynyl aryldiazonium salts. ► Ultra thin robust active functional layers are obtained on carbon substrates. ► Large density of active alcyne terminations available for click chemistry coupling. ► Functional monolayers were built on different types of carbon substrates.

Substituted vinylogous tetrathiafulvalenes (TTFVs) containing two freely moving polyoxyethyl chains were prepared. Investigations of their redox behaviors in organic solvent show that these TTFV could efficiently complex metallic dications such as Pb2+ or Ba2+, leading to considerable modifications of their electrochemical response. As main feature, the molecule senses the association between the TTFV core and the metallic dication through a modification of the molecular motion triggered by the electron transfer. The complexation creates a link between the two parts of the TTFV core, causing considerable changes in the nature of the molecular motion. The resulting behavior is totally unusual as the 2-positively charged TTFV2+ appears to present the highest association constants with the metallic dication.

Multielectronic O2 reduction reactions (ORR) at Pt surface (and at Au surface for comparison purpose) were examined both in water and in organic solvents using a strategy based on radical footprinting and scanning electrochemical microscopy (SECM). Experiments reveal a considerable and undocumented production of OH radicals when O2 is reduced at a Pt electrode. These observations imply that the generally admitted description of ORR as simple competitive pathways between 2-electron (O2 to H2O2) and 4-electron (O2 to H2O) reductions is often inadequate and demonstrate the occurrence of another 3-electron pathway (O2 to OH radical). This behavior is especially observable at neutral and basic pH’s in water and in organic solvents like dimethylformamide or dichloromethane. In view of the high reactivity of OH radical versus organic or living materials, this observation could have important consequences in several practical situations (fuel cells, sensors, etc.) as far as O2 reduction is concerned. This also appears as a simple way to locally produce highly reactive species as exemplified in the present work by the micropatterning of organic surfaces.

The use of catechols, and more specifically of dopamine, as a specific redox mediator for scanning electrochemical microscopy (SECM) investigations was evaluated in the challenging situation of an ultrathin layer deposited on a conductive substrate (carbon materials). Experiments show that dopamine is a well-adapted redox system for SECM in feedback mode and in unbiased conditions. Used as a redox mediator, catechol permits the investigations of modified surfaces without an electrical connection of the sample thanks to fast charge transfer kinetics but with a surface selectivity that does not exist in classical outer-sphere redox mediators. The interest of catechol in SECM as a sensitive redox mediator is exemplified by monitoring several modification steps of an ultrathin (<1 nm) hierarchically porous organic monolayer deposited on carbon substrates. For quantitative analysis, the SECM approach curves using dopamine could simply be characterized with an irreversible electron transfer kinetics model in a large range of pH.

The reactivity of electrogenerated benzyl radicals at carbon surfaces was examined through the cathodic reduction of the corresponding bromide derivatives. 4-Nitrobenzyl bromide and benzyl bromide were reduced in N,N-dimethylformamide (DMF) on highly ordered pyrolytic graphite (HOPG) surfaces. Electroproduced films were examined using electrochemistry, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Experiments show the formation of strongly adherent deposits and the occurrence of electrografting processes. They are based on radical generation and the reaction of the radical with the substrate. As expected, the thickness of the organic film increases with deposition time but the deposit displays a lower compactness than previously reported for the electroreduction of aryl diazonium salts. Interestingly for benzyl derivatives, the reduction potential required for the electrografting could be rendered much more positive by simply using an iodide-type supporting electrolyte.

The electroreduction of functionalized aryldiazonium salts combined with a protection–deprotection method was evaluated for the fabrication of organized mixed layers covalently bound onto carbon substrates. The first modification consists of the grafting of a protected 4-((triisopropylsilyl)ethynyl)benzene layer onto the carbon surface on which the introduction of a second functional group is possible without altering the first grafted functional group. After deprotection, we obtained an ultrathin robust layer presenting high densities of both active ethynylbenzene groups (available for “click” chemistry) and the second functional group. The strategy was successfully demonstrated using azidomethylferrocene to react with ethynyl moieties in the binary film by “click” chemistry, and NO2-phenyl as the second functional group. Two possible modification pathways with different orderings of the various steps were considered to show the influence and importance of the protection–deprotection process on the final surface obtained. Using mild conditions for the grafting of the second layer maintains a concentration of active ethynyl groups similar to that obtained for a one-component monolayer while achieving a high surface concentration of the second modifier. Considering the wide range of functional aryldiazonium salts that could be electrodeposited onto carbon surfaces and the versatility and specificity of the “click” chemistry, this approach appears very promising for the preparation of mixed layers in well-controlled conditions without altering the reactivity of either functional group.

A surface sensitive to reactive oxygen species (ROS) was prepared by reduction of a diazonium salt on glassy carbon electrode followed by the chemical coupling of glutathione (GSH) playing the role of an antioxidant species. The presence of active GSH was characterized through spectroscopic studies and electrochemical analysis after labeling of the −SH group with ferrocene moieties. The specific reactivity of GSH vs ROS was evaluated with scanning electrochemical microscopy (SECM) using the reduction of O2 to superoxide, O2•–, near the GSH-modified surface. Approach curves show a considerable decrease of the blocking properties of the layer due to reaction of the immobilized GSH with O2•– and the passage of GSH to the glutathione disulfide (GSSG). The initial surface could be regenerated several times with no significant variations of its antioxidant capacity by simply using the biological system glutathione reductase (GR)/NADPH that reduces GSSG back to GSH. SECM imaging shows also the possibility of writing local and erasable micropatterns on the GSH surface by production of O2•– at the tip probe electrode.

The reactivities of different phenols and polyphenols versus superoxide ion (O2•−) were investigated as an easy-to-handle electrochemical method for evaluating antioxidant capacities. In view of this application, the O2/O2•− couple and associated reactions between O2•− and polyphenols (or phenols) were examined in an aprotic solvent [dimethylformamide (DMF)] by cyclic voltammetry. Comparisons based on simple criteria (reversibility of the O2 reduction in the presence of the phenolic compound, electron stoichiometry, or apparent kinetic constants) allow discriminations between the possible mechanistic pathways (acid−base or radical reaction type). The results highlight that the proton-transfer and radical-transfer pathways are both present for monophenols and polyphenols, with the relative contributions of the two pathways depending on the phenol structure. In agreement with the literature, polyphenols containing an o-diphenol ring (as in flavonoids) were found to present the highest reactivities.

A versatile method was used to prepare modified surfaces on which metallic silver nanoparticles are immobilized on an organic layer. The preparation method takes advantage, on one hand, of the activated reactivity of some alkyl halides with Ag−Pd alloys to produce metallic silver nanoparticles and, on the other hand, of the facile production of an anchoring polyphenyl acetate layer by the electrografting of substituted diazonium salts on carbon surfaces. Transport properties inside such modified layers were investigated by cyclic voltammetry, scanning electrochemical microscopy (SECM) in feedback mode, and conducting AFM imaging for characterizing the presence and nature of the conducting pathways. The modification of the blocking properties of the surface (or its conductivity) was found to vary to a large extent on the solvents used for surface examination (H2O, CH2Cl2, and DMF).

Electronic properties of electrogenerated Zn-porphyrin layers linked by an electroactive linker and immobilized on a semitransparent ITO electrode were investigated by steady-state SECM in unbiased conditions in view of the numerous possible applications of such surface. This SECM strategy took advantage of the variations of the charge transfer kinetics of the organic redox couple (the mediator used in SECM) on ITO surface with the standard potential of the mediator. After preliminary characterization of nonmodified ITO, analysis of the SECM approach curves recorded with a series of redox mediators allows the characterizations of both film permeability and charge transport inside the organic film in conditions close to a “real optoelectronic device”. Two types of porphyrin films were considered. In the first one, the film was produced by electropolymerization of a modified zinc-β-octaethylporphyrin in which the bipyridinium pendant substituent is first introduced. The second type of film was prepared directly from an in situ electropolymerization method in which the Zn porphyrin is simply oxidized in the presence of 4,4′-bipyridine. Experiments show the occurrence of efficient charge transport inside both films after initial reduction of the electroactive linker. However, the first preparation method leads to films with stronger blocking character versus organic molecules and higher charge injection rates.

Redox-active ferrocenyl-terminated dendrimers have been used to build robust and active interfaces in which the inherent properties of the dendrimers are preserved. The procedure is based on the preparation of a polyarylcarboxylate layer through the controlled reduction of aryldiazonium salts followed by strong adsorption of the dendrimers on the insulating layer. Scanning electrochemical microscopy was used to probe the transport properties as lateral charge diffusion. The experiments demonstrate that charge-transfer communication between the dendrimers inside the layer is fast and efficient, depending on the dendrimer generation.

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.

Transport properties of molecules dissolved in room-temperature ionic liquids are highly sensitive to the charge carried by the molecule because of complex ion−ion interactions that could be tuned by addition of a cosolvent. In this connection, the one-electron reduction of oxygen was used as a probe system for studying the effects of the addition of a cosolvent such as dimethylformamide (DMF) into a pure ionic liquid (triethylbutylammonium bis(trifluoromethylsulfonyl)imide) ([Et3BuN][NTf2]) on the diffusion of charged species versus neutral species. Experimental data about the diffusion coefficients of O2 (DO2) and O2•− (DO2•−) and their ratios (γ = DO2•−/DO2) were extracted using scanning electrochemical microscopy (SECM) in transient mode as a function of the DMF concentration. The ratio γ and both of the diffusion coefficients DO2•− and DO2 were found to increase exponentially with the DMF volume fractions following the same general tendency described for the viscosity. However, DO2•− varies on a much larger range than DO2 (around 1000 times more), and O2•− retains an almost “pure ionic” behavior for higher DMF fractions. All of these results support the occurrence of a sharp transformation in the bonding character of the RTIL cation upon addition of a molecular solvent, as predicted in recent theoretical simulations.

The reactivity of the superoxide anion versus a series of substituted phenols was investigated in a common ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm][TFSI]) and for comparison in dimethylformamide (+0.1 mol·L−1 of Bu4NPF6 as supporting electrolyte). On the whole, the mechanism of the reduction of O2 in the presence of the different phenols was found to be very similar in [BMIm][TFSI] and in DMF: A 2-electron mechanism involving a succession of electrochemical and protonation steps. These steps are accompanied by the production of the corresponding phenolate that was identified through its oxidation potential. The reactivities of the phenols were observed to slightly differ in the two media. A qualitative analysis of the voltammogram allows a classification of the reactivities of the superoxide as a function of the phenols. As previously found in organic solvents, the protonation of superoxide by phenol is an uphill reaction that is rendered possible thanks to a subsequent irreversible electron transfer. Its pKa is estimated to be around 4−5 units lower than that of unsubstituted phenol.

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.

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 γ = Dred/Dox, where Dred and Dox are the diffusion coefficients of the electrogenerated radical anion and of the corresponding neutral molecule, respectively. In this work, measurements of γ 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 γ ratios were determined in an imidazolium-based RTIL and with the charge of the radical anion localized on the nitro group. Conversely, γ tends to unity when the radical anion is fully delocalized or when the nitro group is sterically protected by bulky substituents. The γ 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 γ ratios in a given RTIL but change considerably between ionic liquids with the nature of the cation.

Micropatterned carbon surfaces were prepared by electrochemical soft-lithography and imaged with scanning electrochemical microscopy. The technique allows the characterization of the surface in terms of resistance to molecules permeating the layer. Two types of electrogenerated organic layers were tested: sub-monolayers of tetraethyleneglycol diamine (TGD) and methylphenyl (MP) layers of variable thickness. TGD surfaces were found to provide little isolation and molecules can freely diffuse inside the layer to reach the surface. MP covered surfaces offer a much better resistance to organic molecules and can be almost insulating in the case of multilayer deposits. The blocking was found to be slightly better in water in agreement with the poor affinity of the aromatic layer with water.

The influence of using room temperature ionic liquids on the heterogeneous electron transfer kinetics has been investigated with the example of the reduction of a series of nitro aromatic and aliphatic compounds. The standard electron transfer kinetic rate constants, ks, were measured in two ionic liquids (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and triethylbutylammonium bis(trifluoromethylsulfonyl)imide) and compared with the same data determined in acetonitrile. For the fastest redox couples (nitrobenzene derivatives) for which the outer sphere contribution to the activation energy is the major factor controlling the electron transfer, the values of ks considerably decrease in the ionic liquids (at least 2 orders of magnitude). On the contrary, when the inner sphere contribution is large, for example for the reduction of the 2-methyl-2-nitropropane, the ks shows negligible variations when passing in the ionic liquid. For aromatic molecules, the amplitude of the kinetics slowness is inversely proportional to the steric hindrance introduced around the NO2 group.

The redox properties of a series of substituted vinylogous tertrathiafulvalenes (TTF) prepared by oxidative coupling of 1,4-dithiafulvenes (DTF) have been investigated in acetonitrile and dichloromethane. The different steps of the electrodimerization mechanism have been characterized: fast electron transfer, coupling between two cation-radicals and slow deprotonation. Through the substituent choice of DTF, it is possible to control the relative stabilities of the different redox species of the electrogenerated vinylogous TTF. According to the nature and position of the substituent, the structural changes induced by the steric interactions lead to a compression of potential (where the second electron is easier to remove than the first one), or on the contrary to a large increase of the separation between the first and second oxidation potentials (by comparison with similar molecules without steric hindrance). Density functional modeling calculations and detailed analysis of the electrochemical behavior have been used to rationalize the substituent effect. A good agreement with the occurrence of an EE mechanism in which the electron transfer is concerted with the conformation changes is found. The inner reorganization energies are low (0.35–0.45 eV) allowing a fast passage between the different conformations during the electron transfers.

A global strategy to prepare a versatile and robust reactive platform for immobilizing molecules on carbon substrates with controlled morphology and high selectivity is presented. The procedure is based on the electroreduction of a selected triisopropylsilyl (TIPS)-protected ethynyl aryldiazonium salt. It avoids the formation of multilayers and efficiently protects the functional group during the electrografting step. After TIPS deprotection, a dense reactive ethynyl aryl monolayer is obtained which presents a very low barrier to charge transfer between molecules in solution and the surface. As a test functionalization, azidomethylferrocene was coupled by “click” chemistry with the modified surface. Analysis of the redox activity highlights a surface concentration close to the maximum possible attachment considering the steric hindrance of a ferrocenyl group.

Scanning electrochemical microscopy (SECM) is used for studying the intrinsic photo-electrochemical properties of CdSe/CdS quantum rods. They are deposited on a transparent and non-conductive glass plate and investigated by SECM in feedback and generator–collector modes using a series of redox mediators. The method allows the interrogation of the quantum rods under illumination without the interference of the substrate, notably that due to the electron photo-ejection from the substrate, a process that is inherent to any polarized electrode material. Beside the methodological demonstration that could easily be extended to the investigations of the photo-redox properties of nanoparticles, studies highlight the strong reductive properties of quantum rods under illumination.