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 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

Charge Transfer between Electroactive Species Immobilized on Carbon Surfaces by Aryl Diazonium Reduction. SECM Investigations

Electron transfers in modified polyaryl multilayers containing redox active molecules (ferrocenyl moieties) have been investigated by scanning electrochemical microscopy (SECM) in feedback mode. The modified surfaces were prepared by the electro-reduction of aryl diazonium salts that provides anchoring layers for the immobilization of the electroactive groups. Two types of anchoring films were prepared, the first with aminophenyl and the second with phenylcarboxylic acid groups, allowing us to vary the oxidation level of the electroactive film. Determination of the apparent electron transfer rates between the modified surface and a series of redox mediators displaying increasing standard potentials permits the analysis of different processes involved in the charge transfer, namely, the permeation of the organic molecules (the mediator) and the conduction mechanism. In addition to the first oxidation of the immobilized molecules by the mediator at the solution−film interface, the global oxidation kinetics involves the conduction by charge transfer between grafted ferrocenes and the reverse charge transfer reaction from the film to mediator. This last step that is required to maintain the charge balance could become a kinetic limit for the highest driving force. For the most oxidizing mediator, analyses also suggest that the aromatic layer participates in the charge transfer

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/β-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 β-CD is added after 1 G0 contacts have been generated in water. These results indicate specific adsorption of β-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

Electropolymerization of Polypyrrole by Bipolar Electrochemistry in an Ionic Liquid

Bipolar electrochemistry has been recently explored for the modification of conducting micro- and nanoobjects with various surface layers. So far, it has been assumed that such processes should be carried out in low-conductivity electrolytes in order to be efficient. We report here the first bipolar electrochemistry experiment carried out in an ionic liquid, which by definition shows a relatively high conductivity. Pyrrole has been electropolymerized on a bipolar electrode, either in ionic liquid or in acetonitrile. The resulting polymer films were characterized by scanning electron microscopy and by contact profilometry. We demonstrate that the films obtained in an ionic liquid are thinner and smoother than the films synthesized in acetonitrile. Furthermore, a well-defined band of polypyrrole can be obtained in ionic liquid, in contrast to acetonitrile for which the polypyrrole film is present on the whole anodic part of the bipolar electrode

Wireless Synthesis and Activation of Electrochemiluminescent Thermoresponsive Janus Objects Using Bipolar Electrochemistry

In this work, bipolar electrochemistry (BPE) is used as a dual wireless tool to generate and to activate a thermoresponsive electrochemiluminescent (ECL) Janus object. For the first time, BPE allows regioselective growth of a poly­(<i>N</i>-isopropylacrylamide) (pNIPAM) hydrogel film on one side of a carbon fiber. It is achieved thanks to the local reduction of persulfate ions, which initiate radical polymerization of NIPAM. By controlling the electric field and the time of the bipolar electrochemical reactions, we are able to control the length and the thickness of the deposit. The resulting pNIPAM film is found to be swollen in water at room temperature and collapsed when heated above 32 °C. We further incorporated a covalently attached ruthenium complex luminophore, Ru­(bpy)₃²⁺, in the hydrogel film. In the second time, BPE is used to activate remotely the electrogenerated chemiluminescence (ECL) of the Ru­(bpy)₃²⁺ moieties in the film. We take advantage of the film responsiveness to amplify the ECL signal. Upon collapse of the film, the ECL signal, which is sensitive to the distance between adjacent Ru­(bpy)₃²⁺ centers, is strongly amplified. It is therefore shown that BPE is a versatile tool to generate highly sophisticated materials based on responsive polymers, which could lead to sensitive sensors

Flexible Strategy for Immobilizing Redox-Active Compounds Using in Situ Generation of Diazonium Salts. Investigations of the Blocking and Catalytic Properties of the Layers

A versatile two-step method is developed to covalently immobilize redox-active molecules onto carbon surfaces. First, a robust anchoring platform is grafted onto surfaces by electrochemical reduction of aryl diazonium salts in situ generated. Depending on the nature of the layer termini, −COOH or −NH2, a further chemical coupling involving ferrocenemethylamine or ferrocene carboxylic acid derivatives leads to the covalent binding of ferrocene centers. The chemical strategy using acyl chloride activation is efficient and flexible, since it can be applied either to surface-reactive end groups or to reactive species in solution. Cyclic voltammetry analyses point to the covalent binding of ferrocene units restricted to the upper layers of the underlying aryl films, while AFM measurements show a lost of compactness of the layers after the chemical attachment of ferrocene centers. The preparation conditions of the anchoring layers were found to determine the interfacial properties of the resulted ferrocenyl-modified electrodes. The ferrocene units promoted effective redox mediation providing that the free redox probes are adequately chosen (i.e., vs size/formal potential) and the underlying layers exhibit strong blocking properties. For anchoring films with weaker blocking effect, the coexistence of two distinct phenomena, redox mediation and ET at pinholes could be evidenced

Electrokinetic Assembly of One-Dimensional Nanoparticle Chains with Cucurbit[7]uril Controlled Subnanometer Junctions

One-dimensional (1D) nanoparticle chains with defined nanojunctions are of strong interest due to their plasmonic and electronic properties. A strategy is presented for the assembly of 1D gold-nanoparticle chains with fixed and rigid cucurbit­[<i>n</i>]­uril-nanojunctions of 9 Å. The process is electrokinetically accomplished using a nanoporous polycarbonate membrane and controlled by the applied voltage, the nanoparticle/CB­[<i>n</i>] concentration ratio, time and temperature. The spatial structure and time-resolved analysis of chain plasmonics confirm a growth mechanism at the membrane nanopores

Electropolymerizable Thiophene–Oligonucleotides for Electrode Functionalization

Inserting complex biomolecules such as oligonucleotides during the synthesis of polymers remains an important challenge in the development of functionalized materials. In order to engineer such a biofunctionalized interface, a single-step method for the covalent immobilization of oligonucleotides (ONs) based on novel electropolymerizable lipid thiophene–oligonucleotide (L-ThON) conjugates was employed. Here, we report a new thiophene phosphoramidite building block for the synthesis of modified L-ThONs. The biofunctionalized material was obtained by direct electropolymerization of L-ThONs in the presence of 2,2′-bithiophene (BTh) to obtain a copolymer film on indium tin oxide electrodes. In situ electroconductance measurements and microstructural studies showed that the L-ThON was incorporated in the BTh copolymer backbone. Furthermore, the covalently immobilized L-ThON sequence showed selectivity in subsequent hybridization processes with a complementary target, demonstrating that L-ThONs can directly be used for manufacturing materials via an electropolymerization strategy. These results indicate that L-ThONs are promising candidates for the development of stable ON-based bioelectrochemical platforms

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