Cu(I)-Glutathione Assembly Supported on ZIF-8 as Robust and Efficient Catalyst for Mild CO2 Conversions.

The Cu-glutathione (GSH) redox system, essential in biology, is designed here as a supramacromolecular assembly in which the tetrahedral 18e Cu(I) center loses a thiol ligand upon adsorption onto ZIF-8, as shown by EXAFS and DFT calculation, to generate a very robust 16e planar trigonal single-atom Cu(I) catalyst. Synergy between Cu(I) and ZIF-8, revealed by catalytic experiments and DFT, affords CO2 conversion into high-value-added chemicals with a wide scope of substrates by reaction with terminal alkynes or propargyl amines in excellent yields under mild conditions and reuse at least 10 times without significant decrease in catalytic efficiency.

Electrochemical Properties of Carbon Fibers from Felts.

Electrochemical behaviors of individual carbon fibers coming from carbon felts were investigated using two different redox couples, 1,1'-dimethanolferrocene and potassium ferrocyanide. Electrochemical responses were examined after different oxidation treatments, then simulated and interpreted using the Kissa 1D software and existing models. Our experiments indicate that a crude carbon fiber behaves as an assembly of sites with different electrochemical reactivities. In such case, the Butler-Volmer law is not appropriate to describe the electron transfer kinetics because of the large created overpotential. Oxidation of the fiber erases the effect by increasing the kinetics of the electron transfer probably by a homogenization and increase of the reactivity on all the fiber. Additionally, analysis of the signal shows the large influence of the convection that affects the electrochemical response even at moderate scan rates (typically below 0.1-0.2 V s-1).

SECM Investigation of Carbon Composite Thermoplastic Electrodes.

Thermoplastic electrodes (TPEs) are carbon composite electrodes consisting of graphite and thermoplastic polymer binder. TPE production is a solvent-based method, which makes it easy to fabricate and pattern into complex geometries, contrary to classical carbon composite electrodes. Depending on the composition (carbon type, binder, and composition ratio), TPEs can give excellent electrochemical performance and high conductivity. However, these TPEs are relatively new electrode materials, and thorough electrochemical characterization is still missing to understand and predict why large differences between TPEs exist. We used scanning electrochemical microscopy (SECM) as a screening tool to characterize TPEs. SECM data treatment based on scanning probe microscopy imaging allows a fast and easy comparison of the numerous images, as well as the optimization of the preparation. Experiments suggest that TPEs behave as a network of interacting microelectrodes made by electrochemically active islands isolated between less active areas. Higher carbon content in TPEs is not always indicative of more uniform electrodes with better electrochemical performances. Using various SECM redox probes, it is possible to select a specific graphite or polymer type for the analyte of interest. For example, TPEs made with COC:3569 are the best compromise for general detection, whereas PMMA:11 µm is better suited for catechol-like polyphenol analysis.

Efficient Dechlorination of α-Halocarbonyl and α-Haloallyl Pollutants by Electroreduction on Bismuth.

The electrocatalytic activity of bismuth considered as a low-cost and green electrode material was studied in reductive dechlorination processes. Cyclic voltammetry analyses showed that the Bi electrode exhibited a high catalytic activity to reduce alachlor, a chlorinated herbicide, in the aqueous medium at different pH values. Bulk electrolyses were performed at different potentials and pH values. Alachlor was reduced in deschloroalachlor, its dechlorinated derivative, with a high selectivity (96%) and a current efficiency of 48%. The reductive dechlorination of other chlorinated compounds with an activated carbon atom was then studied, showing that the bismuth electrode catalyzed the electroreduction of chloroacetamides, α-halocarbonyl, and α-haloallyl pollutants. Cyclic voltammetry experiments allowed us to propose a mechanism explaining the high catalytic activity of bismuth to reduce these families of compounds.

Ti-Catalyst Biomimetic Sensor for the Detection of Nitroaromatic Pollutants.

An electrode modified by a Ti complex in Nafion was used in the electrochemical detection of nitroaromatic compounds (NACs). The catalyst reduced nitroaromatic groups by a six-electron process with a high apparent rate constant, k1, which was estimated to be at least 19 000 mol-1 L s-1 by simulation of the first step of the cathodic reduction of 4-nitrophenylacetic acid, which was used as model molecule. The modified electrode was prepared and optimized in terms of Nafion and catalyst concentrations. In the chosen analytical medium, the Ti-catalyst-containing electrode showed good stability in the presence of nitro species. A standard deviation around 15% was calculated on the analysis performed with 28 different modified electrodes, which could be expected for a drop-coating process. We show that the modified electrode detects several nitroaromatic compounds such as nitrophenols, mononitroaniline, and dinitrotoluene with detection limits ranging from 1 × 10-4 to 9 × 10-4 g L-1 (0.2 × 10-6 to 5.1 × 10-6 mol L-1). As an interesting feature, the sensor exhibits good selectivity toward NAC detection because titanocene does not catalyze the reduction of nitroaliphatic compounds.

Investigation of Protective Properties of Organic Layers toward Reactive Oxygen Species.

The antioxidant protective properties of polyaromatic organic layers were evaluated toward reactive oxygen species (ROS) using scanning electrochemical microscopy in a foot-printing strategy. The layers were prepared by electrografting of aryldiazonium salts. Where p-(methyl)phenyl films show only weak protective properties toward ROS, p-(ethynyl)phenyl films evidence efficient protection of the covered surfaces. Applied potentials and electrolytes used during oxygen reduction reaction are critical parameters to control, prevent, or reduce the influence of ROS production and hence enhance the device lifetime.

Molecular Sieving and Current Rectification Properties of Thin Organic Films.

For the purpose of preparing well-organized functional surfaces, carbon and gold substrates were modified using electroreduction of a tetrahedral-shape preorganized tetra-aryldiazonium salt, leading to the deposition of ultrathin organic films. Characterization of the modified surfaces has been performed using cyclic voltammetry, X-ray photoelectron spectroscopy, infrared absorption spectroscopy, ellipsometry, atomic force microscopy, and contact angle measurements. The specific design of the tetra-aryldiazonium salts leads to an intrinsic structuring of the resulting organic films, allowing molecular sieving and current rectification properties toward redox probes in solution.

Photo-electrochemical properties of quantum rods studied by scanning electrochemical microscopy.

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.

Immobilization of Monolayers Incorporating Cu Funnel Complexes onto Gold Electrodes. Application to the Selective Electrochemical Recognition of Primary Alkylamines in Water.

The immobilization of a copper calix[6]azacryptand funnel complex on gold-modified electrodes is reported. Two different methodologies are described. One is based on alkyne-terminated thiol self-assembled monolayers. The other relies on the electrografting of a calix[4]arene platform bearing diazonium functionalities at its large rim and carboxylic functions at its small rim, which is post-functionalized with alkyne moieties. In both cases, the CuAAC electroclick methodology proved to be the method of choice for grafting the calix[6]azacryptand onto the monolayers. The surface-immobilized complex was fully characterized by surface spectroscopies and electrochemistry in organic and aqueous solvents. The Cu complex displays a well-defined quasi-reversible system in cyclic voltammetry associated with the Cu(II)/Cu(I) redox process. Remarkably, this redox process triggers a powerful selective detection of primary alkylamines in water at a micromolar level, based on a cavitary recognition process.

Electrografting of 4-Nitrobenzenediazonium Ion at Carbon Electrodes: Catalyzed and Uncatalyzed Reduction Processes.

Cyclic voltammograms for the reduction of aryldiazonium ions at glassy carbon electrodes are often, but not always, reported to show two peaks. The origin of this intriguing behavior remains controversial. Using 4-nitrobenzenediazonium ion (NBD), the most widely studied aryldiazonium salt, we make a detailed examination of the electroreduction processes in acetonitrile solution. We confirm that deposition of film can occur during both reduction processes. Film thickness measurements using atomic force microscopy reveal that multilayer films of very similar thickness are formed when reduction is carried out at either peak, even though the film formed at the more negative potential is significantly more blocking to solution redox probes. These and other aspects of the electrochemistry are consistent with the operation of a surface-catalyzed reduction step (proceeding at a clean surface only) followed by an uncatalyzed reduction at a more negative potential. The catalyzed reduction proceeds at both edge-plane and basal-plane graphite materials, suggesting that particular carbon surface sites are not required. The unusual aspect of aryldiazonium ion electrochemistry is that unlike other surface-catalyzed reactions, both processes are seen in a single voltammetric scan at an initially clean electrode because the conditions for observing the uncatalyzed reaction are produced by film deposition during the first catalyzed reduction step.

Amine-terminated monolayers on carbon: preparation, characterization, and coupling reactions.

Aminophenyl and aminomethylphenyl monolayers have been electrografted to glassy carbon and pyrolyzed photoresist film from the corresponding diazonium ions using a protection-deprotection strategy based on Boc (tert-butyloxycarbonyl) and Fmoc (fluorenylmethyloxycarbonyl) groups. After grafting and then deprotecting films of Boc-NH-Ar, Fmoc-NH-Ar, and Fmoc-NH-CH2-Ar, depth profiling by atomic force microscopy confirmed that the resulting amine-terminated films were monolayers. In contrast, after deprotection, Boc-NH-CH2-Ar gave a multilayer film. Electroactive carboxylic acid derivatives were coupled to the monolayers through amide linkages. Electrochemical measurements revealed that the deprotected Fmoc-NH-CH2-Ar monolayer gave the highest surface concentration of coupled nitrophenyl and ferrocenyl groups and DFT calculations established that this monolayer has the highest theoretical surface concentration of those examined.

Tunneling dendrimers. Enhancing charge transport through insulating layer using redox molecular objects.

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.

Covalently anchored carboxyphenyl monolayer via aryldiazonium ion grafting: a well-defined reactive tether layer for on-surface chemistry.

Electrografting of aryl films to electrode surfaces from diazonium ion solutions is a widely used method for preparation of modified electrodes. In the absence of deliberate measures to limit film growth, the usual film structure is a loosely packed multilayer. For some applications, monolayer films are advantageous; our interest is in preparing well-defined monolayers of reactive tethers for further on-surface chemistry. Here, we describe the synthesis of an aryl diazonium salt with a protected carboxylic acid substituent. After electrografting to glassy carbon electrodes and subsequent deprotection, the layer is reacted with amine derivatives. Electrochemistry and atomic force microscopy are used to monitor the grafting, deprotection, and subsequent coupling steps. Attempts to follow the same procedures on gold surfaces suggest that the grafted layer is not stable in these reaction conditions.

Locally induced and self-induced "electroclick" onto a self-assembled monolayer: writing and reading with SECM under unbiased conditions.

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.

Bilayer molecular electronics: all-carbon electronic junctions containing molecular bilayers made with "click" chemistry.

Bilayer molecular junctions were fabricated by using the alkyne/azide "click" reaction on a carbon substrate, followed by deposition of a carbon top contact in a crossbar configuration. The click reaction on an alkyne layer formed by diazonium reduction permitted incorporation of a range of molecules into the resulting bilayer, including alkane, aromatic, and redox-active molecules, with high yield (>90%) and good reproducibility. Detailed characterization of the current-voltage behavior of bilayer molecular junctions indicated that charge transport is consistent with tunneling, but that the effective barrier does not strongly vary with molecular structure for the series of molecules studied.

Chemically irreversible redox mediator for SECM kinetics investigations: determination of the absolute tip-sample distance.

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.

Reversible control over molecular recognition in surface-bound photoswitchable hydrogen-bonding receptors: towards read-write-erase molecular printboards.

The synthesis of an anthracene-bearing photoactive barbituric acid receptor and its subsequent grafting onto azide-terminated alkanethiol/Au self-assembled monolayers by using an Cu(I) -catalyzed azide-alkyne reaction is reported. Monolayer characterization using contact-angle measurements, electrochemistry, and spectroscopic ellipsometry indicate that the monolayer conversion is fast and complete. Irradiation of the receptor leads to photodimerization of the anthracenes, which induces the open-to-closed gating of the receptor by blocking access to the binding site. The process is thermally reversible, and polarization-modulated IR reflection-absorption spectroscopy indicates that photochemical closure and thermal opening of the surface-bound receptors occur in 70 and 100 % conversion, respectively. Affinity of the open and closed surface-bound receptor was characterized by using force spectroscopy with a barbituric-acid-modified atomic force microscope tip.

Mixed monolayer organic films via sequential electrografting from aryldiazonium ion and arylhydrazine solutions.

Sequential electrografting at glassy carbon from aryldiazonium salt solutions, or an aryldiazonium salt followed by an arylhydrazine, leads to the formation of covalently attached monolayer films incorporating two modifiers. In the first step, a 4-((triisopropylsilyl)ethynyl)phenyl film is electrografted to the surface, followed by removal of the triisopropylsilyl group to give a submonolayer of phenylethynylene groups. Two general strategies can then be applied to "fill in" the sparse monolayer with a second modifier. In the first route, nitrophenyl groups are grafted to the phenylethynylene-modified surface by the oxidation of 4-nitrophenylhydrazine. Ferrocene can be coupled to the terminal alkyne groups on the surface via a click reaction with azidomethylferrocene; an electrochemical measurement of the amount of immobilized ferrocene demonstrates that the phenylethynylene layer retains close to full reactivity after the second grafting step. In the alternative strategy, ferrocene is coupled to the phenylethynylene layer prior to grafting nitrophenyl groups by the reduction of the 4-nitrobenzenediazonium ion or by the oxidation of 4-nitrophenylhydrazine. For all approaches, the optimization of the grafting conditions gives surface concentrations of ferrocene and nitrophenyl groups that are consistent with those of a mixed monolayer. The stepwise generation of mixed monolayers is also monitored by film thickness measurements by depth profiling using the atomic force microscope. Thickness values are consistent with the proposed film structure in each preparation step.

Surface Modification of Thermoplastic Electrodes for Biosensing Applications via Copper-Catalyzed Click Chemistry.

Cu(I)-catalyzed 1,3-dipolar cycloaddition (CuAAC), also known as click chemistry, has been demonstrated to be highly robust while providing versatile surface chemistry. One specific application is biosensor fabrication. Recently, we developed thermoplastic electrodes (TPEs) as an alternative to traditional carbon composite electrodes in terms of cost, performance, and robustness. However, their applications in biosensing are currently limited due to a lack of facile methods for electrode modification. Here, we demonstrate the feasibility of using CuAAC following the diazonium grafting of TPEs to take advantage of two powerful technologies for developing a customizable and versatile biosensing platform. After a stepwise characterization of the electrode modification procedures was performed, electrodes were modified with model affinity reagents. Streptavidin and streptavidin-conjugated IgG antibodies were successfully immobilized on the TPE surface, as confirmed by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy.

Electrochemical regulation of the band gap of single layer graphene: from semimetal to semiconductor.

As a semimetal with a zero band gap and single-atom-scale thickness, single layer graphene (SLG) has excellent electron conductivity on its basal plane. If the band gap could be opened and regulated controllably, SLG would behave as a semiconductor. That means electronic elements or even electronic circuits with single-atom thickness could be expected to be printed on a wafer-scale SLG substrate, which would bring about a revolution in Moore's law of integrated circuits, not by decreasing the feature size of line width, but by piling up the atomic-scale-thickness of an SLG circuit board layer by layer. Employing scanning electrochemical microscopy (SECM), we have demonstrated that the electrochemically induced brominating addition reaction can open and regulate the band gap of SLG by forming SLG bromide (SLGBr). The SLG/SLGBr/SLG Schottky junction shows excellent performance in current rectification, and the rectification potential region can be regulated by tuning the degree of bromination of SLG. This work provides a feasible and effective way to regulate the band gap of SLG, which would open new applications for SLG in micro-nano electronics and ultra-large-scale integrated circuits (ULSI).