Diazonium Electroreduction Research Articles

Highly Reactive Diazenyl Radical Species Evidenced during Aryldiazonium Electroreduction

We report the experimental reassessment of the widely admitted concerted reduction mechanism for diazonium electroreduction. Ultrafast cyclic voltammetry was exploited to demonstrate the existence of a stepwise pathway, and real-time spectroelectrochemistry experiments allowed visualization of the spectral signature of an evolution product of the phenyldiazenyl radical intermediate. Unambiguous identification of the diazenyl species was achieved by radical trapping followed by X-ray structure resolution. The electrochemical generation of this transient under intermediate energetic conditions calls into question our comprehension of the layer structuration when surface modification is achieved via the diazonium electrografting technique as this azo-containing intermediate could be responsible for the systematic presence of azo bridges in nanometric films.

Strengthening adhesion of polycarbazole films on ITO surface by covalent electrografting of monomer

Strong adhesion of a functional polymer coating to a solid support is a key requirement to achieve its robustness and high performance in different application areas. Herein, a novel carbazole derivative, 9-methyl-9H-carbazole-3,6-diamine (m-Cz-diamine), is synthesized and covalently grafted onto an ITO surface through the diazonium electroreduction method. The functionalization of the ITO surface is confirmed by XPS spectroscopy, water contact angle measurements and cyclic voltammetry analysis, revealing the covalent anchoring of an organic film (m-Cz-ITO). The functionalized ITO surface is then studied for oxidative electropolymerization of m-Cz-diamine and carbazole, showing clear advantages over a bare ITO substrate in terms of polymer film adhesion. The polymer films electrodeposited onto an electrografted ITO surface demonstrate very strong adhesion, as they do not delaminate, either by a scotch tape pulling, storing in air, or immersing under water for two months. On the other hand, the polymer films grown on a bare ITO substrate peel off easily in all the aforementioned stability tests. Therefore, electrografting an organic film onto the ITO surface clearly improves the adhesion of the polymer films on top of it. Moreover, it is interesting to note that the physicochemical properties of the polymer films deposited on the functionalized ITO substrate remain unchanged.

Single-Molecule Junctions with Highly Improved Stability

Single-molecule junctions (SMJs) have been fabricated using layers generated by diazonium electroreduction. This process creates a C-Au covalent bond between the molecule and the electrode. Rigid oligomers of variable length, based on porphyrin derivatives in their free base or cobalt complex forms, have been grafted on the surface. The conductance of the oligomers has been studied by a scanning tunneling microscopy break junction (STM-bj) technique and G(t) measurements, and the lifetime of the SMJs has been investigated. The conductance histograms indicate that charge transport in the porphyrins is relatively efficient and influenced by the presence of the cobalt center. With both systems, random telegraph G(t) signals are easily recorded, showing SMJ on/off states. The SMJs then stabilize and exhibit a surprisingly long lifetime around 10 s, and attenuation plots, obtained by both G(t) and STM-bj measurements, give identical values. This work shows that highly stable SMJs can be prepared using a diazonium grafting approach.

Polypyrrole-Wrapped Carbon Nanotube Composite Films Coated on Diazonium-Modified Flexible ITO Sheets for the Electroanalysis of Heavy Metal Ions.

Highly sensitive multicomponent materials designed for the recognition of hazardous compounds request control over interfacial chemistry. The latter is a key parameter in the construction of the sensing (macro) molecular architectures. In this work, multi-walled carbon nanotubes (CNTs) were deposited on diazonium-modified, flexible indium tin oxide (ITO) electrodes prior to the electropolymerization of pyrrole. This three-step process, including diazonium electroreduction, the deposition of CNTs and electropolymerization, provided adhesively-bonded, polypyrrole-wrapped CNT composite coatings on aminophenyl-modified flexible ITO sheets. The aminophenyl (AP) groups were attached to ITO by electroreduction of the in-situ generated aminobenzenediazonium compound in aqueous, acidic medium. For the first time, polypyrrole (PPy) was electrodeposited in the presence of both benzenesulfonic acid (dopant) and ethylene glycol-bis(2-aminoethylether)-tetraacetic acid (EGTA), which acts as a chelator. The flexible electrodes were characterized by XPS, Raman and scanning electron microscopy (SEM), which provided strong supporting evidence for the wrapping of CNTs by the electrodeposited PPy. Indeed, the CNT average diameter increased from 18 ± 2.6 nm to 27 ± 4.8, 35.6 ± 5.9 and 175 ± 20.1 after 1, 5 and 10 of electropolymerization of pyrrole, respectively. The PPy/CNT/NH2-ITO films generated by this strategy exhibit significantly improved stability and higher conductivity compared to a similar PPy coating without any embedded CNTs, as assessed by from electrochemical impedance spectroscopy measurements. The potentiometric response was linear in the 10−8–3 × 10−7 mol L−1 Pb(II) concentration range, and the detection limit was 2.9 × 10−9 mol L−1 at S/N = 3. The EGTA was found to drastically improve selectivity for Pb(II) over Cu(II). To account for this improvement, the density functional theory (DFT) was employed to calculate the EGTA–metal ion interaction energy, which was found to be −374.6 and −116.4 kJ/mol for Pb(II) and Cu(II), respectively, considering solvation effects. This work demonstrates the power of a subtle combination of diazonium coupling agent, CNTs, chelators and conductive polymers to design high-performance electrochemical sensors for environmental applications.

Versatile electrochemical approaches towards the fabrication of molecular electronic devices.

The concept of molecular electronic devices (MEDs) has evolved since the first theoretical report was published in 1974. This theoretical article laid the foundation for understanding charge-transport phenomena by utilizing either a single molecule or numerous molecules sandwiched between two electrical conductors. Since then, many research groups have engaged in molecular junction fabrication using a variety of molecules including organic, inorganic, and organometallic molecules, polymers, and biomolecules that can mimic the electronic functions of traditional silicon-based devices. To date, most of these molecular junctions have been constructed using well-explored molecular assemblies of thiolated derivatives adsorbed mostly on Au substrates. However, the Au-S bond is not considered to be a true covalent bond in view of its surface bond energy, which is ∼1.9 eV, much less than pure covalent bond energy. Additionally, Au-S interfaces suffer from poor reactivity and instability over time; thus, they are not suitable for real world applications. Therefore, an alternative approach for the fabrication of molecular electronic junctions must be envisioned to address all the difficulties mentioned above. Herein, we summarize the most recent experiential outcomes to demonstrate how electrochemical techniques can be employed to form robust molecular layers on various substrates including Au, ZnO, SnO2, H-terminated Si, and conductive carbon electrodes that are suitable for the fabrication of reliable molecular electronics devices for charge-transport studies. This fascinating electrochemical technique is appropriate for producing not only homostructure but also heterostructure molecular layers with desired thicknesses. We also discuss the pros and cons of the electrochemical process for growing molecular layers in the Conclusion and outlook section.

Ultrathin Molecular Layer Junctions Based on Cyclometalated Ruthenium Complexes

A cyclometalated ruthenium complex has been designed to be immobilized on a surface by diazonium electroreduction. Modified surfaces have been obtained and fully characterized by X-ray photoelectron spectroscopy, electrochemistry, and atomic force microscopy. Molecular junctions consisting of Ru(bpy)2(ppy) oligomer using direct top-coat evaporation are presented.

The role of diazonium interface chemistry in the design of high performance polypyrrole-coated flexible ITO sensing electrodes

Benzene sulfonic acid-doped polypyrrole (PPyBSA) films were electrochemically prepared on aminobenzenediazonium-modified flexible ITO in aqueous media in a simple and efficient way. Diazonium electroreduction resulted in aminophenyl-modified ITO (ITO-NH2), which served as a platform for the electrodeposition of a PPyBSA layer. Without diazonium pretreatment, the PPyBSA coating delaminates readily upon sonication or during ultrahigh vacuum outgassing prior to XPS analysis. In contrast, a pretreatment involving electrografting for 45s at −800mV resulted in excellent adhesion of PPyBSA, which withstood 75min of ultrasonication. The robust ITO-NH2-PPy electrode was employed for the fast, simple and accurate detection of lead (II) by differential pulse voltammetry. The potentiometric response was linear in the Pb(II) concentration range 10−8 to 2.5×10−7mol·L−1 and the detection limit was 9.94×10−10mol·L−1 at S/N=3.This work demonstrates the central role of diazonium chemistry in producing an adhesive aryl monolayer on low-cost, flexible ITO electrodes, allowing the fabrication of robust, high-performance electrochemical-sensing conductive-polymer films.

Ordered Nanoporous Thin Films by Nanosphere Lithography and Diazonium Electroreduction: Simple Elaboration of Ultra‐Micro‐Electrode Arrays

AbstractHighly ordered ultra‐micro‐electrode arrays are obtained using nanosphere lithography combined with electrochemical reduction of an aryl diazonium salt. A template made up of 2D hexagonally close‐packed polystyrene (PS) spheres was first deposited on indium tin oxide or glassy carbon substrates. Then, a thin layer of bisthienylbenzene (BTB) was grafted onto the interstices between the spheres. After dissolution of the PS spheres, the presence of micro‐hole arrays on the substrates was evidenced by atomic force microscopy. The electrochemical response of these micro‐hole arrays was investigated as a function of the size of the spheres used. Depending on their radius and the distance between adjacent holes, they behave as a partially blocked surface or as ultra‐microelectrode arrays (UMEAs).

Unprecedented Self-Organized Monolayer of a Ru(II) Complex by Diazonium Electroreduction.

A new heteroleptic polypyridyle Ru(II) complex was synthesized and deposited on surface by the diazonium electroreduction process. It yields to the covalent grafting of a monolayer. The functionalized surface was characterized by XPS, electrochemistry, AFM, and STM. A precise organization of the molecules within the monolayer is observed with parallel linear stripes separated by a distance of 3.8 nm corresponding to the lateral size of the molecule. Such organization suggests a strong cooperative process in the deposition process. This strategy is an original way to obtain well-controlled and stable functionalized surfaces for potential applications related to the photophysical properties of the grafted chromophore. As an exciting result, it is the first example of a self-organized monolayer (SOM) obtained using diazonium electroreduction.

Grafting of a peptide probe for Prostate-Specific Antigen detection using diazonium electroreduction and click chemistry

The main objective of this work was to validate a label-free electrochemical method of protein detection using peptides as capture probes. As a proof-of-concept, we used a 7 amino acids sequence (HSSKLQL) specific for Prostate Specific Antigen. We investigated various electrografting conditions of two anilines (2-[(4-aminophenyl)sulfanyl]-8-hydroxy-1,4-naphthoquinone and 4-azidoaniline) further converted in situ into their corresponding diazonium salts on glassy carbon electrodes. It was demonstrated that the best method to obtain a mixed layer is the simultaneous electroreduction of the two diazonium salts. 4-azidoaniline was used to covalently immobilize the ethynyl-functionalized peptide probe by click coupling, and the hydroxynaphthoquinone derivative plays the role of electrochemical transducer of the peptide-protein recognition. The proteolytic activity of PSA towards a small peptide substrate carrying streptavidin at its distal end was also investigated to design an original sensing architecture leading to a reagentless, label free, and “signal-on” PSA sensor. Without optimization, the limit of quantification can be estimated in the nM to pM range.

Grafting π-Conjugated Oligomers Incorporating 3,4-Ethylenedioxythiophene (EDOT) and Thiophene Units on Surfaces by Diazonium Electroreduction

The electrochemical reduction of diazonium salts, generated in situ, from 2-(4-aminophenyl)-3,4-ethylenedioxythiophene and two new amino functionalized π-conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) and thiophene units, has been investigated. It coats the electrodes (glassy carbon (GC), gold or ITO) with an ultrathin organic layer (less than 10 nm thickness). The X-ray photoelectron spectroscopy (XPS) investigations confirm the presence of the starting oligomers deposited on the surface. As an important result, EDOT-based oligomer grafting was achieved on those surfaces which may be of general use as adhesion primer layers in all devices using PEDOT type materials. Furthermore, the coating is electroactive and the electrochemical investigations exhibit redox signal at potentials close to that obtained for short oligoEDOT in solution. The electrochemical responses of the modified GC electrodes were further studied in the presence of various reversible redox probes, showing that the ...

Electrografting of conductive oligomers and polymers using diazonium electroreduction

This paper describes the attachment of conjugated oligomers onto electrode surface through the reduction of diazonium compounds. In this connection some properties of conjugated oligomers and of layers grafted through diazonium electroreduction will first be briefly presented. The electrochemical behavior of conjugated oligomers grafted on a surface using diazonium electroreduction will then be discussed.

Electrografting via Diazonium Chemistry: From Multilayer to Monolayer Using Radical Scavenger

A simple strategy to avoid the formation of polyaryl layer during the functionalization of carbon surface by diazonium electroreduction is presented. The approach proposes to directly act on the polymerization mechanism by the use of a radical scavenger. The kinetic gap between the surface coupling and the multilayer formation is exploited to prevent the growth of the layer without interfering with the grafting. The well-known 4-nitrobenzenediazonium electrografting was used to demonstrate the possibility of reaching a monolayer surface coverage with an excess of DPPH (2,2-diphenyl-1-picrylhydrazyl). Experimental conditions were varied to validate the efficiency of the grafting limitation and the radical capture was confirmed by isolation of the aryl radical/DPPH coupling product.

Direct Observation of Large Quantum Interference Effect in Anthraquinone Solid-State Junctions

Quantum interference in cross-conjugated molecules embedded in solid-state devices was investigated by direct current-voltage and differential conductance transport measurements of anthraquinone (AQ)-based large area planar junctions. A thin film of AQ was grafted covalently on the junction base electrode by diazonium electroreduction, while the counter electrode was directly evaporated on top of the molecular layer. Our technique provides direct evidence of a large quantum interference effect in multiple CMOS compatible planar junctions. The quantum interference is manifested by a pronounced dip in the differential conductance close to zero voltage bias. The experimental signature is well developed at low temperature (4 K), showing a large amplitude dip with a minimum >2 orders of magnitude lower than the conductance at higher bias and is still clearly evident at room temperature. A temperature analysis of the conductance curves revealed that electron-phonon coupling is the principal decoherence mechanism causing large conductance oscillations at low temperature.

Organic Electrodes Based on Grafted Oligothiophene Units in Ultrathin, Large-Area Molecular Junctions

Molecular junctions were fabricated with the combined use of electrochemistry and conventional CMOS tools. They consist of a 5-10 nm thick layer of oligo(1-(2-bisthienyl)benzene) between two gold electrodes. The layer was grafted onto the bottom electrode using diazonium electroreduction, which yields a stable and robust gold-oligomer interface. The top contact was obtained by direct electron-beam evaporation on the molecular layers through masks defined by electron-beam lithography. Transport mechanisms across such easily p-dopable layers were investigated by analysis of current density-voltage (J-V) curves. Application of a tunneling model led to a transport parameter (thickness of ~2.4 nm) that was not consistent with the molecular thickness measured using AFM (~7 nm). Furthermore, for these layers with thicknesses of 5-10 nm, asymmetric J-V curves were observed, with current flowing more easily when the grafted electrode was positively polarized. In addition, J-V experiments at two temperatures (4 and 300 K) showed that thermal activation occurs for such polarization but is not observed when the bias is reversed. These results indicate that simple tunneling cannot describe the charge transport in these junctions. Finally, analysis of the experimental results in term of "organic electrode" and redox chemistry in the material is discussed.

Hydroxynaphthoquinone Ultrathin Films Obtained by Diazonium Electroreduction: Toward Design of Biosensitive Electroactive Interfaces

Electroactive 2-(phenylsulfanyl)-8-hydroxy-1,4-naphthoquinone has been electrodeposited via the reduction of the corresponding diazonium salt on Au electrodes. Surface characterizations by X-ray photoelectron spectroscopy (XPS) and infrared reflection-absorption spectroscopy (IRRAS) reveal that the mechanism of film deposition follows an aryl radical formation and its immobilization on the electrode surface. Electrochemical study shows that the surface coverage can be finely tuned (thickness between one and four layers) by adjusting the potential and the deposition time. By managing the potential applied when reducing diazonium in potentiostatic mode, the formed layer could mediate or not charge transfer. This is the first time that the films obtained by diazonium process are demonstrated to act as mediators in the growth process. Hence, with potentials higher than the formal potential of quinone group, very thin and homogeneous layers are obtained, whereas thicker films are formed when more cathodic potentials than that of quinone are applied. The possibility to manage the charge-transfer kinetics, the thickness, and the homogeneity of electroactive deposits is interesting in the scope of designing electrochemical transducers.

New Generation of Diazonium based on Heterocyclic Amines : Electrochemical Investigations

This work is devoted to the study of the electrochemical grafting onto platinum ultramicroelectrode (UME) through diazonium electroreduction using a heteroaromatic compound, without phenyl groups. The grafting was performed using the electrochemical reduction of 2-aminoterthiophenyldiazonium cation, generated in situ. It leads to the attachment of an electroactive organic layer onto the UME surface. Following that the electrochemical response of the modified UME in the presence of various reversible redox couples shows that the attached layer acts as a conductive switch. It behaves as a barrier to electron transfer when the standard redox potential is below 0.5 V vs. SCE; in this case the oxidation wave of the redox probe is shifted to more positive values. However, for more oxidizing redox probes the layer can be considered as transparent and no barrier effect is observed. Finally, scanning electrochemical microscopy (SECM) can be performed using this electroswitchable modified platinum.

Grafting Oligothiophenes on Surfaces by Diazonium Electroreduction: A Step toward Ultrathin Junction with Well-Defined Metal/Oligomer Interface

The functionalization of electrode materials through diazonium electroreduction using a heteroaromatic compound, without phenyl groups, has been investigated for the first time. The electrochemical reduction of 2-aminoterthiophenyldiazonium cation, generated in situ, coats the electrode (glassy carbon (GC), gold or platinum) with an ultrathin organic layer, shown by X-ray photoelectron spectroscopy (XPS) of that deposited on gold to consist of terthiophene or oligothiophene. The coating is electroactive at potential close to that of terthiophene in solution. The electrochemical response of the modified GC electrode in the presence of various reversible redox couples shows that the attached layer acts as a conductive switch. It behaves as a barrier to electron transfer when the standard redox potential is below 0.5 V/SCE; in this case diode-like behavior is observed. However, for more oxidizing redox probes the layer can be considered as transparent and no barrier effect is observed. The layer deposited on a platinum ultramicroelectrode (UME) behaves similarly to that obtained on the large GC electrode. Scanning electrochemical microscopy (SECM) can be performed using this electroswitchable modified platinum UME which can act as a filter toward competitive redox exchange pathways.