One-step electrochemical modification of carbon nanotubes by ruthenium complexes via new diazonium salts

The Positional Effect of an Immobilized Re Tricarbonyl Catalyst for CO2 Reduction

The storage of renewable energy through the conversion of CO2 to CO provides a viable solution for the intermittent nature of these energy sources. The immobilization of rhenium(I) tricarbonyl molecular complexes is presented through the reductive coupling of bis(diazonium) aryl substituents. The heterogenized complex was characterized through ultra-visible, attenuated total reflectance, infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy to probe the electronic structure of the immobilized complex. In addition, studies of cyclic voltammetry, controlled potential electrolysis, and electrochemical impedance spectroscopy were conducted to examine the CO2 reduction activity. The structure and CO2 reduction performance were compared with a previously reported immobilized rhenium(I) tricarbonyl molecular complex to probe the effect of varying the tethering of the aryl substituent from the 5,5'-position to the 4,4'-position of the 2,2'-bipyridine backbone. The immobilized complex on carbon cloth at the 4,4'-position provided excellent selectivity (FECO > 99%) and maximum TONCO and TOFCO values of 3359 and 0.9 s-1, respectively, without the addition of a Bro̷nsted acid source. A nonaqueous flow cell demonstrated the stability of this complex during a 5 h electrolysis. Tethering at the 4,4'-position, compared to the 5,5'-position, yielded lower overall activity for CO2 reduction and was attributed to the difference in growth morphology and formation of aggregations, due to Re-Re dimer formation and π-π stacking interactions within the metallopolymer matrix. For carbon cloth substrates, an optimized catalyst loading was determined to be 44.6 ± 11 nmol/cm2.

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.

Tailoring the performance of electrochemical biosensors based on carbon nanomaterials via aryldiazonium electrografting

Carbon nanomaterials (CNs) offer some of the most valuable properties for electrochemical biosensing applications, such as good electrical conductivity, wide electrochemical stability, high specific surface area, and biocompatibility. Regardless the envisioned sensing application, endowing CNs with specific functions through controlled chemical functionalization is fundamental for promoting the specific binding of the analyte. As a versatile and straightforward method of surface functionalization, aryldiazonium chemistry have been successfully used to accommodate in a stable and reproducible way different functionalities, while the electrochemical route has become the favourite choice since the deposition conditions can be readily controlled and adapted to the substrate. In particular, the modification of CNs by electrochemical reduction of aryl diazonium salts is established as a powerful tool which allows tailoring the chemical and electronic properties of the sensing platform. By outlining the stimulating results disclosed in the last years, this article provides not only a comprehensively review, but also a rational assessment on contribution of aryldiazonium electrografting in developing CNs-based electrochemical biosensors. Furthermore, some of the emerging challenges to be surpassed to effectively implement this methodology for in vivo and point of care analysis are also highlighted.

Long-Range Charge Transport in Diazonium-Based Single-Molecule Junctions

Thin layers of cobalt and ruthenium polypyridyl-oligomers with thicknesses between 2 and 8 nm were deposited on gold by electrochemical reduction of diazonium salts. A scanning tunneling microscope was used to create single-molecule junctions (SMJs). The charge transport properties of the Au-[Co(tpy)₂]_n-Au (n = 1-4) SMJs do not depend markedly on the oligomer length, have an extremely low attenuation factor (β ∼ 0.19 nm^-1), and do not show a thickness-dependent transition between two mechanisms. Resonant charge transport is proposed as the main transport mechanism. The SMJ conductance decreases by 1 order of magnitude upon changing the metal from Co to Ru. In Au-[Ru(tpy)₂]_n-Au and Au-[Ru(bpy)₃]_n-Au SMJs, a charge transport transition from direct tunneling to hopping is evidenced by a break in the length-dependent β-plot. The three different mechanisms observed are a clear molecular signature on transport in SMJs. Most importantly, these results are in good agreement with those obtained on large-area molecular junctions.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Wavelength selective separation of metal ions using electroactive ligands

Photo-induced electron transfer to an irreversibly electroactive ligand is introduced as a novel method for separating meta ions.

Covalent Functionalization of Organic Nanoparticles Using Aryl Diazonium Chemistry and Their Solvent-Dependent Self-Assembly

A simple method for covalent functionalization of Fréchet-type dendron nanoparticles (FDNs) using tris-bipyridylruthenium(II) is described. Covalent functionalization is achieved by chemically reducing the diazo derivative of a ruthenium(II)bipyridine complex in the presence of FDNs wherein the radical species generated gets covalently linked to the nanoparticle surface. Simplicity, rapidity, and robustness are the advantages offered by the present approach. The nanoparticles, post functionalization, were characterized using transmission electron microscopy, thermogravimetric analysis, and infrared, energy-dispersive X-ray, UV-visible, and nuclear magnetic resonance spectroscopic techniques. Depending on the solvent, the ruthenium complex-linked FDN displays a range of morphologies, including nanoparticles, fiber-networks, and nanocapsules. In the nanocapsules and fiber-networks observed in organic solvents, the ruthenium complex is confined within the interior domain of the aggregate, whereas in the nanoparticles observed in water, it is present on the periphery. The formation of predictable morphologies in different solvents plays a key role in using such self-assembled structures for various applications such as sensing, catalysis, and light harvesting. Characterization of these nanoaggregates using different spectroscopic and microscopic techniques is also described.

Electrografted monolayer based on a naphthalene diimide–ruthenium terpyridine complex dyad: efficient creation of large-area molecular junctions with high current densities

Robust monolayers with multiple redox states were used to create large-area molecular junctions (MJ) with a high yield of operating devices. These MJs show high current densities and rectifications properties.

Physical Realization of a Supervised Learning System Built with Organic Memristive Synapses

Multiple modern applications of electronics call for inexpensive chips that can perform complex operations on natural data with limited energy. A vision for accomplishing this is implementing hardware neural networks, which fuse computation and memory, with low cost organic electronics. A challenge, however, is the implementation of synapses (analog memories) composed of such materials. In this work, we introduce robust, fastly programmable, nonvolatile organic memristive nanodevices based on electrografted redox complexes that implement synapses thanks to a wide range of accessible intermediate conductivity states. We demonstrate experimentally an elementary neural network, capable of learning functions, which combines four pairs of organic memristors as synapses and conventional electronics as neurons. Our architecture is highly resilient to issues caused by imperfect devices. It tolerates inter-device variability and an adaptable learning rule offers immunity against asymmetries in device switching. Highly compliant with conventional fabrication processes, the system can be extended to larger computing systems capable of complex cognitive tasks, as demonstrated in complementary simulations.

Evidence of Porphyrin-Like Structures in Natural Melanin Pigments Using Electrochemical Fingerprinting

Eumelanins are extended heterogeneous biopolymers composed of molecular subunits with ambiguous macromolecular topology. Here, an electrochemical fingerprinting technique is described, which suggests that natural eumelanin pigments contain indole-based tetramers that are arranged into porphyrin-like domains. Spectroscopy and density functional theory calculations suggest that sodium ions undergo occupancy-dependent stepwise insertion into the core of porphyrin-like tetramers in natural eumelanins at discrete potentials.

Versatile functionalization of carbon electrodes with a polypyridine ligand: metallation and electrocatalytic H+ and CO2 reduction

A carbon electrode is functionalized with a polypyridine ligand and subsequently metallated to catalyze the electroreduction of H⁺ and CO₂.

X-RAY PHOTOELECTRON SPECTROSCOPY AND SCANNING ELECTROCHEMICAL MICROSCOPY STUDIES OF BRANCHED MULTIWALLED CARBON NANOTUBE PAPER MODIFIED BY ELECTROCHEMICAL GRAFTING AND CLICK CHEMISTRY

Modification of nanomaterials through electrochemical grafting is a useful approach to introduce linking groups on to the surface of these structures. This work shows the possibility of applying electrochemical grafting to branched multiwalled carbon nanotube paper with an electrical resistance of 0.1 ohm-cm, and subsequent reaction of the grafted 4-azidobenzenediazonium with ethynylferrocene through the Sharpless click chemistry reaction. A comparison is made between this paper electrode and adsorbed single-walled carbon nanotubes on a glassy carbon electrode, with electrochemistry, X-ray photoelectron spectroscopy and scanning electrochemical microscopy used for characterization.

Elucidation of the mechanism of redox grafting of diazotated anthraquinone

Redox grafting of aryldiazonium salts containing redox units may be used to form exceptionally thick covalently attached conducting films, even in the micrometers range, in a controlled manner on glassy carbon and gold substrates. With the objective to investigate the mechanism of this process in detail, 1-anthraquinone (AQ) redox units were immobilized on these substrates by electroreduction of 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate. Electrochemical quartz crystal microbalance was employed to follow the grafting process during a cyclic voltammetric sweep by recording the frequency change. The redox grafting is shown to have two mass gain regions/phases: an irreversible one due to the addition of AQ units to the substrate/film and a reversible one due to the association of cations from the supporting electrolyte with the AQ radical anions formed during the sweeping process. Scanning electrochemical microscopy was used to study the relationship between the conductivity of the film and the charging level of the AQ redox units in the grafted film. For that purpose, approach curves were recorded at a platinum ultramicroelectrode for AQ-containing films on gold and glassy carbon surfaces using the ferro/ferricyanide redox system as redox probe. It is concluded that the film growth has its origin in electron transfer processes occurring through the layer mediated by the redox moieties embedded in the organic film.

Electrochemical Functionalization of Single-Walled Carbon Nanotubes Films Obtained by Electrophoretic Deposition

In this paper, we report an electrode coated with single-walled carbon nanotubes (SWCNTs) thin-film prepared by the electrophoretic deposition (EPD) technique. SWCNTs electrodes are fabricated on indium tin oxide (ITO) glass substrates using a mixture of CNTs and a cationic detergent tetraoctylammonium bromide (TOAB) in tetrahydrofuran (THF) by applying a negative voltage to the ITO glass plate. The functionalization of these nanotubes is then achieved via electrochemical reduction of aryl diazonium salts, in a manner similar to that employed for functionalization of other carbon surfaces. A variety of diazonium salts have been used, including those that provide moieties conducive to further elaboration after attachment to the nanotubes.

Redox grafting of diazotated anthraquinone as a means of forming thick conducting organic films

Thick conductive layers containing anthraquinone moieties are covalently immobilized on gold using redox grafting of the diazonium salt of anthraquinone (i.e., 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate). This grafting procedure is based on using consecutive voltammetric sweeping and through this exploiting fast electron transfer reactions that are mediated by the anthraquinone redox moieties in the film. The fast film growth, which is followed by infrared reflection absorption spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, ellipsometry, and coverage calculation, results in a mushroom-like structure. In addition to varying the number of sweeps, layer thickness control can easily be exerted through appropriate choice of the switching potential and sweep rate. It is shown that the grafting of the diazonium salt is essentially a diffusion-controlled process but also that desorption of physisorbed material during the sweeping process is essentially for avoiding blocking of the film due to clogging of the electrolyte channels in the film. In general, sweep rates higher than 0.5 V s(-1) are required if thick, porous, and conducting films should be formed.

Highly stable ECL active films formed by the electrografting of a diazotized ruthenium complex generated in situ from the amine

The electrodeposition of the electrochemiluminescent (ECL) ruthenium complex, bis(2,2'-bipyridyl)(4'-(4-aminophenyl)-2,2'-bipyridyl)ruthenium(II), [Ru(bpy)(2)(apb)](2+), via the in situ formation of a diazonium species from aqueous media is reported. Surface characterization undertaken using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) determined that the layer is bound to the substrate via azo bonding. The layer displays good ECL activity and is stable over a long period of time. The excellent potential of this system for ECL sensing applications is demonstrated using the well-known ECL coreactant 2-(dibutylamino)ethanol (DBAE) as a model analyte, which can be detected to a level of 10 nM with a linear range between 10(-8) and 10(-4) M.

Formation of multinuclear metal-terpyridyl complexes covalently bound to carbon substrates

Multinuclear complexes consisting of metal ions and a bis(terpyridyl) ligand were covalently bound to carbon substrates. The bonding of the complexes is initiated by the bonding of phenylterpyridine (PT) on the substrates using its in-situ-generated diazonium derivative, followed by stepwise coordination of the metal ions and the ligand on it. The bonding of the PT and the formation of the multinuclear complexes were confirmed by XPS, AFM, and CV measurements. The heterogeneous rate constant (k) at the Co complex-substrate interface was evaluated by chronoamperometry (CA). The estimated high k = (2.9-3.6) x 10(3) s(-1)) would be attributed to the C-C bond at the interface without interrupting the conjugation. These multinuclear complexes bound to the carbon substrates can facilitate electron transfer from redox species such as enzymes.

The electrochemical signature of functionalized single-walled carbon nanotubes bearing electroactive groups

We report the modification and characterization of single-walled carbon nanotubes (SWCNTs) in view of molecular sensing applications. We found that ultrasonicated SWCNTs present sticking properties that make them adhere on electrode surfaces. This allows excellent characterization of SWCNTs by cyclic voltammetry (CV) before and after chemical functionalization with diazonium salts bearing electroactive groups. Bare SWCNTs presented distinct invariant shapes in CV, used as control curves, in comparison with functionalized SWCNTs for which specific signatures corresponding to the presence of grafted molecules were identified. According to the electronic substituents in the para position of the diazonium salts, divergent behaviours were observed for the grafting reactions. Diazonium salts having electrowithdrawing groups could be grafted without electrochemical induction whereas those bearing electron donating groups required a cathodic potential to generate the formation of the radical species.