Halogen bonding effect on electrochemical anion oxidation in ionic liquids.

Three Ionic liquids (ILs) based on an imidazolium core have been compared and used as solvents for the oxidation of various anions. Electrochemical experiments as well as NMR titrations and X-ray diffraction analyses unambiguously confirm the crucial role of non-covalent halogen bonding on the oxidation potentials and consequently the electrochemical window of the respective ILs.

Comparative study of non-covalent interactions between cationic N-phenylviologens and halides by electrochemistry and NMR: the halogen bonding effect.

Five N-phenylviologen (PV2+) derivatives have been synthesized and their electrochemical behavior in the presence of halide anions has been studied. Further investigations were carried out by 1H and 19F NMR spectroscopy at different chloride concentrations. This is the first time a systematic study combines cyclic voltammetry and NMR spectroscopy in order to analyse the contribution of halogen bonding among the various non-covalent interactions between iodinated N-phenylviologens. The results show strong evidence for a significant "halogen bonding effect" in the interaction between halides and the iodo-tetrafluoro-phenylviologen PV2+-C6F4I. A significant influence of halogen bonding on reduction potentials of the novel halogen bond donor PV2+-C6F4I has been evidenced resulting in the first example of "inverse redox switching" of an XB-donor being partially deactivated upon reduction. Furthermore the particular binding properties of the perfluorinated derivative PV2+-C6F5 towards chloride are discussed considering a possible contribution of π-anion interaction in solution.

Characterization and Subsequent Reactivity of an Fe-Peroxo Porphyrin Generated by Electrochemical Reductive Activation of O2.

Reductive activation of O2 is achieved by using the [FeIII(F20TPP)Cl] (F20TPP = 5,10,15,20-tetrakis(pentafluorophenyl) porphyrinate) porphyrin through electrochemical reduction of the [FeIII(F20TPP)(O2•-)] superoxo complex. Formation of the [FeIII(F20TPP)(OO)]- peroxo species is monitored by using low-temperature electronic absorption spectroscopy, electron paramagnetic resonance, and cyclic voltammetry. Its subsequent protonation to yield the [FeIII(F20TPP)(OOH)] hydroperoxo intermediate is probed using low-temperature electronic absorption spectroscopy and electron paramagnetic resonance.

Rational design of a redox-labeled chiral target for an enantioselective aptamer-based electrochemical binding assay.

A series of redox-labeled L-tyrosinamide (L-Tym) derivatives was prepared and the nature of the functional group and the chain length of the spacer were systematically varied in a step-by-step affinity optimization process of the tracer for the L-Tym aptamer. The choice of the labeling position on L-Tym proved to be crucial for the molecular recognition event, which could be monitored by cyclic voltammetry and is based on the different diffusion rates of free and bound targets in solution. From this screening approach an efficient electroactive tracer emerged. Comparable dissociation constants Kd were obtained for the unlabeled and labeled targets in direct or competitive binding assays. The enantiomeric tracer was prepared and its enantioselective recognition by the corresponding anti-D-Tym aptamer was demonstrated. The access to both enantiomeric tracer molecules opens the door for the development of one-pot determination of the enantiomeric excess when using different labels with well-separated redox potentials for each enantiomer.

Switching on/off the chemisorption of thioctic-based self-assembled monolayers on gold by applying a moderate cathodic/anodic potential.

An in situ and real-time electrochemical method has been devised for quantitatively monitoring the self-assembly of a ferrocene-labeled cyclic disulfide derivative (i.e., a thioctic acid derivative) on a polycrystalline gold electrode under electrode polarization. Taking advantage of the high sensitivity, specificity, accuracy, and temporal resolution of this method, we were able to demonstrate an unexpectedly facilitated formation of the redox-active SAM when the electrode was held at a moderate cathodic potential (-0.4 V vs SCE in CH3CN), affording a saturated monolayer from only micromolar solutions in less than 10 min, and a totally impeded SAM growth when the electrode was polarized at a slightly anodic potential (+0.5 V vs SCE in CH3CN). This method literally allows for switching on/off the formation of SAMs under "soft" conditions. Moreover the cyclic disulfide-based SAM was completely desorbed at this potential contrary to the facilitated deposition of a ferrocene-labeled alkanethiol. Such a strikingly contrasting behavior could be explained by an energetically favored release of the thioctic-based SAM through homolytic cleavage of the Au-S bond followed by intramolecular cyclization of the generated thiyl diradicals. Moreover, the absence of a discernible transient faradaic current response during the potential-assisted adsorption/desorption of the redox-labeled cyclic disulfide led us to conclude in a potential-dependent reversible surface reaction where no electron is released or consumed. These results provide new insights into the formation of disulfide-based SAMs on gold but also raise some fundamental questions about the intimate mechanism involved in the facilitated adsorption/desorption of SAMs under electrode polarization. Finally, the possibility to easily and selectively address the formation/removal of thioctic-based SAMs on gold by applying a moderate cathodic/anodic potential offers another degree of freedom in tailoring their properties and in controlling their self-assembly, nanostructuration, and/or release.

Synthesis and characterization of oligonucleotide conjugates bearing electroactive labels.

Oxime bond formation has been applied to the preparation of oligonucleotides labeled with electrochemical ferrocene and viologen labels. Aminooxy functionalized ferrocene and viologen derivatives were prepared by a straightforward route and efficiently conjugated with aldehyde containing oligonucleotides either at 3' or 5' end. Both labels were found to not disturb the recognition properties of the oligonucleotide. The versatility of the method was further demonstrated by preparing bi-functionalized conjugates with a disulfide at 3' end and an electrochemical label at 5' end.

Simple and highly enantioselective electrochemical aptamer-based binding assay for trace detection of chiral compounds.

A new electrochemical methodology is reported for monitoring in homogeneous solution the enantiospecific binding of a small chiral analyte to an aptamer. The principle relies on the difference of diffusion rates between the targeted molecule and the aptamer/target complex, and thus on the ability to more easily electrochemically detect the former over the latter in a homogeneous solution. This electrochemical detection strategy is significant because, in contrast to the common laborious and time-consuming heterogeneous binding approaches, it is based on a simple and fast homogeneous binding assay which does not call for an aptamer conformational change upon ligand binding. The methodology is here exemplified with the specific chiral recognition of trace amounts of l- or d-tyrosinamide by a 49-mer d- or l-deoxyribooligonucleotide receptor. Detection as low as 0.1% of the minor enantiomer in a nonracemic mixture can be achieved in a very short analysis time (<1 min). The assay finally combines numerous attractive features including simplicity, rapidity, low cost, flexibility, low volume samples (few microliters), and homogeneous format.

Do molecular conductances correlate with electrochemical rate constants? Experimental insights.

We measured single-molecule conductances for three different redox systems self-assembled onto gold by the STMBJ method and compared them with electrochemical heterogeneous rate constants determined by ultrafast voltammetry. It was observed that fast systems indeed give higher conductance. Monotonous dependency of conductance on potential reveals that large molecular fluctuations prevent the molecular redox levels to lie in between the Fermi levels of the electrodes in the nanogap configuration. Electronic coupling factors for both experimental approaches were therefore evaluated based on the superexchange mechanism theory. The results suggest that coupling is surprisingly on the same order of magnitude or even larger in conductance measurements whereas electron transfer occurs on larger distances than in transient electrochemistry.

Bright green fluorescence of Asian paper wasp nests.

An exceptionally bright fluorescent biomatter was discovered when exploring, with UV-A light, the nests of several oriental paper wasp species of the genus Polistes, a genus of diurnal social insects. Fluorescence spectra of the cocoon cap membranes revealed narrow emission bands in the green range of the visible spectrum. Large Stokes shifts of around 160 nm and high fluorescence quantum yields of up to 35% were measured. Transmission spectra were recorded in order to estimate the contribution of the fluorescence to the visible light transmitted through the cocoon cap membrane. The nest fluorescence of the Vietnamese wasps was compared with a European and an American species. Potential biological functions of these interesting fluorescence properties of the studied biomaterial are discussed. The discovery of this striking example of a fluorescent terrestrial biomaterial may contribute to the debate on adaptive biological functions of natural fluorescence and falls in line with the growing interest in biodiversity and bio-inspiration.

Electroactive benzothiazole hydrazones and their [Mo6O19]2- derivatives: promising building blocks for conducting molecular materials.

The electroactive benzothiazole hydrazone AMBTH-H(2), a new member of the 2,2'-azino-bis(N-alkylbenzothiazole) family, was synthesised in a five-step procedure and characterised by using X-ray diffraction along with two intermediates and the N-methylbenzothiazole hydrazone MBTH-H(2). Both AMBTH-H(2) and MBTH-H(2) were coupled to [Mo(6)O(19)](2-) in acetonitrile in the presence of dicyclohexylcarbodiimide and dimethylaminopyridine to give two new diazoalkane-hexamolybdates, which were isolated as tetrabutylammonium salts and characterised by using IR, UV/Vis and NMR spectroscopies, cyclic voltammetry and, for one of them, X-ray diffraction. The packing arrangement molecules in crystals of AMBTH-H(2), the redox features of the AMBTH-hexamolybdate hybrid together with a good electronic communication between the organic pi system and the molybdenum centres make these compounds very promising blocks for the synthesis of conducting molecular materials.

Electrochemically assisted fabrication of metal atomic wires and molecular junctions by MCBJ and STM-BJ methods.

Atomic wires (point contacts) and molecular junctions are two fundamental units in the fields of nanoelectronics and devices. This Minireview introduces our recent approaches aiming to develop versatile methods to fabricate and characterize these unique metallic and molecular structures reliably. Electrochemical methods are coupled with mechanically controllable break junction (EC-MCBJ) or scanning tunneling microscopy (STM) break junction (EC-STMBJ) methods to fabricate metallic point contacts and metal/molecule/metal junctions. With the designed electrodeposition method, the metal of interest (e.g. Au, Cu, Fe or Pd) is deposited in a controlled way on the original electrode pair, on a chip for MCBJ or on the STM tip, to make the metallic contact. Then, various metal atomic wires and molecular junctions can be fabricated and characterized systematically. Herein, we measured the quantized conductance through the construction of histograms of these metal atomic point contacts and of single molecules including benzene-1,4-dithiol (BDT), ferrocene-bisvinylphenylmethyl dithiol (Fc-VPM), 4,4'-bipyridine (BPY), 1,2-di(pyridin-4-yl)ethene (BPY-EE), and 1,2-di(pyridin-4-yl)ethane (BPY-EA). Finally, we briefly discussed the future of EC-MCBJ and EC-STM for nanoelectronics and devices, for example, for the formation of heterogeneous metal-based atomic point contacts and molecular junctions.

The fabrication and characterization of adjustable nanogaps between gold electrodes on chip for electrical measurement of single molecules.

This work reports on a new method to fabricate mechanically controllable break junctions (MCBJ) with finely adjustable nanogaps between two gold electrodes on solid state chips for characterizing electron transport properties of single molecules. The simple, low cost, robust and reproducible fabrication method combines conventional photolithography, chemical etching and electrodeposition to produce suspended electrodes separated with nanogaps. The MCBJ devices fabricated by the method can undergo many cycles in which the nanogap width can be precisely and repeatedly varied from zero to several nanometers. The method improves the success rate of the MCBJ experiments. Using these devices the electron transport properties of a typical molecular system, commercially available benzene-1,4-dithiol (BDT), have been studied. The I-V and G-V characteristic curves of BDT and the conductance value for a single BDT molecule established the excellent device suitability for molecular electronics research.

Electrochemically driven interfacial halogen bonding on self-assembled monolayers for anion detection.

Electrochemically driven interfacial halogen bonding between redox-active SAMs and halide anions was quantitatively studied for the first time. The halogen bond donor properties were switched on by electrochemically controlling the oxidation state of the adsorbates. Experimental data and simulation show high binding enhancement towards halide anions compared to homogeneous systems.

Theory and practice of enzyme bioaffinity electrodes. Direct electrochemical product detection.

The use of enzyme labeling techniques to convert biorecognition events into high sensitivity electrochemical signals may follow two different strategies. One, in which the current is the electrocatalytic response of a redox couple serving as cosubstrate to a redox enzyme label and another that consists in the detection of an electrochemically active product of the enzyme label. The theoretical relationships that link, in the latter case, the electrochemical current response to the amount of recognized labeled target analyte are established for steady-state diffusion-convection chronoamperometric regimes. Two governing parameters thus emerge. One measures the Michaelis-Menten competition in the enzyme kinetics. The other characterizes the competition between the enzymatic kinetics and the diffusion of the substrate. The electrochemical response is finally related to the labeled target analyte concentration in solution through the recognition isotherm. The direct electrochemical product detection thus provides a route to the characteristics of the recognition isotherm, which serves as a calibration curve in analytical applications. The establishment of further theoretical relationships allows one to surmise the increase in sensitivity that may be obtained by using cyclic voltammetry instead of steady-state chronoamperometry in standard electrochemical cells or by accumulation of the enzyme-product in cells of small volume/surface ratios. The theoretical predictions are tested with the example of the avidin-biotin recognition process in a system that involves alkaline phosphatase as enzyme label and 4-amino-2,6-dichlorophenyl phosphate as substrate, generating 4-amino-2,6-dichlorophenol as electrochemically active product. The advantages of the dichloro-substitution are discussed. The theoretical analysis is a requisite for a rational and realistic discussion of the analytical performances of the steady-state chronoamperometric and cyclic voltammetric approaches. These are shown to compare favorably with the best heterogeneous bioaffinity assays so far reported.

Revealing molecular self-assembly and geometry of non-covalent halogen bonding by solid-state NMR spectroscopy.

We report a new spectroscopic fingerprint of intermolecular contacts in halogen bond-driven self-assembling aggregates and a precise determination of intermolecular NI distances in microcrystalline samples.

Self-assembled monolayer formation of a (N5)Fe(ii) complex on gold electrodes: electrochemical properties and coordination chemistry on a surface.

A coordinatively unsaturated FeII complex bearing a pentadentate ligand (N,N',N'-tris(2-pyridyl-methyl)-1,2-diaminoethane) functionalized with a cyclic disulfide group has been prepared in order to graft reactive metal entities as self-assembled monolayers (SAMs) on gold electrodes. Prior to grafting, exogenous ligand exchange has been investigated by cyclic voltammetry (CV) in solution, showing that the nature of the first coordination sphere (N5)FeII-X (X = Cl-, OTf-, MeCN, acetone) can be tuned, thanks to the control of the chemical conditions. The FeII complex has been immobilized on gold electrodes by spontaneous (passive) adsorption as well as by an electro-assisted method. The resulting SAMs were characterised by XPS and AFM analyses. CV experiments implementing these SAMs as working electrodes showed that the first coordination sphere of the grafted FeII complex can be controlled by adjusting the chemical conditions, similarly to the studies in a homogeneous solution. Finally, the supported FeII complex proved to be reactive with superoxide generated at the electrode surface by reduction of dissolved dioxygen. Under the employed conditions, leaking of the metal complex was not observed.

Electrochemical controlling and monitoring of halogen bond formation in solution.

Cyclic voltammetry has been used for the first time to probe and to control the formation of non-covalent halogen bonding (XB) via redox switching. These results strongly encourage the use of electrochemistry as an economical and precisely controllable tool for the investigation of XB in solution.

Water-soluble, redox-active organometallic calcium chelators.

This paper describes a new series of organometallic water-soluble chelators combining a redox moiety (ferrocene) and a selective Ca2+ chelator (BAPTA) separated by an ethynyl bridge. We report the synthesis and characterization of organometallic derivatives of the BAPTA chelator featuring one (2a) and two ferrocenyl (2b) moieties. Single crystal X-ray structural analysis on these chelators revealed unexpected conformations for the ferrocenyl substituent with respect to the phenyl ring of the BAPTA unit. DFT calculations on a model system of the ferrocenyl-ethynyl-BAPTA molecule were carried out to evaluate the energy separation between the two limiting conformations observed experimentally in the solid state, and to check the effective electronic communication between the binding pocket and the redox probe. The binding affinity of 2a­b for Ca2+, as probed by UV-Vis and cyclic voltammetry, revealed distinct behaviors in the presence of a metal ion depending on whether BAPTA is substituted by one or two ferrocenyl groups.

An organometallic derivative of a BAPTA ligand: towards electrochemically controlled cation release in biocompatible media.

The combination of a ferrocenyl moiety with BAPTA provides a novel, water-soluble, redox-active chelator. This chelator behaving as a conformational sensor exhibits an unexpected electrochemical response with high affinity and selectivity for calcium.