Bernd Schöllhorn

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.

The tailoring of crystal structures via the self-assembly of organic coordination compounds by N⋯I non-covalent halogen bonds: co-crystals of sterically hindered N-heterocycles and 1,4-diiodo-tetrafluorobenzene

The N-containing heteroaromatics 2,3,5,6-tetramethylpyrazine (tmpyr), 2,2′-bipyridine (2,2′-bpy), 2,4′-bipyridine (2,4′-bpy) and o-phenanthroline (phen) were each cocrystallized with 1,4-diiodo-tetrafluoro-benzene (TFDIB). The effect of the sterically constrained aza cycles on supramolecular assembly and the crystal structure determining interactions is discussed. The N⋯I non-covalent halogen bonds are the main directing interactions responsible for the observed structures. Nevertheless, steric hindrance of the donor sites increases the impact of weaker intermolecular interactions such as CH⋯F bonding and π–π interactions provoking changes in the coordination behaviour of the donor modules. In spite of steric constraints, the co-crystals tmpyr·TFDIB (1) and 2,2′-bpy·TFDIB (2) exhibit polymeric structures consisting of infinite one-dimensional chains of alternating electron donors (D) and acceptors (A). In contrast, the structures of the cocrystals 2,4′-bpy·TFDIB (3) and phen·TFDIB (4) reveal a packing of termolecular [A1D2] complexes. It follows that the structure type of such halogen bonded supramolecular assemblies can be tuned by choosing the appropriate precursors.

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.

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.

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.

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.

Directed synthesis of a halogen-bonded open porphyrin network†

A strategy for the elaboration of a halogen-bonded porphyrin network is reported. The progressive introduction of geometric constraints via the modulation of building blocks and self-assembly via strong and directional halogen bonding led successfully to the construction of an open porphyrin network with nano-sized tubular channels.

Electrochemically active phenylenediamine probes for transition metal cation detection

A novel family of tetraalkyl-p-phenylenediamine (TAPD)-based ligands has been efficiently prepared by reductive amination of heterocyclic aldehydes. The redox properties of these electrochemical active ligands change dramatically upon complexation of the transition metal cations Zn2+, Ni2+ and Cd2+ leading to large oxidation potential shifts of up to 950 mV depending on the nature of the ligand. Complexes with a metal to ligand ratio of 1 ∶ 2 were formed and 113Cd NMR revealed an octahedral coordination sphere of the metal. All pyridyl derivatives show a distinct chemoselectivity (Zn2+ > Cd2+ > Ni2+). The thiophenyl containing derivatives display a particularly high selectivity for zinc cations (Zn2+ ≫ Ni2+, Cd2+).

Intramolecular charge effects in the electrochemical oxidation of aminoxyl radicals

The redox potentials of a series of aromatic nitroxides derived from tert-butyl phenyl nitroxide are determined by cyclic voltammetry in non-aqueous solution. It is shown that the first oxidation potential as well as the reduction potential strongly depend on the electron donating and withdrawing substituents of the compounds. The first results on the influence of a negative charge in proximity to the aminoxyl group on the redox properties of aromatic nitroxides, are reported. A remarkably strong intramolecular “charge effect” is probably responsible for the observed high potential shift of 560 mV in the case of the carboxylate 4b.

Competitive assay of 2,4-dichlorophenoxyacetic acid using a polymer imprinted with an electrochemically active tracer closely related to the analyte

2-Chloro-4-hydroxyphenoxyacetic acid was used as an electroactive tracer in a ligand displacement assay involving molecularly imprinted polymer particles for the analysis of the herbicide 2,4-dichlorophenoxyacetic acid by means of linear scan voltammetry at a disposable screen-printed electrode.