Mechanism of oxygen reduction by metallocenes near liquid|liquid interfaces

Dicarboxylate-containing and fully substituted ferrocene with rapid dissolvability, high solubility, good stability, and moderate formal potential for mediated electrochemical detection

An electron mediator with rapid dissolvability and high solubility in aqueous electrolyte solutions is essential for point-of-care testing based on mediated electrochemical detection. However, most ferrocenyl (Fc) compounds have slow dissolvability and poor solubility owing to high hydrophobicity of the Fc backbone. Moreover, many Fc compounds have poor stability and nonoptimal formal potential (). Herein, we present an Fc compound, Fc8m2c, which exhibits rapid dissolvability, high solubility, good stability, and moderate along with its high electron-mediation rate. The of Fc8m2c (0.17 V vs. Ag/AgCl) is tuned by two electron-withdrawing acyl substituents and eight electron-donating methyl substituents. Two pendant carboxylate groups of Fc8m2c allow for rapid dissolvability and high solubility (0.63 M in water), whereas full substitution in its two cyclopentadienyl ligands facilitates good chemical stability against decomposition in the presence of dissolved O₂ and ambient light. A moderate enables the application of a potential of 0.07 V at which electrochemical background currents are low and also contributes toward resisting the decomposition of both Fc8m2c and Fc8m2c+. Fc8m2c provides a high electron-mediation rate constant (2.4 × 10⁶ M^-1 s^-1) in glucose detection using glucose dehydrogenase. When Fc8m2c is applied to a glucose sensor, the calculated detection limit is ∼0.1 mM with a measurement period of 5 s. Considering that the normal concentration of glucose in serum is between 3.9 and 6.6 mM, the detection limit is sufficiently low. These results show that Fc8m2c is an excellent electron-mediator candidate for sensitive and rapid glucose detection.

Electrodeless Synthesis of Low Dispersity Au Nanoparticles and Nanoclusters at an Immiscible Micro Water/Ionic Liquid Interface

Owing to their biocompatibility, optical, and catalytic properties, Au nanoparticles (NPs) have been the subject of much research. Since smaller NPs have enhanced catalytic properties and NP morphology greatly impacts their effectiveness, controlled and reproducible methods of generating Au NPs are still being sought. Herein, Au NPs were electrochemically generated at a water|ionic liquid (w|IL) immiscible micro-interface, 25 µm in diameter, using a redox active IL and compared to results at a water|oil (w|o) one. The liquid|liquid interface is advantageous as it is pristine and highly reproducible, as well as an excellent means of species and charge separation. In this system, KAuCl4 dissolved in the aqueous phase reacts under external potential control at the water|P8888TB (tetraoctylphosphonium tetrakis(pentafluorophenyl)borate) with trioctyl(ferrocenylhexanoyl)phosphonium tetrakis(pentafluorophenyl)borate (FcIL), an electron donor and redox active IL. FcIL was prepared with a common anion to P8888TB, which greatly enhances its solubility in the bulk IL. Simple ion transfer of AuCl4− and AuCl(4−γ)(OH)γ− at the w|P8888TB micro-interface were characterized voltammetrically as well as their heterogeneous electron transfer reaction with FcIL. This interfacial reaction generates Au NPs whose size can be thermodynamically controlled by modifying the pH of the aqueous phase. Critically, at low pH, nanoclusters, <1.7 nm in diameter, were generated owing to inhibited thermodynamics in combination with the supramolecular fluidic nature of the IL microenvironment that was observed surrounding the as-prepared NPs.

The Solvent Effect on H2 O2 Generation at Room Temperature Ionic Liquid|Water Interface

H₂ O₂ is a versatile chemical and can be generated by the oxygen reduction reaction (ORR) in proton donor solution in molecular solvents or room temperature ionic liquids (IL). We investigated this reaction at interfaces formed by eleven hydrophobic ILs and acidic aqueous solution as a proton source with decamethylferrocene (DMFc) as an electron donor. H₂ O₂ is generated in colorimetrically detectable amounts in biphasic systems formed by alkyl imidazolium hexafluorophosphate or tetraalkylammonium bis(trifluoromethylsulfonyl)imide ionic liquids. H₂ O₂ fluxes were estimated close to liquid|liquid interface by scanning electrochemical microscopy (SECM). Contrary to the interfaces formed by hydrophobic electrolyte solution in a molecular solvent, H₂ O₂ generation is followed by cation expulsion to the aqueous phase. Weak correlation between the H₂ O₂ flux and the difference between DMFc/DMFc⁺ redox potential and 2 electron ORR standard potential indicates kinetic control of the reaction.

Ion Selective Detection Based on the Nuances of the Kinetic Fingerprint for Ion Transfer at Soft Interfaces

A novel approach has been developed for the selective determination of cations or anions based on the application of Fourier transformed staircase sinusoidal voltammetry (FT-SC-SV) in combination with the interface between two immiscible electrolyte solutions (ITIES) in the four-electrode configuration. The electrochemistry at the ITIES provides a very simple yet sensitive platform for the detection of a broad spectrum of redox inactive ions and even the neutral (bio)molecules that can be charged (e.g., protonated in appropriate pH). FT-SC-SV employs a complex potential excitation, i.e., a large-amplitude sine wave superimposed onto a dc bias potential that is stepped/scanned throughout the potential window. The response current is subsequently analyzed in the frequency domain by FT. Although the ions have close standard/formal transfer potential, discrimination and selective detection can be achieved by the higher harmonics. Feasibility and reliability of the proposed approach were verified with two pairs of ions that have very close transfer potentials across the ITIES and were chosen as the model systems. Besides, the additivity of the ionic current magnitude on concentration measured either in the mixture of ionic analytes or in individually prepared solutions containing the separate ionic analyte was tested. The experimental results prove that the principle of additivity holds. Compared with the traditional voltammetry, FT-SC-SV is simpler and more efficient in discrimination and quantification of apparently indistinguishable ion transfer from the viewpoint of thermodynamics. This demonstration may provide a new way for analytical detection of a broad range of redox inactive ions in terms of both fundamentals and applications.

Catalytic Activity of Alkali Metal Cations for the Chemical Oxygen Reduction Reaction in a Biphasic Liquid System Probed by Scanning Electrochemical Microscopy

Chemical reduction of dioxygen in organic solvents for the production of reactive oxygen species or the concomitant oxidation of organic substrates can be enhanced by the separation of products and educts in biphasic liquid systems. Here, the coupled electron and ion transfer processes is studied as well as reagent fluxes across the liquid|liquid interface for the chemical reduction of dioxygen by decamethylferrocene (DMFc) in a dichloroethane-based organic electrolyte forming an interface with an aqueous electrolyte containing alkali metal ions. This interface is stabilized at the orifice of a pipette, across which a Galvani potential difference is externally applied and precisely adjusted to enforce the transfer of different alkali metal ions from the aqueous to the organic electrolyte. The oxygen reduction is followed by H2 O2 detection in the aqueous phase close to the interface by a microelectrode of a scanning electrochemical microscope (SECM). The results prove a strong catalytic effect of hydrated alkali metal ions on the formation rate of H2 O2 , which varies systematically with the acidity of the transferred alkali metal ions in the organic phase.

Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts

The oxygen reduction reaction (ORR) is a key component of biological processes and energy technologies. This Review provides a comprehensive report of soluble molecular catalysts and electrocatalysts for the ORR. The precise synthetic control and relative ease of mechanistic study for homogeneous molecular catalysts, as compared to heterogeneous materials or surface-adsorbed species, enables a detailed understanding of the individual steps of ORR catalysis. Thus, the Review places particular emphasis on ORR mechanism and thermodynamics. First, the thermochemistry of oxygen reduction and the factors influencing ORR efficiency are described to contextualize the discussion of catalytic studies that follows. Reports of ORR catalysis are presented in terms of their mechanism, with separate sections for catalysis proceeding via initial outer- and inner-sphere electron transfer to O₂. The rates and selectivities (for production of H₂O₂ vs H₂O) of these catalysts are provided, along with suggested methods for accurately comparing catalysts of different metals and ligand scaffolds that were examined under different experimental conditions.

Mediated water electrolysis in biphasic systems

The concept of efficient electrolysis by linking photoelectrochemical biphasic H₂ evolution and water oxidation processes in the cathodic and anodic compartments of an H-cell, respectively, is introduced. Overpotentials at the cathode and anode are minimised by incorporating light-driven elements into both biphasic reactions. The concepts viability is demonstrated by electrochemical H₂ production from water splitting utilising a polarised water-organic interface in the cathodic compartment of a prototype H-cell. At the cathode the reduction of decamethylferrocenium cations ([Cp₂*Fe^(III)]⁺) to neutral decamethylferrocene (Cp₂*Fe^(II)) in 1,2-dichloroethane (DCE) solvent takes place at the solid electrode/oil interface. This electron transfer process induces the ion transfer of a proton across the immiscible water/oil interface to maintain electroneutrality in the oil phase. The oil-solubilised proton immediately reacts with Cp₂*Fe^(II) to form the corresponding hydride species, [Cp₂*Fe^(IV)(H)]⁺. Subsequently, [Cp₂*Fe^(IV)(H)]⁺ spontaneously undergoes a chemical reaction in the oil phase to evolve hydrogen gas (H₂) and regenerate [Cp₂*Fe^(III)]⁺, whereupon this catalytic Electrochemical, Chemical, Chemical (ECC') cycle is repeated. During biphasic electrolysis, the stability and recyclability of the [Cp₂*Fe^(III)]⁺/Cp₂*Fe^(II) redox couple were confirmed by chronoamperometric measurements and, furthermore, the steady-state concentration of [Cp₂*Fe^(III)]⁺ monitored in situ by UV/vis spectroscopy. Post-biphasic electrolysis, the presence of H₂ in the headspace of the cathodic compartment was established by sampling with gas chromatography. The rate of the biphasic hydrogen evolution reaction (HER) was enhanced by redox electrocatalysis in the presence of floating catalytic molybdenum carbide (Mo₂C) microparticles at the immiscible water/oil interface. The use of a superhydrophobic organic electrolyte salt was critical to ensure proton transfer from water to oil, and not anion transfer from oil to water, in order to maintain electroneutrality after electron transfer. The design, testing and successful optimisation of the operation of the biphasic electrolysis cell under dark conditions with Cp₂*Fe^(II) lays the foundation for the achievement of photo-induced biphasic water electrolysis at low overpotentials using another metallocene, decamethylrutheneocene (Cp₂*Ru^(II)). Critically, Cp₂*Ru^(II) may be recycled at a potential more positive than that of proton reduction in DCE.

Redox Electrocatalysis of Floating Nanoparticles: Determining Electrocatalytic Properties without the Influence of Solid Supports

Redox electrocatalysis (catalysis of electron-transfer reactions by floating conductive particles) is discussed from the point-of-view of Fermi level equilibration, and an overall theoretical framework is given. Examples of redox electrocatalysis in solution, in bipolar configuration, and at liquid-liquid interfaces are provided, highlighting that bipolar and liquid-liquid interfacial systems allow the study of the electrocatalytic properties of particles without effects from the support, but only liquid-liquid interfaces allow measurement of the electrocatalytic current directly. Additionally, photoinduced redox electrocatalysis will be of interest, for example, to achieve water splitting.