Hydrogen or hydrogen peroxide can be generated in liquid−liquid biphasic systems, where the organic phase contains sufficiently strong electron donor (whose redox potential is lower than the potential of reversible hydrogen electrode). H 2 O 2 generation with acidified aqueous phase occurs prior to H 2 evolution when oxygen is present. No other organic solvent than highly toxic 1,2-dichloroethane (DCE) has been reported in biphasic system for H 2 or H 2 O 2 generation. In this work, we have used trifluorotoluene (TFT) instead of carcinogenic DCE, and studied these reactions in TFT−water biphasic system. To evaluate H 2 flux, scanning electrochemical microscopy potentiometric approach curves to the TFT−water interface were recorded. H 2 O 2 was detected voltametrically at a microelectrode located in the vicinity of the interface. H 2 and H 2 O 2 are formed and both reactions occur also in the absence of a hydrophobic salt in the organic phase. Their thermodynamics was discussed on the basis of Gibbs energies determined electrochemically with droplet-modified electrodes. The results show that DCE can be replaced by a noncarcinogenic solvent and the biphasic system for H 2 and H 2 O 2 generation can be simplified by elimination of the uncommon hydrophobic salt from the organic phase.

The charge transfer processes across the interface between two immiscible electrolyte solutions (ITIES) can be employed for energy storage and conversion, solvent extraction, or sensing or in life sciences. Among them are catalytic reactions, which have only been recently studied. Here H 2 O 2 generation is studied with decamethylferrocene (DMFc) as electron donor at the interface between tetrahexylammonium perchlorate solution in 1,2dichloroethane (1,2-DCE) and aqueous HClO 4 . These conditions are unfavorable for proton transfer across ITIES because of positive Galvani potential difference. Voltammetry with 1,2-DCE droplet modified electrode shows that DMFc oxidation is accompanied by ClO 4 − insertion into the organic phase. The reaction progress was followed by UV−vis spectroscopy, voltammetry, and scanning electrochemical microscopy (SECM). In the first and last method, horseradish peroxidase was used as catalyst. It is concluded that O 2 is reduced to H 2 O 2 at the liquid|liquid interface not only under conditions when proton transfer to organic phase is strongly favored, namely, when Galvani potential difference is negative (Angew. Chem., Int. Ed. 2008, 47, 4675−4678).

Nanoscale pH evaluation is a prerequisite for understanding the processes and phenomena occurring at solid-liquid, liquid-liquid, and liquid-gas interfaces, e.g., heterogeneous catalysis, extraction, partitioning, and corrosion. Research on the homogeneous processes within small volumes such as intracellular fluids, microdroplets, and microfluidic chips also requires nanometer scale pH assessment. Due to the opacity of numerous systems, optical methods are useless and, if applicable, require addition of a pH-sensitive dye. Potentiometric probes suffer from many drawbacks such as potential drift and lack of selectivity. Here, we present a voltammetric nanosensor for reliable pH assessment between pH 2 and 12 with high spatial resolution. It consists of a pyrolytic carbon nanoelectrode obtained by chemical vapor deposition (CVD) inside a quartz nanopipette. The carbon is modified by adsorption of syringaldazine from its ethanolic solution. It exhibits a stable quasi-reversible cyclic voltammogram with nearly Nernstian dependency of midpeak potentials (-54 mV/pH). This sensor was applied as a probe for scanning electrochemical microscopy (SECM) in order to map pH over a platinum ultramicroelectrode (UME), generating hydroxide ions (OH(-)) by the oxygen reduction reaction (ORR) at a diffusion-controlled rate in aerated phosphate buffered saline (PBS). The results reveal the alkalization of the electrolyte close to the oxygen reducing electrode, showing the insufficient buffer capacity of PBS to maintain a stable pH at the given conditions.

H2O2 is produced at the interface between a room-temperature ionic liquid with decamethylferrocene as an electron donor and an acidic aqueous solution. The electron donor can be regenerated electrochemically.

Although electron transfer reactions at liquid–liquid interfaces have been thoroughly studied in the presence of deliberately added potential determining ions or phase transfer catalysts, little is known about these reactions in the absence of the above species. Here, we report the formation of hydrogen peroxide (H2O2) in a liquid–liquid two-phase system with only an electron donor (decamethylferrocene) solution in trifluorotoluene (TFT) and a proton donor solution (HClO4) in water. To detect H2O2, we used fluorescent microscopy and scanning electrochemical microscopy (SECM). We applied a potential sweep program to the SECM probe to overcome the electrode deactivation. To provide insight into the reaction rate and mechanism, we fitted SECM results to finite elements simulations. From the results of UV–vis spectroscopy, we determined a H2O2 partition coefficient of 0.03 and the standard Gibbs energy of H2O2 transfer from water to TFT as 9.5 kJ/mol. The most important conclusion from this work is that the studied system provides conditions for spontaneous transfer of protons from the aqueous to the organic phase, even in the absence of deliberately added potential determining ions or phase transfer catalysts.

Scanning electrochemical microscopy potentiometric determination of local hydrogen concentration and its flux next to the liquid|liquid interface was demonstrated. This method is based on the shift of open circuit potential of Pt-based reversible hydrogen electrode. The detection system was verified with a system generating hydrogen under galvanostatic conditions. Then, it was applied to aqueous|1,2-dichloroethane interface where hydrogen is produced with decamethylferrocene as electron donor.

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