We describe a method of observing collisions of single femtoliter (fL) oil (i.e., toluene) droplets that are dispersed in water on an ultramicroelectrode (UME) to probe the ion transfer across the oil/water interface. The oil-in-water emulsion was stabilized by an ionic liquid, in which the oil droplet trapped a highly hydrophobic redox probe, rubrene. The ionic liquid also functions as the supporting electrolyte in toluene. When the potential of the UME was biased such that rubrene oxidation would be possible when a droplet collided with the electrode, no current spikes were observed. This implies that the rubrene radical cation is not hydrophilic enough to transfer into the aqueous phase. We show that current spikes are observed when tetrabutylammonium trifluoromethanesulfonate or tetrahexylammonium hexafluorophosphate are introduced into the toluene phase and when tetrabutylammonium perchlorate is introduced into the water phase, implying that the ion transfer facilitates electron transfer in the droplet collisions. The current (i)-time (t) behavior was evaluated quantitatively, which indicated the ion transfer is fast and reversible. Furthermore, the size of these emulsion droplets can also be calculated from the electrochemical collision. We further investigated the potential dependence on the electrochemical collision response in the presence of tetrabutylammonium trifluoromethanesulfonate in toluene to obtain the formal ion transfer potential of tetrabutylammonium across the toluene/water interface, which was determined to be 0.754 V in the inner potential scale. The results yield new physical insights into the charge balance mechanism in emulsion droplet collisions and indicate that the electrochemical collision technique can be used to probe formal ion transfer potentials between water and solvents with very low (ε < 5) dielectric constants.

Flavonols, a class of flavonoids, are present in flowers, fruits and vegetables. They are jointly responsible for antioxidant activity as free radical acceptors. The redox behaviour of myricetin, quercetin, isorhamnetin, fisetin, morin and kaempferol is investigated using cyclic and differential pulse voltammetry. Quantum chemical calculations of proton affinities and electron transfer enthalpies were performed to identify possible reactive sites and radical species to compare them with measured oxidation potentials of the flavonols. Regarding to their chemical structure, these flavonols showed an oxidation order: myricetin > quercetin > isorhamnetin > fisetin > morin > kaempferol.

[a] 1IntroductionThec ombination of an electrochemical flow cell (EC) coupled online with electrospray ionisation-mass spectrometry (ESI-MS)i sa ne xcellentt ool for mechanistic electrochemical studies and for the identification of the generated short-lived products.I ntroducing EC/ESI-MS over 20 years ago [1],t he development of modernt echniques and instruments has led to analytical methods with increased sensitivity,s peed and accuracy. More recently, an umber of studies have investigated important metabolic oxidation reactions usingEC/ESI-MS [2][3][4][5][6][7][8][9].Using electrospraya sam ild ionisation methodt hat generates little or no fragmentation, the non-volatile,p olar, thermally labile and, especially, high molecular weight analytes can be transferredf rom solution into the gas phase for mass analysis.M oreover, no heating of the solvent is required, and aw ider variety of solvents ystems can be used. The coupling facilitates the identification of electrochemically generated products present in smalls amplev olumes and allows the detection of unstable products when short transferc apillaries are used betweent he electrochemical cell and the masss pectrometer. EC/ESI-MS is as uitable tool to investigate the oxidative behaviour of manyr edox active substances, such as flavonoids,t hat possess shortlived oxidationp roducts and intermediates undergoing follow-up reactions in activated states.Flavonols,aclass of flavonoids,a re presenti nf lowers, fruits and vegetables and are jointly responsible for antioxidanta ctivity as free radical acceptors [10][11][12][13].T hey consist of two benzene rings( Aa nd B) linkedb y ap yrane ring (C),w hich is characterisedb yahydroxyl group at the C 3 position and by aC 2 -C 3 doubleb ond in ring C. Thev ariable amounts and positions of hydroxyl groupsi ntheb ackbone result in many possible naturally occurring flavonol aglycones.S elected species are shown in Fig. 1.These substances can be studied by electrochemical methodsa st hey easily undergo oxidation, expressed by their low oxidation potentials.I narecent publication, we compared different flavonols usingc yclic and differential pulse voltammetry.B ased on theoretical and mechanistic considerations,a sw ell as structural properties,w er anked these compounds according to their ease of oxidation [14].EC/ESI-MS offers auniquet ool to verify the species postulated in the previous study. In the EC,t he substances can be nearly completely converted usingalarge working electrode surface area and low flow rates to achieve ah igh conversion efficiency.H owever, theoxidised species should be rapidly transported to the mass spectrometert od etect the short-lived intermediates that are generated, thus minimising the number of signals in the mass spectrum. Low flow ratesl ead to al onger residence time of the substances in the flow cell:t his enables the occurrence of follow-up reactions of the oxidised species, limiting the detection of short-lived intermediates,d espite the high sensitivity of MS.A lthough, an initial i...

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The electrochemical initiated heterocoupling of 3‐methylcatechol and n‐hexylamine was investigated. The oxidation of 3‐methylcatechol was performed in an electrochemical flow cell with glassy carbon as the working electrode. As a result, the two‐electron, two‐proton oxidised chinone intermediate undergoes a C‐N coupling reaction in the presence of an amine (Michael addition). This mono coupling product can undergo a second two‐electron, two‐proton oxidation depending on acidic or basic conditions and substrate ratios. This flow cell was coupled on‐line with electrospray ionisation mass spectrometry to identify the possible coupling products. Higher substrate concentrations were performed off‐line as first scale‐up experiments in a two‐step procedure.