Wojciech Adamiak

Anomalous effect of flow rate on the electrochemical behavior at a liquid|liquid interface under microfluidic conditions.

We have investigated the oxidation of ferrocene at a flowing organic solvent|aqueous electrolyte|solid electrode junction in a microfluidic setup using cyclic voltammetry and fluorescent laser scanning confocal microscopy. At low flow rates the oxidation current decreases with increasing flow, contrary to the Levich equation, but at higher flow rates the current increases linearly with the cube root of the flow rate. This behavior is explained using a simple model postulating a smallest effective width of the three-phase junction, which after fitting to the data comes to be ca. 20 μm. The fluorescence microscopy reveals mixing of the two phases close to the PDMS cover, but the liquid|liquid junction is stable close to the glass support. This study shows the importance of the solid|liquid|liquid junctions for the behavior of multiphase systems under microfluidic conditions.

Generation–Collection Electrochemistry Inside a Rotating Droplet

In this work, we explore generation-collection electrochemistry in a rotating droplet hydrodynamic system, where a 70 μL droplet containing a redox active species (ferrocyanide) is sandwiched between an upper rotating rod and bottom nonmoving generator and collector planar electrodes. In such a system, we studied the effect of the counter electrode reaction on the recorded generator current, and the effect of the generator-collector distance (ranging from 3 to ca. 500 μm) on the collection efficiencies obtained at rotation rates ranging from 50 to 1100 rpm. We found that the counter electrode reaction competes with the collector reaction for the regeneration of the electroactive species; thus, collection efficiencies of 100% are probably impossible to obtain with this system geometry. We found that the collection efficiency increases with the droplet rotation rate and decreases with the generator-collector distance. The highest collection efficiency we obtained is 62% for the generator-collector distance of 3 μm, which is more than two times higher than that for typical bulk experiments with a commercial rotating ring disk electrode. We show that the increased collection efficiency can be successfully used in epinephrine detection for filtering out signals from ascorbic acid and uric acid interferents.