Laser Scanning Microscopy Images Microjets Impinging Electrodes

Richard Gaughan

To enable the creation of better models of electrochemical interactions, Patrick R. Unwin and colleagues at University of Warwick in Coventry, UK, are investigating novel microjet geometries that project a stream of solution against an electrode, creating steady-state conditions in which they can study rapid electron-transfer kinetics. To interpret the results properly, the flow must be well characterized. They report that confocal laser scanning microscopy is up to the task.

In a demonstration of the approach, the investigators took advantage of the strong pH-dependence of fluorescein to image the flow. They immersed a 100-μm-diameter microjet nozzle and surrogate electrode — uncharged plates for the experiments — in a PTFE cell filled with a buffer solution of potassium biphthalate at a pH of 3. The fluid reservoir for the microjet was filled with fluorescein mixed in a borax buffer solution of pH 8.5. The fluorescein injected through the microjet would fluoresce as normal, while the response of the dye that diffused into the acidic buffer would be extinguished.

The experimental setup incorporated a Zeiss Axioplan 2 laser scanning microscope with a water-immersion objective, coupled to an optical window in the PTFE cell. An argon-ion laser served as a source of 488-nm excitation light, which was scanned over a 650 × 650-μm plane. The confocal technique minimized the out-of-plane background, so the experimenters were able to capture clean images of the flow at a given Z-plane. By imaging several Z-planes, they could produce three-dimensional reconstructions of the flow patterns.

Experimental confirmation

They examined flow patterns at various rates of flow and with surrogate electrodes of different sizes. Traditional models predict that low rates of flow from the nozzle will yield low rates of spreading on the electrode, while higher rates of flow should keep the injected solution in a stream of approximately the same diameter as the nozzle until it hits the electrode, at which point it creates a hydrodynamic boundary layer. The fluorescein images confirmed these predictions.

Of particular interest to the scientists was a flow feature — a vortex that swirls the injected solution back toward the microjet — at intermediate rates of flow that are predicted by newer finite-element models. The technique also revealed the presence of this recirculation.

Analytical Chemistry, online Jan. 26, 2006, doi:10.1021/ac051692i.