Historically, we have made contributions to electrochemical detection, including signal-to-noise ratio theory and experiment. While we still really like the electrochemical detector for its sensitivity and selectivity, it does have a couple ofproblems that we don't like. It requires a complex cell that needs to be coddled, there can be changes in sensitivity over time, and it is susceptible to interferences from things like temperature changes and static electricity that don't dramatically affect other detectors. We reasoned that an electrochemical detector that had the physical attributes of laser-induced fluorescence would be effective. Thus, we designed a simple flow-based detector using metal-ligand complexes (e.g., tris(bipyridyl)Os(III) and the Ru analog as well as others) that are oxidizing agents that become photoluminescent upon reduction. We call the technique "photoluminescence following electron transfer" or PFET.

PFET scheme. The Os(II) complex is oxidized online by a porous electrode. Photoluminescence occurs when the complex is reduced by an analyte like dopamine (red).

PFET scheme. The Os(II) complex is oxidized online by a porous electrode. Photoluminescence occurs when the complex is reduced by an analyte like dopamine (red).

A neutral 3 kD fluorescent dextran is injected into an organotypic hippocampal slice culture and also into the membrane on which the culture lives. Under a fluorescence microscope, a field is applied (+ on left) to drive electroosmosis. The dye moves in the field at a measurable velocity. Repeating this experiment with several dyes, dextrans as well as others, with known electrophoretic mobility leads to a consensus electroosmotic velocity and zeta potential.

A neutral 3 kD fluorescent dextran is injected into an organotypic hippocampal slice culture and also into the membrane on which the culture lives. Under a fluorescence microscope, a field is applied (+ on left) to drive electroosmosis. The dye moves in the field at a measurable velocity. Repeating this experiment with several dyes, dextrans as well as others, with known electrophoretic mobility leads to a consensus electroosmotic velocity and zeta potential.

We are also pursuing ways to pass substrates of enzyme reactions through tissue with good spatial resolution. The objective is to measure enzyme activity, particularly the activity of ectoenzymes, in the extracellular space. The application is explained on the neurochemistry page. A problem is how to pass the fluid through a tissue, or perfuse the tissue, reproducibly and without bias. Using pressure could work, but pressure-induced flow velocity is dramatically influenced by the size and geometry of the extracellular space. On the other hand, electroosmosis is not. Would electroosmosis work in brain tissue? We determined that it does. The zeta potential is in the range of 20-25 mV which is quite substantial. We have completely characterized the process of using electroosmosis to drive fluid through brain tissue, specifically organotypic hippocampal slice cultures and now use it routinely for the determination of ectopeptidase activity.