EVALUATION OF AMPEROMETRIC SENSORS CONTAINING ENZYMES IMMOBILIZED IN HYDROGELS THAT INCORPORATE THE REDOX MEDIATOR [Os(bpy)2(py)Cl]+/2+

Quantification of extracellular levels of neurotransmitters in the brain with a high degree of quality and reliability has been a fundamental challenge for analytical chemists for years. With the ability to characterize how neurotransmitter levels change in response to the administration of different pharmacological agents, it is possible to learn about the mechanisms by which drugs elicit their effect. Characterizing neurotransmitter levels in diseased states enables mapping of a disease or treatment, and may guide the development of novel therapies. In the early 1990's, redox hydrogels composed of a poly(4-vinylpyridine) backbone with pendent osmium-centered redox complexes ([Os(bpy)2(py)Cl]+/2+, where bpy = 2,2'-bipyridine and py = a pyridine ring of the polymer backbone) were found to be successful at mediating electron transfer between horseradish peroxidase and an electrode. Since that time, a number of amperometric sensors utilizing this mediator have been developed to detect analytes such as hydrogen peroxide, glucose, glutamate, choline, lactate, etc. The performance of these sensors has been characterized in vitro and evaluated in vivo; however, an overall lack of reproducibility has limited the usefulness of these sensors. My findings demonstrate several factors that contribute to the poor reproducibility of these sensors include: 1) precipitation of the redox polymer and enzymes, 2) reactions of exposed enzyme metal centers with hydroquinones (e.g., dopamine and DOPAC), 3) reaction of oxidized [Os(bpy)2(py)Cl]+/2+ with hydroquinones, 4) the ability of hydroquinone/quinone to mediate electron transfer between horseradish peroxidase and the electrode, and 5) the accumulation of an insulative hydroquinone oxidation product in the hydrogel which disconnects horseradish peroxidase from the [Os(bpy)2(py)Cl]+/2+ mediator, and neighboring [Os(bpy)2(py)Cl]+/2+ sites from one another. To prepare sensors that resist interference from hydroquinones, it is necessary to minimize the flux of hydroquinones into the hydrogel/enzyme layer, and maintain the integrity of enzymes entrapped in the redox hydrogel. Herein, I describe the use of thick Nafion films and SDS to produce sensors for monitoring cationic and neutral analytes in vivo