Design of biosensor arrays for drug development and diagnostic applications has become a major activity in bioelectrochemistry. Achieving efficient electron transfer processes to produce reliable, reproducible, and sensitive output signals is a critical goal for such devices. This paper describes key electron transfer processes in our development of sensors for the chemistry of genotoxicity, defined here as resulting from the damage and oxidation of DNA by chemicals and their metabolites. This research is driven mainly by the need to screen for the potential genotoxicity of new drug and environmental chemical candidates, about 30% of which have toxicity issues that do not manifest until human clinical trials. A major issue is metabolic chemistry driven mainly by cytochrome (cyt) P450s and other enzymes, and we needed to produce metabolites in our arrays. Metabolic chemistry involves electron-transfer catalysis of organic reactions, usually involving oxygen transfer in the case of cyt P450s. We have found ways to efficiently transfer electrons into cyt P450s in thin films via their natural reductase electron-transfer partners to catalyze metabolic reactions electrochemically via mechanisms that are the same as in our livers and other organs. Optimizing the electrode-driven electron transfer chain from electrode to reductase to cyt P450 has allowed us to replace expensive NADPH as reducing agent and simplify our genotoxicity arrays.

We also needed a way to detect the resulting metabolite-related DNA damage in sensors combining thin LbL films of metabolic enzymes and DNA in which the metabolites are generated and react with DNA. We found that guanines in DNA chains can react in an electrocatalytic process involving electron transfer to a Ru or Os metallopolymer (RuPVP) to output visible light via electrochemiluminescence (ECL). A complex electron transfer sequence drives an electrocatalytic process that oxidizes Ru^II to Ru^III to oxidize guanine moieties in the DNA and produces visible light. The Os^II/Os^III redox couple can be used in a similar ways to design sensor arrays selective for DNA oxidation.

Electrocatalytic Cyt P450 oxidations, ECL generation and microfluidics are combined in our most sophisticated genotoxicity sensor arrays. When DNA is adducted by metabolites, its double-stranded structure is disrupted and Ru^III sites in the film gain better access to guanine moieties to produce more ECL light. The primary oxidation product in DNA oxidation is 8-oxo-guanine, which in a second step is selectively oxidized by Os^III complexes to give ECL at a lower applied potential that for Ru^III/Ru^II. Examples of metabolite-related DNA damage and oxidation, including the genotoxic potential of e-cigarettes, will be discussed.