A smartphone-based, calibration-free Nafion-modified SPCE electrochemical sensor for clinical validation of 6-thioguanine in erythrocyte lysate and serum
Abstract
The immunosuppressive prodrugs azathioprine and 6-mercaptopurine necessitate therapeutic drug monitoring of their active metabolite, 6-thioguanine (6-TG), due to their narrow therapeutic window and substantial risk of severe adverse effects. Conventional chromatographic techniques are accurate but impractical for point-of-care (POC) applications. Conversely, existing electrochemical sensors, despite their widespread use, suffer from an over-reliance on nanomaterial modifications to enhance analytical performance and face persistent challenges associated with calibration. This work resolves a long-standing knowledge gap by providing the first experimental validation of the 6-TG redox mechanism on a Nafion-modified screen-printed carbon electrode (SPCE). Through dithiothreitol intervention and electrochemical kinetic analysis, we confirm that 6-TG oxidation proceeds via a disulfide bond-mediated, four-electron, four-proton multi-step pathway. Critically, elucidation of this mechanism reveals a consistent kinetic framework, which directly inspires a universal-slope (US) calibration-free strategy wherein the calibration slope becomes a stable parameter, obviating frequent recalibration for POC applications. Leveraging this mechanistic insight, we constructed a smartphone-assisted electrochemical sensor on the same Nafion/SPCE platform for on-site detection of 6-TG in erythrocyte lysate and serum. The sensor exhibits a linear response from 0.1 to 15 μM in erythrocyte lysate,…
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