Glucose oxidase (GOx) is immobilised on the working electrode. Each
enzyme turnover oxidises one glucose molecule to gluconolactone while its
FAD cofactor is reduced, then re-oxidised by handing the electron pair
onward. In the natural O₂ pathway the electron pair reduces O₂ to
H₂O₂, which then has to diffuse to the electrode and be oxidised there —
two slow steps. A redox mediator (e.g. ferrocene) instead shuttles
the electron pair straight from the enzyme's FAD centre to the electrode,
skipping O₂ entirely, so the current tracks the enzyme reaction faster and
needs a lower operating potential.
turnover rate v = Vmax·[glucose] / (Km + [glucose]) (Michaelis–Menten)
I_glucose = n·F·A · v (Faraday's law)
I_total = I_glucose + I_interferent(mode, level)
- Glucose concentration — substrate available to GOx; current saturates at high concentration once every active site is occupied (Km ≈ 6 mM here).
- O₂ / H₂O₂ vs mediator — the mediator pathway reaches a higher plateau current and responds faster because electron transfer no longer waits on dissolved O₂ or H₂O₂ diffusion.
- Interferents — ascorbic and uric acid oxidise directly at the bare electrode. The H₂O₂ pathway needs a higher electrode potential, so interferents contribute a bigger false current there; the mediator's lower operating potential suppresses most of it.
The right-hand strip chart traces current over time; the calibration
chart plots the same current against glucose concentration — the curve a
real test-strip factory measures to convert current into a blood-glucose
reading.