Amperometric Detection of Aqueous Silver Ions by Inhibition of Glucose Oxidase Immobilized on Nitrogen-Doped Carbon Nanotube Electrodes

An amperometric glucose biosensor based on immobilization of glucose oxidase on nitrogen-doped carbon nanotubes (N-CNTs) was successfully developed for the determination of silver ions. Upon exposure to glucose, a steady-state enzymatic turnover rate was detected through amperometric oxidation of the H₂O₂ byproduct, directly related to the concentration of glucose in solution. Inhibition of the steady-state enzymatic glucose oxidase reaction by heavy metals ions such as Ag⁺, produced a quantitative decrease in the steady-state rate, subsequently creating an ultrasensitive metal ion biosensor through enzymatic inhibition. The Ag⁺ biosensor displayed a sensitivity of 2.00 × 10⁸ ± 0.06 M^–1, a limit of detection (σ = 3) of 0.19 ± 0.04 ppb, a linear range of 20–200 nM, and sample recovery at 101 ± 2%, all acquired at a low-operating potential of 0.05 V (vs Hg/Hg₂SO₄). Interestingly, the biosensor does not display a loss in sensitivity with continued use due to the % inhibition based detection scheme: loss of enzyme (from continued use) does not influence the % inhibition, only the overall current associated with the activity loss. The heavy metals Cu²⁺ and Co²⁺ were also detected using the enzyme biosensor but found to be much less inhibitory, with sensitivities of 1.45 × 10⁶ ± 0.05 M^–1 and 2.69 × 10³ ± 0.07 M^–1, respectively. The mode of GOx inhibition was examined for both Ag⁺ and Cu²⁺ using Dixon and Cornish-Bowden plots, where a strong correlation was observed between the inhibition constants and the biosensor sensitivity