Supported Microwires for Electroanalysis: Sensitive Amperometric Detection of Reduced Glutathione

A carbon microfiber (7 μm diameter) is employed herein as an electroanalytical sensor. The fabricated sensor is cheap, is disposable, and requires only 150 μL of samples. The carbon fiber is surface-mounted onto an inert surface to overcome the problems of the fragility of the microwire and the possible interference of convective force due to the nonrigid nature of the wires, as well as to improve the reproducibility in length and the amperometric responses. As the cylindrical electrode is supported on a surface, the diffusion of redox-active species to the electrode is partially blocked by the substrate. A theoretical model is developed to account for this hindered diffusion. The mass-transport regime is altered from "linear" at very short time, where the amperometric responses of the supported microwire closely resemble that of an isolated free-standing cylinder (current ∝ electrode area), to "convergent" at long time where its response now tends toward that of a hemicylinder of equal radius. The model is validated using chronoamperometry and cyclic voltammetry of an ideal outer-sphere redox probe, reversible ferrocene methanol oxidation. The fabricated microwire electrode is further applied to the system of irreversible 2-nitro-5-thiobenzoate oxidation used in the detection of reduced glutathione (GSH). The microwire electrode shows significantly higher ratio of Faradaic to non-Faradaic currents as compared to microdisk, macrodisk or carbon nanotube modified electrodes. Using the fabricated microwire, GSH can be detected with the sensitivity of 0.7 nA μM^-1 and the limit of detection of 0.5 μM (3 s_B/m).