The Characterization and Optimization of an IrOx-based Glucose Biosensor
Creators: name: Sebastian, Holly
Publisher: name: Graduate Studies
Year: 2014
Subjects: subject: Chemistry--Physical
subject: Nanoparticle
subject: Glucose biosensor
subject: Glucose Oxidase
subject: Oxygen independent
subject: Direct electron transfer
subject: Nafion
subject: Iridium

description: Persons who live with diabetes must continuously monitor their blood glucose levels (BGLs) throughout the day to ensure their BGLs stay within the normal range. For this reason, glucose sensors must be extremely reliable, robust, accurate, reproducible and sensitive. However, glucose sensor designs face obstacles, such as poor accuracy and reproducibility, which is often the result of O2-dependency and poor biocompatibility. Therefore, the primary focus of this thesis has been to optimize and characterize a biocompatible Ir oxide (IrOx) nanoparticle (NP)/glucose oxidase (GOx) based glucose biosensor, with the goal of achieving an accurate and reproducible O2-independent glucose response. Two approaches were used for the fabrication of this IrOx-based glucose sensor. In the first, a sensing ink consisting of three key components, IrOx NPs, GOx and Nafion®, and a water/ethanol solvent was aliquot-deposited on a Au substrate. In the second method, GOx molecules were immobilized directly to a Au or carbon substrate by covalently linking GOx to a diazonium-derived film deposited on the substrate. An aliquot of the ethanolic Ir ink was then (assumed to be) intercalated into the film. Both films were dried, and the Ir was subsequently electrochemically oxidized to IrOx. The first goal of sensor construction using aliquot-deposition was to understand the effect of each of the ink components on their distribution in the dried films and the interconnectedness particularly of the IrOx NPs, then establishing the effect of these variables on the glucose response. The goal of diazonium sensor construction was to optimize the immobilization matrix, and obtain uniform film morphologies that exhibit sensitive and reproducible O2-independent glucose signals. IrOx NP films (fabricated from ethanolic inks) exhibited uniform film morphologies, and rapid electron transfer kinetics, while films fabricated from aqueous or GOx-containing inks were aggregated and thus easily dislodged from the Au. The addition of 1 wt. % Nafion® to the ink enhanced IrOx NP stability and redox kinetics. The GOx regeneration route could be significantly affected by manipulating the film component ratios. Direct mediation was favored by altering the GOx:IrOx ratio, as well as the water and Nafion® content in the ink. Film thickness and ink sonication prior to film deposition also influences the GOx regeneration route, as well as film reproducibility and sensitivity. To prevent IrOx aggregation (observed for films fabricated from water or GOx-containing inks), GOx was immobilized (diazonium chemistry) to the current collector, and the ethanolic Ir ink was then (assumed to be) intercalated into the film. GOx regeneration took place via the desired IrOx-mediation, independent of the amount of O2 in solution. By increasing the surface coverage of the diazonium-derived layer (to a maximal coverage) and the amount of IrOx NPs in the film, the glucose signal was enhanced, but was not comparable to those obtained using the aliquot method. Overall, it is clear that our sensor sensitivity, reproducibility and O2-independence are quite good relative to other published work, improved here by optimization of the sensing film morphology and stability Further, the goal of achieving an O2-independent glucose signal was essentially met by avoiding film aggregation initiated by the non-uniform drying tendency of the aqueous and ethanolic phases in the ink.