Effects of the surface chemistry and structure of carbon nanotubes on the coating of glucose oxidase and electrochemical biosensors performance

Glucose oxidase (GOx) has been immobilized on multiwall and herringbone carbon nanotubes (NTs) for glucose biosensing. Emphasis has been pointed in the effect of the structure and surface chemistry in the biosensor performance. Functionalization with carboxylic moieties renders a better performance than amino groups, and it also improves the performance with respect to that of bare NTs. No relationship was found between sensitivity to glucose and the amount of charge coming from the direct electron transfer between NTs and the flavin adenine dinucleotide (FAD) group of GOx. Based on these results, oxidized herringbones and multiwall NTs were used to prepare electrochemical sensors with different GOx loadings that were tested at positive and negative potentials. At positive potentials, the sensing mechanism is based on the oxidation of H2O2, and differences in sensitivity are related to the amount of active enzyme loaded on the carbon nanotubes surface. This amount seems to be enhanced by the presence of carboxylic moieties on oxidized NTs, as well as by the homogeneous surface structure and higher surface area of multiwall NTs. In this sense, the homogeneous structure of multiwall NTs seems to be more important than their larger surface area. At negative potentials, the detection mechanism is driven by oxygen consumption during the glucose oxidation. Thanks to the aforementioned reasons, multiwall NTs provide a similar sensibility and working range as herringbone NTs while using half the amount of GOx. These results point out the huge impact of NTs structure and surface chemistry upon the activity of GOx for electrochemical sensors.