Xylose-insensitive direct electron transfer biosensor strip with single-walled carbon nanotubes and novel fungal flavin adenine dinucleotide glucose dehydrogenase

Commercial biosensor strip for self-blood sugar monitoring for diabetes is type of mediated electron transfer (MET), where an electron transfer mediator is used. Here, a xylose-insensitive direct electron transfer (DET) glucose biosensor strip is developed for the first time. It overcomes the disadvantages of MET counterparts. A DET formation in which single-walled carbon nanotubes (SWCNTs), debundled using sodium cholate (SC) surfactant, are positioned near the reaction center pocket of flavin adenine dinucleotide glucose dehydrogenase (FAD-GDH) is constructed. Concomitantly, it is found that thymol (TM) increases the DET current seven times compared to when it is not used. TM does not have a specific redox potential and does not work without carbon nanotubes (CNTs), but assisted DET between the CNTs and FAD-GDH. Therefore, it is not an electron transfer mediator but a DET enhancement chemical. Long-term storage tests show that GDH/TM/SWCNT-SC electrodes are not naturally reduced unlike MET biosensor electrodes. The 'bionanoink,' composed of glycan chain rich novel fungal FAD-GDH, SWCNTs, SC, and TM in aqueous solution is suitable for the fabrication of a DET biosensor strip that is insensitive to xylose. The resulting DET biosensor strip had the required sample volume of 900 nL (1.5 × 0.1 × 6.0 mm), a measurement range of 0-600 mg/mL, measurement time of 10 s, the ability for repeated measurements, and a 5% coefficient of variation in real sample measurements. A low cost biosensor will be widely spread, consequently, will contribute to the realization of a healthy society.