Enhanced glucose detection using enzyme-immobilized ZnO/ZnS core/sheath nanowires

The role of ZnS shell was explored in the enhancement of fluorescent glucose sensing of enzyme-conjugated ZnO/ZnS core/sheath nanowires. High-density ZnO nanowires were vertically grown on Si substrates by carbothermal method and they were coated with almost monolayer ZnS nanocrystals to obtain ZnO/ZnS core/sheath nanowire structures. The surface of ZnO nanowires and ZnO/ZnS nanowires was modified with mercapto-acetic acid (MAA) and MAA-terminated nanowires were activated by esterification of n-hydroxysulfo-succinimide (Sulfo-NHS). Glucose oxidase (GOx), an enzyme, could be covalently combined with NHS-terminated nanowires to form ZnO-MAA-GOx and ZnO/ZnS-MAA-GOx bioconjugates. Due to the surface passivation of ZnO by ZnS sheath, ZnO/ZnS-MAA-GOx showed increased band-edge emission compared to ZnO-MAA-GOx. Glucose detection characteristics were investigated monitoring the change in PL intensity (∼380 nm) and the results were compared to each other. Both samples showed almost linear increase in PL intensity with glucose concentration from ∼3.51 to ∼24.1 mM. However, ZnO/ZnS-MAA-GOx showed about two-times higher sensitivity to glucose and lower detection limit (0.14 mM) compared to ZnO-MAA-GOx (0.23 mM). This is most possibly due to more effective electron injection into the conduction band of ZnO nanowires, arising from the passivation of surface defects and surface dangling bonds of ZnO by ZnS sheath formation. Also, both ZnO-MAA-GOx and ZnO/ZnS-MAA-GOx showed fast glucose response time of <10 and <5 s, respectively.