Electrochemical modification at multiwalled carbon nanotube electrodes with Azure A for FADglucose dehydrogenase wiring: Structural optimization to enhance catalytic activity and stability

Electrochemical grafting is a suitable technology for fabricating electrode surfaces with new chemical functionalities whilst maintaining the bulk properties of the electrode, and electrochemical amine oxidation and diazonium salt reduction are two widely used techniques to achieve this end. Herein, we report the electrochemical reductive grafting of Azure A onto multiwalled carbon nanotube (MWCNT) electrodes for the efficient wiring of flavin adenine dinucleotide (FAD) dependent glucose dehydrogenase. The diazonium salt of Azure A is formed in situ and subsequently grafted onto the electrode surface through electrochemical reduction. The formal potential of the resultant Azure-A-modified electrode shifted to −0.05 V vs. Ag/AgCl upon radical coupling to the MWCNT electrode. Electron transfer from FAD buried in the protein shell to the electrode via Azure A was then observed in the presence of glucose in the buffer solution. This study focused on the important effect of CNT mass loading on Azure-A loading as well as bioelectrocatalytic activity and storage stability. The three-dimensional porous structure of the MWCNT electrode was determined to be favorable for the immobilization of flavin adenine dinucleotide dependent glucose dehydrogenase and efficient electron transfer via the Azure-A functionalities. The optimized 300 µg CNT-loaded modified electrode on glassy carbon (3 mm diameter) retains its initial activity for 3 d and 25% of its initial activity after 10 d. Furthermore, we show that grafted Azure A is stably immobilized on the MWCNTs for 1 month; therefore, the limiting stability factor is enzyme leaching and/or deactivation.