Immobilization of avidin on the functionalized carbon nanotubes

Immobilization of avidin was carried out by functionalizing the multi-wall carbon nanotubes (MWCNTs). Treatment with nitric acid and sulfuric acid mixture (1:3, optimized ratio) leads to the functionalization of nanotubes as observed from Fourier transform infrared absorption spectroscopy (FTIR) measurements. Avidin was coupled with the solution of N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylamino propyl)-carbodiimide hydrochloride (EDC) and then immobilized on nanotubes. X-ray photoelectron spectroscopic (XPS) studies show a shift in the peak position of C 1s towards lower energy side and changes in the bands with treatment/immobilization. The shoulders and shift in photoelectron peak positions indicate the destruction of the graphite structure of the surface layer. SEM images showed that after functionalization, the nanotubes are seen with open ends, granular surface and are joined together. This indicates that after treatment, the CNTs reactivity increased at the ends as well as at the sidewalls. It is believed that the NHS often assist the carbodiimide coupling in the presence of EDC, reacts with the amine function to yield the amide bond. The carbodiimide catalyzes the formation of amide bonds between carboxylic acids and amines by activating carboxyl. The reaction of complex containing avidin can form a covalent bond with functionalized carbon nanotubes as observed from FTIR and XPS measurements.