Synthesis and characterization of enzyme–magnetic nanoparticle complexes: effect of size on activity and recovery

The influence of particle size on the activity and recycling capabilities of enzyme conjugated magnetic nanoparticles was studied. Co-precipitation and oxidation of Fe(OH)2 methods were used to fabricate three different sizes of magnetic nanoparticles (5 nm, 26 nm and 51 nm). Glucose oxidase was covalently bound to the magnetic nanoparticles by modifying the surfaces with 3-(aminopropyl)triethoxysilane (APTES) and a common protein crosslinking agent, glutaraldehyde. Analysis by Transmission Electron Microscopy (TEM) showed that the morphology of the magnetic nanoparticles to be spherical and sizes agreed with results of the Brunauer, Emmett, and Teller (BET) method. Magnetic strength of the nanoparticles was analyzed by magnetometry and found to be 49 emu g−1 (5 nm), 73 emu g−1 (26 nm), and 85 emu g−1 (51 nm). X-ray photoelectron spectroscopy (XPS) confirmed each step of the magnetic nanoparticle surface modification and successful glucose oxidase binding. The immobilized enzymes retained 15–23% of the native GOx activity. Recycling stability studies showed approximately 20% of activity loss for the large (51 nm) and medium (26 nm) size glucose oxidase-magnetic nanoparticle (GOx-MNP) bioconjugate and about 96% activity loss for the smallest GOx-MNP bioconjugate (5 nm) after ten cycles. The bioconjugates demonstrated equivalent total product conversions as a single reaction of an equivalent amount of the native enzyme after the 5th cycle for the 26 nm nanoparticles and the 7th cycle for the 51 nm nanoparticles.