Immobilization of NRRL 3631 lipase and characterization of silica-coated magnetite (Fe O) nanoparticles

Introduction and purpose
The uncoated magnetite (M) and silica-coated magnetite (MS) nanoparticles have been suggested as carriers for the immobilization of enzymes to improve their activity and stability. The objective of this study was to demonstrate the potential use of magnetic nanoparticles in bioengineering applications, using NRRL 3631 lipase as the model enzyme.
Materials and methods
The magnetite (FeO) particles were synthesized by the chemical coprecipitation technique, that is, Massart’s process with minor modifications, using stable ferrous and ferric salts with ammonium hydroxide as the precipitating agent. The uncoated and coated magnetite nanoparticles for immobilizing the lipase were characterized according to the particle sizes, as measured from the transmission electron microscope images. The infrared and X-ray powder diffraction spectra can well explain the bonding interaction and crystal structures of various samples, respectively.
Results and conclusion
Different concentrations of silica-coated magnetite (MS) nanoparticles were used as cross-linking agents. A silica concentration of 1% was proven to be more suitable, with an immobilization efficiency of 96%. The transmission electron microscope images revealed the diameters of the uncoated magnetite particles to be 10–16 nm and those of the coated particles to be about 11 nm. The optimal pH and temperature of the immobilized lipase were 5–6 and 40πC, respectively. There was a slight decrease in the residual activity of the immobilized lipase at 60πC for 1 h. The kinetic constants and were determined to be 250 U/mg protein and 20 mmol/l, respectively, for the immobilized lipase. The residual activity of the immobilized lipase remained over 51% despite being used repeatedly seven times. It can be concluded that FeO magnetic nanoparticles and silica-coated magnetite (MS) nanoparticles have been successfully prepared with excellent properties using the chemical coprecipitation technique with some modifications. The silica coating appeared to be effective in protecting the magnetite from being converted to other oxide species. The results of the X-ray powder diffraction indicate that the composites were in the nanoscopic phase. The resulting immobilized lipase had better resistance to pH and temperature inactivation compared with free lipase and exhibited good reusability.