3.2. Adsorption mechanism
To study the adsorption mechanism, zeta potentials at different
pH between supports and lipase were measured, and the results
are shown in Fig. 4. The isoelectric point (IEP) of magnetic silica
nanoparticles is pH 2.7 similar to previous findings [39]. While
for the magnetic silica coated with ionic liquids, IEP increased to
pH 9.2. Ionic liquids changed surface electrokinetics of the magnetic
supports from anionic to cationic, which is consistent with
the findings that imidazolium cation of ionic liquids can form complex
with negatively charged surface [40]. The resultant positive
surface at neutral pH has strong affinity with negatively charged
lipase, whose IEP is 4.5 [41]. The immobilization of lipase on magnetic
nanoparticles supported ionic liquids was conducted by ionic
adsorption.