Fabrication of poly(ethylene glycol)-based hydrogels entrapping enzyme-immobilized silica nanoparticles

In this study, we immobilized enzymes by combining covalent surface immobilization and hydrogel entrapment. A model enzyme, glucose oxidase (GOX), was first covalently immobilized on the surface of silica nanoparticles (SNPs) via 3-aminopropyltriethoxysilane (APTES), and the resultant SNP-immobilized enzyme was physically entrapped within photopolymerized hydrogels prepared from two different molecular weights (MWs) (575 and 8000 Da) of poly(ethylene glycol)(PEG). The hydrogel entrapment resulted in a decrease in reaction rate and an increase in apparent K_m of SNP-immobilized GOX, but these negative effects could be minimized by using hydrogel with a higher MW PEG, which provides higher water content and larger mesh size. The catalytic rate of the PEG 8000 hydrogel was about ten times faster than that of the PEG 575 hydrogel because of enhanced mass transfer. Long-term stability test demonstrated that SNP-immobilized GOX entrapped within hydrogel maintained more than 60% of its initial activity after a week, whereas non-entrapped SNP-immobilized GOX and entrapped GOX without SNP immobilization maintained less than 20% of their initial activity. Incorporation of SNPs into hydrogel enhanced the mechanical strength of the hydrogel six-fold relative to bare hydrogels. Finally, a hydrogel microarray entrapping SNP-immobilized GOX was fabricated using photolithography and successfully used for quantitative glucose detection.