Stability and reusability of free and immobilized enzyme; a stability studies free lipase (■, black filled square), lipase-CuFe2O4 dense NPs (●, red filled circle) and lipase-CuFe2O4 hollow NPs (▲, blue filled triangle), b reusability of lipase immobilized on hollow and dense CuFe2O4 nanoparticles (color figure online)