BACKGROUND To immobilize protein firmly and maintain its native conformation, a 'nano molecular cage' was designed to encapsulate bovine serum albumin (BSA) on the surface of poly(methyl methacrylate) (PMMA) magnetic microspheres by cross-linking poly(ethylene imine) (PEI) chains. RESULTS The amount of BSA adsorbed on PMMA@PEI microspheres decreased significantly from 70.6 to 7.6 mg g(-1) microsphere after desorption under high ionic strength. A similar phenomenon could also be observed for BSA adsorbed on PMMA@PEI-CB (Cibacron Blue F3GA) microspheres. In contrast, the amount of BSA encapsulated in molecular cages only decreased a little after desorption. Furthermore, conformational analysis by Fourier transform infrared spectroscopy (FTIR) showed that the contents of alpha-helices and beta-sheets decreased slightly by 3.3 and 0.3% respectively on encapsulating BSA in molecular cages. Furthermore, the effects of various factors, including glutaraldehyde amount, cross-linking time and temperature, on BSA encapsulation were investigated. CONCLUSION The results demonstrated that molecular cages could not only immobilize protein firmly but also maintain its native conformation. Therefore molecular cage encapsulation is suggested to be a potentially valuable protocol in enzyme immobilization. (c) 2019 Society of Chemical Industry