Adsorption of cholesterol oxidase and entrapment of horseradish peroxidase in metal-organic frameworks for the colorimetric biosensing of cholesterol.

The co-immobilization of two enzymes onto single support commonly exhibits low efficiency due to the competition against limited sites. Water-stable metal-organic frameworks (MOFs) [i.e., PCN-333(Al)] with a high surface area and ultra-large cavities were employed to efficiently adsorb cholesterol oxidase (ChOx) and encapsulate horseradish peroxidase (HRP), respectively. The prepared PCN-333/ChOx&HRP was characterized through SEM, XRD, confocal microscopy, N2 adsorption isotherms, and thermal gravity analysis (TGA). The high surface area and high concentration of mesoporous cages resulted in the high loadings of both ChOx and HRP. The absorbed ChOx and the encapsulated HRP presented excellent activities without additional chemical modification. The immobilized enzymes were stable against protease digestion, organic solvents, temperature changes, and pH variation. Thus, a colorimetric biosensor for cholesterol detection was fabricated depending on cascade catalytic reactions of the immobilized bi-enzymes. An extended linear range from 0.0 to 40.0 μM with a low detection limit of 0.6 μM was obtained using the biosensor. The co-immobilization of the enzymes onto the surface and into the mesopores of MOFs provided a new and excellent platform for the development of highly stable and sensitive colorimetric biosensors.