Impact of the size effect on enzymatic electrochemical detection based on metal-organic frameworks.

Metal-organic frameworks (MOFs) hold a great promise as immobilization carriers for enzymes and other biomolecules, owing to their enhanced stability, selectivity and controllability. However, enzyme-MOF complexes usually lead to a decrease in the apparent enzyme activity and apparent substrate affinity as a result of the constrained structure of the enzyme and the mass transfer limitation of the substrate, respectively. These results consequently impede the applications of enzyme-MOF complexes in biocatalysis and biosensing. In this study, zeolitic imidazolate framework-8 (ZIF-8) was synthesized to immobilize cytochrome c (Cyt c) via a one-step co-precipitation process under mild conditions. By adjusting the molar ratio of precursors, enzyme-MOF composites with different sizes from 100 nm to 1.3 μm were prepared. The decreased size of the prepared MOFs generated an increase in substrate affinity (with an over 50% decrease in the Michaelis constant Km) and a 6.4-fold improvement in the apparent enzyme activity with a 6.26-fold increase in the enzymatic electrochemical detection sensitivity compared with native Cyt c. The enzyme-MOF composites were coated on a screen-printed electrode for the sensitive and fast detection of H2O2, which is the most common representative of reactive oxygen species in cellular environments, showing the potential for the construction of efficient biosensors with applications in biomedicine.