Enzyme immobilization on ZIF-67/MWCNT composite engenders high sensitivity electrochemical sensing

Metal-organic frameworks (MOFs) as a type of potential matrix for enzyme immobilization have attracted much interest. However, the sensitivity of most MOF-enzymatic electrochemical biosensors has not been satisfactory so far due to the poor conductivity and low apparent affinity of MOFs. In this work, new strategies have been presented: (i) using MOFs that themselves possess catalytic properties for substrates, so the synergistic catalysis by combining the MOFs and immobilized enzymes can be reinforced; (ii) introducing hydrophilic carbon nanomaterials and preparing MOF/carbon composites, not only the conductivity of the matrix but also the apparent affinity of immobilized enzyme for hydrophilic substrate can be remarkably enhanced. Horseradish peroxidase (HRP) and zeolite imidazolate framework-67(Co)/multi-walled carbon nanotube composite (ZIF-67(Co)/MWCNT) were adopted as the model enzyme and the immobilized matrix to illustrate and examine the proposed strategies. Compared with single component (ZIF-67(Co) or MWCNTs) based HRP electrochemical biosensors, the composite electrochemical biosensor displayed increased apparent substrate affinity (Michaelis constant K m reduced by ~75%) and 1.3-fold increased sensitivity in the detection of H 2 O 2 . The sensitivity (315   μA   mM −1   cm −2 ) was also much larger than that of most previously reported MOF-HRP electrochemical biosensors. The proposed strategies are envisaged to be useful for other MOF-enzymes systems, which will be beneficial to the development of biocatalysis and biosensing.