Porous nanosheet-based ZnO microspheres for the construction of direct electrochemical biosensors

Nanosheet-based ZnO microsphere with porous nanostructures was synthesized by a facile chemical bath deposition method followed by thermal treatment, which was explored for the construction of electrochemical biosensors. Spectroscopic and electrochemical researches revealed the ZnO-based composite was a biocompatible immobilization matrix for enzymes with good enzymatic stability and bioactivity. With advantages of nanostructured inorganic–organic hybrid materials, a pair of stable and well-defined quasi-reversible redox peaks of hemoglobin was obtained with a formal potential of −0.345 V (vs. Ag/AgCl) in pH 7.0 buffer. Facilitated direct electron transfer of the metalloenzymes with an apparent heterogeneous electron transfer rate constant (ks) of 3.2 s−1 was achieved on the ZnO-based enzyme electrode. Comparative studies demonstrated the nanosheet-based ZnO microspheres were more effective in facilitating the electron transfer of immobilized enzyme than solid ZnO microspheres, which may result from the unique nanostructures and larger surface area of the porous ZnO. The prepared biosensor displayed good performance for the detection of H2O2 and NaNO2 with a wide linear range of 1–410 and 10–2700 μM, respectively. The entrapped hemoglobin exhibits high peroxidase-like activity for the catalytic reduction of H2O2 with an apparent Michaelis-Menten constant (KMapp) of 143 μM. The nanosheet-based ZnO could be a promising matrix for the fabrication of direct electrochemical biosensors, and may find wide potential applications in biomedical detection and environmental analysis.