Abstract
Biomorphic calcium phosphate (CaP) microspheres with hierarchical porous structure were synthesized using natural cole pollen grains as templates and were further employed for the immobilization of horseradish peroxidase (HRP). Scanning electron microscopy and Fourier transform infrared spectroscopy revealed (a) the porous structure of the CaP microspheres, (b) the effective immobilization, and (c) the retention of the conformation of HRP on CaP. The immobilized HRP was placed on a glassy carbon electrode where it underwent a direct, fully reversible, and surface-controlled redox reaction with an electron transfer rate constant of 1.96 s−1. It also exhibits high sensitivity to the reduction of H2O2. The response to H2O2 is linear in the 5.00 nM to 1.27 μM concentration range, and the sensitivity is 30357 μA⋅mM−1⋅cm−2. The detection limit (at an SNR of 3) is as low as 1.30 nM. The apparent Michaelis–Menten constant (KMapp) of the immobilized enzyme is 0.92 μM. This new CaP with hierarchical porous structure therefore represents a material that can significantly promote the direct electron transfer between HRP and an electrode, and is quite attractive with respect to the construction of biosensors.
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