Direct Electrochemistry of Myoglobin in Titanate Nanotubes Film

Titanate nanotubes (TNT) were proven to be efficient support matrixes for the immobilization of myoglobin (Mb). A comparative study was performed using the corresponding analogue, nanocrystalline anatase TiO2 (TNP). UV-visible absorption and FT-IR spectra show that Mb was not obviously denatured in TNT film in contrast to the significant denaturation of Mb in TNP film. Cyclic voltammetry and square wave voltammetry measurements were carried out using the MbTNT or Mb-TNP cast film-covered basal plane pyrolytic graphite electrode. The Mb-TNT film gave a well-defined, nearly reversible redox couple with the apparent formal peak potential (Ep) of 0.239 V (vs Ag/AgCl) in pH 5.5 buffer, whereas a relatively smaller, quais-reversible redox pair with Ep of -0.263 V was observed for the Mb-TNP film. The amounts of electroactive Mb in TNT film and TNP film were 15 and 10%, respectively. Moreover, the Mb-TNT film exerted facile direct electron transfer with the apparent heterogeneous electron-transfer rate constant (kET) of 86 +- 7 s-1, almost 4 times the 22 +- 5 s-1 value for the Mb-TNP membrane and higher than other Mbentrapped films reported. Additionally, the Mb-TNT film demonstrates good electrocatalytic reduction of hydrogen peroxide with a detection limit of 0.6 M, much lower than the 3.0 M value for the Mb-TNP electrode and other Mb-related film-modified electrodes reported so far. The MbTNT film exhibits higher peroxidase-like activity with the apparent Michaelis-Menton constant (KM) of 140 M, significantly lower than the 1300 M value for the Mb-TNP film. The functional hydroxyl group and the surface charge as well as tubular morphology of TNT are important factors to stabilize the bound protein.