Direct electrochemistry of hemoglobin at vertically-aligned self-doping TiO2 nanotubes: A mediator-free and biomolecule-substantive electrochemical interface

The present work is dedicated to making the best of vertically-aligned TiO2 nanotubes (TNTs) array to serve as a prospectively ideal “vessel” for protein immobilization and biosensor applications. The TNTs fabricated by electrochemical anodizing possess the advantageous of perpendicular alignment and tailored tubular architecture, as well as the good biocompatibility and hydrophilicity. But the electron-transfer resistance of the as-grown (AG-) TNTs is too large for the direct electron transfer and electrochemical biosensing. A simple strategy on controllable electrochemical reduction treatment of TNTs is adopted on it, leading TNTs in situ self-doped with Ti(III), which makes the Ti(III)–TNTs much better conductivity while the tubular and crystal structure of TNTs array still well maintained. Results show that the TNTs can be used as a super vessel for rapid and substantive immobilization of hemoglobin (Hb), with a large surface electroactive Hb coverage (Γ*) of 1.5 × 10−9 mol cm−2. The enhanced direct electron transfer of Hb is commendably observed on the Ti(III)–TNTs/Hb biosensor with a couple of well-defined redox peaks compared with the AG-TNTs/Hb. The biosensor further exhibits fast response, high sensitivity and stability for the amperometric biosensing of H2O2 with the detection limit of 1.5 × 10−6 M, and the apparent Michaelis–Menten constant of 1.02 mM.