Is the iron center important? Comparison of the electrochemistry between poly-phen-NH₂ and poly[Fe(phen-NH₂)₃²⁺] modified electrodes

5-Amino-1,10-phenanthroline (denoted phen-NH₂) and its iron(II) complex, Fe(phen-NH₂)₃²⁺, were prepared as polymer modified electrodes for electrochemical studies. According to the cyclic voltammograms and in situ electrochemical quartz-crystal-microbalance spectra (EQCM) recorded for Fe(phen-NH₂)₃²⁺, phen-NH₂ and 1,10-phenanthroline (denoted phen), Fe(phen-NH₂)₃²⁺ can be immobilized on the Pt-sputtered crystal electrode via an anodic polymerization. This polymerization is likely to be initiated by the oxidation of the amino group in the ligand. Although phen-NH₂ can also be polymerized on the electrode surface, the resulting polymer film is less stable compared with its iron derivative. The poly[Fe(phen-NH₂)₃²⁺] electrode showed a significant sensitivity to hydrogen peroxide, leading to a linear calibration curve up to 10 mM at pH 5. The detection limits reached a level of 10 μM. The rate constant (pseudo-first-order) of the reaction between the reduced poly[Fe(phen-NH₂)₃²⁺] and H₂O₂ was determined to be 470 M^-1 s^-1 at this pH. Due to this catalytic property, a glucose sensor was developed. Although experiments suggested that some catalytic sites in the polymer film might be buried by the bulky enzyme (GOx), this modified electrode showed a significant ability in the detection of glucose. The linear sensitivity covered a range of 0.1-60 mM at pH 5. The detection limits reached a level of 0.1 mM.