Highly Sensitive Hydrogen Peroxide Biosensor Based on Tobacco Peroxidase Immobilized on p-Phenylenediamine Diazonium Cation Grafted Carbon Nanotubes: Preventing Fenton-like Inactivation at Negative Potential

Herein, we present a novel electrode platform for H2O2 detection based on the immobilization of recombinant Tobacco Peroxidase (r-TOP) onto graphite electrodes (G) modified with p-phenylenediamine (p-PD) diazonium cation grafted multi-walled carbon nanotubes (MWCNTs). The employment of both p-phenylenediamine moieties and covalent cross-linking by using glutaraldehyde allowed us to enhance the sensitivity, stability, and selectivity toward H2O2 detection, as well as preventing enzyme inactivation due to the electro-Fenton reaction. This reaction continuously produces hydroxyl radicals, whose high and unselective reactivity is likely to reduce drastically the operating life of the biosensor. The protection against the electro-Fenton reaction is mainly ascribed to a beneficial enzyme immobilization leading to a correct orientation achieved through cross-linking the enzyme in combination with interaction between the uncoupled -NH2 groups (mainly uncharged at pH 7, considering a pKa of 4.6) available on the electrode surface and the enzyme. In particular, the electrode based on the r-TOP/p-PD/MWCNTs/G platform showed a lower limit of detection of 1.8 μM H2O2, an extended linear range between 6 and 900 μM H2O2, as well as a significant increase in sensitivity (63.1±0.1 μA mM−1 cm−2) compared with previous work based on TOP. Finally, the r-TOP/p-PD/MWCNTs/G electrode was tested in several H2O2 spiked food samples as a screening analytical method for the detection of H2O2.