H₂O₂ Detection at Carbon Nanotubes and Nitrogen-Doped Carbon Nanotubes: Oxidation, Reduction, or Disproportionation?

Abstract

The electrochemical behavior of hydrogen peroxide (H₂O₂) at carbon nanotubes (CNTs) and nitrogen-doped carbon nanotubes (N-CNTs) was investigated over a wide potential window. At CNTs, H₂O₂ will be oxidized or reduced at large overpotentials, with a large potential region between these two processes where electrochemical activity is negligible. At N-CNTs, the overpotential for both H₂O₂ oxidation and reduction is significantly reduced; however, the reduction current from H₂O₂, especially at low overpotentials, is attributed to increased oxygen reduction rather than the direct reduction of H₂O₂, due to a fast chemical disproportionation of H₂O₂ at the N-CNT surface. Additionally, N-CNTs do not display separation between observable oxidation and reduction currents from H₂O₂. Overall, the analytical sensitivity of N-CNTs to H₂O₂, either by oxidation or reduction, is considerably higher than CNTs, and obtained at significantly lower overpotentials. N-CNTs display an anodic sensitivity and limit of detection of 830 mA M^–1 cm^–2 and 0.5 μM at 0.05 V, and a cathodic sensitivity and limit of detection of 270 mA M^–1 cm^–2 and 10 μM at −0.25 V (V vs Hg/Hg₂SO₄). N-CNTs are also a superior platform for the creation of bioelectrodes from the spontaneous adsorption of enzyme, compared to CNTs. Glucose oxidase (GOx) was allowed to adsorb onto N-CNTs, producing a bioelectrode with a sensitivity and limit of detection to glucose of 80 mA M^–1 cm^–2 and 7 μM after only 30 s of adsorption time from a 81.3 μM GOx solution