Direct electron transfer of cytochrome C and its electrocatalytic properties on multiwalled carbon nanotubes/ciprofloxacin films

In this study, stable and homogenous thin films of multiwalled carbon nanotubes (MWCNTs) were obtained on conducting surface using ciprofloxacin (CF, fluoroquinolone antibiotic) as an effective-dispersing agent. Further, MWCNTs/CF film modified electrodes (glassy carbon and indium tin oxide-coated glass electrode) are used successfully to study the direct electrochemistry of proteins. Here, cytochrome C (Cyt-C) was used as a model protein for investigation. A MWCNTs/CF film modified electrode was used as a biocompatible material for immobilization of Cyt-C from a neutral buffer solution (pH 7.2) using cyclic voltammetry (CV). Interestingly, Cyt-C retained its native state on the MWCNTs/CF film. The Cyt-C adsorbed MWCNTs/CF film was characterized by scanning electron microscopy (SEM), UV–visible spectrophotometry (UV-vis) and CV. SEM images showed the evidence for the adsorption of Cyt-C on the MWCNTs/CF film, and UV–vis spectrum confirmed that Cyt-C was in its native state on MWCNTs/CF film. Using CV, it was found that the electrochemical signal of Cyt-C was highly stable in the neutral buffer solution and its redox peak potential was pH dependent. The formal potential (−0.27 V) and electron transfer rate constant (13 ± 1 s⁻¹) were calculated for Cyt-C on MWCNTs/CF film modified electrode. A potential application of the Cyt-C/MWCNTs/CF electrode as a biosensor to monitor H₂O₂ has been investigated. The steady-state current response increases linearly with H₂O₂ concentration from 2 × 10⁻⁶ to 7.8 × 10⁻⁵ M. The detection limit for determination of H₂O₂ has been found to be 1.0 × 10⁻⁶ M (S/N = 3). Thus, Cyt-C/MWCNTs/CF film modified electrode can be used as a biosensing material for sensor applications.