Electrochemical Behavior of Flavin Adenine Dinucleotide Adsorbed onto Carbon Nanotube and Nitrogen-Doped Carbon Nanotube Electrodes

Abstract

Flavin adenine dinucleotide (FAD) is a cofactor for many enzymes, but also an informative redox active surface probe for electrode materials such as carbon nanotubes (CNTs) and nitrogen-doped CNTs (N-CNTs). FAD spontaneously adsorbs onto the surface of CNTs and N-CNTs, displaying Langmuir adsorption characteristics. The Langmuir adsorption model provides a means of calculating the electroactive surface area (ESA), the equilibrium constant for the adsorption and desorption processes (K), and the Gibbs free energy of adsorption (ΔG°). Traditional ESA measurements based on the diffusional flux of a redox active molecule to the electrode surface underestimate the ESA of porous materials because pores are not penetrated. Techniques such as gas adsortion (BET) overestimate the ESA because it includes both electroactive and inactive areas. The ESA determined by extrapolation of the Langmuir adsorption model with the electroactive surface probe FAD will penetrate pores and only include electroactive areas. The redox activity of adsorbed FAD also displays a strong dependency on pH, which provides a means of determining the pKa of the surface confined species. The pKa of FAD decreases as the nitrogen content in the CNTs increases, suggesting a decreased hydrophobicity of the N-CNT surface. FAD desorption at N-CNTs slowly transforms the main FAD surface redox reaction with E1/2 at -0.84 V into two new, reversible, surface confined redox reactions with E1/2 at -0.65 and -0.76 V (vs Hg/Hg2SO4), respectively (1.0 M sodium phosphate buffer pH = 6.75). This is the first time these redox reactions have been observed. The new surface confined redox reactions were not observed during FAD desorption from nondoped CNTs.

FAD-sequestering proteins protect mycobacteria against hypoxic and oxidative stress

“…mV/pH unit, respectively) with no redox-linked pK a observed in the range of 3.6 -7.3. This contrasts again with electrontransfer proteins, such as flavodoxins, that have a redox-linked pK a between pH 4.6 and 6.8 (25,27) (Fig. 5b).…”

Direct Electrochemistry of Glucose Oxidase on Novel Free-Standing Nitrogen-Doped Carbon Nanospheres@Carbon Nanofibers Composite Film

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Tuning the reactivity of nanostructured indium tin oxide electrodes toward chemisorption

“…Such non ideality was previously reported with flavin-modified electrodes. [21][22][23] It may result from inhomogeneities in the adsorbate populations and/or surface binding sites, or from unfavorable lateral interactions between the adsorbed FMN molecules.…”