Direct electron transfer between single-walled carbon nanotube and fructose dehydrogenase

In this paper, we report the direct electron transfer (DET) reaction between debundled single-walled carbon nanotubes (SWCNTs) and the enzyme flavin adenine dinucleotide (FAD)-dependent fructose dehydrogenase (FDH). An anionic surfactant, sodium cholate (SC), was used to debundle the SWCNTs and disperse them in an aqueous solution, and the experimental conditions were optimized. The DET electrode, FDH/SWCNT-SC, was fabricated by the layer-by-layer drop-casting technique. Cyclic voltammetry experiments were conducted, and the experimental results revealed the presence of a high level of fructose concentration-dependent current (FDC). In the control experiment, FDC was not observed with either multi-walled carbon nanotubes (MWCNTs) or graphene flakes. The reported dimension of the FDH molecule is ∼7 nm, which is larger than the diameter of an individual SWCNT (1 nm) but smaller than the diameter of an individual MWCNT (10 nm) and the dimensions of a graphene flake (∼100 nm square). Only individual SWCNTs can reside in close proximity (within the distance for DET) of the cofactors FAD and heme c, both of which remain deeply embedded within the protein shell. The FDH/SWCNT-SC electrode had a sensitivity of 27 μA mM-1 cm-2, a linear dynamic range of 1.4-18.5 mM, and a detection limit of 2.1 μM. The electrode, combined with the enzyme invertase (IVT), can detect the total concentration of fructose and sucrose in samples (using the IVT/FDH/SWCNT-SC electrode). Therefore, fructose and sucrose can be separately quantified when both FDH/SWCNT-SC and IVT/FDH/SWCNT-SC electrodes are used. The amounts of fructose and sucrose in real food samples (100% fruit juice in the present case) determined using these electrodes agreed well with the amounts obtained from high-performance liquid chromatography (HPLC) experiments.