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.

中文翻译：

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单壁碳纳米管与果糖脱氢酶之间的直接电子转移

在本文中，我们报告了解束的单壁碳纳米管 (SWCNT) 与酶黄素腺嘌呤二核苷酸 (FAD) 依赖性果糖脱氢酶 (FDH) 之间的直接电子转移 (DET) 反应。使用阴离子表面活性剂胆酸钠 (SC) 将 SWCNT 分散并将其分散在水溶液中，并对实验条件进行了优化。DET电极FDH/SWCNT-SC是通过逐层滴铸技术制造的。进行了循环伏安法实验，实验结果表明存在高水平的果糖浓度依赖性电流（FDC）。在对照实验中，使用多壁碳纳米管 (MWCNT) 或石墨烯薄片均未观察到 FDC。报道的 FDH 分子的尺寸为 ∼7 nm，它大于单个 SWCNT 的直径（1 nm）但小于单个 MWCNT 的直径（10 nm）和石墨烯薄片的尺寸（约 100 nm 正方形）。只有单个 SWCNT 可以与辅因子 FAD 和血红素 c 紧密接近（在 DET 的距离内），这两个因子都深深嵌入蛋白质外壳中。FDH/SWCNT-SC 电极的灵敏度为 27 μA mM ^-1 cm ^-2 ，线性动态范围为 1.4–18.5 mM，检测限为 2.1 μM。该电极与转化酶 (IVT) 结合使用，可以检测样品中果糖和蔗糖的总浓度（使用 IVT/FDH/SWCNT-SC 电极）。因此，当同时使用 FDH/SWCNT-SC 和 IVT/FDH/SWCNT-SC 电极时，果糖和蔗糖可以分别量化。使用这些电极测定的真实食物样品（在本案例中为 100% 果汁）中的果糖和蔗糖含量与从高效液相色谱 (HPLC) 实验中获得的含量非常吻合。