the cyclic voltammograms of the HRP-AuNCs/MWCNTs/CFUME (a), HRP/MWCNTs/CFUME (b), MWCNTs/CFUME (c), HRP-AuNCs/CFUME (d), HRP/CFUME (e) and CFUME (f)

A pair of small redox peaks appear in the cyclic voltamogram (CV) of the HRP/MWCNTs/CFUME (b) and MWCNTs/CFUME (c), and the anodic and cathodic peak potentials are located at about 0 V and - 0.1 V, respectively. This is attributed to the reduction and oxidation of carboxylic acid groups at the surface of MWCNTs. These groups were reduced to -CH2OH coupled with four electrons. Moreover, the background current was very large compared with that of bare CFUME (curve e) because of the increased surface charge [16]. A pair of stable and well-defined redox peaks with regard to Fe(III) to Fe(II) conversion of the immobilized HRP were observed on HRP-AuNCs/MWCNTs/CFUME (a). The anodic and cathodic peak potentials were - 0.10 V and - 0.21 V (vs. Ag/AgCl) at 100 mV/s, respectively. The formal potential (Eo’), obtained by averaging the potential values of the anodic and cathodic peaks (Epa and Epc), was - 0.16 V with a peak potential separation of approximately 110 mV, indicating a quasi-reversible process [17] The peak current did not change after 100 cyclic scans. This indicated that the HRP-AuNCs/MWCNTs film on the electrode surface was very stable. HRP/MWCNTs/CFUME (b) did not show a characteristic peak of HRP, which indicates that the AuNCs acting as the molecular electric wire can help to transfer the electron between the active center of the enzyme and the electrode. HRP-AuNCs/CFUME (d) did not show obvious redox peaks, and the current was much lower. This may be because the HRP-AuNCs could barely be absorbed on the bare CFUME without the aid of the MWCNTs. The enlarged interface area and good conductivity of the MWCNTs, which provides many efficient paths for direct electron conduction between the HRP-AuNCs and the CFUME, enhanced the current signals [18].

HRP/MWCNTs/CFUME（b）与MWCNTs/CFUME（c）的循环伏安图（cyclic voltamogram, CV）中出现一对小型氧化还原峰，其阳极与阴极峰电位分别约为0 V与-0.1 V，该现象归因于多壁碳纳米管（multi-walled carbon nanotubes, MWCNTs）表面羧基的还原与氧化反应，此类羧基可被还原为羟甲基（-CH2OH），并伴随4个电子的转移。此外，相较于裸CFUME（曲线e），其背景电流显著更高，这源于表面电荷的增加[16]。在HRP-AuNCs/MWCNTs/CFUME（a）上可观察到一对稳定且清晰的氧化还原峰，对应固定化辣根过氧化物酶（horseradish peroxidase, HRP）中Fe(III)向Fe(II)的转化。在100 mV/s的扫描速率下，其阳极与阴极峰电位分别为-0.10 V与-0.21 V（相对于Ag/AgCl参比电极）。通过对阳极峰电位（Epa）与阴极峰电位（Epc）取平均得到的形式电位（Eo’）为-0.16 V，峰电位差约为110 mV，表明该过程为准可逆过程[17]。经100次循环扫描后，峰电流未发生变化，这说明电极表面的HRP-AuNCs/MWCNTs薄膜具有极佳的稳定性。HRP/MWCNTs/CFUME（b）未出现HRP的特征峰，这表明作为分子导线的金纳米簇（gold nanoclusters, AuNCs）可介导酶活性中心与电极之间的电子传递。HRP-AuNCs/CFUME（d）未呈现明显的氧化还原峰，且电流强度显著更低，这可能是因为在无多壁碳纳米管辅助的情况下，HRP-AuNCs几乎无法吸附于裸CFUME表面。多壁碳纳米管所具备的增大的界面面积与优异导电性，为HRP-AuNCs与CFUME之间的直接电子传导提供了大量高效通路，从而增强了电流信号[18]。