Effect of Carbon Nanotubes on Direct Electron Transfer and Electrocatalytic Activity of Immobilized Glucose Oxidase

Abstract

Carbon nanotubes (CNTs) are excellent supports for electrocatalysts because of their large surface area, excellent electronic conductivity, and high chemical and structural stability. In the present study, the activity of CNTs on direct electron transfer (DET) and on immobilized glucose oxidase (GO_X) is studied as a function of number of walls of CNTs. The results indicate that the GO_X immobilized by the CNTs maintains its electrocatalytic activity toward glucose; however, the DET and electrocatalytic activity of GO_X depend strongly on the number of inner tubes of CNTs. The GO_X immobilized on triple-walled CNTs (TWNTs) has the highest electron-transfer rate constant, 1.22 s^-1, for DET, the highest sensitivity toward glucose detection, 66.11 ± 5.06 μA mM^-1 cm^-2, and the lowest apparent Michaelis-Menten constant, 6.53 ± 0.58 mM, as compared to GO_X immobilized on single-walled and multiwalled CNTs. The promotion effect of CNTs on the GO_X electrocatalytic activity and DET is most likely due to the electron-tunneling effect between the outer wall and inner tubes of TWNTs. The results of this study have general implications for the fundamental understanding of the role of CNT supports in DET processes and can be used for the better design of more effective electrocatalysts for biological processes including biofuel cells and biosensors.

Figure 1

TEM micrographs of (A) CNT-1, (B) CNT-2, (C) CNT-3, and (D) CNT-4.

Figure 2

CVs recorded at (A) bare GC electrode and CNT-modified electrodes and (B) GO_X and GO_X/CNT-modified GC electrodes in an N₂-saturated 0.1 M PBS solution at a scan rate of 50 mV s^–1.

Figure 3

CVs recorded at (A) GO_X/CNT-1, (B) GO_X/CNT-2, (C) GO_X/CNT-3, and (D) GO_X/CNT-4-modified GC electrodes in an N₂-saturated 0.1 M PBS solution at different scan rates from inner to outer curves: 10, 25, 50, 100, 150, 200, 250, 300, 350, and 400 mV s^–1. Inset shows the linear dependence of I_pa and I_pc on the scan rate.

Figure 4

CVs recorded at (A) GO_X/CNT-1-, (B) GO_X/CNT-2-, (C) GO_X/CNT-3-, and (D) GO_X/CNT-4-modified GC electrodes in an N₂- and O₂-saturated 0.1 M PBS solution at a scan rate of 50 mV s^–1.

Figure 5

CVs of (A) GO_X/CNT-1, (B) GO_X/CNT-2, (C) GO_X/CNT-3, and (D) GO_X/CNT-4-modified GC electrodes in an O₂-saturated 0.1 M PBS solution at different concentrations of glucose (with 0, 100, 200, 300, 400, 600, 800, 1000, 1200, 1400, 1600, 1800, 2000, 2200, 2400, and 2600 μL of 0.1 M glucose solution into 40 mL of PBS solution from outer to inner curves).

Figure 6

Amperometric responses of (A) GO_X/CNT-1-, (B) GO_X/CNT-2-, (C) GO_X/CNT-3-, and (D) GO_X/CNT-4-modified GC electrodes to successive additions of 100 μL of 0.1 M glucose to 40 mL of 0.1 M PBS solution. Solution was stirred at 150 rpm. Applied potential: −0.48 V vs Ag/AgCl.

Figure 7

Current changes obtained from the amperometric curves of about the four electrodes in the different glucose concentrations in 0.1 M PBS solution. Solution was stirred at 150 rpm. Applied potential: −0.48 V vs Ag/AgCl.

Figure 8

Plots of electrocatalytic activity of GO_X/CNT-modified GC electrodes as a function of the number of inner tubes of CNT substrates.

Figure 9

Scheme of the DET and electrocatalytic activity of GO_X immobilized on TWNTs via the electron-tunneling mechanism between the outer wall and inner tubes of TWNTs.