Figure 2. (A) Surface morphology of electrodes with different coating materials studied using scanning electron micrography. The SEM of (ⅰ) Bare CP, (ⅱ) Nafion-coated CP, (ⅲ) Nafion/MWCNT-coated CP, and (ⅳ) Nafion/MWCNT/GOx-coated CP. (B) Investigation of variations in cyclic voltammetry graphs based on the electrode coating material: (i) Nafion, (ⅱ) Nafion/GOx, (ⅲ) Nafion/MWCNT/GOx.
A glucose-sensing system using a three-electrode method was developed to measure the concentration of glucose by comparing the reaction of current flowing through the WE and the CE with the RE. Depending on the material and coating of the electrode, the potential at which the enzyme and the solution reacted in the WE varied (36). In addition, the sensitivity and selectivity of the sensor were determined. The design of the WE and the degree of coating of the surface are important. The surface SEM images of the WE with the bare, Nafion, Nafion/MWCNT, and Nafion/MWCNT/GOx coating solutions for the 3D-printed CP electrode for the glucose biosensor are shown in Fig. 2A. As can be seen in Fig. 2A(i), when the CP electrode was not treated, the surface was non-uniform and had sparse holes. As can be seen in Fig. 2A(ⅱ), Nafion was uniformly coated on the surface of the electrode and served as a protective film. MWCNT grains attached gradually (Fig. 2A(ⅲ)), and MWCNT and GOx adhered well to the electrode surface (Fig. 2A(ⅳ)), causing an oxidation–reduction reaction in the electrode well. As can be seen in the CV diagrams of Nafion, Nafion/GOx, and Nafion/MWCNT/GOx coating solutions (Fig. 2B), in the electrode coated with only Nafion, a reduction reaction did not occur well. In contrast, in the CV graph of the electrode coated with Nafion/GOx, an oxidation-reduction reaction occurred continuously; however, a clear oxidation–reduction peak was not observed. As can be seen from the CV graph of the electrode coated with Nafion/MWCNT/GOx to which MWCNT was added, a clear oxidation peak was observed at around ±0.6 V, indicating that the solution and enzyme were well oxidized and reduced at around ±0.6 V. Moreover, the functioning of the glucose sensor was checked by measuring the peak at a potential of 0.6 V, as usually measured in conventional electrochemical glucose biosensors (37).