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).