3. Results and discussion
3.1. Typical response curves of the sensor system
Concentrations of each standard glucose solution were measured
using the enzyme sensor, and Fig. 2A shows the results.
When the sensor was transferred from buffer solution to glucose
solution, sensor response (oxygen concentration) increased transiently
from the baseline, then gradually decreased. This was
attributed to dissolved oxygen in the buffer solution in the hollow
container being oxidized by the glucose oxidase, and this
reduction in dissolved oxygen was detected by the sensor. In
addition, the transient increase in response was caused when the
sensor touched the air during transfer to the buffer or solution.
Sensor response decreased with increasing glucose concentration
due to the fact that oxygen is consumed by the action of
glucose oxidase (Fig. 2A). Fig. 2B shows the response curve at
a glucose concentration of 10.8 mg dl−1. In the present study,
based on oxygen decrease 1 or 3 min after oxygen concentration
returned to baseline, slope of the regression line was calculated
using the least squares method to determine oxygen reduction
per unit time. With the above-mentioned methods, 1 and 3 min
sensor outputs were compared using standard glucose solutions
(3.6–18.0 mg dl−1). Little difference was observed between the
two values. As the objective of the present study was to measureblood glucose levels by directly inserting the sensor into
fish, testing time should be as short as possible. In subsequent
experiments, 1 min oxygen reduction was utilized as analytical
signal (mg l−1 min−1) of the sensor. Fig. 3 shows the relationship
between analytical signal and concentration in standard glucose
solutions. At glucose concentrations of 1.8–19.3 mg dl−1, a