3.5. The preserve time of the test strips
The paper strips packaged in black plastic bag were preserved
in fridge at −20 ◦C. In order to figure out the preservation period, standard glucose solution with the concentration of 7 × 10−3 M were employed to examine the effectiveness. The relative standard deviation was found to be 9.7% and retained 87% of its initial intensity
for glucose after 60 days. The intensity of color was decreased with storage time, but it would not influence the detection accuracy due to the calibration advantage of tree-shaped paper. And the paper strips can be preserved for more than 60 days, which are stable for storage or long-distance transport.