3.5. Storage stability and lifetime
The stability and lifetime of the biosensor were also tested by consecutive measurements of the current response to a 1 mM fructose solution. As most enzymes lose their activity if not stored in the refrigerator (4 C), the fructose biosensor was carefully stored at 4 C when not in use. Fig. 8 shows the biosensor lifetime under dry and wet storage conditions at 4 C. Under dry storage condi-tions, the biosensor lost about 20% of its initial activity after 1 day of 8 h/day operation and more than 80% after 1 week. Under wet conditions, the biosensor retains almost 100% of its original response after 3 days of 8 h/day operation and about 90% after 10 days. Under wet conditions with a use of a maximum 8 h/day
operation and storage at 4 C, the biosensor is able to retain 80% of original response even up to about 4 months. The stability presented for the proposed biosensor is quite superior to most of the amperometric biosensors for fructose described in the literature (Damar & Demirkol, 2011). This result is consistent with the fact that under wet conditions the biosensor is kept in a closed bottle with cotton wet with buffer solution and this allows the maintenance of a higher water content in the immobilised hydrogel layer.