*2.3. Preparation of Gelatin/ZnO Fibers*

For preparation of gelatin/ZnO fibers, gelatin (12.5%, w/v) was added into 2,2,2-trifluoroethanol with constant stirring at 45 ◦C until it was completely dissolved. Then, ZnO particles (0%, 0.10%, 0.25%, respectively) were evenly dispersed in the ethanol solution by ultrasonic. In the electrospinning process, gelatin solution and ZnO particles solution were added into 50 mL syringes respectively, in which the diameter of the syringe needle containing gelatin was 1.25 mm and that containing ZnO particles was 0.80 mm. The distance from the drum collector to the needle tip was 15 cm, and the applied voltage was 17 kV. The gelatin fibers and ZnO particles were spun with a flow rate of 5 and 2 mL·h−1, respectively. In addition, the speed of reciprocating motion platform and drum collector were 55 and 70 r·min−1, respectively. After electrospinning, the fibers were placed on the porous ceramic shelf of the desiccator, and the ethanol solution containing 1% formaldehyde solution was added at the bottom. Then, the desiccator was put into the oven at 25 ◦C for 48 h. After that, the cross-linked fibers were put into the oven at 40 ◦C for 24 h to remove excess formaldehyde. GZ0', GZ1' and GZ2' represent the gelatin/ZnO fibers, in which the concentration of ZnO particles solution is 0%, 0.1% and 0.25% in the electrospinning process, respectively. GZ0, GZ1 and GZ2 represent GZ0', GZ1' and GZ2' after cross-linking.
