3.1.2. Characterization of Gelatin/ZnO Fibers

SEM measurements were performed to study morphology of the fibers before and after crosslinking. As shown in Figure 3a–c, 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. The average diameters of GZ0', GZ1' and GZ2' are about 6.22, 5.67 and 7.32 μM. The pure gelatin fiber (GZ0') exhibits a uniform smooth surface, and the fiber thickness is consistent. With the addition of ZnO particles in electrospinning process, ZnO particles can be evenly dispersed on the surface of gelatin fibers (Figure 3b,c), but no particles are found inside GZ1' and GZ2' (indicated by blue arrow in Figure 4). The morphology of the fibers remains basically unchanged. Furthermore, as a potential wound dressing, gelatin/ZnO fibers need to have good physical and chemical stability in aqueous solution. Thus, formaldehyde was chosen as the cross-linking agent. GZ0, GZ1 and GZ2 (Figure 3d–f) represent the GZ0', GZ1' and GZ2' after crosslinking, respectively. Figure 5 shows the degradation process of GZ2 in PBS at 37 ◦C. It can be seen that the degradation of GZ2 is basically completed after five days, which indicates that GZ2 has good water resistance. In addition, Figure 3d–e shows that the average diameter of the nanofibers increases after the cross-linking, and the fibers

are curly (indicated by red arrow above). Meanwhile, the fibers become denser and fuse at some intersection points (shown by yellow arrow above).

**Figure 3.** SEM images of (**a**) GZ0', (**b**) GZ1', (**c**) GZ2', (**d**) GZ0, (**e**) GZ1 and (**f**) GZ2.

**Figure 4.** The cross sections SEM images of (**a**) GZ1', (**b**) GZ2'.

**Figure 5.** Degradation process diagram of GZ2 in phosphate buffer saline (PBS) at 37 ◦C.

To ascertain the chemical structures of gelatin/ZnO fibers, FTIR was first tested. As shown in Figure 6a, the spectra of GZ1 and GZ2 represent three characteristic peaks at 1629, 1531 and 1236 cm−<sup>1</sup> corresponding to amide I, amide II and amide III, respectively. The amide I band is mainly attributed

to the tensile vibration of -C=O, and the amide II and III bands are caused by the bending vibration of -NH and the stretching vibration of -C-N, respectively [34]. A weak absorption peak at 407 cm−<sup>1</sup> in the GZ1 and GZ2 spectra is consistent with the characteristic absorption peak of ZnO (as mentioned in Section 3.1.1), which belongs to the stretching vibration of the Zn-O bond. In addition, due to the reaction between the aldehyde group of formaldehyde and the amino lysine residue of gelatin, the stretching vibration peak of the imide group (-CH=N) appears at 1448 cm−<sup>1</sup> [35]. Furthermore, Figure 6b shows the XRD spectra of gelatin/Zn fibers as well as the ZnO particles. GZ0 has a broad diffraction peak at 2θ = 20◦ [36]. Unfortunately, no ZnO peaks are found in the GZ1 and GZ2 spectra, which may be due to the low content of ZnO particles on the surface of the gelatin [24]. In order to determine the types of elements contained in gelatin/ZnO fibers, EDX was carried out. The selected area for mapping is a part of GZ2, in which the ZnO particles are evenly loaded on the surface. Figure 6c,d represent EDX analysis and element mapping of GZ2, respectively. GZ2 shows the presence of C, N, O and Zn elements. The appearance of C and N elements is due to the presence of these two elements in gelatin molecules. The Zn element is caused by the presence of the element in ZnO particles. In addition, the O element exists both in gelatin molecules and ZnO particles. Moreover, Zn mapping of GZ2 verifies that ZnO particles are evenly dispersed on the surface of gelatin fibers.

**Figure 6.** (**a**) FTIR spectra of ZnO particles, GZ0, GZ1 and GZ2, (**b**) XRD patterns of ZnO particles, GZ0, GZ1 and GZ2, (**c**) energy dispersive X-ray spectroscopy (EDX) analysis of GZ2, and (**d**) EDX mapping of GZ2.

TGA was carried out to study the thermal stability of gelatin/ZnO fibers. As shown in Figure 7, TGA curve of ZnO particles shows 9.02% weight lost at 500 ◦C, caused by the evaporation of physically adsorbed water and decomposition of residual acetate. Meanwhile, the TGA curves of GZ0, GZ1 and GZ2 are basically consistent, and the thermal decomposition process is divided into two stages [37,38]. In the first stage (30–224 ◦C), the weight lost is approximately 8.97%, which is caused by the evaporation

of physically adsorbed water. The second stage is related to the decomposition of gelatin, and with the increase of ZnO content, the weight loss decreases gradually at the end of the decomposition process (500 ◦C). Obviously, the presence of ZnO has little effect on the thermal decomposition process of gelatin fibers, and the gelatin/ZnO fibers have good thermal stability below 224 ◦C.

**Figure 7.** Thermogravimetric analysis (TGA) curve of ZnO, GZ0, GZ1 and GZ2.
