## 3 Results and Discussion  
The color of ESM changed from white to brown-yellow after GO nanosheets were coated on the surface of ESM (GO/ESM in Figure 1), indicating that GO nanosheets were combined effectively with ESM. The color of ESM changed from white to wine red after the in situ synthesis of gold nanoparticles (AuNP/ESM in Figure 1). The ESM sample became black-purple when the AuNPs were in situ-synthesized onto the GO-modified ESM (Au/GO/ESM in Figure 1). The AuNP-treated ESM was further coated with GO, which showed a dark color (GO/AuNP/ESM in Figure 1). It should be noted that the color of the GO/ESM became darker during the in situ heating synthesis of AuNPs, implying that GO may be reduced by heat in the presence of ESM, which is similar to the GO reduction on the cotton surface at high temperature [31]. To investigate the optical properties of the ESM samples, UV-Vis diffusion reflectance absorbance spectra were measured (Figure 2). The pristine ESM displayed absorption in the UV region due to its protein composition. The GO/ESM showed higher absorption in the whole wavelength range compared to the pristine ESM because of the coating of GO nanosheets. The curve of AuNP/ESM presented an absorption band around 540 nm, which is assigned to the LSPR mode of AuNPs on the ESM surface [32]. A distinct absorption band around 540 nm was observed in the UV-Vis diffusion reflectance absorbance spectra of AuNP/GO/ESM and GO/AuNP/ESM, indicating that the GO coating did not obviously change the LSPR properties of AuNPs. The complex of GO and AuNPs enhanced the UV-Vis absorption of the ESM samples. The spectral data indicates that GO nanosheets were coated on ESM surface, and AuNPs were in situ-synthesized on ESM after heat treatment. Surface morphologies of the ESM samples were observed by SEM. Pristine ESM displays 3D nonwoven network with porous structures (Figure 3a). The average diameter of the fibers in ESM was measured to be 2.00 \(\upmu\)m (Figure 3b). After treatment with GO, numerous wrinkle nanosheets were found wrapped around ESM fibers, which indicates that GO has been effectively coated onto the networks of ESM (Figure 3c,d). No GO nanosheets were observed in the pores in the 3D structures of ESM, implying direct interaction between ESM fibers and GO nanosheets (Figure 3c). The oxygen-containing groups on the GO nanosheets interacted with the amino and carboxyl groups from amino acids on ESM, which leads to the effective coating of GO nanosheets around the fibrous structures of ESM. As can be seen from the SEM images of AuNP/ESM (Figure 3e,f), a dense coating of nanoparticles was observed on the surface of the ESM network fibers. The average size of the nanoparticles was measured to be 21.15 nm (Figure 3f). The SEM images clearly demonstrate that AuNPs were synthesized onto the ESM surface. The diverse amino acids existing in ESM could possess a reducing effect [33; 34], which may result in the reduction of Au ions and then the formation of AuNPs on the ESM by heat treatment [35]. A large number of AuNPs was also observed in the SEM images of AuNP/GO/ESM  
Figure 1: Optical image of eggshell membrane (ESM) samples. Figure 2: UV-Vis diffusion reflectance absorbance spectra of the treated ESM samples. (Figure 3g) and GO/AuNP/ESM (Figure 3h). The presence of GO nanosheets did not affect the in situ synthesis of AuNPs. In the composite films, wrinkled GO nanosheets and spherical AuNPs were observed obviously on the fibrous structures of ESM. The average sizes of AuNPs on AuNP/GO/ESM and GO/AuNP/ESM were 26.76 and 22.47 nm, respectively. There was no obvious size change in the AuNPs after the assembly of GO on the AuNP/ESM. Nevertheless, this is compared with an increased AuNPs size when Au was in situ-synthesized onto GO/ESM. GO may promote the synthesis of AuNPs due to the active groups on the surface of GO nanosheets. The surface chemical elements of different ESM samples were inspected by XPS.