## 3 Results and Discussion
XPS survey spectra of the ESM samples displayed peaks at 284.8, 399.68, 163.62, and 531.28 eV, corresponding to C1s, N1s, S2p, and O1s, respectively. These are the elements of protein-based materials (Figure 4a). Furthermore, there is a peak at 83.86 eV assigned to Au4f, appearing in the XPS survey curves of AuNP/GO/ESM and AuNP/ESM, implying that AuNPs were prepared on the surface of ESM samples. After peak fitting, the C1s XPS band of pristine ESM can be fitted into four peaks at 284.39, 284.94, 286.05, and 287.79 eV (Figure 4a), which are ascribed to C-C, C-N, C-O, and C=O, respectively [36; 37]. The C1s XPS band of AuNP/ESM also shows four fitting peaks at 284.65, 285.86, 287.08, and 288.22 eV (Figure 4b). The synthesis of AuNPs led to a slight increase in the binding energy of carbon bonds. It is clearly observed that the proportion of C-C increased, and the proportion of C-O and C=O decreased in AuNP/ESM compared with pristine ESM (Figure 4c), which could be attributed to aldehydation of ESM. The AuNPs may be formed through the reduction of Au ions by aldehydation of ESM [38]. Figure 3: SEM images of different ESM samples: (**a**,**b**) pristine ESM, (**c**,**d**) GO/ESM, (**e**,**f**) AuNP/ESM, (**g**) AuNP/GO/ESM, and (**h**) GO/AuNP/ESM. The coating of GO on the surface of ESM was further confirmed by Raman scattering spectroscopy. As shown in Figure 5a, two Raman bands were observed at 1349 and 1600 cm\({}^{-1}\), which corresponded to the in-plane bond stretching motion of C sp\({}^{2}\) atoms (G band) and the breathing modes of benzenoid rings (D band) of GO [39]. The result further demonstrated the successful coating of GO nanosheets on ESM. The composition and thermal stability of ESM samples were tested by TGA. The weight loss of the samples below 100 \({}^{\circ}\)C is due to the evaporation of the moisture inside samples (Figure 5b). A major weight loss was observed for all the samples and was found from 240 \({}^{\circ}\)C to 400 \({}^{\circ}\)C, which was attributed to the decomposition of ESM. There was only a slight weight loss with a further increase in the temperature, which indicates the thermal decomposition of the ESM samples was nearly complete at around 400 \({}^{\circ}\)C. The residual percentages for pristine ESM, GO/ESM, AuNP/ESM, AuNP/GO/ESM, and GO/AuNP/ESM were 22.73%, 22.31%, 35.92%, 36.39%, and 38.37%, respectively. The ESM samples with the coating of both the AuNPs and GO nanosheets showed higher residual percentages compared to other samples, implying that more nanoparticles remained in the residuals. Figure 4: XPS spectra of ESM samples: (**a**) survey curves, and C1s XPS curve of (**b**) pristine ESM and (**c**) AuNP/ESM. Figure 5: (**a**) Raman scattering spectrum of GO/ESM. (**b**) TGA curves of different ESM samples. The photothermal effect of the ESM samples was investigated by being subjected to simulated sunlight irradiation. Figure 6 displays the infrared thermal images of the ESM samples before and after light irradiation for 10 min under the xenon lamp. The surface temperatures of the ESM samples increased greatly under the light irradiation. The temperature of the pristine ESM increased from 23.4 to 33.6 \({}^{\circ}\)C after light irradiation of 10 min. This is compared with a much higher temperature achieved by GO/ESM, AuNP/ESM, AuNP/GO/ESM, and GO/AuNP/ESM under the same conditions, which was 43.1, 43.5, 47.3, and 49.0 \({}^{\circ}\)C, respectively. The thermal images show that the heat distribution of the pristine ESM was over the whole sample as well as the other irradiated area. With photothermal conversion materials, the GO and AuNPs coated ESM showed a concentrated small heat distribution area in the middle of the samples, which suggests that the ESM coated with GO and AuNPs could be used for localized sunlight driven heating. The temperatures of ESM samples varied slightly as the irradiation time was further prolonged (Figure 7), implying that the photothermal conversion reached equilibrium within 10 min.