## 3 Results and Discussion  
For comparison, AuNPs were first fabricated through a standard one-pot single-step synthesis process in the presence of sodium citrate, apple extracts (which are typically rich in natural pectin), and with the precursor, a HAuCl\({}_{4}\) water solution. The freeze-drying method was used to make powdered-like AuNPs compared to the redispersion ability of both fabricated AuNPs. Compared with the AuNPs prepared through the standard method of using sodium citrate as the reducing agent and stabilizer, the AuNPs fabricated using our biogenic (green) approach exhibited a less resolved and red-shifted absorption band centered at 532 nm (Figure 1a; spectrum I). The spectral analysis results indicated that the AuNPs fabricated using the green method exhibited a broader particle size distribution and morphology than did those fabricated using the standard method (Figure 1b, spectrum I). Furthermore, the AuNPs were purified by collecting precipitates through high-speed centrifugation. The AuNPs were completely accumulated and collected after centrifugation. In both cases, no localized surface-plasmon resonance (LSPR) absorption bands were observed in the supernatant (Figure 1a,b, spectrum II). The AuNPs fabricated using the green method was redispersed in water at the same concentration, and two characteristic LSPR (as-prepared and redispersed)absorption bands were observed and determined to be superimposable (Figure 1a, spectra I and III). However, the sodium-citrate-stabilized AuNPs could not recover their redispersion ability (i.e., recover their LSPR absorption bands; Figure 1b, spectra I, and III); this thus indicates the instability of sodium-citrate-capped AuNPs in water solution and demonstrate that such AuNPs cannot be applied in the food industry. AuNPs are sensitive to environmental fluctuations such as changes in pH, ionic strength, and temperature. When AuNPs are capped with a thin layer of stabilizing molecules, high-speed centrifugation may lead to irreversible aggregation. This thus explains why the sodium-citrate-capped AuNPs lost their redispersion ability in the water. However, the AuNPs prepared through the green method using pectin as the promoter exhibited excellent stability; this may be because the gel-like pectin provided solid support to stabilize the AuNPs against irreversible aggregation. Figure 2a illustrates the variations in the absorbance of AuNPs dissolved in the water solution and heated under 80 \({}^{\circ}\)C for 200 min. The absorbance for the green-synthesized AuNPs remained constant after these AuNPs were heated. By contrast, the characteristic LSPR absorption peak of the sodium-citrate-capped AuNPs decreased gradually in the thermal environment. This finding suggests that the pectin-capped AuNPs were more stable than the sodium-citrate-capped AuNPs. Figure 2b presents the thermogravimetric analysis (TGA), Thermo Electron Corporation (Newington, NH, USA), results for the pectin-capped AuNPs, revealing a remarkable weight loss before 500 \({}^{\circ}\)C; this thus confirms that these AuNPs comprised 90% and 10% organic and inorganic contents, respectively. Accordingly, the primary constituents of the AuNPs were pectin and other organic components, and the secondary constituents were Au nanoclusters used for decoration. The morphology of the pectin-capped AuNPs deposited on a silicon substrate was analyzed using SEM, as shown in Figure 3, revealing a gel network engendered by the apple extract, which prevents the AuNPs further assembling into large aggregates under the thermal environment. In addition, elemental mapping revealed the distributions of both carbon and gold in a selected area of the SEM image (Figure 3). Instead of the fabrication of a thin organic layer on the nanoparticle surface, the gold nanoclusters were embedded within the apple extracts to form inorganic-organic hybrid materials.