###### Abstract  
We have developed a \"green\" method for fabricating gold nanoparticles (AuNPs) through biogenic approaches. The proposed method has the advantages of facile preparation under ecofriendly conditions. AuNPs encapsulated in carbon nanosheets, and exhibiting high thermal stability, were fabricated by autoclaving pectin-capped AuNPs, which were subsequently collected through high-speed centrifugation and redispersed in aqueous solution. The 1,1-diphenyl-2-picrylhydraz (DPPPH) radical scavenging assay indicated that our prepared AuNPs exhibited more prolonged antioxidant capacity than pristine apple extracts. Electron paramagnetic resonance (EPR) spectra showed that approximately 80% of DPPH radicals were scavenged by the pectin-capped AuNPs at a concentration of 3 mg/mL. According to our results, AuNPs prepared through biogenic approaches have potential use in the food industry. green synthesis; pectin; gold nanoparticle; antioxidant; thermal stability +
Footnote †: journal: Applied Physics Letters  
12 March  
extracts, such as sugars, amines, phenolic compounds, and flavonoids, can initiate the reduction of gold ions; these biochemicals also help stabilize gold colloids [17; 18; 19; 20; 21]. Among the biogenic resources, pectin is a naturally occurring gel present in fruits such as citrus peels, oranges, apples, cherries, and plums. Pectin can serve as a biological agent for preparing gold nanoparticles (AuNPs) with bioactive functions [22; 23]. Pectin is mainly composed of a complex set of polysaccharides and is rich in polymers of d-galacturonic acid (GalA) with \(\alpha\)(1-4) linkages. In general, the polygalacturonic acid chain of pectin is partly esterified with methyl groups, and free acid groups may be partly or fully neutralized with sodium, potassium, or ammonium ions. The ratio of esterified GalA groups to total GalA groups is termed as the degree of esterification (DE). Pectin can be classified into low- and high-methoxy groups according to the corresponding DE values. Pectin not only exhibits gelling, thickening, and emulsion-stabilizing properties that are commonly used in the food industry but also has potential pharmaceutical applications such as controlled drug delivery, detoxification, and antioxidant. In this study, pectin extracted from red apples (_Malus punila_) was utilized as the biogenic resource to synthesize AuNPs. The antioxidant components of apples, including polyphenols, can induce the reduction of gold ions and initiate crystal growth. Polysaccharides present in pectin can provide a gel network for stabilizing nanoparticles. Accordingly, the pectin-capped AuNPs prepared in this study exhibited remarkable thermal stability under hydrothermal carbonization and consequently formed a hybrid nanostructure of AuNPs encapsulated in carbon nanosheets. Previous studies have used complex fabrication processes, for example sintering, sputtering, multi-steps, and complex chemicals [24; 25; 26; 27; 28; 29; 30; 31] to obtain AuNPs with high thermal stability. However, the lack of bioactive capping agents hinders the practical application of such processes in food preservation [32]. In this study, we analyzed the absorption spectra and morphology of pectin-capped AuNPs with a defined size and found that the characteristic absorption remained unaltered after the AuNPs were redispersed into the aqueous solution. The particle size of pectin-capped AuNPs changed from 10 nm to 7 nm after the AuNPs heating in an autoclave. The 1,1-diphenyl-2-picrylhydraz (DPPH) radical scavenging assay was used to determine the antioxidant activity of AuNPs encapsulated with carbon nanosheets, indicating that our prepared AuNPs exhibited more prolonged antioxidant capacity than pristine apple extracts. The reason for using the DPPH assay is for comparison with electron paramagnetic resonance (EPR) spectra where the EPR results showed that approximately 80% of DPPH radicals were scavenged by the pectin-capped AuNPs at a concentration of 3 mg/mL.