Uncovering the Phytochemical Basis and the Mechanism of Plant Extract-Mediated Eco-Friendly Synthesis of Silver Nanoparticles Using Ultra-Performance Liquid Chromatography Coupled with a Photodiode Array and High-Resolution Mass Spectrometry

Plant extract is a complex concoction of several phytochemicals, for instance, phenolics, sugars, flavonoids, xanthones, and several others. In general, it is said that hydroxyl-rich phenolics act as reducing agents for metal ions, but little is discussed about the stabilizing ligands of metal nanoparticles (NPs). Thus, despite the popularity of plant extract-mediated synthesis of NPs, the phytochemical basis of the process and the exact mechanism are still unclear. Herein, a systematic study was carried out to unveil the mechanism of St. John's wort (Hypericum perforatum (H. perforatum) L.) extract-mediated synthesis of silver (Ag) NPs. Among the phytochemicals present in the extract, phenolic acids and flavonoids are shown to be involved in the reduction of Ag+ ions, while xanthones and phloroglucinols act as capping agents, and naphthodianthrones were involved in both steps. Analysis of the postreaction residues by ultra-performance liquid chromatography coupled with a photodiode array and high-resolution mass spectrometry showed a sharp decrease in the concentration of various secondary metabolites present in the extract, of which large amounts of xanthones and phloroglucinols were recovered from the NPs. Consequently, the low polar fraction of H. perforatum extract, rich in xanthones and phloroglucinols, and the pure compounds of these classes (mangiferin and hyperforin) did not reduce Ag+ ions. In contrast, the polar fraction of the extract, rich in flavonoids and naphthodianthrones, and the pure compounds of these classes (kaempferol-3-glucoside, quercetin, quercetin-3-glucoside, hypericin, pseudohypericin, and protohypericin) were able to reduce Ag+ ions. Overall, the present study contributes significantly to the understanding of green synthesis of NPs by plant extracts.