The M. charantia aqueous leaf extract contains primary metabolites
along with flavonoids, alkaloids, tannins, phytosterols, etc.
[15–17]. In particular, flavonoids are strong reducing agents and
are contributed to the reduction of Ag+ ions to nanoparticles. As flavonoids
(poly-phenols) are powerful reducing agents and they may
also directly scavenge molecular species of active oxygen, this
antioxidant activity of flavonoids emanates from their ability to
donate electrons or hydrogen atoms. The plausible mechanism of
AgNPs formation may be suggested as the flavonoids are oxidized
during the reduction of Ag+ to AgNPs (Fig. 1). According to Moran
et al. [18], the general chelating ability of phenolic compounds
are probably related to the high nucleophilic character of the aromatic
rings rather than to specific chelating groups within the
molecule. The flavonoid backbone (ketoform) reduces Ag+ and converts
to enolform ensuing in Ag0 formation. All the constituents of
leaf extract are not only cogently reduce silver salt but also exhibit
excellent tenacity against agglomeration. In contrast, proteins and
enzymes present in the leaf extract are possibly facilitating the formation
of pure AgNPs through reduction of Ag+ ions and their
stabilization. The protein molecules undergo fragmentation i.e.
some of the peptide bonds will break resulting in peptides formation.
As the carboxylate group present in the proteins has greater
affinity to act as surfactant by forming a protein layer on nanoparticles,
it stabilizes AgNPs ultimately.