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In this study, a natural extract from the plant Radix hedysari (ER) was proven to be useful for the green synthesis of silver nanoparticles (AgNPs) and ER-AgNPs in particular. Considering the complexity of plant extracts, high-pressure liquid chromatography (HPLC) was effective at confirming the components of ER, surprisingly, formononetin was found to comprise more than 85% of the total contents. Since formononetin was confirmed to be the primary component of ER, this compound was also used as the basis to synthesize AgNPs designated FMT-AgNPs that was highly pure and of high quality. The FMT-AgNPs functionalized by formononetin were optimized for further study. A simple, fast and sensitive colorimetric method detect formothion was achieved using FMT-AgNPs. A wide linear range from 0.1 μM to 10 μM was established with a low limit of detection (LOD) of 14.6 nM. Since formononetin performed multiple roles during the synthesis, it was confirmed that the mechanism of detection was based on aldol condensation, intermolecular hydrogen bonds and electrostatic attraction between formononetin and formothion. Finally, the green synthesized FMT-AgNPs were further applied in typical fruit and vegetable samples, indicating that FMT-AgNPs were potentially highly useful. [Display omitted] • The plant extract (ER) and formononetin was utilized to green synthesize silver nanoparticles (ER-AgNPs/FMT-AgNPs). • FMT-AgNPs were proven to be pure and of high quality. • The relevant synthesis or mechanism of detection was clearly explained. • FMT-AgNPs were sensitive and selective for the detection of formothion. • FMT-AgNPs were applied to detect formothion in typical vegetable and fruit samples. [ABSTRACT FROM AUTHOR]