Novel approach to gold ion detection in aqueous solutions using silver nanoparticles synthesized from the seed extract of mangrove associate plant Thespesia populnea

This study presents a novel, eco-friendly colorimetric sensor for the sensitive and selective detection of gold(III) ions, utilizing silver nanoparticles synthesized using Thespesia populnea seed kernel extract (AgNP-TPK). The green synthesis yielded stable nanoparticles, confirmed by UV–Vis, HR-TEM, XRD, DLS, and zeta potential analyses, with excellent colloidal stability lasting over six months. The sensing mechanism involves a galvanic replacement reaction where Au(III) oxidizes AgNPs, forming gold nanoparticles—visibly indicated by a distinct colour change and a hypochromic shift in the SPR peak. The sensor demonstrated strong analytical performance, with a low detection limit of 0.442 ppm and quantification limit of 1.339 ppm. Selectivity studies revealed negligible interference from common mineral mimics such as pyrite and chalcopyrite. Real sample validation using industrial runoff from a BIS-certified hallmarking facility showed significant spectral and visual changes only in the presence of gold ions, confirming the sensor's practical applicability. These findings position AgNP-TPK as a versatile, field-deployable tool for rapid gold detection in environmental monitoring, mining operations, and forensic investigations, while also promoting sustainable nanomaterial development.