Silver nanoparticle-enhanced UV–Vis assay for rapid detection of Escherichia coli in milk

This study reports a novel silver nanoparticle (AgNP)-enhanced ultraviolet–visible (UV–Vis) spectrophotometric assay for rapid and sensitive detection of Escherichia coli in milk samples. Conventional E. coli detection methods are time-consuming and require specialized equipment, limiting accessibility in many settings. The assay exploits the localized surface plasmon resonance (LSPR) of AgNPs, enabling detection within 20 min. AgNPs synthesized with trisodium citrate were characterized by atomic force microscopy (AFM) and UV–Vis spectrophotometry, showing a distinct LSPR peak at 421 nm. When mixed with E. coli suspensions, the peak shifted to 298 nm, lying between that of E. coli suspension alone (289 nm) and AgNPs alone (421 nm), indicating nanoparticle binding to bacterial surfaces. The assay demonstrated strong linearity for concentrations from 1.5 × 10³ to 1.5 × 10⁷ CFU/mL, with a detection limit of 3.47 × 10² CFU/mL, indicating good sensitivity. Specificity tests with Staphylococcus aureus verified accuracy. Application to milk samples showed quantitative overestimation relative to culture methods, attributable to matrix interference, though both methods achieved 100% agreement in qualitative detection. This rapid, sensitive, and specific assay is promising for use in resource-limited settings. With further optimization, it could serve as a valuable platform for bacterial contamination screening, enhancing food safety and public health surveillance.