Rapid, quantitative lateral flow immunoassay using polystyrene-gold composite nanoparticles and CIELAB analysis for on-site detection of Listeria monocytogenes in food samples.

Sensitive, enrichment-free detection of Listeria monocytogenes in food matrices is still challenging. Traditional lateral flow immunoassays (LFIA) come with limited sensitivity and often require lengthy pre-enrichment steps, which is a roadblock for rapid and on-site testing. In this work, a novel LFIA platform was developed that uses surface-modified polystyrene-gold nanoparticle composites (Ps-AuNPs) as enhanced colorimetric labels. The resulting signal enhancement enables the detection of L. monocytogenes without pre-enrichment steps. The assay also uses International Commission on Illumination Lab (CIELAB) color space-based image analysis pipeline to quantify colorimetric signals. CIELAB calculates perceptually uniform color differences (ΔE) and thus reduces subjective bias compared to conventional RGB analysis. The signal enhancement protocol resulted in an assay limit of detection (LOD) of 34 CFU/ml in PBS buffer and 452 CFU/ml in spiked romaine lettuce without any enrichment. The composite nanoparticle labels have excellent stability and retain performance over one month of ambient storage. The reported LFIA assay addresses the sensitivity gap in conventional systems and demonstrates practical applicability for rapid, cost-effective, and field-ready monitoring of L. monocytogenes contamination in real food samples.