Antibody-Functionalized Gold Nanoparticles as a Highly Sensitive Two-Step Colorimetric Biosensor for Detecting Salmonella Typhimurium in Food

The standard laboratory methods used to detect Salmonella are time-consuming, leading to unavoidable delays, which can be costly (e.g., food checks at customs) or even dangerous when quick alarms are needed (e.g., for potential outbreaks). To address this problem, we propose a rapid two-step colorimetric immunoassay. In the first step, gold nanoparticles interact with the sample, while in the second step, an optimized amount of saline solution is added to induce cluster formation among the unbound nanoparticles, providing a color change that inversely correlates with the bacteria concentration. The nanoparticles were functionalized using a technique based on UV activation of the antibodies, resulting in oriented antibodies with antigen-binding regions in the proximity of the surface. While a color change can be observed with the naked eye at relatively higher bacterial concentrations, a more quantitative approach based on UV–visible spectrum measurements allowed us to achieve a limit of detection of 1 cfu/mL in chicken samples. The test can be completed in 15 min and performed on-site, paving the way for widespread applications of our biosensor in the field of food safety.