Rapid Broad-Spectrum Colorimetric Detection of Bacteria Using Polylysine-Mediated Gold Nanoparticle Aggregation

Rapid on-site detection of bacterial contamination remains an urgent need in public health and environmental monitoring. Herein, we report a one-step, label-free colorimetric sensor for broad-spectrum bacterial detection based on a competitive electrostatic mechanism. Cationic poly-l-lysine (PLL) induces the aggregation of anionic, citrate-capped gold nanoparticles (AuNPs), resulting in a distinct red-to-blue color change. In the presence of bacteria, their negatively charged surfaces adsorb PLL, thereby inhibiting AuNP aggregation and maintaining the red color of the dispersion. This strategy enables the visual detection of various Gram-positive and Gram-negative bacteria within 10 min. Using Escherichia coli (E. coli) as a model analyte, the assay reached a detection limit of 720 CFU/mL under the optimized conditions (5 μg/mL PLL, 3 min incubation, 20 mM phosphate buffer at pH 7.5) and exhibited a wide detection range from 102 to 107 CFU/mL (R2 > 0.99). The assay also performed reliably in spiked tap water and drinking water samples, with recoveries of 82-96% and CVs below 10%, demonstrating its potential as a rapid, low-cost screening tool for bacterial contamination in low-matrix water samples.