A low pH-based rapid and direct colorimetric sensing of bacteria using unmodified gold nanoparticles

Herein we report a novel strategy for the detection of bacteria using unfunctionalized gold nanoparticles (AuNPs), which was utilized as a colorimetric sensor. The UV-vis absorbance of AuNPs showed red shift due to the interactions between bacteria and AuNPs in the high acidic environment, producing a distinct color change which can be visually detected by naked-eye. The proposed low pH-based colorimetric assay was studied with seven types of foodborne bacteria, and the detection limit was found to be 1.6 × 10⁷ CFU/mL, 3.3 × 10⁵ CFU/mL, 4.5 × 10⁶ CFU/mL, 5.8 × 10⁶ CFU/mL, 2.8 × 10⁵ CFU/mL, 4.4 × 10⁷ CFU/mL and 6.6 × 10⁶ CFU/mL for strains Staphylococcus aureus, Shigella flexneri, Pseudomonas aeruginosa, Vibrio parahaemolyticus, Bacillus subtilis, Escherichia coli O157:H7, and Salmonella typhimurium respectively. The result can be observed by naked-eye within 5 min, the color changed from red to purple, blue and colorless with increasing the concentration of the bacteria, indicated the assay have the ability to differentiate bacteria of different concentrations. This work demonstrates that low pH-based colorimetric assay using unfunctionalized AuNPs for the directly detection of untreated bacteria is fast, simple and visual, has the potential for applications in bacterial diagnostics, especially the detection of pathogenic bacteria in food.