Smartphone-assisted colorimetric detection of Salmonella typhimurium based on the catalytic reduction of 4-nitrophenol by β-cyclodextrin-capped gold nanoparticles

Salmonella typhimurium (S. typhimurium) is one of the most common pathogens in the environment, such as in drinking water and soil. Herein, an on-site detection method was developed by combining silver-coated magnetic nanoparticles (Fe3O4@Ag NPs) with the β-cyclodextrin-capped gold nanoparticles (β-CD-Au NPs) to achieve sensitive detection of S. typhimurium. After they formed a sandwich structure in the presence of S. typhimurium, the 4-nitrophenol was reduced to 4-aminophenol based on the nitro-reductase activity of β-CD-Au NPs. The naked eyes were able to observe the color change from yellow to colorless. Under optimal conditions, the detection range of S. typhimurium was 10–107 CFU mL−1, and the limit of detection (LOD) was 10 CFU mL−1. The total detection time was 90 min, showing satisfactory performance in real samples. We combined a smartphone app with the colorimetric method, making it possible to semi-quantitatively detect S. typhimurium by analyzing the grey value. In conclusion, this assay detects S. typhimurium in environmental samples, offering an accurate and sensitive detection method without sophisticated equipment.