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

On-site colorimetric detection of Salmonella typhimurium.

Rapid qualitative and quantitative detection of Salmonella typhimurium (S. typhimurium) takes an important role in ensuring food safety. Herein, a colorimetric assay aptasensor for S. typhimurium utilizing intrinsic peroxidase-like activity of gold nanoparticles embedded spherical covalent organic framework and the affinity and specificity of S. typhimurium-aptamer has been explored. This aptasensor can capture the S. typhimurium via the selective binding effect of aptamer, and the catalytically active sites were shielded. As a result, the colorimetric signals of the 3,3',5,5'-tetramethylbenzidine-H2O2 system were turned off. Under optimum conditions, the aptasensor gave a linear response over the range of 10 to 107 CFU/mL for S. typhimurium. The detection limit of 7 CFU/mL was obtained within 45 min and was effectively applied to detect S. typhimurium in milk and lake water samples with recoveries in the range from 96.4 to 101.0%. More importantly, combined with a self-developed smartphone-based image analysis system, the proposed aptasensor can be used for point-of-care testing applications.