Sample-in-answer-out colorimetric detection of Salmonella typhimurium using non-enzymatic cascade amplification

Rapid and sensitive screening of pathogens is a key to prevent the outbreak of foodborne illnesses. Herein, a new colorimetric immunoassay was proposed based on the release of Ag⁺ ions from AgNPs and the inhibition of PtNPs, and its supporting microfluidic platform was developed to automatically perform the whole bacterial detection procedure using Raspberry Pi and smartphone App. First, the immune AgNPs and magnetic nanobeads (MNBs) were used to conjugate with Salmonella typhimurium. Then, H₂O₂ was used to etch the AgNPs for release Ag⁺ ions. Finally, the colorimetric signal was greatly diminished because of the specific and efficient inhibition of Ag⁺ toward the peroxidase-like activity of the PtNPs. This colorimetric immunoassay showed a good specificity and an ultralow detection limit of 16.8 CFU/mL, which was about 3 orders of magnitude improvement compared with conventional ELISA, and the averaged recovery for the spiked chicken samples was 95.6%. The combination of this immunoassay with this microfluidic platform might be promising for rapid and sensitive screening of foodborne pathogens to ensure food safety.