Paper chip-based colorimetric assay for detection of Salmonella typhimurium by combining aptamer-modified Fe3O4@Ag nanoprobes and urease activity inhibition

A rapid and sensitive colorimetric assay is described for  Salmonella typhimurium ( S. typhimurium ) detection using urea/phenol red impregnated test paper. Aptamer-modified Fe 3 O 4 @Ag multifunctional hybrid nanoprobes (apt-Fe 3 O 4 @Ag NPs) were used to specifically captured S. typhimurium ; the nanoprobes were quickly etched by H 2 O 2 to form Ag + . The generated Ag + can inhibit the urease-catalyzed hydrolysis reaction of urea to produce NH 4 + . Consequently, the as-prepared test paper displayed a yellow color. In the presence of  S. typhimurium , the target bacteria can cause aggregation of apt-Fe 3 O 4 @Ag NPs, and the deposited Ag on the nanoprobe’s surface is shielded against H 2 O 2 -induced oxidative decomposition leading to reduced Ag + production. The catalytic activity of urease cannot be inhibited completely by inadequate amount of Ag + . An obvious color change from yellow to pink can be monitored directly using our test paper as a result of increased NH 4 + . The entire assay procedure could be completed within 1 h. A limit of detection of 48 cfu/mL is achieved with a linear range of 1 × 10 2 to 1 × 10 6  cfu/mL. The recoveries of  S. typhimurium spiked in pure milk samples were 92.48–94.05%. Schematic diagram of the proposed colorimetric assay for S. typhimurium detection based on etching of bifunctional apt-Fe 3 O 4 @Ag NPs and inhibiting catalytic activity of urease by Ag + . A color change from yellow to pink can be observed and correlated to the concentration of S. typhimurium.