A composite prepared from carboxymethyl chitosan and aptamer-modified gold nanoparticles for the colorimetric determination of Salmonella typhimurium.

An aptamer-based assay is described for the determination of Salmonella typhimurium (S. typh). Carboxymethyl chitosan was loaded with amino-modified aptamer against S. typh, and then adsorbed on gold nanoparticles by electrostatic interaction to form a composite that acts as the molecular recognition element. In the presence of S. typh, it will be bound by the aptamer, and this changes the structure of the recognition element. On addition of salt solution, the gold nanoparticles agglomerate so that the color of the solution changes from red to blue. S. typh can be detected via measurement of the absorbance at 550 nm. Absorbance increases linearly with the logarithm of the S. typh concentration in the range from 100 to 109 cfu·mL-1. The limit of detection is 16 cfu·mL-1. The specificity and practicability of the assay were evaluated. The recoveries of S. typh from spiked milk samples are between 92.4 and 97.2%. The analytical results are basically consistent with those of a plate counting method. Graphical abstract Schematic representation of the colorimetric assay for Salmonella typhimuium (S. typh) using carboxymethyl chitosan (CMCS)-aptamer (Apt)-gold nanoparticles (AuNPs) composites.