A robust and facile colorimetric aptasensor for the detection of Salmonella Typhimurium based on the regulation of Fe3O4@Cu@PCPy yolk-shell nanozyme activity

Due to their superparamagnetism and enzyme-like activity, iron oxide (Fe 3 O 4 ) nanozymes can be readily used for sample pretreatment and the generation of detection signals, and have, thus, attracted much attention in the field of bioanalysis and diagnosis. However, the low catalytic activity of Fe 3 O 4 nanozymes does reduce the sensitivity of Fe 3 O 4 -based methods, limiting their application. In this study, Fe 3 O 4 @Cu@poly(pyrrole-2-carboxylic acid) yolk-shell nanozymes (Fe 3 O 4 @Cu@PCPy YSNs) were synthesized using a facile approach and selective chemical etching technology . Compared with Fe 3 O 4 nanozymes, the Fe 3 O 4 @Cu@PCPy YSNs demonstrated a three-fold increase in the peroxidase-like activity, good dispersity and strong superparamagnetism. In addition, the flower-shaped structure of aptamer-complementary strand (Apt-CS) conjugates was designed on the surface of the Fe 3 O 4 @Cu@PCPy YSNs, which effectively inhibited their peroxidase-like activity by creating a physical barrier that hindered the access of substrates to the center of the Fe 3 O 4 @Cu@PCPy YSNs. Based on this principle, a robust and facile colorimetric aptasensor was developed for detecting Salmonella Typhimurium. The flower-shaped Apt-CS were dissociated in the presence of S . Typhimurium, promoting the recovery of Fe 3 O 4 @Cu@PCPy YSN catalytic activity. Under optimized conditions, this proposed aptasensor successfully detected S . Typhimurium in a linear range of 3 to 3 × 10 6  CFU/mL, achieving a detection limit of 1 CFU/mL. Finally, the feasibility of this novel aptasensor was further validated by three actual samples, with recoveries of between 84.3% and 102%, thereby demonstrating the huge potential of the proposed aptasensor for detecting S . Typhimurium in foods.