Ultrasensitive colorimetric detection of Salmonella typhimurium using a magnetic graphene oxide-assisted DNAzyme aptasensor.

A magnetic graphene oxide assisted-DNAzyme aptasensor is presented designed and fabricated for simple colorimetric detection of Salmonella Typhimurium in contaminated water. Superparamagnetic Fe₃O₄ nanoparticles were directly synthesized on graphene oxide sheets and then a DNAzyme-probe (G5Zyme) immobilized on sheets. The catalytic core of G5Zyme consists of an exposed guanine-rich loop, which becomes active only upon forming a duplex with the Salmonella aptamer. In this activated state, it catalyzes the oxidation of TMB in the presence of hydrogen peroxide, producing a distinct colorimetric signal. In negative samples (healthy water without Salmonella), free aptamers detach the G5Zyme from graphene oxide sheets and hybridize with it, thereby activating the enzyme leads to produce a color. In contrast, in positive samples containing Salmonella, the aptamers preferentially bind to the bacteria, and following magnetic separation, the G5Zyme is removed from the reaction system, resulting in the suppression of color development. The nanobiosensor enabled highly sensitive detection of Salmonella (1 CFU/mL in water), demonstrating excellent specificity and robust reproducibility within 3 h. Its performance surpassed conventional culture methods and was comparable to PCR assays without requiring DNA extraction.