A label-free fluorescence/colorimetric dual-mode sensor for ultrasensitive detecting Staphylococcus aureus through the alkaline phosphatase activity.

Rapid and accurate detection of Staphylococcus aureus (S. aureus) is significant to ensure food safety and curb the spread of the epidemic. Conventional single-mode optical sensors for detecting S. aureus are susceptible to interference from complex sample matrices. We propose a label-free fluorescence/colorimetric dual-mode sensor for S. aureus through its intrinsic alkaline phosphatase (ALP) activity. The biosensor is based on carbon dot-loaded manganese dioxide nanosheets (CDs@MnO2) nanocomposites. In the presence of S. aureus, ALP catalyzed the hydrolysis of ascorbic acid 2-phosphate magnesium ester, yielding ascorbic acid. The resulting ascorbic acid suppresses the sensor’s oxidase-like activity and mitigates the inner filter effect, thereby reducing the absorbance of substrate and restoring fluorescence. This method achieved a low detection limit of 2 cfu mL‒1 and was successfully applied to milk, milk powder, pork, and lettuce samples, yielding recoveries of 90.7%‒110% with relative standard deviations below 5.9%.