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[Display omitted] • A colorimetric sensor for S. aureu s detection based on HCR and G-quadruplex DNAzyme. • Cascade amplification was successfully used to detect S. aureu s in milk samples. • Combining the sensor with the APP we developed, on-site detection can be achieved. • Providing a new reference for the detection of pathogens in milk. • Expanding the application of HCR combined with G-quadruplex in pathogens detection. As a foodborne pathogen, Staphylococcus aureus (S. aureus) poses a serious threat to the dairy industry. A colorimetric sensor based on hybridization chain reaction (HCR) and G-quadruplex for the detection of S. aureu s is reported. The aptamer of S. aureus was hybridized with the target probe and fixed on the magnetic particles. When S. aureus was present, aptamers preferentially coupled with S. aureus. The released target probe triggered the HCR to form long-chain DNAs with multiple G-quadruplex. S. aureus can be quantitatively detected by measuring the catalytic signal of G-quadruplex. Cascade amplification enabled rapid and sensitive detection of S. aureus with a detection limit of 4.4 cfu·mL−1. Meanwhile, combining this method with the smartphone application (APP) developed by us can achieve on-site detection. In addition, the sensor has a good performance in the detection of S. aureus in milk samples. This method provides a new reference for detecting pathogens in milk and expands the application of HCR combined with G-quadruplex in pathogens detection. [ABSTRACT FROM AUTHOR]