Enzyme-free and label-free detection of Staphylococcus aureus based on target-inhibited fluorescence signal recovery

In this work, a one-step fluorometric strategy based on nanometal surface energy transfer (NSET) between carbon dots (CDs) and gold nanoparticles (AuNPs) was developed for facile detection of Staphylococcus aureus (S. aureus). The fluorescence of CDs was quenched up to 63.5% by AuNPs due to nucleic acid hybridization in the presence of linker DNA, which contained the complementary sequences of S. aureus-specific aptamer, and the fluorescence signal was in the "off" state. Upon aptamer addition, the CDs was released from linker DNA through the strong competitiveness of aptamer, leading to the notable fluorescence recovered. Once S. aureus is introduced, aptamer preferentially bind to the bacterial surface and cannot hybridize with complementary sequences in the linker DNA, resulting in the fluorescence signal with "off" state. Based on these findings, the performance and reliability of the fluorescence-based assay were evaluated. Compared to direct hybridization of complementary DNA on the surface of CDs and AuNPs, our sensing strategy has enhanced detection limit (10 cfu⋅mL^-1) and improved linear range (10 to 10⁶ cfu⋅mL^-1) for S. aureus. Therefore, our proposed enzyme-free and label-free strategy may become a promising method for ease of operation, sensitive and selective S. aureus detection.