<p>A label-free surface-enhanced Raman scattering (SERS) sensor using optimal bimetallic Au@Ag nanocuboids (Au@Ag NCs) was developed for ultrasensitive detection of florfenicol residue in eggs. The Raman characteristic peaks of florfenicol detection were identified at 1145&#xa0;cm⁻¹ and 1595&#xa0;cm⁻¹ which were consistent with the theoretical Raman spectrum of florfenicol calculated by density functional theory. The Au@Ag NCs were optimized by changing the aspect ratio of Au nanorods and the thickness of Ag nanocuboids, achieving an enhancement factor of 1.69 × 10⁶ for SERS signals. Under optimal conditions, the label-free SERS sensor demonstrated a&#xa0;broad linear range of 0.01&#xa0;mg/kg to 100&#xa0;mg/kg for florfenicol in real sample. The limit of detection of florfenicol in eggs was as low as 0.0410&#xa0;μg/kg, and the recovery was 94.83% ~ 108.24%. Therefore, this label-free SERS sensor offers a rapid and sensitive approach for the trace detection of florfenicol residues in food safety monitoring.</p> Graphical abstract <p></p>