Bicycled Hairpin Assembly-Mediated SERS/Colorimetric Dual-Mode Sensor for the Detection of Dexamethasone in Milk and Pork

The presence of dexamethasone (DEX) residues and the illegal addition of DEX to food have garnered widespread public concern over recent decades. Based on the bicycled hairpin assembly (BHA) mechanism and the catalytic oxidase effect of urchin-shaped Au–Ag alloy loaded with Pt nanoparticles (Au@Ag@Pt), an accurate and ultrasensitive surface-enhanced Raman scattering (SERS)/colorimetric dual-mode method for DEX detection is pioneered. When DEX is introduced into the sensor system, the BHA cycle begins. The Fe₃O₄ nanosphere loaded with Au nanoparticles (Fe₃O₄@Au) surface facilitates the anchoring of a large amount of Au@Ag@Pt, resulting in synergistic signal amplification and facile magnetic separation from the complex system. Finally, the collected product effectively catalyzes the conversion of 3,3′,5,5′-tetramethylbenzidine (TMB) to the oxidized product, producing a strong SERS response, an ultraviolet–visible absorption signal, and color deepening. Our dual-mode platform achieves ultrasensitive detection of DEX, achieving a detection limit (LOD) of 6.61 × 10^–17 M within the broad range of 10^–16 to 10^–9 M for SERS and a LOD of 8.81 × 10^–13 and 6.98 × 10^–13 M within the range of 10^–12–10^–5 M for the colorimetric method. Additionally, this platform shows high selectivity and satisfactory recovery rates for DEX in milk and pork samples, with recoveries of 92.3%–106.5% (RSDs <8.35%) for SERS and 95.2%–107.6% (RSDs <8.81%) for colorimetric analysis. This study demonstrates that by combining the BHA amplification process with multifunctional Au@Ag@Pt and Fe₃O₄@Au substrates, the DEX content in actual food samples can be accurately determined using a dual-mode approach that combines the high sensitivity of SERS with the simplicity of colorimetric analysis.