An AuNP-labeled triple-helix molecular switch for ultrasensitive, on-site detection of tetracycline in milk via fiber-optic localized surface plasmon resonance biosensing

To meet the urgent demand for monitoring tetracycline residues in milk—a critical food for vulnerable groups a novel fiber-optic Localized Surface Plasmon Resonance (LSPR) biosensor was engineered using gold-nanoparticle-labeled DNA triple-helix switches. The sensor operates via a target-activated disassembly: tetracycline binding dissociates the immobilized triple-helix, releasing gold nanoparticles and inducing a measurable LSPR blue shift. This active displacement amplifies the signal via localized refractive-index changes, enabling rapid, label-free detection. The sensor achieved excellent performance, with a linear range of 5–100 ng/mL and a low detection limit of 0.28 ng/mL. When applied to real milk samples, it delivered excellent recovery rates of 96.75%–104.15% (RSD 0.05) compared to HPLC reference analysis. Combining high sensitivity with a modular, engineering-flexible design, this platform not only provides a practical solution for on-site tetracycline screening in food safety but also holds significant potential for adapting to other antibiotics and environmental targets, extending its utility beyond dairy monitoring. • A DNA triple-helix molecular switch enables label-free tetracycline sensing. • Target-induced AuNP pair formation drives plasmonic coupling for signal amplification. • Achieves an ultralow LOD of 0.28 ng/mL, well below regulatory limits in milk. • Portable fiber-optic LSPR platform allows rapid on-site detection. • Validated in real milk with high recovery and no sample pretreatment.