Label-free fluorescent aptasensor for sensitive detection of oxytetracycline based on CRISPR/Cas12a and G-quadruplex...

Oxytetracycline (OTC), a widely used broad-spectrum antibiotic, frequently persists in food and the environment due to overuse, posing risks of resistance, allergy, and toxicity. The World Health Organization has set strict limits for OTC residues, highlighting the need for rapid and reliable detection. Conventional assays, though accurate, remain costly and technically demanding, underscoring demand for simpler alternatives. We report a label-free fluorescent aptasensor integrating CRISPR/Cas12a with G-quadruplex (G4) DNAzyme catalysis, using rhodamine B (RhB) as a reporter. In the absence of OTC, the aptamer binds the activator strand, allowing G4 DNAzyme formation that quenches RhB fluorescence. OTC binding liberates the activator, triggering Cas12a cleavage of G4 DNA, thereby preventing DNAzyme formation and preserving fluorescence. This switch provides efficient signal amplification with low background. The assay achieved a detection limit of 0.3 nM and enabled accurate quantification of OTC in river water and milk samples. This platform offers a cost-effective, label-free strategy for antibiotic residue monitoring, eliminating dependence on nanomaterials or external labeling. Its validated sensitivity and robustness in real samples underscore its potential for practical applications in food safety and environmental surveillance.