Self-constrained DNAzyme for aptamer-based and sensitive label-free fluorescent assay of sarafloxacin via signal amplification cascades

The convenient and sensitive assay of antibiotic residuals is important for monitoring food safety and human health. Here, with a new self-constrained metal ion-dependent DNAzyme and hybridization chain reaction (HCR) signal amplifications, a simple label-free and highly sensitive aptamer-based fluorescent sensing assay for sarafloxacin is established. The target sarafloxacin molecules bind the aptamers in the duplex DNA probes to release the complementary strands, which cyclically activate the catalytic activity of the self-constrained DNAzymes to cut the substrate sequences to liberate many ssDNA strands. Subsequent HCR of two hairpins is initiated by these ssDNA strands for yielding long dsDNAs with lots of G-quadruplexes, which intercalate thioflavin T dye to generate drastically magnified fluorescence for non-label and sensitive sarafloxacin detection at the detection limit of 29 pM. Also, a highly selective assay of low levels of sarafloxacin in diluted milk samples has been demonstrated, showing the great potential of our approach for the development of versatile sensitive and convenient aptasensors for monitoring different antibiotics.