Enzyme-free fluorescent biosensor for the detection of DNA based on core–shell Fe3O4 polydopamine nanoparticles and hybridization chain reaction amplification

A novel, highly sensitive assay for quantitative determination of DNA is developed based on hybridization chain reaction (HCR) amplification and the separation via core–shell Fe 3 O 4 polydopamine nanoparticles (Fe 3 O 4 @PDA NPs). In this assay, two hairpin probes are designed, one of which is labeled with a 6-carboxyfluorescein (FAM). Without target DNA, auxiliary hairpin probes are stable in solution. However, when target DNA is present, the HCR between the two hairpins is triggered. The HCR products have sticky ends of 24   nt, which are much longer than the length of sticky ends of auxiliary hairpins (6   nt) and make the adsorption much easier by Fe 3 O 4 @PDA NPs. With the addition of Fe 3 O 4 @PDA NPs, HCR products could be adsorbed because of the strong interaction between their sticky ends and Fe 3 O 4 @PDA NPs. As a result, supernatant of the solution with target DNA emits weak fluorescence after separation by magnet, which is much lower than that of the blank solution. The detection limit of the proposed method is as low as 0.05   nM. And the sensing method exhibits high selectivity for the determination between perfectly complementary sequence and target with single base-pair mismatch. Importantly, the application of the sensor for DNA detection in human serum shows that the proposed method works well for biological samples.