Ultrasensitive visual detection of DNA with tunable dynamic range by using unmodified gold nanoparticles and target catalyzed hairpin assembly amplification.

A simple and novel strategy for enzyme-free ultrasensitive DNA detection platform has been present here based on gold nanoparticles (AuNPs) colorimetry and target catalyzed hairpin assembly amplification. Three hairpin auxiliary probes (H1, H2, and H3) are designed with signal-stranded DNA (ssDNA) sticky ends which could effectively stabilize AuNPs against salt-induced aggregation. However, a cascade of assembly steps with H1, H2, and H3 are activated in the presence of the target DNA, followed by a disassembly step in which H3 displaces the target DNA from the complex, freeing the target DNA to catalyze the self-assembly of additional branched junctions. The formed branched junction consisted with dsDNA is stiffer, and cannot prevent salt-induced AuNPs aggregation, corresponding to a red-to-blue color change. The result can be read out by naked eyes or UV-vis spectrometer. The detection limit of this method is 0.1 pM by naked eyes, and this result is comparable or even better than enzyme or hybridization chain reaction (HCR) based amplification AuNPs colorimetric assays. Moreover, the dynamic range of sensor could be tuned by using different concentration of hairpins. Importantly, this strategy provides a versatile ultrasensitive detection platform for the DNA and related filed targets including metal ions, small molecules, proteins, cells et al. by combining with specific DNAzymes and aptamers.