Target-triggered quadratic amplification for label-free and sensitive visual detection of cytokines based on hairpin aptamer DNAzyme probes.

The employment of DNAzyme probes for visual biodetections has received increasing interest recently due to the simple nature of this type of assay. However, achieving high sensitivity and detecting targets beyond nucleic acids remain two major challenges in DNAzyme-based visual detections. In this work, based on a new quadratic amplification strategy, we developed a sensitive and visual detection method for cytokines by using hairpin aptamer DNAzyme probes. The target cytokine, interferon γ (IFN-γ), associates with the aptamer sequences and unfolds the hairpin structure of the probes, leading to simultaneous recycling of the target IFN-γ (assisted by Bst-polymerase) and the DNA sequences (aided by λ exonuclease) to achieve quadratic amplification. This quadratic amplification results in the generation of numerous peroxidase-mimicking DNAzymes, which cause significantly intensified color change of the probe solution for highly sensitive detection of IFN-γ by the naked eye down to 50 pM. The proposed visual sensing method shows also high selectivity toward the target IFN-γ and can be performed in homogeneous solutions with using completely unmodified, synthetic aptamer DNAzyme probes. These distinct advantages of our developed assay protocol make it a potential platform for detecting various types of biomolecules with careful probe designs.