Developing a colorimetric nucleic acid-responsive DNA hydrogel using DNA proximity circuit and catalytic hairpin assembly.

The development of powerful techniques for sensitive detection of nucleic acids has attracted much attention for fabricating accurate biosensors in various fields, such as genomics, clinical diagnostics, and forensic sciences. Up to now, different systems have been introduced, the majority of which are expensive, time-consuming, and relatively low selectivity/limit of detection. These limitations caught our attention to fabricate a nucleic acid responsive system by combining three layers of signal amplification strategy, namely a split proximity circuit (SPC), a catalytic hairpin assembly (CHA), and a DNA hydrogel. Herein, by SPC operation, two initiators and a target strand were assembled and activated the CHA reaction in the presence of three 5'-cytosine (C)-rich hairpins. Then, produced C-rich embedded three-way junction structures could form i-motif structures under acidic environment followed by a transition from sol to gel state. To acquire a quantitative and colorimetric measurement, gold nanoparticles (GNPs) were used that encapsulated and sediment by the gel formation. The resulting platform detected the target with a limit of detection of 1 pM and considerable selectivity.