Lighting Up Fluorescent Silver Clusters via Target-Catalyzed Hairpin Assembly for Amplified Biosensing

Isothermal enzyme-free nucleic acid circuits have been developed for carrying out diverse functions ranging from dictate biocomputing to amplified biosensing. Catalytic hairpin assembly (CHA), the catalyzed cross-opening of two hairpin substrates by an initiator, has attracted increasing attention because of its facile design and high amplification capacity. The complex labeling and frequent photobleaching of a conventional fluorescent CHA biosensor still remains a challenge that needs to be solved. Herein, we constructed a new label-free and enzyme-free isothermal CHA lighting up AgNCs strategy for amplified nucleic acid assay by integrating the interfacially and spatially sensitive feature of DNA-templated fluorescent silver nanoclusters (DNA-AgNCs) and the high signal amplification capability of the CHA circuit. In this strategy, one polyguanine-grafted hairpin and the other AgNCs-capturing hairpin were engineered as assembly constitutes, which were kinetically impeded from cross-hybridizations without target. However, in the presence of target, the CHA-catalyzed assembly of two functional hairpins was successively progressed and concomitantly accompanied by an efficient accommodation of AgNCs to the polyguanine-elongated dsDNA product, leading to highly efficient AgNCs-lighting up and to the generation of an amplified fluorescence signal. As a simple mix-and-detect strategy, the isothermal enzyme-free CHA-mediated lighting up AgNCs (CHA-AgNCs) system provided a facile visualization way for amplified detection of DNA with a detection limit of 20 pM, which was comparable to or even better than some enzyme-involved amplification methods. The homogeneous CHA-AgNCs system can be used as a general sensing platform and be easily adapted for analyzing other biologically important analytes, for example, microRNA (miRNA), by introducing the sensing module consisting of an auxiliary hairpin through an easy-to-integrate procedure. By taking advantage of the signal amplification features of CHA and the robust AgNCs-lighting up procedure, we anticipate that the CHA-lighting up AgNCs system can provide an important tool for biomedicine and bioimaging applications and thus should hold great promise in clinical diagnoses and treatment fields.