Label-Free Fluorescent DNA Dendrimers for microRNA Detection Based On Nonlinear Hybridization Chain Reaction-Mediated Multiple G-Quadruplex with Low Background Signal.

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Various fluorescent sensing systems for miRNA detection have been developed, but they mostly contain enzymatic amplification reactions and label procedures. The strict reaction conditions of tool enzymes and the high cost of labeling limit their potential applications, especially in complex biological matrices. Here, we have addressed the difficult problems and report a strategy for label-free fluorescent DNA dendrimers based on enzyme-free nonlinear hybridization chain reaction (HCR)-mediated multiple G-quadruplex for simple, sensitive, and selective detection of miRNAs with low-background signal. In the strategy, a split G-quadruplex (3:1) sequence is ingeniously designed at both ends of two double-stranded DNAs, which is exploited as building blocks for nonlinear HCR assembly, thereby acquiring a low background signal. A hairpin switch probe (HSP) was employed as recognition and transduction element. Upon sensing the target miRNA, the nonlinear HCR assembly of two blocks (blocks-A and blocks-B) was initiated with the help of two single-stranded DNA assistants, resulting in chain-branching growth of DNA dendrimers with multiple G-quadruplex incorporation. With the zinc(II)-protoporphyrin IX (ZnPPIX) selectively intercalated into the multiple G-quadruplexes, fluorescent DNA dendrimers were obtained, leading to an exponential fluorescence intensity increase. Benefiting from excellent performances of nonlinear HCR and low background signal, this strategy possesses the characteristics of a simplified reaction operation process, as well as high sensitivity. Moreover, the proposed fluorescent sensing strategy also shows preferable selectivity, and can be implemented without modified DNA blocks. Importantly, the strategy has also been tested for miRNA quantification with high confidence in breast cancer cells. Thus, this proposed strategy for label-free fluorescent DNA dendrimers based on a nonlinear HCR-mediated multiple G-quadruplex will be turned into an alternative approach for simple, sensitive, and selective miRNA quantification.