A DNA tetrahedral nanostructure actuates a "signal-off" fluorescent biosensor for ultrasensitive detection of mercury...

The performance of fluorescent Hg²⁺ biosensors is fundamentally limited by the high background noise inherent in conventional "signal-on" designs. Here, we overcome this limitation by engineering a "signal-off" nanosensor based on a programmable tetrahedral DNA nanostructure (TDN). This scaffold is precisely functionalized with vertex quenchers and fluorophore-labeled T-rich aptamers, generating a maximized initial signal. Target-induced T-Hg²⁺-T coordination folds the aptamer into a hairpin, forcibly bringing the fluorophore near the quencher via an efficient FRET mechanism to achieve significant quenching. Beyond a static support, the TDN serves as an active signal amplifier, with its pre-organizing power critically enhancing sensitivity. The resultant sensor achieves an ultra-low detection limit (1.1 nM), excellent selectivity, stability, and accurate Hg²⁺ quantification in environmental waters. This work establishes a new design paradigm, highlighting the transformative role of DNA nanostructures as dynamic components in advanced biosensing.