<p> Cytosine-rich and poly(adenine)-tailed tetrahedral DNA framework (TDF) is designed as template (A₈-TDF) for anchoring silver nanoclusters (AgNCs) and igniting the dual-color fluorescence of AgNCs. The resultant DNA-AgNCs simultaneously emits red and green fluorescence, and the quantum yield of red fluorescence is as high as 44.8%. The red fluorescence can be selectively quenched by Hg²⁺ within 2 min due to the redox reaction between Hg²⁺ and Ag⁰ as well as the subsequent Ag amalgamation. Meanwhile, the green fluorescence is enhanced. Thus a ratiometric fluorescence sensor is constructed for rapid and visual Hg²⁺ detection&#xa0;based on the synchronous change of two fluorescence signals. In addition, the resultant AgNCs probe also provides ratiometric colorimetric sensing of Hg²⁺ and simultaneously shows a visible color change from pink to yellow along with increasing content of Hg²⁺. The detection limits of ratiometric fluorescence and colorimetric modes are 0.24 nM and 1.32 nM, respectively. The method has been applied to the determination&#xa0;of Hg²⁺ in water samples, and recoveries ranged between 94.1 and 108.7%. This study not only provides a new guideline to modulate the fluorescence emission of AgNCs by scientifically designing terminal DNA sequence but also provides a facile and efficient single-probe and dual-mode ratiometric sensing platform for the routine determination and field monitoring of Hg²⁺.</p> Graphical Abstract <p></p>