**2. Materials and Methods**

#### *2.1. DNA Sequence Design and Sample Preparation*

Our designed *M-DNA* sensor was a duplex formed by the CCTG MDB strand (5 - CCTG CCTG-3 ) and its complementary strand (5 -CAGG CAGG-3 ), which were named *CCTG2* and *CAGG2*, respectively. As a control, a self-complementary 8-bp duplex formed by 5 -GCAGCTGC-3 was used. The high-performance liquid chromatography (HPLC) purified DNA samples were purchased from Sangon Biotech (Shanghai, China), and they were further purified in our laboratory using diethylaminoethyl sephacel anion exchange column chromatography and Amicon Ultra-4 centrifugal filter devices. The ultra-violet (UV) absorbance at 260 nm was measured for DNA quantitation.

#### *2.2. Preparation of SYBR Green I (SGI) and Metal Ion Stock Solutions*

SGI (10,000×) was purchased from Beijing Solarbio Science and Technology Co., Ltd. (Beijing, China) and diluted using DMSO to a final concentration of 100× or 10× as the stock solution. It is noted that SGI 1× was equivalent to a concentration of 1.96 μM. The analytical-grade AgNO3, KCl, LiCl, CaCl2, MgCl2, MnCl2, CoCl2, CuSO4, BaCl2, and NiSO4 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China) and dissolved using DI water to a final concentration of 50 μM as the stock solutions.

## *2.3. NMR Experiments*

To monitor the binding of Ag+ to the CCTG MDB, NMR experiments were performed using a Bruker AVANCE NEO 400 MHz spectrometer. One-dimensional (1D) 1H NMR experiments were conducted at 25 ◦C using the excitation sculpting pulse sequence to suppress the water signal.
