*2.4. Circular Dichroism (CD) Experiments*

CD experiments were performed using a Chirascan V100 CD spectrometer with a bandwidth of 1 nm at room temperature, unless otherwise specified. The CD samples (~100 μL) were placed in a cuvette of 0.5 mm path length, and the CD spectra were collected from 200 to 350 nm with a step size of 1 nm. For each sample, three sets of scans were acquired, and an average value was taken. CD spectra were background-corrected using the corresponding buffer solution.

#### *2.5. Fluorescence Experiments*

Fluorescence experiments, except for the kinetic study of Ag<sup>+</sup> sensing, were performed using a Shimadzu RF-6000 spectrometer at room temperature. The fluorescence samples (~2 mL) were placed in a 10 mm four-sided glazed quartz cuvette, and the fluorescence spectra were collected from 512 to 650 nm with a step size of 1 nm. Fluorescence intensity was recorded at 520 nm with an excitation wavelength of 492 nm. The excitation and emission band widths were 5 nm. For a kinetic study of Ag+ sensing, fluorescence experiments were performed using an Edinburgh FLS1000 photoluminescence spectrometer at room temperature. The sample containing a DNA sensor in the absence of Ag+ (~2.5 mL) was first placed in a 10 mm four-sided glazed quartz cuvette, and the fluorescence intensity at 520 nm was recorded from 0 to 180 s with a step time of 2 s. Ag<sup>+</sup> was then added to this sample, and the fluorescence intensity was immediately recorded from 0 to 180 s with a step time of 2 s. The excitation and emission band widths were 2 nm.

The detailed sample conditions for NMR, CD, and fluorescence experiments are stated in the figure legends.

#### **3. Results**
