## 3 Results and Discussion  
Construction and Characterization of Molecular Game-Theoretical System Based on Molecular Interactions  
We first constructed an artificial molecular system based on molecular interactions among polythymine oligonucleotide T33, metal ions (Hg\({}^{2+}\)), and CoOOH nanosheets (Figure 1). FAM-labeled T33 random coils in aqueous solution had strong fluorescence (normalized F = 1.0 a.u.) at 520 nm. Due to its unique fluorescence quenching and molecular adsorption capacities (Figure S1) [37], when positively charged hexagonal CoOOH nanosheets (TEM and SEM images in Figure S2A) were added into the solution, the negatively charged T33 adsorbed onto the surface of CoOOH nanosheets, resulting in fluorescence quenching (normalized F = 0.67 a.u., Figure 1a). This result may be attributed to the electrostatic adsorption (Figure S3) of T33 on CoOOH nanosheets and the effective fluorescence resonance energy transfer (FRET) between FAM and CoOOH nanosheets (Figure S2B) [38; 39]. Only in the presence of Hg\({}^{2+}\) did T33 change its conformation from a random coil to a T-Hg\({}^{2+}\)-T hairpin structure, resulting in slight fluorescence quenching (normalized F = 0.96 a.u., Figure 1b). In the simultaneous presence of CoOOH and Hg\({}^{2+}\), Hg\({}^{2+}\) may combine with T33 on the CoOOH nanosheets to form a T-Hg\({}^{2+}\)-T hairpin structure, which increases the tightness of the bond of the three components, resulting in stronger quenching (normalized F = 0.3 a.u., Figure 1c). Atomic force microscopy (AFM), UV-Vis absorption spectroscopy, and CD) spectroscopy were used to characterize the above-mentioned molecular interaction process. The AFM results showed that the thickness of the CoOOH nanosheets was 12-16 nm (Figure 2A, \(n\) = 15). The thickness of the T33-CoOOH composites increased to 15-19 nm (Figure 2B, \(n\) = 15), indicating that T33 was adsorbed on the CoOOH nanosheets. The thickness of the T33-CoOOH-Hg\({}^{2+}\) mixture increased to 21-23 nm (Figure 2C, \(n\) = 15). Figure 3A shows the absorption spectra of T33 and T33-CoOOH in the presence of various concentrations of Hg\({}^{2+}\). The optical density decreased as Hg\({}^{2+}\) concentration increased and their absorption spectra displayed a slight redshift (264\(\rightarrow\)270 nm, Figure 3A). In Figure 3B, CD spectra of T33 and T33-CoOOH in the presence of various concentrations of Hg\({}^{2+}\) are shown. After the addition of Hg\({}^{2+}\), the positive band at 278 nm reduced. The experimental results clearly indicate the formation of T-Hg\({}^{2+}\)-T pairs [40]. By further comparison with the fluorescence changes of free fluorescein, T3, and T33-CoOOH complex, at the different concentrations of Hg\({}^{2+}\), the fluorescence  
Figure 1: Schematic illustration and fluorescence emission spectra change of an artificial molecular system based on molecular interactions among polythymine oligonucleotide T33, metal ions (Hg\({}^{2+}\)), CoOOH nanosheets. (**a**–**c**) the formation of T33–CoOH (**a**); T-Hg\({}^{2+}\)–T pairs (**b**); T33–CoOOH–Hg\({}^{2+}\) (**c**). T33: 100 nM, CoOOH nanosheets: 4.69 \(\upmu\)g/mL, Hg\({}^{2+}\): 10 \(\upmu\)M. The excitation and emission wavelength are 485 and 520 nm. The fluorescence intensity is normalized. Buffer: 5 mM Tris-HCl buffer solution (pH 7.4); FAM: carboxylfluorescein; CoOOH: cobalt oxyhydroxide. of free fluorescein and T33 slightly decreased with increasing concentration of Hg\({}^{2+}\) (Figure S4), indicating that the CoOOH nanosheets play an important role in enhancing the sensitivity of Hg\({}^{2+}\) detection. We further investigated fluorescence responses of the T33-CoOOH complex to T33's fully complementary DNA A33. As a result, the formation of a T33-A33 duplex DNA also lead to similar quenching (normalized F from 0.67 to 0.51 a.u., Figure 3C). Thymine residues in T33 preferentially bind to Hg\({}^{2+}\) through covalent N-Hg bonds [40].