## 4 Conclusions  
In summary, we demonstrated a proof-of-concept molecular game-theoretical system (molecular prisoner's dilemma) that relies on the formation of the T-Hg\({}^{2+}\)-T hairpin structure specifically induced by Hg\({}^{2+}\) and fluorescence quenching and molecular adsorption capacities of CoOOH nanosheets, resulting in fluorescence intensity and distribution change in polythymine oligonucleotide T33. The bait molecule T33 interacted with two molecular players, CoOOH and Hg\({}^{2+}\), with different states (absence = silence and presence = betrayal) regarded as strategies, and created conflicts (sharing or self-interest) of fluorescence distribution of T33, quantifiable in a 2 \(\times\) 2 payoff matrix. The molecular game-theoretical system based on T33 and CoOOH was used for sensing Hg\({}^{2+}\) in the range of 20 to 600 nM with a detection limit of 7.94 nM (3\(\sigma\)) and for determination of Hg\({}^{2+}\) in pond water. Although the demonstrated molecular game system is relatively simple, it has high versatility and broad applicability and can be extended to various other molecular systems, such as RNA/DNA/peptide-based systems, nano-based systems, host-guest supramolecular systems, and solid-phase molecular self-assembly systems. Inspired by the proof-of-concept for the feasibility of molecular game theory, a variety of molecular decision-making platforms will be developed, which will help promote molecular information processing and generating novel molecular intelligent decision systems for environmental monitoring and molecular diagnosis and therapy.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Added (nM)** & **Found (nM)** & **Recovery (\%)** & **RSD (\%)** \\ \hline \multirow{2}{*}{Pond water} & 84.00 & 90.96 & 108.29 & 0.59 \\  & 155.00 & 162.64 & 104.93 & 1.15 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Determination of Hg\({}^{2+}\) in real water samples (\(n=3\)).  
**Supplementary Materials:** The following are available online at [[http://www.mdpic.com/2076-3417/8/12/2530/s1](http://www.mdpic.com/2076-3417/8/12/2530/s1)]([http://www.mdpic.com/2076-3417/8/12/2530/s1](http://www.mdpic.com/2076-3417/8/12/2530/s1)), Figure S1: Comparison of fluorescence quenching ability of CoOOH to different fluorescence dyes. Figure S2: Characterization of CoOOH nanosheets. Figure S3: Comparison of \(\zeta\)-potential for CoOOH, T33-CoOOH, and T33-CoOOH-Hg\({}^{2+}\) mixtures. Figure S4: Comparison of fluorescence changes in free fluorescein, T33, and T33-CoOOH complexes at different concentrations of Hg\({}^{2+}\). Figure S5: Optimization of the concentration of CoOOH nanosheets. Figure S6: Optimization of reaction time. Figure S7: Competition experiment and the interference of anions. Figure S8: T33-CoOOH complexes for detection of A33 DNA. Table S1: Comparison of the detection limit and linear ranges of other reported Hg\({}^{2+}\) assays.  
**Author Contributions:** [PERSON] designed the methods, [PERSON] and [PERSON] performed the experiments, and optimized the parameters; [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] optimized the process and analyzed the data, [PERSON] and [PERSON] wrote the paper; [PERSON] and [PERSON] performed review and editing. The first two authors contributed equally to this paper.  
**Funding:** This work was supported by the National Natural Science Foundation of China (No. 21505042), Scientific and Technological Plan Project of Changsha of China (No. KQI707010 and KQI802046), Hunan Provincial Natural Science Foundation of China (No. 2016J)3084), the Research Foundation of Education Bureau of Human Province (No. 15K084), the Cooperative Innovation Center of Engineering and New Products for Developmental Biology of Hunan Province (No. 20134486), and College Student Innovative Experiment Project of Hunan Province (No. 2017106).  
**Conflicts of Interest:** The authors declare no conflict of interest.