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3、 簽字日期： 年 月 日 ABSTRACT Subnano metal clusters, which consist of several to tens of metal atoms and possess dimensions smaller than 1 nm, usually exhibit unique catalytic properties due to their high surface

4、 to volume ratio and large fraction of under-coordinated surface atoms. Aiming to elevate catalytic performance, reduce cost and fulfill the requirement of green chemistry and eco-friendliness, this research focuses on t

5、he preparation of highly catalytic subnano Pd and Ag-Pd bimetallic clusters with polymorphic DNA as templates in aqueous solution at ambient temperature. Meanwhile, the influences of DNA conformation on the structure and

6、 properties of metal clusters were investigated systematically. Firstly, highly active subnano Pd clusters were synthesized using i-motif DNA Im1 as the template. It is indicated that Pd1~Pd5 clusters are generated at [P

7、d]/[base] ratio of 0.2, Pd8~Pd9 clusters at [Pd]/[base] ratio of 0.5 and large nanoparticles with the size about 2.6 nm are formed at [Pd]/[base] ratio of 2.0. The i-motif-stabilized Pd8~Pd9 clusters show high catalytic

8、activity towards the reduction of 4-nitrophenol with relative rate constant value of 2034 min-1 (mM Pd)-1, which is much larger than that of Pd nanocatalysts templated by G-quadruplex or duplex DNA and peptides or other

9、polymers reproted in literatures. Experimetal characterizations and DFT calculations disclose that the unique structure of i-motif with consecutive hemiprotonated CH+· C pairs can efficiently ligate Pd ions on the N

10、3 sites of cytosines and the synthesized Pd clusters consist of metallic Pd atoms as well as positively charged Pd that is ligated by nucleobases, which is capable of facilitating the activation of nitryl group of 4-nitr

11、ophenol. Secondly, the size, oxidation state and acitivity of Pd clusters were finely tuned through changing the loop sequces of Im1 to Im-AAA, Im1-TTA and Im1-TTT. At [Pd]/[base] ratio of 0.2, the nuclearities of domin

12、ating Pd clusters templated by these four DNA are smaller than 6 and the percentages of positive Pd are higher than 70%, which result in extremely low activity towards the reduction of 4-nitrophenol. At [Pd]/[base] ratio

13、 of 0.5, Both of Im1 and Im1-TTA generate Pd8 as the main cluster, which consists of about 50% positive Pd and shows high activity towards the same reaction. In contrast, Im1-AAA produces Pd10 as the major species with 6