精确控制合成Au36(SR)24金原子簇(SR = SPh,SC6H4CH3, SCH(CH3)Ph,SC10H7)

近十几年来，具有原子精确的金原子簇(Au_nL_m)逐渐发展成一种新型可靠的金纳米材料。在本研究中，报道一种简单实用合成脂肪或者芳香巯基保护Au₃₆(SR)₂₄金原子簇的方法。通过“尺寸聚焦”方法，成功地获得Au₃₆(SCH(CH₃)Ph)₂₄，Au₃₆(SC₆H₄CH₃)₂₄，Au₃₆(SPh)₂₄及Au₃₆(SC₁₀H₇)₂₄等金原子簇。这些原子簇通过UV-Vis光谱，电喷雾(ESI)和基质辅助激光解析飞行时间(MALDI)质谱以及TGA等表征进行了进一步的确定。同时发现在UV-Vis光谱中，芳香巯基保护Au₃₆(SR)₂₄金原子簇发生了明显的红移现象；例如与Au₃₆(SCH(CH₃)Ph)₂₄原子簇相比，萘巯基保护的Au₃₆(SC₁₀H₇)₂₄原子簇在570 nm左右的吸收峰发生了13 nm的位移。

Controlled Synthesis of Au36(SR)24 (SR = SPh,SC6H4CH3, SCH(CH3)Ph,and SC10H7) Nanoclusters

In the past decade, gold nanoclusters of atomic precision have been demonstrated as novel and promising materials for potential applications in catalysis and biotechnology because of their optical properties and photovoltaics. The Au₃₆(SR)₂₄ nanocluster is one of the most well-known clusters, which is directly converted from the Au₃₈(SR)₂₄ cluster through a "ligand-exchange" process. It consists of an Au₂₈ kernel with a truncated face-centered cubic (FCC) framework exposing the (111) and (100) facets. Here we report a simple protocol to prepare Au₃₆(SR)₂₄ nanoclusters, ligated by aliphatic and aromatic thiolate ligands (SR = SPh, SC₆H₄CH₃, SCH(CH₃)Ph, and SC₁₀H₇) via a "size-focusing" process. First, polydisperse Au nanoparticles were synthesized and isolated, and then reacted under harsh conditions in the presence of excess thiol ligands at relatively high etching temperatures (80 ℃). The as-synthesized Au₃₆(SR)₂₄ nanoclusters were characterized and analyzed by UV-Vis absorption spectroscopy, electrospray ionization (ESI), and matrix-assisted laser desorption ionization (MALDI) mass spectrometry, as well as thermogravimetric analysis (TGA). All the Au₃₆(SR)₂₄ nanoclusters showed two step-like optical absorption peaks at ~370 and 580 nm in the UV-vis spectra. Only one strong set of nanocluster-ion peaks centered at an m/z of 10517.0 was observed in the ESI mass spectrum of the Au₃₆(SCH(CH₃)Ph)₂₄ nanocluster. This could be assigned to the Au₃₆(SCH(CH₃)Ph)₂₄Cs]⁺ species, and was a strong indicator of the high purity of the as-obtained Au₃₆ cluster samples produced on a small scale. The TGA profile showed 31.67% organic weight loss of the nanocluster, matching well with the expected theoretical value of 31.71%. The Au-SR bond in the gold nanoclusters was broken at ~180 ℃ in a normal air atmosphere. Fragments of the Au₃₆(SR)₂₄ clusters capped with different thiolate ligands, which were mainly caused by the strong laser intensity during the analysis, were detected in the MALDI mass spectra. This interesting phenomenon was also observed in the case of Au₂₅(SR)₁₈, and could be due to the inherent properties of the Au-SR bond on the surface of the gold nanoclusters. Finally, the optical properties of the Au₃₆(SR)₂₄ nanoclusters were found to be influenced by the capping thiolate ligands. Compared to the UV-Vis spectrum of the Au₃₆(SCH(CH₃)Ph)₂₄ cluster, the optical spectra of the other three Au₃₆ clusters were red-shifted (~3 nm for Au₃₆(SPh)₂₄, 5 nm for Au₃₆(SC₆H₄CH₃)₂₄, and 13 nm for the (Au₃₆(SNap)₂₄ clusters). This shift could be explained by the electron transfer occurring from the electron-rich aromatic ligands to the Au kernel. The electron transfer capacity followed the order ―SNap > ―SC₆H₄CH₃ > ―SPh > ―SCH(CH₃)Ph. Overall, this study demonstrates the effectiveness and promising application of ligand engineering for tailoring the electronic properties of Au nanoclusters.