One-Pot Synthesis of Au25(SG)18 2-and 4-nm Gold Nanoparticles and Comparison of Their Size-Dependent Properties

A one-pot synthesis of glutathione (denoted as -SG) capped gold nanoparticles, including Au-25(SG)(18) (ca. 1 nm in diameter) 2- and 4-nm particles is reported. These nanoparticles are isolated by methanol-induced precipitation with a controlled amount of added methanol. Except for their particle size, these nanoparticles have an identical chemical composition (i.e., gold and -SG content), synthetic history, and surface conditions, which allows for precise comparison of their size-dependent properties, in particular the magnetic property as this could be attributed to contamination by trace iron impurities. Specifically, the structure, optical, and magnetic properties of these gold nanoparticles are compared. A trend from non-fcc (fcc = face centered cubic) Au-25(SG)(18) nanoclusters (ca. 1 nm) to 2- and 4-nm fcc-crystalline Au nanocrystals is revealed. The Au-25(SG)(18) nanoparticles resemble molecules and exhibit multiple optical absorption peaks ascribed to one-electron transitions, whereas the 4-nm nanoparticles exhibit surface plasmon resonance at around 520 nm related to the collective excitation of conduction electrons upon optical excitation. The transition from the non-fcc cluster state to the fcc crystalline state occurs at around 2 nm. Interestingly, both 2- and 4-nm particles exhibit paramagnetism, whereas the Au-25(SG)(18) (anionic) clusters are diamagnetic. The information attained on the evolution of the properties of nanoparticles from nanoclusters to fcc-structured nanocrystals is of major importance and provides insight into structure-property relationships.