Green Gold: Au30(S-t-C4H9)18 Molecules

Here, we report the synthesis and separation of Au-30(tert-thiol)(18) (tert-thiol = tert-butanethiol and 1-adamantanethiol) from a mixture of sizes of bulky-ligated nanoparticles. With precisely 30 gold metal atoms and 18 tertiary butyl ligands, this new 30 Au atom molecule is assigned a formula based on results obtained in high-resolution electrospray ionization (ESI) mass spectrometry. UV-vis-NIR spectroscopy shows a distinct electronic transition featured at 620 nm, with a valley in the green wavelength 520-570 nm region, explaining the green appearance. This lack of absorbance in the green region is uncommon, and therefore, metal nanoparticles of green color are extremely rare. Its optical band gap, 1.76 eV is much different than the 1.3 eV reported for Au-25(SR)(18). We report its reproducible direct synthesis (similar to 10 mg) in a one-pot reaction, with two different ligands. Because of the unique optical properties and altered-discrete sizes, there is a fundamental need for structural analysis of this nanoparticle.