Ag11(SG)7: A New Cluster Identified by Mass Spectrometry and Optical Spectroscopy

We report a one-step and high yield synthesis of a redluminescent silver cluster with the molecular formula, Ag11(SG)7 (SG: glutathionate) via reduction of silver ions by sodium borohydride in the presence of the tripeptide, glutathione (GSH). The as-prepared cluster shows prominent absorption features at 485 and 625 nm in its UV−vis absorption spectrum. Aging of the as-prepared cluster solution led to the disappearance of the 625 nm peak, followed by broadening of the 485 nm peak to give three maxima at ∼487, 437, and 393 nm in its absorption spectrum. These peaks remain unchanged even after polyacrylamide gel electrophoresis (PAGE), where a single band was observed confirming high purity of the cluster formed. Electrospray ionization mass spectrometry (ESI MS) reveal the composition of the cluster to be Ag11(SG)7 with multiple sodium attachments to the ligand to give −3 and −2 charged species. These compositions match well with their calculated isotope patterns. Extensive MS/MS was performed to understand the fragmentation. Potential atomic structures are discussed based on density functional theory calculations and comparisons for optical absorption spectra using Ag11(SCH3)7 as the model. Photoluminescence of this cluster was selectively quenched in the presence of Hg(II) and Cu(II) separately. Detection limit was found to be below their permissible limits in drinking water set by US EPA. Ag11(SR)7 cluster is reported for the first time.