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

We report a one-step and high yield synthesis of a red-luminescent 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