Characterisation, degradation and regeneration of luminescent Ag₂₉ clusters in solution

Luminescent Ag clusters are prepared with lipoic acid (LA) as the ligand. Using a combination of mass spectrometry, optical spectroscopy and analytical ultracentrifugation, the clusters are found to be highly monodisperse with mass 5.6 kDa. We assign the chemical composition [Ag₂₉(LA)₁₂]^3- to the clusters, where LA likely binds in a bidentate fashion. The Ag₂₉ clusters show slow degradation, retaining their deep red emission for at least 18 months if stored in the dark. Purification or exposure to light results in faster degradation. No other cluster species are observed during the degradation process. Once degraded, the clusters could easily be regenerated using NaBH₄, which is not usually observed for thiolate-capped Ag clusters.