Mass Spectrometry of Small Bimetal Monolayer-Protected Clusters

Monolayer-protected clusters were prepared by procedures like those yielding Au25L18 (where L = -SCH2CH2Ph -SC2Ph) but using, instead, mixtures of Au and Pd salts, as starting materials, with the intent of creating and characterizing Au25-xMxL18 clusters. Isolation of small nanoparticle product followed by partial ligand exchange to introduce thiolated poly(ethylene glycol) (SPEG = -S(CH2CH2O)(5)CH3) into the nanoparticle ligand shell enabled characterization of the Au25-xMxL18 content by positive mode electrospray ionization mass spectrometry (ESI-MS). For synthetic feed mole ratios of Au:Pd of 9:1 and 13:12, electrospray spectra of the PEGylated MPCs showed that the reaction and isolation produce a mixture of Au-25(SC2Ph)(18) and a mono-Pd nanoparticle Au24Pd(SC2Ph)(18). A higher proportion of the mono-Pd nanoparticle is produced by the 13:12 mole ratio, and also when the thiol:metal ratio was lowered, according to ESI-MS and MALDI-TOF-MS. As the nanoparticle mixture is enriched, by solvent fractionations. in Au24Pd(SC2Ph)(18) relative to Au-25(SC2Ph)(18), the distinctive optical and electrochemical signatures of Au-25(SC2Ph)(18) are replaced by Au24Pd(SC2Ph)(18) nanoparticle responses, which are very different, even though only one Au atom is replaced by a Pd atom.