Phase Transfer Induced Enhanced Stability of Monolayer Protected Silver Quantum Clusters

Mercaptosuccinic acid (MSA) protected silver clusters (Ag@MSA) were synthesized and their stability was monitored both in an aqueous and in a toluene phase. In the former, the pH of the cluster solution was tuned between 3 and 10 using ammonium acetate buffer, and the cluster stability was tested. The clusters were unstable at all measured pH values, whereas in the toluene phase, by using sterically hindered counter ion (tertraoctylammonium, TOA), the stability of the clusters was improved significantly. The phase transferred clusters in the toluene phase showed very high stability over months at room temperature. Transmission electron microscopy images show an average cluster size of similar to 2.05 nm. Size exclusion chromatography and Electrospray Ionisation Mass Spectrometry (ESI-MS) measurements were employed to analyse the phase transferred clusters. Importantly, Ag(4)MSA(4) (-) type stable fragments were observed in ESI-MS measurements. The [Ag-8(MSA)(8)(TOA)(5)](2-) was the largest cluster identified by ESI-MS.