Facile Exchange of Ligands on the 6-Mercaptopurine-Monolayer Protected Gold Clusters Surface

We have investigated the exchange of 6-mercaptopurine (6MP) molecules by 11-mercaptoundecanoic acid (MUA) and 11-mercapto-1-undecanol (MUOH) on the surface of monolayer protected gold clusters (6MP-MPCs) of 2.4 nm diameter. After the addition of MUA or MUOH to the 6MP-MPC solutions the absorption band at 312 nm due to 6MP is suddenly observed. The monitoring of this signal allows us the study of the place-exchange reactions. The final products obtained from the exchange at different molecular ratios are characterized by IR spectroscopy. On the other hand, the ligand exchange reaction is slowed down upon increase nanoparticle size (6MP-AuNPs of 13 nm diameter) and, as expected, in two-dimensional surfaces (either 2D-polyfaceted or 2D-single crystal surfaces). Thus, it is concluded that the presence of defects in the monolayer due to the existence of a high ratio of edge and corner sites make easier the exchange of the 6MP by MUA or MUOH molecules being extremely difficult the exchange from the terraces that allow the organization of the molecules in the monolayer through lateral intermolecular interactions. Finally, the study of exchange in 2D-surfaces by means of electrochemical techniques evidence the important role of the stability of binding on the exchange kinetics.