The Role of Oxidation during the Synthesis of Silver-Glutathione Monolayer-Protected Clusters.

The synthesis of metal monolayer-protected clusters (MPCs) is still not well understood. It was recently shown that the mechanism of MPC formation involves sequential growth, wherein small MPCs form first and then grow into progressively larger sizes. The sequential growth model does not entirely explain all experimental observations, however. For example, the evolution of MPC product sizes is found to be a non-monotonic function of reaction kinetics, whereas the sequential growth model predicts monotonic behavior. Size evolution of MPCs is studied during synthetic reactions for a wide range of kinetics and it is found that all syntheses began with the sequential growth of MPCs but also found that growth transitioned to degradation if reduction kinetics are fast enough to give way to ambient oxidation. It is identified that MPCs can degrade via oxidation during syntheses and in a manner that is opposite to sequential growth, namely by forming smaller known MPC species from larger MPC species. This sequential degradation process therefore played an important role in determining final MPC products for reactions with fast reduction kinetics. Together, complementary oxidative and reductive processes provide a more complete description of MPC synthesis as well as new tools for controlling metal MPC synthesis.

Sequential Growth as a Mechanism of Silver-Glutathione Monolayer-Protected Cluster Formation.

Silver monolayer-protected clusters (MPCs) are an important new class of small metal nanoparticles with discrete sizes and unique properties that are eminently tunable; however, a fundamental understanding of the mechanisms of MPC formation is still lacking. Here, the basic mechanism by which silver-glutathione MPCs form is established by using real-time in situ optical measurements and ex situ solution-phase analyses to track MPC populations in the reaction mixture. These measurements identify that MPCs grow systematically, increasing in size sequentially as they transform from one known species to another, in contrast to existing models. In the new sequential growth model of MPC formation, the relative stability of each species in the series results in thermodynamic preferences for certain species as well as kinetic barriers to transformations between stable sizes. This model is shown to correctly predict the outcome of silver MPC synthetic reactions. Simple analytic expressions and simulations of rate equations are used to further validate the model and study its nature. The sequential growth model provides insights into how reactions may be directed, based on the interplay between relative MPC stabilities and reaction kinetics, providing tools for the synthesis of particular MPCs in high yield.