Metal nanoparticles surrounded by a monolayer of n-alkanethiols, also known as monolayer-protected clusters (MPCs), are versatile materials used for a broad spectrum of applications ranging from material science to biology (R. W. Murray et al. Acc. Chem. Res. 2000, 33, 27-36). However, the synthesis of MPCs is a thermodynamic consequence of nucleation, growth, and ligand-based termination, and therefore it results in a heterogeneous population of metal-core sizes. Because the study of these interesting materials requires highly size-monodisperse fractions, MPCs must be purified by repeated extraction from organic solvents. We sought to develop an alternative synthetic approach for preparing MPCs that would directly result in size-monodisperse materials, thereby avoiding the need for subsequent purification.

This new approach relies on kinetic, rather than thermodynamic, control of particle size. It is based on our finding in 1998 that highly monodisperse monometallic, bimetallic, and semiconductor nanoparticles can be prepared within dendrimer templates (R. M. Crooks et al. J. Am. Chem. Soc. 1998, 120, 4877-4878). This synthetic strategy involves two steps. First, the interior of dendrimers (a class of monodisperse, spherical polymers) are load with metal ions. Second, the metal ion/dendrimer composite is chemically reduced. These two steps result in formation of dendrimer-encapsulate nanoparticles (DENs) having sizes up to 3 nm in diameter.

To prepare MPCs, research group members Dr. Joaquin C. Garcia Martinez and Dr. Robert W. J. Scott simply prepared DENs, capped the encapsulated nanoparticles with alkylthiols, and then extracted them into an organic phase. The general approach is shown in Figure 1. Specifically, 1.7 nm-diameter Pd nanoparticles were prepared within fourth-generation poly(amidoamine) (PAMAM) dendrimer hosts, and then a toluene solution containing an n-alkanethiol is added to the aqueous DEN solution. When this two-phase system is stirred, n-alkanethiols presumably self-assemble onto the surface of the Pd nanoparticles, extract them from the dendrimer, and transport them to toluene phase. This process is easy to follow just by visual examination of the color change of each phase: after shaking for 5 minutes the aqueous phase turns from brown to colorless and the toluene phase turns from colorless to brown (Figure 2a corresponds to the left side of Figure 1 and Figure 2b corresponds to the right side of Figure 1).