(675d) Synthesis and Characterization of Accessible Active Sites in Ligand Stabilized Metal Nanoparticles

The design and synthesis of hybrid organic-inorganic nanoparticle catalysts is currently limited by an understanding of surface chemistry of accessible portions of an organically-capped metal surface. This in turn is often hampered by the lack of reliable chemisorption probes for accessible surface as in the case of gold. Here, we report the synthesis of gold nanoparticles postsynthetically modified with a calixarene macrocycle, and the measurement of accessible metal in these systems. Nanoparticles capped with calixarene phosphine ligand demonstrate improved colloidal stability relative to unmodified, surfactant-stabilized gold nanoparticles in solution as well as when supported on the surface of TiO₂. The accessible gold surface in these nanoparticles is measured quantitatively for the first time for a calixarene-modified nanoparticle, using a newly developed fluorescence methodology involving 2-naphthalenethiol as a relevant chemisorption probe molecule. The fluorescence of 2-naphthalenethiol reveals that calixarene ligand steric bulk as well as surface coverage control amount of accessible surface on the metal nanoparticle.