Switchable control of nanoparticle surface accessibility using dynamic covalent chemistry

Abstract

Metal nanoparticles have exciting potential as catalysts for several chemical transformations, often displaying unique reactivity that is unprecedented by the analogous bulk material or organometallic complexes. In contrast to established substrate-supported nanoparticle catalysts, colloidally stable nanoparticles present opportunities for rational optimisation of catalytic activities by combining favourable features of both homogeneous and heterogeneous catalysts. However, achieving high colloidal stability, which requires a surface-passivating coating, while allowing easy access to catalytic sites on the nanoparticle surface remains a key unmet challenge. Some existing strategies rely on mixed monolayers of strong and weakly bound surface ligands, with no clear route to achieving high stability and high reactivity simultaneously; other approaches use ligands with complicated and synthetically demanding structures. Recently, dynamic covalent hydrazone exchange performed within a nanoparticle-bound monolayer has been used to reversibly control nanoparticle properties. This thesis presents the application of dynamically exchangeable steric bulk at the periphery of nanoparticle-bound monolayers to facilitate accessibility to the nanoparticle core without compromising colloidal stability. A series of nanoparticles stabilised by hydrazone-functionalised ligands of differing steric bulk have been synthesised to understand the relationship between ligand peripheral steric bulk and monolayer density. The results reflect the difficulty in deconvoluting the cause and effect of several inter-related factors, primarily the impact of ligand steric bulk on nanoparticle colloidal stability and how this impacts monolayer density. In general, when the density of nanoparticle-bound monolayers is at saturation for a given ligand, nanoparticles bound with sterically bulky ligands consistently have lower monolayer densities compared to nanoparticles coated with less bulky ligands. A method has also been developed to assess the accessibility to nanoparticles surfaces. Using dithiothreitol to displace nanoparticle-bound ligands results in aggregation, which can be monitored using UV-vis spectroscopy. The rate of dithiothreitol-induced nanoparticle aggregation is affected by both monolayer density and the ligand peripheral steric bulk. In general, nanoparticles with dense monolayers showed slow rates of dithiothreitol-induced aggregation. By exploiting the dynamic nature of the hydrazone bond, the monolayer periphery can be tuned via the exchange of aldehydes with differing sizes. The consequent effects on surface accessibility were investigated, revealing that decreasing the steric bulk at the monolayer periphery results in greater accessibility to the nanoparticle core surface. The impact of changing monolayer peripheral bulk on nanoparticle catalytic activity was also investigated. Catalytic activity was generally higher for mixed-ligand monolayer-stabilised nanoparticles with a high proportion of less sterically bulky peripheral units. However, issues maintaining colloidal stability during the catalytic reactions investigated are still to be solved.