Complementary hydrazone-based dynamic covalent nanoparticles

Abstract

The extraordinary and unique properties exhibited by monolayer-stabilised metal nanoparticles suggest exciting potential applications. Surface-bound molecules stabilize the material in colloidal form, but also define a whole host of physicochemical properties and provide a means to link nanoparticles with any number of other components. Post-synthetic strategies for functionalizing nanoparticle-bound monolayers are therefore critical for virtually all applications. However, established methods each have limitations. The emerging concept of dynamic covalent nanoparticle building blocks provides a transformative strategy for achieving responsive and adaptive surface-engineering of nanomaterials.

Previously, dynamic covalent hydrazone exchange has been employed to reversibly alter gold nanoparticle-bound monolayers using electrophilic molecular modifiers (aldehydes), establishing the possibility of using dynamic covalent reactions to manipulate nanoparticle-bond functionality. This thesis takes several steps towards developing this approach into a general strategy for divergent modification of nanoparticle surface functionality.

Exploiting the directional nature of the hydrazone bond, a complementary family of dynamic covalent nanoparticles having the electrophilic species tethered to the nanoparticle surface, was created. The scope of the dynamic covalent nanoparticle strategy is thus significantly expanded, allowing reversible post-synthetic functionalization using nucleophilic exchange units. Using solution-state NMR spectroscopy, hydrazone exchange kinetics for these two sets of complementary nanoparticles were investigated, revealing how the surface-confined reactivity compares to bulk solution and also significant differences in reactivity between the complementary pair of nanoparticles.

The reversible nature of dynamic covalent reactions allows each member of the complementary family of nanoparticle building block to be assembled in a predictable and controlled way, governed by simple abiotic molecular systems. Furthermore, the complementary reactivity of these two systems provides access to binary nanoparticle assemblies without requiring any molecular linkers. Finally, a detailed understanding about surface-confined chemical reactivities offers the opportunity to explore self-sorting behaviour of complementary nanoparticles.

Dynamic covalent exchange can be used to not only switch nanoparticle solvent compatibility between widely differing solvents (from hexane to water), but also to progressively tune solubility across the entire continuum between these extremes. Indeed, molecular-level control over surface-confined reactions, allows to produce a self-consistent family of kinetically stable nanoparticles with different mixed-ligands monolayer compositions, providing a unique platform to study structure–property relationships on the nanoscale.