Towards self-catalytic dynamic covalent nanoparticles

Abstract

Monolayer-stabilised nanoparticles have found applications in various areas due to their unique properties. Manipulation of the monolayer bound to the nanoparticle surface can be used to control and functionalise nanoparticles. Developing post-synthetic strategies for functionalising nanoparticle-bound monolayers are essential for virtually all applications. However, traditional established methods each have drawbacks. The Kay group has applied dynamic covalent chemistry to modify nanoparticle-bound molecules, which overcomes many of the disadvantages of traditional methods.

The reactivity of dynamic covalent reactions of nanoparticle-bound molecules can be affected by both external and intra-monolayer environments. In this thesis, stimuli-responsiveness and intra-monolayer catalysis of dynamic covalent exchange on nanoparticle-bound monolayer have been investigated. To achieve this, nanoparticles stabilised with an acetal functionalized monolayer were firstly prepared. The nanoparticle-bound acetal can be converted to aldehyde readily, and the conversion can be tracked in situ with NMR spectroscopy, enabling kinetics analysis of this process. Moreover, partial conversions form nanoparticle-bound acetal to aldehyde have been realized by stopping the conversion when the composition is as expected, as indicated by NMR spectroscopy. The ability to make mixed monolayer of acetal/aldehyde provides possibility to install both catalytic and exchangeable units on nanoparticles at controllable ratios.

Previously, kinetics of dynamic covalent exchange of hydrazones on nanoparticle-bound monolayers has been investigated. In this thesis, Hydrazones were introduced as exchangeable units to nanoparticle-bound monolayers by reacting nanoparticle-bound acetal/aldehyde with hydrazides. The conversion form acetal to hydrazone in the monolayer, being trackable by NMR spectroscopy, can be stopped at certain intermediate states to make mixed monolayers of acetal/hydrazone, providing possibility to install catalytic units starting from reactions with acetals in the mixed monolayers.

Imines can also be introduced to nanoparticle-bound monolayers as exchangeable units. Imines are usually more labile compared to hydrazones, and thus can be employed to demonstrate the adaptive features of dynamic covalent nanoparticles. Mixed nanoparticle-bound monolayer consists of two imines were produced be reacting nanoparticle-bound aldehyde with two amines with different nucleophilicity. Changing pH of the reaction solution can induce the constitutional reorganization of the mixed monolayer of two imines, which can be characterized in real-time by NMR spectroscopy.

Catalytic units may be installed to the nanoparticle-bound monolayer as a permanent component by reductive amination. Conditions for realisation of nanoparticle-bound have been investigated. Alcohols may also be introduced as a component to promote dynamic covalent reactions on the same monolayer. Therefore, alcohols were introduced to the monolayer by direct synthesis and kinetics of acetal hydrolysis were investigated. These are some preliminary explorations regarding self-catalytic dynamic covalent nanoparticles, but provide methodologies for further investigations.