Lee, TC; (2012) Cucurbit[n]uril-metal nanoparticle composites. Doctoral thesis , University of Cambridge.

Abstract

The research presented in this dissertation pioneers a new area of study in CB[n]-metal nanoparticle (CB[n]-NP) supramolecular assemblies. While many of the supramolecular motifs are only functional in organic media, cucurbit[n]urils (CB[n]s), as a result of their water solubility and unique multi-functional macrocyclic structure, are promising as a series of robust supramolecular building blocks for both self-assembly of the CB[n]-NP organic-inorganic composite frameworks and the subsequent molecular recognition of small organic molecules in aqueous media. While various ways in integrating CB[n]s with metal nanoparticles have been investigated, two major systems are presented here, namely, portal binding (Type III) and indirect binding (Type IV) composites. Chapter 1 introduces metal nanoparticles based on their unique properties and the corresponding applications in photonics, catalysis and as nano-scaffolds. After a brief description of various supramolecular motifs, macrocyclic molecules will be highlighted as a series of robust supramolecular building blocks. Motivations and challenges in incorporating supramolecular self-assembling and molecular-recognition motifs into nanomaterials are then discussed and illustrated by specific examples in the literature. Precedent examples of composites between macrocyclic molecules and metal nanoparticles are critically reviewed in the latter part of the chapter. Chapter 2 describes the synthesis and characterisation of aqueous metastable gold nanoparticles (AuNPs*), which were discovered to be a facile intermediate in the fabrication of CB[n]-AuNPs. Although similar syntheses have been reported, there is no report on investigating the “metastability” of these AuNPs* or optimising different parameters for practical uses. In the absence of any additional stabilising ligands, AuNPs* are only stabilised by a layer of adsorbed chloride ions, which can then be easily displaced by a wide variety of water-soluble ligands, including weakly bound CB[n]s. This serves as an important foundation for the subsequent research in Type III CB[n]-AuNP systems which will be discussed in detail in the next two chapters. Chapter 3 focuses on the synthesis and characterisation of Type III CB[n]-AuNP supramolecular composites, in which the CB[n] molecules are dynamically capped by AuNPs on either or both of their portals. The CB[n]-AuNPs can then further self-assemble and form well-defined dynamic aggregates in aqueous media, with a controllable ratio between singly and doubly capped CB[n]s. The first report of Type III CB[n]-AuNP in the literature is presented here, contributing to the early development of this field of study which is now being pursued together with a number of research groups around the world. Owing to the high rigidity and well-defined molecular geometry of such molecules, CB[n]s can act as a precise sub-nanometer junction between plasmonically active AuNPs within the dynamic composites. Chapter 4 focuses on discussing the plasmonic properties of these Type III CB[n]-AuNP systems, and their applications in surface-enhanced Raman scattering (SERS) spectroscopy. Through the aid of computational simulation, we are able to assign each major peak in the Raman and SERS spectra, as well as understand their systematic shifts across the CB[n] homologues. Armed with such fundamental knowledge, we then further employ the SERS-technique to study the aggregation kinetics and the corresponding evolution of plasmonic behaviours of the Type III CB[n]-AuNP systems. The precise and well-defined plasmonic junctions, created by doubly AuNP-capped CB[n]s, allow aggregation kinetics to be studied in the finest details, which is unprecedented in any previous reports. Furthermore, hydrophobic cavity of the CB[n] hosts offers the possibility for small organic guest molecules to be encapsulated right at the heart of these plasmonic hot spots, leading to a self-calibrated molecular-recognition-based in situ SERS sensing system. An initial example of this powerful sensing system is presented at the end of the chapter. Chapter 5 highlights an alternative promising strategy in building up dynamic hierarchical materials in different solvents. The first part of this chapter describes a novel method of dispersing AuNPs within a polymer matrix. Thiol-terminated ureido-pyrimidinone (UPy)-functionalised polymers are attached to AuNPs, creating a polymeric shell with quadruple hydrogen-bonding units on the periphery. The second part of the chapter focuses on Type IV CB[8]-NP supramolecular composites, in which CB[8] and metal nanoparticles are indirectly tethered via host-guest complexation with guests that are covalently anchored on the surface of nanoparticles. While CB[8], a larger homologue in the CB[n] family, can simultaneously accommodate two complementary guests in its cavity, it can act as a robust and versatile “supramolecular handcuff” that ligates appropriately functionalised molecules and nano-objects together in a reversible manner. Dynamic hierarchical composites based on self-assembly between guest-functionalised AuNPs and a water-soluble polymer with pendant complementary guests is presented.

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