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NANOStructure Photochemistry via Hot Electron driven REactions

Periodic Reporting for period 1 - NANOSPHERE (NANOStructure Photochemistry via Hot Electron driven REactions)

Reporting period: 2015-03-15 to 2017-03-14

The self-assembly of metal nanoparticles into higher order nanostructures is a high impact area of research for many reasons. Firstly, the chemical fabrication and assembly of organised structures is an essential prerequisite for understanding how complexity arises in living systems. A second aspect driving research in this direction is related to the harnessing and control of light at the nanoscale. Thirdly, these systems can generate high-energy ‘hot’ electrons. Oscillating electric fields surrounding nanoparticles, termed plasmons, can decay into high-energy electrons, which have sparked intense research in plasmon-induced chemical reactions and catalysis. Finally, these new and complex, composite materials exhibit unusual optical properties, which lend themselves to applications, such as plasmon-enhanced solar light harvesting and photocatalysis, ultrasensitive chemical and biological sensors, optical circuitry and metamaterials.
The spacing between nanoparticles within an assembled nanoparticle structure ultimately determines the structures optical properties, as well as the strength of the optical near field that can be focussed between particles. It is therefore crucial that inter-particle spacing be controlled with utmost accuracy. Whilst synthetic routes for controlling nanoparticle size and shape have dramatically improved during the past two decades, the development of methods for controlling inter-particle spacing in assembled nanostructures has remained a fundamental scientific challenge.
Although nanoparticles and assembled nanoparticle structures offer orders of magnitude increases in performance in a wide variety of applications, such applications have not as-of-yet benefited from nanostructure incorporation due to the difficulty in assembling nanostructures reproducibly with accurate control of inter-particle spacing, despite intense research. This research proposal aimed to utilise the unique macrocyclic host-guest chemistry of cucurbit[n]urils in conjunction with metal nanoparticles to demonstrate a novel and malleable approach to nanoparticle self-assembly, resulting in structures that will be used in light-driven chemical reactions and advanced molecular sensing.
The Marie Curie Fellow has performed research that has married the areas of nanotechnology and supramolecular host-guest chemistry. In the Figure 1(a), the structure of CB[n] macrocycles can be seen. These macrocycles can bind to the surfaces of gold nanoparticles through their carbonyl lined portals. In this fellowship, it was demonstrated that host-guest chemistry can be utilised to modulate the optical properties of gold nanoparticles that are aligned on the top of a gold film (Figure 1(b)). With CB[7] placed in the gap between the gold nanoparticle and gold film, the scattering peak of the system (650 nm), arising due to the antenna plasmon mode, can be split into two peaks, by the presence of methylene blue in the CB[7] macrocycle. This represents strong coupling between the optical response of the gold nanoparticle and the dye, at room temperature and has been published in the journal Nature.
Furthermore, CB[8] has been used to assembled hydrogel networks that incorporate CePO4 nanowires (NWs). It was demonstrated that the incorporation of wires into this system augments the rheological nature of the gel, and thus reinforcing it. The supramolecular hydrogel used was comprised of methyl viologen-functionalised poly(vinyl alcohol) (PVA-MV), hydroxyethyl cellulose with naphthyl moieties (HEC-Np) and CB[8] macrocyclic hosts. It was demonstrated that gel structure can be effectively enhanced by the framework supporting effects of CePO4 NWs and additional hydrogen bonding interactions between the NWs and the PVA-MV/HEC-Np polymers. The high aspect ratio NWs serve as a “skeleton” for the network, providing extra physical crosslinks, resulting in a single continuous phase hybrid supramolecular network with improved strength, presenting a general approach to reinforce soft materials.
Thermo-responsive materials are generating significant interest on account of the sharp and tunable temperature deswelling transition of the polymer chain. Such materials have shown promise in drug delivery devices, sensing systems, and self-assembly. Incorporation of nanoparticles (NPs), typically through covalent attachment of the polymer chains to the NP surface, can add additional functionality and tunability to such hybrid materials. The Fellow has demonstrated that the aggregation of PNIPAm-coated AuNPs, and likely other such materials, relies on the size and concentration of unbound “free” PNIPAm in solution, contray to popular belief (Figure 1(d)). It is this unbound polymer that also leads to an increase in solution turbidity, a characteristic that is typically used to prove nanoparticle aggregation.
The Fellow has also applied CB[n] macrocycles to gas encapsulation, reduction chemistries via hot electron generation and chemical sensing applications and the details of these outcomes will be published in due course.
It has been demonstrated previously that CB[n]s can be utilised in producing photonic nanoarchitectures, as CB[n]s make effective, rigid linkers between metal nanoparticles. The work conducted during this fellowship goes beyond this as the Fellow has shown that the optical properties of assembled gold nanostructures can be manipulated through the inclusion of guests inside CB[n] macrocycles when using them as assembly agents. Fluorescent dye molecules, when placed in an optical cavity, can experience an environment that changes how they are coupled to the surrounding light field. In the weak-coupling regime, the extraction of light from the emitter is enhanced. But more profound effects emerge when single-emitter strong coupling occurs: mixed states are produced that are part light, part matter forming building blocks for quantum information systems and for ultralow-power switches and lasers. Such cavity quantum electrodynamics has until now only been accessible through the use of low temperatures and complicated fabrication methods, compromising its use. Here, by scaling the cavity volume to less than 40 cubic nanometres and using host–guest chemistry to align one to ten protectively isolated methylene-blue molecules, the Fellow has shown that the strong-coupling regime can be reached at room temperature and in ambient conditions. It is envisaged that numerous applications will stem from this work, including single-photon emitters, photon blockades, quantum chemistry, nonlinear optics, and tracked or directed molecular reactions.
Furthermore, the Fellow has produced a review article that covers the vast scientific areas that the study of CB[n] host-guest chemistry has affected. These areas include polymer chemistry, hydrogel formation, nanoparticle and bulk surface functionalisation, gas encapsulation, catalysis and many more. In addition, comprehensive tables were produced that include guest molecules specific to CBs 5, 6, 7 and 8, along with binding constants and methods of determination. This represents the first time that such information has been available in one place. Given that the application of CB[n] host-guest chemistry is driven by the types of guest molecules that CB[n]s can accommodate, it is envisaged that these tables will encourage the discovery of new guest molecules, and thus new applications, for CB[n] macrocycles.