Strategies for colloidal metal nanocrystal synthesis in deep eutectic solvents
Authors:   , Adhikari Laxmi 
Published: September 2022

[EMBARGOED UNTIL 6/1/2023] Noble-metal nanocrystals have a wide range of applications in a variety of areas, including catalysis, electronics, optical sensing, and biomedicine. Despite the fast-growing number of reports on synthetic techniques for morphology-controlled synthesis of nanoparticles, there still exists a gap between academic studies and their industrial applications due to the lack of ability of the traditional nanoparticle synthetic methods to produce the nanocrystals in large quantities while still maintaining nanoparticle quality. High-concentration nanoparticle synthesis is still an unattained goal in traditional solvents due to their limited metal salt solubilities. Although salt dissolution is not an issue for aqueous phase syntheses, these routes lead to particles with poor homogeneity and colloidal stability. Designing a sustainable synthetic medium capable of high metal salt dissolution while retaining control over particle morphology is thus a critical requirement for process intensification of nanoparticles, especially to advance the utility of these nanoscale materials for industrial applications. Deep eutectic solvents (DESs) are an emerging class of media well known for their ionic liquid-like properties including high thermal stabilities, low vapor pressure, tunable polarities and charge densities, useful electrochemical potential windows, and high coordinating ability with an added benefit of biodegradability, low toxicity, and cost-effectiveness. DESs are usually formed by a complex hydrogen-bond interaction between a quaternary ammonium or phosphonium salt with a hydrogen bond donor which frustrates the individual crystallinity resulting in a mixture with a depressed melting point compared to its parent components. These typically viscous, clear liquids with tunable physicochemical features are highly relevant in nanometal applications, including colloidal metal nanoparticle synthesis. This dissertation reports strategies for colloidal metal nanocrystal synthesis in deep eutectic solvents and aims to enlighten how the designer aspect of DESs can be exploited to create unconventional DESs for nanoscale tasks. Chapter 1 introduces these novel media and explores research reporting strategies used for metal colloid synthesis in these media. In Chapter 2, we have developed for the first time a facile route towards organosoluble silver nanoparticles (AgNPs) in a task-specific, halide-free, DES using a simple and convenient wet chemical reduction route involving microwave (MW) heating. This work illustrates how the designer nature of DESs can be utilized to create unconventional DESs designed for nanoapplications. For Chapter 3, we have demonstrated the promise of metallic DESs as fluidic precursors for advancing the production of silver metal nanocrystals to large quantities. Chapter 4 highlights the promise of dimethylammonium nitrate-polyol DESs for the purpose of high-volume colloidal metal nanocrystal production, including their utility in a continuous flow set up. This chapter emphasizes the potential of DESs as high-performance sustainable media for various nanoapplications including the high throughput routes towards metal nanoparticle process intensification. In Chapter 5, we have presented some ideas worth pursuing for addressing the current limitations of DESs in metal nanoparticle synthesis to pave a surfactantless greener route towards anisotropic nanoparticle synthesis.