Deep Eutectic Solvents: Sustainable Media for Nanoscale and Functional Materials

Abstract

Deep eutectic solvents (DESs) represent an alternative class of ionic fluids closely resembling room-temperature ionic liquids (RTILs), although, strictly speaking, they are distinguished by the fact that they also contain an organic molecular component (typically, a hydrogen bond donor like a urea, amide, acid, or polyol), frequently as the predominant constituent. Practically speaking, DESs are attractive alternatives to RTILs, sharing most of their remarkable qualities (e.g., tolerance to humidity, negligible vapor pressure, thermostability, wide electrochemical potential windows, tunability) while overcoming several limitations associated with their RTIL cousins. Particularly, DESs are typically, less expensive, more synthetically accessible (typically, from bulk commodity chemicals using solvent/waste-free processes), nontoxic, and biodegradable. In this Account, we provide an overview of DESs as designer solvents to create well-defined nanomaterials including shape-controlled nanoparticles, electrodeposited films, metal-organic frameworks, colloidal assemblies, hierarchically porous carbons, and DNA/RNA architectures. These breakthroughs illustrate how DESs can fulfill multiple roles in directing chemistry at the nanoscale: acting as supramolecular template, metal/carbon source, sacrificial agent (e.g., ammonia release from urea), and/or redox agent, all in the absence of formal stabilizing ligand (here, solvent and stabilizer are one and the same). The ability to tailor the physicochemical properties of DESs is central to controlling their interfacial behavior. The preorganized "supramolecular" nature of DESs provides a soft template to guide the formation of bimodal porous carbon networks or the evolution of electrodeposits. A number of essential parameters (viscosity, polarity, surface tension, hydrogen bonding), plus coordination with solutes/surfaces, all play significant roles in modulating species reactivity and mass transport properties governing the genesis of nanostructure. Furthermore, DES components may modulate nucleation and growth mechanisms by charge neutralization, modification of reduction potentials (or chemical activities), and passivation of particular crystal faces, dictating growth along preferred crystallographic directions. Broad operational windows for electrochemical reactions coupled with their inherent ionic nature facilitate the electrodeposition of alloys and semiconductors inaccessible to classical means and the use of cosolvents or applied potential control provide under-explored strategies for mediating interfacial interactions leading to control over film characteristics. The biocompatibility of DESs suggests intriguing potential for the construction of biomolecular architectures in these novel media. It has been demonstrated that nucleic acid structures can be manipulated in the ionic, crowded, dehydrating (low water activity) DES environment-including the adoption of duplex helical structures divergent from the canonical B form and parallel G-quadruplex DNA persisting ne...

Efficient Cleavage of Lignin–Carbohydrate Complexes and Ultrafast Extraction of Lignin Oligomers from Wood Biomass by Microwave‐Assisted Treatment with Deep Eutectic Solvent

“…DESs are generally prepared by the self‐association of hydrogen‐bond donors (HBD) and hydrogen‐bond acceptors (HBA), which form a liquid driven by strong hydrogen‐bonding interactions that reduce the ability of the parent compounds to crystallize (Figure 1 a) 28, 29. Generally, a DES is obtained by mixing a quaternary ammonium halide salt with a neutral organic HBD to form a complex with halide ions and solvent molecules 30, 31, 32. DESs exhibit several advantages, such as easy and cheap fabrication, no need for purification, and insensitivity to water, and have been widely accepted.…”

Exploring the bulk-phase structure of ionic liquid mixtures using small-angle neutron scattering

“…1,[20][21][22][23][24][25][26][27][28][29] The most appropriate model is strongly related to the type of IL under study, as there are many distinct classes. Indirect evidence from viscosity, conductivity and Raman spectroscopy proposed longrange ordering in ILs from as early as 2000.…”

Facile fabrication of nickel nanostructures on a copper-based template via a galvanic replacement reaction in a deep eutectic solvent

“…18 The intriguing solvent properties of DESs such as polarity, surface tension, and highly ordered hydrogen bonding, potentially provide an environment for the generation of nanostructured materials. [19][20][21] Moreover, the capacity to tune redox properties in DESs 22 enables the possibility to obtain Galvanic exchange processes that are difficult to realize in aqueous solutions.…”