Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure−property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.

Organosoluble silver nanoparticles (AgNPs) have been synthesized for the first time in a task-specific, halide-free, deep eutectic solvent (DES) using a simple and convenient wet chemical reduction route involving microwave (MW) heating with oleylamine (OAm) acting as a surfactant and reducing agent. Nanoparticle formation is extremely rapid and occurs within 30 s of microwave heating at 100 °C. The effects of various reaction parameters (e.g., synthesis temperature, MW irradiation time, maximal MW power, water content of the medium) on the size and uniformity of the prepared AgNPs have been elucidated in this study. The produced colloidal AgNPs were characterized using UV–vis spectroscopy and transmission electron microscopy (TEM), with the aim of identifying reaction parameters simultaneously achieving optimal particle yield and colloid uniformity. This work illustrates how the versatile nature of DESs can be exploited to create unconventional DESs designed for nanoscale tasks for which conventional (e.g., halide-containing) DESs may be poorly suited, further expanding the repertoire of these solvents as sustainable media for various nanoapplications.

We have investigated the textural properties, electrochemical supercapacitances and vapor sensing performances of bamboo-derived nanoporous carbon materials (NCM). Bamboo, an abundant natural biomaterial, was chemically activated with phosphoric acid at 400 °C and the effect of impregnation ratio of phosphoric acid on the textural properties and electrochemical performances was systematically investigated. Fourier transform-infrared (FTIR) spectroscopy confirmed the presence of various oxygen-containing surface functional groups (i.e. carboxyl, carboxylate, carbonyl and phenolic groups) in NCM. The prepared NCM are amorphous in nature and contain hierarchical micropores and mesopores. Surface areas and pore volumes were found in the range 218–1431 m2 g−1 and 0.26–1.26 cm3 g−1, respectively, and could be controlled by adjusting the impregnation ratio of phosphoric acid and bamboo cane powder. NCM exhibited electrical double-layer supercapacitor behavior giving a high specific capacitance of c.256 F g−1 at a scan rate of 5 mV s−1 together with high cyclic stability with capacitance retention of about 92.6% after 1000 cycles. Furthermore, NCM exhibited excellent vapor sensing performance with high sensitivity for non-aromatic chemicals such as acetic acid. The system would be useful to discriminate C1 and C2 alcohol (methanol and ethanol).

Nanoporous activated carbons (AC) have been prepared from low-cost agro-waste corncob powder by phosphoric acid activation and investigated for their electrochemical supercapacitor and sensing properties. Surface areas and pore volumes are found in the range of 690–1288 m2 g−1 and 0.49–1.64 cm3 g−1, respectively and could be controlled by adjusting the weight ratio of corncob and phosphorous. The corncob-derived AC showed excellent electrochemical performance giving a maximum specific capacitance ca. 340.8 F g−1 at a scan rate of 5 mV s−1. At relatively a high scan rate of 100 mV s−1 the specific capacitance of 133.7 F g−1 was obtained. About 96% capacitance retention rate was achieved even after 1000 cycles demonstrating potential usages of the materials in high-performance supercapacitor electrodes. Furthermore, our AC showed excellent solvent vapor sensing performance with high selectivity for ammonia.

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We report on a simple, rapid, and room-temperature undergraduate-level experiment for gold nanoparticle synthesis, analysis, and utilization as a catalyst for the sodium-borohydride-assisted reduction of methylene blue and Congo red dyes. Typical procedures toward gold nanoparticle formation require an elaborate, lengthy synthesis (some requiring 1 h for completion of nanoparticle formation and ripening), whereas faster methods frequently yield inhomogeneous or catalytically inferior gold nanoparticles. This situation limits the usefulness of gold nanoparticles as rudimentary nanochemistry teaching tools in time-restricted undergraduate laboratories. The rapid gold nanoparticle synthesis presented herein utilizes ascorbic acid or sodium borohydride as dual reducing/capping agents to produce catalytically active gold nanoparticles at the “speed of mixing” under ambient conditions, allowing preparation and characterization to be performed within a single laboratory period. Furthermore, whereas most gold nanoparticle syntheses require rigorously cleaned glassware to avoid contamination and irreproducibility, the devised undergraduate experiment reported here alleviates this burden by employing disposable reaction vessels. Thus, this undergraduate-level laboratory experiment is designed to provide a meaningful introduction to metal nanoparticle synthesis and application while simultaneously being accommodated by a typical 2–4 h laboratory period, providing much-needed teaching assistance in the growing field of nanomaterials education.

Conventional colloidal nanocrystal synthesis is conducted in batch formats which produce materials on a scale far below what is required for industrial applications. As a practical solution to this challenge, we show herein that a task-specific argentous deep eutectic solvent (DES) comprising a 1:4 mixture of silver triflate with acetamide can be used as a fluidic metal precursor to push the production of silver nanocrystals to large quantities. This approach employs oleylamine as cosolvent, reductant, and colloid capping agent and is shown to quickly produce large amounts of high-quality monodispersed silver nanocrystals (AgNCs) in a simple microwave-mediated reduction (2 min reaction at 200 °C). The large initial silver concentration ([Ag]0) of the DES-based reaction medium in turn allows a high colloidal concentration of AgNCs to be accomplished. For an [Ag]0 of 0.50 M, the highest silver concentration which gave uniform AgNCs a colloidal concentration of 54 mg mL–1 was attained, equivalent to the production of ∼58 g of AgNCs per kilogram of DES reaction medium. Interestingly, although low concentrations of metal have been historically preferred in order to suppress uncontrolled nanocrystal growth, aggregation, and fusion, a “size-focusing” effect is evident in the highly concentrated regime. Indeed, for an [Ag]0 of 0.30 M, the reaction yielded 15.7 ± 2.1 nm AgNCs (13% relative standard deviation), which were more uniform than AgNCs made in the [Ag]0 = 0.02–0.26 M range. Notably, little loss in nanocrystal quality occurred under more intensified process conditions (i.e., higher silver concentration) until 0.63 M [Ag]0 was reached. This work highlights the promise of metallic DESs for the purpose of high-volume colloidal nanocrystal production by novel yet rational synthetic strategies in future nanomanufacturing efforts.

Significant process intensification of nanoparticle synthesis is required to meet the demands of industrial applications. Unfortunately, conventional nanoparticle synthetic methods (i.e., those using traditional solvents) are largely unsustainable long term due to inefficient reagent, solvent, personnel, and time utilization. As one possible path forward, we demonstrate the benefits of a dimethylammonium nitrate−polyol deep eutectic solvent (DES) system paired with oleylamine for the rapid and facile production of significant quantities of high-quality silver and gold nanoparticles dispersible in hydrophobic organic solvents such as toluene. We investigate three potential polyols as the hydrogen bond donor of the DES (i.e., triethylene glycol, ethylene glycol, glycerol), provide a temperature semioptimization, and investigate product quality using both batch and continuous flow reaction formats. These DESs possess extraordinary metal dissolution properties and are capable of producing monodispersed coinage metal colloids at remarkably high concentrations (i.e., 1000 and 400 mM silver and gold content, respectively) in essentially a quantitative yield. As an illustration of the consequence of this high metal concentration, a 1 mL reaction employing 1000 mM AgNO 3 produces 107 mg of silver nanoparticles within minutes, some 1000-to 4000-fold higher than a conventional aqueous synthesis performed using the same reaction volume. Likewise, translation to continuous flow millifluidic synthesis at a moderate flow rate of 0.24 mL min −1 at the same metal loading yields 26 mg of silver nanoparticles per minute. This amounts to 37.4 g of isolable nanoparticles daily, a substantial output, which can be amplified further by operating multiple continuous flow reactors in parallel. Finally, we demonstrate the efficient phase transfer of the oleylamine-stabilized gold nanoparticles from nonpolar solvent (toluene) into aqueous solution by applying a ligand-exchange surface reaction using 11-mercaptoundecanoic acid with full preservation of nanoparticle size, morphology, monodispersity (8.8% RSD), and colloidal stability. This research points to an exceptional future for DESs as high-performance fluids in scalable, sustainable, and flow processes toward the intensification of nanomanufacturing efforts.