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Research Article pubs.acs.org/journal/ascecg

Biocompatible Deep Eutectic Solvents Based on Choline Chloride: Characterization and Application to the Extraction of Rutin from Sophora japonica Bing-Yi Zhao,† Pei Xu,† Fu-Xi Yang,† Hong Wu,† Min-Hua Zong,*,‡ and Wen-Yong Lou*,†,‡ †

Lab of Applied Biocatalysis, School of Light Industry and Food Sciences, and ‡State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: The development of novel green solvents has been one of the hottest subjects in green chemistry. Deep eutectic solvents (DESs) have logically and naturally emerged in the search for more biocompatible and biodegradable solvents. In this study, some basic physical properties, including viscosity, conductivity, and density, of 20 DESs prepared from choline chloride and various hydrogen-bond donors were investigated systematically. In addition, the biocompatibility of the tested DESs was qualitatively and quantitatively evaluated using two Gram-positive (Staphylococcus aureus and Listeria monocytogenes) and two Gramnegative (Escherichia coli and Salmonella enteritidis) bacteria. A closed bottle test was used to assess the biodegradability of these DESs. The results demonstrated that these choline chloride-based DESs were excellent solvents with extremely low toxicity and favorable biodegradability. Finally, DESs were used to extract a flavonoid (rutin) from the flower buds of Sophora japonica. An extraction efficiency of 194.17 ± 2.31 mg·g−1 was achieved using choline chloride/triethylene glycol containing 20% water. The excellent properties of DESs indicate their potential as promising green solvents for the extraction of rutin with favorable prospects for wide use in the field of green technology. KEYWORDS: Deep eutectic solvent (DES), Choline chloride, Physical properties, Biocompatibility, Biodegradability, Rutin

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INTRODUCTION

found that the type of salt and HBD had a significant effect on the studied properties.3 Despite the advantages of ILs, some aspects definitely challenge their development. One is that some by-products (e.g., water and salt) generate in the preparation process, which is rarely mentioned.4,5 Another is related to their biodegradability and bioaccumulation. Consequently, there is increasing focus on their potential influence on the environment.6,7 As for DESs, their ecological footprint has not yet been thoroughly investigated and few relevant studies have been published.8,9 Therefore, the label “green” for DESs should be used with caution and it is essential to determine their biodegradation potential. DESs have been used in many fields, such as organic reactions, electrochemical, nanoparticles, and drugs.1,10−13 However, only a few studies have focused on the use of DESs for the extraction of bioactive compounds.14−19 Rutin is a kind of flavonoid and can be used to treat hypertension and cerebral hemorrhage.20 Rutin is abundant in the flower buds of

Deep eutectic solvents (DESs) are emerging as alternatives to conventional ionic liquids (ILs) and organic solvents, attracting attention in many fields due to their unique advantages. As promising solvents, DESs not only retain the excellent merits of ILs, but also overcome their shortcomings. They have the merits such as low vapor pressure, nonflammability, simple preparation, easy purification, and low price. A DES, a eutectic mixture, is generally composed of two or three cheap and safe components that are capable of associating with each other through hydrogen-bond interactions.1 In most cases, the DES is prepared by mixing a quaternary ammonium salt with a hydrogen-bond donor (HBD) which has the ability to form a hydrogen bond with the halide anion of the quaternary ammonium salt. Workers have combined different starting materials to synthesize solvents with eutectic behavior, and there are a large number of reports on their physical properties. Dai prepared DESs from a wide range of natural products and investigated the molecular interactions using nuclear magnetic resonance spectroscopy.2 Mukhtar et al. measured the basic physical parameters of novel phosphonium-based DESs and © 2015 American Chemical Society

Received: July 2, 2015 Revised: September 24, 2015 Published: September 27, 2015 2746

DOI: 10.1021/acssuschemeng.5b00619 ACS Sustainable Chem. Eng. 2015, 3, 2746−2755

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Structure of DESs based on choline chloride.

rutin from the flower buds of Sophora japonica (Figure S1, Supporting Information), for the first time, was explored. These discoveries will serve as a bridge between knowledge of the environmental fate of DESs and bright prospects for application in human health.

Sophora japonica, and traditionally extracted by hot water, methanol, and ethanol with relatively low extraction efficiency, due to its low solubility in these solvents. To extend the applications of DESs in the extraction of bioactive natural products, it is of great interest to attempt to extract rutin using these novel solvents which are able to significantly damage the cell wall, release product, and enhance the solubility of rutin. In this work, our team rationally designed and prepared a series of DESs based on choline chloride and natural, renewable products (Figure 1). Their physical properties, biocompatibility, and biodegradability were studied systematically. In addition, utilization of the prepared DESs for the extraction of

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MATERIALS AND METHODS

Chemicals. Choline chloride (ChCl) (≥98% mass fraction purity) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Urea, acetamide, ethylene glycol, glycerol, 1,4butanediol, triethylene glycol, xylitol, D-sorbitol, p-toluenesulfonic acid, oxalic acid, levulinic acid, malonic acid, malic acid, citric acid, tartaric 2747


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ACS Sustainable Chemistry & Engineering Table 1. Physical Parameters of DESs

a

DESs

salt/HBD molar ratio

ChCl/urea ChCl/acetamide ChCl/ethylene glycol ChCl/glycerol ChCl/1,4-butanediol ChCl/triethylene glycol ChCl/xylitol ChCl/D-sorbitol ChCl/p-toluenesulfonic acidb ChCl/oxalic acidc ChCl/levulinic acid ChCl/malonic acid ChCl/malic acid ChCl/citric acidb ChCl/tartaric acid ChCl/xylose/water ChCl/sucrose/water ChCl/fructose/water ChCl/glucose/water ChCl/maltoseb/water

1:2 1:2 1:2 1:2 1:4 1:4 1:1 1:1 1:1 1:1 1:2 1:1 1:1 1:1 2:1 1:1:1 5:2:5 5:2:5 5:2:5 5:2:5

water content wt %

viscositya Pa·s

conductivitya μS·cm−1

densitya g/cm3

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.214 0.127 0.025 0.177 0.047 0.044 3.867 13.736 0.183 0.089 0.119 0.616 11.475 45.008 66.441 0.887 3.939 0.598 0.584 3.122

1287 2710 9730 1647 2430 1858 172.6 63.3 1138 2350 1422 732 41.4 18.4 14.3 1092 147.2 1399 2820 421

1.1879 1.0852 1.1139 1.1854 1.0410 1.1202 1.2445 1.2794 1.2074 1.2371 1.1320 1.2112 1.2796 1.3313 1.2735 1.2505 1.2737 1.2095 1.2094 1.2723

1.89 2.83 3.79 1.68 2.87 2.47 1.21 1.10 5.85 6.68 2.55 3.36 1.72 4.06 1.35 9.85 5.43 9.35 9.35 9.47

0.01 0.02 0.01 0.01 0.01 0.03 0.01 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.05 0.02 0.03 0.02 0.06 0.01

Determined at 30 °C. bMonohydrate. cDehydrate. CFU mL−1 in each well, and the tested DESs were added at concentrations of 8−52 mM, at 2 mM intervals, with a final volume of 100 μL. An inoculum without DES addition was used as a control. The plates were lightly shaken, sealed, and incubated at 37 °C for 20 h and then the absorbance at 600 nm was measured on a microplate reader (Tecan Infinite 200 PRO). Each test was repeated four times. The minimal inhibitory concentration (MIC, mM) was the lowest concentration of DES solution that prevented the growth of a microorganism after a specified incubation period. After incubation, the bacterial suspension in the plate was cultured and observed. The minimal bactericidal concentration (MBC, mM), the lowest concentration of DES solution required to kill ≥99.9% of the test bacterium, was also determined. Biodegradability. The biodegradability of the as-prepared DESs was determined according to the closed bottle test.23 The standard for this method is 60% theoretical oxygen demand for the reference substance in a 14 d window within the 28 d period of the experiment. A solution of each DES (3 mg L−1) in mineral medium was inoculated individually into a fresh lake water sample at a concentration of 1 mL· L−1. An inoculum without DES addition was used as the control and sodium benzoate was used as the reference substance. The mineral medium was composed of 8.50 mg L−1 KH2PO4, 21.75 mg L−1 K2HPO4, 33.40 mg L−1 Na2HPO4·2H2O, 0.5 mg L−1 NH4Cl, 27.50 mg L−1 CaCl2, 22.50 mg L−1 MgSO4·7H2O, and 0.25 mg L−1 FeCl3·6H2O. The bottles were kept at 25 °C in the dark for 28 d and the biological oxygen demand was measured every 7 d. Each assay was performed in triplicate to ensure accuracy, and the reported result is the average value with a relative standard deviation of 0.58%. Extraction of Rutin from S. japonica. Powdered S. japonica buds (1.00 g) were mixed with 10 mL of solvent in a flask. Twenty different DESs containing 20% water were used as the solvents with methanol/ water (60%, v/v) and ethanol/water (60%, v/v) as the controls. The flask was placed in a water bath at 55 °C with continuous stirring for 20 min, and the mixture was centrifuged at 12000g for 10 min to remove the solids. The supernatant was diluted, filtered through a 0.45 μm nylon membrane and analyzed by HPLC. Each extraction was performed in triplicate to ensure accuracy, and the reported result is the average value with a relative standard deviation of 0.16%. The direct separation of rutin from the extraction solution of DES was carried out using column chromatography with a column (15 mm × 500 mm) packed with wet AB-8 macroporous resin. The bed volume (BV) of resin was 40 mL. A 20 mL aliquot of DES extraction

acid, xylose, sucrose, fructose, glucose, and maltose (all ≥99% mass fraction purity) were all purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, China). Rutin (≥98% mass fraction purity) was purchased from Shanghai Yuanye Biotechnology Co., Ltd. (Shanghai, China). S. japonica bud was purchased from ZhanJiang Yizhou Medicines Co., Ltd. (Guangdong, China). All other chemicals were of analytical grade. Preparation of DESs. DESs were synthesized by mixing choline chloride and HBDs at a defined molar ratio (see Table 1) and heating at 100 °C for 2−4 h at an atmospheric pressure under constant stirring until a stable homogeneous liquid was formed.21 Sugar-based DESs were prepared using the same conditions but under a nitrogen atmosphere. All the prepared DESs were allowed to cool to room temperature and dried in a vacuum oven at 50 °C for 24 h. The solvents were stored in sealed laboratory vials and kept in a desiccator. Physical Properties. The water contents of the samples were measured using a Metrohm Karl−Fischer (model 890) titrator. The viscosities of the DESs were measured with a HAAKE RheoStress 600 at 100 Hz from 25−80 °C at a rate of 5 °C min−1. The conductivity of all samples was measured with a conductivity meter (Shanghai Leici DDS-307A) at a preset temperature. The densities of all samples were determined using a 5 cm3 pycnometer calibrated with deionized water at 30 °C. As for the determination of viscosities, conductivity, and densities, all the DESs except sugar-based DESs were dried at 100 °C to minimize the water content. All measurements were performed at constant temperature. The relative standard deviation for all the tests were less than 1%. Biocompatibility. The tested strains were incubated in LB medium consisting of nutrient broth (18g L−1) and agar (15g L−1) at 37 °C for 12 h. A 6 mm filter paper was soaked with DES and equilibrated for 12 h in a closed vessel before applying to seeded plates. The plates were cultivated at 37 °C for 24 h, and then the diameters of the inhibition zones were measured and recorded. All experiments were performed in triplicate to ensure accuracy and the reported result is the average value with a relative standard deviation of 0.6%. A WST-1 assay, a modification of the classical MTT test, was used to quantitatively determine cellular biocompatibility of DESs.22 After enrichment, the concentration of the bacterial suspension was calibrated to 0.5 McIntosh turbidity containing 1 × 108 CFU mL−1 and diluted with nutrient broth to 1 × 106 CFU mL−1. Four types of bacteria were seeded in 96-well plates at a concentration of 5 × 105 2748


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Figure 2. Effect of temperature on the viscosity of DESs: (a) amine-based DESs; (b) alcohol-based DESs; (c) acid−based DESs; (d) sugar-based DESs. solution was injected at the flow rate of 3BV/h and washed with sufficient deionized water first, and then eluted with ethanol/water (70/30,v/v) at the flow rate of 4.5BV/h. The ethanolic fraction was collected and analyzed with HPLC. The solution was concentrated and dried under vacuum to get the solid rutin. HPLC Analysis. Samples were analyzed by RP-HPLC on a 4.6 mm × 250 mm (5 μm) Zorbax SB-C18 column (Agilent Technologies Industries Co. Ltd.) using a Waters HPLC system consisting of two Waters 1525 pumps and a Waters 2489 UV detector set at 254 nm. The mobile phase was a mixture of methanol, water, and phosphate acid (35/65/0.3, v/v/v). The flow rate was 1.0 mL min−1 and the injection volume was 10 μL. The peak was detected at 16.25 min according to the standard rutin which was observed at 16.34 min (Figure S2, Supporting Information).

obtained for a DES composed of choline chloride and urea (with melting points of 302 °C and 133 °C, respectively) in a ratio of 1:2.28 The significant reduction of melting point was due to interactions between the HBD molecule and the halide anions of choline chloride (Figure S3, Supporting Information).29 The smaller the charge and the larger the size of the ion, the less energy is required to break the bond.30 It was therefore supposed that the HBD worked as a special agent that interacted with the anionic species to increase its effective size, which conversely reduced interaction with the cation, resulting in the depression of melting point.31−33 Researchers could tailor the properties of DESs to make them ideal candidates relying on their water content, viscosity, conductivity, and density. Table 1 shows that the water contents of many DESs were alcohol-based DESs > acid−based DESs. The highest and the lowest biodegradability were found to be 97.1% and 69.3% for ChCl/urea and ChCl/triethylene glycol, respectively. The values for ChCl/urea and ChCl/glycerol were 81.2% and 83.2% in 14 d, respectively, and the biodegradation of sugar-based DESs reached about 70%. Moreover, the biodegradability of DESs and reference substance were faster in the first 14 d period than in the last 14 d period. The biodegradation values for 20 different DESs were far higher than conventional imidazole and pyridine ILs.53,54 The components of the tested DESs contributed significantly to the high degrees of biodegradation. Generally, biodegradable compounds first cross the cell wall in a variety of ways, such as free diffusion, facilitated diffusion, or active transport, and are then oxidized enzymatically. Intermediate products are metabolized to water and carbon dioxide or transformed into constituent materials of the cell. This is the primary reason for the good biodegradability of amine- and sugar-based DESs. Long-chain substances are less amenable to transmembrane transport, resulting in the relatively low biodegradation of ChCl/triethylene glycol. There are also exclusive carriers for the transportation of choline salt anion on the bacterial cell membrane, so choline chloride is easily transported. In addition, the hydroxyl, carboxyl and amino groups of the DESs are potential sites for enzyme reactions and likely to increase their degradation. Moreover, the acidic HBDs are readily metabolized by microorganisms, leading to the 70%− 80% biodegradation rate observed for acid-based DESs. The potential for choline amino acid ILs as biodegradable solvents has been acknowledged previously. Hou et al. assessed the biodegradability of 18 types of cholinium-based amino acid ILs and concluded that the ILs could be denoted as “readily biodegradable” based on their high level of biodegradability (62%−87%).23 Wen evaluated the biodegradability of 8 DESs based on choline chloride and choline acetate, and two of them had the biodegradability of about 80%.55 Our research may

31.3 mM, respectively.23 The benign biocompatibility of these ChCl-based DESs is attributed to the combination of nontoxic choline chloride with natural, renewable HBDs. It was confirmed by Frade et al. that viability was dependent on the concentration of ILs, but the ChCl-based solvents are generally nontoxic or of low toxicity.52 Differences in toxicity of the various ChCl-based DESs can be related to the different HBDs since they share the same hydrogen-bond acceptor. The higher toxicity of DESs containing organic acids might be partly explained by the change of pH (Table 3) because environmental pH change can alter cellular proliferation and metabolic properties. The ChCl/ p-toluenesulfonic acid DES, which contained a benzene ring structure, had the lowest toxicity. The toxicity of the acid− based DESs decreased with elongation of the carbon chain (Table 3, entries 2 and 4). However, this effect was not evident when comparing entries 2, 3, and 6, which had similar inhibitory effects, possibly due to the introduction of an acetyl group and polar carboxyl group. As shown in entries 5 and 7, the addition of an extra hydroxyl group in the HBD resulted in a slight increase in antibacterial activity. ChCl/oxalic acid, ChCl/levulinic acid, and ChCl/citric acid had the highest toxicity, while the lowest toxicity was observed for ChCl/ malonic acid and ChCl/p-toluenesulfonic acid. So the toxicity of the various ChCl-based DESs was associated with pH and the HBA compounds. Biodegradability. The biodegradability of the DESs, important for understanding their environmental impact and fate, was evaluated according to the closed bottle test. DESs were added to aqueous medium containing oxygen. Then the mixture was inoculated with lake microorganisms, and the biodegradation value was determined at a defined time interval (Table 4). On the basis of the Closed Bottle Test, the biodegradability of sodium benzoate was 62.8% on the 14th day of this research, so the method was valid. The biodegradability for all the tested DESs was >69.3% after 28 d, therefore all of them could be considered as biodegradable green solvents. The 2752


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based DESs were lower than those with amine- or alcoholbased DESs, except for ChCl/levulinic acid and ChCl/malonic acid. In view of the high cost of levulinic acid, the optimal extraction solvent was ChCl/triethylene glycol. By using the above-described separation method, the recovery of the extracted rutin form DES was 99.3 mg/g. In addition to the viscosity of the DES, the effect of the structure of the HBD should also be considered. Rutin is a flavonoid, which can be considered as a HBD. Therefore, the HBDs and rutin compete for the chloride anion. If a molecule of HBD has sufficient hydrogen-bond donor groups or branches, it can envelope the chloride anion, resulting in considerable steric hindrance and preventing interactions between rutin and chloride anion. This is a possible explanation for the high extraction efficiencies of ChCl/triethylene glycol and ChCl/levulinic acid. Therefore, a favorable solvent should have the correct distance between HBD groups and chloride anion.

encourage a shift of attention from classical imidazolium and pyridinium ILs to DESs from natural sources such as amines, alcohols, organic acids, and sugars. It is worth noting that some “readily biodegradable” classical ILs with long alkyl chains have also been reported to be highly toxic because of their lipophilic character. Conversely, ILs with short alkyl side chains were safe but suffered from reduced biodegradability.7 However, this conflict between toxicity and biodegradability was not apparent in the DESs tested in this study since low toxicity was associated with good biodegradability. Extraction of Rutin from S. japonica. In the pioneering work of the potential application of natural deep eutectic solvents incellular metabolism and physiology by Choi et al.,56 it was found that DESs had a strong ability to dissolve the flavonoid rutin, as well as other cellular metabolic compounds, indicating 50 to 100 times higher solubility of rutin than that in water. In that case, however, the use of DESs for extraction of rutin from a plant was not involved. Therefore, it was of great interest to investigate the extraction of the valuable rutin from the flower buds of Sophora japonica with the as-prepared choline chloride-based DESs. Twenty DESs at a defined molar ratio (Figure 1) were tested on the basis of previous studies. The viscosities of the DESs were generally high, which hampered mass transfer from powder to solution. Therefore, water (20%, v/v) was added to the DES (except for sugar-based DESs) for successful extraction of rutin. Solvents including 60% ethanol/water and 60% methanol/water were selected as reference extraction solvents.57 Clear superiority in extraction amount of rutin was observed with the evaluated DESs compared with 60% methanol and 60% ethanol. As shown in Figure 5, the amount of rutin extracted by ChCl/triethylene glycol (194.17 ± 2.31 mg) and ChCl/

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CONCLUSIONS In this work, the physical properties (water content, viscosity, conductivity, density) of a variety of DESs were studied in detail and the hole theory was used to explain the intrinsic microscopic links between viscosity, conductivity, and density. The qualitative study on biocompatibility of the DESs revealed that amine-, alcohol- and sugar-based DESs were benign solvents, while the acid-based DESs were harmful to Gramnegative (E. coli and S. enteritidis) and Gram-positive (S. aureus and L. monocytogenes) bacteria. Quantitative research on the biocompatibility of the acid-based DESs illustrated that even though they have antimicrobial activity, they might still be referred to as “green solvents” because of their low toxicity compared with traditional solvents and ILs. Additionally, all DESs were environmentally friendly solvents with biodegradation >69%, classified as “readily biodegradable”. Moreover, it was evident that rutin could be readily and efficiently extracted with ChCl/triethylene glycol solution, indicating that DES would have great potential to extract rutin from Sophora japonica.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00619. The chemical structure of rutin; HPLC analysis of standard rutin and extracted rutin; the interaction of a HBD with the quaternary ammonium salt choline chloride; the thermal properties of the DESs or individual substances (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-20-87111452. E-mail: [email protected]. *Tel.: +86-20-22236669. Fax: +86-20-22236669. E-mail: [email protected].

Figure 5. Effect of various DESs on the amount of rutin extracted from S. japonica (solid/liquid ratio of 1 g/10 mL, 55 °C, 20 min).

Notes

The authors declare no competing financial interest.

levulinic acid (197.80 ± 3.14 mg) was higher than that by other solvents. The extraction efficiencies of amine- and alcoholbased DESs were superior to those of sugar-based DESs, because the latter had a higher viscosity. Rutin is readily soluble in alkaline solvent, so the extraction efficiencies of the acid−

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ACKNOWLEDGMENTS We wish to thank the National Natural Science Foundation of China (21336002; 21222606; 21376096), the Key Program of 2753


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solvents: application to flavonoid extraction from Flos sophorae. Green Chem. 2015, 17 (3), 1718−1727. (19) Helalat-Nezhad, Z.; Ghanemi, K.; Fallah-Mehrjardi, M. Dissolution of biological samples in deep eutectic solvents: an approach for extraction of polycyclic aromatic hydrocarbons followed by liquid chromatography-fluorescence detection. J. Chromatogr. A 2015, 1394, 46−53. (20) Afanas’Eva, I. B. Enhancement of antioxidant and antiinflammatory activities of bioflavonoid rutin by complexation with transition metals. Biochem. Pharmacol. 2001, 61 (6), 677−684. (21) Florindo, C.; Oliveira, F. S.; Rebelo, L. P. N.; Fernandes, A. M.; Marrucho, I. M. Insights into the Synthesis and Properties of Deep Eutectic Solvents Based on Cholinium Chloride and Carboxylic Acids. ACS Sustainable Chem. Eng. 2014, 2 (10), 2416−2425. (22) Mosmann, T. Rapid colorimetric assay for cellular growth and survival: Application to proliferation and cytotoxicity assays. J. Immunol. Methods 1983, 65 (1−2), 55−63. (23) Hou, X.-D.; Liu, Q.-P.; Smith, T. J.; Li, N.; Zong, M.-H. Evaluation of toxicity and biodegradability of cholinium amino acids ionic liquids. PLoS One 2013, 8 (3), e59145. (24) Maugeri, Z.; Dominquez de Maria, P. Novel choline-chloridebased deep-eutectic-solvents with renewable hydrogen bond donors: levulinic acid and sugar-based polyols. RSC Adv. 2012, 2 (2), 421−425. (25) Hayyan, A.; Mjalli, F. S.; AlNashef, I. M.; Al-Wahaibi, Y. M.; AlWahaibi, T.; Hashim, M. A. Glucose-based deep eutectic solvents: Physical properties. J. Mol. Liq. 2013, 178 (0), 137−141. (26) Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. Deep eutectic solvents formed between choline chloride and carboxylic acids: Versatile alternatives to ionic liquids. J. Am. Chem. Soc. 2004, 126 (29), 9142−9147. (27) Li, C.; Wang, C.; Yang, Z. Production and separation of phenols from biomass-derived bio-petroleum. J. Anal. Appl. Pyrolysis 2010, 89 (2), 218−224. (28) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Novel solvent properties of choline chloride/urea mixtures. Chem. Commun. 2003, 1 (1), 70−71. (29) Francisco, M.; van den Bruinhorst, A.; Kroon, M. C. LowTransition-Temperature Mixtures (LTTMs): A New Generation of Designer Solvents. Angew. Chem., Int. Ed. 2013, 52 (11), 3074−3085. (30) Avalos, M.; Babiano, R.; Cintas, P.; Jiménez, J. L.; Palacios, J. C. Greener Media in Chemical Synthesis and Processing. Angew. Chem., Int. Ed. 2006, 45 (37), 3904−3908. (31) Abbott, A. R.; Capper, G.; Gray, S. Design of improved deep eutectic solvents using hole theory. ChemPhysChem 2006, 7 (4), 803− 806. (32) D’Agostino, C.; Harris, R. C.; Abbott, A. P.; Gladden, L. F.; Mantle, M. D. Molecular motion and ion diffusion in choline chloride based deep eutectic solvents studied by H-1 pulsed field gradient NMR spectroscopy. Phys. Chem. Chem. Phys. 2011, 13 (48), 21383− 21391. (33) Abbott, A. P.; Harris, R. C.; Ryder, K. S. Application of hole theory to define ionic liquids by their transport properties. J. Phys. Chem. B 2007, 111 (18), 4910−4913. (34) Pena-Pereira, F.; Kloskowski, A.; Namieśnik, J. Perspectives on the replacement of harmful organic solvents in analytical methodologies: a framework toward the implementation of a generation of eco-friendly alternatives. Green Chem. 2015, 17 (7), 3687−3705. (35) Yue, D.; Jing, Y.; Ma, J.; Yao, Y.; Jia, Y. Physicochemical properties of ionic liquid analogue containing magnesium chloride as temperature and composition dependence. J. Therm. Anal. Calorim. 2012, 110 (2), 773−780. (36) Bandrés, I.; Montaño, D. F.; Gascón, I.; Cea, P.; Lafuente, C. Study of the conductivity behavior of pyridinium-based ionic liquids. Electrochim. Acta 2010, 55 (7), 2252−2257. (37) Gorke, J. T.; Srienc, F.; Kazlauskas, R. J. Hydrolase-catalyzed biotransformations in deep eutectic solvents. Chem. Commun. 2008, 10, 1235−1237.

Guangdong Natural Science Foundation (S2013020013049), and the Fundamental Research Funds for the Chinese Universities (2015PT002; 2015ZP009) for partially funding this work.

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