Hydrophobic deep eutectic solvents for the extraction of organic and inorganic analytes from aqueous environments

Hydrophobic deep eutectic solvents for the extraction of organic and inorganic analytes from aqueous environments

Trends in Analytical Chemistry 118 (2019) 853e868 Contents lists available at ScienceDirect Trends in Analytical Chemistry journal homepage: www.els...

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Trends in Analytical Chemistry 118 (2019) 853e868

Contents lists available at ScienceDirect

Trends in Analytical Chemistry journal homepage: www.elsevier.com/locate/trac

Hydrophobic deep eutectic solvents for the extraction of organic and inorganic analytes from aqueous environments Jeongmi Lee*, Dasom Jung, Keunbae Park School of Pharmacy, Sungkyunkwan University, Suwon, 16419 Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 20 July 2019

New-generation solvents termed deep eutectic solvents (DESs) are attracting increasing attention as ecofriendly solvents in analytical chemistry. Recently, a new subclass of DESs called hydrophobic DESs (hDESs) has been reported. hDESs are generally immiscible with water and have high extraction efficiency for nonpolar analytes; thus, they have been suggested as potential extraction media to replace toxic organic solvents or expensive hydrophobic ionic liquids. Since the first introduction of hDESs in 2015, a growing number of studies on the application of hDESs in sample preparation methods have been reported. The present review provides an overview on the preparation and physicochemical properties of hDESs, followed by applications of hDESs in the extraction of organic and inorganic analytes from aqueous environments. In this review, up-to-date studies of conventional (liquideliquid extraction) and miniaturized (liquid-phase microextraction) scale processes will be discussed with a focus on work up to January 2019. © 2019 Elsevier B.V. All rights reserved.

Keywords: Hydrophobic deep eutectic solvents Green analytical chemistry Liquideliquid extraction Liquid-phase microextraction Aqueous sample

1. Introduction Since their introduction as novel media by Abbot et al. [1], deep eutectic solvents (DESs) have attracted increasing attention from researchers in a variety of chemical fields. These non-conventional solvents are analogs of ionic liquids (ILs) and share unique physicochemical properties with ILs, namely their low volatility, nonflammability, and designability [2,3]. Moreover, DESs possess additional advantages; they can be synthesized simply and easily from two or more inexpensive and biodegradable components that function as a hydrogen bond acceptor (HBA) or a hydrogen bond donor (HBD) [2,3]. In agreement with the principles of green analytical chemistry, DESs have been extensively investigated as eco-friendly extraction media to replace conventional organic solvents. In the last few years, the number of articles published on this topic has increased rapidly, and several review articles have been published [4,5]. The applications of DESs to extraction processes involved in analytical method development, the isolation of bioactive compounds, and the removal of pollutants have been previously reviewed [6,7]. More specific reviews on the analytical applications of DESs as

* Corresponding author. E-mail addresses: [email protected], [email protected] (J. Lee). https://doi.org/10.1016/j.trac.2019.07.008 0165-9936/© 2019 Elsevier B.V. All rights reserved.

extraction solvents in liquid and solid samples have also been published [5,8]. A subclass of DESs called natural deep eutectic solvents (NADESs), which are formed from cellular primary metabolites, has been applied to similar analytical methods and reviewed in detail [4,9]. In most of the studies mentioned in the aforementioned reviews, the DESs used were hydrophilic and water-miscible, and the analytes of interest were extracted from solid or non-aqueous liquid samples that could be phase separated from the DESs. The hydrophilic DES property is anticipated because of their innate hydrogen-bonding ability [10]. In analytical chemistry, a wide repertoire of extraction solvents is desirable to cover the diverse range of analytes in various types of sample matrices. For example, hydrophobic solvents could enhance the extraction efficiency of nonpolar analytes from aqueous solutions, but hydrophilic DESs that are unstable in an aqueous environment would not be suited to this purpose. Recently, a new subclass of DESs with hydrophobic properties, namely hydrophobic DESs (hDESs), has been reported by the Kroon [11] and the Marrucho groups [12]. These hDESs consist of DL-menthol with natural acids [12] or tetraalkyl quaternary ammonium salts with decanoic acid [11]. Being generally immiscible with water, these hDESs have been suggested as potential extraction media to replace toxic organic solvents or expensive hydrophobic ILs, and study of these compounds has led to an expansion in the range of applications of hDESs in the analytical extraction field, as evidenced by the rapid

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increase in publications concerning hDESs, which will be discussed in this review. The present review provides an overview of the preparation and physicochemical properties of hDESs, followed by applications of hDESs in the extraction of organic and inorganic analytes from aqueous environments. A summary of up-to-date studies in the conventional (liquideliquid extraction, LLE) and miniaturized (liquid-phase microextraction, LPME) scales is presented within the timeframe of up to January 2019. 2. Synthesis and physicochemical properties of hDESs The synthesis of hDESs has been carried out using poorly watersoluble components. Depending on their HBA classes, they can be largely categorized as either tetraalkyl-quaternary-ammoniumbased or monoterpene-based solvents. There are hDESs containing other types of HBAs, and these also will be described. The HBAs and HBDs used in the preparation of hDESs are listed in Table 1. 2.1. Tetraalkyl-quaternary-ammonium-based hDESs The first hDESs reported by van Osch et al. were composed of various tetraalkyl quaternary ammonium salts as the HBA, whereas the HBD was fixed with a long chain carboxylic acid (decanoic acid) [11]. The quaternary ammonium salts were long chain tetraalkyl halides, which impart greater hydrophobicity compared to that of short chain ammonium salts such as cholinium, including tetrabutylammonium (N4444), trioctylmethylammonium (N88881), tetraheptylammonium (N7777), and tetraoctylammonium (N8888) salts. The water solubility of the prepared solvents was evaluated from the water content of the water-saturated hDESs, and it was found that the water solubility was 1.8e6.9%. The authors showed that the hDESs could recover volatile fatty acids from aqueous solutions via LLE. Since this study, a variety of other long chain acids and alcohols have been tested as HBDs following the same strategy [13e17]. Saturated (hexanoic acid, octanoic acid, and decanoic acid) or unsaturated (oleic acid) acids and fatty alcohols (butanol, octanol, dodecanol, and octadecenol) have been reported to form hDESs with tetraalkylammonium salts in this class. Ibuprofen, a typical nonsteroidal anti-inflammatory drug (NSAID) containing a eCOOH group, has also been used as a HBD in combination with N7777-Cl [18]. N4444-Br was subsequently used as a HBA, and acetic, propionic, butyric, acrylic, octanoic, decanoic, and oleic acids were tested as its counterpart, but only the last three could form hDESs [16]. A study by Cao et al. reported a ternary hDES composed of N8881-Cl, octanol, and octanoic acid at a 1:2:3 molar ratio [19]. Reports on the physicochemical properties of the quaternaryammonium-based hDESs are quite limited to date, and the majority of studies of this category of solvents have focused on their applications [14,16,19e23], which will be described in Sections 3 and 4. The viscosities of quaternary-ammonium-based hDESs are high, ranging from 228 to 1139 mPa s at 293.15 K (Table 2). The Coulombic charge interactions in the salt are responsible for the high viscosity, as similarly found in ILs and DESs, which are also salts [3,6]. However, with increasing temperature, the viscosity decreases significantly, and this is consistent with observations for hydrophilic DESs [24]. For example, the viscosity of one hDES (N7777-Cl:decanoic acid, 1:2 molar ratio) dropped from 228 to 54.32 mPa s from 293.15 to 323.15 K, respectively. The melting temperatures (Tms) for the hDESs in this category are available only for a limited number of solvents. However, their Tm values appear to be sufficiently low, similar to those of most hydrophilic DESs [7] because all of these solvents could be used with ease as the extraction medium in LLE and LPME at room temperature (Tables 2

and 3). Specifically, one hDES, N8881-Cl:decanoic acid (1:2), has been frequently adopted as the extractant in LPME methods [20e23,25,26], and its Tm is 0.05 C (Table 2) [11]. The density of the extractant is an important property affecting extractions based on two liquid phases because it determines whether the extractant can be collected from the upper or lower phase. Hydrophilic DESs have, in general, a higher density than water, ranging between 1.01 and 1.63 g cm3 at around room temperature [2]. In contrast, none of the quaternary-ammoniumbased hDESs had a density higher than water (Table 2), and the density decreased slightly as the temperature increased. Accordingly, the analyte-enriched DES phase was obtained from the upper liquid phase when this category of solvents was applied to the LLE [11,18] and LPME methods [14,15,17,20,22,23,27]. Being generally hydrophobic, the quaternary-ammonium-based hDESs show varying water solubility. Unsurprisingly, the hydrophobicity of the individual components is reflected by the water contents of the water-saturated hDESs, and hDESs containing longer fatty acids or a longer tetraalkyl chain, or both, exhibit lower water solubility. For example, for the DESs containing decanoic acid, the water content was in the range of 7e20%, and an N8888based solvent exhibited the lowest water content [11], whereas, among the N4444-based hDESs, the water content was between 4 and 7% and the lowest water content was observed in an oleic acidbased solvent [16].

2.2. Terpene-based hydrophobic DESs Around the same time that the tetraalkyl-quaternaryammonium-based hDESs were reported [11], Ribeiro et al. synthesized a different category of hDESs [12]. Menthol, which is a cheap, natural monoterpene that has very low water solubility, has been used in the pharmaceutical field for the formation of eutectic mixtures with other terpenes or therapeutic compounds [12]. Ribeiro et al. produced a series of hDESs by combining menthol as a HBA with organic acids of short (pyruvic, acetic, and lactic acids) or long chain (dodecanoic acid) lengths as HBDs. In this type of combination, the H-bonding interactions are weak, resulting in much lower viscosities compared to those of the usual hydrophilic DESs [3] and quaternary-ammonium-based hDESs [11]. Differentiation between the H-bond accepting or donating properties for the DES components is pointless for the terpene-based DESs; nonetheless, menthol is considered as a HBA for convenience in this review. Because of the innate hydrophobic property of menthol, the resulting DESs were hydrophobic and immiscible with water (Table 2). Their water solubility was measured to be within the range of 1.2e1.6%, which is much lower than those of quaternaryammonium-based DESs [12]. Afterwards, a series of menthol-based hDESs was reported in combination with fatty acids of which the carbon chain length varied from C8 to C18 (octanoic, decanoic, dodecanoic, tetradecanoic, hexadecanoic, and octadecanoic acid) [28e30]. Phenolic compounds such as phenyl salicylate (also known as salol) have also been used as HBDs [31], and two therapeutic drugs, diclofenac and ketoprofen, have been shown to form eutectic solvents with menthol, although the molar ratios were not specified [32]. Thymol is a naturally occurring aromatic counterpart of menthol (Table 1). It has been used in hDESs as a hydrophobic HBA in several studies [29,30,33]. All the hydrophobic fatty acids that were incorporated into the menthol-based hDESs could form hDESs with thymol. However, the molar ratios between thymol and its counterparts were usually different from those of menthol-based solvents (Tables 2 and 3). Camphor, a bicyclic monoterpene ketone, could also be employed as the counterpart of thymol [33].

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Table 1 HBAs and HBDs used in the preparation of hDES. IUPAC name/Common name

Chemical structure

Abbreviation

HBAs 5-Methyl-2-propan-2- ylcyclohexan-1-ol/Menthol

5-Methyl-2-propan-2-ylphenol/Thymol

2-(Diethylamino)-N-(2,6-dimethylphenyl)acetamide/Lidocaine

Dodecanoic acid

(3R)-3-Hydroxy-4-(trimethylazaniumyl)butanoate/L-Carnitine

2-(Trimethylazaniumyl)acetate/Betaine

2-Hydroxyethyl(trimethyl)azanium; chloride/Choline chloride

Ch-Cl

1-Dioctylphosphoryloctane/Trioctylphosphine oxide

P888O

Tetrabutylazanium; bromide/Tetrabutylammonium bromide Tetrabutylazanium; chloride/Tetrabutylammonium chloride Tetraheptylazanium; chloride/Tetraheptylammonium chloride Tetraoctylazanium; bromide/Tetraoctylammonium bromide Tetraoctylazanium; chloride/Tetraoctylammonium chloride Methyl(trioctyl)azanium; bromide/Methyltrioctylammonium bromide Methyl(trioctyl)azanium; chlorideMethyltrioctylammonium chloride

N4444-Br N4444-Cl N7777-Cl N8888-Br N8888-Cl N8881-Br N8881-Cl

Trihexyl(tetradecyl)phosphonium; chloride

P66614-Cl

Dichlorozinc/Zinc chloride HBDs Acetic acid

ZnCl2

Acetamide

1,1,1,3,3,3-Hexafluoropropan-2-ol/Hexafluoroisopropanol

2-Hydroxypropanoic acid/Lactic acid

(continued on next page)

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Table 1 (continued ) IUPAC name/Common name 2-Oxopropanoic acid/Pyruvic acid

Undec-10-enoic acid/10-Undecylenic acid

(Z)-Octadec-9-enoic acid/Oleic acid

Hexanoic acid

Octanoic acid

Nonanoic acid

Decanoic acid

Dodecanoic acid

Tetradecanoic acid

Hexadecanoic acid

Octadecanoic acid

Butan-1-ol/1-Butanol

Octan-1-ol/1-Octanol

Chemical structure

Abbreviation

J. Lee et al. / Trends in Analytical Chemistry 118 (2019) 853e868

857

Table 1 (continued ) IUPAC name/Common name Dodecan-1-ol/1-Dodecanol

(Z)-Octadec-9-en-1-ol/Oleyl alcohol

Phenol

4-Chlorophenol

1,7,7-Trimethylbicyclo[2.2.1]heptan-2-one/Camphor

1-Phenylethanol

2-[4-(2-Methylpropyl)phenyl]propanoic acid/Ibuprofen

2-(3-Benzoylphenyl)propanoic acid/Ketoprofen

Phenyl-2-hydroxybenzoate/Salol

2-[2-(2,6-dichloroanilino)phenyl]acetic acid/Diclofenac

Chemical structure

Abbreviation

858

Table 2 Physicochemical properties of some selected hDESs. DES HBD

Molar ratio

N4444-Cl N7777-Cl N8881-Cl N8881-Br N8888-Cl N8888-Br Menthol

Decanoic acid

1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:1 1:2 2:1 1.5:1 1.5:1 3:1 4:1 1:1 1:1 1:1 1:1 11:9 3:1 4:1 9:1 1:2 1:3 1:2 1:3 1:3 1:3 1:2 1:2 1:4 1:1 1:2

Menthol

Menthol Thymol

Thymol

Betaine

Pyruvic acid Acetic acid Lactic acid Dodecanoic acid Octanoic acid Decanoic acid Dodecanoic acid Tetradecanoic acid Salol Camphor 10-Undecylenic acid Decanoic acid Dodecanoic acid Tetradecanoic acid Hexadecanoic acid Octadecanoic acid Hexafluoroisopropanol

L-Carnitine

Dodecanoic acid

Lidocaine

Octanoic acid Nonanoic acid Decanoic acid Decanoic acid

P888O

Phenol

Density (g cm3)

Viscosity (mPa s)

Tm ( C)

Note

Ref.

0.9199 0.8996 0.8939 0.9456 0.8921 0.9331 0.999 0.9350 1.0380 0.8970 0.8887 0.8855 0.8823 0.8812 1.07 0.9873 0.9457 0.9437 0.9107 0.9240 0.9255 0.9357 1.476 1.525 1.503 1.505 0.9040 0.9010 0.8980 0.9624 0.9461 0.910 0.935

368.54 227.96 1138.73 814.53 654.18 889.57 44.637 11.296 370.860 33.058 7.80 9.43 12.40 14.21 e 25.8 13.2 11.2 7.01 7.16 7.54 6.88 76 46 698 149 8.223 10.115 12.886 352.5 197.5 54.00 16.47

11.95 16.65 0.05 8.95 1.95 8.95 ea e e e e e e e 22.28 44.0 11.0 17.0 e e e e 39.4 34.7 18.7 17.2 9.0 9.0 18.0 e e 5.9 33.0 (visual observation)

Density and viscosity were measured at 293.15 K.

[11]

Density and viscosity were measured at 293.15 K.

[12]

Density and viscosity were measured at 313.15 K.

[30]

Temperature for density measurement was not specified. Density and viscosity were measured at room temperature.

[31] [33]

Density and viscosity were measured at 318.15 K.

[30]

Viscosity was measured at 298.15 K. Temperature for density measurement was not specified.

[47]

Density and viscosity were measured at 293.15 K.

[35]

Density and viscosity were measured at 293.15 K.

[34]

Density and viscosity measured at 293.15 K.

[10]

Abbreviations: HBA, hydrogen bond acceptor; HBD, hydrogen bond donor; N4444-Cl, tetrabutylammonium chloride; N7777-Cl, tetraheptylammonium chloride; N8881-Br, trioctylmethylammonium bromide; N8881-Cl, trioctylmethylammonium chloride; N8888-Br, tetraoctylammonium bromide; N8888-Cl, tetraoctylammonium chloride; P888O, trioctylphosphine oxide. a Not available.

J. Lee et al. / Trends in Analytical Chemistry 118 (2019) 853e868

HBA

Table 3 Applications of hDESs in liquid-liquid extraction of organic and inorganic analytes in aqueous environment. DES

Sample type (Amount)

Analyte (LOD)

Analysis method

Note

Ref.

2 mL

Ginkgo biloba leaves (0.2 g)

Flavonoids Terpene trilactones Procyanidine Polyprenyl acetates

HPLC-UV Colorimetric method

[19]

2 mL

Model aqueous solution (2 mL)

Two mL of hydrophilic DESs (choline chloride-levulinic acid 1:2, choline chloridemalonic acid 1:2) were also used. It was SLLE where two DES phases were involved. Density was measured at 298.15 K. Solvents' water miscibility was examined.

1 mL

Model aqueous solution (1 mL)

e

Model aqueous solution

0.8971

e

Model aqueous solution

1:3 1:3 1:2 1:3

0.9040 0.9010 0.8980 e

2 mL

Model aqueous solution (2 mL)

0.2e0.5 mL

Lipsticks, eye shadows (2 g)

Decanoic acid

1:2 1:3 1:4

0.9624 0.9540 0.9461

10 g

Model aqueous solution (10 mL)

Decanoic acid Ibuprofen Oleic acid Dodecanoic acid Decanoic acid Decanoic acid Hexanoic acid Phenol

1:2 7:3 1:2 2:1 1:2 1:2 1:2 1:1 1:2

0.8907 0.8920 0.8670 0.8940 e e e 0.910 0.935

e

Model aqueous solution

e

1 mL

HBD

Molar ratio

Density (g cm3)

Amount

N8881-Cl

Octanol, octanoic acid

1:2:3

ea

N4444-Cl N8881-Cl N7777-Cl N8888-Cl N8881-Br N8888-Br Menthol

Decanoic acid

Pyruvic acid Acetic acid Lactic acid Dodecanoic acid Octanoic acid Decanoic acid Dodecanoic acid

1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:1 1:2 2:1 1:1 1:1 2:1

0.9168 0.8954 0.8907 0.8889 0.9422 0.9298 0.999 0.9350 1.0380 0.8970 e e e

Menthol

Dodecanoic acid

2:1

Dodecanoic acid

ZnCl2

Octanoic acid Nonanoic acid Decanoic acid Acetamide

Lidocaine

N7777-Cl

Menthol

Menthol P66614-Cl N8888-Br P888O

Acetic acid Propionic acid Butyric acid

HPLC-UV

[11]

UV-Vis

Density was measured at 293.15 K.

[12]

UV-Vis

N4444-Cl was attempted to synthesize hDESs, but the formed solvents were found unstable in water.

[28]

NMR

Density was measured at 293.15 K.

[53]

UV-Vis

Density was measured at 293.15 K.

[35]

Metals Pb (0.66 mg L1) Cd (0.86 mg L1)

F-AAS

[49]

Alkali and transition metals Co(II) Ni(II) Fe(II) Mn(II) Zn(II) Cu(II) Na(I) K(I) Li(I) Metals 111 In (III)

IC-CD

Solid samples were acid digested and dissolved in aqueous solution before extraction. Density was measured at 293.15 K.

Model aqueous solution

Metals 99m TcO-4

Radioactivity gamma counter

Model aqueous solution (1 mL)

Metals UO2þ 2 (0.5 mg mL1)

UV-Vis

Caffeine Tryptophan Isophthalic acid Vanillin Pesticides Imidacloprid Acetamiprid Nitenpyram Thiamethoxam Lower alcohols Ethanol 1-Propanol 1-Butanol Biosphenol A

Radioactivity gamma counter

Density was measured at 298.15 K.

[34]

J. Lee et al. / Trends in Analytical Chemistry 118 (2019) 853e868

HBA

[18]

[48]

[10]

(continued on next page)

859

2-(5-Bromo-2-pyridylazo)5-(diethylamino)phenol was added for color development. Density was measured at 293.15 K.

860

Table 3 (continued ) DES

Sample type (Amount)

Analyte (LOD)

Analysis method

Note

Ref.

HBD

Molar ratio

Density (g cm3)

Amount

Menthol

Octanoic acid Decanoic acid Dodecanoic acid Tetradecanoic acid Hexadecanoic acid Octadecanoic acid Octanoic acid Decanoic acid Dodecanoic acid Tetradecanoic acid Hexadecanoic acid Octadecanoic acid Decanoic acid

3:2 3:2 3:1 4:1 17:3 9:1 21:29 1:1 11:9 3:1 4:1 9:1 1:2

0.8735 0.8705 0.8677 0.8665 0.8666 0.8662 0.9020 0.9031 0.8992 0.9126 0.9140 0.9240 e

e

Model aqueous solution

Metals Cu(II)

UV-Vis ICP-OES

Density was measured at 333.15 K.

[29]

0.2e1 mL

Model aqueous solution (0.2e1 mL, 1:1 vol. ratio)

ICP-MS

Effect of water was studied for electrical conductivity.

[52]

Octanoic acid Decanoic acid Dodecanoic acid Tetradecanoic acid Hexadecanoic acid Octadecanoic acid Decanoic acid Dodecanoic acid Tetradecanoic acid Hexadecanoic acid Octadecanoic acid

1.5:1 1.5:1 3:1 4:1 17:3 9:1 1:1 11:9 3:1 17:3 9:1

0.8887 0.8855 0.8823 0.8812 0.8814 0.8810 0.9147 0.9107 0.9240 0.9255 0.9357

e

e

Metals Cr(VI) Cu(II) Ni(II) Cr(III) None

None

Density was measured at 313.15 K and 318.15 K for menthol-based and thymol-based hDESs, respectively. It was focused on the characterization of hDESs.

[30]

Thymol

N4444-Cl

Menthol

Thymol

Abbreviations: F-AAS, flame atomic absorption spectroscopy; HPLC-UV, high performance liquid chromatography-ultraviolet detection; IC-CD, ion chromatography-conductivity detection; ICP-MS, inductively coupled plasmamass spectrometry; ICP-OES, inductively couple plasma-optical emission spectroscopy; N4444-Cl, tetrabutylammonium chloride; N7777-Cl, tetraheptylammonium chloride; N8881-Br, trioctylmethylammonium bromide; N8881-Cl, trioctylmethylammonium chloride; N8888-Br, tetraoctylammonium bromide; N8888-Cl, tetraoctylammonium chloride; P66614-Cl, trihexyltetradecylphosphonium chloride; P888O, trioctylphosphine oxide; SLLE, solid-liquid-liquid extraction; UV-Vis, ultraviolet-visible spectrophotometry. a Not available.

J. Lee et al. / Trends in Analytical Chemistry 118 (2019) 853e868

HBA

J. Lee et al. / Trends in Analytical Chemistry 118 (2019) 853e868

A low-viscosity extractant is always desirable for extraction because the viscosity is directly associated with the mass transport between phases. In this sense, the terpene-based hDESs are preferable to the quaternary-ammonium-based hDESs, given that the former have much lower viscosities (45 mPa s at 293.15 K) than the latter. One hDES that is an exception is the menthol:lactic acid (1:2) system, which has a viscosity of 371 mPa s (Table 2); however, its viscosity significantly drops to 68 mPa s upon water saturation, which will be similar to real experimental conditions for aqueous sample applications [12]. As observed for the DESs reported [3], the viscosity of the menthol- and thymol-based hydrophobic DESs decreased dramatically as the temperature increased [12,30]. For instance, the DES composed of menthol and decanoic acid (3:2) changed its viscosity from 24.68 to 4.58 mPa s from 293.15 to 333.15 K, respectively [30]. In general, terpene-based hDESs are lighter than water (Table 2) with the exceptions being menthol:lactic acid (1:2) [12] and menthol:salol (1:1) [31], both of which have a density slightly higher than 1.0. However, the density of the former solvent decreased from 1.038 to 0.921 g cm3 when the solvent became saturated with water at 293.15 K [12]. Accordingly, the hDES extractant should be sought in the upper phase of the aqueous environment. In the case of menthol:salol (1:1), for which the exact density has not been reported, the extracted DES phase settled at the bottom of the tube after centrifugation [31]. In general, the menthol-based hDESs exhibited slightly lower densities than the thymol-based DESs when the HBD components were the same (Table 2) [30,33]. The water solubility of the menthol- and thymol-based hDESs decreased with increasing hydrophobicity of the alkyl chain length of the acid, having water contents between 0.3% and 2.5% [29,30]. 2.3. Other types of hDESs Decanoic acid, which has been used as a HBD in the tetraalkylquaternary-ammonium- and terpene-based hDESs, formed a different type of hDES on the incorporation of lidocaine [34]. The produced hDESs were lighter than water (Table 2). The water solubility was relatively high (7e20%), where higher portions of decanoic acid resulted in lower water solubility. Further, the density increased with water saturation and temperature decrease and remained lower than that of water at all temperatures. Although the viscosity of the solvents was rather high (198 mPa s at 293.15 K), the water saturation resulted in a slight to moderate reduction in viscosity depending on the solvents' water solubility [34]. Dodecanoic acid has been combined with other fatty acids, i.e., octanoic, nonanoic, and decanoic acid, to produce less viscous hDESs that have water contents of 1.4% or lower in the watersaturated state [35]. As expected based on the water content, the longer fatty acid chains produced more hydrophobic solvents; the hDES composed of decanoic acid and dodecanoic acid (1:2) contained water at 0.5% when saturated with water. These solvents are the most hydrophobic DESs reported so far. Their densities and viscosities remained almost unchanged after water saturation because of their very low water solubilities, but they showed a decreasing trend in density and viscosity with increasing temperature. Farajzadeh et al. reported the synthesis of a hDES from 4chlorophenol and choline chloride (Ch-Cl) in a 2:1 molar ratio [36e38]. Although Ch-Cl is a polar salt and is one of the most commonly used HBAs to synthesize hydrophilic DESs [3], the resulting DES was moderately hydrophobic and water-immiscible. Its density was 1.21 g cm3 [38], and, thus, it was enriched in the bottom phase [36,38]. Phenylethanol:Ch-Cl (4:1) is a water-

861

immiscible hDES that is lighter than water [39]. In a similar fashion, phenol was incorporated as a hydrophobic HBD in DES formation with Ch-Cl at various molar ratios (1:1 to 1:4). Unlike the 4-chlorophenol-based solvent, however, the phenol-based DESs appeared to have higher water solubility because a small portion of these solvents could be soluble in water by up to about 7% (Table 4). In the strict sense, the phenol-based DESs may not be regarded as hDESs, and phenol prevents these solvents from being considered green. However, they were still included in other types of hDESs in this review because they can form a separate upper phase with the use of aprotic solvents such as tetrahydrofuran (THF) in many LPME methods [25,26,40e46]. Betaine and L-carnitine are structurally similar to Ch-Cl. On using hexafluoroisopropanol as a HBD, hDESs are formed in 1:2 and 1:3 ratios; these hDESs exhibited the highest density (ca. 1.5 g cm3) of all the hDESs reported (Table 2). The betaine-based DESs were much less viscous than the L-carnitine-based DESs because L-carnitine contains one extra eOH group, which induces stronger hydrogen bonding with hexafluoroisopropanol [47]. Recently, Gilmore et al. prepared hDESs in which trioctylphosphine oxide (P888O) and phenol were incorporated at different molar ratios as HBA and HBD, respectively [10]. The resulting DESs, which remained liquid at room temperature, could form liquideliquid biphasic systems with aqueous solutions. Their density, which is lighter than water, decreased with increasing P888O content. Their viscosity was lower than that of the quaternary-ammonium-based hDESs, but it decreased with increasing temperature and decreasing P888O content. Further, a phosphonium cation with a long tetraalkyl chain length, trihexyltetradecylphosphonium chloride (P66614-Cl), formed a hDES with decanoic acid at a 1:2 molar ratio [48]. In a very recent study by Kazi et al. a metal salt, ZnCl2, and acetamide were combined in a 1:3 ratio as a HBA and a HBD, respectively, and could be used as a hydrophobic extraction solvent in an aqueous environment [49]. 3. Applications of hDESs in two-liquid-phase extraction from aqueous environments In analytical chemistry, a two liquid-phase extraction can be conducted either in the traditional LLE format or in the more recent LPME format, depending on the volume scale of the extraction solvent. The applications of hDESs in LLE are limited to those on LPME, which is in accordance with attempts to make analytical chemistry “green”; thus, LPME is preferred over LLE. Studies on the hDES-based LLE have usually focused on the synthesis of hDESs and the characterization of their physicochemical properties. Many of their applications, which, to date, have been more proof-ofconcept, have been demonstrated using model aqueous solutions, and the development of novel quantitative analytical methods has not been pursued (Table 3). In contrast, studies on LPME methods with a focus on method development have been more prevalent, and this is similar to hydrophobic ILs, which are popularly employed in LPME methods [50]. This could be a consequence of the current trend in analytical chemistry towards miniaturization [51]. Hydrophobic DESs have been applied to diverse LPME techniques and have been described with several analytical figures of merit (Table 4). 3.1. Liquideliquid extraction using hDESs Reports on hDES-based LLE are summarized in Table 3. The amount of hDESs used is typically on the milliliter-scale at the same volume as the samples. As listed in the table, the quaternary-ammonium-based hDESs could extract metals,

Extraction method DES

Analyte (LOQ)

Analysis Note method

Ref.

132

Fruit juice (10 mL) Vegetables (10 mL)

GCFID

Acetonitrile was used as dispersive solvent. Density was measured at 293.15 K.

[37]

1.476 1.49 1.525 1.503 1.542 1.505 Lighter than water

102 101 98 100 97 100 50

Tea beverages Fruit juices (5 mL)

HPLCUV

Acetonitrile was used as dispersive solvent.

[47]

HPLCUV

FeCl3 in ethanol was used for extraction solvent dispersion and phase separation.

[17]

1:2

ec

190

GCFID

No dispersive solvent was used. Extraction solvent dispersion was facilitated by air (gas) agitation.

[36]

Phenylethanol

1:4

Lighter than 250 water

Human plasma Pharmaceutical waste water (6 mL)

Pesticides Haloxyfop-R-methyl (1.5 ng mL1) Oxadiazon (3.8 ng mL1) Clodinafop-propargyl (1.4 ng mL1) Diclofop-methyl (3.2 ng mL1) Fenazaquin (8.2 ng mL1) Fenoxaprop-P-ethyl (11 ng mL1) Pyrethroid pesticides (tea/fruit) Transfluthrin (0.53/0.57 ng L1) Fenpropathrin (0.31/0.39 ng L1) Fenvalerate (0.32/0.41 ng L1) Ethofenprox (0.20/0.23 ng L1) Bifenthrin (0.20/0.25 ng L1) Benzoylureas Triflumuron (0.11 mg L1)a Hexaflumuron (0.11 mg L1)a Flufenoxuron (0.21 mg L1)a Lufenuron (0.35 mg L1)a Pesticides Penconazole (0.75 ng L1) Hexaconazole (0.84 ng L1) Diniconazole (1.3 ng L1) Tebuconazole (1.9 ng L1) Diazinon (4.2 ng L1) Fenazaquin (2.7 ng L1) Clodinafop-Propargyl (3.9 ng L1) Haloxyfop-R-methyl (1.4 ng L1) Bromopropylate (0.71 ng L1) Amphetamine-type stimulants Amphetamine (15 ng mL1) Methamphetamine (8 ng mL1)

HPLCUV

[39]

Octanoic acid

2:1 1:1 1:2 2:1 1:1 1:2 2:1 1:1 1:1 1:1 1:1 1:2

Lighter than 100 water

Swimming pool water River water Wastewater (8 mL)

No dispersive solvent was used. Extraction solvent dispersion was facilitated by air agitation. No dispersive solvent was used. Extraction solvent dispersion was facilitated by air agitation.

e

1:2

Lighter than 600 water

Tea beverages Carbonated drinks Fruit juices Lactobacillus beverages (10 mL) Food samples Fruity pastel Smarties Ice cream Candy Jelly (8 mL)

HBD

Molar ratio Density (g cm3)

Volume (mL)

DLLME

Ch-Cl

4-Chlorophenol

1:2

1.21

DLLME

Betaine

DLLME-SFO

Hexafluoroisopropanol 1:2 1:2.5 1:3 L-Carnitine 1:2 1:2.5 1:3 N8881-Cl 1-Dodecanol 1:1

AA-DLLME

Ch-Cl

4-Chlorophenol

AA-DLLME

Ch-Cl

AA-DLLME

Menthol

Decanoic acid

AA-DLLME

N8881-Cl

Dodecanoic acid Octanoic acid Decanoic acid Octanoic acid Decanoic acid Decanoic acid

EA-DLLME

N8881-Cl

Decanoic acid

N4444-Cl

100

River water Well water Swimming pool water (8 mL) Fresh vegetable juice Fresh, packaged fruit juice (5 mL)

Benzophenone UV filters Benzophenone (0.3 ng mL1) 2,4-Dihydroxybenzophenone (0.3 ng mL1) 2,20 ,4,40 -Tetrahydroxybenzophenone (0.5 ng mL1) 2-Hydroxy-4-methoxybenzophenone (0.2 ng mL1) 2,20 -Dihydroxy-4, 40 -dimethoxybenzophenone (0.3 ng mL1) 4-Hydroxybenzophenone (0.3 ng mL1) Synthetic dyes Lemon yellow (0.016 ng mL1)a Sunset yellow (0.028 ng mL1)a Carmine (0.026 ng mL1)a Brilliant blue (0.104 ng mL1)a Synthetic dyes Sunset yellow (2.92 ng mL1)a Brilliant blue (2.02 ng mL1)a

HPLCUV

HPLCPDA

UV-Vis

[13]

No dispersive solvent was [21] used. Extraction solvent dispersion was facilitated by air agitation. [23] Extraction solvent dispersion was facilitated by effervescence, which was achieved using sodium bicarbonate and acetic acid.

J. Lee et al. / Trends in Analytical Chemistry 118 (2019) 853e868

Sample type (volume)

HBA

N4444-Br

862

Table 4 Applications of hDESs in liquid-phase microextraction of organic and inorganic analytes in aqueous environment.

Menthol

Salol

1:1

1.07

0.6 g extraction mixture containing 50 mL DES

Water from different locations near the explosives (40 mL)

SA-DLLME

N8881-Br

Decanoic acid

1:2

e

40

Wastewater River water (10 mL)

UA-DLLME

Thymol

Camphor

7:3 3:2 1:1b 7:3 3:2 1:1 1:2 1:3 1:4 3:2 1:1 1:2 1:3

0.9698 0.9732 0.9873 0.9598 0.9515 0.9457 0.9395 0.9375 0.9353 0.9515 0.9437 0.9365 0.9272

200

Industrial effluents (10 mL)

10-Undecylenic acid

Decanoic acid

Nitroaromatic compounds 2-Nitrotoluene (0.10 mg L1) 3-Nitrotoluene (0.10 mg L1) Nitrobenzene (0.10 mg L1) 1,2-Dinitrobenzene (0.13 mg L1) 1,3,5-Trinitrobenzene (0.13 mg L1) 2,6-Dinitrotoluene (0.13 mg L1) 2,4-Dinitrotoluene (0.17 mg L1) 2,4,6-Trinitrotoluene (0.17 mg L1) Dyes Methylene blue (5 ng mL1)a

PAHs Naphthalene (0.029 mg L1) Biphenyl (0.030 mg L1) Acenaphthylene (0.025 mg L1) Fluorene (0.026 mg L1) Anthracene (0.013 mg L1) 9-Methyl anthracene (0.11 mg L1) Fluoranthene (0.018 mg L1) Pyrene (0.016 mg L1) Benz[a]anthracene (0.024 mg L1) Chrysene (0.014 mg L1) Benzo[b]fluoranthene (0.022 mg L1) Benzo[k]fluoranthene (0.027 mg L1) Perylene (0.013 mg L1) Benzo[a]pyrene (0.024 mg L1) Indeno[1,2,3-cd]pyrene (0.027 mg L1) Benzo[ghi]perylene (0.063 mg L1) UV filters Benzophenone (0.5 ng mL1) 2,4-Dihydroxybenzophenone (0.5 ng mL1) 2-Hydroxy-4-methoxybenzophenone (1.0 ng mL1) PAHs Naphthalene (21.8 ng L1) Fluorene (4.0 ng L1) Phenanthrene (11.3 ng L1) Anthracene (2.4 ng L1) Fluoranthene (14.1 ng L1) Pyrene (3.0 ng L1)

HPLCUV

UV-Vis

GCMS

N8881-Cl

Decanoic acid

1:1 1:2 1:3 1:4 1:5

Lighter than 30 water (mg)

Swimming pool water River water (8 mL)

UA-DLLME-SFO

N4444-Br

Oleic acid Decanoic acid Octanoic acid

1:2 1:2 1:2

0.959 0.957 0.974

Environmental water samples (20 mL)

VA-DLLME

N8881-Br

Decanoic acid

1:2

Lighter than 35 water

Human urine Apple juice Rain water (10 mL)

Aldehydes (Urine/juice/water) Malondialdehyde (108.94/55.25/75.42 ng mL1) Formaldehyde (51.78/83.60/53.29 ng mL1)

HPLCUV

VA-DLLME

N4444-Br

Hexanoic acid Octanoic acid Decanoic acid 1-Butanol

1:1 1:1 1:1 1:1

Lighter than 50 water

Tap water Wastewater Seafood market water (5 mL)

Antibiotics Levofloxacin (0.05 mg mL1) Ciprofloxacin (0.08 mg L1)

HPLCUV

80

HPLCUV

No dispersive solvent was used. Extraction solvent dispersion was facilitated by ultrasonic irradiation.

[20]

HPLCFLD

No specified temperature was informed for density measurement. Extraction solvent dispersion was facilitated by ultrasonic irradiation. DES phase floated and solidified after centrifugation. No dispersive solvent was used. Extraction solvent dispersion was facilitated by vortexing. Analytes were derivatized with 2,4dinitrophenylhydrazine. No dispersive solvent was used. Extraction solvent dispersion was facilitated

[16]

[27]

[14]

(continued on next page)

863

UA-DLLME

[31] Extraction solvent dispersion was facilitated by effervescence, which was achieved using sodium dihydrogen phosphate and sodium carbonate. DES phase was solidified and settled in the bottom after centrifugation No dispersive solvent was [22] used. Extraction solvent dispersion was facilitated by shaker shaking. DES phase was back extracted to aqueous phase after pH adjustment. [33] Acetonitrile was used as dispersive solvent. Extraction solvent dispersion was facilitated by ultrasonic irradiation. Density was measured at RT.

J. Lee et al. / Trends in Analytical Chemistry 118 (2019) 853e868

EA-DLLME-SS

864

Table 4 (continued ) Extraction method DES HBA

HBD

Molar ratio Density (g cm3) 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 2:1 1:2 1:3

N8881-Cl

A-ELLME

Ch-Cl

Phenol

AA-ELLME

Ch-Cl

Phenol

1:1 1:2 1:3 1:4 1:3

AA-ELLME

Ch-Cl

Phenol

UA-ELLME

Ch-Cl

Phenol

N8881-Cl

Analyte (LOQ)

Analysis Note method

Ref.

by vortexing. DESs were tested for water stability.

Lighter than 150 water (mg)

Well water Tap water Lake water Saliva Urine (5 mL)

Nitrite (1.0 mg L1)

HPLCPDA

e

50

Caffeine (0.1 mg L1)

HPLCUV

e

400

Green tea Cola Energy drink (1 mL) Kang'ai injection (8 mL)

HPLCUV

THF was used for phase separation. Air facilitated emulsification. Fe3O4 nanoparticles were used for easy removal of DES phase.

[40]

1:4

e

600

Foods (CRM) Water (30 mL)

Ginsenosides S-Rh1 (258.8 ng mL1) S-Rg2 (284.1 ng mL1) ReRh1 (358.8 ng mL1) Rg6 (287.2 ng mL1) F4 (318.5 ng mL1) Rk3 (155.4 ng mL1) Rh4 (55.5 ng mL1) R-Rg3 (428.5 ng mL1) Rk1 (133.1 ng mL1) Metals Pb(II) (1.98 ng L1)

GFAAS

[41]

1:2 1:3 1:4

e

100

Tap water Industrial wastewater (1.5 mL)

BTE and PAHs Benzene (21.0 mg L1) Toluene (22.0 mg L1) Ethylbenzene (2.90 mg L1) Biphenyl (2.30 mg L1) Fluorene (2.20 mg L1) Phenanthrene (0.30 mg L1) Anthracene (0.06 mg L1) Pyrene (0.26 mg L1)

HPLCUV

THF was used for phase separation. Air agitation facilitated dispersion of aggregated DES droplets. THF was used for phase separation. Ultrasonic irradiation facilitated dispersion of aggregated DES droplets.

No dispersive solvent was [15] used. Extraction solvent dispersion was facilitated by vortexing. Nitrite underwent diazotization-coupling reaction with p-nitroaniline and diphenylamine. [46] THF was used for phase separation. EME was automated.

[43]

J. Lee et al. / Trends in Analytical Chemistry 118 (2019) 853e868

VA-DLLME

1-Octanol 1-Dodecanol Octadecenol Hexanoic acid Octanoic acid Decanoic acid 1-Butanol 1-Octanol 1-Dodecanol Octadecenol Oleic acid

Volume (mL)

Sample type (volume)

Chrysene (0.70 mg L1) Benzo[a]pyrene (0.28 mg L1) Metals Co(II) (3.60 mg L1)

e

500

Pharmaceutical supplements Tea (10 mL)

Phenol

1:2 1:3 1:4

e

450

Tap water Industrial wastewater (10 mL)

Ch-Cl

Phenol

1:1 1:2 1:3

e

1000

UA-ELLME

Ch-Cl

Phenol

1:1 1:2 1:3 1:4

e

500

Metals Water samples (25 mL) As(III) (33 mg L1) Tap water Mineral water River water Lake water Food and environmental samples (500 mg) Foods (mushroom, fish, tea, rice) Soil Sediment Fish water Anti-parasites (15 mL) Malachite green (11.8 mg L1)

UA-ELLME

Ch-Cl

Phenol

e

500

N8881-Cl N4444-Cl

Decanoic acid Decanoic acid

1:1 1:2 1:3 1:2 1:2

VA-ELLME

Ch-Cl

Phenol

1:2 1:3 1:4

e

400

HF-LPME

Ch-Cl

Phenylethanol

1:4

e

40

Ch-Cl

Phenol

N4444-Cl N8881-Cl

Decanoic acid

UA-ELLME

Ch-Cl

UA-ELLME

Tap water Mineral water Cow's milk Mixed fruit juice Grape fruit Orange fruit Sheep milk Yogurt Honey Rgg Canned fish Mushroom (25 mL) Turmeric root herbal tea Turmeric products (10 mL)

Human plasma urine Pharmaceutical waste water (10 mL)

Metals Cr (VI) (18.2 mg L1)

F-AAS

F-AAS

ETAAS

UV-Vis

Metals Se(IV) (15.4 ng L1) Se(VI)

ET-AAS

Curcumin (9.44 mg L1)

UV-Vis HPLCUV

Antiarrhythmic agents Propranolol (0.8 ng mL1) Carvedilol (1.5 ng mL1) Verapamil (0.8 ng mL1) Amlodipine (2.5 ng mL1)

HPLCUV

[25] THF was used for phase separation. Ultrasonic irradiation facilitated dispersion of aggregated DES droplets. 1-Nitroso-2naphthol was used as complex formation ligand. Samples were wet digested and dissolved in water. [54] THF was used for phase separation. Ultrasonic irradiation facilitated dispersion of aggregated DES droplets. Cr(VI) formed hydrophobic complex with diethyldithiocarbamate. [44] THF was used for phase separation. Ultrasonic irradiation facilitated dispersion of aggregated DES droplets. Sodium diethyldithiocarbamate was used as a chelating agent.

THF was used for phase separation. Ultrasonic irradiation facilitated dispersion of aggregated DES droplets. THF was used for phase separation. Ultrasonic irradiation facilitated dispersion of aggregated DES droplets.

THF was used for phase separation. Vortexing facilitated dispersion of extraction solvent. hDES was used for supported liquid membrane in three-phase (liquid-liquid-liquid) microextration.

[45]

[26]

J. Lee et al. / Trends in Analytical Chemistry 118 (2019) 853e868

1:2 1:3 1:4 1:2 1:2

UA-ELLME

[42]

[55]

865

(continued on next page)

Abbreviations: AA, air-assisted; Ch-Cl, choline chloride; A-EME, automated emulsification microextraction; BTE, benenze, toluene, ethylbenzene; DEMF, deep eutectic mixture formation; DLLME, dispersive liquid-liquid microextraction; EA, effervescence-assisted; ELLME, emulsification liquid-liquid microextraction; ET-AAS, electrothermal atomic absorption spectroscopy; F-AAS, flame atomic absorption spectroscopy; GC-FID, gas chromatography-flame ionization detection; GC-MS, gas chromatography-mass spectrometry; GF-AAS, graphite furnace atomic absorption spectroscopy; HF-LPME, hollow fiber liquid-phase microextraction; HPLC-FLD, high performance liquid chromatography-fluorescence detection; HPLC-UV, high performance liquid chromatography-ultraviolet detection; LLME, liquid-liquid microextraction; N4444-Br, tetrabutylammonium bromide; N4444-Cl, tetrabutylammonium chloride; N8881-Br, trioctylmethylammonium bromide; N8881-Cl, trioctylmethylammonium chloride; NSAIDs, non-steroidal anti-inflammatory drugs; PAHs, polycyclic aromatic hydrocarbons; SA, shaker shaking-assisted; SFO, solidification of floating organic droplets; SS, solidification of settled droplets; TA, temperature-assisted; THF, tetrahydrofuran; UA, ultrasound-assisted; UV-Vis, ultraviolet-visible spectrophotometry; VA, vortex-assisted. a Limit of detection (LOD). b DES in bold was selected in the optimized conditions. c Not available.

HPLCUV NSAIDs Diclofenac (150 ng mL1) Ketoprofen (50 ng mL1) Lighter than Menthol (50 mg) Human urine water (8 mL) Not specified Diclofenac Ketoprofen LLME-in situ DEMF Menthol

HPLCUV PAHs Acenaphthene (1.6 ng mL1) Phenanthrene (0.91 ng mL1) Anthracene (0.61 ng mL1) Pyrene (3.0 ng mL1) Ch-Cl (0.12 g) Phenol (0.20 g) e 1:2 4-Chlorophenol Ch-Cl TA-LLME

HBA

HBD

Molar ratio Density (g cm3)

Volume (mL)

Tap water River water Industrial wastewater (10 mL)

Analysis Note method Analyte (LOQ) Sample type (volume) Extraction method DES

Table 4 (continued )

[38] Upon heating, two components added to aqueous samples formed hDES, which then functioned as the extraction solvent. NSAIDs formed DESs with [32] menthol in situ via H-bond between OH of menthol and COOH of NSAIDs and were phase separated.

J. Lee et al. / Trends in Analytical Chemistry 118 (2019) 853e868

Ref.

866

including 111In(III) [18], 99mTcO-4 [48], and Cr(VI) [52], from aqueous solutions. The first two radioactive analytes were measured using a radioactivity gamma counter, whereas the last metal ion was quantified using inductively coupled plasmaemass spectrometry (ICP-MS). Small organic acids that were quantified using high-performance liquid chromatographyeultraviolet detection (HPLC-UV) could be extracted in this class of hDESs [11]. Cao et al. suggested a two-phase DES system to extract and fractionate analytes of diverse polarity, which were hydrophobic polyprenyl acetates and partially hydrophilic components (flavonoids, terpene trilactones, and procyanidine) from ginkgo leaves [19]. In their study, the ternary N8881-Cl:octanol:octanoic acid (1:2:3) hDES could be phase separated not only from the Ch-Clbased hydrophilic DESs containing levulinic acid or malonic acid but also from the solid ginkgo leaves. Terpene-based hDESs have also been applied to extract various organic analytes by LLE. Four model molecules, caffeine, tryptophan, isophthalic acid, and vanillin, which are bioactive compounds with varying octanolewater partition coefficients, could be extracted into menthol-based hDESs [12]. Hydrophobic DESs containing menthol and fatty acids were applied to the LLE of four different neonicotinoid pesticides [28], and the concentrations of the analytes were determined using ultraviolet-visible spectrophotometry (UV-Vis). Similarly, the lower alcohols, ethanol, propanol, and butanol, could be enriched in the menthol-based hDES, and the analyte concentrations were determined using nuclear magnetic resonance spectroscopy [53]. In addition, the extraction of inorganic metals has been reported: 111In(III) [18] and Cu(II) [29] could be transferred to menthol and thymol-based hDESs. The copper ion content was quantified using UV-Vis and ICP coupled to optical emission spectroscopy (ICP-OES) [29]. There have been a few reports of the extraction of metals from model aqueous solutions using other types of hDESs. For example, alkali and transition metals were extracted by a hDES containing lidocaine and decanoic acid [34], whereas the heavy metals lead and cadmium were extracted to a ZnCl2:acetamide (1:3) hDES [49]. In the case of UO2þ 2 , which was extracted in a hDES consisting of P888O and phenol, its concentrations could be measured using a spectroscopic probe for quantification by UV-Vis [10]. 3.2. Liquid-phase microextraction using hDESs Numerous types of analytes and samples have been analyzed using hDES-based LPME methods. The most frequently analyzed sample type is environmental water [13e17,20,22,27,31,33,38,39,43,44,54]. Other types of samples present in aqueous form were also analyzed including foods (mostly juice and beverage) [21,23,26,27,36, 38,41,42,44,46,47], pharmaceutical supplements (aqueous extracts of herbal medicine and wet acid digested tablet) [25,40], and biological fluids (plasma, saliva, and urine) [27,32,39,55]. The organic analytes, pre-concentrated and purified from aqueous samples, were analyzed using various kinds of instruments, where HPLC-UV systems were the most often employed [13,14,17,20,27,31,32,38e40, 42,43,46,47,55]. The extracted phase was injected to an HPLC system without dilution [31,38e40,42,47,55] or with dilution in methanol [14,15,21,32] or ethanol [17,27]. HPLC coupled to photodiode array detection (PDA) [15,21] or fluorescence (FL) [16] has also been used. Some studies have used UV-Vis [22,23,42,45]. Unlike HPLC, there have been only a few studies reporting the use of gas chromatography (GC), which has been typically coupled to flame ionization detector (FID) [36,37] or MS [33]. The extracted hDES phase was directly injected to the GC system in these studies, although the compatibility of hDESs with the system and the effects of repeated injection on the system remain to be addressed. Metals were analyzed using atomic absorption spectrometry (AAS) combined with various atomization methods

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[25,26,41,44,54]. Similar to conventional organic solvents, many different types of LPME techniques have been developed employing hDESs as the extraction solvent. Summaries on the individual studies are provided in Table 4. Typical dispersive liquideliquid microextraction (DLLME) methods have been reported. In Farajzadeh et al.'s report [37], a hDES composed of Ch-Cl and 4-chlorophenol (1:2), which meets the requirements of an extraction solvent for DLLME (that is, waterimmiscibility and density heavier than water) was the extractant, and acetonitrile, which is miscible with both the hDES and water, was the disperser. The sedimented phase was enriched with the hDES containing six different pesticides, of which the levels in fruit juice samples were then determined using GC-FID. Similarly, heavy hDESs where hexafluoroisopropanol was incorporated as a HBD were used in a DLLME method with acetonitrile as a disperser [47]. Specifically, five pyrethroid pesticides could be extracted from beverages and juices and analyzed by HPLC-UV. Without a dispersive solvent, water-immiscible hDESs can be still dispersed with the assistance of an external physical disturbance, such as ultrasound irradiation, air agitation, vortexing, and shaker shaking, which are denoted AA-, UA-, VA-, and SA-DLLME methods, respectively. Various hDESs from all three categories described in Section 2 have been used as the extractant with these techniques. AA-DLLME methods have been the most frequently developed, whereas the analytes of interest included pesticides [36], amphetamine-type stimulants [39], benzophenone UV filters [13], and synthetic dyes [21]. VA-DLLME methods have been reported for antibiotic [14] and nitrite [15] analysis. In a study by Wang et al. extraction based on a hDES containing N8881-Cl and decanoic acid was achieved by UA-DLLME [20], which allowed three benzophenones to be pre-concentrated from swimming pool water and river water samples for subsequent HPLC-UV analysis. Makos et al. also employed UA-DLLME; however, acetonitrile was used as the dispersive solvent, for GC- MS analysis of polyaromatic hydrocarbons (PAHs) from industrial effluents [33]. When SADLLME was applied to determine methylene blue in environmental water samples, the analyte extracted in the hDES phase (N8881-Cl:decanoic acid, 1:2) was back-extracted to the aqueous phase after pH adjustment [22]. In some studies, the collected hDES phase was further solidified into a floating droplet at low temperatures, which is known as the solidification of floating organic droplets (SFO) method [16,17]. Yousefi et al. facilitated the dispersion of N4444-Br:decanoic acid (1:2) by ultrasonic irradiation without using a disperser [16]. Yang et al. reported [17] that FeCl3 in ethanol can be used as a “dispersive-demulsified solvent”; specifically, ethanol first assisted the dispersion of N8881-Cl:1-dodecanol (1:1), and, then, the use of FeCl3 as a demulsifier enabled phase separation prior to SFO. Another dispersion method via effervescence has been combined with a DLLME technique where water-immiscible hDESs were employed [23,31]. The generation of carbon dioxide bubbles from an effervescent agent such as sodium bicarbonate [56] helped distribute the hDESs. The N8881-Cl:decanoic acid (1:2) hDES could be enriched with two synthetic dyes in the top phase by an effervescence-assisted (EA)-DLLME method [23]. In the method of Nedaei et al. denoted EA-DLLME-SS, the menthol:salol (1:1) hDES, which is slightly heavier than water, could be solidified and settled (termed SS) at 20 C after the extraction of eight nitroaromatic compounds. In the studies described above, the dispersion of the hDES extractant in the aqueous environments was achieved using a dispersive solvent or external method because of its water immiscibility. However, the group of DESs consisting of Ch-Cl and phenol have moderate solubility in water; therefore, dispersion or emulsification of the extractant can be achieved with relative ease

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[43]. An aprotic organic solvent was added for the separation between the DES and the aqueous phases. THF is most frequently used as the phase separation solvent, which was called an emulsifying [25,40,42e45,54] or a demulsifying [41] solvent, although both refer to the same function. This type of extraction technique can be regarded en bloc as emulsification liquid-liquid microextraction (ELLME), although this technique has been given different names in the various studies. ELLME has been used in UA- [25,26,43e45,54], VA- [42], and AA-ELLME [40,41] methods depending on how the emulsification was achieved. The ultrasonic wave irradiation treatment appears to be very efficient for producing emulsification of the phenol-based DES and the aqueous samples. The UA-ELLME methods were found to be effective for organic and inorganic analytes. For example, they were used for the analysis of aromatic pollutants [43] and an anti-parasitic agent, malachite green [45], as well as a variety of metals including Co(II) [25], Cr(VI) [54], As(III) [44], and Se(IV) [26] employing Ch-Cl:phenol combined at various molar ratios. In all of these ELLME methods, the extracted DES phase was collected at the upper phase by centrifugation. In a study by Li et al. [40], AA-ELLME could enrich nine ginsenosides from a pharmaceutical injection, and Fe3O4 magnetic nanoparticles were used for the easy removal of the DES phase without centrifugation. In a study by Shishov et al. [46], an automated ELLME method (AELLME) to quantify caffeine in beverages was developed using Ch-Cl:phenol (1:3) and THF as the extractant and phase separation solvent, respectively. A Ch-Cl:phenylethanol (1:4) hDES was employed as the middle hydrophobic liquid phase in a three-phase hollow fiber (HF)-LPME [55]. The hDES was the supported liquid membrane between the donor and acceptor liquid phases. By using this extraction method in combination with HPLC-UV, the authors could determine the levels of four antiarrhythmic agents in human biological fluids (plasma and urine) and environmental water. Farajzadeh et al. reported the analysis of four PAHs from environmental water [38]. The hDES was not synthesized before extraction; instead, the two components, Ch-Cl and 4-chlorophenol, were separately added to environmental water samples in a 1:2 ratio and formed a hDES upon heating and functioned as an efficient extraction solvent. A similar approach was used by Shishov et al. to analyze NSAIDs in human urine [32]. Menthol added to urine samples formed hDESs with diclofenac and ketoprofen in situ, which were easily phase separated for further analysis by HPLC-UV. 4. Concluding remarks Extraction is a crucial and often unavoidable step in analytical processes, especially when the concentrations of analytes are low or the sample matrices are complex. Choosing the most appropriate extraction solvent is very important for analytical performance. In addition, the employment of eco-friendly solvents is consistent with the principles of green analytical chemistry. This accounts for the rapidly increasing number of studies conducted on the applications of hDESs in sample preparation methods. Compared to the case of water-miscible hydrophilic DESs, for which applications are limited to solid or non-aqueous liquid samples, hDESs can be applied to extract a variety of nonpolar analytes from aqueous solutions. A variety of organic and inorganic analytes present in aqueous environments could be efficiently extracted into numerous kinds of hDESs and subsequently quantified using the pertinent analytical instruments. In particular, hDESs have been applied in different types of LPME for real sample analysis. Given that the LPME techniques are continuously evolving and that more variable hDESs will be synthesized, it is evident that the number of studies on the application of hDESs for analytical extraction methods will

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continue to rise. One of the challenges expected in hDES-based LPME techniques is automation, as has been commonly encountered in LPME techniques such as DLLME [57]. Although the moderately high viscosity and density of hDESs allows easy phase separation, a hDES that is too viscous would not be tolerated because its dispersion in the sample and handling in narrow tubing would be very limited [58]. Studies of the synthesis of hDESs and characterization of their physicochemical properties are still limited compared to those concerning hydrophilic DESs, and no focused studies are available yet on the toxicity of hDESs. Above all, more diverse hDESs must be prepared consisting of non-toxic, low-cost, and biodegradable components; naturally occurring components will be preferable as found in NADESs. The polarity of hDESs is a key property for understanding the extraction capability and water-miscibility of the solvents; however, this area has been little studied, and, thus, a systematic and comprehensive investigation of the polarity of various hDESs will be of great value. In general, the dry and watersaturated hDESs contained varying amounts of water. In particular, the water content has a significant effect on the extraction process, as well as its efficiency, because it sometimes greatly affects the properties of hDESs such as viscosity and density. Therefore, more careful characterization of the physicochemical properties of new hDESs must be carried out in accordance with the water content for a better understanding and appropriate application of hDESs. Acknowledgements This study was financially supported by a research grant NRF2017R1A2B2004823 from the National Research Foundation of Korea. References [1] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Chem. Commun. (2003) 70e71. [2] Q.H. Zhang, K.D. Vigier, S. Royer, F. Jerome, Chem. Soc. Rev. 41 (2012) 7108e7146. [3] E.L. Smith, A.P. Abbott, K.S. Ryder, Chem. Rev. 114 (2014) 11060e11082. [4] M. Espino, M.D. Fernandez, F.J.V. Gomez, M.F. Silva, Trac. Trends Anal. Chem. 76 (2016) 126e136. [5] S.C. Cunha, J.O. Fernandes, Trac. Trends Anal. Chem. 105 (2018) 225e239. [6] F. Pena-Pereira, J. Namiesnik, ChemSusChem 7 (2014) 1784e1800. [7] B. Tang, H. Zhang, K.H. Row, J. Sep. Sci. 38 (2015) 1053e1064. [8] A. Shishov, A. Bulatov, M. Locatelli, S. Carradori, V. Andruch, Microchem. J. 135 (2017) 33e38. [9] M.D. Fernandez, J. Boiteux, M. Espino, F.J.V. Gomez, M.F. Silva, Anal. Chim. Acta 1038 (2018) 1e10. [10] M. Gilmore, E.N. McCourt, F. Connolly, P. Nockemann, M. Swadzba-Kwasny, J.D. Holbrey, ACS Sustain. Chem. Eng. 6 (2018) 17323e17332. [11] D.J.G.P. van Osch, L.F. Zubeir, A. van den Bruinhorst, M.A.A. Rocha, M.C. Kroon, Green Chem. 17 (2015) 4518e4521. [12] B.D. Ribeiro, C. Florindo, L.C. Iff, M.A. Coelho, I.M. Marrucho, ACS Sustain. Chem. Eng. 3 (2015) 2469e2477. [13] D. Ge, Y. Zhang, Y. Dai, S. Yang, J. Sep. Sci. 41 (2018) 1635e1643. [14] W. Tang, Y. Dai, K.H. Row, Anal. Bioanal. Chem. 410 (2018) 7325e7336. [15] K. Zhang, S. Li, C. Liu, Q. Wang, Y. Wang, J. Fan, J. Sep. Sci. 42 (2018) 574e581.

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