Analytica Chimica Acta 716 (2012) 28–33
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Ionic liquids based simultaneous ultrasonic and microwave assisted extraction of phenolic compounds from burdock leaves Zaixiang Lou, Hongxin Wang ∗ , Song Zhu, Shangwei Chen, Ming Zhang, Zhouping Wang State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, Wuxi 214122, PR China
a r t i c l e
i n f o
Article history: Received 15 November 2010 Received in revised form 2 March 2011 Accepted 4 March 2011 Available online 10 March 2011 Keywords: Ionic liquids Simultaneous ultrasonic and microwave assisted extraction technique Burdock leaves Phenolic compounds
a b s t r a c t The ionic liquids based simultaneous ultrasonic and microwave assisted extraction (IL-UMAE) technique was ﬁrst proposed and applied to isolate compounds. The ionic liquids comprising a range of four anions, ﬁve 1-alkyl-3-methylimidazolium derivatives were designed and prepared. The results suggested that varying the anion and cation both had apparent effects on the extraction of phenolics. The results also showed that irradiation power, time and solid–liquid ratio signiﬁcantly affected the yields. The yields of caffeic acid and quercetin obtained by IL-UMAE were higher than those by regular UMAE. Compared with conventional heat-reﬂux extraction (HRE), the proposed approach exhibited higher efﬁciency (8–17% enhanced) and shorter extraction time (from 5 h to 30 s). The results indicated ILUMAE to be a fast and efﬁcient extraction technique. Moreover, the proposed method was validated by the reproducibility and recovery experiments. The ILUMAE method provided good recoveries (from 96.1% to 105.3%) with RSD lower than 5.2%, which indicated that the proposed method was credible. Based on the designable nature of ionic liquids, and the rapid and highly efﬁcient performance of the proposed approach, ILUMAE provided a new alternative for preparation of various useful substances from solid samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Burdock, Arctium lappa L. is a popular vegetable in China and Japan. It has been extensively studied for its components of root and seed due to their antioxidant properties and other biochemical activities [1,2]. However, there is less information about burdock leaf (Chinese traditional medicine), which is rich in phenolic components . In vitro and epidemiological studies strongly suggest that food containing phenolic compounds has potential protective effects against many diseases [4–6]. These compounds could be used as antibacterial, antimutagenic and anti-inﬂammatory agents . Due to their biological effects, increasing attention is being paid on phenolics. Extraction is one of the most imperative steps in the evaluation of phenolic compounds from plant. The capability of a number of extraction techniques have been investigated, such as solvent extraction  and enzyme-assisted extraction . However, these extraction methods have drawbacks to some degree . For example, solvent extraction is time consuming and enzyme in enzymeassisted extraction is easy to denature [9,11,12]. Ultrasonic is one of the most industrially used methods to enhance mass transfer phenomena [13–19]. Meanwhile, microwave assisted extraction
∗ Corresponding author. Tel.: +86 510 85917795; fax: +86 510 85876799. E-mail addresses: [email protected]
(Z. Lou), [email protected]
(H. Wang). 0003-2670/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2011.03.012
heats the extracts quickly and signiﬁcantly accelerates the extraction process [20–22]. Simultaneous ultrasonic/microwave assisted extraction (UMAE) coupled the advantage of microwave and ultrasonic, presenting many advantages . In analytical chemistry, increasing attention has been paid on ionic liquids (ILs). The potential of ILs in both academic and industrial ﬁelds is related to their unique properties, such as negligible vapor pressure, tunable viscosity, good thermal stability and miscibility with water and organic solvents, as well as strong extractability for various compounds, mainly depend on their special structures [24,25]. Compared to classical organic solvents, ILs generally consist of different inorganic or organic cations and numerous anions . Due to the non-ﬂammable, non-volatile nature of ILs, they become an excellent choice for the development of safer processes. In sample pretreatment, ILs have been successfully applied in various extraction, owing to the distinct features [27,28]. Furthermore, ILs are regarded as “designer solvents”, because their polarity, hydrophobicity, viscosity and other chemical and physical properties can be selected by choosing the cationic or the anionic constituent. Because of this tunable nature, their potential applications were signiﬁcantly increased [29–31]. It also has been found that ILs as solvents and co-solvents can efﬁciently absorb microwave energy [31,32]. Therefore, combining UMAE approach with the use of an aqueous ILs solution as extracting phase would be a new and complementary technique. However, the extraction of active compounds by ILs based UMAE approach has never been reported.
Z. Lou et al. / Analytica Chimica Acta 716 (2012) 28–33
Fig. 1. Flow scheme for the analytical procedure based on ILUMAE.
Because of the tunable nature of ILs, a series of ILs were designed and synthesized. In this work, a new, rapid, effective, validated and environmental friendly ionic liquids based simultaneous ultrasonic/microwave assisted approach for extraction of three phenolic compounds from burdock leaves was developed, and the efﬁciency was compared with those of the conventional reference methods. 2. Materials and methods 2.1. Materials Burdock leaves, provided by Xuzhou Wangda Farm and Sideline Products Co., Ltd. The cleaned leaves were frozen dried, and then crushed into powder in a grinder (ZSJD, Kaichuang mechanism Co., Ltd., China) with a size range of 0.5–1.2 mm. The resulted powder was kept in a vacuum dryer until use. The same powder was always employed in experiments. Caffeic acid, quercetin, and chlorogenic acid were obtained from Sigma (Shanghai, China). 2.2. Preparation of ILs chloride ([C4 MIM]Cl), 1-Butyl-3-methylimidazolium 1-butyl-3-methylimidazo-lium bromide ([C4 MIM]Br), 1butyl-3-methylimidazolium tetraﬂuoroborate ([C4 MIM][BF4 ]) and 1-butyl-3-methylimidazolium dihydrogen phosphate ([C4 MIM][H2 PO4 ]) were prepared as the literature method [33–35], the same procedure was used as for 1-ethyl-3-methylimidazolium bromide ([C2 MIM]Br), 1-hexyl-3-methylimidazolium bromide ([C6 MIM]Br) and 1-octyl-3-methylimidazolium bromide ([C8 MIM]Br). After the ILs were obtained, they were dried at 80 ◦ C
Fig. 2. Effect of ILs anions and cations (a) and [C4 MIM]Br concentrations (b) on the extraction of phenolic compounds. Sample: 5.0 g, extractant volume: 100 mL; irradiation power: 400 W; extraction time: 30 s.
for 12 h under vacuum and then checked by 1 H NMR and 13 C NMR spectra on INOVA 500 NMR spectrometry at room temperature. The characterization of NMR spectroscopy showed that there were no organic impurities in ILs. 2.3. Extraction 2.3.1. IL-based simultaneous ultrasonic/microwave assisted extraction IL-based simultaneous ultrasonic/microwave assisted extraction (IL-UMAE) experiment was carried out with a simultaneous ultrasonic and microwave extracting apparatus (CW-2000, Shanghai Xintuo Microwave Instrument Co. Ltd., China). The dried powder (5 g) of burdock leaves was mixed with solvent. Extraction process was performed in the apparatus chamber with simultaneous different microwave power and a ﬁxed ultrasonic power of 50 W. 2.3.2. Regular UMAE Regular UMAE was carried out with the same apparatus. The dried powder (5 g) of burdock leaves was mixed with 100 mL 70% ethanol (the optimal extractant). Extraction process was performed in the apparatus chamber.
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Table 1 Some physicochemical properties of the studied ILs. Cation
Solubility in H2 O
[C4 SO3 HMIM]Br
[C4 MIM]H2 PO4
H2 PO4 −
2.3.3. Conventional reference extraction method The dried powder (5 g) of burdock leaves was extracted with 100 mL 70% ethanol (which was optimized as the best extractant). The extraction was carried out at 60 ◦ C, 250 rpm (the optimal condition according to our preliminary experiments). All samples were prepared and analyzed in triplicate . 2.4. Quantiﬁcation of compounds Analytical chromatographic analysis was performed using liquid chromatography Agilent Technologies series 1100 (USA), which was equipped with a reversed-phase column, symmetry C18 (250 mm × 4.6 mm) (Waters, USA). A ﬂow scheme for the analytical procedure based on ILUMAE was shown in Fig. 1. The samples were eluted with a gradient system consisting of solvent A (1% acetic acid, v/v) and solvent B (acetonitrile:methanol, 1:5, v/v), used as the mobile phase, with a ﬂow rate of 0.8 mL min−1 . The injection vol-
Fig. 3. Effect of extraction time on the extraction efﬁciency of phenolics with 1.5 M [C4 MIM]Br.
ume was 5 L. The gradient system started from 100% A at 0 min, to 80% A at 10 min, 60% A at 15 min, 50% A at 20 min, and 40% A at 25 min. The peaks of the phenolic compounds were monitored at 280 nm. UV–vis absorption spectra were recorded on-line from 200 to 700 nm during the HPLC analysis. The compounds present in samples were identiﬁed by comparing their retention times and ultraviolet spectra with standard compounds. Conﬁrmation was performed by co-injection, and comparing the ultraviolet spectra of standards and samples. The yield of each compound was expressed as mg per gram of burdock leaves on dry weight basis. 3. Results and discussion 3.1. Inﬂuence of ILs on the extraction of compounds In addition to the strong solvent dissolving power, the ionic liquids can efﬁciently absorb microwave energy, thus ILs were employed as solvents and co-solvents in microwave assisted extraction of analytes [27,34]. Moreover, the structure of ILs has signiﬁcant effect on its physicochemical properties, affecting the extraction efﬁciency of target compounds . To evaluate the performance of ILs in the UMAE of phenolic compounds from burdock leaves and ﬁnd out the optimal one, the effects of anion and cation on the extraction of phenolics were investigated. 3.1.1. Anion effect As shown in Table 1, in the series of ILs, the identity of the anion greatly inﬂuences the water miscibility. Therefore, the 1butyl-3-methylimidazolium ILs with four kind of different anions (Cl− , Br− , BF4 − , H2 PO4 − ) were studied. The effect of anion on the extraction of phenolics was shown in Fig. 2a. The results suggested that the anions of ILs inﬂuenced the yields of phenolic compounds. For the 1-n-butyl-3-methylimidazolium based ionic liquids with Br− , Cl− , BF4 − and H2 PO4 − , the results indicated that Br− , Cl− and BF4 − were more efﬁcient than H2 PO4 − in the UMAE of chlorogenic acid and caffeic acid. For the extraction of quercetin, [C4 MIM]Br and [C4 MIM]BF4 were statistically more
99.65 96.07 103.87 0.028 0.012 0.013
1.196A 0.261AB 0.231A 3.92 4.75 4.28
103.16 104.60 96.15
Recovery (%) Recovery (%)
Operation under optimized conditions. The same operation conditions except the extracting phase. c See experimental section for operation conditions. The total extraction time is 5 h. Means in the same row with different letters are signiﬁcantly different (p < 0.05).
1.27B 0.256A 0.242A
SD Mean SD
3.87 4.72 5.06 105.33 96.33 96.42
0.032 0.011 0.015 1.293B 0.281B 0.272B
Observed values (mg g
0.038 0.013 0.012
) Observed values (mg g
Regular UMAE (70% ethanol, n = 3)b ILUMAE ([C4 MIM]Br, n = 3)
Conventional method Proposed methodsa Compounds
Table 2 Comparative study of extraction efﬁciency under different extraction methods.
3.1.3. Concentration effect The concentration effect of [C4 MIM]Br on the extraction of phenolic compounds from burdock leaves was shown in Fig. 2b. In Fig. 2b, it can be found that the extraction yields of phenolic compounds from burdock leaves were increased with the increase of [C4 MIM]Br concentration. When the concentration of [C4 MIM]Br solution was below 1.50 mol L−1 , the extraction yields of quercetin increased more rapidly than that of caffeic acid, while that of chlorogenic acid increased slowly. The reasons were related to the solubility of the phenolic compounds in extraction solvents. Chlorogenic acid is soluble in water, but caffeic acid is poorly soluble, and quercetin is insoluble. When [C4 MIM]Br was added, it dramatically improved the extraction yields of the phenolic compounds, especially quercetin, which was due to the solvation power and strong interactions of [C4 MIM]Br. Moreover, [C4 MIM]Br improved the transfer efﬁciency of microwave energy, and thus also increased the extraction yields of the target compounds. The solution of [C4 MIM]Br with a concentration of 1.50 mol L−1 gained higher yields of caffeic acid than those with lower concentrations, though the difference was not signiﬁcant. When the concentration of IL solution was 1.50 mol L−1 , the yield of quercetin was signiﬁcantly higher (p < 0.05) than the yields obtained at lower IL concentrations. When the concentration of [C4 MIM]Br was increased from 1.5 and 2.0 mol L−1 , the yields of the three phenolic compounds were nearly stable. The extraction efﬁciency of [C4 MIM]Br solution at
Observed values (mg g−1 )
HRE (70% ethanol, n = 3)c
3.1.2. Effect of the alkyl chain length With the same anion, the effects of alkyl chain length and substituted groups of the alkyl chain of 1-alkyl-3-methylimidazolium cations on the extraction efﬁciency of ILs were evaluated. As shown in Fig. 2a, increasing alkyl chain length from ethyl to butyl dramatically increased the extraction efﬁciency. The average yields of chlorogenic acid, caffeic acid and quercetin increased from 1.030, 0.221 and 0.220 mg g−1 to 1.293, 0.276 and 0.272 mg g−1 , respectively. However, when the alkyl chain length increased from hexyl to octyl, the extraction efﬁciency of ILs was slightly decreased. The length of the alkyl chain could affect the water miscibility of the ILs [35,39], and the extraction efﬁciency might be promoted. Furthermore, the effect of the substituted groups of the alkyl chain on the extraction of phenolics was investigated. The results indicated that the extraction ability of the IL without sulfonic group on the 1-alkyl chain was about 30–40% higher than that of the IL with sulfonic group on the 1-alkyl chain. It was probably due to the changes in the water miscibility and relative dissolving ability of the ILs . Overall, in the extraction of chlorogenic acid, the ability of [C4 MIM]Br, [C6 MIM]Br and [C8 MIM]Br was not signiﬁcantly different from each other. For caffeic acid, the ILs with hexyl chain and butyl chain were statistically more efﬁcient (p < 0.05) than other two ILs. In the extraction of quercetin, [C4 MIM]Br showed signiﬁcantly higher efﬁciency than did other ILs. According to the above results, the [C4 MIM]Br was selected as the subsequent extraction solvent.
Chlorogenic acid Caffeic acid Quercetin
efﬁcient (p < 0.05) than others. This result suggested the efﬁciency of phenolics extraction was anion-dependent, and it was similar to the reported studies [36–38]. It was probably due to the stronger multi-interactions including –, ionic/charge- charge and hydrogen bonding between the ILs ([C4 MIM]Br and [C4 MIM]BF4 ) solution and phenolics . The extraction efﬁciency of the solutions of [C4 MIM]Br and [C4 MIM]BF4 were not signiﬁcant different from each other. However, in production of [C4 MIM]Br and [C4 MIM]BF4 , [C4 MIM]BF4 needs more steps. Therefore, in consideration of cost, [C4 MIM]Br was more appropriate for the extraction of phenolic compounds from burdock leaves.
4.76 3.17 2.96
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Z. Lou et al. / Analytica Chimica Acta 716 (2012) 28–33
Fig. 4. Effect of microwave power and time on phenolic yields with 1.5 M [C4MIM]Br and heat-reﬂux extraction (HRE).
the two concentrations was not signiﬁcantly different. Given the extraction results, 1.5 mol L−1 was the appropriate concentration. 3.2. Effect of simultaneous ultrasonic and microwave treatment on the yields of the phenolic compounds 3.2.1. Treating time In IL based simultaneous ultrasonic/microwave assisted extraction, burdock leaves were added with a ratio of solid (g): solvent (mL) of 1:20, in a beaker and treated in a simultaneous ultrasonic and microwave extracting apparatus with a microwave power of 300 W and a ﬁxed ultrasonic power of 50 W. The effect of simultaneous ultrasonic/microwave treatment and its extraction time on the extraction of three compounds was shown in Fig. 3. For all detected acids under study, the extraction rate of the compounds was all very high during the ﬁrst 30 s of ILUMAE. For example, after ILUMAE for 30 s, the yields of chlorogenic acid, caffeic acid and quercetin increased from 0.932 to 1.251, 0.185–0.219, 0.173–0.238 mg g−1 , respectively. When the extraction time was beyond 30 s, the changes of the compounds yields were no longer signiﬁcant (p < 0.05). Hence, the optimal extraction time for ILUMAE was selected to be 30 s. In the investigation, when extraction time up to 40 s, the yields of quercetin started to decrease slightly with the increasing extraction time. This indicated long extraction time and high temperature caused by microwave may have resulted in degradation of some phenolic acids. As has been reported before [40,18,41], the degradation of phenolic compounds also might occur under ultrasound. However, the yields of chlorogenic acid and caffeic acid did not declined with increased extraction time. One of the possible reasons for this phenomenon is that the stability of these compounds suffering from high temperature and intense shaking of ultrasonic was different. This may be partly ascribed to the differences of their chemical structures. 3.2.2. Effect of microwave power The microwave power had a signiﬁcant effect on the extraction of the compounds from burdock leaves (Fig. 4). By 30 s simultaneous ultrasonic/microwave assisted extraction, when the microwave power was increased from 200 W to 400 W coupled with a ﬁxed ultrasonic power of 50 W, the yield of the compounds increased very fast. For example,
when the microwave power increased from 200 W to 400 W for 30 s in ILUMAE, the yield of chlorogenic acid increased from 1.208 to 1.285 mg g−1 , 0.205–0.271 mg g−1 in caffeic acid, 0.197–0.276 mg g−1 in quercetin, respectively. Beyond 400 W, the yields of the compounds leveled off, and the changes of the yields were no longer signiﬁcant. 3.2.3. Effect of solvent to solid ratio As shown in Fig. 5, the yields of the compounds by ILUMAE were found to increase with the increase of solvent to solid ratio and then fall down at the high ratios. The larger solvent to solid ratio means a larger concentration difference between the interior of the plant cells and the external solvent, which favors mass transfer. But in microwave-assisted extraction, a higher solvent volume may give lower yield . As shown in Fig. 5, the yields of the three phenolic compounds increased signiﬁcantly (p < 0.05) when the solvent to solid ratio increased from 15 to 20 mL g−1 . When the solvent to solid ratio was higher than 20 mL g−1 , there were no signiﬁcant changes in the yields. Hence, the liquid to solid ratio of 20 mL g−1 seems to be appropriate.
Fig. 5. Effect of solvent to solid ratio on the extraction of phenolic compounds with 1.5 M [C4 MIM]Br.
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3.3. Comparison of the proposed ILUMAE approach with the conventional methods The effect of extraction time on the yields of phenolic compounds in HRE was shown in Fig. 4. The extraction efﬁciency increased when extraction time was changed from 1 h to 5 h. No signiﬁcant increase in yields was observed when it was increased to 6 h. Therefore, the optimal extraction time for HRE was 5 h. A comparison was made between the proposed ILUMAE approach and the conventional HRE method under optimal working conditions. The results in Table 2 indicated that the proposed approach could dramatically improve the extraction efﬁciency (8–17% enhanced) of the three target compounds. The yields of chlorogenic acid and quercetin obtained by ILUMAE were significantly higher (p < 0.05) than those obtained by HRE. And the total extraction time of ILUMAE (30 s) was signiﬁcantly shorter than that of HRE (5 h). In order to further evaluate the effect of ILs, the IL based UMAE method was then compared with the regular UMAE process under the same UMAE operation conditions [43,44]. As shown in Table 2, in the extraction of caffeic acid and quercetin, the efﬁciency of regular UMAE was statistically lower than that obtained by the ILUMAE approach. The results indicated that the proposed IL-based UMAE procedure provided a new, rapid and effective approach for the extraction of the phenolic compounds from plant leaves or other materials. The superiority of ILUMAE is mainly due to the excellent microwave-absorbing ability and strong dissolving ability of ionic liquids. Moreover, there is almost no solvent release in this approach. 3.4. Analytical performance of the proposed method In this study, the proposed IL-based UMAE approach was evaluated through determining some parameters such as reproducibility and recovery under the optimized conditions. For the proposed and the conventional methods, the reproducibility study was carried out on repetitive experiments (2.0 g sample for each extraction). The relative standard deviation (RSD) of the proposed approach was between 2.3% and 5.6%. Similar results were obtained by Du et al. , who reported that the RSDs of precisions for polyphenolic compounds ranged from 1.4 to 6.8%. The relative standard deviation in reproducibility study was lower than 5% in the study of Cao et al.  who studied the extraction of piperine from white pepper. Further more, recoveries were evaluated by standard-addition method, and the extracts were spiked with known quantities of standards. Results showed that the recoveries were in the range of 96.07–105.33% for the phenolic compounds with RSDs lower than 5.2%. Similarly, Cao et al.  reported that the recoveries of the added standard were 97–106% with relative standard deviations of 1.8–2.5% in the determination of papaverin using an aqueous two-phase system as a pretreatment approach. Cao et al.  found that the recovery values were between 93.5% and 98.7% with RSD of 0.59–4.21% for piperine. In the study of Masque et al. , the recoveries in tap water were lower, between 67% for phenol and 86% for eight polar analytes. The reproducibility and recovery proved that the present method was credible. 4. Conclusion An ILUMAE method has been proposed for the extraction and quantiﬁcation of phenolic compounds from burdock leaves. The results indicated that the yields of caffeic acid and quercetin obtained by ILUMAE were higher than those by regular UMAE. Compared to conventional HRE methods, the new approach provided
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