Ultrasonic-Assisted Extraction of Procyanidins Using Ionic Liquid Solution from Larix gmelinii Bark

An ionic liquid-based ultrasonic-assisted extraction method has been developed for the effective extraction of procyanidins from Larix gmelinii bark. So as to evaluate the performance of ionic liquids in ultrasonic-assisted extraction process, the effects caused by changes in the anion and the alkyl chain length of the cation on the extraction efficiency were investigated in this paper. e results indicated that the characteristics of anions had remarkable effects on the extraction efficiency of procyanidins, and 1-butyl3-methylimidazolium bromide ([Bmim]Br) aqueous solution was the best among the investigated ionic liquids. e optimum conditions for the extraction were as follows: [Bmim]Br concentration 1.25M, soak time 3 h, solid-liquid ratio 1 : 10, ultrasonic power 150W, and ultrasonic time 30min. is work not only introduces a simple, green, and highly efficient sample preparation method for extraction of procyanidins from L. gmelinii bark, but also reveals the tremendous application potential of ionic liquids.

Larix gmelinii is a deciduous tree primarily distributed in northeast, China, north Sakhalin, and east Siberia. L. gmelinii bark containing numerous procyanidins are extremely useful natural products. In recent years, procyanidins have been found in large quantities in L. gmelinii bark and have been recognized as a multipurpose natural component with great economic potential and environmental value, attracting the increasing attention of people [14][15][16][17].
e extraction of procyanidins from L. gmelinii bark has been accomplished by several extraction methods in the past. ese include heating re�ux extraction [15,16] and homogenated extraction [17] with water, methanol, ethanol, acetone, acetic ether, and some mixtures as solvents [15][16][17]. However, the main disadvantage of traditional extraction lies in the complicated working procedure which increases the cost; repeated distillations prolong the heating time and accelerate oxidation of the extract. Moreover, these organic solvents used are problematic in the extraction of procyanidins because of their toxicity, volatility, and �ammability. To overcome the above-mentioned problems, environment friendly techniques become attractive following the development of the "Green Chemistry. " Much wider attention has been given to applications of ultrasound-assisted extraction (UAE) [18] and microwave-assisted extraction (MAE) [19,20]. Among the two methods, UAE can more easily be scaled up for commercial production [18]. And the UAE is one of the promising extraction techniques that can offer high reproducibility in a shorter time, simpli�ed manipulation, reduced solvent consumption and temperature, and lowerenergy input, which has been widely used to extract analytes from many matrixes [21,22]. Ultrasound enhancement of extraction is attributed to the disruption of cell walls, particlesize reduction, and the enhancement on the mass transfer of the cell content to the solvent caused by the collapse of the bubbles produced by cavitations [23,24]. e UAE is expeditious, inexpensive, efficient, and an environmental protection alternative to conventional extraction techniques, which is also a well-established method in the processing of plant material and in the extraction of analytes from different parts of plants.
Ionic liquids, also known as molten salts, which are composed of organic cations and inorganic or organic anions, are liquid near room temperature (or by convention below 100 ∘ C) [25]. ey have been proposed as greener alternatives to traditional organic solvents due to their unique characteristics such as good stability, negligible vapor pressure, wide liquidus range, good dissolving, and extracting ability, which have been attributed mainly to their nonmolecular nature [26][27][28]. In comparison with conventional organic solvents, ionic liquids could alleviate environmental pollution and improve the selectivity and the extraction efficiencies of compounds in separation technologies and sample pretreatment processes [29][30][31][32][33]. Ionic liquids as solvents are of promising potential in the application of the preparation of various useful substances from plant samples such as alkaloids [18,19,34,35], stilbene [36], quinines [37], lignans [38,39], and coumarins [40,41]. As alternative solvents, the experimental results have indicated that ILs are promising solvents which are available in a simple and efficient technique for sample preparation and separation added in the introduction. e aim of this work is to develop an effective, rapid, and environment friendly ionic liquid-based ultrasonic-assisted approach for the extraction of procyanidins from L. gmelinii bark and to compare the results with conventional extraction methods. Herein, we describe our investigations on the performance of various ionic liquids with different cations and anions in an ionic liquid-based ultrasonic-assisted extraction (ILUAE) method. It was found that parameters including the ionic liquid concentration, soak time, solid-liquid ratio, and ultrasonic power and time were in�uential on the extraction efficiency, and these parameters were optimized systematically. L. gmelinii bark was provided by Mohe Forestry (Heilongjiang, China) and authenticated by Professor Shaoquan Nie from the State Engineering Laboratory for Bioresource Eco-Utilization, Northeast Forestry University, China. e bark was dried at room temperature for a month and then was powdered into a homogeneous size and then sieved (60-80 mesh). e same batch of samples was used here in the experiments.

Ultrasonic-Assisted Extraction Apparatus.
For the ultrasonic-assisted extraction experiments, an ultrasonic bath was used as an ultrasonic source. KQ-250DB ultrasonic bath (Kunshan, Jiangsu, China) was used in the extraction step. e bath was a rectangular container (23.5 × 13.3 × 10.2 cm), to which 50 kHz transducers were annealed at the bottom. e bath power rating was 250 W on the scale of 40%-100%. e temperature control achieved by the replacement of inlet and outlet water to avoid water temperature rises.

Ionic Liquid-Based
Ultrasonic-Assisted Extraction. 0.5 g of dried sample powder was mixed with 5 mL of the various ionic liquid aqueous solutions in a 25 mL �ask. e �ask was then partially immersed in the ultrasonic bath, which contained 2.5 L of water. e suspension was extracted by UAE. e cation and anion of the ionic liquid, concentration of selected ionic liquid, soak time, solid-liquid ratio, and ultrasonic power and time were systematically optimized in this work to obtain the best extraction efficiency. Aer each extraction, the extract was �ltered through a 0.45 m nylon membrane (Guangfu Chemical Reagents Co., Tianjin, China) prior to the analysis. e extraction efficiency was expressed as the observed values of procyanidins, and the maximum amount in each curve was taken to be 100%.

Reference and Conventional Extraction Method.
Pure water, 1.25 M sodium chloride, and 80% ethanol were selected for use as reference solvents in the UAE of procyanidins from L. gmelinii bark. e extraction experiments were operated under the optimized conditions except for solvent type. 0.5 g of sample powder was mixed with 5 mL of the above solvents and soaked for 3 h. e suspension was extracted for 30 min by UAE. Ultrasonic power and the solidliquid ratio were 150 W and 1 : 10, respectively. e extract was �ltered through a 0.45 m microporous membrane for analysis.
80% ethanol was selected as solvent in conventional heat re�ux extraction (HRE) and maceration extraction (ME). e main technical parameters used were the same as above except extraction time and temperature 4 h and 85 ∘ C for HRE and 24 h and 25 ∘ C for ME, respectively.

Vanillin-HCl Method Quantitative Analysis.
Procyanidins in the extract solution were determined by the standard vanillin-HCl method [42] using (+)-catechin as standard. Brie�y, to 1.0 mL of the extract solution in a brown tube, 9.0 mL of 2% vanillin/HCl-methanol reagent (2 g vanillin dissolved in 12 N HCl-methanol (1 : 2) solution to get �nal volume of 100 mL) was added, immediately capped, mixed for 10 seconds, and incubated at 19-21 ∘ C for 15 min. Absorbance of this solution was measured by spectrophotometer (UV-2550, Shimadzu, Japan) at 500 nm (reference: water) ( SOLUTION ). Procyanidins content was calculated from the value of ( SOLUTION )-( BLANK ) by using working curve.
Working curve was obtained as follows: 1, 2, and 3 mg of (+)-catechin was dissolved in water to a �nal volume of 10 mL (the standard solution). 1.0 mL of each standard solution was taken in a brown tube and 9.0 mL of 2% vanillin/HClmethanol reagent was added, immediately capped, mixed for 10 seconds and incubated at 19-21 ∘ C for 15 min. Absorbance of this solution was measured at 500 nm by spectrophotometer (reference: water) ( CAL ). In case of blank, water was used instead of standard solution ( BLANk ). Working curve was obtained with correcting values: ( CAL )-( BLANK ). e working curve was constructed for procyanidins: 0.0052 0.0 , ( 2 0.997 ), where Absorbance (Abs) and Concentration of reference substance ( g mL − ). A good linearity was found for absorbance in the range of 0.107 Abs-1.034 Abs.
2.6. HPLC Qualitative Analysis. e way ANOVA test was used to calculate the signi�cance of the differences of extraction efficiency for the procyanidins. e results of spectrophotometric analysis were expressed as means of extraction efficiency ± SD.

Screening of the Ionic Liquid-Based Extracting Solvent.
e structure of ionic liquids had a signi�cant in�uence on their physicochemical properties, which might have greatly affected the extraction efficiency of target analytes [43]. e optimal ionic liquid for extraction was sought and the general trends observed are described below.

Anion Effect.
Some papers indicate the important in�uence of the cation part in different properties. For the series of ionic liquids studied here, the water miscibility of the ionic liquid was important to the extraction efficiency. N-Methylimidazolium based ionic liquids with seven different anions (Cl − , Br − , BF − , NO 3 − , HSO − , Ac − , and OH − ) were studied and differences in their extraction efficiency were readily apparent, as shown in Figure 1(a). All of the ionic liquids tested were sufficiently miscible in any proportion with water. e results showed that the ionic liquids based on Br − and HSO − were the more efficient of the liquids tested, with Br − showing the best results. e hydrogen bonding and hydrophobic interactions of [Bmim]Br and [Bmim]HSO caused the stronger solvation interactions with procyanidins. With the addition of ionic liquids, the extraction yields of procyanidins were improved greatly. is result indicates that extraction efficiency of procyanidins is anion dependent, which is similar to previous studies [43,44].

Effect of the Alkyl Chain Length of the Ionic Liquid
Cation. Using the same anion of Br − a series of 1-alkyl-3-methylimidazolium cations including Emim , Bmim , Hmim , Omim , and Dmim were evaluated, and the results are shown in Figure 1(b); the results implied that, for procyanidins, extraction efficiency increased slightly with the increasing alkyl chain length from ethyl to butyl. e alkyl chain length of cation was increased from butyl to dodecyl while the extraction efficiency decreased rather than increased. It could be attributed to the increase of the alkyl chain length in the cation moiety leading to larger steric clash. Having optimized both the anion and cation of the ionic liquid, [Bmim]Br was selected for subsequent extraction parameter optimization studies.

Concentration Effect. e optimum [Bmim]Br concentration in aqueous solution for UAE of procyanidins extraction was sought by carrying out extractions with [Bmin]Br solutions of different concentrations (from 0.25 to 1.25 M).
Based on the results shown in Figure 1(c), it can be seen that the extraction efficiency increased in the [Bmin]Br concentration range of 0.25-1.25 M. We propose that the high viscosity of the solvent at high ionic liquid concentrations may lead to poor penetration of the solvent into the plant tissue and high ionic liquids consumption. 1.25 M [Bmin]Br solution was therefore selected as the optimal ionic liquid concentration.

3.5.
Optimization of the UAE Parameters. e univariate method was used to optimize the following parameters: soak time, solid-liquid ratio, and ultrasonic power and time.   [Bmim]Br on the extraction of procyanidins from L. gmelinii bark at room temperature. It demonstrated the substantial increase in extraction efficiency obtained aer soaking the bark. To extract procyanidins from the cellular structure, the solvent must have access to the cellular compartments, where the procyanidins are located. An intact cell structure restricts accessibility of the solvent to the procyanidins, while ultrasound treated cells have a more open, fragmented structure, which facilitates efficient extraction. e increase in extraction efficiency of the procyanidins aer soaking with the solvent is probably because of increased diffusion of the solvent into the cellular structure allowing improved solubilization of the procyanidins. e extraction efficiency of procyanidins increased signi�cantly when the soak time was 0-3 h, however; longer soak times did not lead to further increases in efficiency. Hence 3 h was chosen as the optimal soak time.
e solid-liquid ratio is a crucial factor and was also studied to optimize extraction efficiency. Large solvent volumes could make the procedure difficult and lead to unnecessary waste, while small volumes may lead to incomplete extraction. A series of experiments were carried out with different solid-liquid ratios (1 : 6, 1 : 8, 1 : 10, 1 : 12, and 1 : 14 g mL −1 ) to evaluate the effect of the solid-liquid ratio. As shown in Figure  2(b), the extraction efficiency increased evidently with the increase of the solvent volume for solid-liquid ratio of up to 1 : 10. Higher solvent volumes, however, did not signi�cantly improve the extraction efficiency. us, a solid-liquid ratio of 1 : 10 was adopted as the optimal solid-liquid ratio in this study.

Ultrasonic Power and Time.
Ultrasonic power is believed to be the driving force for the complete dispersion of [Bmim]Br into the solid sample. To examine the effect of the ultrasonic power on the extraction efficiency, experiments were carried out at 100, 150, 200, and 250 W, respectively. e UAE time was maintained constant throughout this experiment at 30 min. Figure 2(c) shows the effect of ultrasonic power on extraction efficiency. With the ultrasonic power increasing from 100 to 150 W, the extraction efficiency of the procyanidins increased. However, when the ultrasonic power increased above 150 W, no obvious change could be observed at higher ultrasonic power. It meant that the ultrasonic power of 150 W was large enough to ensure the dispersion of [Bmim]Br. us, the ultrasonic power of UAE was set at 150 W in the following experiments. e in�uence that the time ultrasonic was applied to the sample on the extraction efficiency of the alkaloids was examined over a range of 10 to 60 min, and the results are shown in Figure 2(d). ey show that the extraction efficienciy of procyanidins increased when the ultrasonic time was increased from 10 to 30 min. When the variable was changed from 30 to 60 min, slight improvements were observed. e extraction efficiency was low during the �rst 20 min of ultrasonication, indicating that more time was needed for ultrasound to disrupt the cell walls and aid the release of the procyanidins into the solvent. Prolonged application of ultrasound, of more than 30 min, did not result in any further signi�cant improvement in extraction efficiency. It was found that more than 98% of the procyanidins content extracted during the �rst 30 min of UAE. e application of ultrasonic for 30 min was therefore selected for all subsequent experiments.
Reverse-phase HPLC was also used to analyze the composition of procyanidins. e HPLC apparatus was a Waters 717 automatic sample handling system series HPLC system (Waters Corporation, Milford, USA), consisting of a Waters 1525 bin pump with a steel column heater module controlling the column temperature, and a Waters 2487 UV-detector monitored by a Waters Millenium 32 chromatography manager soware. Chromatographic separation was performed on a HiQ sil-C18 reversed-phase column (4.6 mm × 250 mm, 5 m, KYA TECH). e elution conditions were as follows: �ow rate 1.0 mL min −1 , column temperature 24 ∘ C, injection volume 10 L, and solvent A: methanol, solvent B: 0.5% (v/v) phosphoric acid in water. e elution pro�le was: 0 min 18% A in B, 0-10 min 18% to 24% A in B, 10-15 min 24% A in B, 15-65 min 24% to 64% A in B, 65-75 min 100% A (wash-out), and 75-90 min 18% A in B (reconditioning). e detection wavelengths were set at 280 nm.   included pure water extraction and sodium chloride solution extraction. Water is the most common and inexpensive solvent and is therefore oen selected as a cosolvent in various extraction process. We compared the extraction capacities of ionic liquid solutions with pure water. As can be seen from Table 1, the extraction efficiency of the procyanidins was only 19.62 ± 0.76% with water, while that obtained when using 1.25 M [Bmim]Br was 100.00 ± 4.89%. e main contributor to procyanidins extraction efficiency was therefore the ionic liquid rather than water in the ionic liquid-water system. e procyanidins extraction efficiency achieved using 1.25 M NaCl solution was only 17.47±0.85%. e solvent effect of the ionic liquid was therefore more important in achieving high extraction efficiencies than the salt effect derived from NaCl. Hence, salt effects do not play a major role in improving the extraction of procyanidins.
In the present study, UAE, HRE, and ME techniques were compared for their efficiency in the extraction of procyanidins from L. gmelinii bark. e extraction efficiency of the procyanidins obtained under six different extraction methods using optimal conditions is summarized in Table 1. e extraction times used for UAE, HRE, and ME were 0.5, 4, and, 24 h, respectively. e extraction temperature of HRE was 85 ∘ C, while the extraction temperature used for UAE and ME was room temperature (25 ∘ C). e procyanidins extraction efficiency obtained using ILUAE methods was higher than those achieved using 80% ethanol HRE or ME methods.
3.8. HPLC Qualitative Analysis. e amount of the procyanidins studies presented in L. gmelinii bark extracts was qualitatively analyzed by using the chromatographic methodology. e chromatograms of HPLC of samples extracted with [Bmim]Br and 80% ethanol were shown in Figure  3. As can be seen from Figure 3, the relative contents of these characteristic procyanidin fraction ((+)-catechin, (−)epicatechin, procyanidin dimers B2 and B4, and procyanidin trimer C1) in the [Bmim]Br extraction solution had improved distinctly, and [Bmim]Br had good effect on the extraction of procyanidins. It is clear that ILUAE represents an efficient method for the extraction of procyanidins from L. gmelinii bark.

Conclusions
In this work, we propose a novel extracting method for procyanidins from L. gmelinii bark based on the use of ionic liquids in UAE followed by Vanillin-HCl method analysis and quanti�cation. e UAE conditions were optimized in detail. Considering the effect of both anion and cation, [Bmim]Br was selected for the subsequent evaluation. e optimum conditions for the extraction were as follows: [Bmim]Br concentration 1.25 M, soak time 3 h, solid-liquid ratio 1 : 10, ultrasonic power 150 W, and ultrasonic time 30 min. Under this condition, satisfactory extraction efficiency of the procyanidins was obtained. Relative to other methods, the proposed approach provided higher extraction efficiency and obviously reduced energy consumption time.
e method may also prove useful in the development of energy saving and environment friendly extraction methods for procyanidins from other plant materials.