Imidazolium ionic-liquid-modified phenolic resin for solid-phase extraction of thidiazuron and forchlorfenuron from cucumbers

An imidazolium ionic-liquid-modified phenolic resin (ILPR) was synthesized using 3-aminophenol as a functional monomer, glyoxylic acid as a green cross-linker, and polyethylene glycol 6000 as a porogen.

Journal of Chromatography A 1623 (2020) 461192 Contents lists available at ScienceDirect Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma Imidazolium ionic-liquid-modified phenolic resin for solid-phase extraction of thidiazuron and forchlorfenuron from cucumbers Pengfei Li a, Yanke Lu a, Jiangxue Cao a, Mengyuan Li a, Chunliu Yang a,∗, Hongyuan Yan a,b,∗ a b Key Laboratory of Medicinal Chemistry and Molecular Diagnosis, College of Public Health, Hebei University, Baoding, 071002, China Key Laboratory of Analytical Science and Technology of Hebei Province, College of Pharmaceutical Science, Hebei University, Baoding, 071002, China a r t i c l e i n f o Article history: Received 21 February 2020 Revised 30 April 2020 Accepted 30 April 2020 Available online May 2020 Keywords: Imidazolium ionic liquid Phenolic resin Solid-phase extraction Benzoylurea plant hormone Cucumber a b s t r a c t An imidazolium ionic-liquid-modified phenolic resin (ILPR) was synthesized using 3-aminophenol as a functional monomer, glyoxylic acid as a green cross-linker, and polyethylene glycol 60 0 as a porogen The obtained ILPR showed better extraction of benzoylurea plant hormones thidiazuron and forchlorfenuron than the unmodified phenolic resin because the imidazolium IL provides more interaction modes with the analytes ILPR, as a tailored adsorbent for solid-phase extraction, was coupled with highperformance liquid chromatography (ILPR–SPE–HPLC) for the simultaneous determination of thidiazuron and forchlorfenuron in cucumbers Good linearity of the ILPR–SPE–HPLC method was obtained, ranging from 0.0100 to 5.00 μg g−1 with a correlation coefficient (r) ≥ 0.9999 The recoveries of spiked samples ranged from 91.4% to 100.7% with a relative standard deviation of ≤ 6.0% Introduction Widely used in crop production in many countries, thidiazuron (TDZ) and forchlorfenuron (CPPU) are benzoylurea plant hormones that regulate plant growth and development and promote fruit quality [1–4] However, several studies have shown that they may interfere with the endocrine system and could be harmful to human genes [5] Since the maximum residue limits (MRLs) for TDZ and CPPU in fruits and vegetables are strictly controlled at 50 μg kg−1 in many countries [6], there is an urgent need to develop sensitive, accurate methods to detect trace levels of these compounds To date, a number of methods have been developed based on liquid chromatography [7,8], liquid chromatographytandem mass spectrometry [9–11], gel chromatography-gas chromatography/mass spectrometry [12], Raman spectroscopy [6], ion mobility spectrometry [13], and electrophoresis [14,15] Although these methods have their own advantages, all suffer from impurity interference due to the complex sample matrices [16] Therefore, a simple and effective sample pretreatment method would be very desirable for complicated samples before instrumental analysis Solid-phase micro-extraction, solid-phase extraction (SPE), magnetic solid-phase extraction, and matrix solid-phase dispersion ∗ Corresponding authors E-mail addresses: yangchunliu@hbu.edu.cn (C Yang), yanhy@hbu.edu.cn (H Yan) https://doi.org/10.1016/j.chroma.2020.461192 0021-9673/© 2020 Elsevier B.V All rights reserved © 2020 Elsevier B.V All rights reserved [17–24] are the most widely used pretreatment techniques because they not only separate and purify simultaneously, but also are economical, simple, and fast [25,26] In a sense, it is crucial to develop new adsorbents with higher adsorption selectivities and excellent adsorption capacities, which can improve the efficiencies of these methods [17,21] In recent years, phenolic resins have been used in the field of separation science owing to their high porosity, excellent thermal stability, and low cost of raw materials [27–29] However, the reported traditional phenolic resins function through a single type of adsorption interaction Furthermore, the formaldehyde used as the cross-linker in the preparation of those phenolic resins is harmful to the environment and human health; it can induce respiratory irritation, allergic reaction, and cancer, even at concentrations slightly higher than nature levels [29,30] It would be desirable to develop innovative resin adsorbents with high adsorption capacity, multiple adsoption interactions, and green synthesis process To this end, we considered the use of glyoxylic acid (H(CO)CO2 H), a biodegradable, natural component of plants, which could serve as a cross-linking agent Ionic liquids (ILs) are molten salts consisting of inorganic anions and organic cations [31] Generally low in toxicity, recyclable, and functionalizable [32], they have been widely used in the extraction and separation fields [16,33–36] The ILs used in adsorbent synthesis are expected to participate in multiple types of molecular interactions [37,38], which would not only improve the adsorption selectivity for the desired adsorbent but P Li, Y Lu and J Cao et al / Journal of Chromatography A 1623 (2020) 461192 also increase its adsorption capacity Du et al synthesized imidazolium IL-functionalized poly(ethylene glycol dimethacrylate-covinylimidazole) microspheres that showed excellent adsorption capacity for thymopentin [39] Hence, we expected that modification of a resin adsorbent with an imidazolium IL would enrich its adsorption interactions and improve its adsorption capacity In this work, glyoxylic acid as a green cross-linker was employed to synthesize an imidazolium IL-modified phenolic resin (ILPR), avoiding the use of the highly toxic formaldehyde crosslinker in the preparation of a traditional phenolic resin We then applied the obtained ILPR as a tailored SPE adsorbent, coupling it with HPLC (ILPR–SPE–HPLC) to extract and detect trace TDZ and CPPU in cucumbers The ILPR–SPE–HPLC method combined the advantages of the multiple interactions of the IL, high hydrophilicity and porosity of the phenolic resin, and the economy and simplicity of SPE, and was applied for the extraction and determination of TDZ and CPPU in foodstuff samples Experimental 2.1 Chemicals and reagents Acetic acid was obtained from Tianjin Guangfu Fine Chemical Co., Ltd TDZ, CPPU, glyoxylic acid, and 1-chlorohexane were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd Polyethylene glycol 60 0 (PEG 60 0), trifluoroacetic acid, and imidazole were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd Ethyl acetate was obtained from Tianjin Beichen Reagent Factory 3-Aminophenol and 2-bromoethanol were obtained from Beijing J&K Scientific Co., Ltd Ultra-pure water was filtered through a membrane filter (0.45 μm) before use 2.2 Instruments and conditions Fourier-transform infrared spectra (FT-IR) of the ILPR were obtained with a Vertex70 FTIR spectrometer (Bruker, Karlsruhe, Germany) Elementary analyses were performed on Thermo Flash 20 0 elementary analyzer (Thermo Fisher Scientific, USA) Bromine (Br) element analysis was performed on IC20 0 ion chromatograph (Dionex, USA) The 13 C nuclear magnetic resonance (NMR) spectra was recorded on a Bruker AVANCE III 400 WB spectrometer (Bruker, Germany) The surface morphology of the ILPR was investigated by scanning electron microscopy (Phenom Pro, Eindhoven, Netherlands) The chromatographic system employed was an UltiMate-30 0 liquid chromatograph (Thermo Fisher Scientific, USA) equipped with an Eclipse Plus C18 column (4.6 mm × 150 mm, 3.5 μm), Chromeleon 7.2 workstation, and UV detector with a wavelength of 278 nm The mobile phase was water–acetonitrile (60:40, v/v, with 0.1% trifluoroacetic acid) with a flow rate of 1.0 mL min−1 The injection volume was 20 μL, and the column temperature was set at 25 °C 2.3 Preparation of imidazolium ionic-liquid-modified phenolic resin Imidazole (6.80 g) and 1-chlorohexane (6.00 g) were mixed in ethyl acetate (40 mL) in a 100 mL flask and stirred for 72 h at 70 °C The product was washed three times with water (10 mL) to remove unreacted reagents The ethyl acetate was then removed at 35 °C using a rotary evaporator The residue was subsequently vacuum-dried at 50 °C until a constant weight was obtained Then, this material (1.50 g) and 2-bromoethanol (1.50 g) were mixed with ethyl acetate (20 mL) in a hydrothermal kettle and reacted at 120 °C for h After cooling to room temperature, the bottom IL layer was removed 3-Aminophenol (0.327 g), PEG 60 0 (0.30 g), and the IL (0.828 g) were added to flask A with acetonitrile (20 mL) and stirred until a clear solution was formed Then, concentrated sulfuric acid (1.5 mL) was added to flask A Glyoxylic acid (0.653 g) was dissolved in acetonitrile (20 mL) in flask B The contents of flask A were mixed with B and the mixture was stirred at 45 °C for 30 min; thereafter, the temperature was increased to 75 °C for 24 h After washing the cooled reaction mixture with ethanol and deionized water, the residue was vacuum-dried to obtain the ILPR The phenolic resin without IL modification (PR) was synthesized using an identical method, except for the addition of the IL and H2 SO4 2.4 ILPR–SPE process The ILPR (20.0 mg) was placed into an empty SPE column (6 cm × cm) between two polyethylene screen plates Then, the ILPR column was activated with methanol (2.0 mL) followed by water (2.0 mL) Subsequently, the sample solution (1.0 mL) was loaded and the column was washed with water (1.0 mL) and eluted with methanol/acetic acid (9:1 v/v, 1.5 mL) The eluate was collected and evaporated to dryness under a nitrogen stream and redissolved with the mobile phase (0.50 mL) for HPLC To achieve full extraction of the analytes by the adsorbent, combined with the absorption amount of ILPR and the flow rate of other literature [6,21], the flow rate was set to two drops per minute To control the flow rate at this level, a rubber bulb with an iron frame was used during the SPE process Briefly, the tip of the rubber bulb was tightly inserted into the SPE column, while the head was clamped using a clip, and the flow rate was adjusted by controlling the force of the clip 2.5 Preparation of cucumber samples Cucumber samples (25.0 g) obtained from the farmers’ markets in Baoding were homogenized using a homogenizer, and the solid residues were precipitated by centrifugation at 150 0 rpm for 15 To precipitate the sample matrix, the juice was mixed twice with lead acetate solution (16 wt%, 1.5 and 0.50 mL portions) The sample was then centrifuged and the supernatant was freeze-dried overnight The residue was dissolved in methanol (20 mL), passed through a 0.45 μm membrane, and evaporated to dryness Finally, the mixture was redissolved with doubledeionized water (20 mL) for HPLC Results and discussion 3.1 Characteristics of the ILPR and PR A schematic illustrations of the ILPR synthesis is shown in Fig The glyoxylic acid crosslinker and imidazolium IL were condensed by an esterification reaction, and the IL-modified crosslinker was reacted with the 3-aminophenol monomer to form the ILPR The positively charged imidazole ring was introduced in the ILPR by modifying the IL, which increased its electrostatic attraction to the analytes and improved the adsorption capacity The amounts of TDZ and CPPU adsorbed by the ILPR are obviously higher than those of the unmodified PR (Fig 2A) The ILPR adsorbs a larger amount of CPPU than TDZ, which may be due to the electrostatic interaction between the positive charge carried on the IL and the electronegative chlorine atom on CPPU, which promotes its adsorption Hydrogen bonding also plays an important role The SEM images in Fig 2C and D reveal obvious differences between the ILPR and PR The morphology of PR is revealed as stacked microparticles that are approximately spherical In contrast, the ILPR presents a fluffy porous structure with a rough surface and tiny through pores, which are mainly ascribed to the P Li, Y Lu and J Cao et al / Journal of Chromatography A 1623 (2020) 461192 Fig Schematic illustration of the ILPR synthesis route sticky imidazolium IL Compared with PR, the ILPR adsorbent exhibits excellent features, including a rough surface that provids numerous binding sites which should be beneficial for interaction with the target molecules In addition, the tiny through pores in the ILPR could reduce the mass transfer resistance of the analytes, which should be conducive to rapid extraction The FT-IR spectra of the ILPR and PR are shown in Fig 2B A broad peak corresponding to the O–H stretching vibration is observed at 3397 cm−1 The adsorption band at 2920 cm−1 is due to the asymmetric stretching vibration of C–H, while that at 1718 cm−1 is attributed to the C=O stretching vibration of glyoxylic acid A peak corresponding to the C=C vibrations of 3aminophenol appears at 1629 cm−1 Typically, the peaks at 1082 and 837 cm−1 are ascribed to the symmetric stretching vibrations of the imidazole ring and C–H of the aromatic ring These results indicate that the IL was successfully introduced into the PR, which would enable the generation of hydrogen bonds and electrostatic interactions between the adsorbent and analytes The obtained ILPR was confirmed by 13 C NMR, as shown in Fig 2E, the major signals are carboxylic ester (δ in 173.97 ppm), aromatic ring (δ in around 100 and 121 ppm), imidazole ring (δ in around 137 ppm), alkyl (δ in around 50 ppm), and ether (δ in 70.75 ppm) These results are consistent with the results of FT-IR, indicating that the IL was successfully introduced into the PR Elemental analysis was used to characterize the elemental composition of the ILPR The results show that ILPR is primarily composed of C (54.21%), O (28.45%) In addition, trace amount of Br (0.17%) is detected, indicating that the IL was successfully introduced into the PR 3.2 Adsorption performance of the ILPR The adsorption thermodynamics of the new adsorbent was studied by mixing ILPR (5.00 mg) with various concentrations of sample solution (2.0 mL; 5.00, 10.0, 20.0, 30.0, 40.0, 60.0, or 80.0 μg mL−1 ) at different temperatures After shaking for 12 h and centrifuging, the supernatants were analyzed by HPLC The isotherms for TDZ and CPPU adsorption at different temperatures are presented in Fig 3A and B, which show that the amounts of TDZ and CPPU adsorbed on the ILPR increase with increasing initial analyte concentration at the same temperature Moreover, the adsorption capacity of the ILPR decreases with increasing temperature, suggesting that adsorption occurs via an exothermic process To evaluate the adsorption mechanism of ILPR, the adsorption kinetics was evaluated by mixing the ILPR (5.00 mg) and a standard solution of each analyte (2.0 mL; 40.0 μg mL−1 ) in a 10 mL centrifuge tube, and then shaking at 350 rpm and 25 °C for 2, 5, 10, 30, 60, 120, or 180 The supernatants were analyzed by HPLC, and the obtained adsorption data were fitted with various kinetics models [21] The adsorption amount was calculated using Eq.(1), where ci (μg mL−1 ) represents the initial concentration, ce (μg mL−1 ) represents the concentration of the standard solution at equilibrium, and V (mL) and W (g) represent the solution volume and weight of the sorbent, respectively As shown in Eqs.(2) and (3), k1 (min−1 ) and k2 (g mg−1 min−1 ) represent pseudo-firstorder and pseudo-second-order rate constants, respectively, and Qe (mg g−1 ) and Qt (mg g−1 ) are the adsorption capacities of ILPR for TDZ and CPPU at equilibrium and time t, respectively Qe = (Ci − Ce ) × V W (1) ln(Qe − Qt ) = lnQe − k1 t (2) t t = + Qt Qe K2 Qe (3) The linear fittings of the kinetics models are shown in Fig 3C and D, and the data are listed in Table The R2 value of the quasi-second-order equation is obviously higher than that of the other, indicating that the process of adsorption by ILPR may operate via chemisorption or strong surface complexation rather than mass transfer [21] 3.3 Optimization of ILPR-SPE process The parameters affecting the extraction performance of the ILPR-SPE, including the sample loading volume, and type and volume of washing solvent and eluent, were next optimized As P Li, Y Lu and J Cao et al / Journal of Chromatography A 1623 (2020) 461192 Fig Absorption amounts (A), FT-IR spectra (B), and SEM images of PR (C) and ILPR (D), and 13 C NMR spectra of ILPR (E) Table Kinetic parameters for ILPR Pseudo-first order Analytes Qe,cal (μg mg−1 ) k1 (min−1 ) R2 Pseudo-second order Qe,cal (μg mg−1 ) k2 (g mg−1 min−1 ) R2 TDZ CPPU 0.9675 0.9493 7.4985 11.7495 0.9909 0.9911 3.3242 4.6886 0.0071 0.0076 shown in Fig 4A, the recovery of analytes decreases as the loading volume of the sample increases, which is due to the large loading volume breaking through the adsorption amount of the adsorbent Therefore, 1.0 mL of loading solution was selected for further processing The washing solvent plays a critical role in the removal of coadsorbed interferents during the extraction process, while ensuring that the adsorption interaction between the analytes and ad- 0.0221 0.0150 sorbent is not destroyed In this work, five washing solvents were investigated, with water providing the lowest loss rate of TDZ and CPPU (Fig 4B) From the perspective of the purification effects, most of the interfering substances originating from the sample matrix are washed out from the SPE column effectively After optimization, the washing solvent was established as 1.0 mL water As demonstrated in Fig 4C, five elution solvent systems were investigated, with methanol/acetic acid (9:1 v/v) exhibiting the P Li, Y Lu and J Cao et al / Journal of Chromatography A 1623 (2020) 461192 Fig Amounts of TDZ (A) and CPPU (B) adsorbed by ILPR, and kinetics plots for the pseudo-first order (C) and pseudo-second order rate equations for ILPR (D) Table Parameters for the ILPR–SPE–HPLC method Analyte Linearity (μg g−1 ) Correlation coefficient (r) Calibration plot (y = ax+b) LOD (μg g−1 ) LOQ (μg g−1 ) RSD (%) Intra-day Inter-day TDZ CPPU 0.0100–5.00 0.0100–5.00 0.9999 0.9999 y = 1.3967x+0.0006 y = 1.6596x−0.0162 0.00195 0.00169 0.00651 0.00564 1.32 0.66 4.41 4.73 Table Spiked recoveries for the ILPR–SPE–HPLC method 0.0500 (μg g−1 ) Analyte Recovery (%) RSD (%) 1.00 (μg g−1 ) Recovery (%) RSD (%) 5.00 (μg g−1 ) Recovery (%) RSD (%) TDZ CPPU 95.1 100.7 5.4 4.6 91.4 98.1 0.9 0.2 91.4 96.1 6.0 2.4 highest recovery Because TDZ and CPPU are protonated under acidic conditions, the electrostatic attraction and hydrogen bonding interactions with the ILPR are weakened After volume optimization, 1.5 mL methanol/acetic acid (9:1 v/v) was used for further investigations 3.4 Validation of the ILPR–SPE–HPLC method The ILPR–SPE–HPLC method was validated in terms of its linearity, limit of detection (LOD), limit of quantitation (LOQ), precision, accuracy, and spiked recovery Calibration curves were obtained using nine spiked levels of TDZ and CPPU in the range of 0.0100– 5.00 μg g−1 , with correlation coefficients (r) of ≥0.9999 (Table 2) The LODs and LOQs, calculated according to LOD = Sb/m and LOQ = 10 Sb/m (where m is the calibration slope and Sb is the standard deviation [40]), were 0.00195 and 0.00169 μg g−1 , and 0.0 0651 and 0.0 0564 μg g−1 for TDZ and CPPU, respectively The accuracy and precision of the method were evaluated by performing three replicate measurements (5.00 μg mL−1 ) on the same day (n = 3) and three consecutive days, while their intra-day and interday precisions expressed as relative standard deviations (RSDs) are in the ranges 0.66–1.32% and 4.41–4.73% for TDZ and CPPU, respectively Finally, the recoveries are 91.4–100.7% (RSD ≤ 6.0%) (Table 3), which were determined at three spiked levels (0.0500, 1.00, and 5.00 μg g−1 ) 3.5 Detection of TDZ and CPPU in cucumber samples The feasibility of the ILPR–SPE–HPLC method was evaluated using five cucumber samples obtained from the farmers’ markets P Li, Y Lu and J Cao et al / Journal of Chromatography A 1623 (2020) 461192 Fig Optimization of ILPR–SPE procedure (A: Loading volume, B: washing solvent; C and D: elution solvents) Table Comparison of the present method with reported methods Method SPE SPE DLLME QuEChERS QuEChERS SPE SPE Detection SERS HPLC HPLC-DAD LC–MS/MS LC–MS/MS IMS HPLC Sample Grapes, kiwi Grapes, pitaya River water Grapes Fruits Fruit juices Cucumber Absorbent (mg) 100 100 — — — 150 20.0 Linearity Recovery (%) −1 30.0–300 ng mL 30.0–200 × 103 ng g−1 1.00–100 ng mL−1 0.100–50.0/1.00–500 ng mL−1 5.00–500 ng mL−1 10.0–400 ng mL−1 10.0–5.00 × 103 ng g−1 78.9–87.9 72.4–94.9 91–101 75.6–109.0 79.9–109.1 80–115 91.4–100.7 LOD −1 15.0 ng mL 16.1 ng g−1 0.500 ng mL−1 — 0.300–0.400 ng g−1 2.00 ng mL−1 1.69–1.95 ng g−1 RSD (%) Ref 8.1–13.2 0.18–3.53 6.4 1.2–11.4 1.1–10.4 7.6 0.2–6.0 [6] [7] [8] [10] [11] [13] This work SERS: surface-enhanced raman spectroscopy; HPLC-DAD: HPLC-diode array detection; DLLME: dispersive liquid–liquid microextraction; IMS: ion mobility spectrometry in Baoding, China In one of the cucumbers, a trace of TDZ (i.e., 43.5 ng g−1 ) was detected, which is below the maximum residue limit (50.0 ng g−1 ) Fig shows that all interferences from the cucumber matrix were effectively removed and no impurity peaks exist near the retention times of the analytes, indicating that the proposed ILPR–SPE–HPLC method is an effective extraction and isolation process for the accurate determination of trace levels of TDZ and CPPU in cucumbers 3.6 Method comparison with reference methods A comparison of the present method with reported methods is shown in Table The developed ILPR–SPE–HPLC method uses less absorbent, and further, affords a lower LOD compared to other methods that use SPE as a pretreatment technique Compared with LC−MS/MS, the developed ILPR–SPE–HPLC method exhibits a higher LOD, but the expensive instrumentation of the former lim- P Li, Y Lu and J Cao et al / Journal of Chromatography A 1623 (2020) 461192 References Fig Chromatograms of spiked sample (A) and cucumber-derived sample (B) its its application for routine analysis In addition, the recoveries for TDZ and CPPU by the proposed method are similar to those of the reference methods Therefore, the proposed ILPR–SPE–HPLC method could be employed for the analysis of trace CPPU and TDZ in cucumbers Conclusion In this work, a new type of ILPR employing glyoxylic acid as a green cross-linker was prepared and used as a special SPE adsorbent for the extraction of TDZ and CPPU from cucumbers Due to hydrogen bonding and electrostatic interactions, the ILPR obviously increased the extraction efficiency and adsorption capacity compared to the unmodified PR The developed ILPR–SPE–HPLC method was employed to successfully extract and detect TDZ and CPPU in cucumber Therefore, the ILPR can serve as a potential SPE adsorbent and is expected to be used for the separation and determination of benzoylurea plant hormones in cucumber samples Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper CRediT authorship contribution statement Pengfei Li: Methodology, Conceptualization Yanke Lu: Data curation, Validation Jiangxue Cao: Software, Formal analysis Mengyuan Li: Writing - review & editing Chunliu Yang: Visualization, Project administration Hongyuan Yan: Conceptualization, Methodology, Supervision Acknowledgments This work is supported by the Natural Science Foundation of Hebei Province (B2018201270, H2019201288), the National Natural Science Foundation of China (21575033), the Talent Engineering Training Foundation of Hebei Province (A201802002), and the Post-graduate’s Innovation Fund Project of Hebei University (hbu2019ss073, hbu2020ss004) [1] A Valverde, A Aguilera, C Ferrer, F Camacho, A Cammarano, Analysis of forchlorfenuron in vegetables by LC/TOF-MS after extraction with the buffered QuEChERS method, J Agric Food Chem 58 (2010) 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