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.
Trang 1Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/chroma
Pengfei Li a , Yanke Lu a , Jiangxue Cao a , Mengyuan Li a , Chunliu Yang a , ∗ , Hongyuan Yan a , b , ∗
a Key Laboratory of Medicinal Chemistry and Molecular Diagnosis, College of Public Health, Hebei University, Baoding, 071002, China
b 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 5 May 2020
Keywords:
Imidazolium ionic liquid
Phenolic resin
Solid-phase extraction
Benzoylurea plant hormone
Cucumber
a b s t r a c t
Animidazoliumionic-liquid-modifiedphenolicresin (ILPR)was synthesizedusing3-aminophenolasa functionalmonomer,glyoxylicacid asagreencross-linker,and polyethyleneglycol6000asaporogen The obtainedILPR showed betterextraction ofbenzoylurea planthormones thidiazuronand forchlor-fenuronthantheunmodifiedphenolicresinbecausetheimidazoliumILprovidesmoreinteractionmodes with the analytes ILPR, as a tailored adsorbent for solid-phase extraction, was coupled with high-performanceliquidchromatography(ILPR–SPE–HPLC)forthesimultaneousdeterminationofthidiazuron andforchlorfenuronincucumbers.GoodlinearityoftheILPR–SPE–HPLCmethodwasobtained,ranging from0.0100to5.00μgg−1 withacorrelationcoefficient(r )≥ 0.9999.Therecoveriesofspikedsamples rangedfrom91.4%to100.7%witharelativestandarddeviationof≤ 6.0%
© 2020 Elsevier B.V All rights reserved
1 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
hu-man genes [5] Since the maximum residue limits (MRLs) for
TDZ and CPPU in fruits and vegetables are strictly controlled at
50 μ g kg−1in many countries [6] , there is an urgent need to
de-velop 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
chromatography-tandem mass spectrometry [9–11] , gel chromatography-gas
chro-matography/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),
mag-netic 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)
[17–24] are the most widely used pretreatment techniques because they not only separate and purify simultaneously, but also are eco-nomical, simple, and fast [ 25 , 26 ] In a sense, it is crucial to de-velop new adsorbents with higher adsorption selectivities and ex-cellent 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 formalde-hyde used as the cross-linker in the preparation of those pheno-lic 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 ad-sorption capacity, multiple adsoption interactions, and green syn-thesis process To this end, we considered the use of glyoxylic acid (H(CO)CO2H), a biodegradable, natural component of plants, which could serve as a cross-linking agent.
Ionic liquids (ILs) are molten salts consisting of inorganic an-ions and organic cations [31] Generally low in toxicity, recy-clable, 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
https://doi.org/10.1016/j.chroma.2020.461192
0021-9673/© 2020 Elsevier B.V All rights reserved
Trang 22 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
imi-dazolium IL-functionalized poly(ethylene glycol dimethacrylate- co
vinylimidazole) microspheres that showed excellent adsorption
ca-pacity for thymopentin [39] Hence, we expected that modification
of a resin adsorbent with an imidazolium IL would enrich its
ad-sorption interactions and improve its adsorption capacity.
In this work, glyoxylic acid as a green cross-linker was
em-ployed to synthesize an imidazolium IL-modified phenolic resin
(ILPR), avoiding the use of the highly toxic formaldehyde
cross-linker 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
ad-vantages of the multiple interactions of the IL, high hydrophilicity
and porosity of the phenolic resin, and the economy and
simplic-ity of SPE, and was applied for the extraction and determination of
TDZ and CPPU in foodstuff samples.
2 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
pur-chased from Shanghai Aladdin Biochemical Technology Co., Ltd.
Polyethylene glycol 60 0 0 (PEG 60 0 0), trifluoroacetic acid, and
imi-dazole were purchased from Tianjin Kemiou Chemical Reagent Co.,
Ltd Ethyl acetate was obtained from Tianjin Beichen Reagent
Fac-tory 3-Aminophenol and 2-bromoethanol were obtained from
Bei-jing J&K Scientific Co., Ltd Ultra-pure water was filtered through a
membrane filter (0.45 μ m) before use.
2.2 Instruments and conditions
Fourier-transform infrared spectra (FT-IR) of the ILPR were
ob-tained with a Vertex70 FTIR spectrometer (Bruker, Karlsruhe,
Ger-many) Elementary analyses were performed on Thermo Flash
20 0 0 elementary analyzer (Thermo Fisher Scientific, USA) Bromine
(Br) element analysis was performed on IC20 0 0 ion
chromato-graph (Dionex, USA) The 13C nuclear magnetic resonance (NMR)
spectra was recorded on a Bruker AVANCE III 400 WB
spec-trometer (Bruker, Germany) The surface morphology of the
ILPR was investigated by scanning electron microscopy
(Phe-nom Pro, Eindhoven, Netherlands) The chromatographic system
employed was an UltiMate-30 0 0 liquid chromatograph (Thermo
Fisher Scientific, USA) equipped with an Eclipse Plus C18 column
(4.6 mm × 150 mm, 3.5 μ m), 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 μ L, 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 5 h After cooling to room temperature, the bottom
IL layer was removed.
3-Aminophenol (0.327 g), PEG 60 0 0 (0.30 0 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 sul-furic 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
H2SO4.
2.4 ILPR–SPE process
The ILPR (20.0 mg) was placed into an empty SPE column (6 cm × 1 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’ mar-kets in Baoding were homogenized using a homogenizer, and the solid residues were precipitated by centrifugation at 150 0 0 rpm for 15 min To precipitate the sample matrix, the juice was mixed twice with lead acetate solution (16 wt%, 1.5 and 0.50 mL por-tions) 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 μ m membrane, and evaporated
to dryness Finally, the mixture was redissolved with double-deionized water (20 mL) for HPLC.
3 Results and discussion
3.1 Characteristics of the ILPR and PR
A schematic illustrations of the ILPR synthesis is shown in Fig 1 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 at-traction 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 2 A) The ILPR adsorbs
a larger amount of CPPU than TDZ, which may be due to the elec-trostatic 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 2 C and D reveal obvious differences between the ILPR and PR The morphology of PR is revealed as stacked microparticles that are approximately spherical In con-trast, the ILPR presents a fluffy porous structure with a rough sur-face and tiny through pores, which are mainly ascribed to the
Trang 3Fig 1 Schematic illustration of the ILPR synthesis route
sticky imidazolium IL Compared with PR, the ILPR adsorbent
ex-hibits 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 2 B.
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
gly-oxylic acid A peak corresponding to the C = C vibrations of
3-aminophenol 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 13C NMR, as shown in
Fig 2 E, 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
El-emental analysis was used to characterize the elemental
composi-tion 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 μ g 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 tem peratures
are presented in Fig 3 A and B, which show that the amounts of
TDZ and CPPU adsorbed on the ILPR increase with increasing ini-tial analyte concentration at the same temperature Moreover, the adsorption capacity of the ILPR decreases with increasing tempera-ture, 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 stan-dard solution of each analyte (2.0 mL; 40.0 μ g 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 min 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 i ( μ g mL−1) represents the initial concentration, e
( μ g mL−1) represents the concentration of the standard solution
at equilibrium, and V (mL) and W (g) represent the solution vol-ume and weight of the sorbent, respectively As shown in Eqs.(2 ) and (3) , k1(min−1) and k2(g mg−1min−1) represent pseudo-first-order 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
t
Qt = 1
K2Qe2
+ Q t
e
(3)
The linear fittings of the kinetics models are shown in Fig 3 C and D, and the data are listed in Table 1 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 oper-ate 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 vol-ume of washing solvent and eluent, were next optimized As
Trang 44 P Li, Y Lu and J Cao et al / Journal of Chromatography A 1623 (2020) 461192
Fig 2 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 1
Kinetic parameters for ILPR
Analytes
Q e,cal ( μg mg −1 ) k 1 (min −1 ) R 2 Q e,cal ( μg mg −1 ) k 2 (g mg −1 min −1 ) R 2
shown in Fig 4 A, 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
pro-cessing.
The washing solvent plays a critical role in the removal of
co-adsorbed interferents during the extraction process, while
ensur-ing that the adsorption interaction between the analytes and
ad-sorbent is not destroyed In this work, five washing solvents were investigated, with water providing the lowest loss rate of TDZ and CPPU ( Fig 4 B) From the perspective of the purification effects, most of the interfering substances originating from the sample ma-trix are washed out from the SPE column effectively After opti-mization, the washing solvent was established as 1.0 mL water.
As demonstrated in Fig 4 C, five elution solvent systems were investigated, with methanol/acetic acid (9:1 v/v) exhibiting the
Trang 5Fig 3 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 2
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
Table 3
Spiked recoveries for the ILPR–SPE–HPLC method
Analyte
0.0500 ( μg g −1 ) 1.00 ( μg g −1 ) 5.00 ( μg g −1 ) Recovery (%) RSD (%) Recovery (%) RSD (%) Recovery (%) RSD (%)
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
optimiza-tion, 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
linear-ity, limit of detection (LOD), limit of quantitation (LOQ), precision,
accuracy, and spiked recovery Calibration curves were obtained
us-ing nine spiked levels of TDZ and CPPU in the range of 0.0100–
5.00 μ g g−1, with correlation coefficients ( ) of ≥0.9999 ( Table 2 ).
The LODs and LOQs, calculated according to LOD = 3 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 μ g g−1, and 0.0 0651 and 0.0 0564 μ g g−1 for TDZ and CPPU, respectively The accuracy and precision of the method were evaluated by perform-ing three replicate measurements (5.00 μ g mL−1) on the same day ( n = 3) and three consecutive days, while their intra-day and inter-day precisions expressed as relative standard deviations (RSDs) are
in the ranges 0.66–1.32% and 4.41–4.73% for TDZ and CPPU, re-spectively 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 μ g g−1).
3.5 Detection of TDZ and CPPU in cucumber samples
The feasibility of the ILPR–SPE–HPLC method was evaluated us-ing five cucumber samples obtained from the farmers’ markets
Trang 66 P Li, Y Lu and J Cao et al / Journal of Chromatography A 1623 (2020) 461192
Fig 4 Optimization of ILPR–SPE procedure (A: Loading volume, B: washing solvent; C and D: elution solvents)
Table 4
Comparison of the present method with reported methods
SPE HPLC Cucumber 20.0 10.0–5.00 × 10 3 ng g −1 91.4–100.7 1.69–1.95 ng g −1 0.2–6.0 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 5 shows that all interferences from the
cu-cumber 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 4 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
Trang 7lim-Fig 5 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.
4 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
ad-sorbent for the extraction of TDZ and CPPU from cucumbers Due
to hydrogen bonding and electrostatic interactions, the ILPR
ob-viously increased the extraction efficiency and adsorption
capac-ity 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
deter-mination of benzoylurea plant hormones in cucumber samples.
Declaration of Competing Interest
The authors declare that they have no known competing
finan-cial 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:
Visual-ization, Project administration Hongyuan Yan: Conceptualization,
Methodology, Supervision.
Acknowledgments
This work is supported by the Natural Science Foundation of
Hebei Province ( B2018201270 , H2019201288 ), the National
Nat-ural Science Foundation of China ( 21575033 ), the Talent
Engi-neering Training Foundation of Hebei Province ( A201802002 ), and
the Post-graduate’s Innovation Fund Project of Hebei University
( hbu2019ss073 , hbu2020ss004 )
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