Hydrophilic molecularly imprinted phenol-amine-formaldehyde resins published by Ural Federal University eISSN 2411-1414 chimicatechnoacta.ru ARTICLE 2023, vol. 10(3), No. 202310310 DOI: 10.15826/chimtech.2023.10.3.10 1 of 7 Hydrophilic molecularly imprinted phenol-amine-formaldehyde resins Yuliya Yu. Petrova a * , Elena V. Bulatova a , Dmitry O. Zelentsov a , Yuliya G. Mateyshina b a: Institute of Natural and Technical Sciences, Surgut State University, Surgut 628412, Russia b: Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch of the Russian Academy of Sciences, Novosibirsk 630090, Russia * Corresponding author: petrova_juju@surgu.ru This paper belongs to the RKFM'23 Special Issue: https://chem.conf.nstu.ru/. Guest Editors: Prof. N. Uvarov and Prof. E. Aubakirov. Abstract Hydrophilic molecularly imprinted resins (MIR), which are produced us- ing hydrophilic monomers such as phenols, aldehydes, melamine or urea, have recently attracted increasing attention for use in separation and pre- concentration. Among their obvious advantages are good sorption capac- ity, high recovery and selectivity, as well as their reusability in aqueous solutions. In this work we applied the bulk molecular imprinting method to produce quercetin-imprinted phenol-amino-formaldehyde resin. For this purpose, phloroglucinol and melamine solutions were mixed with for- maldehyde and then polyethylene glycol and quercetin (Qu) were added to the obtained solution as a porogen and a template, respectively. The mix- ture was stirred under heating, then left in the thermostat for a continuous time. The optimum ratio of phloroglucinol to melamine was 3:1. The aver- age molecular mass of porogen (Mw) varied between 4000–10000 Da. The obtained MIR were eluted with ethanol-water mixture (4:1, v/v) in the Soxhlet extractor for 36 h to remove the template. The MIR were charac- terized by FTIR-spectroscopy, laser diffraction spectroscopy and differen- tial thermal analysis. The maximum recovery and sorption capacity of MIR synthesized in the presence of a porogen with Mw 10000 were 47% and 4.7 μmol Qu/g, respectively. The maximum imprinting factor was 1.41. The sorption kinetics of quercetin by a non-imprinted resin (NIR) is best de- scribed by a pseudo-second-order model, while MIR has a mixed pseudo- first-second-order mechanism. Keywords molecular imprinting hydrophilic resins sorption rebinding quercetin Received: 04.07.23 Revised: 26.07.23 Accepted: 16.08.23 Available online: 23.08.23 Key findings ● We obtained a molecularly imprinted hydrophilic phloroglucinol-melamine-formaldehyde resin for the sorption con- centration of quercetin. ● The sorption capacity for quercetin by the molecularly-imprinted sample was ~5 µmol/g, and the imprinting factor was 1.41. ● The quercetin rebinding process was in compliance with the pseudo-second-order model and the Freundlich model. © 2023, the Authors. This article is published in open access under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction Molecularly imprinted polymers (MIP) are widely used in analytical chemistry and other fields for selective extrac- tion, separation and preconcentration of target molecules. One of the significant disadvantages of traditional organic solvent synthesized MIP is the insufficiently selective bind- ing of organic analytes from aqueous matrices. This is caused by the structural changes of the MIP and, conse- quently, its binding sites due to the swelling of the polymer in polar solvents. Hydrophilic molecularly imprinted resins (MIR), which are produced using hydrophilic monomers such as phenols, aldehydes, melamine or urea, have recently at- tracted increasing attention of researchers [1–16]. The high density of hydrophilic functional groups in MIR http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2023.10.3.10 mailto:petrova_juju@surgu.ru http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0003-3702-2249 https://orcid.org/0000-0003-3514-3872 https://orcid.org/0009-0007-6153-8380 https://orcid.org/0000-0002-1880-7182 https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2023.10.3.10&domain=pdf&date_stamp=2023-08-23 Chimica Techno Acta 2023, vol. 10(3), No. 202310310 ARTICLE 2 of 7 DOI: 10.15826/chimtech.2023.10.3.10 (hydroxyl, amino and imino groups, as well as ether bonds) promotes the formation of complexes with tem- plate molecules through multiple hydrogen bonds, π–π– and electrostatic interactions, which makes molecular im- printing more effective. MIR have been widely used in sep- aration and preconcentration [17, 18]. Resorcinol-formal- dehyde resins were used to extract sulfonamide antibiot- ics [19] and aminophenol-formaldehyde resins were used to determine perphluorocarboxylic acids, chlorophenols, benzoic acids, dyes, plant growth regulators and for selec- tive absorption of tetramethylpyrazine (ligustrazine) in traditional Chinese herb [20–23], etc. Among the obvious advantages of MIR are good adsorption capacity, high re- covery and selectivity, as well as their reusability in aque- ous solutions. In this work, the molecular imprinting method was used to produce quercetin-imprinted phloroglucinol-melamine- formaldehyde resins [24]. Polyethylene glycol of varied av- erage molecular mass was used as a porogen to increase adsorption capacity of MIR. Quercetin has numerous biological and pharmacological activities, including antiviral, anti-inflammatory, antialler- gic and antitumor properties [25]. Quercetin is introduced into the composition of some pharmaceutical drugs and di- etary supplements; however, its recovery from plant ex- tracts is extremely difficult. So, it is important to develop selective, fast and simple methods for separation and de- termination of quercetin in plant extracts. 2. Materials and Methods 2.1. Chemicals and materials We used phloroglucinol (≥99.0%, Sigma-Aldrich, China); melamine (99%, Acros Organics, United Kingdom); formal- dehyde (37.6%, Merck, Germany); polyethylene glycol (av- erage molecular mass, Mw: 4000, 6000, and 10000 Da, Pan- reac); quercetin (99.3%, Sigma-Aldrich); acetic acid (99.9%, ECROS, Russia); acetonitrile for HPLC (>99.9%, Cryochrome, Russia); ethanol (95%, RFC, Russia), etc. A standard solution of 5.00∙10–4 mol/L of quercetin was prepared by dissolving it in 95% ethanol; calibration solu- tions of 1.00∙10–6–7.50∙10–5 mol/L were prepared by dilut- ing the standard solution with ultrapure water (18 MΩ∙cm, Aqualab UVOI-"MF"-1812 water purification system, Medi- ana-Filter, Russia). 2.2. Instrumentation The resin particle size distribution was studied by laser dif- fraction (SALD-2300 particle size analyzer with a SALD- BC23 batch cell unit, Shimadzu, Japan). A Spectrum 100 FTIR spectrometer (Perkin Elmer, USA) with a Quest ATR accessory (Specac, UK) was used to record IR spectra in the attenuated total reflection (ATR) mode. The differential thermal analysis (TGA/DSC) was performed with a Mettler Toledo TGA/DSC 3+ Star System at a heating rate of 10 °C/min under nitrogen atmosphere with a flow rate of 50 mL/min. The total specific surface area was determined by thermal desorption of gas-adsorbate (BET) using an an- alyzer of specific surface area of dispersed and porous ma- terials ThermoSorb TPD 1200 (Catakon, Russia). The con- centration of quercetin in solutions was measured by UV- vis spectroscopy (UV-2600, Shimadzu, Japan). 2.3. Synthesis of phloroglucinol-melamine-for- maldehyde resins (PMFR) Molecularly imprinted resins (MIR) were prepared by pol- ycondensation of monomers in an ethanol-water mixture (ethanol:water ratio 5:4, v/v) [24]. For this purpose, phloroglucinol (3 mmol) and formaldehyde (12 mmol) were dissolved in 4.5 ml of ethanol-water mixture at room tem- perature under ultrasound and magnetically stirred for 30 min (solution A). Melamine (1 mmol) and formaldehyde (2 mmol) were dissolved in 1.5 mL of ethanol-water mixture under heating to 80 °C and stirred continuously until com- plete dissolution (solution B). After cooling solution B to room temperature, it was mixed with solution A. Polyeth- ylene glycol (0.025 mmol) as a porogen and quercetin (0.16 mmol) as a template were added to the obtained pre- polymerization mixture. The mixture was stirred under heat (40 °C) for 30 min and incubated in an air thermostat at 60 °C for 2 h, then at 80 °C for 24 h. Quercetin was not added to the non-imprinted resin (NIR) sample. After dry- ing, the PMFR samples were ground for 1 min either in a laboratory ball mill ML-1 (EcON, Russia) or in an agate mortar. To remove the template after synthesis, the samples were eluted with an ethanol:water mixture (9:1, v/v) by Soxhlet extraction (500 mg, 125 mL, 36 h) and then dried at 60 °C. The concentration of quercetin was monitored by spectrophotometry (λ 373.6 nm), and the apparent degree of template removal was calculated as the proportion of quercetin eluted from the PMFR MIR samples relative to the amount added during the MIR synthesis. 2.4. Adsorption experiments The kinetics of quercetin (template) rebinding were studied under static adsorption conditions using 10 μmol/L aqueous solution of quercetin. 50 mg of MIR or NIR sample was placed in the studied quercetin solution (~50 mL), then al- iquots (3.0 mL) of this solution were taken every 5–15 min to determine quercetin by spectrophotometry at 367.6 nm wavelength. To study rebinding isotherms, an adsorption experiment was carried out with 5–70 μmol/L quercetin so- lutions at 27 °C (TS-1/80 SPU dry-air thermostat, Russia): 10 mg of MIR or NIR sample was placed in 10 ml of querce- tin solution and incubated for 1 day with periodic stirring until equilibrium was reached. The adsorption properties of MIR and NIR and the effi- ciency of molecular imprinting were characterized by cal- culating the recovery (R, %), sorption capacity (q, μmol/g) and imprinting factor (IF) as the ratio of the MIR sorption capacity to the NIR sorption capacity at the current time t. https://doi.org/10.15826/chimtech.2023.10.3.10 https://doi.org/10.15826/chimtech.2023.10.3.10 Chimica Techno Acta 2023, vol. 10(3), No. 202310310 ARTICLE 3 of 7 DOI: 10.15826/chimtech.2023.10.3.10 The kinetics of quercetin rebinding by MIR and NIR samples was studied using pseudo-first and pseudo-second order models [6, 21, 26]. The Langmuir and Freundlich models [9, 21, 26–28] were applied to describe the mechanism of the rebinding. Linearized equations were used to test model compliance: the Scatchard equation and the logarithmic form of the Freundlich equation. In the first case, the ad- sorption process was characterized by calculating the effec- tive binding constant Ka and the maximum sorption capac- ity qmax. In the second case, the adsorption coefficient β and the empirical constant n were used as a measure of hetero- geneity of binding sites. 3. Results and Discussion In this work, three samples of quercetin-imprinted phenol- amino-formaldehyde resins (MIR) were obtained by poly- condensation of melamine and phloroglucinol (phloroglu- cinol:melamine ratio 3:1) in the presence of formaldehyde, varying the average molecular mass of polyethylene glycol (porogen) 4000, 6000 and 10000 Da: PMF 3-1-4K, PMF 3- 1-6K and PMF 3-1-10K respectively. Non-imprinted samples (NIR) were obtained in the absence of quercetin. Extraction with an ethanol-water mixture (ethanol:water ratio 4:1, v/v) by the Soxhlet method for 36 h was chosen to remove the template from the obtained MIR samples as a method providing the maximum apparent degree of template re- moval (Table 1). Laser diffraction, FTIR-spectroscopy, and differential thermal analysis were used to characterize the obtained PMFR samples. It was shown (Figure 1) that the median particle diameter of PMF MIR increases with the average molecular mass of the porogen from 21.8 (Mw 4000) to 67.9 μm (Mw 10000). In the FTIR spectra (Figure 2) out-of-plane deformation bands (δoop) of N–H bonds in the region 811–814 cm–1, C–O–C stretching region 1000–1111 cm–1, NH deformation vibrations in aromatic amines 1272–1370 cm–1, stretch vi- brations (ν) of C=C of aromatic ring (1614, 1450 cm–1) and C=N of triazine (1550, 1350 cm–1) and stretching region 3200–3400 cm–1 of O–H and N–H groups were identified. The results of the differential thermal analysis in an in- ert medium (N2) showed that the resins are thermally sta- ble up to 200 °C, and in the temperature range 250–450 °C (Figure 3, TGA and DTA-curves of thermogravimetric and differential thermal analysis, respectively) they are ther- mally decomposed, accompanied by an exothermic effect (Figure 3, DSC-curve of differential scanning calorimetry). The recovery and sorption capacity of PMFR during the rebinding of quercetin increase with increasing the average molecular mass of polyethylene glycol (Table 2). Thus, the maximum recovery and sorption capacity of PMF 3-1-10K MIR synthesized in the presence of a porogen with an aver- age molecular mass of 10000 Da reached ~43% and 4.71 μmol/g of quercetin, respectively. The maximum IF was 1.41 (in 90 min). The MIR obtained in this work are stable after removal of the template in a Soxhlet apparatus using various elution solvents (Table 2) and subsequent drying at 60 °C. Using 3- 1-6K MIR as an example, it was shown that they can be re- used after 2–3 elutions with ethanol-water mixture (2–3 cy- cles). In this case, the decrease in the sorption capacity dur- ing the rebinding of quercetin was no more than 8%. The kinetics of quercetin rebinding under static sorption conditions are better described by the pseudo-second-order model for most MIR samples (Table 3, Figure 4a), while for PMF 3-1-10K MIR a mixed mechanism was observed (Table 4, Figure 4b). It should be noted that kinetic models of re- binding for the samples with the highest Mw of the porogen (PMF 3-1-10K MIR) confirmed (Tables 3, 4) the efficiency of molecular imprinting, as qe(MIR) > qe(NIR). Table 1 Optimization of elution solvent. PMFR Eluent K (%)a PMF 3-1-6K EtOH:H2О (4:1, v/v) 22 МеOH:H2О (4:1, v/v) 18 EtOH:HАcb (9:1, v/v) 17 МеOH:HАc (9:1, v/v) 16 ACN:H2O (1:1, v/v) 10 a Apparent degree of template removal; b HAc – acetic acid. Figure 1 Median diameter (D50) of PMFR (white bars – NIR, gray bars – MIR). Figure 2 ATR FTIR-spectra of PMFR: PMF 3-1-4K MIR (1) and PMF 3-1-4K NIR (2), PMF 3-1-6K MIR (3) and PMF 3-1-6K NIR (4), PMF 3-1-10K MIR (5) и PMF 3-1-10K NIR (6). https://doi.org/10.15826/chimtech.2023.10.3.10 https://doi.org/10.15826/chimtech.2023.10.3.10 Chimica Techno Acta 2023, vol. 10(3), No. 202310310 ARTICLE 4 of 7 DOI: 10.15826/chimtech.2023.10.3.10 Figure 3 TGA (black), DSC (red) and DTA (blue) curves of PMF 3- 1-10K MIR (N2, 10 °C/min). The isotherms of quercetin rebinding by PMFR samples are better described by the Freundlich model (Table 5), which confirms the heterogeneity of the surface and differ- ent types of binding centers (the empirical coefficient n is 0.5–0.6). At the same time, the empirical coefficient β, which characterizes the adsorption of quercetin, is 1.5– 3.0 times higher for MIR samples than for NIR samples. This can be explained by the formation of molecular imprints that are complementary to the template molecules. The compli- ance with the Freundlich model correlates well with the re- sults of the specific surface area obtained by BET (Table 6). The relevant task of this work is to apply the advantages of the prepared MIR for selective adsorption of target bio- molecules from biological matrix (plant extract). In this experiment, it takes quercetin (template) and rutin as the target flavonols, their diluted solutions as a sample matrix to simulate the selective adsorption of flavonols from the biological matrix (ethanol:water extract). The effect of con- centration was studied by adding quercetin or rutin in the range of 7.5∙10–6–7.0∙10–5 mol/L (Table 7). No significant change in the imprinting factor of quercetine was observed as the concentration further increased from 7.5∙10–6 to 7.0∙10–5 mol/L. So, it can be concluded that MIR can selec- tively adsorb target flavonol quercetin in diluted etha- nol:water extract without the matrix interference of the bi- ological matrix. It was noted (Table 7) that PMF 3-1-4K MIR selectively adsorbs quercetin, but PMF 3-1-10K MIR adsorbs rutin more selectively than quercetin as a template. Thus, quercetin can be considered as a dummy template for the selective recovery of rutin. The results proved the specific selectivity of obtained PMF MIR, and are expected to be fur- ther applied to the imprinting and selective recognition of more flavonoids so as to play an important role in plant analysis. 4. Limitations In this work, samples of quercetin-imprinted phloroglu- cinol-melamine-formaldehyde resins with an insufficiently high recovery (~43%) were obtained, which is probably due to the small pore size and low permeability. Therefore, the following studies will be aimed at increasing the sorption capacity of MIR samples by optimizing the amount of poro- gen. At the same time, it is important to control the effi- ciency of molecular imprinting (IF at least 1.5). Table 2 Sorption parameters of PMFR (50 mL of 10 μmol∙L–1 quercetin solution, 50 mg of PMFR). PMFR Rmax (%) qmax (μmol/g) IFmax MIR NIR MIR NIR PMF 3-1-4K 21.6 21.7 2.50 2.58 1.13a PMF 3-1-6K 41.4 48.1 4.63 5.42 1.13b PMF 3-1-10K 42.6 41.1 4.71 4.69 1.41c a 15 min; b 35 min; c 90 min. Table 3 Pseudo-second-order kinetics parameters of PMFR. PMFR q24h (μmol/g) k2 (g∙μmol –1∙min–1) qe (μmol/g) R 2 PMF 3-1-4K MIR 2.50 7.9∙104 1.47 0.9290 NIR 2.58 4.2∙105 1.60 0.9616 PMF 3-1-6K MIR 4.63 2.2∙104 1.92 0.8664 NIR 5.42 6.3∙103 3.87 0.8167 PMF 3-1-10K MIR 4.71 4.1∙103 3.15 0.7729 NIR 4.69 2.2∙104 1.72 0.8604 Table 4 Pseudo-first-order kinetics parameters of PMFR. PMFR q24h (μmol/g) k1 (min –1) qe (μmol/g) R 2 PMF 3-1-6K MIR 4.63 3.2∙10–3 4.17 0.9406 NIR 5.42 6.7∙10–3 4.86 0.5760 PMF 3-1-10K MIR 4.71 5.5∙10–3 4.62 0.9432 NIR 4.69 2.5∙10–3 4.16 0.7472 https://doi.org/10.15826/chimtech.2023.10.3.10 https://doi.org/10.15826/chimtech.2023.10.3.10 Chimica Techno Acta 2023, vol. 10(3), No. 202310310 ARTICLE 5 of 7 DOI: 10.15826/chimtech.2023.10.3.10 5. Conclusions Therefore, the molecular imprinting methodology allowed obtaining a hydrophilic phloroglucinol-melamine-formal- dehyde resin for the sorption concentration of quercetin. It was shown that in the presence of polyethylene glycol (porogen) with an average molecular mass of 10000 Da, the sorption capacity of quercetin by the molecularly-imprinted sample was ~5 µmol/g, the specific surface area was 288.1 m2/g, and the imprinting factor was 1.41. It was shown that recovery of quercetin (21–43%) is slightly lower, but the imprinting factor (1.4) is not inferior to that of the similar methods for extracting phenolic compounds [26] and dyes [29] with resorcinol-melamine-formaldehyde and phenol-formaldehyde resins, as well as quercetin with surface-imprinted polymers [30, 31]. Modeling of the kinet- ics and isotherms of quercetin rebinding by the samples, obtained in the presence of polyethylene glycol of different average molecular mass, showed compliance with the pseudo-second-order model and the Freundlich model de- scribing an inhomogeneous surface. Figure 4 Kinetic adsorption curves of PMF 3-1-6K MIR (a) and PMF 3-1-10K MIR (b) for quercetin rebinding: experimental curve (1), pseudo-first-order model (2), pseudo-second-order model (3). Table 5 Modeling of quercetin sorption isotherms by Freundlich. PMFR β n R2 PMF 3-1-6K MIR 4.5∙10–3 0.5 0.9462 NIR 2.7∙10–3 0.5 0.9759 PMF 3-1-10K MIR 1.1∙10–2 0.6 0.9179 NIR 3.7∙10–3 0.5 0.9008 Table 6 Surface characteristics of the PMFR. PMFR SBET (m 2/g) V (cm3/g) Pore size (nm) MIR NIR MIR NIR MIR NIR PMF 3-1-6K 288.1 285.9 0.17 0.15 2.44 2.08 Table 7 Effect of the сoncentration of quercetin (Qu) and rutin (Rut) at the range of 7.5∙10–6–7.0∙10–5 mol/L on the imprinting factor (IF). MIR IF (n = 8, P = 0.95a) Qu Rut PMF 3-1-4K 1.160.06 0.780.09 PMF 3-1-6K 1.180.08 1.200.04 PMF 3-1-10K 1.420.05 1.680.05 a confidence interval ● Supplementary materials No supplementary materials are available. ● Funding The work was carried out within the framework of the state task of the ISSCM SB RAS (project №121032500065-5). ● Acknowledgments The authors are grateful to D.A. Lazarev, an expert of the Center for Collective Use of Surgut State University, and D.R. Mukhutdinov, M.M. Gasanova, and U.V. Novokshanova for assistance in conducting the experiments. ● Author contributions Conceptualization: Yu.Yu.P. Formal Analysis: E.V.B., Yu.G.M. Funding acquisition: Yu.G.M., Yu.Yu.P. Investigation: E.V.B., D.O.Z., Yu.G.M. Methodology: Yu.Yu.P., E.V.B., Yu.G.M. Project administration: Yu.Yu.P. Resources: Yu.Yu.P., Yu.G.M. Supervision: Yu.Yu.P. Validation: E.V.B., Yu.Yu.P. Visualization: E.V.B., D.O.Z. Writing – original draft: Yu.Yu.P., E.V.B. Writing – review & editing: D.O.Z., Yu.Yu.P. ● Conflict of interest The authors declare no conflict of interest. ● Additional information Author IDs: Yuliya Yu. Petrova, Scopus ID 6603754153; Elena V. Bulatova, Scopus ID 57193926543; Yuliya G. Mateyshina, Scopus ID 6506782050. Websites: Surgut State University, https://int.surgu.ru/; https://doi.org/10.15826/chimtech.2023.10.3.10 https://doi.org/10.15826/chimtech.2023.10.3.10 https://www.scopus.com/authid/detail.uri?authorId=6603754153 https://www.scopus.com/authid/detail.uri?authorId=57193926543 https://www.scopus.com/authid/detail.uri?authorId=6506782050 https://int.surgu.ru/ Chimica Techno Acta 2023, vol. 10(3), No. 202310310 ARTICLE 6 of 7 DOI: 10.15826/chimtech.2023.10.3.10 Institute of Solid State Chemistry and Mechanochemis- try, Siberian Branch of the Russian Academy of Sciences, http://www.solid.nsc.ru/en/. References 1. Lu Y, Li P, Yang C, Han Y, Yan H. 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