Preparation of Molecularly Imprinted Poly (methacrylic acid) Grafted

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Preparation of Molecularly Imprinted Poly(methacrylic acid) Grafted on Iniferter-Modified Multiwalled Carbon Nanotubes by LivingRadical Polymerization for 17β-Estradiol Extraction Maiyara C. Prete,† Dayana M. Dos Santos,† Luciane Effting,† and César R. T. Tarley*,†,‡ †

Chemistry Department, State University of Londrina (UEL), 86051-990 Londrina, PR Brazil Chemistry Institute, Bioanalytical National Institute of Science and Technology (INCT), State University of Campinas (UNICAMP) 13083-970 Campinas, SP Brazil

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 04/01/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: The synthesis of a nanocomposite based on molecularly imprinted poly(methacrylic acid) (MIP) grafted on an iniferter-modified carbon nanotube (MIP/MWCNT) by living-radical polymerization and its application for 17β-estradiol (E2) adsorption is described. The nonimprinted nanocomposite (NIP/MWCNT) and the polymers MIP and NIP synthesized in the absence of carbon nanotubes were also prepared and characterized by FT-IR, TGA, nitrogen adsorption−desorption measurements, and SEM and TEM techniques. From TEM images, a narrow layer of MIP at nanoscale onto MWCNT surface was formed, as expected by iniferter-controlled synthesis. The adsorption capacity of MIP/MWCNT toward E2 was higher than MIP. In addition, relative selectivity coefficients (k′) higher than one unit were obtained from competitive adsorption studies in the presence of estrone, 17α-ethynylestradiol, and bisphenol A, thereby demonstrating that MIP/MWCNT is more selective toward E2 when compared to NIP/MWCNT. Equilibrium adsorption of E2 was reached at 120 min and the maximum adsorption capacity of the nanocomposite was found to be 40.0 mg g−1, showing higher or similar performance when compared with others adsorbent materials described on the literature for E2. Thermodynamic parameters suggest that the adsorption process is spontaneous and of exothermic nature. The performance of MIP/MWCNT as a new packing adsorbent material was evaluated for E2 extraction in solid-phase extraction (SPE) procedure, which was performed by loading the sample at pH 7.0 through 50.0 mg of MIP/MWCNT packed into SPE cartridge. Taking into account the high elution value of 96 ± 3.6% obtained by using acetone:chloroform (1:1 v/v), as well as the selectivity of adsorbent, we can infer that MIP/MWCN/T nanocomposite shows interesting analytical potentiality for E2 preconcentration from natural water samples. are the most widely applied for E2 determination.7 However, their sensitivity is usually insufficient for direct determination of these contaminants at a very low concentration level in complex matrix environmental samples.8 Therefore, a sample pretreatment prior to chromatographic analysis is usually necessary. Solid-phase extraction is the most common technique for environmental water sample pretreatment due to its advantages including high recovery, short extraction time, high enrichment factor, low cost, low consumption of organic solvents and a wide variety of available adsorbents.9 Many materials have been used as adsorbents for pollutant removal in the field of water remediation, such as commercial activated carbons, natural materials, agricultural wastes,

1. INTRODUCTION Endocrine disruptor compounds (EDCs) are emerging contaminants that can affect the operation or function of the endocrine system by mimicking or blocking the natural hormonal system causing overproduction or underproduction of hormones.1 EDCs are continually introduced into the aquatic environment through different anthropogenic sources, which can result in toxic and adverse effects on aquatic organisms and consequently on humans.2 They are classified into several categories, including hormones (natural and synthetic estrogen or steroids).3 17β-Estradiol (E2), a natural female estrogen, has been widely detected in the aquatic environment as well as in drinking water.4−6 Therefore, the monitoring of E2 in environmental samples is extremely important for controlling the exposure of this compound and evaluating its effect on human and animal health. Chromatographic techniques such as gas chromatography (GC) and high-performance liquid chromatography (HPLC) © XXXX American Chemical Society

Special Issue: Latin America Received: October 31, 2018 Accepted: March 14, 2019

A

DOI: 10.1021/acs.jced.8b01010 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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the same time and retain functionality.39 The term “living radical polymerization” was first used in a paper by Otsu et al. in 1982, which described the mechanism of an initiator called “iniferter”.40 An iniferter is a radical initiator which simultaneously performs the initiation, chain-transfer, and termination in the polymerization process, which gives rise to its name.41 It decomposes into two radicals under UV irradiation. The reactive radical is immobilized on a support surface, which can initiate the polymerization. The other nonreactive radical remains in solution and can only carry out termination and chain-transfer reactions.42 Despite grafting-to, polymerization is considered the most common approach adopted for grafting process of MIP onto CNTs surface; by using the grafting-from method, the polymerization in the solution can be remarkably reduced, thus forming a narrow polymer film at nanoscale level onto CNTs surface.32,37 Furthermore, considering that the binding sites are mainly created on the surface of MIPs, polymers synthesized by surface imprinting methods have a significantly lower amount of nonspecific binding sites, thus providing high adsorption and low response time.32,43 The immobilization of MIP on CNTs via living-radical polymerization induced by an iniferter was first reported by Lee and Kim (2009).36 In the last few years, only a few works have explored this synthesis approach37,44 and to the best of our knowledge no study for E2 extraction has been reported before. Herein, this work aims to report the synthesis of a molecularly imprinted poly(methacrylic acid) grafted on MWCNT surface by living-radical polymerization using sodium diethylcarbamate (DDTC) as a photoactive iniferter and its potentialities as an adsorbent for E2 extraction in an aqueous medium.

inorganic materials, ion-exchange resins, and industrial byproducts.10 However, to enhance the adsorption properties, stability, and selectivity, the design and synthesis of novel adsorbents materials have drawn more attention.11−15 Nanocomposites based on carbon materials have been widely explored recently in the literature as adsorbents for a broad variety of pollutants.16−18 Carbon nanotubes (CNT) have attracted great attention due to their unique properties such as chemical stability, high electrical conductivity, mechanical strength, large specific surface area, and high thermal stability.19 They can be divided according to the number of graphite sheets rolled into a seamless cylinder into single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs).20 CNTs have strong interactions with other molecules due to their hexagonal arrays of carbon atoms on their surface. Therefore, CNTs show great potential as a solid-phase extraction adsorbent.21 The use of CNTs as a sorbent for organic compound removal and SPE applications have been widely reported.8,22−28 However, the main disadvantage of CNTs is the lack of selectivity toward the target analyte. One way to overcome this drawback is to combine the properties of CNTs with molecularly imprinted polymers (MIPs) by exploiting the surface imprinting techniques. MIPs are synthetic polymers containing specific recognizing sites of a target molecule, interacting with them in a selective way.29 These specific sites are formed by copolymerization process involving cross-linkers and functional monomers and an imprinted molecule based usually on noncovalent bonds.30 After polymerization, the template molecule is removed and the specific site is available for the association of the target molecule.29,31 Therefore, MIPs have been widely used as a very efficient material in SPE procedure due to their intrinsic selectivity, mainly in complex samples as environmental matrices, where contaminants are present in a mixture.30 Owing to its simplicity, one of the most common procedures for MIPs synthesis is the bulk polymerization, but this procedure suffers from obtaining irregular particles in size and a reduced number of selective binding sites. Thus, the combination of CNTs and MIPs in one single material makes possible to obtain imprinted nanocomposite with the high surface area, high imprinted sites/volume ratio due to the nanoscale and the quick mass transfer of analyte toward the selective binding sites, unlike the MIP synthesized by the bulk method.32 MIPs can be immobilized on CNT surface by means of two main strategies called “grafting-to” and “grafting-from”.32 Grafting-to strategy involves the attachment of vinyl or acrylate groups via either physisorption or covalent bond formation on the support surface. These groups are polymerized by free radical polymerization in the presence of thermal or photoinitiators.33−35 However, as the radical is formed in the bulk of the solution, the polymerization reaction might not only occur at the surface of CNTs, resulting in a nonhomogenous material.36 On the other hand, grafting-from approach consists of covalently attaching initiators called iniferter (initiationtransfer-termination) on the support functionalized surface and then grafting polymers in situ by living/controlled radical polymerization.32,37,38 This method relies on pure polymerization reactions which occurs by an initiator molecule and stops only when there are no more monomers available, being free of terminations by impurities and secondary transfers reactions. Also, the polymer chains begin to grow essentially at

2. EXPERIMENTAL SECTION 2.1. Reagents and Solutions. For nanocomposite and polymer synthesis, the following reagents were used: H2SO4 (Sigma-Aldrich, 95−95%), HNO3 (Sigma-Aldrich, > 90%), MWCNTs, supplied by CNTs Co. Ltd. Yeonsu-Gu, Incheon, Korea 93% 10−40 nm diameter and length of 5−20 mm, methacrylic acid (MAA, Acros Organics, ≥99.5%), trimethylolpropane trimethacrylate (TRIM, Sigma-Aldrich), toluene (HPLC grade, Sigma-Aldrich, 99.9%), sodium diethylcarbamate (DDTC, Merk, ≥99.9%), ethanol (Sigma-Aldrich, 99.9%), 3-chloropropyltrimethoxysilane (CPTMS, Sigma-Aldrich, 97.0%), and 2,20- azobis(isobutyronitrile) (AIBN, Sigma-Aldrich, 98%). Analytical Sigma-Aldrich grade chloroform (CHCl3, ≥99.8%) and HPLC-grade methanol (CH3OH, ≥99.9%) were purchased from Vetec (Rio de Janeiro, Brazil). HPLC-grade acetonitrile (ACN, ≥99.9%) was obtained from J.T. Baker (Philipsburg, NJ, U.S.A.). Acetic acid (HAc, ≥99.8%) was obtained from Sigma-Aldrich. Dichloromethane (CH2Cl2, ≥99,8%) was obtained from Biotec (São Paulo, Brazil). Estrogens with high purity (>99%), estrone (E1), 17βestradiol (E2) and 17α-ethinylestradiol (EE2), and bisphenol A (BPA) were purchased from Sigma-Aldrich. Stock solutions were prepared in methanol and stored at −20 °C. The humic acid powder was acquired from Sigma-Aldrich and it was dissolved in a 1.0 mol L−1 KOH solution. All tested solutions were prepared using ultrapure water obtained from an ELGA PURELAB system (High Wycombe, Bucks, U.K..) and filtered through a 0.22 mm poly(tetrafluoroethylene) (PTFE) membrane previously to chromatographic injection. All B



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Scheme 1. Schematic Procedure for the Synthesis of MIP/MWCNT Nanocomposite by Living-Radical Polymerization Induced by a Photoactive Iniferter Radical

glassware was washed with diluted Extran purchased from Merk and rinsed with ultrapure water and acetone to prevent contamination. 2.2. Equipment. Estrogens and BPA determination was carried out using a high-performance liquid chromatograph and a diode array detector (Shimadzu, Tokyo, Japan LC20AD/T) with diode array detector (DAD, Shimadzu, Japan) operating at λmax285 nm with a C18 column (Phenomenex, 250 mm × 4.5 mm, particle size 5 μm) and a guard column (Phenomenex, 4.0 mm × 30 mm i.d., 5 μm in particle size) in gradient mode using acetonitrile (solvent A) and Milli-Q water (solvent B) as follow: 35% of A until 3 min; from 4 to 7 min 55% of A; 55% of A until 20 min. Morphologic characteristics of the nanocomposites were evaluated by field emission gun scanning electron microscope (FEG-SEM) by Tescan model MIRA 3. Before the analysis, the samples were placed in an aluminum sample port fixed by a carbon tape and metalized

with gold. Analyses by transmission electron microscope (TEM) were performed using a JEOL JEM-1400 microscope at an accelerating voltage of 80 kV (Tokyo, Japan). The samples were dispersed in propanol under sonication for 20 min, followed by deposition of slurry onto a copper grid and dried under vacuum. Analysis of surface area, pore diameter, and size were performed using a Quantachrome Nova1 1200e (Odelzhausen, Germany) equipment in which the material sample was heated to 180 °C for 4 h under vacuum and submitted to nitrogen adsorption. The surface area of the materials was obtained by multipoint BET (Brunauer− Emmett−Teller) method while its average pore diameter and size were determined by BJH (Barrett−Joyner−Halenda) method. Functional groups of material were determined by infrared spectrometry performed at 4000−400 cm−1 region using a Fourier transform infrared spectrophotometer model 8300 Shimadzu (Tokyo, Japan) using KBr pellets conventional C



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and final concentrations of compounds (mg L−1), V is the solution volume (mL), and m is the nanocomposite (MIP/ MWCNT or NIP/MWCNT) mass (mg). K d(E2) and Kd(interfering) are the distribution coefficients of E2 and concomitant compound, respectively. kMIP and kNIP are selectivity coefficients of MIP/MWCNT and NIP/MWCNT, respectively45

method. Thermogravimetric analysis (TGA) was performed using a TGA 4000 Perkin Elmer1 (Massachusetts, U.S.A.) equipment by heating 10 mg of each sample from 30 to 900 °C at a rate of 10 °C min−1 under nitrogen atmosphere. Thermodynamics studies were performed using a shaker with a thermostatic bath Marconi MA 093/1 with temperatures varying from 25 to 45 °C. A Metrohm1 pH 827 lab digital pHmeter (Herisau, Switzerland) was used for pH measurements. 2.3. Grafting of Photoactive Iniferter on MWCNT. First, pristine MWCNT was oxidized by acid treatment according to literature data.34 In a round-bottom flask, 600 mg of MWCNT was mixed with 80.0 mL of HNO3/H2SO4 (3:1 v/v) for 2 h at 65 °C. The obtained material (MWCNTox) was washed with water until pH ≅ 7.0 to remove acid excess and oven-dried at 35 °C. After this procedure, CPTMS molecules were immobilized on MWCNTox surface. For this task, 700 mg of CPTMS dissolved in 10.0 mL of toluene was mixed with 500 mg of MWCNTox for 4 h under heating at 80 °C. MWCNToxCPTMS was then washed with ethanol and dried in an oven at 35 °C. After dried, 400 mg of MWCNTox-CPTMS was functionalized with the iniferter photoactive radical by mixing it with 700 mg of DDTC dissolved in 10.0 mL of ethanol at 50 °C for 24 h. The final material (functionalized MWCNToxi) was then washed and dried at the same conditions mentioned before. 2.4. Graft Polymerization of the MIP on the Surface of Functionalized MWCNToxi. MIP synthesis involving iniferter grafting at functionalized MWCNToxi was carried out in according to previous studies with some modifications.36 In a round-bottom flask, 300 mg of functionalized MWCNToxi were dispersed in 50.0 mL of toluene. Next, 34.8 mmol of MAA, 11.7 mmol of TRIM, and 7.0 mmol of E2 as a template were added to the dispersion. Polymerization was carried by UV light exposure (365 nm) in a UV chamber composed of 4 Hg lamps (8 w each) under stirring for 3 h. The nonimprinted polymer (NIP-MWCNT) was prepared following the same procedure, except by the addition of the E2 molecule as a template. MIP without MWCNT grafting was also prepared. For this task, 34.8 mmol of MAA in 50.0 mL toluene, 11.7 mmol of TRIM, 7.0 mmol of E2, and 100 mg of AIBN was mixed in a round-bottom flask. The mixture was bubbled with N2 gas for 10 min, the flask was sealed, and kept in an oil bath at 60 °C for 24h. NIP was also synthesized as described above in the absence of a template molecule. Materials synthesized were washed using MeOH/HAc solution (4:1 v/v) for template and reagent excess removal. Schematic procedure for the grafting of MIP on functionalized MWCNTox is depicted in Scheme 1. 2.5. Selectivity Studies. The selectivity of MIP/MWCNT toward E2 was evaluated by means of competitive adsorption using binary solutions of E2/interfering molecules (1:1 w/w). Estrone (E1) and 17α-Ethynylestradiol (EE2) and the industrial compound bisphenol A (BPA) were evaluated as potentially interfering compounds. The adsorption experiments were carried out by stirring 10.0 mg of MIP/MWCNT 20.0 mL of 20 mg L−1 binary solutions at pH 7.0 (10% MeOH) during 120 min. Afterward, the supernatant was filtered through a PTFE hydrophilic filter (0.22 μm) membrane and monitored by HPLC-DAD. The distribution (Kd), selectivity (k), and the relative selectivity coefficients (k′) were determined by the eqs 1−3, where Ci and Cl are the initial

ij C − Cf yz V (mL) zz × Kd = jjj i zz j C m (g) f k {

k=

(1)

kd(E2) kd(interfering)

k′ =

(2)

kMIP kNIP

(3)

2.6. Adsorption Kinetics. The effect of the contact time on E2 adsorption onto MIP/MWCNT nanocomposite was evaluated by means of batch experiments. For this task, 10 mg of the nanocomposite was mixed with 20.0 mL of a 20 mg L−1 E2 solution at pH 7.0 (10% v/v MeOH) for times ranging from 0.5 to 250.0 min. After stirring time, the supernatant was filtered through a PTFE hydrophilic filter (0.22 μm) membrane and monitored by HPLC-DAD. The amount of E2 adsorbed in the material (mg g−1) was calculated according to eq 4 Qt =

(Ci − Cf ) × V (L) m (g )

(4) −1

where Qt is the amount of BPA (mg g ) adsorbed in the polymer at determined time (min); Ci and Cf are the initial and final (supernatant) concentrations of E2, respectively (mg L−1); V is the volume of solution (L); and m is the mass of the nanocomposite (g). Adsorption mechanism and rate-controlling can be better understood by applying kinetics models such as pseudo-first and pseudo-second-order, Elovich and intraparticle diffusion. Pseudo-first order model predicts that the active sites of the sorbent material have equal energies and that the adsorption rate of the analyte is proportional to the number of unoccupied sites.46 On the other hand, the pseudo-second-order model assumes that the active sites of the adsorbent material have heterogeneous energies and the rate of adsorption is proportional to the square of the number of unoccupied sites.46 These models are described by eqs 5 and 6, respectively, where Qt refers to the amount of analyte adsorbed in time t and Qe at the equilibrium time (mg g−1), k1 is the velocity constant (min−1) and k2 is the rate constant (g mg−1 min−1). Q t = Q e(1 − e k1t ) Qt =

(5)

Q e 2k 2t [k 2(Q e)t + 1]

(6)

Elovich model is complementary to the pseudo-secondorder model, therefore it also predicts the existence of active sites in the sorbent with different energies.47 It is described by eq 7, where α is the initial adsorption rate (mg g−1 min−1) and β is the constant related to the coverage surface extension and the activation energy (g mg−1) D


Journal of Chemical & Engineering Data Qt =

1 1 ln(αβ) + n t β β

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Langmuir−Freundlich model, also known as the Sips model, originated from the combination of the Langmuir and Freundlich models. This model is reduced to the Freundlich model when a lower concentration of adsorbate is employed in building isotherm, while behaves like the Langmuir model when a higher concentration of adsorbate is used.51 Single site Langmuir−Freundlich model assumes the presence of sites with homogeneous energies. However, despite being very useful this model does not consider the availability of twoadsorption sites with different affinities toward adsorbate.54 Thus, the dual site Langmuir−Freundlich model is more complete due to the possibility to describe adsorption binding interactions considering adsorption sites with different affinities and due to more fitting parameters used.55 At low concentrations, the sorbate interacts with the site of greatest affinity and, in high concentrations, interacts with the sites of lower affinity. Single and dual Langmuir−Freundlich model are described in eqs 11 and 12, respectively, where K1,2 is the equilibrium constant (L mg−1) representing sorbent−sorbate affinity56

(7)

Intraparticle diffusion model is described by eq 8, where Qt is the amount of solute on the surface of the sorbent in time t (mg g−1), kid is the constant intraparticle diffusion rate (mg g−1 min1/2), t is the time (min) and C is the line interception, which is related to the boundary layer thickness (mg g−1).48 This model evaluates the influence of diffusion phenomenon in the adsorption process and generally shows multilinearity, indicating three processes. The first step is associated with the adsorption at a site on the surface (internal or external) of the adsorbent an is often assumed to be extremely rapid. The second step is the diffusion itself of the adsorbate molecules to an adsorption site either by a pore diffusion process through the liquid-filled pores or by a solid surface diffusion mechanism. The last step is when adsorption equilibrium is reache.49,50 Q t = k id t1/2 + C

(8)

2.7. Adsorption Isotherm. The construction of adsorption isotherms, apart from making possible to obtain insight into the presence of binding sites with different energies, gives information on maximum adsorption capacity of the adsorbent. For this task, batch experiments were carried out by mixing 10 mg of the MIP/MWCNT nanocomposite with 20.0 mL of E2 solutions at pH 7.0 in concentrations ranging from 9.0 to 250 mg L−1 (10% MeOH) for 120 min. After this time, the mixture was centrifuged for 5 min at 5000 rpm, then the supernatant was filtered through a PTFE hydrophilic membrane (0.22 μm) and injected in the chromatographic system. The amount of E2 adsorbed in the material (mg g−1) was calculated according to eq 4. In order to achieve a better understanding of E2 adsorption mechanism in the nanocomposite and obtain information about its binding sites energies, nonlinear isotherm models including Langmuir, Freundlich, Langmuir−Freundlich single and dual site were applied to the data. Langmuir model predicts homogeneous monolayer adsorption. In its formulation, the adsorbed layer is one molecule in thickness and the adsorption can only occur at a finite number of identical and equivalent active sites.51 On the other hand, the Freundlich model assumes that as the analyte concentration increases, the amount of analyte adsorbed on the surface of the sorbent also increases, thus occurring an infinite amount of adsorption theoretically. This model also assumes that adsorption could occur via multiple layers rather than a monolayer.52 These models are represented by eqs 9 and 10, respectively, where Qe is the amount adsorbed (mg g−1); Ceq is the concentration of the sorbate in the solution at equilibrium (mg L−1); b is the maximum adsorption capacity (mg g−1); KL refers to the equilibrium constant of Langmuir (L mg−1), KF refers to the equilibrium constant of Freundlich (mg g−1) (L g −1 ) and n is the empirical constant of Freundlich (dimensionless), referring to the heterogeneity of adsorption sites53 Qe =

Qe =

Qe =

1/ n Q e = KF × Ceq

1 + (K1 × Ceq)n b1 × (K1 × Ceq)n1 1 + (K1 × Ceq)n1

(11)

+

b2 × (K 2 × Ceq)n2 1 + (K 2 × Ceq)n2

(12)

2.8. Thermodynamic Studies. In order to study the effect of temperature on E2 adsorption onto the nanocomposite, isotherms at different temperatures (25, 30, 35, and 40 °C) were performed at the same conditions as the adsorption isotherm described in Section 2.7. Thermodynamics parameters such as enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) regarding the adsorption of E2 onto the MIP/ MWCNT nanocomposite were investigated from 25.0 mg L−1 E2 solutions. The parameters ΔH and ΔS were calculated from the slope and intercept, respectively, of Van’t Hoff plot of ln Kd versus 1/T, according to eq 13 ln Kd = −

ΔH 1 ΔS × × R T R

(13) −1

where distribution coefficient (Kd) (L g ) is defined as the ratio between the amount adsorbed (Qe) (mg g−1), and the supernatant concentration (Ceq) (mg L−1), R represents the universal gas constant (R = 8.314 J mol−1 K−1) and T is the temperature in Kelvin.57,58 Gibbs free energy of adsorption process was then determined using eq 14. ΔG = ΔH − T ΔS

(14)

2.9. Optimization of Elution Solvent and pH in SPE Procedure. In order to obtain the better elution solvent for E2 desorption, 50.0 mg of the MIP/MWCNT nanocomposite was packed in an empty polyethylene syringe cartridge (3 mL capacity) containing between two polyethylene frits. On the bottom of the cartridge, glass wool was also placed to avoid losses of adsorbent during sample loading. A vacuum manifold was used to facilitate the extraction experiment. The cartridge was first conditioned with 6.0 mL of acetone, 6.0 mL of methanol, and 6.0 mL of water. Then, 8.0 mL of a 500 μg L−1 E2 solution (pH 7.0) were percolated in the cartridge at a flow rate of 1.0 mL min−1. It was verified that no E2 was detected on cartridge effluent. After that, aliquots of 8.0 mL of solvents with different polarities and compositions (MeOH/HAc;

KL × b × Ceq (1 + KL × Ceq)

b × (K1 × Ceq)n

(9) (10) E



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MAA. The band in the region of 2990−2940 cm−1 is attributed to symmetric and asymmetric C−H stretching vibrations from CH2 and CH3 from the cross-linking reagent TRIM.55 A very well-defined band at 1730 cm−1 is assigned to stretching vibration of CO from MAA.61 The shoulder at 1636 cm−1 can be attributed to CC stretching from unreacted vinyl groups or overlap by OH deformation (water).62 The signals at 1260 and 1150 cm−1 can be attributed to the stretching of C− O from the ester group and carboxylic acid. The signals at 1470 and 1390 cm−1 can be ascribed, respectively, to C−H from methylene and methyl groups.54 One should note that the spectral similarity of nanocomposites and MWCNT indicates that only a narrow layer of MIP was synthesized on the MWCNT surface as will be further demonstrated by TEM images, justifying the low detectability of the signals by FT-IR. Although the nanocomposites present low amount of binding sites with regard the polymers, the grafting of MIP on iniferterMWCNT gives rise to an adsorbent material with high imprinted sites/volume ratio and low amount of nonspecific binding sites compared to the ones synthesized by the free radical polymerization method. Thermogravimetric curves and their derivative curves are shown in Figure 2a and Figure 2b, respectively, which was very useful to confirm the presence of MIP synthesized at MWCNT surface. As observed, pristine MWCNT has high thermal stability and no significant mass loss event. On the other hand, MIP/MWCNT presented a mass loss of approximately 10% at a maximum temperature of 515 °C, which refers to the decomposition of the MAA/TRIM polymeric chain.55 Therefore, it can be confirmed the presence of approximately ∼10% of MIP grafted at MWCNT surface. Textural properties of pristine MWCNT, MIP/MWCNT and MIP including surface area, pore volume and diameter are shown in Table 1. As observed, pristine MWCNT shows higher surface area, and after the grafting of the polymer only a slight decrease was observed. Such an outcome is somewhat expected considering the nature of the grafting process grafting-from herein used in which the polymerization in the solution is avoided and preserves the morphological features of the nanostructure substrate. Previous studies reported by our research group have pointed out a significant decrease in the surface area of nanocomposite when grafting to approach has been used.63 The higher surface area of MIP/MWCNT compared to NIP/MWCNT can be attributed to the presence of a template molecule in the synthesis. In this case, most likely the solubility of template-monomer is lower in the porogenic solvent, which makes the removal of solvent easier and then giving rise to an adsorbent with a higher surface area. The polymers MIP and NIP synthesized by bulk method showed very low surface area, which suggests that binding sites are lesser available toward E2. Furthermore, all materials were considered mesoporous, having pores with diameters between 2 and 50 nm, according to the IUPAC definition.64 SEM images for MWCNT, MIP/MWCNT, and MIP are shown in Figure 3. It can be observed that the nanocomposite filaments (Figure 3(b)) have similar morphological features of MWCNT (Figure 3(a)). Once again, such outcomes corroborate with FT-IR and textural data, which only a narrow layer of MIP was synthesized at MWCNT surface. The morphological features of MIP (Figure 3c) and MIP/ MWCNT are too much different each other. MIP present agglomerate of particles, which is not observed in the nanocomposite images. Such feature is clearly observed from

chloroform; methanol; acetone; acetonitrile, dichloromethane; acetone:chloroform; and acetone:dichloromethane) were evaluated for E2 desorption. When necessary, the solvent percolated was evaporated under N2 and reconstituted with 8.0 mL of the mobile phase. Before chromatographic analysis, extracts were filtered using a PTFE hydrophilic membrane (0.22 μm). After established the best elution solvent, the influence of pH on E2 adsorption was also evaluated.

3. RESULTS AND DISCUSSION 3.1. Characterization. The FT-IR spectra of materials are depicted in Figure 1. For the pristine MWCNT, MIP/

Figure 1. FT-IR spectra of (a) pristine MWCNT, MIP/MWCNT, and NIP/MWCNT and (b) MIP and NIP.

MWCNT, and NIP/MWCNT spectra (Figure 1a), it is observed an intense absorption band at 3430 cm−1 that can be attributed to the (O−H) stretching vibration from carboxyl group from MAA on nanocomposites surface and water physically adsorbed.34 For the nanocomposites, a band at 1630 cm−1 was attributed to CO stretching of quinone groups on the surface of MWCNT generated from oxidation treatment. Such an outcome indicates that the surface of MWCNT was not fully covered with a narrow layer of MIP. The low intense band at 1350 cm−1 is attributed to the in-plane vibration of (O−H) groups, both related to the carboxylic acid.34 The lowintensity signals at 620 and 480 cm−1 are ascribed to the Si−O stretching vibrations from CPTMS functionalization.59,60 For the MIP and NIP polymers (Figure 1b), the broad band at 3550 cm−1 is also assigned to the (O−H) stretching vibration from adsorbed water and from carboxyl group from F



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TEM images for MWCNT and the nanocomposite MIP/ MWCNT (S1), in which the polymeric network is homogeneously dispersed over the surface of MWCNT. In those images, it was observed that the MWCNT and nanocomposite filaments have approximately 6 and 11 nm of diameter respectively; therefore a thin MIP film of 5 nm is formed in the MWCNT surface. It is important to notice that the absence of aggregates on nanocomposite images confirms that the polymerization occurred only on MWCNT surface and not in solution, as expected when involving iniferter “grafting-from” approach. 3.2. Evaluation of Synthesized Materials’ Adsorption Performance. In order to compare the adsorption performance of the MWCNT, MIP/MWCNT, and MIP toward E2 molecule, 20.0 mL of a 20 mg L−1 E2 solution at pH 7.0 (10% MeOH) was stirred with 10.0 mg of each material for 120 min. After this time, the supernatant was filtered through a PTFE hydrophilic filter (0.22 μm) membrane and monitored by HPLC-DAD. The amount of E2 adsorbed in the material (mg g−1) was calculated according to eq 4. Figure 4 shows the

Figure 4. Comparison of E2 adsorption into MIP, MIP/MWCNT, and pristine MWCNT.

Figure 2. (a) Thermogravimetric analysis and (b) derivative curves of pristine MWCNT and MIP/MWCNT.

amount of E2 adsorbed (mg g−1) in each material. It is possible to observe that MIP/MWCNT and MWCNT adsorbed similar amount of E2, which was approximately 3-fold more E2 than MIP. Despite MWCNT showing similar adsorption of E2 compared to MIP/MWCNT, the carbonaceous material does not present selectivity. In addition, as the surface area of MIP/ MWCNT, as already mentioned, is lower than MWCNT, it seems that adsorptive performance of nanocomposite is ascribed to binding sites available in the MIP. Furthermore, the combination of MIP with MWCNTs in one single material provides higher surface area when compared to MIP, leading to more available binding sites, as previously observed on textural data. Therefore, by using the grafting from approach, the transfer mass rate of analyte toward the selective binding sites is higher in comparison to MIP synthesized by the bulk method. 3.3. Selectivity Studies. In order to evaluate the imprinting effect on the MIP/MWCNT selectivity, competitive adsorption experiments using the nonimprinted nanocomposite (NIP/MWCNT) were carried out. Interfering molecules E1 and EE2 estrogens were selected as structural analogs of the E2 template, while BPA, a monomer used in the manufacture of polycarbonate plastics and epoxy resins, is also

Table 1. Surface area, Pore Volume, and Diameter for Pristine MWCNT, MIP-MWCNT, and MIP materials pristine MWCNT MIP/MWCNT NIP/MWCNT MIP NIP

surface area (m2 g−1)

pore volume (cm3 g−1)

pore diameter (nm)

185.02

0.564

3.38

137.10 103.60 4.13 3.18

0.588 0.920 0.015 0.015

3.36 3.06 12.23 4.38

Figure 3. SEM images with 50000 times of magnification of (a) MWCNT, (b) MIP/MWCNT, and (c) MIP.

G



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considered as an EDC. Their molecular structures are shown in supporting materials (S2). Figure 5 shows the relative

Table 2. Kinetic Parameters for E2 Adsorption onto MIP/ MWCNTa parametersb

models pseudo-first-order Qt = Qe(1 − ek1t) pseudo-second-order

Qt =

Q e2k 2t [k 2(Q e)t + 1]

Elovich

Qt =

1 β

ln(αβ) +

1 β

ln t

intraparticle diffusion Qt = kidt1/2 + C

k1 (min−1); Qe (mg g−1) R2 k2 (g mg−1 min−1); Qe (mg g−1) R2 α (min−1 mg g−1); β (g mg−1) R2 C (mg g−1) kid (mg g−1 min−1/2) R2

0.133 34.64 0.9053 0.006 36.42 0.9526 35.05 0.17 0.9777 3.79 27.14 6.064 0.816 0.9880 0.8870

Qe(experimental): 36.0 mg g−1. bk1 = Pseudo-first order constant (min−1); k2 = Pseudo-second order constant (g mg−1 min−1); Qe = amount of E2 adsorbed at equilibrium time (mg g−1); α = initial adsorption rate (min−1 mg g−1); β = is related to the extent of surface coverage (g mg−1); C = constant related to the boundary layer thickness (mg g−1); kid = internal diffusion coefficient (mg g−1 min−1/2). a

Figure 5. Relative selectivity coefficient (k′) for the binary solutions of E2/E1, EE2, and BPA.

selectivity coefficient (k′) regarding each interfering molecule, which is superior to 1.0. This means that the MIP/MWCNT have a good recognition selectivity and binding affinity for the E2 molecule considering the competitive adsorption in the presence of molecules with similar structure and/or binding sites.65 Therefore, it can be inferred that the presence of the thin MIP film on the MWCNT, in addition to maintaining the adsorptive properties of the nanotube, gives the nanocomposite selectivity toward E2 molecule. 3.4. Adsorption Kinetics. Figure 6 shows the effect of the contact time on E2 adsorption by MIP/MWCNT nano-

second-order model provided a better fit than the pseudo-firstorder model. Therefore, it can be inferred that the active sites of the nanocomposite have different energies and that the rate of adsorption is proportional to the square of the number of unoccupied sites. Elovich model was also well-fitted to the data, thus corroborating the information given by pseudosecond-order model.46 Intraparticle diffusion model showed two linear plots before equilibrium time. The constant related to the thickness of the boundary layer (C, mg g−1) for the first linear plot is far from zero, which means that apart from intraparticle diffusion into pores of material represented by the second plot the adsorption process is also controlled by both the quick external surface adsorption limited by boundary layer in a shorter time.47 3.5. Adsorption Isotherms. Isotherm of E2 adsorption onto MIP/MWCNT nanocomposite is shown in Figure 7. The maximum Qexp of E2 was found to be 40.0 mg g−1. In order to investigate with more details, the E2 adsorption phenomena onto the nanocomposite, as well as to obtain information about the binding sites affinities, nonlinear isotherm models of Langmuir, Freundlich, Langmuir−Freundlich single and dual site were applied to the experimental data and their fittings are also shown in Figure 7. Isotherm models parameters are exposed in Table 3. Langmuir−Freundlich dual site model showed the best fit to the experimental data considering the regression coefficients values (R2) and the lower root-mean-square error (RMSE). This model assumes the presence of two different binding sites with different affinities toward the analyte, represented by parameter K1 and K2 (L mg−1).55 Parameters b1 and b2 are related to the amount of E2 adsorbed in each site (mg g−1). Therefore, it is observed that E2 adsorption is higher (b1) into the site with higher affinity (K1). High-affinity sites can be attributed to the H-bond between the proton of E2 molecule in its molecular form and the oxygen of MAA. Low-affinity

Figure 6. Study of the influence of contact time on E2 adsorption onto MIP/MWCNT nanocomposite and fitting curves of pseudofirst-order (blue) and pseudo-second-order (red) models.

composite. The equilibrium time was reached at 120 min while the experimental amount of E2 adsorbed (Qexp) was found to be 36.0 mg g−1. Adsorption process mechanism was evaluated by means of kinetic models applied to the experimental data, including nonlinear models of pseudo-first and pseudo-secondorder (Figure 6) and linear models of Elovich and intraparticle diffusion (Figure S3a,b, respectively). The respective parameters of these models are presented in Table 2. Considering the values of the regression coefficient (R2), as well as the Qexp and Qe predicted by the model, the pseudoH



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Table 4. Comparison of Adsorption Capacity and Equilibrium Time of Other Materials Used for E2 Adsorption Described in the Literature adsorbents SWCNTsb66 Fe3O4/graphene oxide (2:1)67 Molecularly imprinted hollow spheres68 activated carbon69 few-layered graphene oxide70 magnetic graphene oxide71 graphene oxide72 MWCNTc73 MWCNTc/COFeO274 MIP/MWCNT (this work)

equilibrium timea (min)

adsorption capacity (mg g−1)

240 600 15

26.7 19.48 12.12

NI 480 480 120 120 120 120

67.1 149.4 85.80 32.5 25.0 61.79 40.0

a

Figure 7. Adsorption isotherm of E2 on MIP/MWCNT and fit of Langmuir (green), Freundlich (blue), Single (pink) and Dual-site (red) Langmuir−Freundlich models.

c

3.6. Thermodynamic Studies. Figure 8 shows the adsorption isotherms obtained at different temperatures. It

Table 3. Isotherm Parameters Calculated from E2 Adsorption onto MIP/MWCNTa parametersb

model

−1

Langmuir

Qe =

KL × b × Ceq (1 + KL × Ceq)

Freundlich Qe = KF × Ceq1/n

single-site Langmuir−Freundlich

Qe =

b × (K1 × Ceq)n 1 + (K1 × Ceq)n

dual-site Langmuir−Freundlich

Qe =

b1 × (K1 × Ceq)n1 1 + (K1 × Ceq)n1

+

NI = Not Informed. bSWCNT = Single-walled carbon nanotubes. MWCNT = Multiwalled carbon nanotubes.

b2 × (K2 × Ceq)n2 1 + (K2 × Ceq)n2

KL (L mg ) b (mg g−1) R2 RSME KF (mg g−1) (L g−1) n R2 RSME K1 (L mg−1) b1 (mg g−1) n1 R2 RSME K1 (L mg−1) b1 (mg g−1) n1 K2 (L mg−1) b2 (mg g−1) n2 R2 RSME

3.384 43.63 0.9379 2.154 31.94 0.3088 0.9384 2.035 1.094 63.81 0.582 0.9528 1.991 5.334 25.86 7.493 0.9519 14.14 6.219 0.9931 0.9626

Figure 8. Adsorption isotherm of E2 onto MIP/MWCNT at different temperatures (25, 30, 35, and 40 °C).

can be observed that at low concentrations, the temperature does not affect the adsorption significantly. However, at higher concentrations, the adsorption decreases with the increase of temperature. The maximum adsorption capacities were found to be 46.0, 44.0, 40.0, and 37.0 mg g−1 for respective temperatures 25, 30, 35, and 40 °C. In order to determine whether the adsorption process is spontaneous and favorable, values of energy and entropy must be taken into account. Thermodynamic parameters of enthalpy (ΔH) and entropy (ΔS) were calculated from the slope and intercept of the Van’t Hoff’s plot (Figure S4) according to eq 13 and Gibbs free energy (ΔG) was calculate by eq 14 for all temperatures. The values obtained are shown in Table 5. The negative values of ΔG obtained for the evaluated temperatures indicate that adsorption process is spontaneous in nature and thermodynamically favorable. Moreover, ΔG values has fewer negatives with increasing temperature. Negative value of enthalpy indicates that the adsorption process of E2 onto the nanocomposite is exothermic. These results imply that the adsorption is favored at low temper-

a Qexp = 40.0 mg g−1. bKL = Langmuir equilibrium constant (L mg−1); KF = Freundlich equilibrium constant (mg g−1) (L g−1); K1 and K2 = equilibrium constant (L mg−1); b, b1, and b2 = maximum adsorption capacity (mg g−1); n, n1, and n2 = Freundlich empiric constant; RSME = root square mean error.

sites can be related to the π−π interactions of the rings of the E2 molecule with MWCNT. Table 4 compares equilibrium time and adsorption capacity of MIP/MWCNT nanocomposite synthesized in this work with other adsorbents described on literature for E2 removal. In general, it is observed that MIP/MWCNT nanocomposite show lower or similar equilibrium time and higher or similar adsorption capacity. Furthermore, in addition to these features, the nanocomposite synthesized shows better selective performance toward E2 molecule compared to other adsorbent materials. I



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Table 5. Experimental Thermodynamics Parameters for the E2 Adsorption onto MIP/MWCNT ΔG (kJ mol‑1) −8.13 −7.30 −6.47 −5.65

(298.15 (303.15 (308.15 (313.15

K) K) K) K)

ΔH (kJ mol‑1)

ΔS (K‑1 J mol‑1)

−57.58

−165.61

atures, which is supported by the isotherms in Figure 8. This can be due to the weakening of the physical bonding between the organic compounds and the active sites of the adsorbent as the temperature increase. Also, the solubility of E2 increases with temperature, resulting in stronger interactions between E2 and solvent than between E2 and the nanocomposite.75 Furthermore, according to the magnitude of ΔG and ΔH values, it can be inferred that the adsorption of E2 onto the MIP/MWCNT was a physisorption process.76,77 Negative entropy indicates that the adsorption process decreases the system disorder and suggests that enthalpy contributes more than entropy in producing negatives ΔG values. Similar results are found in the literature for the adsorption of organic molecules onto carbonaceous materials,56,78,79 and for the adsorption of E2.80,81 3.7. Optimization of Elution Solvent and pH. Figure 9 presents the percentage of E2 elution by applying different

Figure 10. Percentage (%) of E2 adsorption into MIP/MWCNT cartridge in function of solution pH.

interactions with MAA monomer and stack interactions with MWCNT. In this sense, pH 7.0 was chosen as optimum, which is of great interest since the extraction procedure can be applied using natural waters. In order to check the feasibility of MIP/MWCNT toward E2 adsorption in real samples, lake water samples were collected at Igapó Lake, Londrina, City in ambar flasks, acidified with 0.5 mol L−1 H2SO4, filtered through cellulose acetate filter 0.45 μm, and the pH adjusted to 7.0. Moreover, with the influence of humic acid at 1.0 mg L−1 concentration, the usual amount in natural water samples82 on E2 adsorption was also evaluated. These samples were subjected to extraction on MIP/MWCNT where no interferences and matrix effect were noticed since the E2 adsorption percentage was equal to 99.8%. It is important to note that all solid-phase experiments were carried out using the same cartridge (approximately 50 cycles), thus indicating that the material is stable and present good reusability.

4. CONCLUSIONS In the present work, the synthesis of a nanocomposite based on molecularly imprinted poly(methacrylic acid) grafted on an iniferter-modified carbon nanotube (MIP/MWCNT) for E2 extraction was reported. From the characterization techniques including FTIR, TGA, adsorption/desorption of N2, SEM, and TEM, it was noticed the formation of thin MIP film at nanoscale on the surface of MWCNT, thus confirming the success of the synthesis based on the grafting from approach. From kinetics and isotherms studies, it could be inferred that the E2 adsorption mechanism consists of both surface adsorption and intraparticle transport recognized by the intraparticle diffusion model. Additionally, the E2 adsorption occurs in two binding sites of heterogeneous energies, attributed to the carbonyl groups from MAA and π−π interactions, as predicted by pseudo-second-order and Dualsite Langmuir−Freundlich models. From thermodynamic studies, it was concluded that the adsorption process is spontaneous and exothermic, characteristic of physisorption processes. For final remarks, taking into account the results of the adsorptive performance of the MIP/MWCNT, including relative low equilibrium time, high adsorption capacity, the intrinsic selectivity of the imprinted nanocomposite, and the

Figure 9. Percentage of E2 elution (%) from MIP/MWCNT cartridge using different elution solvents or a mixture of solvents (v/v).

solvents and mixtures. It is observed that with chloroform and acetone, 80 ± 9 and 78 ± 12 percentage of recovery was achieved, respectively. However, the combination of these two solvents (1:1 v/v) increases the elution percentage to 96 ± 3.6. Since E2 retention in MIP/MWCNT can be both by H-bonds and hydrophobic interactions, the mixture of polar (acetone) and nonpolar (chloroform) solvents provided a suitable elution. The influence of pH on E2 adsorption toward the nanocomposite was evaluated and is presented in Figure 10. It was obtained adsorptions percentages of 70.0% ± 3.6 for pH 3, 64.0% ± 6.8 for pH 5, and 92.5% ± 3.5 for pH 7. Since the pKa of E2 molecule is 10.766 and pKa of MAA is 4.7,55 at pH 7.0 the E2 molecule is in its neutral form, while the functional monomer MAA is ionized. Therefore, as stated before, E2 adsorption into the nanocomposite may occur by H-bond J



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promising SPE procedure results, it might be emphasized that new studies based on living-radical polymerization induced by an iniferter deserve to be expanded for new target molecules with a focus on the development of solid-phase extraction methods.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b01010. TEM images of pristine MWCNT and MIP/MWCNT, structure of interfering molecules, application of Elovich and intraparticle diffusion kinetic models ,and the Van’t Hoff plot from thermodynamic studies (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +55 4396280744. E-mail: [email protected]. ORCID

César R. T. Tarley: 0000-0002-8685-1556 Present Address

́ M.C.P., D.M.D.S., and C.R.T.T.: Instituto de Quimica de São Carlos, Universidade de São Paulo (USP), CEP 13560−970 São Carlos, SP, BR. Funding

The authors acknowledge the financial support and fellowships ́ of Coordenação de Aperfeiçoamento de Nivel Superior (CAPES) (Project Pró - Forenses 3353/2014 Grant 23038.007082/2014-03), Conselho Nacional de Desenvolví mento Cientifico e Tecnológico (CNPq) (Grants 481669/ 2013-2, 305552/2013-9, 472670/2012-3), Fundaçaõ Araucária do Paraná (163/2014), SETI do Paraná, and Instituto ́ Nacional de Ciência e Tecnologia de Bioanalitica (INCT) (FAPESP Grant 2014/50867-3 and CNPq Grant 465389/ 2014-7). Notes

The authors declare no competing financial interest.

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REFERENCES

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