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magnetic chitosan nanocomposite for selective extraction of zinc ions from different food samples
Accepted Manuscript Analytical Methods Synthesis, characterization, and application of a Zn (II)-imprinted polymer grafted on graphene oxide/magnetic chitosan nanocomposite for selective extraction of zinc ions from different food samples Elahe Kazemi, Shayessteh Dadfarnia, Ali Mohammad Haji Shabani, Mansoureh Ranjbar PII: DOI: Reference:
S0308-8146(17)31035-X http://dx.doi.org/10.1016/j.foodchem.2017.06.053 FOCH 21275
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
6 March 2017 3 June 2017 7 June 2017
Please cite this article as: Kazemi, E., Dadfarnia, S., Haji Shabani, A.M., Ranjbar, M., Synthesis, characterization, and application of a Zn (II)-imprinted polymer grafted on graphene oxide/magnetic chitosan nanocomposite for selective extraction of zinc ions from different food samples, Food Chemistry (2017), doi: http://dx.doi.org/10.1016/ j.foodchem.2017.06.053
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Synthesis, characterization, and application of a Zn (II)-imprinted polymer grafted on graphene oxide/magnetic chitosan nanocomposite for selective extraction of zinc ions from different food samples
Elahe Kazemi, Shayessteh Dadfarnia٭, Ali Mohammad Haji Shabani, Mansoureh Ranjbar Department of Chemistry, Yazd University, Safaieh, 89195-741, Yazd, Iran
* Corresponding author. Tel: +983531232667; fax: +983538210644 E-mail addresses: [email protected] (S. Dadfarnia), [email protected] (A. M. Haji Shabani), [email protected] (E. Kazemi)
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Abstract A novel Zn(II) imprinted polymer was synthesized via a co-precipitation method using graphene oxide/magnetic chitosan nanocomposite as supporting material. The synthesized imprinted polymer was characterized by Fourier transform infrared spectrometry (FTIR) and scanning electron microscopy (SEM) and applied as a sorbent for selective magnetic solid phase extraction of zinc followed by its determination by flame atomic absorption spectrometry. The kinetic and isothermal adsorption experiments were carried out and all parameters affecting the extraction process was optimized. Under the optimal experimental conditions, the developed procedure exhibits a linear dynamic range of 0.5–5.0 µg L-1 with a detection limit of 0.09 µg L-1 and quantification limit of 0.3 µg L-1. The maximum sorption capacity of the sorbent was found to be 71.4 mg g-1. The developed procedure was successfully applied to the selective extraction and determination of zinc in various samples including well water, drinking water, black tea, rice, and milk.
Keywords: Magnetic solid phase extraction; Flame atomic absorption spectroscopy; Ion imprinted polymer; Graphene oxide; Chitosan; Zinc
Running title: Synthesis & analytical application of novel Zn(II)-imprinted polymer
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1. Introduction Zinc is one of the most important essential trace elements and considered to be a vital micronutrient as it has shown a wide range of biochemical functions in all living organisms. Zinc deficiency in the human body causes several disorders including growth retardation, diarrhea, eye and skin lesions, immunity depression and malfunction of wound healing (Scherz & Kirchhoff, 2006). On the other hand, an excessive intake of zinc can be toxic and harmful and lead to various chronic and acute adverse effects (Zhao, Han, Zhang, & Wang, 2007). Accordingly, trace amount determination of zinc in different matrices is of great importance. Various analytical techniques including inductively coupled plasma optical emission spectrometry (Feist & Mikula, 2014), voltammetry (Behnia, Asgari, & Feizbakhsh, 2015), spectrophotometry (Pourreza & Naghdi, 2014; Ribas, Tóth, & Rangel, 2017), and induced plasma-atomic emission spectrometry (Ozbek & Akman, 2016) have been employed for the determination of zinc. Furthermore, based on our literature survey flame atomic absorption spectrometry is the most frequently used technique for this purpose (Behbahani, Salarian, Bagheri, Tabani, Omidi, & Fakhari, 2014; Lemos, Bezerra, & Amorim, 2008; Roushani, Abbasi, Khani, & Sahraei, 2015; Shakerian, Dadfarnia, & Haji Shabani, 2012; Shamsipur, Rajabi, Pourmortazavi, & Roushani, 2014). However, the trace level determination of metal ions in complex matrices commonly necessitate a step of sample preparation before instrumental analysis. The application of imprinted polymers (IIPs) as cost-effective and robust smart materials with the peculiar recognition properties in trace or ultra-trace analysis provide significant breakthroughs in separation or preconcentration chemistry. Bulk and precipitation polymerization are the most commonly utilized method for the preparation of imprinted polymers. However, some major
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drawbacks have been reported in the resultant polymers. For example, heterogeneous distribution of the binding sites inside their structure leads to poor site accessibility of the target and therefore undesirable sorption/desorption kinetics as well as slow mass transfer (Su, Li, Li, Liu, Lei, Tan, et al., 2015). It has proven that controlling the position of the binding sites on the material’s surfaces via surface imprinting can efficiently resolve these problems. This methodology is based on the synthesis of the imprinted polymer on the surface of a supporting material (Deng, Qi, Deng, Zhang, & Zhao, 2008). Among various supporting materials, Fe3O4 nanoparticles individually (Kazemi, Haji Shabani, & Dadfarnia, 2015) or incorporated into other nano or micro materials (Qiu, Luo, Sun, Lu, Fan, & Li, 2012) has enjoyed wide range of application as it provides large external surface area to volume ratio as well as facilitating the separation of the sorbent via external magnetic field. Graphene oxide (GO) with possessing extremely large surface area, extraordinary physical and mechanical stability and especially plentiful presence of hydroxyl, epoxide, and carboxylic functional groups which enables its surface chemical modification has also attracted great attention in preparation of imprinted polymers (Ning, Peng, Li, Chen, & Xiong, 2014; Qiu, Luo, Sun, Lu, Fan, & Li, 2012). There are some other reports using chitosan in the preparation of imprinted polymers. Chitosan is a biocompatible and biodegradable hydrophilic polymer with abundant hydroxyl and amino functional groups which is known to be one of the most abundant natural amino-polysaccharide (Chang, Zhang, Ying, Li, Lv, & Ouyang, 2010; Wang, Wang, Wu, Li, Zhu, Zhu, et al., 2014). In recent years, the research interest has turned into integrating materials with different properties in order to provide multi imprinting site, enhanced surface area, and sorption capacity. In this regards, the integration of graphene oxide and chitosan which occurred through the special interaction between the epoxy groups of GO and primary amine
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groups of chitosan as well as the H-bonding between them have been utilized in preparation of imprinted polymers (Deng, Xu, & Kuang, 2014; Han, Yan, Chen, & Li, 2011). However, based on our literature review there is only two works including our previous study dealing with the triplicate combination of chitosan, magnetic nanoparticles and graphene oxide in preparation of imprinted polymers (Barati, Kazemi, Dadfarnia, & Haji Shabani, 2017; Duan, Li, Wang, Wang, Li, & Luo, 2015). Up to now, several kinds of Zn(II)-imprinted polymers have been synthesized and applied for selective extraction of zinc from different samples which all of them have been prepared using the traditional methods (Behbahani, Salarian, Bagheri, Tabani, Omidi, & Fakhari, 2014; Behnia, Asgari, & Feizbakhsh, 2015; Roushani, Abbasi, Khani, & Sahraei, 2015; Shakerian, Dadfarnia, & Shabani, 2012; Shamsipur, Rajabi, Pourmortazavi, & Roushani, 2014; Zhao, Han, Zhang, & Wang, 2007). The aim of the present study was to prepare a sorbent exhibiting high selectivity, high sorption capacity and significantly fast mass transfer for zinc ions with simple and ease of separation. For this purpose, a novel zinc imprinted polymer on supporting material exploiting the unique characteristic of graphene oxide, chitosan and magnetic nanoparticles was synthesized and its performance for selective separation, preconcentration, and extraction of Zn2+ ions from different matrices was evaluated. The structure and binding properties of the synthesized polymer were thoroughly studied and all the important parameters influencing the extraction of zinc ions were investigated and optimized. Finally, the performance of the synthesized polymer and feasibility of the developed methods for the selective extraction and determination of zinc from water and different food samples was evaluated.
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2. Experimental 2.1. Reagents and materials Ethylene glycol dimethacrylate (EGDMA), acrylic acid (AA), styrene, glutaraldehyde (25 wt.%), zincon (C20H16N4O6S), ferric chloride (FeCl3.6H2O), ferrous chloride (FeCl2.4H2O), graphite powder, ethylenediaminetetraacetic acid (EDTA), histidine (C6H9N3O2) and all solvents used in this study were of analytical reagent grade and purchased from Merck Company (Darmstadt, Germany). Chitosan with an average molecular weight of 350 kDa and deacetylation degree
of
>75%
was
obtained
from
Sigma-Aldrich
(Missouri,
USA).
2,
2'
-
Azobisisobutyronitrile (AIBN) was prepared from ACROS Company (New Jersey, USA). All solutions throughout the study were prepared using double distilled water. A stock solution of zinc at 1000 mg L-1 was prepared by dissolving an appropriate amount of Zn(NO3)2.6H2O in double distilled water. Working solutions were prepared through serial dilutions of the stock solution with doubly distilled water.
2.2. Instrumentation An Analytik Jena novAA 300 atomic absorption spectrometer (model 330, Jena, Germany) equipped with a Zn-hollow cathode lamp (HCL) and an air-acetylene flame atomizer was used for determination of zinc ions. The operation conditions were adjusted according to the manufacturer’s recommendation as follows: HCL wavelength: 213.9 nm, Lamp current: 4.0 mA, spectral bandwidth: 0.5 nm. The pH measurements were carried out using a digital pH meter, Metrohm model 827 (Herisau, Switzerland), equipped with a combined glass calomel electrode. The magnetic phase separation was carried out by means of a strong magnet (1.2 T, 10 cm × 5 cm × 2 cm).
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2.3. Synthesis of the imprinted sorbent In the first step for the synthesis of magnetic ion imprinted polymer, Fe3O4 nanoparticles, and graphene oxide were separately synthesized via co-precipitation method (Tahmasebi & Yamini, 2012) and a modified Hummers method (Kazemi, Dadfarnia, & Haji Shabani, 2015), respectively. Next, to prepare the magnetic composite of graphene oxide/chitosan (GO/Chm), an accurately weighted amount of pure chitosan (2.0 g) was dissolved in 100.0 mL of acetic acid solution (2.0% v/v), and mixed by sonication for 30 min. 0.75 g of Fe3O4 nanoparticles was added to the mixture and stirred for 2 h. Thereafter, 30.0 mL of glutaraldehyde (25.0 wt.% in water) and 1.5 g of graphene oxide was added to the mixture and the pH was adjusted to 9.0– 10.0 by the addition of ammonia solution. The mixture was stirred at 80 ºC. After 1 h, the precipitate of GO/Chm nanocomposites was collected using an external magnet and dried under vacuum at 60 ºC (Travlou, Kyzas, Lazaridis, & Deliyanni, 2013). The prepared magnetic nanocomposite (GO/Chm) was then modified with acrylic acid. For this purpose, 1.0 g of the nanocomposite was added to 100.0 mL of ethanol under sonication. 10.0 mL of acrylic acid was added and the mixture was stirred at the rate of 300 rpm for 2h. At the end of this process, the precipitate of the acrylic acid modified GO/Chm nanocomposite was collected by the aid of an external magnet and dried under vacuum at 50 ºC (Liu, Han, Guan, Wang, Liu, & Zhang, 2011). The imprinted polymer supported on the magnetic nanocomposite of graphene oxide/chitosan (IIP-GO/Chm) was synthesized through coprecipitation polymerization. For this purpose, 1.0 mmol zincon was dissolved in 60.0 mL mixture solution of ethanol/acetonitrile (1:2 v/v). Then, 1.0 mmol Zn(NO3)2.6H2O was added and the mixture was stirred for 1h to allow
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completion of the dark blue complex formation between zinc and zincon. 1.0 g of the previously prepared nanocomposite was added to the mixture and mixed by mechanical stirring for another hour. Then, 32.0 mmol of EGDMA and 8.0 mmol styrene was added and stirred for a few second to achieve a homogenized solution. 50.0 mg of AIBN were added, and the mixture was transferred to an ice bath (0 ºC) and purged with nitrogen for 15 min. The reaction flask was sealed and thermally polymerized in a water bath at 60 ºC for 24 h. The obtained magnetic polymer was collected using an external magnet, washed with ethanol and distilled water, and dried at 60 ºC under vacuum. At the final step, the template ion was removed by repetitive extraction with EDTA solution (0.005 mol L-1, pH = 5.5) until no analyte was detected by flame atomic absorption spectrometry. The product which was turned from blue to purple due to leaching was collected, dried and stored for further use. Non-imprinted polymer (NIP) was synthesized using the same procedure, without the presence of zinc ions. Fig. S1 shows the schematic representation of the synthesis process.
2.4. Real sample preparation The water samples including drinking water and well water were filtered through a 0.45 mm pore filter. The amount of zinc in 100.0 mL of these samples was determined using the developed procedure. The rice sample, purchased from a local supermarket, was cleaned, rinsed with double distilled water, finely grounded and oven-dried at 60 ºC. 100.0 mg of the pre-treated sample was transferred to 100.0 mL beaker. 10.0 mL of concentrated nitric acid (65% w/w) was added and the mixture was heated to dryness. The residue was cooled to the ambient temperature, then 3.0 mL of H2O2 (30%, w/w) was added and again heated to dryness. The residue was then dissolved
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in 10.0 mL of double distilled water and filtered. After adjustment of pH using 2.5 mL borate buffer (pH= 8.5), the filtrate was diluted to 100.0 mL with double distilled water and treated according to the developed procedure (Abbasi-Tarighat, Shahbazi, & Niknam, 2013). The black tea sample was prepared from a local supermarket. The tea sample was rinsed with cold distilled water, oven-dried at 60 ºC and grounded. 10.0 mL of concentrated nitric acid (65% w/w) was added to 100.0 mg of the sample and the mixture was heated to dryness. The resultant residue was dissolved in 10.0 mL of double distilled water and filtered. After adjustment of pH using 2.5 mL borate buffer (pH = 8.5), the filtrate was diluted to 100.0 mL with distilled water and treated according to developed procedure (Dadfarnia, Haji Shabani, Shirani Bidabadi, & Jafari, 2010). The milk sample was prepared as follows: 10.0 mL of concentrated nitric acid followed by 2.0 mL of 30% (w/w) hydrogen peroxide were added to 0.15 g of the accurately weighed dried milk sample. The mixture was heated at 100 ºC for 15 minutes, and allowed to cool at room temperature. Afterward, the mixture was filtered through a 0.45 mm pore filter, the pH was adjusted to 8.5 by the addition of 2.5 mL of borate buffer, transferred to a 100.0 mL volumetric flask and the volume was adjusted with distilled water (Machado, Bergmann, & Pistón, 2016). The resulting solution was analyzed according to the given general procedure.
2.5. General procedure The extraction of zinc ions using the prepared sorbent was carried out by the batch experiments as follows: 20.0 mg of the synthesized magnetic ion imprinted polymer was added to 100.0 mL of sample/standard solution containing no more than 0.5 µg zinc and the pH was adjusted to 8.5 by the addition of 2.5 mL of borate buffer. The mixture was stirred mechanically
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for 10 minutes to complete the extraction process. Thereupon, the sorbent was held by an external magnet and the supernatant was decanted. 0.5 mL of EDTA solution (0.005 mol L-1, pH = 5.5) was then added to the sorbent and the mixture was sonicated for 5 minutes to desorb the retained zinc ions. The desorbing solution was separated by the aid of an external magnet and introduced to atomic absorption spectrometry for quantification.
2.6. Adsorption studies To study the binding and adsorption properties of the synthesized ion imprinted polymer, the batch mode sorption experiments were carried out as follow: 20.0 mg of the prepared sorbent was equilibrated with 100.0 mL of the zinc solutions with a different initial concentration within the range of 50.0–300.0 mg L-1 at the optimized conditions. At certain time intervals up to 6h, the concentration of zinc remaining in the solution was determined by flame atomic absorption spectrometry. The amount of zinc retained on the sorbent (qt) at the time of t was calculated using a mass balance relationship as follow: =
− V W Where Co is the initial concentration of zinc and Ct (mg L-1) represent the remaining
concentration of zinc in the solution at time t. V represent the volume of solution (L) and W is the sorbents mass (g). The results of this experiments are depicted in Fig. S2. As it can be seen, the retained amount of Zn2+ ion per unit mass of the polymer was increased almost linearly with the increase in the initial concentration of the zinc ions and gradually reached a plateau indicating the saturation of the active binding sites on the synthesized IIP-GO/Chm. Furthermore, the adsorption equilibrium in solutions with different initial concentrations of zinc was achieved after about 2.5 h, representing the significantly high rate of mass transfer. 10
3. Results and discussion 3.1. Sorbent characterization 3.1.1. FTIR analysis The correct synthesis of Fe3O4 nanoparticles, graphene oxide, the magnetic composite of graphene oxide/chitosan and modified magnetic nanocomposite by zinc-imprinted polymer was investigated using the FTIR spectroscopy (Fig. 1). The spectrum of pure Fe3O4 nanoparticles (Fig. 1a) shows the characteristic peak at 540 cm−1 corresponding to the Fe-O stretching mode of the Fe3O4 lattice. The characteristic absorption peaks of graphene oxide corresponding to C=O stretching of the carboxyl group at 1740 cm-1, C=C stretching vibration of the remaining sp2 character at 1620 cm-1, C-O-C stretching of the epoxy at 1248 cm-1 and C-O-H stretching of the hydroxyl group at 1051 cm-1 can be seen in Fig. 1b. The spectrum of magnetic composite of graphene oxide/chitosan (Fig. 1c) exhibits the characteristic peaks of Fe3O4 nanoparticles at 600 cm-1 and the C=O stretching corresponding to carboxyl group of graphene oxide is downshifted to 1712 cm-1 which is the exhibition of hydrogen bonding between the carboxyl group in the graphene oxide and NH2 in chitosan. The appearance of carboxyl peak at 1400 cm-1 indicates the formation of (-COO- +H3N-R). Furthermore, the appearance of two characteristic peaks at 1633 and 1566 cm-1 which can be ascribed to the C–O stretching vibration of –NHCO– and the N–H bending of NH2 confirms the successful preparation of magnetic nanocomposite of chitosan grafted on the graphene oxide. The spectra of zincon and zinc-zincon complex (Fig. 1d and 1e) show the main characteristic absorption peaks of zincon at 1042 cm-1 corresponding to S=O stretching vibration, at 1107 cm-1 corresponding to C-N stretching vibration, at 1200 cm-1 corresponding to C-O or O-
11
H bending vibration of benzene ring, at 1471 cm-1 corresponding to stretching vibration of benzene ring, at 1580 cm-1 corresponding to N=N stretching vibration and at 1700 cm-1 attributed to C=O stretching. These peaks are all observed in the spectrum of the complex (Fig. 1e) with a slight shift. The peak at 1603 cm-1 is attributed to the bending vibration of N-H which is disappeared in the spectrum of complex due to interaction with zinc. The IR spectrum of the imprinted polymer before and after leaching is demonstrated in Fig. 1f and 1g. As it can be seen, these spectra exhibit similar features of absorption peaks at 1154 cm-1 corresponding to S=O stretching vibration of zincon, 1296 cm-1 corresponding to stretching vibration of phenolic group in zincon and 1724 cm-1 attributed to stretching vibration of C=O in the polymer backbone. The peak observed at 1638 cm-1 is attributed to the amide group in the supporting material which is slightly shifted due to interaction with acrylic acid in the functionalization process. Accordingly, the successful synthesis of ion imprinted polymer and incorporation of zincon in its structure as well as the stability of polymer in all leaching process can be concluded.
3.1.2. SEM analysis The morphology of the Fe3O4 nanoparticles, graphene oxide, the magnetic nanocomposite of graphene oxide-chitosan and IIP coated GO/Chm was characterized by scanning electron microscopy. Figure 2a shows the synthesized spherical nanoparticles of Fe3O4 and Fig.2b. shows the typical flake-like shape of graphene oxide. The SEM of the magnetic nanocomposite of GO/Chm (Fig. 2c) shows the successful aggregation of magnetic nanoparticles and chitosan on the surface of graphene oxide. Fig. 2d illustrates the highly porous threedimensional structure as well as the spherical shape of IIP-GO/Chm. This evidence confirms the successful synthesis of IIP coated GO/Chm. 12
3.1.3. Adsorption Isotherms The adsorption isotherms provide the interactive behavior between the solute and the adsorbent. Thus, the analysis of isotherm data is important in estimating the adsorption capacity as well as describing the affinity and adsorbent surface properties. In this study, Langmuir (Langmuir, 1916), Freundlich (Surikumaran, Mohamad, & Sarih, 2014) and Temkin (Barati, Kazemi, Dadfarnia, & Haji Shabani, 2017) models were used to assess the binding properties of the synthesized IIPs-GO/Chm. The studied models expressed by the following equations: Langmuir:
=
+
Freundlich:
= +
Temkin:
= + =
Where qe (mg g-1) and Ce (mg L-1) are the amount of zinc adsorbed at equilibrium and the equilibrium concentration of zinc in the solution, respectively. qm and b are the Langmuir constant corresponding to the maximum monolayer capacity (mg g-1), and the sorption energy, respectively. The Langmuir model assumes that the sorbent has a homogeneous surface with similar and energetically equivalent binding sites which provide monolayer adsorption. The Freundlich model is commonly utilized to describe multilayer adsorption onto heterogeneous surfaces. Kf (mg g-1) and n are the Freundlich constants, and 1/n represent the surface heterogeneity or exchange intensity. A value of 1/n smaller than 1.0 accounts for a favorable adsorption. The Temkin model illustrates the homogeneous distribution of binding energies up to some maximum and postulates the linear decrease of the heat of the sorption of all molecules in 13
the layer with the surface coverage due to the interactions between the solute molecules and the adsorbent. KT is the equilibrium binding constant (mol-1) ascribed to the maximum binding energy, and the constants A and b represent the heat of sorption and the heat of adsorption respectively. The obtained diagrams for Langmuir, Freundlich and Temkin models are illustrated in Fig. S3 and their corresponding parameters including values of R2 are listed in Table S1. The applicability of the studied models was judged by evaluating the correlation coefficients, R2 values. As shown in Table S1, the R2 value of the Langmuir isotherm was greater and more proximate to unity (0.999) than that of the other models. Therefore, the experimental data of zinc adsorption onto IIP-GO/Chm fitted well to the Langmuir isotherm model indicating the homogeneous solid surface of the synthesized ion imprinted polymer and the regular monolayer adsorption of zinc molecules (Barati, Kazemi, Dadfarnia, & Haji Shabani, 2017). Furthermore, based on the data summarized in Table S1, the maximum predictable sorption capacity of the adsorbent for zinc ions was found to be 71.4 mg g-1.
3.1.4. Adsorption kinetics study To investigate the mechanism of zinc adsorption onto the surface of the IIP-GO/Chm, four kinetic models including the pseudo-first-order, pseudo-second order (Barati, Kazemi, Dadfarnia, & Haji Shabani, 2017), intraparticle diffusion (Cheung, Szeto, & McKay, 2007) and Elovich (Özacar & Şengil, 2005) were used to explore the experimental data. These models are expressed by the following equations: Pseudo-first-order: Pseudo-second order:
!
" log − = log − #.%%
&
=!
' '
+
14
"
Intraparticle diffusion: = () * ' + +
Elovich: = , ln./ + , ln *
qe and qt which are the same in all equations represent the amounts of zinc ion (mg g-1) sorbed at the equilibrium time and time t, respectively. k1 (min-1) and k2 (g mg-1 min-1) are the pseudo-first order and the pseudo-second order rate constant of sorption, respectively. The Kid and I are the intraparticle diffusion rate constant (mg g-1 min-1/2) and the intercept determined from the plot of qt versus t1/2, respectively. In Elovich equation α is the initial sorption rate constant (mg g-1 min1
) and β represent the amount of the surface coverage and the activation energy of chemisorption
(g mg-1). Fig. S4, 5, 6 and 7 shows the experimental data fitted with pseudo-first-order, pseudosecond order, Elovich, and intraparticle diffusion, respectively. As it can be concluded from data summarized in Table S2, the correlation coefficient (R2) values of the pseudo-second-order model are higher than that’s of other models and closer to unity. Therefore, the pseudo-second order equation which suggests the chemisorption of solute onto the sorbent was more suitable to describe the adsorption of zinc ions into the polymer.
3.2. Optimization of extraction conditions In order to have the most desirable extraction performance and achieve satisfactory results, all parameters affecting the extraction efficiency of zinc using the synthesized ion imprinted polymer was investigated and optimized. All optimization experiments were performed in triplicates and the results were averaged.
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3.2.1. Effect of pH The sample solution pH exerts a profound influence on the overall performance of the SPE procedure, as it can affect the sorptive uptake of the target analyte by changing the surface chemistry of both sorbent and analyte. The driving force for adsorption of zinc ions onto the synthesized ion imprinted polymer is the ability of the zinc in the formation of a stable chelate with N-containing groups existing in zincon. By considering the structure of zincon (Fig. 3a) and its pKa values (Fig. 3b) it is expected that the best complex formation between zinc and zincon take place at alkaline medium (Säbel, Neureuther, & Siemann, 2010). With this perception, the influence of the sample solution pH on the extraction of zinc ions was investigated in the pH range of 4.0–11.0. The pH of the sample solution was adjusted using nitric acid or ammonia solution as well as phosphate buffer (6.0–8.0) and borate buffer (8.0–10.0). The results represented in Fig. 4a showed that as expected the maximum extraction recovery is achieved in the pH range of 7.0–9.0. The decrease in extraction recoveries at lower pH can be explained by protonation of the ligand which leads to incomplete complexation and retention of the analyte by the sorbent, whereas at higher pH, zinc ions are prone to the precipitation as hydroxide. Accordingly, pH of 8.5 was selected as the optimum pH.
3.2.2. Optimization of desorption condition In order to assure the achievement of satisfactory recovery of retained zinc ions, the effect of type of desorbing solution on the extraction performance was investigated. For this purpose, 1.0 mL of different potential desorbing solution including EDTA (0.05 mol L-1), histidine (0.05 mol L-1), HNO3 (1.0 mol L-1), H2SO4 (1.0 mol L-1) and CH3COOH were examined. As it can be concluded from the results represented in Fig. 4b, EDTA (0.05 mol L-1)
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provided the most effective desorption of the retained zinc ions which can be explained by the competition of the EDTA with zincon in the complex formation with zinc. As the complexation and consequently the desorption capability of EDTA toward zinc may be affected by the pH, a series of EDTA solutions with pHs varying in the range of 2.0–10 was used for desorption of zinc. The results implied that the maximum extraction recovery was obtained in the pH range of 4.5–10.0. Furthermore, the effect of concentration of EDTA in the range of 5 × 10-4 – 0.05 mol L-1 was investigated and 0.005 mol L-1 was chosen as the optimum concentration. The minimum volume of desorption solution required to efficiently desorb the analyte was also investigated by varying its volume in the range of 0.3 to 1.5 mL. The quantitative results were achieved by 0.5 mL of EDTA. Accordingly, 0.5 mL EDTA solution with a concentration of 0.005 mol L-1 and pH of 5.5 was selected for the desorption stage.
3.2.3. Effect of time The adsorption process of the target analyte must be done for enough time to achieve equilibrium. The optimum time for adsorption of zinc defined as the time between the addition of the sorbent to the sample solution and its separation for the sample was determined by application of different time intervals from 1 to 30 minutes. Based on the results (Fig. 4c), 10 minutes was selected as the optimum adsorption time. The effect of desorption time was also investigated in the range of 3-20 minutes and 5 minutes was found to be sufficient for quantitative desorption of analyte.
3.2.4. Effect of amount of sorbent, sample volume and ionic strength
17
The dependency of extraction efficiency to the amount of sorbent was studied by changing the amount of IIP-GO/Chm from 5.0 to 30.0 mg. The results showed that 20.0 mg of the prepared sorbent was adequate to achieve satisfactory results. Consequently, 20.0 mg of IIPGO/Chm was utilized in the subsequent studies. To investigate the breakthrough volume of the sample solution, 0.5 µg of zinc was extracted from different volumes of sample solutions (25.0–200.0 mL). The results revealed that up to 100.0 mL of the sample solution, the extraction process was quantitative. Accordingly, in order to achieve a higher preconcentration factor, 100.0 mL of the sample solution was selected for the extraction of analyte in the following experiments. The influence of the ionic strength on the extraction efficiency of zinc ions was investigated by conducting the extraction process in the presence of different amount of sodium chloride (0-1.5 mol L-1). It was observed (Fig. 4d) that the presence of salt up to 1.0 mol L-1 did not interfere with the extraction process. However, the extraction efficiency decreased at higher concentration of salt. This can be related to the competition of Na+ ions at high concentration with zinc for the sorption sites or the hindrance of mass transfer due to the increase in the viscosity of the sample in the presence of high amount of salt. Accordingly, the following experiments were performed without the addition of salt.
3.3. Selectivity and reusability of the sorbent The selectivity of the synthesized IIP-GO/Chm toward zinc ions was investigated by considering competitive extraction of zinc in the presence of other ions including some transition metals such as Cu2+, Co2+, Ni2+ and Cd2+ which are capable of forming a complex with zincon. The tolerance limit was taken as the maximum concentration of the ions which caused an error
18
not exceeding ±5%. The results summarized in Table S3 showed that the examined ions at the given mole ratio did not interfere in the extraction and determination of zinc ions. Accordingly, the developed procedure using IIP-GO/Chm exhibits high selectivity toward zinc. In order to evaluate the renewability and reusability of the synthesized sorbent, a set of experiments were performed by using the same sorbent repeatedly. For this purpose, the synthesized IIP-GO/Chm was subjected to several adsorption and desorption cycles, under the same experimental conditions. Based on the obtained results (Fig. S8a), the IIP-GO/Chm was stable and could be regenerate without any significant loss of analytical performance up to 9 adsorption/desorption cycle. Furthermore, the analytical performance of IIP-GO/Chm was compared with NIP-GO/Chm by application of these polymers for the extraction of zinc ions under the similar condition. As it can be concluded from Fig. S8b, the application of IIPGO/Chm result in much higher extraction efficiency indicating the higher affinity of the synthesized IIP-GO/Chm toward zinc ions due to imprinting effect.
3.4. Analytical performance The developed procedure was validated by determination of quality parameters including linear dynamic range, coefficient of determination (R2), limit of detection, preconcentration factor, and precision. Under the optimal concentration, the method showed a good linear correlation of analytical signal to the zinc concentration within the range of 0.5–5.0 µg L-1 with a regression equation of A (signal) = 0.019 CZn (µg L-1) + 0.028 and correlation coefficient of R2 = 0.999. The limit of detection of 0.09 µg L-1 and limit of quantification of 0.3 µg L-1 were obtained based on 3Sb/m and 10Sb/m (Sb is the standard deviation of the blank and m is the slope of the calibration graph), respectively. Five replicate measurement of zinc solutions at the
19
concentration of 2.0 µg L-1 gave a relative standard deviation of 2.7%. The enrichment factor, ascribed to the ratio of the maximum volume of the sample solution (100.0 mL) to the final volume of the extract (0.5 mL) was 200. A comparison of the analytical performance of the current procedure with other literature is provided in Table 1. It is obvious that the developed procedure achieved better or comparable precision, higher preconcentration factor and with one exception (Lemos, Bezerra, & Amorim, 2008) the lower detection limit in comparison with other previously reported methods. Furthermore, it is noteworthy that the capacity of the synthesized ion imprinted polymer based on graphene oxide/magnetic chitosan is significantly higher than most of the other zinc imprinted polymers which confirm the superiority of the polymer synthesized by surface imprinting methodology in comparison with traditional methods.
3.5. Application To verify the feasibility of the method, the developed procedure was applied to the determination of zinc ions in real samples including well water, drinking water, black tea, rice, and milk. The accuracy of the procedure was assessed through the recovery experiments from samples spiked with the known amount of zinc as well as a comparison of the results with data obtained from independent analysis using electrothermal atomic absorption spectrometry (ETAAS). The results summarized in Tables 2 demonstrate that the recoveries of the spiked sample are in a satisfactory range of 96.0-106.0 and at 95% confidence levels there is no significant differences between the results of the current method and ETAAS analysis. These results signify the capability and suitability of the developed procedure for the determination of zinc in a wide variety of matrices.
20
4. Conclusion The present study reports the synthesis and analytical application of a novel Zn(II) imprinted polymer grafted on graphene oxide/magnetic chitosan. The synthesized polymer was well characterized using FTIR and SEM. The Langmuir isotherm model fitted well with experimental data demonstrating the homogeneous solid surface of the synthesized ion imprinted polymer and the regular monolayer adsorption of zinc molecules. The adsorption kinetic studies showed that the adsorption process followed a pseudo-second-order kinetic model indicating chemisorption of target analyte onto the sorbent. The synthesized ion imprinted polymer exhibited high selectivity, high sorption capacity and significantly fast mass transfer which can be attributed to the extremely large surface area and multi imprinting sites of the magnetic chitosan/graphene oxide. The magnetic field sensitivity enables the simple, rapid and efficient separation of the sorbent thereby shortening the separation and improving the efficiency of the extraction process. In summary, the specific recognition capability, high adsorption capacity, high preconcentration factor, low detection limit and good precision and accuracy signifies that the developed procedure is a promising methodology for the selective and accurate determination of zinc in a wide variety of real samples.
Conflict of interest The authors declare no conflict of interest.
21
References Abbasi-Tarighat, M., Shahbazi, E., & Niknam, K. (2013). Simultaneous determination of Mn2+ and Fe3+ as 4,4′[(4-cholorophenyl)methylene] bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) complexes in some foods, vegetable and water samples by artificial neural networks. Food Chemistry, 138(2–3), 991-997. Barati, A., Kazemi, E., Dadfarnia, S., & Haji Shabani, A. M. (2017). Synthesis/characterization of molecular imprinted polymer based on magnetic chitosan/graphene oxide for selective separation/preconcentration of fluoxetine from environmental and biological samples. Journal of Industrial and Engineering Chemistry, 46, 212-221. Behbahani, M., Salarian, M., Bagheri, A., Tabani, H., Omidi, F., & Fakhari, A. (2014). Synthesis, characterization and analytical application of Zn(II)-imprinted polymer as an efficient solid-phase extraction technique for trace determination of zinc ions in food samples. Journal of Food Composition and Analysis, 34(1), 81-89. Behnia, N., Asgari, M., & Feizbakhsh, A. (2015). Sub-nanomolar detection of zinc on the ionimprinted polymer modified glassy carbon electrode. Journal of Environmental Chemical Engineering, 3(1), 271-276. Chang, Y., Zhang, L., Ying, H., Li, Z., Lv, H., & Ouyang, P. (2010). Desulfurization of gasoline using molecularly imprinted chitosan as selective adsorbents. Applied Biochemistry and Biotechnology, 160(2), 593-603. Cheung, W. H., Szeto, Y. S., & McKay, G. (2007). Intraparticle diffusion processes during acid dye adsorption onto chitosan. Bioresource Technology, 98(15), 2897-2904. Dadfarnia, S., Haji Shabani, A. M., Shirani Bidabadi, M., & Jafari, A. A. (2010). A novel ionic liquid/micro-volume back extraction procedure combined with flame atomic absorption
22
spectrometry for determination of trace nickel in samples of nutritional interest. Journal of Hazardous materials, 173(1–3), 534-538. Deng, P., Xu, Z., & Kuang, Y. (2014). Electrochemical determination of bisphenol A in plastic bottled drinking water and canned beverages using a molecularly imprinted chitosan– graphene composite film modified electrode. Food Chemistry, 157, 490-497. Deng, Y., Qi, D., Deng, C., Zhang, X., & Zhao, D. (2008). Superparamagnetic highmagnetization microspheres with an Fe3O4 @SiO2 Core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins. Journal of the American Chemical Society, 130(1), 28-29. Duan, H., Li, L., Wang, X., Wang, Y., Li, J., & Luo, C. (2015). β-Cyclodextrin/chitosanmagnetic graphene oxide-surface molecularly imprinted polymer nanocomplex coupled with chemiluminescence biosensing of bovine serum albumin. RSC Advances, 5(84), 6839768403. Feist, B., & Mikula, B. (2014). Preconcentration of heavy metals on activated carbon and their determination in fruits by inductively coupled plasma optical emission spectrometry. Food Chemistry, 147, 302-306. Han, D., Yan, L., Chen, W., & Li, W. (2011). Preparation of chitosan/graphene oxide composite film with enhanced mechanical strength in the wet state. Carbohydrate Polymers, 83(2), 653658. Kazemi, E., Dadfarnia, S., & Haji Shabani, A. M. (2015). Dispersive solid phase microextraction with magnetic graphene oxide as the sorbent for separation and preconcentration of ultratrace amounts of gold ions. Talanta, 141, 273-278.
23
Kazemi, E., Haji Shabani, A. M., & Dadfarnia, S. (2015). Synthesis and characterization of a nanomagnetic ion imprinted polymer for selective extraction of silver ions from aqueous samples. Microchimica Acta, 182(5-6), 1025-1033. Langmuir, I. (1916). The constitution and fundamental properties of solids and liquids. Part I. Solids. Journal of the American Chemical Society, 38(11), 2221-2295. Lemos, V. A., Bezerra, M. A., & Amorim, F. A. C. (2008). On-line preconcentration using a resin functionalized with 3,4-dihydroxybenzoic acid for the determination of trace elements in biological samples by thermospray flame furnace atomic absorption spectrometry. Journal of Hazardous materials, 157(2–3), 613-619. Liu, B., Han, M., Guan, G., Wang, S., Liu, R., & Zhang, Z. (2011). Highly-controllable molecular imprinting at superparamagnetic iron oxide nanoparticles for ultrafast enrichment and separation. The Journal of Physical Chemistry C, 115(35), 17320-17327. Machado, I., Bergmann, G., & Pistón, M. (2016). A simple and fast ultrasound-assisted extraction procedure for Fe and Zn determination in milk-based infant formulas using flame atomic absorption spectrometry (FAAS). Food Chemistry, 194, 373-376. Ning, F., Peng, H., Li, J., Chen, L., & Xiong, H. (2014). Molecularly imprinted polymer on magnetic graphene oxide for fast and selective extraction of 17 β-estradiol. Journal of Agricultural and Food Chemistry, 62(30), 7436-7443. Özacar, M., & Şengil, İ. A. (2005). A kinetic study of metal complex dye sorption onto pine sawdust. Process Biochemistry, 40(2), 565-572. Ozbek, N., & Akman, S. (2016). Method development for the determination of calcium, copper, magnesium, manganese, iron, potassium, phosphorus and zinc in different types of breads by microwave induced plasma-atomic emission spectrometry. Food Chemistry, 200, 245-248.
24
Pourreza, N., & Naghdi, T. (2014). Combined cloud point-solid phase extraction by dispersion of TiO2
nanoparticles
in
micellar
media
followed
by
semi-microvolume
UV–vis
spectrophotometric detection of zinc. Talanta, 128, 164-169. Qiu, H., Luo, C., Sun, M., Lu, F., Fan, L., & Li, X. (2012). A chemiluminescence array sensor based on graphene-magnetite-molecularly imprinted polymers for determination of benzenediol isomers. Analytica Chimica Acta, 744, 75-81. Ribas, T. C. F., Tóth, I. V., & Rangel, A. O. S. S. (2017). A solid phase extraction flow injection spectrophotometric method for the zinc determination in plants. Microchemical Journal, 130, 366-370. Roushani, M., Abbasi, S., Khani, H., & Sahraei, R. (2015). Synthesis and application of ionimprinted polymer nanoparticles for the extraction and preconcentration of zinc ions. Food Chemistry, 173, 266-273. Säbel, C. E., Neureuther, J. M., & Siemann, S. (2010). A spectrophotometric method for the determination of zinc, copper, and cobalt ions in metalloproteins using zincon. Analytical Biochemistry, 397(2), 218-226. Scherz, H., & Kirchhoff, E. (2006). Trace elements in foods: Zinc contents of raw foods-A comparison of data originating from different geographical regions of the world. Journal of Food Composition and Analysis, 19(5), 420-433. Shakerian, F., Dadfarnia, S., & Haji Shabani, A. M. (2012). Synthesis and application of nanopore size ion imprinted polymer for solid phase extraction and determination of zinc in different matrices. Food Chemistry, 134(1), 488-493. Shamsipur, M., Rajabi, H. R., Pourmortazavi, S. M., & Roushani, M. (2014). Ion imprinted polymeric nanoparticles for selective separation and sensitive determination of zinc ions in
25
different matrices. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 117, 24-33. Su, X., Li, X., Li, J., Liu, M., Lei, F., Tan, X., Li, P., & Luo, W. (2015). Synthesis and characterization of core–shell magnetic molecularly imprinted polymers for solid-phase extraction and determination of Rhodamine B in food. Food Chemistry, 171, 292-297. Surikumaran, H., Mohamad, S., & Sarih, N. (2014). Molecular imprinted polymer of methacrylic acid functionalised β-cyclodextrin for selective removal of 2,4-dichlorophenol. International Journal of Molecular Sciences, 15(4), 6111. Tahmasebi, E., & Yamini, Y. (2012). Facile synthesis of new nano sorbent for magnetic solidphase extraction by self assembling of bis-(2,4,4-trimethyl pentyl)-dithiophosphinic acid on Fe3O4@Ag core@shell nanoparticles: Characterization and application. Analytica Chimica Acta, 756, 13-22. Travlou, N. A., Kyzas, G. Z., Lazaridis, N. K., & Deliyanni, E. A. (2013). Functionalization of graphite oxide with magnetic chitosan for the preparation of a nanocomposite dye adsorbent. Langmuir, 29(5), 1657-1668. Wang, Y., Wang, E., Wu, Z., Li, H., Zhu, Z., Zhu, X., & Dong, Y. (2014). Synthesis of chitosan molecularly imprinted polymers for solid-phase extraction of methandrostenolone. Carbohydrate Polymers, 101, 517-523. Zhao, J., Han, B., Zhang, Y., & Wang, D. (2007). Synthesis of Zn(II) ion-imprinted solid-phase extraction material and its analytical application. Analytica Chimica Acta, 603(1), 87-92.
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Figure captions Fig. 1. FT-IR spectra of magnetic nanoparticles (a) graphene oxide (b) graphene oxide/magnetic chitosan nanocomposite (c) zincon (d) zinc-zincon complex (e) Zn(II) imprinted polymer grafted on graphene oxide/magnetic chitosan before (f) and after (g) leaching. Fig. 2. SEM images of magnetic nanoparticles (a) graphene oxide (b) graphene oxide/magnetic chitosan nanocomposite (c) Zn(II) imprinted polymer grafted on graphene oxide/magnetic chitosan (d). Fig. 3. The structure of zincon and complex formation between zinc and zincon (a). The pKa values and ionization process of zincon (b). Fig. 4. The effect of pH (a) type of desorbing solution (b) sorption time (c) and ionic strength of
the sample solution (d) on the extraction efficiency of zinc. Conditions: sample volume, 100.0 mL; zinc concentration, 2.0 µg L-1 ; amount of sorbent, 20.0 mg; desorbing solution volume, 0.5 mL; desorption time, 5 min.
27
- The first synthesis of Zn(II) ion imprinted polymer grafted on GO/Chm. - Development of a novel MSPE/FAAS method for the determination of Zinc in food samples. - Achievement of high selectivity, high sorption capacity & fast mass transfer. - The capability of detection of zinc ions with a low detection limit of 0.09 µg L-1.
28
29
30
Fig. 1
Fig. 2
31
Fig. 3
32
Fig. 4
33
Table 1 Comparison of analytical characteristics of the present method and some previously reported methods for the extraction and determination of zinc.
Analytical Sorbent
Matrix
Sorbent capacity
system
-1
(mg g )
LOD
RSD PF
(µg L-1)
Ref (%)
IIP
FAAS
Water, cereals
2.73
0.8
118
3.4
(Shakerian et al., 2012)
IIP
FAAS
Milk, Rice, tea, water
22.11
1.0
100
3.0
(Roushani et al., 2015)
IIP
FAAS
Water, Juice, syrup
0.13
0.33
120
2.7
(Shamsipur et al., 2014)
IIP
FAAS
Vegetable, Meat, Milk, Water
68.6
0.15
78
2.8
(Behbahani et al., 2014)
ICP-OES
Fruits
2.40
1.5
80
1.3
(Feist et al., 2014)
Functionalized resin
FAAS
Standard reference materials
-
0.077
62
3.4
(Lemos et al., 2008)
IIP -GO/Chm
FAAS
Milk, Rice, tea, water
71.4
0.09
200
2.7
This work
Activated carbon
LOD; Limit of detection, PF; Preconcentration factor, RSD; Relative standard deviation
34
Table 2
Determination of zinc in different water and food samples.
Zinc
Recovery (%)
Sample
Well water (µg L-1)
Drinking water (µg L-1)
Black tea (µg g-1)
Rice (µg g-1)
Milk (µg g-1)
ET-AAS
Experimental t٭
Added
Found
0
N.D
-
N.D
-
1.0
1.05 ± 0.05
105.0
-
-
2.0
1.95 ± 0.13
97.5
-
-
0
1.00 ± 0.01
-
0.97 ± 0.04
1.26
1.0
2.06 ± 0.07
106.0
-
-
2.0
2.95 ± 0.09
97.5
-
-
0
2.00 ± 0.02
-
2.05 ± 0.08
1.05
1.0
2.96 ± 0.08
96.0
-
-
2.0
3.94 ± 0.14
97.0
-
-
0
1.01 ± 0.02
-
1.09 ± 0.05
2.56
1.0
1.98 ± 0.05
97.0
-
-
2.0
2.96 ± 0.12
97.5
-
-
0
1.16 ± 0.05
-
1.22 ± 0.03
1.78
1.0
2.12 ± 0.07
96.0
-
-
2.0
3.18 ± 0.08
101.0
-
-
٭The t for degree of freedom of 4 at 95% confidence level is 2.78, N.D: not detected
35
S0308-8146(17)31035-X http://dx.doi.org/10.1016/j.foodchem.2017.06.053 FOCH 21275
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
6 March 2017 3 June 2017 7 June 2017
Please cite this article as: Kazemi, E., Dadfarnia, S., Haji Shabani, A.M., Ranjbar, M., Synthesis, characterization, and application of a Zn (II)-imprinted polymer grafted on graphene oxide/magnetic chitosan nanocomposite for selective extraction of zinc ions from different food samples, Food Chemistry (2017), doi: http://dx.doi.org/10.1016/ j.foodchem.2017.06.053
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Synthesis, characterization, and application of a Zn (II)-imprinted polymer grafted on graphene oxide/magnetic chitosan nanocomposite for selective extraction of zinc ions from different food samples
Elahe Kazemi, Shayessteh Dadfarnia٭, Ali Mohammad Haji Shabani, Mansoureh Ranjbar Department of Chemistry, Yazd University, Safaieh, 89195-741, Yazd, Iran
* Corresponding author. Tel: +983531232667; fax: +983538210644 E-mail addresses: [email protected] (S. Dadfarnia), [email protected] (A. M. Haji Shabani), [email protected] (E. Kazemi)
1
Abstract A novel Zn(II) imprinted polymer was synthesized via a co-precipitation method using graphene oxide/magnetic chitosan nanocomposite as supporting material. The synthesized imprinted polymer was characterized by Fourier transform infrared spectrometry (FTIR) and scanning electron microscopy (SEM) and applied as a sorbent for selective magnetic solid phase extraction of zinc followed by its determination by flame atomic absorption spectrometry. The kinetic and isothermal adsorption experiments were carried out and all parameters affecting the extraction process was optimized. Under the optimal experimental conditions, the developed procedure exhibits a linear dynamic range of 0.5–5.0 µg L-1 with a detection limit of 0.09 µg L-1 and quantification limit of 0.3 µg L-1. The maximum sorption capacity of the sorbent was found to be 71.4 mg g-1. The developed procedure was successfully applied to the selective extraction and determination of zinc in various samples including well water, drinking water, black tea, rice, and milk.
Keywords: Magnetic solid phase extraction; Flame atomic absorption spectroscopy; Ion imprinted polymer; Graphene oxide; Chitosan; Zinc
Running title: Synthesis & analytical application of novel Zn(II)-imprinted polymer
2
1. Introduction Zinc is one of the most important essential trace elements and considered to be a vital micronutrient as it has shown a wide range of biochemical functions in all living organisms. Zinc deficiency in the human body causes several disorders including growth retardation, diarrhea, eye and skin lesions, immunity depression and malfunction of wound healing (Scherz & Kirchhoff, 2006). On the other hand, an excessive intake of zinc can be toxic and harmful and lead to various chronic and acute adverse effects (Zhao, Han, Zhang, & Wang, 2007). Accordingly, trace amount determination of zinc in different matrices is of great importance. Various analytical techniques including inductively coupled plasma optical emission spectrometry (Feist & Mikula, 2014), voltammetry (Behnia, Asgari, & Feizbakhsh, 2015), spectrophotometry (Pourreza & Naghdi, 2014; Ribas, Tóth, & Rangel, 2017), and induced plasma-atomic emission spectrometry (Ozbek & Akman, 2016) have been employed for the determination of zinc. Furthermore, based on our literature survey flame atomic absorption spectrometry is the most frequently used technique for this purpose (Behbahani, Salarian, Bagheri, Tabani, Omidi, & Fakhari, 2014; Lemos, Bezerra, & Amorim, 2008; Roushani, Abbasi, Khani, & Sahraei, 2015; Shakerian, Dadfarnia, & Haji Shabani, 2012; Shamsipur, Rajabi, Pourmortazavi, & Roushani, 2014). However, the trace level determination of metal ions in complex matrices commonly necessitate a step of sample preparation before instrumental analysis. The application of imprinted polymers (IIPs) as cost-effective and robust smart materials with the peculiar recognition properties in trace or ultra-trace analysis provide significant breakthroughs in separation or preconcentration chemistry. Bulk and precipitation polymerization are the most commonly utilized method for the preparation of imprinted polymers. However, some major
3
drawbacks have been reported in the resultant polymers. For example, heterogeneous distribution of the binding sites inside their structure leads to poor site accessibility of the target and therefore undesirable sorption/desorption kinetics as well as slow mass transfer (Su, Li, Li, Liu, Lei, Tan, et al., 2015). It has proven that controlling the position of the binding sites on the material’s surfaces via surface imprinting can efficiently resolve these problems. This methodology is based on the synthesis of the imprinted polymer on the surface of a supporting material (Deng, Qi, Deng, Zhang, & Zhao, 2008). Among various supporting materials, Fe3O4 nanoparticles individually (Kazemi, Haji Shabani, & Dadfarnia, 2015) or incorporated into other nano or micro materials (Qiu, Luo, Sun, Lu, Fan, & Li, 2012) has enjoyed wide range of application as it provides large external surface area to volume ratio as well as facilitating the separation of the sorbent via external magnetic field. Graphene oxide (GO) with possessing extremely large surface area, extraordinary physical and mechanical stability and especially plentiful presence of hydroxyl, epoxide, and carboxylic functional groups which enables its surface chemical modification has also attracted great attention in preparation of imprinted polymers (Ning, Peng, Li, Chen, & Xiong, 2014; Qiu, Luo, Sun, Lu, Fan, & Li, 2012). There are some other reports using chitosan in the preparation of imprinted polymers. Chitosan is a biocompatible and biodegradable hydrophilic polymer with abundant hydroxyl and amino functional groups which is known to be one of the most abundant natural amino-polysaccharide (Chang, Zhang, Ying, Li, Lv, & Ouyang, 2010; Wang, Wang, Wu, Li, Zhu, Zhu, et al., 2014). In recent years, the research interest has turned into integrating materials with different properties in order to provide multi imprinting site, enhanced surface area, and sorption capacity. In this regards, the integration of graphene oxide and chitosan which occurred through the special interaction between the epoxy groups of GO and primary amine
4
groups of chitosan as well as the H-bonding between them have been utilized in preparation of imprinted polymers (Deng, Xu, & Kuang, 2014; Han, Yan, Chen, & Li, 2011). However, based on our literature review there is only two works including our previous study dealing with the triplicate combination of chitosan, magnetic nanoparticles and graphene oxide in preparation of imprinted polymers (Barati, Kazemi, Dadfarnia, & Haji Shabani, 2017; Duan, Li, Wang, Wang, Li, & Luo, 2015). Up to now, several kinds of Zn(II)-imprinted polymers have been synthesized and applied for selective extraction of zinc from different samples which all of them have been prepared using the traditional methods (Behbahani, Salarian, Bagheri, Tabani, Omidi, & Fakhari, 2014; Behnia, Asgari, & Feizbakhsh, 2015; Roushani, Abbasi, Khani, & Sahraei, 2015; Shakerian, Dadfarnia, & Shabani, 2012; Shamsipur, Rajabi, Pourmortazavi, & Roushani, 2014; Zhao, Han, Zhang, & Wang, 2007). The aim of the present study was to prepare a sorbent exhibiting high selectivity, high sorption capacity and significantly fast mass transfer for zinc ions with simple and ease of separation. For this purpose, a novel zinc imprinted polymer on supporting material exploiting the unique characteristic of graphene oxide, chitosan and magnetic nanoparticles was synthesized and its performance for selective separation, preconcentration, and extraction of Zn2+ ions from different matrices was evaluated. The structure and binding properties of the synthesized polymer were thoroughly studied and all the important parameters influencing the extraction of zinc ions were investigated and optimized. Finally, the performance of the synthesized polymer and feasibility of the developed methods for the selective extraction and determination of zinc from water and different food samples was evaluated.
5
2. Experimental 2.1. Reagents and materials Ethylene glycol dimethacrylate (EGDMA), acrylic acid (AA), styrene, glutaraldehyde (25 wt.%), zincon (C20H16N4O6S), ferric chloride (FeCl3.6H2O), ferrous chloride (FeCl2.4H2O), graphite powder, ethylenediaminetetraacetic acid (EDTA), histidine (C6H9N3O2) and all solvents used in this study were of analytical reagent grade and purchased from Merck Company (Darmstadt, Germany). Chitosan with an average molecular weight of 350 kDa and deacetylation degree
of
>75%
was
obtained
from
Sigma-Aldrich
(Missouri,
USA).
2,
2'
-
Azobisisobutyronitrile (AIBN) was prepared from ACROS Company (New Jersey, USA). All solutions throughout the study were prepared using double distilled water. A stock solution of zinc at 1000 mg L-1 was prepared by dissolving an appropriate amount of Zn(NO3)2.6H2O in double distilled water. Working solutions were prepared through serial dilutions of the stock solution with doubly distilled water.
2.2. Instrumentation An Analytik Jena novAA 300 atomic absorption spectrometer (model 330, Jena, Germany) equipped with a Zn-hollow cathode lamp (HCL) and an air-acetylene flame atomizer was used for determination of zinc ions. The operation conditions were adjusted according to the manufacturer’s recommendation as follows: HCL wavelength: 213.9 nm, Lamp current: 4.0 mA, spectral bandwidth: 0.5 nm. The pH measurements were carried out using a digital pH meter, Metrohm model 827 (Herisau, Switzerland), equipped with a combined glass calomel electrode. The magnetic phase separation was carried out by means of a strong magnet (1.2 T, 10 cm × 5 cm × 2 cm).
6
2.3. Synthesis of the imprinted sorbent In the first step for the synthesis of magnetic ion imprinted polymer, Fe3O4 nanoparticles, and graphene oxide were separately synthesized via co-precipitation method (Tahmasebi & Yamini, 2012) and a modified Hummers method (Kazemi, Dadfarnia, & Haji Shabani, 2015), respectively. Next, to prepare the magnetic composite of graphene oxide/chitosan (GO/Chm), an accurately weighted amount of pure chitosan (2.0 g) was dissolved in 100.0 mL of acetic acid solution (2.0% v/v), and mixed by sonication for 30 min. 0.75 g of Fe3O4 nanoparticles was added to the mixture and stirred for 2 h. Thereafter, 30.0 mL of glutaraldehyde (25.0 wt.% in water) and 1.5 g of graphene oxide was added to the mixture and the pH was adjusted to 9.0– 10.0 by the addition of ammonia solution. The mixture was stirred at 80 ºC. After 1 h, the precipitate of GO/Chm nanocomposites was collected using an external magnet and dried under vacuum at 60 ºC (Travlou, Kyzas, Lazaridis, & Deliyanni, 2013). The prepared magnetic nanocomposite (GO/Chm) was then modified with acrylic acid. For this purpose, 1.0 g of the nanocomposite was added to 100.0 mL of ethanol under sonication. 10.0 mL of acrylic acid was added and the mixture was stirred at the rate of 300 rpm for 2h. At the end of this process, the precipitate of the acrylic acid modified GO/Chm nanocomposite was collected by the aid of an external magnet and dried under vacuum at 50 ºC (Liu, Han, Guan, Wang, Liu, & Zhang, 2011). The imprinted polymer supported on the magnetic nanocomposite of graphene oxide/chitosan (IIP-GO/Chm) was synthesized through coprecipitation polymerization. For this purpose, 1.0 mmol zincon was dissolved in 60.0 mL mixture solution of ethanol/acetonitrile (1:2 v/v). Then, 1.0 mmol Zn(NO3)2.6H2O was added and the mixture was stirred for 1h to allow
7
completion of the dark blue complex formation between zinc and zincon. 1.0 g of the previously prepared nanocomposite was added to the mixture and mixed by mechanical stirring for another hour. Then, 32.0 mmol of EGDMA and 8.0 mmol styrene was added and stirred for a few second to achieve a homogenized solution. 50.0 mg of AIBN were added, and the mixture was transferred to an ice bath (0 ºC) and purged with nitrogen for 15 min. The reaction flask was sealed and thermally polymerized in a water bath at 60 ºC for 24 h. The obtained magnetic polymer was collected using an external magnet, washed with ethanol and distilled water, and dried at 60 ºC under vacuum. At the final step, the template ion was removed by repetitive extraction with EDTA solution (0.005 mol L-1, pH = 5.5) until no analyte was detected by flame atomic absorption spectrometry. The product which was turned from blue to purple due to leaching was collected, dried and stored for further use. Non-imprinted polymer (NIP) was synthesized using the same procedure, without the presence of zinc ions. Fig. S1 shows the schematic representation of the synthesis process.
2.4. Real sample preparation The water samples including drinking water and well water were filtered through a 0.45 mm pore filter. The amount of zinc in 100.0 mL of these samples was determined using the developed procedure. The rice sample, purchased from a local supermarket, was cleaned, rinsed with double distilled water, finely grounded and oven-dried at 60 ºC. 100.0 mg of the pre-treated sample was transferred to 100.0 mL beaker. 10.0 mL of concentrated nitric acid (65% w/w) was added and the mixture was heated to dryness. The residue was cooled to the ambient temperature, then 3.0 mL of H2O2 (30%, w/w) was added and again heated to dryness. The residue was then dissolved
8
in 10.0 mL of double distilled water and filtered. After adjustment of pH using 2.5 mL borate buffer (pH= 8.5), the filtrate was diluted to 100.0 mL with double distilled water and treated according to the developed procedure (Abbasi-Tarighat, Shahbazi, & Niknam, 2013). The black tea sample was prepared from a local supermarket. The tea sample was rinsed with cold distilled water, oven-dried at 60 ºC and grounded. 10.0 mL of concentrated nitric acid (65% w/w) was added to 100.0 mg of the sample and the mixture was heated to dryness. The resultant residue was dissolved in 10.0 mL of double distilled water and filtered. After adjustment of pH using 2.5 mL borate buffer (pH = 8.5), the filtrate was diluted to 100.0 mL with distilled water and treated according to developed procedure (Dadfarnia, Haji Shabani, Shirani Bidabadi, & Jafari, 2010). The milk sample was prepared as follows: 10.0 mL of concentrated nitric acid followed by 2.0 mL of 30% (w/w) hydrogen peroxide were added to 0.15 g of the accurately weighed dried milk sample. The mixture was heated at 100 ºC for 15 minutes, and allowed to cool at room temperature. Afterward, the mixture was filtered through a 0.45 mm pore filter, the pH was adjusted to 8.5 by the addition of 2.5 mL of borate buffer, transferred to a 100.0 mL volumetric flask and the volume was adjusted with distilled water (Machado, Bergmann, & Pistón, 2016). The resulting solution was analyzed according to the given general procedure.
2.5. General procedure The extraction of zinc ions using the prepared sorbent was carried out by the batch experiments as follows: 20.0 mg of the synthesized magnetic ion imprinted polymer was added to 100.0 mL of sample/standard solution containing no more than 0.5 µg zinc and the pH was adjusted to 8.5 by the addition of 2.5 mL of borate buffer. The mixture was stirred mechanically
9
for 10 minutes to complete the extraction process. Thereupon, the sorbent was held by an external magnet and the supernatant was decanted. 0.5 mL of EDTA solution (0.005 mol L-1, pH = 5.5) was then added to the sorbent and the mixture was sonicated for 5 minutes to desorb the retained zinc ions. The desorbing solution was separated by the aid of an external magnet and introduced to atomic absorption spectrometry for quantification.
2.6. Adsorption studies To study the binding and adsorption properties of the synthesized ion imprinted polymer, the batch mode sorption experiments were carried out as follow: 20.0 mg of the prepared sorbent was equilibrated with 100.0 mL of the zinc solutions with a different initial concentration within the range of 50.0–300.0 mg L-1 at the optimized conditions. At certain time intervals up to 6h, the concentration of zinc remaining in the solution was determined by flame atomic absorption spectrometry. The amount of zinc retained on the sorbent (qt) at the time of t was calculated using a mass balance relationship as follow: =
− V W Where Co is the initial concentration of zinc and Ct (mg L-1) represent the remaining
concentration of zinc in the solution at time t. V represent the volume of solution (L) and W is the sorbents mass (g). The results of this experiments are depicted in Fig. S2. As it can be seen, the retained amount of Zn2+ ion per unit mass of the polymer was increased almost linearly with the increase in the initial concentration of the zinc ions and gradually reached a plateau indicating the saturation of the active binding sites on the synthesized IIP-GO/Chm. Furthermore, the adsorption equilibrium in solutions with different initial concentrations of zinc was achieved after about 2.5 h, representing the significantly high rate of mass transfer. 10
3. Results and discussion 3.1. Sorbent characterization 3.1.1. FTIR analysis The correct synthesis of Fe3O4 nanoparticles, graphene oxide, the magnetic composite of graphene oxide/chitosan and modified magnetic nanocomposite by zinc-imprinted polymer was investigated using the FTIR spectroscopy (Fig. 1). The spectrum of pure Fe3O4 nanoparticles (Fig. 1a) shows the characteristic peak at 540 cm−1 corresponding to the Fe-O stretching mode of the Fe3O4 lattice. The characteristic absorption peaks of graphene oxide corresponding to C=O stretching of the carboxyl group at 1740 cm-1, C=C stretching vibration of the remaining sp2 character at 1620 cm-1, C-O-C stretching of the epoxy at 1248 cm-1 and C-O-H stretching of the hydroxyl group at 1051 cm-1 can be seen in Fig. 1b. The spectrum of magnetic composite of graphene oxide/chitosan (Fig. 1c) exhibits the characteristic peaks of Fe3O4 nanoparticles at 600 cm-1 and the C=O stretching corresponding to carboxyl group of graphene oxide is downshifted to 1712 cm-1 which is the exhibition of hydrogen bonding between the carboxyl group in the graphene oxide and NH2 in chitosan. The appearance of carboxyl peak at 1400 cm-1 indicates the formation of (-COO- +H3N-R). Furthermore, the appearance of two characteristic peaks at 1633 and 1566 cm-1 which can be ascribed to the C–O stretching vibration of –NHCO– and the N–H bending of NH2 confirms the successful preparation of magnetic nanocomposite of chitosan grafted on the graphene oxide. The spectra of zincon and zinc-zincon complex (Fig. 1d and 1e) show the main characteristic absorption peaks of zincon at 1042 cm-1 corresponding to S=O stretching vibration, at 1107 cm-1 corresponding to C-N stretching vibration, at 1200 cm-1 corresponding to C-O or O-
11
H bending vibration of benzene ring, at 1471 cm-1 corresponding to stretching vibration of benzene ring, at 1580 cm-1 corresponding to N=N stretching vibration and at 1700 cm-1 attributed to C=O stretching. These peaks are all observed in the spectrum of the complex (Fig. 1e) with a slight shift. The peak at 1603 cm-1 is attributed to the bending vibration of N-H which is disappeared in the spectrum of complex due to interaction with zinc. The IR spectrum of the imprinted polymer before and after leaching is demonstrated in Fig. 1f and 1g. As it can be seen, these spectra exhibit similar features of absorption peaks at 1154 cm-1 corresponding to S=O stretching vibration of zincon, 1296 cm-1 corresponding to stretching vibration of phenolic group in zincon and 1724 cm-1 attributed to stretching vibration of C=O in the polymer backbone. The peak observed at 1638 cm-1 is attributed to the amide group in the supporting material which is slightly shifted due to interaction with acrylic acid in the functionalization process. Accordingly, the successful synthesis of ion imprinted polymer and incorporation of zincon in its structure as well as the stability of polymer in all leaching process can be concluded.
3.1.2. SEM analysis The morphology of the Fe3O4 nanoparticles, graphene oxide, the magnetic nanocomposite of graphene oxide-chitosan and IIP coated GO/Chm was characterized by scanning electron microscopy. Figure 2a shows the synthesized spherical nanoparticles of Fe3O4 and Fig.2b. shows the typical flake-like shape of graphene oxide. The SEM of the magnetic nanocomposite of GO/Chm (Fig. 2c) shows the successful aggregation of magnetic nanoparticles and chitosan on the surface of graphene oxide. Fig. 2d illustrates the highly porous threedimensional structure as well as the spherical shape of IIP-GO/Chm. This evidence confirms the successful synthesis of IIP coated GO/Chm. 12
3.1.3. Adsorption Isotherms The adsorption isotherms provide the interactive behavior between the solute and the adsorbent. Thus, the analysis of isotherm data is important in estimating the adsorption capacity as well as describing the affinity and adsorbent surface properties. In this study, Langmuir (Langmuir, 1916), Freundlich (Surikumaran, Mohamad, & Sarih, 2014) and Temkin (Barati, Kazemi, Dadfarnia, & Haji Shabani, 2017) models were used to assess the binding properties of the synthesized IIPs-GO/Chm. The studied models expressed by the following equations: Langmuir:
=
+
Freundlich:
= +
Temkin:
= + =
Where qe (mg g-1) and Ce (mg L-1) are the amount of zinc adsorbed at equilibrium and the equilibrium concentration of zinc in the solution, respectively. qm and b are the Langmuir constant corresponding to the maximum monolayer capacity (mg g-1), and the sorption energy, respectively. The Langmuir model assumes that the sorbent has a homogeneous surface with similar and energetically equivalent binding sites which provide monolayer adsorption. The Freundlich model is commonly utilized to describe multilayer adsorption onto heterogeneous surfaces. Kf (mg g-1) and n are the Freundlich constants, and 1/n represent the surface heterogeneity or exchange intensity. A value of 1/n smaller than 1.0 accounts for a favorable adsorption. The Temkin model illustrates the homogeneous distribution of binding energies up to some maximum and postulates the linear decrease of the heat of the sorption of all molecules in 13
the layer with the surface coverage due to the interactions between the solute molecules and the adsorbent. KT is the equilibrium binding constant (mol-1) ascribed to the maximum binding energy, and the constants A and b represent the heat of sorption and the heat of adsorption respectively. The obtained diagrams for Langmuir, Freundlich and Temkin models are illustrated in Fig. S3 and their corresponding parameters including values of R2 are listed in Table S1. The applicability of the studied models was judged by evaluating the correlation coefficients, R2 values. As shown in Table S1, the R2 value of the Langmuir isotherm was greater and more proximate to unity (0.999) than that of the other models. Therefore, the experimental data of zinc adsorption onto IIP-GO/Chm fitted well to the Langmuir isotherm model indicating the homogeneous solid surface of the synthesized ion imprinted polymer and the regular monolayer adsorption of zinc molecules (Barati, Kazemi, Dadfarnia, & Haji Shabani, 2017). Furthermore, based on the data summarized in Table S1, the maximum predictable sorption capacity of the adsorbent for zinc ions was found to be 71.4 mg g-1.
3.1.4. Adsorption kinetics study To investigate the mechanism of zinc adsorption onto the surface of the IIP-GO/Chm, four kinetic models including the pseudo-first-order, pseudo-second order (Barati, Kazemi, Dadfarnia, & Haji Shabani, 2017), intraparticle diffusion (Cheung, Szeto, & McKay, 2007) and Elovich (Özacar & Şengil, 2005) were used to explore the experimental data. These models are expressed by the following equations: Pseudo-first-order: Pseudo-second order:
!
" log − = log − #.%%
&
=!
' '
+
14
"
Intraparticle diffusion: = () * ' + +
Elovich: = , ln./ + , ln *
qe and qt which are the same in all equations represent the amounts of zinc ion (mg g-1) sorbed at the equilibrium time and time t, respectively. k1 (min-1) and k2 (g mg-1 min-1) are the pseudo-first order and the pseudo-second order rate constant of sorption, respectively. The Kid and I are the intraparticle diffusion rate constant (mg g-1 min-1/2) and the intercept determined from the plot of qt versus t1/2, respectively. In Elovich equation α is the initial sorption rate constant (mg g-1 min1
) and β represent the amount of the surface coverage and the activation energy of chemisorption
(g mg-1). Fig. S4, 5, 6 and 7 shows the experimental data fitted with pseudo-first-order, pseudosecond order, Elovich, and intraparticle diffusion, respectively. As it can be concluded from data summarized in Table S2, the correlation coefficient (R2) values of the pseudo-second-order model are higher than that’s of other models and closer to unity. Therefore, the pseudo-second order equation which suggests the chemisorption of solute onto the sorbent was more suitable to describe the adsorption of zinc ions into the polymer.
3.2. Optimization of extraction conditions In order to have the most desirable extraction performance and achieve satisfactory results, all parameters affecting the extraction efficiency of zinc using the synthesized ion imprinted polymer was investigated and optimized. All optimization experiments were performed in triplicates and the results were averaged.
15
3.2.1. Effect of pH The sample solution pH exerts a profound influence on the overall performance of the SPE procedure, as it can affect the sorptive uptake of the target analyte by changing the surface chemistry of both sorbent and analyte. The driving force for adsorption of zinc ions onto the synthesized ion imprinted polymer is the ability of the zinc in the formation of a stable chelate with N-containing groups existing in zincon. By considering the structure of zincon (Fig. 3a) and its pKa values (Fig. 3b) it is expected that the best complex formation between zinc and zincon take place at alkaline medium (Säbel, Neureuther, & Siemann, 2010). With this perception, the influence of the sample solution pH on the extraction of zinc ions was investigated in the pH range of 4.0–11.0. The pH of the sample solution was adjusted using nitric acid or ammonia solution as well as phosphate buffer (6.0–8.0) and borate buffer (8.0–10.0). The results represented in Fig. 4a showed that as expected the maximum extraction recovery is achieved in the pH range of 7.0–9.0. The decrease in extraction recoveries at lower pH can be explained by protonation of the ligand which leads to incomplete complexation and retention of the analyte by the sorbent, whereas at higher pH, zinc ions are prone to the precipitation as hydroxide. Accordingly, pH of 8.5 was selected as the optimum pH.
3.2.2. Optimization of desorption condition In order to assure the achievement of satisfactory recovery of retained zinc ions, the effect of type of desorbing solution on the extraction performance was investigated. For this purpose, 1.0 mL of different potential desorbing solution including EDTA (0.05 mol L-1), histidine (0.05 mol L-1), HNO3 (1.0 mol L-1), H2SO4 (1.0 mol L-1) and CH3COOH were examined. As it can be concluded from the results represented in Fig. 4b, EDTA (0.05 mol L-1)
16
provided the most effective desorption of the retained zinc ions which can be explained by the competition of the EDTA with zincon in the complex formation with zinc. As the complexation and consequently the desorption capability of EDTA toward zinc may be affected by the pH, a series of EDTA solutions with pHs varying in the range of 2.0–10 was used for desorption of zinc. The results implied that the maximum extraction recovery was obtained in the pH range of 4.5–10.0. Furthermore, the effect of concentration of EDTA in the range of 5 × 10-4 – 0.05 mol L-1 was investigated and 0.005 mol L-1 was chosen as the optimum concentration. The minimum volume of desorption solution required to efficiently desorb the analyte was also investigated by varying its volume in the range of 0.3 to 1.5 mL. The quantitative results were achieved by 0.5 mL of EDTA. Accordingly, 0.5 mL EDTA solution with a concentration of 0.005 mol L-1 and pH of 5.5 was selected for the desorption stage.
3.2.3. Effect of time The adsorption process of the target analyte must be done for enough time to achieve equilibrium. The optimum time for adsorption of zinc defined as the time between the addition of the sorbent to the sample solution and its separation for the sample was determined by application of different time intervals from 1 to 30 minutes. Based on the results (Fig. 4c), 10 minutes was selected as the optimum adsorption time. The effect of desorption time was also investigated in the range of 3-20 minutes and 5 minutes was found to be sufficient for quantitative desorption of analyte.
3.2.4. Effect of amount of sorbent, sample volume and ionic strength
17
The dependency of extraction efficiency to the amount of sorbent was studied by changing the amount of IIP-GO/Chm from 5.0 to 30.0 mg. The results showed that 20.0 mg of the prepared sorbent was adequate to achieve satisfactory results. Consequently, 20.0 mg of IIPGO/Chm was utilized in the subsequent studies. To investigate the breakthrough volume of the sample solution, 0.5 µg of zinc was extracted from different volumes of sample solutions (25.0–200.0 mL). The results revealed that up to 100.0 mL of the sample solution, the extraction process was quantitative. Accordingly, in order to achieve a higher preconcentration factor, 100.0 mL of the sample solution was selected for the extraction of analyte in the following experiments. The influence of the ionic strength on the extraction efficiency of zinc ions was investigated by conducting the extraction process in the presence of different amount of sodium chloride (0-1.5 mol L-1). It was observed (Fig. 4d) that the presence of salt up to 1.0 mol L-1 did not interfere with the extraction process. However, the extraction efficiency decreased at higher concentration of salt. This can be related to the competition of Na+ ions at high concentration with zinc for the sorption sites or the hindrance of mass transfer due to the increase in the viscosity of the sample in the presence of high amount of salt. Accordingly, the following experiments were performed without the addition of salt.
3.3. Selectivity and reusability of the sorbent The selectivity of the synthesized IIP-GO/Chm toward zinc ions was investigated by considering competitive extraction of zinc in the presence of other ions including some transition metals such as Cu2+, Co2+, Ni2+ and Cd2+ which are capable of forming a complex with zincon. The tolerance limit was taken as the maximum concentration of the ions which caused an error
18
not exceeding ±5%. The results summarized in Table S3 showed that the examined ions at the given mole ratio did not interfere in the extraction and determination of zinc ions. Accordingly, the developed procedure using IIP-GO/Chm exhibits high selectivity toward zinc. In order to evaluate the renewability and reusability of the synthesized sorbent, a set of experiments were performed by using the same sorbent repeatedly. For this purpose, the synthesized IIP-GO/Chm was subjected to several adsorption and desorption cycles, under the same experimental conditions. Based on the obtained results (Fig. S8a), the IIP-GO/Chm was stable and could be regenerate without any significant loss of analytical performance up to 9 adsorption/desorption cycle. Furthermore, the analytical performance of IIP-GO/Chm was compared with NIP-GO/Chm by application of these polymers for the extraction of zinc ions under the similar condition. As it can be concluded from Fig. S8b, the application of IIPGO/Chm result in much higher extraction efficiency indicating the higher affinity of the synthesized IIP-GO/Chm toward zinc ions due to imprinting effect.
3.4. Analytical performance The developed procedure was validated by determination of quality parameters including linear dynamic range, coefficient of determination (R2), limit of detection, preconcentration factor, and precision. Under the optimal concentration, the method showed a good linear correlation of analytical signal to the zinc concentration within the range of 0.5–5.0 µg L-1 with a regression equation of A (signal) = 0.019 CZn (µg L-1) + 0.028 and correlation coefficient of R2 = 0.999. The limit of detection of 0.09 µg L-1 and limit of quantification of 0.3 µg L-1 were obtained based on 3Sb/m and 10Sb/m (Sb is the standard deviation of the blank and m is the slope of the calibration graph), respectively. Five replicate measurement of zinc solutions at the
19
concentration of 2.0 µg L-1 gave a relative standard deviation of 2.7%. The enrichment factor, ascribed to the ratio of the maximum volume of the sample solution (100.0 mL) to the final volume of the extract (0.5 mL) was 200. A comparison of the analytical performance of the current procedure with other literature is provided in Table 1. It is obvious that the developed procedure achieved better or comparable precision, higher preconcentration factor and with one exception (Lemos, Bezerra, & Amorim, 2008) the lower detection limit in comparison with other previously reported methods. Furthermore, it is noteworthy that the capacity of the synthesized ion imprinted polymer based on graphene oxide/magnetic chitosan is significantly higher than most of the other zinc imprinted polymers which confirm the superiority of the polymer synthesized by surface imprinting methodology in comparison with traditional methods.
3.5. Application To verify the feasibility of the method, the developed procedure was applied to the determination of zinc ions in real samples including well water, drinking water, black tea, rice, and milk. The accuracy of the procedure was assessed through the recovery experiments from samples spiked with the known amount of zinc as well as a comparison of the results with data obtained from independent analysis using electrothermal atomic absorption spectrometry (ETAAS). The results summarized in Tables 2 demonstrate that the recoveries of the spiked sample are in a satisfactory range of 96.0-106.0 and at 95% confidence levels there is no significant differences between the results of the current method and ETAAS analysis. These results signify the capability and suitability of the developed procedure for the determination of zinc in a wide variety of matrices.
20
4. Conclusion The present study reports the synthesis and analytical application of a novel Zn(II) imprinted polymer grafted on graphene oxide/magnetic chitosan. The synthesized polymer was well characterized using FTIR and SEM. The Langmuir isotherm model fitted well with experimental data demonstrating the homogeneous solid surface of the synthesized ion imprinted polymer and the regular monolayer adsorption of zinc molecules. The adsorption kinetic studies showed that the adsorption process followed a pseudo-second-order kinetic model indicating chemisorption of target analyte onto the sorbent. The synthesized ion imprinted polymer exhibited high selectivity, high sorption capacity and significantly fast mass transfer which can be attributed to the extremely large surface area and multi imprinting sites of the magnetic chitosan/graphene oxide. The magnetic field sensitivity enables the simple, rapid and efficient separation of the sorbent thereby shortening the separation and improving the efficiency of the extraction process. In summary, the specific recognition capability, high adsorption capacity, high preconcentration factor, low detection limit and good precision and accuracy signifies that the developed procedure is a promising methodology for the selective and accurate determination of zinc in a wide variety of real samples.
Conflict of interest The authors declare no conflict of interest.
21
References Abbasi-Tarighat, M., Shahbazi, E., & Niknam, K. (2013). Simultaneous determination of Mn2+ and Fe3+ as 4,4′[(4-cholorophenyl)methylene] bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) complexes in some foods, vegetable and water samples by artificial neural networks. Food Chemistry, 138(2–3), 991-997. Barati, A., Kazemi, E., Dadfarnia, S., & Haji Shabani, A. M. (2017). Synthesis/characterization of molecular imprinted polymer based on magnetic chitosan/graphene oxide for selective separation/preconcentration of fluoxetine from environmental and biological samples. Journal of Industrial and Engineering Chemistry, 46, 212-221. Behbahani, M., Salarian, M., Bagheri, A., Tabani, H., Omidi, F., & Fakhari, A. (2014). Synthesis, characterization and analytical application of Zn(II)-imprinted polymer as an efficient solid-phase extraction technique for trace determination of zinc ions in food samples. Journal of Food Composition and Analysis, 34(1), 81-89. Behnia, N., Asgari, M., & Feizbakhsh, A. (2015). Sub-nanomolar detection of zinc on the ionimprinted polymer modified glassy carbon electrode. Journal of Environmental Chemical Engineering, 3(1), 271-276. Chang, Y., Zhang, L., Ying, H., Li, Z., Lv, H., & Ouyang, P. (2010). Desulfurization of gasoline using molecularly imprinted chitosan as selective adsorbents. Applied Biochemistry and Biotechnology, 160(2), 593-603. Cheung, W. H., Szeto, Y. S., & McKay, G. (2007). Intraparticle diffusion processes during acid dye adsorption onto chitosan. Bioresource Technology, 98(15), 2897-2904. Dadfarnia, S., Haji Shabani, A. M., Shirani Bidabadi, M., & Jafari, A. A. (2010). A novel ionic liquid/micro-volume back extraction procedure combined with flame atomic absorption
22
spectrometry for determination of trace nickel in samples of nutritional interest. Journal of Hazardous materials, 173(1–3), 534-538. Deng, P., Xu, Z., & Kuang, Y. (2014). Electrochemical determination of bisphenol A in plastic bottled drinking water and canned beverages using a molecularly imprinted chitosan– graphene composite film modified electrode. Food Chemistry, 157, 490-497. Deng, Y., Qi, D., Deng, C., Zhang, X., & Zhao, D. (2008). Superparamagnetic highmagnetization microspheres with an Fe3O4 @SiO2 Core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins. Journal of the American Chemical Society, 130(1), 28-29. Duan, H., Li, L., Wang, X., Wang, Y., Li, J., & Luo, C. (2015). β-Cyclodextrin/chitosanmagnetic graphene oxide-surface molecularly imprinted polymer nanocomplex coupled with chemiluminescence biosensing of bovine serum albumin. RSC Advances, 5(84), 6839768403. Feist, B., & Mikula, B. (2014). Preconcentration of heavy metals on activated carbon and their determination in fruits by inductively coupled plasma optical emission spectrometry. Food Chemistry, 147, 302-306. Han, D., Yan, L., Chen, W., & Li, W. (2011). Preparation of chitosan/graphene oxide composite film with enhanced mechanical strength in the wet state. Carbohydrate Polymers, 83(2), 653658. Kazemi, E., Dadfarnia, S., & Haji Shabani, A. M. (2015). Dispersive solid phase microextraction with magnetic graphene oxide as the sorbent for separation and preconcentration of ultratrace amounts of gold ions. Talanta, 141, 273-278.
23
Kazemi, E., Haji Shabani, A. M., & Dadfarnia, S. (2015). Synthesis and characterization of a nanomagnetic ion imprinted polymer for selective extraction of silver ions from aqueous samples. Microchimica Acta, 182(5-6), 1025-1033. Langmuir, I. (1916). The constitution and fundamental properties of solids and liquids. Part I. Solids. Journal of the American Chemical Society, 38(11), 2221-2295. Lemos, V. A., Bezerra, M. A., & Amorim, F. A. C. (2008). On-line preconcentration using a resin functionalized with 3,4-dihydroxybenzoic acid for the determination of trace elements in biological samples by thermospray flame furnace atomic absorption spectrometry. Journal of Hazardous materials, 157(2–3), 613-619. Liu, B., Han, M., Guan, G., Wang, S., Liu, R., & Zhang, Z. (2011). Highly-controllable molecular imprinting at superparamagnetic iron oxide nanoparticles for ultrafast enrichment and separation. The Journal of Physical Chemistry C, 115(35), 17320-17327. Machado, I., Bergmann, G., & Pistón, M. (2016). A simple and fast ultrasound-assisted extraction procedure for Fe and Zn determination in milk-based infant formulas using flame atomic absorption spectrometry (FAAS). Food Chemistry, 194, 373-376. Ning, F., Peng, H., Li, J., Chen, L., & Xiong, H. (2014). Molecularly imprinted polymer on magnetic graphene oxide for fast and selective extraction of 17 β-estradiol. Journal of Agricultural and Food Chemistry, 62(30), 7436-7443. Özacar, M., & Şengil, İ. A. (2005). A kinetic study of metal complex dye sorption onto pine sawdust. Process Biochemistry, 40(2), 565-572. Ozbek, N., & Akman, S. (2016). Method development for the determination of calcium, copper, magnesium, manganese, iron, potassium, phosphorus and zinc in different types of breads by microwave induced plasma-atomic emission spectrometry. Food Chemistry, 200, 245-248.
24
Pourreza, N., & Naghdi, T. (2014). Combined cloud point-solid phase extraction by dispersion of TiO2
nanoparticles
in
micellar
media
followed
by
semi-microvolume
UV–vis
spectrophotometric detection of zinc. Talanta, 128, 164-169. Qiu, H., Luo, C., Sun, M., Lu, F., Fan, L., & Li, X. (2012). A chemiluminescence array sensor based on graphene-magnetite-molecularly imprinted polymers for determination of benzenediol isomers. Analytica Chimica Acta, 744, 75-81. Ribas, T. C. F., Tóth, I. V., & Rangel, A. O. S. S. (2017). A solid phase extraction flow injection spectrophotometric method for the zinc determination in plants. Microchemical Journal, 130, 366-370. Roushani, M., Abbasi, S., Khani, H., & Sahraei, R. (2015). Synthesis and application of ionimprinted polymer nanoparticles for the extraction and preconcentration of zinc ions. Food Chemistry, 173, 266-273. Säbel, C. E., Neureuther, J. M., & Siemann, S. (2010). A spectrophotometric method for the determination of zinc, copper, and cobalt ions in metalloproteins using zincon. Analytical Biochemistry, 397(2), 218-226. Scherz, H., & Kirchhoff, E. (2006). Trace elements in foods: Zinc contents of raw foods-A comparison of data originating from different geographical regions of the world. Journal of Food Composition and Analysis, 19(5), 420-433. Shakerian, F., Dadfarnia, S., & Haji Shabani, A. M. (2012). Synthesis and application of nanopore size ion imprinted polymer for solid phase extraction and determination of zinc in different matrices. Food Chemistry, 134(1), 488-493. Shamsipur, M., Rajabi, H. R., Pourmortazavi, S. M., & Roushani, M. (2014). Ion imprinted polymeric nanoparticles for selective separation and sensitive determination of zinc ions in
25
different matrices. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 117, 24-33. Su, X., Li, X., Li, J., Liu, M., Lei, F., Tan, X., Li, P., & Luo, W. (2015). Synthesis and characterization of core–shell magnetic molecularly imprinted polymers for solid-phase extraction and determination of Rhodamine B in food. Food Chemistry, 171, 292-297. Surikumaran, H., Mohamad, S., & Sarih, N. (2014). Molecular imprinted polymer of methacrylic acid functionalised β-cyclodextrin for selective removal of 2,4-dichlorophenol. International Journal of Molecular Sciences, 15(4), 6111. Tahmasebi, E., & Yamini, Y. (2012). Facile synthesis of new nano sorbent for magnetic solidphase extraction by self assembling of bis-(2,4,4-trimethyl pentyl)-dithiophosphinic acid on Fe3O4@Ag core@shell nanoparticles: Characterization and application. Analytica Chimica Acta, 756, 13-22. Travlou, N. A., Kyzas, G. Z., Lazaridis, N. K., & Deliyanni, E. A. (2013). Functionalization of graphite oxide with magnetic chitosan for the preparation of a nanocomposite dye adsorbent. Langmuir, 29(5), 1657-1668. Wang, Y., Wang, E., Wu, Z., Li, H., Zhu, Z., Zhu, X., & Dong, Y. (2014). Synthesis of chitosan molecularly imprinted polymers for solid-phase extraction of methandrostenolone. Carbohydrate Polymers, 101, 517-523. Zhao, J., Han, B., Zhang, Y., & Wang, D. (2007). Synthesis of Zn(II) ion-imprinted solid-phase extraction material and its analytical application. Analytica Chimica Acta, 603(1), 87-92.
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Figure captions Fig. 1. FT-IR spectra of magnetic nanoparticles (a) graphene oxide (b) graphene oxide/magnetic chitosan nanocomposite (c) zincon (d) zinc-zincon complex (e) Zn(II) imprinted polymer grafted on graphene oxide/magnetic chitosan before (f) and after (g) leaching. Fig. 2. SEM images of magnetic nanoparticles (a) graphene oxide (b) graphene oxide/magnetic chitosan nanocomposite (c) Zn(II) imprinted polymer grafted on graphene oxide/magnetic chitosan (d). Fig. 3. The structure of zincon and complex formation between zinc and zincon (a). The pKa values and ionization process of zincon (b). Fig. 4. The effect of pH (a) type of desorbing solution (b) sorption time (c) and ionic strength of
the sample solution (d) on the extraction efficiency of zinc. Conditions: sample volume, 100.0 mL; zinc concentration, 2.0 µg L-1 ; amount of sorbent, 20.0 mg; desorbing solution volume, 0.5 mL; desorption time, 5 min.
27
- The first synthesis of Zn(II) ion imprinted polymer grafted on GO/Chm. - Development of a novel MSPE/FAAS method for the determination of Zinc in food samples. - Achievement of high selectivity, high sorption capacity & fast mass transfer. - The capability of detection of zinc ions with a low detection limit of 0.09 µg L-1.
28
29
30
Fig. 1
Fig. 2
31
Fig. 3
32
Fig. 4
33
Table 1 Comparison of analytical characteristics of the present method and some previously reported methods for the extraction and determination of zinc.
Analytical Sorbent
Matrix
Sorbent capacity
system
-1
(mg g )
LOD
RSD PF
(µg L-1)
Ref (%)
IIP
FAAS
Water, cereals
2.73
0.8
118
3.4
(Shakerian et al., 2012)
IIP
FAAS
Milk, Rice, tea, water
22.11
1.0
100
3.0
(Roushani et al., 2015)
IIP
FAAS
Water, Juice, syrup
0.13
0.33
120
2.7
(Shamsipur et al., 2014)
IIP
FAAS
Vegetable, Meat, Milk, Water
68.6
0.15
78
2.8
(Behbahani et al., 2014)
ICP-OES
Fruits
2.40
1.5
80
1.3
(Feist et al., 2014)
Functionalized resin
FAAS
Standard reference materials
-
0.077
62
3.4
(Lemos et al., 2008)
IIP -GO/Chm
FAAS
Milk, Rice, tea, water
71.4
0.09
200
2.7
This work
Activated carbon
LOD; Limit of detection, PF; Preconcentration factor, RSD; Relative standard deviation
34
Table 2
Determination of zinc in different water and food samples.
Zinc
Recovery (%)
Sample
Well water (µg L-1)
Drinking water (µg L-1)
Black tea (µg g-1)
Rice (µg g-1)
Milk (µg g-1)
ET-AAS
Experimental t٭
Added
Found
0
N.D
-
N.D
-
1.0
1.05 ± 0.05
105.0
-
-
2.0
1.95 ± 0.13
97.5
-
-
0
1.00 ± 0.01
-
0.97 ± 0.04
1.26
1.0
2.06 ± 0.07
106.0
-
-
2.0
2.95 ± 0.09
97.5
-
-
0
2.00 ± 0.02
-
2.05 ± 0.08
1.05
1.0
2.96 ± 0.08
96.0
-
-
2.0
3.94 ± 0.14
97.0
-
-
0
1.01 ± 0.02
-
1.09 ± 0.05
2.56
1.0
1.98 ± 0.05
97.0
-
-
2.0
2.96 ± 0.12
97.5
-
-
0
1.16 ± 0.05
-
1.22 ± 0.03
1.78
1.0
2.12 ± 0.07
96.0
-
-
2.0
3.18 ± 0.08
101.0
-
-
٭The t for degree of freedom of 4 at 95% confidence level is 2.78, N.D: not detected
35