O R I G I N A L A R T I C L E
Synthesis and characterization of novel nanofiber based
calixarene and its binding efficiency towards chromium
and uranium ions
Fatih O¨ zcan1,2•Mevlu¨t Bayrakcı3• S¸eref Ertul1
Received: 19 January 2016 / Accepted: 11 March 2016 / Published online: 18 March 2016 Ó Springer Science+Business Media Dordrecht 2016
Abstract The objective of this study is to obtain nanofibrous precursor based calixarene with high ion adsorption capacity by electrospinning of blended solution of polyacrylonitrile (PAN) and upper rim functionalized calix[4]arene bearing N-methylglucamine (Calix-NMG). The obtained electrospun fibers were characterized using fourier transform infrared (FT-IR-ATR), scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectrometry (EDX) and thermogravimetric analyses (TGA and DSC). Analysis indicated that preparation of nanofi-bers based Calix-NMG was successfully achieved. The ion binding studies exhibited that the nanofiber based Calix-NMG could be efficiently used for the binding of chromate anions and uranium cations. Nanofiber based calix[4]arene with N-methylglucamine chelating groups may be a good candidate as a filter material for treatment of a large quantity of wastewater owing to their very large surface area as well as both the inclusion and donor–acceptor complexation capability of all surfaces associated with the calixarene skeleton.
Keywords Calixarene Nanofiber N-methylglucamine Uranium Chromate
Introduction
Nanotechnology has gained interest in recent years from both public and private sector organizations. Therefore, nanotechnology has offered several novel products, which have superior properties making them valuable for a wide range of applications. One of the major successes of nan-otechnology has been nanofibers. Nanofibers can be man-ufactured by electrospinning which is a technique to obtain fiber structures that can range in diameter from nanometers to micrometers [1–4]. Electrospun fibers can be made of many types of polymers and composite combinations that can be utilized to imitate properties of native extracellular matrix or to optimize various chemical, mechanical, elec-trical, architectural, and biological properties. Polymeric nanofibers have enormous specific surface area and high flexibility. As a result, nanofiber skeletons have large sur-face-to-volume ratio, micropores, and high porosity [5]. Owing to the special properties of nanofibers, they are used in a wide range of applications such as battery separators, aerospace, transistors, drug delivery systems, capacitors, energy storage, information technology, filtration, super absorbents, as scaffolds for tissue engineering, wound dressings, fuel cells and many electronic applications [6–
16], Highly purified air, water, gasses, chemicals, biologi-cal agents without contaminants are needed intensively in food, pharmaceuticals and biotechnology industries. Therefore, compared the microfibers, nanofibrous media undoubtedly hold great advantage in filtration and nanofi-ber structures can act as protective materials against environmental and infectious agents in hospitals, offices and homes. From this point of view functionalized nano-fibers will be most important topic and used as affinity membranes for filtering heavy metals and toxic anions that are difficult to purify by conventional purification methods. & Mevlu¨t Bayrakcı
mevlutbayrakci@gmail.com
1 Department of Chemistry, Faculty of Science, Selcuk
University, 42075 Konya, Turkey
2 Advanced Technology Research and Application Center,
42075 Konya, Turkey
3 Department of Bioengineering, Faculty of Engineering,
Karamanog˘lu Mehmetbey University, Karaman 70200, Turkey
Furthermore, it is estimated that the current nanofiber market worldwide may be $400 million and will be worth more than $1 billion by 2020 [17]. This increase also shows the importance of the investigations on functionalized nanofibers. Careful examination of the literature reveals that considerable work has been reported on the design and synthesis of nanofibers with supramolecules as cyclodex-trins and their analytical applications [18, 19]. But very little information has appeared in the literature concerning nanofibers derived from calixarene supramolecular and their possible applications [15, 20]. Calixarenes are often referred to as the third generation of supramolecular receptors after crown ethers and cyclodextrins [21, 22]. Calixarenes are cyclic oligomers made of several phenolic units bound with methylene bridges [23–26]. The phenolic OH groups of the calixarene lower rim can be further functionalized to give various ionic receptors for anions, cations, as well as hosts of neutral molecules [27–29]. This excellent skeleton enables calixarenes to act as ‘‘molecular baskets’’ towards neutral or ionic guests. Many different studies about the chromate and dichromate anions (Cr2O27 /HCr2O7) from contaminated water have been
reported, but the toxic effect of chromate is not well doc-umented despite its importance as an environmental pol-lutant [30]. Chromium is one of the most toxic pollutants generated from industrial applications [31]. Chromium generally appears both in trivalent Cr(III) and hexavalent Cr(VI) forms in aqueous media. Especially, trivalent chromium is considered as an essential micronutrient for human, plant and animal metabolism and less toxic than hexavalent chromium which is extremely toxic and car-cinogenic to living organism [32, 33]. Depending on pH and the total concentration of the solution, Cr(VI) is existed in different oxy-anion forms. Cr(VI) is unstable and shows very oxidizing behaviors in the presence of the electron donor in acidic medium. At lower pH values, the dominant form of the chromium is HCrO4 and only CrO24 ions exist above pH 7. The synthesis of supports and hosts for specific anions as dichromate is an important goal. Because Cr(VI) has well known effects on environment and living organism, it is necessarily to remove Cr(VI) from wastewater. Owing to the oxide moieties, the dichromate ions are known as oxyanions. These oxide moieties are potential sites for hydrogen bonding to the host molecule. Pyridine, amino and imino derivatives of calixarene are extremely efficient extractant for oxoanions [34]. Nowa-days many researchers are interested in surface modifica-tion of all kinds of materials such as nanoparticles [35], resin [36] and silica gel [37] with calixarene compounds to obtain new composite materials. Therefore, simple nano-fibers containing calixarenes may result in composites having a range of useful and interesting properties. In
addition, the presence of functionalized calixarene in polymeric matrix provides the functional properties of solid supports. Electrospinning of nanofibers with cal-ixarene units is of particular interest, since in this way, nanofibers with specific function groups can be produced. The polystyrene structures based N-methylglucamine (NMG) have been developed for boron removal from irrigation water [38]. Along with boron removal, (NMG) containing several macroporous resins have been studied for its affinity towards oxoanions of Ge(IV), V(V), Mo(VI), As(V), As(III), W(VI), Se(IV), Se(VI), and Sn(IV) [39], uranium sorption studies were not observed. N-methyl-glucamine (NMG) has a high interaction capability with different types of species owing to the presence chelating tertiary amine and vic diol groups that interacted with different ion species. Therefore, there is a growing interest on (NMG)-based support materials by surface modification methods. Compared the support materials, nanofibers mainly including higher durability in acidic environment, large surface-to-volume ratio, micropores, high porosity and flexibility are excellent support materials especially for separation science. At this context, it is a good tentative idea to combine the multifunctional character of calixarene skeleton based NMG and PAN nanofibers. Even though surface modifications of fibers by calixarene compounds were reported [15,20], to the best of our knowledge, the synthesis of nanofibers with calix [4] arene containing N-methylglucamine chelating groups and their extraction studies were not reported previously. With this in mind, we set out to synthesize PAN nanofiber based calix [4] arene containing N-methylglucamine chelating groups at upper rim of calixarene skeleton and explore its ion binding properties towards chromate anions and uranium cations.
Experimental
General1H and 13C NMR spectra were obtained using a Varian
400 MHz spectrometer operating at 400 MHz. IR spectra were recorded on a Perkin-Elmer spectrum 100 FT-IR spectrometer (ATR). Thermogravimetric analysis (TGA) data were obtained with a Setaram SETSYS thermal ana-lyzer. SEM images were received using a Zeiss LS-10 field emission SEM instrument. UV–Visible spectra were recorded on Jenway 6105 and Shimadzu160A UV–Visible recording spectrophotometers. Millipore Milli-Q Plus water purification system is used for the distilled water. For the pH measurements, An Orion 410Aþ pH meter was used. All of the reagents used in this study were obtained from analytical grade and used without further purification.
Arsenazo III, uranyl acetate dihydrate were purchased from Fluka. Standard stock solution of 0.9787 g/mL ura-nium(VI) was prepared by dissolving the appropriate amounts of uranyl acetate dihydrate in deionized water. A stock arsenazo III solution (0.01 %) was prepared by dis-solving reagent. Adjusting the pH values of the working solutions was carried out using 5 M of sodium acetate buffer to determination of UO2þ2 in aqueous solution. Thin layer chromatography (TLC) was performed using silica gel on glass TLC plates (silica gel H, type 60, Merck).
Synthesis
5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrahydroxycalix [4] arene (1), 25,26,27,28-tetrahydroxycalix [4] arene (2) and 5,17-bis[(N-methylglucamine)methyl]-25,26,27,28-te-trahydroxycalix [4] arene (3) compounds were synthesized according to the literature procedures [21–23,40,41].
Electrospinning
Fibers were electrospun as reported in literature [15]. The polymer solution were prepared by dissolving calixarene compounds in DMF which is containing 20 % (w/v) PAN. The concentration of calixarenes was 20 wt% with respect to the PAN concentration in DMF. The polymer solution was held in a horizontally plastic syringe fitted with a metallic needle of 0.7 mm inner diameter. A stainless steel electrode was immersed in the solution and connected to a high voltage power supply. A metal plate coated with aluminum foil placed opposite served as a counter elec-trode. The applied voltages between the needle tip and collector were set at 20 kV with a tip-to-collector distance of 15 cm. The electrospinning temperature and the relative humidity were 25°C and 50 %, respectively.
Adsorption experiment
The sorption capacities of the synthesized Calix-NMG/ PAN were determined by the following technique. Aqueous solution (10 mL) of Na2Cr2O7 or UO2(CH3COO)22H2O
with 1.0 9 10-4M (for Na2Cr2O7) concentration and
1.15 9 10-5M (for UO2(CH3COO)22H2O) and 25 mg of
the sorbent were pipetted in a stoppered flask that was shaken at 175 rpm and 25°C for 1 h. The sorbent was separated before measurements. The residual dichromate and uranium concentration of aqueous solute was deter-mined spectrophotometrically by UV–Vis analyses at 346 nm for dichromate [23] anions and 652 nm for ura-nium cations [42]. The effect of pH was studied by adjusting the pH of aqueous solutions using diluted HCl and KOH solutions at 25°C. The experiments were
performed three times. From the blank experiments data it was observed that no dichromate extraction occurred in the absence of the Calix-NMG/PAN nanofibers. The percent extraction (E %) was calculated through the absorbance of the aqueous phase measured using the following expression:]
Extraction E%¼ Að 0 AÞ=A0 100 ð1Þ
where A0and A are the initial and final concentrations of
the dichromate ion before and after the extraction, respectively (Eq.1).
Results and discussion
Synthesis of calix[4]arene bearing N-methylglucamine
In this study, we aimed to design a new calix[4]arene nanofiber structure decorated with N-methylglucamine chelating units and explore its sorption behaviors towards dichromate and uranium ions. For this purpose, the required starting materials, p-tert-butylcalix[4]arene 1 and calix[4]arene 2 were synthesized by following the proce-dure available in the literature [43] as shown in Scheme1. N-methylglucamine groups were attached to the upper rim of calix[4]arene skeleton 2 by the substitution reaction so-called Mannich reaction. In this reaction calix[4]arene 2 was interacted with a secondary amine N-methylglucamine and formaldehyde to obtain the cone conformer calix[4]-arene based N-methylglucamine (Calix-NMG) 3 in the presence of AcOH in THF. All of the structures were in the cone conformation in solution.
The synthesis and characterization of nanofibers based calixarene
In this report, PAN nanofibers based Calix-NMG were obtained by electrospinning as shown in Scheme 2. Elec-trospinning technique was used for the modification of PAN nanofibers with calixarene molecules bearing N-methylglucamine units because of the absence of the any reactive groups of the PAN polymer backbone which are capable of covalent bound formation with calixarene skeleton. Two different PAN nanofibers with and without calixarene were also electrospun for the comparison study. SEM, FT-IR (ATR) and TGA-DSC spectroscopic tech-niques were used for the characterization of the newly prepared fiber structures. Morphological changes of fiber structures after modification of PAN nanofibers with and without calixarene derivative were investigated by scan-ning electron microscopy (SEM) analysis.
SEM images of the pure PAN and Calix-NMG/PAN nanofibers were obtained from the homogeneous solution of calixarene derivative and PAN as shown in Fig.1. From the SEM images, it is clearly observed that bead-free nanofibers were produced with diameters mostly ranging from 274 to 290 nm. The surface morphologies of Calix-NMG/PAN nanofibers were obviously different from the unmodified PAN nanofibers. While the surface of the unmodified PAN nanofibers was both smooth and uniform, the surfaces of the Calix-NMG/PAN nanofibers seen rough feasibly due to presence of the calixarene molecules onto PAN nanofibers. In brief, the rough and irregular surface of modified PAN nanofibers showed that the modification of of PAN nanofibers with calixarene molecules was occurred successfully. The presence of calixarene derivatives in the PAN nanofibers was also confirmed by Fourier transform infrared spectroscopy (FT-IR) studies.
The FT-IR (ATR) spectra of nanofiber based Calix-NMG are shown in Fig.2and the FT-IR spectrum of pure PAN nanofiber is also given for comparison. The over-lapping of absorption peaks of calixarene bearing N-methylglucamine units and PAN makes the identification of the individual components in the nanofiber structures rather complicated. One can observe the widening and the overlapping of the bands in the FT-IR spectra of the nanofibers. The FT-IR spectra do not indicate any appearance of new signals or disappearance of existing signals which is assigned to the possible covalent bond formation between calixarene and polyacrylonitrile. This
observation is consistent with the absence of the free reactive groups on polyacrylonitrile polymer scaffold. In Fig.2a, typical bands stretching vibrations of CH and CH2
groups, CN and CH/CH2deformation vibration signals for
PAN was observed around 2920–2930, 2242 and 1250–1453 cm-1 respectively [19]. In case of FT-IR spectrum of Calix-NMG/PAN (Fig.2b), the new and broad band at 3300–3500 cm-1, indicating phenolic hydroxyl groups of calixarene skeleton was observed for the Calix-NMG/PAN compared to the spectrum of pure PAN. In addition, new peak appeared at 1058 cm-1, which can be assigned to the amine C–N stretching vibrations [44]. All these results provide clear evidence the presence of the calixaren molecules onto PAN nanofibers.
TGA and DSC analysis are used to observe the thermal behavior of prepared PAN nanofibers. In Fig.3, the major weight loss for the PAN nanofibers was recorded at around 290–350 °C which is consistent with the main degradation temperature reported for the PAN nanofibers. The weight loss from 365 to 440°C was mainly caused by decompo-sition of the carbon–carbon main chains. Compared to pure PAN nanofibers, it is clearly seen that Calix-NMG/PAN nanofibers have demonstrated slightly higher degradation temperature. This situation exhibited that the incorporation of the calixarene molecules onto the fiber structure resulted in higher thermal stability.
Exothermic and endothermic peaks, exhibiting thermal stabilizing, cyclization of nitrile group and melting pro-cess of PAN nanofibers were observed in DSC curves Scheme 1 Schematic
illustration of the synthesis of calix[4]arene derivatives. (i) Formaldehyde, NaOH, diphenyl ether; (ii) AlCl3,
phenol, toluene, rt, 3 h; (iii) N-methylglucamine,
(Fig.3). An exothermic peak attributable multiple com-plex chemical reactions, such as dehydrogenation, cyclization and crosslinking reactions of PAN was seen around 291°C for pure PAN nanofiber [44], while in the presence of calixarene molecules this weak exothermic peak shifted to some extent. The residue of thermal decomposition at 555°C is around 41 %, probably con-sisting of ash. All of these data confirmed the presence of the calixarene molecules onto the PAN nanofibers. In addition to the characterization experiments mentioned above, the sorption behavior of the Calix-NMG/PAN nanofiber gave valuable information about the modifica-tion of the PAN nanofiber upon the addimodifica-tion of calixarene molecules based N-methylglucamine units. If the cal-ixarene molecules with chelating group had not been attached to the nanofiber structure, it would not have shown any sorption behavior [20]. (As will be shown later, the pure PAN nanofiber had no oxoanion sorption capability whereas it was significantly adsorbed oxoanions after modification with calixarene molecules bearing N-methylglucamine groups)
Adsorption studies
Dichromate anion sorption studies
The extraction efficiency of nanofibers at different pH values toward chromate ions are studied and obtained results are given in Table1. From the previous study, it is well known that pure PAN nanofibers have no adsorption capacity toward chromate anions [45]. Whereas excellent adsorption percentage using calixarene-modified nanofiber was observed for chromate ions.
From the chromate extraction experiments especially at low pH values, it was determined that Calix-NMG/PAN is more effective solid support and could be used as adsorbent for chromate ions. The maximum percentage of dichromate ions extracted was 87 % for Calix-NMG/PAN when the pH of the aqueous solution was 1.5 and they attained minimum level of 41 % for Calix-NMG/PAN when the pH of the aqueous solution was increased to 4.5. Furthermore, the presence of the chromium on the surface of the nanofibers after dichromate extraction is supported by the EDX Scheme 2 Schematic of the electrospinning setup and synthesis of PAN nanofiber based calix[4]arenebearing N-methylglucamine chelating groups
results. EDX analysis of Calix-NMG/PAN shows that 3.90 % of the chromium was found at surface of the Calix-NMG/PAN which is having main elements such as carbon, nitrogen and oxygen before extraction experiments. This increase at lower pH value can be explained by the fact that the Calix-NMG/PAN can be protonated in acidic condi-tions due to the presence of the tertiary amine groups of
N-methylglucamine units at para position of calixarene skeleton and can easily form complexes with chromate anions by electrostatic interactions and hydrogen bonding. Furthermore, the presence of the vic diols groups of the N-methylglucamine units also support the sorption behavior of the nanofibers structure via hydrogen bonding between oxoanion and vic diol groups as well as ion-dipol Fig. 1 SEM images of PAN
nanofibers. a Calix-NMG/PAN (1 lm), b Calix-NMG/PAN (200 nm) c pure PAN (1 lm) and d pure PAN (200 nm)
Fig. 2 The FT-IR (ATR) spectra of prepared nanofibers. apure PAN, b Calix-NMG/ PAN nanofibers
interaction between Na?ion and diol groups. According to our knowledge the data obtained in extraction Calix-NMG/ PAN can be attributed to a number of reasons. Calix nanofibers possess a tertiary amine and vic diols, facili-tating hydrogen bonding with the dichromate anion. The next reason is that PAN nanofibers based calixarene bear-ing N-methylglucamine units have a more stable and functionalized groups of the calixarene skeleton, and also upon the nature of the aggregations of the ions around the calixarenes skeleton. On the other hand, the slight increase in chromate binding was observed at 3.5 and 4.5 pH values. From the our knowledge, this increase at these pH values is probably due to formation of an ion-pair interaction between sodium ion (Na2Cr2O7) and N-methylglucamine
units of Calix-NMG/PAN as well as more rigid and appropriate structure of calixarene derivatives in the polymeric matrix. Furthermore, it is expected that cal-ixarene skeleton having chelating groups as N-methylglu-camine would geometrically be more suited for effectively interaction with dichromate anions. Compared the pub-lished literature results about dichromate extraction by solid materials such as magnetite nanoparticle and epoxy resin with N-methylglucamine units [40,41], Calix-NMG/ PAN showed excellent extraction results. These results are
probably due to the special and large surface area, high porosity, microporesity and high flexibility properties of the fiber structures of Calix-NMG/PAN. In aqueous solu-tions having a lower pH, the dichromate will be primarily in its protonated form HCr2O7. Compared the dianionic
Cr2O27 and monoanionic HCr2O7 form, monoanion will
have a smaller free hydration energy. As a result, monoanionic HCr2O7 form is easily transferred from the
aqueous phase to the organic phase owing to this smaller hydration energy. Another advantage of monoanionic form HCr2O7 is also the only one sodium ion needs to be
co-extracted to maintain charge balance, whereas for Cr2O27
two sodium ions are extracted, with additional loss of hydration energy [46]. Furthermore, the interfering effect of other anions such as Cl-, NO3, SO24 and CH3COO-on
dichromate anion extraction for the selectivity properties of Calix-NMG/PAN was examined. Obtained results showed that the extraction of chromate ions was not affected by the presence of some selected anions, such as Cl-, NO3, SO24 and CH3COO-although they used more concentrated than
the concentration of chromate anions. As a results, fiber type of solid supports with protonable groups as pyridine in solvent extraction processes were found to be very useful in the purification of waters contaminated with the chro-mate anions.
Uranium sorption studies
The increasing usage of nuclear reactors for large-scale energy production leads to radioactive contamination; hence, research concerning the separation of U(VI) ions from water has become a critical environmental issue in the last decade [47,48]. For the extraction of uranium (U) from seawater and brines, a number of chelating polymeric adsorbents were employed. Micro and macroporous poly-mers having different chelating groups were studied for the Fig. 3 TG and DSC curves of PAN nanofibers (TG lines and DSC dashed lines)
Table 1 Percentage extraction of dichromate anions by extractants pure PAN and Calix-NMG/PAN at different pH values
Nanofibers pH
1.5 2.5 3.5 4.5
Pure PAN 1.1 ± 0.1 – – –
Calix-NMG/PAN 87 ± 0.3 72 ± 0.3 64 ± 0.2 41 ± 0.1 Aqueous phase, (metal dichromate) = 1 9 10-4M; solid phase, 25 mg (fiber) at 25°C, for 1 h
Averages and Standard deviations calculated for data obtained from three independent extraction experiments
uranium uptake from seawater [49, 50]. The adsorption percentage of uranium anions onto pure PAN and Calix-NMG/PAN at different pH values are shown in Table2. The results showed that Calix-NMG/PAN nanofibers are suitable for the separation of uranium ions from aqueous solution. The uranium separation was 90 % for Calix-NMG/PAN nanofiber, based on triplicate analysis at pH 4.5. The extractability of uranium ions dropped to slightly lower level with an increasing pH values (from 4.5 to 8.5). This indicates that effectively the applicability of the Calix-NMG/PAN nanofibers in the higher pH range is limited.
The sorption capability of Calix-NMG/PAN nanofibers was compared with that of pure PAN nanofibers. The pure PAN nanofibers exhibited around 5.5 % sorption; on the other hand, Calix-NMG/PAN nanofibers showed higher sorption capacity. The presence of the calixarene mole-cules with N-methylglucamine chelating groups on the surface of the fiber structures converted hydrophobic properties of the fiber structure to hydrophilic, and the sorption of uranium ions increased due to the chelating ability of the N-methylglucamine units. Comparing the pure PAN nanofiber and Calix-NMG/PAN nanofiber, obtained results show that Calix-NMG/PAN nanofiber with N-methylglucamine chelating units are suitable extractant for uranium cations. Moreover, the sorption results indi-cated that the complexation of the uranium cation depends on the structural properties of the receptors, such as sta-bility or rigidity and hydrogen binding asta-bility. Therefore, this improved performance for Calix-NMG/PAN nanofiber compared to pure PAN nanofiber, is attributed to more rigid structure of calixarene skeleton owing to the support material and the presence of the two N-methylglucamine units that may form a good chelating site and help generate a suitable geometry for the uranium ions. In addition, surface of the functionalized fiber Calix-NMG/PAN is dominated by vic diols, and tertiary amine groups. In acidic media these groups would normally be expected to undergo protonation. Thus, at lower pH values, the chemical forms of the fiber surface and the uranium ions do not support an electrostatic hypothesis of sorption. The fixation of the UO22? ions might therefore be expected to proceed via
formation of surface complexes that possess a coordinative
nature, with the lone pairs of vic diols and tertiary amine groups on the surface of fiber structures playing the main role in interaction with the uranium species. With a pH of 4.5, the adsorption capacity of uranium by the prepared Calix-NMG/PAN reached its maximum value and the sorption of uranium ions decreased at pH values greater than 6. Approximately 85 % of the U(VI) exists in UO2þ2 chemical forms at 4.5 pH value. In pH greater than 6, hydrolysis of UO2þ2 may also affect adsorption. Extraction experiments showed a little decreasing for the adsorption of uranium ions by Calix-NMG/PAN nanofiber over the pH range of 7–8.5. This decrease was paralleled by a reduction in the aqueous concentration of UO2þ2 in favor of UO2
(OH)? and (UO2)3(OH) 5?
.It is well-known that uranium may form a series of aqua-complexes, such as UO2(OH)?,
UO2
ð Þ2ðOHÞ 2þ
2 and UOð 2Þ3ðOHÞ þ
5 ions through a
hydrol-ysis process [51].
Conclusion
This study demonstrated that PAN nanofibers based cal-ixarene bearing N-methylglucamine units Calix-NMG/ PAN produced by electrospinning are an effective sorbent for the removal of chromate anions and uranium cations. In the fabrication process, PAN nanofibers based calixarene bearing N-methylglucamine units were first produced with the aim to develop functional nanofibers. The Calix-NMG/ PAN fiber network provided higher ion binding ability due to the very high surface area, porosity, flexibility and microporesity of PAN nanofibers and surface associated with calixarene molecules containing N-methylglucamine chelating groups. Calixarenes are already being used in variety areas such as pharmaceuticals, catalyst, filtrations and controlled drug delivery systems, therefore, having nanofiber structures might hopefully open up the possibil-ities and extend the use of calixarenes in these fields or in other functional systems. Moreover, our findings may contribute to the fabrication of new functional nanofibers from other types of calixarene and/or other supramolecular systems via electrospinning. The good adsorption capaci-ties of the by Calix-NMG/PAN have been shown in labo-ratory studies, and indicate the potential of these fiber Table 2 Percentage extraction
of uranium ions by pure PAN and Calix-NMG/PAN at different pH values Nanofibers pH 4.5 5.5 7.0 8.0 8.5 Pure PAN 5.5 ± 0.1 4.9 ± 0.3 4.3 ± 0.2 3.3 ± 0.2 3.2 ± 0.2 Calix-NMG/PAN 92 ± 0.3 85 ± 0.2 74 ± 0.3 67 ± 0.3 60 ± 0.3 Aqueous phase, (uranium acetate) = 1.15 9 10-5M; solid phase, 25 mg (fiber) at 25°C, for 1 h Averages and Standard deviations calculated for data obtained from three independent extraction experiments
structures towards cationic and anionic ion species. Therefore, the design of adsorbents with not only the high adsorption properties of nanofibers but also the packing and durability advantages of the fiber skeletons would be a key strategy for preparing practical and efficient ion extraction materials. In addition, a detailed study dealing with iso-therms and sorption mechanisms is underway.
Acknowledgments The authors gratefully would like to thank Selcuk University and Karamanog˘lu Mehmetbey University Research Foundation (Project No: 25-M-15) for financial support.
References
1. McMurtrey, R.J.: Patterned and functionalized nanofiber scaf-folds in 3-dimensional hydrogel constructs enhance neurite out-growth and directional control. J. Neural Eng. 11, 066009 (2014) 2. Smith, L.A., Ma, P.X.: Nano-fibrous scaffolds for tissue
engi-neering. Colloids Surf. B 39, 125–131 (2004)
3. Huang, Z.M., Zhang, Y.Z., Kotaki, M., Ramakrishna, S.: A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos. Sci. Technol. 63, 2223–2253 (2003)
4. Megelski, S., Stephens, J.S., Chase, D.B., Rabolt, J.F.: Micro-and nanostructured surface morphology on electrospun polymer fibers. Macromolecules 35, 8456–8466 (2002)
5. Ramakrishna, S., Fujihara, K., Teo, W.E., Yong, T., Ma, Z., Ramaseshan, R.: Electrospun nanofibers: solving global issues. Mater. Today 9, 40–50 (2006)
6. Zhang, Z.Y., Shao, C.L., Li, X.H., Zhang, L., Xue, H.M., Wang, C.H., Liu, Y.C.: Electrospun nanofibers of ZnO - SnO2
hetero-junction with high photocatalytic activity. J. Phys. Chem. C 114, 7920–7925 (2010)
7. Zhu, P.N., Nair, A.S., Peng, S.J., Yang, S.Y., Ramakrishna, S.: Facile fabrication of TiO2–graphene composite with enhanced
photovoltaic and photocatalytic properties by electrospinning. ACS Appl. Mater. Inter. 4, 581–585 (2012)
8. Wang, X.Y., Kim, Y.G., Drew, C., Ku, B.C., Kumar, J., Samuelson, L.A.: Electrostatic assembly of conjugated polymer thin layers on electrospun nanofibrous membranes for biosensors. Nano Lett. 4, 331–334 (2004)
9. Yu, Y., Gu, L., Zhu, C.B., vanAken, P.A., Maier, J.: Tin nanoparticles encapsulated in porous multichannel carbon microtubes: preparation by single-nozzle electrospinning and application as anode material for high-performance Li-based batteries. J. Am. Chem. Soc. 131, 15984–15985 (2009) 10. Viet, A.L., Reddy, M.V., Jose, R., Chowdari, B.V.R.,
Ramakr-ishna, S.: Nanostructured Nb2O5polymorphs by electrospinning
for rechargeable lithium batteries. J. Phys. Chem. C 114, 664–671 (2010)
11. Huang, C., Tang, Y., Liu, X., Sutti, A., Ke, Q., Mo, X., Wang, X., Morsi, Y., Lin, T.: Electrospinning of nanofibres with parallel line surface texture for improvement of nerve cell growth. Soft Matter 7, 10812–10817 (2011)
12. Wang, X., Chen, X., Yoon, K., Fang, D., Hsiao, B.S., Chu, B.: High flux filtration medium based on nanofibrous substrate with hydrophilic nanocomposite coating. Environ. Sci. Technol. 39, 7684–7691 (2005)
13. Sharma, N., Mckeown, S.J., Ma, X., Pochan, D.J., Cloutier, S.G.: Structure-property correlations in hybrid polymer-nanoparticle electrospun fibers and plasmonic control over their dichroic behavior. ACS Nano 4, 5551–5558 (2010)
14. Sill, T.J., von Recum, R.A.: Electrospinning: applications in drug delivery and tissue engineering. Biomaterials 29, 1989–2006 (2008)
15. Chen, M., Wang, C., Fang, W., Wang, J., Zhang, W., Jin, G., Diao, G.: Electrospinning of calixarene-functionalized polyacry-lonitrile nanofiber membranes and application as an adsorbent and catalyst support. Langmuir 29, 11858–11867 (2013) 16. Bhat G, Lee Y (2003) Recent advancements in Electrospun
nanofibers. In Proceedings of the twelfth international symposium of Processing and Fabrication of Advanced materials, Ed TS Srivatsan & RA Vain, TMS
17. Bhat, G.: Polymeric nanofibers: recent technology advancements stimulating their growth. J. Text. Sci. Eng. 5, 186 (2015) 18. Celebioglu, A., Uyar, T.: Cyclodextrin nanofibers by
electro-spinning. Chem. Commun. 46, 6903–6905 (2010)
19. Kayaci, F., Uyar, T.: Encapsulation of vanillin/cyclodextrin inclusion complex in electrospun polyvinyl alcohol (PVA) nanowebs: prolonged shelf-life and high temperature stability of vanillin. Food Chem. 133, 641–649 (2012)
20. Bayrakcı, M., O¨ zcan, F., Ertul, S.: Synthesis of calixamide nanofibers by electrospinning and toxic anion binding to the fiber structures. Tetrahedron 71, 3404–3410 (2015)
21. Gutsche, C.D., Muthukrishnan, R.: Calixarenes. 1. Analysis of the product mixtures produced by the base-catalyzed condensa-tion of formaldehyde with para-substituted phenols. J. Org. Chem. 43, 4905–4906 (1978)
22. Gutsche CD. (1989.) In Calixarenes, monographs in supramolecular chemistry; Stoddart JF (ed.) The Royal Society of Chemistry: Cambridge, UK,
23. Bayrakci, M., Ertul, S., Sahin, O., Yilmaz, M.: Synthesis of two new p-tert-butylcalix [4] arene b-ketoimin derivatives for extraction of dichromate anion. J. Incl. Phenom. Macrocycl. Chem. 63, 241–247 (2009)
24. Bayrakci, M., Yigiter, S.: Synthesis of tetra-substituted calix [4] arene ionophores and their recognition studies toward toxic arsenate anions. Tetrahedron 69, 3218–3224 (2013)
25. Baruah, U., Gogoi, N., Majumdar, G., Chowdhury, D.: b-Cy-clodextrin and calix[4]arene-25,26,27,28-tetrol capped carbon dots for selective and sensitive detection of fluoride. Carbohyd. Polym. 117, 377–383 (2015)
26. Ertul, S., Bayrakci, M., Yilmaz, M.: Removal of chromate and phosphate anion from aqueous solutions using calix [4] aren receptors containing proton switchable units. J. Hazard. Mater. 181, 1059–1065 (2010)
27. Ozcan, F., Ersoz, M., Yilmaz, M.: Preparation and application of calix[4]arene-grafted magnetite nanoparticles for removal of dichromate anions. Mater. Sci. Eng. C Mater. Biol. Appl. 29, 2378–2383 (2009)
28. Bayrakci, M., Ertul, S., Yilmaz, M.: Transportation of poorly soluble drug molecules from the organic phase to the aqueous phase by using phosphorylated calixarenes. J. Chem. Eng. Data 56, 4473–4479 (2011)
29. Ozmen, M., Ozbek, Z., Buyukcelebi, S., Bayrakci, M., Ertul, S., Ersoz, M., Capan, R.: Fabrication of langmuir–blodgett thin films of calix [4] arenes and their gas sensing properties: ınvestigation of upper rim para substituent effect. Sens. Actuat. B Chem. 190, 502–511 (2014)
30. Zheng, Y., Wang, W., Huang, D., Wang, A.: Kapok fiber ori-ented-polyaniline nanofibers for efficient Cr(VI) removal. Chem. Eng. J. 191, 154–161 (2012)
31. Viti, C., Giovannetti, L.: The impact of chromium contamination on soil heterotrophic and photosynthetic microorganisms. Ann. Microbiol. 51, 201–213 (2001)
32. Katz, S.A., Salem, H.: The toxicology of chromium with respect to its chemical speciation: a review. J. Appl. Toxicol. 13, 217–224 (1993)
33. Wang, J., Pana, K., He, Q., Cao, B.: Polyacrylonitrile/polypyrrole core/shell nanofiber mat for the removal of hexavalent chromium from aqueous solution. J. Hazard. Mater. 244, 121–129 (2013) 34. Bayrakci, M., Ertul, S., Yilmaz, M.: Synthesis of di-substituted
calix[4] arene-based receptors for extraction of chromate and arsenate anions. Tetrahedron 65, 7963–7968 (2009)
35. Li, H.B., Qu, F.G.: Synthesis of CdTe quantum dots in sol - gel-derived composite silica spheres coated with calix[4]arene as luminescent probes for pesticides. Chem. Mater. 19, 4148–4154 (2007)
36. Oshima, T., Saisho, R., Ohe, K., Baba, Y., Ohto, K.: Adsorption of amino acid derivatives on calixarene carboxylic acid impreg-nated resins. Reac. Funct. Polym. 69, 105–110 (2009)
37. Yigiter, S., Bayrakci, M., Ertul, S., Yilmaz, M.: Synthesis of novel silica gel immobilized-calix[4] arene amide ionophores and their anion binding abilities toward phosphate and chromate anions. J. Appl. Polym. Sci. 124, 3831–3839 (2012)
38. Yoshimura, K., Miyazaki, Y., Ota, F., Matsuoka, S., Sakashita, H.: Complexation of boric acid with the N-methyl-D-glucamine group in solution and in crosslinked polymer. J. Chem. Soc., Faraday Trans. 94, 683–689 (1998)
39. Schilde, U., Uhlemann, E.: Separation of several oxoanions with a special chelating resin containing methylamino-glucitol groups. Reac. Polym. 20, 181–188 (1993)
40. Sayin, S., Ozcan, F., Yilmaz, M.: Synthesis and evaluation of chromate and arsenate anions extraction ability of a N-methyl-glucamine derivative of calix[4]arene immobilized onto magnetic nanoparticles. J. Hazard. Mater. 178, 312–319 (2010)
41. Sayin, S., Ozcan, F., Memon, S., Yilmaz, M.: Synthesis and oxoanions (dichromate/arsenate) sorption study of N-methylglu-camine derivative of calix[4]arene immobilized onto poly [(phenyl glycidyl ether)-co-formaldehyde. J. Incl. Phenom. Macrocycl. Chem. 67, 385–391 (2010)
42. Niazi, A., Ghasemi, N., Goodarzi, M., Ebadi, A.: Simultaneous spectrophotometric determination of uranium and thorium using
arsenazo III by H-point standard addition method and partial least squares regression. J. Chin. Chem. Soc. 54, 411–418 (2007) 43. Gutsche, C.D., Nam, K.C.: Calixarenes. 22. Synthesis, properties,
and metal complexation of aminocalixarenes. J. Am. Chem. Soc. 110, 6153–6162 (1998)
44. Wei, Y.T., Zheng, Y.M., Chen, J.P.: Functionalization of regen-erated cellulose membrane via surface initiated atom transfer radical polymerization for boron removal from aqueous solution. Langmuir 27, 6018–6025 (2011)
45. Wang, J., Luo, C., Qi, G., Pana, K., Cao, B.: Mechanism study of selective heavy metal ion removal with polypyrrole functional-ized polyacrylonitrile nanofiber mat. Appl. Surf. Sci. 316, 245–250 (2014)
46. Memon, S., Yilmaz, A., Roundhill, D.M., Yilmaz, M.: Synthesis of polymeric calix(4)arene dinitrile and diamino-derivatives: exploration of their extraction properties towards dichromate anion. J. Macromol. Sci. A 41, 433–447 (2004)
47. Sadeghi, S., Azhdari, H., Arabi, H., Moghaddam, A.Z.: Surface modified magnetic Fe3O4nanoparticles as a selective sorbent for
solid phase extraction of uranyl ions from water samples. J. Hazard. Mater. 215, 208–216 (2012)
48. Horzum, N., Shahwan, T., Parlak, O., Demir, M.M.: Synthesis of amidoximated polyacrylonitrile fibers and its application for sorption of aqueous uranyl ions under continuous flow. Chem. Eng. J. 213, 41–49 (2012)
49. Sodaye, H., Nisan, S., Poletiko, C., Prabhakar, S., Tewari, P.K.: Extraction of uranium from the concentrated brine rejected by inte-grated nuclear desalination plants. Desalination 235, 9–32 (2009) 50. Gibert, O., Valderrama, C., Peterko´va, M., Cortina, J.L.:
Evalu-ation of selective sorbents for the extraction of valuable metal ions (Cs, Rb, Li, U) from reverse osmosis rejected brine. Solvent Extr. Ion Exch. 28, 543–562 (2010)
51. Toth, L.M., Begun, G.M.: Raman spectra of uranyl ion and its hydrolysis products in aqueous nitric acid. J. Phys. Chem. 85, 547–549 (1981)
![illustration of the synthesis of calix[4]arene derivatives. (i) Formaldehyde, NaOH, diphenyl ether; (ii) AlCl 3 , phenol, toluene, rt, 3 h; (iii) N-methylglucamine,](https://thumb-eu.123doks.com/thumbv2/9libnet/4553489.82887/4.892.340.811.83.483/illustration-synthesis-derivatives-formaldehyde-diphenyl-phenol-toluene-methylglucamine.webp)
