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Supramolecular hydrogel based on cyclodextrin modified GO as a potent natural organic matter absorbent
European Polymer Journal 92 (2017) 126–136
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Supramolecular hydrogel based on cyclodextrin modified GO as a potent natural organic matter absorbent
MARK
⁎
Masoumeh Parsamanesh, Abbas Dadkhah Tehrani , Yaghoub Mansourpanah Department of Chemistry, Faculty of Science, Lorestan University, Khoramabad, Iran
AR TI CLE I NF O
AB S T R A CT
Keywords: Hydrogel Graphene oxide Supramolecular polymer Water purification Cyclodextrin
New nanocomposite hydrogel was synthesized using biocompatible and biodegradable material. Firstly α-cyclodextrin functionalized with mercaptoacetic acid (MAA) and then attached to the surface of graphene oxide (GO) by ene-click reaction. Also starch (St) functionalized with myristic acid (MA) by esterification reaction. In the next step host-guest interactions formed between α-CD cavities and MA molecules. Due to this interaction graphene oxide nanosheets crosslinked to each other and a stable nanocomposite hydrogel formed successfully. These new synthesized nanomaterials were characterized by spectroscopic methods such as IR spectroscopy, UV–vis spectroscopy, X-ray diffraction (XRD) technique, scanning electron microscopy, thermogravimetric analysis (TGA) and Raman spectroscopy. FE-SEM analysis confirmed that prepared nanocomposite hydrogel has a well-defined and interconnected 3D porous network with pore sizes in the range of submicrometer to several micrometers. This nanocomposite hydrogel has beneficial application in different fields such as water purification. This newly synthesized nanomaterial is sensitive to external stimuli. We illustrated that addition of adamantane to the system as an external stimulus, result in destroying host-guest interactions between α-CD and StMA because adamantane is a strong competitor for α-CD cavity in comparison with MA molecule. This also confirms that host-guest complex formation between α-CD and MA is the driving force for hydrogel formation. The obtained supramolecular hydrogel was used for water purification and the results showed that this product has a good ability for removal of organic materials such as humic acid (HA) from contaminated water.
1. Introduction The past decades have witnessed the intensely growing research interest of polymer hydrogels, which are natural or synthesized aggregations of three-dimensional polymeric networks of hydrophilic cross-linked macromolecules [1–3]. Nowadays, hydrogels from natural polysaccharides, such as Chitin, chitosan, starch, alginate, hyaluronic acid, and cellulose have been intensively studied, and successfully fabricated. However, most of the hydrogels consist of the polysaccharides-based hydrogels suffer from the lack of mechanical performance, which has limited their beneficial applications in different fields. Therefore, to improve their mechanical properties and add new functional features, hybrid hydrogels of polymers and nanomaterials have recently been widely studied [1,4–6]. It has been previously reported that incorporation of nanomaterials with characteristic physical properties into a hydrogel, plays a significant role in determining the mechanical properties of the overall hydrogel structure. Carbonaceous materials such as carbon nanotube and graphene are examples which have been used for this purpose [7,8]. Graphene, a single layer of graphite has attracted ⁎
Corresponding author. E-mail address: [email protected] (A.D. Tehrani).
http://dx.doi.org/10.1016/j.eurpolymj.2017.05.001 Received 8 February 2017; Received in revised form 25 April 2017; Accepted 1 May 2017 Available online 03 May 2017 0014-3057/ © 2017 Elsevier Ltd. All rights reserved.
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great interest in the field of composite materials all over the world because of its integrated functionalities and its advantage in many application fields [9–11]. However, graphene contains only sp2 carbon atoms, which limits its application to composites. The extreme insolubility of graphene in water due to the lack of polar groups in its molecular structure is another obstacle in incorporating it within supramolecular hydrogels. A facile method for increasing the hydrophilicity and reactivity of graphene is the synthesis of graphene oxide nanosheets through oxidative exfoliation from cheap graphite which avoid these disadvantages [1,5,12–14]. GO owing to its large specific area and abundant functional groups is an important derivative of graphene, which has been widely used in reinforced biopolymer materials [15–17]. GO composite hydrogels could be synthesized by chemical cross-linking with surface functionalized GO as the cross-linker or physical cross-linking between polymers and GO nanosheets for instance through interactions such as hydrogen bonding, π-π stacking, and electrostatic interaction. These strong interactions between GO and the polymer matrix result in higher mechanical performance, thus preventing the disintegration of composite hydrogels in the swollen state [2,18–20]. Furthermore, polymer/GO composite hydrogels, compared to pure polymer hydrogels, show an increase in tensile strength, breaking elongation, and compressive strength [18,21,22]. Several researchers have reported the use of GO as a 2D cross-linker for polysaccharide-based hydrogels, such as starch, alginate, agarose, cellulose and chitosan, mainly aiming for the reinforcement of mechanical properties. Recently Han and Yan prepared a supramolecular hydrogel of chitosan and GO by noncovalent interactions [12,23]. Hosseinzadeh and Ramin synthesized starch-g-poly (acrylic acid-co-acrylamide)/graphene oxide superabsorbent nanocomposites [24]. Zhang et al. successfully prepared a new kind of nanocomposite hydrogel by introducing GO into the PAM/CMC(polyacrylamide (PAM)/carboxymethyl cellulose sodium (CMC) nanocomposite) hydrogels followed by ionically crosslinking of aluminum ions [25]. The special soft-wet structure makes hydrogels capable of absorbing and retaining water without disintegrating, which has enabled them to be widely employed for many applications, such as, wastewater treatment for removal of a range of heavy metals, and organic contaminants. Because of the Porous structure of hydrogels, the contaminant solution could diffuse rapidly into the polymeric network and interact with its functional groups [2,26,27]. Chen et al. prepared GO–chitosan (GO–CS) composite hydrogels as new broad-spectrum adsorbents for water purification by three-dimensional (3D) self-assembly of GO sheets promoted by different types of crosslinking agents [22]. Gonzalez et al. presented a novel hybrid material for wastewater treatment composed by chitin and graphene oxide nanosheets [28]. Although it has been shown that GO itself and also polymeric hydrogels can be used as dye adsorbents, the presence of GO, increases adsorption capacity of GO-based hydrogels because of its high specific surface area and presence of a large number of polar functional groups such as carboxyl, hydroxyl and epoxy groups which could adsorb a variety of metal ions and organic pollutants such as natural organic matters (NOMs) through coordination, hydrogen bonding, π–π stacking interaction and electrostatic interaction [5,8,26,29]. Humic acid, the main component of natural organic materials (NOMs), is responsible for an undesirable color and has been implicated in bacterial growth in water. When HA contaminated water is treated with disinfectants such as chlorine-based compounds, carcinogenic organic compounds are formed. Therefore the elimination of HA from aqueous solution is of great practical significance for water treatment [5,30,31]. HA is composed of a skeleton of alkyl and aromatic units with different functional groups, such as carboxylic acid, phenolic hydroxyl, ketone, and quinone groups and could be absorbed on the surface of GO nanosheets through polar groups, and aromatic molecules [30,32,33]. Furthermore according to the result of Song and coworkers, modification of GO surface with CD molecules can effectively enhance its sorption capacity because CD molecules can form strong complexes with organic pollutants. They showed that CD modified GO can be used as a promising material in the enrichment of hexavalent uranium and HA from wastewater in hexavalent uranium and humic substances obtained by environmental pollution cleanup [5]. In this research, we firstly functionalized α-CD molecules with mercaptoacetic acid. Then attached this molecules to the surface of GO by ene-click reaction. Also starch biopolymer functionalized with myristic acid by esterification reaction. In the last step hostguest interactions formed between α-CD cavities and MA molecules. Due to this interaction GO nanosheets crosslinked to each other and a stable nanocomposite hydrogel formed successfully. These newly synthesized nanomaterials were characterized by spectroscopic measurement methods such as IR spectroscopy, UV–vis spectroscopy, XRD technique, scanning electron microscopy, TGA and Raman spectroscopy. The prepared hydrogel has applications in different fields such as water purification and we consider its ability to remove humic acid from contaminated water. Furthermore this new nanomaterial is sensitive to external stimuli which could be considered by destroying host-guest interactions between α-CD-SH and St-MA with addition of external stimuli. The adamantane molecule added to the prepared system as an external stimulus and destroyed host-guest interactions by competing with MA for α-CD cavity.
2. Experimental 2.1. Materials Starch, myristic acid, trimethylsilyl chloride (TMSCl), thionyl chloride, graphite, sulfuric acid (H2SO4), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), phosphoric acid (H3PO4), hydrochloric acid (HCl), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), adamantane, mercaptoacetic acid, α-cyclodextrin (α-CD), humic acid and acetone were purchased from Sigma-Aldrich. A dialysis bag was provided from Spectrum Company with 3.5 kDa MWCO. The other chemicals used in the study were of analytical grade. Deionized water was used in all experiments.
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2.2. Instruments The FT-IR analysis was performed using an FT-IR Bruker-Tensor 320 spectrometer. All of the products were mixed with analytical grade KBr at a weight ratio of 1/100 mg. Absorption spectra of samples in solution were recorded by a Shimadzu UV–visible 1650 PC spectrophotometer with a cell of 1.0 cm path length. Morphology and structure of synthesized materials were investigated using an LEO 440i scanning electron microscope under vacuum at an operating voltage of 10 kV. The instrument equipped with an EDX (energy-dispersive X-ray microanalysis) system with a sufficient sensitivity for detection of corresponding elements atomic numbers. Dried samples used for SEM experiment were coated with a thin layer of gold by sputtering for 15 s. The patterns of X-ray diffraction of the synthesized nanomaterials were recorded by a Halland Philips Xpert X-ray powder diffraction (XRD) diffractometer. (Cuk, radiation, λ = 0.154056 nm) at a scanning speed of 2°/min from 10° to 100 (2θ). TGA measurements were recorded by a STA 409 apparatus (Linei) at a temperature from 0 to 800 °C with a 10 °C/min heating rate under argon gas. Raman spectra were recorded on dispersive Raman Microscope with ʎexc = 785 nm and high spatial & spectral resolution (Spectral Resolution: < 3 cm−1). 2.3. Methods 2.3.1. Preparation of α-CD-SH via esterification reaction The esterification of α-cyclodextrin was carried out according to the published procedure with some modifications [34]. MAA (0.014 ml) and TMSCl (0.023 ml) were placed in a round bottom flask and stirred for 30 min at room temperature. Then α-CD (0.2 g) was added to them and the obtained mixture was stirred at room temperature for 72 h. After this time, the mixture of the reaction was cooled to 0 °C and precipitated in acetone. Then separated by centrifuge at 6000 rpm for 3 min and washed with cold acetone three times. The purified product (α-CD-SH) was obtained after drying as a white powder. 2.3.2. Preparation of GO GO was synthesized by Hummers’ method according to the reported procedure in the literature [35]. 2.3.3. Preparation of GO-α-CD via ene-click reaction In this research we used a polar solvent, DMF, to promote thiol-ene ‘‘click’’ reaction. For this purpose, firstly GO (0.02 g) and αCD-SH (0.1 g) was dissolved in DMF (10 ml) and sonicated for 30 min. The obtained solution was stirred at 100 °C for 48 h. Then cooled to room temperature and dialyzed against distilled water. The purified product (GO-α-CD) obtained after drying as a black compound. 2.3.4. Preparation of St-MA by esterification reaction Starch biopolymer was modified with MA according to the reported procedure in the literature [36] with a brief modification. MA (0.22 gr) was added to a round-bottomed flask. Then thionyl chloride (0.29 ml) was added to flask slowly and the mixture was refluxed in the water bath for 5 h. In the next step, starch (0.15 g) dissolved in DMSO was added to the above mixture. The mixture of the reaction was stirred at room temperature for 72 h. After this time the product precipitated in ethanol. The purified product (StMA) obtained after drying as a semitransparent compound. 2.3.5. Preparation of nanocomposite hydrogel via host-guest interactions For this purpose GO-α-CD (0.05) was dissolved in distilled water (10 ml). Then St-MA (0.5 g) was added to this solution. The mixture of reaction was sonicated for 30 min and stirred at room temperature for 24 h. After this time, the mixture of reaction stored at room temperature for 1 week for preparation of nanocomposite hydrogel. The obtained hydrogel then washed with water to obtain a purified product (GO-α-CD/St-MA). 2.3.6. Removal capability of HA from contaminated water by (GO-α-CD/St-MA) hydrogel UV–Vis spectroscopy was applied to monitor the removal process of HA from contaminated water after adding GO-α-CD/St-MA hydrogel to the HA solution. Control assay also performed using starch biopolymer to consider the relevance of using the whole system as adsorbent. HA removal was carried out by immersing 0.015 g of hydrogel and starch biopolymer into 10 ml of HA solution with 250 ppm concentration at 25 °C in a thermostatic bath under constant stirring. Each experiment was repeated three times. The amount of removed HA was evaluated using a UV–Vis spectrometer at kmax = 254 nm. The removal efficiency (RE %) of HA by hydrogel was calculated according to the following equation:
RE% = C 0 C t /C 0 × 100 where, C0 is the initial HA concentration (mg L−1) and Ct is the remaining HA concentrations in the solution at t time. 3. Result and discussion New nanocomposite hydrogel consisting of CD-modified graphene oxide and modified starch biopolymer were prepared. Hostguest interactions between CD-modified graphene oxide and polymers carrying guest moieties result in supramolecular graphene hydrogels formation. For the preparation of nanocomposite hydrogel, we chose GO-α-CD as one side of system and St-MA as the other 128
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Scheme 1. Synthetic route of GO-α-CD/St-MA nanocomposite hydrogel.
side of system. A host-guest complex formed between α-CD at the surface of GO and MA at the surface of the starch which resulted in formation of GO-α-CD/St-MA hydrogel [37]. To consider sensitivity of prepared hydrogel to the external chemical stimuli, adamantane added to the system as an external stimulus. We observed that in the presence of adamantane molecules, the formed host-guest complex between α-CD and MA destroyed because adamantane is a strong competitor for α-CD cavity in comparison with MA molecules (Scheme 1). Furthermore, the prepared supramolecular hydrogel has a porous structure which could absorbed many type of contaminations. We used this property for removal of HA from contaminated water and showed that this product has a good ability for water purification. Synthesized nanomaterials characterized by usual spectroscopy and microscopy methods such as IR spectroscopy, UV–vis spectroscopy, X-ray diffraction (XRD) technique; scanning electron microscopy, thermogravimetric analysis (TGA) and Raman spectroscopy.
3.1. Characterization 3.1.1. FT-IR analysis Fourier transform infrared (FTIR) spectroscopy was used to characterize all of synthesized products. In all of spectrums the % of transmittance is plotted as a function of wavenumber (cm−1). IR spectrums of all products are shown in Fig. 1. IR spectrum of MA is shown in Fig. 1a. In this spectrum, absorption bands at 2918 cm−1 and 2850 cm−1 are attributed to the symmetrical and asymmetrical stretching of eCH2 functional groups respectively. The C]O stretching vibration of myristic acid is appeared at 1703 cm−1 (Fig. 1a). Fig. 1b. shows the FT-IR spectrum of starch biopolymer. In this spectrum, the peaks at 3415 cm−1 and 1647 cm−1 are attributed to the stretching and bending vibrations of hydrogen bonding OH group of starch. Also the peaks of CeO stretching groups appeared at 1000–1300 cm−1 (Fig. 1b) [36]. IR spectrum of St-MA is shown in Fig. 1c. In this spectrum, absorption bands at 3375 cm−1 2924 cm−1, 1647 cm−1 and , 1000–1300 cm−1 are attributed to the OeH, CeH, CeC, and CeO stretching groups respectively (Fig. 1c). The absence of any absorbance band in the region of 1710 cm−1 corresponding to the carbonyl stretch of unreacted fatty acid and 1780 cm−1 associated with carbonyl group of fatty acid chlorides and appearance of new carbonyl peak at 1724 cm−1 corresponding to the C]O stretching groups of esterified starch confirmed that MA attached to OeH functional groups of starch by the esterification reaction [36]. Fig. 1d shows the FT-IR spectrum of α-CD-SH. In this spectrum, the peaks at 3354 cm−1, 2928 cm−1 and 1000–1300 cm−1 are attributed to the OeH, CeH, and CeO stretching groups respectively (Fig. 1d). The peaks at 2460 cm−1 and 1728 cm−1 which are attributed to 129
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Fig. 1. FT-IR spectra of MA (a), St (b), St-MA (c), α-CD-SH (d), GO (e), GO-α-CD (f) and GO-α-CD/St-MA (e).
the SeH and C]O functional groups, confirm the esterification reaction of α-CD with MAA [34]. Fig. 1e shows IR spectrum of GO. The peaks at 3421 cm−1, 1728 cm−1, 1631 cm−1 and 1037 cm−1 are attributed to the OH, C]O, C]C, and CeO stretching groups respectively (Fig. 1e) [38]. In the IR spectrum of GO-α-CD, the peaks at 3384 cm−1, 2931 cm−1 and 1724 cm−1 are attributed to the OeH, CeH and C]O, stretching groups respectively (Fig. 1f). The CeC and CeO stretching vibrations of cyclodextrin molecules appeared at 1658 cm−1 and 1000–1300 cm−1 [39,40]. Also the S-H vibrations of α-CD-SH disappeared in this spectrum because S-H functional groups of αCD-SH react with surface functional groups of GO by ene-click reaction which result in attachment of α-CD-SH to the surface of GO 130
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Fig. 2. UV–vis spectra of GO, α-CD-SH, GO-α-CD, St-MA and GO-α-CD/St-MA.
and C-S covalence bounds formation. As mentioned above, St-MA could interact with GO-α-CD by host-guest complex formation between MA and α-CD which result in supramolecular hydrogel formation. Absorption bands at 3458 cm−1, 2925 cm−1, 1735 cm−1 and 1000–1300 cm−1 in the IR spectrum of GO-α-CD/St-MA hydrogel (Fig. 1g) are corresponding to the OH, CeH, C]O and CeO stretching vibrations respectively. 3.1.2. UV–vis spectroscopy analysis UV–vis experiments were used to investigate the preparation of these new nanomaterials. Fig. 2 shows UV–vis spectra of GO, αCD-SH, GO-α-CD, St-MA and GO-α-CD/St-MA. GO shows an absorption peak at 232 nm corresponds to the π-π∗ transition of the aromatic C]C and a shoulder around 300 nm attributed to n-π∗ transitions of the carbonyl groups. The α-CD-SH spectrum has two characteristic peak in 258 nm and 289 nm which are corresponded to the n-σ∗ and n-π∗ transitions. The n-π∗ transitions in this spectrum which correspond to the carbonyl groups of MAA confirmed that MAA reacted with α-CD by the esterification reaction. Maximum absorptions appeared in GO-α-CD spectrum at around 254 nm and 273 nm which are attributed to the π-π∗ and n-π∗ transitions. The absorption spectrum of St-MA display absorbance bands centered on 277 nm, correspond to n-π∗ transitions of carbonyl groups of MA. This evidence emphasize that MA reacted with OH functional groups of St by esterification reaction. As can be seen from the figure, absorption spectrum of GO-α-CD/St-MA, shows absorbance bands centered at 271 nm and 328 nm which is attributed to the π-π∗ and n-π∗ transitions. 3.1.3. Raman spectroscopy analysis Raman spectroscopy was utilized for further investigation of carbon structure of GO, GO-α-CD and GO-α-CD /St-MA. Raman spectra of these products are shown in Fig. 3. The spectrum of GO (Fig. 3a) displays two dominant peaks D-band (1322 cm−1) and Gband (1591 cm−1) which are attributed to sp3 carbon atoms of the disordered structure and in-plane vibration of the sp2 carbon atoms, respectively. The spectral shifts which observed during reaction steps could be attributed to the disturbance of the GO structure caused by the physical or chemical interactions between functional groups of GO and polar groups of α-CD-SH and St-MA [10]. For example D peak shows a red shift from 1322 cm−1 to 1313 cm−1 in GO-α-CD (Fig. 3b) which is corresponded to the interaction of S-H functional groups of α-CD-SH with functional groups of GO by ene-click reaction. St-MA can form an inclusion complex with α-CD molecules at the surface of GO. The ratios of the intensity of D band to G band (ID/IG) can be used to evaluate the defects in graphene materials (sheets) [41]. The ID/IG ratio increased from 1.073 for GO to 1.086 for GO-α-CD while this ratio increased to 1.165 for GO-α-CD/St-MA. These slight increase of ID/IG are ascribed to the insertion of α-CD-SH and St-MA into GO layers which result in increased disordered in carbon structure of GO-α-CD and GO-α-CD/St-MA. 3.1.4. X-ray diffraction X-ray diffraction as an important tool was used to investigate crystal structure of materials. The crystal structures of prepared GO, GO-α-CD and GO-α-CD/St-MA hydrogel are obtained by XRD measurements (Fig. 4). The XRD pattern of GO (Fig. 4a) shows an intense diffraction peak at 2θ = 10.81° (attributed to an interlayer spacing of 0.817 nm) which is corresponds to the (001) plane and indicates that GO has been fully oxidized. In XRD pattern of GO-α-CD (Fig. 4b) the GO peak shifted to a lower angle (2θ = 8.78°) corresponds to an interlayer distance of 1.006 nm. The shift of GO peak to lower angle is due to attachment of α-CD-SH to the surface of GO which result in increasing in interlayer spacing of GO nanosheets. Furthermore new peaks appeared at 2θ = 19.73° and 26.70° in XRD pattern of GO-α-CD which further confirm the presence of α-CD-SH at the surface of GO. In the XRD pattern of GO-α-CD/StMA hydrogel (Fig. 4c), the GO diffraction peak disappeared and the crystal structure of GO changes after forming the GO-α-CD/St-MA 131
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Fig. 3. Raman spectra of GO (a), GO-α-CD (b) and GO-α-CD/St-MA (c).
Fig. 4. X-ray diffraction patterns of GO (a), GO-α-CD (b) and GO-α-CD/St-MA (c).
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Fig. 5. TGA thermograms of GO (a), GO-α-CD (b) and GO-α-CD/St-MA (c).
hydrogel. As shown in Fig. 4c, the XRD pattern of GO-α-CD/St-MA hydrogel only contains a very broad peak at 2θ = 26.8° which corresponds to the (002) plane of exfoliated GO. Interlayer spacing of GO-α-CD/St-MA hydrogel decreased to 0.332 nm. This evidence confirm that St-MA interact with GO nanosheets which result in formation of poorly ordered GO sheets along their stacking direction and decreasing in interlayer spacing of GO nanosheets [42].
3.1.5. TGA measurements The composition and changes in thermal stability of GO, GO-α-CD and GO-α-CD/St-MA hydrogel were further investigated by TGA (Fig. 5). The experiments were performed from 0 to 800 °C under a nitrogen atmosphere at a heating rate of 10 °C min−1. GO is thermally unstable and has three stages of mass loss upon the increase of temperature. The first mass loss (11.81%) which is occurred below 100 °C was mainly attributed to the evaporation of saturated water entrapped in GO nanosheets. The second major mass loss (42.29%) which is occurred between 110 and 210 °C, could be assigned to the decomposition of oxygen-containing functional groups. The further mass loss (14.68%) which occurred at higher temperature is due to the combustion of carbon skeleton in the GO. The TGA curve of GO-α-CD shows two mass loss at around 100 °C (2.8%) and at 130–800 °C (64.29%) corresponding to the volatilization of stored water and decomposition of oxygen-related functional groups respectively. As can be seen from the figure, attachment of α-CD to the surface GO could increase its decomposition temperature. The TGA curve of GO-α-CD/St-MA exhibits three slower mass losses during the stepwise thermal degradation process. The first mass loss below 100 °C (3.94%) is due to the removal of adsorbed water. The second mass loss at 135–230 °C (10.42%) is corresponding to the decomposition of oxygen-related functional groups and the third mass loss which occurred at 240–360 °C (39.03%) is due to the destruction of crosslinked St-MA backbones. As can be seen from the figure, the weight loss of GO-α-CD/St-MA is smaller than GO while its decomposition temperature is greater than GO. This evidence confirms that St-MA polymer which forms inclusion complexes with α-CD at the surface of GO increased the thermal stability of GO-α-CD/St-MA hydrogel. Fig. 6 shows the digital photos of the mixture of GO-α-CD and St-MA in water before hydrogel formation (Fig. 6a), after hydrogel formation (Fig. 6b) and after its destruction with addition of adamantane (Fig. 6c) respectively. The figure shows that there is no
Fig. 6. Photographs of mixtures of GO-α-CD and St-MA in water before hydrogel formation (a), after hydrogel formation (b) and after destruction with addition of adamantane (c).
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Fig. 7. FE-SEM images of GO-α-CD/St-MA (a–c), EDX spectra and elemental maps of GO-α-CD/St-MA (d–g).
hydrogel at the beginning of an experiment. But after few days due to the host-guest complex formation between St-MA and GO-α-CD a stable hydrogel can be formed in which GO nanosheets homogeneously dispersed in hydrogels. Then, adamantane molecule was added to this system as an external stimulus to investigate the destruction of host-guest interaction between St-MA and GO-α-CD. As can be seen from the figure, the introduction of adamantane to this system results in the destruction of hydrogel because adamantane in comparison with St-MA is a strong competitor for interaction with GO-α-CD therefore destruct the interaction between GO-α-CD and St-MA and formed new host-guest interaction with α-CD at the surface of GO which results in hydrogel destruction.
3.1.6. Field emission-scanning electron microscopy (FE-SEM) analysis FE-SEM observations were used for characterization of the microstructure of the hydrogels. FE-SEM image of GO-α-CD/St-MA hydrogel is shown in Fig. 7a–c. As can be seen from the figure, GO-α-CD/St-MA hydrogel has a well-defined and interconnected 3D porous network with pore sizes in the range of submicrometer to several micrometers. 3D network formation occurred due to the host-guest interaction between α-CD molecules at the surface of GO-α-CD and the MA linked to the St-MA polymer which results in cross-linking of GO nanosheets and hydrogel formation. Well information about the elements existing in the sample and also the mass concentration of the elements can be determined by Energy dispersive X-ray spectroscopy (EDX). The EDX spectrum of GO-α-CD/StMA hydrogel and its corresponding element mapping images are shown in Fig. 7d–g. As can be seen from these figures, the GO-α-CD/ St-MA hydrogel contains three kinds of elements (C, O, S) indicating the presence of α-CD-SH at the surface of GO nanosheets which promote hydrogel formation.
3.1.7. HA removal study The ability of nanocomposite hydrogel for removal of HA was monitored by UV–Vis spectroscopy in room temperature. In order to probe the relevance of using the whole system as adsorbent, control assays was also performed using starch biopolymer as adsorbent. UV–vis spectra and also digital photos of HA solution before HA removal (Fig. 8Aa and Ba) and after HA removal (Fig. 8Ab and Bb using starch and Ac and Bc using nanocomposite hydrogel) are shown in Fig. 8. The figure clearly shows that the intensity of the maximum adsorption peak of HA solution declined intensely within 24 h after addition of nanocomposite hydrogel. While the HA solution after addition of starch biopolymer show a slight decrease in maximum adsorption peak. Digital photos clearly confirm this evidence and show that the prepared nanocomposite hydrogel in comparison with starch biopolymer has a good ability for removal of HA from contaminated water due to the presence of modified GO and also the porous structure of prepared hydrogel which result in diffusion of HA solution into the soft-wet and porous structure of hydrogels so that HA could be absorbed on the surface of GO nanosheets through polar groups and aromatic molecules. HA removal capability of prepared nanocomposite hydrogel for a period of 24 h is also presented in Fig. 9. As can be seen from figure, after 24 h, 75.83% of HA was absorbed by this system through a π–π stacking and hydrogen bonding interaction between HA and GO based nanocomposite hydrogel. 134
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Fig. 8. UV–vis spectra (A) and Photographs (B) of HA solution before adsorption (a) after adsorption with Starch biopolymer (b) and after adsorption with nanocomposite hydrogel (c).
Fig. 9. Removal efficiency of nanocomposite hydrogel.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Supramolecular hydrogel based on cyclodextrin modified GO as a potent natural organic matter absorbent
MARK
⁎
Masoumeh Parsamanesh, Abbas Dadkhah Tehrani , Yaghoub Mansourpanah Department of Chemistry, Faculty of Science, Lorestan University, Khoramabad, Iran
AR TI CLE I NF O
AB S T R A CT
Keywords: Hydrogel Graphene oxide Supramolecular polymer Water purification Cyclodextrin
New nanocomposite hydrogel was synthesized using biocompatible and biodegradable material. Firstly α-cyclodextrin functionalized with mercaptoacetic acid (MAA) and then attached to the surface of graphene oxide (GO) by ene-click reaction. Also starch (St) functionalized with myristic acid (MA) by esterification reaction. In the next step host-guest interactions formed between α-CD cavities and MA molecules. Due to this interaction graphene oxide nanosheets crosslinked to each other and a stable nanocomposite hydrogel formed successfully. These new synthesized nanomaterials were characterized by spectroscopic methods such as IR spectroscopy, UV–vis spectroscopy, X-ray diffraction (XRD) technique, scanning electron microscopy, thermogravimetric analysis (TGA) and Raman spectroscopy. FE-SEM analysis confirmed that prepared nanocomposite hydrogel has a well-defined and interconnected 3D porous network with pore sizes in the range of submicrometer to several micrometers. This nanocomposite hydrogel has beneficial application in different fields such as water purification. This newly synthesized nanomaterial is sensitive to external stimuli. We illustrated that addition of adamantane to the system as an external stimulus, result in destroying host-guest interactions between α-CD and StMA because adamantane is a strong competitor for α-CD cavity in comparison with MA molecule. This also confirms that host-guest complex formation between α-CD and MA is the driving force for hydrogel formation. The obtained supramolecular hydrogel was used for water purification and the results showed that this product has a good ability for removal of organic materials such as humic acid (HA) from contaminated water.
1. Introduction The past decades have witnessed the intensely growing research interest of polymer hydrogels, which are natural or synthesized aggregations of three-dimensional polymeric networks of hydrophilic cross-linked macromolecules [1–3]. Nowadays, hydrogels from natural polysaccharides, such as Chitin, chitosan, starch, alginate, hyaluronic acid, and cellulose have been intensively studied, and successfully fabricated. However, most of the hydrogels consist of the polysaccharides-based hydrogels suffer from the lack of mechanical performance, which has limited their beneficial applications in different fields. Therefore, to improve their mechanical properties and add new functional features, hybrid hydrogels of polymers and nanomaterials have recently been widely studied [1,4–6]. It has been previously reported that incorporation of nanomaterials with characteristic physical properties into a hydrogel, plays a significant role in determining the mechanical properties of the overall hydrogel structure. Carbonaceous materials such as carbon nanotube and graphene are examples which have been used for this purpose [7,8]. Graphene, a single layer of graphite has attracted ⁎
Corresponding author. E-mail address: [email protected] (A.D. Tehrani).
http://dx.doi.org/10.1016/j.eurpolymj.2017.05.001 Received 8 February 2017; Received in revised form 25 April 2017; Accepted 1 May 2017 Available online 03 May 2017 0014-3057/ © 2017 Elsevier Ltd. All rights reserved.
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great interest in the field of composite materials all over the world because of its integrated functionalities and its advantage in many application fields [9–11]. However, graphene contains only sp2 carbon atoms, which limits its application to composites. The extreme insolubility of graphene in water due to the lack of polar groups in its molecular structure is another obstacle in incorporating it within supramolecular hydrogels. A facile method for increasing the hydrophilicity and reactivity of graphene is the synthesis of graphene oxide nanosheets through oxidative exfoliation from cheap graphite which avoid these disadvantages [1,5,12–14]. GO owing to its large specific area and abundant functional groups is an important derivative of graphene, which has been widely used in reinforced biopolymer materials [15–17]. GO composite hydrogels could be synthesized by chemical cross-linking with surface functionalized GO as the cross-linker or physical cross-linking between polymers and GO nanosheets for instance through interactions such as hydrogen bonding, π-π stacking, and electrostatic interaction. These strong interactions between GO and the polymer matrix result in higher mechanical performance, thus preventing the disintegration of composite hydrogels in the swollen state [2,18–20]. Furthermore, polymer/GO composite hydrogels, compared to pure polymer hydrogels, show an increase in tensile strength, breaking elongation, and compressive strength [18,21,22]. Several researchers have reported the use of GO as a 2D cross-linker for polysaccharide-based hydrogels, such as starch, alginate, agarose, cellulose and chitosan, mainly aiming for the reinforcement of mechanical properties. Recently Han and Yan prepared a supramolecular hydrogel of chitosan and GO by noncovalent interactions [12,23]. Hosseinzadeh and Ramin synthesized starch-g-poly (acrylic acid-co-acrylamide)/graphene oxide superabsorbent nanocomposites [24]. Zhang et al. successfully prepared a new kind of nanocomposite hydrogel by introducing GO into the PAM/CMC(polyacrylamide (PAM)/carboxymethyl cellulose sodium (CMC) nanocomposite) hydrogels followed by ionically crosslinking of aluminum ions [25]. The special soft-wet structure makes hydrogels capable of absorbing and retaining water without disintegrating, which has enabled them to be widely employed for many applications, such as, wastewater treatment for removal of a range of heavy metals, and organic contaminants. Because of the Porous structure of hydrogels, the contaminant solution could diffuse rapidly into the polymeric network and interact with its functional groups [2,26,27]. Chen et al. prepared GO–chitosan (GO–CS) composite hydrogels as new broad-spectrum adsorbents for water purification by three-dimensional (3D) self-assembly of GO sheets promoted by different types of crosslinking agents [22]. Gonzalez et al. presented a novel hybrid material for wastewater treatment composed by chitin and graphene oxide nanosheets [28]. Although it has been shown that GO itself and also polymeric hydrogels can be used as dye adsorbents, the presence of GO, increases adsorption capacity of GO-based hydrogels because of its high specific surface area and presence of a large number of polar functional groups such as carboxyl, hydroxyl and epoxy groups which could adsorb a variety of metal ions and organic pollutants such as natural organic matters (NOMs) through coordination, hydrogen bonding, π–π stacking interaction and electrostatic interaction [5,8,26,29]. Humic acid, the main component of natural organic materials (NOMs), is responsible for an undesirable color and has been implicated in bacterial growth in water. When HA contaminated water is treated with disinfectants such as chlorine-based compounds, carcinogenic organic compounds are formed. Therefore the elimination of HA from aqueous solution is of great practical significance for water treatment [5,30,31]. HA is composed of a skeleton of alkyl and aromatic units with different functional groups, such as carboxylic acid, phenolic hydroxyl, ketone, and quinone groups and could be absorbed on the surface of GO nanosheets through polar groups, and aromatic molecules [30,32,33]. Furthermore according to the result of Song and coworkers, modification of GO surface with CD molecules can effectively enhance its sorption capacity because CD molecules can form strong complexes with organic pollutants. They showed that CD modified GO can be used as a promising material in the enrichment of hexavalent uranium and HA from wastewater in hexavalent uranium and humic substances obtained by environmental pollution cleanup [5]. In this research, we firstly functionalized α-CD molecules with mercaptoacetic acid. Then attached this molecules to the surface of GO by ene-click reaction. Also starch biopolymer functionalized with myristic acid by esterification reaction. In the last step hostguest interactions formed between α-CD cavities and MA molecules. Due to this interaction GO nanosheets crosslinked to each other and a stable nanocomposite hydrogel formed successfully. These newly synthesized nanomaterials were characterized by spectroscopic measurement methods such as IR spectroscopy, UV–vis spectroscopy, XRD technique, scanning electron microscopy, TGA and Raman spectroscopy. The prepared hydrogel has applications in different fields such as water purification and we consider its ability to remove humic acid from contaminated water. Furthermore this new nanomaterial is sensitive to external stimuli which could be considered by destroying host-guest interactions between α-CD-SH and St-MA with addition of external stimuli. The adamantane molecule added to the prepared system as an external stimulus and destroyed host-guest interactions by competing with MA for α-CD cavity.
2. Experimental 2.1. Materials Starch, myristic acid, trimethylsilyl chloride (TMSCl), thionyl chloride, graphite, sulfuric acid (H2SO4), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), phosphoric acid (H3PO4), hydrochloric acid (HCl), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), adamantane, mercaptoacetic acid, α-cyclodextrin (α-CD), humic acid and acetone were purchased from Sigma-Aldrich. A dialysis bag was provided from Spectrum Company with 3.5 kDa MWCO. The other chemicals used in the study were of analytical grade. Deionized water was used in all experiments.
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2.2. Instruments The FT-IR analysis was performed using an FT-IR Bruker-Tensor 320 spectrometer. All of the products were mixed with analytical grade KBr at a weight ratio of 1/100 mg. Absorption spectra of samples in solution were recorded by a Shimadzu UV–visible 1650 PC spectrophotometer with a cell of 1.0 cm path length. Morphology and structure of synthesized materials were investigated using an LEO 440i scanning electron microscope under vacuum at an operating voltage of 10 kV. The instrument equipped with an EDX (energy-dispersive X-ray microanalysis) system with a sufficient sensitivity for detection of corresponding elements atomic numbers. Dried samples used for SEM experiment were coated with a thin layer of gold by sputtering for 15 s. The patterns of X-ray diffraction of the synthesized nanomaterials were recorded by a Halland Philips Xpert X-ray powder diffraction (XRD) diffractometer. (Cuk, radiation, λ = 0.154056 nm) at a scanning speed of 2°/min from 10° to 100 (2θ). TGA measurements were recorded by a STA 409 apparatus (Linei) at a temperature from 0 to 800 °C with a 10 °C/min heating rate under argon gas. Raman spectra were recorded on dispersive Raman Microscope with ʎexc = 785 nm and high spatial & spectral resolution (Spectral Resolution: < 3 cm−1). 2.3. Methods 2.3.1. Preparation of α-CD-SH via esterification reaction The esterification of α-cyclodextrin was carried out according to the published procedure with some modifications [34]. MAA (0.014 ml) and TMSCl (0.023 ml) were placed in a round bottom flask and stirred for 30 min at room temperature. Then α-CD (0.2 g) was added to them and the obtained mixture was stirred at room temperature for 72 h. After this time, the mixture of the reaction was cooled to 0 °C and precipitated in acetone. Then separated by centrifuge at 6000 rpm for 3 min and washed with cold acetone three times. The purified product (α-CD-SH) was obtained after drying as a white powder. 2.3.2. Preparation of GO GO was synthesized by Hummers’ method according to the reported procedure in the literature [35]. 2.3.3. Preparation of GO-α-CD via ene-click reaction In this research we used a polar solvent, DMF, to promote thiol-ene ‘‘click’’ reaction. For this purpose, firstly GO (0.02 g) and αCD-SH (0.1 g) was dissolved in DMF (10 ml) and sonicated for 30 min. The obtained solution was stirred at 100 °C for 48 h. Then cooled to room temperature and dialyzed against distilled water. The purified product (GO-α-CD) obtained after drying as a black compound. 2.3.4. Preparation of St-MA by esterification reaction Starch biopolymer was modified with MA according to the reported procedure in the literature [36] with a brief modification. MA (0.22 gr) was added to a round-bottomed flask. Then thionyl chloride (0.29 ml) was added to flask slowly and the mixture was refluxed in the water bath for 5 h. In the next step, starch (0.15 g) dissolved in DMSO was added to the above mixture. The mixture of the reaction was stirred at room temperature for 72 h. After this time the product precipitated in ethanol. The purified product (StMA) obtained after drying as a semitransparent compound. 2.3.5. Preparation of nanocomposite hydrogel via host-guest interactions For this purpose GO-α-CD (0.05) was dissolved in distilled water (10 ml). Then St-MA (0.5 g) was added to this solution. The mixture of reaction was sonicated for 30 min and stirred at room temperature for 24 h. After this time, the mixture of reaction stored at room temperature for 1 week for preparation of nanocomposite hydrogel. The obtained hydrogel then washed with water to obtain a purified product (GO-α-CD/St-MA). 2.3.6. Removal capability of HA from contaminated water by (GO-α-CD/St-MA) hydrogel UV–Vis spectroscopy was applied to monitor the removal process of HA from contaminated water after adding GO-α-CD/St-MA hydrogel to the HA solution. Control assay also performed using starch biopolymer to consider the relevance of using the whole system as adsorbent. HA removal was carried out by immersing 0.015 g of hydrogel and starch biopolymer into 10 ml of HA solution with 250 ppm concentration at 25 °C in a thermostatic bath under constant stirring. Each experiment was repeated three times. The amount of removed HA was evaluated using a UV–Vis spectrometer at kmax = 254 nm. The removal efficiency (RE %) of HA by hydrogel was calculated according to the following equation:
RE% = C 0 C t /C 0 × 100 where, C0 is the initial HA concentration (mg L−1) and Ct is the remaining HA concentrations in the solution at t time. 3. Result and discussion New nanocomposite hydrogel consisting of CD-modified graphene oxide and modified starch biopolymer were prepared. Hostguest interactions between CD-modified graphene oxide and polymers carrying guest moieties result in supramolecular graphene hydrogels formation. For the preparation of nanocomposite hydrogel, we chose GO-α-CD as one side of system and St-MA as the other 128
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Scheme 1. Synthetic route of GO-α-CD/St-MA nanocomposite hydrogel.
side of system. A host-guest complex formed between α-CD at the surface of GO and MA at the surface of the starch which resulted in formation of GO-α-CD/St-MA hydrogel [37]. To consider sensitivity of prepared hydrogel to the external chemical stimuli, adamantane added to the system as an external stimulus. We observed that in the presence of adamantane molecules, the formed host-guest complex between α-CD and MA destroyed because adamantane is a strong competitor for α-CD cavity in comparison with MA molecules (Scheme 1). Furthermore, the prepared supramolecular hydrogel has a porous structure which could absorbed many type of contaminations. We used this property for removal of HA from contaminated water and showed that this product has a good ability for water purification. Synthesized nanomaterials characterized by usual spectroscopy and microscopy methods such as IR spectroscopy, UV–vis spectroscopy, X-ray diffraction (XRD) technique; scanning electron microscopy, thermogravimetric analysis (TGA) and Raman spectroscopy.
3.1. Characterization 3.1.1. FT-IR analysis Fourier transform infrared (FTIR) spectroscopy was used to characterize all of synthesized products. In all of spectrums the % of transmittance is plotted as a function of wavenumber (cm−1). IR spectrums of all products are shown in Fig. 1. IR spectrum of MA is shown in Fig. 1a. In this spectrum, absorption bands at 2918 cm−1 and 2850 cm−1 are attributed to the symmetrical and asymmetrical stretching of eCH2 functional groups respectively. The C]O stretching vibration of myristic acid is appeared at 1703 cm−1 (Fig. 1a). Fig. 1b. shows the FT-IR spectrum of starch biopolymer. In this spectrum, the peaks at 3415 cm−1 and 1647 cm−1 are attributed to the stretching and bending vibrations of hydrogen bonding OH group of starch. Also the peaks of CeO stretching groups appeared at 1000–1300 cm−1 (Fig. 1b) [36]. IR spectrum of St-MA is shown in Fig. 1c. In this spectrum, absorption bands at 3375 cm−1 2924 cm−1, 1647 cm−1 and , 1000–1300 cm−1 are attributed to the OeH, CeH, CeC, and CeO stretching groups respectively (Fig. 1c). The absence of any absorbance band in the region of 1710 cm−1 corresponding to the carbonyl stretch of unreacted fatty acid and 1780 cm−1 associated with carbonyl group of fatty acid chlorides and appearance of new carbonyl peak at 1724 cm−1 corresponding to the C]O stretching groups of esterified starch confirmed that MA attached to OeH functional groups of starch by the esterification reaction [36]. Fig. 1d shows the FT-IR spectrum of α-CD-SH. In this spectrum, the peaks at 3354 cm−1, 2928 cm−1 and 1000–1300 cm−1 are attributed to the OeH, CeH, and CeO stretching groups respectively (Fig. 1d). The peaks at 2460 cm−1 and 1728 cm−1 which are attributed to 129
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Fig. 1. FT-IR spectra of MA (a), St (b), St-MA (c), α-CD-SH (d), GO (e), GO-α-CD (f) and GO-α-CD/St-MA (e).
the SeH and C]O functional groups, confirm the esterification reaction of α-CD with MAA [34]. Fig. 1e shows IR spectrum of GO. The peaks at 3421 cm−1, 1728 cm−1, 1631 cm−1 and 1037 cm−1 are attributed to the OH, C]O, C]C, and CeO stretching groups respectively (Fig. 1e) [38]. In the IR spectrum of GO-α-CD, the peaks at 3384 cm−1, 2931 cm−1 and 1724 cm−1 are attributed to the OeH, CeH and C]O, stretching groups respectively (Fig. 1f). The CeC and CeO stretching vibrations of cyclodextrin molecules appeared at 1658 cm−1 and 1000–1300 cm−1 [39,40]. Also the S-H vibrations of α-CD-SH disappeared in this spectrum because S-H functional groups of αCD-SH react with surface functional groups of GO by ene-click reaction which result in attachment of α-CD-SH to the surface of GO 130
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Fig. 2. UV–vis spectra of GO, α-CD-SH, GO-α-CD, St-MA and GO-α-CD/St-MA.
and C-S covalence bounds formation. As mentioned above, St-MA could interact with GO-α-CD by host-guest complex formation between MA and α-CD which result in supramolecular hydrogel formation. Absorption bands at 3458 cm−1, 2925 cm−1, 1735 cm−1 and 1000–1300 cm−1 in the IR spectrum of GO-α-CD/St-MA hydrogel (Fig. 1g) are corresponding to the OH, CeH, C]O and CeO stretching vibrations respectively. 3.1.2. UV–vis spectroscopy analysis UV–vis experiments were used to investigate the preparation of these new nanomaterials. Fig. 2 shows UV–vis spectra of GO, αCD-SH, GO-α-CD, St-MA and GO-α-CD/St-MA. GO shows an absorption peak at 232 nm corresponds to the π-π∗ transition of the aromatic C]C and a shoulder around 300 nm attributed to n-π∗ transitions of the carbonyl groups. The α-CD-SH spectrum has two characteristic peak in 258 nm and 289 nm which are corresponded to the n-σ∗ and n-π∗ transitions. The n-π∗ transitions in this spectrum which correspond to the carbonyl groups of MAA confirmed that MAA reacted with α-CD by the esterification reaction. Maximum absorptions appeared in GO-α-CD spectrum at around 254 nm and 273 nm which are attributed to the π-π∗ and n-π∗ transitions. The absorption spectrum of St-MA display absorbance bands centered on 277 nm, correspond to n-π∗ transitions of carbonyl groups of MA. This evidence emphasize that MA reacted with OH functional groups of St by esterification reaction. As can be seen from the figure, absorption spectrum of GO-α-CD/St-MA, shows absorbance bands centered at 271 nm and 328 nm which is attributed to the π-π∗ and n-π∗ transitions. 3.1.3. Raman spectroscopy analysis Raman spectroscopy was utilized for further investigation of carbon structure of GO, GO-α-CD and GO-α-CD /St-MA. Raman spectra of these products are shown in Fig. 3. The spectrum of GO (Fig. 3a) displays two dominant peaks D-band (1322 cm−1) and Gband (1591 cm−1) which are attributed to sp3 carbon atoms of the disordered structure and in-plane vibration of the sp2 carbon atoms, respectively. The spectral shifts which observed during reaction steps could be attributed to the disturbance of the GO structure caused by the physical or chemical interactions between functional groups of GO and polar groups of α-CD-SH and St-MA [10]. For example D peak shows a red shift from 1322 cm−1 to 1313 cm−1 in GO-α-CD (Fig. 3b) which is corresponded to the interaction of S-H functional groups of α-CD-SH with functional groups of GO by ene-click reaction. St-MA can form an inclusion complex with α-CD molecules at the surface of GO. The ratios of the intensity of D band to G band (ID/IG) can be used to evaluate the defects in graphene materials (sheets) [41]. The ID/IG ratio increased from 1.073 for GO to 1.086 for GO-α-CD while this ratio increased to 1.165 for GO-α-CD/St-MA. These slight increase of ID/IG are ascribed to the insertion of α-CD-SH and St-MA into GO layers which result in increased disordered in carbon structure of GO-α-CD and GO-α-CD/St-MA. 3.1.4. X-ray diffraction X-ray diffraction as an important tool was used to investigate crystal structure of materials. The crystal structures of prepared GO, GO-α-CD and GO-α-CD/St-MA hydrogel are obtained by XRD measurements (Fig. 4). The XRD pattern of GO (Fig. 4a) shows an intense diffraction peak at 2θ = 10.81° (attributed to an interlayer spacing of 0.817 nm) which is corresponds to the (001) plane and indicates that GO has been fully oxidized. In XRD pattern of GO-α-CD (Fig. 4b) the GO peak shifted to a lower angle (2θ = 8.78°) corresponds to an interlayer distance of 1.006 nm. The shift of GO peak to lower angle is due to attachment of α-CD-SH to the surface of GO which result in increasing in interlayer spacing of GO nanosheets. Furthermore new peaks appeared at 2θ = 19.73° and 26.70° in XRD pattern of GO-α-CD which further confirm the presence of α-CD-SH at the surface of GO. In the XRD pattern of GO-α-CD/StMA hydrogel (Fig. 4c), the GO diffraction peak disappeared and the crystal structure of GO changes after forming the GO-α-CD/St-MA 131
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Fig. 3. Raman spectra of GO (a), GO-α-CD (b) and GO-α-CD/St-MA (c).
Fig. 4. X-ray diffraction patterns of GO (a), GO-α-CD (b) and GO-α-CD/St-MA (c).
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Fig. 5. TGA thermograms of GO (a), GO-α-CD (b) and GO-α-CD/St-MA (c).
hydrogel. As shown in Fig. 4c, the XRD pattern of GO-α-CD/St-MA hydrogel only contains a very broad peak at 2θ = 26.8° which corresponds to the (002) plane of exfoliated GO. Interlayer spacing of GO-α-CD/St-MA hydrogel decreased to 0.332 nm. This evidence confirm that St-MA interact with GO nanosheets which result in formation of poorly ordered GO sheets along their stacking direction and decreasing in interlayer spacing of GO nanosheets [42].
3.1.5. TGA measurements The composition and changes in thermal stability of GO, GO-α-CD and GO-α-CD/St-MA hydrogel were further investigated by TGA (Fig. 5). The experiments were performed from 0 to 800 °C under a nitrogen atmosphere at a heating rate of 10 °C min−1. GO is thermally unstable and has three stages of mass loss upon the increase of temperature. The first mass loss (11.81%) which is occurred below 100 °C was mainly attributed to the evaporation of saturated water entrapped in GO nanosheets. The second major mass loss (42.29%) which is occurred between 110 and 210 °C, could be assigned to the decomposition of oxygen-containing functional groups. The further mass loss (14.68%) which occurred at higher temperature is due to the combustion of carbon skeleton in the GO. The TGA curve of GO-α-CD shows two mass loss at around 100 °C (2.8%) and at 130–800 °C (64.29%) corresponding to the volatilization of stored water and decomposition of oxygen-related functional groups respectively. As can be seen from the figure, attachment of α-CD to the surface GO could increase its decomposition temperature. The TGA curve of GO-α-CD/St-MA exhibits three slower mass losses during the stepwise thermal degradation process. The first mass loss below 100 °C (3.94%) is due to the removal of adsorbed water. The second mass loss at 135–230 °C (10.42%) is corresponding to the decomposition of oxygen-related functional groups and the third mass loss which occurred at 240–360 °C (39.03%) is due to the destruction of crosslinked St-MA backbones. As can be seen from the figure, the weight loss of GO-α-CD/St-MA is smaller than GO while its decomposition temperature is greater than GO. This evidence confirms that St-MA polymer which forms inclusion complexes with α-CD at the surface of GO increased the thermal stability of GO-α-CD/St-MA hydrogel. Fig. 6 shows the digital photos of the mixture of GO-α-CD and St-MA in water before hydrogel formation (Fig. 6a), after hydrogel formation (Fig. 6b) and after its destruction with addition of adamantane (Fig. 6c) respectively. The figure shows that there is no
Fig. 6. Photographs of mixtures of GO-α-CD and St-MA in water before hydrogel formation (a), after hydrogel formation (b) and after destruction with addition of adamantane (c).
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Fig. 7. FE-SEM images of GO-α-CD/St-MA (a–c), EDX spectra and elemental maps of GO-α-CD/St-MA (d–g).
hydrogel at the beginning of an experiment. But after few days due to the host-guest complex formation between St-MA and GO-α-CD a stable hydrogel can be formed in which GO nanosheets homogeneously dispersed in hydrogels. Then, adamantane molecule was added to this system as an external stimulus to investigate the destruction of host-guest interaction between St-MA and GO-α-CD. As can be seen from the figure, the introduction of adamantane to this system results in the destruction of hydrogel because adamantane in comparison with St-MA is a strong competitor for interaction with GO-α-CD therefore destruct the interaction between GO-α-CD and St-MA and formed new host-guest interaction with α-CD at the surface of GO which results in hydrogel destruction.
3.1.6. Field emission-scanning electron microscopy (FE-SEM) analysis FE-SEM observations were used for characterization of the microstructure of the hydrogels. FE-SEM image of GO-α-CD/St-MA hydrogel is shown in Fig. 7a–c. As can be seen from the figure, GO-α-CD/St-MA hydrogel has a well-defined and interconnected 3D porous network with pore sizes in the range of submicrometer to several micrometers. 3D network formation occurred due to the host-guest interaction between α-CD molecules at the surface of GO-α-CD and the MA linked to the St-MA polymer which results in cross-linking of GO nanosheets and hydrogel formation. Well information about the elements existing in the sample and also the mass concentration of the elements can be determined by Energy dispersive X-ray spectroscopy (EDX). The EDX spectrum of GO-α-CD/StMA hydrogel and its corresponding element mapping images are shown in Fig. 7d–g. As can be seen from these figures, the GO-α-CD/ St-MA hydrogel contains three kinds of elements (C, O, S) indicating the presence of α-CD-SH at the surface of GO nanosheets which promote hydrogel formation.
3.1.7. HA removal study The ability of nanocomposite hydrogel for removal of HA was monitored by UV–Vis spectroscopy in room temperature. In order to probe the relevance of using the whole system as adsorbent, control assays was also performed using starch biopolymer as adsorbent. UV–vis spectra and also digital photos of HA solution before HA removal (Fig. 8Aa and Ba) and after HA removal (Fig. 8Ab and Bb using starch and Ac and Bc using nanocomposite hydrogel) are shown in Fig. 8. The figure clearly shows that the intensity of the maximum adsorption peak of HA solution declined intensely within 24 h after addition of nanocomposite hydrogel. While the HA solution after addition of starch biopolymer show a slight decrease in maximum adsorption peak. Digital photos clearly confirm this evidence and show that the prepared nanocomposite hydrogel in comparison with starch biopolymer has a good ability for removal of HA from contaminated water due to the presence of modified GO and also the porous structure of prepared hydrogel which result in diffusion of HA solution into the soft-wet and porous structure of hydrogels so that HA could be absorbed on the surface of GO nanosheets through polar groups and aromatic molecules. HA removal capability of prepared nanocomposite hydrogel for a period of 24 h is also presented in Fig. 9. As can be seen from figure, after 24 h, 75.83% of HA was absorbed by this system through a π–π stacking and hydrogen bonding interaction between HA and GO based nanocomposite hydrogel. 134
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Fig. 8. UV–vis spectra (A) and Photographs (B) of HA solution before adsorption (a) after adsorption with Starch biopolymer (b) and after adsorption with nanocomposite hydrogel (c).
Fig. 9. Removal efficiency of nanocomposite hydrogel.
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