Self-Assembled Hydrogels Based on Poly-Cyclodextrin and Poly-Azobenzene Compounds and Applications for Highly Efficient Removal of Bisphenol A and Methylene Blue.

The excellent physical and chemical properties of cyclodextrin polymer (poly-CD)/azobenzene-modified polyacrylic acid (PAA-Azo) binary composite hydrogels have been designed and prepared. The prepared hydrogels were subjected to a variety of characterizations, including scanning electron microscopy, ultraviolet spectroscopy, circular dichroism spectroscopy, infrared spectroscopy, rheological properties, and specific surface area tests. It was found that the obtained hydrogels have the cross-linked three-dimensional porous network nanostructures, and the formed composite poly-CD/PAA-Azo hydrogel can basically be shear thinned and have good recovery performance. A process of gel-sol transition can occur when the gel has a stimulatory response under UV light irradiation. In addition, such excellent properties of hydrogels exhibit different mechanisms in the adsorption of organic molecules that are harmful to the environment, such as bisphenol A (BPA) and methylene blue (MB). The polymeric hydrogel serves as novel adsorbent agents to adsorb BPA via host-guest interaction and anchor MB via electrostatic interaction and hydrogen bonding.

Introduction

Cyclodextrins (CDs) are macrocyclic oligosaccharides with the special structure of hydrophobic interior cavity and hydrophilic exterior ring.[1] β-Cyclodextrin is composed of seven repeated monomers of glucopyranose units linked by α-(1,4) glycosidic linkages. Due to moderate internal diameter, low price, and negligible toxicity, β-cyclodextrin-based self-assembled materials have been intensively investigated in many fields, such as controlled release drug carriers, self-healing hydrogels, and electrochemical sensors.[2−4] However, native cyclodextrins are unable to incorporate certain hydrophilic compounds or large molecules. To overcome these limitations and extend the inclusion capacity, the syntheses of cyclodextrin polymers are necessary.[5,6] Cyclodextrin polymers could be prepared by reacting cyclodextrins with cross-linking agents, such as epichlorohydrin, carbonyl compounds (e.g., diphenyl carbonate, dimethyl carbonate, carbonyldiimidazole), and organic dianhydrides (e.g., pyromelitic anhydride).[7−10] In contrast to CDs, cyclodextrin polymers (poly-CD) exhibited higher efficiency to accommodate high-weight molecules and demonstrated higher stability constants since all of the cyclodextrin units co-operatively participated in the formation process of inclusion complexes.[11−13]

Polymeric hydrogel is a series of essential soft matter with cross-linked network structure. Stimuli-responsive polymeric hydrogels are significant at present and have potential applications as “smart” materials in many areas owing to well responsiveness to environmental stimuli, such as light, pH, temperature, or solvents.[14−16] Compared to the fixed networks constructed by chemical cross-linking, the stimuli-responsive hydrogels showed temporary physical network, which could be inversely transformed into solutions by adjusting the environmental factors.[17]

Recently, the supramolecular self-assembled hydrogels via host–guest interactions between cyclodextrins polymers and guest molecules have attracted much attention. The hydrophobic cavity of cyclodextrins can obtain host–guest inclusion complexes with suitable hydrophobic guest molecules through hydrophobic interactions, electrostatic interactions, van der Waals forces, and dipole–dipole interactions. Hence, the host–guest interaction via cyclodextrin can be utilized to form hydrogel as adsorbent agents to anchor and remove organic molecules from harmful environment. In addition, bisphenol A (BPA) is one of hydrophobic contaminants, which comes from food packaging materials, including epoxy resins and polycarbonates mainly used for water bottles and tin internal coatings.[18] BPA could be removed using cyclodextrin-based hydrogel through the host–guest interaction to form inclusion complexes. Besides, methylene blue (MB) is a kind of organic dye from aqueous solution, which is harmful to the environment. Now many excellent research works have been achieved about the synthesis of novel composite materials for MB removal. For examples, Jale et al. reported the synthesis of carbonized peanut shell as low-cost adsorbent for adsorption of MB.[19] Yildiz and co-workers investigated monodisperse Pt/Rh@GO nanocomposites and their adsorption performances for MB with adsorption capacity of 346.79 mg/g.[20] Han et al. reported the preparation of flower-like MoS2 nanosheet-based nanostructure and superior dye-adsorption performance.[21] Sert et al. investigated the synthesis of monodisperse Vulcan carbon-supported Pt nanoparticles via microwave-assisted method and application for MB removal with remarkable adsorption capacity of 271.15 mg/g.[22]

In this present work, we synthesized cyclodextrin polymers (poly-CD) by cross-linking agents and fabricated photosensitive supramolecular polymeric hydrogel via poly-cyclodextrin (poly-CD) and azobenzene-branched poly(acrylic acid) copolymer (PAA-Azo) to investigate the light-responsive properties of polymeric hydrogel after UV irradiation. These new sol–gel switching hydrogel materials through host–guest interactions could be utilized as light-operated switch and self-healing materials. Moreover, these polymeric hydrogels served as excellent adsorbent agents to remove BPA via hydrophobic interaction and MB via electrostatic interaction and hydrogen bonding, which demonstrated potential applications in dye removal and wastewater treatment.

Results and Discussion

Preparation and Characterization of Hydrogels

The photographs of as-obtained poly-CD/PAA-Azo composite hydrogels are showed in Figure . It was observed that all hydrogels perform good gelation stability, and the gel with lower concentration of poly-CD becomes more transparent and clearer. To explore the internal structure of the gels, the morphological and spectral characterizations were performed. Figure shows the X-ray diffraction (XRD) patterns of poly-CD and PAA-Azo as well as the five groups of hydrogels. The XRD pattern of pure PAA-Azo has a broad peak with centered position of 23°, which is mainly attributed to the alkyl main chain in the PAA molecule. The XRD pattern of poly-CD shows many diffraction peaks between 2θ values of 10–25° indicating the amorphous state of the cyclodextrin polymer. For the five groups of hydrogels, the obtained XRD patterns of Gel-A and Gel-B are similar to poly-CD, indicating that the concentrations of poly-CD in the hydrogels were excessive. Compared to the formed Gel-C, Gel-D, and Gel-E, the diffraction peaks assigned to poly-CD almost disappeared and there appeared the characteristic diffraction peak at 2θ value of 19.4°, which indicated that the special interaction occurred in the hydrogels (Table ).

Figure 1

Photographs of hydrogels: Gel-A, Gel-B, Gel-C, Gel-D, Gel-E (from left to right). Photograph courtesy of “Yagui Gao”. Copyright 2018.

Figure 2

XRD patterns of poly-CD, PAA-Azo, and composite hydrogels with different concentration ratios.

Table 1

Concentration Ratios of Poly-CD and PAA-Azo in Hydrogels

	poly-CD concn (mg/mL)/volume (mL)	PAA-Azo concn (mg/mL)/volume (mL)
Gel-A	100:0.8	25:1
Gel-B	50:0.8	25:1
Gel-C	25:0.8	25:1
Gel-D	12.5:0.8	25:1
Gel-E	6.25:0.8	25:1

The Fourier-transform infrared (FT-IR) spectra of the five groups of hydrogels are shown in Figure . The characteristic peak at 3350 cm–1 was mainly owing to the stretching vibration peaks of −OH, −COOH, and H–OH groups. As for poly-CD, the peak at 2915 cm–1 was assigned to the −CH2 group, which is due to the introduction of the epichlorohydrin as a cross-linking agent. And the peaks at 1650, 1150, and 1095 cm–1 originated from C=H bonds, C=O bonds, and C–OH bonds, respectively, which indicated the good hydrophilicity of poly-CD.[23−25] The characteristic peaks at 1015 and 910 cm–1 are attributed to the vibration peaks of the C–O–C bond and the α-1,4 glycosidic bond on the cyclodextrin backbone, respectively. In addition, the above characteristic peaks also appeared in the FT-IR spectra of hydrogels with different concentrations, and the newly appearing characteristic peaks at 1730 and 1660 cm–1 were attributed to the stretching vibration peaks of carbonyl and amide bonds, respectively. Therefore, the present obtained FT-IR data demonstrated the successful synthesis of composite materials between poly-CD and PAA-Azo, which occurs through the linkage of amide bonds. It should be noted that due to the component of poly-CD solution with maximum concentration of 100 mg/mL, the formed Gel-A demonstrate cloudy state with a few aggregates, which can be seen in photograph of hydrogels in Figure . The concentration ratio of the Gel-E was the minimum gelation concentration between poly-CD and PAA-Azo. Thus, the formed Gel-B and Gel-E were selected as representatives for the next characterization investigation.

Figure 3

FT-IR spectra of poly-CD, PAA-Azo, and composite hydrogels with different concentration ratios.

It could be seen that the micro/nanosized morphologies of the Gel-B and Gel-E hydrogels are demonstrated in Figure . The cross-linked network-like hydrogel system could be formed under the dispersion medium, and the microscopic size changed from several micrometers to hundreds of nanometers. Comparison of Gel-B and Gel-E, it was found that the pore structure existed both in the Gel-B and Gel-E. Due to the poly-CD concentration in Gel-E was lower than that in Gel-B, so the more porous structures and larger specific surface area were appeared in Gel-E than that in Gel-B. The porous microstructures of the hydrogels were further investigated by utilizing the nitrogen adsorption–desorption isotherms, and the pore size distributions of poly-CD/PAA-Azo hydrogels were calculated by the Barrett–Joyner–Halenda (BJH) method shown in Figure . In the range of relative pressures from 0 to 1, Gel-E shows the hysteresis loops of typical IV isothermal curves at the p/p0 = 0.3–0.7, indicating that the mesoporous structures exist in the Gel-E.[26−28] Thus, Gel-E demonstrated more pores and larger specific area than Gel-B. In addition, the BJH method was used to calculate the specific surface areas of the two groups of gels, as shown in Table , where the physical properties of nitrogen adsorption and desorption are shown. It can be clearly observed that the specific surface areas of hydrogels in Gel-B and Gel-E are 66.148 and 68.184 m2/g, respectively, whereas the average pore diameters are 4.886 and 5.225 nm, respectively, and the average pore volumes are 0.077253 and 0.086408 cm3/g. The higher specific surface areas and larger pore diameters and pore volumes can be expected to enhance the enveloped capacity of the organic molecules.

Figure 4

Scanning electron microscopy (SEM) images of as-obtained poly-CD/PAA-Azo composite hydrogels: (a) Gel-B and (b) Gel-E.

Figure 5

Nitrogen adsorption–desorption isotherms (a) and pore size distributions (b) of the poly-CD/PAA-Azo hydrogels.

Table 2

Physical Data of the Obtained Gel-B and Gel-E Hydrogels

sample	specific surface area (m2/g)	average pore diameter (nm)	pore volume (cm3/g)
Gel-B	66.148	4.886	0.077253
Gel-E	68.184	5.225	0.086408

Figure demonstrates the thermograms of the prepared hydrogels under nitrogen conditions. It could be observed that the first weight loss of four curves was around at 100 °C. This can be normally explained by the removal and evaporation of surface-adsorbed water molecules. From the thermogravimetric curves of PAA-Azo, it can be seen that PAA-Azo mainly loses weight in two stages of 190 and 480 °C, which is mainly owing to the decomposition of the alkyl backbone of PAA molecules and the azobenzene group organics. As for poly-CD, the weight loss of 62% at 300 °C was attributed to the thermal decomposition of cyclodextrin oligosaccharides.[29−33] In addition, from the thermogravimetric plots of poly-CD/PAA-Azo hydrogels of Gel-B and Gel-E, it can be seen that there is approximately 10% weight loss between 100–300 °C, which is attributed to poly-CD molecules and the thermal decomposition of alkyl chains in PAA-Azo molecules; and the mass loss approximately 55% between 300 and 400 °C was mainly attributed to the inclusion complexes of the composite hydrogels became thermal decomposition.

Figure 6

Thermogravimetric (TG) curves of poly-CD, PAA-Azo, Gel-B, and Gel-E.

The rheological behavior of two kinds of obtained hydrogels was characterized, considering the dependence of storage modulus (G′) and loss modulus (G″) on angular frequency (ω). The storage modulus G′ represents the elastic part of stored energy in material and also becomes the elastic modulus; the loss modulus G″ reflects the viscosity and energy loss of the material and also becomes the viscous modulus. Comparing Figure a,b, the G′ and G″ values of the Gel-B and Gel-E are equal at shear stresses of 1.02 and 0.43%, respectively. This shows that the Gel-B has relatively wider viscoelastic region and shear strength relative to the Gel-E. At a shear stress of 0.001%, the frequency-dependent oscillating shear rheological behavior of the obtained gels was measured in Figure c,d. It was found that G′ dominated the detected frequency range, exhibiting the true gel-state behavior.[34−37] From the Figure e,f curves, it can be seen that the two groups of gels can basically be shear thinned and have good recovery performance.

Figure 7

Rheological characterizations of Gel-B (a, c, and e) and Gel-E (b, d, and f) hydrogels.

The photoisomerization performances of the azobenzene group were studied using the minimum gel formation concentration of Gel-E, as shown in Figure . Figure a shows the Gel-E placed in a quartz cell, and the hydrogel would be exposed to UV light at different time intervals with 365 nm UV lamp in dark conditions. The UV–vis spectra were measured with the irradiation time of 0 s, 15 s, 30 s, 1 min, 5 min, and 10 min, as shown in Figure c. Observing the UV–vis spectra of the Gel-E before irradiation, it can be found that the absorption peaks appear at 221, 282, and 433 nm. The characteristic peak at 348 nm was attributed to the π–π* transfer of the azobenzene trans-isomer, whereas the n−π* transfer of the cis-isomer was at 438 nm. With the increase of the irradiation time, the intensity of the π–π* transfer peak gradually decreased and the position of the peak became slowly blue shifts, at the same time, the intensity of the n−π* transfer peak gradually increased. The equilibrium state was basically reached after 5 min of irradiation, indicating that the cis and trans isomers of azobenzene group almost reached equilibrium at this time. Figure d is a comparison of circular dichroism (CD) spectra before and after UV light illumination. It could be seen that the hydrogel had a good chiral signal before illumination, indicating that the azobenzene group and the β-CD group in the poly-CD/PAA-Azo hydrogel have the host–guest reaction. The host–guest recognition had formed in a hydrogel system with a stable cross-networking structure. Compared with the CD spectra of the 10 min UV light, the chiral signal was weaker and almost disappeared. This indicated that UV light make the azobenzene group change from trans-isomer to cis-isomer. The conformational conversion of isomer caused the partial host–guest interactions of azobenzene group and β-CD groups to be disassembled, and the stable system of hydrogels was conversed to a sol state macroscopically, as shown in the photograph of Figure b.

Figure 8

Photographs of the Gel-E group hydrogel before (a) and after (b) exposure to UV light as well as the corresponding UV–vis spectra (c) and CD spectra (d). Photograph courtesy of Yagui Gao. Copyright 2018.

Adsorption Performances toward Dye Removal

On the basis of the above characterization analysis, we know that the obtained hydrogels have porous nanostructure and large specific surface area. So, the adsorption properties of present obtained gels to bisphenol A and organic dyes were researched. The Gel-E was chosen as the typical adsorbent to selectively remove MB and BPA mainly owing to the larger specific surface area and more porous structures. The adsorption kinetics was carefully investigated by fitting experimental data with the pseudo-first-order model and pseudo-second-order model adsorption equation, as shown in Figure . Classical kinetic models were utilized to show the above adsorption mechanism as follows:

Figure 9

Kinetic adsorptions of (a) q versus t plots and (b) t/q versus t plots for BPA and MB.

The pseudo-first-order model can be showed by eq The pseudo-second-order model can be showed by eq where qe represents the adsorption capacity at equilibrium qe and q represents the adsorption capacity at time t and the k1 and k2 values represent the kinetic rate constants.[38,39] The kinetic results are calculated and summarized in Table , and demonstrated that the pseudo-two-order model showed a higher correlation coefficient (R2 > 0.99) in MB adsorption process, whereas the pseudo-first-order model seemed more accurate (R2 > 0.99) in BPA adsorption process. Thus, it was hypothesized that Gel-E exhibited the different mechanisms in removal of MB and BPA. In addition, the fitted removal efficiency of Gel-E for MB reached 85.3248 mg/g, whereas for the BPA system, the calculated removal efficiency showed value of 20.7297 mg/g. It should be noted that from previous reports, the removal efficiencies of MB from different composite materials, including sandwiched Fe3O4/carboxylate graphene oxide nanostructure, polydopamine sheathed electrospun nanofibers, and diamond based core–shell nanocomposites, showed the value range of 34–40 mg/g.[40−42] In addition, some two-component supramolecular gels based on glutamic acid component and graphene oxide hydrogels demonstrated the maximum removal values of 16.898 and 334.448 mg/g for MB removal.[43,44] Thus, present obtained composite gel materials exhibited excellent removal capacities for MB molecules. Moreover, durability and regeneration of absorbent materials seemed very important in real industrial application. Different composite systems mentioned above can be reused several times or recycled in a controlled way, demonstrating long use in wastewater purification.[45,46] However, as for present composite hydrogel materials, due to the mass losses of poly-CD and PAA-Azo components in the regeneration process by washing with organic solvents, the durability and reusability performance do not seem promising.

Table 3

Kinetic Parameters of Obtained Hydrogel Gel-E for MB and BPA Removal at 298 K

	pseudo-first-order model			pseudo-second-order model
Gel-E	qe (mg/g)	R2	K1 (min–1)	qe (mg/g)	R2	K2 (g/(min mg))
BPA	19.0055	0.99334	1.634 × 10–2	20.7297	0.92056	1.3274 × 10–3
MB	85.3248	0.96126	3.2181 × 10–3	84.1750	0.99914	5.4157 × 10–3

It is well known that cyclodextrin-based compounds also have a good affinity for the binding of BPA mainly due to host–guest interaction, so they can be utilized as specific host molecules for anchoring and removal of BPA from wasterwater.[47] On the other hand, MB belong to a positively charged organic molecule, which interacts with many adsorbents through electrostatic attraction and hydrogen binding.[41,42] Therefore, present prepared hydrogel materials could remove both kinds of organic molecules with good removal efficiency, which was reasonably attributed to different removal mechanisms. As shown in Figure , the prepared gels served as adsorbent agents to anchor MB via electrostatic interaction and hydrogen bonding by functional carboxyl groups linked in molecular skeletons, which demonstrated high removal efficiency mainly due to porous structures and numerous chemical active sites to anchor MB molecules. In addition, the differences of assembly modes in the Gel-E and BPA are exhibited in Figure b. BPA molecules could be embedded on the cavity of CD in gel systems via host–guest interactions, showing a versatile adsorption process. It can be expected that the current obtained composite hydrogel materials can be used for potential extensive applications in fields of nanocomposite materials and environmental engineering, demonstrating new clues for the design and preparation of β-CD-based hydrogels.

Figure 10

Schematic illustration of the Gel-E adsorption processes of MB (a) and BPA (b).

Conclusions

In summary, new composite hydrogel materials based on poly-CD and PAA-Azo polymers were prepared, and the dye removal capacities of the hydrogels were studied. Various poly-CD/PAA-Azo composite hydrogels with different concentrations ratios were prepared, demonstrating gel–sol conversion process due to the cis–trans isomerization of azobenzene group. The maximum removal efficiency of present obtained gel reached 85.3248 mg/g for MB and 20.7297 mg/g for BPA, demonstrating excellent anchoring capacities. The prepared gels served as adsorbent agents to anchor MB via electrostatic interaction and hydrogen bonding by functional carboxyl groups linked in molecular skeletons. And the removal of BPA molecules could be attributed to host–guest interactions with versatile adsorption process. Present research work showed new exploration of composite hydrogels used as composite absorbent materials for applications in environmental engineering and wastewater treatment.

Experimental Section

Materials

β-Cyclodextrin (β-CD, 98%), epichlorohydrin (99%), 4-aminoazobenzene (N-Azo), and poly(acrylic acid) (PAA, average MW ∼ 450 000) were obtained from Alfa Aesar (Tianjin, China) Chemicals, Aladdin Reagent Chemicals (Shanghai, China), and TCI Shanghai Chemicals without further purification. N-(3-Dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide were obtained from Sigma-Aldrich without further purification. Methylene blue (MB) and bisphenol A (BPA) were purchased from Beijing Chemicals with analytical reagent grade. The used other reagents, such as toluene and isopropanol, were obtained from Sinopharm Chemical Reagent Co. Ltd. with analytical reagent grade. All aqueous solutions were obtained with water from a double-stage Millipore Milli-Q Plus purification system.

Preparation of Hydrogels

First, poly(β-cyclodextrin) (poly-CD) was synthesized according to the literature.[48−50] The final white product solid was obtained after freeze–drying process. The azobenzene-branched poly(acrylic acid) copolymer (PAA-Azo) was prepared according to our previous work.[51] Then, aqueous poly-CD solutions with various concentrations (100, 50, 25, 12.5, and 6.25 mg/mL) were prepared and continuously stirred at room temperature until poly-CD molecules was completely dissolved into ultrapure water. Aqueous PAA-Azo solution (10 mg/mL) was also prepared and fully dissolved by sonication for 2–3 min. Then, 1 mL of PAA-Azo solutions was mixed with the different concentrations 0.8 mL of poly-CD solutions, respectively. After sonication for 30 min, the gelation states were formed. The detailed formulations are shown in Table , and the prepared samples of hydrogels were named as Gel-A, Gel-B, Gel-C, Gel-D, and Gel-E. It should be mentioned that the concentration ratio of the Gel-E was also the minimum gelation concentration between poly-CD and PAA-Azo.

Adsorption Experiments

Adsorption performances were performed by utilizing two typical organic molecules, methylene blue (MB) and bisphenol A (BPA). The UV–vis absorption spectra were monitored for the adsorption process at wavelengths of 276 nm (BPA) and 632 nm (MB) using a 752-type UV–vis spectrometer (Sunny Hengping scientific instrument Co., Ltd., Shanghai, China). Required solution of BPA was prepared from 1 mg/mL of stock prepared in 5% dimethyl sulfoxide solution. About 1 mL of as-obtained poly-CD/PAA-Azo hydrogel was added to 20 mL of BPA solution with 25 mg/L concentration at room temperature. Corresponding to the hydrogel was added to 100 mL of MB solution (5 mg/L) at room temperature. The absorbances were monitored at different time intervals and then calculated and fitted using the calibration curves.

Characterization

The xerogels were acquired via FD-1C-50 lyophilizer (Beijing Boyikang Experimental Instrument Co., Ltd., China) to completely remove water over 2–3 days. The morphology of the hydrogels was characterized via a field-emission scanning electron microscope (SEM) (S-4800II, Hitachi, Japan) with 5–15 kV accelerating voltage. X-ray diffraction patterns were investigated on an X-ray diffractometer (SMART LAB, Rigaku) using Cu Kα X-ray radiation. FT-IR spectra were carried out by a Fourier infrared spectroscopy (Thermo Nicolet Corporation) by the conventional KBr disk tablet method. The specific surface areas and pore diameter distributions were measured by using Brunauer–Emmett–Teller measurements (NOVA 4200-P). Thermogravimetries (TG) were conducted by a Netzsch STA 409 PC Luxxsi multaneous thermal analyzer (Netzsch Instruments Manufacturing Co, Ltd, Germany) in argon gas atmosphere. UV–vis absorption spectra were investigated on a Shimadzu UV-2550 system (Shimadzu Corporation, Japan). Circular dichroism (CD) spectra were obtained by a JASCO J-810 CD spectrometer. Dynamic rheology experiments were measured with an Anton Paar MCR302 rheometer at room temperature.