Article pubs.acs.org/JAFC
Multiresponsive Hydrogels Based on Xylan-Type Hemicelluloses and Photoisomerized Azobenzene Copolymer as Drug Delivery Carrier Xuefei Cao,† Xinwen Peng,*,† Linxin Zhong,† and Runcang Sun*,†,‡ †
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China Institute of Biomass Chemistry and Technology, Beijing Forestry University, Beijing, 100083, China
‡
S Supporting Information *
ABSTRACT: Stimulus-responsive hydrogels, which can undergo significant physicochemical changes in response to various physical or chemical stimuli, have drawn wide attention in many fields. In this study, novel photoresponsive hydrogels prepared by free radical copolymerization of xylan-type hemicellulose methacrylate with 4-[(4-acryloyloxyphenyl)azo]benzoic acid (AOPAB) were investigated, which showed multiresponsive behaviors to pH, water/ethanol alternating solutions, and light. The swelling ratios of the prepared hydrogels in distilled water decreased from 9.8 to 2.2 g/g with AOPAB content increase from 2% to 16%. The hydrogel displayed rapid swelling and deswelling performance in water and ethanol alternating solutions. Additionally, under UV irradiation the trans-conformation of azobenzene in the hydrogel would generally convert into the cisconformation and resulted in the hydrophilic/hydrophobic balance variation of the hydrogel. Therefore, the hydrogel loaded with vitamin B12 (VB12) showed a higher drug cumulative release rate under UV irradiation than that without UV irradiation. KEYWORDS: xylan-type hemicelluloses, hydrogel, multiresponse, azobenzene, drug delivery
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lignocellulosic materials.17 Söderqvist Lindblad et al.18 prepared biodegradable hemicellulose-based hydrogels by the radical polymerization of 2-hydroxyethyl methacrylate with modified hemicelluloses. Peng et al.11,19 reported that ionic xylan-rich hemicellulose hydrogels showed multiple responses to pH, ions, and organic solvents and excellent adsorption ability to heavy metal ions. Li and Pan20 reviewed the hydrogels based on hemicelluloses and stated that hemicelluloses with hydrophilic functional groups were a good feedstock for hydrogels, which had broad prospects in agriculture, petroleum oil drilling, biomedical engineering, and daily life.20 Recently, polysaccharide-based multiresponsive hydrogels with light responsive abilities are of great interest for controlled drug and protein delivery systems. Light irradiation has no or little harmful effect on the activity of most proteins, which can be controlled conveniently.21,22 Azobenzene compounds are well-known for their reversible trans−cis photoisomerization, which have been utilized in self-assembly micelles, 23 photonics,24 optical storage,25 and photodriven devices.26 Zhao et al.27 reported that a hydrogel constructed from deoxycholic acid modified β-cyclodextrin derivative and azobenzene-branched poly(acrylic acid) copolymer showed obvious gel-to-sol and sol-to-gel phase transitions in response to the UV irradiation. Chen et al.28 synthesized carboxylic azobenzene-based gels with multiresponsive behaviors to temperature, solvent polarity, and light stimuli, which was gelled based on the formation of H-aggregation and hydrogen bonds. The photoresponsive hydrogels have great potential in light-controlled encapsulation or release of molecules.
INTRODUCTION Hydrogel is a hydrophilic network of polymer chains, which can be swelled in water medium and form a colloidal gel with a degree of flexibility very similar to that of natural tissue.1 Stimulus-responsive hydrogels can undergo significant changes in their physicochemical properties in response to physical or chemical stimuli such as temperature, solvent composition, light, pH, and ions,2 which have drawn much attention in various fields such as drug delivery,2 pharmaceutical formulations,3 tissue engineering,4 and biomimetic materials.5 In the past few years, the synthesis and application of hydrogels have been extensively investigated.6−8 Theoretically, any water-soluble or hydrophilic polymerizable monomers and their polymers can be used to prepare hydrogels via chemical or physical cross-linking. Actually, the majority of stimulusresponsive hydrogels are constructed from synthetic polymers, especially (meth)acrylate derivatives and their copolymers.7 These hydrogels prepared from synthetic polymers show a much higher swelling ability and a significant stimulusresponsive behavior than those from natural polymers.9 However, the swelling ability and stimulus-responsive behavior of those natural polymer based hydrogels are mainly related to their introduced hydrophilic group types and their contents, as well as the cross-linking degree.10,11 Polysaccharides are the most abundant and renewable natural polymers in the world with a great amount of hydrophilic hydroxyl groups along the polysaccharide backbone. Hydrogels from polysaccharides possess specific advantages in environment-friendly products12,13 and health fields,14,15 because of their excellent biocompatibility, nontoxicity, biodegradation, and ready availability.16 Hemicelluloses, especially xylan, are the second most abundant polysaccharides just behind cellulose in nature, which can be separated as byproducts from various kinds of © 2014 American Chemical Society
Received: Revised: Accepted: Published: 10000
August 22, 2014 September 25, 2014 September 26, 2014 September 26, 2014 dx.doi.org/10.1021/jf504040s | J. Agric. Food Chem. 2014, 62, 10000−10007
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mmol) and NaOH (2.52 g, 63 mmol) mixture in 30 mL of H2O was added dropwise at 0−5 °C. The mixture was stirred for 2 h, and the NaOH solution was added to precipitate the product, which was then washed with water and dried in a vacuum oven at 50 °C for 24 h. The structure of the orange product obtained was determined using 1H NMR. 1H NMR (DMSO-d6): 7.01 (d, 2H), 7.85 (d, 2H), 7.89 (d, 2H), 8.12 (d, 2H). The 1H NMR spectrum of HPAB is illustrated in Figure S1 in the Supporting Information. AOPAB was prepared by the reaction between acryloyl chloride and HPAB according to the method reported by Haitjema et al.31 HPAB (3.63 g, 15 mmol) and triethylamine (5.06 g, 50 mmol) were dissolved in dry tetrahydrofuran (THF, 50 mL) and cooled in an ice water bath. A solution of acryloyl chloride (4.50 g, 50 mmol) in dry THF (15 mL) was added over 2 h, and the temperature was fixed at 0−5 °C and stirred for 48 h. At the end of reaction, the mixture was added dropwise to water. After filtration, the yellow precipitate was first dissolved in boiling acetic acid and subsequently crystallized during cooling down of the solution. The structure of the yellow product obtained was determined using 1H NMR. 1H NMR (DMSO-d6): 6.22 (d, 1H), 6.46 (dd, 1H), 6.60 (d, 1H), 7.47 (d, 2H), 8.00 (dd, 4H), 8.16 (d, 2H). The 1H NMR spectrum of AOPAB is illustrated in Figure S2 in the Supporting Information. Preparation of Xylan-Type Hemicellulose-graf t-photoisomeric Azobenzene Hydrogels. HMA (1.0 g) and AOPAB (0.02, 0.04, 0.08, 0.12, and 0.16 g) were added into the test tube with 20 mL of DMSO, respectively. The mixture was shaken vigorously on a vortex mixer until completely homogeneous. Then AIBN (0.02 g) was added into the mixture as an initiator with nitrogen bubbling for 15 min. After mixing, the test tube was placed in a water bath at 60 °C for 24 h. The resultant hydrogels were cut into a single slice about 2 mm thick and first immersed in ethanol for 3 days and then dialyzed in deionized water for 1 week. The prepared hydrogels were freeze-dried in a freeze drier at −50 °C. The conditions of synthesizing the hydrogels are illustrated in Table 1.
In this paper, modified hemicellulose-graf t-carboxylic azobenzene hydrogels were prepared, which showed multiresponsive behaviors to light, pH, and water/ethanol alternating solutions. The chemical structures of the xylan-type hemicellulose derivative, azobenzene monomers, and the synthesized hydrogels were investigated using FT-IR and 1H and 13C NMR. The morphologies of the resulting hydrogels were characterized by using SEM. The water swelling ratios and swelling/deswelling behaviors of the hydrogels in different solvents were investigated. Furthermore, the in vitro drug release behaviors of the synthesized hydrogels were also studied by using vitamin B12 (VB12) as a model drug.
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MATERIALS AND METHODS
Materials. Xylan-type hemicelluloses were extracted from bamboo (Dendrocalamus membranaceus Munro, DmM) according to our previous work.29 The obtained xylan-type hemicelluloses mainly consisted of xylose (86.58%), arabinose (5.65%), glucose (1.97%), mannose (1.75%), galactose (0.69%), and glucuronic acid (2.80%). Glycidyl methacrylate (GMA), p-aminobenzoic acid, acryloyl chloride, 4-dimethylaminopyridine (DMAP), and azobis(isobutyronitrile) (AIBN) were purchased from Aladdin Reagent Co., Ltd., Shanghai, China. Other chemicals and solvents were of analytical reagent grade and used as received. Methods. The scheme for the preparation of xylan-type hemicellulose-based hydrogel is illustrated in Scheme 1. The experimental details are given below.
Scheme 1. Scheme for Preparation of Xylan-Type Hemicellulose-Based Hydrogel
Table 1. Synthesis Conditions of the Prepared Hydrogels
a
samples
HMA/g
AOPAB/wt %a
AIBN/g
H1 H2 H3 H4 H5
1.0 1.0 1.0 1.0 1.0
2 4 8 12 16
0.02 0.02 0.02 0.02 0.02
The mass ratio of AOPAB to HMA.
Characterizations. The composition of neutral sugars and uronic acids in the xylan-type hemicelluloses was detected by highperformance anion-exchange chromatography (HPAEC, Dionex ICS3000, U.S.) with pulsed amperometric detector, a Carbopac PA20 column (4 × 250 mm, Dionex), and a guard PA-20 column (3 × 30 mm, Dionex). 18 mM NaOH was used as mobile phase at a flow rate of 0.4 mL/min.32 FT-IR spectra of xylan-type hemicelluloses, HMA, and prepared hydrogels were recorded on a Bruker Tensor 27 spectrophotometer in the range of 4000−400 cm−1 with a resolution of 4 cm−1. A KBr disk containing 1% finely ground sample was used for measurement. 1H NMR spectra of HPAB and AOPAB and 13C NMR spectra of xylan-type hemicelluloses and HMA were carried out on a Bruker DRX-400 spectrometer operating at 400.12 MHz for 1H NMR and 100.62 MHz for 13C NMR. Xylan-type hemicelluloses in 1.0 mL of D2O and HPAB, AOPAB, HMA in 1.0 mL of DMSO-d6 was used for NMR measurement (20 mg for 1H NMR, 80 mg for 13C NMR), respectively. A scanning electron microscope (ZEISS EVO 18, Germany) was used to record the morphologies of the hydrogels. The accelerating voltage of the instrument was maintained at 10.00 kV. Ultraviolet visible spectroscopy (S-3100, Scinco) was used to measure the time-dependent light sensitive behavior of the hydrogel and drug concentration in PBS. Measurement of Time- and pH-Dependent Swelling Ratios. The swelling ratio of the hydrogel in distilled water as a function of
Synthesis of Hemicellulose Methacrylate (HMA). HMA was synthesized according to the method reported by De Smedt et al.30 10.0 g of dried xylan-type hemicelluloses was added into the flask with 90 mL of DMSO under magnetic stirring. The mixture was stirred vigorously at 90 °C for 0.5 h. After that, the temperature was adjusted to room temperature and 2.0 g of DMAP was added. Then 3.60 g of GMA was added to the solution corresponding to a ratio of 1 mol of GMA per 3 mol of anhydroxylose units. The reaction was carried out at room temperature for 48 h. At the end of the reaction, the mixture was added dropwise into 500 mL of ethanol under vigorous stirring. The separation, purification, and dehydration of HMA were performed according to the process reported in the literature.30 Synthesis of 4-[(4′-Hydroxy)phenylazo]benzoic Acid (HPAB) and 4-[(4-Acryloyloxyphenyl)azo]benzoic Acid (AOPAB). HPAB was synthesized according to the method reported by Chen et al.28 A solution of NaNO2 (4.14 g, 60 mmol) in 60 mL of H2O was added dropwise to a solution of 4-aminobenzoic acid (8.23 g, 60 mmol) in 24 mL of HCl (37%) in an ice water bath. The solution was stirred for 30 min and diluted with 300 mL of ice water. Then phenol (5.92 g, 63 10001
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time was measured as follows. Preweighed dried hydrogel (m0) was immersed into excessive distilled water, and the mass of the wet hydrogel (mt) was determined at regular intervals after removal of the surface water with filter paper. The swelling experiments were performed twice to check the reproducibility. The swelling ratio (SR) was calculated by the following equation:
SR = (mt − m0)/m0
375 nm to determine the concentration of VB12. The drug cumulative release profiles were plotted as a function of time.
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RESULTS AND DISCUSSION FT-IR Analysis of Xylan-Type Hemicelluloses, HMA, and Prepared Hydrogels. The FT-IR spectra of xylan-type hemicelluloses, HMA, and the prepared hydrogel (H3) were recorded and are illustrated in Figure 1. The strong and wide
(1)
where m0 and mt were the weights of dried and swollen hydrogels, respectively. The pH sensitivity of the hydrogel was investigated by immersing about 1.0 g of sample into phosphate buffer solutions of pH 2.0, 4.0, 5.9, 7.4, 9.0, and 10.8 for 48 h at 37 °C to reach equilibrium. Since the SR of the hydrogel is also influenced by the ionic strength of the solution, the ionic strengths of these phosphate buffer solutions with various pH values were adjusted to 0.1 M with NaCl. The SR values of the hydrogels in various pH buffer solutions were measured using a similar method for water absorption and calculated according to eq 1. Swelling and Deswelling Behaviors of Hydrogel in Different Buffer Solutions. The swelling and deswelling behaviors of the hydrogel in response to pH were carried out in different buffer solutions (pH 2.0 and 7.4). The swollen samples in water were first immersed in buffer solution of pH 7.4 for 30 min and weighed. Then the swollen samples were soaked in the buffer solution with pH 2.0 for another 30 min and weighed again. The procedure was repeated to record the swelling and deswelling performances of the hydrogel in buffer solutions of different pH values. Swelling and Deswelling Behaviors of Hydrogel in Water/ Ethanol Alternating Solutions. The swelling and deswelling behaviors of the hydrogel in response to water/ethanol alternating solutions were carried out as follows. The swollen samples in water were first immersed in ethanol and weighed per 100 s. After 10 min, the swollen samples were soaked in water and weighed per 100 s for another 10 min. The procedure was repeated to record the swelling and deswelling performances of the hydrogel in different solvents. Light Response Behavior of Hydrogel. Dried hydrogel was first ground into a fine powder by using a Mikro-dismembrator S homogenizer (Sartorius, Goettingen, Germany) for light response test. Then it was transferred into plenty of distilled water for 48 h. The hydrogel was recovered by filtration and washed with plenty of distilled water. Next the hydrogel was dispersed into a certain amount of distilled water again. UV irradiation was carried out in an ultraviolet analyzer, which could emit UV light at 365 nm. The UV/vis spectra of the hydrogels as a function of irradiation time were recorded by ultraviolet visible spectroscopy (S-3100, Scinco). The Maximum Drug Release and in Vitro Drug Release Behavior of Hydrogel. The maximum drug release was carried out by swelling the preweighed hydrogel in sufficient vitamin B12 (VB12) solution (0.5 mg/mL). After swelling in the solution for 48 h, the hydrogel was taken out and freeze-dried. To determine the maximum drug release, the hydrogel was first ground into fine powder. Then, the powder was put into a certain amount of water to maximize the drug release. The solution was filtered through a 0.22 μm filter, and the filtrate was determined by measuring its absorbance at 375 nm using a UV spectrophotometer to calculate the maximum drug release of the hydrogel. The drug concentrations were calculated based on the determined calibration curves. In this work, VB12 was used as a model hydrophilic drug to investigate the release behavior of the synthesized hydrogel in vitro. First, 20 mL of PBS (pH 7.4 or pH 2.2) was added into 50 mL of transparent tube and maintained in an incubator at 37 ± 0.3 °C with or without UV irradiation at 365 nm. Then, a certain amount of dried VB12 loaded hydrogel was put into the PBS solution. The hydrogel was immersed in 20 mL of PBS (pH 7.4 or pH 2.2) and shaken at 100 rpm at 37 °C. At specific time intervals, 1.0 mL of PBS solution was removed and replaced by 1.0 mL of fresh PBS solution. After centrifugation at 10,000 rpm for 10 min, the supernatant of the PBS solution was collected and stored at −20 °C for further analysis. The collected supernatants were detected on a UV spectrophotometer at
Figure 1. FT-IR spectra of xylan-type hemicelluloses, HMA, and the synthesized hydrogel (H3).
absorption at 3423 cm−1 is attributed to the hydroxyl stretching vibration, and the bands at 2926 and 2880 cm−1 are related to the C−H asymmetric and symmetrical vibrations of alkanes, respectively. These signals in the spectra are originated from the anhydrosugar unit of xylan-type hemicelluloses. The strong bands in the region 1200−1000 cm−1 in xylan-type hemicelluloses are mainly from the ring vibrations and C−O−C glycosidic bond vibration overlapped with C−OH group stretching vibration.33 The absorption band at 1634 cm−1 in the spectrum of xylan-type hemicelluloses is assigned to the bending mode of the absorbed water. However, the band at 1637 cm−1 in the spectrum of HMA is much sharper than that in xylan-type hemicelluloses. This is very likely related to the CC in HMA, which shows the sharp CC stretching signals at around 1630 cm−1. Furthermore, HMA displays much stronger bands at 1327, 1300, and 1180 cm−1 than xylan-type hemicelluloses, which originate from the C−H in-plane bending vibration of alkenes and C−O−C stretching of carboxylic esters. The small sharp band at 812 cm−1 in HMA is related to the CH2 out of plane bending of alkenes, and it can be seen that this band is absent in the xylan-type hemicelluloses. Two small absorption signals at 1553 and 1493 cm−1 in the synthesized hydrogel originate from the aromatic skeletal vibration of aromatic structures. Signals at 1327, 1300, and 812 cm−1 related to the CC groups in hydrogel are much lower than those in HMA, suggesting that, during the preparation of the hydrogels, most CC groups of HMA react with AOPAB by free radical reaction. These signal changes observed in the 10002
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HMA, respectively. These signals are all assigned and shown in Figure 2. In addition, the signal splitting of C7 (126.05 and 125.18 ppm) and C1 (103.16, 101.77, and 99.63) is likely caused by the substituted position difference of acrylate in the xylan backbone, and a similar phenomenon had been reported in other hemicelluloses derivatives.34 Morphology of Hydrogels. Figure 3 shows the SEM images of the cross section of the freeze-dried hydrogels with different contents of AOPAB. Honeycomb-like architecture could be observed in H1−H4, while cracklike structure can be found in H5 due to its high crossing density. Furthermore, as the content of AOPAB increased, the observed pore volume decreased significantly. The decrease of hydrogel pore volume was further confirmed by the swelling behavior later. This is likely related to the reactivity ratio of HMA and AOPAB. There are still visible residual CC groups in the FT-IR spectrum of the hydrogel after reaction at 60 °C for 24 h, suggesting that the poor self-polymerization rate of HMA and AOPAB might be responsible for this phenomenon. It was assumed that the copolymerization between HMA and AOPAB could have led to an increase in cross-linking density and then caused the pore volume decrease. Time-Dependent Swelling Behaviors of Hydrogels. The swelling ability is one of the most important factors of a hydrogel, which is correlated with many of its properties, such as permeability, drug loading, and mechanical characteristics.35 Therefore, the swelling ability of the prepared hydrogels with different contents of AOPAB in deionized water was measured as a function of time and is illustrated in Figure 4. The swelling ratios of the dried hydrogels increased at a fast rate during the first 3 h and then achieved a swelling equilibrium about 4 h later. The maximum swelling degrees of these hydrogels (from H1 to H5) were 9.8, 8.4, 6.7, 2.8, and 2.2 g/g in deionized water, respectively, which were in accordance with the pore volume decrease of hydrogels with the increase of AOPAB content, as observed in their SEM images. It indicated that AOPAB content showed a significant effect on the swelling process of the hydrogels. As the dried hydrogels were immersed in water, partial carboxylic groups (−COOH) originating from AOPAB were gradually ionized into carboxylate groups (−COO−). The formation of anion− anion electrostatic repulsion forces within the three-dimensional polymer structure caused the hydrogel network sustained expansion. As the carboxylic group further reached its
FT-IR spectra preliminarily confirmed the successful synthesis of HMA and the target hydrogel. 13 C NMR Analysis of Xylan-Type Hemicelluloses and HMA. In this work, HMA is an important intermediate in the preparation of xylan-type hemicellulose-based hydrogel. The introduction of CC groups into hemicelluloses makes HMA easy to polymerize with AOPAB. In order to ensure the accurate structure of HMA, 13C NMR spectra of xylan-type hemicelluloses and HMA were performed and are illustrated in Figure 2.
Figure 2. 13C NMR spectra of xylan-type hemicelluloses and HMA.
As seen in the 13C NMR spectrum of xylan-type hemicelluloses, five strong signals at 101.73, 76.43, 74.27, 68.25, and 63.02 ppm originating from the main 1,4-linked β-Dxylopyranosyl units can be observed apparently, which are assigned to the C1, C2, C3, C4, and C5 positions of the xylan backbone, respectively. In the 13C NMR spectrum of HMA, except the signals from xylan backbone, signals from acrylate groups also appear. Chemical shifts at 165.84, 136.23, 126.05 (125.18), and 18.08 ppm originate from C6, C8, C7, and C9 of
Figure 3. SEM images of the cross section of the freeze-dried hydrogels with different contents of AOPAB. 10003
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SEM images. However, most carboxyl groups exist in the −COO− form in alkaline solution, thus the electrostatic repulsion forces will dominate the solution absorption capacity of the hydrogel. High AOPAB content will lead to high Seq value of hydrogel. Therefore, the hydrogel with a higher content of AOPAB showed a stronger pH responsive behavior. Swelling and Deswelling Behaviors of Hydrogel in Buffers. Reusability is another important factor that is associated with the applications of hydrogel. In this study, the swelling and deswelling behaviors of the hydrogel (H5) in the buffer solutions with pH 2.0 and 7.4 at 37 °C are shown in Figure 6. When the swollen hydrogel in the buffer of pH 7.4
Figure 4. Swelling behavior of the hydrogels in water as a function of time.
ionization equilibrium, the swelling of the hydrogel also achieved a maximum. pH-Dependent Swelling Behaviors of Hydrogels. Stimulus-responsive behavior is an important property of a hydrogel. Figure 5 illustrates the equilibrium swelling ratios
Figure 6. Swelling and deswelling ability of the hydrogel (H5) in the buffer solutions of pH 2.0 and 7.4.
was transferred into the solution of pH 2.0, the swelling ratio of H5 decreased from ∼6.2 g/g to ∼3.9 g/g. According to the results from pH-dependent swelling test, the carboxylates (−COO−) would be generally protonated into carboxyl groups with the pH decrease and further resulted in the deswelling of the hydrogel. When the hydrogel was put into the solution of pH 7.4 again, these carboxyl groups would be ionized into −COO− again and the electrostatic repulsion forces induced the expansion of the hydrogel network and finally led to the reswelling of the hydrogel. After three swelling and deswelling cycles, no obvious decrease in swelling could be observed. A similar result was also reported in other hemicellulose-based poly(acrylic acid) hydrogel.11 Swelling and Deswelling Behaviors of Hydrogel in Water and Ethanol Alternating Solutions. To investigate the response behavior of the hydrogel in organic solvent and its recycle performance, the swelling and deswelling behaviors of the hydrogel (H1) in distilled water and ethanol alternating solutions are shown in Figure 7. As can be seen, the swollen hydrogel in distilled water rapidly deswelled in ethanol. When the shrunken hydrogel was put into distilled water again, it would expand again within a short period. It was reported that the swelling capacity of the hydrogel in organic solvents was closely related to the solubility parameter of the solvent and dielectric constant as well as the interaction between the polar groups of solvents and the ionic groups in the polymer network.11,36,37 The Hildebrand solubility parameter and the relative dielectric constant of water and ethanol are 23.4 and 12.9 cal1/2 cm−3/2, respectively.38 According to the Hildebrand equation, to dissolve a polymer in a solvent, the solubility parameter values must be close to each other.37 To our best knowledge, almost all hydrogels with hydrophilic groups
Figure 5. Equilibrium swelling ratios of the obtained hydrogels as a function of pH.
(Seq) of the obtained hydrogels with different amounts of AOPAB in solutions of different pH values. The equilibrium swelling ratio of all hydrogels increased with the pH of the initial solution. It was assumed that as the pH increased from 2.0 to 10.8, more −COOH groups in the hydrogel were ionized into −COO− groups. The anion−anion electrostatic repulsion forces were strengthened and the hydrogel networks were expanded gradually, thus resulting in the high swelling ratio of the hydrogel at the high pH value. In the pH range of 2.0 to 7.4, the hydrogels containing a lower content of AOPAB showed higher Seq values. A similar result was found in their water absorption test. However, as compared with Seq in pH 2.0−7.4, the hydrogels showed opposite swelling capacities as the pH increased to 9.0 and 10.8. It was observed that in the solution of pH 9.0 and 10.8, the higher the AOPAB content was, the lower the Seq value was. The Seq value of H5 in pH 10.8 was 4.15 times higher than that in pH 2.0, while this value was only 1.46 for H1. It can be deduced that, at lower pH value, almost all carboxyl groups exist in the COOH form, and the solution absorption capacity mainly depends on the pore volume of the hydrogel. The Seq values are well in line with the pore changes observed in their 10004
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irradiation time, while the absorption band at around 430 nm (n−π* transition of cis-azobenzene) appeared with very weak intensity, which can be observed in the top right corner of Figure 8. The signal change suggested the trans−cis photoisomerization of the azobenzene units in the synthesized hydrogel. Furthermore, a previous report showed that trans−cis photoisomerization of the azobenzene would change the hydrophilic/hydrophobic balance of material.40 Therefore, the light response behavior of synthesized hydrogel might have potential in drug delivery application. Drug Release Behaviors of Hydrogel. In order to investigate whether the synthesized hydrogel had special drug release behavior, vitamin B12 (VB12) was used as a model drug to record its in vitro release behavior at 37 °C. The influences of release medium pH and UV irradiation on the release behaviors of the prepared hydrogel were tested. The cumulative release data were summarized and are plotted in Figure 8. As seen in Figure 8, the initial release of VB12 was very fast in the first 2 h, and then the release of VB12 slowed down and moved toward release equilibrium. Additionally, pH value of medium and UV irradiation both had obvious effects on the release of VB12. Over the releasing process, the cumulative release value of VB12 in neutral medium was always higher than that in acidic medium. Without UV irradiation, the final cumulative release rates of the hydrogel in acidic medium and neutral medium were 78.2% and 89.3%, respectively. This is very likely related to the difference in swell ratios of the hydrogel in different mediums. Because the SR value of the hydrogel in neutral medium was much higher than that in acidic medium, the diffuse of drug was much faster. As previously mentioned, the synthesized hydrogel showed a light response behavior under the UV irradiation. The thermodynamically stable trans-conformation of azobenzene structure would convert into the cis-conformation under UV irradiation.23,40 However, as the cis-conformation of azobenzene was exposed to visible light or deposited in the dark, it would transfer into the trans-conformation again, resulting in a shift of the hydrophilic/hydrophobic balance arising from the trans−cis isomerization.40 From Figure 9, it was found that the cumulative release of hydrophilic VB12 from the hydrogel under UV irradiation was higher than that without UV irradiation. It can be explained by following reasoning. As the VB12 loaded hydrogel was placed into the release medium, due to the
Figure 7. Swelling and deswelling behavior of the hydrogel (H1) in water/ethanol alternating solutions.
showed higher swelling ratios in water than in organic solvent,11,36 suggesting that the solubility parameters of these hydrogels were much closer to that of water. Furthermore, ethanol possesses a much lower dielectric constant than water, which can significantly reduce the electrical double layer thickness and the dissociation abilities of the carboxyl groups.39 Therefore, the synthesized hydrogel showed much lower swelling ratio in ethanol than in water. In addition, during the swelling and deswelling recycles, the adsorption and desorption process is reversible, which makes the repeated utilization of hydrogels possible. Time-Dependent Light Response Behavior of Hydrogel. To investigate the light responsive behavior of hydrogel, finely ground hydrogel suspended in deionized water after being irradiated with UV light for different periods was subjected to UV/vis spectroscopy. The UV/vis spectra of the hydrogel solutions (H3) after UV irradiation were recorded and are illustrated in Figure 8. It was reported that the maximum
Figure 8. UV/vis spectra of the hydrogel (H3) solution after UV irradiation for 0, 30, 60, and 90 min, respectively (from the top down).
absorption peak of trans-azobenzene appeared at around 350 nm, which was associated with π−π* transition of transazobenzene, while cis-azobenzene showed a maximum absorption band near 440 nm, which was assigned to the n−π* transition of cis-azobenzene.23,27 As seen in Figure 8, the absorption at around 340 nm originating from transazobenzene (π−π* transition) decreased with increasing UV
Figure 9. Vitamin B12 (VB12) in vitro cumulative release behavior of the synthesized hydrogel (H3) at 37 °C. 10005
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different concentrations of VB12 in the hydrogel and the release medium, the VB12 started to gradually diffuse toward the release medium accompanied by the expansion of the hydrogel. However, the UV irradiation would change the hydrophilic/ hydrophobic balance and make the hydrogel more hydrophobic, which would drive the hydrophilic VB12 into the release medium and finally result in a higher cumulative release rate. To conclude, a novel hemicellulose-based hydrogel with multiresponse behaviors to pH, solvent, and light was prepared by free radical copolymerization of xylan-type hemicellulose glycidyl methacrylate with 4-[(4-acryloyloxyphenyl)azo]benzoic acid. The swelling ratio of the synthesized hydrogel ranged from 2.2 to 9.8 g/g in distilled water, which decreased generally with the increase of AOPAB content. The swelling capacity was closely associated with the strong electrostatic repulsion among carboxylic groups (−COO−). The stimulusresponse behaviors indicated that the hydrogel could be easily controlled by changing solvent pH and solvent type. Furthermore, the prepared hydrogel also showed good swelling and deswelling recycling performance. Results from light response test showed that the trans-conformation of azobenzene in hydrogel would generally convert into the cisconformation under UV irradiation. Furthermore, this light response hydrogel was applied as a drug delivery device to monitor its drug release behavior, and results indicated that the hydrogel under UV irradiation showed higher drug cumulative release rate than that without UV irradiation.
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ASSOCIATED CONTENT
S Supporting Information *
1
H NMR spectra of HPAB and AOPAB. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(X.P.) Tel/fax: +86 20 87111861. E-mail: fexwpeng@scut. edu.cn. *(R.S.) Tel/fax: +86 20 87111861. E-mail:
[email protected]. Funding
The project is supported by the National Natural Science Foundation of China (21404043), Research Fund for the Doctoral Program of Higher Education (20130172120024), Guangdong Natural Science Foundation (S2013040015055), Science and Technology Project of Guangzhou City in China (2014J2200063), and Fundamental Research Funds for the Central Universities. Notes
The authors declare no competing financial interest.
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