Preparation of Hybrid Hydrogel Containing Ag Nanoparticles by a

Jul 6, 2012 - Preparation of Bimetallic Nanoparticles Using a Facile Green Synthesis ... Yongjun Men , Xiyan Du , Jianan Shen , Leli Wang , Zhengping ...
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Preparation of Hybrid Hydrogel Containing Ag Nanoparticles by a Green in Situ Reduction Method Bihua Xia, Qianling Cui, Fang He, and Lidong Li* School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China S Supporting Information *

ABSTRACT: In this Article, large and uniform Ag nanoparticle-containing hybrid hydrogels were prepared by in situ reduction of Ag ions in cross-linked tapioca dialdehyde starch (DAS)−chitosan hydrogels. In the hybrid hydrogels, chitosan was chosen as a macromolecular cross-linker because of its abundant source and good biocompatibility. The hybrid hydrogel showed good waterswelling properties, which could be controlled by varying the ratio of chitosan to tapioca DAS in the hydrogel. The reductive aldehyde groups in the cross-linked hydrogels could be used to reduce Ag ions to Ag nanoparticles without any additional chemical reductants. Interestingly, by controlling the reduction conditions such as the tapioca DAS concentration, aqueous AgNO3 concentration, reaction time, and aqueous ammonium concentration, Ag nanoparticles with different sizes and morphologies were obtained. Because of their biocompatibility, degradable constituents, mild reaction conditions, and controlled preparation of Ag nanoparticles, these tapioca DAS−chitosan/Ag nanoparticle hybrid hydrogels show promise as functional hydrogels.

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INTRODUCTION Hydrogels are important hydrophilic soft materials that have properties similar to those of natural tissue, such as good swelling behavior in water, and are suitable to a variety of applications in pharmaceutical and medical science.1−5 Hydrogels need to have excellent biocompatibility for use in these applications.6,7 For this reason, many functional hydrogels have been prepared from natural polymers such as polysaccharides and proteins, and from approved synthetic polymers.8−10 Among the natural polymers, starch is an ideal material to prepare functional hydrogels because of its biodegradability, biocompatibility, easy modification, and abundance.11,12 Under oxidizing conditions, starch can be converted to dialdehyde starch (DAS), which is easier to modify or cross-link with amino-containing cross-linkers.13−15 To avoid the introduction of toxic diamine chemicals, chitosan, an amino-containing natural polymer, has been used as macromolecular cross-linker to prepare imine linked DAS−chitosan hydrogels.16,17 Tapioca starch can be extracted from cassava root tubers in high yield.18 In industry, it is mainly used as biodegradable plasticizers, as biopolymer films,19 and in biodegradable packing materials.20 Because it is inexpensive and biocompatible, tapioca starch would be useful for hybrid hydrogel preparation. However, the tapioca starch-based hydrogels for biocompatible materials have scarcely been investigated. © 2012 American Chemical Society

The unique internal microenvironments of functional hydrogels are good matrixes for in situ preparation of metal nanostructures.21−28 Recently, hybrid hydrogels with Ag nanoparticles have attracted attention for their potential applications as antimicrobial materials and hydrogel-supported catalysts.29−31 Ag nanoparticle-containing hybrid hydrogels have been prepared by chemical,32,33 photoinduced,34 and microwave-assisted reduction methods,35 with chemical reduction methods being the most common. However, these reduction procedures involve addition of toxic chemical reductants such as sodium borohydride.36,37 Although glucose38 and ascorbic acid have been used as nontoxic reductants to prepare Ag nanoparticle-containing hybrid hydrogels,39 additional nontoxic methods are required for preparation of hybrid hydrogels.40 There are a large number of excess aldehyde groups in DAS−chitosan hydrogels that are not involved in the cross-linking between the DAS and chitosan, which are formed by a Schiff base reaction.41 When the tapioca DAS−chitosan gels were immersed into aqueous ammonia, if the Ag ions was also introduced into this system, then the ammoniated Ag ions can be formed inside the hydrogels. Because of the existing free Received: March 21, 2012 Revised: July 3, 2012 Published: July 6, 2012 11188

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immersed in 2% AgNO3 aqueous solution at 25 °C in the dark for 24 h, so Ag ions could be absorbed into the hydrogels. The hydrogels then were immersed in aqueous ammonia (NH3:H2O = 1:5, v/v) to ammoniate the Ag ions, which could then be readily reduced to Ag nanoparticles. Using this procedure, uniform Ag nanoparticles were produced in an in situ manner in the large tapioca DAS−chitosan hydrogels (Figure 2b). A series of hybrid hydrogels were prepared to study the influence of the experimental parameters, such as tapioca DAS content, ammonia concentration, reduction time, and Ag ions concentration on the hydrogels. Swelling Capacities of the Tapioca DAS−Chitosan Hydrogels. After the tapioca DAS hydrogels were left at room temperature for 3 h, they were placed in a set volume of deionized water, and the change in volume was measured (0 h). After the hydrogels were immersed in deionized water for 24 h, the volumes of the swollen hydrogels were recorded as described above. The swelling capacities were obtained by comparing the volumes of the tapioca DAS−chitosan hydrogels at 0 and 24 h. Cytotoxicity Assay by MTT Method. Hela cervical carcinoma cells were seeded into 96-well plates at an intensity of 8 × 104 cells mL−1 and maintained for 24 h in DMEM medium. Different amounts of 60% (w/w) tapioca DAS−1% (w/w) chitosan hydrogels (2.0, 4.0, 6.0, 8.0,10.0 μg) were then added into the medium, and the cells were incubated in the DMEM for another 24 h at 37 °C. After the medium was poured out, 100 μL of freshly prepared MTT (1 mg mL−1 in PBS) was added to each well and incubated for 4 h. After the MTT medium solution was removed, the cells were then lysed by adding 100 μL of DMSO. The plate was gently shaken for 5 min, and the absorbance of purple formazan at 520 nm was monitored using a spectra MAX 340PC plate reader.

aldehyde groups inside the hydrogels, Ag ions can be reduced to Ag nanoparticles. Metal nanoparticle formation by the DAS−chitosan hydrogel reduction matrixes is in situ achieved and does not require additional chemical reductants. In the present work, we reported the preparation of covalently cross-linked tapioca DAS−chitosan hydrogels with excellent swelling, biocompatibility properties, and their utility to form Ag nanoparticle-containing hybrid hydrogels with the aldehyde groups in the tapioca DAS backbone acting as reductants. The size and morphology control and the microstructure of the prepared Ag nanoparticles were also investigated.

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EXPERIMENTAL SECTION

Materials and Instrumentation. Tapioca starch was purchased from Sigma Aldrich. Chitosan (degree of acetylation of 80−95%, Mw = 200 000 g/mol, PDI = 1.5, monomodal distribution), was purchased from Sinopharm Chemical Reagent Co. Ltd. Silver nitrate was purchased from Shantou Xilong Chemical Plant Co. Ltd. Sodium periodate, hydrochloric acid, sodium hydroxide, and acetic acid were analytical grade products and purchased from Beijing Chemical Reagent Co. Ltd. All of these reagents were used without further purification unless stated otherwise. The TEM and HR-TEM images were recorded by a JEM 2100 transmission electron microscope with an accelerating voltage of 120 kV. The samples of prepared DAS− chitosan/Ag nanoparticle hybrid hydrogels were transferred onto carbon-coated grids and allowed to adsorb for 1 h. The hybrid hydrogels then were removed, and only a thin layer of hydrogel remained on the carbon film for TEM observations. The FTIR spectra of tapioca starch, tapioca DAS, chitosan, and the tapioca DAS− chitosan hydrogel were obtained from samples as KBr pellets using a Nicolet 170SX FT-IR spectrometer. An average of eight scans were collected with a resolution of 4 cm−1. The thermal property analysis curves of DAS−chitosan/Ag nanoparticles hybrid hydrogel were measured by the TGA/DTA thermal system (WRT-2C) at the heating rate of 10 °C/min. Preparation of DAS. Tapioca starch and sodium periodate (1:0.5 molar ratio) were dissolved in water with vigorous mechanical stirring.42 The pH of the solution was kept at 7.0, and the reaction temperature was set at 37 °C. After 12 h, the slurry was filtered and the product was washed with deionized water five times, and then dried in a vacuum at 50 °C for 24 h and at 120 °C for 24 h to obtain a gray powder. Figure 1 shows the basic structural units of tapioca starch and DAS.

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RESULTS AND DISCUSSION The tapioca starch was first functionalized by introducing aldehyde groups to the backbone of the tapioca starch chain by an oxidation process. This allowed the starch to be used for preparation of cross-linked hydrogels with amino-containing species. The tapioca starch was oxidized by a periodate method at low pH.42 Initially, we attempted to use FTIR spectroscopy to monitor the oxidation process and the resulting product, and expected appreciable changes in the range of 1600−1800 cm−1 that corresponds to the carbonyl vibration. The FTIR spectra of the oxidized tapioca starch were illustrated in Figure 3a. As reported by Varma et al.,43 no appreciable change was identified as the characteristic vibration band of aldehyde between 1600 and 1800 cm−1 except one significant band change at 1631 cm−1, which could be attributed to carboxylate ion (COO−) due to overoxidation of aldehydes, or could be a result of adsorbed bound water. In the literature,44 a mechanism was proposed to elucidate the absence of the expected absorption band of aldehyde groups on celluloses, in which aldehydes were easy to react with adjacent OH groups to form hemiacetals. However, the hemiacetals can react with amines in the acidic/ basic condition to form Schiff base bonds. The aldehyde groups in tapioca DAS make it possible to prepare a cross-linked hydrogel with amino-containing species by the Schiff base reaction. Chitosan was selected as the aminocontaining cross-linker because of its biocompatibility. The homogeneous tapioca DAS−chitosan hydrogel prepared by reaction of the chitosan solution with the tapioca DAS solution was analyzed by FTIR spectroscopy (Figure 3b). The appearance of a new band at 1640 cm−1 for CN bending and wagging and disappearance of an absorption band of −NH2 at 1383 cm−1 indicated the formation of Schiff base bonds, which resulted in the cross-linkage between the tapioca DAS hydrogel and chitosan. The cross-linking process was illustrated in Figure 4.

Figure 1. Chemical structures of tapioca starch (a) and dialdehyde starch (b). Preparation of Tapioca DAS−Chitosan Hydrogels. Chitosan was mixed with deionized water, followed by the addition of a small volume of acetic acid. The pH of the solution was then adjusted to the desired level using sodium hydroxide. A set volume of chitosan solution was added to a tapioca DAS solution, and the mixture was vigorously stirred at 45 °C for 10 h. The temperature then was increased to 90 °C for 20 min. A large piece of tapioca DAS−chitosan hydrogels was obtained after cooling the mixture to room temperature (Figure 2a). Preparation of Tapioca DAS−Chitosan/Ag Nanoparticle Hybrid Hydrogels. The tapioca DAS−chitosan hydrogels were first 11189

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Figure 2. Photograph of tapioca DAS−chitosan hydrogel (a) and tapioca DAS−chitosan/Ag nanoparticle hybrid hydrogel (b).

Figure 3. FTIR spectra of tapioca starch and tapioca DAS (a) and FTIR spectra of chitosan and the tapioca DAS−chitosan hydrogel (b).

DAS−chitosan hydrogels with different degrees of cross-linking were measured by comparing their volumes after they were initially placed in deionized water and after they had been immersed in the deionized water for 24 h. Figure 5a shows the swelling capacities of tapioca DAS−chitosan hydrogels as a function of the chitosan mass fraction (0.5−2.5%). The volumes of the hydrogels gradually increased from 0 to 24 h, and reached equilibrium after immersion for 24 h. Interestingly, the swelling capacity depended on the chitosan content, which increased with increasing chitosan content. Hydrogels with 0.5% chitosan were about 3 times larger at 24 h than at 0 h, while those with 2.5% chitosan were about 12 times at 24 h than at 0 h. This can be attributed to the additional hydrophilic chitosan polymers in the hydrogel system, which enhances the swelling behavior of the tapioca DAS−chitosan hydrogel. These results show the swelling capacity of the hydrogel can be controlled by the chitosan content. The swelling capacity was also dependent on the tapioca DAS content of the hydrogel. Figure 5b shows the volumetric changes of tapioca DAS−chitosan hydrogels with different tapioca DAS mass fractions after immersion for 24 h. The

Figure 4. Schiff base cross-linking of the tapioca DAS−chitosan hydrogel.

The prepared tapioca DAS−chitosan hydrogels showed very good swelling capacities. The swelling capacities of tapioca

Figure 5. (a) Swelling capacity of tapioca DAS−chitosan gels as a function of chitosan content. (b) Swelling capacity of tapioca DAS−chitosan gels as a function of tapioca DAS content (mass fraction): (A) 10% tapioca DAS, 1% chitosan; (B) 20% tapioca DAS, 1% chitosan; (C) 40% tapioca DAS, 1% chitosan; (D) 60% tapioca DAS, 1% chitosan; (E) 70% tapioca DAS, 1% chitosan; and (F) 80% tapioca DAS, 1% chitosan. 11190

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hydrogels. Reduction then occurred spontaneously to form Ag nanoparticle-containing hybrid hydrogels within several hours. As shown in the TEM image in Figure 7a, Ag nanoparticles (ø 500−600 nm) were obtained in the 40% tapioca DAS−1% chitosan gel. Because the aldehyde groups acted as reductants in the system, we investigated if the aldehyde content could influence the size and morphology of the prepared Ag nanoparticles. Therefore, hybrid hydrogels formed from 60% tapioca DAS−1% chitosan and 80% tapioca DAS−1% chitosan were also studied. Ag nanoparticles (ø 150−400 nm) were obtained in the 60% tapioca DAS−1% chitosan hydrogel (Figure 7b). In the 80% tapioca DAS−1% chitosan hydrogels, smaller Ag nanoparticles (average ø 150 nm) formed (Figure 7c). These results showed the Ag nanoparticles became smaller as the tapioca DAS mass fraction increased. This could be attributed to a larger number of aldehyde groups reducing and stabilizing Ag nanoparticles in a smaller area than a smaller number of aldehyde groups, which would limit the growth of the Ag nanoparticles. By tuning the tapioca DAS mass fraction of the tapioca DAS−chitosan hydrogel, the size of the prepared Ag nanoparticles can be effectively controlled. The influence of Ag ion’s concentration in the system was also investigated. The following experiments were performed. The tapioca DAS−chitosan hydrogels were immersed in 2% (w/w), 10% (w/w), and 50% (w/w) AgNO3 aqueous solution, respectively, and the reduction was performed to prepare different hybrid hydrogels. The TEM images of obtained hybrid hydrogels were shown in Figure 8. In Figure 8a, most of the prepared Ag nanoparticles’ size is about 250−400 nm with an Ag ion concentration of 2%. When the concentration of Ag ions was increased to 10%, the prepared Ag nanoparticles become smaller (ø 50−200 nm) (Figure 8b). When the concentration of Ag ions was extended to 50%, the size of the prepared Ag nanoparticles is only about 10−50 nm (Figure 8c). The above experiments indicate that the concentration of Ag ions has a big influence on the size of Ag nanoparticles. The size of Ag nanoparticles will become smaller with increasing concentration of Ag ions. Because of the increasing concentration of Ag ions, on the contrary, less Ag ions could be ammoniated in the tapioca DAS−chitosan gels when the concentration of aqueous ammonia is fixed to only 2% (w/w). In this case, the smaller size of Ag nanoparticles will be formed more easily. From the results, we think that maybe the silver nanoculsters with smaller size and fluorescent properties could be also produced for the biological application in the future after tuning the production condition.45 During the reduction process in the tapioca DAS−chitosan hydrogels, the immersion time in aqueous ammonia is an important factor. This is related to ammonization of the Ag ions and could be use to control the size and morphology of the prepared Ag nanoparticles. Tapioca DAS−chitosan hydrogels

swelling capacities of hydrogels with tapioca DAS mass fractions of 10%, 20%, and 40% were 4, 2.5, and 1.6, respectively. Within the tapioca DAS mass fraction range from 10% to 40%, the swelling capacity gradually decreased. This may be attributed to the dependence of the density of the hydrogel structure on the swelling behavior. When the tapioca DAS mass fraction was increased to >50%, the swelling capacity was stable. This indicates that high tapioca DAS mass fractions (>50%) have little influence on the swelling behavior of the DAS−chitosan hydrogels. Biocompatibility is another important property of hydrogels for applying in pharmaceutical and medical science. To demonstrate our new tapioca DAS−chitosan hydrogels have high biocompatibility, we study the cytotoxicity of the 60% (w/ w) tapioca DAS−1% (w/w) chitosan hydrogels using a 3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cell-viability assay. The absorbance of MTT at 520 nm is dependent on the degree of activation of the cells. Cell viability then was expressed by the ratio of absorbance of the cells coculture with tapioca DAS−chitosan hydrogels to that of the cells incubated with culture medium only. As shown in Figure 6, the cell viability was more than 90% after 24 h coculture of

Figure 6. Cell viability results after incubation of hela cells with various amounts of 60% (w/w) tapioca DAS−1% (w/w) chitosan hydrogels (2.0, 4.0, 6.0, 8.0, 10.0 μg). The cell viability calculated relative to that of the cells without adding 60% (w/w) tapioca DAS−1% (w/w) chitosan hydrogels is defined as a viability of 1.

Hela cervical carcinoma cells when the weight of adding tapioca DAS−chitosan hydrogels was up to 10 μg. These results indicated that the tapioca DAS−chitosan hydrogels have excellent biocompatibility. The tapioca DAS−chitosan hydrogels contained many aldehyde groups, which could reduce Ag ions to Ag nanoparticles. Therefore, the tapioca DAS−chitosan crosslinked hydrogels could be used as reduction matrixes for in situ preparation of Ag nanoparticles inside the hydrogels to form uniform hybrid hydrogels. The tapioca DAS−chitosan hydrogels were immersed in aqueous AgNO3 and aqueous ammonia to absorb ammoniated Ag ions into the tapioca DAS−chitosan

Figure 7. TEM images of silver nanoparticles dispersed in tapioca DAS−chitosan gels with different tapioca DAS mass fractions: (a) 40% tapioca DAS, 1% chitosan; (b) 60% tapioca DAS, 1% chitosan; (c) and 80% tapioca DAS, 1% chitosan. The scale bar is 200 nm. 11191

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Figure 8. TEM images of silver nanoparticles produced after immersion of the hydrogel in AgNO3 aqueous solution at the following concentrations: (a) 2% (w/w), (b) 10% (w/w), (c) 50% (w/w). The scale bar is 200 nm.

Figure 9. Typical TEM images of silver nanoparticles produced after immersion of the hydrogel in aqueous ammonia for (a) 4 h and (b) 44 h. The scale bar is 200 nm.

Figure 10. TEM images of silver nanoparticles prepared by reduction in aqueous ammonia at the following concentrations: (a) 1:5 (ammonia:H2O, v/v); (b) 1:2 (ammonia:H2O, v/v); and (c) pure aqueous ammonia (25% solution). The scale bar is 200 nm.

Figure 11. (a) High-resolution TEM image of silver nanoparticle, and (b) the SAED pattern of silver nanoparticle.

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Langmuir were immersed in aqueous ammonia (1:5 v/v) for different times to study the influence of the immersion time. After an immersion time of 4 h, Ag nanoparticles with a size of 150−180 nm formed (Figure 9a). When the immersion time was extended to 44 h, the size of obtained Ag nanoparticles increased to about 200 nm, and the nanoparticles aggregated in the hydrogel matrix (Figure 9b). These results indicate that a prolonged immersion time can enhance ammonization of the Ag ions and increase the size and density of the prepared Ag nanoparticles. After immersion for 44 h, full ammonization was achieved, and the concentration of ammoniated Ag ions reached a maximum, which resulted in the formation of larger nanoparticles with high density in the tapioca DAS−chitosan hydrogel matrix. The concentration of the ammonia solution was also studied in the preparation of Ag nanoparticles in the tapioca DAS− chitosan hydrogels. Figure 10 shows the TEM images of Ag nanoparticles prepared using 1:5 (NH3:H2O, v/v), 1:2 (NH3:H2O, v/v), and pure aqueous ammonia (25% aqueous solution) with a reaction time of 4 h. The Ag nanoparticles prepared using the (1:5, v/v) aqueous ammonia ranged in size from 150 to 180 nm (Figure 10a). When the ammonia concentration was increased (1:2, v/v), the prepared Ag nanoparticles were smaller (ø 80−150 nm) (Figure 10b). Interestingly, few Ag nanoparticles were found in the hydrogels when pure aqueous ammonia was used (Figure 10c), which indicates that high concentrations of ammonia are not appropriate in the preparation of Ag nanoparticles. This may be attributed to the enhanced mobility of Ag ions in the high concentration aqueous ammonia, which will move Ag ions to the solution phase. This would reduce the quantity of Ag ions in the hydrogel and, correspondingly, the preparation of Ag nanoparticles. To investigate the detailed microstructure of the in situ reduction of Ag nanoparticles in hybrid hydrogel, the HRTEM and SAED pattern experiments were performed. From the HRTEM images in Figure 11, the clear and uniform lattice fringes confirmed that the Ag nanoparticles have the same crystalline patterns and well-defined morphologies. The lattice spacing of 0.236 nm corresponds to (111) planes of silver. The results show that the dominant faces of Ag nanoparticles are (111). The selected-area electron diffraction study (SAED) pattern was obtained by directing the electron beam perpendicular to the Ag nanoparticles. The diffraction spots pattern shown in the Figure 11b confirmed that the Ag nanoparticles by in situ reduction in the tapioca DAS−chitosan hydrogel are well crystalline, and its face is indexed to (111) planes. Both the HRTEM image and the SAED pattern confirmed that the as-prepared Ag nanoparticles are well crystals. The thermal properties of tapioca DAS−chitosan/Ag nanoparticles hybrid hydrogel were measured by TGA and DTA experiments. From the TGA curve (Figure S1 in the Supporting Information), it can be seen that the sample started to decompose quickly at about 200 °C, and the decomposition rate became slower when the temperature was higher than 300 °C. This rate change may be caused by the decomposition of Ag nanoparticles. Overall, TGA results show a loss of 40% up to 566 °C. The DTA curves (Figure S2 in the Supporting Information) showed an endothermic peak at 389 °C and an intense exothermic peak at 443 °C, which mainly can be attributed to the evaporation of water and crystallization of Ag nanoparticles.

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CONCLUSION

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ASSOCIATED CONTENT

Article

A series of large and uniform tapioca DAS−chitosan hydrogels were prepared on the basis of the cross-linking of aldehyde and amino groups. The content of tapioca DAS and chitosan crosslinker could be used to control the swelling capacity of the hydrogel, respectively, and the hydrogels were also demonstrated to have high biocompatibility. Because the tapioca DAS−chitosan hydrogels contained a large amount of aldehyde groups, they were used as reduction matrixes for the preparation of Ag nanoparticles. Interestingly, the size and morphology of the prepared Ag nanoparticles were tuned by varying the reduction conditions, such as the tapioca DAS content of the hydrogel, concentration of the aqueous AgNO3, concentration of the aqueous ammonia, and immersion time of the hydrogel in the aqueous ammonia. The Ag nanoparticles also proved to have uniform crystalline microstructure. One of the advantages of this reduction system is that the reduction process is performed by the aldehyde groups in the hydrogels and does not require addition of chemical reductants. This research could be used to develop biocompatible tapioca DAS− chitosan hydrogels and a green approach for the preparation of large and uniform nanoparticle-containing hybrid hydrogel materials.

S Supporting Information *

For thermal property analysis, TGA and DTA graphs (Figures S1 and S2) of tapioca DAS−chitosan/Ag nanoparticles hybrid hydrogel. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 82377202. Fax: +86 10 82375712. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This research was supported by the National Natural Science Foundation of China (20904003, 90923015), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-11-0576), the Program for Changjiang Scholars and Innovative Research Team in University, and the Fundamental Research Funds for the Central Universities of China (FRF-TP-09-011B, FRF-TP-09006A).

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dx.doi.org/10.1021/la302011x | Langmuir 2012, 28, 11188−11194