Synthesis of Silver Nanoparticles Templated by TEMPO-Mediated

Aug 4, 2009 - The synthesis of silver nanoparticles incorporates 2,2,6,6-tetramethylpiperidine-1-oxyradical (TEMPO)-mediated oxidation to introduce ...
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Biomacromolecules 2009, 10, 2714–2717

Synthesis of Silver Nanoparticles Templated by TEMPO-Mediated Oxidized Bacterial Cellulose Nanofibers Shinsuke Ifuku,*,† Manami Tsuji,† Minoru Morimoto,‡ Hiroyuki Saimoto,† and Hiroyuki Yano§ Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University 4-101 Koyamac-cho Minami, Tottori, Japan, Research Center for Bioscience and Technology, Tottori University 4-101 Koyamac-cho Minami, Tottori, Japan, and Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan Received June 19, 2009; Revised Manuscript Received July 18, 2009

We have prepared silver nanoparticles on the surface of bacterial cellulose (BC) nanofibers. The synthesis of silver nanoparticles incorporates 2,2,6,6-tetramethylpiperidine-1-oxyradical (TEMPO)-mediated oxidation to introduce carboxylate groups on the surface of BC nanofibers. An ion exchange of the sodium to the silver salt was performed in AgNO3 solution, followed by thermal reduction. By using oxidized BC nanofibers as a reaction template, we have prepared stable silver nanoparticles with a narrow size distribution and high density through strong ion interactions between host carboxylate groups and guest silver cations, which have been investigated by scanning electron microscopy, UV-visible spectroscopy, and a small-angle X-ray scattering method.

Introduction Metal nanoparticles have attracted much research attention with regard to their potential applications in electronic, catalytic, biomedical, and sensor materials due to their size-dependent optical, electronic, and chemical properties.1-5 It is generally difficult to disperse metallic nanoparticles in a solvent, as metal nanoparticles tend to aggregate due to their high surface energy. Therefore, addition of a surfactant or surface modification of metal nanoparticles is usually required to stabilize metal nanoparticles in a solvent.6-8 The other effective approach to prevent aggregation is immobilization of metal nanoparticles through hybridization with an organic template. Moreover, such hybrid materials have strong advantages such as mechanical strength, optical activity, and catalytic properties. However, immobilization on a solid substrate is still a major challenge in the field of metal nanoparticles, because the immobilization process is sensitive to balances between nucleation and crystal growth. Careful examination is therefore needed to control the particle size and distribution.9 Recently, hybrid nanocomposites consisting of a high-performance polymer such as polyimide containing fine metal nanoparticles with controlled sizes were reported.10-12 However, it is difficult to fully take advantage of quantum size effects of metal nanoparticles, because these nanoparticles are embedded in polymers. Therefore, development of the immobilization method of metal nanoparticles on a solid surface is important. Bacterial cellulose (BC) is an indigenous dessert food known as a “nata-de-coco” produced by Acetbacter xylinum.13,14 BC consists of ribbon-shaped nanofibers approximately 10 nm thick and 50 nm wide structured in a weblike network and is made up of a bundle of cellulose microfibrils 4 nm thick and 4 nm wide. Because the cellulose microfibrils are aggregates of semicrystalline extended cellulose chains, their Young’s modu* To whom correspondence should be addressed. Phone and Fax: +81857-31-5592. E-mail: [email protected]. † Graduate School of Engineering, Tottori University. ‡ Research Center for Bioscience and Technology, Tottori University. § Kyoto University.

lus is 138 GPa,15 their tensile strength is estimated to be at least 2 GPa,16 and their thermal-expansion coefficient in the axial direction is as small as 0.1 × 10-6 1/K.17 BC nanofibers therefore have excellent physical properties. If BC nanofibers could play the role of a template, they would be an excellent candidate for organic supports to stably fabricate metallic nanoparticles with a high surface-to-volume ratio, high mechanical toughness, and low thermal-expansion properties. These properties are important for electronic, chemical, catalytic, and biomedical applications to enhance their activity, durability, and heat-resistance properties. Moreover, sheet-shaped BC supports can make it easy to handle nanosized metallic particles. There have been some reports regarding the application of solid cellulose as a template, including BC for metal nanoparticles. For preparation, these studies have used the electrostatic interactions between metallic ions and dipole moments of cellulose molecules and the nanospace in the cellulose fibers that behave as nanoreactors.5,18,19 However, these methods are too weak to anchor metallic ions on cellulose fibers, resulting in low yield and sparse distribution of metallic nanoparticles. Therefore, a stronger interaction between guest metal ions and host cellulose fibers is required to prepare densely immobilized metal nanoparticles with higher yield. Cellulose has broad potential in the design of advanced polymeric materials because of its linear (1,4)-β-glucan structure with three reactive hydroxyl groups per anhydroglucopyranose unit.20 Recently, catalytic oxidation by 2,2,6,6-tetramethylpyperidine-1-oxy radical (TEMPO) has been widely studied to introduce carboxylate groups into cellulose.21,22 C6 primary hydroxyl groups of cellulose molecules are selectively oxidized by TEMPO-mediated oxidation, and the corresponding polyuronic acids are obtained. Advantages of the application of TEMPO-mediated oxidation to BC nanofibers are mainly as follows: (1) Oxidation can proceed under mild aqueous conditions. (2) The crystallinity and crystal size of BC nanofibers are unchanged during the oxidation.23 (3) A carboxylate moiety can be introduced into the BC nanofiber surface.

10.1021/bm9006979 CCC: $40.75  2009 American Chemical Society Published on Web 08/04/2009

Synthesis of Silver Nanoparticles

Carboxylate groups along the entire nanofiber surface can be used as host compounds to quantitatively introduce guest metal ions by an ion-exchange reaction, which should bring about salt formation with the regular arrangement.24,25 After the ion-exchange process, a reduction reaction of metal salt on the nanofiber surface can lead to fine metal particles. We have demonstrated herein a facile fabrication of silver nanoparticles on the surface of TEMPO-oxidized BC nanofibers, which serve as efficient templates to support metal nanoparticles.

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Scheme 1. Synthesis of Silver Nanoparticles from Bacterial Cellulose Nanofibers

Experimental Section Materials. All chemicals were purchased from Kanto Reagents or Wako Chemicals, and used as received. BC pellicles were a kind gift from Dr. Y. Kuwana (Fujicco Co., Ltd.). The bacterial strain Acetobacter xylinum FF-88 was incubated for 10 days in a static culture containing 5% (v/v) coconut milk (nitrogen content, 0.8%; lipid, 30%) and 8% (w/v) sucrose, adjusted to pH 3.0 by acetic acid. The BC fiber content in the pellicles was approximately 1% (v/v). BC pellicles, cut into approximately 10 × 10 × 1 cm thick pieces, were boiled in 2% NaOH for 2 h to remove the bacterial cell debris. The residue was thoroughly washed under running tap water for 2 days and then was compressed to a 1 mm thickness by pressing at 2 MPa and 20 °C for 2 min to roughly remove the water bulk. The compressed BC pellicles were soaked in acetone, and the water in the sample was replaced repeatedly by acetone. The sample piece was hot-pressed at 2 MPa and 80 °C for 4 min to obtain a dried BC sheet. TEMPO-Mediated Oxidation of Bacterial Cellulose. TEMPOmediated oxidation was performed according to the procedure described in ref 22 TEMPO (1.25 mg) and sodium bromide (12.5 mg) were dissolved in distilled water (37.5 mL), and a dried bacterial cellulose sheet (530 mg, 10 × 10 cm) was immersed in the solution. Then, 11% sodium hypochlorite solution (5.0 mmol/g of cellulose) was added to start the oxidation. The pH of the solution was maintained to be 10.5 by adding 0.5 M NaOH overnight at room temperature. After the oxidation, the reaction was quenched by the addition of 0.5 mL of ethanol. The oxidized BC (Cell-Na+) was washed thoroughly with distilled water and then ethanol. The sample was then dried at 65 °C overnight. Deposition of Silver Nanoparticles onto Cellulose Nanofibers. To exchange metal cations from Na+ to Ag+ of oxidized BC, AgNO3 was added (1.0 mol/COONa) in 30 mL of water, and an ion exchange was performed by immersing the Cell-Na+ sample in the dark at r.t. overnight, followed by a thorough rinsing with pure water. Thermal reduction of silver salt (Cell-Ag+) was carried out at 100 °C for 1 h under atmospheric conditions. Measurements. The carboxylate content of the TEMPO-mediated oxidized BC sheet was measured by the electric conductivity titration method.26 Water (92 mL) and 0.01 M NaCl (8.3 mL) were added to the 0.5 g of dried BC sheet. Then, 0.1 M HCl was added to the mixture to set the pH value of 2.7 under shaking. A 0.05 M NaOH solution was added at a rate of 0.1 mL/min up to pH 11. The carboxylate content of the BC sample was determined from the conductivity curves. UV-visible absorption spectra were recorded on a JASCO V-550 photometer by using the diffusion reflectance method. Infrared spectra of a series of BC sample were recorded with an FT-IR spectrometer (Spectrum One, Perkin-Elmer Japan Co., Ltd.) equipped with an ATR attachment (Universal ATR, Perkin-Elmer Japan Co., Ltd.). All the spectra were obtained by an accumulation of 16 scans, with a resolution of 4 cm-1 at 400-4000 cm-1. For SEM observation, the BC sheet samples were coated with platinum by an ion-sputter coater and were observed with a field-emission scanning electron microscope (JSM6700F, JEOL Ltd.). Small-angle X-ray scattering (SAXS) was carried out using an Ultima IV. The scattering angle was regulated with a parabolic graded multilayer mirror. The range of measurements was 0.1-2.0°, the step used was 0.005°, and the counting time was 2 s/step. The diameter and the distribution of the obtained silver nanoparticles

were evaluated by SAXS using an analyzing code NANO-Solver (Rigaku). The principle of the procedure is described in reference.27

Results and Discussion A synthetic route for silver nanoparticles templated by BC nanofibers is shown in Scheme 1. The BC sheet was treated by the TEMPO/NaBr/NaClO system under aqueous conditions. The primary hydroxyl groups at the C6 position of cellulose molecules are regioselectively oxidized by TEMPO-mediated oxidation. When 5.0 mmol NaClO per gram cellulose was used for oxidation, the carboxylate content was found to be 0.84 mmol per gram cellulose, which was determined by the electric conductivity titration method. Although the content corresponds, on average, to one carboxylate group per seven anhydro glucose units, carboxylate moiety is mainly introduced into BC nanofiber surface. The carboxylate content under the reaction condition seems to be a reasonable value compared to previous report.26 The sodium salt of the oxidized BC (Cell-Na+) was exchanged to the silver salt to provide Cell-Ag+ in the aqueous solution of AgNO3 with a concentration of 1.0 mol/COONa. The ion-exchange reaction was performed in the dark, and there was no color change of the BC sheet. The reduction of CellAg+ was carried out by heating at 100 °C. The color of the BC sheet changed drastically from white to brown in response to thermal treatment. Figure 1 shows the UV-visible diffuse reflectance spectra of the BC derivatives before and after thermal reduction of Cell-Ag+ at 100 °C. Although the Cell-Ag+ sample has no absorption band, a broad reflection band was observed with a maximum absorbance of 443 nm after thermal reduction. This peak is obviously due to the surface plasmon resonance of silver nanoparticles, which corresponds to the brown color.5,24 This phenomenon indicates that thermal treatment was effective

Figure 1. UV-visible spectra of (a) before and (b) after thermal reduction of silver carboxylate groups of TEMPO oxidized BC.

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Figure 2. FT-IR spectra of a series of bacterial cellulose derivatives: (a) original BC, (b) TEMPO-oxidized BC with sodium salt, (c) TEMPOoxidized BC with silver salt, and (d) silver nanoparticles on TEMPOoxidized BC.

for reducing carboxylate silver salts on BC fibers. When an excess of AgNO3 was used (10.0 mol/COONa) for ion exchange, followed by washing with water to remove excess AgNO3, the UV-visible spectrum of the Cell + Ag sample remained the same as the spectrum of the sample exchanged by an equivalent amount of AgNO3 (data not shown). This result indicates that by using an equimolar amount of AgNO3, sodium salts of CellNa+ were equivalently exchanged to silver salts against the carboxylate anions. Ion-exchange behavior of the carboxylate groups has already been reported by Saito and Isogai, and silver ions were found to be introduced quantitatively into the TEMPO-oxidized cellulose as counterions of the carboxylate groups.25 Figure 2 shows FT-IR-ATR spectra of the series of the BC derivative. Original BC nanofibers have no absorption band from 1500 to 1900 cm-1 (Figure 2a). In Figure 2b, an absorption band at 1601 cm-1 derived from the carbonyl groups appeared in response to the TEMPO-mediated oxidation of cellulose nanofibers, indicating that hydroxyl groups at the C6 position of cellulose molecules were converted to sodium carboxylate. In the present study, after treatment with an aqueous solution of AgNO3, the CdO vibrations of the carboxylate anion were slightly shifted to lower wave numbers from 1601 to 1590 cm-1, suggesting that sodium salt was changed to silver salt (Figure 2c). Thermal treatment of Cell-Ag+ was carried out at 100 °C. The peak due to the CdO vibrations of carboxylate anion at 1590 cm-1 disappeared, and the band at 1727 cm-1, which is likely due to the CdO stretching vibration of the carboxylic acid group, appeared as shown in Figure 2d. This band shift from 1590 to 1727 cm-1 indicates that silver ions of Cell-Ag+ were reduced to zero valence metals during thermal treatment on BC nanofibers. Thus, these spectra indicate that sodium salts were fully replaced by silver anions and that the silver ions were completely reduced by thermal treatment at 100 °C for 1 h. The silver particle/BC nanocomposite was coated with an approximately 2 nm layer of platinum by an ion sputter coater and observed by SEM to reveal the structure of the hybrid material. Figure 3 shows SEM images of TEMPO-mediated oxidized BC (Cell-Na+) and silver particles deposited on BC nanofibers (Cell + Ag). In Figure 3a, oxidized BC fibers (CellNa+) consist of 50-100 nm wide ribbons that look the same as the original BC nanofibers.20 In contrast, in Figure 3b,c, the

Figure 3. SEM images of (a) BC after TEMPO-mediated oxidation (Cell-Na+), and (b,c) silver nanoparticles on BC nanofibers (Cell + Ag) at different magnifications. The length of the scale bars are 100 nm, respectively.

surface of BC nanofibers was entirely covered with nanosizedparticles, and the structure of the BC nanofibers was maintained during the exchange from sodium to silver cations and the subsequent thermal reduction. These images demonstrate that carboxylate groups on the BC served as effective host functional groups to introduce guest metallic ions with strong ionic interactions. The silver ions aggregate with each other by thermal reduction, resulting in fine silver nanoparticles immobilized onto nanofibers with a higher density growth than that obtained by conventional methods.5,18,19 The resulting nanoparticles seem uniform in size, primarily in the range of 10-20 nm, including a 2 nm thick platinum coating layer. There should be silver nanoparticles less than 10 nm on BC fibers, but it is difficult to see the smaller nanoparticles due to the limiting resolution of FE-SEM. Recently, SAXS analysis has been found to be an efficient method for estimating the accurate average particle size and its distribution.13,27 We applied a SAXS method to estimate the size distribution of the entire sample of silver nanoparticles. The SAXS scattering intensity profiles and the results of curve fitting for the silver nanoparticles on the BC nanofibers are shown in Figure 4, and the correlation between the diameter and the distribution of the silver nanoparticles is shown in Figure

Synthesis of Silver Nanoparticles

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and biomedical applications. This preparation method involves an ion-exchange reaction of the sodium to the silver salt and its thermal reduction, leading to the production of fine silver particles with a controlled size distribution. Furthermore, a strong ion interaction is effective for introducing guest silver cations to host carboxylate groups, resulting in immobilization of silver nanoparticles with high density. We consider that this simple and facile approach would be applicable for preparing various metallic nanoparticles. Acknowledgment. This work was financially supported by the Hosokawa Powder Technology Foundation. We are deeply grateful to Koichi Udo and Yoji Moritani, Rigaku Co., Ltd. for the SAXS measurement.

References and Notes Figure 4. SAXS profile for the silver nanoparticles on BC nanofibers: (a) scattering profile and fitting curve; (b) residue.

Figure 5. Particle size distribution of silver nanoparticles.

5. The average particle size evaluated by NANO-Solver was 13.1 nm, and the normalized variance of the nanoparticles, which represents degree of dispersion, was 52.9%. The average particle size of 13.1 nm seems similar to that observed by SEM (Figure 3). The particle size would be controlled by adjusting the DS of the carboxylate groups introduced in BC nanofibers, and this will be investigated in the future. The SEM and SAXS results indicated that silver nanoparticles could be prepared on BC nanofibers with controlled size distribution due to the prevention of aggregation of nanoparticles.

Conclusions Silver nanoparticles were prepared on TEMPO-mediated oxidized BC nanofibers, which were used as reaction templates with a high surface-to-volume ratio and high mechanical toughness, which are important factors for electronic, catalytic,

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