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Quasi-One-Dimensional Arrangement of Silver Nanoparticles Templated by Cellulose Microfibrils Min Wu,*,† Shigenori Kuga,‡ and Yong Huang† State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing 100080, China, and Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The UniVersity of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan ReceiVed May 25, 2008. ReVised Manuscript ReceiVed June 25, 2008 We demonstrate a simple, facile approach to the deposition of silver nanoparticles on the surface of cellulose microfibrils with a quasi-one-dimensional arrangement. The process involves the generation of aldehyde groups by oxidizing the surface of cellulose microfibrils and then the assembly of silver nanoparticles on the surface by means of the silver mirror reaction. The linear nature of the microfibrils and the relatively uniform surface chemical modification result in a uniform linear distribution of silver particles along the microfibrils. The effects of various reaction parameters, such as the reaction time for the reduction process and employed starting materials, have been investigated by transmission electron microscopy (TEM) and ultraviolet-visible spectroscopy. Additionally, the products were examined for their electric current-voltage characteristics, the results showing that these materials had an electric conductivity of approximately 5 S/cm, being different from either the oxidated cellulose or bulk silver materials by many orders of magnitude.
Introduction Metallic nanoparticles have attracted strong interest for their potential applications as electronic,1 catalytic,2,3 and biomedical4 materials based on quantum confinement effects and their large relative surface areas. Here, the difficulty encountered in metallic nanoparticle preparation is their tendency to aggregate in liquiddispersed states owing to their high surface energy. One strategy to prevent aggregation is the controlled deposition of metal particles through hybridization with an organic template ensuring a well-defined spatial distribution leading to ordered assemblies. The reported examples of templates include synthetic polymers,5,6 surfactants,7,8 DNA,9 and certain rodlike viruses.10-12 In the present work, we demonstrate a simple, facile approach to the deposition of silver nanoparticles along the native cellulose microfibrils. This process was found to give a relatively uniform linear arrangement of silver nanoparticles as described below. Native cellulose, which is readily available from abundant biological sources, arises as crystalline microfibrils, typically * Corresponding author. E-mail:
[email protected]. Tel: +86-1082618573. Fax: +86-10-62559373. † The Chinese Academy of Sciences. ‡ The University of Tokyo. (1) Hermanson, K. D.; Lumsdon, S. O.; Williams, J. P.; Kaler, E. W.; Velev, O. D. Science 2001, 294, 1082–1086. (2) Zhou, Z.; Wang, S.; Zhou, W.; Wang, G.; Jiang, L.; Li, W.; Song, S.; Liu, J.; Sun, G.; Xin, Q. Chem. Commun. 2003, 394–395. (3) Campell, D. T.; Parker, S. C.; Starr, D. E. Science 2002, 298, 811–814. (4) Sondi, I.; Salopek-Sondi, B. J. Colloid Interface Sci. 2004, 275, 177–182. (5) Bhattacharjee, R. R.; Mandal, T. K. J. Colloid Interface Sci. 2007, 307, 288–295. (6) Zou, X. F.; Chen, S. Y.; Zhang, D. Y.; Guo, X. F.; Ding, W. P.; Chen, Y. Langmuir 2006, 22, 1383–1387. (7) Krichevshi, O.; Tirosh, E.; Markovich, G. Langmuir 2006, 22, 867–870. (8) Xiong, Y. J.; Xie, Y.; Yang, J.; Zhang, R.; Wu, C. Z.; Du, G. J. Mater. Chem. 2002, 12, 3712–3716. (9) Gu, Q.; Cheng, C. D.; Gonela, R.; Suryanarayanan, S.; Anabathula, S.; Dai, K.; Haynie, D. T. Nanotechnology 2006, 17, R14–R25. (10) Lee, S. W.; Mao, C.; Flynn, C. E.; Belcher, A. M. Science 2002, 296, 892–895. (11) Tseng, R. J.; Tsai, C.; Ma, L.; Ouyang, J.; Ozkan, C. S.; Yang, Y. Nat. Nanotechnol. 2006, 1, 72–77. (12) Nam, K.; Kim, T. D.; Yoo, P. J.; Chiang, C; Meethong, N.; Hammond, P. T; Chiang, Y.; Belcher, A. M. Science 2006, 312, 885–888.
3-30 nm wide depending on the origin.13 Although higher plants produce thin microfibrils (3 to 4 nm) constituting organized tissues of cell walls, microbial and animal celluloses consist of wider (10-50 nm), highly crystalline, and readily dispersible microfibrils.14 If such cellulose microfibrils serve as templates, then the quasi-one-dimensional arrangement of metallic nanoparticles with well-controlled sizes may be prepared. The surfaces of cellulose microfibrils have many hydroxyl groups that can be utilized for chemical modification. In the present work, to provide reducing functionality for metal deposition, the surfaces of cellulose microfibrils were oxidized by periodate. It cleaves the C2-C3 bond of the anhydroglucose unit and gives two aldehyde groups per glucopyranose unit. Whereas the product, so-called dialdehyde cellulose (DAC),15 is soluble in hot water,16 it stays water-insoluble at room temperature.17,18 Because periodate oxidation proceeds from the surface of crystalline microfibrils, the choice of proper conditions can lead to the preparation of microfibrils coated with aldehyde groups. Utilizing this functionality, we examined the deposition of silver particles by the Tollens (silver mirror) reaction.19 The procedure gave, as described below, the deposition and quasione-dimensional arrangement of silver nanoparticles on cellulose microfibrils. This phenomenon may be useful for preparing electric, catalytic, and sensing materials with novel functionalities. (13) Brown, R. M. In Cellulose: Molecular and Structural Biology; Brown, R. M., Jr., Saxena, I. M., Eds.; Springer: Dordrecht, The Netherlands, 2007; p 19. (14) Helbert, W.; Nishiyama, Y.; Okano, T.; Sugiyama, J. J. Struct. Biol. 1998, 124, 42–50. (15) Nevell, T. P. In Methods in Carbohydrate Chemistry; Whistler, R. L., Ed.; Academic Press: New York, 1963; Vol. 3, p 164. (16) Kim, U.-J.; Wada, M.; Kuga, S. Carbohydr. Polym. 2004, 56, 7–10. (17) Gal’braikh, L. S.; Rogovin, Z. A. In Cellulose and Cellulose DeriVatiVes; Bikales, N. M., Segal, L., Eds.; Wiley-Interscience: New York, 1971; Part V, p 894. (18) Mester, L. J. Am. Chem. Soc. 1955, 77, 5452–5453. (19) Yin, Y.; Li, Z. Y.; Zhong, Z.; Gates, B.; Xia, Y.; Venkateswaran, S. J. Mater. Chem. 2002, 12, 522–527.
10.1021/la801602k CCC: $40.75 2008 American Chemical Society Published on Web 08/05/2008
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Scheme 1. Periodate oxidation of cellulose
Experimental Section Cellulose microfibril suspensions were prepared via acid hydrolysis as reported before.20 The fibrous shell of Halocynthia roretzi (sea squirt) was treated with 5% (w/w) KOH and 0.3% (w/w) NaClO2(aq) successively and repeatedly (usually three times). The water-washed material was suspended in water and mechanically disintegrated by a stator-rotor-type homogenizer and treated with 4 M HCl at 70 °C for 4 h to give a cellulose microfibril suspension. After removing the water-soluble species (HCl and hydrolysis products) by repeated centrifugation (5000 rpm, 5 min) and dialysis, we obtained watersuspended cellulose microfibrils. To provide reducing functionality for metal deposition, the surfaces of cellulose microcrystals were oxidized by periodate to form aldehyde groups. Twenty milliliters of the cellulose microcrystal suspension of solid content of 0.2% was mixed with 0.1 g of NaIO4. The mixture was stirred gently at 25 °C for 120 h in the dark. The oxidized microcrystals were recovered by dialysis. The actual degree of oxidation (DS) was evaluated by the consumption of iodate through the decrease in UV absorption at 280 nm. Here, DS refers to the bulk value, and the oxidized groups are considered to be localized to surfaces of cellulose microcrystals. Silver ammonia aqueous solution (Ag(NH3)2(ag)) was prepared by adding 1 mL of 25% ammonia to 25 mL of a 4% (w/w) silver nitrate solution. The final stage of ammonia addition was performed dropwise to give a clear solution. The silver ammonia solution (7 mL) was added to 1 mL of the cellulose microcrystal suspension of 0.1% solid content, and the mixture was heated to 100 °C for about 5 min, resulting in brown coloration. The mixture was then cooled and subjected to dialysis to remove remaining silver species and byproduct. Transmission electron microscopy (TEM) images of nanoparticles were taken with a JEOL 2010F FEG TEM at 200 kV acceleration. The samples were mounted on a carbon-coated copper grid and examined at room temperature. UV-vis spectra were measured with a Shimadzu UV-1601PC UV-vis spectrophotometer. The samples were placed in a 1-cm-thick quartz cuvette, and spectra were recorded at room temperature. Scanning electron microscopy (SEM) images of a thin film of the Ag-stained microfibrils were taken with a Hitachi S4000 scanning electron microscope.
Results and Discussion Oxidation of Cellulose Microfibrils. The oxidation of cellulose by periodic acid is a well-known, characteristic reaction. This reaction cleaves the C2-C3 bond of the anhydroglucopyranoside moiety and gives two aldehyde groups (Scheme 1). Kim et. al have reported the loss of Whatman CF11 crystallinity by periodate oxidation.21 In spite of the loss of crystallinity, the dialdehyde cellulose remains water-insoluble at room temperature even when the oxidation is complete.21 Because partial oxidation can be used to introduce aldehyde functionality onto the surface of cellulose microcrystals, the DS in this work was controlled (20) Araki, J.; Wada, M.; Kuga, S.; Okano, T. Colloids Surf., A 1998, 142, 75–82. (21) Kim, U.-J.; Wada, M.; Kuga, S. Biomacromolecules 2000, 1, 488–492. (22) Slistan-Grijalvaa, A.; Herrera-Urbinab, R.; Rivas-Silvac, J. F; A’valosBorjad, M.; Castillo´ n-Barrazad, F; Posada-Amarillase, A. Physica E 2005, 27, 104–112.
Figure 1. TEM images of tunicate cellulose microfibrils (a) before and (b) after periodate oxidization.
Figure 2. TEM images of silver particles deposited onto periodateoxidized cellulose (DAC) microfibrils. (a, b) Typical appearance of silver-stained microfibrils. (c) Some parts show the highly uniform size and arrangement of silver particles. (d) High-magnification images of individual silver particles (scale bar 5 nm).
at around 0.3 by adjusting the quantity of periodate added and the reaction time. Figure 1 shows TEM images of the obtained tunicate cellulose microfibrils before and after periodate oxidation. The micrographs show that the typical crystallites, 2-5 µm long and 20 nm wide with straight linear morphology, were well maintained. Silver Nanoparticles Deposited onto Periodate-Oxidized Cellulose Microfibrils. The silver-stained cellulose microfibrils were mounted on a carbon-coated copper grid and examined by TEM. The images show interesting features of silver deposition on the cellulose microfibrils (Figure 2). Instead of continuous coverage of the surface by silver, the microfibrils were found to
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Figure 3. . UV-vis spectra of a DAC suspension before and after reaction with silver ammonia solution.
be carrying fairly uniform-sized silver particles. This phenomenon demonstrates that the aldehyde groups on the microfibrils served as a reducing agent for silver ions. The particle size ranged from 5 to 25 nm but was mostly in the range of 10-15 nm. The individual particles are polycrystalline as shown by the highmagnification images (Figure 2d). It is also remarkable that the particle intervals are fairly uniform (Figure 2c). In contrast, when the silver reduction was performed by a dissolved reducing agent (glucose) in the presence of nonoxidized cellulose microcrystals, only large aggregates of silver particles were formed, together with weak mirror formation on the glass wall. This difference indicates that the mode of silver reduction is strongly dependent on the form of reducing agent: the dissolved species versus the substrate-immobilized species. In the latter case, the generation of silver atoms would take place in the vicinity of the substrate, and the silver atoms must find the locus of deposition nearby, instead of traveling long distances and forming large particles or a silver mirror. This result demonstrates again that the aldehyde groups on the microfibrils served as a reducing agent for silver ions. In addition, the DAC nanocrystal serves as a stabilizing template for silver nanoparticles and prevents the further aggregation of silver particles. Figure 3 shows UV-vis absorption spectra of a DAC suspension before and after reaction with silver ammonia solution. Although the starting DAC suspension mixed with silver ammonia
Wu et al.
solution showed no absorption band, the suspension after reaction gave an absorption band with a peak at 408.5 nm. This peak arises from the surface plasmon absorption of silver nanoparticles.22 It can also be seen that the plasmon absorption band is fairly sharp, indicating the narrow particle size distribution of silver particles. Factors Influencing Nanoparticle Assembly. To investigate the effect of reaction conditions on the assembly process, the time and temperature of the reaction were varied. Figure 4a illustrates the absorption spectra of silver nanoparticles for a reaction time of 0 to 58 min on a DAC microfibril surface with a AgNO3 concentration of 3.88 wt % and a reaction temperature of 50 °C. Silver nanoparticles give a strong surface plasmon band in the visible region with a wavelength between 423 and 448 nm corresponding to a reaction time between 8 and 58 min. The maximum absorbance of these bands increases as the reaction time increases up to 20 min and then decreases as the reaction time increases up to 58 min. At the same time, the position of the plasmon band shifts to higher wavelengths as the reaction time increases. The plasmon bands of silver particles increase in intensity, indicating an increase in the amount of reduced silver, whereas the absorbance of the plasmon band decreases and undergoes a red shift attributed to the growth of the resultant silver nanoparticles. Therefore, longer reaction time up to 20 min caused an increase in the number of silver particles, but they tended to aggregate thereafter. Figure 4b shows the surface plasmon absorption spectra of silver nanoparticles reduced at different temperatures. The maximum absorbance of their surface plasmon band obviously increases as the temperature increases. At the same time, the absorbance of the plasmon band undergoes a slight blue shift. This may be because with the increase in temperature the expended time for the same silver nanoparticle size would be short. Resultant silver nanoparticles do not have enough time to grow at high temperature as opposed to their growth at low temperature. These results suggest that the size and assembly of the silver nanoparticles can be controlled by adjusting the reaction time and temperature for assembly processes. Besides the Ag(NH3)2-DAC system, silver nitrate (AgNO3) could also be reduced by DAC or cellulose microfibrils (CM) to form silver nanoparticles. Figure 5 shows TEM images of Ag(NH3)2 and AgNO3 reduced by DAC and unoxidized cellulose microfibrils, respectively. The reducing ability as judged from coloration and siver particle population in the micrographs was in the order : DAC/Ag(NH3)2 > DAC/AgNO3 > CM/Ag(NH3)2
Figure 4. UV-vis spectra of silver nanoparticles reduced by DAC at different (a) reaction times and (b) temperatures.
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Figure 5. TEM images of (a) Ag(NH3)2 reduced by DAC, (b) AgNO3 reduced by DAC, (c) Ag(NH3)2 reduced by cellulose microfibrils, and (d) AgNO3 reduced by cellulose microfibrils. Figure 7. Current-voltage curve of a Ag-stained DAC microfibril film.
silver nanoparticles with uniform particle size, supported by the DAC microfibrils. Figure 7 shows the I-V diagram of the film specimen (2.0 mm wide, 12 m thick, and 6.0 mm span) measured under ambient conditions. The behavior was nearly ohmic, giving an electrical conductivity of 5.4 S/cm. Slight nonlinearity was probably due to heat generation. Because cellulose is not conductive even after oxidation, this conductivity obviously arose from silver nanoparticles. The conductivity was many orders of magnitude lower than that of bulk silver, 6.3 × 106 S/cm, falling in the semiconductor range. This anomalous behavior probably results from the small size and poor mutual contacts of the silver nanoparticles. A clarification of the mechanism of semiconductivity needs further study.
Conclusions
Figure 6. SEM image of the thin film of Ag-stained DAC microfibrils.
> CM/AgNO3. Also, the particle size uniformity was the highest for DAC/Ag(NH3)2 (Figure 5a). These results indicate that the introduction of aldehyde groups affects the deposition and arrangement of silver nanoparticles. Although cellulose has aldehyde groups at each chain end, their number is several orders smaller than that of aldehyde groups formed by chemical oxidation; therefore, the contribution of reducing ends to silver reduction is considered to be negligible for DAC. Current-voltage Characteristics of the Thin Film of AgStained Microfibrils. Because such an arrangement of conductive nanoparticles can bring about anomalous electronic behavior, we examined the current-voltage (I-V) behavior of the thin film of Ag-stained microfibrils. Figure 6a is an SEM image of the surface of a thin dried film of Ag-stained microfibrils used for the electrical measurement. The Figure shows densely packed
We found that silver nanoparticles could be self-assembled on periodate-oxidized cellulose microcrystals, giving rise to a quasione-dimensional arrangement of silver particles. The product was semiconductive, with σ ≈ 5 S/cm. Our preliminary tests have shown that the density and size of silver could be controlled by varying the conditions of silver reduction. Also, the direct use of silver nitrate (AgNO3) instead of the silver ammonia reagent could give similar results. Such materials are potentially useful in electronic, chemical sensing, and catalytic materials. Acknowledgment. This work was supported by the National Natural Science Foundation of China (no. 50773086 and no. 50521302). We are deeply grateful to Professor Xuedong Bai, Institute of Physics, Chinese Academy of Sciences, for TEM micrographs. LA801602K
