5218
Langmuir 2000, 16, 5218-5220
Preparation of Highly Positively Charged Silver Nanoballs and Their Stability Tetsu Yonezawa,* Shin-ya Onoue, and Nobuo Kimizuka* Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan Received February 8, 2000. In Final Form: April 10, 2000 Reduction of AgNO3 by sodium borohydride in the presence of quaternary ammnoium disulfide gave a stable aqueous dispersion of silver nanoballs. The obtained Ag nanoballs showed a strong plasmon absorption peak, indicating that they consisted of metal Ag, although the counteranion of the stabilizer was Br-. They are obtainable in a powder form by reprecipitation and can be kept without any change under air. Redispersibility of the obtained powder into water was high, and even in slightly acidic condition, they can be kept stably for more than a week. The stabilization mechanism is also discussed.
Research on metal particles in nanometer order (nanoballs) has been conducted extensively from expectation to obtain novel functions arising from the “quantum size effect”.1 Many preparative methods of metal nanoballs, especially gold ones,1a,b,2 and their unique properties and applications have been reported.3 To realize nanodevices with these unique properties, designed architecture of metal nanoballs with varied metal species should be very important. For this purpose, two-dimensional2c-g,j and onedimensional arrays4 of metal nanoballs were constructed, especially by the use of positively chaged particles and bilayer membranes and DNA as anionic templates.5 In contrast, reports on preparation of silver nanoballs are limited,6 and long-chain alkanethiols have been mostly (1) (a) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 1179. (b) Bradley, J. S. In Clusters and Colloids; Schmid, G., Ed.; VCH: Weinheim, 1994; pp 459-544. (c) Schmid, G.; Ba¨umle, M.; Geerkens, M.; Heim, I.; Osemann, C.; Sawitowski, T. Chem. Soc. Rev. 1999, 28, 179. (d) Fendler, J. H. Chem. Mater. 1996, 8, 1616. (e) Storhoff, J. J.; Mirkin C. A. Chem. Rev. 1999, 99, 1849. (f) Keating, C. D.; Musick, M. D.; Keefe, M. H.; Natan, M. J. J. Chem. Educ. 1999, 76, 949. (g) Schmid, G.; Ba¨umle, M.; Geerkens, M.; Heim, I.; Osemann, C.; Sawitowski, T. Chem. Soc. Rev. 1999, 28, 179. (h) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (2) (a) Brust M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655. (c) Yonezawa, T.; Onoue, S.; Kunitake, T. Adv. Mater. 1998, 10, 414. (d) Schmid, G.; Ba¨umle, M.; Beyer, N. Angew. Chem., Int. Ed. 2000, 39, 181. (e) Spatz, J. P.; Mo¨ssmer, S.; Mo¨ller, M. Chem. Eur. J. 1996, 2, 1552. (f) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (g) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609. (h) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462. (i) Kimura, K.; Chen, S. Langmuir 1999, 15, 1075. (3) (a) Hayat, M. A., Ed. Colloidal Gold; Academic Press: San Diego, CA, 1991. (b) Stupp, S. I.; Braun, P. V. Science 1997, 277, 1242. (c) Hoestler, M. J.; Murray, R. W. Curr. Opin. Colloid Interface Sci. 1997, 2, 42. (d) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959. (e) Dorgi, M.; Gomez, J.; Osifchin, R.; Andres, R. P.; Osifchin, R. G. Phys. Rev. B 1995, 52, 9071. (f) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (4) (a) Braun, E.; Eischen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (b) Francis, G. M.; Goldby, I. M.; Kuipers, L.; von Issendorff, B.; Palmer, R. E. J. Chem. Soc., Dalton Trans. 1996, 665. (5) (a) Yonezawa, T.; Onoue, S.; Kunitake, T. Chem. Lett. 1999, 1061. (b) Yonezawa, T.; Onoue, S.; Kunitake, T. Kobunshi Ronbunshu 1999, 56, 855. (6) (a) Patil, V.; Sastry, M. Langmuir 1998, 14, 2707. (b) Chen, S.; Kimura, K. Chem. Lett. 1999, 1169. (c) Wang, Z. L.; Harfenist, S. A.; Vezmar, I.; Whetten, R. L.; Bentley, J.; Evans, N. D.; Alexander, K. B. Adv. Mater. 1998, 10, 808.
used as stabilizers.2j,7 To our best knowledge, there has been no report on silver nanoballs with a cationic surface. This is probably due to undesired formation of silver halides, when halogens are employed as counterions. In this study, we have devised a novel strategy for the preparation of monodispersed, highly cationic silver nanoballs. We employed the reduction of Ag+ by sodium borohydride. As the stabilizer 12 (bis(11-trimethylammonioundecanoylaminoethyl)disulfide dibromide)5a contains bromide anions as the counteranion, Ag+ should be transformed to AgBr in the solution. Thus, a strong reductant, such as NaBH4, was employed to prepare Ag(0) metal atoms. Furthermore, borohydride also reduces disulfide to thiols, which have higher affinity to metals than disulfides. 12 was injected into the aqueous solution of AgNO3 (5.0 mmol L-1, 30 mL) with various S/Ag ratios. The solution becomes slightly turbid in white because of the formation of AgBr. Immediately after the dissolution of 12, aqueous NaBH4 (0.4 mol L-1, 5 mL) was injected dropwise into the solution under vigorous stirring. The white turbid dispersion turned into a clear dark yellow solution, and apparently the reduction was successfully completed. No precipitates were found, and the dispersion was stable for weeks. Figure 1 shows a transmission electron micrograph and the size distribution of 1-stabilized Ag nanoballs prepared by NaBH4 reduction at various S/Ag ratios. The nanoballs have a spherical form. At higher S/Ag ratios (>1.0), the nanoballs become uniform and the average diameter decreased with the increase of S/Ag ratio. This phenomenon was also observed in the case of alkanethiol-stabilized Au nanoballs.8 The smallest size we could obtain in this study was 33 Å. On the contrary, at lower S/Ag ratios (
