Biomacromolecules 2004, 5, 2340-2346
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Synthesis of Chitosan-Stabilized Gold Nanoparticles in the Absence/Presence of Tripolyphosphate Haizhen Huang and Xiurong Yang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun Jilin 130022, P. R. China Received May 14, 2004; Revised Manuscript Received July 23, 2004
Gold nanoparticles were prepared by reducing gold salt with a polysaccharide, chitosan, in the absence/ presence of tripolyphosphate (TPP). Here, chitosan acted as a reducing/stabilizing agent. The obtained gold nanoparticles were characterized with UV-vis spectroscopy and transmission electron microscopy. The results indicated that the shape and size distribution of gold nanoparticles changed with the molecular weight and concentration of chitosan. More interestingly, the gelation of chitosan upon contacting with polyanion (TPP) can also affect the shape and size distribution of gold nanoparticles. By adding TPP to chitosan solution before the reduction of gold salt, gold nanoparticles have a bimodal size distribution, and at the same time, polygonal gold particles were obtained in addition to spherical gold nanoparticles. Introduction Metal nanoparticles have attracted much attention for their unusual chemical and physical properties, such as catalysis, novel electronic, optic, and magnetic properties, and application in biotechnology.1 The synthesis of nanoparticles with different chemical composition, size distribution, and controlled monodispersity is an important area of research in nanotechnology. Concerning the synthesis of metal nanoparticles, especially gold nanoparticles, many methods have existed in the literature.2 In the latest several years, biomolecules and bioorganisms were also used in the synthesis of nanomaterials.3 A novel biological method for the synthesis of gold and silver nanoparticles using the fungus Verticillium was reported by Mukherjee et al. Exposure of the fungal biomass to aqueous AuCl4- or Ag+ ions resulted in the intracellular reduction of the metal ions and formation of gold and silver nanoparticles of good monodispersity.4 Recently, Raveendran et al.5 reported the preparation of starched silver nanoparticles using a reducing sugar, β-Dglucose, as the reducing agent and starch as a capping agent, which provided a method for “green” nanoparticle preparation. Gold nanoparticles were prepared using many methods, as indicated in this article. But to the best of our knowledge, this is the first time use of a polysaccharide, chitosan, in the synthesis of gold nanoparticles. Recently, Qi et al.6 reported the synthesis of gold nanoparticles using sucrose, which is another kind of sugar. Chitosan has been used as a protecting agent by Esumi in preparation of gold nanoparticles.3b But we found that chitosan is more than a protecting agent and that gold salt can be reduced to zerovalent gold nanoparticles by chitosan itself without any additional reducing agent. Gold nanoparticles with different size distribution could be * Corresponding author. Fax: +86-431-5689711. E-mail: xryang@ ciac.jl.cn.
obtained by using different molecular weight chitosan as reducing/stabilizing agent. Chitosan, with excellent biodegradable and biocompatible characteristics, is a naturally occurring polysaccharide. Using chitosan for the reduction and stabilization of formed gold nanoparticles will not introduce any environmental toxicity or biological hazards. With the increasing interest in the minimization or total elimination of waste and the implementation of sustainable processes through the adoption of 12 fundamental principles of green chemistry,7 the development of biological and biomimetic approaches for the growth of advanced materials is desirable. And this method for the preparation of gold nanoparticles may be amenable to a cleaner large-scale industrial production. Due to the unique polymeric cationic character of chitosan and its gel- and film-forming properties, chitosan has been extensively examined in the pharmaceutical industry for its potential use in the development of controlled release drug delivery systems.8 The polycationic nature of chitosan results in polycondensation in the presence of anionic molecules. Tripolyphosphate (TPP) is a typical polyanion that can diffuse into chitosan droplets or films freely to form ionically cross-linked chitosan beads or films.9 Many works have been done on the formation of chitosan nanoparticles based on the ability of chitosan to undergo a liquid gel transition due to the ionic interaction with TPP.10 Given that chitosan is a cationic polymer, our study was based on inducing the gelation of chitosan by controlling its interaction with the counterion TPP. In this sense, it is known that the inter- and intramolecular linkages created between TPP and the positively charged amino groups of chitosan are responsible for the success of the gelation process, which resulted in a suspension of chitosan nanoparticles in a certain concentration of chitosan and TPP. More interestingly, this gelation did not prevent the reduction of gold salt into zerovalent gold nanoparticles by chitosan. And gold nanoparticles with bimodal size distribution could be obtained
10.1021/bm0497116 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/03/2004
Chitosan-Stabilized Gold Nanoparticles
Biomacromolecules, Vol. 5, No. 6, 2004 2341
when gold salts were exposed to the mixture of chitosan/ TPP. In addition to spherical gold nanoparticles, polygonal gold crystals were also obtained. Experimental Section Materials. HAuCl4 was purchased from Aldrich and used without further purification. Medium molecular weight chitosan (poly-(1,4-β-D-glucopyranosamine), 400 000 g/mol) with a degree of deacetylation of 100% was purchased from Fluka, and lower molecular weight chitosan (poly-(Dglucosamine),
