Sakai Toshio, Alexandridis Paschalis
Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-4200, USA.
J Phys Chem B. 2005 Apr 28;109(16):7766-77. doi: 10.1021/jp046221z.
Spontaneous formation and efficient stabilization of gold nanoparticles with an average diameter of 7 approximately 20 nm from hydrogen tetrachloroaureate(III) hydrate (HAuCl4.3H2O) were achieved in air-saturated aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer solutions at ambient temperature in the absence of any other reducing agent. The particle formation mechanism is considered here on the basis of the block copolymer concentration dependence of absorption spectra, the time dependence (kinetics) of AuCl4- reduction, and the block copolymer concentration dependence of particle size. The effects of block copolymer characteristics such as molecular weight (MW), PEO block length, PPO block length, and critical micelle concentration (cmc) are explored by examining several PEO-PPO-PEO block copolymers. Our observations suggest that the formation of gold nanoparticles from AuCl4- comprises three main steps: (1) reduction of metal ions by block copolymer in solution, (2) absorption of block copolymer on gold clusters and reduction of metal ions on the surface of these gold clusters, and (3) growth of metal particles stabilized by block copolymers. While both PEO and PPO blocks contribute to the AuCl4- reduction (step 1), the PEO contribution appears to be dominant. In step 2, the adsorption of block copolymers on the surface of gold clusters takes place because of the amphiphilic character of the block copolymer (hydrophobicity of PPO). The much higher efficiency of particle formation attained in the PEO-PPO-PEO block copolymer systems as compared to PEO homopolymer systems can be attributed to the adsorption and growth processes (steps 2 and 3) facilitated by the block copolymers. The size of the gold nanoparticles produced is dictated by the above mechanism; the size increases with increasing reaction activity induced by the block copolymer overall molecular weight and is limited by adsorption due to the amphiphilic character of the block copolymers.
在空气饱和的聚环氧乙烷 - 聚环氧丙烷 - 聚环氧乙烷（PEO - PPO - PEO）嵌段共聚物水溶液中，于室温且无任何其他还原剂的条件下，实现了由三水合四氯金（III）酸（HAuCl4·3H2O）自发形成平均直径约为7至20纳米的金纳米颗粒，并能高效稳定这些颗粒。本文基于吸收光谱的嵌段共聚物浓度依赖性、AuCl4-还原的时间依赖性（动力学）以及颗粒尺寸的嵌段共聚物浓度依赖性，探讨了颗粒形成机制。通过研究几种PEO - PPO - PEO嵌段共聚物，探索了嵌段共聚物特性如分子量（MW）、PEO嵌段长度、PPO嵌段长度和临界胶束浓度（cmc）的影响。我们的观察结果表明，由AuCl4-形成金纳米颗粒包括三个主要步骤：（1）溶液中的嵌段共聚物还原金属离子；（2）嵌段共聚物吸附在金簇上并在这些金簇表面还原金属离子；（3）由嵌段共聚物稳定的金属颗粒生长。虽然PEO和PPO嵌段都对AuCl4-还原（步骤1）有贡献，但PEO的贡献似乎占主导。在步骤2中，由于嵌段共聚物的两亲性（PPO的疏水性），嵌段共聚物吸附在金簇表面。与PEO均聚物体系相比，PEO - PPO - PEO嵌段共聚物体系中颗粒形成效率更高，这可归因于嵌段共聚物促进的吸附和生长过程（步骤2和3）。所产生的金纳米颗粒的尺寸由上述机制决定；尺寸随着嵌段共聚物总分子量引起的反应活性增加而增大，并受限于嵌段共聚物两亲性导致的吸附作用。
