Large Scale Synthesis of Near-Monodisperse Gold Nanorods and their Assembly into 3D Anisotropic Single Crystals

本研究项目由国家科学基金会材料研究部固体与材料化学(SSMC)计划资助，旨在研究各向异性金纳米结构的动力学控制合成及其在水介质中的胶体结晶。虽然已知了许多球形颗粒的3D晶体，但还没有由棒状积木组成的类似系统。这是因为几乎所有现有的纳米棒合成方法都无法控制其长度，这往往使它们不适合远程3D结晶。种子介导的金纳米棒的合成是这种反应产生长度相当明确的棒的罕见例子。然而，这种方法目前是不可扩展的，不能提供足够数量的纳米棒来进行全面的结晶研究。这个项目将开发一条路线，可以将合成规模扩大到四个数量级。提出的方法的关键是通过减少表面残留的金离子来均匀放大预制纳米棒。初步研究结果表明，如果还原速度非常低，则可以实现这一目标，这允许完全抑制随机成核事件。一旦生产出大量近单分散的纳米棒，将对其结晶成3D单晶进行系统的研究。该项目将确定各种参数的作用，如棒材的尺寸分布和纯度、它们与底层底物的相互作用以及溶剂挥发速度。尤其重要的是在结晶过程中必须存在于溶液中的CTAB表面活性剂的作用。当确定了结构和物理变量的最佳组合时，胶体单晶的周期性阵列将组装在光刻图案化的基板上。将进行沿纳米棒轴线和垂直于纳米棒轴线的晶体的光学、电学和机械性能的测量，以评估其方向相关的矢量性质。非技术摘要在过去的十年中，人们对形状可控的金纳米粒子的兴趣急剧增长。然而，它们通常太难制造和/或提纯。目前黄金纳米棒的商业价格是散装黄金价格的7000多倍。因此，发展更高效的大规模合成将解决它们的可及性问题，这是它们在抗癌治疗、军事设备和隐形斗篷技术中实际应用的主要瓶颈。更好地理解控制非球形粒子组装成大晶体的机制将提供具有方向相关性质的新型纳米材料。在该项目过程中产生的新科学知识将通过信息共享技术和Web2.0通信广泛传播。将在YouTube和莱斯大学的网站上发布视频材料，其中详细演示了金纳米棒的合成，并通过光学和电子显微镜对3D晶体进行实时成像。尤其重要的是与当地少数民族学生众多的中学的科学教师的互动。PI和他的研究生将利用他们在分子图形学方面的丰富经验，对纳米结构和胶体组件进行3D可视化，以便在目前与莱斯大学合作的休斯顿公立学校创建一种独特的活动类型。此外，还计划在科学和艺术的交叉点开展一项令人兴奋的外展活动，其中将包括与休斯顿美术馆的合作互动。

TECHNICAL SUMMARY This research project, supported by the Solid State and Materials Chemistry (SSMC) Program in the Division of Materials Research, National Science Foundation aims to study kinetically-controlled syntheses of anisotropic gold nanostructures and their colloidal crystallization in aqueous media. While numerous 3D crystals of spherical particles are known, there are no analogous systems composed of rod-like building blocks. This is because nearly all existing syntheses of nanorods cannot control their length, which often makes them unsuitable for long range 3D crystallization. Seed-mediated synthesis of gold nanorods is a rare example of the reaction producing rods that are fairly well-defined in terms of their length. However, this method is currently non-scalable and cannot offer a sufficient quantity of nanorods to conduct a comprehensive study of their crystallization. This project will develop a route that can scale the synthesis up to four orders of magnitude. The key of the proposed approach is based on uniform amplification of preformed nanorods by reducing residual gold ions on their surface. Preliminary findings show that this goal can be achieved if the rate of reduction is very low, which allows for complete suppression of random nucleation events. Once the large quantities of near-monodisperse nanorods are produced, their crystallization into 3D single crystals will be systematically studied. The project will determine the role of various parameters such as size distribution and purity of rods, their interaction with the underlying substrates, and the rate of solvent evaporation. Of particular importance will be the role of CTAB surfactant that must be present in solution during crystallization. When the best combination of structural and physical variables is identified, periodic arrays of colloidal single crystals will be assembled on lithographically patterned substrates. Measurements of optical, electrical, and mechanical properties of crystals along and perpendicular to the axes of nanorods will be performed in order to assess their direction-dependent vectorial nature.NON-TECHNICAL SUMMARY Significant interest in gold nanoparticles with controlled shapes has grown dramatically in the past decade. However, they are often too difficult to make and/or purify. The current commercial price of gold nanorods is more than 7,000 times the price of bulk gold. Therefore, a development of more efficient large-scale synthesis will resolve the issue of their accessibility, which is the main bottleneck of their real-life applications in anticancer therapy, military devices, and invisible cloak technology. Better understanding of mechanisms that govern the assembly of non-spherical particles into large crystals will offer novel types of nanomaterials with direction-dependent properties. The new scientific knowledge generated in the course of this project will be widely disseminated via information sharing techniques and Web2.0 communications. Video materials containing a detailed demonstration of the synthesis of gold nanorods and real-time imaging of 3D crystals by optical and electron microscopy will be posted on the YouTube and Rice University web sites. Of particular importance will be the interactions with science teachers from local middle schools that have a large population of minority students. The PI and his graduate students will use their extensive experience with molecular graphics for 3D visualization of nanostructures and colloidal assemblies in order to create a unique type of activity in Houston public schools that currently collaborate with Rice University. In addition, an exciting outreach activity is planned at the intersection of science and art, which will involve collaborative interactions with the Museum of Fine Arts, Houston.