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

本课题由国家自然科学基金材料研究部固体与材料化学（SSMC）项目资助，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。虽然已知有许多球形颗粒的三维晶体，但还没有类似的由棒状积木组成的系统。这是因为几乎所有现有的纳米棒合成都无法控制其长度，这往往使它们不适合远距离3D结晶。种子介导的金纳米棒合成是一种罕见的反应产生棒的例子，其长度相当明确。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。该项目将开发一条路线，可以将合成规模扩大到四个数量级。该方法的关键是通过减少纳米棒表面残留的金离子来均匀放大纳米棒。初步研究结果表明，如果还原速率非常低，可以完全抑制随机成核事件，则可以实现这一目标。一旦大量近单分散的纳米棒被生产出来，它们的结晶将被系统地研究成三维单晶。该项目将确定各种参数的作用，如棒的尺寸分布和纯度，它们与底层基质的相互作用，以及溶剂蒸发的速度。特别重要的是CTAB表面活性剂的作用，它在结晶过程中必须存在于溶液中。当结构变量和物理变量的最佳组合被确定后，胶体单晶的周期性阵列将被组装在光刻图案衬底上。沿着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将被执行，以评估其方向依赖的矢量性质。在过去的十年中，对具有可控形状的金纳米颗粒的重大兴趣急剧增长。然而，它们通常很难制造和/或纯化。目前，金纳米棒的商业价格是散装黄金价格的7000多倍。因此，更高效的大规模合成技术的发展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颗粒组装成大晶体的机制将提供具有方向依赖特性的新型纳米材料。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。视频材料包含了金纳米棒合成的详细演示，以及通过光学和电子显微镜对3D晶体进行实时成像，这些视频材料将被发布在YouTube和莱斯大学的网站上。特别重要的是与当地有大量少数民族学生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方面的丰富经验，对纳米结构和胶体组装进行3D可视化，以便在休斯顿公立学校中创建一种独特的活动类型，目前与莱斯大学合作。此外，还计划在科学与艺术的交叉点开展一项令人兴奋的推广活动，这将涉及与休斯敦美术博物馆的合作互动。

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.