Molaei Mehdi, Atefi Ehsan, Crocker John C
Chemical and Biomolecular Engineering, University of Pennsylvania, 220 South 33rd Street, Philadelphia, Pennsylvania 19104, USA.
Phys Rev Lett. 2018 Mar 16;120(11):118002. doi: 10.1103/PhysRevLett.120.118002.
The complex rotational and translational Brownian motion of anisotropic particles depends on their shape and the viscoelasticity of their surroundings. Because of their strong optical scattering and chemical versatility, gold nanorods would seem to provide the ultimate probes of rheology at the nanoscale, but the suitably accurate orientational tracking required to compute rheology has not been demonstrated. Here we image single gold nanorods with a laser-illuminated dark-field microscope and use optical polarization to determine their three-dimensional orientation to better than one degree. We convert the rotational diffusion of single nanorods in viscoelastic polyethylene glycol solutions to rheology and obtain excellent agreement with bulk measurements. Extensions of earlier models of anisotropic translational diffusion to three dimensions and viscoelastic fluids give excellent agreement with the observed motion of single nanorods. We find that nanorod tracking provides a uniquely capable approach to microrheology and provides a powerful tool for probing nanoscale dynamics and structure in a range of soft materials.
各向异性粒子复杂的旋转和平移布朗运动取决于其形状及其周围环境的粘弹性。由于金纳米棒具有强烈的光散射特性和多样的化学性质，它们似乎能提供纳米尺度下流变学的终极探针，但尚未证明在计算流变学时所需的足够精确的取向跟踪。在这里，我们用激光照射暗场显微镜对单个金纳米棒成像，并利用光偏振来确定其三维取向，精度优于一度。我们将单个纳米棒在粘弹性聚乙二醇溶液中的旋转扩散转化为流变学，并与大量测量结果取得了很好的一致性。将早期各向异性平移扩散模型扩展到三维和粘弹性流体，与单个纳米棒的观测运动取得了很好的一致性。我们发现，纳米棒跟踪为微流变学提供了一种独特的有效方法，并为探测一系列软材料中的纳米尺度动力学和结构提供了一个强大的工具。
