Fu Xuewen, Chen Bin, Tang Jau, Zewail Ahmed H
Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA.
Sci Adv. 2017 Aug 25;3(8):e1701160. doi: 10.1126/sciadv.1701160. eCollection 2017 Aug.
Dynamics of active or propulsive Brownian particles in nonequilibrium status have recently attracted great interest in many fields including artificial micro/nanoscopic motors and biological entities. Understanding of their dynamics can provide insight into the statistical properties of physical and biological systems far from equilibrium. We report the translational dynamics of photon-activated gold nanoparticles (NPs) in water imaged by liquid-cell four-dimensional electron microscopy (4D-EM) with high spatiotemporal resolution. Under excitation of femtosecond laser pulses, we observed that those NPs exhibit superfast diffusive translation with a diffusion constant four to five orders of magnitude greater than that in the absence of laser excitation. The measured diffusion constant follows a power-law dependence on the laser fluence and a linear increase with the laser repetition rate, respectively. This superfast diffusion of the NPs is induced by a strong random driving force arising from the photoinduced steam nanobubbles (NBs) near the NP surface. In contrast, the NPs exhibit a superfast ballistic translation at a short time scale down to nanoseconds. Combining with a physical model simulation, this study reveals a photoinduced NB propulsion mechanism for propulsive motion, providing physical insights into better design of light-activated artificial micro/nanomotors. The liquid-cell 4D-EM also provides the potential of studying other numerical dynamical behaviors in their native environments.
非平衡状态下活性或推进性布朗粒子的动力学最近在包括人工微/纳米马达和生物实体在内的许多领域引起了极大兴趣。对其动力学的理解可以为远离平衡的物理和生物系统的统计特性提供见解。我们报告了通过具有高时空分辨率的液池四维电子显微镜（4D-EM）成像的水中光子激活金纳米颗粒（NPs）的平移动力学。在飞秒激光脉冲激发下，我们观察到这些NPs表现出超快扩散平移，其扩散常数比无激光激发时大4至5个数量级。测得的扩散常数分别遵循对激光能量密度的幂律依赖关系以及随激光重复率的线性增加。NPs的这种超快扩散是由NP表面附近光诱导蒸汽纳米气泡（NBs）产生的强大随机驱动力引起的。相比之下，NPs在短至纳秒的时间尺度上表现出超快弹道平移。结合物理模型模拟，本研究揭示了推进运动的光诱导NB推进机制，为光激活人工微/纳米马达的更好设计提供了物理见解。液池4D-EM还提供了在其原生环境中研究其他数值动力学行为的潜力。
