The quivering motion of small suspended particles as they zigzag through a liquid was first observed by the botanist Robert Brown back in 1827. He found that each moving step of the particle (pollen in his case) was independent of the previous one. This random motion, now known as passive Brownian motion, allows substances to disperse and mix. Albert Einstein, in his seminal 1905 paper, showed that Brownian motion comes about thanks to collisions of particles with molecules of the surrounding medium.

In recent years, researchers have been studying “far-from-equilibrium” active Brownian motion for micro- and nanomachines that might be employed in future biomedical applications. Here, an external driving force could push the Brownian object out of equilibrium and propel it. Such out-of-equilibrium systems also exist in nature and some examples include bacteria and motile cells. Artificial motors inspired by these biological machines could be used to deliver nano-sized objects to target positions for applications in non-invasive microsurgery and drug delivery, for example.

The problem is that the superfast propulsive motion of Brownian objects, especially nanoscale ones, is still poorly understood for lack of the right high-spatiotemporal-resolution liquid-phase imaging techniques. Conventional liquid-cell electron microscopy, for example, is good but the images obtained are either static or are obtained too slowly because of the millisecond response of the detector.

Not limited by the response time of the detector

Researchers at Caltech led by Ahmed Zewail (who sadly passed away last year) recently developed a technique called 4D-EM that was not limited by the response time of the detector but their first microscope was only able to work in the solid state. The same team has now successfully improved the technique, which they call liquid-cell 4D-EM, so that it works in the liquid phase and it has been used to image the superfast translation and rotation of single gold nanoparticles in water as the particles are excited with femtosecond laser pulses.

The researchers observed a four-to-five-fold increase in the diffusion rate of the nanoparticles as they applied the laser pulses. This is equivalent to diffusion at nanosecond speeds. The motion is driven by the formation of photoinduced nanobubbles on the surface of the particles that then propel the particles, they say. These bubbles can be directly visualized using 4D-EM as the particles move. Applying repetitive exciting pulses produces heat, which leads to smaller nanobubbles merging to create larger, visible ones that make the nanoparticles move in different ways.

“4D-EM is an ultrafast imaging technique that combines ultrafast lasers and transmission electron microscopy and is based on a pump-probe working mechanism,” explains team member and lead author of the study Xuewen Fu. “We use two ultrashort laser-pulse beams. The first is used to trigger the motion of the particles in the liquid while a precisely timed electron pulse generated by the second ultrashort laser pulse is used to image the transient state of the dynamical process.”

Towards improved light-powered micro- and nanorobots

“To image the superfast diffusion process of nanoparticles in liquid, we designed a liquid cell that sandwiches an ultrathin (hundreds of nanometres) layer of liquid and integrates it into the 4D-EM. This liquid cell can work in high vacuum and is transparent to both ultrashort laser pulses and electron pulses, which allows for both high-spatial-resolution imaging with a single weak electron pulse and the effective photoexcitation of the nanoparticles immersed in the liquid.”

The nanobubbles' propulsion mechanism that we observed using liquid-cell 4D-EM could be helpful for understanding physical and biological systems out-of-equilibrium and also provide insights for designing future light-powered micro- and nanorobots working in complex liquid environments, he told

The researchers, reporting their work in Science Advances, are now busy looking at how they can control and use such light-powered micro- and nanomotors in real-world applications. “We will also be trying to improve the spatiotemporal resolution of our advanced liquid-phase 4D-EM imaging technique and use it to explore dynamical processes occurring in chemicals, materials and biological transformations occurring in liquids,” adds Fu. “Such transformations include physical and chemical reactions of individual molecules and nanocrystals as well as the conformational dynamics of biomolecules.”