Optical Tweezers Combined with X-Rays

Scientists have found a way to use optical tweezers by employing lasers, a mirror and a light modulator to anchor a crystal in solution. The tweezers have made it possible to conduct X-ray diffraction measurements of a crystal suspended in solution. (Source: R. Horn, ANL)

Under­standing how chemical reactions happen on tiny crystals in liquid solutions is central to a variety of fields, including materials synthesis and hetero­geneous catalysis, but obtaining such an under­standing requires that scientists observe reactions as they occur. By using coherent X-ray diffrac­tion techniques, scientists can measure the exterior shape of and strain in nanocrystalline materials with a high degree of precision. However, carrying out such measurements requires precise control of the position and angles of the tiny crystal with respect to the incoming X-ray beam. Tradi­tionally, this has meant adhering or gluing the crystal to a surface, which in turn strains the crystal, thus altering its structure and potentially affecting reactivity.

Now, scientists at the U.S. Department of Energy’s Argonne National Laboratory and the University of Chicago have developed a new technique that combines the power of nanoscale ​tractor beams with high-powered X-rays, enabling them to position and mani­pulate crystals in solution using only light and without any contact with substrate. While ordinary optical tweezers involve a single focused laser beam, the holo­graphic optical tweezers used in the study involve lasers precisely modified with a spatial light modulator.

These lasers are reflected off a mirror to create an inter­ference pattern of ​hotspots that are both more localized than a simply focused laser beam and have rapidly recon­figurable locations. The electric field gradient of these focused hotspots attracts the polarizable crystal and holds it in place. With a pair of tweezers engaged – each at one end of the crystal – the scientists could manipulate the semi­conductor microcrystal in three dimensions with high precision in the presence of a liquid solution and without exposing it to other surfaces.

Usually, when people look at micro­crystals using X-ray diffraction, they’re glued onto a sample holder, which causes a distortion,” said Linda Young. ​But now, with optical tweezers, you can capture a single particle in its native state in solution and watch its structural evolution. In principle, you can add reactants, capture disso­lution or reaction and monitor changes at an atomic level.” By gaining the ability to manipulate the sample using only light, Young and her colleagues were able to take advan­tage of the coherent X-rays produced by Argonne’s Advanced Photon Source (APS). Using Bragg coherent diffraction imaging (CDI), the researchers were able to examine the crystal’s structure under real conditions and from a number of different angles.

By pairing optical tweezers with Bragg CDI, scientists now have a new way to explore materials in liquid media, explained Brookhaven National Laboratory (BNL) scientist Yuan Gao. Our discovery comes from a combi­nation of different techniques including pairing lasers with the coherent beam from the APS,” he said. ​To make the experiment work, we needed the nano­fabrication technique at the Center for Nanoscale Materials to make the sample cell as well.” According to Young, the technique might be useful for a wide range of future studies, including nuclea­tion and crystal growth. ​Typically, people look at isolated nanocrystalline samples in air or in vacuum. We wanted to be able control such objects in the liquid phase. For example, we wanted to be able to watch catalysis or crystalli­zation unfold in real time with the precision that is afforded by X-ray crystallography,” she said.

Gao pointed to the stability afforded by the optical tweezers as a primary advantage for future coherent X-ray experiments. ​Coherent diffraction is very sensitive to position and orienta­tion of the sample, and this experiment demonstrated the possi­bilities of this new technique,” he said. Because of the stability of the technique, inve­stigators were able to obtain coherent diffrac­tion data, which allowed them to reconstruct the sample with sub-nanometer accuracy, revealing sub-nano­meter scale defects and grain boundaries within the ostensibly crystalline ZnO micro­crystal.

As we look toward the upgrade of the APS, which will increase the brightness of the X-ray beams by orders of magnitude, these measurements will be much faster and provide even more exciting insight into how samples change in time,” added Ross Harder, an Argonne physicist at the APS. Eventually, the researchers would like to extend the technique to capture the ultrafast evolution of the crystal when it is excited by a laser pulse, said University of Chicago chemistry professor Norbert Scherer. This is the first step in achieving our larger ambition, which is to visualize the time-dependent structural dynamics of how the lattice changes,” he said. (Source: ANL)

Reference: Y. Gao et al.: Three-dimensional optical trapping and orientation of microparticles for coherent X-ray diffraction imaging, Proc. Nat. Ac. Sc. 116, 4018 (2019). DOI: 10.1073/pnas.1720785116

Link: Chemical Sciences and Engineering (L. Young), Argonne National Laboratory ANL, Lemont, USA

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