Researchers from the University of Technology Sydney (UTS) have deployed the existing principles of optical tweezer technology, which enables the manipulation and assemblage of nanoparticles, as a base for a technique that allows them to manipulate particles possessing the same refractive properties as those of the background environment in a given setting. Optical tweezing relies on a difference in the refractive properties between trapped nanoparticles and the surrounding environment. The new method, shown as a proof of concept, involves doping nanocrystals with rare-earth metal ions and trapping the nanoparticles at low energy levels and with high levels of efficiency. “Traditionally, you need hundreds of milliwatts of laser power to trap a 20-nanometer gold particle,” said Xuchen Shan, first co-author on the study describing the technique and a Ph.D. candidate in the UTS School of Electrical and Data Engineering. “With our new technology, we can trap a 20-nanometer particle using tens of milliwatts of power.” The method is also highly sensitive — key to ensuring efficiency in operation. “Our optical tweezers also achieved a record high degree of sensitivity or ‘stiffness’ for nanoparticles in a water solution,” Shan said. “Remarkably, the heat generated by this method was negligible compared with older methods, so our optical tweezers offer a number of advantages.” In medicine, the researchers said the advancement is a cursory step for realizing the optical manipulation of, for example, intracellular structures, as well as performing nanoscale biomechanics measurements. “The ability to push, pull, and measure the forces of microscopic objects inside cells, such as strands of DNA or intracellular enzymes, could lead to advances in understanding and treating many different diseases such as diabetes or cancer,” said Fan Wang, leading co-author on the study. A downside of traditional mechanical microprobing for the manipulation of cells is that the process is invasive, Wang said. The positioning resolution is low, too; these microprobes can only measure cell membrane stiffness, as opposed to the force of molecular motor proteins inside a cell. The rare-earth metal ion doping process of nanocrystals controlled both the refractive properties of the nanoparticles, as well as their luminescence. The resonance of ions in nanocrystals creates a strong optical trapping force. Courtesy of Fan Wang. The prospect of developing such a highly efficient nanoscale force probe advances the researchers toward development of a force probe that can be labeled to specifically target intracellular structures and organelles. This would enable the optical manipulation of the intracellular structures, said Peter Reece, also a leading study co-author, from the University of New South Wales. The research was published in Nature Nanotechnology (www.doi.org/10.1038/s41565-021-00852-0).