A research team affiliated with UNIST has made a significant breakthrough in uncovering the potential of ultra-photostable avalanching nanoparticles (ANP). Their study demonstrates that such particles can perform unlimited photoswitching, leading to new advancements in fields like optical probes, 3D optical memory, and super-resolution microscopy.
This breakthrough has been achieved through the efforts of Professor Yung Doug Suh and his research team in the Department of Chemistry at Ulsan National Institute of Science and Technology (UNIST), in collaboration with researchers from Columbia University and Berkeley Lab. Additionally, two national labs in South Korea also participated in this research endeavor.
In 2021, lanthanide-doped nanoparticles made waves—or rather, an avalanche—when Changwan Lee, who was then a PhD student in Jim Schuck's lab at Columbia Engineering, instigated an extreme light-producing chain reaction from ultrasmall crystals developed at the Molecular Foundry at Berkeley Lab. Now, those same crystals are back with a new feature: they can be intentionally and indefinitely controlled to blink on and off.
"We've found the first fully photostable, fully photoswitchable nanoparticle—a holy grail of nanoprobe design," said P. James Schuck, Associate Professor of Mechanical Engineering.
This exceptional material was synthesized in the laboratories of Emory Chan and Bruce Cohen at Berkeley, along with two national labs in South Korea. Professor Suh's lab was also a leading group of this research team.
The Holy Grail: A Simple, Stable Light Switch
Organic dyes and fluorescent proteins are widely used in various applications, such as optical memory, nanopatterning, and bioimaging. These molecules have contributed to significant advancements in these fields over the years, even earning a Nobel Prize in Chemistry in 2014. However, they suffer from limited lifespans due to their tendency to blink randomly upon illumination and eventually fade away permanently—a process known as "photobleaching."
In contrast, lanthanide-doped nanoparticles exhibit exceptional photostability. Schuck's lab has been working with them for over 15 years without observing any deaths among them until one day in 2018 when Lee and PhD student Emma Xu noticed a crystal going dark before turning back on again. Upon researching literature about lanthanide optical fibers that can be "photodarkened" or "photobrightened," Lee discovered mentions dating back three decades ago which suggested it was possible to control this blinking behavior.
In their paper, published today in Nature, the team has successfully achieved their objective. They were able to darken and brighten their nanoparticles over a thousand times across various ambient and aqueous environments using near-infrared light without showing any indications of degradation.
"We can turn these particles, which don't otherwise photobleach, off with one wavelength of light and back on with another, simply using common lasers," Lee said. It is worth noting that near-infrared light can deeply penetrate both biological tissue and inorganic materials with minimal phototoxicity or scattering.
Weird Results Brighten Future Applications
Regarding potential applications, the research team demonstrated how these particles could be utilized to write and rewrite patterns onto 3D substrates. This advancement might eventually enhance high-density optical data storage and computer memory capabilities. UNIST Professor Suh mentioned that "This indefinite, bidirectional photoswitching nanocrystal could yield an all-optical quantum memory device for storing the vast amount of data produced by quantum computers—think of CD-ROMs and CD-RWs, but faster and much more precise with giant data storage capacity."
Furthermore, these particles have potentially infinite resolving power which relies on the number of photons produced by a probe under a super-resolution nanoscope. Lee achieved sub-Angstrom precision in just a few hours using equipment available in Professor Suh's lab.
The team attributes photoswitching observed in the current work to atomic crystal defects too small for even advanced electron microscopes to visualize. These defects shift the particle's avalanche threshold up or down and can be manipulated using different light wavelengths to increase or decrease signal brightness.
In addition to pursuing potential applications in optical memory, super-resolution microscopy, bioimaging, and biosensing, the team employs nanoparticle synthesis robots at the Molecular Foundry along with machine learning and computational models to enhance these crystals further. They also aim to investigate whether they can produce other nanoparticles displaying similar photoswitchable properties.
Cohen expressed surprise over this entire study as since his 2009 paper, happened to be also reported by 2009 Prof. Suh's paper published independently and simultaneously, their team and Prof. Suh had been claiming that this class of nanoparticles does not switch on and off while studying precisely that here. One significant outcome they have discovered about these nanoparticles is how crucial it sometimes is to embrace unusual results by saying "One of the things we've found with these nanoparticles is to embrace weird results."
Journal Reference
Changhwan Lee, Emma Z. Xu, Kevin W. C. Kwock, et al., "Indefinite and Bidirectional Near Infrared Nanocrystal Photoswitching," Nature, (2023).
DOI: https://doi.org/10.1038/s41586-023-06076-7
