Nanocrystals Shine On and Off Infinitely
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In 2021, lanthanide-doped nanoparticles create waves — or rather, avalanches — when Changwan Lee, then a PhD student in Jim Schuck’s lab at Columbia Engineering, triggers an extreme light-producing chain reaction from ultra-small crystals developed at Molecular. Foundry at Berkeley Laboratory. Those same crystals are back again with a flicker that is now controllable on purpose and without limit.
“We have discovered the first fully phototransmissible and fully imageable nanoparticles—the holy grail of nanoprobe design,” says Schuck, professor of mechanical engineering.
This unique material was synthesized in the laboratories of Emory Chan and Bruce Cohen at the Molecular Foundry, the Lawrence Berkeley National Laboratory, as well as in a national laboratory in South Korea. The research team also included Yung Doug Suh’s lab at the Ulsan National Institute of Science and Technology (UNIST).
The Holy Grail: Simple and Stable Light Switch
Existing organic dyes and fluorescent proteins used in applications such as optical memory, nanopatterning, and bioimaging have yielded breakthroughs over the years (and earned the Nobel Prize in Chemistry in 2014), but these molecules have a limited lifespan. Once lit, most of them will start flashing randomly and will eventually become permanently dark, or “photobleach”.
In contrast, the lanthanide-doped nanoparticles exhibited remarkable photostability. In more than 15 years of working with them in his lab, Schuck noted that they had never seen one die. Until one fateful day in 2018 when Lee and PhD student Emma Xu observed a crystal darken, then light up again. Lee dug into the literature and found mention of 30-year-old lanthanide optical fibers that could be “photo-darkened” and “photo-illuminated”—suggesting that the flickering behavior could be controlled.
In a new paper published today at Natural, the team did just that. Using near-infrared light, they darkened and brightened their nanoparticles more than a thousand times in different ambient and aqueous environments with no sign of degradation.
“We can turn off these particles, which are not photobleachs, with one wavelength of light and come back with another, using just an ordinary laser,” said Lee. In particular, near-infrared light can penetrate deep into inorganic materials and biological tissues with minimal scattering or phototoxicity.
Weird Results Brighten Future Applications
Looking into potential applications, the team demonstrated how particles could be used to write—and rewrite—patterns onto 3D substrates, which could one day enhance high-density optical data storage and computer memory.
“These infinite two-way photo-altering crystals could produce an all-optical quantum memory device for storing vast amounts of data generated by a quantum computer—think CD-ROMs and CD-RWs, but faster and much more precise,” said Suh.
The particles also offer infinite resolving power, which depends on the number of photons produced by a probe under a super-resolution nanoscope. Using equipment in Suh’s lab, Lee achieved sub-angstrom precision in just a few hours.
The team believes that the photowitching observed in the current work ultimately results from defects in atomic crystals that are too small to be visualized even with the most advanced electron microscopes. This defect shifts the particle avalanche threshold up or down and can be switched by different wavelengths of light to make the signal dimmer or brighter.
In addition to pursuing potential applications in optical memory, super-resolution microscopy, and bioimaging and biosensing, the team used nanoparticle synthesis robots at the Molecular Foundry, advanced computational models, and machine learning to enhance current crystals even further and explore whether they could. synthesize other types of nanoparticles with similar photoswitchable properties.
“This whole study is surprising,” Cohen said. “We’ve been saying since our 2009 paper that this class of nanoparticles can’t be turned on and off, but that’s what we’re learning here. One of the things we found with these nanoparticles was receiving strange results.”
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