Novel nanoparticles could enable humans to see near-infrared light

A team of scientists at the University of Massachusetts Medical School (UMass Worcester; Worcester, MA) injected a special type of nanoparticles into the eyes of mice to give them the ability to see near-infrared (near-IR) light. The scientists reported progress in making versions of these nanoparticles that could someday give built-in night vision to humans.

"When we look at the universe, we see only visible light," says Gang Han, Ph.D., the project's principal investigator, who presented the research team's work at the American Chemical Society (ACS) Fall 2019 National Meeting & Exposition taking place August 25-29 in San Diego, CA. "But if we had near-infrared vision, we could see the universe in a whole new way. We might be able to do infrared astronomy with the naked eye, or have night vision without bulky equipment."

The eyes of humans and other mammals can detect light between the wavelengths of 400 and 700 nm. Near-IR light, on the other hand, has longer wavelengths (750 nm to 1.4 µm). Thermal imaging cameras can help people see in the dark by detecting near-IR radiation given off by organisms or objects, but these devices are typically bulky and inconvenient. Recognizing this limitation, Han and his colleagues wondered whether they could give mice near-IR vision by injecting a special type of nanomaterial, called upconversion nanoparticles (UCNPs), into their eyes. These nanoparticles, which contain the rare-earth elements erbium and ytterbium, can convert low-energy photons from near-IR light into higher-energy green light that mammalian eyes can see.

In work published earlier in 2019, the researchers targeted UCNPs to photoreceptors in mouse eyes by attaching a protein that binds to a sugar molecule on the photoreceptor surface. Then, they injected the photoreceptor-binding UCNPs behind the retinas of the mice. To determine whether the injected mice could see and mentally process near-IR light, the team conducted several physiological and behavioral tests. For example, in one test, the researchers placed the mice into a Y-shaped tank of water. One branch of the tank had a platform that the mice could climb on to escape the water. The researchers trained the mice to swim toward visible light in the shape of a triangle, which marked the escape route. A similarly lit circle marked the branch without a platform. Then, the researchers replaced the visible light with near-IR light. "The mice with the particle injection could see the triangle clearly and swim to it each time, but the mice without the injection could not see or tell the difference between the two shapes," says Han.

Although the UCNPs persisted in the mice's eyes for at least 10 weeks and did not cause any noticeable side effects, Han wants to improve the safety and sensitivity of the nanomaterials before he contemplates trying them out in humans. "The UCNPs in our published paper are inorganic, and there are some drawbacks there," Han says. "The biocompatibility is not completely clear, and we need to improve the brightness of the nanoparticles for human use."

Now, the team is experimenting with UCNPs made up of two organic dyes, instead of rare-earth elements. "We've shown that we can make organic UCNPs with much improved brightness compared with the inorganic ones," Han says. These organic nanoparticles can emit either green or blue light. In addition to having improved properties, the organic dyes could also have fewer regulatory hurdles.

One of the next steps for the project might be translating the technology to dogs. "If we had a super dog that could see near-IR light, we could project a pattern onto a lawbreaker's' body from a distance, and the dog could catch them without disturbing other people," Han says. The technology could also have important medical applications, such as treating diseases of the eye. "We're actually looking at how to use near-IR light to release a drug from the UNCPs specifically at the photoreceptors," Han says.