Could science ever grant us superhuman abilities? A new study suggests that this, indeed, possible after researchers used a nanoparticle solution to grant ordinary mice the ability to see in near-infrared.
"When we look at the universe, we see only visible light," said Gang Han, the project's principal investigator and a researcher at the University of Massachusetts Medical School.
"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."
We are able to see because rods and cones -- photoreceptor cells in the retina -- absorb photons of light and send corresponding electric signals to the brain. But not all light is absorbed due to the fact that some wavelengths are too short or too long.
Most mammals, including people, can only see in a narrow range of the electromagnetic spectrum, called visible light. The visible spectrum extends from 380 nanometers to 740 nanometers, which is outside the infrared spectrum whose wavelengths extend from 800 nanometers all the way one millimeter.
In order to 'see' objects that give off infrared light, we had to invent specialized devices such as telescopes or thermal cameras. The latter, for instance, are equipped with detectors that can translate infrared radiation by assigning each temperature a shade of a color. Colder temperatures are often given a shade of blue, purple, or green, while warmer temperatures can be assigned a shade of red, orange, or yellow.
But would it be possible to detect infrared without any equipment? The researchers at the University of Massachusetts Medical School seem to think so.
For their study, Han and colleagues injected a special type of nanoparticles called upconversion nanoparticles (UCNPs) into the eyes of ordinary mice. The nanoparticle solution contains the rare-earth elements erbium and ytterbium, which can convert low-energy infrared photons into higher-energy green light that is visible.
In order to test whether the mice could truly detect infrared, the researchers devised a series of physiological and behavioral tests.
During one such test, the mice were placed in a Y-shaped tank of water, where one of the branches had an opening through which the rodents could escape. The escape route was marked with visible light in the shape of a triangle, while the other blocked end was signaled with a similarly lit circle.
After a couple of rounds of training, the visible light was replaced with near-infrared.
"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.
A single injection of nanoparticles in the eyes of the mice bestowed infrared vision for up to 10 weeks. Although there was a minor side effect (a cloudy cornea), it disappeared in less than a week. Tests found no damage to the retina’s structure, suggesting that the procedure is safe.
However, before the same could be done to humans, there's still a lot of work to do. For one, there are biocompatibility uncertainties since the UCNPs are inorganic. Han would like to replace them with 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," he says.
Besides enabling human beings to see beyond our natural capabilities, the procedure could be useful in medicine to correct human red color vision deficits or to trigger drug release upon contact with infrared light.
The researchers will present their findings at the American Chemical Society (ACS) Fall 2019 National Meeting & Exposition.
