Infrared Mice. Wait, What?

Perhaps the most curious thing about this story, of how mice were made to see infrared, is that it represents a collaboration amongst three Chinese universities plus the University of Massachusetts Medical School. Wait, what? So our leading universities are engineering our soldiers plus the Chinese to see each other glow in the dark?

The human visual spectrum spans red through violet. Red light has longer wavelength and smaller energy per photon; violet has shorter wavelength and higher energy. Recall how the protagonist of Brain Plague had one mutated gene that enabled her to see infrared, the wavelength beyond red color, shading into thermal radiation (heat). What Chrys could see might look something like this:

What animals can see infrared? Some snakes, mosquitoes and bedbugs can detect infrared. Humans can detect infrared photons—but only as bursts of many photons coming together, so pairs of them can add up. To see individual infrared photons is not possible for mammals, because we are “warm-blooded.” Because we maintain a higher temperature, our thermal energy generates “noise”; that is, random firing of the opsin proteins that absorb each photon.

But the Chinese and Massachusetts investigators made use of an optical trick with nanoparticles. The nanoparticles show a form of fluorescence called “upconversion.”

Fluorescence, the simple kind, involves a material that absorbs light at a short wavelength (lambda 1); dissipates some of its energy as heat (broken line); then emits the rest of its energy as a photon of longer wavelength (lambda 2) with lesser energy. For example, absorb blue, emit red.

Suppose however your detector absorbs infrared: Could it emit light at a shorter wavelengh, which your retina can see? Not by plain fluorescence. But upconversion means that the electron cloud absorbing the first infrared photon then absorbs a second one, and is raised to an even higher energy state. Now the electron cloud can release all its energy in one photon of shorter wavelength (lambda 2).

So the researchers injected upconversion nanoparticles into the retinas of mices. The nanoparticles coated the mouse photoreceptor cells, where they could absorb infrared and emit light in the visible range (green). When the mice were given green light (535 nanometers wavelength) they produced electrical signals. And when the nanoparticle-injected mice were exposed to near-infrared light (980 nm) them also produced electrical signals–in response to the green light emitted by upconversion of the nanoparticles.

The infrared response was imperfect, but it was good enough for the mice to distinguish patterns and shapes formed by infrared. Perhaps this kind of approach could engineer new kinds of color vision in the future. If you don’t mind your retina getting injected.