By adding fluorescent dyes to DNA and then spinning the DNA strands into nanofibers, researchers at the University of Connecticut have made a new material that emits bright white light. The material absorbs energy from ultraviolet light and gives off different colors of light–from blue to orange to white–depending on the proportions of dye it contains.
The researchers, led by chemistry professor Gregory Sotzing, create white-light-emitting devices by coating ultraviolet (UV) light-emitting diodes (LEDs) with the material. They are even able to fine-tune the white color tone to make it warm or cold, as they report in a paper published online in the journal Angewandte Chemie.
The new material could be used to make a novel type of organic light bulb. The light emitters should also be longer-lasting because DNA is a very strong polymer, Sotzing says. “It’s well beyond other polymers [in strength],” he notes, adding that it lasts 50 times longer than acrylic.
The color-tunable DNA material relies on an energy-transfer mechanism between two different fluorescent dyes. The key is to keep the dye molecules separated at a distance of 2 to 10 nanometers from each other. When UV light is shined on the material, one dye absorbs the energy and produces blue light. If the other dye molecule is at the right distance, it will absorb part of that blue-light energy and emit orange light.
By changing the ratio of the two dyes, the researchers can alter the combined color of light that the material gives off. Varying the amount of dye also lets them make finer tweaks. For example, by increasing the proportion of dye in the DNA from 1.33 percent to 10 percent, they can change the white light from cool to warm. “As you go across the white spectrum, if you want a soft yellow-type light or blue-type light, you can get these very easily with the DNA system,” Sotzing says.
Others have used nanostructured materials such as silica nanoparticles and block copolymers–self-assembled materials containing two linked polymer chains–to get the right spacing between the two dyes. But, says David Walt, a chemistry professor at Tufts University, “the advantage in the present system seems to be that the DNA fibers orient the dyes in an optimum way for efficient [fluorescence energy transfer] to occur.” Furthermore, when larger amounts of dye are used in the other materials, they start to aggregate. This has two effects: it decreases energy transfer between them, dimming the light output, and it also prevents precise color tuning.
To make the fibers, Sotzing and his colleagues make a solution of salmon DNA and mix in the two types of dye. The solution is pumped slowly out from a fine needle, and a voltage is applied between the needle tip and a grounded copper plate covered with a glass slide. As the liquid jet comes out, it dries and forms long nanofibers that are deposited on the glass slide as a mat. The researchers then spin this nanofiber mat directly on the surface of an ultraviolet LED to make a white-light emitter.
During the fiber-spinning process, the two different dye molecules automatically attach themselves to two different locations on the DNA. The researchers have found in previous work that the nanofiber mats produce 10 times brighter light than thin films of the dye-containing DNA.
“It’s really very cool [work], and I think that it has practical promise,” says Aaron Clapp, a professor of chemical and biological engineering at Iowa State University. “[But] it seems like an overly dramatic way of doing it.”
Clapp speculates that instead of relying on energy transfer between the two fluorescent dyes, you could just change their ratios and get the colors you want.
However, each dye would then require a different input energy source as opposed to just one UV source, Sotzing points out. What’s more, energy transfer between two dyes gives better control over the color of the output light.
Walt says that it may be possible to use the first dye to transfer energy to multiple dyes and get an even wider range of colors. “The results reported here suggest DNA-[energy transfer] light emitters are promising,” Walt says, “but the ultimate utility will depend on factors such as lifetime and power efficiency.”
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