To further investigate the optical properties of a DNA-based lipid complex that is widely used in current DNA thin-film research, researchers developed a refinement process to minimize the relative bound water content and control binding of the surfactant onto the DNA backbone. Organic thin films made from DNA, fine-tuned to provide control over the material’s optical and thermal properties, could be used in biomedical and other photonic devices. The researchers worked with films made from salmon DNA. Researchers have learned to control the index of refraction in organic thin films made out of DNA. Courtesy of Kyunghwan Oh, Yonsei University. The research team, from Yonsei University, believes that DNA could be used for organic photonic devices in the same way that silicon is used for inorganic devices. DNA could, for example, be used to make waveguides similar to silica fibers to carry light within the body, said researcher Kyunghwan “Ken” Oh. To make a thin film from DNA, researchers must first dissolve DNA in water, then dissolve that mixture into an organic solvent. The liquid is placed on a surface that spins the material so that it is evenly spread. The solvent evaporates, leaving the film behind. Because DNA doesn't readily dissolve, researchers must first mix it with a solution of water and the surfactant cetyltrimethylammonium (CTMA). This mixture produces a precipitate that can then be dissolved into the solvent and spin-coated. While researchers have used this procedure for years, their results have been inconsistent. “We noticed that . . . the refractive index and material properties varied in a wide range, so we were very curious about it,” said Oh. “And we found out the fabrication process was a little bit different from research group to research group.” The Yonsei team learned that by controlling the amount of water and CTMA in their mixture, they could fine-tune the refractive index of the thin film, depending on whether they added droplets of water and DNA to the CTMA solution, or added water and CTMA to the DNA bath. Oh describes the DNA strand as a rope, with sites along it to which the CTMA can bind. “If you drop this rope into a CTMA bath, there are tons of CTMA available, so you can soak the rope with the CTMA,” he said. “On the other hand, if you drop CTMA onto a large batch of DNA, the ‘rope’ may not get completely wet; that is, there are areas of DNA without CTMA attached.” The team followed the same approach — i.e., controlling the amount of water and CTMA mixed together — to control the thermal properties of the film. This, in turn, allowed control over how much the refractive index changed when the film was heated or cooled. Control of the film’s thermal properties also allowed the film to be used as a temperature sensor, as changes in the light passing through it could be linked to changes in temperature. In fine-tuning a method for using DNA to create thin films, the team was able to achieve a range of refractive indexes four times greater than that available with silicon. With a greater index difference between core and cladding, the team could make optical fibers as thin as 3 μm in diameter, compared to a minimum diameter of 10 μm in silicon. The thinner organic fiber allows for a smaller spot size for the light that is coming through, which could be useful in applications that must carefully target light. “If you have a small target, you should have a sharper arrow,” said Oh. Devices made with organic materials could be more flexible and more compatible with living tissue than silicon, as well as being more environmentally friendly. Potential applications could include photodynamic therapy. The films could also be useful in optogenetics and for making biosensors. The team is exploring other methods to control the optical properties of DNA. Oh and his lab hope to develop a set of fundamental principles and processes that would allow manufacturers to build a wide range of optical devices, including a new generation of wearable sensors. The research was published in Optical Materials Express, a publication of OSA, the Optical Society (doi: 10.1364/OME.7.003796).
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