Tunable Laser Determines Best UV Wavelengths for Germ Eradication

While awaiting full access to their labs due to COVID-19 restrictions, scientists at the National Institute of Standards and Technology (NIST) have taken the time to report on research conducted in 2012 on the disinfection of drinking water using ultraviolet (UV) light. The research was published online in July 2020 by the Review of Scientific Instruments.

The NIST scientists developed a system that uses a portable, tunable laser to deliver a well-controlled UV beam, which a University of Colorado Boulder (UC Boulder) team then used to test UV wavelengths to learn which were most effective at disinfecting water.

Water irradiation systems typically have used a UV lamp that emits most of its UV light at a single wavelength (254 nm). At the time of the study, interest was growing in a disinfection lamp that could emit UV light at many different wavelengths.

“We discovered in the mid-2000s that polychromatic UV sources were more effective for virus inactivation — specifically because these lamps produced UV light at low wavelengths, under 230 nm,” Karl Linden, a UC Boulder environmental engineer, said. “But it was hard to quantify how much more effective and what the mechanisms of that effectiveness were.”

A scientist places a water sample onto a custom-made platform before a test. Each water sample contains microorganisms such as the parasite Giardia lamblia and adenoviruses, both of which can make humans sick. Courtesy of T. Larason/NIST.

The controllable source for the UV light beam allowed the UC Boulder scientists to test each microorganism under different wavelengths. The tunability of the beam source enabled the scientists to produce a beam with an extremely narrow bandwidth — less than a single nm — over a wide range of wavelengths, in this case from 210 nm to 300 nm. The laser’s portability made it transportable to the lab where the research was taking place. The CU Boulder team also used a NIST-calibrated UV detector to measure the light before and after it hit the sample.

Because different UV wavelengths require different reflective materials, the NIST researchers used mirrors with different reflective coatings that could be swapped out between test runs. A diffuser was used to spread out the beam so that the light was uniform across the entire sample.

The CU Boulder scientists created a series of graphs that showed how different germs responded to UV light of different wavelengths — the first such data for some of the microbes. The NIST setup allowed greater precision, the scientists said, and the experiments led to some unexpected results. For example, some viruses exhibited increased sensitivity as wavelengths decreased below 240 nm. But for other pathogens, such as Giardia, UV sensitivity was about the same even as the wavelengths got lower.

“The results from this study have been used quite frequently by water utility companies, regulatory agencies, and others in the UV field working directly on water and also air disinfection,” UC Boulder environmental engineer Sara Beck said.

Although the system was designed for water sample testing, the NIST researchers believe it could be used to explore how well UV light kills germs on solid surfaces and in the air. For example, in an effort to reduce hospital-acquired infections, some medical centers blast rooms with a sterilizing beam of UV radiation carried in by robots. But there are no real standards for use of these robots, the researchers said. “For devices that irradiate surfaces, there are a lot of variables. How do you know they’re working?” researcher Tom Larason said. A system like the one developed by NIST could be useful for developing a standard way to test different models of disinfection bots. Another potential project could be to examine the effect of sunlight on the novel coronavirus, both in the air and on surfaces, Larason said.

The original team of collaborators hopes to use the laser system for other future projects related to water disinfection. “The sensitivity of microorganisms and viruses to different UV wavelengths is still very much relevant for current water and air disinfection practices, especially given the development of new technologies as well as new disinfection challenges, such as those associated with COVID-19 and hospital-acquired infections,” Beck said.

The research was published in the Review of Scientific Instruments (www.doi.org/10.1063/5.0016500).