Lens-Free Fluorometer Can Monitor Water Quality in Low-Resource Settings

Researchers from the Phutung Research Institute, the University of São Paulo, and the University of York showed that a lensless fluorometer is generally better than a lensed system for monitoring unsafe bacteria levels in water.

“This was an important finding because lenses account for a significant share of the costs of optical systems, and their bulk and weight make it difficult to create practical, portable devices,” research team leader Ashim Dhakal said. “Our analysis revealed that using a light source, detectors, and sample sizes that are all as large and as close to each other as possible produces a stronger signal, leading to better performance for water quality monitoring.”

Researchers developed a fluorescence detection system that can provide highly sensitive detection of harmful microorganisms in drinking water, without using any lenses. They are converting the technology into a lensless, dip-in, handheld system that could be useful for testing water quality in the traditional natural stone spouts used by people in Kathmandu Valley in Nepal. Courtesy of Rijan Maharjan and Ashim Dhakal, Phutung Research Institute.

The lens-free fluorometer could provide a simple, low-cost way to monitor water quality in resource-limited settings. “In developing countries, unsafe water sources are responsible for more than one million deaths each year,” Dhakal said.

The researchers developed a theoretical framework for studying the efficiency of a lensless fluorometer with an extended source and detector and then compared the lensless approach with a conventional lensed approach.

They designed a lensless fluorescence system using detectors and LEDs that are 1 to 2 mm² in size and produced UV light. The fluorometer used the UV light from the LEDs to excite proteins in harmful microbes and detect the fluorescence that resulted from excitation. A study of the excitation and collection of fluorescence signals revealed that the lensless system produced a fluorescence signal that was almost double the strength of a lensed system. Several factors restricted the performance of the lensed system, including a limited numerical aperture and the finite imaging distance required between the source and detector, the lenses, and the sample.

While optical lenses are commonly used in cameras, microscopes, and telescopes, they often reduce performance in situations that do not require images. “Today’s fluorometers typically use costly lenses that are made of specialty UV-transparent glass and require precise positioning,” Dhakal said. “We show that eliminating the lenses not only reduces the device cost, size, and weight but also provides better performance, given that we are not aiming for imaging here.”

In addition to promising low-cost, real-time water quality monitoring for developing countries and disaster areas, the new approach could be useful when water safety results are needed quickly. Current methods used to assess microbial contamination in water require culturing the water samples and then quantifying harmful bacteria. This can take over 18 hours, making it impractical when immediate confirmation of water safety is needed. This is also a key reason why water surveillance is ineffective in developing countries, where the required human resources, infrastructure, and reagents are not readily available.

“We hope that our work will facilitate the development of simpler and cost-effective, yet highly efficient sensing paradigms for drinking water,” Dhakal said.

The researchers are developing a pocket-sized version of the lensless fluorometer for field testing. Before the fluorometer can be put to broad use, the team must demonstrate that it can withstand the harsh environments found in multiple scenarios. The team also plans to incorporate measurements of multiple parameters into the device to ensure that it meets the requirements for detecting specific types of bacterial contamination.

The lensless fluorometer is sensitive enough to detect fluorescent proteins from bacteria in water down to levels of less than one part per billion. These results meet the World Health Organization’s criteria for detecting fecal contamination in drinking water.

“Our system is already highly useful because the sensitive and accurate measurement of concentration of bacterial proteins that it provides is directly related to efficiency of water treatment, the dose of disinfectants required for disinfection, and the likelihood of bacterial proliferation in a recontamination event,” Dhakal said.

The research was published in Optica (www.doi.org/10.1364/optica.527289).