PATAN, Nepal, August 23, 2024 – Researchers from the Phutung Research Institute, the University of São Paulo and the University of York have shown that a lensless fluorometer is generally better than a lens system for monitoring unsafe levels of bacteria in water.

“This was an important finding because lenses account for a significant portion of the cost of optical systems and their size and weight make it difficult to make practical, portable devices,” said Ashim Dhakal, leader of the research team. “Our analysis found that using a light source, detectors and sample sizes that are all as large and as close together as possible produces a stronger signal, resulting in better performance in water quality monitoring.”

Researchers have developed a fluorescence detection system that enables highly sensitive detection of harmful microorganisms in drinking water without the use of lenses. They are converting the technology into a lens-free, portable submersible system that could be useful for testing water quality in the traditional natural stone sinks used by people in the Kathmandu Valley in Nepal. Image courtesy of Rijan Maharjan and Ashim Dhakal, Phutung Research Institute.

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

The researchers developed a theoretical framework to investigate the efficiency of a lensless fluorometer with an extended source and detector and then compared the lensless approach with a conventional lens approach.

They developed a lensless fluorescence system with detectors and LEDs from 1 to 2 mm² large and produced UV light. The fluorometer used the UV light from the LEDs to excite proteins in harmful microbes and detect the fluorescence produced by the excitation. An investigation of the excitation and detection of fluorescence signals showed that the lensless system produced a fluorescence signal that was almost twice as strong as a lens system. Several factors limited the performance of the lens system, including a limited numerical aperture and the limited required imaging distance between the source and detector, the lenses and the sample.

While optical lenses are commonly used in cameras, microscopes and telescopes, they often compromise performance in situations where images are not needed. “Today’s fluorometers typically use expensive lenses made of special UV-transmissive glass that must be precisely positioned,” Dhakal said. “We show that eliminating the lenses not only reduces device cost, size and weight, but also provides better performance because we are not targeting images here.”

The new approach not only promises real-time, low-cost water quality monitoring in developing countries and disaster areas, but could also be useful when water safety results are needed quickly. Current methods for assessing microbial contamination of water require culturing the water samples and then quantifying harmful bacteria. This can take over 18 hours and is therefore impractical when immediate confirmation of water safety is required. This is also a major reason why water monitoring is ineffective in developing countries, where the necessary human resources, infrastructure and reagents are not readily available.

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

The researchers are developing a pocket-sized version of the lensless fluorometer for field testing. Before the fluorometer can be deployed on a large scale, the team must demonstrate that it can withstand the harsh environmental conditions in different scenarios. The team also plans to incorporate measurements of multiple parameters into the device to ensure it meets the requirements to detect specific types of bacterial contamination.

The lensless fluorometer is sensitive enough to detect fluorescent proteins from bacteria in water down to concentrations 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 very useful because the sensitive and accurate measurement of the concentration of bacterial proteins that it enables is directly related to the efficiency of water treatment, the dose of disinfectant required for disinfection and the likelihood of bacterial proliferation in the event of recontamination,” said Dhakal.

The study was published in optics (www.doi.org/10.1364/optica.527289).