Vietnamese doctor makes sensor to detect ammonia gas

Dr. Nguyen Chung, 35 years old, a Vietnamese researcher at RMIT (Australia), the main author, is the only Vietnamese member, along with scientists from RMIT University (Australia), the University of Melbourne and the Center of Excellence for Photonic Nanosystems (TMOS) of the Australian Research Council (ARC) to conduct the research.

The sensor, which consists of a transparent and ultra-thin tin oxide film, can easily detect ammonia at much lower levels than similar technologies. The device acts like an electronic “nose” that can detect even the smallest amounts of ammonia. The sensor can also differentiate between ammonia and other gases with greater accuracy than other technologies.

Dr. Nguyen Chung said the research idea was born in early 2022. The project took nearly a year to complete most of the measurements in May 2023. The research was published in the leading journal of materials science, Advanced Functional Materials, in November 2023.

The presence of ammonia in the air changes the resistance of the tin oxide film in the sensor, meaning that the higher the ammonia concentration, the greater the change in the device's resistance, said Dr. Nitu Syed, who led the research. The team conducted experiments with the sensor in a specially designed device to test its ability to detect ammonia at different concentrations (5 - 500 ppm) and conditions, including temperature. They also tested the device's accuracy in distinguishing ammonia from other gases, including carbon dioxide and methane. The results showed that the sensor can detect ammonia at very low concentrations, so it could be designed to detect ammonia in breath to warn of potential health disorders.

To make the sensor, the team used a low-cost and easily scalable technique to attach ultra-thin tin oxide to a substrate. This technique is possible even on flexible materials - which are often difficult to fabricate with other methods. The team obtained a tin oxide film from the surface of molten tin at 280 degrees Celsius. This film is 50,000 times thinner than paper. "Our method requires only a single synthesis step, without the need for any toxic solvents, vacuum environments or bulky and expensive equipment," said Dr Ylias Sabri, from the School of Engineering at RMIT University (Australia).

Dr. Nguyen Chung said that this miniaturized sensor offers a safer and more compact solution for detecting toxic gases compared to existing techniques. Current ammonia detection methods provide accurate measurements but require expensive laboratory equipment with qualified technicians, as well as many samples and elaborate preparation. This process is often time-consuming and not portable due to the bulky size of the equipment. In addition, the production of ammonia sensors requires expensive and complex processes to prepare sensitive material layers for sensor fabrication.

The team’s new sensor can instantly tell whether ammonia levels are safe or dangerous in the environment. “This tin oxide attachment technique can be scaled up, so it opens up the opportunity for mass production at a more affordable cost,” he said.

He said the team had difficulty characterizing the material due to the ultra-thin nature of the material (the team's tin oxide is only 2 nanometers thick). Setting up a reliable gas measurement system was also a challenge. However, the collaboration of many researchers and experts from RMIT University and the University of Melbourne helped the team overcome the challenge.

The team hopes to work with industry partners to further develop and engineer the next version of the sensor to optimize its performance. The team's approach is compatible with existing manufacturing processes in the silicon industry, making it suitable for mass production.

An estimated 235 million tonnes of ammonia are produced globally each year to meet the needs of a variety of industries. Ammonia is being touted as an efficient way to store hydrogen for green fuel production, and ammonia production is expected to increase in the coming years.

However, exposure to high concentrations of ammonia can lead to chronic lung disease and irreversible organ damage. Ammonia leaks during transportation and plant operations are potentially dangerous and can be fatal. Therefore, the ability to detect ammonia effectively and reliably is essential to ensure safety. The gas is also found in human breath and can be used as an important "biomarker" for diagnosing many diseases such as kidney and liver disorders.

Dr. Nguyen Chung received his master's degree in chemical engineering from Ho Chi Minh City University of Technology in 2014. He is currently a research fellow at the School of Engineering, RMIT University, with research interests primarily focused on liquid metal chemistry and the synthesis of 2D materials for use in electronic and sensor applications.