Image credit: TMOS
Researchers at TMOS, the Australian Research Council Centre of Excellence for Transformative Meta-Optical Systems, have developed a tiny nitrogen dioxide sensor that could help protect the environment from vehicle pollutants that cause lung disease and acid rain. The sensor, which is made from an array of nanowires, is just one-fifth of a millimeter per side, making it easy to incorporate into a silicon chip.
According to research published in Advanced Materials, the new sensor requires no power source as it runs on its own solar-powered generator. The sensor could be installed in vehicles, and an alarm would sound and alerts would be sent to your phone if it detects dangerous levels of nitrogen dioxide emitted from the exhaust.
In addition to detecting nitrogen dioxide, the nanowire sensor could also be adapted to detect other gases, such as acetone. This could be used as a non-invasive breath test for ketosis, including diabetic ketosis, which could save countless lives.
Compared to existing gas detectors, which are bulky and slow and require a trained operator, the new device can quickly and easily measure less than one part per billion. The TMOS prototype used a USB interface to connect to a computer.
Nitrogen dioxide is a dangerous pollutant to humans even in small concentrations. It is a common pollutant from cars and is created indoors by gas stoves. The new device is designed with a PN junction – the engine of a solar cell – in the shape of a nanowire. The PN junction sits on a base and an ordered array of thousands of nanowire solar cells spaced about 600 nanometers apart form the sensor.
The device is made from indium phosphide, with the base doped with zinc to form the P part, and the N section at the tip of the nanowires, doped with silicon. The middle part of each nanowire was undoped, separating the P and N sections. When light falls on the device, it causes a small current to flow between the N and P sections. However, if the intrinsic middle section of the PN junction is touched by any nitrogen dioxide, which is a strong oxidizer that sucks away electrons, this will cause a dip in the current. The size of the dip allows the concentration of nitrogen dioxide in the air to be calculated.
Dr. Zhe Li, a postdoctoral fellow in EME, conducted numerical modeling and showed that the PN junction’s design and fabrication are crucial to maximizing the signal. The characteristics of nitrogen dioxide – strong adsorption, strong oxidization – make it easy for indium phosphide to distinguish it from other gases. The sensor could also be optimized to detect other gases by functionalizing the indium phosphide nanowire surface.
TMOS Chief Investigator Professor Lan Fu, leader of the research group, said, “The ultimate aim is to sense multiple gases on the one small chip. As well as environmental pollutants, these sensors could be deployed for healthcare, for example, for breath tests for biomarkers of disease.”
“The tiny gas sensor is easily integratable and scalable. This, combined with meta-optics, promises to achieve multiplexing sensors with high performance and multiple functionalities, which will enable them to fit into smart sensing networks. TMOS is a network of research groups across Australia dedicated to progressing this field,” said Fu.
The development of nanowire sensors is an important step forward in the Internet of Things (IoT), a network of connected devices that can communicate with each other and with humans. IoT is already being used in various industries, including healthcare, transportation, and agriculture. With the addition of nanowire sensors, IoT will become even more effective in detecting environmental and health risks, helping to protect human health and the environment.
