Carbon nanotube sensor efficiently measures oxygen in gas mixtures under light

Oxygen is essential for life and a reactive player in many chemical processes. Accordingly, methods that accurately measure oxygen are relevant for numerous industrial and medical applications: They analyze exhaust gases from combustion processes, enable the oxygen-free processing of food and medicines, monitor the oxygen content of the air we breathe or the oxygen saturation in blood.

Oxygen analysis is also playing an increasingly important role in environmental monitoring.

"However, such measurements usually require bulky, power-hungry, and expensive devices that are hardly suitable for mobile applications or continuous outdoor use," says Máté Bezdek, Professor of Functional Coordination Chemistry at ETH Zurich. His group uses molecular design methods to find new sensors for environmental gases.

In the case of oxygen, Bezdek's group has now succeeded. In a study published in the journal Advanced Science, the researchers have presented a light-activated high-performance sensor that can precisely detect oxygen in complex gas mixtures and also has the relevant properties for use in the field.

An uncompromising all-arounder

Lionel Wettstein, Ph.D. student in Bezdek's group and first author of the study, explains, "Conventional measurement methods often compromise high sensitivity at the expense of other criteria."

For example, there are sensors that react very sensitively to oxygen, but consume a lot of power and are disturbed by environmental factors such as humidity. Others are tolerant of interfering gases but are less sensitive and are quickly consumed.

"Stationary devices, complex samples and high costs also limit the possible applications," says Wettstein.

The new sensor, on the other hand, is a practical all-arounder: It is very sensitive, can detect molecules of oxygen among a million gas particles, and does so reliably even at higher concentrations. It is also selective, i.e., it tolerates moisture and other interfering gases, and has a long service life. Finally, it is tiny, yet inexpensive, easy to use, and consumes very little power.

This makes the miniaturized sensor interesting for portable devices and mobile real-time measurements in the field—for example, analyzing car exhaust fumes or the early detection of spoiled food. The detector is also suitable for the continuous monitoring of lakes, rivers and soils using distributed sensor networks.

"The oxygen content in these ecosystems is an important indicator of ecological health," says Wettstein.

Sensing molecules with nanotubes

In order to achieve the desired properties, Bezdek's group specifically designed the sensor from molecular components. It belongs to the class of chemiresistors: these are tiny electrical circuits with an active sensor material that interacts directly with the molecule to be analyzed, thereby changing its electrical resistance.

"The big advantage is that this signal can be measured very easily," Bezdek says.

The researchers chose a composite of titanium dioxide and carbon nanotubes as the basis for the sensor material. Titanium dioxide can be used as a chemical resistor but has the disadvantage that it typically only works at very high temperatures.

"For this reason, we have incorporated carbon nanotubes into the composite material," Bezdek continues.

The nanotubes form the energy-saving platform—they ensure that the sensor reaction takes place at room temperature and does not require heating. Finally, to ensure that the sensor material can reliably distinguish oxygen from other gases, the team was inspired by dye-sensitized solar cells, in which special dye molecules called photosensitizers collect light energy and convert it into electrical current.

The researchers have transferred this functional principle to their sensor: In the presence of green light, the photosensitizer transfers electrons to the composite material made of titanium dioxide and nanotubes. This activates the material, making it specifically sensitive to oxygen.

"In contrast to other gases, oxygen hinders this charge transfer in the activated sensor, which changes its resistance," says Wettstein.

From the lab to field application

The researchers have already submitted a patent application for the sensor and are now looking for industrial partners to further develop the technology. Durable and reliable sensors that specifically measure oxygen in gas mixtures are estimated to have an annual market volume of approximately 1.4 billion US dollars.

The team is currently working on expanding its sensor concept beyond oxygen to include other environmental gases that play an important ecological role.

"Our sensor material has a modular structure, and we want to explore how changing its chemical composition can enable the detection of other target molecules," says Bezdek.

One of the current topics in his group is the detection of nitrogen-based pollutants that lead to over-fertilization in agriculture and pollute soil and water.

"To reduce the ecological footprint of the agricultural sector, we need sensors that enable precise fertilization of fields," says Bezdek.