All-optical Sensor Safely Detects Volatile Chemicals

University of Navarra scientists in Spain and partners from European photonics innovation hub ACTPHAST 4R are collaborating to develop technology capable of detecting lethal and highly explosive chemicals in industrial settings. The team is working to build a demonstrator, relying exclusively on photonics and capable of employing optical sensing technology that can recognize toxic and explosive solvents in chemical plants.

The new device uses optical fibers to monitor air quality, offering an alternative to sensor devices with electrical (and potentially flammable) components. In addition to providing a solution in areas where electricity is forbidden, the technology aims to improve the probability of safe chemical monitoring procedure.

The impetus for the work stems from the current lack of technology to perform, in real time, automatic checks in fuel tanks for volatile organic compounds (VOCs). These chemicals, typically held in giant tanks, evaporate quickly at room temperature and can explode when they interact with friction or static electricity. This poses a severe risk to plant workers tasked with monitoring chemical vapor or liquid chemicals present in the holding tanks by climbing inside the tanks themselves. Though electronic sensor technology does exist to perform these measurements and monitor the potentially harmful chemicals, the sensors need to be heated beyond 150 °C to achieve functionality.

Electronic sensors also use metallic oxides that are unsafe to use in the shipping of chemic solvents; when exposed to VOCs, semiconductor metallic oxides exhibit a change in electric resistivity. Even if they are used safely, they can be slow operating and do not deliver results in real time.

The new system instead focuses on determining the interaction between the cladding modes and the sensitive coating, and the ability of that interaction to produce an independent, readable signal in the presence of toxic substances.

“An air sensor using optical fibers for the purpose of VOC detection is a major scientific breakthrough concept,” said Cesar Elosua Aguado, lead researcher on the demonstrator project, from the University of Navarra. “We still use metallic oxides, but our system looks at the refractive index of the sensing material, rather than changes in electrical conductivity.”

Zinc oxide coats the sensor surface, reacting when it is exposed to a harmful material. Cladding modes are components of the optical signal that travels by way of a distributed Bragg reflector around (as opposed to directly through) the cladding, allowing interaction with the surrounding media to take place.

The sensor is therefore, in effect tuned to a toxic substance. The only molecules captured along the sensor, Aguado said, are those of harmful gasses. The extent of the reactivity to the gas, known as “selectivity,” depends on the distinguishing properties of each present VOC molecules. The reaction mechanism between the metallic oxide and the VOC is a reversible redox chemical reaction, in fact, and as the selectivity of those materials is low, they react to a wide range of solvents with differing sensitivities. It means that sensors with different responses combine to form a specific pattern for each potentially present VOC.

Eventually, the sensor response system will train an AI system to identify specific samples and their defining qualities, Aguado said.

Aguado noted that the device is in a “pre-prototype demonstrator phase.” Though not yet rolled out, it has the potential to incorporate technology targeting specific industrial application, he said.

ACTPHAST 4R is supported by the European Commission and the Photonics Public-Private-Partnership under the Horizon 2020 program for innovation actions. It combines the technologies and capabilities of 24 European photonics competence centers.