Tin oxide fibres provide key to exhaled-breath diagnostic sensor

The sensor works through the arrangement of hierarchical tin oxide (SnO₂) fibres that are assembled from wrinkled thin SnO₂ nanotubes.

Il-Doo Kim, Associate Professor of Materials Science and Engineering Department at the Korea Advanced Institute of Science and Technology (KAIST), and his research team have detailed their research in a paper entitled, Thin-Wall Assembled SnO₂ Fibres Functionalized by Catalytic Pt Nanoparticles and their Superior Exhaled Breath-Sensing Properties for the Diagnosis of Diabetes, in Advanced Functional Materials.

In the paper, the research team presented a morphological evolution of SnO₂ fibres, dubbed micro phase-separations, which takes place between polymers and other dissolved solutes when varying the flow rate of an electrospinning solution feed and applying a subsequent heat treatment.

The morphological change results in nanofibres that are shaped like an open cylinder inside which thin-film SnO₂ nanotubes are layered and then rolled up.

A number of elongated pores ranging from 10nm to 500nm in length along the fibre direction were formed on the surface of the SnO₂ fibres, allowing exhaled gas molecules to easily permeate the fibres.

The inner and outer wall of SnO₂ tubes is evenly coated with catalytic platinum (Pt) nanoparticles.

According to the research team, highly porous SnO₂ fibres, synthesised by eletrospinning at a high flow rate, showed five-fold higher acetone responses than that of the dense SnO₂ nanofibres created under a low flow rate. The catalytic Pt coating shortened the fibres’ gas response time too.

The breath analysis for diabetes is largely based on an acetone breath test because acetone is one of the specific volatile organic compounds (VOC) produced in the human body to signal the onset of particular diseases. The biomarkers predict certain diseases such as acetone for diabetes, toluene for lung cancer, and ammonia for kidney malfunction.

Breath analysis for medical evaluation has attracted attention because it is less intrusive than conventional medical examination, as well as fast and convenient, and environmentally friendly, leaving almost no biohazard wastes.

Various gas-sensing techniques have been adopted to analyse VOCs including gas chromatography-mass spectroscopy (GC-MS), but these techniques are reportedly difficult to incorporate into portable real-time gas sensors because the testing equipment is bulky and expensive, and their operation is more complex. Metal-oxide based chemiresistive gas sensors, however, offer greater usability for portable real-time breath sensors.

In a statement, Il-Doo Kim said, ‘Catalyst-loaded metal oxide nanofibres synthesised by electrospinning have a great potential for future exhaled breath sensor applications. From our research, we obtained the results that Pt-coated SnO₂ fibres are able to identify promptly and accurately acetone or toluene even at very low concentration less than 100 parts per billion (ppb).’

The exhaled acetone level of diabetes patients exceeds 1.8 parts per million (ppm), which is two to six-fold higher than that (0.3-0.9ppm) of healthy people.

A highly sensitive detection that responds to acetone below 1ppm, in the presence of other exhaled gases as well as under the humid environment of human breath, is important for an accurate diagnosis of diabetes.

The research team has now been developing an array of breathing sensors using various catalysts and a number of semiconducting metal oxide fibres, which will offer patients a real-time easy diagnosis of diseases.