The abnormality in the concentration of certain trace gases in human breath, so-called biomarkers, could provide clues to diagnose corresponding diseases. For example, elevated isoprene is a result of cholesterol metabolic disorders, acetone is the biomarker of type-1 diabetes and NO is related to asthma. Non-invasive human breath analysis for disease diagnostics requires selective sensors that respond rapidly and with extreme sensitivity to specific biomarker gases.

On the other side, tungsten trioxide (WO₃) is a very important semiconducting metal oxide which has shown great potential in gas sensing applications. WO₃ exists in a series of stable solid phases at different temperatures from α phase to ϵ phase and an unstable hexagonal phase (h-WO₃). However, except extensively studied y-WO₃, the properties of other phases are still not fully known, esp. ϵ-WO₃ and h-WO3 whose structures are different from other phases.

This dissertation discusses the development of several selective biomarker sensors based on room temperature (RT) stable ϵ-WO₃ and h-WO₃ nanostructured materials.

Ferroelectric ϵ-WO₃ nanoparticles were synthesized using the flame spray pyrolysis method. Although the ϵ-WO₃ polymorph vanishes during heat treatment in pure WO₃ products, chromium dopants were utilized to stabilize this phase. The resistive sensor based on 10at%Cr doped ϵ-WO₃ nanoparticles was found to be very sensitive and selective to low concentrations of acetone (0.2-1ppm) compared to a series of interfering gases at 400°C. The proposed explanation for the materials selectivity to acetone is the likely interaction between the surface dipole of ferroelectric ϵ-WO3 nanoparticles and the highly polar acetone gas molecules.

Open structured h-WO₃ nanoparticles were produced by acid precipitation method. It was found that h-WO ₃ is very sensitive to NO_x compared to other gases at 150°C due to the open tunnel structure of h-WO₃. Such selectivity is lost at 350°C. Instead, the material is very sensitive and selective to isoprene gas at 350°C. A p-n transition was found when the working temperature of the sensor increased from RT to 350°C which could be related to the excessive surface oxygen of the product.

Finally, a handheld exhaled breath analyzer prototype has been developed for non-invasive disease diagnosis. Real-time monitoring of the gas concentration is demonstrated, making this invention a revolutionary, non-invasive, diabetes diagnostic tool.