Extremely sensitive and selective NO probe based on villi-like WO ₃ nanostructures for application to exhaled breath analyzers

Self-assembled WO₃ thin film nanostructures with 1-dimensional villi-like nanofingers (VLNF) have been synthesized on the SiO₂/Si substrate with Pt interdigitated electrodes using glancing angle deposition (GAD). Room-temperature deposition of WO₃ by GAD resulted in anisotropic nanostructures with large aspect ratio and porosity having a relative surface area, which is about 32 times larger than that of a plain WO₃ film. A WO₃ VLNF sensor shows extremely high response to nitric oxide (NO) at 200 C in 80% of relative humidity atmosphere, while responses of the sensor to ethanol, acetone, ammonia, and carbon monoxide are negligible. Such high sensitivity and selectivity to NO are attributed to the highly efficient modualtion of potential barriers at narrow necks between individual WO₃ VLNF and the intrinsically high sensitivity of WO ₃ to NO. The theoretical detection limit of the sensor for NO is expected to be as low as 88 parts per trillion (ppt). Since NO is an approved biomarker of chronic airway inflammation in asthma, unprecedentedly high response and selectivity, and ppt-level detection limit to NO under highly humid environment demonstrate the great potential of the WO₃ VLNF for use in high performance breath analyzers.