Extremely Sensitive and Selective NO Probe Based on Villi-like WO₃ Nanostructures for Application to Exhaled Breath Analyzers

Abstract

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 WO3 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.

Perspectives on oblique angle deposition of thin films: From fundamentals to devices

“…and methane at mild temperatures (150°C) [470]. Similarly, tungsten oxide (WO 3 ) nanocolumns have been used as conductometric sensors of NO, NO 2 and ethanol [471][472][473]. To illustrate the possibilities and lay-out of a typical OAD conductimetry sensor device, Fig.…”

“…To illustrate the possibilities and lay-out of a typical OAD conductimetry sensor device, Fig. 5.10 summarizes some of the results obtained with one of these WO 3 sensors [471] in the detection of NO. This particular system consists of MS-OAD WO 3 films grown on interdigitated Pt electrodes.…”

“…(F) Response of OAD and plain sensors as a function of NO concentration at 200°C. Theoretical detection limits (DL) of both sensors are also presented[471].…”

NO sensing by single crystalline WO3 nanowires

“…Owing to its high response, WO 3 , an ntype semiconductor with a bandgap of 2.8 eV, is one of the most promising materials for the sensing of NO x gases. Moon et al [9] synthesized one-dimensional villi-like nano-fingers of WO 3 on SiO 2 /Si substrate with Pt interdigitated electrodes, and found that the sensor showed high response to NO at 200 °C. Chen et al [10] improved the low-temperature response and selectivity of WO 3 -based sensors by modifying the WO 3 nano-plates with Ag nanoparticles.…”

Gigantically enhanced NO sensing properties of WO3/SnO2 double layer sensors with Pd decoration

“…ZnO is easy to synthesize as various nanostructures, but as a sensor its selectivity is poor. Among these n-type semiconductors, WO 3 has the best sensitivity to NO, but a long recovery time inhibits the application of WO 3 in many cases [2,[16][17][18][19] other elements and forming composite with other materials are the strategies commonly used [20][21][22][23][24][25]. It is generally agreed that the noble metal decoration enhances both the sensitivity and responding speed [26][27][28][29].…”