(390d) Si:WO3 Sensors for Highly Selective Detection of Acetone for Easy Diagnosis of Diabetes by Breath Analysis

             Non-invasive detection of illnesses by human breath analysis¹ is an emerging field of medical diagnostic representing a rapid, economic and simple alternative to standard blood analysis and endoscopy. The bulk matrix of the breath is a mixture of nitrogen, oxygen, carbon dioxide, water vapor and inert gases. The remaining small fraction consists of more than 1000 volatile traces with concentrations ranging from parts per trillion (ppt) to parts per million (ppm)¹. Some endogenous compounds, including inorganic gases (e.g. NO, CO) and volatile organic compounds (VOCs; e.g. ethane, pentane, ammonia, acetone, ethanol), can be assigned to specific pathologies and thus are utilized as breath markers.¹ In particular, acetone is a selective breath marker to type-1 diabetes.¹

          Recently, chemo-resistive detectors based on semiconductor metal oxide films have been applied to the analysis of several breath markers.² These simple devices have low fabrication cost, offer high miniaturization potential, sensitivity and sufficient LOD (ppb concentrations).³ Among the large number of sensing metal oxides,³ WO₃ and in particular its e-phase is promising for selective and quantitative detection of acetone in ppb concentrations. This is attributed to the spontaneous electric dipole moment of the e-phase that increases the interaction with analytes having high dipole moment (e.g. acetone).⁴ Nevertheless, such WO₃ detectors, have been tested only in ideal conditions (dry air) without accounting for the effect of water vapor which is a major component of the human breath. Furthermore, it is well known that water vapor interferes with the sensing mechanism of semiconductor metal oxides (e.g. SnO₂) decreasing their sensitivity (e.g. to ethanol) ⁵ and leading to an unreliable response.

                         Here, flame spray pyrolysis has been utilized to synthesize and direct deposit WO₃ nanoparticles onto sensor substrate. The e-WO₃ phase content was enhanced and thermally stabilized by co-synthesis of SiO₂. Furthermore, the crystal and grain size was tailored by the Si-content. In this way, the sensitivity and thermal stability were increased. This should result in increased long term stability and thus avoid baseline drift. The resulting nanocomposite optimally combines the high selectivity of the e-phase of WO₃ and the high sensitivity of small grains.

The optimal operating parameters for the detection of acetone have been determined.⁵ Chemo-resistive gas-sensors based on tungsten oxide nanoparticles showed high sensitivity to acetone down to 20 ppb in ideal (dry air) and in real condition (90% RH).⁶ Figure 1 shows the resistance of a Si-doped WO₃ sensor at realistic conditions and with 20, 50 and 80 ppb acetone at 400 °C. Detection of such low concentration of acetone has not been reported for chemo-resistive gas sensors yet. The cross-sensitivity to ethanol and water vapors during acetone detection by e-WO₃ based gas-sensors was low, creating unique opportunities for using such sensors for non-invasive detection of illnesses (e.g. diabetes) by human breath analysis, which is an emerging field of medical diagnostics, representing a rapid, economic and simple alternative to standard blood analysis and endoscopy.

Figure 1. Si-doped WO₃ sensor exposed to different ultra low acetone concentrations (20, 50 and 80 ppb) at realistic conditions.⁶

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