Enhancement Acetone Sensing Performances of SnO2 Nanorod Arrays By Decorating Noble Metal Catalysts
Authors: Jang Yeonji , Jeong Hwaebong , Lee Jun Ho , Choe Yongsahm , Lee Wooyoung 
Published: May 2020

Introduction
Breath analysis using gas chromatography (GC) to identify more than 200 compounds in the human breath came into practice in the 1970s [1]. Endogenous compounds such as inorganic gases (NO, CO) and VOCs (acetone, ethanol, ammonia, ethane and pentane) in the human breath serve as a biomarker for several diseases [2]. Breath acetone is related to diabetes [3], fasting [4], fat metabolism [5], and other numerous diseases [6].
The breath acetone concentration is measured by gas chromatography-mass spectrometry (GC-MS), selected ion flow tube mass spectrometry (SIFT-MS), ion mobility spectrometry (IMS), and proton transfer reaction mass spectrometry (PTR-MS), which are commonly used in the laboratory. As the conventional tools for breath analysis are disadvantageous in terms of cost, portability, and the complexity, so we developed a cheap, and portable breath analyzer.
Method
The breath acetone analyzer comprised a sampling loop, a packed column, three solenoid valves, a mini-sized pump, and a sensor based on SnO2 nanorod (NR) arrays. SnO2 is one of the metal oxide materials that shows high sensitivity to small amount of gases and has strong durability. The mixed gas was passed through the column for the separation of acetone by the difference of interaction with stationary phase. Acetone is separated from the exhaled breath by the packed column within 100 seconds. The operation temperature of the column is maintained at 30 oC. The separated acetone gas was detected by the SnO2-based sensors.
The SnO2 NR arrays were synthesized by using glancing angle deposition method on SiO2 wafers. And various kinds of catalyst such as Au, Pd, Pt were deposit on the surface of SnO2 NR arrays. The tests performed at various temperatures (154 - 434 oC) optimized the operation temperature of the breath acetone analyzer system within the boundary. The breath acetone analyzer was used to detect air balanced acetone gas (50 ppm). At optimized operation temperature, the air balanced acetone gas was used to confirm the detection limits of SnO2 NR arrays-based sensors. The resistance of the SnO2 NR arrays –based sensor was converted to a sensor signal (log(R)) by the breath acetone analyzer. The response of the SnO2 NR arrays sensor was defined as:
Sensor Signal: ∆(log(R)) = log(R)max – log(R)min

, where log(R)max is the maximum resistance before exposure of the acetone and log(R)min is the minimum resistance at acetone exposure.

Results and Conclusions

The response of the sensor increased with the operating temperature and decreased after an optimal point. Optimal temperature for bare SnO2 NR, Pd-coated SnO2, Pt-coated SnO2 NR, and Au-coated SnO2 NR were 315 oC, 275 oC, 315 oC and 434 oC, respectively. The Figure 1 shows response of bare and catalysts decorated SnO2 NR arrays for various concentrations of acetone. It exhibits the effect of various metal catalysts on sensing properties. The Au-coated SnO2 NR arrays had superior acetone sensing properties than other sensors. The response of the SnO2 NR array based sensors increased with increasing acetone concentration. Au-coated SnO2 NR array sensors could detect acetone as low as 0.1 ppm at 434 oC, which is sufficient to analyze breath acetone (~ 0.5 ppm for normal cases). Depending on the type of gases, there should be a difference in time due to the difference of interaction between the stationary phase and gas. Therefore, we will conduct experiments with mixed gas samples and confirm selectivity of the device. In conclusion, we demonstrated that Au metal catalyst exhibited the largest enhancement among metal decorated SnO2 NR sensors for the detection of acetone.

References

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Figure 1