Acetone contained in our exhaled breath is a metabolic product of the breakdown of body fat and is expected to be a good indicator of fat-burning. Typically, gas chromatography or mass spectrometry are used to measure low-concentration compounds in breath but such large instruments are not suitable for daily use by diet-conscious people. Here, we prototype a portable breath acetone analyzer that has two types of semiconductor-based gas sensors with different sensitivity characteristics, enabling the acetone concentration to be calculated while taking into account the presence of ethanol, hydrogen, and humidity. To investigate the accuracy of our prototype and its application in diet support, experiments were conducted on healthy adult volunteers. Breath acetone concentrations obtained from our prototype and from gas chromatography showed a strong correlation throughout the experiments. Moreover, body fat in subjects with a controlled caloric intake and taking exercise decreased significantly, whereas breath acetone concentrations in those subjects increased significantly. These results prove that our prototype is practical and useful for self-monitoring of fat-burning at home or outside. Our prototype will help to prevent and alleviate obesity and diabetes.

Analysis of gases emitted from human skin and contained in human breath has received increasing attention in recent years for noninvasive clinical diagnoses and health checkups. Acetone emitted from human skin (skin acetone) should be a good indicator of fat metabolism, which is associated with diet and exercise. However, skin acetone is an analytically challenging target because it is emitted in very low concentrations. In the present study, zeolite was investigated for concentrating skin acetone for subsequent semiconductor-based analysis. The adsorption and desorption characteristics of five zeolites with different structures and those hydrophobicities were compared. A hydrophobic zeolite with relatively large pores (approximately 1.6 times larger than the acetone molecule diameter) was the best concentrator of skin acetone among the zeolites tested. The concentrator developed using zeolite was applied in a semiconductor-based gas sensor in a simulated mobile environment where the closed space was frequently collapsed to reflect the twisting and elastic movement of skin that would be encountered in a wearable device. These results could be used to develop a wearable analyzer for skin acetone, which would be a powerful tool for preventing and alleviating lifestyle-related diseases.

1. Introduction Acetone is a metabolic product of the breakdown of body fat and is exhaled through our breath and skin with very low concentrations. Frequent self-monitoring of skin acetone is expected to be a powerful tool to prevent and alleviate lifestyle-related diseases because skin acetone associates with diet, aerobic exercise, obesity and diabetic control [1]. However, typical concentrations of skin acetones are several tens of parts per billion which is too low to detect with small-sized sensors such as semiconductor-based gas sensors. We proposed to use zeolite as a concentrator of skin acetone to relieve the requirements for the detector sensitivity (Figure 1) and investigated the feasibility of a single type of zeolite [2]. However, it still remains unclear which zeolite is the best to act as a concentrator of skin acetone because various types of zeolite exist. The purpose of this study is to investigate and to compare adsorption/desorption characteristics of various types of zeolite with different structure and hydrophobicity toward acetone gas. 2. Experimental & Results We chose five types of zeolite with different structure and hydrophobicity; FER-156 (synthesized by authors), HISIV3000 (Union Showa K.K., Japan), 390HUA [2], 385HUA, and 350HUA (Tosoh Corp., Japan) as shown in Table 1. Zeolite is a porous material and pore sizes of the above five zeolites are comparable to the diameter of acetone molecules (~ 4.6 Å). To investigate the impact of structural difference, we first compared adsorption/desorption characteristics of FER-156, HISIV3000, and 390HUA. Pure acetone gas containing 40.8 ng of acetones was generated in a closed space with 16.9 mL of a glass vial container and the amount of acetones adsorbed/desorbed into/from each zeolite was analyzed by using a conventional gas chromatography. We found that the rates of adsorbed acetones are almost the same among the three zeolites but 390HUA zeolite has much better desorption performance especially when desorption at low temperature is desirable (Figure 2). These results presumably derive from the difference of pore sizes of the zeolites; 390HUA zeolite with larger pores tends to desorb adsorbed acetones more easily. To investigate the impact of hydrophobic difference, we next compared adsorption/desorption characteristics of Y-type zeolites, 390HUA, 385HUA, and 350HUA, by conducting the same experimental methods described above. Note that the value of SiO2/Al2O3denotes the hydrophobicity and high value corresponds to the hydrophobic zeolite. We found that the rates of adsorbed acetones are almost the same among the three zeolites but 390HUA zeolite has much better desorption performance especially when desorption at low temperature is desirable (Figure 3). These results indicate that hydrophilic zeolites have strong adsorption force toward polar molecules such as acetones. 3. Conclusion We proved that hydrophobic zeolite with relatively larger pores is suitable for the acetone concentrator. Considering that 390HUA zeolite can adsorb/desorb skin-emitted acetones in a feasible manner [2], 390HUA could be the most promising zeolite toward self-monitoring of fat metabolisms. References 1. T. Tsuda, T. Ohkuwa, and H. Itoh, Gas Biology Research in Clinical Practice, pp.125-132 (2011). 2. Y. Yamada, S. Hiyama, T. Toyooka, H. Onoe, and S. Takeuchi, Proc. MicroTAS’13, Freiburg, Germany, Oct. 2013.