Characterization of Various Types of Zeolite Acting As a Concentrator of Skin-Emitted Acetone Toward Self-Monitoring of Fat Metabolisms

Abstract

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.