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Ultrasensitive Detection of Methylmercaptan Gas Using Layered Manganese Oxide Nanosheets with a Quartz Crystal Microbalance Sensor Yuki Tokura, Gentoku Nakada, Yukari Moriyama, Yuya Oaki, Hiroaki Imai, and Seimei Shiratori Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02738 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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Analytical Chemistry

Ultrasensitive Detection of Methylmercaptan Gas Using Layered Manganese Oxide Nanosheets with a Quartz Crystal Microbalance Sensor Yuki Tokura,† Gentoku Nakada,† Yukari Moriyama,† Yuya Oaki,† Hiroaki Imai† and Seimei Shiratori*,†

†

Center for Material Design Science, School of Integrated Design Engineering, Keio

University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522 Japan

Corresponding Author: [email protected]

ABSTRACT Methylmercaptan (MM) is a marker of periodontal disease, however the required sensitivity for MM is ppb, which has been challenging to realize with a simple sensor. Here, we report the capability to detect MM at concentrations as low as 20 ppb using layered manganese oxide nanosheets with a quartz crystal microbalance sensor. The sensing capabilities of the manganese oxide nanosheets are promoted by adsorbed water present on and between the nanosheets. The strong adsorption of MM to the sensor, which is necessary for the high sensitivity, leads to significant hysteresis in the response on cycling due to irreversible adsorption. However, the sensor can be readily re-set by heating to 80 °C, which leads to high reproducible response to MM vapor at low concentrations. A key aspect of this sensor

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design is the high selectivity towards MM in comparison to other compounds such as ethanol, ammonia, acetaldehyde, acetic acid, toluene and pyridine. This layered nanosheets design for high sensitive sensors, demonstrated here for dilute MM, holds significant promise for addressing needs to identify sulfur-compounds associated for environmental protection and medical diagnostics.

KEYWORDS Thiols, QCM, gas sensor, nanosheets, manganese oxide, catalyst

INTRODUCTION The development of sensors or detectors for methylmercaptan (MM) is urgently required for environmental monitoring and protection. MM is a volatile sulfur compound similar to hydrogen sulfide and dimethylsulfide. It is released in exhaust gas, waste liquid from a factory,1-3 and rotten food in a kitchen, and has a negative impact on human health at an exposure level of several ppb. Furthermore, a high concentration of MM is related to the intensity of the odor in human breath. For example, this gas is known as a marker for the diagnosis of periodontal disease, that is, the concentration of MM in the breath of patients that have periodontal disease increases.4-5 Therefore, the detection of MM in human breath is a useful method for the diagnosis of periodontal disease and health management in daily life. Because of the requirements in the environmental and medical fields discussed above, MM needs to be detected at concentration between 10 ppb (normal human) and 500 ppb (Japanese patient with periodontal disease).5 Moreover, gas sensors that are highly selective, user friendly, reversible, and can detect MM at a low concentration are strongly required for

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environmental monitoring and self-diagnosis of dental diseases. Various methods have been proposed for measuring low concentrations of MM, such as gas chromatography,6-9 chemoresistive sensors,10-13 optical sensors,14-17 and biosensors.18-20 However, for practical use, these methods have disadvantages such as low sensitivity, low selectivity, large device size, and high cost. For example, though gas chromatography6-9, optical bio-sniffer17,

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and sensing system with optical sensor15,

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achieved high sensitivity to MM, it was difficult for them to measure the concentration of MM in situ because of their system size. Conversely, quartz crystal microbalance (QCM) sensors, which are inexpensive and compact devices, have the potential for sensitive and selective detection of target gases because of the ability to measure the change in mass at the nanogram scale after the interaction between the sensing film coated on the QCM21-29 and the gases.30-33 The QCM sensors are expected to be applied to portable gas sensor that can be used easily and anywhere in daily life. The crystal of a QCM connected to the oscillation circuit oscillates stably at a constant frequency, and the mass of adsorbed gas on the electrode is related to the decrease in the frequency of the crystal. In this way, a QCM can be used to measure the mass of adsorbed materials on the surface at the nanogram level, as shown by Sauerbrey’s equation.34 In a previous report, Seyama et al. reported a QCM sensor with a sensing film consisting of polyethylene, D-phenylalanine, or D-histidine that can detect 84 ppb MM.32 However, sensors with a higher sensitivity are required to manage human breath because the concentration of MM in normal human breath is 10 ppb. Furthermore, we fabricated polyelectrolyte multilayers of poly(acrylic acid) and poly(allylamine hydrochloride) with a Ag ion on a QCM.33 This sensing film responded to 20 ppb MM. Although this QCM detector with a sensing film had a high sensitivity for

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detecting MM at concentration of 20 ppb, it cannot be used to detect MM repeatedly. To solve the challenge of performing highly sensitive and selective detection of target gases repeatedly, nanosheets, which have a two-dimensional structure, have been reported to have the potential to improve the sensitivity of gas sensors. Two-dimensional materials such as graphene, graphene oxide, metal oxide, layered compounds and organic-inorganic compounds have attracted much attention in energy storage, battery, optical device, catalysis and chemical sensors because of large surface area, controllable electronic properties by controlling thickness of nanosheets, flexibility, surface activity and optical transparency.35-37 Among them, gas sensors containing nanosheets can be used to detect low concentrations of volatile organic compounds gas, H2S gas, or other toxic gases by increasing the specific surface area and interaction between the nanosheets and the target gases.37-41 However, to the best of our knowledge, sensors with reversible sensing characteristics and high sensitivity and selectivity have not been reported yet. In our previous study, we fabricated birnessite-type manganese oxide nanosheets by using a one-pot synthesis with disodium dihydrogen ethylenediamine tetraacetate dehydrate (EDTA) in an aqueous solution.42 These nanosheets had a layered crystal structure that was built up from layered MnO6 octahedra with water molecules and cations between the layers.43-46 Therefore, MM could adsorb in the interlayer of the layered nanosheets as well as on the surface of those nanosheets. In this work, we fabricated a QCM sensor with manganese oxide nanosheets by using a simple method of dropping a dispersion of manganese oxide nanosheets into water on a QCM, and detected MM at a ppb concentration (the detection limit was 20 ppb). Owing to the combination of the mass measurement of the adsorbed gas by QCM and MM

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adsorption in the interlayer of layered manganese oxide nanosheets, the fabricated QCM sensor with a layered manganese oxide nanosheets exhibited great performances toward MM sensing; the sensor was highly sensitive and highly selective. Interestingly, this sensor exhibited a reproducible response to MM through the use of heat treatment after adsorption of the MM because the sensing mechanism was based on dissolution and oxidation; MM dissolved in water and compounds generated by oxidation of MM were evaporated by heat treatment. The design of layered nanosheets for highly sensitive detection will promote the fabrication of sensors for sulfur compounds and will be potentially applied for self-diagnosis as health management in our daily lives as well as for toxic gas monitoring systems in factories or in our living environment.

Scheme 1. Schematic illustrations of the approach for highly sensitive detection of methylmercaptan (MM). 5



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EXPERIMENTAL SECTION Materials. Manganese chloride tetrahydrate (MnCl2 ･4H2O, Kanto Chemical Co, Inc., Tokyo, Japan), EDTA (Kanto Chemical Co, Inc., Tokyo, Japan), and sodium hydrate (NaOH, Junsei Chemical Co, Ltd., Tokyo, Japan) were used for the fabrication of layered manganese oxide nanosheets. A 10 MHz AT-cut quartz crystal with an Ag electrode (QCM, Crystal Sunlife Company, Tokyo, Japan) was used as the substrate. The MM (203 ppm, standard gas is nitrogen, Taiyo Nippon Sanso, Tokyo, Japan), nitrogen gas (N2, Toyoko Kagaku Co., Ltd., Kanagawa, Japan), oxygen gas (O2, Toyoko Kagaku Co., Ltd., Kanagawa, Japan) cylinders, and air gas from an air compressor (SLP-22EB, ANEST IWATA Co., Ltd, Kanagawa, Japan) were used in the MM adsorption experiments. Fabrication of sodium manganese oxide nanosheets. MnCl2･4H2O was dissolved in pure water to obtain a solution with a concentration of 20 mM (400 mL). EDTA (5 mmol) was added to this solution and stirred until the materials were completely dissolved. An aqueous solution of NaOH (400 mL) was adjusted to 0.2 M and added to the MnCl2･4H2O and EDTA solution at once. This solution was left to stand for 5 days at 25 °C. After 5 days, the precipitate was separated by centrifugation at a rotation speed of 3000 rpm for 10 min, rinsed with ethanol and pure water, and dried at room temperature. Fabrication of the sensing film on QCM. To chemically clean the surface of the QCM, it was immersed in KOH and isopropyl alcohol solution (1M, KOH/H2O/IPA = 1:40:60 volume ratio) with ultrasonic agitation for 3 min and ultrasonically rinsed three times with pure water for 1 min. After the rinsing, the QCM was dried by air blowing. The sodium manganese oxide nanosheet precipitates were dispersed in pure water by ultrasonication.

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The concentration of this solution was 0.06 wt%. This solution (10 µL) was applied on the surface of the QCM and dried at room temperature until the water droplet disappeared. Finally, the QCM with the sensing film was heated at 60 °C for 1 h to remove any residual water. Gas sensing measurements. Figure S1 shows the schematic diagram of the gas sensing experiments. The response experiments were conducted in two types of experimental systems, a static system and a flow system. In the static system, the prepared QCM sensor with a measured frequency of the quartz crystal was placed in an acrylic chamber (volume, 10 L) equipped with an air circulating fan and closed. After the frequency (baseline) stabilized, MM was injected into the chamber by syringe. The concentration of MM in the chamber was controlled by the amount of gas that was injected. The frequency of the QCM was measured by a frequency counter and was output to a personal computer. In the flow system, the prepared QCM was placed in a 2×1.5×2 cm sensor cell. In this system, air gas, N2 gas, and O2 gas diluted with N2 were used as carrier gases. The dry carrier gas was humidified by bubbling it through water. The RH in the sensor cell was controlled by the flow rate of the dry carrier gas and humid carrier gas; the total flow rate of the carrier gas was fixed at 500 cc per min. The MM from the gas cylinder was diluted by these carrier gases. The flow rate of MM from the gas cylinder was 10 cc per min, which was approximately 2.5 ppm as measured by the detector tube. All the flow rates were controlled by a mass flow controller. As in the static system, the frequency of the QCM was output to a personal computer. Analysis of the manganese oxide nanosheets and the sensing film formed by the nanosheets. The crystal structure was characterized by using an X-ray diffractometer

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(XRD, D8 ADVANCE eco, Bruker AXS K. K., Tokyo, Japan) with CuKα radiation. The morphology of the fabricated sensing film was observed by field emission scanning electron microscopy (FE-SEM, Sirion, FEI Company, Tokyo, Japan) and field emission transmittance electron microscopy (FE-TEM, FP 5360/22, E. A. Fischione Instruments, Inc. Pennsylvania, United States). The surface modification before and after exposure to MM was observed by using X-ray photoelectron spectroscopy (XPS, JPS-9010TR, JEOL, Tokyo, Japan) with a MgKα laser and Raman spectroscopy (inVia confocal Raman microscope, Renishaw, Gloucestershire, United Kingdom).

RESULTS AND DISCUSSION Characteristics of the manganese oxide nanosheets sensing film. As shown in the XRD patterns of the fabricated manganese oxide nanosheets, (0 0 1) and (0 0 2) peaks were mainly observed in the fabricated nanosheets, which indicated that a crystalline structure was laminated with a manganese oxide (MnO6) and sodium ion in the direction of the c axis (Figure 1 (a) and (b)). Moreover, the pattern of the fabricated nanosheets was almost the same as the pattern of birnessite-type manganese oxide nanosheets, which had a layered crystal structure.42 From these XRD patterns, it was calculated that the interlayer distance between the manganese oxide nanosheets was 0.712 nm. Therefore, the adsorption of MM is considered to occur not only on the surface but also in the interspace between these nanosheets because the distance between the interlayer space was larger than the molecular size of MM.47

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Figure 1. XRD patterns of (a) birnessite-type manganese oxide (Na0.55Mn2O4･H2O) and (b) the fabricated manganese oxide nanosheets.42 The weak hallo around 2θ = 25°is caused by the sample holder consisting of silica glass.

We observed the surface morphology of bare QCM and QCM coated with manganese oxide nanosheets (QCM-NS) (Figure 2 (a) ~ (d)) by FE-SEM. Moreover, a FE-TEM image of the fabricated manganese oxide nanosheets is shown in Figure 2 (e). According to these images, the manganese oxide nanosheets did not aggregate and were uniformly coated on the QCM electrode. Nanosheets with a width of several µm and a thickness of several nm were observed. In our previous study, the birnessite-type manganese oxide nanosheets had a thickness of approximately 10 nm and a width of 2–5 µm. Therefore, the nanosheets fabricated on the QCM electrode were the same size as those in the previous study.42

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Figure 2. FE-SEM images of (a) bare QCM and (b) manganese oxide nanosheets coated on QCM (QCM-NS), and (c), (d) magnified image of (a) and (b). (e) FE-TEM image of manganese oxide nanosheets.

We investigated the response of the fabricated sensor to MM in a static system (Figure S1). Only the QCM-NS responded to 20 ppb MM, which is approximately the concentration in normal human breath (Figure 3). Moreover, the relative humidity (RH) sensor in the sensor cell did not respond to MM after injection of MM (inset in Figure 3). This result indicated that the RH did not change during the measurement before and after the injection of MM. These results demonstrated that the QCM-NS had high sensitivity to MM. As explained later, we consider that the response principle of the sensitive film to MM is owing to the adsorption of MM into the interspace between the manganese oxide nanosheets and the oxidation reaction of MM by the manganese oxide nanosheets, which is

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also enhanced by oxygen.

Figure 3. The response of the QCM-NS and the noncoated Ag electrode QCM to 20 ppb MM in a static system. The inset shows the RH of the system detected by the humidity sensor.

Sensing properties of the manganese oxide nanosheets sensing film. There are various important factors that need to be considered for the design of gas sensors, such as sensitivity, reproducibility of fabrication of sensing film on QCM, relationship between concentration of target gas and sensor response, repeatable response to MM, and selectivity. The QCM-NS had a high sensitivity to MM because it responded to a low MM concentration of 20 ppb (Figure 3). Moreover, we confirmed the reproducibility of the fabrication of the QCM-NS (Figure S2). From these results, each sensing film fabricated by the same method showed almost the same response to 20 ppb MM. In addition to 11



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sensitivity and reproducibility of fabrication, we investigated other sensing properties of the QCM-NS. The response of the QCM-NS for 20 min in a static system showed a nonlinear curve to the concentration of MM because the amount of MM dissolved in water and its oxidation gradually reached a saturation point in the high concentration region (Figure 4(a)). Therefore, in our case, this curve was found to fit to a Langmuir adsorption isotherm.48, 49 This adsorption model is usually used when the number of adsorption sites is limited and the adsorption is saturated in the high concentration region of the target gas. This model is defined by the following formula: =

,

(1)

in which r is the response of the QCM sensor, rmax is the saturation response, c is the concentration of the sensing gas, and K is an equilibrium constant. This model can be transformed into the following:

= + ,

(2)

in which a1 and a2 are constants shown by the following. =

=

,

,

(3) (4)

Because this isotherm was linear (R2 = 0.87), there was a good relationship between the concentration of the MM and the response of the QCM-NS according to the Langmuir adsorption isotherm model (Figure 4(b)). From this adsorption model and equation (3) and (4), the saturation response of QCM-NS (rmax) was 50 Hz and the equilibrium constant (K) was 0.011 ppb-1. When the rmax is large, it means that the sensor response has large and the fabricated sensor has wide detection range. Whereas, when K has larger value, it means

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sensor response approaches saturation response at the lower concentration side. Therefore, the response speed is large, but the detection range is narrow. Conversely, when K has smaller value, it means the sensor response approaches the saturation response at the higher concentration side, so the detection range is wide, but response speed is slow. From the above points, although the response speed of the QCM-NS to MM was slow, the wide range concentration (20-1000 ppb) of MM was calculated from the response of the QCM-NS by using this model as a calibration curve.

Figure 4. (a) Relationship between the concentration of MM and the response of the QCM sensor with a sensing film of manganese oxide nanosheets and (b) fitting line to the Langmuir isotherm of (a).

Here, we investigated the repeatability of the QCM-NS in a flow system (the experimental system is described in Figure S1 and a detailed experimental method for the repeatability experiments is provided in Figure S3). The response of the QCM-NS did not return to the baseline when the flow of MM was stopped in the system (Figure S4). In this case, MM dissolved in water or the products generated by the oxidation of MM did not desorb from the manganese oxide nanosheet sensing film. Therefore, the sensor response 13



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decreased with increasing detection time. However, the response of the sensor was almost the same after heat treatment of the sensing film with a heat dryer (approximately 80 °C for 3 min) after adsorption of the MM gas (Figure 5 (a)). This phenomenon might be caused by desorption of MM and evaporation of the products generated by the oxidation of MM (details of the expected mechanism are provided in the next section). For example, dimethyldisulfide, which is one of the products generated by the oxidation of MM, is a liquid at room temperature and has a boiling point of approximately 100 °C. Thus, dimethyldisulfide was evaporated by heat treatment with the heat dryer. These results demonstrated that the response of the QCM-NS was repeatable after heat treatment. Finally, we investigated the selectivity of this QCM-NS by estimating the so-called response constant (response after 20 min / concentration of gas; Hz/ppm). The response constant of MM (1 ppm), ethanol, ammonia, acetaldehyde, acetic acid, toluene, and pyridine (all gases were 2 ppm except for MM) were estimated by measuring the time-dependent sensor response of each gas (Figure S5). The response constant of MM was approximately 48 Hz/ppm, whereas those of other gases were negligible (< 10 Hz/ppm) (Figure 5(b)). These small responses might be generated by physical adsorption (electrostatic force) on the surface of nanosheets or dissociation in water. In the case of detection of MM, it was considered that the sensor response to the change in mass on the QCM was caused by adsorption (dissolution) of MM in water and the products generated by the oxidation of MM, described more detail later. From these results, the QCM-NS sensor showed high selectivity to MM over ethanol (alcohol), ammonia (amine), acetaldehyde (aldehyde), toluene and pyridine (aromatic compounds), and acetic acid (carboxylic acid). These results demonstrate the potential for breath sensor applications

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because the sensor did not respond to other gases that are present in human breath (ethanol, ammonia, pyridine, and acetic acid).

Figure 5. (a) Repeatability experiments of adsorption and desorption of MM by heat treatment with a heat dryer after flow of MM in the flow system. (b) The response constants of MM, ethanol, ammonia, acetaldehyde, acetic acid, toluene, and pyridine estimated by the sensor response after 20 min / concentration of gases.

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The response process of the manganese oxide nanosheets to methylmercaptan gas. We investigated the response process of the QCM-NS sensor to MM. To explore the adsorption mechanism, we investigated the response of the manganese oxide nanosheet sensing film to MM when the RH of the baseline and carrier gas was changed in the sensor cell of the flow system. Furthermore, the manganese oxide nanosheet sensing film was analyzed by XPS and Raman spectroscopy before and after the exposure to MM. First, we investigated the response of the QCM-NS after changing the carrier gas and RH in the sensor cell (Figure 6). In the case of each carrier gas, the sensor responses increased as the RH baseline in the sensor cell increased. In addition, there was a small response under dry condition when N2 gas or 50% O2 gas diluted with N2 were used as carrier gases (red lines in Figure (a) and (c)). These small responses might be caused by the physical adsorption through van der Waals forces and electrostatic forces between the MM and the manganese oxide nanosheets.50 In humid conditions, different results were obtained depending on the carrier gases. When the carrier gas was air, the sensor response to MM in wet condition (50 %RH) was more than double compared with the case when the carrier gas was N2 in wet condition (47 %RH) (black lines in Figure 6 (a) and (b)). Moreover, the sensor response to MM in air with 35 %RH as a carrier gas was almost the same as that in a mixture of N2 and O2 with 32 %RH (blue lines in Figure 6 (b) and (c)). From these results, the presence of water and oxygen in the air was correlated to the response of the sensor to MM because the response under air and diluted O2 flow was more than the response under N2 flow.

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Figure 6. Response to 2.5 ppm MM when the relative humidity (RH) of sensor cell was changed in flow system. (a) N2 carrier gas, (b) air carrier gas and (c) N2 and O2 carrier gas.

To further investigate the response process, the sensing film was analyzed by XRD, XPS, and Raman spectroscopy. The XRD patterns of the precipitates of manganese oxide nanosheets did not change before and after exposure to 2 ppm MM for 1 day (Figure S6 in the Supporting Information). Therefore, the crystal structure of manganese oxide did not change before and after the exposure to MM. From the result of the XPS spectra of the powder of manganese oxide nanosheets in the manganese (2p1/2, 2p3/2) binding energy,50, 51 both of the manganese peaks were slightly shifted to a lower binding energy after exposure to 2 ppm MM for 1 day (Figure

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7(a)). These results demonstrated that manganese oxide reacted with MM and was reduced to divalent or trivalent manganese ions.50, 51 The sodium peak (1s) showed a similar shift to a lower binding energy (Figure 7(b)). Moreover, a sulfur peak (2p3/2) was not observed (Figure S7 in the Supporting Information), indicating that the MM adsorbed on the manganese oxide nanosheets and products generated by the oxidation of MM desorbed under vacuum conditions.33

Figure 7. XPS spectra of the precipitates of the manganese oxide nanosheets in (a) the manganese (2p) and (b) sodium (1s) binding energy range before and after exposure to 2 ppm MM for 1 day.

Peaks at 583 cm-1 and 642 cm-1 were observed in the Raman spectra of the sensing film formed by layered manganese oxide nanosheets before and after exposure to MM, which were assigned to the Mn-O vibration along the chains of the MnO2 basal plane and the symmetric stretching vibration of the Mn-O bond in the octahedral MnO6, respectively (Figure S9 in the Supporting Information).45, 46 After the exposure to MM, new peaks were

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observed at 2923 cm-1 and 689 cm-1, which were assigned to the C-H stretching mode and C-S symmetric stretching mode, respectively, although the intensity of the peaks was low.52, 53

These peaks might be derived from MM or products generated by the oxidation of MM.

From these results, it was confirmed that the manganese oxide nanosheets were reduced, and MM or products generated by the oxidation of MM existed on the manganese oxide nanosheets sensing film after exposure to MM. In summary, the structural analysis indicated that oxygen and water could be correlated to the sensor response of the QCM-NS, and that manganese oxide was reduced, and S-H and C-H bonds were observed after exposure to MM for a one day. Based on these results, the sensor response of the QCM-NS to MM could be attributed to (1) the dissolution of MM in water on the manganese oxide nanosheets, (2) the oxidation/reduction reaction between MM and the oxygen on the manganese oxide nanosheets, and/or (3) the oxidation/reduction reaction between MM and the manganese oxide nanosheets. Manganese oxide could catalyze the oxidation of MM with oxygen. Various materials for the adsorption and oxidation of MM have been reported.47, 54-56 Activated carbon with urea have been reported for the adsorption and oxidation of MM; water on the activated carbon played a role in the dissociation of MM, leading to its oxidation by oxygen.54 In another study, the manganese oxide showed catalytic behavior in the oxidation of MM.56 Furthermore, birnessite-type manganese oxide was reported to catalyze the oxidation of various materials because of oxygen activation and oxygen transfer through complex manganese species (Mn3+ and Mn4+) of MnO6.43, 44, 57, 58 Therefore, a possible explanation of the sensor response mechanism was the dissolution of MM in water and its oxidation (Scheme 2). Only a small sensor response was observed under dry conditions without

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oxygen because MM was adsorbed on the manganese oxide by physical adsorption (Scheme 2(a)). In humid conditions without oxygen, the sensor response was mainly owing to the dissolution of MM in water and its oxidation to new compounds such as dimethyldisulfide and methanol (which are liquids) by manganese oxide in addition to physical adsorption and dissolution of MM in water (Scheme 2(b)). In humid conditions with oxygen, the MM dissolved in water and was oxidized by oxygen to new compounds in addition to the adsorption described above (Scheme 2(c)). In these reaction, manganese oxide acted as a catalyst through the Mn3+/Mn4+ matrix in MnO6 of the birnessite manganese oxide nanosheets. Thus, the sensor response to the change in mass was caused by adsorption (dissolution) of MM by water and the products generated by the oxidation of MM. Although it will be necessary to further investigate the response mechanism, this specific response process by a layered structure and Mn3+/Mn4+ matrix of manganese oxide nanosheets will broaden the possibility of fabricating highly sensitive and selective, wide range concentration measurable sensor for sulfur compounds compared with other sensing system (Table S1). Moreover, for the practical use, it will be useful for application as a breath sensor for the management of human breath, which requires the use of a sensor in which water does not interfere because this sensor had a higher response in humid conditions. Also, in the case of environmental monitoring, it is desirable to use the QCM-NS under high humid environment (35 % or higher) to achieve high sensitivity because the QCM-NS showed higher response to MM in humid condition than that in dry condition. In the future, for the use of the QCM-NS in dry condition such as desert, it may be necessary to devise measurement condition such as humidifying the sensor head.

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Whereas, the sensor response to MM was higher as the oxygen concentration was higher (Figure 6). However, there is no influence of oxygen to use the QM-NS for breath or environmental monitoring because concentration of oxygen in human breath is about 15 to 20 % and that in air is about 20 %. The influence of oxygen was not as large as the influence of humidity because the QCM-NS responded to MM in nitrogen gas and humidity (oxygen is about 0 %) as shown in Figure 6. As described above, it is considered that widespread use is possible by some devises in the human living environment.

Scheme 2. Scheme of possible response mechanism of the QCM sensor with layered manganese oxide nanosheets (QCM-NS) to MM.

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CONCLUSIONS In this work, a QCM sensor with a layered structure of manganese oxide nanosheets was fabricated. The sensor responded to 20 ppb MM, which corresponds to high sensitivity near the human detection threshold and has the potential to be applied for environmental monitoring and as a breath sensor. This sensor had a layered structure and a Mn3+/Mn4+ complex and showed excellent sensor characteristics such as high sensitivity and selectivity toward MM over ethanol (alcohol), ammonia (amine), acetaldehyde (aldehyde), toluene and pyridine (aromatic compounds), and acetic acid (carboxylic acid), repeatability after heat treatment with a dryer, and an easy calculation of MM concentration because of the excellent relationship between the target gas concentration and sensor response. The sensing mechanism for the sensing of MM was attributed to the possible dissociation and oxidation of MM on the manganese oxide nanosheets and in the interlayer spacing of this material. This QCM sensor with layered nanosheets will be widely applied for the self-diagnosis of human breath as well as for monitoring of the environment. Because of the excellent sensing characteristics and properties, the sensor reported in this work could form the basis for the design of new materials and devices for gas sensors.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/XXXXXXX. Schemes of gas sensing experiments, reproducibility of response of

QCM-NS, procedure of repeat gas experiment, repeatability experiments of adsorption and desorption of MM without heat treatment after flow of MM in the flow system, selectivity of QCM-NS to other gases, XRD patterns of manganese oxide nanosheets after exposure to 22


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MM, XPS narrow-scan spectra of manganese oxide nanosheets in S (2p) peak region after exposure to MM, XPS wide-scale spectra of manganese oxide nanosheets after exposure to MM, Raman spectra of manganese oxide nanosheets after exposure to MM and comparison of sensing performance in this work and other sensing system.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Fax: +81-45-566-1602

Author Contributions

Y. T. conceived and carried out the experiments. Y. T. designed equipment and wrote the paper. G. N. and Y. M. provided experimental support and support in data analysis. Y. O. and H. I. provided experimental support and commented on the manuscript. S. S. supervised the project and commented on the manuscript.

ACKNOWLEDGMENT We are deeply grateful to Dr. Kyu-Hong Kyung, Dr. Kouji Fujimoto and Dr. Yoshio Hotta, whose insightful comments and suggestions were of inestimable value for our study. We thank Prof. Bryan D. Vogt for useful discussions. We also thank Dr. Kengo Manabe and Dr. Mizuki Tenjimbayashi for valuable comments which were an enormous help.

Funding Sources

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This work was partially supported by the SENTAN project from the Japan Science and Technology Agency (JST).

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