Exhaled VOCs sensing properties of WO3 nanofibers functionalized by Pt and IrO2 nanoparticles for diagnosis of diabetes and halitosis
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- Shin, J., Choi, S., Youn, D. et al. J Electroceram (2012) 29: 106. doi:10.1007/s10832-012-9755-y
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This work presents a simple synthetic route to produce WO3 nanofibers functionalized by catalytic Pt and IrO2 nanoparticles and their superior acetone and H2S sensing characteristics, demonstrating the potential use of Pt and IrO2 nanoparticles in applications as sensors of biomarkers of diabetes and halitosis, respectively, in exhaled breath. The individual WO3 fiber, calcined at 500 °C, was composed of small nanoparticles with a size distribution in the range of 30–100 nm. Networks of WO3 fibers exhibited a high surface-to-volume ratio and unique morphologies, thus facilitating efficient gas transport into the entire fiber layers. Pt (4–7 nm) and Ir (4–8 nm) nanoparticles were synthesized by polyol methods and were used as additives to decorate the surface of the WO3 fibers. After a heat treatment, those catalyst particles were partially or fully oxidized to Pt/PtOx and IrO2, respectively. To investigate the advantages of Pt-decorated WO3 fibers (Pt-WO3) and IrO2-decorated WO3 (IrO2-WO3) fibers as acetone (CH3COCH3) and H2S sensing materials, respectively, we carried out gas-sensing measurements in a highly humid atmosphere (RH 75 %) similar to that of an oral cavity. The Pt-WO3 fibers showed a high acetone response (Rair/Rgas = 8.7 at 5 ppm) at 350 °C and a superior H2S response (Rair/Rgas = 166.8 at 5 ppm) at 350 °C. Interestingly, IrO2-WO3 fibers showed no response to acetone, while the gas response to H2S exhibited temperature-insensitivity, which has never been reported in any other work. Thus, the highly selective cross-response between H2S and acetone was successfully achieved via the combination of IrO2 particles on WO3 fibers. This work demonstrates that accurate diagnosis of diabetes and halitosis by sensing exhaled breath can be realized through the use of electrospun WO3 fibers decorated with Pt and IrO2 catalysts.
KeywordsElectrospinningNanofibersExhaled breath sensorsWO3PtIrO2DiabetesHalitosis
Recently, exhaled breath sensors have been attracted much attention due to their ability to provide facile diagnosis of diabetes [1, 2], halitosis [3–5], and certain diseases through the simple detection of exhaled volatile organic compounds (VOCs), typically acetone and H2S. For instance, the acetone level in breath is closely related to diabetes, because incomplete management of glucose in the blood of diabetes patients can result in the production of higher levels of acetone (>1.8 ppm) in exhaled breath compared to that of healthy people (<0.9 ppm) . In addition, H2S can be produced by the micro-bacterial metabolism of amino acids and proteins in the digestive tract [7, 8] and can be used as a target gas for diagnosis of halitosis, or oral malodor. The main advantages of exhaled breath sensors are simplicity of operation, non-invasiveness, a low cost, and rapid detection. While many diabetes patients must undergo periodic blood sugar testing, the use of real-time exhaled breath sensors for monitoring acetone levels can significantly reduce both the pain felt during these tests and the time required to conduct them. In addition, quantitative measurements of H2S in exhaled breath for malodor intensity monitoring purposes would be beneficial for preventing the relapse of this and other diseases and would allow the formulation of proper drug therapies. In this sense, the most challenging issues related to the development of exhaled breath sensors include optimizing the designs of materials and structures with high gas sensitivity and the selection of specific target gases.
To obtain outstanding gas-sensing performance, numerous studies have been conducted in an effort to develop highly sensitive metal oxide chemiresistive sensors using one-dimensional (1-D) nanostructures such as nanowires , nanotubes , nanorods , or nanowhiskers . Among the different strategies for producing 1-D sensing materials, electrospinning offers several unique advantages, including a simple fabrication process and reproducible device performance due to the characteristics of the fiber network . In particular, nanofibrous metal oxide layers display unique polycrystalline morphologies; i.e., the nanofibers are composed of tiny nanoparticles that give rise to a large surface-to-volume ratio, leading to the effective gas modulation of the fiber-based sensing layers . Specifically, the bimodal architecture, which includes large pores separating the nanoscale fibers and small pores that form among densely packed nanoparticles, is one of the unique morphological advantages of electrospun metal-oxide fibers.
Thus far, a number of electrospun metal-oxide nanofibers have been synthesized and studied. These metal-oxide nanofibers include SnO2 , TiO2 , ZnO , MoO3 , In2O3 , Zn2SnO4  and WO3 . Among the various types of metal oxides, n-type WO3, with a wide band gap of 2.6 eV, is considered as a prospective candidate material for sensing VOCs due to its good electrical properties, excellent gas-sensing characteristics and its nontoxicity [22, 23]. Compared to other oxides, the interesting features of WO3 are closely related to its stoichiometric phase transition and specific affinity toward certain gases, as changes in its phase lead to distinguishable variations in its electrical and optical properties [24–26]. Thus far, several gas sensor works using electrospun WO3 fibers have been reported. Examples include those by Wang et al. , Santucci et al. , Gouma et al. , Zhang et al. , and Kim et al. . All measurements in this studies were conducted in a dry air condition for the detection of environmental gases such as NOx, COx (x=1,2), C2H4 and NH3. Although some articles related to exhaled breath sensors using WO3 nanoparticles [32–34], thin film  and nano-rods  have been published, to the best of our knowledge, there are no reports about the potential use of electrospun WO3 fibers, particularly those decorated with catalytic additives, as an exhaled breath sensor for the diagnosis of diabetes and halitosis.
Thus, for the first time, we report here the facile synthesis and superior acetone and H2S sensing properties of WO3 fibers functionalized by Pt and IrO2 particles. The unique sensitization effects of Pt and IrO2 catalysts anchored onto the surface of WO3 fibers are clearly discussed in terms of their superior response and cross-sensing properties against acetone and H2S. The temperature-independent gas response characteristics of IrO2-WO3 against H2S gas in the range of 350–500 °C are highlighted. Based on these results, we expect that the process of the facile pattern recognition of acetone and H2S in exhaled breath can be simplified through the use of metal-oxide fibers functionalized by specific catalysts, thus real-time diagnosis of diabetes and halitosis can be facilitated.
2.1 Synthesis of WO3 fibers
All chemicals were used as received without any purification. Polycrystalline WO3 fibers were prepared by the electrospinning of WCl6 precursors dissolved in a polyvinylpyrrolidone (PVP) solution and a subsequent calcination step in an air atmosphere. In a typical procedure, 1.5 g of WCl6 and 1.25 g of PVP (Mw = 1,300,000 g mol−1) were dissolved in 10 g of dimethyl formamide (DMF) and 0.2 g of acetic acid. After vigorous stirring at 500 rpm for 10 h at room temperature, the WCl6/PVP solution was loaded into a plastic syringe connected to a 21-gauge needle at a constant flow rate (5 μl min−1). High voltage (11.5 kV) and a constant distance (15 cm) were maintained between the syringe tip and the collecting stainless steel foil, which was wrapped around a grounded rotating cylinder (100 rpm). The collected WCl3/PVP composite fibers were heat-treated at 500 °C for 3 h in an electric furnace (Vulcan 3-550, Ney) to ensure the complete burning out of polymers and to induce crystallization of the WO3 fibers. The heating rate was fixed at 4 °C min−1 and the cooling rate was maintained at 20 °C min−1.
2.2 Synthesis of Pt nanoparticles
Pt catalytic nanoparticles were synthesized by a polyol method . 0.5 g of H2PtCl6 was dissolved in 5 ml ethylene glycol (EG) solution. Then, the H2PtCl6/EG solution was slowly injected into a three-neck round flask filled with 45 ml of EG in a heated oil bath (150 °C). Subsequently, 20 ml of PVP (0.5 g, 10,000 g mol−1) dissolved in an EG solution was added to the flask at a rate of 2 ml min−1. The color of the solution turned suddenly from yellow to black. After 1 h, acetone was added to the solution at a 5:1 volume ratio of acetone to the Pt solution. To isolate the Pt particles from the solvent, the mixed solution was centrifuged at 3000 rpm for 5 min and washed with deionized (DI) water multiple times. The obtained black powders were dispersed in ethanol, resulting in the formation of a Pt colloidal solution, which was directly used as the Pt additive source.
2.3 Synthesis of Ir nanoparticles
To decorate the surface of the WO3 fibers with the IrO2 catalyst, Ir catalytic nanoparticles were synthesized by the polyol method described above. 0.5 g of H2IrCl6 was dissolved in 75 mL EG under magnetic stirring at room temperature. The H2IrCl6 solution was then heated to 100 °C at a rate of 1 °C min−1 for 1 h. The color of the solution changed gradually from dark to light brown. To obtain colloidal Ir particles, the synthesized solution was centrifuged at 3000 rpm for 10 min and was then washed with DI water multiple times. Ir nanoparticles were dispersed in ethanol, and these were directly used as the Ir additive source. Finally, IrO2 catalysts were formed after high-temperature calcination of the Ir-decorated WO3 fibers.
2.4 Microstructural characterizations
The microstructural characteristics of the as-spun WCl6/PVP fibers and calcined WO3 fibers were investigated using a scanning electron microscope (Field Emission SEM, Magellan400, FEI) and a transmission electron microscope (FE-TEM 200KV, Tecnai). The characterization of the crystal structures of the calcined WO3 fibers was carried out by X-ray diffractometry (XRD, D/MAX-2500 series, RIGAKU; with CuKα radiation (λ = 1.54 Å)). The morphological distribution of the Pt and IrO2 particles on the WO3 fibers and the oxidation states were identified by high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS, Sigma Probe, Thermo VG Scientific), respectively.
2.5 Fabrication of sensors
To investigate the acetone and H2S gas-sensing properties of pristine WO3, Pt-WO3 fibers, and IrO2-WO3 fibers, three different fibers, which were mixed with a polymeric binder (PVAc), were coated onto Al2O3 substrates patterned with two Au electrodes accompanied with an electric heater on the back side. In this study, colloidal Pt and Ir nanoparticles dispersed in ethanol were mixed with WO3 fibers using a pestle and a mortar and were dropped onto the sensing substrate, compressed on a pre-heated hot plate (80 °C) for 30 min. Next, the WO3, Pt-WO3 and IrO2-WO3 fibers on each sensor substrate were heat-treated at 500 °C for 2 h to eliminate the PVAc and thus provide good electrical contact between the network of the WO3 fibers and the Au electrodes. During this process, Ir nanoparticles were oxidized to IrO2 as well.
2.6 Gas-sensing test
Exhaled breath sensing tests against the two target gases (acetone and H2S) at various concentrations (5–1 ppm) were performed to evaluate the acetone and H2S sensing performance of the WO3, Pt-WO3, and IrO2-WO3 fibers in an operating temperature range of 300–500 °C. To identify the gas response characteristics of our sensors in a highly humid atmosphere, humid air was injected to the testing chamber, and a relative humidity of 75 % was maintained (RH 75 %). Prior to the sensor test, networks of WO3, Pt-WO3, and IrO2-WO3 fibers were stabilized for 48 h until a constant resistivity level was obtained. The target gases were then introduced into the testing chamber for 10 min. This was followed by purging with air for 10 min to clear the target gas.
The responses of the sensors were evaluated by measuring the resistivity changes using a 16-channel multiplexer (34902A, Agilent) combined with a data acquisition system (34972A, Agilent). The temperature of the sensors was controlled by a DC power supply (E3647A, Agilent) with applied bias through a heater.
3 Results and discussion
A list of initial resistivities (Rair) under humid air atmosphere of pristine WO3, Pt-WO3, and IrO2-WO3 fibers as a function of operating temperatures
The acetone responses of pristine WO3 fibers and Pt-WO3 fibers at various temperatures in the range of 300–500 °C are summarized in Fig. 5(c). Pt-WO3 fibers demonstrate declining response behaviors after an increase in the operating temperature, while pristine WO3 fibers exhibit increasing response behaviors after an increase in the operating temperature. It was determined that the responses of pure WO3 fibers gradually increase from 300 °C and reach a maximum at an operating temperature in the range of 400–450 °C. In contrast, Pt-WO3 fibers exhibit their maximum responses at 350 °C, and showing dramatic decreases in these levels as the temperatures increase beyond this point. The reason behind the different optimum operating temperatures is the Pt catalyst effect, which modifies the operating temperature, or generally lowers the optimum temperature and enhances the sensor sensitivity [47, 48]; at elevated temperatures, the chemisorbed oxygen, which is driven by the chemical sensitization of Pt, can easily be vaporized and dissociated, thereby lowering the resistivity and resulting in decreased responses of Pt-WO3 fibers.
On the other hand, the increased response of pure WO3 fibers with an increase in the temperature can be explained by the thermally excited electrons which jump from the valence band to the conduction band. This promotes enhanced resistivity changes in presence of reducing gases, i.e., acetone and H2S. It was also revealed that the IrO2-WO3 fibers do not react with acetone at appreciable levels. In other words, at high-temperature oxygen desorption reaction is predominant, especially for Pt-WO3 fibers with excessive amounts of adsorbed oxygen. Thus, the oxygen desorption process is hence the more thermodynamically favorable process, causing the gas responses at high temperatures for Pt-WO3 fibers to be greatly reduced.
Figure 5(d) shows the linear extended acetone response of Pt-WO3 fibers at 350 °C. The calculated acetone response (Rair/Rgas) of the Pt-WO3 fibers was 1.27 at 0.3 ppm, which can distinguish diabetes patients whose acetone concentrations exceed 1.8 ppm as compared to healthy people (<0.9 ppm).
The degradation of the sensor response of the Pt-WO3 fibers along with an increase in the operating temperature was identical to that shown in the acetone sensing result (Fig. 6(c)). Compared to the highest response at 350 °C, the relative H2S responses of Pt-WO3 fibers are greatly reduced to 1.5 % of the sensor response at 500 °C. For the IrO2-WO3 fibers, the highest response (Rair/Rgas = 7.55, 5 ppm) to H2S was achieved at 400 °C and the relative response rates compared to the maximum value remained almost constant (>90 %) regardless of the temperature.
Figure 6(d) shows the linear extended H2S response of the Pt-WO3 fibers at 350 °C. Despite the steep slope, the calculated H2S response (Rair/Rgas = 1.27) of the Pt-WO3 fibers at 0.3 ppm can be utilized to detect exhaled H2S, which commonly requires a detection level as low as 0.6 ppm.
Polycrystalline WO3 fibers were synthesized by electrospinning followed by calcination at 500 °C and were used to prepare chemiresistive sensors for exhaled breath analyses of diabetes and halitosis by detecting acetone and H2S in a humid atmosphere (RH 75 %). To provide enhanced gas-sensing properties, Pt (4–7 nm) and Ir (4–8 nm) nanoparticles prepared by polyol synthesis were added to the surface of the WO3 fibers. Whereas well-dispersed Pt particles on WO3 fibers were revealed by TEM images, slightly agglomerated IrO2 particles on WO3 fibers were obtained after the post-heat-treatment step. The oxidation behaviors of both Pt and IrO2 particles during the post-heat-treatment process were confirmed by HRTEM and XPS analyses. The p-type semiconductor property of PtO (0.86 eV) and the n-type semiconductor property of IrO2 (2.34 eV) resulted in totally distinct sensor properties which can give rise to certain selectivity to acetone and H2S driven by chemical sensitization and electrical sensitization, respectively. Moreover, the Pt-WO3 fibers showed a 2.9-fold (Rair/Rgas = 7.96 versus 2.74) higher acetone response at 300 °C and, more importantly, a superior H2S response, by 19.5-fold (Rair/Rgas = 106.43 versus 5.45), compared to pristine WO3 fibers. The superior H2S sensing capability of Pt-WO3 fibers is clearly beneficial to those seeking to diagnosis halitosis. Although the acetone response of the WO3 fibers ceased when they were decorated with IrO2 particles, the benefit of the IrO2 catalyst was its exclusive selectivity to H2S gas. To be specific, IrO2-WO3 fibers which exhibit high selectivity between acetone and H2S can be utilized as a standard reference for H2S and acetone gases. This means that IrO2-WO3 fibers can effectively detect H2S gas without any disturbance caused by acetone in exhaled breath. Considering the unique gas response characteristics of Pt-WO3 and IrO2-WO3 fibers as exhaled breath sensors, it is possible to build a sensor set for accurate diagnosis of halitosis and diabetes. The interesting part of this research is that it represents the first finding of temperature-insensitive responses of IrO2 as a catalyst. This can lead to the creation of sensors with valuable properties, especially sensors that are utilized in a wide temperature range. We anticipate that further works will be related to the optimization and control of the catalyst quantities and the unprecedented unique properties of IrO2 catalysts.
This work was supported by a grant from the Ministry of Research, Korea and the Ministry of Science & Technology, Israel. This work was also supported by the Engineering Research Center Program from Korean National Research Foundation.