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Novel templating route using Pt infiltrated block copolymer micro-particles for catalytic Pt functionalized macroporous WO3 nanofibers and its application in breath pattern recognition Seon-Jin Choi, Kang Hee Ku, Bumjoon J. Kim, and Il-Doo Kim ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00422 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on September 8, 2016
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Novel templating route using Pt infiltrated block copolymer micro-particles for catalytic Pt functionalized macroporous WO3 nanofibers and its application in breath pattern recognition Seon-Jin Choi,†,‡,∥ Kang Hee Ku,§,∥ Bumjoon J. Kim,*,§ and Il-Doo Kim*,‡ †
Applied Science Research Institute and ‡Department of Materials Science and Engineering and §Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305–701, Republic ∥ of Korea. S.J.C and K.H.K contributed equally to this work. KEYWORDS Chemical sensors, electrospinning, WO3 nanofibers, block copolymer, catalyst, pattern recognition
ABSTRACT: We propose a new route for transferring catalysts onto macroporous metal oxide nanofibers (NFs) using metallic nanoparticles (NPs) infiltrated block copolymer submicron spheres as sacrificial templates. Pt decorated polystyrene-b-poly(4vinylpyridine) (PS-b-P4VP) copolymers microparticles (Pt-BCP MPs), produced from oil-in-water emulsions, were uniformly dispersed within electrospun PVP/W precursor composite NFs. The macropore-loaded WO3 NFs (macroporous Pt-WO3 NFs), which are additionally functionalized by Pt NPs (10 nm), were achieved by decomposition of polymeric components and oxidization of W precursor after high-temperature calcination. In particular, macropores with the similar size distribution (50–300 nm) with BCP MPs were also formed on interior and exterior of WO3 NFs. Chemical sensing performance of macroporous Pt-WO3 NFs was investigated for pattern recognition of simulated breath gas components at highly humid ambient (95% RH). The result revealed that superior hydrogen sulfide sensitivity (Rair/Rgas = 834.2 ± 20.1 at 5 ppm) and noticeable selectivity were achieved. In addition, H2S pattern recognition against other chemical components (acetone, toluene, and methyl mercaptan) was clearly identified without any overlapping of each pattern. This work demonstrates the potential application of BCP-templated maroporous Pt-WO3 NFs in exhaled breath analysis for non-invasive monitoring of physical conditions.
Block copolymers (BCPs) are intensively studied for the nanostructure self-assembly in wide range of research fields including nanolithography,1-4 biocompatible drug delivery,5, 6 flexible devices,7 and energy generators8, 9 by facilitating their ordered structures such as lamellae, cylinders, and other complex bicontinuous gyroid structures.10 Taking advantage of BCP selfassembly, well-organized thin film as well as various dimensional nanostructures have been demonstrated as the next generation functional devices.11-16 Recently, various functional nanoparticles (NPs) were selectively decorated on the BCP micro- and nano-structures by infiltration of metallic precursors and subsequent chemical reduction to form metallic NPs.17-20 For example, dot or fingerprint-like BCP microspheres decorated with inorganic Au and Pt NPs were demonstrated by infiltration of inorganic precursors such as HAuCl4·3H2O and HPt2Cl6·6H2O, respectively.20-22 Such metallic NP infiltrated BCP micro-and nanostructures are advantageous in consideration that these structures can be employed as sacrificial hard templates to form metallic NP-loaded metal oxide nanostructures. The potential utilization of functional metallic NPdecorated BCP structures was firstly demonstrated using zero dimensional (0D) microparticles by coating the surface with inorganic SnO2 thin-layer and subsequent calcination, resulting in
decomposition of BCP microparticle templates and transferring the NPs to the inner layers of SnO2 to form Au NP-loaded 0D SnO2 hollow microspheres.23 Beside 0D metal oxide spheres, 1D nanostructure is attractive considering its unique shape and network configuration for specific application. Furthermore, the surface morphology as well as composition of the 1D nanostructures can be carefully controlled using the metallic NP infiltrated BCP templates. For the morphological modification, Choi et al demonstrated porous 1D WO3 nanofibers (NFs) using polystyrene colloid templates during electrospinning process and subsequent calcination.24 However, compositional modification such as catalyst functionalization was not achieved due to the use of pure polymeric templates. Recently, metallic NP decorated polystyrene templates were introduced in the electrospinning process to functionalize catalytic NPs on the surface of 1D metal oxide NFs as well as generate pores.25-27 However, the catalytic NP decorated polystyrene templates exhibited non-uniform shape and distribution of the NPs. On the other hand, one of the remarkable candidates as hard template is metallic NP infiltrated BCP microparticles (MPs), because overall shape and surface structures can be controlled to generate high surface area (i.e. regular dot patterned raspberry-like shape), and various catalysts can be introduced densely into the desired loca-
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tion of the MPs. Although few reports proposed BCP as the templates to fabricate metal oxide nanostructures,28-30 single-step synthesis of macroporous 1D metal oxide NFs functionalized with catalytic NPs has yet been demonstrated using functional NP infiltrated BCP MPs, which can be potentially applicable in high performance sensors such as biomarker molecule sensing. In this work, for the first time, we proposed Pt NP infiltrated BCP microparticles (Pt-BCP MPs) to synthesize catalytic Pt functionalized macroporous 1D WO3 NFs assisted by electrospinning technique. Pt-BCP MPs were successfully synthesized by oil-inwater emulsion technique. The unique structure of Pt-BCP MPs exhibited well-defined surface morphologies and Pt catalysts were introduced periodically into the MPs. By utilizing Pt-BCP MPs as hard templates, we achieved well-distributed catalytic functionalization on the surface of metal oxide as well as macroscale pores after the electrospinning and subsequent thermal calcination. The focus of this study is to demonstrate the novel synthetic method of Pt-functionalized 1D macroporous WO3 NFs and investigate their selective pattern recognition of exhaled biomarker species for daily monitoring of physical conditions.
Experimental section Preparation of Pt-BCP MPs. Pt-decorated BCP MPs were fabricated through the oil-in-water emulsion technique, as described in our previous studies.20-23 The polystyrene-bpoly(4-vinylpyridine) (PS-b-P4VP, the number-average molecular weight (Mn) = 235 kg mol–1, fPS = 0.81, PDI = 1.18 from Polymer Sources, Inc.) copolymers were dissolved in chloroform to produce a 1 wt% polymer solution. The polymer solution (0.5 mL) was emulsified in DI water (4.5 mL) containing 1 wt% Pluronic F108 (PEO-b-PPO-b-PEO, 15 kg mol–1, Aldrich) using a homogenizer for 2 min at 25,000 rpm. The organic solvent was evaporated at 40 °C under 150 mbar for 20 min by rotary pump, and the sample was annealed at 95 °C for 24 h. Then, the sample was washed with DI water to remove remaining surfactants by repeated centrifugations performed at 10,000 rpm for 20 min. To load Pt into the P4VP domains, Pt precursor (K2PtCl4, Aldrich) solution was added to the BCP MP dispersion in a 1:1 molar ratio as compared to P4VP units. The mixture was stirred at room temperature for 24 h, and purified by washing with DI water and repeated centrifugations at 10,000 rpm for 20 min. The obtained Pt-BCP MPs were redispersed in DI water. Synthesis of 1D Pt-WO3 NFs. Macroporous WO3 NFs functionalized by Pt NPs were synthesized by electrospinning technique and sacrificial templating route. The electrospinning solution was prepared by dissolving 0.05 g of ammonium metatungstate hydrate [(NH4)6H2W12O40·xH2O] and 0.0625 g of PVP (Mw = 1,300,000 g mol–1) in 0.375 g of DI water. Then, Pt-BCP MPs were homogeneously dispersed in the electrospinning solution. The electrospinning was proceeded with the composite electrospinning solution at the flow rate of 2 mL min–1 using a syringe pump and a constant DC voltage of 15 kV between the nozzle (21 gauge) and the stainless steel foil, employed as grounded collector. After
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electrospinning, 1D nanofibrous composite structure comprising of W precursor/PVP decorated with Pt-BCP MPs was synthesized. The composite as-spun NFs were calcined at 500 °C for 1 h in ambient air to obtain Pt-WO3 NFs. To investigate the optimum gas sensing characteristics of Pt-WO3 NFs quantitatively, the concentration of Pt NPs on WO3 NFs was varied to be 0.008 wt%, 0.042 wt%, and 0.083 wt%, which was confirmed by inductively coupled plasma (ICP) analysis (Supporting Information, Figure S1). Pristine WO3 NFs were synthesized without Pt-BCP MP templates as a reference sample for gas sensing characterization. Characterization. Field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEOL 2000FX), and EDS measurements (JEOL) were used to observe the surface and internal structure of Pt-BCP MPs and Pt-WO3 NFs. The samples were prepared by drop-casting suspensions onto silicon wafers and TEM grids coated with a 20 nm thick carbon film, respectively. X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific K-alpha) analysis was performed to investigate chemical binding states of Pt-WO3 NFs. Fabrication of sensors. The Pt-WO3 NFs as well as pristine WO3 NFs were coated on sensor substrates. Sensor substrates were prepared using alumina (Al2O3) with the dimension of 2.5 mm × 2.5 mm and the thickness of 2 mm (Supporting Information, Figure S2a). Parallel Au electrodes were pattered on the front side of the alumina substrate for sensing electrodes. In addition, Pt electrodes were patterned at the back side of the alumina substrate for heating electrodes to control the temperature of sensors. To coat the sensing layers on the alumina substrates, a 3 mg of the calcined NFs was dispersed in a 50 µL of ethanol. For the homogeneous solution, the mixture solutions were sonicated for 5 min. Then, a 5 µL of the mixture solution was drop-coated on the sensor substrates on a hot plate at 60 °C using micropipette. After drying the solvent, all the sensors were transferred to the testing equipment for gas sensing characterization. The thickness of the sensing layer was approximately 121 µm, which was confirmed by SEM (Supporting Information, Figure S2b). Multilayered electrospun nanofibers were deposited on the sensor substrate (in the inset of Figure S2). Evaluation of gas sensing performance. The gas sensing characteristics of all the sensors were evaluated in a specialized gas sensor testing system described elsewhere.31, 32 The sensors were stabilized for 6 h in the baseline air. Considering the breath sensor application, the humidity level in the baseline air was maintained at 95% RH. The analyte gases such as hydrogen sulfide (H2S), acetone (CH3COCH3), toluene (C6H5CH3), and methyl mercaptan (CH3SH) were used as simulated breath biomarkers. The concentration of these analytes was controlled in the range of 0.1–5 ppm, mixed in the baseline air with the flow rate of 1000 sccm. Cyclic exposures of 10 min to the analyte gas followed by 10 min in the baseline air were performed. The resistance of the sensor was measured using a data acquisition system (34972A, Ag-
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ilent) with a 16 channel multiplexer (34902A, Agilent). The measured resistance was converted into the resistance ratio of Rair/Rgas (where Rair is the sensor resistance in the baseline air and Rgas is the resistance measured during exposure to the test gas) and the resistance ratio was defined as response. The operating temperatures of the sensors were controlled by applying voltage to the microheater at the back side of the sensor substrate using a DC power supply (E3647A, Agilent).
Results and discussion Scheme 1 presents illustrations of Pt decorated BCP microparticle (Pt-BCP MP) and synthetic process of Pt functionalized macroporous WO3 nanofibers (Pt-WO3 NFs) using electrospinning technique. Firstly, the Pt-BCP MPs were synthesized by an oil-water emulsion of polystyrene-b-poly(4-vinylpyridine) (PS-bP4VP) BCPs followed by infiltration of Pt precursor into the BCP MPs under acidic condition (Scheme 1a). The obtained Pt-BCP MPs were homogeneously dispersed in electrospinning solution comprising of polyvinylpyrrolidone (PVP) and W precursor dissolved in deionized (DI) water (Scheme 1b). The electrospinning was performed using the composite solution to form 1D nanofibrous structure (Scheme 1c). As a result, PVP/W precursor composite NFs were obtained with uniformly decorated Pt-BCP MPs (Scheme 1d). Finally, Pt-WO3 NFs were achieved after calcination to decompose polymeric compounds and oxidize W precursor (Scheme 1e). During the calcination process, macropores (>50 nm) were generated due to the decomposition of BCP MPs. In addition, Pt NPs were successfully transferred to the WO3 NFs after decomposition of the Pt-BCP MPs templates. The synthesized Pt-WO3 NFs were characterized to evaluate H2S sensing performance and selective pattern recognition of interfering components for breath analysis.
uniform surface morphologies of P4VP-Pt hybrid dots. During infiltration of Pt precursors under acidic conditions, P4VP domains in BCP MPs were selectively swollen toward water, and formed uniform arrangement of small hemispheres which were ordered on the tens of nanometer scale.20, 21, 33 The sizes of the PtBCP MPs range from 80 to 460 nm with an average size of 310 ± 28 nm. In addition, P4VP-Pt hybrid dots on the MPs exhibited the average size of 42 ± 5.1 nm. Furthermore, the size of Pt NPs on the BCP MPs was confirmed by high-resolution TEM (HRTEM), which showed the diameters in the range of 2–10 nm (Figure 1c). However, crystal structure of Pt NPs on BCP MPs could not be confirmed due to the amorphous nature of Pt NPs (Supporting Information, Figure S3). The compositional elemental mapping using energy dispersive X-ray spectroscopy (EDS) revealed carbon-based templates with uniform Pt decoration on BCP MPs (Figure 1d-f). The Pt-BCP MPs are effective templates because they provide single-step synthesis of porous 1D structure of WO3 NFs with catalytic Pt functionalization by catalyst transferring technique during the electrospinning and subsequent calcination processes.
Figure 1. (a) SEM and (b) TEM images of Pt-BCP MPs. (c) High resolution TEM (HRTEM) image of Pt-BCP MPs. (d) Magnified TEM image of Pt-BCP MPs. EDS elemental mapping images of Pt-BCP MPs for (e) C and (f) Pt.
Scheme 1. Schematic illustrations of (a) Pt decorated PS-b-P4VP microparticle (Pt-BCP MP), (b) composite electrospinning solution comprising of Pt-BCP MPs, W precursor, and PVP dissolved in DMF, (c) electrospinning of the composite solution, (d) as-spun Pt-BCP MP-loaded W precursor/PVP composite nanofibers (NFs), and (e) Pt functionalized macroporous WO3 nanofibers (Pt-WO3 NFs) after calcination.
Microstructural characterization. Microstructures of Pt-BCP MPs and Pt-WO3 NFs were observed using SEM and TEM. As shown in Figure 1a and b, raspberry-like MPs were produced with
The pristine WO3 NFs were firstly synthesized without addition of Pt-BCP MPs. Nanofibrous structure with smooth surface morphology was observed with the as-spun W precursor/PVP composite NFs with the average diameter of 977.9 nm (Figure 2a). The as-spun composite NFs were calcined to decompose PVP and oxidize W precursor, forming WO3 NFs (Figure 2b). Slightly reduced average diameter of 887.2 nm was observed due to the shrinkage of NFs during the calcination step. In contrast to the pristine as-spun composite NFs, rough surface morphology was observed for the Pt-BCP MP decorated W precursor/PVP composite NFs (Figure 2c), indicating homogeneous embedding of PtBCP MPs in the composite NFs during electrospinning. After calcination at high-temperature (500 °C, 1 h), rough surface morphology with open pores was formed on the surface of WO3 NFs (Figure 2d). The magnified SEM image clearly presents aggregated macroscale (50–300 nm) pores on the surface of WO3 NFs due to the decomposition of multiple BCP MPs (Figure 2e). TEM
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analysis revealed that the macroscale pores were formed not only on the surface, but also inside of the WO3 NFs (Figure 2f). The porous structure is advantageous considering its facile gas penetration into the sensing layer as well as enlarged surface area for gas reactions. In addition, catalytic Pt NPs on WO3 NFs, which were transferred from Pt-BCP MPs after calcination, can contribute to higher activation for the decomposition of analyte molecules as well as surface reactions. HRTEM image confirmed wellcrystalized WO3 structure with crystal planes of (002), (200), and (112), which correspond to the interplanar distances of 3.836 Å, 3.616 Å, and 3.069 Å (Figure 2g). Moreover, crystalline Pt (111) was observed with the interplanar distance of 2.26 Å. Selected area electron diffraction (SAED) pattern revealed the polycrystalline structure of Pt-WO3 NFs with the crystal planes of (020), (112, and (202), which are partially observed in HRTEM image (Figure 2h). However, crystal planes related to Pt NPs were not observed in the SAED pattern mainly due to the low content of Pt compared with the WO3 component. To confirm successful transfer of the Pt NPs along the WO3 NFs and examine the distribution of Pt NPs along the WO3 NFs, EDS analysis was performed. As shown in EDS elemental mapping images (Figure 2i), homogeneously distributed Pt components were clearly observed in the fibrous structure including W and O components.
Figure 2. SEM images of (a) as-spun W precursor/PVP composite NFs, (b) pristine WO3 NFs after calcination at 500 °C, (c) Pt-BCP MPs decorated as-spun W precursor/PVP composite NFs, (d) Pt-WO3 NFs after calcination at 500 °C, and (e) magnified SEM image of Pt-WO3 NFs. (f) TEM image of Pt-WO3 NFs. (g) HRTEM image of Pt-WO3 NFs. (h) Selected area electron diffraction (SAED) pattern of Pt-WO3 NFs, and (i) EDS elemental mapping of Pt-WO3 NFs.
Analyte sensing characterization. The analyte sensing performances were examined using the pristine WO3 NFs and PtWO3 NFs with various concentrations of Pt NPs in the range of 0.008–0.083 wt%, by tuning loading amounts of Pt-BCP MPs in WO3 NFs solution (Figure 3). The simulated analyte concentration was maintained in the range of 0.1–5 ppm at a highly humid
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ambient (95% RH) and the operating temperatures were controlled in the range of 250–450 °C). Figure 3a shows dynamic response transition of sensors toward H2S at 350 °C. The result revealed that unprecedentedly high response (Rair/Rgas) of 825.3 was observed with 0.042 wt% Pt functionalized macroporous WO3 NFs (hereafter, 0.042 wt% Pt-WO3 NFs) at 5 ppm, which was over 32-fold improved response as compared with response (Rair/Rgas= 25.3) of the pristine WO3 NFs. In addition, the 0.042 wt% Pt-WO3 NFs was the optimum condition for H2S sensing considering the decreased responses of 365.9 and 647.2 with 0.008 wt% and 0.042 wt% Pt functionalized WO3 NFs, respectively. To confirm the optimum H2S sensing characteristics, temperature dependent response properties were evaluated in the temperature range of 250–450 °C (Figure 3b). All the sensors using Pt-WO3 NFs exhibited the optimum operating temperature at 350 °C. In addition, all the sensors exhibited stable response characteristics according to narrow standard deviations to multiple H2S sensing evaluations. The highest response was observed with the 0.042 wt% Pt-WO3 NFs showing the average response of 834.2 ± 20.1. The minimum detectable H2S concentration, i.e., limit of detection (LOD), was investigated by measuring the responses at sub-ppm level of H2S concentrations (Supporting Information, Figure S4). The 0.042 wt% Pt-WO3 NFs exhibited very low LOD of 100 ppb with the noticeable response of 1.5. The selective detection properties of pristine WO3 NFs and 0.042 wt% Pt-WO3 NFs were investigated (Figure 3c). The 0.042 wt% Pt-WO3 NFs exhibited highly H2S selective sensing performance with superior response of 834.2 ± 20.1 with minor responses towards interfering analytes such as acetone (Rair/Rgas = 49.2 ± 0.6), methyl mercaptan (Rair/Rgas = 18.0 ± 0.8), and toluene (Rair/Rgas = 1). The outstanding H2S sensing properties of Pt-WO3 NFs were demonstrated by comparing with the recent studies using diverse catalyst functionalized WO3 composite sensing layers (Table 1). H2S sensing mechanism. The dramatically improved H2S sensing property is discussed based on catalytic effect. Generally, the basic sensing principle of SMO-based sensing material is resistivity changes depending on the concentrations of chemisorbed oxygen species, i.e., O2–, O2– and O–, on the surface of SMO. In the present study, WO3 NFs exhibiting n-type sensing property were utilized as a sensing material, where electrons are majority carriers. In the baseline air ambient, high resistivity of WO3 NFs was observed due to the formation of chemisorbed oxygen species, thickening the surface depletion layer by trapping electrons from the conduction band. When reducing analyte such as H2S is exposed to the sensor, decreasing resistance change was observed due to the surface reactions between the chemisorbed oxygen species and H2S molecules, which resulted in thinning of the surface depletion layer by releasing the trapped electrons back to the conduction band. Therefore, resistivity changes can be achieved by the modulation in thickness of surface depletion layer on WO3 NFs. As the resistivity changes occur by the surface reactions, sensing layer with large surface area is advantageous for high analyte sensitivity. In addition, porous structure can enhance the reaction sites on the surface of WO3 NFs, promoting facile diffusion into the sensing layer. In this regard, BCP MPs templates played a crucial role in forming macroporous WO3 NFs to improve H2S sensing performance.
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In addition, Pt NPs can dissociate oxygen molecules to generate large amount of chemisorbed oxygen species on WO3 NFs. The increased surface oxygen species can be confirmed by the enhanced baseline resistance of Pt-WO3 NFs (119.8 MΩ) compared with the baseline resistance (1.4 MΩ) of pristine WO3 NFs (Figure 4). For this reason, large resistivity changes can be achieved with Pt-WO3 NFs by effectively eliminating the surface oxygen species when H2S is injected to the sensor. Electronic sensitization can also enhance the resistance changes during the analyte sensing due to the effective electron transfer. Higher work function of Pt (5.65 eV)34 than WO3 (4.56 eV)35 can induce electron transfer from WO3 to Pt, thereby thickening the depletion layer of WO3 NFs. Therefore, large resistance changes can be achieved by thinning of the depletion layer when H2S gas is injected to the sensor. Furthermore, additional depletion layers can be formed between Pt catalyst and WO3 NFs due to the formation of p-n junction.36 As confirmed by XPS (Supporting Information, Figure S5), Pt NPs were partially oxidized during the calcination process leading to the formation of PtO and PtO2. These PtOx components exhibit p-type property, whereas WO3 shows n-type property. For this reason, increased depletion layer by the p-n junction formation can effectively enhance the resistance changes during the H2S exposure. The p-n junction formation and the increased resistance changes were also evidenced by dynamic resistance transition of Pt-WO3 NFs toward H2S (Figure 4). Based on all of these reasons, synergistic effect of chemical and electronic sensitizations can remarkably improve the H2S sensing performance of PtWO3 NFs. In addition to the sensing performance, stable sensing characteristics were obtained, considering its reversible H2S response with consistence recovery of baseline resistance (Figure 4).
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Figure 3. (a) Dynamic response transition in a concentration range of 1–5 ppm and (b) temperature dependent response transition in the temperature range of 250–450 °C of pristine WO3 NFs, 0.008 wt % Pt-WO3 NFs, 0.042 wt % Pt-WO3 NFs, and 0.083 wt % Pt-WO3 NFs. (c) Selective H2S detection property of pristine WO3 NFs and 0.042 wt% Pt-WO3 NFs toward interfering analytes at 5 ppm with the operating temperature of 350 °C.
Catalytic Pt NPs on WO3 NFs can further enhance the resistivity changes by chemical and electronic sensitization. During the chemical sensitization, analyte molecules can be dissociated by Pt NPs and transferred to the surface WO3 NFs, which is known as spill-over process, thereby accelerating the chemical reaction between the chemisorbed oxygen species and dissociated H2S.32
Figure 4. Dynamic resistivity changes of 0.042 wt% Pt-WO3 NFs and pristine WO3 NFs during the cyclic H2S exposures in the concentration range of 1–5 ppm at 350 °C.
Pattern recognition using simulated gases. To demonstrate pattern recognition of chemical species, we performed principal component analysis (PCA) using three different sensors, i.e., pristine WO3 NFs, 0.008 wt% Pt -WO3 NFs, and 0.042 wt% Pt-WO3 NFs, toward four different analytes (H2S, acetone, toluene, and methyl mercaptan) in the concentration range of 1–5 ppm (Figure 5). Interestingly, all four analytes were classified in a twodimensional space without overlapping. This result suggests that each analyte component can be distinguished clearly by pattern recognition using PCA. In particular, the two volatile sulfuric
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compounds (VSC), i.e., H2S and CH3SH, were clearly identified by PCA analysis. It has been known that H2S and CH3SH are closely related to the biomarkers of intra-oral halitosis.37 Therefore, accurate identification of VSC biomarkers can be possible for intra-oral halitosis to monitor physical condition and perform adequate medical treatment. Moreover, the pattern recognition was successfully achieved with only three sensors. In this regard, a number of different chemical species can be selectively patterned in the graphical region by establishing characteristic sensor arrays
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H2S because the concentration is the 100% recognizable level by an odor panel.37 Different response values were obtained using the sensor arrays, i.e., pristine WO3 NFs and 0.008 wt% Pt-WO3 NFs. (Supporting Information, Figure S6). In addition, classification between the healthy human breath and simulated halitosis breath was successfully achieved by PCA, which demonstrated the potential application of Pt-WO3 NFs for diagnosis of halitosis by breath analysis (Figure 6).
Conclusions
Figure 5. Principal component analysis (PCA) for pattern recognition of analyte components such as H2S, acetone, toluene, and methyl mercaptan using pristine WO3 NFs, 0.008 wt% Pt-WO3 NFs, and 0.042 wt% Pt-WO3 NFs. Camera image of the three sensors in the inset.
In this work, we developed a novel single-step synthetic method for producing macroporous WO3 NFs functionalized with Pt NPs using sacrificial templates of Pt-BCP MPs during the electrospinning process. The raspberry-shaped Pt-BCP MPs with densely packed Pt NPs were introduced in the electrospinning solution to form open pores on the WO3 NFs as well as functionalize Pt catalyst after subsequent calcination process. The Pt NPs were successfully transferred to the macroporous WO3 NFs during the decomposition of BCP MPs. As a result, dramatically improved H2S sensing property was observed with unprecedentedly high response of 834.2 ± 20.1 at 5 ppm in a humid ambient (95% RH) using 0.042 wt% Pt-WO3 NFs. In addition, highly selective H2S sensing property was obtained with minor responses (Rair/Rgas
