Hierarchical SnO 2 fi bers assembled from wrinkled thin tubes are synthesized by controlling the microphase separation between tin precursors and polymers, by varying fl ow rates during electrospinning and a subsequent heat treatment. The inner and outer SnO 2 tubes have a number of elongated open pores ranging from 10 nm to 500 nm in length along the fi ber direction, enabling fast transport of gas molecules to the entire thin-walled sensing layers. These features admit exhaled gases such as acetone and toluene, which are markers used for the diagnosis of diabetes and lung cancer. The open tubular structures facilitated the uniform coating of catalytic Pt nanoparticles onto the inner SnO 2 layers. Highly porous SnO 2 fi bers synthesized at a high fl ow rate show fi ve-fold higher acetone responses than densely packed SnO 2 fi bers synthesized at a low fl ow rate. Interestingly, thin-wall assembled SnO 2 fi bers functionalized by Pt particles exhibit a dramatically shortened gas response time compared to that of un-doped SnO 2 fi bers, even at low acetone concentrations. Moreover, Pt-decorated SnO 2 fi bers signifi cantly enhance toluene response. These results demonstrate the novel and practical feasibility of thin-wall assembled metal oxide based breath sensors for the accurate diagnosis of diabetes and potential detection of lung cancer.

We report on the heterogeneous sensitization of metal-organic framework (MOF)-driven metal-embedded metal oxide (M@MO) complex catalysts onto semiconductor metal oxide (SMO) nanofibers (NFs) via electrospinning for markedly enhanced chemical gas sensing. ZIF-8-derived Pd-loaded ZnO nanocubes (Pd@ZnO) were sensitized on both the interior and the exterior of WO NFs, resulting in the formation of multiheterojunction Pd-ZnO and ZnO-WO interfaces. The Pd@ZnO loaded WO NFs were found to exhibit unparalleled toluene sensitivity (R/R = 4.37 to 100 ppb), fast gas response speed (∼20 s) and superior cross-selectivity against other interfering gases. These results demonstrate that MOF-derived M@MO complex catalysts can be functionalized within an electrospun nanofiber scaffold, thereby creating multiheterojunctions, essential for improving catalytic sensor sensitization.

Sensitive detection of acetone and hydrogen sulfide levels in exhaled human breath, serving as breath markers for some diseases such as diabetes and halitosis, may offer useful information for early diagnosis of these diseases. Exhaled breath analyzers using semiconductor metal oxide (SMO) gas sensors have attracted much attention because they offer low cost fabrication, miniaturization, and integration into portable devices for noninvasive medical diagnosis. However, SMO gas sensors often display cross sensitivity to interfering species. Therefore, selective real-time detection of specific disease markers is a major challenge that must be overcome to ensure reliable breath analysis. In this work, we report on highly sensitive and selective acetone and hydrogen sulfide detection achieved by sensitizing electrospun SnO2 nanofibers with reduced graphene oxide (RGO) nanosheets. SnO2 nanofibers mixed with a small amount (0.01 wt %) of RGO nanosheets exhibited sensitive response to hydrogen sulfide (Rair/Rgas = 34 at 5 ppm) at 200 °C, whereas sensitive acetone detection (Rair/Rgas = 10 at 5 ppm) was achieved by increasing the RGO loading to 5 wt % and raising the operation temperature to 350 °C. The detection limit of these sensors is predicted to be as low as 1 ppm for hydrogen sulfide and 100 ppb for acetone, respectively. These concentrations are much lower than in the exhaled breath of healthy people. This demonstrates that optimization of the RGO loading and the operation temperature of RGO-SnO2 nanocomposite gas sensors enables highly sensitive and selective detection of breath markers for the diagnosis of diabetes and halitosis.

Thin-walled WO(3) hemitubes and catalytic Pt-functionalized WO(3) hemitubes were synthesized via a polymeric fiber-templating route and used as exhaled breath sensing layers for potential diagnosis of halitosis and diabetes through the detection of H(2)S and CH(3)COCH(3), respectively. Pt-functionalized WO(3) hemitubes with wall thickness of 60 nm exhibited superior acetone sensitivity (R(air)/R(gas) = 4.11 at 2 ppm) with negligible H(2)S response, and pristine WO(3) hemitubes showed a 4.90-fold sensitivity toward H(2)S with minimal acetone-sensing characteristics. The detection limit (R(air)/R(gas)) of the fabricated sensors with Pt-functionalized WO(3) hemitubes was 1.31 for acetone of 120 ppb, and pristine WO(3) hemitubes showed a gas response of 1.23 at 120 ppb of H(2)S. Long-term stability tests revealed that the remarkable selectivity has been maintained after aging for 7 months in air. The superior cross-sensitivity and response to H(2)S and acetone gas offer a potential platform for application in diabetes and halitosis diagnosis.

The increase of surface area and the functionalization of catalyst are crucial to development of high-performance semiconductor metal oxide (SMO) based chemiresistive gas sensors. Herein, nanoscale catalyst loaded CoO hollow nanocages (HNCs) by using metal-organic framework (MOF) templates have been developed as a new sensing platform. Nanoscale Pd nanoparticles (NPs) were easily loaded on the cavity of Co based zeolite imidazole framework (ZIF-67). The porous structure of ZIF-67 can restrict the size of Pd NPs (2-3 nm) and separate Pd NPs from each other. Subsequently, the calcination of Pd loaded ZIF-67 produced the catalytic PdO NPs functionalized CoO HNCs (PdO-CoO HNCs). The ultrasmall PdO NPs (3-4 nm) are well-distributed in the wall of CoO HNCs, the unique structure of which can provide high surface area and high catalytic activity. As a result, the PdO-CoO HNCs exhibited improved acetone sensing response (R/R = 2.51-5 ppm) compared to PdO-CoO powders (R/R = 1.98), CoO HNCs (R/R = 1.96), and CoO powders (R/R = 1.45). In addition, the PdO-CoO HNCs showed high acetone selectivity against other interfering gases. Moreover, the sensor array clearly distinguished simulated exhaled breath of diabetics from healthy people's breath. These results confirmed the novel synthesis of MOF templated nanoscale catalyst loaded SMO HNCs for high performance gas sensors.