Superhydrophilic, Underwater Superoleophobic, and Highly

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24533−24543

Superhydrophilic, Underwater Superoleophobic, and Highly Stretchable Humidity and Chemical Vapor Sensors for Human Breath Detection Xuewu Huang,† Bei Li,† Ling Wang,† Xuejun Lai,‡ Huaiguo Xue,† and Jiefeng Gao*,†,§ †

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225002, China College of Materials Science and Engineering, Key Laboratory of Guangdong Province for High Property and Functional Polymer Materials, South China University of Technology, Guangzhou 510641, P. R. China § State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R. China

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S Supporting Information *

ABSTRACT: Humidity and chemical vapor sensors have promising applications in the field of environment protection, human healthcare, and so forth. It is still challenging to develop sensor materials that can serve as both humidity and chemical vapor sensors with high sensitivity, low detection limit, and excellent stretchability, repeatability, and reliability. In this study, a flexible, stretchable, and conductive nanofiber composite (CNC) with superhydrophilicity and underwater superoleophobicity is prepared by acidified carbon nanotube (ACNT) decoration onto the thermoplastic polyurethane (PU) nanofiber surface. ACNT introduction increases both the Young’s modulus and tensile strength and almost maintains the superelasticity of the PU nanofibrous membrane. The as-obtained CNC could be used to detect both moisture and chemical vapors. When used as the humidity sensor, ACNTs can absorb surrounding water molecules and thus increase their resistance. On the other hand, the PU can be swollen by different chemical vapors, which can, to a different extent, damage the conductive network inside the composite and cause the increase of the composite resistance. The CNC can be integrated with a mask for real-time detection of human respiration. The CNC-based chemical vapor sensor possesses low detection limit, quick response, good selectivity, and excellent recyclability (even in a high humid environment) and has potential applications in monitoring biomarker gases from human breath. KEYWORDS: superhydrophilic, underwater superoleophobicity, electrically conductive, nanofiber composite, humidity sensor, chemical vapor sensor

1. INTRODUCTION Humidity and chemical vapor sensors have wide applications in many fields such as flexible electronics, environment protection, and respiratory diseases diagnosis.1−8 Metal oxides such as copper oxide and tungsten trioxide have been used as high-performance gas sensors,9−13 and the mechanism is based on the charge transfer caused by the chemical reactions between the target gases and adsorbed oxygen species that can finally lead to the change of sensor resistance.14,15 Twodimensional graphene oxide (GO) is often used to prepare the humidity sensor because it can absorb surrounding water moisture. The oxygen-containing functional groups onto the GO surface react with water molecules, resulting in variation of its electrical conductivity.16−18 However, the metal oxide and GO are in the form of powders and usually coated to a specific substrate by dip coating or spin coating.19−21 In many cases, the weak interaction between the sensing materials and the substrate causes unstable sensing performance. To address this © 2019 American Chemical Society

issue, the electrically conductive nanofillers are usually incorporated into polymers to prepare nanocomposite-based humidity or chemical vapor sensors with good flexibility.22−25 A typical example is the nanocomposite composed of polyvinyl alcohol (PVA) and nanofillers such as carbon nanotubes (CNTs) and GO. Zhou et al.26 employed a wet-spinning method to fabricate CNT/PVA filaments, whose conductivity can be tuned by varying the CNT concentration in the composite. When used as the humidity sensor, the filament displayed a quick and large increase in resistance after exposure to the moisture. The water molecules were absorbed into the filament, making the PVA partially swollen. As a result, the distance between the CNTs increases, giving rise to great increase of the resistance. In terms of the chemical vapor Received: March 11, 2019 Accepted: June 18, 2019 Published: June 18, 2019 24533

DOI: 10.1021/acsami.9b04304 ACS Appl. Mater. Interfaces 2019, 11, 24533−24543

Research Article

ACS Applied Materials & Interfaces

respiration. Also, the CNC can detect various chemical vapors (even in a high humidity environment) with a quick response and low detection limit. The stretchable CNC-based humidity and chemical vapor sensor has potential application in detection of people’s respiration, biomarker gases from human breath, and toxic gases leakage.

sensing performance, the adsorption of the chemical vapors could cause the matrix swollen and thus damage the conductive network of the composite, leading to the resistance increase. On the contrary, the de-adsorption of these vapors makes the conductive network recovery, and the resistance tends to decrease to its original value.27 Although PVA is sensitive to the humidity, because of its excellent hydrophilicity, its conductive composite is easily swollen by the water vapor, losing the size stability and hence the durability especially in the practical use. Therefore, the polymer composite should be hydrophilic and hygroscopic but cannot be swollen or even dissolved in the moisture. It is also desired that the conductive composite is sensitive to the chemical vapor, that is, the composite serves as both the humidity and chemical vapor sensors, which have wide applications in environment protection, human healthcare, and so on.28−30 For instance, there are different kinds of volatile organic compounds in human breath, known as the biomarkers, which can provide important information for the diagnosis of the diseases.31,32 It is quite important if the sensor materials can detect the chemical vapors in a high humid environment.33,34 To facilitate the adsorption of water or the chemical vapors, the interconnected porous structure in the composite is usually required, and the vapors can quickly de-adsorb after the composite is removed from the vapor environment, ensuring good recyclability. Also, the polymer composite should be able to sustain the mechanical deformation especially in the application of wearable devices that require sufficient flexibility, stretchability, and skin affinity.35−41 To this end, nanofibrous composite membrane is a good candidate for high performance humidity and chemical vapor sensors because of its large specific surface area and unique porous structure. The nanofiber composite is usually fabricated by mixing the nanofillers with the polymer solution, followed by electrospinning. However, the nanofillers are easily aggregated, which is not only detrimental to the smooth electrospinning but can also decrease the nanofibrous membrane mechanical properties. In addition, the nanofillers are wrapped by an insulating polymer, thereby deteriorating the sensing performance of the nanofiber composite. Up to now, it is still challenging to prepare a nanofiber composite with relatively low nanofiller concentration, which can be used as both humidity and chemical vapor sensors possessing excellent flexibility, stretchability, durability, and recyclability. Herein, we propose a facile and simple method to prepare superhydrophilic, stretchable, and electrically conductive nanofiber composite (CNC) based on the ultrasonicationinduced acidified CNT (ACNT) decoration onto the surface of the thermal plastic polyurethane (PU) nanofibers. The ACNT decoration endows the porous membrane with superhydrophilicity and underwater superoleophobicity, which can be maintained even after numerous cyclic tests. The ACNTs introduction greatly increases both the tensile strength and Young’s modulus while still maintaining outstanding stretchability. The obtained CNC can be used to detect both the humidity and chemical vapors, which is never reported in the previous literature. The CNC displays excellent sensitivity in a broad relative humidity (RH) range from 11 to 95% and excellent recyclability and reliability. As a proof of concept, the CNC-based humidity sensor is integrated with a mask for real-time detection of human respiration, providing accurate respiration signals such as rate and depth of

2. EXPERIMENTAL SECTION 2.1. Materials. PU, a thermoplastic elastomer, was purchased from Shanghai Kaisheng Plastic Co., Ltd. Different organic reagents, including N,N-dimethylformamide (DMF), tetrahydrofuran (THF), n-hexane, chloroform, n-hexadecane, methylbenzene, were provided by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Multiwalled CNTs (purity > 99%, 10−20 mm in length and 10−30 nm in diameter) were obtained from Hengqiu Science and Technology Co., Ltd., Suzhou, China. Sulphuric acid (98%), nitric acid (98%), absolute ethyl alcohol, lithium chloride, magnesium chloride, sodium bromide, and potassium sulphate were also received from Sinopharm Chemical Regent Co., Ltd. Diesel oil was obtained from China Petrochemical Group Co., Ltd. Deionized water was home-made in our laboratory. High purity nitrogen gas was supplied by Nanjing Special Gas Co., Ltd. All of the reagents and materials were used without further purification. 2.2. Acidification of CNTs. First, 2.0 g of CNTs was mixed with 30 mL of HNO3 and 120 mL of H2SO4 in a 250 mL three-neck flask, and then the flask was placed in an ultrasonic bath for 30 min to obtain uniformly dispersed CNTs. Next, the flask was transferred into an oil bath, and the solution was subject to reflux at 60 °C with vigorous stirring. After reaction for 4 h, the CNT solution was cooled down to room temperature in an ice-water bath. Then deionized water was added into the solution, which then experienced vigorous stirring in the ice-water bath. Subsequently, the diluted solution was centrifuged to obtain solid residues which was repeatedly washed with deionized water until the supernatant liquid became neutral. After being dried in a vacuum oven at 80 °C for 24 h, the ACNTs were finally obtained. 2.3. Fabrication of the PU Nanofiber Membrane. A PU nanofiber mat was prepared by electrospinning a PU solution with a concentration of 14 wt %. Specifically, a certain number of PU pellets were dissolved in a mixture solvent containing DMF and THF with the weight ratio of 4:1. Subsequently, the mixture was magnetically stirred at 60 °C for 6 h to obtain a homogeneous solution. Then, the solution was sucked into a plastic syringe that was connected with a stainless needle (22G) for electrospinning. The rotating drum was grounded and covered with an aluminum foil to collect the electrospun nanofibers. The distance between the top of the needle and the aluminum foil was 12 cm, and the applied voltage was 15.0 kV. The feeding rate was controlled at 1 mL h−1, and the temperature and humidity were 25 °C and 40%, respectively. 2.4. Preparation of a Conductive PU/ACNTs Nanofiber Mat. First, ACNTs were dispersed in a mixed solution composed of water and ethanol with a volume ratio of 1:4. The concentration of ACNTs was 1 mg mL−1. Then, the suspension experienced ultrasonication (20 kHz, 950 W at 20% efficiency) (JY98-IID, Ningbo Scientz Biotechnology Co., Ltd., Ningbo, China) for 30 min. The as-prepared electrospun PU nanofiber mat was immersed into the ACNT solution that subsequently underwent ultrasonication for different times, during which some ACNTs in the solution were gradually decorated onto the nanofiber surface. In order to prevent the quick evaporation of water and ethanol during the ultrasonication, the suspension was placed into an ice-water bath. After rinsing with ethanol and deionized water, the PU nanofiber composite mat was obtained after dried in an oven at 60 °C for 6 h. For convenience of description, CNC-X is used to represent the nanofiber composite prepared under different ultrasonication times (X, min). Unless otherwise specified, the nanofiber composite used for characterization and sensing performance test is CNC-30. 2.5. Humidity Sensing and Chemical Vapor Sensing Measurement. The humidity sensing performance of the as24534


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ACS Applied Materials & Interfaces prepared hydrophilic membrane was measured by a homemade testing apparatus. The membrane with a rectangle shape (length × width: 2 cm × 1 cm) was hung in a sealed conical flask, and the two ends of the membrane were connected with two copper wires by using the silver paste. The two copper wires were connected with a high resistance meter to record the resistance variation. The detailed information about the humidity sensing test is schematically shown in Scheme 1. Before all the sensing tests, the valves 1 and 4 are open,

3. RESULTS AND DISCUSSION Figure 1a shows the schematic demonstration for the fabrication of the conductive and stretchable nanofibrous

Scheme 1. Home-Made Apparatus for Humidity and Chemical Vapor Sensing Measurement

Figure 1. (a) Schematic demonstration of preparation of the CNC and the sensing mechanisms for humidity and chemical vapor sensors, respectively. (b) Photograph of the as-prepared CNC-30 with excellent flexibility and one water droplet on the CNC surface (inset); (c) photograph of the CNC-30 and water droplets on the CNC surface (inset) under different strain; (d) SEM image of CNC30. (e) Cross section of the SEM image of the CNC-30.

while the valves 2 and 3 are closed, and the air in the conical flask containing the sample was exhausted by the high purity nitrogen. For the humidity sensing test, the valves 2, 3, and 4 are open (valve 1 is closed), and the high purity nitrogen flowed into different saturated salt solutions and carried the moisture to the sample chamber, namely, the conical flask. The different humidity was achieved by choosing different saturated solutions of lithium chloride, magnesium chloride, sodium bromide, sodium chloride, and potassium sulphate, corresponding to the RH of 11, 33, 59, 75, and 95%, respectively.42,43 To remove the moisture in the flask, the valves 2 and 3 are closed, while the valves 1 and 4 are open. For the chemical vapor sensing test, all the valves were closed, and a certain amount of volatile solvent was dripped into the conical flask from the inlet throat by using a pipette. Once the solvent had been fed into the flask, the inlet throat was immediately closed, and the solvent was quickly evaporated to the vapors. After a certain time, the chemical vapor is excluded out of the flask by opening the valves 1 and 4. The sensing behavior of the nanofiber composite in a mixed moisture and chemical vapors was measured by combining the two abovementioned testing procedures. Note that the variation of resistance is in-situ recorded by the resistance meter during all the sensing tests. 2.6. Characterization. The surface morphology of the PU nanofiber mat and its composite was characterized by field emission scanning electronic microscopy (SEM, Zeiss_Supra 55, Germany) operated at 5 kV. All of the samples were spurted with a layer of gold to increase the surface conductivity; it prevents discharging during the SEM test. The morphology of the specimens was also observed by a transmission electron microscope (Tecnai 12, Philips, Holland) with an accelerating voltage of 20 kV. The functional groups of the nanofiber composite were characterized by a microscopic infrared spectrometer (Cary 610/670, Varian, USA) in an attenuated total reflection mode with the wavenumber from 400 to 4000 cm−1. The Raman spectrum was conducted by a laser confocal Raman spectrometer (inVia, Renishaw, Britain) in the range of 400−4000 cm−1. The electrical conductivity of the nanofiber composite was measured by a four-probe resistance meter (RTS-9, Guangzhou, China), and five different places of each specimen were tested in order to obtain the average conductivity. The contact angles (CAs) were measured by using an optical CA measuring device (OCA 40, DataPhysics, Germany), and at least three different places on the sample surface were chosen for the measurement. The mechanical properties of the nanofiber composites with a dumbbell shape (20 × 4 × 0.5 mm3) were tested by using an electronic universal testing machine (Instron Co. Ltd., USA). The test was operated at room temperature, and the crosshead speed was fixed at 100 mm min−1.

membrane with superhydrophilicity and underwater superoleophilicity. The ACNTs were anchored onto to the electrospun PU nanofiber surface under the assistance of ultrasonication, and both the interfacial sintering and hydrogen bonding between the viscoelastic elastomer nanofiber and the ACNTs are responsible for the CNT decoration.44−46 The obtained CNC can serve as both humidity and chemical vapor sensors. When used as the humidity sensor, ACNTs can absorb the surrounding water molecules, causing the electrons transfer from H2O to ACNTs. As a result, the holes in the ACNTs that are responsible for the conduction are reduced, which leads to the increase of their resistance. On the other hand, the PU can be swollen by different chemical vapors, which can, to a different extent, damage the conductive network inside the composite and cause the increase of the composite resistance. Note that the resistance can return to its original value after the water or chemical vapor is de-adsorbed from the materials. The polymer nanofibers could keep the structure integrity under the continuous impact from the ACNTs because the ultrasonication-driven ACNT decoration is an instantaneous process. The pristine PU nanofiber mat is white and the nanofiber surface is smooth (Figure S1). After decorated by ACNTs, the nanofiber mat turns to black. Fortunately, the obtained CNC possesses excellent flexibility and stretchability (Figure 1b,c), which is inherited from the superelastic PU nanofibrous membrane. The nanofiber composite possesses a nanofiber core and ACNT shell, enhancing the nanofiber surface roughness (Figure 1d,e). The abundant oxygen-containing groups on the ACNT surface and the hair-like structure on the nanofiber surface greatly improve the wettability of membrane, that is, the CNC becomes almost superhydrophilic (the inset in Figure 1b,c). At the same time, the ACNT-decorated nanofibers become conductive, and they connect each other to produce numerous 24535


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Figure 2. FTIR spectra (a) and Raman spectra (b) of PU nanofiber mat and its composite.

Figure 3. (a) Variation of static water CA and underwater oil (chloroform) CA and (b) underwater CAs of different oils for the CNC prepared under different ultrasonication time. (c) Variation of the static water CAs and underwater oil (chloroform) CAs of the CNC-30 under different strains. (d) Typical stress−strain curves of the PU nanofiber mat and its composites.

the hydrogen bonding between the hydroxyl or carboxyl of ACNTs and the N−H groups of PU. The Raman spectrum could be used to characterize graphitization of the carbon materials. As shown in Figure 2b, raw CNT shows a band at 1340 cm−1 (D band), corresponding to the disorder structure, while the two bands at 1583 cm−1 (G) and 2689 cm−1 (2D) are attributed to the in-plane C−C bond stretching. The D band and G band for ACNTs upshift to 1350 and 1591 cm−1, respectively. On the other hand, the 2D band shifts from 2689 cm−1 down to 2677 cm−1, indicating that the microstructure may be partially damaged after the acidification. For the CNC, the characteristic absorption peaks of PU at 2930 and 1616 cm−1 disappear, while absorption peaks at 2661, 1587, and 1350 cm−1 are present. Also, the G band and 2D band of ACNTs shift by 4 and 16 cm−1 toward the low frequency, implying certain interaction between PU and ACNTs. Generally, the intensity ratio of the D peak to G peak (ID/ IG) is used to quantify the graphitization. For raw CNTs, the value of ID/IG is 0.99, while it becomes 1.08 for ACNTs, corresponding to a decreased graphitization caused by the damaged sp2 structure of the carbon.51 In other words, surface defects were introduced into the CNTs during the progress of acidification. After being anchored onto the PU nanofibers, the

conductive pathways, forming a conductive network inside the composite. The functional groups of the ACNTs as well as the interaction between the ACNTs and the polymer nanofibers are investigated by Fourier transform infrared (FTIR) and Raman spectrum, as displayed in Figure 2. For raw CNTs, the peaks at 1646 and 1395 cm−1 should be attributed to the C C bond and O−H bending vibration, respectively. Three new peaks located at 3436, 1716, and 1042 cm−1 are present for ACNTs, and they are assigned to the stretching vibration of the O−H band, CO band, and C−O band, respectively, suggesting the successful introduction of −COOH and −OH onto the CNT surface.47,48 For the PU nanofiber membrane, a weak peak at 3334 cm−1 represents the characteristic N−H stretching band of urethanes. In addition, peaks at 1716 and 1528 cm−1 belong to the vibration adsorption peaks of the free and hydrogen-bonded carbonyl groups in the urethane linkage (−H−N−COO−). Moreover, the N−H in-plane bending vibration is verified from the peak at 1595 cm−1, and the peak at 1532 cm−1 is associated with the combination of N−H bending and C−H stretching.49,50 After ACNTs are anchored onto the PU nanofiber surface, no obvious peak shift is observed, except that the characteristic peaks of 3334 cm−1 for the PU nanofiber shifts to 3326 cm−1 for the CNC, indicating 24536


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ACS Applied Materials & Interfaces ACNTs possess a decreased ID/IG of 1.15, presumably because of the increased defects induced by the ultrasonication. As mentioned, ACNTs are gradually assembled on the nanofiber surface, and its density and thus electrical conductivity could be controlled by the ultrasonication time. Naturally, more ACNTs would be anchored onto the PU nanofiber surface with prolonging ultrasonication time. At a short ultrasonication time of 0.5 min, a small number of ACNTs are present on the nanofiber surface (Figure S1a), while the nanofiber surface is almost fully covered by the ACNTs at the ultrasonication of 5 min (Figure S1b). With further increase of the ultrasonication time, many entangled ACNTs are observed on the nanofiber surface (Figure S1c). Figure S1d shows the conductivity of the CNC as a function of the ultrasonication time. It can be found that the conductivity increased from 5.5 × 10−7 S m−1 for CNC-1 to 0.21 S m−1 for CNC-10 and 7.44 S m−1 for CNC-40, respectively. The influence of the ultrasonication time on the material wettability including the CA and underwater oil CA (OCA) is displayed in Figure 3. The CA decreases from about 30° for CNC-5 to 15° for CNC-30, while the underwater OCA increases from 157.7 ± 0.9° to 159.7 ± 1.8°, respectively (Figure 3a). Here, the wettability is strongly related with the nanofiber surface topology, and the hierarchical structure-induced high surface roughness is beneficial to the enhancement of surface wettability (both hydrophilicity and underwater hydrophobicity). The CNC possesses excellent underwater superoleophobic behavior, and the underwater OCAs for various oils such as toluene, chloroform, and hexane exceed 150°, as exhibited in Figure 3b. For example, the underwater OCAs of CNC-30 can reach 159.7 ± 1.8°, 153.6 ± 1.8°, 153.5 ± 0.8°, and 153.1 ± 2.0° for chloroform, toluene, hexadecane, and hexane, respectively. It is worth noting that the underwater superoleophobicity could be maintained during stretching, and they are kept at around 155° when the strain varies from 0 to 100% (see the blue curve in Figure 3c), displaying outstanding reliability and durability. Interestingly, the water CA shows continuous decrease with the strain. The CA declines from the original 15° to 10° at the strain of 10%, exhibiting superhydrophilicity. The CA further decreased to be 0° when the strain is beyond 50%, as shown in the red curves in Figure 3c, that is, the CNC can be quickly wetted and the water is easily diffused into the CNC. The strain-induced superhydrophilicity may be attributed to the fact that the stretching creates a more hierarchical microstructure that is also responsible for the maintenance of underwater superoleophobicity. The detailed discussion about the microstructure evolution of the nanofiber composite under different strain will be discussed in the following section (Figure 4). Apart from the enhancement of conductivity and surface wettability, the ACNT introduction can improve the mechanical properties of the polymer nanofibrous membrane. Figure 3d shows the typical stress−strain curves of the polymer nanofiber mat and its composite. Compared with the pristine PU nanofiber mat, the nanofiber composites possess both enhanced tensile strength and Young’s modulus and still display excellent stretchability, although the elongation at break slightly declines. The detailed mechanical properties are summarized in Table S1. The average Young’s modulus values of CNC-10 and CNC-30 are 2.55 and 2.14 MPa, respectively, which are 1.48 and 1.24 times higher than those of the PU nanofiber membrane (1.72 MPa). The CNC-10 possesses a higher tensile strength than CNC-30 (10.41 and 9.00 MPa,

Figure 4. SEM images of CNC-30 under different strains: 0 (a), 30 (b), 50 (c), and 100% (d).

respectively). Because the ACNT decoration is completed under ultrasonication, the ACNTs are finally uniformly distributed on the nanofiber surface. There also exists the hydrogen bonding between the hydroxyl groups of ACNTs and the amide groups of PU. ACNTs’ uniform distribution and strong interfacial interaction promote the effective stress transfer, leading to the improved tensile strength and Young’s modulus. Furthermore, the ACNT-decorated nanofiber tends to align along the stretching direction (see the red arrows in Figure 4c,d), and the interfacial contact area is hence increased, which also contributes to the enhancement of the mechanical property. In addition, many ACNTs ends are exposed after the stretching, as shown from the blue circles in Figure 4c,d. The protruded ACNTs ends could, to a certain extent, increase the nanofiber surface roughness, giving rise to decrease of the CA and increase of the underwater OCAs. As known, the raw CNTs display some hydrophobicity. After acidification, oxygen-containing groups, like hydroxyl and carboxyl groups, were successfully grafted onto the CNT surface. These active sites endow the CNTs with hydrophilicity and are thus able to absorb surrounding water molecules. There exists weak hydrogen bonding between the adsorbed water molecules and the oxygen-containing groups, and the conductivity of ACNTs is accordingly changed. Figure 5 shows the sensing performance of the CNC-30 under different RH. Obviously, periodical and repeatable curves are present during the cyclic sensing test. In each cyclic test, the resistance increases when the material is exposed to moisture, while it decreases and tends to return to its original value after the moisture is removed. It is also found that the response intensity (RI = ΔR/R0 (%), where ΔR is the resistance variation relative to the original value R0) increased with the increase of RH. The RI is 5, 11, 16, 20, and 29% with the RH of 11, 33, 59, 75, and 95%, respectively. The humidity sensing mechanism has been schematically demonstrated in Figure 1a. Specifically, most of the carbon atoms in ACNTs possess three σ bands and one additional π band by adopting a sp2 hybridization. The formed π bands play a significant role in determining the electrical conductivity of ACNTs. When the H2O contacts the C atom of ACNTs, its electron cloud shifts toward the C atom. This phenomenon macroscopically presents that the electrons transfer from H2O molecules to ACNTs. It has been reported that there would be 0.3 e− transferring from H2O to CNTs when a H2O molecule was 24537


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Figure 5. Humidity test of CNC at the RH of 11 (a), 33 (b), 59 (c), 75 (d), and 95% (e). (f) Resistance response intensity of the CNC at different RH.

Figure 6. Humidity sensing performance of the CNC under different strains: (a) 0, (a) 30, (b) 50, and (c) 100% at a fixed RH of 59%. (d) RI of the CNC under different strain.

absorbed on the CNTs.52−54 In most instances, a CNT is a typical p-type semiconductor, and its conductivity originates from the “hole conduction”. Thus, the electrons transferred from H2O molecules would neutralize part holes of ACNTs, reducing the quantity of holes, which would finally lead to the increase of the resistance of the CNC. Note that the swelling of PU nanofiber caused by water could be ignored because of the great difference of the solubility parameter between H2O (δ = 47.9 J1/2 cm−2/3) and PU (δ = 20.5 J1/2 cm−2/3). At a low RH, a small quantity of water molecules was absorbed on the ACNTs, which causes transfer of a small number of electrons and thus slight increase of the resistance of the CNC. With the increase of the RH, ACNTs can capture more H2O molecules, corresponding to a continuously enhanced RI. In practical application, the humidity sensor was inevitably subjected to external forces such as stretching and bending, and it is desirable that the humidity sensors work under

mechanical deformations with a stable signal output. As mentioned above, the as-obtained superhydrophilic CNC possesses excellent stretchability. Figure 6 shows the resistance response of the CNC under different strains at a fixed RH of 59%. A stable and recyclable resistance response is observed, and the RI of the nanofiber composite increases with the strain. As discussed before, the strain could induce the enhancement of the hydrophilicity, and CA drops from around 15° to almost 0° when the CNC is stretched by 50%. As a consequence, the increased hydrophilicity could promote the water molecule adsorption onto the ACNT surface, giving rise to the enhanced RI. Moreover, the stable sensing performance and the RI could still be maintained after 100 cyclic uniaxial stretching at a strain of 50% (Figure S2), displaying outstanding repeatability and durability. The CNC-based humidity sensor can be used for human respiration. As a proof of concept, the CNC is fixed inside a 24538


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Figure 7. (a) Photograph of a volunteer wearing the breathing mask, and the CNC-based humidity sensor is fixed inside the mask; (b) resistance response of the sensor to fast, normal, and deep breathing; (c) resistance response to mouth and nose breathing; and (d) sensing performance during the breath and apnea processes.

Figure 8. (a) Chemical vapor sensing property of CNC-30 toward different organic vapors with a fixed concentration of 30 ppm. (b) Resistance response of CNC-30 at different acetone concentrations.

the chemical vapors, during which the polymer fibers are swollen and de-swollen and the conductive network are hence damaged and recovered, leading to the increase and decrease of the resistance during the cyclic sensing test. As known, the vapor sensing behavior is mainly influenced by the solubility parameters of both the polymer and the solvent vapor and the saturated vapor pressure. Figure 8a shows the chemical vapor sensing performance of the CNC exposed to different organic vapors at a low concentration of 30 ppm. Clearly, there is an increase for the resistance of the CNC after exposed to the vapors, while the resistance can return to its original value (ΔR/R0 = 0) when the vapors were expelled from the chamber by nitrogen, displaying outstanding reproducibility. Periodic sensing signals with different RI are observed for the detection of different chemical vapors. As shown in the inset curve of Figure 8a, the CNC possesses the largest response against methanol vapor while lowest response against heptane with the RI of 25 and 2%, respectively.55−57 As is known, the chemical vapor adsorption and deadsorption could induce the swelling and de-swelling of the polymer matrix, and thus the interaction between the solvent vapor and the polymer plays a key role determining the sensing performance, and it can be expressed based on the Flory− Huggins interaction parameter, χ12 (see eq 1)58

medical breathing mask for a respiration sensor, as shown in Figure 7a. During the respiratory process, the gas containing the water vapors exhaled from the nose goes through the CNC sensor, resulting in the resistance variation of the material. Figure 7b shows the resistance response of the humidity sensor for normal, fast, deep breathing. The normal breath with a frequency of 0.2 Hz displays a regular signal with the RI of around 1.5%. For the fast breathing with a frequency of 0.3 Hz, the RI slightly drops to 1.2%. While for deep breathing, the sensor shows the highest RI of 2.0%, indicating that more water molecules in the exhaled gas was adsorbed on the CNC sensor. Interestingly, the as-prepared sensor can also be used to detect nose breathing and mouth breathing, as shown in Figure 7c. Compared with mouth breathing, nose breathing displays a higher frequency while a lower RI. Moreover, to evaluate the airflow effect on the humidity sensor, the apnea process was carried out during breathing (Figure 7d). The resistance of the CNC keeps almost unchanged during the apnea process, making it promising in detection of the sleep apnea symptom. As mentioned, in addition to the humidity detection capability, the CNC can serve as a chemical vapor sensor, which has potential applications in monitoring toxic gas leakage and some biomarkers in people’s respiration. The mechanism is based on the adsorption and de-adsorption of 24539


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Figure 9. Chemical vapor sensing performance of CNC (a) in a 10 ppm acetone vapor under different strains and (b) in acetone vapors with different concentrations under a fixed strain of 30%.

Figure 10. Cyclic resistance response to the mixture of humidity (RH = 95%) and different chemical vapors, (a) heptane, (b) acetone, (c) toluene, (d) THF, and (e) methanol. The vapor concentration was fixed at 50 ppm. (f) RI contributed from different chemical vapors.

V0 χ12 = 1 (δ1 − δ2)2 RT

large molar volume (147.5 L mol−1). Surprisingly, the methanol has the largest RI of 25%, which may be caused by the strong interaction between methanol and ACNTs such as the hydrogen bonding, despite the weakest interaction between the methanol and the PU. Like the water molecules, the methanol could transfer the electrons to the ACNT, leading to the large increase of the resistance. Figure 8b shows the responsivity of the nanofiber composite exposed to the acetone vapor with different concentrations. It can be found that the RI displays a continuous increase with the vapor concentration. The detection limit is as low as 10 ppm, at which the distinguished sensing signals with the RI of around 2% can be observed. The RI can reach 15% at the vapor concentration of 100 ppm. A higher vapor concentration corresponds to a larger vapor pressure, leading to more severe damage to the conductive network by the separation of ACNTs anchored nanofibers and hence a higher RI. Similar with the humidity sensor, it is desired that the chemical vapor sensors can work under mechanical deformations because they often undergo external forces especially in the practical applications. Figure 9a shows the resistance variation of the CNC stretched to different strains against acetone vapor. Interestingly, the stretching endows the material with a higher sensitivity, and the RI increases from its original value of 2−5% at a strain of 100%. More interfacial area is exposed to the vapor when the CNC is stretched, promoting vapor adsorption and thus the resistance response. This strain-induced enhancement of the

(1)

In eq 1, V01 is the molar volume of the solvent (cm3 mol−1), T is the temperature (K), R refers to the ideal vapor constant (8.314 J·mol−1), and δ1 and δ2 are the solvent and polymer solubility parameters J1/2 cm−3/2, respectively. To achieve a high solubility of polymer in the solvent, a low χ12 is required. It can be concluded that a low V01 and a small difference between δ1 and δ2 are beneficial to the enhancement of the interaction. In addition to the χ12, the vapor sensing performance is affected by the saturated vapor pressure that can influence the separation between the conductive nanofibers during the vapor adsorption. The detailed information about the physical properties of the polymer (PU) and solvent including their solubility parameters, molar volume, and saturated vapor pressure are summarized in Table S2. The solubility parameters of acetone and THF are both 20.5 J1/2 cm−3/2, which is the same with that of PU. By comparison, the heptane and toluene possess lower solubility parameters of 15.2 and 18.2 J1/2 cm−3/2, respectively, while methanol has a much higher value of 29.7 J1/2 cm−3/2. In view of the interaction between the PU and the solvent, the nanofiber composite should be responsive strongly to acetone and THF while insensitive to heptane and methanol. It is reasonable that heptane has the lowest RI because of its low solubility parameter and saturated vapor pressure (5.3 kPa) as well as 24540


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maintained during stretching. The introduction of ACNTs enhances both the Young’s modulus and tensile strength while maintaining excellent stretchability of the PU nanofibrous membrane. The CNC can serve as both humidity and chemical vapor sensors. The CNC displays high sensitivity in a broad RH range from 11 to 95% and excellent recyclability and reliability. A CNC-based humidity sensor can be integrated with a mask for real-time detection of human respiration, providing accurate respiration signals such as rate and depth of respiration. Also, the CNC can be used to monitor different chemical vapors (even in a high humidity environment) with a quick response and low detection limit. The stretchable CNCbased humidity and chemical vapor sensor has potential application in detection of people’s respiration, biomarker gases from human breath, and toxic gases leakage.

signal intensity could decrease the detection limit, as shown in Figure 9b. Even if the vapor concentration drops down to 1 ppm, the stable and periodic response with the RI of around 2% is found. In most cases, the chemical vapor is in a humid atmosphere. As mentioned, a small amount of volatile organic compounds exists in the people’s breath. In particular, the concentration of some chemical vapors is higher for a patient. As a result, these chemical vapors serve as biomarkers that can provide useful information for the diagnosis of those diseases. To simulate the atmosphere of people’s breath, a low concentration of chemical vapors is mixed with the moisture, and the CNC is used to detect the volatile vapors in the mixture. Figure 10 shows the sensing performance of the CNC toward different chemical vapors with a fixed concentration of 50 ppm in a high humidity of 95%. As seen, no obvious resistance response is found after the heptane is introduced. On the other hand, the CNC is responsive to all other vapors in the high humid environment, displaying a continuous increase of the resistance. The responsivity for different vapors is summarized in Figure 10f. For example, RI for the acetone in the high humidity environment is about 15%, while the methanol introduction brings in the largest response with the RI of around 40%.59,60 Figure 11 shows the sensing performance of the CNC exposed

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04304.

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SEM images of (a) CNC-0.5, (b) CNC-5, (c) CNC-40, and (d) electrical conductivity of CNC as the ultrasonication time; results of mechanical properties of different nanofiber mats; humidity sensing property after 100 cycles of uniaxial strain in a 59% RH environment, the strain and strain rates fixed at 50% and 30 mm min−1, respectively; and physical properties of the polymer or solvents (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jiefeng Gao: 0000-0002-6038-9770

Figure 11. Resistance response of the CNC in the mixture of humidity (RH = 95%) and acetone vapor with different concentrations.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Natural Science Foundation of China (nos. 51873178, 51503179, 21673203), the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (no. sklpme20184-31), Qing Lan Project of Jiangsu province, the China Postdoctoral Science Foundation (no. 2016M600446), the Jiangsu Province Postdoctoral Science Foundation (no. 1601024A), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Innovation Program for Graduate Students in Universities of Jiangsu Province (no. KYCX18_2364).

to different acetone concentrations. Compared with the resistance response in pure acetone vapor, the detection limit in the high humid environment is slightly increased because the high RH level may shield the signals from the chemical vapor sensing at a low acetone concentration of 10 ppm, that is, the signal results from the humidity-induced resistance response. With the increase of the acetone concentration to 30 ppm, a low resistance response is observed. When the concentration reaches 100 ppm, the RI from the acetone vapor increases to 70%. Thus, the nanofiber composite can be used for detection of different chemical vapors. In the real-life application, chemical vapors can be identified by comparison between the sensing behavior of the unknown and known (standard) chemical vapors. For example, the nanofiber composite could serve as an alarming apparatus that outputs reliable signals of the resistance change if the leakage of some chemical vapors regardless of in a dry or humid atmosphere.

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4. CONCLUSIONS In conclusion, ACNTs are successfully anchored onto the electrospun PU nanofiber with the assistance of ultrasonication, and the obtained CNC possesses superhydrophilicity and underwater superoleophobicity that can be 24541


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