Flexible Room-Temperature NH3 Sensor for Ultrasensitive, Selective

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Flexible Room-Temperature NH3 Sensor for Ultrasensitive, Selective, and Humidity-Independent Gas Detection Hua-Yao Li, Chul-Soon Lee, Do Hong Kim, and Jong-Heun Lee* Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea

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S Supporting Information *

ABSTRACT: Ammonia (NH3) is an irritant gas with a unique pungent odor; subparts per million-level breath ammonia is a medical biomarker for kidney disorders and Helicobacter pylori bacteria-induced stomach infections. The humidity varies in both ambient environment and exhaled breath, and thus humidity dependence of gas-sensing characteristics is a great obstacle for real-time applications. Herein, flexible, humidity-independent, and room-temperature ammonia sensors are fabricated by the thermal evaporation of CuBr on a polyimide substrate and subsequent coating of a nanoscale moisture-blocking CeO2 overlayer by electronbeam evaporation. CuBr sensors coated with a 100 nm-thick CeO2 overlayer exhibits an ultrahigh response (resistance ratio) of 68 toward 5 ppm ammonia with excellent gas selectivity, rapid response, reversibility, and humidity-independent sensing characteristics at room temperature. In addition, the sensing performance remains stable after repetitive bending and long-term operation. Moreover, the sensors exhibit significant response to the simulated exhaled breath of patients with H. pylori infection; the simulated breath contains 50 ppb NH3. The sensors thus show promising potential in detecting sub-parts per million-level NH3, regardless of humidity fluctuations, which can open up new applications in wearable devices for in situ medical diagnosis and indoor/outdoor environment monitoring. KEYWORDS: gas sensor, ammonia, CeO2-coated CuBr, humidity dependence, exhaled breath analysis, selectivity, medical diagnosis

1. INTRODUCTION Ammonia (NH3), which is a natural gas, presents throughout the atmosphere from the ancient times of earth history and was probably a source component of water and life beings on earth. Today, most of atmospheric ammonia is generated by ammonification and combustion.1 Ammonification is mainly performed by bacteria and fungi’s metabolic activities, which decompose organic nitrogen from animals and plants. Combustion from the chemical industry, which produces fertilizers and other chemicals, generates ammonia to the atmosphere. Ammonia, even lower than 50 ppm (∼35 mg/ m3), can severely irritate human respiratory organs, skin, and eyes.2 The long-term permissible exposure limit of ammonia in indoor areas is therefore set at 25 ppm.3 Furthermore, in the presence of moisture, NH3 is known to spontaneously react with SO2 or NO2 generated by the combustion of fossil fuels to form ammonium sulfate or ammonium nitrate, respectively, which are major sources of airborne pollutants, such as PM 2.5 (particulate matter smaller than 2.5 μm).4 These inhalable particles cause adverse respiratory and cardiovascular health effects.5 Meanwhile, ammonia is also generated by human metabolic activity. Ammonia in exhaled breath is a diagnostic biomarker for disturbed urea balance caused by kidney disorder or ulcers caused by Helicobacter pylori bacteriainduced stomach infections.6−9 NH3 concentration in the exhaled breath of end-state renal disease (ESRD) patients (mean 4.88 ppm; range 0.82−14.7 ppm) is higher when © XXXX American Chemical Society

compared to the breath of healthy control patients (mean 0.96 ppm; range 0.425−1.8 ppm).10 NH3 concentration in the exhaled breath of H. pylori-positive patients is 8 times higher (from 0.05 to 0.4 ppm) when compared to the concentration in the breath of healthy patients after urea dose. This is because the conversion of urea to ammonia and bicarbonate is promoted by H. pylori.11 Therefore, selective detection of parts per million- and sub-parts per million-level NH3 is of great significance for environmental monitoring and medical diagnosis.12 Metal oxide semiconductors such as SnO2,13 WO3,14 and MoO315 have been explored to detect parts per million-level ammonia owing to their advantageous features, such as easy synthesis, low cost, facile integration, and ability to detect a wide range of target gases. However, ammonia sensors based on these oxide chemiresistors should be operated at elevated temperature (200−400 °C), which leads to high power consumption and complex thermally insulating sensor structures. This is an unavoidable obstacle for wearable room-temperature sensors. Recently, transition metal dichalcogenides (TMDCs, such as MoS216−18 and WS219), conducting polymers (such as polypyrrole (PPy),20 poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) (PEDOT:PSS),21 and Received: June 2, 2018 Accepted: July 27, 2018 Published: July 27, 2018 A

DOI: 10.1021/acsami.8b09169 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of the sensor synthesis procedure. (b) Schematic diagram of pristine and CeO2-coated CuBr sensors. (c) Images of a pristine CuBr sensor (left) and CuBr sensor coated with a 100 nm-thick CeO2 overlayer (right). (d) Flexibility of a CuBr sensor coated with a 100 nm-thick CeO2 overlayer.

thermal evaporation, and a nanoscale CeO2 thin film was uniformly coated on the CuBr layer by electron beam (ebeam) to prevent the humidity impact on gas-sensing characteristics. The CeO2-coated CuBr flexible sensor showed excellent humidity-independent ammonia-sensing properties with ultrahigh response, selectivity, excellent reversibility, and rapid response/recovery at room temperature. Such unprecedentedly good sensing performance was well maintained even after repetitive bending and long-term stability testing. In addition, the sensor showed a noticeable response to the simulated breath of patients with H. pylori infection, which contained 50 ppb NH3, confirming its promising potential in medical diagnosis.

polyaniline (PANI)),22 carbon nanotubes (CNTs),23−28 and graphene derivatives29,30 have demonstrated the potentials of ammonia sensing at room temperature (RT). However, their response, response/recovery speed, reversibility, and selectivity are still inadequate for practical applications. Although optical gas analyzers based on the inherent absorption spectrum of ammonia show high accuracy, selectivity, and low detection limit, their implantation is limited by their high cost, long response time, and complex structure.31 The goal of advanced gas sensor is being lightweight, flexible, and stretchable, which can be well implemented in wearable and portable devices for consumer applications.32,33 Integration of electron device with flexible substrate (such as paper,34,35 poly(ethylene terephthalate),36−38 and polyimide (PI)39,40) is the best way to achieve this goal. Copper(I) bromide (CuBr) is a room-temperature Cu+ ion conductor with a conductivity of 3 × 10−8 S/cm.41,42 The Cu+ can react with NH3 to form Cu(NH3)2+, which immobilizes the copper(I) ions and decreases the electrical conductivity.43,44 For wearable sensor applications, sensitive, selective, rapid, and reversible detection of NH3 using a flexible sensor structure at room temperature is essential. The humidityindependent NH3 sensing characteristics are also mandatory because ambient humidity changes dynamically according to the variation of location, season, day and night, and weather. Although earlier studies have ever demonstrated the possibility of CuBr as ammonia-sensing materials,45−48 the studies neither investigated the humidity dependence of gas-sensing behaviors46,48 nor achieved the humidity-independent gas-sensing characteristics.45,47 Moreover, no flexible CuBr-based ammonia sensor was reported. For medical diagnosis, trace concentrations of NH3 in exhaled breath should be measured in highly humid atmospheres (>80% relative humidity (RH)), and the humidity-independent ammonia sensor can simplify the sensor system significantly because the preconditioning of exhaled gas through the dehumification process can be neglected.12 Nevertheless, the humidity dependence of NH3-sensing characteristics in CuBr-based flexible sensors, which can be operated at room temperature, and their removal have never been investigated systematically. In this study, highly porous and nanostructured CuBr flexible sensors were fabricated on PI flexible substrates by

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. CuBr films were fabricated by thermal evaporation. CuBr powder (0.1 g, Sigma-Aldrich) was placed at the center of a quartz tube, while the PI substrate (12 × 8 mm2) with Pt interdigital electrodes (IDEs) on its top surface (electrode width of 50 μm and separation of 100 μm) was located 10 cm downstream from the source. After evacuation to a pressure of ∼9 × 10−2 Torr using a rotary pump, the furnace temperature was increased to 500 °C within 15 min. CuBr films were grown for 90 min in an Ar environment (flow rate of 200 standard cubic centimeter per min, sccm). Later, the tube furnace was allowed to cool down to room temperature naturally. CeO2 films were then coated on the CuBr films using an e-beam evaporator (Rocky Mountain Vacuum Tech.). The base pressure of the chamber was maintained at 10−6 Torr. The applied electron-beam voltage and current were set at 7.0 kV and 40 mA, respectively, leading to a deposition rate of 1.0 Å/s. Different furnace temperatures (400 and 600 °C) and different thicknesses of the CeO2 film (10, 50, 100, and 150 nm) were chosen for comparison (Figure 1). All of the synthesis processes were conducted in nonaquatic conditions to prevent the oxidation of Cu+. 2.2. Characterization. The crystal structures of the materials used to fabricate sensors were investigated using an X-ray diffraction (XRD) instrument (Rigaku D/MAX-2500V/PC) with Cu Kα (λ = 1. 542 Å) radiation. The morphologies of the sensor materials were characterized using a field emission scanning electron microscope (FE-SEM, FEI Inspect F50) and a transmission electron microscope (TEM, FEI, Talos F200X). X-ray photoelectron spectroscopy (XPS) spectra were analyzed using ULVAC-PHI X-TOOL. Diffuse reflectance infrared Fourier transform (DRIFT) spectra were recorded with a VERTEX 70-FTIR spectrometer equipped with a SMART collector and an mercury cadmium telluride detector. B


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Figure 2. SEM images of (a) pristine CuBr and (b) CuBr coated with a 100 nm-thick CeO2 overlayer. HRTEM images of (c) pristine CuBr and (d) CuBr coated with a 100 nm-thick CeO2 overlayer. (e) Magnified image of the circled region in (d). (f) EDS element mapping of CuBr coated with a 100 nm-thick CeO2 overlayer. (Growth temperature of CuBr: 500 °C.) 2.3. Gas-Sensing Measurement. The gas-sensing measurements were carried out in a small volume (7 cm3) of quartz tube to avoid time delay during the change of the atmosphere (Supporting Information, Figure S1). The sensor was placed in it, and the gas concentrations were controlled by changing the mixing ratio of the parent gases (NH3, NO2, trimethylamine, acetone, benzene, ethanol, toluene, and xylene; the concentrations of all of the gases were maintained at 5 ppm using a dry synthetic air balance) and dry synthetic air. The humidity in the tube was controlled by changing the mixing ratio of fully humid air and dry air, and the humidity level was checked using a humidity sensor. Total flow rate of gas was fixed at 500 mL/min, and the four-way value was used to change the atmosphere. The direct current two-point probe resistance of the sensor was measured using a Keithley 6487 picoammeter with a computer interface.

3. RESULTS AND DISCUSSION 3.1. Material Characterization. The CuBr was deposited on a flexible PI substrate (12 × 8 mm2) with platinum (Pt) interdigital electrodes (IDEs) by thermal evaporation at 400, 500, and 600 °C (Figure 1). The CuBr film grown at 400 °C showed relatively dense structures. The CuBr films grown at 500 and 600 °C exhibited porous microstructures (Supporting Information, Figure S2). The diameters of more than 100 CuBr particles were measured from SEM image, and average diameters of CuBr particles grown at 400, 500, and 600 °C were 728 ± 132, 647 ± 146, and 3379 ± 988 nm, respectively. Thus, considering both porosity and particle size, 500 °C was chosen as the optimal growth temperature (Figure 2a). The C


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Figure 3. (a) Sensing transients of pristine CuBr sensors grown at 400, 500, and 600 °C and exposed to 5 ppm NH3 at room temperature (dry atmosphere). Gas responses of (b) pristine CuBr sensor and CuBr sensors coated with (c) 10 nm of CeO2, (d) 50 nm of CeO2, (e) 100 nm of CeO2, and (f) 150 nm of CeO2 to 5 ppm NH3 at various humidity levels. (Growth temperature of CuBr in (b)−(e): 500 °C.)

Figure 4. (a) Response−recovery curve of the CuBr sensor coated with a 100 nm-thick CeO2 overlayer to 0.02−5 ppm NH3 at RH 80%. (b) NH3 responses of the sensors reported in the literature and the CuBr sensor coated with a 100 nm-thick CeO2 overlayer (the dark blue region indicates the detecting range of H. pylori infections, while the light cyan region indicates the detecting range for ESRD). (c) Selectivity of the CuBr sensor coated with a 100 nm-thick CeO2 overlayer at 80% RH (gas concentration: 5 ppm). (d) Response−recovery curves of the CuBr sensor coated with a 100 nm-thick CeO2 overlayer upon exposure to a healthy person’s breath and the simulated breath of a patient with H. pylori infection; the simulated breath contained 50 ppb NH3.

D


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ACS Applied Materials & Interfaces Table 1. Comparison of Different NH3 Sensors Reported in the Literature13−15,17−30,45−48 materiala Au−SnO213 Cr−WO314 Si−MoO315 MoS2 thin film17 Co3O4/MoS218 WS219 Au/PPy20 PEDOT:PSS nanowires21 SiO2/PANI22 tetrapodal-ZnO−CNT23 ZnO−CNT24 PANI/MWCNT25 PANI/MWCNTs/MoS226 ITO−SWCNT27 SnO2/SWCNT28 3D R-GO29 BPB/R-GO30 CuBr45 CuBr46 CuBr47 CuBr48 CeO2−CuBr (this work)

response (Rg/Ra or Ra/Rg)

concn (ppm)

operating conditionb

humidity conditionc

flexible sensor?

20 3 1.5 1.002 1.65 3 1.08 1.7 5 37.5 ∼60 1.155 1.4 1.023 1.1 1.6 1.02d 5 7 5.5d 6 68

10 5 1 1 5 NA 10 1.6 10 10 10 2 6 3 0.5 10 5 1 5 5 1 5

212 °C 350 °C 400 °C RT (recov with UV) RT RT (with 633 nm light) RT RT RT RT RT RT RT RT RT RT RT RT RT RT RT RT

80% RH NA 90% RH dry air 40% RH dry air dry air dry air 30% RH 30% RH 30% RH dry air 40% RH dry air dry air dry air dry, 25−95% RHe dry air dry air dry, 33−97% RHf NA dry, 20−80% RH

no no no no no no no no yes no no no no yes no no yes no no no no yes

a

Polypyrrole, PPy; poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate), PEDOT:PSS; polyaniline, PANI; multiwalled carbon nanotube, MWCNT; indium-tin oxide, ITO; single-walled carbon nanotube, SWCNT; reduced graphene oxide, R-GO; bromophenol blue, BPB. bRoom temperature, RT. cRelative humidity, RH. dAcquired in dry air. eSensor response shifted as humidity increased. fSensor response affected by humidity.

CuBr was identified as the γ-phase with a zinc blende structure (JCPDS no. 06-0292) according to X-ray diffraction (XRD) analysis (Supporting Information, Figure S3). The CeO2 films with thicknesses of 10, 50, 100, and 150 nm were coated by ebeam evaporation on the deposited CuBr layers. The coated CeO2 layers exhibited a cubic structure (JCPDS no. 34-0394; see Supporting Information, Figure S3). The surface of the CuBr layer becomes smoother after coating with CeO2 (Figure 2b). The interplanar spacing of 0.2012 nm along the perpendicular directions in the high-resolution TEM (HRTEM) image (Figure 2c) of pristine CuBr was consistent with the (220) d-space of CuBr. CuBr coated with 100 nmthick CeO2 exhibited a semi-core−shell structure (Figure 2d). The presence of the CeO2 phase in the shell was confirmed by high-resolution lattice image (Figure 2e) and energy dispersive X-ray spectroscopy (EDS) elemental mapping (Figure 2f). 3.2. Gas-Sensing Characteristics. The dynamic sensing transients of pristine CuBr sensors exposed to 5 ppm ammonia at room temperature under a dry atmosphere are shown in Figure 3a. The gas response (S) is defined as Rg/Ra, where Rg and Ra are the resistances of the sensor in gas and air, respectively. The response and recovery times are defined as the times required to reach 90% variation of the sensor resistance upon exposure to the analyte gas or air. The pristine CuBr sensor fabricated at 500 °C among three sensors (400, 500, and 600 °C) exhibited the highest response of 220 toward 5 ppm of ammonia. This can be explained by the highly porous structure and relatively small particle size of the CuBr sensing film. The response and recovery times of this sample were 210 and 7 s, respectively, which are relatively short for a roomtemperature sensor. Because ambient humidity dynamically changes according to weather, season, and location and is nearly saturated at high

levels (RH >80%) in exhaled breath, maintaining a consistent sensor performance regardless of humidity variation is very important for practical applications.49 The responses of the pristine CuBr sensor synthesized at 500 °C to 5 ppm NH3 at different humidity levels are depicted in Figure 3b. The baseline sensor resistance increased with increasing humidity from dry to RH 80%. The response to 5 ppm NH3 in humid air was higher than that in dry air. CeO2 overlayers with varying thicknesses were coated on the CuBr layer to eliminate the effect of humidity (Figure 3c−f). Overall, the response to NH3 in dry air decreased with an increase in the thickness of the CeO2 overlayer (Figure 3b−f). This is attributed to a decrease in the active CuBr sensing area due to the coating of inactive CeO2. In CuBr sensor coated with a 10 nm-thick CeO2 overlayer (Figure 3c), the shape of initial sensing transient upon ammonia exposure at RH 80% (see violet line) is different from those at dry and RH 20−60%, and the humidity dependence of gas response is not simple. This indicates that the thickness of the CeO2 overlayer is insufficient to protect the CuBr sensor from highly humid atmosphere. However, the humidity dependence of baseline sensor resistance and gas response tended to decrease with an increase in the thickness of the CeO2 overlayer (Figure 3d) and completely disappeared in the sensors coated with 100 and 150 nm-thick CeO2 layers (Figure 3e,f). Considering the gas response and humidityindependent gas-sensing characteristics together, the CuBr sensor coated with a 100 nm-thick CeO2 layer was selected as the optimal one. The 90% response and recovery times of the CuBr sensor coated with 100 nm-thick CeO2 in dry atmosphere were 112 and 74 s, respectively. The response−recovery curves of the CuBr sensor coated with 100 nm-thick CeO2 layer and exposed to different NH3 E


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Figure 5. (a) Response−recovery curve of the CuBr sensor coated with a 100 nm-thick CeO2 overlayer to 5 ppm NH3 under various bending conditions at RH 80%. (b) Baseline resistance and response of the CuBr sensor coated with a 100 nm-thick CeO2 overlayer to 5 ppm NH3 at different bending angles. (c) Response−recovery curve of the CuBr sensor coated with a 100 nm-thick CeO2 overlayer before and after 2000 bending cycles at 45°.

water-induced degradation of the sensing surface. Although CuBr2 aqueous solution had been replaced with ethyl ether and methanol solutions in two of the compared studies,47,48 the sensors were thick or dense films with low gas accessibility. In contrast, CuBr film in the present study is porous and the particle size of CuBr is the smallest among all of the CuBr sensors in the literature.45−48 Thus, the unprecedentedly high NH3 response observed in this investigation can be attributed to the highly crystalline, fine, and gas-accessible CuBr particles grown by the moisture-free vapor-phase method. Moreover, the interaction between the CuBr sensing surface and moisture was further blocked by the e-beam-coated CeO2 overlayer. This ensures a more stable and moisture-resistant CuBr film. The sensor showed very high selectivity to NH3 with negligible cross-responses to acetone, benzene, CO, ethanol, formaldehyde, trimethylamine, toluene, and xylene (Figure 4c). Accordingly, the CuBr sensor coated with a 100 nm-thick CeO2 in the present study exhibited a high NH3 response, rapid sensing speed, high selectivity, good reversibility as well as humidity-independent gas-sensing characteristics. These sensing characteristics are superior to those of other competing sensors reported in the literature in many aspects. As mentioned above, the long-term permissible exposure limit for ammonia in an indoor environment is 25 ppm.3 The high response of the developed sensor (68) to 5 ppm NH3 clearly indicates that it can be used to monitor indoor atmosphere, regardless of humidity variation. In medical diagnosis, the examination of human exhaled breath is considered a noninvasive, convenient, and attractive method,

concentrations were measured under RH 80% (Figure 4a). The sensor showed rapid, reversible, and reliable sensing transients, although the response time becomes slightly longer when NH3 concentration is lower than 0.1 ppm. Note that the sensor can detect 20 ppb NH3 with a substantial response of 1.2. Furthermore, the sensor showed a linear relationship between response and NH3 concentration in the double logarithmic scale, which facilitated signal processing. A survey of the available literature shows that the highest response to NH3 reported thus far was achieved with metal oxide chemiresistors, which operated at elevated temperatures.13 Note that the NH3 responses of the present CuBr sensor coated with a 100 nm-thick CeO2 are among the highest values in the literature (Table 113−15,17−30,45−48 and Figure 4b). Other room-temperature NH3 sensors based on TMDCs, conducting polymers, and carbon-related materials17−30 exhibit responses significantly lower than ours. Among all of the reported sensors, five of them22−24,26,30 were tested in humid conditions, and their gas responses shifted substantially when tested in a humid atmosphere. Compared to other CuBr sensors in early works,45−48 the NH3 response in the present work is significantly higher. It should be noted that the CuBr sensors reported previously were mostly synthesized using a wet chemical method, in which copper metal reacts with a CuBr2 aqueous solution.45,46 As water molecules have a high affinity to CuBr, Cu(H2O)n+ is easily formed in the wet chemical product, which in turn decreases the gas response. It should be emphasized that moisture-free thermal evaporation of CuBr, as demonstrated in this study, can block possible F


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Figure 6. XPS spectra of the Cu 2p levels of (a) pristine CuBr sensor and (b) CuBr sensor coated with a 100 nm-thick CeO2 overlayer. The sensors were stored in dry air and 100% RH for 48 h. (c) DRIFT spectra of pristine CuBr and CuBr coated with a 100 nm-thick CeO2 overlayer and stored in RH 80%.

To confirm the potential of the present sensor for medical diagnosis, the gas-sensing characteristics to simulated breath containing trace ammonia were evaluated. A healthy person’s exhaled breath was collected using a 2 L Tedlar bag and injected into the sensor chamber. The obtained signal is represented by the black line in Figure 4d. It can be seen that after a “healthy gas” injection, the sensor resistance exhibited very little fluctuation, indicating the absence of ammonia. Then, we injected 20 mL of 5 ppm NH3 into the Tedlar bag and rested it for 1 h to stabilize in order to mimic the exhaled breath of a patient with H. pylori infection, which contains 50 ppb NH3 (Figure 4d). After injection of the simulated “illness gas” into the sensor chamber, the sensor resistance increased significantly (red line in Figure 4d). This clearly demonstrates that the CuBr sensor coated with a 100 nm-thick CeO2 layer can be used for medical diagnosis. PI was applied as a substrate to fabricate flexible sensors (Figure 1d). The sensing transients were measured under various bending conditions (Figure 5). At the high-angle bending (e.g., +85° and −85°), the weak reduction of NH3 response was observed (Figure 5b). This can be explained by the decrease of connectivity between CuBr particles, which is supported by the slight increase of sensor resistance at high bending angle. However, overall, the baseline resistance and gas response remained nearly the same in both positive and negative bending modes. Even after bending for 2000 times at 45°, the gas response was constant (Figure 5c). Only three previously reported studies (Table 1 and Figure 4b) demonstrated flexible NH3 sensors,22,27,30 but their responses are much lower than that of the current sensor. Note that none of them was based on CuBr. These results confirm that the sensor developed in this study is stable against bending and hence can be used in wearable devices. 3.3. Gas-Sensing Mechanism and Discussions. CuBr is a typical copper ion conductor at room temperature. The major carrier is copper(I) ions (Cu+). When ammonia is injected, Cu+ ions react with ammonia molecules to form

especially for children, pregnant women, and elderly persons.12,50 However, approximately over 200 different gases as well as a large amount of water vapor is exhaled from the breath, which give a great challenge in disease diagnosis.12,51,52 Ammonia in exhaled breath has been confirmed as a biomarker of H. pylori bacteria-induced stomach infections and kidney disorders. Conventional noninvasive breath diagnosis of H. pylori uses labeled 13C or 14C urea.11 A high concentration of labeled CO2 is a measure of the H. pylori existence because urea is decomposed to CO2 and NH3 by H. pylori. However, labeled urea is either expansive (13C) or radioactive (14C). Besides the CO2 increase, the concentration of NH3 is increased by 8 times (0.05−0.4 ppm) in H. pylori-positive patients after urea dosing. The CuBr sensor coated with a 100 nm-thick CeO2 layer in the present study showed a high response of ∼2−10 (Figure 4b left dark blue region) with respect to 0.05−0.4 ppm ammonia with excellent selectivity (Figure 4c) and negligible humidity dependence and hence can be used to examine the presence of H. pylori. In this case, a normal urea dose is sufficient for diagnosis, which is inexpensive and can be safely used to test pregnant women and children. Higher concentrations of NH3 can be detected in the breath of ESRD patients because of the excess ammonia and ammonium in the blood caused by renal failure; these are exhaled out due to alveolar gas exchange in the lungs.9 High blood urea nitrogen and creatinine are indicators of kidney dysfunction, which increases the concentration of ammonia in exhaled breath.53 NH3 concentration in the exhaled breath of ESRD patients ranges from 0.82 to 14.7 ppm, while that in healthy controls ranges from 0.425 to 1.8 ppm.10 The high NH3 responses (10−100) of the CuBr sensor coated with a 100 nm-thick CeO2 layer in this concentration region (Figure 4b right light cyan region) are sufficient to differentiate between healthy controls and patients. In this case, the breath diagnosis provides the facile evaluation of renal disease, which is friendly to all of the people. G


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ACS Applied Materials & Interfaces Cu(NH3)2+, which immobilizes Cu+ ions and thus increases the resistance.46,47 Cu+ + 2NH3 → Cu(NH3)+2

(1) +

When ammonia gas is removed, Cu(NH3)2 decomposes, releasing copper(I) ions and leading to a decrease in the resistance. However, copper(I) ions can also react with water molecules, which immobilizes copper(I) ions, resulting in an interference to NH3 sensing.54,55 Cu+ + nH 2O → Cu(H 2O)+n

(2) Figure 7. Schematic structure of CuBr sensor coated with CeO2 overlayer and ammonia-sensing mechanism without moisture interference.

Furthermore, copper(I) can be easily oxidized to copper(II) in a humid environment. To investigate the changes in the sensing materials under humid conditions, the XPS spectra of the Cu 2p level of pristine CuBr sensor stored in dry air and 100% RH humidity air for 48 h were obtained (Figure 6a). The peaks at 931.5 ± 0.3 and 951.5 ± 0.3 eV correspond to Cu+ 2p3/2 and 2p1/2, respectively; meanwhile, the peaks at binding energies of 933.2 ± 0.3 and 943.0 ± 0.3 eV correspond to Cu2+ 2p3/2 and 2p1/2, respectively.56−58 There was an obvious peak corresponding to Cu2+ in the pattern of pristine CuBr stored in a water-saturated atmosphere (lower figure in Figure 6a), which confirmed the irreversible oxidation of Cu(I) to Cu(II). CeO2 was introduced to eliminate the effect of humidity through the following reaction, which can take place at room temperature.59−61 4Ce 4 + + 2H 2O → 4Ce3 + + 4H+ + O2

(3)

O−2

(4)

+ 2H 2O → 4OH + e

−

OH + Ce3 + + H+ → Ce 4 + + H 2O

(5)

O2 + e− → O−2

(6)

atmosphere decreases from 220 to 19 with the thickening of CeO2 overlayer (Figure S4a). If all of the CuBr particles are completely encapsulated with inactive CeO2, no gas response will be obtained. The low but still substantial ammonia response (19) of the CuBr sensor coated with 150 nm CeO2 (Figures S4a, 3f), therefore, suggests that the surface of CuBr is not completely covered with CeO2 overlayer and a part of the uncoated CuBr surface participates in ammonia-sensing reaction. This is feasible considering the porous and nonflat CuBr film consisting of coarse particles (Figure S2c,d). The ammonia response values measured in dry, RH 20%, RH 40%, RH 60%, and RH 80% atmospheres (insets in Figure 3b−f) were statistically analyzed (Figure S4b). The standard deviation values of ammonia responses normalized by mean values (σ/m) were significantly high (0.16−0.22) in pristine CuBr sensor and CuBr sensors coated with 10 or 50 nm-thick CeO2 layer, whereas those values became significantly low (0.04 and 0.07) when the CuBr sensors are coated with 100 and 150 nm-thick CeO2 layers. This means that the CeO2 overlayer should be at least 100 nm thick to remove the humidity interference. The optimum thickness of CeO2 coating (100 nm) in the present study can be interpreted as the compensation between gas response and humidity dependence of gas-sensing characteristics.

4+

As CeO2 is the top layer, Ce will react with water molecules instead of Cu+, which blocks the interaction between moisture and CuBr. The XPS results also prove that Cu2+ is hardly observed in the CuBr sensor coated with a 100 nm-thick CeO2 layer even after being stored in a water-saturated atmosphere for 48 h (lower figure in Figure 6b). The details of the CeO2 humidity-independent effect can be explained as follows: (A) Ce4+ ions are reduced to Ce3+ when in contact with water molecules (eq 3). (B) The hydroxyl groups formed in eq 4 are scavenged by Ce3+, resulting in eq 5. (C) CeO2 is known to be an excellent oxygen reservoir. The stored oxygen is reionized by the captured electrons on the surface (eq 6). Diffuse reflectance infrared Fourier transform (DRIFT) spectra were adopted to investigate the surface reactions of pristine CuBr and CuBr coated with a 100 nm-thick CeO2 layer in an RH 80% atmosphere (Figure 6c). The absorption bands at ∼3060 and 3340 cm−1 in the spectrum of pristine CuBr in humid conditions are related to Cu(H2O)n+,55 which confirms that eq 2 took place, resulting in an interference to NH3 sensing. With CeO2 coating, the bands related to Cu(H2O)n+ disappeared. The absorption bands at 3300−3500 and 3600−3700 cm−1 are related to OH groups and adsorbed water molecules on ceria,62−64 which confirms that eqs 3−5 occurred. The DRIFT spectra also proved that the CeO2 film can effectively protect the CuBr layer from water molecules. The schematic structure of the CuBr sensor coated with CeO2 overlayer and ammonia-sensing mechanism is illustrated in Figure 7. Note that the response to 5 ppm ammonia in dry

4. CONCLUSIONS The humidity-independent gas-sensing improves the precision of environmental monitoring sensors because ambient humidity is dynamically changing and simplifies the exhaled breath analysis significantly because the gas pretreatment step such as dehumidification can be removed. And flexible sensor without external heating facilitates the realization of wearable devices. In this study, CeO2-coated CuBr porous films on PI substrates have been developed as flexible, ultrasensitive, selective, reversible, rapid, and humidity-independent NH3 sensors that can be operated at room temperature. To fabricate these sensors, initially, porous and nanoscale CuBr particles were deposited on a flexible PI substrate by thermal evaporation, after which a CeO2 overlayer was deposited by e-beam evaporation. The CeO2 layer plays the role of blocking the interaction between moisture and CuBr. The sensor could detect low concentrations of ammonia, down to ∼20 ppb. Further, we successfully demonstrated its potential for the diagnosis of H. pylori infections using a simulated illness gas containing 50 ppb NH3. The sensor resistance and gas response remained constant, regardless of the variation in humidity from dry to RH 80% due to the presence of the CeO2 overlayer, which confers protection from moisture. FurtherH


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more, the sensor characteristics were negligibly affected by repetitive bending. Thus, these sensors can pave a new way for devising wearable devices for reliable environmental monitoring and in situ medical diagnosis from exhaled breath.

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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.8b09169. Experimental setup to measure gas-sensing characteristics; SEM images of pristine CuBr synthesized at 400− 600 °C; X-ray diffraction patterns of pristine and CeO2coated CuBr films; ammonia response of CuBr sensors in dry atmosphere and humidity-dependent variation of ammonia responses as a function of CeO2 overlayer thickness (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-2-3290-3282. Fax: +82-2-928-3584. ORCID

Jong-Heun Lee: 0000-0002-3075-3623 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the National Research Foundation of Korea (NRF), which was funded by the Korean government (Ministry of Education, Science, and Technology (MEST), Grant No. 2016R1A2A1A05005331).

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REFERENCES

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Flexible Room-Temperature NH3 Sensor for Ultrasensitive, Selective

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