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Flexible, disposable cellulose paper based MoS2-Cu2S hybrid for wireless environmental monitoring and multifunctional sensing of chemical stimuli Parikshit Sahatiya, Anand Kadu, Harshit Gupta, P Thanga Gomathi, and Sushmee Badhulika ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00245 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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ACS Applied Materials & Interfaces

Flexible, disposable cellulose paper based MoS2-Cu2S hybrid for wireless environmental monitoring and multifunctional sensing of chemical stimuli Parikshit Sahatiya1, Anand Kadu1, Harshit Gupta2, P Thanga Gomathi1, and Sushmee Badhulika1* 1

Department of Electrical Engineering, Indian Institute of Technology Hyderabad Hyderabad, 502285, India

2

Department of Electrical Engineering, Indian Institute of Technology Ropar, Punjab

*Corresponding author: E-mail: [email protected]; Telephone: 040-23018443 Fax 04023016032

Abstract Multifunctional sensors responding to different chemical stimuli fabricated using functional nanomaterials still remains a challenge due to the usage of same sensor multiple times for different sensing and unreliable frontend processing of the sensing data. This challenge is intensified by the lack of suitable techniques for fabricating disposable sensors which are integrated to smartphone with a dedicated application developed for each sensing. A novel MoS2/Cu2S hybrid grown on disposable cellulose paper by hydrothermal method is reported for its utilization in humidity, temperature and breath sensing and ethanol adulteration wherein the data can be wirelessly transmitted to smartphone with dedicated application module for each sensing. The sensor can be utilized for particular sensing and then can be disposed avoiding the need for utilizing the same sensor for different sensing thereby increasing accuracy of the sensing data. The fabricated sensor sensing mechanism is explained for each stimuli in terms of change in the transport properties of MoS2/Cu2S hybrid. Development of such unique hybrid materials for wireless disposable multifunctional sensor is a great step ahead in flexible and wearable electronics having potential applications in medical, security, Internet of things etc. Keywords: MoS2/Cu2S hybrid, humidity sensing, breath sensing, ethanol adulteration, smartphone integrated sensor etc.

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1. Introduction Flexible and disposable multifunctional sensors that can sense physical and chemical stimuli are important in the field of flexible and wearable electronics and hold great potential for applications in health care, sensors, Internet of Things (IoT), environmental monitoring etc.

1-3

However, the major issue with the multifunctional sensing is the complex frontend processing of the data which overlaps with the other sensing data. To address this issue, variety of analysis are performed such as pattern recognition, principal component analysis (PCA) etc. These methods are not very reliable resulting in measuring false data. Moreover, the functional materials used for such sensor applications have excellent chemical and electronic properties and hence the sensor responds to different chemical and physical stimuli. In such situations, it is difficult to analyze the data for different sensors using the same functional materials. Also, utilizing the same sensor fabricated with functional nanomaterial for different sensing applications often leads to unreliable results. Flexible, low cost and disposable sensors are ideal choice for developing such multifunctional sensor platforms which are integrated to smartphone with a dedicated application developed for a particular sensing. The user can perform the desired sensing wherein the sensor data can be wirelessly transmitted to smartphone, followed by which the sensor can be disposed. The development of such low cost, disposable sensors will be a major step in the flexible and wearable electronics wherein the real time sensing data can be wirelessly transmitted to smartphone thereby enabling remote sensing. Smartphones are the most preferred and widely used electronic devices because of their large storage capacity, strong processing unit and user friendly touch screen.4-5 Because of the recent advances in smartphone technology, their affordability and accessibility to all demography, has resulted in development of smartphone based sensors for point of care diagnostics to receive and


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display signals generated from sensors.6-8 However, till date, there are no reports on simultaneous monitoring of environmental and chemical stimuli and developing a dedicated user friendly android application. Hence there is an urgent need to develop such systems which could take remote sensing to next level and easy for general audience to read and interpret data. In search of functional materials for multisensory applications, 2D MoS2 has gained significant interest among other 2D materials due to its exciting electronic and chemical properties.9 The ability to tune bandgap of MoS2 by thickness modulation has opened up numerous opportunities for its use in electronic applications. Further, the compatibility of MoS2 with different flexible substrates makes it a versatile material suitable for flexible and wearable sensors. There are various method to fabricate MoS2 devices on flexible substrates which include chemical vapor deposition, mechanical exfoliation etc.10-11 While the former is plagued by the complexity of transfer process which induces device to device variation in performance is plagued by the complexity of transfer process which induces device to device variation in performance, the latter results in small lateral size and less yield. Thus direct growth of MoS2 on flexible substrates still remains a challenge. Recently, our group demonstrated the direct growth of large 2D MoS2 on different flexible substrates using hydrothermal method and utilized it various electronic applications with performance comparable to MoS2 devices fabricated using sophisticated cleanroom technology.12-14 To further expand the scope of applications that pristine MoS2 offers, hybrids of MoS2 have been synthesized and utilized in energy storage, electrochemical sensors, photodetectors etc.15-16 Nanostructured transition metal chalcogenides have gained interest due to their importance in field of sensing and electronic applications.17 Among metal chalcogenides, Cu2S is a p type semiconductor with bandgap of ~ 1.2 eV and has been utilized for a variety of applications in the field of energy harvesting, sensors, electronic



and optoelectronic applications.18-20 Since Cu2S is a sulfur based metal chalcogenide and has lattice parameters close of MoS2, the synergistic effect allows for the growth of MoS2/Cu2S hybrids. Even though there are literature on synthesizing Cu2S by variety of chemical routes, reports on hybrids of MoS2/Cu2S are few and the interesting applications it offers remain unexplored. In this work, we demonstrate, for the first time, flexible MoS2/Cu2S hybrid for its utilization in humidity, temperature, and breath sensing and ethanol adulteration and integrated it to microcontroller wherein the data acquired was transferred to smartphone through Bluetooth communication thereby enabling remote sensing. Dedicated application for each sensing was developed which can be could be selected by a simple user interface. Detailed chemical characterizations were performed to confirm the formation and understand the growth dynamics of MoS2/Cu2S hybrids. The fabricated sensor was further utilized for real time diagnosis of hydration level of human body using breath. Detailed sensing mechanism is explained for environmental monitoring (temperature and humidity) and different chemical stimuli (breath and ethanol) analyzing the transport properties of MoS2/Cu2S. The method presented here is a major step in the field of multifunctional sensing. As per the authors’ knowledge, this is the first report on growing MoS2/Cu2S hybrid on flexible cellulose paper substrate and utilizing it for wireless multifunctional sensors. 2. Results and Discussion Two step hydrothermal method was employed for the growth of MoS2-Cu2S on cellulose paper wherein MoS2 was initially grown on cellulose paper followed by the growth of Cu2S on MoS2cellulose paper. One step in situ synthesis of MoS2-Cu2S was not possible as MoS2 needs elevated heating and takes longer time (20 hours) as compared to the formation of Cu2S which


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takes only few hours. Growth time for Cu2S was optimized at 6 hours such that there is a discrete distribution of Cu2S over MoS2 so as to have synergistic effect of both MoS2 and Cu2S for sensing. Increasing the hydrothermal time leads to the thick layer deposition of Cu2S thereby shielding entire MoS2. Detailed explanation regarding the growth of MoS2/Cu2S on cellulose paper is discussed in Experimental section and the schematics for the same is as shown in Figure 1. To study the crystal structure of MoS2/Cu2S grown on cellulose paper, XRD was performed as shown in figure 2a. The well-defined peaks of Cu2S can be well indexed to face centered cubic Cu2S (JCPDS no- 84-1770).18 It should be noted that no peaks related to Cu or CuO was observed suggesting the high phase purity of Cu2S. The XRD peaks pertaining to MoS2 reveal hexagonal phase of MoS2 (JCPDS no – 37-1492). Peaks corresponding to (100) and (110) planes are observed and the missing peak at 2θ = 14° can be attributed to the few layer MoS2 (less than 5 layers).12 To study the morphology of the as grown MoS2/Cu2S on cellulose paper, FESEM analysis was performed. Figure 2b shows the growth of MoS2 on cellulose paper where microfibers like structures of cellulose paper is observed suggesting that the growth of MoS2 on cellulose paper does not degrade the properties of cellulose paper substrate. The growth of Cu2S nanoparticles on MoS2/cellulose paper can be clearly observed in figure 2c. Further, discrete random distribution of Cu2S on MoS2/cellulose paper is clearly observed in high magnification FESEM image as shown in figure 2d. Figure 2e shows further high magnification image wherein spherical morphology of Cu2S and nanosheets morphology of MoS2 is clearly visible. The low sampling depth and high surface-sensitivity renders X-Ray Photoelectron spectroscopy (XPS) as a powerful technique to probe the surface states and chemical composition of the



sample. Figure 3d highlights the survey spectrum of MoS2-Cu2S wherein Molybdenum, Sulfur, copper, oxygen and carbon are present as the primary elements in the sample. The presence of a peak for carbon at 284 eV is due to the utilization of cellulose paper as the substrate for growth of MoS2-Cu2S. Mo 3d and S 2p peaks in the survey provide important information about the phases present in synthesized MoS2 and hence their respective spectral features upon deconvolution is presented in figure 3a and figure 3c. The presence of two prominent peaks at 228.7 eV and 232 eV in the Mo 3d spectra signifies the presence of Mo(IV) 3d5/2 and Mo(IV) 3d3/2 in the prepared sample supporting the fact that the synthesized MoS2 had a large density of MoS2 in +4 oxidation state.21 However, the presence of two doublets upon deconvolution of the above peaks at 228.7, 232 eV and 229.6, 232.9 eV with a spacing of 0.9 eV proves the co-existence of both 1T and 2H phase of MoS2.22. In addition, the peak present at 235.2 eV corresponds to a small amount of unreacted MoO3 on the surface of MoS2.23 Similarly, the broad peak in figure 3c displays the S 2p spectrum, which on further deconvolution revealed the presence of peaks at 161.7 and 163.2 eV suggesting the presence of S 2p

3/2

and S 2p

1/2

levels.24 Besides, the peaks at 932.4 eV (Cu 2p3/2) and 952.4 eV (Cu 2p1/2)

confirms the formation of monovalent copper sulfide (Chalcocite), which is in accordance with the NIST XPS database. The presence of an auger peak at 569.6 eV, as depicted in figure 3b, indicating the presence of Copper in (I) state validates the formation of Cu2S. Moreover, the absence of any satellite peaks in the Cu2p spectra denotes that the synthesized material were devoid of other crystallographic phases of copper.25-26

Humidity sensor


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The fabricated flexible MoS2/Cu2S hybrid was utilized as humidity sensor wherein the sensor was exposed to different relative humidities and corresponding resistance change was measured. Saturated salt solutions were prepared for different relative humidity environment required for the testing of the fabricated sensor where the humidity was monitored by commercially available humidity sensor (AM2302 DHT22). The experiments were performed in ambient temperature of 27°C. The electrical resistance of the MoS2/Cu2S was measured using Keithley 2450 connected to PC through RS-232 interface. Further, the measurements were also performed by measuring the response through Arduino board and transmitting the data wirelessly via Bluetooth to smartphone. Experimental set up for the humidity sensing is as shown in figure 4a. Figure 4b shows the IV characteristics of MoS2/Cu2S under different relative humidity conditions wherein as the RH increases, increase in the current of MoS2/Cu2S device was observed. This can be attributed to the electron donor nature of the water molecules which donates electrons to MoS2/Cu2S thereby increasing the current. Further temporal response of MoS2/Cu2S with different RH was measured and is as shown in figure 4c. 46 % increment in normalized resistance was observed for 78% RH. Similar experiments were repeated with pristine MoS2 wherein increment in the current was observed when the device was exposed to different RH. Figure showing the graph of both pristine MoS2 and MoS2/Cu2S under 78% RH can be found in SI wherein it was found that MoS2/Cu2S was more sensitive to humidity when compared to pristine MoS2. (Graphs showing the response under different RH for pristine MoS2 can be found in Supporting Information (SI) as figure S1). Figure 2d shows the calibration plot of the fabricated humidity sensor wherein linear fit suggests the sensitivity of the MoS2/Cu2S humidity sensor to be 0.57/RH (%).



To understand the working principle of the humidity sensor, sensing mechanism for pristine MoS2 under different RH is initially explained which can be attributed to the resistance modulation due to the absorption of water molecules. MoS2 is n type material and the water molecules arising due to the increase in humidity are electron donor species. When pristine MoS2 is exposed to higher RH environment, water molecules get adsorbed on MoS2 and donates electrons to MoS2 which increases the majority carriers of MoS2 thereby increasing current. As the water molecule gets adsorbs on the MoS2 surface, there is downward band bending of n type MoS2 allowing for the easy flow of electrons towards the metal contact. As soon as the device is bought back to the initial RH environment, water molecule gets desorbed from the MoS2 surface which creates an upward band bending thereby retaining the initial resistance value. But the inclusion of p type Cu2S alters the sensing mechanism which is due to the creation of local p-n heterojunction. It should be noted that due to the discrete distribution of Cu2S on MoS2 as evident from FESEM image (figure 2e) main conduction occurs along the MoS2 and hence the metal contacts are fabricated on MoS2. Also, to attain an equilibrium state, p type Cu2S will transfer electron to n type MoS2 thereby acting as an electron absorber which increases the potential barrier and increases the resistance thereby intensifying the resistance modulation due to RH. This was verified by measuring IV characteristic of MoS2/Cu2S and comparing it with pristine MoS2 and was found that with the inclusion of Cu2S, the resistance of the sensor increased when compared to pristine MoS2. As the RH increases, the water molecules gets adsorbed on the n type MoS2 as well on p type Cu2S. Since water molecules are electron donor species, it will increase the charge carrier concentration in n type MoS2 and will decrease the charge carrier concentration in Cu2S which results in decrease in the potential barrier created thereby increasing current. Another explanation to response of the MoS2/Cu2S sensor towards


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humidity can be attributed to the 1T/2H mixed phase and some amount of unreacted MoO3 which induces defects as evident from the XPS spectra (figure 3). These defects are due to the sulfur vacancies and play an important role in catalytic property of MoS2. The water molecule due to the increase in the humidity will get adsorb on the sulfur vacancies and donating electron to MoS2 thereby increasing current. Schematic showing the sensing mechanism for both pristine MoS2 and MoS2/Cu2S is as shown in figure 4e.

Breath sensor Motivated with the high sensitive response of MoS2/Cu2S towards various RH, the fabricated sensor was further explored for its utilization in breath sensing as the humidity is expected to be different while inhalation and exhalation of air (usually higher).

The experiments were

performed with a 21 year old healthy adult placing the device at a distance of 4 cm from the nose as shown in figure 5a. Figure 5b shows the response of the sensors for 60 seconds wherein during exhalation, the resistance dropped and while inhalation the resistance of the sensor tries to go to its initial value. It should be noted that the recovery time of the sensor is low due to the defects induced in MoS2/Cu2S hybrid during the hydrothermal synthesis on cellulose paper. The resistance further decreases upon next successive breath and hence the sensors do not have a stable baseline. However, stable baseline is not important in this application as the breath counts depend upon the number of peaks. Further the sensor was investigated for the response of the subject taking deep breaths as shown in figure 5c. It was observed that under normal breath conditions, 20 breath counts per minute was recorded. The subject was asked to modulate the breath pattern by repeatedly inhalation and exhalation of breath. Interestingly, the sensor was able to detect the changes in the breathing pattern as shown in figure 5d. As soon as the subject



stops breathing, the sensor tries to regain its initial current baseline. Characterization of breath is a simple yet effective tool in diagnosing health of an individual. Also, it has become powerful non-invasive tool in diagnosing various diseases such as asthma, diabetes etc.27-28 While monitoring of specific analytes for disease/disorder is important, it is also important to monitor humidity of the exhaled breath as it can provide information on the individual’s health such as dehydration. There are reports which monitor dehydration through other indices,29 but monitoring dehydration through humidity is simple and remains few and unexplored. To do so, we have performed experiments monitoring the subject’s nasal breath and not mouth breath as nasal breath can give information about lung hydration levels. The experiment was performed for 3 individuals and the sensor measured the breathing pattern in terms of resistance change. To verify the exact humidity levels, commercial humidity sensor was also integrated which gives out the reading of exact RH change. The subject was made to breathe normally and the data was recorded as shown in figure 5e. The subject was then made to run for 20 minutes continuously and data was recorded after relaxing after a minute such that the subject attains stable breathing rate. Interestingly, the decrease in the resistance for the latter case was higher when compared to the resistance recorded for normal breath (without running) which can be attributed to the dehydration of the subject after running. The commercial humidity sensor also displayed decrease in the RH level when measured on subject after running. The measurements were repeated after every 60 minutes and interestingly the humidity level of the subject was back to its initial RH value after 120 minutes of running. Also the change in resistance of the fabricated sensor regained its normalized resistance value after 120 minutes as shown in figure 5f. This can be attributed to the fact that lungs regain their hydration levels which is an indication of a healthy individual. It is known that human lungs remains wet and is prone to be dry thereby reducing the


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RH in the exhaled breath. However, it is interesting to note that in our study, RH of the breath was completely regained in 2 hours indicating that lungs regenerates its hydration levels by various metabolic activities of human body. Sensing mechanism of the breath sensor would be on similar lines as already discussed in humidity sensing part wherein the increase in the RH decreases the potential barrier between MoS2/Cu2S thereby decreasing resistance.

Ethanol (EtOH) adulteration To further expand the scope of applications that MoS2/Cu2S offers, it was used for ethanol adulteration. Detection of water content in ethanol is vital as water is totally soluble in ethanol and is commonly used as an adulterant in chemical and pharmaceuticals industries and bio fuel cells.30-31 Most of the available methods for detection of ethanol adulteration are based on gas chromatography and Infrared spectroscopy which are costly and energy inefficient.32 However, irreversibility is a major problem in these systems. In this work, we utilized simple chemiresistive MoS2/Cu2S based sensor for the detection of water in ethanol. The samples were prepared by adding various concentrations of water in ethanol (% v/v). The electrical resistance of the MoS2/Cu2S was measured using Keithley 2450 connected to PC through RS-232 interface. The change in the current levels was monitored by drop casting 5µL of adulterated solution on the flexible MoS2/Cu2S device. Figure 6a shows the temporal response of the fabricated sensor wherein different water/EtOH concentrations were drop casted and the modulation in device current was monitored. It was observed that current decreases as the concentration of water in ethanol increases wherein the fabricated sensor was able to detect as low as 2% water in ethanol (v/v). Similar experiments were repeated for 2 more devices and the results are as shown in figure 6b where it can be observed that the sensor is more sensitive to lower concentrations of



water in ethanol and as the concentration of water in ethanol increases, the response of the sensor eventually saturates. Linear fitting the calibration plot (figure 6b) suggest that the sensitivity of the fabricated MoS2/Cu2S device was -0.02/% of water towards ethanol adulteration. Further, the results of the EtOH sensing were wirelessly transmitted on the smartphone thereby enabling user friendly detection of EtOH adulteration. The sensing mechanism of the sensor can be explained in terms of oxidation of ethanol with water wherein water acts as a catalyst and leads to the release of H2 gas.33 The reaction for the same is as follows CH3CH2OH + nH2O CH3CHO + H2 + nH2O Hydrogen from OH group of ethanol gets transferred to the water molecule, while the hydrogen that is bonded with water is released to form H2 molecule. These low concentration water molecules chemisorb on the active sites of MoS2/Cu2S surface via dissociative chemisorption process as follows: H2O + Mo + O 2OH-Mo + V +2eWhere, O is lattice sulfur and V is sulfur vacancy adjacent to the sulfur site. Next H+ dissociated from the water molecules reacts with doubly ionized oxygen to form OH- species H+O2- OHThese OH- ions are responsible for decrease in the current. Also, H2 molecule released may adsorbed on MoS2/Cu2S and since H2 is an electron acceptor, it increases the potential barrier between MoS2/Cu2S thereby decreasing current. As the concentration of water in ethanol increases, more H2 molecules are released thereby further decreasing the current. Apart from that, charged species in water might be the cause for the decrease in current. Moreover it is also reported that adding water in ethanol in air leads to the higher production of H2.34 In this case,


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addition of water to ethanol was done under ambient conditions which might have led to higher production of H2 resulting in highly sensitive response of the fabricated sensor towards ethanol. The results presented here not only quantifies the amount of water mixed with ethanol but also open avenues for the possibility of H2 detection with MoS2.

Temperature sensor MoS2/Cu2S was further utilized as a temperature sensor. Figure 7a shows the IV characteristics of MoS2/Cu2S in the range of 25°C to 120°C wherein increment in the current was observed demonstrating negative temperature coefficient of resistance (TCR). The measurements were performed by increasing the temperature of the slab of probe station to 120°C and then cooling it in steps till 25°C. Prior to each measurements waiting period of 20-25 seconds was observed so as to stabilize the temperature. The results indicate the fabricated sensor to have a wide dynamic range of detection and hence can be used in industrial applications where high temperatures monitoring are required. TCR of the fabricated sensor was 2 x 10-5 °C-1 defined by α= 1/R0 × (dR/dT), where R0 is the initial resistance. The calculated TCR value was found to be higher than commercial available Pt based temperature sensors. Higher the TCR value, more sensitive is the sensor towards temperature variations. TCR value calculated for pristine MoS2 was (data in SI as figure S2) which was less when compared to MoS2/Cu2S demonstrating that MoS2/Cu2S is more sensitive to temperature changes when compared to pristine MoS2. This could be attributed to the decrease in the potential barriers of MoS2/Cu2S upon increase in temperature. Activation energy of MoS2/Cu2S sensor was calculated to be 86.7 meV by plotting the linearly fitted ln (R) versus inverse of temperature as shown in figure 7c. Greater the activation energy, more sensitive is the sensor towards temperature changes. To demonstrate the real time application of the fabricated



temperature sensor, it was subjected to flame and the temporal response of the sensor was measured. Interestingly, when the flame was lit, increase in the current was observed and as soon as the flame was extinguished, the device regains its initial baseline current. The same was repeated for 6 cycles and the as seen in figure 7d the results were repeatable. This suggests that the fabricated sensor could be used in flame detection which finds potential applications in early fire detection in commercial and non-commercial infrastructures. Majority of the flame sensors are based on chemical and physical properties of flames such as smoke emission, light etc which might lead to false alarm.35 The present approach displays a simple approach wherein conductivity of a highly sensitive functional material modulates when exposed to flame. To demonstrate the robustness and reliability of the fabricated multifunctional sensor, the sensor was subjected to bending cycles wherein the device was bend (both tensile and compressive strain) and bought to its initial position which was then tested for humidity sensing, ethanol adulteration and temperature sensor. Negligible change in the performance was observed even after 500 bending cycles suggesting the reliability and robustness of the fabricated sensor. Graph showing the response of the sensor towards humidity, temperature and ethanol adulteration cab be found in SI as figure S3. Also, the sensor parameters for humidity, temperature and ethanol adulteration can be found in SI as Table S1. Further, it is important to measure the sensor signal simultaneously for multi-sensor platform and hence temperature and humidity measurements were performed simultaneously wherein as the temperature increases, decrease in the humidity values were observed. This can be attributed to the fact that as temperature increases, the amount of water vapor the air can hold increases which decreases the relative humidity. The temperature was varied and corresponding humidity and resistance values of the fabricated sensor were measured. The graph showing the temperature


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variation and the corresponding relative humidity values and the change in resistance of the fabricated sensor can be found in SI as figure S4. When compared the values of the change in resistance of the fabricated sensor with the calibration plot of the fabricated humidity sensor, the values were found to be comparable suggesting the reliable performance of the fabricated sensor under different temperatures. Since exhaled breath also works on the principle of humidity change, the above experiments also confirms the reliability of breath sensor under different temperatures. Wireless integration of sensor The as fabricated sensor was interfaced with microcontroller wherein the data acquired was transferred to smartphone via Bluetooth. Dedicated android based application was coded wherein it had individual modules for each sensing (humidity, temperature, breath analysis and ethanol adulteration). The data acquired by the sensor (resistance change values) was then transferred to smartphone via Bluetooth wherein another dedicated application would convert the data into graph thereby enabling easy readout for the user. Figure 8a displays the schematic of the circuit diagram showing the integration of sensor with smartphone via Arduino Uno board. Simple resistive circuit was employed for measuring the unknown resistance, which, in this case is the fabricated sensor. Arduino board was employed as the controller. The continuous resistance change data of the sensor can be accessed from Arduino board programming which was then used for plotting graphs. To transfer the sensor data acquired by Arduino board, Bluetooth module (HC-05) which utilizes easy serial port protocol for transparent wireless serial connection setup was used. An android programming application was developed on the smartphone for receiving the sensor real time data and plot the responses on the smartphone screen. Figure 8b shows the frontend display of the android app wherein four modules namely



Humidity, temperature, breath analysis and ethanol adulteration are clearly observed. The user can select the dedicated module and perform sensing. Figure 8 c, d, e shows the corresponding real time sensor data acquired by the smartphone for humidity, temperature and ethanol adulteration respectively. The data was measured by continuously varying the respective stimuli. Details regarding the connection and the code for the Android application can be found in the SI as figure S5.

There are various studies which report smartphone based wireless sensors for the potential point of care diagnostic applications. Adiguzel et al., reported wireless breath sensor for detection of various diseases including lung cancer.36 Apart from biomarkers detection, physical parameters such as temperature has been studied as diagnostic tool for human health condition.2 Liu et al., reported smartphone based VOCs detection and further applied for breath sensing.6 Mogera et al., studied the hydration levels of lung by the use of breath analysis.27 However, the fabrication cost of such individual sensors is huge and thus are not well suited for applications in flexible and wearable IoT applications. Moreover, the sensors are utilized for sensing single entity thereby limiting the scope of applications. There are reports on multifunctional sensors using functional nanomaterials, but frontend processing data and the ability of the functional material to change its transport properties to different chemical stimuli often leads to falsified data. Further, utilizing the same fabricated sensor for multifunctional sensing often leads to unreliable data. In this report, we demonstrate the fabrication of flexible and disposable multifunctional sensor which is integrated to smartphone for real time remote sensing. The cost of each sensor is ~0.02$ which allows the user to use new sensor every time a sensing of a different chemical stimuli is required thereby increasing the accuracy. Further, the sensor being fabricated on


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cellulose paper allows for easy disposability. The synthesis procedure can is entirely scalable and can be utilized to grow large area MoS2 on cellulose paper. The growth area depends upon the size of the autoclave and hence larger the size of hydrothermal autoclave, larger MoS2-cellulose paper can be achieved. Also, the synthesis method demonstrated here is versatile and can be used to to various synthesize different functional materials.

3. Conclusion In summary, we demonstrate, for the first time, the growth of MoS2-Cu2S on disposable cellulose paper and utilize it for wireless multifunctional sensing of humidity, temperature, human breath and ethanol adulteration. The sensor was integrated to smartphone with a dedicated application developed for each sensing module. Sensitivity of humidity sensor was calculated to be 0.6/%RH TCR of the temperature sensor was found to be 2 x 10-5 °C-1 with an activation energy of 86.7 meV. The sensor was able to detect as low as 2% of water in ethanol (v/v %). The sensor was applied for real time monitoring of hydration level of lungs by RH analysis of breath sensing (before and after rigorous exercise) and it was found that the lungs of a healthy individual was able to regain its moisture after 120 minutes after running. Further, utilizing the temperature sensing property of the sensor, fabricated device was tested for real time flame detection with highly repeatable performance. The successful development of such wireless disposable multifunctional sensors is a major step ahead in flexible electronics for IoT applications.

4. Experimental Section



Fabrication of paper-based MoS2-Cu2S photodetector was carried out using two-step hydrothermal process. MoS2 was grown on cellulose paper substrate followed by the growth of Cu2S on MoS2 grown paper. Growth of MoS2 on cellulose paper: Cellulose paper (5 cm x 5cm) was utilized as the substrate for the hydrothermal growth of MoS2. The seed solution was prepared by mixture of 10mM of sodium molybdate (Na2MoO4.2H2O) and 20mM of thiourea (CH4N2S) in deionized (DI) water. The paper substrate was dipped in asprepared seed solution for 1 h followed by drying at 80 °C. A nutrient solution comprising of 50mM sodium molybdate and 100 mM thiourea was stirred in DI water for 30 min. Thereafter, the seed-coated cellulose paper and the nutrient solution were transferred to the hydrothermal reactor and was maintained at 200 °C for 20 h. The reactor was allowed to cool down naturally and the resultant black colored cellulose paper was dried at 80 °C.

Growth of Cu2S on MoS2-cellulose paper MoS2 grown on cellulose paper was utilized as a substrate for the growth of Cu2S. The seed solution was prepared by dispersing 20mM of copper acetate and 10mM of sodium sulfide. The MoS2-cellulose paper was dipped in as prepared seed solution for 1 h followed by drying at 80°C. A nutrient solution for hydrothermal was prepared by mixing 100 mM of copper acetate and 50mM of sodium sulfide. Thereafter, the seed coated MoS2-cellulose paper and the nutrient solution were transferred to the hydrothermal autoclave and was maintained at 180°C for 6 h. The hydrothermal reactor was then allowed to cool down naturally and Cu2S grown on MoS2cellulose paper was dried at 80°C.


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Fabrication of multifunctional sensor The as obtained Cu2S on MoS2-cellulose paper was cut into desired dimension (7mm x 7mm) followed by defining contacts by silver paste. Schematic demonstrating the complete synthesis and fabrication procedure is as shown in figure 1

Materials and characterization Sodium molybdate, and Thiourea were purchased from Sigma Aldrich and were used as received. Structural characterization was investigated using X’pert PRO X-Ray Diffraction (XRD) with Cu Kα radiation. Raman spectra were obtained from Raman spectrometer (Senterra inVia opus, Bruker) having an excitation wavelength of 532 nm. Field Emission Scanning Electron Microscopy (FESEM) analysis was performed by ZEISS Ultra-55 SEM to study morphology.

UV-visible-NIR

spectra

was

obtained

using

LAMBDA

UV/Vis/NIR

spectrophotometers (PerkinElmer). The electrical measurements were carried out with Keithley 4200 and 2450 SCS instrument. Arduino Uno board, Bluetooth module (HC-05), smartphone, humidity and temperature sensor were utilized for smartphone readout of the sensor. To confirm the humidity of the saturated salt solutions, it was monitored by commercial available humidity sensor (AM2302 DHT22). ASSOCIATED CONTENT Supporting Information



Page 20 of 37

Comparative temporal response of MoS2 and MoS2/Cu2S for 78% RH, comparative TCR values for MoS2 and MoS2/Cu2S, bending studies for humidity sensor, ethanol adulteration and temperature sensor, Table containing sensor parameters such as sensitivity, limit of detection and TCR, graph of humidity sensing with varying temperature and screenshot of the layout of the developed android code. Author information *

E-mail: [email protected]

Notes: Authors declare no competing financial interests.

Acknowledgements A part of the reported work (characterization) was carried out at the IITBNF, IITB under INUP which is sponsored by DeitY, MCIT, Government of India. SB acknowledges Seed grant, IITH. References 1. Gong, S., Schwalb, W., Wang, Y., Chen, Y., Tang, Y., Si, J.,& Cheng, W. A wearable and

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Figure 1. Schematics demonstrating the two step hydrothermal method for growth on MoS2 on cellulose paper followed by the growth of Cu2S on MoS2-cellulose paper.

Figure 2. a) XRD of MoS2/Cu2S grown on cellulose paper b) FESEM image of MoS2 grown on cellulose paper c) FESEM image of Cu2S grown on MoS2-cellulose paper d,e) High magnification image of Cu2S on MoS2/cellulose showing the flake like structure of MoS2 and Cu2S nanoparticles.




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Figure 3. a) Deconvolution spectra for Mo2d b) Deconvolution spectra for Cu2p c) Deconvolution spectra for S2p d) XPS spectra of MoS2/Cu2S on cellulose paper




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Figure 4. a) Schematic of the humidity sensing set up b) IV characteristic of MoS2/Cu2S on cellulose paper upon exposure to different RH levels c) Temporal response of MoS2/Cu2S on cellulose paper upon exposure to different RH levels d) Calibration plot of MoS2/Cu2S for humidity sensing e) Schematic illustrating the sensing mechanism of MoS2/Cu2S under different RH levels.


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Figure 5. a) Photographic image showing the breath sensing set up b) Temporal response of MoS2/Cu2S under normal breathing for 60 seconds c) Temporal response of MoS2/Cu2S under deep breathing d) Temporal response of MoS2/Cu2S under breath modulation (breath-stopbreath) e) Graph showing the humidity levels of breath of healthy individual before running and



after running f) Graph showing the normalized resistance change in MoS2/Cu2S device when exposed to breath of healthy individual before and after running.

Figure 6. a) Temporal response of MoS2/Cu2S when exposed to different (v/v %) of water in ethanol b) Calibration plot of MoS2/Cu2S when exposed to different (v/v %) of water in ethanol (N=3)


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Figure 7. a) IV characteristic of MoS2/Cu2S under different temperature (30°C to 120°C) b) Graph of resistance change v/s temperature c) Arrhenius plot (ln R) v/s inverse of temperature for MoS2/Cu2S device d) Temporal response of MoS2/Cu2S under repeated exposure to flame



Wireless integration of sensor.

Figure 8. a) Schematic of the circuit diagram for wireless integration of sensor b) Main application screen showing four modules namely Humidity, breath analysis, Ethanol adulteration and Temperature c) Real time sensor data acquired by smartphone for humidity sensing d) for temperature sensing and e) % of water in ethanol.


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Flexible, disposable cellulose paper based MoS2 ... - ACS Publications

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