Multifunctional Sensor Based on Porous Carbon Derived from Metal

Flexible and sensitive sensors that can detect external stimuli such as pressure, temperature, and strain are essential components for applications in...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 3986−3993

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Multifunctional Sensor Based on Porous Carbon Derived from Metal−Organic Frameworks for Real Time Health Monitoring Xin-Hua Zhao,†,‡ Sai-Nan Ma,‡ Hui Long,†,‡ Huiyu Yuan,†,‡ Chun Yin Tang,†,‡ Ping Kwong Cheng,†,‡ and Yuen Hong Tsang*,†,‡ †

The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen, Guangdong, People’s Republic of China Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Flexible and sensitive sensors that can detect external stimuli such as pressure, temperature, and strain are essential components for applications in health diagnosis and artificial intelligence. Multifunctional sensors with the capabilities of sensing pressure and temperature simultaneously are highly desirable for health monitoring. Here, we have successfully fabricated a flexible and simply structured bimodal sensor based on metal−organic frameworks (MOFs) derived porous carbon (PC) and polydimethylsiloxane (PDMS) composite. Attributed to the porous structure of PC/PDMS composite, the fabricated sensor exhibits high sensitivity (15.63 kPa−1), fast response time (0.11 °C−1) and fast response time (∼100 ms), indicating its potential application in sensing temperature. All of these capabilities indicate its great potential in the applications of health monitoring and artificial skin for artificial intelligence system. KEYWORDS: metal−organic frameworks (MOFs), porous carbon, pressure sensor, temperature sensor, real time health monitoring

1. INTRODUCTION Flexible and wearable electronic devices are in high demand because of their applications in health diagnosis and artificial intelligence.1−4 In particular, pressure sensors, which convert applied forces into electrical signals, have been widely explored.5−7 By detecting the subtle pressures related to human activities such as pulse pressure and facial expressions, it is possible to monitor the health conditions of the human body.8−11 For practical applications, high sensitivity, flexibility, and stability are required for pressure sensors. To date, great efforts have been made to optimize the materials and device structures to achieve high-performance pressure sensors. First, different kinds of active materials including active carbon,12 graphene,13−15 metal nanoparticles,16 metal nanowires,17 and widely studied conductive polymers18 such as poly(3,4-ethylenedioxythiophene−poly(styrenesulfonate) (PEDOT:PSS) have been introduced into pressure sensors to function as the conductive fillers for the merits of their high con ductivity, high stability, and low cost. Second, to obtain highly flexible and stretchable devices, elastomer composites such as polydimethylsiloxane (PDMS) and polyurethane (PU) have been widely used in the development of pressure-sensing matrices. It has been demonstrated that preparing the elastomer and conductive materials composite as the sensing layer is a promising method to fabricate high-performance pressure sensors.19−23 Nano- or microporous structures have been employed to increase the sensitivity and improve the response speed of the sensors.24−26 © 2018 American Chemical Society

Apart from the individual pressure-sensitive sensors, multifunctional sensing platforms that can simultaneously detect pressure and temperature stimuli are highly desired in health diagnosis such as detecting pulse pressure and temperature of artery vessels at once and analyzing the influence of temperature on the pulse pressure.27,28 Besides, multifunctional sensors are of great importance in the application of artificial fingers which are required to feel the temperature and discern surface texture and hardness of the objects at the same time.29−31 Recently, great efforts have been paid in the development of such kind of multiple sensors.32−35 For instance, by using ferroelectric and graphene, Park et al. developed a flexible and highly sensitive e-skin which can sense pressure, vibration, and temperature simultaneously. Meanwhile, their sensors have demonstrated the abilities of simultaneous monitoring of pulse pressure and temperature of artery vessels, precise detection of acoustic sounds, and discrimination of various surface textures.36 Despite the great effort being devoted to the development of these multifunctional sensors, the sophisticated fabrication process and the structure complexity are hindering their practical applications. To meet the requirements of practical applications, a simple and cost-effective approach for Received: November 6, 2017 Accepted: January 5, 2018 Published: January 5, 2018 3986

DOI: 10.1021/acsami.7b16859 ACS Appl. Mater. Interfaces 2018, 10, 3986−3993

Research Article

ACS Applied Materials & Interfaces

copper conducting wires were attached to the ITO electrodes to complete the fabrication of the bimodal sensor device. 2.4. Material and Device Characterizations. The crystallographic information on the as-prepared samples was characterized by X-ray diffraction (XRD, Bruker, D8 Phaser) using Cu Kα radiation (λ = 1.5418 Å) and Raman spectroscopy (Reishaw 2000). Thermogravimetric analysis (TGA) above room temperature for MOF-5 was performed on Mettler Toledo TGA/DSC3+ at a rate of 10 °C min−1 under a N2 atmosphere. N2 adsorption/desorption isotherms were measured at 77 K, using an ASAP 2020 instrument (Micromeritics). Before the experiments, the samples were degassed under vacuum at 150 °C for 3 h. The surface area, pore volume, and pore-size distribution of the samples were determined on the basis of the N2 adsorption/desorption isotherm. The morphologies and microstructures of the samples were investigated by the scanning electron microscopy (SEM, Tescan VEGA3). The current responses of the pressure sensor to external pressure were measured using a computer controlled testing system including a motorized test stand and force gauge (Mark-10). The pressure sensor was attached to a human wrist with the assistance of tape to monitor the wrist pulse waves. For the measurement of wrist pulse after physical exercise, the device was attached to the human wrist in the whole process of running; then, the signals of the wrist pulses were measured immediately when the running was finished. The applied voltage between two electrodes was 3 V for all of the electrical measurements, and the electrical measurement was carried out by the Agilent 4155C Semiconductor Parameter Analyzer in ambient condition. The current responses to temperatures from 23 to 120 °C were measured by placing the as-prepared sensor on a hot plate, before each measurement; the temperature was increased by 10 °C and then maintained as a constant for 10 min to ensure thermal equilibrium between the sensor and hot plate. By abruptly loading a water droplet at 70 °C onto the surface of the sensor, the response time for sensing temperature was determined.

fabrication of the multifunctional sensors with high sensitivity and good flexibility is essential. Metal−organic frameworks (MOFs) are a class of crystalline materials assembled by metal ions as the nodes and organic ligands as struts.37−39 Due to their high surface area and permanent porosity, MOFs have been used as sacrificial templates and precursors to fabricate porous and functional nanostructured materials, such as carbonaceous materials,40 metal oxides,41 and carbon-wrapped metal nanoparticles.42 These nanostructured materials have been investigated in a wide range of applications, including chemical sensors,43,44 gas adsorption,45 heterogeneous catalysis,46,47 energy storage devices,48−50 and gas separation.51,52 As a class of carbon materials, MOF-derived porous carbon (PC) is environmental friendly, conductive, and nontoxic. It could be an ideal material for fabricating sensors because of their outstanding properties of high porosity, low density, and excellent electrical stability. However, to the best of our knowledge, only a pressure sensor based on MOF-derived nanomaterials (copper 7,7,8,8-tetracyano-p-quinodimethane (CuTCNQ)) has been reported so far.53 The bimodal sensors that can detect both the pressure and temperature based on the MOF derived nanostructures have not been reported. In this study, we fabricated a flexible and simply structured bimodal sensor with the MOF-derived PC and PDMS composite that can sense both pressure and temperature. Due to the porous structure of PC/PDMS composite, the fabricated sensor exhibits high sensitivity, fast response time, and good stability for sensing both pressure and temperature. Furthermore, the application of the sensor device in detecting wrist pulse in real time has been demonstrated, indicating its great potential in a real time health monitoring application.

3. RESULTS AND DISCUSSION 3.1. Characterization of Sensor Based on PC/PDMS Composite. Figure 1a shows the key fabrication steps of the PC and PC/PDMS based pressure sensor device. As proven by the TGA measurement (Figure S1), the MOF-5 was decomposed into ZnO and porous carbon at temperatures higher than 550 °C. Therefore, the 3D PC was obtained by thermal decomposition of MOF-5 at 800 °C and acid etching. The PC/PDMS composite was prepared by a mechanical mixing process. The detailed fabrication process of the PC/PDMS based pressure sensor is depicted in Figure S2. The surface morphology of as-prepared MOF-5 precursor and the derived ZnO@C and PC materials was explored by SEM. As shown in Figure S3, the MOF-5 crystals show cubic shape with the particle size of 50−80 μm. After the MOF-5 precursor was thermally decomposed into ZnO@C, the crystal shape was well maintained and no significant size shrinkage was observed. Compared with the neat surface of MOF-5, the ZnO@C products exhibit a porous nanostructure with a rough surface. After the removal of ZnO, more defects and holes of PC are generated compared with those of ZnO@C. Furthermore, the top-view and cross-sectional view of the PC/PDMS film were also characterized by SEM as depicted in Figure 1b,c, respectively. As shown in Figure 1b,c, the cubic-shaped PC particles are well dispersed in the PDMS matrix and the thickness of the film is determined to be about 700 μm. Additionally, cavities with feature sizes ranging from tens to hundreds of microns can be found within the whole film. Meanwhile, microscale pores are distributed on the rough surface of the film. The XRD measurement was performed to explore the detailed crystal structures and composition of different samples.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals including zinc nitrate hexahydrate (Zn(NO3)2·6H2O), terephthalic acid (H2BDC), N,N-dimethylformamide (DMF), acetic acid (HAc), and polydimethylsiloxane (PDMS, Sylgard 184) were obtained from Sigma-Aldrich and used without further purification. The indium tin oxide (ITO, 100 nm) coated poly(ethylene terephthalate) (PET, 125 μm) sheets were purchased from Sigma-Aldrich, washed with ethanol, and blown with dry nitrogen gas flow for further use. 2.2. Preparation of MOF-Derived PC Material. First, the MOF-5 crystals were prepared by a hydrothermal method.54 In detail, a mixture of Zn(NO3)2·6H2O (0.595 g, 2 mmol) and H2BDC (0.111 g, 0.667 mmol) in 20 mL of DMF was sonicated at room temperature for 10 min. Then, the obtained mixture was sealed and placed in the oven at 135 °C for 24 h. Then, white crystals were obtained by filtration, washing with DMF, and drying in vacuum for 12 h. Second, the as-synthesized MOF-5 crystals were thermally treated at 800 °C under Ar atmosphere with a heating rate of 5 °C/min to pyrolyze the organic species. After the thermal annealing at 800 °C for 2 h, the material was cooled to room temperature to obtain the black-colored cubic ZnO@C nanoparticles. Finally, the ZnO@C nanoparticles were dipped into a 0.5 M HAc aqueous solution for 12 h to remove ZnO, obtaining the PC material. 2.3. Fabrication of Sensor Device. To prepare the PC/PDMS composite, the PDMS prepolymer was made by mixing the base monomer with the curing agent (the weight ratio of base monomer to curing agent was 10:1), followed by thoroughly stirring for 5 min. After that, the PC was added into the PDMS prepolymer, obtaining a PC/PDMS hybrid composite with a weight concentration of 30 wt %. To fabricate the PC/PDMS composite film, the PC/PDMS composite was cast onto the ITO-coated PET substrate with a doctor blade, followed by thermal treatment on a hot plate at 90 °C for 1 h. Finally, another piece of ITO-coated PET substrate was covered on top, and 3987

DOI: 10.1021/acsami.7b16859 ACS Appl. Mater. Interfaces 2018, 10, 3986−3993

Research Article

ACS Applied Materials & Interfaces

Figure 1. Fabrication procedure and structure characterization of the PC/PDMS based sensor. (a) Schematic diagram to show the fabrication procedure of the sensor. (b) Top-view SEM image of PC/PDMS composite film. The inset shows the SEM image of a single PC particle. (c) Crosssectional SEM image of the PC/PDMS composite film. (d) XRD patterns of the as-synthesized ZnO@C and PC material. (e) Raman spectra of PC material.

high relative pressure (P/P0 > 0.9), which probably originates from large meso- and macropores between PC particles. Meanwhile, the inset in Figure S8 showed that the PC/PDMS material has relatively broad mesopores with a maximum frequency near 5 nm. 3.2. Performance of the Device as a Pressure Sensor. The basic working principle of the PC/PDMS based pressure sensor is illustrated in Figure S9. The sensing operation depends on the variation of contact area both inside the composite film and between the composite film and the electrodes in response to external pressure. As discussed above, numerous meso- and micropores are exhibited in the PC/PDMS composite film. In the initial state without an external pressure (P = P0), the resistance of the conductive composite and the contact resistance between PC and the electrodes are high because of the small contact area between the PC fillers and between PC and the electrodes. When an external pressure is applied (P > P0), more conductive pathways are formed for the current flow. The increased conductive pathways may result from the following two possible processes: (a) the pores inside the PC/PDMS film are compressed to increase the contact area between neighboring PC particles, reducing the resistance, and (b) the increased contact area between the ITO-coated PET substrates and PC/PDMS film further decreases the contact resistance. After removing the external pressure, the contact areas between neighboring PC and between PC/PDMS film and electrodes relax to the initial state, and the resistance simultaneously increases to the initial value. Thus, the pressure sensing capability of the PC/PDMS composite mainly comes from the pores both inside and between the PC particles and the variation of contact area between the PC/PDMS composite and ITO-coated PET substrates.

As illustrated in Figure S4, the diffraction peaks of the as-prepared MOF-5 sample match well with the simulated XRD pattern of crystalline MOF-5 without any impurities. After the thermal treatment in Ar, the MOF-5 was completely transformed into ZnO quantum dots and amorphous carbon. As depicted in Figure 1d, all of the measured peaks are in accordance with the hexagonal wurtzite-type ZnO (JCPDF no. 36-1451) without any characteristic peaks of carbon detected, and we may attribute the absence of characteristic peaks of carbon to the shielding effect of the strong signal of ZnO peaks. However, after ZnO was removed by a 0.5 M HAc aqueous solution, a peak located at 23°, which corresponds to the 002 diffraction plane of carbon (JCPDS no. 41-1487), appeared and no ZnO peaks were detected (Figure 1d). The disappearance of ZnO peaks indicates the complete removal of ZnO and confirms the purity of PC. The 3D PC material was further analyzed by Raman spectra. As shown in Figure 1e, two peaks at 1350 and 1580 cm−1 are observed, which correspond to the disordered carbon (D bond) and graphic carbon (G bond), respectively. The ratio of D to G band intensity (ID/IG) is about 1.01, implying the abundant defects existing in the PC material. The surface areas and the pore-size distribution of MOF-5, ZnO@C, PC, and PC@PDMS were analyzed by N2 adsorption/desorption isotherms experiments according to the Barrett− Joyner−Halenda model. On the basis of the N2 adsorption/ desorption result in Figures S5−S8, the MOF-5, ZnO@C, PC, and PC/PDMS exhibit BET surface area of 493.058, 161.496, 582.186, and 377.256 m2 g−1 and a total pore volume of 0.273, 0.194, 0.611, and 0.292 cm3 g−1, respectively. As shown in Figure S8, the PC/PDMS material shows relatively large uptakes of nitrogen at low relative pressure (P/P0 < 0.1), and this is followed by a plateau region and a steep increase of adsorbed nitrogen at 3988

DOI: 10.1021/acsami.7b16859 ACS Appl. Mater. Interfaces 2018, 10, 3986−3993

Research Article

ACS Applied Materials & Interfaces

Figure 2. Sensitivity, response time, and durability of PC/PDMS composite based pressure sensor. (a) Multiple cycles of current responses under different pressures in a range of 60−2000 Pa. (b) The sensitivity in the range of 0−2000 Pa, showing a sensitivity of 15.63 kPa−1. (c) The current responses to a loading and unloading of a pressure of 20 Pa; the insets show magnified images of the response edges during loading and unloading. The response and relaxation times are determined to be 60 and 75 ms, respectively. (d) Durability test (2000 cycles) under a pressure of 1 kPa. The inset shows a magnified 9 cycles at the middle stage of the durability test.

pressure are obtained, resulting in a high sensitivity. Figure 2c shows the step response of the pressure sensor with loading− unloading a small pressure of 20 Pa, which indicates that the sensor has a low limit of detection (LOD) of 20 Pa. The magnified current to pressure responses are depicted in the insets of Figure 2c, which shows a fast response time of 60 ms and a fast relaxation time of 75 ms upon loading and unloading the external pressure, respectively. The fast response and relaxation time enables the PC/PDMS based pressure sensor device to monitor the wrist pulses in real time. The long-term durability is another important parameter of the pressure sensor. To investigate the durability of the as-prepared pressure sensor device, 2000 loading−unloading cycles were performed on the device under a pressure of 1 kPa at 1 Hz to monitor the corresponding current change in real time, as shown in Figure 2d. It is found that no significant change is observed in current amplitude during 2000 loading−unloading cycles. The inset of Figure 2d presents the magnified curve of the current responses to the dynamic pressure at the middle stage of the durability test, and the nearly identical peaks indicate a good durability and repeatability. This result suggests that the PC/PDMS based pressure sensor is highly stable in long-term and repeated operations. In the PC/PDMS composite film, PC fillers are uniformly distributed in the PDMS elastomer, and the strong bond between the PC and PDMS enables the pressure sensor device to have a good durability. Additionally, we investigated the reproducibility and ambient of sensor devices as shown in Figure S10. Figure S10a,b shows the reproducibility of eight devices measured at a pressure of 200 Pa and 1 kPa, respectively. The result demonstrates that the devices prepared by our fabrication have a good reproducibility. We measured the device, which had been stored in an ambient condition for 6 months, to investigate the ambient stability of the device. As shown in Figure S10c, the wrist pulse tested by the

To explore the pressure sensing properties of the PC/PDMS based sensor, the current responses to different external pressures (60, 200, 400, 900, and 2000 Pa) were measured at 3 V as shown in Figure 2a. It is obvious that the sensor responds to the pressure repeatedly with stable current signals under each external pressure, and the current increases gradually with the increase of external pressures. All of these experimental results indicate an excellent operational stability and reproducibility of the device. For practical applications, a high sensitivity of the pressure sensor is required. Typically, the pressure sensitivity S is defined as S = (ΔI/I0)/ΔP, where I0 is the initial current without applied pressure, ΔI is the change in current (Ip − I0), and ΔP is the change in applied pressure.55 In order to obtain the sensitivity of the as-fabricated PC/PDMS based pressure sensor, the current change versus the pressure variation in the low pressure regime (0−2000 Pa) was measured and demonstrated in Figure 2b. It is found that ΔI/I0 increases approximately linearly with the increased pressure, which is beneficial for the practical applications.56 A least-squares fit of the data gives S = 15.63 kPa−1, which is comparable or even better than those of previously reported results.21−23 The high sensitivity of the device enables the PC/PDMS based pressure sensor to detect the subtle wrist pulses (vide infra). This high sensitivity probably originates from the porous structures of PC/PDMS composite and the rough surface of the PC/PDMS film. Without external pressure, due to the porous structures of the PC/PDMS composite and the rough surface of the PC/PDMS, there are very few current pathways inside the pressure sensor, a very low I0 passing through the device. An obvious deformation is obtained with a small pressure applied on the pressure sensor device due to the porous structures inside the composite film and the rough surface of the PC/PDMS, and more current pathways form inside the pressure sensor device. Because of the porous structures of the PC/PDMS composite and the rough surface of the PC/PDMS, a very small I0 and large ΔI with a small 3989

DOI: 10.1021/acsami.7b16859 ACS Appl. Mater. Interfaces 2018, 10, 3986−3993

Research Article

ACS Applied Materials & Interfaces

Figure 3. Wrist pulse monitoring using the fabricated pressure sensor based on PC/PDMS composite. (a, c) The measurement results of the wrist pulse under normal condition and after running. The inset shows the photograph of the as-fabricated sensor being placed on the radial artery of the wrist. (b, d) The picked wrist pulses under normal condition (from a) and after running (from c).

demonstrate the application of MOF-5 in pressure sensing only. However, for the wearable electronics and e-skin, sensing the temperature is of equal importance. In our study, we demonstrate the application of MOF-5 derived PC for the application in pressure and temperature sensing simultaneously. 3.3. Performance of the Device as a Temperature Sensor. Apart from the pressure sensing, the temperatureresistivity characteristics of the PC/PDMS composite were measured to explore its ability of sensing temperature. The current− voltage (I−V) curves of the composite as a function of temperature from 23 to 120 °C are depicted in Figure 4a. The result shows that the current decreases with increasing temperature, showing a positive temperature coefficient (PTC) effect. This result is well consistent with other reported temperature sensors based on a conductive materials−polymer composite.57−59 In these situations, the PTC effect could be attributed to the thermal expansion of the polymer, which further results in the breakdown of conducting paths between conductive fillers. The PTC effect of temperature sensors is highly desired in self-regulating heaters, overcurrent, or overtemperature protectors. Therefore, it is of great importance to fabricate temperature sensors with strong PTC effect and high sensitivity. In order to investigate the sensitivity of the PC/PDMS composite for sensing temperature, the measured resistance variations subject to temperatures are presented in Figure 4b. It is well-known that the temperature coefficient of resistance (TCR) parameter could be calculated by α = (ΔR/R0)/ΔT, where R0 is the initial resistance at room temperature, ΔR is the change of resistance (R − R0), and ΔT is the change in temperature (T − T0).36 As depicted in Figure 4b, the results indicate a linear model between 23 and 50 °C and the TCR is estimated to be 0.11 °C−1. Meanwhile, an exponential model is observed when the temperature is over 60 °C and the value of ΔR/R0 at 120 °C is 171.3, which further confirms the strong PTC effect of PC/PDMS composite. Additionally, the response time was determined by immediately dropping a water droplet onto the surface of a composite film.

device has a good signal-to-noise ratio, which means the device has an excellent ambient stability. To demonstrate the potential of our PC/PDMS composite based pressure sensor for the application in health monitoring devices, we attached the sensor to a human wrist to detect the arterial wrist pulses in real time. Figure 3a,c shows the real time current responses of the sensor to the arterial wrist pulses of the volunteer both under normal condition and after running. It is notable that the pressure sensor can distinguish the difference between these two different conditions. The heart rates of the volunteer increased from 67 beats per minute (bpm) to 92 bpm after running (each peak represents a pulse). As shown in the magnified waveform in Figure 3b,d, two systolic blood pressure peaks (SBP1 and SBP2) together with a diastolic blood pressure (DBP) are clearly detected in a typical radical artery pulse pressure wave. SBP1 is the early systolic peak pressure; SBP2 is the late systolic peak pressure, and DBP represents the reflected wave from the lower part of the human body. The value of (SBP1 − DBP/SBP2 − DBP) × 100% is defined as the systolic augmentation index (AI) which is one of the most commonly used parameters for diagnosing vascular aging and stiffness problems related to the cardiovascular diseases.10 From the obtained waveforms, the average AI is determined to be 59% and 50% under normal condition and after running, respectively, which suggests that the 32 year old volunteer does not have the vascular aging problem. The decreased value of AI after running may be due to the dilated muscular arteries.53 All the above results indicate that the discriminated subtle differences in pulse pressure waveforms are significantly correlated to the physiological condition of the human body, which further demonstrates the great potential of the as-prepared PC/PDMS pressure sensor to be used as a wearable device for real time health monitoring. The previously reported CuTCNQ based pressure sensor53 and the device realized by our approach both show a high performance in sensitivity, response time, and detection limit, which indicates the potential applications of the MOF-derived materials for pressure sensing applications. In the previous report, they 3990

DOI: 10.1021/acsami.7b16859 ACS Appl. Mater. Interfaces 2018, 10, 3986−3993

Research Article

ACS Applied Materials & Interfaces

Figure 4. Temperature sensing performance of the PC/PDMS composite. (a) Current−voltage (I−V) curves at various temperatures. (b) Relative resistance change as a function of temperature. (c) The current response to water droplets at room temperature and 70 °C; the inset shows the photograph of the water droplet on the PC/PDMS film. (d) The response time to a water droplet at 70 °C.

As shown in Figure 4c, when a water droplet at room temperature was dropped on the film, no significant change in current occurred. However, when a water droplet at 70 °C fell on the film, a sharp decrease in current was observed, which is from the sharp expansion of the composite film, showing a fast response to the temperature change. The fast response time is estimated to be 100 ms (Figure 4d). The high performance in sensing temperature enables the as-prepared sensor for artificial e-skin, which can mimic human skin sensitivity and build connectivity with the external environment. Besides, the sensor was demonstrated to monitor human body temperature in real time. Compared with the temperature sensors using other conductive filler−polymer composites,59 the porous and ordered structures of MOF-derived PC material avoid the aggregation of conductive filler during the temperature variation. Furthermore, the uniformly dispersed PC particles in the PDMS matrix ensure their stability.

has been illustrated to be a promising candidate for developing multifunctional electronic skin, which can respond to multiple external stimuli. Since MOF-derived materials have been widely studied in the application of gas sensors,60 we aim to fabricate sensors that can detect not only physical stimuli but also chemical species in the future.

4. CONCLUSIONS In summary, we have successfully fabricated a flexible and simply structured bimodal sensor based on MOF-derived PC and PDMS composite. The 3D PC material was synthesized by thermal decomposition of MOF-5 and acid etching, and the PC/PDMS composite was obtained by a simple mechanical mixing process. Attributed to the rich pores both in the cubic-shaped PC particles and between the PC particles, the as-fabricated sensor shows excellent properties in sensing both pressure and temperature. For pressure sensing, it exhibits high sensitivity (15.63 kPa−1), fast response time (2000 cycles). Meanwhile, it can detect human motions such as subtle wrist pulses in real time. Furthermore, the as-prepared device sensor based on the PC/PDMS composite exhibits a good response to the temperature variation with high sensitivity (>0.11 °C−1) and fast response time (∼100 ms). All of these capabilities indicate its great potential in the application of health monitoring and artificial intelligence. Because of the attractive performance of the MOF-derived PC material and the facile processing approach of the PC/PDMS composite, this composite

Corresponding Author

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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.7b16859. Detailed fabrication procedure of PC/PDMS based sensor device; SEM images of MOF-5, ZnO@C, and porous carbon (PC); powder XRD spectra of MOF-5; pressure sensing mechanism of PC/PDMS composite (PDF)

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AUTHOR INFORMATION

*E-mail: [email protected]. ORCID

Huiyu Yuan: 0000-0002-5302-3519 Yuen Hong Tsang: 0000-0001-5632-5224 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 61605174), Shenzhen Science and Technology Innovation Commission (Project No.: JCYJ20170303160136888), and the Research Grants Council of Hong Kong, China (Project Number: GRF 152109/16E PolyU B-Q52T).

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REFERENCES

(1) Kim, D. H.; Lu, N.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S.; Wu, J.; Won, S. M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T.; Chowdhury, R.;

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DOI: 10.1021/acsami.7b16859 ACS Appl. Mater. Interfaces 2018, 10, 3986−3993

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DOI: 10.1021/acsami.7b16859 ACS Appl. Mater. Interfaces 2018, 10, 3986−3993