Article Cite This: ACS Sens. 2018, 3, 327−333
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High Performance Colorimetric Carbon Monoxide Sensor for Continuous Personal Exposure Monitoring Chenwen Lin, Xiaojun Xian,* Xingcai Qin, Di Wang, Francis Tsow, Erica Forzani, and Nongjian Tao*
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Center for Bioelectronics and Biosensors, The Biodesign Institute, Arizona State University, Tempe, Arizona 85287, United States
ABSTRACT: Carbon monoxide (CO) is a highly poisonous gas, which can cause serious health risk. CO monitoring helps protect us from excessive exposure at home and in the workplace, and reduce occupation-related health risks for workers. Conventional electrochemical and metal oxide semiconductors (MOS) based CO sensors have been widely used, but the drawbacks such as poor selectivity and calibration burden also limit their applications, e.g., as wearable exposure monitors. Aiming at the reliable, miniaturized, and easy-to-use personal exposure device development, we report a colorimetric CO sensing platform, which achieves a detection limit of 1 ppm, dynamic range of 0−500 ppm, and high selectivity to CO over common interferents in air, such as CO2, NO2, SO2, and O3. This optical sensing platform can be expanded to other air pollutants by adding other chemical sensing probes. We believe the new sensing platform we introduced can provide a potential high performance sensing unit for wearable personal exposure assessment devices. KEYWORDS: carbon monoxide, colorimetry, wearable device, personal exposure monitor, gas sensor
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standards for public health and welfare protection with the time-weighted average (TWA) of 9 ppm (ppm) measured over 8 h, and 35 ppm measured over 1 h,1,4 and World Health Organization (WHO) sets TWA exposure limits of 10 ppm for 8 h, 25 ppm for 1 h, 50 ppm for 30 min, and 90 ppm for 15 min.5 Although there is an increasing awareness of the threat of CO poisoning, reports have shown that unintentional nonfirerelated CO poisoning still leads to estimated 15,000 emergency department visits and approximately 500 deaths annually in the United States alone.6 Therefore, reliable CO exposure monitoring is of great importance. Monitoring CO concentration in industrial settings, workplace, and home is of critical importance to alert the occupants of an emergency, identify the CO generation source, minimize personal exposure, assist the ventilation operation, or even map the CO distribution in the community. Thus, a CO monitor is like a guardian who can warn us of this colorless and odorless silent killer and keep us out of harm’s way.
ir quality has a major impact on our health. Monitoring air quality helps protect us from exposure to harmful gases at home and in the workplace, reduce health risks of millions of patients who suffer from chronic respiratory diseases, and minimize occupational safety threats for workers. It is also important for exposure−diseases relationship study, identification of sources of pollutants, warning of environmental disease triggers, and development of air quality management plan. According to the United States Environmental Protection Agency (EPA), there are a total of six “criteria pollutants”, including particulate matter (PM), lead (Pb), ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2) and nitrogen dioxide (NO2).1 Carbon monoxide (CO) is a well-known silent killer as it is toxic but colorless, odorless, and notoriously difficult to detect. It is generated by the incomplete oxidation during combustion from furnaces, stoves, heaters, and automobiles. Personal exposure of CO causes various symptoms depending on CO exposure duration and concentration. Mild and moderate CO poisoning causes symptoms similar to the flu, resulting in a headache, fatigue, shortness of breath, nausea, and dizziness. More severe symptoms can be triggered progressively by high level of CO poisoning, including mental confusion, vomiting, loss of muscular coordination, loss of consciousness, and ultimately death.2,3 EPA lists two primary © 2018 American Chemical Society
Received: September 28, 2017 Accepted: January 4, 2018 Published: January 4, 2018 327
DOI: 10.1021/acssensors.7b00722 ACS Sens. 2018, 3, 327−333
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palladium salts as catalyst, which involves both oxidation (sensing) and reduction (recovery) processes, we use the palladium salts as a reactant only for sensing and avoid any recovery procedures, which can significantly reduce the time demanding to finish one detection cycle. (2) Instead of involving multiple chemicals in the recipe to make the sensor reversible for year-long lifetime, we just use K2Pd(SO3)2 in the recipe without involving other chemicals and make the sensor chip disposable for daily use with optimized lifetime. This extremely simple recipe can simplify the sensor chip production procedure and reduce the cost. (3) An active built-in sample delivery system rather than passive diffusion or manually operated external pump is used to deliver the gas into the detection chamber at a constant flow rate, which can dramatically increase the sensitivity and reduce the response time. The pump can also be programmed to have an adaptive sampling time to ensure the sensitivity and save sensor chip lifetime. All these innovations contribute to the new colorimetric CO sensing platform the capability for fast and continuous CO monitoring with a response time of 20 s. There are other important features that make the proposed CO sensor unique and attractive. First, an ultralow detection limit could be achieved, 1 ppm in our system compared to 15 ppm in traditional SIR CO sensor, which better satisfies the EPA standard of 9 ppm TWA over 8 h. Second, the sensor chip is expandable and configurable. By increasing the number of chemical arrays on the sensor chip, other gaseous pollutants, such as NO2 and O3, can also be detected on the same optical detection system for different applications. And third, the asprepared sensor chip will be precalibrated during production, which avoids putting the calibration burden on the users. On our colorimetric CO sensing platform, a detection limit of 1 ppm has been achieved and a dynamic range from 0 to 500 ppm has been demonstrated. The selectivity study has revealed that the CO sensor is immune to common interferents in ambient air, such as CO2, NO2, SO2, and O3. We believe the new sensing platform we introduced could overcome the drawbacks of the traditional CO sensors and provide a potentially high performance sensing unit for wearable personal exposure devices.
A typical application of CO sensors is in the form of a subunit in the fire alarm fixed on the ceiling together with a smoke detection. Besides the stationary monitoring, a wearable device for personal exposure assessment has drawn more and more attentions since people engage in activities in different places, e.g. sleeping in the room, traveling in the car, eating in the cafeteria, working in the office, or exercising in the gym. Furthermore, as for the needs of efficient air quality monitoring, a highly integrated device which can monitor all of the listed pollutants simultaneously is highly preferred. While recent advances in sensor technologies are impressive, and some, including electrochemical sensors,7−10 colorimetric sensors,11,12 metal oxide gas sensors,13 and UV absorption sensors,14,15 are commercially available, none of them can provide an integrated solution to detect all of the common air pollutants simultaneously, continuously, and accurately. We believe the optical detection platform can serve as an ideal candidate for multiple pollutants detection since PM can be reliably detected by light scattering techniques and other gaseous pollutant can be detected on a colorimetric sensor array. In this work, we have demonstrated a highly sensitive colorimetric carbon monoxide sensor for reliable and continuous personal exposure monitoring. The sensing technique developed in this work can be used either as a sensing unit in the multiple analytes array, or as a portable standalone CO monitor. Conventional commercial CO sensing technologies are usually based on electrochemical cells, metal oxide semiconductors (MOS), and colorimetric sensors, which have been intensively studied and well developed. But at the same time, they suffered from their own limitations. Electrochemical sensors are often interfered by pressure changes and require periodic calibration for accurate reading, which add a burden to the users. MOS sensors have low selectivity and require high operation temperature (usually from 200 to 500 °C) with high power consumption.16,17 Colorimetric sensors detect the color change induced by a specific chemical reaction between the target analyte and the sensing probe. As an alternative method for CO detection, colorimetric sensing has been of interests recently for its high sensitivity, selectivity, and simplicity. The sensing probes are the keys to the performance of the colorimetric CO sensors. Generally, the chemicals in the sensing probes play two major roles: to serve as the catalyst for the reaction between CO and chemicals and to “visualize” the reaction by inducing color change.18 A wide range of sensing probes for CO detection have been explored,19−23 and the progressive innovations have led to commercial CO monitoring products on the market, such as detector tubes,12 and solidstate infrared (SIR) CO sensor.24 The palladium salts are commonly used in these products as the catalyst for CO oxidation and the molybdenum compounds are used as color indicator.25,26 The humidity effect was addressed via numerical compensation or additional humidity control system.27,28 Though these conventional CO detectors have wide applications, there is one major drawback which hinders them from personal exposure assessment: lack of the capability for instantaneous CO level monitoring. A typical SIR CO sensor has a response time of 45 min, which is mainly due to the passive CO sampling and long recovery time.29 In order to overcome the drawbacks of conventional colorimetric sensing technologies and create a new generation of colorimetric CO sensors that can provide instantaneous and continuous CO monitoring for personal exposure use, we come up with the following innovations. (1) Instead of using
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EXPERIMENTAL SECTION
Materials. Potassium disulfitopalladate (II) (K2Pd(SO3)2) was purchased from Alfa Aesar. Ultrapure water (18 MΩ) was produced via an ELGA Purelab Ultra RO system. Silica gel substrates were purchased from Sorbent Technologies (Silica G TLC Plates, Polyester Backed, 20 × 20 cm2, 250 μm thickness, 60 Å mean pore diameter of silica gel, pore volume: ∼0.75 mL/g, specific surface area: ∼500 m2/g). Carbon monoxide standard gas (1000 ppm, balanced with nitrogen) was purchased from Cross company. The various concentrations of CO gases were prepared in 4 L Tedlar bags (Custom Sensor Solutions Inc.) by dilution with clean air (Praxair Certified Breath Air). CO Sensor Chip Preparation. The CO sensor chip was prepared by immobilizing the sensing element that reacts with CO selectively on the porous silica gel substrate. The solution of the sensing element was prepared by dissolving 80 mg of K2Pd(SO3)2 in 10 mL of hot DI water. The silica paper was cut into 5 × 5 mm2 pieces with a laser cutter and washed with DI water and ethanol three times each. The 30 μL of K2Pd(SO3)2 solution was then drop-casted onto the 5 × 5 mm2 silica gel substrate and vacuum-dried for 1 h. The drop-casting process was repeated four times to achieve a better sensitivity of the asprepared CO sensor chip. The porous silica gel substrate was selected to maximize the loading capacity of the sensing materials, which can increase both the sensitivity and sensing capacity. 328
DOI: 10.1021/acssensors.7b00722 ACS Sens. 2018, 3, 327−333
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Figure 1. (a) Schematic of the CO sensing setup. CO sample gas is delivered at 1 L/min from the Tedlar air bag to the sensing channel where the reference chip (gray) and CO sensor chip (yellow) are located. The white LED illuminates the sensor chip and the transmission light is detected by the CMOS imager. (b) UV−vis spectrum of sensing probe on silica paper during continuous exposure of CO. Insets (I), (II), and (III) show the images of CO sensor chip under different dosage of CO exposure, from low to high.
Figure 2. (a) Optical responses of the CO sensor to different concentrations (0, 50, 100, 200, 300, 400, and 500 ppm) of dry CO gas samples (after baseline correction). (b) Calibration of sensor response to CO. Absorbance change (ΔAbs (au/s)) was calculated from slope difference between baseline and sampling periods (R2 = 0.9822, slope = (4.202 ± 0.252) × 10−6 au/s·ppm). Error bars represent the standard deviation. (c) Optical responses of the CO sensor to lower concentrations (0, 1, 5, and 10 ppm) of dry CO gas samples (after baseline correction). (d) Calibration of sensor response to CO (R2 = 0.9889, slope = (1.7879 ± 0.152)× 10−5 au/s·ppm). Apparatus. The CO sensing platform was composed of a sensing chamber, an optical system, and an active sample delivery system. The sensing chamber was made by CNC milling (MAXNC-10) using Acetron, which has a gas inlet and outlet, and a sensor slot that can position and align the sensor chip with the optical detection system. The optical detection system consisted of a CMOS imager (Logitech C905) for capturing the color change of the sensor chip and a white LED (LEDtronics Inc.) connected to a LED driver (CL520N3-G-ND) for stable illumination for the sensing chamber. The active sample delivery system was assembled by connecting an air pump (Parker, T3EP-1ST-05-3FFP-b) via a Teflon tubing, and the flow rate of which
was maintained at 1 L/min. The concentrations of CO gas sample were confirmed with a reference CO meter (Extech CO10). Methods. The air pump was connected to a mechanical three-way valve for switching between dry clean air and CO sample gas. The gases were stored in Tedlar bags and delivered to the sensing chamber at a flow rate of 1 L/min. A 60 s purging of dry clean air to the system was used to establish a stable baseline, and then CO sample gas at a particular concentration was injected into the system for 20 s. The CO gas reacted with the sensing probes on the sensor chip during the sampling period, resulting in a color change, which can be detected by the optical system mounted on the sensing chamber. To avoid the 329
DOI: 10.1021/acssensors.7b00722 ACS Sens. 2018, 3, 327−333
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Figure 3. (a) Cross-sensitivity of CO sensor with other common interferents (21% oxygen, 100 ppb NO2, 100 ppb ozone, 100 ppb SO2, 200 ppb HCHO, 1000 ppm CO2, 200 ppb NO). Relative response calculated by the ratio of CO sensor response to the interferents and CO sensor response to 10 ppm of CO. (b) CO sensor response during continuous monitoring of 10 ppm of CO at 75% RH (60 s purging and 20 s sampling). The variation of sensor response among different sampling periods was less than 6%. influence of any signal drift from the optical detection system, a blank silica gel substrate (5 × 5 mm) was used as a reference in all measurements for signal correction. A homemade Matlab program was used to define sensing and reference regions in the areas monitored by the CMOS imager, and the intensities of the defined regions were recorded for signal processing.30−32 The schematic diagram of the CO sensing platform is shown in Figure 1a. When running the test, the sensor cartridge was assembled in the detection chamber, and the optical detection module continuously recorded the absorbance (−log(Isensing/Ireference)) change as a function of time during the entire cycle of test. The slope of the optical absorbance curve difference between the sensing and purging periods was calculated, which was correlated with the concentration of CO in the gas sample.
when the CO gas is injected into the detection chamber, regardless of the CO concentrations in the range of 0−500 ppm. This fast reaction rate originates from the large negative free energy change of this chemical reaction and the low activation energy barrier at room temperature. It should be noticed that the slope of the absorbance change, which is the absorbance change before and after the injection of CO divided by the corresponding exposure time (ΔAbs = Absorbance change/time, unit = au/s), increases with the CO concentrations in the gas samples. And further data analysis shows that these slope values have good linear relationship with CO concentration. In the lower CO concentration range (0−10 ppm), the sensor has higher sensitivity of (1.7879 ± 0.152) × 10−5 au/s·ppm, while in the higher CO concentration range, the sensor shows a relative lower sensitivity of (4.202 ± 0.252)× 10−6 au/s·ppm, as plotted in Figure 2b and d. This linear sensing behavior can be explained according to the model we described in the previous work.30 Briefly, when the active sites of the sensing elements on the substrate are excessive, the reaction rate depends on the concentration of the analyte in the gas phase.
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RESULTS AND DISCUSSION Sensing Mechanism. CO detection is based on a selective chemical reaction between CO molecules and the sensing elements on the porous substrate: CO + K 2Pd(SO3)2 → Pd + CO2 + SO2 + K 2SO3
(1)
The chemical reaction happens rapidly when the CO gas is introduced into the reaction chamber. The sensing probes on the substrate change color gradually from yellow to black when the exposure time increases, as shown in Figure 1b. The adsorption spectrum clearly indicates that the absorption peak is in the range of 500−700 nm, which is due to the formation of Pd nanoparticles during this selective chemical reaction, and keeps increasing with the exposure time. Though the spectra show a more sensitive absorbance change in the range of 500− 700 nm and a higher sensitivity for CO detection could be achieved by using a green LED, aiming at building a universal sensing platform to cover multiplex detection, a white LED was used as the light source in the current system. Since the chemical reaction between CO and K2Pd(SO3)2 can cause significant color change on the sensor chip, with the assistance of a white LED as the light source, a CMOS imager is used to monitor the absorbance change, as illustrated in Figure 1a. Dynamic Range, Detection Limit, and Sensitivity of the CO Sensor. To achieve the goal of continuous CO monitoring, the chemical reaction between the analyte and the sensing probe should be fast enough to achieve a fast response time. The response curves of the sensor to different concentrations of CO have been investigated to evaluate the reaction kinetics of this gas−solid phase reaction. According to Figure 2a and c, the absorbance signal changes spontaneously
R = KCCO
(2)
According to eq 2, where K is a constant that related to surface adsorption and CCO is the concentration of CO, the CO concentration will be proportional to the absorbance slope since the reaction rate (R) can be represented by the slope of the optical absorbance−time plot, which is consistent with the experimental results. We hypothesized the different adsorption and reaction behaviors in the lower and higher CO concentration ranges may contribute to the discontinuity of linear sensor response at 10 ppm. Under low concentration, CO molecules are mainly adsorbed and reacted with sensing elements on the top surface of the substrate, while under high concentration, CO molecules can further diffuse into the porous substrate and react with sensing elements deep inside. It should be mentioned that conventional colorimetric CO sensor cannot reach a detection limit bellow 15 ppm,29 while our CO sensor can easily discriminate different concentrations of CO bellow 10 ppm with a detection limit of 1 ppm, as shown in Figure 2c and d. The above sensitivity and detection limit were achieved under the sampling condition of 20 s injection time and 1 L/min flow rate. If needed, these specs can be further 330
DOI: 10.1021/acssensors.7b00722 ACS Sens. 2018, 3, 327−333
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Figure 4. (a) Optical responses of the CO sensor to 100 ppm CO gas samples with 0%, 25%, 50%, and 75% RH (after baseline correction). (b) Sensor responses to CO with various concentrations (100, 200, 300, and 400 ppm) under different relative humidity levels (0%, 25%, 50%, and 75% RH).
improved by applying a longer sampling time or higher flow rate. The flat baseline in the response curves suggested that there is no chemical reaction during the purging period (Figure 2a and c). Once the CO injection is switched off, the absorbance curve becomes flat as the chemical reaction stops. This transition is not as sharp as the injection, which may due to the residual CO trapped in the porous matrix of the supporting materials on the sensor substrate. Though the purging and sensing periods last for 60 and 20 s, respectively, in the current experiments, the time can be further reduced to 10 and 5 s instead, which will shorten the response time of the sensor to 15 s for a single measurement. Considering the composition of the ambient air in home, office, or industrial settings are relatively stable for most cases, the subminute response time is sufficient for hazardous gas monitoring purpose. Selectivity, Reproducibility, and Continuous Monitoring. The beauty of the colorimetric chemical sensor is that high selectivity can be achieved by applying chemical reactions that are specific to the targeting analyte. This feature differentiates it from other sensing approaches, such as electrochemical sensing, metal oxide sensing, and infrared absorption sensing. Due to the unique coordination and reduction reaction between CO and K2Pd(SO3)2, the sensing probes coated on the substrate is very selective to CO while immune to other common interferents in the ambient air. We have tested the sensor response to the following gases at their typical concentration found in the ambient air: 21% O2, 100 ppb NO2, 100 ppb O3, 100 ppb SO2, 200 ppb HCHO, 1000 ppm CO2, and 200 ppb NO, and compared the response to 10 ppm of CO. None of them show a considerable response, with the signal fluctuations less than 5% of the signal from 10 ppm of CO (Figure 3a). This high selectivity can avoid the need of scrubbers to remove the potential interferents as in conventional sensor devices in order to achieve acceptable accuracy. It should be mentioned that the detection limit of our colorimetric CO sensor can be as low as sub-ppm level when a longer sampling time is used. By programming the circuit accordingly, an adaptive sampling concept can be used to make the sampling time adjustable, which gives longer sampling time when the analyte concentration is low to achieve better sensitivity while making the sampling time shorter when the analyte concentration is high to save sensing probes and increase sensor lifetime.33 Conventional colorimetric sensors are usually for one-time use only, since most of the sensing probes will be consumed during the detection, like the colorimetric gas detector tubes,
pH test papers, and humidity indicators. This is because (1) there is no control of the mass transportation of the exposure of analyte to the sensing probes, usually based on passive diffusion; and (2) the qualitative or quantitative detection of the analyte is based on visual checking with human eyes, and thus, reliable detection depends on the tremendous amount of chemical reaction between analytes and sensing probes. In order to save the sensing probes while improving the detection limit and sensitivity, we apply a miniaturized pump to actively deliver the sample gas at a constant flow rate within a short period of time and use the CMOS imager to sensitively and reliably monitor the color development during the detection. This design has made it possible to use the same sensor for multiple and continuous measurements. As shown in Figure 3b, multiple consecutive injections have been performed on the same sensor chip by alternating the purging and sensing period. The reproducibility of the consecutive measurement is evaluated by calculating the variability of five sensor responses to 10 ppm of CO at 75% RH, and the variation is less than 6%, indicating excellent sensing reliability and capacity. The consecutive sensing plots also indicate that the sensing mechanism is not reversible. When the injection stops, the optical intensity also stops changing until a new injection happens. The irreversibility comes from the high selectivity of the reaction and high stability of the product, both of which are essential for the high performance of the colorimetric CO sensor. Results indicated in Figure 3b also imply that the continuous CO monitoring can be achieved even with a single sensor chip. We have tested the sensing capacity of the sensor chip we prepared and it can reach 8 ppm·h, a dose the sensor can take before its sensitivity drop by 10%. Assuming the CO concentration in the ambient air is 10 ppm (WHO standard, 10 ppm (8 h mean)) and the sampling time is 20 s, the CO sensor chip can be used 150 times before replacement. Humidity Tolerance. Humidity is a major interferent to many kinds of chemical sensors, including electrochemical sensors, metal oxide sensors, infrared sensors, and colorimetric sensors. We also investigated the humidity tolerance of our CO sensor. The sensing performance does depend on the humidity of the ambient air, but this dependence is predictable and correctable. As shown in Figure 4a, under different humidity levels, the sensor shows different responses to the same concentration of CO: the higher the humidity, the higher the sensor signal. However, once the humidity level is fixed, the sensor signal always shows a linear dependence on the CO 331
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concentration, though the slope and the intercept are different (Figure 4b). From these results, we speculate that the water molecule can accelerate the chemical reaction between CO and K2Pd(SO3)2, maybe through a catalytic pathway. In a conventional colorimetric CO sensor, the humidity effect was addressed either via numerical compensation or additional humidity control system.27,28 Considering the size limitation of a wearable personal exposure device, using a low-cost humidity sensor for humidity effect correction could be a reasonable and practical way. As for the numerical compensation approach, usually, a humidity sensor is implemented in the system and the humidity reading is accounted to determine the actual CO concentration. But due to the slow response (45 min) of the conventional colorimetric sensing techniques, it is rather difficult to correct humidity interference in a highly accurate way because of the mismatch of response time between the CO sensor and the humidity sensor. The fast response time of our CO colorimetric sensor can overcome this drawback and dramatically improve the accuracy of numerical humidity compensation. Besides the humidity compensation, a cheap thermistor can be easily integrated to the circuit for monitoring real-time ambient temperature and an algorithm can be implemented to compensate the temperature effect based on the Arrhenius equation.
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CONCLUSION By introducing an optical detection system (including a light source and a CMOS imager), an active sample delivery system, and a flat sensor chip geometry, a high performance CO sensing platform has been successfully designed and constructed. The fast, specific, and irreversible colorimetric reaction between CO and K2Pd(SO3)2 provides the chemical sensor with fast response, high selectivity, and high reliability. The high loading capacity of sensing probes on the porous substrate, the sensitive optical detector, and the optimized sampling rate make the sensor suitable for continuous personal CO exposure monitoring. Our tests have demonstrated that the CO sensor has a detection limit of 1 ppm, dynamic range of 0−500 ppm, detection capacity of 8 ppm·h, and high selectivity to CO against common interferents in air, such as CO2, NO2, SO2, and O3. The sensing technique developed in this work is universal and either can be used as a sensing unit in the multiple analytes array, or can be designed into a portable standalone CO monitor.
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Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Chenwen Lin: 0000-0001-6573-5997 Nongjian Tao: 0000-0002-5206-153X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Institute of Environmental Health Sciences (Project Number: 1R43ES025095-01). 332
DOI: 10.1021/acssensors.7b00722 ACS Sens. 2018, 3, 327−333
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