detecting co2 using nanowire chemiresistive sensor for

44th International Conference on Environmental Systems ICES-2014-044 13-17 July 2014, Tucson, Arizona

Detecting CO2 Using Nanowire Chemiresistive Sensor for Monitoring Air Quality in Enclosed Space Habitat

Maksudul M. Alam,1 Mohammad Mushfiq,2 Uma Sampathkumaran3 and Kisholoy Goswami4

InnoSense LLC, 2531 West 237th Street, Suite 127, Torrance, CA 90505-8245

and

Eric L. Brosha5

Materials Physics and Applications Division, Sensors and Electrochemical Devices, Los Alamos National Laboratory, Los Alamos, NM 87545

Monitoring carbon dioxide (CO2) concentration within an enclosed space habitat is critical to ensuring the safety of astronauts and their overall well-being in space because CO2 produced through respiration can accumulate rapidly within closed spaces. If not properly managed, the space crew could experience increased respiratory rate, headaches and hyperventilation, impaired vision and hearing, and decreased cognitive abilities. Currently, non-dispersive infrared (NDIR) spectroscopy is considered the most sensitive system (for a calibrated condition of 21.1 °C and 1 atmosphere) for detecting CO2 during manned space flight. However, measurements are not reliable and error rate increases with variation of temperature, pressure and composition of gas. A more robust and accurate sensor system is required for reliable measurement and monitoring of CO2 under temperature/pressure variations. InnoSense LLC (ISL) explores the development of a polymer nanowire sensor as a potential robust and real-time air-quality monitoring system for accurate, reliable and sensitive detection of CO2 in enclosed spaces. The nanowire sensor is a chemiresistive junction composed of two solid state electrodes bridged by conducting polymer materials in the form of ~70–150 nm diameter wires. The electron transport properties of the sensor change upon exposure to CO2. The nanowire sensors can detect CO2 in the 0–10,000 ppm range with high sensitivity (~50 ppm) and selectivity in the presence of 0–80% relative humidity and temperatures from 0–60 °C. The sensor response time to CO2, across all measured concentration ranges, was 2 minutes. The sensors are capable of detecting CO2 reversibly and self-regenerate to baseline signal within 30 minutes upon CO2 removal.

Nomenclature A = Amplifier ADC = Analog-digital conversion CO2 = Carbon dioxide COTS = Commercial off-the-shelf CP = Conducting polymer DAQ = Data acquisition DC = Direct current DI = Deionized water G = Gain

1 Deputy Director, R&D, InnoSense LLC, 2531 West 237th Street, Suite 127, Torrance, CA 90505. 2 Research Engineer, R&D, InnoSense LLC, 2531 West 237th Street, Suite 127, Torrance, CA 90505. 3 Vice President, R&D, InnoSense LLC, 2531 West 237th Street, Suite 127, Torrance, CA 90505. 4 President and Chief Technology Officer, InnoSense LLC, 2531 West 237th Street, Suite 127, Torrance, CA 90505. 5 Staff Scientist, Materials Physics and Applications Division, Sensors and Electrochemical Devices, Los Alamos National Laboratory, Los Alamos, NM 87545.

International Conference on Environmental Systems

2

GPIO = General purpose input/output I-V = Current voltage ISL = InnoSense LLC ISS = International Space Station M = Mole MCA = Major Constituent Analyzer MFC = Mass flow controller min = Minute mL = Milliliter mmHg = Millimeter of mercury nA = Nanoampere NDIR = Non-dispersive infrared NI = National Instruments nm = Nanometer PANI = Polyaniline PolyNAMS = Polymer nanowire microelectronic sensor ppm = Parts per million PPy = Polypyrrole PT = Polythiophene R2 = Regression value RBn = Precision resistor Rg = Single resistor SEM = Scanning electron microscope UHP = Ultrahigh purity V = Volt VDC = Volts direct current

I. Introduction arbon dioxide (CO2) is a by-product of metabolism, as metabolic loads increase more CO2 is produced. At rest, a person generates approximately 200 mL of CO2 per minute. Under maximal exercise, CO2 production can

exceed 4.0 L/Min.1 If expired CO2 is allowed to accumulate, it can have adverse effects on health and performance. Terrestrial studies suggest that initial symptoms develop at or above 2% CO2 (15 mmHg), with headache and dyspnea upon exertion. Visual disturbances and tremor appear at 6% CO2 (45 mmHg), unconsciousness at 7 to 9% CO2 (53-68 mmHg), and eventually death with the lowest published concentration at 9% (68 mmHg) in 5 min or 17% CO2 for 1 min (128 mmHg).2

Two primary systems have been used to monitor CO2 in the U.S. modules on the International Space Station (ISS): the service module gas analyzer and the major constituent analyzer (MCA). The service module gas analyzer is biased low by approximately 1 mmHg to 2.5 mmHg based on data from air sampling. The MCA, while less rugged, is accurate and samples via collection vents from each U.S. module.

Space platforms have unique challenges in providing habitable environments. They are essentially closed systems with significant power, mass, and volume constraints. Consequently, a number of CO2 removal technologies are under development for human spacecraft and spacesuits. Terrestrially, technology development requires precise performance characterization to qualify promising air revitalization equipment. On-orbit, instrumentation is required to identify and eliminate unsafe conditions. This necessitates accurate in situ CO2 monitoring in enclosed space habitat.

3 Both systems are limited by their fixed sampling locations. The lack of air convection in microgravity makes ventilation and accurate monitoring difficult as the recorded value at fixed sampling sites may not reflect the environment immediately surrounding the crew. In addition to these systems, non-dispersive infrared (NDIR) spectroscopy is being considered as a low-cost sensitive detection technique. A number of commercial off-the-shelf (COTS) NDIR sensors are readily available for CO2 detection.4,5,6 However, the accuracy of COTS sensors are highly application specific. Absorption lines undergo broadening through a variety of mechanisms that diminish the sensitivity and precision of these sensors.7

C

Spectral lines have a contribution to width which arises from the Heisenberg uncertainty principle as it applies to spectroscopic measurements. Specifically, excited states have a finite lifetime governed by a decay process such that absorption measurements produce ensemble-averaged results. Random collisions perturb the energy levels of the components of a gas sample. This is referred to as “pressure-broadening” and is strongly influenced by system


3

pressure where high pressure increases the likelihood of molecular interaction. Pressure broadening is also composition dependent since kinetic excitation is influenced by the momenta of the molecules involved in an excitation event. In addition, thermal energy imparts random Doppler velocities to each molecule which serves to ‘blur’ a line over a spectral range. This indicates that detection is also influenced by system temperature. In a recent report8

II. Working Principle of Nanowire Sensor

it has been mentioned that “the ISS and Space Shuttle were designed for 14.7 pounds per square inch (psia), and the Shuttle can also operate at a pressure of 10.2 psia. For extra-vehicular activity (EVA), pressures of 4.3 psia are under consideration while the spacesuit must operate at 8.0 psia to interface with vehicles and as high as 23.1 psia under contingency operations such as decompression sickness mode. Moreover, atmospheric composition is dictated by the Environmental Control and Life Support System (ECLSS) and can range from low to high relative humidity with several orders of magnitude higher CO2 partial pressure than exist on Earth.” Although COTS NDIR sensors are attractive from a testing stand-point as they are low cost and exact; however, detection over a wide range of pressures, temperatures, and gas

compositions poses a challenge to employing these sensors. To support the transport of crewed missions to distant planets, monitoring CO2 and controlling the life support process requires a sensor device that is: (1) highly accurate and precise, (2) reliable, (3) operational over a wide range of pressures/temperatures/gas compositions, (4) low-cost, (5) low system volume and mass, and (6) operational over a long lifetime. This system must minimize maintenance and the use of expendables. In this paper, we report the development of a polymer nanowire microelectronic sensor (PolyNAMS) coupled to a low-power data logger system that meets these requirements and will provide an essential tool for in situ CO2 monitoring in confined spacecraft and spacesuits. Figure 1 shows a schematic design of a PolyNAMS.

Conducting polymers (CPs) are distinguishable from other polymers in that their conjugated backbone allows for the flow of electrons (current).9 This intrinsic ability to conduct and transfer an electric current is very different from the insulating properties of traditional polymers. CPs are able to maintain other polymer-associated physical characteristics and are not plagued by problems associated with other semiconducting or conducting materials. This dualism has translated into the use of CPs in a wide range of applications, including sensors and biosensors.10

9

The most widely studied CPs are polypyrrole (PPy), polyaniline (PANI), polythiophene (PT), and derivatives, because of their environmental and thermal stability, their straightforward synthesis, and availability of low-cost raw-materials and/or monomers. ,10,11,12,13

11

CPs are interesting materials for chemical sensing because their conductivity can increase by over 10 orders of magnitude upon exposure to analyte gases. Exposure occurs via the change in ionic species, charge carrier and transport mechanism. Figure 2A shows an example for the PANI case. This process can be reversed by dedoping. ,12,13

In recent years, CP-based nanostructured materials in thin films and nanowires have been utilized in numerous electronic sensors because of their light weight, large surface area, adjustable transport properties, chemical specificity, low cost, processing ease, tunable conductivities, material flexibility, and readily scalable production.11,14,15

12

Thus, these nanomaterials have become prime candidates for replacing conventional bulk materials in micro- and nano-electronic devices, and chemical sensors in the context of two-terminal and three-terminal architectures. ,13,16

Figure 1. (A) Schematic rendition of PolyNAMS. The sensor consists of polymer nanowires, directly grown onto patterned microelectrode junction devices, and electronic components. Each CO2 sensor will be coupled with a reference sensor for making accurate measurements. (B) Sensor response for measuring CO2 after interacting with polymer nanowires of the sensor.


4

The CP growth occurred between two solid state electrodes, which caused the transport properties of the sensor to change upon exposure to the analyte gas CO2. Their conjugated backbone allowed for the flow of electrons, enabling the ions present in the polymer to interact with the CO2.9 This caused a transformation between two ionic states and changes in the carrier and transport mechanisms thereby changing the electrical characteristics.12,13 Thus, by applying a constant bias or potential difference across the junction, a detectable change in current level is noticed when the analyte gas is introduced. To increase the CO2 sensitivity of the polymer nanowire sensor, we used the functionalized aniline electroactive monomer to create polymer nanowires that are specific to CO2. The resulting polymer becomes highly sensitive to CO2 (Fig. 2B). The formation of the carbamate group in the presence of CO2 is a reversible reaction. Therefore, at higher concentrations of CO2, the reaction is driven towards the formation of the carbamate. While in lower concentrations, the reaction is driven back towards the formation of the original polymer. These changes can be detected by

monitoring the current level of the electrode junctions of the sensor devices.

III. Microelectrode Junction Device Preparation, Nanowire Growth, Characterization and Sensor Fabrication

A. Fabrication of Microelectrode Junction Device InnoSense LLC (ISL) designed patterned microscale electrode junction devices that could be used to

electrochemically develop polymer nanowires. The design was such that the nanowires would be incorporated into an electrical circuit without templates or a complicated production method. The patterned electrode we designed (Fig. 3) had a 55 nm thick layer of gold on a 15 nm of chrome. This entire pattern was deposited on a silicon substrate coated with silicon dioxide. The two electrode pads contained four (4) gold electrodes or “fingers.” The gold “fingertips” of each electrode had a 2 µm gap between them. For fabricating microelectrode junction devices, we designed a photomask with AutoCAD 2008 2D software. The design was implemented by Photo-Sciences, Inc.

(Torrance, CA) who fabricated the quartz-chrome photomask. The photomask was then used by REVTEK Inc (Torrance, CA) to fabricate the electrode junctions. A scanning electron microscope (SEM) image of the electrode junction is included in Fig. 3.

Each silicon wafer usually contains several hundred electrode junction devices. Thus, before the electrochemical polymerization of the nanowires, the electrode junction devices were separated from each other using a diamond scribe. The surfaces were thoroughly cleaned by soaking in a series of solutions and sonication. Before the electrochemical polymerization of the nanowires, the device surfaces were activated in a piranha solution (70% concentrated sulfuric acid, 30% hydrogen peroxide) for 10 min, followed by deionized (DI) water rinse and drying in a clean nitrogen (N2) jet. A wire was bonded to each electrode pad by both soldering and using conductive silver epoxy. Finally, the wire bonding site and electrode pad was encapsulated using a non-conducting epoxy layer, which restricted the electrochemical polymerization to the gold fingers. The wire bonded device with the two epoxy layers can be seen in Fig. 4. As the figure shows, the non-conducting epoxy layer encapsulates the wire bonding site and the gold electrode pad along with a significant portion of the gold electrodes. This

Figure 2. (A) Polyaniline interacts with analyte gas reversibly (transformation occurs between two ionic states, thereby changing electrical characteristics). (B) Polymer of the functionalized aniline and its reversible reaction with the analyte gas CO2.

Figure 3. An image of a fabricated electrode junction device (top), and an SEM image of a 2 µm gap between the gold electrodes (bottom).


5

allows the polymer nanowires to be generated only in the exposed region of the gold electrodes and within the 2 µm gap.

B. Processing Electroactive Monomer and Growth of Polymer Nanowires

The two monomers used were aniline and a unique functionalized amine-modified aniline known as 2-(2 aminoethyl) aniline. The structures of the monomers can be seen in Figs. 5(A and B). Aniline was acquired commercially and purified at ISL by distillation using a rotary evaporator. Figure 5C shows that the purified liquid is transparent. The functionalized aniline was acquired as a dark grey powder. A solution of the monomer in 1.0 M perchloric acid or 1.0 M hydrochloric acid was used to generate the conducting nanowires. The concentration of the monomer was varied from 0.1 M to 0.5 M to generate different nanowire densities. We used a small volume flask filled with ~16 mL monomer solution, in which the wire-bonded electrode junction device was submerged. One side of the device acted as the working electrode. For the counter electrode, a platinum coil was used. The platinum coils had 10–12 turns and a wire diameter of 0.25 mm. A silver/silver chloride reference electrode was used to monitor the reaction voltage. The solution was purged with nitrogen for 10 min prior to starting the electrochemistry.

Nitrogen was constantly flowed into the flask during the experiment to maintain a neutral and

non-oxidative environment above the solution. An oxidative potential was applied to one side of the electrode junction device, and ground to the platinum coil. A Princeton Applied Research model 263A-1 potentiostat (Fig.

6A) was used to generate the potential difference. This oxidized the aniline monomers and triggered a chain reaction resulting in the formation of polyaniline. The electrochemical flask setup is shown in Fig. 6B.

To control the polymer growth, which ensures proper morphology and nanowire structure development, the electrochemistry was carried out in three stages. For the first stage a constant current of 50 nA was applied for 30 min. It was during this stage that the monomer was oxidized, and the seeding began. The second and third stages each had duration of 2.8 hours. For the second stage, the current was held constant at 25 nA and for the third stage, at 12.5 nA. These two stages allowed the polymer to propagate and elongate over the 2 µm gap of the electrode junction. The low current ensured

Figure 6. (A) Electro-polymerization setup using a potentiostat and (B) electrochemical (EC) cell with the three electrodes dipped in the electroactive monomer solution.

Figure 5. (A) Structure of aniline monomer (B) Structure of 2-(2-aminoethyl) aniline monomer (C) Aniline solution before and after purification by distillation.

Figure 7. A voltage versus time graph for the three stages of the electropolymerization process.

Figure 4. (A) Two epoxy layers bind the wires to the electrode surface. (B) The size of the electrode junction device compared to a penny.


6

the nanoscale diameter of the polymer nanowires. During electro-polymerization, the voltage of the electrochemical cell was monitored and was seen to vary with the three stages applied. The graph in Fig. 7 shows the result of the electro-polymerization process. A similar plot line was seen for all devices. The entire electro-polymerization process was approximately 6 hours. After the device was soaked in DI water for 10 min to remove any salt residue from the solution, it was dried overnight at 70 °C before we imaged and characterized the nanowire properties.

C. Characterization of Polymer Nanowires After the electro-polymerization process was completed, the polymer nanowire was first characterized by

examining the current-voltage (I-V) curve of the nanowire device. Before electro-polymerization, the electrode junction device was an open circuit, and hence no current could flow through. After electro-polymerization, if the

nanowires had bridged the 2 µm gap of the electrode junction and formed a closed circuit, then current flowed through the device. This allowed us to quickly determine if the electrochemical process had been successful. We inferred that a high density of nanowires in the gap allowed a high current to flow through because of a lowered resistance, versus a low density device that would only allow a few microamperes of current to flow. Using the I-V curve analysis, we categorized the devices into batches. The five batches of devices were separated according to their current level measured at 2 V. The categories were: <1 µA, 1-10 µA, 10-100 µA, 100-500 µA, 500 µA-1 mA. In order to confirm that higher current level nanowire devices had denser polymer nanowire network, we analyzed the nanowire devices with SEM. These micrographs were obtained with a Hitachi Field Emission Scanning Electron Microscope at Photometrics Inc. (Huntington Beach, CA). For SEM images, the working voltage was 5 kV. We used magnifications of 30,000x and 40,000x to examine the nanowire structures and morphology. We used the measurement function of ImageJ software to analyze the diameter of the polymer nanowires as seen in the SEM images.

Figure 8A shows the results of I-V curve analysis of a nanowire device with a 2V current level of 28 µA. The graph shows the I-V curve of the nanowire device before and after the electro-polymerization. Since it is an open circuit before the presence of polymers, no current flows through regardless of the applied bias. Figure 8(B–D) shows SEM images of nanowire devices. We examined the 2 µm gap region of the device to determine if the polymer nanowires had bridged the gap between the electrodes. In the images, the 2 µm gap has been marked, and the presence of polymer nanowires is clearly visible. The nanowires were formed when the monomer was

Figure 9. (A) I-V curves of a sensor device with current level ~0.12 µA at 2.0V and (B) its SEM image. (C) I-V curves of another sensor device with current level of ~260 µA and (D) its SEM image of polymer nanowires (diameters ~50–120 nm) showing nanowire network at the 2 µm gap.

Figure 8. (A) I-V curve of the sensor device measured before and after electro-polymerization. (B–D) SEM images of polymer nanowires (diameters ~50–120 nm) shown bridging the 2 µm gap.


7

oxidized and seeding began. A few bridging nanowires and a slight nanowire network formation was visible. I-V curves and their corresponding SEM images of two other sensor devices are shown in Fig. 9. The results show that nanowire density increased with increasing device current. For example, the high current device with current level ~260 µA at 2.0V (Fig. 9C) shows the formation of denser polymer nanowire network (Fig. 9D), whereas low current device with current level ~0.12 µA at 2.0V (Fig. 9A) shows the formation of very few nanowires at the 2 micron gap (Fig. 9B). However, the diameters of the nanowires measured were between 50 nm and 120 nm, which is the desired morphology.

IV. Sensor Electronics and CO2 Sensor Performance

A. Sensor Electronics and Hardware for Data Acquisition We have designed and developed circuit and signal processing

electronics that will interface with PolyNAMS and acquire the response data, while minimizing noise, drift and temperature effect. The circuit used for the nanowire sensors is a balanced bridge design which uses one sensor for reference and a second sensor for measurement. The entire circuit is designed on a circuit board that measures 0.9 in. x 1.7 in. Figure 10 is an image of the completed circuit board.

Circuit Performance. Initially, the overall performance of the circuit design had to be tested to make certain that it was stable over a range of temperatures. To do this, the bridge was formed using four fixed 30.1 kOhm resistors. These were 0.1% precision, thin-film resistors. The precision reference source was a Linear Technology LT6656 3.000 VDC chip. The DC offset voltage out of the instrumentation amplifier was set at half the precision reference voltage, or 1.500 volts.

Two boards were constructed using the same components except for the quad-operational amplifier. One board was built using a Linear Technology LT6005 and the second was built using an Analog Devices AD8504. Thus, the temperature performance could be compared. The boards were placed in a temperature chamber and operated from -50 °C to +75 °C while recording the analog output voltage (Fig. 11). The results demonstrate that the basic circuit design is extremely stable over the tested range of temperatures; it exhibited no drift over the full temperature range.

B. Sensor Performance The nanowire sensors were tested using the setup illustrated in

Fig. 12. To generate the desired CO2 concentration, a certified CO2 gas tank, of known concentration (balance dry N2), and a certified ultra high

purity (UHP) N2 gas tank were connected to a mass flow controller (MFC). These MFCs were then connected to a laboratory PC controlled data acquisition (DAQ) system with LabVIEW software, which allowed us to vary the CO2 concentration. The varying concentrations of CO2 were achieved by using certified tanks of 100 ppm CO2, 1000 ppm CO2, 5000 ppm CO2 and 10,000 ppm CO2. The signal and I-V curve of the device was measured on an Agilent semiconductor

Figure 11. Baseline temperature data of two signal processing electronic circuit boards.

Figure 10. A 0.9 in. x 1.7 in. completed electronic circuit board based on bridge-configuration.

Figure 12. Schematic of a test setup to evaluate sensor performance in the presence of CO2. The CO2 concentration is controlled by the MFC and the DAQ-LabVIEW system. The sensor device is placed in a test chamber with a gas inlet and outlet, and the semiconductor parameter analyzer is used to measure the signal change.


8

parameter analyzer. Since dry gas was used in the setup, the sensors were initially tested at 0% relative humidity (RH) and room temperature (~22 °C).

For all tests performed at the laboratory, the flow rate was maintained at 500 mL/min. First, the test chamber containing the sensor was purged with N2 for at least 10 min to establish a base line. Once the device attained a steady I-V curve, CO2 was allowed to flow through the chamber at the same flow rate for 10 min, after which the I-V curve of the device was measured. For every concentration, three readings were taken to eliminate errors in the testing process. The concentration of CO2 was then increased and the measurement process was repeated. We calculated the change in signal (ΔI) as the difference between the current level at 2 V in a pure N2 environment (𝑰𝑵𝟐) and the current level at 2 V in specific concentration of CO2 (𝑰𝑪𝑶𝟐). The equation used to calculate change in signal or ΔI for the sensors was:

∆𝑰 = �𝑰𝑵𝟐 − 𝑰𝑪𝑶𝟐� (1)

The ΔI data was collected by measuring the device’s I-V curves from 0 to 2 V and acquiring the signal value at the 2 V point. In cases where the standard error was too small to be graphed, the error bars indicating ±5% of the value were used.

We tested several PolyNAMS in varying concentrations of CO2 (from 0 to 1000 ppm) at room temperature under dry N2 by measuring I-V curves from 0 to 2 V for each concentration of CO2. Test results of two representative PolyNAMS for sensing CO2 are shown in Figs. 13 and 14. All PolyNAMS showed similar response to CO2. The

Figure 13. (A) I-V curves of PolyNAMS device (MED 1-3) with increasing CO2 concentrations at room temperature. (B) A zoomed in view of Figure 14A. (C) Change in PolyNAMS response as a function of CO2 concentration up to 1000 ppm. Each point is the average of three measurements, and the error bars indicate ±5% of the value. R2 indicates the regression value for a quadratic fit.

Figure 14. (A) I-V curves of PolyNAMS device (MED 1-4) with increasing CO2 concentrations at room temperature. (B) A zoomed in view of Figure 15A. (C) Change in PolyNAMS response as a function of CO2 concentration up to 1000 ppm. Each point is the average of three measurements, and the error bars indicate ±5% of the value. R2 indicates the regression value for a quadratic fit.


9

PolyNAMS device current decreased with increasing concentrations of CO2, whereas the change in signal (ΔI) increased as the CO2 concentration increased (Figs. 13C and 14C). We observed quadratic fits for the PolyNAMS witt regression values (R2) of 0.9955 and 0.9951.

Using the same setup, but with a gas tank of higher CO2 concentration, we then examined the response of sensors to 5000 ppm CO2. Sensors with similar current levels of 70 µA and 80 µA were used. These results are summarized in Fig. 15. Both sensors showed a highly linear response with good linear trend line fit (R2 > 0.9). We then examined the sensors’ response up to 10,000 ppm CO2. Two sensors with initial device current levels of 18µA and 20 µA were tested. Figure 16 shows these results. Both sensors showed a significant linear response to CO2 with signal changes of approximately 2 µA and the trend line had a good fit.

C. CO2 Measurement under Varying Temperatures We evaluated the PolyNAMS device performance at temperatures from 10 °C to 60 °C. Tests at higher

temperatures were performed to see how the sensors would perform in real life environments. The setup in Fig. 12 was modified to control the temperature in the test chamber with a hot plate. The temperatures within the chamber were constantly monitored with a thermocouple. To achieve temperatures lower than room temperature, ice packs replaced the hot plate. Temperature was held constant (±2 °C) for the duration of all tests, and flow rate was always maintained at 500 mL/min. All these tests were performed in dry gas (<5% RH) to negate any influence of humidity.

The sensors were tested from 10–60 °C in 10 °C increments (Fig. 17). The CO2 concentration was increased in 200 ppm steps from 0 to 1000 ppm. The response to CO2 was linear, except in the case of tests at 20 °C and 40 °C where there was a large signal change after initial CO2 exposure and a linear response thereafter. During testing at 50 °C, a larger response was seen for 200 ppm CO2 than 400 ppm CO2. However, this may have been due to experimental error, since it was not seen at any other temperatures. The sensor did display a linear increase in current level (or decrease in resistance) with temperature, which is a property of semi-conductors. However, this effect can be accounted for by generating a calibration curve for the sensors.

Figure 15. (A) Response of a 70 µA sensor from 0 to 5000 ppm CO2. (B) Response of the 80 µA sensor from 0 to 5000 ppm CO2. Each data point is the average of three measurements, and the error bars indicate ±5% of the value. R2 indicates the regression value for the trend line.

Figure 16. (A) Response of an 18 µA sensor when tested from 0 to 10,000 ppm CO2. (B) Response of a 20 µA sensor when tested from 0 to 10,000 ppm CO2. Each data point is the average of three measurements, and the error bars indicate ±5% of the value. R2 indicates the regression value for the trend line.


10

To further examine the effects of temperature on the sensors while detecting high CO2 concentrations, we tested a sensor from 0 to 10,000 ppm CO2 at 22 °C and 60 °C. Figure 18 shows the results of the tests. The sensor was able to detect up to 10,000 ppm CO2 at room temperature (22 °C) and 60 °C. The responses were linear.

Figure 17. The sensor functioned at low and high temperatures. It detected up to 1000 ppm CO2 concentration at (A) 10 °C, (B) 20 °C, (C) 30 °C, (D) 40 °C, (E) 50 °C and (F) 60 °C. Each point is the average of three measurements, and the error bars indicate ±5% of the value. R2 indicates the regression value for the trend line.

Figure 18. The sensor successfully detected up to 10,000 ppm CO2 at temperatures (A) 22 °C (room temperature) and (B) 60 °C. The signal response was linear. Each point is the average of three measurements, and the error bars indicate ±5% of the value. R2 indicates the regression value for the trend line.


11

D. CO2 Measurement under Varying Humidity The PolyNAMS devices were tested under 0 to 80% RH conditions. Dry gas was used to control the

concentrations of CO2, thus the humidity was created by modifying the test setup (see Fig. 12) with a Miller Nelson Test Atmosphere Generator. The mixed CO2 gas that was output from the MFCs was passed through the Miller Nelson Test Atmosphere generator, which allowed us to control the RH of the gas from 0% to 80%. The temperature was maintained at 22 °C to eliminate any cross-effects of temperature variation. Inside the test chamber, the temperature was monitored with a thermocouple. We allowed each sensor to equilibrate under N2 in the humid conditions. Once the sensor had stabilized the semiconductor, a parameter analyzer was used to measure the I-V curve of the sensor. After this, the concentration of CO2 was increased and, the device was exposed to it for 10 min before once again measuring the I-V curve. The RH was held constant for the duration of all tests.

We tested PolyNAMS sensors up to 10,000 ppm CO2 at 0%, 20%, 40% and 80% RH (Fig. 19). The sensors had base current levels ~18 µA in normal conditions, and when exposed to CO2 displayed a change in signal up to 3 µA for 10,000 ppm CO2. The sensor was able to detect CO2 at all humidity levels tested, and the response showed a high degree of linearity. Thus, it can be concluded that the nanowire sensors were successfully able to detect from 0 ppm to 10,000 ppm CO2, even in the presence of high levels of humidity.

E. Reversibility Measurement The small signal processing electronic circuit board designed by ISL (see Fig. 10) was easily connected to the

PolyNAMS. For reversibility testing, the CO2 flow was controlled with MFCs and the same test chamber was used. In order to connect the sensor to the custom circuit, we soldered the wires which extend from sensor to the circuit board. The circuit was powered with 5 V output from a National Instrument (NI) DAQ card (Model: NI USB-6009). A LabVIEW VI (Virtual Instrument) was used to visualize the sensor’s response.

Since the nanowire bridge requires a reference sensor, we used a stock resistor that matched the PolyNAMS resistance. The resistor was placed outside the test chamber with the circuit board while the sensor was placed in the test chamber. The voltage measured is the ratio of the voltage of the sensor and the resistor. Thus, when the sensor was exposed to CO2 in the test chamber, any changes in the measured voltage were due to the interaction with CO2.

Figure 19. Results of 0 to 10,000 ppm CO2 testing of PolyNAMS at (A) 0% RH (B) 20% RH (C) 40% RH (D) 80% RH. The device consistently sensed up to 10,000 ppm CO2 without saturation. Each point is the average of three measurements; the error bars indicate ±5% of the value. R2 indicates the regression value for the trend line.


12

The test methodology adopted for this portion of testing was to expose the PolyNAMS to N2 followed by CO2. The concentration of CO2 was increased with each test. The gases were calibrated to flow at approximately 500 mL/min. The N2 was held for 20 min followed by the CO2 for 10 min before repeating the steps with a higher concentration. This process was repeated at least 3 times in succession. The signal (voltage) vs. time graph generated during these tests enabled us to analyze the PolyNAMS response.

The reversibility was tested for 400 ppm (Fig. 20A), 600 ppm (Fig. 20B), 200–800 ppm (Fig. 21A), and 100–1000 ppm (Fig. 21B) CO2. The response of the PolyNAMS was both reversible and repeatable for all measured concentrations of CO2. The PolyNAMS returned to the original base line after approximately 2–5 min in a nitrogen environment. There is a drift in signal for these PolyNAMS. We will investigate thoroughly to understand the cause of signal drift and to mitigate this effect.

F. Interference Gases The sensor’s selectivity to CO2 in comparison to other interference gases such as methane (CH4) and oxygen

(O2) was tested. In order to test this effect, the setup in Fig. 12 was used. First a CO2 gas tank was used, and then the CO2 was replaced by CH4 and subsequently 20% O2. The change in signal (ΔI) of a sensor when exposed to methane and CO2 was compared. Figure 22 shows the observed results of three sensor devices. They were exposed to up to 1000 ppm of CH4 and compared in parallel to similar CO2 concentrations. None of the three tested sensors showed a noticeable response to methane, while they all had a significant response to CO2, which increased with higher current levels. Based on this, we can conclude that the sensors are not responsive to CH4.

To further confirm sensor results, and to examine the effect of O2 on the sensor, we performed a reversibility test on PolyNAMS devices with intermittent exposure to CO2. The test was performed on the same sensor, but with intermittent exposures to 1000 ppm CH4, and again with 20% O2. The observed results are summarized in Fig. 23. The electrode junction was maintained at a constant bias of 1 V, while the signal was monitored over a 5 hour testing period. The shaded regions of the graph indicate the duration for which the analyte gas was exposed to the

Figure 20. The reversibility of PolyNAMS was tested in a nitrogen background for: (A) 400 ppm and (B) 600 ppm CO2. The response to CO2 was reversible in the presence of N2.

1.50

1.52

1.54

1.56

1.58

0 1000 2000 3000 4000 5000

Volta

ge (V

)

N2

N2

N2

400 ppmCO2

400 ppmCO2

400 ppmCO2

under N2

Time (sec)

A

1.55

1.56

1.57

1.58

1.59

1.6

1.61

1.62

0 1000 2000 3000 4000 5000

Volta

ge (V

) 600 ppmCO2

600 ppmCO2

600 ppmCO2

N2

N2

N2

MED 1-5

under N2

Time (sec)

B

Figure 21. The reversibility of PolyNAMS was tested in a nitrogen background for: (A) 200 – 800 ppm and (B) 100–1000 ppm CO2. The response to CO2 was reversible in the presence of N2.

1.50

1.54

1.58

1.62

1.66

1.70

0 5000 10000 15000

Volt

age

(V)

Time (sec)

200 ppmCO2

N2

N2

N2N2 N2

N2 N2

400 ppmCO2

400 ppmCO2

600 ppmCO2

600 ppmCO2

800 ppmCO2

800 ppmCO2

under N2

A

1.66

1.68

1.70

1.72

1.74

0 2000 4000 6000 8000 10000

Volta

ge (V

)

N2

N2N2

N2N2 N2

100 ppmCO2

200 ppmCO2

400 ppmCO2

600 ppmCO2

800 ppmCO2

1000 ppmCO2

under N2

Time (sec)

B


13

device surface. The sensor showed a significant response in the presence of CO2. But there was no detectable change in the presence of either methane or oxygen. The same drift was noticed in the sensor that had been observed in previous tests both with the semiconductor parameter analyzer and ISL’s constructed measurement system. In a future study, we will examine the effects of other interference gases, and the effect of the interference gases in varying temperatures and humidity levels. This will allow us to thoroughly characterize the nature of the sensors and their ability to function in confined environments.

V. Conclusion Nanowires are a manifestation of nanomaterials offering high surface-to-volume ratio for developing low-cost,

field deployable sensors. We grew polymer nanowire electrochemically using functionalized groups. By carefully designing patterned metal electrodes on a substrate and manipulating the bias voltage, nanowires form a bridge between electrodes offering good electrical connection. We successfully developed a small/compact signal processing electronic circuitry that can acquire sensor responses for the detection of CO2 under various experimental conditions. PolyNAMS showed a CO2 detection capability ranging from 0 ppm to 10,000 ppm with a linear response at temperatures ranging from 10 °C to 60 °C and relative humidity ranging from 0% to 80%. PolyNAMS reversibly detects CO2 at different concentrations with a response time of 2 min to CO2 exposure. The PolyNAMS sensors are also capable of responding over a wide dynamic range, and can detect CO2 in the presence of interfering gases including moisture, methane, oxygen, etc.

Acknowledgments This work was supported through a STTR Grant (DE-SC0008210) from the Department of Energy-Office of

Science. Authors gratefully acknowledge the support and guidance of the program manager Dr. Joshua Hull.

References

1Williams, W. J., “Physiological responses to oxygen and carbon dioxide in the breathing environment,” NIOSH Public Meeting, Pittsburgh, PA, September 17, 2009.

2Environmental Protection Agency (EPA). “Carbon dioxide as a fire suppressant: examining the risks,” Report

Figure 22. The three sensors with current levels (A) 5 μA (B) 20 μA (C) 90 μA did not respond when exposed to CH4 (black line) particularly when compared to the response to CO2 (red line).

Figure 23. (A) Sensor responses with 1000 ppm CO2. The same sensor did not show any response when exposed to (B) 20% O2 or (C) 1000 ppm CH4. The shaded region of each graph represents the duration for which the sensor device was exposed to the analyte gas, and remaining time in a N2 environment.


14

EPA430-R-00-002, 2000.

3 Felker, P., “Assessment of crew reports on ISS carbon dioxide symptoms,” Internal Report, 2003. 4McDermitt, D., Welles, J., and Eckles, R., “Effects of temperature, pressure and water vapor on gas phase infrared

absorption by CO2,” LI-COR Inc, 1993. 5Welles, J. and McDermitt, D., “Measuring carbon dioxide in the atmosphere,” Agronomy, 2005, Vol. 47, pp. 287. 6Burch, D., Gryvnak, D., Patty, R., and Bartky, C., “Absorption of Infrared Radiant Energy by CO2 and H2O. IV. Shapes of

Collision-Broadened CO2 Lines,” Journal of the Optical Society of America, 1969, Vol. 59, No. 3, pp. 267–280. 7Gordley, L., Marshall, B., and Allen Chu, D., “LINEPAK: Algorithms for modeling spectral transmittance and radiance,”

Journal of Quantitative Spectroscopy and Radiative Transfer, 1994, Vol. 52, No. 5, pp. 563–580. 8Swickrath, M. J., Anderson, M., McMillin, S., Broerman, C., “Application of commercial non-dispersive infrared

spectroscopy sensors for sub-ambient carbon dioxide detection,” American Institute of Aeronautics and Astronautics 2012, pp. 1–14. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120006020.pdf

9Skotheim, T. A., and Reynolds, J. R., Handbook of Conducting Polymers, Conjugated Polymers: Processing and Applications. 3rd ed., CRC Press: Boca Raton, 2007.

10Rahman, M. A., Kumar, P., Park, D., and Shim, Y., “Electrochemical sensors based on organic conjugated polymers,” Sensors, 2008, Vol. 8, pp. 118–141.

11MacDiarmid, A. G., “Synthetic metals: a novel role for organic polymers,” Angew Chimica Acta International Edition, 2001, Vol. 40, pp. 2581–2590.

12Virji, S., Huang, J. X., Kaner, R. B., and Weiller, B. H., “Polyaniline nanofiber gas sensors: examination of response mechanisms,” Nano Letters, 2004, Vol. 4, pp. 491–496.

13Choi, S. J., and Park, S. M., “Electrochemical growth of nanosized conducting polymer wires on gold using molecular templates,” Advanced Materials, 2000, Vol. 12, pp. 1547–1549.

14Shirakawa, H., “The discovery of polyacetylene film: The dawning of an era of conducting polymers.” Angew. Chem., Int. Ed. 2001, Vol. 40, pp. 2575–2580.

15 Park, S., Lim, J.H., Chung, S.W., and Mirkin, C.A., “Self-assembly of mesoscopic metal-polymer amphiphiles.” Science, 2004, Vol. 303, pp. 348–351.

16Bartlett, P. N., and Astier, Y., “Microelectrochemical enzyme transistors.” Chemical Communications 2000, 105–112.

detecting co2 using nanowire chemiresistive sensor for

Documents