CO2 sensing at room temperature using carbon nanotubes coated core fiber Bragg grating B. N. Shivananju, S. Yamdagni, R. Fazuldeen, A. K. Sarin Kumar, G. M. Hegde et al. Citation: Rev. Sci. Instrum. 84, 065002 (2013); doi: 10.1063/1.4810016 View online: http://dx.doi.org/10.1063/1.4810016 View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v84/i6 Published by the AIP Publishing LLC.
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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 065002 (2013)
CO2 sensing at room temperature using carbon nanotubes coated core fiber Bragg grating B. N. Shivananju,1 S. Yamdagni,1 R. Fazuldeen,2 A. K. Sarin Kumar,2 G. M. Hegde,3 M. M. Varma,3,4 and S. Asokan1,5,6,a) 1
Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, India Advanced Materials Research and Applications, Honeywell Technology Solutions, Bangalore, India 3 Center for Nano Science and Engineering (CeNSE), Indian Institute of Science, Bangalore, India 4 Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore, India 5 Applied Photonics Initiative, Indian Institute of Science, Bangalore, India 6 Robert Bosch Centre for Cyber Physical Systems, Indian Institute of Science, Bangalore, India 2
(Received 6 March 2013; accepted 27 May 2013; published online 18 June 2013) The sensing of carbon dioxide (CO2 ) at room temperature, which has potential applications in environmental monitoring, healthcare, mining, biotechnology, food industry, etc., is a challenge for the scientific community due to the relative inertness of CO2 . Here, we propose a novel gas sensor based on clad-etched Fiber Bragg Grating (FBG) with polyallylamine-amino-carbon nanotube coated on the surface of the core for detecting the concentrations of CO2 gas at room temperature, in ppm levels over a wide range (1000 ppm–4000 ppm). The limit of detection observed in polyallylamineamino-carbon nanotube coated core-FBG has been found to be about 75 ppm. In this approach, when CO2 gas molecules interact with the polyallylamine-amino-carbon nanotube coated FBG, the effective refractive index of the fiber core changes, resulting in a shift in Bragg wavelength. The experimental data show a linear response of Bragg wavelength shift for increase in concentration of CO2 gas. Besides being reproducible and repeatable, the technique is fast, compact, and highly sensitive. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4810016] I. INTRODUCTION
Detection of carbon dioxide (CO2 ) at room temperature is necessary and important in a wide range of applications such as for monitoring global warming, indoor and outdoor air quality control, air quality monitoring in mines, process control in food industry, etc.1–3 Since it is chemically inert, the sensing of CO2 gas is a challenging task for the scientific community.4 Researchers have been working to develop highly sensitive, selective, portable, low power consuming, cost effective, and highly stable room temperature CO2 sensors with fast response and recovery times based on field effect transistors, infrared absorption, photo acoustics, surface acoustic waves, etc.,5–7 each having its own advantages and disadvantages. Metal oxides and polymers are some of the commonly used gas-sensing materials.8, 9 Recently, gas sensing based on nanomaterials, such as carbon nanotubes (CNTs), nanoparticles, nanofibers, and nanowires have been suggested widely due to their extremely high surface-to-volume ratio and hollow structure which is ideal for absorption of gas molecules, etc.10, 11 In particular, CNTs which are cylindrical carbon molecules with a diameter of a few nanometers have unique electrical, thermal, mechanical, chemical, and optical properties. CNTs can be classified into two types: Single-Walled Carbon Nanotubes (SWCNTs) and Multi-Walled Carbon Nanotubes (MWCNTs) which have found potential applications such as gas sensors, biosensors, solar cells, photovoltaic cells, and transistors.12 Researchers have shown that polya) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0034-6748/2013/84(6)/065002/7/$30.00
mer/CNT composite gas sensors are more sensitive, selective, and highly stable, thereby increasing the lifetime of polymers and enabling sensing at room temperature.11 Fiber Bragg Grating (FBG) sensors have been exploited recently, for a variety of sensing applications, due to their many desirable advantages such as high sensitivity, compact form, inherent multiplexing capability, multi-functionality, long term stability, immunity to electromagnetic interference, etc. A FBG sensor consists of a periodic modulation in the refractive index of the core of a single mode optical fiber. When light is guided along the core of the FBG, it gets reflected by successive grating planes; the contributions of reflected light from different grating planes add constructively for a particular wavelength (λB ),13 if the following condition is satisfied: λB = 2neff ,
(1)
where neff is the effective refractive index of the core and is the grating periodicity. FBGs have been used to sense temperature,14 strain,15 load,16 refractive index,17, 18 micro fluidic refractive index,19 humidity,20–22 and also gases such as hydrogen23 and ammonia.24 In strain or temperature sensing, the grating periodicity and/or the effective refractive index changes with strain/temperature, causing a shift in the Bragg wavelength. By etching the clad and core, FBG sensors can also be made sensitive to optical phase changes on the interface between the fiber core and the surrounding medium.17, 18 By controlling the etch depth, one can control the interaction of the propagating optical field with the bulk medium and
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FIG. 1. (a) The shift in the FBG Bragg wavelength with etching time in a solution of HF at 40%. (b) Scanning Electron Microscope (SEM) image of the etched FBG.
restrict the interaction with the surface to probe phenomena such as surface binding of molecular species or conformational changes in molecular layers immobilized on the surface of the fiber core. In this work, by combining the high surface to volume ratio and other advantages of polymer-CNT composites, and the high multiplexing ability and remote detection capabilities of optical fibers, we have demonstrated a novel CO2 gas sensor based on nanocomposites of polyallylamine (PAA)-aminoCNT film coated on the surface of a FBG sensor, etched to the core (referred to as core-FBG sensor) with ppm level sensitivity at room temperature. Here, the adsorption of CO2 gas molecules by the nanocomposites of PAA-amino-CNTs coated on the fiber core changes the effective refractive index, neff , of the fiber core. This in turn results in the shift in Bragg wavelength which corresponds to the concentration of the CO2 adsorbed by the nanotubes. The details of our sensing method are given in Secs. II A–II D. II. MATERIALS AND METHODS A. Fabrication of etched FBG sensors
FBGs can be fabricated by many methods; in the present work, the phase mask method is used to fabricate fiber Bragg grating sensors.25 A UV beam (KrF excimer laser) at 248 nm wavelength passes through the phase mask (pitch = 1064 nm) over a length of 3 mm to form an interference pattern in the core of a photo sensitive optical fiber (Fibercore, SM1500 with a core diameter 4.2 μm and cladding diameter 80 μm) placed immediately behind the phase mask. This process results in the photo-imprinting of a refractive index modulation (Bragg grating) in the fiber core. To increase the interaction of the propagating optical field in the fiber core with the surrounding medium, we chemically etch the clad completely to fabricate core FBG using an aqueous solution of hydrofluoric acid (HF) at 40% concentration.26 The etching of the fiber can be monitored by the shift in Bragg wavelength as shown in Figure 1(a). The etching process is stopped when the
Bragg wavelength shifts by a pre-set amount (∼6 nm). This ensures that the thickness of the FBG has reached core level as shown in the Scanning Electron Microscope (SEM) image of the fiber taken after the wet-chemical etching (Figure 1(b)). The sensitivity of the core-FBG sensors depends strongly on the fiber diameter, and the etching process described above ensures uniformity between multiple sensors.
B. Verification of bulk refractometric sensing with the etched FBGs
The shift in the Bragg wavelength of the FBG etched to the core is related only with the variation of the refractive index of the surrounding medium,17, 18 whereas an unetched FBG acts as a reference sensor to compensate for potential temperature drifts. To verify the performance of the core-FBGs to changes in surrounding refractive index, we exposed the etched FBG sensors to various liquids with different refractive indices (n), namely, water (n = 1.33), ethanol (n = 1.36), and propanol (n = 1.38). It is seen from Fig. 2(a) that the core-FBG shows significant changes in the Bragg wavelength while the un-etched FBG sensor does not respond. Figure 2(b) shows the repeatable shift in Bragg wavelength when the etched FBG and un-etched FBG are dipped in water and ethanol periodically.
C. PAA-amino-CNT functionalization of etched FBG sensors
The core-FBG sensors are coated with nanocomposites of PAA-amino-CNTs which were prepared from polyallylamine hydrochloride (MW = 15000), dimethylformamide, deionizated water (Aldrich), and amino-CNTs functionalized with amino groups (Nanocyl, Belgium). The synthesis of this nanocomposite is explained in detail in Ref. 4. The selection of PAA for making composites with amino-CNTs is based on the Hard Soft Acid Base (HSAB) rule. According to this theory, a hard Lewis base prefers to bond to a hard Lewis acid,
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FIG. 2. (a) The shift in Bragg wavelength as a function of the refractive index of different liquids (water, ethanol, and propanol) showing that the etched FBG and un-etched FBG sensors respond to changes in surrounding refractive index. (b) The repeatable shift in Bragg wavelength when the etched FBG and un-etched FBG are dipped in water and ethanol periodically.
and a soft Lewis base prefers to bond to a soft Lewis acid.27 PAA is a hard base and CO2 is a hard acid. Thus, according to the HSAB rule, these nanocomposites can interact with CO2 molecules at room temperature by means of the amino groups which exist in the backbone of each polymer. This interaction is an acid-base equilibrium, which is reversible and leads to the formation of carbamates. The advantage of PAA-aminoCNTs nanocomposites over PAA amino group for CO2 gas sensing is that an amino group in carbon nanotubes enhances their sensitivity. This is owing to the size of CNTs which enables many amino groups to be present at the surface of layer of the sensor. CNTs also improve the mechanical properties and can increase the lifetime of polymers due to their antioxidant character.4, 10 D. Detection setup
The schematic of the core-FBG sensor with PAA-aminoCNTs coating for CO2 gas sensing is shown in Figure 3. The experimental setup for CO2 sensing consists of a mass flow controller and an 8-channel multi-gas controller. In this installation, gas-flows at part per million levels could be achieved. In order to nullify the effects of humidity, the CO2 sensing is carried out in dry air (80% N2 and 20% O2 ) as the baseline. The gas chamber consists of a small air tight metallic box, which could accommodate FBG sensors, having an option to insert a temperature compensated humidity detector to monitor the relative humidity (RH) during experiments. A controlled flow of gases is achieved through the test chamber and the data are gathered using the 4-channel FBG interrogation system (Micron Optics SM130) with 1 pm resolution and a sampling rate of 1 kHz. The polymer’s affinity towards water molecule can have a negative effect on the sensitivity of the sensor and in such a case the effect of humidity will dominate than that due to the measurand (CO2 gas in our case). It has been reported that polymer materials with amino functional group have a tendency to absorb water molecules.28 The studies on
polyethyleneimine (PEI) and similar compounds for sensing CO2 gas using SAW devices have indicated that the presence of humidity masks the CO2 gas sensing signal.29 To overcome this effect, we have maintained a constant humidity level in our experiments. The approximate RH is kept around 47% throughout the experiment so that Bragg wavelength shift is caused by CO2 gas concentration variation and not due to humidity changes. III. RESULTS AND DISCUSSION
The measurements have been conducted at room temperature (∼25 ◦ C) and at a constant humidity (RH 47%) with different CO2 concentrations. The test sequence consists of repeated exposures of the sensor to gas with different CO2 concentrations balanced with pure N2 . The experimental results in different concentrations of CO2 gas with respect to time are shown in Figure 4(a). When the concentration of CO2 is increased, the Bragg wavelength is shifted towards the shorter wavelength side. The shift in the Bragg wavelength is due to the change in the effective refractive index of the core due to the adsorption of CO2 in the PAA-amino-CNTs coating. It has been reported earlier that the relative permittivity εr of the nanotubes shifts to lower value when exposed to reducing gases like CO2 .30 Since the relative permittivity εr is proportional to effective refractive index neff of the fiber core, this results in the shift in Bragg wavelength to shorter wavelengths corresponding to the concentration of the CO2 molecule adsorbed by the nanotubes. The shift in the base line is due to relative humidity change when different concentration of CO2 gas is purged in the sensor chamber. Even though we have kept the RH constant around 47%, there is some humidity change in decimal place whenever a different CO2 gas concentration is purged in the test chamber which causes base line shift during the experiment. This phenomenon is also observed in amino-group functionalized polymers for CO2 sensing.4 Figure 4(b) shows the Bragg wavelength shift versus different CO2 concentrations between 1000 ppm and
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FIG. 3. Schematic of the experimental setup for CO2 gas sensing using PAA-amino-CNTs coated on core-FBG.
FIG. 4. (a) The Bragg wavelength shift of PAA-amino-CNTs coated core-FBG for various CO2 concentrations operated at room temperature. (b) The Bragg wavelength shift as a function of CO2 concentration, calculated from (a), after base line subtraction, showing good linearity in the measured interval.
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FIG. 5. Bragg wavelength variation when the PAA composite sensor is exposed to 1000 ppm CO2 . The limit of detection calculated based on noise level is about 75 ppm.
FIG. 6. The reproducible characteristics of PAA-amino-CNTs functionalized core-FBG pulsed at 1000 ppm thrice to assess short-term repeatability of the sensor.
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4000 ppm after subtracting the base line. It can be seen that the sensor shows good sensitivity in the whole range of concentrations studied and the Bragg wavelength shift versus CO2 concentration exhibits a linear trend. The response (the time during which the sensor is exposed to the analyte) and recovery time (the time taken by the sensor to return to its base line) of the core-FBG sensor for CO2 sensing are found to be 3.07 min and 2.95 min, respectively, as shown in Figure 5. A Bragg wavelength shift of ∼6 pm is observed when the PAA-amino-CNTs composite coated core-FBG sensor is exposed to 1000 ppm CO2 gas concentration. Based on the noise level of the detection limits, the limit of detection calculated is about 75 ppm. Repeatability is another important desirable feature of a sensing system. Figure 6 shows the reproducibility characteristics of PAA-amino-CNTs functionalized core-FBG sensors. In this experiment, the sensor is exposed to 1000 ppm concentration of CO2 in triplicate. It can be seen from Figure 6 that there is a negligible drift in the base line or on the Bragg shift upon repeated exposure of the sensor with same concentration of CO2 (1000 ppm). The data shown in Fig. 6 correspond to short-term repeatability of the sensor. To assess the long-term repeatability of the sensor, we have performed the repeatability test described above on the same sensor with a difference of one month between the first trial and the second trial. The results of this long-term repeatability experiment are shown in Figure 7(a). Figure 7(b) shows the Bragg wavelength shift versus different CO2 concentrations between 1000 ppm and 4000 ppm after subtracting the base line. It can be seen from Figs. 7(a) and 7(b) that there is no significant difference in the performance of the sensor even after one month indicating good stability of the polymerCNT nanocomposite. Both the trials show a good sensitivity in the whole range of concentrations, and the Bragg wavelength shift with CO2 concentration is found to be linear in both the trials. FBGs are known to respond to changes in temperature resulting in temperature cross sensitivity in sensing
FIG. 7. (a) The long-term repeatability characteristics of PAA-amino-CNTs functionalized core-FBG sensor measured after one month. (b) Shift in Bragg wavelength with different CO2 gas concentration after subtracting the base line.
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FIG. 8. The effect of temperature during CO2 concentration measurement was normalized using an un-coated sensor which is sensitive only to temperature effects and not to surrounding refractive index.
applications. This has been identified as one of the key risks in FBG-based gas detection schemes.31 In order to overcome this problem, we have used an un-coated non-etched FBG sensor as a temperature reference. As shown in Fig. 8, in the presence of CO2 gas, a linear down-shift in the Bragg wavelength has been observed for PAA-amino-CNTs coated core-FBG sensors whereas a non-linear shift in Bragg wavelength is observed in bare FBG which is only sensitive to temperature shifts and not to surrounding refractive index. This clearly demonstrates that the response in PAA-amino-CNTs coated core-FBG sensors is due to the effect of CO2 , and the use of a reference sensor which is not sensitive to surrounding refractive index enables us to normalize out the effects of signal drifts due to temperature. IV. CONCLUSIONS
We have demonstrated a new sensing methodology, comprising of a core-FBG functionalized with PAA-amino-CNTs for measuring CO2 gas with a wide dynamic range from 1000 ppm to 4000 ppm. The experimental data show a linear response for CO2 measurements within the range measured. The limit of detection observed in PAA-amino-CNTs coated etched FBG sensors is about 75 ppm. The sensors show a good short-term as well as long-term repeatability and reproducibility. The sensor sensitivity and the ability to discriminate different gases can be improved by means of a special grating design using a series of multi-sensing heads functionalized with different thin-film receptors. As the coating thickness of the polymer layer affects the sensitivity and response time, there is further scope to study the Bragg wavelength shift as a function of the coating thickness. 1 C.
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