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We developed a highly sensitive side-polished plastic optical fiber coated with chemical synthesized graphene/polyaniline nanocomposite for ammonia gas sensing application. It was found that the optical sensor absorbance spectrum linearly increases with increasing ammonia concentrations. The experimental results revealed that the side-polished fiber sensor coated with graphene/polyaniline nanocomposite exhibited higher performance than the ones coated with only polyaniline. The proposed sensor demonstrated sensitivity, response and recovery times of 132.8 AU/%, 112 s, and 185 s, respectively, at room temperature. The superior sensing performance of the developed fiber sensor indicates its high efficiency for NH3 gas-sensing applications at room temperature.
© 2017 Optical Society of America
1. Introduction
Gaseous air pollutants are ubiquitous in daily human activities, such as agriculture, food processing, industrial coolants, and household detergents [1]. Over the past few decades, the detection of these pollutants has been extensively studied because of their high toxicity and wide use in many fields. Ammonia (NH3) is a colorless flammable gas with a very strong smell that is commonly produced from various sources, such as chemical plants, livestock farming, and motor vehicles [2, 3]. Immediate exposure to 300 ppm NH3 is considered to be dangerous to life and health [3]. The inhalation of large quantities of ammonia can cause harmful effects on human health such as eye irritation, pulmonary edema, and respiratory arrest [4]. Recently in India, five people were killed and over a hundred others complained of breathing problems after they inhaled large amounts of NH3 gas that leaked from an NH3 gas tanker [5]. Therefore, ammonia concentrations should be monitored using reliable, inexpensive sensors that can be operated at room temperature.
Many of the existing ammonia gas sensors are based on metal oxides because of their high selectivity and sensitivity. However, these sensors are limited by several drawbacks, such as high cost, high power consumption, and operating at elevated temperatures [6–8]. The use of conducting polymer polyaniline (PANI) as an active layer for ammonia gas detection has attracted considerable attention because of its several merits, such as low cost, facile synthesis, and its operation at room temperature [9]. However, PANI-based gas sensors suffer poor mechanical strength and lack selectivity [10]. To overcome these drawbacks, PANI/inorganic nanocomposites have been proposed for gas sensing applications, such as PANI/TiO2 nanocomposite for NH3 detection [11] and PANI/In2O3 for H2, NO2, and CO gases-sensing [12]. As a two-dimensional monolayer of sp2−bonded carbon atoms, graphene nanoplatelets (graphene) have been recently regarded as a promising gas-sensing material. Graphene was found to have large surface-to-volume ratio, electron mobility, and strong hydrophilic nature in addition to its high optical absorbance [13].
At present, researchers incorporate graphene with PANI to produce graphene/PANI nanocomposites for chemical sensing application [14, 15]. Wu et al. [16] reported that the graphene/PANI nanocomposite-based conductometric sensor shows approximately five times more sensitivity compared to a sensor solely based on PANI. Furthermore, a graphene/PANI nanocomposite-coated interdigitated transducer for methane detection was fabricated, and the sensor detection range increased proportionally to the mass ratio of graphene to PANI [17]. Graphene/PANI nanocomposites have been recently applied in electrochemical devices [18], supercapacitors, [19]and energy storages [20]. Apart from that, the integration of graphene/PANI nanocomposite for optical gas sensing application is still in its infancy stage.
Optical fiber sensors are highly used for gas detection because of their advantages over conventional electrical sensors. Optical fiber sensors are immune to electromagnetic interference, micro-sized, and nondestructive [21]. In optical fiber gas sensors, it is important to have strong interaction between the sensing layer and light propagates in the fiber core to improve their sensitivity. This can be achieved by modifying the optical fiber by removing part of the fiber structure and coating it with an active gas-sensing layer. Thus, changes in the optical, physical, and chemical properties may occur as a result of the interaction between the gas molecules and the sensing layer coated on the exposed optical fiber [22]. Tien et al. [23] revealed that the fabrication of gas sensors based on side-polished fiber exhibits high sensitivity to its surrounding area because of the long interaction length. Plastic optical fiber (POF) offers numerous advantages, including low cost, ease of manipulation, and high degree of flexibility [24]. The large core of POF provides greater surface area than silica optical fiber, thereby improving the sensor sensitivity [25].
In this paper, we report the successful fabrication and characterization of an ammonia gas sensor featuring a chemically synthesized graphene/PANI nanocomposite as the sensitive layer coated on side-polished POF (SP-POF). The developed graphene/PANI optical gas sensor shows higher sensitivity toward NH3 than that of PANI nanofibers-based gas sensors.
2. Experiment
2.1. SP-POF fabrication
A multimode POF was used to fabricate the proposed NH3 sensor. The POF comprised a polymethyl-methacrylate (PMMA) core diameter of 980 µm and a fluorinated polymer cladding thickness of 20 µm. Prior to the polishing process, 4 cm of the fiber protective jacket was stripped using a fiber stripper. First, the fiber mounted in a circular groove was made on a customized metal block as shown in Fig. 1. Then, finer sandpapers (9 µm and 3 µm) were used to remove the fiber cladding and part of its core. A 1 µm polishing paper was used to ensure the quality of the polished surface. Finally, the SP-POF was cleaned with alcohol and deionized (DI) water to eliminate any contaminants. The achieved sensing region was approximately 2 cm in length and 600 µm in depth. Both ends of the fiber were stripped and polished using sandpaper to improve the coupling of light to the fiber. Figures 2(a) and 2(b) show the microscopic images of the SP-POF (taken with Motic Microscope-BA310E, South Korea) before and after the polishing process. Figure 2(c) reveals that a 1 mm fiber was polished to approximately 600 µm without any obvious damage or defects on the optical fiber.
Fig. 2 Microscopic images of POF (a) before and (b) after polishing process. (c) SEM image for the SP-POF.
2.2. Preparation and deposition of the nanocomposite
The graphene/PANI nanocomposite was synthesized according to the in-situ polymerization method reported by Wu et al. [17]. Figure 3 illustrates the synthesis of graphene/PANI composite. Graphene of 15 mg, 186 mg of aniline, and 1 M hydrochloric acid (HCl) were mixed with 60 mL DI water and magnetically stirred for 20 min in an ice-water bath. Subsequently, a 40 mL of ammonium peroxydisulfate (APS) (0.002 mol) aqueous solution was added to the obtained cooled solution and continued to polymerize in an ice bath with continuous stirring for 24 h. Finally, the produced dark green precipitates were filtered and washed with DI water and dried for 36 h in an oven at 40 °C. The graphene/PANI nanocomposite was dispersed in a 10 mL solution (7 ml DI water and 3 mL ethanol) and sonicated for 2 h via ultrasonication bath. To obtain a complete evaluation of the sensor performance, a preparation process similar to that of the chemically synthesized nanocomposite was used to synthesize PANI only without graphene. The proposed sensing layers were deposit on the fiber by drop-casting method. The coated fibers were heated to 50 °C in the oven for 1 h and left to dry in the air at room temperature. Before the coating process, the SP-POFs were heated in the oven at 50 °C for 10 min to enhance the binding of the nanomaterial onto the polished surface. In this work, chemically synthesized material was named GPC.
2.3. Ammonia sensing setup
The experimental setup of the SP-POF gas sensor is shown in Fig. 4. The proposed NH3 sensor was fixed in a customized chamber and connected from one side to an attenuator used to control the intensity of light and a tungsten-halogen lamp (Ocean Optics™ HL 2000, wavelength range of 360 nm to 2400 nm) through optical fiber cables. From the other side, the SP-POF sensor was connected to a spectrophotometer (Ocean Optics™ USB-4000, spectral range of 200 nm to 1100 nm) used for the absorbance measurement.
The spectrophotometer was connected to a computer via a USB port. The optical response measured with the spectrophotometer was processed and analyzed using the SpectraSuite software (version 6.2) in real time. The absorbance, Aλ, is calculated by the software, as represented by the following mathematical equation:
where Sλ is the light intensity detected at wavelength λ upon exposure to NH3. Dλ represents the intensity that is stored when no light passes though the waveguide, i.e., SP-POF, and Rλ is the reference intensity for the blank fiber at wavelength λ. Meanwhile, the gas chamber was attached to computerized mass flow controllers (MFC), which controls gas flow at 200 sccm. All the experiments were performed at room temperature (25 °C–26 °C) with relative humidity between 40% and 50%.3. Results and discussion
3.1. Material characterization
The surface morphologies of the graphene, pure PANI, and GPC nanocomposites were examined using FESEM, as illustrated in Figs. 5(a)–5(d). Figure 5(a) depicts the agglomerate platelet-like structure of the graphene with sharp corners with an average dimension of 350 nm. As indicated in Fig. 5(b), pristine PANI exhibits a mesh-like structure of nanofibers with an average size of 120 nm. PANI also shows a granular and porous structure. Figure 5(c) reveals that GPC sample is comprised of a graphene nanoplatelets grow on the interconnected PANI nanofibers with the width and length of 80 nm and 600 nm, respectively. PANI nanofibers were tidily grown on the graphene surface, because of the electrostatic attractions that allow the adsorption of aniline monomer onto the graphene surface [16].
Fig. 5 FESEM images of (a) Graphene, (b), PANI, (c and d) GPC nanocomposite of different magnification and (e) 3D topography AFM image for the GPC nanocomposite.
Figure 5(d) demonstrates that PANI finely coated the graphene. The GPC nanocomposite showed not only a lot of pores in its structure but also a significant number of holes owing to the graphene that acted as a support material with high surface area in the formation of the nanocomposite by chemical polymerization. Figure 5(e) depicts the atomic force microscope (NT-MDT Solver NEXT AFM) 3D image of the GPC nanocomposite. The scan boundary area for the AFM analysis was set to a square area of 10 µm × 10 µm. As shown in Fig. 5(e), the average surface roughness was found to be 91.9 nm. This level of roughness indicates that the effect of light scattering on the sensing performance was insignificant [26]. Rough sensing layer is important because it provides easy diffusion of gas molecules into or out of the layer. Thus, the sensing performance parameters of the gas sensors such as response time, sensitivity, and reversibility can be significantly improved.
Figure 6 shows the Raman spectra for the developed GPC nanocomposite and the pure PANI. The Raman spectroscopy analysis was performed with a Raman spectrometer (WITec, Alpha 300R) using laser excitation source with λ=532 nm. The GPC spectrum (shown in Fig. 6(a)) consists of significant characteristic peaks correspond to the graphene (D, G, and G’ (2D)). Compared with PANI (Fig. 6(b)), an increase in the intensity ratio for the three bands in the GPC nanocomposite (shown in Fig. 6(a)) was observed. The D band peak at 1339 cm−1 was attributed to the K-point phonons of A1g symmetry, whereas the G band peak at 1585 cm−1 was assigned to zone center phonons of E2g symmetry [27], with a shoulder at 1635 cm−1 associated to the C–C stretching of the benzenoid ring [28]. The G’ (2D) band was observed around 2761 cm−1 as depicted in Fig. 6(a), whereas it did not obviously appear in PANI spectrum (Fig. 6(b)). The D and G peaks were slightly shifted in the GPC nanocomposite compared to the pure PANI. The slight shift in the bands is an evidence on the formation of interaction between graphene and PANI in the nanocomposite [29]. The C=N stretching vibration of the quinonoid ring and the C–N stretching mode of the polaronic units were found in 1483 cm−1 and 1393 cm−1, respectively [30]. C≡N stretching vibration at 1457 cm−1 was observed, revealing the presence of the PANI structures [31]. C=C stretching in the quinonoid ring can be observed at 1565 cm−1, its location corresponding to the protonation of polymeric products and semi-quinonoid ring formation [32]. The peak at 1180 cm−1 can be attributed to the C–H bending of the quinonoid ring [33]. The C–N stretching vibrations of diverse benzenoid, quinonoid, or polaronic shapes corresponded to the band at 1246 cm−1 [34]. The peaks found at 833 cm−1 and 702 cm−1 corresponded to the C–H bending deformation in the benzenoid ring and bipolaronic C–C ring deformations, respectively [35]. The peaks at 511 cm−1 and 410 cm−1 were linked to out-of-plane deformations of the ring [33]. The peak observed at 573 cm−1 was assigned to the vibration of phenoxazine-type units [36, 37].
3.2. Ammonia gas sensing performance
The SP-POF sensors based on PANI and GPC nanocomposite were fabricated and their sensing performance were tested toward different concentrations of NH3. Figure 7 illustrates the dynamic response of the SP-POF sensor coated with PANI and GPC thin films toward NH3 of different concentrations (0.125% to 1%) balanced in synthetic air over a range of wavelengths between 400 nm–800 nm at 26 °C. It is evident that the absorbance response of both sensors increased proportionally with the NH3 concentration for the specified visible spectrum. In general, the sensors well recovered when synthetic air has pumped in the chamber at room temperature. As illustrated in Fig. 7(a), pristine PANI exhibited a minimal increase in absorbance response (16.6%) upon exposure to 1% NH3 gas. GPC nanocomposite-based sensor shows superior absorbance response (81%) toward 1% NH3 gas as shown in Fig. 7(b). The average (10% to 90%) response time of the SP-POF sensor-based PANI and GPC are measured to be 292 s and 112 s, respectively. The average (90% to 10%) recovery time of the SP-POF sensor-based GPC is found to be 185 s. The response of the developed sensors is faster than the response of the optical fiber sensor reported by Shobin et al. [38] and Raj et al. [22], which is 70 min and 90 min, respectively. In order to observe the repeatability, the proposed sensors were subjected to another sensing cycle (C2), keeping NH3 concentration at 0.125%, as revealed in Figs. 7(a) and 7(b). It was observed that, the sensitivity and response and recovery times were almost the same as in the case of the first cycle (C1). This exhibits the excellent repeatability of the sensor which is very important for accurate detection of NH3.
Fig. 7 Dynamic responses of the (a) PANI and (b) GPC thin films toward different concentrations of NH3 at room temperature.
Based on Fig. 7(a), PANI SP-POF sensor clearly suffers baseline drift upon increase of NH3 concentrations. It is believed that the baseline drifts caused by the residual NH3 molecules still occupied the PANI layer even after purging with synthetic air [26]. Further investigation has performed to determine the PANI based SP-POF sensor sensing behavior. Figure 8 shows the absorbance response of PANI sensor upon different cycles of air and NH3. The purging with the synthetic air for 10 min does not remove all the gas molecules from the PANI sensing layer as shown in Fig. 8(3). In contrast, SP-POF coated with GPC overcomes this issue. The return of the GPC coated SP-POF response to its original baseline upon synthetic air exposure as shown in Fig. 7(b) is the main indicator that the residual NH3 has been removed from the sensor surface. Figure 9 represents the sensitivity of the developed optical sensors that relates to its absorbance unit (AU). The sensitivity is calculated by the following mathematical equation [39]:
where S is the sensitivity, Agas and A0 represent the absorbance of the sensing layer upon NH3 gas exposure and synthetic air, respectively. ∆C is the gas concentration change in %. From Fig. 9(a), the sensitivities of the SP-POF sensor-based PANI and GPC nanocomposite are 21.7 AU/% and 132.8 AU/%, respectively. The GPC and PANI based SP-POF sensors revealed linearity slope for NH3 concentrations of approximately 81.7% and 49.5%, respectively. Figure 9(b) represents the plots of absorbance change as a function of NH3 concentration for PANI and GPC sensing layers over the wavelength range of 500–800 nm. It can be noticed that the response is rising proportionally with the increase of the NH3 concentrations. The GPC sensor limit of detection was calculated to be 0.002246%, which is equal to 22.46 ppm, based on established technique reported by Ammu et al. [40]. Thus, the developed GPC based SP-POF sensor is able to monitor NH3 gas concentration below the gas lowest permissible exposure limit reported by Occupational Safety and Health Administration (OSHA) which is 35 ppm [41].Fig. 8 Absorbance response versus optical wavelength for PANI based SP-POF exposed to NH3 with 1% concentration in synthetic air at room temperature.
Fig. 9 (a) Sensitivity and (b) absorbance response change as function of NH3 concentration of PANI and GPC sensing layers-coated SP-POF ammonia gas sensors.
Based on the sensors responses toward NH3, the GPC based SP-POF shows better performance as compared to PANI based sensor. However, the detailed sensing mechanism is yet to be fully understood. This significant change in the performance of GPC nanocomposite sensor can be ascribed to its surface morphology. According to the FESEM micro-characterization of the GPC, the presence of graphene enhanced the surface to volume ratio as well as the porosity of the GPC nanocomposite as compared with the pristine PANI. This can be contributed by aniline monomers density decreased in the polymerized GPC solution because of the electrostatic attractions. As a result, PANI nanofibers diameter decrease at variance to graphene mass ratio [42]. The GPC nanocomposite surface area is higher than PANI and therefore, there are greater numbers of active sites for NH3 gas adsorption. Consequently, SP-POF sensor coated with GPC nanocomposite film has high sensitivity and fast response [43]. The gas molecules can deeply penetrate the sensing layer and interact with the decayed evanescent wave formed at the interface of SP-POF and the sensing layer due to the attenuated total internal reflection (ATR) via multiple internal reflections along the fiber as illustrated in Fig. 10. Therefore, the sensor performance increased proportionally with the gas concentration.
Fig. 10 Schematic representation of the proposed interaction mechanism between NH3 gas molecules and GPC sensing layer.
The sensing mechanism to explain the interaction of NH3 molecules with PANI layer has been proposed in our previous work [26]. As the emeraldine salt PANI (green) is exposed to ammonia, it is dedoped through deprotonation process. This reaction modifies the PANI from emeraldine salt to emeraldine base form (blue). It is presumed that some amount of evanescent wave that propagate at the surface of the polished region is absorbed by the layer when it is in the emeraldine base form. The increase in ammonia concentration has increased the number of dedoped molecules, resulting in higher absorbance value [26].
Hydrogen (H2) gas was introduced to the GPC sensor to investigate its performance toward different gases. Figure 11 compares the sensor absorbance performance for 0.5% NH3 and 0.5% H2 over the wavelength range of 600 nm to 800 nm. As 0.5% NH3 pumped inside the gas chamber, the absorbance magnitude change found to be 52%, as revealed in Fig. 11(a). When NH3 was replaced with H2 after purging with synthetic air, the absorbance magnitude slightly increased to be 10%. Thus, it can be observed that the absorbance amplitude change is approximately five times higher when the sensor is exposed to NH3 than to H2. PANI (emeraldine salt) have strong interaction with NH3 and make it converted into emeraldine base structure according to the following equation [44]:
On the other hand, H2 interacts with the charged amine group of doped PANI (emeraldine salt) and forming a bridge between nitrogen atoms on two chains [45]. The possible mechanism of selectivity for NH3 is that the H2 gas interaction with PANI is weaker as compared to NH3 due to the fact that different gases have different electron affinity values [46]. As a result, the electron transfer for NH3 that changes the emeraldine salt into its base will be stronger than for H2. This make the absorption amplitude change of ammonia is five time higher than hydrogen. Figure 11(b) shows the absorbance spectrum of the GPC nanocomposite coated SP-POF upon exposure to NH3 and H2 of the same 0.5% concentration. An obvious increase in the absorbance spectrum observed at wavelength ranging from 605 nm to 630 nm. It is believed that this peak belonged to the NH3 absorption peak [47]. The distinguished responses produced by the GPC coated SP-POF sensor when exposed to NH3 and H2 of the same 0.5% concentrations indicate the selectivity potential of the developed sensor.Fig. 11 (a) Dynamic and (b) absorbance response of SP-POF coated GPC nanocomposite toward NH3 and H2.
We fabricated a set of three SP-POF sensors with different values of polished lengths (0.5 cm, 1 cm, and 2 cm) to investigate the effects of the polished region length on the sensor performance. The three sensors were coated with GPC nanocomposite due to its higher gasochromic performance toward NH3 at room temperature. The sensitivities of the SP-POF coated with GPC nanocomposite sensors for the 0.5, 1, and 2 cm polished lengths were 102.15 AU/%, 106.68 AU/%, and 132.80 AU/%, respectively, as shown in Fig. 12. When the polished length increased, the number of reflections along the fiber increased, thus improving the sensor sensitivity due to the increase in the number of ATR. In addition, longer polished area increases the coverage of the sensing layer and thus, higher number of active sites allow more NH3 gas molecules adsorption. As a result, higher sensitivity was measured for the sensor with longer polished area than the ones with shorter polished area. Nevertheless, any additional increase in the polished length may weaken the SP-POF mechanical stability that degraded the sensor sensitivity. The effect of the fiber parameters requires further investigations in order to enhance the sensor sensitivity.
Fig. 12 The sensitivity for SP-POF ammonia gas sensor with three polished lengths (0.5 cm,1 cm and 2 cm) coated with GPC nanocomposite.
4. Conclusion
In this paper, we reported NH3 sensing performance of side-polished optical fiber sensors coated with PANI and GPC nanocomposites. GPC nanocomposite was synthesized using chemical in-situ polymerization method. The optical sensing performance of the gas sensors was studied at room temperature against various concentrations of NH3 without any carrying gases. Upon exposure to different NH3 concentrations at room temperature, the sensor absorbance increases as the concentrations increase. The experimental results revealed that adding graphene into PANI increase the sensor absorbance magnitude six folds from 21.7 AU/% to 132.8 AU/%. The reason can be mainly attributed to the high-surface-to-volume ratio of the GPC nanocomposite compared to PANI. Moreover, fast response and recovery times were observed for the GPC nanocomposite based SP-POF sensor compared to PANI based sensor. The superior sensitivity, repeatability and selectivity of the SP-POF coated with GPC nanocomposite may make it a suitable candidate for NH3 gas-sensing applications such as remote and room temperature operated NH3 leakage sensor at chemical plants.
Funding
Universiti Putra Malaysia (UPM) (501100004530) and Ministry of Higher Education, Malaysia (501100003093).
Acknowledgments
The authors would like to acknowledge Department of Computer & Communication Systems Engineering, Faculty of Engineering, Universiti Putra Malaysia for the research resources.
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