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Abstract
Owing to the unique merits of high sensitivity and large range detection, long-period grating (LPG) optical fiber sensor modified with functional coatings has attracted significant attention recently. As a refractive index sensor, the appropriate choice of the coating properties is critical for exploring the high-performance LPG sensor. In this study, p-type delafossite AgAlO2 (AAO) nanoparticles were proposed as a novel sensitive coating material for the cladding modification of LPG develop a high-sensitivity fiber-optic ammonia sensor. As the resistance/dielectric constant of AAO cladding layer is sensitive to concentration variation of ammonia vapor even at room temperature. Sensing performances of the LPG optical fiber sensor could be substantially enhanced with the presence of AAO cladding layer. A linear sensitivity of 2.07 pm/ppm to ammonia from 25 ppm to 400 ppm was achieved at room temperature. Moreover, the LPG sensor exhibits short response (38 s) and recovery time (50 s), and high selectivity to ammonia compared to several possible interfering vapors.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ammonia is an essential chemical agent that is widely used in industrial production. It is a colorless toxic gas with a characteristic pungent odor. The principal sources of ammonia in the atmosphere are emissions and leaks from industrial processes, vehicle exhausts, and food spoilage [1,2]. Ammonia is a known irritant of the upper respiratory tract’s mucous membranes, nose, and eyes [3]. Thus, it can do harm to the respiratory system. Ingesting excess ammonia can cause pulmonary edema or even death. Further, ammonia is a precursor of secondary particulate matter, which can combine with sulfur and nitrogen oxides to promote the production of fine particulate matter (PM 2.5) [4]. The maximum permissible ammonia concentration for indoor long-term residential areas suggested by Occupational Safety and Health Administration (OSHA) is set at 25 ppm [5]. The presence of 35 ppm of ammonia in the environment will spoil human health within 15 min and lead to acute respiratory diseases [6]. Therefore, in the perspective of environmental pollution detection and personal safety, it is necessary to develop an ultra-sensitive and reliable ammonia sensor.
Various kinds of ammonia sensors have been developed, such as metal-oxide semiconductors (MOSs) sensors [7–9], surface acoustic wave devices [10,11], field-effect transistors [12,13], and optical fiber sensors [14]. The widely used MOSs sensors typically require high operating temperature (∼200 °C) [15] associating with high heating current, which hinders their applications in inflammable and explosive places. On the contrary, optical fiber sensors, composed of a waveguide and coating, analyze adsorbed vapors at room temperature based on optics parameters modulation of the cladding layer due to adsorption/desorption of the analyte gas. Fiber-optics sensing platforms have been attracted considerable attention due to many surprising features such as robust operation in harsh environments, immunity to electromagnetic interference and enabling remote real-time monitoring with no electrical power needed at the sensing point [16,17]. Several optical fiber sensing methods were well-established for ammonia gas concentrations measurement, including quartz crystal microbalance [18], absorption spectroscopy [19], evanescent field interaction [20–22], interferometry [23–25], surface plasmon resonance [26,27], and grating-based index transduction [28,29]. Among the different types of grating-based optical fiber sensors, LPG [30,31] has been employed extensively for refractive index measurement and even for monitoring trace amounts of chemical, gaseous and biological species. It is worth noting that LPG optical fiber sensors provide wavelength-encoded information, which overcomes the referencing problems associated with intensity-based methods [14,32]. Through chemical modification, LPG sensors with appropriate functional coatings to detect ammonia [29], volatile organic compounds (VOCs) [16,33,34], hydrogen [35], dissolved oxygen [36], relative humidity (RH) [32], pH [37], and heavy metal ions [38] have been achieved. For specialize in ammonia sensing applications, ongoing studies aim to enhance their sensitivity or selectivity. Several methods with remarkable results have been proposed, such as cladding etching [39], coupling to higher-order cladding modes working near their dispersion turning point (DTP) [40–42], working in mode transition (MT) through the deposition of nano-scale high refractive index (HRI) layer [43], and a combination of different approaches [34,44,45].
It is known that the appropriate sensitive coating is important to achieve high-performance fiber sensors. In recent years, some attempts to use chemiresistive gas sensing materials, such as carbon nanotubes (CNTs) [46], graphene [47], and their derivatives, intrinsically conducting polymers (ICPs) [6,48] and MOSs [49,50] as sensitive coatings for fiber-optic sensors have confirmed the enhanced sensing performance, which opens a new genre of fiber-optic ammonia sensing. High-performance sensitive coatings for fiber-optic ammonia sensors are of great importance to meet practical application demands.
For the consideration of energy consumption and circuits simplification, sensitive materials working at room temperature are highly preferred. MOSs have been extensively studied as gas sensing materials due to their excellent chemical stability, distinct surface chemical properties and high response. However, only a few MOSs have been studied for their ability to enhance the response of fiber-optic sensors to ammonia gas at room temperature [22,51]. Seeking sensitive coatings appropriate for room temperature fiber-optic sensors is to promote the development of optical fiber sensors. Meanwhile, the integration of MOSs with optical fiber poses a unique set of technical challenges. In our previous studies, we have found that the resistance of delafossite AAO exhibits a sensitive and selective response to ammonia gas at room temperature [52]. This is mainly attributed to the surface reaction between physisorbed water (H2O) and preferentially adsorbed ammonia gas, which leads to a significant increase of ionic conductivity on the surface of delafossite AAO. Considering this preferential adsorption and reaction may also induce a change in its effective refractive index, which causes wavelength shift of the transmission spectrum, delafossite AAO may be a promising sensitive coating for optical fiber sensors.
In this study, a sensitive coating of delafossite AAO nanoparticles was applied to improve the LPG optical fiber sensor’s sensitivity and selectivity to ammonia gas at room temperature. As predicted, the AAO-coated LPG optical fiber sensor’s sensitivity was significantly improved, and a linear sensitivity of 2 pm/ppm to ammonia from 25 ppm to 400 ppm was achieved.
2. Theoretical analysis and simulation of ammonia sensing
The LPG was chosen as an optical transducer for its intrinsic sensitivity to the surrounding refractive index and the possibility of fine-tuning its sensitivity for specific applications [34]. The essence of long-period fiber grating is due to the introduction of periodic dielectric perturbations that destroys the consistency of the optical properties of the fiber and causes energy exchange between the modes. It is assumed that the core mode (LP01) couples only to the axisymmetric cladding modes denoted as LP0ν (ν > 1). Since there are many co-propagating cladding modes in a certain wavelength range, any light whose wavelength satisfying the phase-matching condition will experience loss. As a result, one or more attenuation bands appear at the fiber transmission spectrum. These resonance bands are located at the different dip wavelengths λres satisfying the phase-matching condition.
On the external surface of the LPG, a thin AAO overlayer is uniformly coated along the radial direction to establish a three-cladding structure model consisting of cladding, AAO overlayer, and surrounding medium as shown in the right panel of Fig. 1. Ignore the material absorption loss, where: n1, n2, n3 and n4 are the effective refractive index of the core, cladding, AAO overlayer and surrounding medium respectively, r1 and r2 are the radii of the core and cladding, and r3- r2 represents the thickness of the AAO overlayer.
Fig. 1. Three-cladding structure and the step refractive index model of AAO-coated LPG ammonia sensor.
According to the mode-coupling theory, the mode coupling of the AAO-coated LPG ammonia occurs between the fundamental core mode of a single-mode fiber and the νth-order co-propagating cladding mode. The transmission loss can be represented as [35]:
When the AAO-coated LPG sensor is exposed to a certain concentration of gas, the AAO coating will absorb a certain amount of gas molecules, resulting in neff,cl changes. Thus, the difference between neff,co and neff,cl will also change, and then lead to λres shift in the transmission spectrum of the LPG. Therefore, the effective refractive index value of the AAO coating also can be obtained by measuring the shift of the dip wavelength Δλres. The magnitude of Kcl-co,1v-01 determines the strength of the coupling between the core mode and the cladding mode in the LPG, which changes with the variation of the coating effective refractive index and eventually leads to λres shift in the transmission spectrum of the LPG. If the midpoint of the grating is the origin of the z-axis, and the length of grating is L, the initial boundary conditions of LPG are as follows:
The transmittance T of LPG can be expressed as:
Using the initial boundary conditions, solving the system of partial differential equations Eq. (1) and Eq. (2) yields Aco(L/2) and Aco(-L/2). T can be derived by substituting into Eq. (6).
Optical simulations of the AAO-coated LPG were performed using OptiGrating v.4.2 software by Optiwave. The grating period Λ was determined during the fabrication process and was set at 585 µm. The grating length L was fixed at 2.5 cm. The fiber model was based on the manufacturer’s datasheets for the fiber used in the experimental work (Corning HI1060-Flex fiber). The exact fiber parameters used in the AAO-coated LPG model were summarized in Table 1.
Table 1. Specification of the AAO-coated LPG model
In Fig. 2(a), the numerical transmitted spectrum (red line) of the bare LPG with a modulation amplitude of 3×10−4 is reported. The depth of LP05 cladding mode is much higher and the band is much narrower than that of LP06, which is an excellent feature in the view of interrogation for sensor applications. Considering the investigated wavelength range, in this work LP05 cladding mode was finally chosen as the object of study. Further, the typically transmitted spectrum of the LP05 band corresponding to bare LPG, AAO-coated LPG, and AAO-coated LPG/ammonia using simulation software are shown in Fig. 2(b). The refractive index of the 410 nm-thick AAO coating is a variable that changes with the external ammonia gas, and the other parameters are all held constant. As seen in the graph that AAO coating and ammonia affect the magnitude and position of the attenuated peak of LPG. The simulation results show that the visibility of the LP05 band experiences a decrease with a simultaneous wavelength blue shift in the transmission spectrum when the AAO nanofilm is deposited onto the LPG portion. In addition, the resonance wavelength of the AAO-coated LPG sensor shifts significantly blue with changes in the external ammonia concentration.
Fig. 2. Simulation transmission spectrum: (a) bare LPG on HI1060-Flex fiber with 585 µm period, (b) typical transformation of the core-5th-cladding coupling mode bare LPG, AAO-coated LPG, and AAO-coated LPG/ ammonia.
3. Experimental details
3.1 Preparation and characterization of AAO nanoparticles
The AAO nanoparticles used for ammonia sensing were prepared by a hydrothermal method based on previous literature [52]. All reagents were of analytical grade and used without further purification. First, equimolar quantities of Al (NO3)3·9 H2O and AgNO3 were dissolved in 70 mL deionized H2O at room temperature. Then, an appropriate amount of NaOH was added to adjust the pH to be 8.5. After stirring the solution for 3 h. The obtained suspension was sealed in a 100 mL Teflon-lined autoclave for the reaction at 210 °C for 60 h. Then, the produced precipitates were washed with 1 mol/L nitric acid, 1 mol/mL ammonia solution, and deionized H2O in sequence. Finally, the precipitates were dried in an oven.
X-ray diffractometer (XRD) patterns of AAO nanoparticles were obtained on a Rigaku SmartLab XRD instrument with Cu-Kα source (λ = 1.52Å) operated at a generator voltage of 40 kV and an emission current of 150 mA. The patterns were acquired over 2θ regions of 10∼80° at a sampling scan width of 0.25 °/s was used to identify the crystalline phases. The XRD pattern of the as-synthesized AAO sample is shown in Fig. 3(a). All the peaks match well with the standard pattern of 3R-AgAlO2 phase (JCPDS file Card No. 84-2201).
Fig. 3. (a) XRD pattern, (b) EDS elemental mapping, (c) SEM micrographs, (d) histograms of the particle size distribution, and (e) elemental analysis report for the AAO nanoparticles.
Dimensional morphology and chemical composition of specimen surface was measured using Hitachi SU8000 model field emission scanning electron microscope (FE-SEM) with an electron dispersion spectrum (EDS) accessory. Figures 3(b) and 3(e) show the EDS elemental mapping and elemental analysis report for the selected region of the as-synthesized sample, respectively. It can be seen that all the elemental Ag, Al, and O are homogeneously distributed. The atomic ratio of Ag (14.34%) to Al (16.99%) is 0.84:1. SEM micrographs of AAO nanoparticles are presented in Fig. 3(c). The high-resolution SEM micrograph of AAO nanoparticles in the inset of Fig. 3(c) displays a silkworm chrysalis shape with a layer-by-layer structure. The particle size distribution of AAO was estimated from the low-magnification SEM micrograph analysis by using the Nano Measurer software. The histograms of the particle size distribution for the AAO nanoparticles are presented in Fig. 3(d). The histograms were fitted to the log-normal distribution function and the peak value was taken as average particle size. As can be seen from Fig. 3(d), the particle size ranges from 125 to 575 nm with an average diameter of 247 nm and standard deviations of 7 nm.
3.2 Fabrication of the AAO-coated LPG ammonia sensor
In this work, we used a high-performance thermal LPG made from Corning HI1060-Flex fiber with a cut-off wavelength of 920 nm via point-by-point CO2-laser irradiation. The fiber has a 125 µm-diameter pure silica cladding and a 3.4 µm-diameter Ge-doped silica core. The LPG had a period (Λ) of 585 µm and a total grating length (L) of ∼ 3 cm. Then the fabricated LPG was immersed in diluted hydrofluoric acid for 15 min to etch the fiber cladding around the grating region. Figure S1 shows cross-sections of three cladding etched LPG, verifying that the cladding etching process is reproducible. The etched LPG was washed with distilled H2O and ethanol to remove residues on the gratings’ surface. After that, the LPG was treated with oxygen plasma for 5 min to change the hydrophobic surface into a hydrophilic one to ensured well spreading of the AAO paste on the LPG sensing area.
The AAO paste was prepared by mixing 0.16g as-synthesized AAO nanocrystals, 0.02 g ethyl cellulose M9, and 0.1 g terpineol in 2 mL ethanol. Ethyl cellulose was used as a binder and was dissolved before usage in ethanol to yield 10 wt% solutions. Surfactant terpineol played a role in regulating the viscosity of the paste. For homogeneous dispersion, the paste is ultrasonicated for 30 minutes and subsequently stirred for 10 hours. Then the paste was homogeneously coated on the LPG sensor by a multilayer brushing method [55–59]. Because the film thickness depends on the manual operation, repeated practice is necessary. Before brushing, the bare LPG was fixed on a well-cleaned glass substrate by glue and baked in a vacuum oven with a temperature of 80 °C for 2 h to solidify the glue. Finally, the AAO-coated LPG sensor was sintered in air at 300 °C for 3 h. The SEM cross-sectional image was taken to check for uniformity of the AAO layer coated onto the LPG fiber. As shown in Fig. 4(a), dense AAO particles were homogeneously packed on the fiber surface. The outer diameter of AAO-coated LPG was about 98.50 µm. Figure 4(b) shows the micro-scale SEM picture of the fracture cross-section for the AAO-coated LPG. Because the film is composed of AAO nanoparticles with unevenly distributed grain size, rough surface was observed in Fig. 4(b). The thickness of AAO coating is in several hundred nanometers and a thickness of 410 nm is used for theoretical simulation.
3.3 Experimental setup of ammonia sensing
As shown in Fig. 5, the AAO-coated LPG gas sensor was put in the gas chamber, whose ends were connected to the Superluminescent light-emitting diodes (SLED) broadband light sources emitting light over 1400∼1700 nm range and an optical spectrum analyzer (OSA, Anritsu, MS9740A). During the preparation and static analysis, the spectrum of LPG has been monitored in the wavelength range 1500∼1560 nm by the OSA with a minimum wavelength resolution of 0.03 nm. The volume of the gas chamber is 20 L. The test gases were prepared by drawing out the headspace vapor of 25∼28% ammonia solution into an injector at room temperature. According to the ideal gas law, adjusting the volume of an injected vapor can control the gas concentration. In the experiments, 0.9 ml, 1.8 ml, 3.6ml, 7.2 ml, 10.8 ml, 14.4 ml, 21.6 ml and 28.8ml of saturated vapor were injected sequentially at 25°C, corresponding to a concentration of 25 ppm, 50 ppm, 100 ppm, 200 ppm, 300 ppm, 400 ppm, 600 ppm and 800 ppm of ammonia in the volume, respectively. Whereas, for fast acquisition operation (gas testing) we used OSA in the wavelength range 1540∼1548 nm coordinated by a computer data acquisition system. The OSA is controlled through a host computer that runs a custom LabView software to acquire the transmission spectrum of the grating and track the position of dip wavelength in real-time by Gaussian fit centroid analysis. During the experiments, the pressure inside the gas chamber was the same as the atmospheric pressure, and other potential environmental influences were ignored. By gently removing the glass cover, ammonia gas can be released from the test chamber and expelled out the outside via the vents. The temperature and RH cross-sensitivity is a critical issue for any LPG-based sensing device [60,61]. During the test process, the test room was kept at around 300 K and 35% RH by the air conditioning systems. The RH in the test chamber was monitored by a Testo 605-H1 hygrometer during the test and a variation of around ±1% RH was observed.
4. Result and discussion
4.1 Response to ammonia
Figure 6(a) presents the transmission spectra of the sensors uncoated and coated with AAO nanoparticles. The dip wavelength λres of the transmission spectrum blue shift about 0.69 nm after being coated with AAO nanoparticles due to the increased refractive index of the optical fiber surface. The AAO coating can enhance the evanescent field, as well as the sensitivity to the local refractive index, and make the optical fiber suitable for chemical gas detection. The depth of the LPG attenuation band in the transmission spectrum is attenuated by 3.23 dB due to the deposition of the AAO nanofilm coating. Usually, this effect is underpinned by the imaginary part of the nano-film RI [62]. In Fig. 6(b), the transmission spectra of the AAO-coated LPG-based sensor in different ammonia concentrations at room temperature 35% RH were recorded. It was observed that the dip wavelength shifts from 1545.10 nm to 1543.81 nm as ammonia gas concentrations increasing from 0 to 800 ppm. The spectra blue shift of about 1.29 nm was originated from the reaction of ammonia gas and AAO surface. As in the atmospheric environment, H2O vapors were readily adsorbed on the AAO surface. Upon the exposure of ammonia gas, physisorbed H2O reacts with ammonia gas and then forms volatile ammonium hydroxide NH4OH, which induces the conductivity and refractive index changes of the LPG surface [63,64]. The high volatility of NH4OH ensures the quick response and fast recovery of the sensor. Spectral responses of the AAO-coated LPG sensor were recorded for each measurement at room temperature while reproducible results are demonstrated.
Fig. 6. (a) Variations of transmission spectra with uncoated and coated AAO nanoparticles (b) enlarged spectra detail of the wavelength blue shift as the ammonia concentration increases.
4.2 Sensitivity of ammonia sensing
All responses of the AAO-coated LPG gas sensor were monitored on OSA and its sensitivity was defined as the shift in the dip wavelength (Δλres) per unit change in the ammonia gas concentration (C). As shown in Figs. 7, the LPG sensor with AAO nano-coating was tested under 25∼800 ppm ammonia gas concentrations at room temperature. For the sensor, the second-order polynomial fitting of the dip wavelength shift curve was shown in Fig. 7(a). As is shown in the figure, the sensitivity decreases with the increase in the concentration of ammonia gas. When C is above 400 ppm, Δλres becomes slightly saturated. Because the LPG region of the sensor has a finite surface area, the sensing layer is limited in the physisorbed H2O molecules that can interact with ammonia gas on the AAO surface. For higher ammonia concentration, the dissolution of ammonia gas in physisorbed H2O tends to be saturated thus a lesser Δλres is evident by the saturation behavior of the calibration curve at higher concentrations. The error bars show the standard deviation of Δλres while conducting multiple experiments, which indicates the excellent reliability of the produced sensing probe. A polynomial fitting of the data gives the following relation:
Fig. 7. Shift in the dip wavelength of the AAO-coated LPG sensor when changing ammonia concentrations: (a)25∼800 ppm; (b)25∼400 ppm.
In the range of 25∼800 ppm, the dip shift is 1.08 nm. Thus, the average sensitivity of the AAO-coated LPG gas sensor is −1.39 pm/ppm. A linear approximation can be found in the range of 25 to 400 ppm ammonia gas concentrations, as shown in Fig. 7(b). A linear fitting of the data is given:
The linearity (R2) of the AAO-coated LPG ammonia gas sensor is 0.974, and the concentration-dependence coefficient is −2.07 pm/ppm. A slight hysteresis characteristic can be observed when the concentration was reversed from high to low for gas-sensitive performance testing, as shown in Figure S4.
4.3 Dynamic temporal response and repeatability
The dynamic response curve of the sensor in the ammonia concentration range from 25 to 800 ppm as shown in Fig. 8(a). After the introduction of ammonia gas into the test chamber, the dip wavelength shifted immediately resulting in a steep response curve and then became stable within one minute. By expelling out ammonia gas from the test chamber, the dip wavelength could shift back to the initial state within 2 minutes depending on the ammonia gas concentration. Initially, the AAO nanoparticles showed a linear response, which then saturated at a higher concentration of ammonia. The repeatability of the sensor is evaluated by monitoring whether the dip wavelength of the sensor can shift back to its original wavelength in repeated measurements. Three reversible cycles of the real-time response of the wavelength as a function of time for detection of 800 ppm ammonia gas as shown in Fig. 8(b). It indicated that the sensor has good signal repeatability at room temperature. Because the interaction of physisorbed H2O and ammonia gas is a fully reversible process, it simply forms an unstable complex that readily decomposes into gaseous products when ammonia gas is exhausted, allowing reuse of highly sensitive probe.
Fig. 8. (a) Dynamic response curve of the AAO-coated LPG sensor towards various ammonia concentrations; (b) reversible response of the AAO-coated LPG sensor 800 ppm ammonia gas and (c) response and recovery time of the AAO-coated LPG sensor towards 400 ppm and ammonia gas.
This fast response and recovery characteristics were highly required for practical applications. The response and recovery time were evaluated by the dynamic response curve of the sensor to 400 ppm ammonia gas, as shown in Fig. 8(c). When 400 ppm ammonia gas is injected into the chamber, the response or rise time is defined as the time taken to achieve 90% variation of the sensor signal. Analogously, the recovery or fall time, which is the time taken to drop down to 10% variation, was measured by reversing this process. Results reveal that the AAO-coated LPG sensor showed a rapid response (38 s) and recovery (50 s) to 400 ppm ammonia gas at room temperature. The response time differs depending on the ammonia concentration as shown in Fig. S3. The response and recovery time of the tested ammonia concentration was all listed in Table S1. Note that the measurement of response/recovery time closely relates to the volume of the test chamber. The Smaller chamber facilitates fast diffusion of ammonia into the sensing layer, and thus achieves a more prompt response. Present AAO LPG sensor possesses comparable response and recovery parameters with previous work, as listed in Table 2.
Table 2. A comparison of the response/recovery time and operating concentration of several metal-oxide nanomaterial-based ammonia sensors were reported in the literature
4.4 Selectivity
The selectivity of this AAO coating-based LPG ammonia sensor has also been studied as one of the most critical properties to evaluate. The proposed sensing probe was checked by several possible interfering analytes VOCs such as Ethanol, Toluene, Isopropanol, Acetone, Methanol, and N-hexane in the atmosphere air background. Figure 9 shows the differences in resonance dip wavelength shift of the AAO-coated LPG sensor towards 400 ppm concentrations of different gases. The results confirm that the probe shows the maximum response to ammonia gas. For the other VOCs, minor shifts in dip wavelength are observed due to the weak adsorption and reaction of gases over the surface of AAO nanoparticles. Another possible reason is the higher solubility of ammonia gas compared with the other examined analytes. The response to ethanol may be due to the proton transfer from H3O+ to ethanol molecules. Though ethanol molecules cannot ionize in water, they have a larger proton affinity (188.3 kcal·mol−1) than H3O+ (166.5 kcal·mol−1). The influence of humidity on the sensor was also studied and the drifting plot of the transmission spectrum dip was shown in Fig. S2. The transmission spectrum of the sensor shifts as the relative humidity increases. However, the wavelength shift in the low humidity range of 30∼50% RH was much smaller than that of the high humidity range of 50∼60% RH.
4.5 Sensing mechanism of the AAO-coated LPG with ammonia
The ammonia sensing mechanism of the AAO-coated LPG sensor is also illustrated in Fig. 10. The fiber section along the LPG is wrapped in AAO nanoparticles. Light from a broadband source is emitted into the core of the fiber. When the light passes through the LPG, a part of the light is propagated in the core and the other part from the core into the cladding. The final coupling is back to the core as it enters the single-mode fiber. The coupling is manifested as the coupling of the forward transmitted core mode to the first-order forward transmitted cladding mode. At room temperature, ionic transfer will occur between physisorbed H2O molecules and target gas molecules on AAO surface.
Fig. 10. Schematic illustration of the sensing mechanism of the AAO-coated LPG sensor to ammonia gas.
These chemical reactions can be expressed as
Ammonia is extremely soluble in the adsorbed H2O, which can be converted to NH4+ and OH−, changing the refractive index of the fiber. As a result, the ammonia concentration will result in a significant change in the refractive index of AAO surface. During the transmission of light, the effective refractive index of the core neff,co remains invariant, while the effective refractive index of the cladding mode neff,cl varies with the concentration of ammonia. According to Eq. (4), λres in the transmission spectra of the LPG will shift significantly due to the change in the effective refractive index of the cladding mode neff,cl. Thus, the ammonia concentration can be determined by tracing the dip wavelength shift Δλres.
5. Conclusion
In this paper, AAO nanoparticles were synthesized by a facile hydrothermal synthesis route, and the AAO-coated LPG optical fiber probe was demonstrated for ammonia gas sensing at room temperature. The structural, morphological, and compositional analysis was done to ensure material properties. The AAO nanoparticles show a single Ag-deficient 3R delafossite phase with an average diameter of around 247 nm. By a multilayer brushing method, the AAO nanoparticles were uniformly distributed on the surface of the fiber showing a thickness around 440 nm. The modification in the refractive index/dielectric constant of coated material in the presence of ammonia shifted the transmission spectra spectrum. The AAO-coated LPG sensor exhibited high sensitivity and better selectivity towards ammonia gas owing to the reaction of ammonia gas with the physisorbed H2O on the AAO surface. In the range of 25∼400 ppm, the sensor shows a linear response to ammonia gas with a high sensitivity of −2.07 pm/ppm. Also, the sensor demonstrates good repeatability and a short response and recovery time of 38 s and 50 s, respectively. This high-performance probe may have great potential for rapid detection of ammonia gas.
Funding
National Natural Science Foundation of China (41627803, 62105001); Major Research and Development program of Anhui Province of China (1804a0802219); Science and Technology Plan of Shenzhen (KQTD20170331115422184); Key Technology Research Project of Chinese Academy of Sciences (ZKYXG-2018-04).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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