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Abstract
A sensitive ammonia sensor based on long-period fiber grating (LPFG) is designed and manufactured for the detection of ammonia concentration in water. Femtosecond laser direct writing technology is used to write LPFGs on standard single-mode silica fiber. A thin layer doped with basic dyes is coated on the optical fiber for sensing by using the sol-gel method. The thicknesses of sol-gel layers, which play a key role in the sensitivity of the LPFG sensor, were carefully studied. Experimental results show that LPFG with a functional layer of ∼340 nm has the best sensing performance, and the detection limit is 0.08 ppm. The response time of the sensor is less than one minute, and the sensor has good repeatability with a short recovery time. Compared with other organic molecules and ions in water, the proposed LPFG sensor has not only good reusability, but also selectivity for the detection of ammonia.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ammonia is an important material on Earth: it is not only a part of food, but also a part of life. Ammonia is widely used in fertilizers, nitric acid, plastics, explosives, textiles, pesticides, dyes, and other chemicals. However, it is corrosive [1]. High concentrations of ammonia are harmful and have serious consequences because ammonia has irritating and corrosive effects on the human skin mucosa. Inhalation of low concentrations of ammonia can cause chemical pharyngolaryngitis, chemical pneumonia, and other diseases, while the high concentration of ammonia will be life-threatening after being inhaled by the human body. Therefore, the concentration of ammonia in water and air needs to be detected to prevent harm to the human body and environment [2] [3].
Traditionally, potentiometric electrodes are used to detect ammonia in water because of their high accuracy, sensitivity, and selectivity [4][5]. However, electrode detection has evident disadvantages, such as the need for experienced professionals to use expensive static instruments and the consumption of analytes [6]. Ratiometric fluorescence sensor [7], metal oxide semiconductor detector [8], and optical fiber sensor are also used to measure ammonia [9–11]. Among these sensors, optical fiber sensors have the advantages of anti-electromagnetic interference, low cost, and easy miniaturization. In the classification of optical fiber sensing, functional long-period fiber grating (LPFG) with chemical film stands out as a sensor with wide application prospects [12][13].
LPFG is a diffraction grating formed by periodically modulating the refractive index (RI) of the optical fiber core in the axial direction by a certain method. LPFG can be combined with various biochemical materials to produce optical sensors with good specificity, high sensitivity, and portability [13–16]. Previous studies showed that the transmission spectrum of LPFG is highly sensitive to the change of coating thickness and RI by coating the surface of optical fiber [17–19]. On the basis of this principle, organic vapors [20], sucrose [21], organic solvents [22], PH [23], and biological materials [15] have been successfully tested by LPFG sensors.
To realize the specific sensing of ammonia in water, the fiber surface needs to be coated with a layer of sensing material to make the fiber functional. The additional chemical material coated on the fiber surface can react with ammonia in water physically or chemically to change the optical properties of the coating material, thereby leading to the change of the transmission spectrum of the fiber grating. The concentration of ammonia in water can be detected by detecting the change of the spectrum. Similar to the sensing principle of the surface plasmon resonance fiber sensor, LPFG can also provide highly accurate information on the adsorption or desorption of the measured material according to the surface coating. The advantage of LPFG is that it can be used for affordable and simple manufacture of a sensor that can be used for a variety of environmental tests. In the preparation of functional layers, the advantage of the sol-gel method is that it can produce a porous glassy material that can be easily fixed on optical fiber, thus making it attractive in the field of optical fiber sensing. Sol-gel films, in particular, are now widely regarded as an efficacious, low-cost strategy for the mass production of reversible, robust and portable optical chemical sensors, and biosensors [24–27].
Numerous studies have used fiber optic sensors for the detection of ammonia in air, but relatively few studies have been conducted on the detection of ammonia in water. One possible reason is that detecting ammonia in water has higher demands on the stability and repeatability of the sensor because maintaining the performance of the coating in water is a challenging task. In this study, we have developed an optical sensor for detecting ammonia in water by combining the advantages of the sol-gel method and LPFG sensor. Adsorption of ammonia by the sol-gel coating on the fiber surface produces a change in the RI of the film, which leads to a spectral shift in the transmission response of the fiber optic sensor. In this experiment, the monitoring of ammonia concentration in water based on the variation of LPFG transmission spectra was achieved by proper calibration.
2. Theoretical analysis
LPFG is a periodic modulation of the RI in the fiber core. The RI variation of the LPFG can be typically written as
In short, an LPFG couples light from optical modes propagating in the core to multiple modes in the fiber cladding. Its wavelength is determined by the phase-matching condition
During the propagation of optical fiber, part of electric field will penetrate into the surrounding medium as evanescent wave due to grating. Any change caused by external refractive index change can produce large wavelength shift in resonance. By detecting the change of λ(x), the information of the change of external physical quantity can be obtained. Therefore, any change in the surrounding RI can be monitored by measuring the corresponding change in the LPFG resonance wavelength [14].
Figure 1(a) shows a schematic of the LPFG sensor detecting ammonia in water. The sol-gel coating on the surface of optical fiber is porous and has a certain adsorption capacity, so ammonia molecules are adsorbed into the coating. The combination of dye and ammonia will change the overlay RI, thereby changing the resonance wavelength of LPFG. The ammonia content in water can be obtained by analyzing the shift of the resonance wavelength.
3. Experimental section
3.1 Fabrication of LPFG
Highly sensitive fiber gratings with periodic RI modulation are inscribed on single-mode fibers by a femtosecond laser system. The optical fiber we used is SMF-28 single-mode optical fiber of Corning Company. The core diameter of the fiber is 8.2 µm, the cladding diameter is 125 ± 1 µm and the numerical aperture is 0.14. Optical parametric amplifier system (OPA, Mirra 900 + Legend Elite) has a central wavelength of 800nm and was used to inscribe gratings. LPFG was inscribed along the fiber core by femtosecond laser with repetition rate of 1kHz and energy of 0.8µJ. Femtosecond laser pulses were focused into the fiber core using a 40x objective lens (Olympus, Japan) with the NA of 0.6. The specific parameters of the written LPFG are: the total grating length is about 18 mm, the duty cycle is 50%, λ is about 360 µm, and there are 500 grating elements. The evolution of the transmission spectrum from 1,100 nm to 1,700 nm was monitored in real time during LPFG inscription by using an SC source (YSL, SC-5, China) and a spectrum analyzer (Yokogawa AQ6370D, Japan).
3.2 Sol-gel coating
The production of sol-gels consists of several steps, starting with the use of compounds containing highly chemically active components as precursors These raw materials are then mixed well and undergo a chemical reaction of hydrolysis and condensation in the liquid phase. This results in the formation of a stable and transparent sol-gel system in solution. The solute slowly polymerizes between the aged gel particles and the silica gel formed by this method is a porous matrix. Thus a gel with a three-dimensional network structure is formed, which contains interconnected pores made of silica. In order to ensure the scientific nature of the experiment, the water used in all experiments was distilled water. In this investigation, tetraethyl ortho silicate (TEOS, Macklin, 99+% [gc]) was used as the precursor for the sol preparation. Bromocresol purple (BCP, Macklin, Indicator Grade) has the advantage of being more stable and antioxidant than other acid-base indicators; therefore, it is used as an ammonia-sensitive dye in this case. A magnetic stirrer was employed to mix the sol-gel reagents. The sol-gel films were prepared as shown in Fig. 1(b) through the following steps: (1) 37% HCl was used to acidify 10 ml of deionized water to pH = 1. (2) 45 ml of TEOS was mixed with 55 ml of ethanol for 20 min. (3) 10 ml of acidified water was added to the resultant solution, and mixing was continued for 1 hour. (4) 10 ml of ethanol containing 10 mg of BCP was added, and mixing was continued for 1 hour. (5) During the experiment, the resultant silica sol solution was sealed and kept in the refrigerator. Each fiber sensor was manufactured by moving the translation stage three-dimensionally to make the LPFG part pass through a drop of silica sol solution, thus forming a very thin film on the fiber surface. The process of passing the fiber through the silica droplet from one end of the LPFG sensing area to the other is defined as single-layer coating (one-pass coating). This process allows multiple coatings of the fiber surface by repeating the same steps several times. After the target number of layers was coated, the sensor was first cured at room temperature for 24 hours, heated at 120 °C for 2 hours, and then dried at room temperature for 24 hours. The coated sensor was not immediately used for detection, and it was soaked in water for 24 hours before the experiment. This is because the infiltration of water will also affect the sensing. [28,29].
3.3 Sensing test setup
The experimental setup for this experiment to detect ammonia concentration in water is shown in Fig. 2. The ends of the optical fibers were connected to single-mode FC/PC patch cords by using a fiber optic fusion splicer (Sumitomo Electric Industries, Ltd., Osaka, Japan). The input end of the patch cord was fixed to a supercontinuous spectrum light source (YSL, SC-5, China) ranges from 470 nm to 2400 nm. On the other hand, the output end was fixed to the spectrum analyzer (Yokogawa AQ6370D, Japan). The detection region of the fiber was held in a custom mold. A peristaltic pump pumped the ammonia/water sample into the sensing area at 20 r/min. This allowed the ammonia/water sample to flow slowly around the sensing region. The two ends of the optical fiber were fixed in the mold and straightened, so that the optical fiber in the mold can prevent external environment interference such as vibration and temperature. Finally, the ends of the fiber were secured to the groove by using UV adhesive to prevent the effects of stretching and vibration. The external ambient temperature was maintained at 25 °C throughout the testing process.
4. Results and discussion
4.1 Property of the sol-gel film
To verify that sol-gel has been coated on the surface of optical fiber, the spectra of the sensor before and after coating the sol-gel were recorded. After coating with sol-gel, the resonance wavelength of the sensor exhibits an obvious blue shift, which indicates that the sol-gel coating is fixed on the surface of the optical fiber (Fig. 3(a)). Figure 3(a) also shows the transmission spectrum of the sensor in water after four passes of sol-gel coating.
Fig. 3. (a) Transmission spectrum of LPFG sensor measured before and after coating and while immersed in water. (b) SEM image of the side of LPFG sensing area coated with sol-gel. The SEM image of the end side of LPFG sensing area coated with sol-gel.
The morphology of the silica gel on the fiber surface is an important parameter. A fiber optic sensor was fabricated using the same coating process, and its surface and side morphologies were observed with a scanning electron microscope (SEM). The SEM images of a fiber sensor coated with silica coating (four passes) are shown in Fig. 3(b). The sol-gel coated on the surface of the optical fiber is smooth, and the estimated thickness of the coating is about 340 nm.
4.2 Concentration detection and detection time
The thickness of the sol-gel coating has an important influence on the sensing function of the LPFG sensor. Figure 4(b) shows the logic fitting diagram of wavelength drift when LPFG sensors with different coating numbers detect ammonia with different concentrations in water. The resonance wavelengths of the sensors with four and eight passes of coating both move toward a long wavelength with the increase in the ammonia concentration. The resonance wavelength of the LPFG sensor coated only once has almost no large drift, which shows that excessively thin coating will have little effect on the wavelength drift. Therefore, the manufactured sensor was not suitable for sensing experiments. However, excessively thick coating will also have a negative impact on the sensitivity of the sensor. The figure shows that the wavelength drift of the eight-pass sensor is less than that of the four-pass sensor at different ammonia concentrations.
Fig. 4. (a) Measured spectral response of LPFG sensor (four-pass coating) at different ammonia concentrations in water. (b) Logic fitting diagram of wavelength drift when LPFG sensors with different coating numbers detect ammonia with different concentrations in water.
The above analysis indicates that the LPFG with a coating thickness of around 340 nm has the highest sensing sensitivity; thus, it can be used for ammonia detection. To evaluate the sensitivity of the sensor, we defined the parameter of sensitivity S with reference to other literature [29]. The sensitivity S of the sensor is defined as S = Δλ /C, where Δλ represents the corresponding spectral wavelength shift at different ammonia concentrations and C is the concentration of ammonia in water. Figure 4(a) illustrates the shift of the resonance wavelength in the transmission spectrum of the LPFG sensor at different ammonia concentrations in water. With the increase of the ammonia concentration, the resonance wavelength monotonically moves to the longer wavelength. As can be seen from the graph, the wavelength shift rate at low concentrations is significantly higher than that at high concentrations, which indicates the higher sensitivity of the sensor at low concentration detection. The sensitivity S is estimated to be 0.26 nm/ppm at 10 ppm and 0.05 nm/ppm at 100 ppm. On the basis of the measurements at low concentrations and after calculations, the lowest detection limit of the sensor (LOD) is 0.08 ppm. The calculation of LOD is based on the international general calculation method [30].
The response and recovery time of the sensor is of great importance for field detection. After a period of time after detection, the environmental refractive index of LPFG recovered to the original level after ammonia volatilization. Figure 5(a) shows the real-time resonance wavelength drift plot of this LPFG sensor at three different concentrations. Response time is defined as the time when the sensor reaches 90% full response, and recovery time is defined as the time when the sensor drops to 10% full response. Figure 5(a) shows that the recovery time of the sensor is short, but it takes a long time to recover to the same wavelength as before. On the basis of the previous definitions of response time and recovery time, the response time of this LPFG sensor was estimated to be less than 1 minute, while the recovery time was roughly around 10 minutes. Notably, the evaporation of ammonia took longer than what was actually needed in our experiments because the ammonia solution used in the experiment was highly diluted and no heat was used to assist in the evaporation process.
Fig. 5. (a) Spectral shift diagram of response and recovery of LPFG sensor under different concentration tests. (b) Measurement of ammonia with the same concentration in water by the same LPFG sensor at different times.
The reproducibility of ammonia sensor measurement at low concentration is also studied, and the experimental results are shown in Fig. 5(b). In the first measurement, the manufactured sensor is tested with 10 ppm ammonia water on the first day. After the test, the sensor was placed in air for 24 hours. Then, the sensor was used for the second measurement of ammonia with the same concentration in water, and so on. The test results of different times are shown in Fig. 5(b). The wavelength shifts tested for different times all show the same value, thus proving that our sensor has good reproducibility.
4.3 Selective detection
The specific detection performance of the sensor for the substance to be measured is an important parameter in practical applications [29]. Therefore, the sensor was used to test the sensing performance for common molecules and particles in water. The sensor was tested for the wavelength drift of methanol, ethanol, CaCl2, NaCl, MgCl2, and Na2SO4 in water, and the sensing results are shown in Fig. 6. The figure shows that the sensor responds to ammonia in water only, producing a large wavelength drift, while it basically does not respond to other particles. Therefore, the LPFG sensor fabricated by sol-gel has a good specificity for sensing ammonia in water.
Fig. 6. Experimental results of the specificity of LPFG sensing, including the detection of methanol, ethanol, CaCl2, NaCl, MgCl2, Na2SO4 and ammonia in water.
5. Conclusion
A sensitive ammonia sensor based on LPFG was designed and manufactured to measure ammonia concentration in water. Femtosecond laser direct writing grating technology was used to inscribe LPFGs on standard single-mode silica fiber. The coating doped with basic dyes made by the sol-gel method is coated on optical fiber for sensing. Sol-gel films with different thicknesses were deposited on the surface of the LPFG sensor to improve the sensitivity of the sensor. The experimental results show that the LPFG with four passes of coating has the best sensing performance, and the detection limit is 0.08 ppm. The response time of the sensor is less than one minute, the recovery time is short, and the sensor has good repeatability. Compared with other common ions and organic molecules in water, the proposed sensor also provides good performance in terms of repeatability and good selectivity for sensing ammonia.
Funding
National Natural Science Foundation of China (61935006, 62075107, 62075109, 62090064); K. C. Wong Magna Fund in Ningbo University.
Acknowledgments
Peiqing Zhang thanks the Natural Science Foundation of China and the K. C. Wong Magna Fund in Ningbo University for help identifying collaborators for this work.
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.
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