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Abstract
A high sensitive aqueous ammonia sensor based on tilted fiber Bragg grating (TFBG) had been reported. The sensors were fabricated by a 10 ° TFBG coated by a membrane receptor named as Polyaniline/Graphene oxide on the surface of the fiber. The correlative concentrations of aqueous ammonia were demodulated by global monitoring of the envelope area of cladding modes in the transmitted spectrum of the TFBG. Tests have shown that the proposed sensor can provide a linear and rapid response of aqueous ammonia within 22 seconds, in a concentration range from 1-12 ppm. Moreover, the limit of detection can even reach 0.08 ppm, through the theoretical analysis of our experimental results. The proposed sensor has good performance, is easy to manufacture and of small size, making it a good choice for real-time, in-situ, label-free detection of aqueous ammonia in the future.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ammonia molecules are an essential part of life, the ammonia is also an important analyte used in biotechnology, agriculture and clinical medicine [1,2]. However, excessive accumulation of ammonia can be harmful to organisms and aquatic ecosystems. Direct contact of high-concentration ammonia in water with human skin will cause skin liquefaction and necrosis [3]. Excessive ammonia can cause alkaline burns in the digestive tract, causing ulcers, erosions, and even gastric perforation, while it is inhaled through the gastrointestinal tract. In the natural environment, excessive ammonia will also lead to the eutrophication of water bodies, and the proliferation of algae and microbial microorganisms. In severe cases, the dissolved oxygen in the aquatic ecosystem will decrease, and then fish and other biotic population will die in large numbers accordingly, and then the entire system will be damaged totally [4,5].
Nowadays, various sensors have been developed to monitor the ammonia content, to avoid the harm of superfluous aqueous ammonia to the human body and the natural environment. Such as a polyaniline/multiwalled-carbon-nanotubes (PANI/MWCNTs) based fluorescence ammonia sensor with a limit detection of 0.25 ppm [6], a polyaniline film-based optical transmittance method (LOD 3 ppm) [7], and a colorimetric sensor (LOD 10 ppm) [8]. The functionalized fiber-optic ammonia sensors also have been proposed, which provide comparable sensitivity to the above methods and with a simpler sensing system, including a sol-gel silica based long period fiber grating sensor (LOD 0.08 ppm) [9], a tapered fiber (2.47 nm/ppm) [10], a Fabry-Perot Interferometers (LOD 0.1 ppm) [11].
Among all of the available sensing techniques, fiber-optic-based sensors have been intensively studied and presented multitudinous benefits, including resistance to electromagnetic signals, remote sensing capabilities, compact size and high sensitivity [12–14]. In the classification of fiber-optic sensors, the tilted fiber Bragg grating (TFBG), which with slantwise periodic refractive index modulation in the photosensitive core of the single-mode optical fiber, so that TFBGs are synchronously in possession of the advantages of fiber Bragg grating (to measure the temperature with core mode) and long-period fiber grating (to measure the surrounding refractive index with cladding modes) [15]. As the tilted grating breaks the cylindrical symmetry of the fiber, the core mode will be backscattered and coupled into the cladding to form hundreds of cladding modes, the excited cladding modes can be observed as a high-density comb of narrowband spectral resonances with a Q-factor of 104, which provides outstanding sensitivity response for the chemical and biological sensing [16].
For selective recognition of the diverse samples, a certain layer with specific material should be deposited on the surface of the optical fiber. When the surrounding refractive index (SRI) changes, the spectral resonances with adjacent effective refractive index change accordingly, and the specified analyte can be quantitatively detected by tracing the transmitted spectrum, the functionalized TFBG sensors based on this principle have been extensively demonstrated for biochemical applications [17,18].
Polyaniline (PANI) is a conjugated conducting polymer, which is being intensively investigated as an ammonia detector. For further improving the sensing performance, heterostructures of PANI and nanostructures are applied to ammonia sensing. The nanocomposite combines the characteristics of a large surface area and high porosity of nanomaterials, and has been experimentally proved and theoretically demonstrated that the nanocomposite has higher sensitivity and absorption capacity for ammonia molecules than pure PANI [6,19].
Graphene oxide (GO) has been intensively studied as a derivation of graphene, which not only inherits the properties of graphene, and after oxidation, the oxygen-containing functional groups on it also constitute and increase the outstanding properties such as hydrophilicity, dispersibility, and biocompatibility [20,21]. In addition, the carboxyl groups and hydroxyl groups of the GO strengthen the interaction with PANI, effectively quenching the localization or trapping of the charge carrier. The incorporation of GO and PANI with high surface-to-volume ratio and high carrier mobility, in the meantime, it reduces the signal-to-noise ratio because of the direct path for charge/ion transportation while providing excellent sensitivity and selectivity for ammonia molecules [22,23]. Those aforementioned properties make the nanocomposite of PANI/GO an ideal option fitting with the detection of ammonia in water. At present, ammonia sensors based on nanocomposites of nanomaterials and conductive polymers are mainly relying on electrochemical methods and fluorescent labelling methods [6,24,25], and the PANI/GO optical fiber sensor we built enables the ammonia sensor to control the sample to be measured at 1 mL, remotely controlled and immunity from electromagnetic disturbance.
In this article, we propose and experimentally demonstrate a PANI/GO nanocoating TFBG sensor for aqueous ammonia detection. The presence of PANI/GO nanocomposite on the sensor surface results in sensitivity improvement in terms of the enhancement in surface area and absorption capacity for ammonia molecules, and also makes the sensor with a larger sensing range as the deviation of fiber boundary conditions. Moreover, the sensor has a resolution of 2*10−4 RIU as well as a temperature-insensitive property by utilizing the envelope area interrogation. Our proposed sensor with the integrated merits, such as low cost, high sensitivity, fast response, and small size, make it a good choice for real-time, in-situ, label-free detection of aqueous ammonia.
2. Materials and methods
2.1 Materials
The sodium hydroxide (NaOH), hydrochloric acid (HCl), magnesium chloride (MgCl2), monopotassium phosphate (KH2PO4) and ammonia solution were purchased from BioWo Biochemical company. (3-Aminopropyl) triethoxysilane (APTES, 97%) was obtained from Kuer Bioengineering Co., Ltd., Anhui, China. The ammonium peroxydisulfate (APS) and aniline were purchased from Sigma Aldrich. Graphene Oxide was bought from XFNANO Material Technology Co., Ltd., Jiangsu, China. All experimental reagents are of analytical grade. In addition, the configuration of the liquid ammonia solutions with different concentrations was diluted with deionized water.
2.2 TFBG fabrication
We fabricated the TFBG using the phase mask inscription technique, which is mainly divided into two parts: improving photosensitivity and laser inscription [26,27]. Hydrogen loading is the common, simple and most widely used method for increasing the photosensitivity and reducing the inscription time. There are two ways to inscribe the tilted fiber Bragg gratings, one is to simultaneously adjust the fiber and the phase mask toward the laser beam with a certain angle, and the other is to only rotate the phase mask. The former method is commonly used today, which has the advantage of facilitating the rotation angle without decreasing the inscription depth of the fiber grating [28].
We loaded the single-mode fiber in a hydrogen chamber with a pressure of 1500 psi for one week, which can provide high-quality photosensitivity. After that, a 248-nm KrF excimer laser with a power of 64 mW was adopted to inscribe the tilted grating, the ultraviolet light cylindrically focused onto the fiber through the ±1 diffraction order of phase mask. And then, we scanned the UV beam spatially based on a programmable controlled displacement platform with a scanning speed of 0.01 mm/s, through the phase mask with a pitch of 1078.7 nm (from Ibsen Photonics) and along the treated fiber. The tilt of the grating was achieved by simultaneously rotating the phase mask and fiber with an angle in relative to the plane of fiber cross-section.
In this work, the 10 ° TFBG with a length of 10 mm we used could provide the spectrally dense comb of backward propagating cladding resonance modes for measuring surrounding reflective index (SRI) within the range 1.33 to 1.41 in mixed solutions of glycerin and deionized water. The resonance wavelengths of the cladding modes (${\lambda _{clad,i}}$) and core mode (${\lambda _{core}}$) can be respectively described by the following the grating phase-matching condition [29,30]:
2.3 Surface functionalization
For specific detection of ammonia in water, surface functionalization should be carried out based on the TFBG. To deposit the Polyaniline/Graphene Oxide composite, the silanization technique was employed on the surface treatment of the silica fiber sensor [31]. In this process, the fiber was firstly rinsed several times with ethanol and with deionized (DI) water to keep the fiber surface clean, and then we immersed the cleaned TFBG in 1 M sodium hydroxide solution for 2 hours to produce hydroxyl bonds (-OH) on the fiber surface as shown in Fig. 2. The sensor was rinsed with ethanol and with DI water again for removing the superfluous sodium hydroxide. Thereafter, the treated sensor was immersed in the mixed solution of 97% APTES and ethanol (1:20, v/v) for 30 minutes to generate the saline concoction. After that and rinsing with ethanol, the TFBG was placed in the PANI/ GO mixed solution with the thermal at 80 °C for 2 hours.
The synthesization of PANI/GO nanocomposite was fabricated according to the polymerization method [32,33]. 3.1 g/L of aniline and 0.5 g/L Graphene Oxide in 15 mL DI water were mixed with 5 mL of 1 M hydrochloric acid solution with magnetically stirring for 20 min. Then, after dropping 0.4 g/L APS in 10 mL at room temperature for 6 hours, the reaction precipitate was filtered, washed and sequentially dried at 60 °C for 12 hours. The micrograph is from the field emission scanning electron microscope (Tescan MIRA), as shown in Fig. 3, the nanocomposite layer is uniform employing the aforementioned deposition method. The right side of Fig. 3 is a SEM image magnified 1,500 times, which is enlarged 3 times in comparison with the left image. In addition to observing the flatness of the film using a field emission scanning electron microscope, the stability of the cladding resonances or envelope area of TFBG is monitored in DI water for one hour, to ensure the ideal completion of the functionalized film on the surface of the optical fiber. In this case, the intensity values of the resonances were non-monotonic, and the corresponding standard deviation of them was less than 0.01 dB.
3. Experimental system
3.1 Experimental setup
The experimental setup of the aqueous ammonia fiber sensor is shown in Fig. 4, involving a broadband source (BBS, output power of 7.3 mW) with a wavelength range from 1500 to 1600 nm, an optical spectrum analyzer (OSA, AQ6370D, YOKOGAWA) with a resolution of 0.05 nm for automatically monitoring the experimental response spectrum from the functionalized TFBG. The refractive indices of the solutions were measured with an Abbe refractometer, with an accuracy of 2*10−4.
Fig. 4. Block diagram of the experimental setup. BBS: broadband source; OSA: optical spectrum analyzer.
The surrounding refractive index signal was demodulated by global monitoring of the envelope of the cladding modes in the transmission spectrum, which provide a resolution of 2*10−4 RIU as well as a temperature-insensitive property [34]. The working principle of the PANI/GO coating sensor is presented in Fig. 5. Since the PANI/GO nanocomposite has a higher refractive index (RI) than the cladding of fiber which leaks the cladding modes into the film of PANI/GO, and also owing to the intrinsic optical absorption of the graphene oxide, the envelope area in the spectrum is reduced. However, the nanocoating makes the sensor more sensitive to the external environment in the low-refractive-index region (1.33-1.40 RIU) and widens the range of the refractive index sensing [35,36].
3.2 Sensor interrogation
The different interrogation principle between bare TFBG and TFBG with nanocomposite on the surface is shown in Fig. 6. In general, we trace the cut-off mode rather than the cladding modes in the measurement spectrum to monitor the surrounding refractive index, owing to the better sensitivity of cut-off mode. However, when the surface of the sensor is deposited with a material with a refractive index close to or higher than the fiber cladding, the cut-off mode will not be observed in the transmission spectrum, as shown in Fig. 6(b), the cladding modes become leaky modes because of the deviation of fiber boundary conditions [35,37]. Therefore, the refractive index sensing range is enlarged and available to detect the samples with RI lower than 1.3775. Cladding modes with effective refractive indices less than the SRI are transformed into leaky modes, and the cut-off mode is somewhere in between [38]. The cut-off mode has a certain wavelength in the transmission spectrum, and the effective refractive index is near the external refractive index, as shown in Fig. 6(a), the cut-off modes are labelled with the red symbol “*”.
Fig. 6. Working principle of the sensor. (a) Conventional SRI demodulation method of bare TFBG by tracing cut-off modes; (b) SRI Spectral response with PANI/GO coating. The inset is the zoom-in of some leaky modes.
As the refractive index increases, the extinction ratio of each cladding mode resonance is decreasing, that is, the envelope area will correspondingly decrease, as shown in the inset of Fig. 6(b). The envelope area method for TFBG proposed by C. Caucheteur et al. [34] shows a resolution of 2*10−4 RIU as well as a temperature-insensitive property, the method has been widely studied, including the sensing of liquid level, temperature and so on [39,40]. The value of the envelope area was obtained by calculating the integral area contained in the upper envelope and the lower envelope, which was based on the demodulation algorithm we coded. For the sensor used in this work, we monitored the integral area (with a unit of dB*nm) between 1510.5 nm and 1559 nm to demodulate the external refractive index of the nanocoating TFBG, as shown in Fig. 5. The main function of the demodulation algorithm is to polynomially fit the upper and lower envelopes, and automatically collect the value of the integrated area between the two envelopes. We collected the points of local maximum intensity or local minimum intensity among the above-mentioned range in wavelength, and then the envelope area between the cubic polynomial fitting of selected points was calculated, which can be clearly shown in Fig. 5. The core mode on the right side of the spectrum is sensitive to temperature but not to the SRI, so we can evaluate the stability of the TFBG experimental system by observing the core mode in aquatic experiments, to eliminate crosstalk of temperatures or power fluctuation on refractive index sensing, thus improving the measurement accuracy. And we could explicitly see that the core mode on the right of each spectrum is very stable, which ensures the stability of our sensing system. The core mode and resonances in the envelope area can be applied to self-calibrate the temperature before the calculation of spectral area. Similar to the case of cut-off mode demodulation, the stability of the core mode area is the window to monitor the temperature and power fluctuation. We should point out that the SRI can be also demodulated from the wavelength drift of cladding modes, as shown in the inset of Fig. 6(b), but the SRI sensitivity of this demodulation method is around 1 nm/RIU and with a resolution of 0.25 RIU, which is relatively inferior to the envelope area method (resolution of 10−4 RIU) and unqualified for the high-accuracy sensing [34,41].
4. Results and discussion
4.1 Detection of ammonia
According to the experimental setup presented in Fig. 4, we used the sensor to monitor the aqueous ammonia solutions with different levels, and the corresponding response and recovery process of three ammonia solutions in different concentrations (3, 5 and 7 ppm) were recorded by optical spectrum analyzer every 1 second, which can be observed in Fig. 7(a). We measured step-concentration aqueous ammonia solutions like the work of Noman et al. [42], which demonstrated that our sensor can directly distinguish samples of different concentrations, rather than rinsing the sensor in DI water in advance for each measurement of ammonia [9,10].
Fig. 7. (a) Response and recovery signal of the proposed sensor (in 3, 5 and 7 ppm of ammonia solutions). (b) Linear fit ranging from 0 to 12 ppm. The error bar is the standard deviation from the 5-time measurement.
After sinking the PANI/GO coated TFBG into the ammonia solution, the PANI was deprotonated and led it from an emeraldine salt state (ES) to the emeraldine base state (EB) [7], resulting in variations of dielectric constant. Therefore, coated TFBG acts as a refractometer, and the envelope area in its spectrum changes accordingly. We measured the correlated relationship between ammonia concentration and the signal of our sensor, and Fig. 7 separately demonstrates the response and recovery signal, as well as the linear fitting curve. TFBG sensor was firstly immersed in an aqueous ammonia solution at a concentration of 3 ppm, as shown in the yellow area of Fig. 7(a), followed by a blue and orange area in which the sensor is dipped in the aqueous ammonia solution of 5 ppm and 7 ppm, respectively. We monitored the spectral response process in those ammonia solutions by comparing the absolute values of the relative change with the initial envelope area (104 dB*nm) in 3 ppm ammonia solution, the response time and recovery time of the sensor are about 20 s, the response time and recovery time were calculated by recording the first 90% of the time along the binding or dissociation [9]. Figure 7(b) shows the linear fit of the measured results from six levels of concentration with a slope of 0.3772 dB*nm, which is numerically equal to the sensitivity of the ammonia sensing, and with a good R-squared of 0.9917. The differential envelope area is defined as the absolute value of the difference from the integral area while the nanocoating sensor is immersed in buffer solution, that is, the differential envelope area numerically equals zero when the functionalized TFBG sensor is dipping in DI water.
The limit of detection (LOD) of the proposed aqueous ammonia sensor is 0.08 ppm, utilizing the equation LOD =3σ/S, among them, S is the sensitivity of the ammonia sensing, and σ is the standard deviation of the system noise under the envelope area demodulation method with the unit of dB*nm. We repeatedly immersed the functionalized TFBG five times into the deionized water, which was the buffer of the sensing system for ammonia solutions. The corresponding envelope areas of the spectrum signal were real-time recorded, and the calculated standard deviation σ was 0.01 dB*nm. The repeatability of the nanocoating ammonia sensor was also investigated, after immersing with a mixed solution of alcohol and DI water (7:3, v/v) and then completely drying, the sensor was used to perform sample measurement again. The error bars of Fig. 7(b) and Fig. 8 are based on five times measurements in the same sensor.
4.2 Selective detection
The special identification of ammonia molecules in the ammonia optical fiber sensor is important because it allows for the accurate detection of ammonia in water. The sensor can detect the diverse concentrations of ammonia in water by measuring the SRI on the fiber surface, which is affected by the presence of ammonia. We tested the envelope area drift of calcium chloride (CaCl2), MgCl2, Ethanol, KH2PO4, and sodium sulfate (Na2SO4) in water, and the sensing performance is presented in Fig. 8. It is clear that the PANI/GO nanocoating sensor shows low sensitivity to those compounds in comparison with that to ammonia, which reveals that our functionalized sensor with good selectivity to ammonia molecules.
Table 1 summarizes the performances of our sensor compared with other methods. We can see that the fluorescence and LPFG methods have relatively outstanding LOD and detection range among those works that have been reported, however, their pretreatment is complicated. The sol-gel coated sensors should be immersed in water overnight before the ammonia measurement, to eliminate the penetrating from the water of ammonia solutions [10]. The fluorescent labelling method is inconvenient in terms of the continuous measurement of different concentrations of ammonia solutions, due to the fluorescence labelling before the tests and fluorescence quenching in the experiment [6]. According to Table 1, our sensor has the fastest response time, comparable low LOD and linear detection range, and is easy to manufacture, those merits make it a good option for ammonia detection.
Table 1. Summary of different sensors for measurement of ammonia
5. Conclusion
We have proposed and experimentally demonstrated a nanocomposite functionalized TFBG for aqueous ammonia detection in this work. Experimental results show that our sensor can detect aqueous ammonia in a concentration as low as 1 ppm, and the theoretical LOD can even reach 0.08 ppm. The concentration range of the detection is from 1-12 ppm, and the response time is only about 20 s. The proposed sensor possesses the advantages of simple design, ease of manufacture, low cost and good performance compared to other methods, so the sensor has a potential application in label-free, in-situ, real-time for measurement of aqueous ammonia in future.
Funding
Shenzhen Municipal Science and Technology Innovation Council (SGDX2020110309520303 K-ZGCQ).
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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