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Abstract
We propose a distributed pH sensor based on an optical frequency domain reflectometry using a PEGDA-based pH-sensitive hydrogel coated on a single mode fiber. The volume of hydrogel increased as pH value of the surrounding fluid decreased, which converts the pH value to the axial strain in the fiber. Taking capacity of distributed strain measurement with high spatial resolution in optical frequency domain reflectometry, the pH value of the external medium is distributed measured by the wavelength shifts of the local Rayleigh backscattering spectra. The basic hydrogel with different molecular weight was optimized to balance the sensitivity, the response time and also the stability. In the experiment, the range of the pH value from 2 to 6 was measured with a sampling resolution of 1.7 mm, a sensitivity of -199 pm/pH and a response time of 14 min when the hydrogel coating diameter is 2 mm. Such a distributed pH sensing system has a potential to detect and locate some chemical or biological substances in a large-scale environment.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
pH value is a crucial parameter in several fields of sciences, such as environmental engineering, biomedicine, chemistry, and agriculture [1–3]. In recent years, substantial efforts were invested to improve the performance of pH sensor and reduce the cost. Optical fiber-based pH sensors have been proposed due to their numerous advantages, including small size, immunity to electromagnetic interferences, low-cost, and multiplexing capability. To date, several types of fiber optic-based pH sensors have been reported based on either intensity interrogation or wavelength interrogation. Intensity interrogation based on fluorescence [4–8] can be utilized to determine the pH of surrounding environment. For example, an intensity-based pH sensor was demonstrated using fluorescence material coated on polymer optical fiber and the sensing film showed different fluorescence intensities to different pH solutions in a pH range of 2.5–4.5 [9]. However, intensity-based pH sensors are vulnerable to the fluctuation of source power and other external factors. Compared with the former, wavelength-based pH sensors are more reliable owing to their high stability and high accuracy. For example, pH-sensitive hydrogel coated fiber Bragg grating has been proposed for pH sensing where the Bragg wavelength will be shifted due to the compression of hydrogel [10]. Furthermore, a fiber pH sensor based on a Fabry-Perot interferometer with hydrogel coated on a fiber end-face has been demonstrated [11]. Besides, another type of pH sensors was proposed by detecting the change in the refractive index of coating film [12–15]. However, the methods mentioned above are all point sensors. Few works were reported to realize the distributed pH sensing [16,17]. In Ref. [16], W. C. Michie reported a distributed pH sensor using optical time domain reflectometry. The pH-sensitive hydrogel fitted closely with a multimode fiber and swelled in solutions with different pH values. The swelling of the hydrogel bent the fiber and created a loss of light intensity. The pH value of the external environment can be demodulated by monitoring the intensity loss with a spatial resolution about 1 m.
In 1981, optical frequency domain reflectometry (OFDR) was first demonstrated for the measurement of the Rayleigh backscattered light amplitude in the single mode fiber (SMF) [18]. OFDR provided distributed sensing with a high spatial resolution by measuring the wavelength shift of the Rayleigh scattering spectrum. The wavelength shift can be obtained by implementing the cross-correlation of the reference and measurement signals in spectrum domain. For now, OFDR has been used for distributed strain sensing [19], temperature sensing [20], shape sensing [21], torsion sensing [22], refractive index sensing [23,24], and polarization analysis [25]. In this paper, we demonstrate a distributed pH sensor based on a hydrogel-coated SMF and the OFDR. The pH measurement principle relies on the transformation from hydrogel swelling to axial strain in the SMF. The sensor fabrication method was introduced by coating PEGDA based hydrogel film, followed by the measurement of residual stress during hydrogels curing. The molecular weight of the PEGDA and the coating diameter were optimized to balance the sensitivity, range, and the response time.
2. Materials and methods
2.1 Hydrogel composition
The hydrogels prepolymer mixture consists of acrylic acid, AA (>99%,180-200ppm inhibitor), Poly(ethylene glycol) diacrylate, PEGDA(contains 80-120ppm inhibitor), IRGACURE 2959(>98%), 2-Hydroxy-4-methoxybenzophenone-5-sulfonic acid hydrate, HEMA(98%). All chemical materials without any purification were purchased from Aladdin and Macklin. The gel was prepared by mixing of 4.85% wt.% AA, 94% wt.% PEGDA,1% wt.% IRGACURE 2959, 0.15% wt.% HEMA and was stored in an amber bottle under 4°C to protect against exposure to ultraviolet (UV) light.
2.2 pH solution preparation
A series of test solutions with a pH gradient of 1 in the range of 2.00–7.00 were prepared by citric acid monohydrate (99%-102%) and sodium hydrogen phosphate dodecahydrate (99%), all chemicals diluted with the proper amount of deionized water. The pH value of the solutions may vary slightly under different temperature and humidity condition, we used pH meter (PH818, pH range of 0.00–14.00, the resolution of 0.01 pH) for real-time comparison in the process of preparation.
2.3 Fabrication of pH sensors
The prepared hydrogel solution was coated on the surface of single mode fiber (SMF) as shown in Fig. 1. A SMF was passed through a pipe sleeve. Both fiber and sleeves were fixed on the precision displacement platform. By tuning the three-axis adjustment frame at both ends, the SMF was located in the middle of the hollow sleeve. After that, the hydrogel was injected into the pile sleeve. The hydrogel solution was cured in 2 min using a 365 nm UV lamp with a power density of about 40 mW/cm2. The hollow casing was removed after the hydrogel was completely cured on SMF. We have prepared a series of samples with different coating diameters. Figure 2 shows the microscope images of pH sensors with coating diameters of 510, 800, and 980 µm. The pictures clearly show that SMFs were located at the center of the hydrogel.
Fig. 2. Microscope images of fiber pH sensors with different coating thicknesses. (a) Uncoated; (b) D = 500 µm; (c) D = 800 µm; (d) D = 980 µm.
3. Experimental setup and results
3.1 Sensing principle
Hydrogels made by PEGDA contains a large number of carbonyl groups, and the hydrophilic property of the solidified hydrogel is significantly improved. The basic hydrogel is a good choice for pH sensing in the acidic solution [1]. The carbonyl groups will combine with H+ and ionize in acidic solution, and the volume of hydrogel will increase significantly. The swelling of the hydrogel is increased in solutions with high hydrogen ion concentration thus, the pH information of external environment is transformed from hydrogel swelling to the axial strain in the SMF. The bigger the molecular weight of PEGDA, the larger its internal three-dimensional network structure and that means the greater volume change of the hydrogel in pH sensing.
3.2 Experimental system based on OFDR
Figure 3 shows the structure diagram of OFDR. The linear sweeping light from the tunable laser (Santec, TSL-550) is separated into the main and auxiliary interferometers by a 1:99 coupler. The small fraction of the 1% light is then further split by a 50/50 coupler for constructing an auxiliary Mach-Zehnder interferometer. The delay fiber of this auxiliary interferometer is 10 m. This interferometer functions as an auxiliary interferometer for obtaining the instantaneous frequency of the sweep light source and then compensating the nonlinearity of the sweep process. The 99% light from C1 is future split by a 1:99 coupler to the main Mach-Zehnder interferometer with a local arm and a measurement arm. The measurement arm of the main interferometer is composed of a SMF coated with pH sensitive hydrogel. The measurement arm is used to collect the Rayleigh backscattering light in the SMF through a circulator. All data from both the auxiliary and main interferometers are detected by the photoelectric detectors (Thorlabs, PDB470C), and then acquired by a four-channel data acquisition card (ATS9440) at a sampling rate of 20 MS/s.
Fig. 3. Configuration of the distributed pH sensing based on the OFDR system. TLS: tunable laser source; C1, C2: 1:99 couplers; C3-C5: 50:50 couplers; PC: polarization controller; BPD: balanced photodetector; DAQ: data acquisition; FUT: fiber under test.
In our experiment, the tunable laser is swept from 1520 to 1550 nm at a rate of 100 nm/s for 0.2048 s acquisition time, which gives a determined wavelength range of 20.48 nm. The wavelength sweeping range gives a theoretical spatial resolution of 38.14 µm in the distance domain. 900 data samples are selected for the cross correlation, giving a gauge spatial resolution of 4.7 cm. In order to obtain sufficient sensing points and mitigate spectral leakage when implementing the short-time fast Fourier transformation, we selected the hamming window with 95% overlapping between adjacent windows. Hence, the sampling resolution is 1.7 mm. Our signal processing procedure of pH sensing in details is as following:
- a, The reference and measurement signals with different pH values are acquired separately and are converted to the distance domain by fast Fourier transform.
- b, A certain location signal is selected via a hamming window as the local Rayleigh backscattering.
- c, To increase the pH measurement accuracy, the local Rayleigh scattering signal in distance domain is zero padded.
- d, These selected local signals are converted to the Rayleigh scattering spectrum via the short-time fast Fourier transform.
- e, Cross-correlation is performed between the reference and measurement signals in spectrum domain to obtain the spectral shift. The location of the cross-correlation peak reflects the pH variation of external medium.
The pH measurement results obtained by the cross-correlation of the Rayleigh backscattering spectrum are shown in Fig. 4. Figure 4(a) illustrates the normalized Rayleigh backscattering spectrum before and after immersing the pH sensors in solutions with different pH values. Here, the measurement spectrum was collected in the solution with pH value 6 and the solution with a pH value of 7 is selected as the reference. After cross-correlation calculation of the measurement and reference Rayleigh backscattering spectrum, a clear local spectral shift of 102.6 pm can be found, as shown in Fig. 4(b).
Fig. 4. (a) Rayleigh backscattering spectrum between reference and measurement. (b) Wavelength shift of the local RBS with pH value of 6.
3.3. Experimental results
When the hydrogel is cured by UV light irradiation, the state of the gel will change gradually solidify from liquid to solid and resulting in residual stress. First, we real-time monitored the residual strain generated during hydrogel curing. Figure 5(a) shows the wavelength shift response in the process of hydrogel curing when the coating diameter is 1 mm. Under UV explosion, the gel will gradually shrink and cure, and hence continuously putting shrinkage axial stress on SMF. We can see that the wavelength shifts in curving area increase in the negative direction when the gel is under UV explosion. Both ends of the sensor were fixed by glue during the encapsulation process as shown in Fig. 2. When the gel gradually shrinks and solidifies, the fiber outside the curving area is subjected to tensile strain. Therefore, the wavelength shifts outside the curving area increase along UV exposure time. Figure 5(b) shows the wavelength shift response in the whole process of hydrogels curing. In the early 20 s, due to the temperature rise brought by the UV radiation, the Rayleigh spectrum shows a “red shift”. After 20 s, the gel started to solidify and then the wavelength gradually moved in the opposite direction. After UV exposure for 50 s, the wavelength shift remains the same when coating diameter is 1 mm and that means all the hydrogels were solidified on SMFs. The curing time is about 90 and 120 s for coating diameter of 1.5 and 2 mm, respectively. The final wavelength shifts are -401, -690, -1119 pm for coating diameter of 1, 1.5, and 2 mm. reactively. The comparison results indicate that the larger coating diameter, the bigger wavelength shifts, and hence the larger the residual strain after hydrogels totally solidified on SMFs
Fig. 5. (a) Wavelength response of residual stress during hydrogels curing when coating diameter is 1 mm. (b) Time response of sensors curing process with different coating diameter.
The pH sensing performance is greatly depended on molecular weight of PEGDA. The bigger the molecular weight of PEGDA, the larger its internal three-dimensional network structure and that means the greater volume change of the hydrogel in pH sensing. We investigated the wavelength response of the hydrogel with the different molecular weight of PEGDA. We fabricated pH sensors with 1mm coating diameter, and the molecular weight of PEGDA were 200 and 600. The pH sensors were immersed in solution with pH value of 7 and sensors in the air is selected as the reference. Figure 6(a) shows the wavelength shift of the pH sensors. The wavelength shifts increase as time because the volume change of hydrogels continuously applies axial strain to the fiber. After the hydrogel stops swelling in the solution, the biggest wavelength shift was 495 and 1946 pm when the molecular weight of PEGDA were 200 and 600 shown in Fig. 6(b). The PEGDA with molecular weight of 200 takes longer time to achieve the combination of carbonyl group and H+ in the acidic solution, and hence it takes longer time for wavelength drift to reach stability. Indeed, PEGDA with bigger molecular weight can increase the sensitivity of the pH sensors. However, the hydrogel will fall off the fiber due to the excessive volume change when the gel continues to expand, thus limits the sensing stability and the sensing range of pH. So, we chose PEGDA with molecular weight of 200 to realize pH sensing.
Fig. 6. (a) Measured wavelength shifts of the pH sensor when the molecular weight of PEGDA is 600. (b) Wavelength response of pH sensors with different PEGDA molecular.
The hydrogel-coated pH sensor was evaluated by immersing in solutions with pH values from 2−7. The solution with a pH value of 7 is selected as the reference when performing the OFDR measurement. Figure 7(a) shows the wavelength shift of the pH sensors by the cross-correlation calculation. The wavelength shifts increase when the pH decreases from 6 to 2 in the 6 cm hydrogel coating section. The solution with a smaller pH value has a higher hydrogen ion concentration, the hydrophilic property of the solidified hydrogel is significantly improved and the swelling of the hydrogel is increased in the solution with a smaller pH value, thus more strain is exerted on the pH sensors. The thickness of the hydrogel is not exactly the same along the SMF, so the wavelength shifts on the sensing section at the distance from 2.24 to 2.30 m were slightly different. The non-uniform of the coating thickness will lead to error for distributed pH sensing. Here, the non-uniform of the coating thickness mainly comes from the manual coating method. The uniformity of the coating thickness can be improved to ±10 µm by using the commercially available dip coating equipment [26]. Additionally, we investigated the response time of the pH sensors in solutions. The results in Fig. 7(b) show that the average response time is about 6.9 min.
Fig. 7. (a) Measured wavelength shifts of the pH sensor when coating diameter is 1 mm. (b) Response time (tr) with different pH values.
The sensitivity of the pH sensors with respect to different coating diameters was also investigated. The coating thicknesses of the investigated pH sensors were 1, 1.5, and 2 mm. The pH sensitivity and response time of these three sensors were measured in solutions with pH values from 2 to 6, and all the sensors exhibited a linear response to pH values. As expected, the sensor with the thicker coating diameter has the higher sensitivity but the slower response time, as shown in Fig. 8(a) and Fig. 8(b), respectively. The sensitives of the sensors with coating diameter of 1, 1.5, and 2 mm are 103, 150, and 199 pm/pH, the corresponding response time are 6.9, 9.5, and 14.4 min, respectively. Indeed, the thicker coating hydrogel provides more oxygen atoms which allows more hydrogen ions to be combined with them, and gives a higher sensitivity. On the other hand, the response time is inversely proportional to the coating thickness as it is related to diffusion process.
In order to verify the capability of the distributed pH sensing, we performed another experiment by dividing the sensor into three parts. The middle part keeps in the air, and the other two parts were immersed into the solution with pH from 3 to 6. Figure 9 shows the experimental result of the distributed pH sensing. The wavelength shift values of two side parts are consistent with each other. Such result successfully demonstrates the ability of the distributed pH sensing of the proposed method, which cannot be achieved by traditional single-point pH sensing methods.
4. Conclusion
In this work, we presented a distributed pH sensor using OFDR and hydrogel-coated SMF. The pH sensor was fabricated by coating hydrogel on SMF. It was demonstrated that hydrogels produce different degrees of swelling under different pH value solutions and exert lateral stress on SMF. The pH value of the external medium was measured by the wavelength shifts of the Rayleigh back-reflection spectra using OFDR. In our experiment, we realized a distributed pH sensing with a sampling resolution of 1.7 mm in a 6 cm coating section. In order to balance the sensitivity, sensing range, and response time, we optimized the molecular weight of hydrogel materials and the sensor diameter, and finally we demonstrated a high sensitivity of -199 pm/pH in pH range from 2 to 6. The response time was shortened to 6.9 min. The proposed distributed pH sensor has a potential to detect and locate some chemical or biological substances in a large-scale environment.
Funding
National Natural Science Foundation of China (61975022); Chongqing Talents: Exceptional Young Talents Project (CQYC202005011); National Science Fund for Distinguished Young Scholars (61825501).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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.
References
1. R. Andreas, P. Georgi, K. Stephan, L. Jens, and H.-J. P. Adler, “Review on Hydrogel-based pH Sensors and Microsensors,” Sensors 8(1), 561–581 (2008). [CrossRef]
2. M. I. Khan, K. Mukherjee, R. Shoukat, and H. Dong, “A review on pH sensitive materials for sensors and detection methods,” Microsyst. Technol. 23(10), 4391–4404 (2017). [CrossRef]
3. M. Islam, M. Ali, M. H. Lai, and K. S. Lim, “Chronology of Fabry-Perot Interferometer Fiber-Optic Sensors and Their Applications: A Review,” Sensors 14(4), 7451–7488 (2014). [CrossRef]
4. Q. Cui, O. Podrazky, J. Mrazek, J. Probostova, and I. Kasik, “Tapered-Fiber Optical Sensor for Physiological pH Range,” IEEE Sens. J. 15(9), 4967–4973 (2015). [CrossRef]
5. T. Liu, W. Wang, H. Ding, and D. Yi, “Smartphone-Based Hand-Held Optical Fiber Fluorescence Sensor for On-Site pH Detection,” IEEE Sens. J. 19(20), 9441–9446 (2019). [CrossRef]
6. Z. J. Tang, D. Gomez, C. He, S. Korposh, S. P. Morgan, R. Correia, B. H. Gill, K. Setchfield, and L. L. Liu, “A U-shape fibre-optic pH sensor based on hydrogen bonding of ethyl cellulose with a sol-gel matrix,” J. Lightwave Technol. 39(5), 1557–1564 (2021). [CrossRef]
7. V. Moradi, M. Akbari, and P. Wild, “A fluorescence-based pH sensor with microfluidic mixing and fiber optic detection for wide range pH measurements,” Sens. Actuators, A 297, 111507 (2019). [CrossRef]
8. L. Zhao, G. Q. Li, J. L. Gan, and Z. G. Yang, “Hydrogel Optical Fiber Based Ratiometric Fluorescence Sensor for Highly Sensitive pH Detection,” J. Lightwave Technol. 39(20), 6653–6659 (2021). [CrossRef]
9. X. H. Yang and L. L. Wang, “Fluorescence pH probe based on microstructured polymer optical fiber,” Opt. Express 15(25), 16478–16483 (2007). [CrossRef]
10. X. Cheng, J. Bonefacino, B. O. Guan, and H. Tam, “All-polymer fiber-optic pH sensor,” Opt. Express 26(11), 14610–14616 (2018). [CrossRef]
11. A. A. Noman, J. N. Dash, X. Cheng, C. Y. Leong, H. Tam, and C. Y. Yun, “Hydrogel based Fabry-Pérot cavity for a pH sensor,” Opt. Express 28(26), 39640–39647 (2020). [CrossRef]
12. Z. Ma, Q. Zhang, and C. C. Chan, “A taper-in-taper fiber optic biosensor based on Mach-Zehnder interferometer for human sweat pH detection,” Proc. SPIE 11780, 6–9 (2020). [CrossRef]
13. A. L. Aldaba, A. G. Vila, M. Devliquy, M. L. Amo, C. Caucheteur, and D. Lahem, “Polyaniline-coated tilted fiber Bragg gratings for pH sensing,” Sens. Actuators B: Chem. 254, 1087–1093 (2018). [CrossRef]
14. R. Yan, G. F. Sang, B. Yin, S. H. Wu, M. G. Wang, B. Huo, M. Gao, R. Chen, and H. Yu, “Temperature self-calibrated pH sensor based on GO/PVA-coated MZI cascading FBG,” Opt. Express 29(9), 13530–13541 (2021). [CrossRef]
15. B. B. Gu, M. J. Yin, P. Zhang, J. W. Qiang, and S. L. He, “Low-cost high-performance fiber-optic pH sensor based on thin-core fiber modal interferometer,” Opt. Express 17(25), 22296–22302 (2009). [CrossRef]
16. Fei Lu, Ping Lu, Mudabbir Badar, Ruishu F. Wright, and Michael Buric, “Distributed fiber optic chemical sensor with a temperature compensation mechanism,” Proc. SPIE 11500, 21–27 (2020). [CrossRef]
17. W. C. Michie, B. Culshaw, I. Mckenzie, and M. Konstanakis, “Distributed sensor for water and pH measurements using fiber optics and swellable polymeric systems,” Opt. Lett. 20(1), 103–105 (1995). [CrossRef]
18. W. Eickhoff and R. Ulrich, “Optical Frequency-Domain Reflectometry in Single-Mode Fiber,” Appl. Phys. Lett. 39(9), 693–695 (1981). [CrossRef]
19. G. L. Yin, R. Jiang, and T. Zhu, “In-fiber Auxiliary Interferometer to Compensate Laser Nonlinear Tuning in simplified OFDR,” J. Lightwave Technol. 40(3), 837–843 (2022). [CrossRef]
20. H. J. Zhong, C. L. Fu, P. F. Li, B. Du, C. Du, Y. J. Meng, and Y. P. Wang, “Distributed high-temperature sensing based on optical frequency domain reflectometry with a standard single-mode fiber,” Opt. Lett. 47(4), 882–885 (2022). [CrossRef]
21. C. Shao, G. Yin, L. Lv, M. Liu, I. P. Ikechukwu, H. N. Han, L. Zhou, J. D. Zhang, W. J. Zhai, S. Wang, and T. Zhu, “OFDR with local spectrum matching method for optical fiber shape sensing,” Appl. Phys. Express 12(8), 082010 (2019). [CrossRef]
22. G. L. Yin, L. Lu, L. Zhou, C. Shao, Q. J. Fu, J. D. Zhang, and T. Zhu, “Distributed directional torsion sensing based on an optical frequency domain reflectometer and a helical multicore fiber,” Opt. Express 28(11), 16140–16150 (2020). [CrossRef]
23. Z. Y. Ding, K. Sun, K. Liu, J. F. Jiang, D. Yang, Z. Yu, J. Li, and T. G. Liu, “Distributed refractive index sensing based on tapered fibers in optical frequency domain reflectometry,” Opt. Express 26(10), 13042–13054 (2018). [CrossRef]
24. Z. D. Zhu, D. X. Ba, L. Liu, L. Q. Qiu, and Y. K. Dong, “Temperature-compensated distributed refractive index sensor based on an etched multi-core fiber in optical frequency domain reflectometry,” Opt. Lett. 46(17), 4308–4311 (2021). [CrossRef]
25. T. Feng, Y. Shang, X. Wang, S. Wu, A. Khomenko, X. Chen, and X. Steve Yao, “Distributed polarization analysis with binary polarization rotators for the accurate measurement of distance-resolved birefringence along a single-mode fiber,” Opt. Express 26(20), 25989–26002 (2018). [CrossRef]
26. P. V. N. Kishore, M. Sai Shankar, and M. Satyanarayana, “Detection of trace amounts of chromium (VI) using hydrogel coated Fiber Bragg grating,” Sens. Actuators, B 243, 626–633 (2017). [CrossRef]
