Development of Optical Fiber Sensor for Water Salinity Detection
Corresponding email: punitha@unikl.edu.my
Published at : 31 Oct 2023
Volume : IJtech
Vol 14, No 6 (2023)
DOI : https://doi.org/10.14716/ijtech.v14i6.6650
Nor, M.S.M., Khan, A.A., Mohamad, S., Thirunavakkarasu, P., 2023. Development of Optical Fiber Sensor for Water Salinity Detection. International Journal of Technology. Volume 14(6), pp. 1247-1255
| Muhammad Safwan Mohammed Nor | Communication Technology Section, British Malaysia Institute, Universiti Kuala Lumpur, 53100 Gombak, Malaysia |
| Abdul Ali Khan | Communication Technology Section, British Malaysia Institute, Universiti Kuala Lumpur, 53100 Gombak, Malaysia |
| Suraya Mohamad | Communication Technology Section, British Malaysia Institute, Universiti Kuala Lumpur, 53100 Gombak, Malaysia |
| Punithavathi Thirunavakkarasu | Communication Technology Section, British Malaysia Institute, Universiti Kuala Lumpur, 53100 Gombak, Malaysia |
This project addresses the lack of a real-time, low-cost sensor to detect salt levels in water. The authors aim to develop an optical fiber sensor for water salinity detection. The sensor employs the principles of absorption spectroscopy using a broadband light source and spectrometer to detect changes in the optical spectrum of the sensor in the presence of varying concentrations of sodium chloride ions. A D-shaped sensor is fabricated by modifying the circular structure of a plastic optical fiber. Functionalized carbon nanotubes are drop casted over the D-shaped sensing region. Both uncoated and CNT-coated POF sensors are exposed to different concentrations of sodium chloride in water, and the spectral response is recorded. The results show that the sensors exhibited a strong correlation in their intensity response towards varying concentrations of sodium chloride salt ranging from 0 to 25%. The uncoated sensor had a sensitivity of 31A.U./% salt, and the CNT-coated Sensor had a sensitivity of 114 A.U/% salt. The functionalized CNT layer increased the sensitivity of the POF sensor by approximately 4 times. The outcome of this research provides a cost-effective and reliable method for water salinity detection in industrial and environmental applications.
Carbon Nanotubes (CNT); D-shape optical fiber; Optical fiber sensor; Sodium chloride ions, Sensitivity; Water salinity detection
Water salinity plays a crucial role in numerous fields, including
environmental monitoring, industrial processes, and agriculture. Precise and
continuous measurement of water salinity is essential for effective water
resource management and ensuring optimal conditions for various applications.
However, conventional sensors are affected by electromagnetic interference,
radio frequency interference, and real-time monitoring may also poses potential
risks, such as short circuits when used in an aqueous environment (Arrieta, Barrera and Mendoza, 2023; Abuzairi
et al. 2022; Hardi and Rahman 2020).
To address these constraints, the development of an optical fiber sensor (OFS) for water salinity sensing has emerged as a promising solution. This sensor employs absorption spectroscopy principles and integrates a D-shaped optical fiber sensing zone to detect changes in optical parameters when exposed to dissolved salts. Moreover, the coating of the OFS with carbon nanotubes (CNT) has shown remarkable potential in enhancing sensitivity and improving detection capabilities.
Optical
fiber sensors utilize the interaction between light and the analyte in the
surrounding environment to sense alterations in the surrounding refractive
index (SRI). The versatility of optical fiber sensors enables various sensing
mechanisms such as (absorption-based, reflection-based, evanescent wave, and
surface plasmon resonance (SPR)) sensors (Liu and
Peng, 2021).
Absorption-based sensors rely on the principle that the absorption spectrum of
water changes with salinity. The salinity level can be determined by measuring
the intensity of light transmitted through an optical fiber immersed in the
water sample.
Refractive
index [RI] sensors, on the other hand, exploit the refractive index variations
caused by changes in water salinity (Tan et al.,
2014). Thus making it viable by
measuring the intensity or phase of light reflected or transmitted through the
fiber. Evanescent wave sensors make use of the electromagnetic field extending
beyond the fiber core. When the fiber is exposed to a water sample, the RI of
the evanescent field changes due to salinity variations, leading to alterations
in the transmitted or reflected light. SPR sensors utilize the excitation of
surface plasmons on a thin metal film deposited on the fiber surface.
The interaction between plasmons and the salinity of a water
sample leads to changes in RI, which enables the measurement of water salinity
levels. When designing optical fiber sensors for this purpose, researchers must
consider several key factors. Among these, the choice of fiber is of utmost
importance. In this study, plastic optical fibers were selected due to their
robustness, large core size, and flexibility. Additionally, the selection of
materials for the sensor probe, including coatings or sensing layers, plays a
vital role in enhancing sensitivity and stability.
Sensor geometry and configuration have a significant impact on the
sensor's performance. Various modifications, such as tapered fibers,
long-period gratings, or microstructured fibers, can be used to increase the
interaction between the fiber and the surrounding medium, leading to increased
sensitivity
Great
progress has been made in the field of optical fiber sensors for water salinity
measurement (Flores, Janeiro, and Viegas, 2019).
Researchers have concentrated on the development of novel sensing
configurations, enhancing sensitivity and selectivity, and incorporating
optical fiber sensors with other technologies for multi-parameter measurements,
such as using nanomaterial coatings on the fiber surface to increase
sensitivity and enable selective detection of specific ions in saline water.
However, fiber sensors have several limitations, such as low mechanical
strength, especially in glass fibers, complex modification processes, and low
selectivity without a sensitive film.
Researchers
have also explored the combination of optical fiber sensors with microfluidics,
allowing real-time monitoring of water salinity. This integration offers
benefits such as smaller sample volume requirements, rapid response times, and
the capability to analyze multiple samples simultaneously. CNTs are now popular
as an efficient material for chemical sensing detection due to their
distinctive structural and optical properties, such as strong tensile
characteristics, extremely lightweight, and chemical and thermal stability (Khalaf et al.,2017; Pokhrel et al., 2017). These special characteristics have gained
substantial interest in using CNTs in research areas involving emerging
nanomaterials and their applications. Optical spectroscopy techniques measure
the changes in the RI of the CNTs caused by salt adsorption or desorption
processes. Furthermore, functionalization is
a chemical process that introduces specific functional groups onto the
sidewalls of CNTs. These functional groups can be created using specific
molecules or coatings, and they can enhance the selectivity of CNT-based
sensors towards particular salts, thereby improving the sensor's performance (Norizan et al., 2020).
Overall,
carbon nanotubes provide a promising platform for salt detection, offering high
sensitivity, rapid response times, and the potential for integration into
miniaturized and portable sensing devices. Further research is being conducted
to optimize the sensing performance, selectivity, and stability of CNT-based
salt sensors for a broad range of applications, including environmental
monitoring, agriculture, and healthcare. In this research, we propose a novel
salinity sensor using a D-shaped polymer optical fiber with a carbon nanotube sensing
layer. By optimizing the sensor's sensitivity, accuracy, and real-time
monitoring capabilities, it holds the potential to offer a more efficient and
cost-effective alternative to traditional methods such as microstrip moisture
sensor
In this research, MWCNTs 95%, sodium chloride 99%, nitric acid 65%, and sulfuric acid 95% were purchased from Sigma-Aldrich, and a 1000-micron diameter plastic optical fiber (POF) core and cladding made of (poly methyl methacrylate and fluorine resin) purchased from Mouser Electronics (Malaysia) was used to design a D shaped sensor. The POF was then placed in a v groove to secure it, and a fine file was used to polish the fiber down to the required size, as shown in Figure 1 (a). The POF sensing zone was fabricated to be 1 cm in length. After polishing, the cladding of the POF over the sensing region was totally removed. The side view and cross-section diagrams of the D-shaped POF are depicted in Figure 1(b and c) separately, and the resultant D-shaped sensor is depicted in Figure 1(d).
Figure 1 POF in V
groove for side polishing process a) Aerial view; b) Side view; c) Cross section view d) D-shaped sensor
To functionalize CNTs, acid treatment was employed using commonly
used acids such as nitric acid 
sulfuric acid 
or a mixture of both 
Figure 2 shows the sensing setup for this research. The sensing region of the POF sensor was positioned in a liquid chamber. The POF was connected to a broadband light source at one end and a spectrometer at the other end.
The AvaLight-HAL-S-MINI Tungsten-Halogen Light
Source has a spectral range of (400-2000 nm) and the spectrometer (AvaSpec-ULS2048CL)
has a range of (400-1100) nm. The
resulting data was analyzed using dedicated software on a PC. The sensitivity
of the sensor was evaluated by measuring the response to salt solutions of
varying concentrations ranging from (0-25)%. The measurements were taken after
20 sec for each concentration repeated for both the uncoated fiber and the
CNT-coated fiber. The collected data was further analyzed, compared, and
calculated to determine the sensitivity of the sensor.
The described material and method provide a systematic approach for the fabrication, functionalization, characterization, sensing, optimization, and analysis of the optical fiber sensor, with functionalized carbon nanotubes (CNTs) playing an important part in enhancing the sensor's performance for water salinity detection.
Figure 3 SEM image of CNT on POF with 90x magnification
Figure 4 SEM image of CNT on POF with 30000x magnification
values.
The presence of the 002 peaks suggests a lower degree of alignment among the
nanotubes, with a crystal spacing or d-spacing of 0.34 
, and 100 peak signifies highly aligned nanotubes with a
crystal spacing, or d-spacing of 2.12 . These
results indicate the presence of aligned and unaligned CNTs in the sample.Table 1 EDX analysis of CNT
|
Element |
Weight% |
Atomic% |
|
C K |
97.04 |
97.9 |
|
O K |
2.45 |
1.90 |
|
S K |
0.51 |
0.20 |
|
Totals |
100 |
100 |
Figure 5 XRD analysis of CNT
Figures 6 and 7 present the intensity results of the uncoated and CNT-coated POF sensor towards different concentrations of salt, respectively. The observed trend reveals that as the salt concentration increases, the intensity spectrum exhibits a corresponding increase. This phenomenon can be attributed to the variations in the RI of the surrounding medium as the salt concentration is increased. When the CNT-coated POF sensor is deployed, it can be clearly observed that the changes in intensity are more noticeable for each concentration of salt. Due to the high surface-to-volume ratio of nanotubes, it allows more salt molecules to interact. As the salt concentration increases, the salt molecules bind with the CNTs, causing greater changes to the CNT’s effective RI.
Figure 6 Spectral response of uncoated POF sensor towards salt solutions
Figure 7 Spectral response of CNT-coated POF sensor towards salt solutions
Figure 8 compares the performance of the uncoated and CNT-coated POF sensor. The results clearly demonstrate the superior performance of the
CNT-coated fiber in terms of intensity change when exposed to different salt
concentrations.
The
results indicate that the CNT-coated fiber demonstrates a significantly larger
intensity change compared to the uncoated fiber. This enhanced response is
attributed to the functionalization of CNTs, which leads to an improved
selectivity towards specific salts. The chemical composition of the
functionalized CNTs enables a binding interaction between the salt ions and the
CNTs, resulting in a greater difference in the effective RI between the sensor
surface and the surrounding medium.
The sensitivity of the uncoated and CNT-coated sensors is quantified using equation 4 (Arasu et al. 2016) and summarized in Table 2. It is noteworthy that the CNT-coated sensor exhibits approximately four times higher sensitivity compared to the uncoated sensor. This increased sensitivity is attributed to the unique properties of the functionalized CNTs, which enhance the sensing capability of the fiber. The functionalized CNTs facilitate a more precise and selective detection of salt concentrations by inducing significant variations to the effective RI of the sensor surface.
Figure 8 Comparison
of the intensity shift of the uncoated and CNT coated sensor
Table 2 Sensitivity Comparison
|
Type of
fiber |
Sensitivity
(AU/%salt) |
|
Uncoated
fiber |
31 AU/%salt |
|
Coated CNT
fiber |
114 AU/%salt |
Overall, the results
demonstrate the effectiveness of the CNT-coated fiber sensor in detecting and
quantifying salt concentrations. The uncoated sensor had a sensitivity of
31A.U./% salt, and the CNT-coated Sensor had a sensitivity of 114 A.U/% salt.
The functionalization of the CNTs enhances the sensor's (selectivity and
sensitivity) making it a favorable choice for a range of potential applications
requiring accurate and reliable water salinity detection.
In
conclusion, the development of a D-shaped fiber sensor for water salinity
detection has proven to be successful. The use of CNT coating through the
drop-casting method has significantly enhanced the sensitivity of the sensor,
achieving up to 4 times the sensitivity of the uncoated sensor. The CNT-coated
fiber sensor’s superior sensing capabilities have been established, providing a
better sensing mechanism for water salinity detection. This research opens up
possibilities for further exploration and improvement in the area of optical
fiber sensing. Future work in this area may be focused towards the sensing
layer. More research can be done towards determining the optimum thickness of
the sensing layer that would produce the highest sensitivity. Varying the
sensing layer with other sensitive materials such as graphene, Zinc Oxode,
conductive ploymers as well as nanocomposite layers could be another direction
for further work in this area. This would broaden the variety of materials for
sensor coating and potentially uncovering novel sensing mechanisms. The successful development of the CNT-coated fiber sensor and the
demonstrated improvements in sensitivity hold great promise for various
applications requiring water salinity detection. Industries such as
environmental monitoring, industrial processes, and agriculture can benefit
from the cost-effective, reliable, and real-time monitoring capabilities
offered by this optical fiber sensor. The simplicity and robustness of this
sensor make it easy to deploy in various environmental conditions. The findings of this research contribute to
the advancement of sensor technology and provide a foundation for future
studies in the field of water salinity detection and related applications.
This work was
supported by the Malaysian Ministry of Higher Education (MOHE) through the
Fundamental Research Grant Scheme (FRGS/1/2020/TK0/UNIKL/02/15).
Abuzairi, T., Sumantri,
N.I., Putri, N.A., Andarini, M.V., Lampung, E.J., Sitinjak, D., 2022.
Development of the Sterilization Box for Medical Equipment with an Ozone Gas
Leak Sensor Feature. International Journal of Technology, Volume 13, pp. 1672–1680
Arasu, P.T., Noor, A.S.M.,
Shabaneh, A.A., Yaacob, M.H., Lim, H.N., Mahdi, M.A., 2016. Fiber Bragg grating
assisted surface plasmon resonance sensor with graphene oxide sensing layer. Optics
Communications, Volume 380, pp. 260–266
Arrieta, A., Barrera, I.,
Mendoza, J., 2023. Application of a Polypyrrole Sensor Array Integrated into a
Smart Electronic Tongue for the Discrimination of Milk Adulterated with Sucrose.
International Journal of Technology, Volume 14, pp. 90–99
Deng, X., Shen, Y., Liu, B., Song, Z., He, X.,
Zhang, Q., Ling, D., Liu, D., Wei, D., 2022. Terahertz Metamaterial Sensor for
Sensitive Detection of Citrate Salt Solutions. Biosensors, Volume 12, p.
408
Flores, R., Janeiro, R.,
Viegas, J., 2019. Optical Fibre Fabry-Pérot Interferometer Based on Inline
Microcavities For Salinity and Temperature Sensing. Scientific Reports, Volume 9(1), p. 9556
Hardi, G.W., Rahman, S.F.,
2020. Amperometric Detection of Dopamine based on a Graphene Oxide/PEDOT:PSS
Composite Electrode. International Journal of Technology, Volume 11, pp. 974–983
Jain, S., 2022. Early Detection of Salt and Sugar
by Microstrip Moisture Sensor Based on Direct Transmission Method. Wireless
Pers Communications, Volume 122, pp. 593–601
Khan, A.A.,
Thirunavakkarasu, P.M., Saidin, N.B., Khalaf, A.L., Roslan, F.S., Haider, M.L.,
2022. Characterization of Plain and Zno Coated D-Shaped Plastic Optical Fiber
Sensor Of Different Diameters. In: 8th International Conference on
Engineering and Emerging Technologies, ICEET 2022
Khalaf, A.L., Arasu, P.T., Lim, H.N., Paiman, S., Yusof, N.A., Mahdi, M.A., Yaacob, M.H., 2017. Modified Plastic
Optical Fiber with CNT and Graphene Oxide Nanostructured Coatings for Ethanol Liquid Sensing. Optics
Express, Volume 25(5), pp. 5509-5520
Liu, Y., Peng, W., 2021. Fiber-Optic
Surface Plasmon Resonance Sensors and Biochemical Applications: A Review. Journal of
Lightwave Technology, Volume 39(12), pp. 3781–3791
Norizan, M.N., Moklis,
M.H., Demon, S.Z.N., Halim, N.A., Samsuri, A., Mohamad, I.S., Abdullah, N., 2020. Carbon Nanotubes:
Functionalisation and Their Application in Chemical Sensors. RSC Advances, Volume 10(71), pp. 43704–43732
Pokhrel, L.R., Ettore, N.,
Jacobs, Z.L., Zarr, A., Weir, M.H., Scheuerman, P.R., Kanel, S.R., Dubey, B., 2017. Novel Carbon
Nanotube (CNT)-Based Ultrasensitive Sensors for Trace Mercury (II) Detection in Water: A Review.
Science of the Total Environment, Volume 574, pp. 1379–1388
Tan, Y.C., Ji, W.B., Mamidala, V., Chow, K.K., Tjin, S.C., 2014. Carbon-Nanotube-Deposited Long Period Fiber Grating for Continuous Refractive Index Sensor Applications. Sensors and Actuators B: Chemical, Volume 196, pp. 260–264