monitoring and industrial sectors. However, traditional detection methods often suffer from being intrusive,

expensive and having limited sensitivity. This study presents the development and validation of a portable,

non-invasive and cost-effective saline detection system designed to overcome these challenges. The core of

this system is a novel sensor created from a tapered plastic optical fiber (POF) coated with Zinc Oxide (ZnO)

nanorods, integrated with a NodeMCU ESP8266 for data transmission and real - time monitoring. To optimize

the interaction between light and the surrounding saline solution, POFs were tapered to various waist diameters

(500 µm, 550 µm, 600 µm, 650 µm, and 700 µm) using a combination of chemical and mechanical etching

techniques. Subsequently, ZnO nanorods were grown on the tapered fiber surface via the hydrothermal method

- a critical step for enhancing the sensor’s sensitivity. The sensor’s performance was evaluated using a saline

concentration ranging from 2 g to 10 g. A consistent and predictable inverse relationship was observed, where

sensing.

Saline detection plays a crucial role in various sensing fields; including healthcare, environmental monitoring

and industrial applications [1],[2]. Saline solutions, particularly sodium chloride (NaCl) in water, are

commonly used in medical treatments such as , intravenous (IV) fluids, and in industrial processes like water

treatment and chemical manufacturing [3]-[5]. The ability to accurately measure and monitor saline

concentrations is essential for maintaining proper medical protocols, ensuring the quality of industrial

products, and for safeguarding environmental health.

Traditionally, saline concentration is measured using methods such as conductivity measurement [6] and

them an ideal choice for a range of applications [10]. When coated with materials like zinc oxide (ZnO), which

has unique optical properties, these sensors can detect subtle changes in the refractive index of saline solutions.

Zinc oxide’s ability to interact with the surrounding environment enhances the sensitivity of the sensor,

allowing it to detect even minor variations in saline concentration [11].

Additionally, tapered POFs further improve sensor performance by increasing the interaction area between the

fiber core and the surrounding medium. This design optimizes the sensor's ability to detect small changes in

refractive index, which is essential for accurate saline detection.

The tapered region of the optical fiber is more sensitive to refractive index changes than the non-tapered

sections, as the light propagating through the fiber interacts more with the surrounding medium [12]. When

saline solution surrounds the tapered fiber, changes in the concentration of the saline will alter the refractive

In this paper, section 2 shows our literature review, section 3 displays our set-up, section 4 analyses our results,

followed by discussion and conclusions.

The demand for real-time and remote monitoring of water quality has spurred research and development of

various sensing technologies. Among these, optical fiber sensors have emerged as a promising solution; due to

their numerous advantages - including immunity to electromagnetic interference, high sensitivity, compact size

and capability for remote sensing. This literature review focuses on the key components and recent

advancements related to the development of a ZnO – coated tapered plastic optical fiber (POF) sensor for

salinity, integrated with a NodeMCU ESP8266 for data transmission and real-time monitoring.

This process involves heating and stretching a section of the fiber to create a narrower waist region. The

sensing applications - including the detection of chemical ions, gases and biomolecules.

The performance of an optical fiber can be significantly enhanced by coating the fiber surface with a sensing

material which interacts with the target analyte. Zinc oxide (ZnO) is a wide-bandgap semiconductor with

unique optical and electrical properties, making it an excellent choice of material for these applications. ZnO

nanostructures, such as nanoparticles, nanorods, and thin films, offer a high, which increase the active sensing

area and improve the sensor sensitivity and response time [15].

ZnO is biocompatible, chemically stable, and can be synthesized using various low-cost methods. The sensing

mechanism is typically based on the change in the refractive index or light absorption of the ZnO layer upon

interaction with the analyte. Recent reviews on ZnO nanostructures-based biosensors highlight their potential

for developing highly sensitive and selective sensing devices [16],[17]. Furthermore, the application of ZnO

environmental monitoring and industrial processes. Traditional methods for salinity measurement, such as

conductivity meters, can be bulky, expensive, and susceptible to electromagnetic interference. Optical fiber

sensors offer a viable alternative for real-time and in-situ salinity monitoring because of it small lightweight,

and resistant to corrosion [18]. Several optical fiber sensing techniques have been explored for salinity

The integration of a sensor with the NodeMCU ESP8266, enabling real-time data acquisition for easily

interface with various sensors to collect data on water quality parameters such as pH, turbidity, conductivity

and temperature. Several projects have demonstrated the use of NodeMCU ESP8266 for real-time water

quality monitoring, showcasing its potential for developing affordable and scalable monitoring systems, and

also emphasizing the importance of real-time data effective environmental management [21],[22].

In summary, this literature review reveals a clear and progressive trend from foundational research on

individual components-such as enhancing POF sensitivity through tapering and leveraging the unique

properties of ZnO nano-coatings towards the development of integrated, functional sensing systems. The

significance of this progression lies in its potential to create highly effective and accessible tools for real-time

environmental monitoring. This project capitalizes on the distinct advantages of each component: the low cost

The ease of controlling the tapered region renders the chemical and mechanical etching of POF very popular.

This method involves the employment of acetone, de-ionized (DI) water, as well as sandpaper to polish the

targeted area. Several samples with different waist diameters (500 – 700 µm) have been prepared for

characterization and optimization investigation. In this work, a standard multimode model SH4001 Super Eska

POF fiber (original diameter = 1000 µm) was used. Initially, a cutter blade was employed to cut the fiber

jacket from the center of the fiber with a 3 cm length. Finally, sandpaper of 7000 grit was used in the polishing

process to remove a portion of the core. Micrometers were frequently used to obtain the desired diameter of the

tapered fiber. The surface of the polished POF was then gently wiped with ethyl acetate solution to remove the

microparticles produced by sandpaper before the final cleaning process repeated several times with DI water.

is to enhance the performance of fiber optic sensors. The technique is extremely dependent on the uniformity,

length, diameter, density of the solution and POF core surface action and on the annealing of the nanorods.

Initially, two sets of solutions were prepared: ZnO nanoparticles solution and pH control solution. The first

solution was synthesised by dissolving 4.4 µg of zinc acetate dihydrate [Zn(O2CCH3)2.2H2O] with 20 ml of

ZnO properties via the hydrothermal process.

The growth of the nanorods improves when the pH of the ZnO nanoparticles solution increases to alkaline. It

has also been shown that the pH value could affect the nuclei and environment of ZnO growth. The pH control

solution was dropped on the nanoparticles after 10 minutes. This was achieved using a dropping and stirring

technique, where the ZnO nanoparticles solution was stirred slowly for every single 1 ml pH control solution

drop using a pipet for around 1 minute. Then the process was repeated 20 times until the pH increased from ~4

to ~9. This step was crucial to provide more hydroxyl ions (OH-) in the seeding solution. The mixture was

maintained in an ultrasonic water bath at 60 °C for 3 hours until a change in colour of the solution from clear

to milky became noticeable. Figure 2 (a) depicts the process of POF surface treatment in which the exposed

core was dipped in a mixture of 50ml Tween 80 and 500ml DI water. Next, the fibers used the dip and dry

zinc and oxygen

2. Amara, H., Achou, L., & Djellabi, R. (2024). Improved Temperature Sensitivity of Tapered Fiber

3. Godja, N.-C., & Munteanu, F.-D. (2024). Hybrid nanomaterials: a brief overview of versatile solutions

4. Haider, M. S. U. K., Chen, C., Ghaffar, A., Noor, L. U., Hussain, S., & Liu, M. (2025). High-

5. Hajrasuliha, S., Cassel, D., & Rezainejad, Y. (1991). Estimation of chloride ion concentration in saline

15. Manhas, S., Nautiyal, P., & Negi, S. (2024). A systematic review of IoT-based monitoring system for

16. Millero, F. J., Zhang, J.-Z., Lee, K., & Campbell, D. M. (1993). Titration alkalinity of seawater. Marine

17. Qian, Y., Zhao, Y., Wu, Q.-l., & Yang, Y. (2018). Review of salinity measurement technology based

20. Uniyal, A., Srivastava, G., Pal, A., Taya, S., & Muduli, A. (2023). Recent advances in optical

21. Waqas Alam, M., Sharma, A., Sharma, A., Kumar, S., Mohammad Junaid, P., & Awad, M. (2025).

22. Wei, X., Peng, Y., Chen, X., Zhang, S., & Zhao, Y. (2023). Optimization of tapered optical fiber sensor