| Author: | Wang, Xujie |
| Title: | Ion imprinted polymer utilized fiber optic Mach-Zehnder interferometer based cadmium sensors |
| Advisors: | Yu, Changyuan (EEE) |
| Degree: | M.Sc. |
| Year: | 2025 |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | x, 70 pages : color illustrations |
| Language: | English |
| Abstract: | Fiber-optic Mach-Zehnder interferometers (MZIs) are widely recognized for their high sensitivity, strong anti-interference capability, compact design, and ease of fabrication. The merits make them highly attractive in various research and application fields as well. Meanwhile, cadmium ion (Cd²⁺) is a type of heavy metal ion which poses severe threats to the environment, human health, and ecosystems. Therefore, developing efficient and rapid detection methods for trace-level Cd²⁺ inspection is crucial for regulating its emission and implementing preventive measures. In this dissertation, MZI-based fiber optic sensors with two different coating approaches are demonstrated for cadmium ion monitoring. We have designed an inexpensive, compact and reliable single-mode fiber/ multimode fiber/ no-core fiber/ single-mode fiber (SMNS) interference structure to realize highly sensitive refractive index (RI) detection for later Cd²⁺ detection after achieving functional coating. The setup includes a short section of both multimode fiber (MMF) and no-core fiber (NCF) spliced in between two single-mode fibers (SMFs), the optimal lengths of 25mm NCF and 25 mm MMF are successively determined according to the interference spectrum, main cladding modes and optical field distribution of interferometers based on single-mode fiber/ multimode fiber/ no-core fiber/ single-mode fiber (SNS) and SMNS, respectively. The fabricated sensors offer an average linear RI sensitivity of over 190 nm/RIU in glycerol solutions with increasing RIs ranging from 1.3320 to 1.4235. Additionally, three chitosan (CS) based Cd²⁺ imprinted polymers (CS-Cd²⁺IP) were adopted to accomplish sensitive and selective Cd²⁺ monitoring of SMNS. The coating process involved using CS-Cd²⁺ compounds with thermally attached probes to self-generate CS-Cd²⁺IP with low concentrations of 0.2%, 0.5%, and 1% epichlorohydrin (ECH). The probes demonstrated Cd²⁺ selectively sensing ranges of 0 ppb (parts per billion) to 1 ppb (probe 1), 0.01 ppb to 10 ppb (probe 2), and above 1000 ppb (probe 3) with the limit of detection (LOD) of 0.04 ppb, 0.67 ppb, and 66.67 ppb, respectively. Further validation utilizing probe 2 displayed a linear sensitivity of 0.02773 nm/ppb, along with outstanding reusability and a fast transition time of 20 s for various Cd²⁺ concentrations in its application range. Besides, the insensitivity in the weakly acidic range and the temperature sensitivity of -0.0196 nm/°C were presented as well. Subsequently, we proposed another graphene oxide (GO)-cysteamine (MEA)/CS-Cd²⁺IP bi-layer functional scheme for enhanced Cd²⁺ sensitivity. Three similar SMNS sensors were created by successively dip-coating GO-MEA and CS-Cd²⁺IP to ensure consistent results. The one with the highest sensitivity was -0.1211 nm/ppb among all sensors regarding cadmium levels (0-10 ppb), and it responded in 90 seconds. The strong recognition ability for Cd²⁺ of this probe was further measured in comparison to other heavy metal ions. Moreover, its response across various pH solutions was examined to reduce sensing errors in real scenarios. Additionally, temperature crosstalk was mitigated by evaluating the response of a fiber Bragg grating (FBG). |
| Rights: | All rights reserved |
| Access: | restricted access |
Files in This Item:
| File | Description | Size | Format | |
|---|---|---|---|---|
| 8319.pdf | For All Users (off-campus access for PolyU Staff & Students only) | 3.98 MB | Adobe PDF | View/Open |
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