Principle of pH Measurement Using Fiber Optic Sensors with Polyaniline
Polyaniline-coated fiber optic sensors measure pH by detecting changes in the polymer's refractive index or optical absorption in response to protonation/deprotonation, which modulates the transmitted or reflected light signal.
Sensing Mechanism
Polyaniline (PANI) is a pH-sensitive conducting polymer whose optical properties change with the surrounding hydrogen ion concentration. When PANI is coated onto an optical fiber, either as a thin film or via surface polymerization, the protonation state of the polymer changes with pH, altering its refractive index and absorption spectrum. These changes affect the light propagating through the fiber, either by modulating the evanescent field in unclad or side-polished fibers or by shifting the interferometric fringes in microfiber sensors .
Optical Transduction
- Surface Plasmon Resonance (SPR) Approach: A thin metal layer (e.g., platinum) is deposited on the fiber, followed by a PANI coating. Changes in PANI's refractive index with pH modify the SPR conditions, leading to measurable shifts in resonance wavelength or intensity .
- Interferometric Approach: In microfiber or trenched core-free fibers, the PANI layer interacts with the evanescent field. pH-induced refractive index changes cause wavelength shifts in the interference pattern, which can be precisely measured .
- Transmission/Absorption Monitoring: The optical power transmitted through the fiber changes with PANI's absorption spectrum, providing a direct correlation between light intensity and pH .
Sensor Performance
- Operational Range: PANI-coated fiber sensors can operate over a wide pH range, typically from 1 to 14 for SPR-based sensors and 4.2 to 8.1 for trenched core-free fibers .
- Sensitivity: Sensitivity varies with fiber design and polymer thickness. For example, interferometric microfiber sensors show sensitivities of approximately −0.54 nm/pH in acidic solutions and 0.28 nm/pH in alkaline solutions .
- Response Time and Stability: These sensors exhibit fast response times, high repeatability, and long-term stability, making them suitable for real-time and remote monitoring applications .
Applications
PANI-functionalized fiber optic pH sensors are used in:
- Biomedical monitoring: Measuring pH in biological fluids.
- Environmental monitoring: Detecting pH changes in water or soil.
- Industrial processes: Real-time pH control in chemical or food processing . In summary, the principle relies on pH-induced optical property changes in polyaniline, which are transduced into measurable optical signals via fiber optic configurations, enabling sensitive, fast, and versatile pH detection.
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