Single-Mode Optical Fiber Coated with Carbon-quantum Dots for Chemical Sensing Applications

This study presents the design and fabrication of a single-mode optical fiber-based sensor capable of identifying different concentration levels of acetic acid solutions. Carbon quantum dots were deposited onto the core region of the optical fiber following chemical etching of the cladding layer using hydrofluoric acid (HF). The sensor was evaluated using five acetic acid concentrations (10, 15, 18, 20, and 25%) to determine its sensitivity based on variations in resonance wavelength and optical transmission. When exposed to larger concentrations, the sensor clearly showed a red shift in resonance wavelength, indicating a direct correlation between the optical response of the sensor and the refractive index of the detecting medium. Since traditional methods frequently have low sensitivity and large instrumentation. There is an increasing demand for a small and extremely sensitive approach to detect acetic acid. Spectral and intensity analyses confirmed the sensor's performance, showing a sensitivity measurement of 10 µm/RIU and an established ratio of signal-to-noise of 0.14. Surface Plasmon Resonance (SPR) is the main basis for the sensing mechanism enabled by the relationship between the fiber's evanescent field and the carbon quantum dot coating. The testing results establish that the sensor performs efficiently in detecting liquid chemicals, especially acetic acid and related organic substances. A specially fabricated tapering setup was employed to enhance the sensor’s responsiveness to changes in the surrounding refractive index. A short-tapered segment of single-mode fiber, approximately 4 cm in length, was formed at the center of the sensor. The tapered region, with carefully controlled cladding diameter and length, was exposed to acetic acid solutions with different concentrations in order to monitor variations in the effective refractive index (RI).