Development, Optimization, and Experimental Characterization of a Flexible Fluorescence-Based pH Sensor

The continuous monitoring of pH is of significant interest in biomedical applications, particularly for the development of wearable and non-invasive sensing systems. Fluorescence-based pH sensing represents a promising approach due to its high sensitivity and compatibility with compact and flexible optoelectronic platforms. This thesis presents the development and experimental characterization of a flexible fluorescence-based pH sensing platform, with particular focus on the optimization of the optical and electronic subsystems and on the compensation of the main sources of measurement uncertainty. A flexible printed circuit was fabricated on a PET substrate using direct-write printing technology, integrating the optical components required for fluorescence detection. The optical performance of the excitation source was experimentally optimized through mechanical modification of the LED package and the application of a BaSO₄ reflective coating. The combined modifications increased the measured optical power from approximately 0.54 mW to 0.88 mW, corresponding to an improvement of about 63% compared with the unmodified configuration. To improve the reliability of the sensing platform under realistic operating conditions, compensation strategies for mechanical deformation and temperature variations were investigated. Printed carbon-based strain gauges were characterized and integrated as a half Wheatstone bridge for monitoring substrate bending, while an NTC thermistor was characterized and incorporated for temperature monitoring. A reference photodiode was also introduced to monitor variations in excitation intensity. These elements provide the additional information required to compensate for the main disturbances affecting the optical measurement. Finally, an integrated flexible sensor incorporating the analog front-end and two excitation LEDs was developed to investigate a ratiometric fluorescence measurement architecture. Experimental characterization using HPTS demonstrated an approximately linear variation of the photodiode output with pH over the investigated range. However, the non-ideal optical filtering properties of the flexible substrate introduced different levels of excitation-light leakage for the two wavelengths, limiting the effectiveness of the ratiometric approach. The results therefore indicated that excitation at 465 nm provides the most effective configuration for the proposed flexible platform. Overall, the work demonstrates the feasibility of integrating optical sensing, signal conditioning, and mechanical and thermal monitoring into a flexible printed platform. Although the complete wearable sensor, including the final fluorescent sensing layer and full pH characterization, requires further development, the proposed platform provides a technological basis for compact, low-cost, and flexible fluorescence-based biosensors for continuous biomedical monitoring.