Design of a fluorescence-based microfluidic sensing device for continuous pH measurement

Continuous pH control plays a key role in many sectors, such as agriculture, food processing, chemical production, and water‑quality testing. In hydroponics, for example, accurate pH control is fundamental to maintain nutrient solutions suitable for plant growth. However, while many sensors are available for occasional or manual pH checks, robust solutions for real-time and continuous monitoring are still largely missing. The main objective of this thesis is to develop the prototype of a compact pH‑measurement system, designed for continuous operation and suitable for in-field operation. The proposed device relies on fluorescence analysis, exploiting the relationship between the fluorescence peak of a suitable fluorophore and pH. In particular, the experiments have been carried out using Sodium Fluorescein (Uranine), a polar dye that dissolves easily in aqueous solutions. Initial tests confirmed a clear relationship between fluorescence peak and pH but also revealed significant limitations: the very small quantity of dye compared with the sample volume made dosing highly sensitive, so even small errors caused large measurement deviations. To improve robustness, a new preparation method was developed to relax dosing tolerances, complemented by the design of a passive microfluidic mixing device capable of homogenizing the dye-sample solution without external actuation. The complete prototype is composed of a microfluidic circuit, an electronic control board, and a software for data acquisition and management. The microfluidic circuit has been designed in Autodesk Fusion 360 and simulated in COMSOL Multiphysics, with two inlet ports and a channel geometry engineered to induce chaotic advection and promote diffusion, ensuring efficient passive mixing. The electronic board features the acquisition system, the circuits to drive peristaltic pumps, and for temperature sensing and LED excitation, and a mini-spectrometer for fluorescence detection. A custom PCB was developed in KiCad to integrate these functions. Battery powered and equipped with efficient boost and buck-boost converters, the board drives the pumps in PWM H-bridge mode, reads data from an NTC thermistor and the AS7341 mini-spectrometer, and controls the LED source. All communication and power management are consolidated through a single USB-C port, which also supports WebUSB connectivity to a browser-based interface for cross-platform use, with Bluetooth as an additional option. The data retrieval, processing, and visualization software is enhanced by the integration of machine-learning techniques to improve accuracy and adaptability. This work demonstrates the feasibility of a portable, fluorescence-based pH sensing platform that combines microfluidics, custom electronics, and intelligent data processing. While conceived as a proof of concept, the prototype lays the groundwork for further refinement toward a robust, autonomous, and scalable solution.