Fiber-optic probes for real-time pH monitoring†

While pH determination is a commonplace laboratory practice, conventional commercial pH probes exhibit drawbacks of bulkiness, slow response times, and signal drift. These become particularly limiting in specialized fields like tissue engineering and bio-industrial processing, where unique pH probe specifications surpass the capabilities of standard laboratory equipment. Here, we present the development of compact pH fiber probes by integrating silica optical fiber with a colorimetric pH indicator. Our approach involves cross-linking the pH indicator with a biocompatible synthetic hydrogel matrix, facilitating colorimetric and precise pH measurements. Two distinct designs of optical fiber sensors were devised to cater to a broad spectrum of applications. The first design involved attaching the hydrogel sensor to the fiber tip during the photopolymerization process, while the second design was crafted by folding the hydrogel sensor onto the bare terminal of the fiber using the casting process. The fiber sensor exhibited high sensitivity (17 nm pH⁻¹) within physiological and pathophysiological pH ranges (6–8) when tested in reflection configuration. Validation of the developed fiber sensors was carried out on cancerous tissue phantoms derived from an ovine extracellular matrix. The unique specifications of these fiber sensors position them as promising candidates for applications in tissue engineering, cell growth, and continuous blood pH monitoring.