Continuous in situ monitoring of ammonia in bioprocesses using a novel scattering fiber-optic sensor

This study presents a laboratory proof-of-concept for a novel scattering-based fiber-optic sensor for continuous in situ monitoring of free NH 3 in highly loaded aqueous bioprocess media. The sensing element consists of a thin phenyl-modified sol–gel coating with covalently immobilized bromocresol green on fused silica fiber rods. The experimental observations are consistent with covalent immobilization of the dye within the hybrid matrix, which appears to suppress classical pH-dependent indicator behavior and enables a largely pH-independent response within the biogas-relevant range of 6.8–8.5. Nanoscale structural inhomogeneities within the high-refractive-index sol–gel layer are expected to contribute to weak light scattering and to enhance the effective optical interaction length. Upon exposure to free NH 3 , the sensor shows reversible spectral changes, with the strongest response observed at 748 nm. The sensor exhibited an exponentially saturating calibration behavior between 20 and 200 mg/L free NH 3 at 24 °C, response times of t 90 less than 2 min, and good reversibility over more than 50 cycles. The deliberately minimalist optical setup consists of a white LED, the coated fiber element, and a compact spectrometer. It operated at low power without reagents, membranes, or consumables and achieved an AGREE greenness score of 0.88. Preliminary tests in real biogas digestate suggest promising matrix compatibility for continuous in situ monitoring in anaerobic digestion and related nitrogen-rich bioprocesses.