Optical Trace Ammonia Sensors with Ultra-Fast Response Time and Stability

Abstract

Free ammonia poses a threat to aquatic systems already in very low concentrations (<25 µg·L–1) but is currently challenging to continuously monitor at these levels using effortless and compact analytical tools. Herein, we present an optical chemical sensor which overcomes the stability limitations of ammonia sensors based on pH-sensitive dyes. The new aza-BODIPY dye incorporates a N,N-diisopropylaminophenyl group that induces pH sensitivity via the photoinduced electron transfer effect in the neutral form (pKa of 4.25) without significant alteration of the absorption spectrum. The new indicator dye efficiently absorbs in the red part of the electromagnetic spectrum (λabs = 654 nm, ε = 92,600 L·mol–1·cm–1) and shows moderately strong fluorescence for the cationic form (λem = 689 nm, Φ = 0.24) and no emission from the neutral form. The ammonia sensors are designed by immobilization of the indicator into a hydrophilic matrix (polyurethane hydrogel D4) with porous glass particles along with a protective hydrophobic silicone layer and PTFE membrane on top that eliminates cross-talk to pH. The sensors exhibit a low limit of detection (0.26 µg·L–1) and a limit of quantification (1.23 µg·L–1), along with fast response and recovery times of 18 s and 6–7 min, respectively. The sensor shows excellent reversibility and long-term stability over several weeks. Field tests demonstrate the suitability of new ammonia sensors for fish farming and wastewater treatment. Ammonia concentrations determined are in good agreement with those obtained by a photometric reference method.