A bio-inspired colorimetric sensor based on ethylene glycol - non-close-packed photonic crystals for visual monitoring of ammonia

Portable sensors with high selectivity and sensitivity are essential for ammonia detection. In this work, a colorimetric sensor based on non-close-packed photonic crystals (NPCs) was developed for visual monitoring of ammonia, which had great selectivity, stability, and durability. The initial structural color of the NPCs was customized in the entire color range by tuning the PMMA concentration and acrylamide (AM) content. Ethylene glycol-NPCs (EG-NPCs) with a softer texture were obtained by swelling of the NPCs in 5% glycerol & 30% ethylene glycol solution, and the reflective peak of the EG-NPCs with 50 mg mL⁻¹ AM shifted from 459 nm to 626 nm. Besides, the abundant hydroxyl groups in the EG-NPCs can form hydrogen bonds with ammonia, resulting in a greater decrease in the lattice distance. The reflective peak of the EG-NPCs blue shifted from 626 nm to 482 nm when the ammonia concentration increased from 0 μL cm⁻³ to 0.12 μL cm⁻³, and the δλ = 144 nm, which realized colorimetric sensing in the entire visible color range. The excellent selectivity, stability, and durability of the colorimetric sensors make it possible to sense ammonia in the air, which is critical for the safe production of chemical plants, farms, and other key sites.