Polyelectrolyte multilayer nanothin film coated long period grating fiber optic sensors for ammonia gas sensing

The formation of poly(acrylic acid), PAA, -anchored polyelectrolyte multilayers on an optical fibre long period grating (LPG) by the layer-by-layer (LbL) assembly technique for highly sensitive ammonia gas detection is reported. PAA could act as a receptor for binding of amine compounds, especially ammonia, which would induce changes in the coating properties such as optical thickness (OT), film thickness/density and electrostatic interaction, thus influencing the transmission spectrum of the LPG. The ammonia binding is based on the acid-base interaction to free carboxylic acid groups of PAA. Film morphology and thickness changes due to the binding of ammonia gas, explaining the sensing mechanism, were confirmed through atomic force microscopic (AFM) measurements.