Optical fiber long period grating sensor with a polyelectrolyte alternate thin film for gas sensing of amine odors

The formation of poly(acrylic acid) (PAA) and poly(allyamine hydrochloride) (PAH) multilayers on an optical fiber long period grating (LPG) by the layer-by-layer (LbL) assembly technique for highly sensitive ammonia gas detection is reported. Observation of the LPG's transmission spectrum indicated that the refractive index (RI) of the PAH/PAA alternate layer film was changed on exposure to ammonia, possibly via a change in the structure of the polyelectrolyte multilayers. PAA could act as a receptor for binding of amine compounds including 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. An LPG of period 100 μm with a 7-cycle PAH/PAA coating exposed to ammonia exhibited a limit of detection of 10.7 ppm. Film morphology and thickness changes due to the binding of ammonia gas, explaining the sensing mechanism, were confirmed through atomic force microscopic (AFM) measurements.