Fabrication of sensitive fibre-optic gas sensors based on nano-assembled thin films

A layer-by-layer approach was used to deposit coatings of thickness of order 10 nm onto a multimode optical fibre from which the cladding had been removed, with the aim of demonstrating a fibre-optic ammonia sensor. The film was composed of alternate layers of tetrakis-(4-sulfophenyl) porphine (TSPP) and poly(diallyldimethylammonium chloride) (PDDA) deposited using the electrostatic self assembly process. Exposure of this material to ammonia induces a change in the absorption spectrum, which can be observed in the transmission spectrum of the coated optical fibre. The ammonia sensor shows a linear sensitivity in the concentration range of 0.1–50 ppm and the sensor response and recovery times were less than 30 seconds. The sensor was exposed to different chemical compounds (toluene, pyridine) in order to study the sensors selectivity.