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.