A fibre optic long period grating (LPG) with an nano-assembled mesoporous coating of alternate layers of poly(diallyldimethylammonium chloride) (PDDA) and SiO2 nanospheres was used for the development of a highly sensitive fibre-optic chemical sensor. Sensor fabrication involves a 2-stage process: firstly, the deposition of the base mesoporous thin film (PDDA/SiO2) over an LPG written in the optical fibre using a layer-by layer technique, followed by the infusion of a functional material into the porous film. The refractive index of the base mesoporous coating, determined at a wavelength of 633 nm using ellipsometry, was found to be 1.2. The infusion of the functional material into the coating resulted in a significant change in the RI of the coating, producing a dramatic change in the transmission spectrum of the LPG. The sensing mechanism exploited is based upon chemically induced desorption of the functional material from the mesoporous coating. The sensing of ammonia in aqueous solution was chosen as an example to demonstrate the sensing principle of the LPG sensor. The operation of the sensor was characterized using two functional materials, tetrakis-(4-sulfophenyl)porphine (TSPP) and polyacrylic acid (PAA). The device showed high sensitivity to ammonia with a response time less than 100 s and a limit of detection of 140 ppb when the TSPP infused (PDDA/SiO2) film was employed as a sensitive element.