The polymeric layers were deposited using the ESA layer-by-layer method [11]. In this work the materials involved were poly(diallyidimethyl) ammonium chloride (PDDA), PolyR-478, poly(allylamine) hydrochloride (PAH) and LUDOX? SM-30 SiO2-water colloid. In this case, the PAH and PDDA acted as polycations, and PolyR-478 and SM-30 were the anionic species. The number of overlays of PDDA/PolyR-478 was 14 layers and the number of PAH/SM30 overlays was 14 layers, which confer a total film thickness of less than 300 nanometers. Specifically, although the film thickness is difficult to measure due to the fibre geometry, in previous works it has been demonstrated that each PDDA/PolyR-478 and PAH/SM30 layers had thicknesses of approximately 12 nm and 7 nm, respectively [1, 7].

The thickness overlay is chosen to guarantee that the attenuation band is located where there is good sensitivity and where it does not vanish at the same time. In this work, the good sensitivity band corresponds to ~1,520 nm. In fact, the same effect noticeable when the ambient humidity increases, can be appreciated when the thickness of the coating is getting increased (see Figures 2a and 2b). If the sensor is located near the vanishing area (around 1,500 nm using this LPG), it would be more sensitive to any humidity change, but it would not be possible to detect any change once the peak vanished. On the other hand, if the sensor is placed near the minimum resonant peak, the humidity sensitivity would be almost negligible, because the resonant peak shift would be extremely slow, as experimentally demonstrated in [8].

A halfway decision is more convenient to work out both design features. It is important to stop the building process of the sensitive layer at a point halfway between the minimum value and the value where the resonant peak vanishes. So, there must be a trade-off between sensor sensitivity (far from the minimum because the blue shift dependence is higher) and sensor maximum wavelength displacement detection range. Using the intermediate layer, the wavelength working point can Batimastat be easily located in a more favorable value.Figure 2.a) LPG spectra before and after deposition of PAH/SM30 (only sensitive layer), b) LPG spectra before and after deposition of PDDA/PolyR-478 + PAH/SM30 (with the intermediate layer).In Figure 3 the experimental setup used to do the humidity tests is shown.

The LPG was placed inside a climatic chamber, with temperature and humidity control, and was illuminated using a broad
Gas sensing has become increasingly important as environmental awareness and industrial processes impose greater demands on measurement and monitoring systems [1, 2]. Spectroscopy-based techniques are well suited for monitoring gases as most of them have a characteristic absorption spectrum.