Polystyrene photonic crystals as optical sensors for volatile organic compounds

We have synthesized self-assembled photonic crystals (PCs) based on polystyrene nanospheres having average diameters of 250 nm. Samples were obtained by drop-casting technique and subsequent selfassembly on pre-treated glass substrates to increase the surface wettability. Films showed a very good reflectance band with a maximum at 600 nm. We studied the reflectance peak changes as a function of time in presence of vapours of different alcohols. Specifically, we investigated methanol, ethanol, 1- propanol, isopropanol and n-butanol in order to test the potentiality of the system as optical gas sensor for volatile organic compounds (VOCs). We found a considerable redshift of the reflectance band in the presence of the alcohols that cannot be explained only on the basis of the different refractive index of the solvents. We attributed this behaviour to a cooperative effect due to an increase of the effective refractive index and to a swelling process of the polystyrene nanospheres induced by the contact with the alcohols. A different behaviour was found for water due to the hydrophobic properties of the surface of the polymeric photonic crystals. This property was exploited to test the polystyrene PCs for the measurement of the relative concentration of ethanol vapour in a closed volume exploiting different ethanol/water concentrations for a possible use as breathalyzer. The estimated limit of detection (LOD) of ethanol vapour for our system was 2% (vEthan/vtot) corresponding to 1167 ppm.