Abstract
Photonic crystals (PCs) show reflectance spectra depending on the geometrical structure of the crystal, the refractive index (neff), and the light incident angle, according to the Bragg-Snell law. Three-dimensional photonic crystals (3D-PCs) composed of polymeric sub-micrometer spheres, are arranged in an ordered face cubic centered (fcc) lattice and are good candidates for vapor sensing by exploiting changes of the reflectance spectra. We synthesized high quality polystyrene (PS) 3D-PCs, commonly called opals, with a filling factor f near to the ideal value of 0.74 and tested their optical response in the presence of different concentrations of methanol (MeOH) vapor. When methanol was present in the voids of the photonic crystals, the reflectance spectra experienced energy shifts. The concentration of methyl alcohol vapor can be inferred, due to a linear dependence of the reflectance band maximum wavelength as a function of the vapor concentration. We tested the reversibility of the process and the time stability of the system. A limit of detection (LOD) equal to 5% (v/v0), where v was the volume of methanol and v0 was the total volume of the solution (methanol and water), was estimated. A model related to capillary condensation for intermediate and high methanol concentrations was discussed. Moreover, a swelling process of the PS spheres was invoked to fully understand the unexpected energy shift found for very high methanol content.
Highlights
IntroductionEcosystems, such as air, water, soil, etc., sustain human and animal life providing livelihoods
Ecosystems, such as air, water, soil, etc., sustain human and animal life providing livelihoods.Environmental pollutants, such as heavy metal ions [1,2,3,4,5,6,7], inorganic, and organic compounds [8,9]are released into the ecosystems, and into the entire food chain, producing poisoning effects, which can cause diseases to human and animal beings
We developed a photonic crystal based on self-assembled polystyrene nanospheres, which shows an optical response to methanol vapors
Summary
Ecosystems, such as air, water, soil, etc., sustain human and animal life providing livelihoods. Liquids and vapors of alcohols are highly flammable with obvious risks on work safety [16,17,18] For all these factors, it is extremely important to be able to detect even very small quantities of VOCs. Usually, electrochemical sensors able to detect the presence of VOCs, are difficult to produce and are quite expensive. The working principle of PCs sensors is a variation of the RI of the whole system in the presence of a contaminant, which produces an energy shift of the reflectance peak or a change of its shape, according to Bragg-Snell’s law [47]. We extend previous experiments performed with ethyl alcohol and we used some characterizations and results of that study [29]
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