Organic solvent vapor sensing using porous photopolymer photonic bandgap structures

Significant research efforts have been focused on the development of effective means for the optical detection of organic molecules using porous one-dimensional photonic bandgap (PBG) structures. To date, efforts have been focused on porous silicon microstructures, which are typically created using a controlled electrochemical etching process in a hydrofluoric acid solution. Generally, these sensors rely on changes in the optical resonance that occurs when the porous structure is filled by the analyte of interest and allows for simple and effective optical detection schemes. Here, we present a simple method for the production of polymer Bragg reflection gratings containing periodic porous layers, and we demonstrate optical detection of organic solvent vapors using these structures. To create the structures, a pre-polymer syrup containing a monomer, a photoinitiator, a co-initiator, liquid crystals (LC), and a non-reactive solvent (acetone or toluene) is sandwiched between two pieces of glass, and the periodic structure is then formed by applying an optical interference pattern generated using a simple one-beam laser setup. More importantly, we demonstrate that acetone vapor penetrates the porous structure and induces a change in the effective refractive index of these gratings that result in a shift in the reflection wavelength. This shift is pronounced, and can easily be observed by eye, or detected by optical means. We also demonstrate that this shift depends on the particular type of chemical vapor and vapor concentration, and the detection is reversible and repeatable. Finally, the addition of aminosilane to the pre-polymer syrup is shown to improve the stability of the resulting gratings, suggesting that this photopolymer fabrication technique could be used to create structures suitable for biological applications in aqueous environments.