Periodically nanostructured hydrogels for ethanol vapors sensing

Chemical sensing using optics has been under extensive research all over the world during last decades andmany optical chemical sensors are nowadays finding increasing applications in industry, environmentalmonitoring, medicine, biomedicine and chemical analysis. These optical sensors can be based on variousoptical principles, such as absorbance, reflectance or transmittance, luminescence and fluorescence, coveringdifferent regions of the spectrum (UV, visible, IR, NIR). Optical chemical sensors have several advantagesover conventional electricity-based sensors, in terms of selectivity, immunity to electromagnetic interference,higher sensitivity, and they are also relatively inexpensive and minimally invasive. A wide class of opticalchemical sensors is based on Photonic Crystals (PCs), i.e. regular arrays of materials with different refractiveindices. In particular, they are artificial structures with a periodic dielectric function.In this paper, we present the optical characterization of a polystyrene opal, infiltrated with a stimuliresponsive hydrogel specifically formulated to be sensitive to ethanol (EtOH), also in the presence of water.Stimuli-responsive hydrogels are interesting materials for sensing applications due to thefact that they canchange their volume significantly in response to small alterations of certain environmental parameters. Infact, hydrogels are increasingly considered as responsive materials to generate active inverse opals fortheirability to exhibit significant reversible diffraction shifts as a response of a variety of stimuli, suchastemperature, pH and ionic strength, single molecules binding and mechanical forces.The stimuliresponsivenessmust be accompanied by adequate elasticity and chemical stability forthe inverse opal to beable to survive, without collapsing, to the template removal process byorganic solvents (for polymercolloids) during preparation and to withstand repeated swelling/deswelling cycles when in use, as well aserosion due to prolonged exposure to the swelling medium.While there are interesting studies which report diffraction shifts in a wide region of the visible spectralregion when e.g. a crosslinked 2-hydroxyethyl methacrylate (HEMA) hydrogel is exposed either to pureliquid water or to concentrated ethanol/water liquid solutions, at the best of our knowledge there are noequivalent studies which report on the ability of hydrogel inverse opals tospecifically respond to ethanolvapors when already swollen by water.The hydrogel network should be designed so that it can uptake andretain water, when exposed towater vapor-rich atmospheres, and further swell when the atmosphere which isexposed to isprogressively concentrated of ethanol vapors. For this purpose, 2-hydroxyethyl methacrylate(HEMA) was used as main building block for the network, for its known favorable Flory-Hugginsmixingparameter with ethanol; acrylic acid (AA) at two different ratios was also considered as co-monomerfor itsaffinity toward water and its contribution to hydrogel network mechanicalproperties, due to establishment offurther crosslinking through strong secondary interactions;finally poly-ethylene glycol-200dimethacrylate(PEG200DMA) was used as crosslinking agent. The polymerization process combined a “cold” UV-photocrosslinkingstep and a thermal post-cure.Preliminary swelling studies in the presence of both liquid ethanoland ethanol vapors were carried out on the macrogel analogue as well as a dynamic mechanical thermalanalysis to withdraw usefulinformation on the hydrogels mechanical spectra and validate both thefo