Periodically nanostructured hydrogels for ethanol vapors sensing

Abstract

Chemical sensing using optics has been under extensive research all over the world during last decades and many optical chemical sensors are nowadays finding increasing applications in industry, environmental monitoring, medicine, biomedicine and chemical analysis. These optical sensors can be based on various optical principles, such as absorbance, reflectance or transmittance, luminescence and fluorescence, covering different regions of the spectrum (UV, visible, IR, NIR). Optical chemical sensors have several advantages over 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 optical chemical sensors is based on Photonic Crystals (PCs), i.e. regular arrays of materials with different refractive indices. 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 stimuli responsive 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 can change their volume significantly in response to small alterations of certain environmental parameters. In fact, hydrogels are increasingly considered as responsive materials to generate active inverse opals fortheir ability to exhibit significant reversible diffraction shifts as a response of a variety of stimuli, such astemperature, pH and ionic strength, single molecules binding and mechanical forces.The stimuliresponsiveness must be accompanied by adequate elasticity and chemical stability forthe inverse opal to be able to survive, without collapsing, to the template removal process byorganic solvents (for polymer colloids) during preparation and to withstand repeated swelling/deswelling cycles when in use, as well as erosion due to prolonged exposure to the swelling medium. While there are interesting studies which report diffraction shifts in a wide region of the visible spectral region when e.g. a crosslinked 2-hydroxyethyl methacrylate (HEMA) hydrogel is exposed either to pure liquid water or to concentrated ethanol/water liquid solutions, at the best of our knowledge there are no equivalent studies which report on the ability of hydrogel inverse opals tospecifically respond to ethanol vapors when already swollen by water.The hydrogel network should be designed so that it can uptake and retain water, when exposed towater vapor-rich atmospheres, and further swell when the atmosphere which is exposed 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-Hugginsmixing parameter with ethanol; acrylic acid (AA) at two different ratios was also considered as co-monomerfor its affinity toward water and its contribution to hydrogel network mechanicalproperties, due to establishment of further crosslinking through strong secondary interactions;finally poly-ethylene glycol-200dimethacrylate (PEG200DMA) was used as crosslinking agent. The polymerization process combined a “cold” UV-photocrosslinking step and a thermal post-cure.Preliminary swelling studies in the presence of both liquid ethanol and ethanol vapors were carried out on the macrogel analogue as well as a dynamic mechanical thermal analysis to withdraw usefulinformation on the hydrogels mechanical spectra and validate both the formulation and curingprocess. The most promising of the two formulations was selected to infiltrate a polystyrene (PS)opal structure, which was generated onto pre-etched silica through self-assembly of PS nanoparticles. The periodically nanostructured hydrogel film (Fig.1) was t