Track-etched polycarbonate membranes (TEPM) are commercially available membranes typically used in particle filtration due to the pores present in their surfaces. This porous structure reminds of that of a Fabry-Pérot interferometer made on porous silicon, an optical structure long employed in optical chemical sensing. Because of this morphological similarity, we hypothesized that TEPM could exhibit a similar optical response and thus being useful for creating simpler to fabricate, easily available and low cost chemical sensors. To asses this hypothesis, we investigated the optical response of TEPM in the infrared range, improved it by chemically attaching the membranes to a silicon flat surface and then, performed reflectivity measurements in presence of different concentrations of ethanol.
When exposing a TEPM to a change of the refractive index of the medium it is surrounded by (air) by placing a drop of ethanol, with a higher refractive index, we can observe a shift of its spectrum towards higher wavelengths. This indicates the presence of the solvent, and we could check that the bigger the concentration, the bigger the magnitude of that shift. Furthermore, when the solvent is evaporated the spectrum returns to its initial position, which allowed us to perform different concentration sensing steps using the same sample. These promising results, although early, could indicate the utility of these membranes to easily fabricate cheap chemical sensors and, probably, optical biosensors as their surface can be chemically modified.