Fast optical humidity sensor based on nanostructured hydrogels

The aim of the current work is to improve the response time of an optical readout based humidity sensor. Therefore, we present the application of nanoimprint lithography (NIL) on thin films which are deposited by initiated chemical vapor deposition (iCVD). Hydrogels are polymeric networks with the ability to swell after certain physical conditions change, which makes them very useful as sensing layers for optical devices. In the first step we used iCVD to deposit a humidity responsive hydrogel (here: pHEMA) as a planar thin film on sapphire substrates. To increase the effective surface area, we tried for the first time NIL on our hydrogel thin films with promising results: First, characterization with a SEM showed that NIL allows the design of large homogeneous areas of nanostructures without damaging the sensitive hydrogel thin film and having a great stability at ambient conditions. Second, NIL offers the benefit to build different geometries and sizes of nanostructures based on the requested application. For our first test we selected a simple line array structure, combined with an optical detection method as sensor principle. By choosing a specific structure to wavelength ratio the imprinted nanostructures act as a diffraction grating enabling a fast response time by increasing the effective sensing area. Since in our application the hydrogel works as the sensing element, we observed a humidity dependence behavior by measuring the intensity of the first order diffraction peak. Finally, the response time was a lot faster by using optical detection methods than commercial humidity sensors