Poly (N-Isopropylacrylamide) microgel-based optical devices for humidity sensing

Optical sensors for environmental humidity have been constructed from poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgels. The devices were constructed by first depositing a monolithic layer of pNIPAm-co-AAc microgels on a Au-coated glass substrate followed by the addition of another Au layer on top. The resultant assembly showed visual color, and exhibited multipeak reflectance spectra. We found that the thickness of the device's microgel layer depended on environmental humidity, which corresponded to a change in the device's optical properties. Specifically, at low humidity the microgel layer was collapsed, while it absorbed water from the atmosphere (and swelled) as the humidity increased. Additionally, we investigated how the deposition of the hygroscopic polymer poly (diallyldimethylammonium chloride) (pDADMAC) onto the microgel layer (prior to final Au layer deposition) influenced the devices humidity response. We found that the devices were more sensitive to humidity as the number of pDADMAC layers in the device increased. Finally, we evaluated the device performance at various temperatures, and found that the sensitivity was enhanced at low temperature, although the response was more linear at elevated temperature.