Swelling of Polymer Dielectric Thin Films for Organic-Transistor-Based Aqueous Sensing Applications

Sensors based on organic thin-film transistors (OTFTs) are of considerable interest for chemical and biological detection applications, and the development of highly sensitive, chemically specific, low-cost sensors operating in aqueous environments will have a profound impact. However, the behavior of the dielectric and semiconducting thin films in OFTF-based sensors during underwater operation is not well understood. Here we investigate OTFT-based sensor materials, specifically a polymer dielectric film of cross-linked poly(4-vinylphenol) (x-PVP), used in OTFTs operating in aqueous environments. We show that immersing x-PVP thin films in a 90:10 water methanol (model analyte) solution causes swelling of nearly 30% and a corresponding 300% increase in the film dielectric constant. Hence, to quantify the charge-transport behavior of organic molecules within aqueous environments, this drastic change in the capacitance must be accounted for in sensor material design.