Center for Functional Nanomaterials Seminar

Soft, conductive composite thin films and nanofibers are used in sensors, electrically conductive membranes, wearable textiles, and biomedical devices. One route to constructing such devices is to design a conductive polymer composite, consisting of a polymer matrix for mechanical support and a conductive filler. To produce stable and long-lasting flexible composites, the fillers should be as flexible as the polymer matrix. In the current work, we explored two flexible conductive fillers of high aspect ratios to yield composite materials with different geometries. We employed novel bio-inspired conductive nanowires formed by the self-assembly of in-house synthesized oligopeptides as conductive fillers and compared them with the well-known conjugated polymer, poly(3,4-ethylenedioxythiophene) or PEDOT. We used drop-casting and electrospinning to fabricate thin films and nanofibers, respectively, as the suitability of each geometry depends on specific applications. Evaluating both geometries together lends an understanding of how material geometries and process parameters affect electrical conductivity. We have evaluated the electrical properties of the composites by employing 2-point probe measurements. Additionally, the effects of pH, relative humidity, and temperature on conductivity have also been explored. For PEDOT-based composites, changes in the relative humidity did not impact device conductivity. Also, conductivity did not change significantly when the pH value was increased from 3 to 10, however, decreased by two orders of magnitude when the pH value was increased to 14. We believe our work will provide further insights into PEDOT-based composites, especially porous PEDOT-based fibers, and will inform the field of soft composite biomaterials and bio-inspired materials. On the other hand, for composites based on oligopeptides including naphthalene-capped FFKK (Nap-FFKK) and pyrene-capped FFKK (Pyr-FFKK), the conductivity increased by 5 orders of magnitude when the relative humidity increased from 10% to 90%, making these composites excellent humidity sensors. While these oligopeptides were non-conductive under vacuum, our findings on PEDOT and Pyr-FFKK oligopeptides inspired the synthesis of other novel peptides based on oligothiophenes, specifically tetra- and pentathiophene-capped FFKK. Moreover, despite due to their small molecular weight, we successfully electrospun pure oligopeptide-based fibers. We also proposed and verified, for the first time in literature, the mechanism underlying their electrospinnability. The results from this fundamental study on oligopeptides hold the potential to be extended to other self-assembling small molecules.