  Conjugated polymer-based organic electrochemical transistors (OECTs) are
being studied for applications ranging from biochemical sensing to neural
interfaces. While new conjugated polymers are being developed that can
interface digital electronics with the aqueous chemistry of life, the vast
majority of high-performance, high-mobility organic transistor materials
developed over the past decades are extremely poor at taking up
biologically-relevant ions. Here we incorporate an ion exchange gel into an
OECT, demonstrating that this structure is capable of taking up
biologically-relevant ions from solution and injecting larger, more hydrophobic
ions into the underlying polymer semiconductor active layer in multiple
hydrophobic conjugated polymers. Using poly[2,5-bis(3-tetradecylthiophen-2-yl)
thieno[3,2-b]thiophene] (PBTTT) as a model semiconductor active layer and a
blend of the ionic liquid 1-butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide (BMIM TFSI) and poly(vinylidene
fluoride-co-hexafluoropropylene) (PVDF-HFP) as the ion exchange gel, we
demonstrate more than a four order of magnitude improvement in OECT device
transconductance and a one hundred-fold increase in ion injection kinetics. We
demonstrate the ability of the ion exchange gel OECT to record biological
signals by measuring the action potentials of a Venus flytrap plant. These
results show the possibility of using interface engineering to open up a wider
palette of organic semiconductor materials as OECTs that can be gated by
aqueous solutions.
