Ion-Selective Organic Electrochemical Junction Transistors Based on Poly(3,4-ethylenedioxythiophene) Doped with Poly(styrene sulfonate)

K(+)-, Ca(2+)-, and Ag(+)-selective OEJTs were characterized by measuring the collector/emitter current (l(c)) as a function of time at constant collector/emitter voltage (V(c)) both in absence and presence of applied base voltage (V(B)). K(+)-selective OEJT responds to K(+) in the concentration range 10(-1) M-10(-4) M with 0.1. M NaCl as the background electrolyte. Prolonged conditioning of the K(+)-selective OEJTs in KCl solution caused deterioration in the selectivity of the transistors. The selectivity, however, was regained after keeping the transistors dry for a certain time. In the case of Ca(2+)-selective OEJTs, various membrane compositions and measuring parameters were studied and compared. Factors such as the base voltage and the concentration of the ionophore and the lipophilic salt in the ion-selective membrane (ISM) influenced the performance of the Ca(2+)-selective OEJTs. The best selectivity for Ca(2+) in the concentration range 10(-1) M-10(-4) M (with 0.1 M K(+) as the interfering ion) was achieved by using ca.10 wt% ionophore and 5 wt% lipophilic salt. Ag(+)-selective OEJTs, both in presence and absence of ISM, gave more complicated response than Ca(2+)- or K(+-) selective OEJTs. One reason is that Ag(+) is an electroactive ion that may oxidize the PEDOT: PSS transducer layer even in the absence of the base voltage.