Introducing State Variables in Organic Electrochemical Transistors With Application to Biophysical Systems

Organic electrochemical transistors (OECTs) are transducing devices that, placed in contact with an electrolyte solution, detect the ionic composition of that solution by measuring the channel current I. OECTs enable the streaming of continuously updated zero-to-low latency information and show, therefore, promise for being used as highly efficient biosensors. Nevertheless, apart from simple geometries, decoding such an information may be infeasible. Here, we show how I can be processed to derive a reduced set of two variables that account for most of the information of a system: 1) the modulation m is the current gained by the system compared to its initial value; and 2) the effective time t
_e
is the time over which the response of the system stays above the 65% of its final value. m and t
_e
can be reported in a diagram that is akin to the state space diagrams used in thermodynamics: points in the diagram describe the state of a system at a specific time; trajectories in the diagram describe the time evolution of that system. We show that the total electric charge Q exchanged by the system between two states A and B is independent on the path taken between them. This, in turn, implies that m and t
_e
are state variables of the system. In experiments with Solanum lycopersicum tomato plants, we show how this concept can be used to extract relevant information about a biophysical system without direct knowledge of its internal workings.