Graphene transistors are becoming a promising building block in the large field of Chemical Logic but also for application as biosensors, requiring highly stable materials in aqueous and biological media.
The low-cost fabrication process of reduced Graphene Oxide Electrolyte-Gated Transistors (rGO-EGFET) from homemade formulation of Graphene Oxide (GO) ink¹ is paving the way to the development of cheap and competitive graphene-based logic gates, operating at low-voltage thanks to the thin double layer capacitance forming at the electrolyte/conductive layer interfaces.

In this work, we investigate the conception of a complementary-like inverter from two rGO-EGFET, with different doping states induced either by changing the reduction charge amount or the nature of the electrolyte itself (which could be a biological medium).₂ In both cases, the system can be simply modelled by two resistors in series, and the switch occurs when the value of one resistor becomes significantly lower than the other one, while changing the applied gate voltage.³
The proof of concept of our transduction method has been obtained with connecting two rGO-EGFET with different aqueous electrolytes; we noticed therefore a switch operating around 1V from the High state to the Low state of the logic gate.

On the other side, measuring electrolyte losses from sweat is of interest, in particular for monitoring sports activities and prevent e.g., dehydration. In such application, an alert when a given electrolyte (eg., K+) reaches a threshold value is more valuable than continuous quantification.
A compelling application of our rGO-based logic gate consists in building a proper ion-sensor through the functionalization of the gates with ionophore-based membrane: A sufficient change of the Gate/Electrolyte capacitance, induced by the presence of a targeted molecule, could lead to a change of the input voltage and thus to the switch of the logic state. As an application here, in order to monitor the K+ concentration in human or artificial sweat, the valinomycin ionophore is added into a membrane functionalizing the gate contacts. The switch between the two logic states is then depending on a threshold concentration of K+ ions, that can be considered as the analogic input signal of our ion-sensitive logic gate, which could then trigger another action.

References:

1. Vasilijević, S., Mattana, G., Anquetin, G., Battaglini, N. & Piro, B. Electrochemical tuning of reduced graphene oxide in printed electrolyte-gated transistors. Impact on charge transport properties. Electrochimica Acta 371, 137819 (2021).
2. Son, M. et al. Low-Power Complementary Logic Circuit Using Polymer-Electrolyte-Gated Graphene Switching Devices. ACS Appl. Mater. Interfaces 11, 47247–47252 (2019).
3. Traversi, F., Russo, V. & Sordan, R. Integrated complementary graphene inverter. Appl. Phys. Lett. 94, 223312 (2009).