Isacco Gualandi1 Marta Tessarolo2 3 Federica Mariani1 Tobias Cramer2 Domenica Tonelli1 Erika Scavetta1 Beatrice Fraboni2

1, University of Bologna, Bologna, , Italy
2, University of Bologna, Bologna, , Italy
3, Università di Bologna, Bologna, , Italy

Great research efforts are devoted to the development of portable and wearable sensors as they could have a broad impact on real life in different fields such as point-of-care medical applications and environmental monitoring due to the ubiquity of smart technologies and wireless communication networks,. The crucial bottleneck is the development of new smart materials capable to effectively convert the chemical information into an electrical signal. Amplified transduction of ionic or electrochemical signals is nowadays achieved in sensors that consist of three electrodes operating in a transistor configuration (see Figure 1A): Source and Drain electrodes which drive an electronic current through a semiconducting channel that is coupled to the gate electrode through an ionically conductive analyte solution.
In this contribution, we propose a nano-composite material based strategy to integrate the amplified electrochemical response and sensitivity of a three-terminal organic electrochemical transistor (OECT) into a simpler two terminal device, thus paving the way for a new generation of smart sensors. To do it, we design, synthesize and exploit a new composite material based on Ag/AgCl nanoparticles and PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate)). The Ag/AgCl gate electrode, which is the transducer of the OECT, is embedded into the semiconducting polymer in the form of nanoparticles and, as a consequence, the sensor combines an intrinsically amplified response with a simple two terminal electrical connection. In order to demonstrate our strategy, we investigate the novel composite material by electrostatic force microscopy, scanning electron microscopy and electrochemical impedance spectroscopy and show that the spontaneous interaction between the NPs and Cl- ions present in the sample solution is directly coupled to the charge transfer process into the semiconductor, so inducing a fast modulation of the channel conductivity. Consequently, the current flowing in the channel is directly related to the logarithm of Cl- ions concentration. We show two application examples that demonstrate the efficacy and the robustness of our approach. The first regards a real-time, portable sensor for in-situ detection of salinity in water. The second is a textile sensor, obtained on a cotton yarn, for real-time sweat monitoring. The here presented sensors show an excellent reliability, as demonstrated by comparing the results obtained analyzing real-life samples with our sensors and with standard chemical analyses. Finally, our approach was successfully used to fabricate novel sensors for Br-, I- and S2-, demonstrating the widespread applicability of such devices.