Organic electrochemical transistors (OECTs) are attracting a great deal of interest for biosensing and bioelectronics applications. However, their device physics is not yet well-understood. In this paper, we focus on the effect of the gate electrode material on the response of OECTs. We studied OECTs made from the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate), and which utilized halide electrolytes. We demonstrate that OECTs with Ag gate electrodes show larger current modulation compared to OECTs with Pt gate electrodes. This effect is due to a change in the OECT regime of operation from capacitive, in case of a Pt gate electrode, to Faradaic, in the case of an Ag electrode.

This perspective deals with the coupling of ionic and electronic transport in organic electronic devices, focusing on electrolyte-gated transistors. These include electrolyte-gated organic field-effect transistors (EG-OFETs) and organic electrochemical transistors (OECTs). EG-OFETs, based on molecules and polymers, can be operated at low electrical bias (about 1 V or below) and permit unprecedented charge carrier densities within the transistor channel. OECTs can be operated in aqueous environment as efficient ion-to-electron converters, thus providing an interface between the worlds of biology and electronics. The exploration and the exploitation of coupled ionic and electronic transport in organic materials brings together different disciplines such as materials science, physics, chemistry, electrochemistry, organic electronics and biology.

Selective detection of bioanalytes in physiological fluids, such as blood, sweat or saliva, by means of low-cost and non-invasive devices, is of crucial importance to improve diagnosis and prevention in healthcare. To be really useful in everyday life a sensing system needs to be handy, non-invasive, easy to read and possibly wearable. Only a sensor that satisfies these requirements could be eligible for applications in healthcare and physiological condition monitoring. Herein an organic electrochemical transistor has been investigated as a simple, low-cost and e-textile biosensor, fully integrated on a single cotton yarn. The biosensor has been used for real-time detection of adrenaline, selectively compared to the saline content in human physiological fluids. The sensing mechanism is based on the oxidation of adrenaline at the Pt-gate electrode surface, with the formation of adrenaline-quinone and adrenochrome. Two independent organic electrochemical transistors, characterized by different gate-electrode materials, detect saline and adrenaline concentrations, respectively, in real human sweat. Measurements performed in real-time mode show the complete independence of adrenaline detection from NaCl and, hence, guarantee the simultaneous monitoring of both concentrations. The oxidation of adrenaline has been studied by means of absorption spectroscopy in air, with either silver or platinum working electrodes. Our results confirm that the oxidation reaction driven by the Pt-electrode leads to the formation of adrenochrome, while with the Ag-electrode the oxidation is similar to the spontaneous one occurring in air. The cotton-based biosensor shows the possibility of monitoring human performances (hydration and stress) in situ and using a non-invasive approach, opening new unexplored opportunities in healthcare, fitness and work safety.

We use a seeded supersonic molecular beam to control the kinetic energy of pentacene (${\mathrm{C}}_{22}{\mathrm{H}}_{14}$) during deposition and growth on Ag(111). Highly ordered thin films are grown at low substrate temperatures ($\sim 200\mathrm{K}$) at kinetic energies of a few electron volts, as shown by low energy He diffraction and x-ray reflectivity spectra. In contrast, deposition of thermal molecules yields only amorphous films. Growth at room or higher temperature substrates yields films of poorer quality irrespective of the depositing beam energy. We find that after the first wetting layer is completed, a new ordered phase is formed, whose in-plane lattice spacings match one of the bulk crystal planes. The high quality of the films can be interpreted as the result of local annealing induced by the impact of the impinging high-energy molecules.

A single natural cotton fiber has been functionalized with poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) conductive polymer by a simple soaking process and used as a channel of an organic electrochemical transistor (OECT), directly interfaced with a liquid electrolyte in contact with an Ag wire gate. The device shows a stable and reproducible current modulation and has been demonstrated to be very effective for electrochemical sensing of NaCl concentration in water. The single wire cotton fiber OECT results to be a simple and low cost device, which is very attractive for wearable electronics in fitness and healthcare.