Investigation on the Mechanism of Bioelectrical Transduction at the Organic/Electrolyte Interface in Perylene-Based O-CST

Nowadays, neurological disorders are common pathologies able to affect memory and ability to perform daily activities. The development of innovative bioelectronic devices for monitoring nerve cells activity in vitro and in vivo is required. The flow of ions through the ion channels present in the plasma membrane can control the physiology of nerve cells by their bioelectrical activity. Thus, neural interfaces able to transduce and interact with electrical signals from the biological world are of primary importance. We have already demonstrated that primary neurons can adhere, grow and differentiate on a suitably engineered perylene-based field-effect transistor platform, while maintaining their firing properties even after a prolonged time of cell-culturing. [1] Moreover, we implemented the field-effect transistor platform called organic cell stimulating&sensing transistor device (O-CST) for stimulating the neuronal cells and record the bioelectrical activity.[2] It is evident that a clear understanding of the microscopic mechanisms at the basis of bioelectrical transduction is mandatory for mastering the functionality of the organic electronic device, and thus enhancing the signal-to-noise ratio and the selectivity of the extracellular recording. Here, we propose and experimentally validate an effective model for describing the ionic charge accumulation and flux at the region at the boundary between the physiological solution and the perylene layer in an electrolytic condenser device structure. We demonstrate that the cross-correlation between microscopic and macroscopic investigation tools is the key factor in describing the working principles of ionic transduction at the organic functional interface. We implemented Electrical Impedance Spectroscopy (EIS) for inferring the working principles of the sensing mechanism of the O-CST architecture by introducing a suitable and non-trivial equivalent circuit. Furthermore, we support this macroscopic model with Electrostatic Force Microscopy (EFM) charge distribution maps of the organic layer surface after biasing the device in wet conditions. The use of the EFM allows us to investigate the charge carrier distribution at the solution/organic interface by varying the applied bias protocols and, at the same time, correlate the surface potential maps with the topographical imaging obtained from atomic force microscopy. By this top-down approach, it is straightforward to discriminate the different processes occurring at the interphase between the electrolyte and the organic layer, such as double-layer charging and Faradaic charge-transfer reactions, which rule the iono-electrical transduction mechanism. [1] S. Toffanin, V. Benfenati et al., J. Mater. Chem. B, 2013, 1, 3850 [2] V. Benfenati, S. Toffanin et al., Nat. Mat. 12, 2013, 672-680