Direct Combined Determination of Ion Transport and Exchange in Organic Bioelectronic Devices

Organic bioelectronic devices such as organic electronic ion pumps or organic electrochemical transistors are small electronic devices with a high potential for, for example, stimulation and correction of nervous activity (treatment of Parkinson's disease, suppression or complete cessation of epileptic seizures, regeneration of damaged nerve connections), detection and influence of cellular tissues (cardiac pacemaker, heart rate monitor, sweat or blood composition), and, last but not least, the targeted dosing of medicine in time and space (treatment of cancer, depression, diabetes), i.e. with significant minimization of side effects. Both of these devices are based on ion exchange and ion transport in organic semiconductors and ion-selective materials, which are mostly polymeric substances with the unique ability to conduct ions in addition to electrons, unlike classical devices, they are thus capable of much more efficient information transfer between living tissue and devices than the devices commonly used today.
However, despite significant progress in research, the disadvantage of these new bioelectronic devices is still their relatively low sensitivity, repeatability, and biocompatibility, which prevents their mass use in everyday life. Moreover, the understanding of ion transport and ion exchange is derived mostly based on nonspecific or non-real-time detection methods (e.g. mass spectrometry or purely electrical readouts). However, a precise understanding of the role of ions (e.g. drift and especially diffusion and its effect on electronic mobility) is therefore critical for understanding the principle of operation of organic (bio)electronic devices.
Nevertheless, the overall extent of the exchanged ions (especially the number of exchanged ions) is still not fully elaborated. Here, we report on a new approach based on direct combined determination of ionic transport and exchange using optical, electrical, and pH detection. We use absorbance measurement of the pH indicator (fluorescence of ion-selective fluorescent probe) to monitor a change in the concentration of protons (specific ions) that are transported/exchanged and correlate this change with electrical output. To support this measurement, a pH change is continuously measured in the case of proton transport and is correlated with the optical and electrical readout. Through the correlation of electrical output, pH, and data from optical measurements, we can calculate the delivered proton concentrations. Determining the amount of transported/exchanged ions is important not only from the point of view of characterizing new organic semiconductors or ion-exchanged membranes but also for studying the mechanism of interaction of bioelectronic devices with cells and living tissues and thus for the development of new bioelectronic devices and new drug development.