Processing of conducting polymers for biologica applications
Organic electroactive materials are used to produce flexible and easily processable electronic devices, such as organic light-emitting diodes, transistors and photovoltaic cells. Alongside these well-established applications, organic electroactive materials have been introduced in bioelectronics, where electronic signals are translated into ionic biosignals and vice versa. Examples of bioelectronic devices are sensors based on organic electrochemical transistors (OECTs).
The objective of this project is to provide a deeper understanding of the working mechanism of OECTs by exploring how the device electrical characteristics depend on the processing of the conducting polymer film.
Conducting polymer films will be deposited by different techniques, to explore how film processing affects the doping/dedoping processes. We will use spin-coating and vapor phase polymerization. We expect to obtain films with different doping/dedoping characteristics, which will shed light on the role of film processing on the ability of the conducting film to accommodate ions. The conducting polymer of choice for films to be processed by spin-coating is PEDOT:PSS. PEDOT:PSS is used because of its high conductivity, biocompatibility, and chemical stability. VPP will yield films of PEDOT doped with p-toluenesulfonate (or tosylate, TOS). PEDOT:PSS and PEDOT:TOS are expected to lead to different operation mechanisms of operation in OECTs. In PEDOT:PSS the dopant anions (PSS-) are essentially immobile, since they are part of a polymer chain. Therefore the OECT current modulation is controlled mainly by incorporation of electrolyte cations. In PEDOT:TOS the dopant ions (TOS-) can be released during dedoping, therefore the dedoping process is a result of both cation incorporation and anion release.
Spin-coated PEDOT:PSS films will be obtained from commercial aqueous suspensions, which need to be mixed with an organic compound (secondary dopant) to increase film conductivity. After spin-coating, the films will be annealed to temperatures higher than 100 °C to remove the excess of water and solvents. Although the role of secondary dopants is still under debate, it has been ascertained that their presence is required to achieve high conductivity. Typically employed secondary dopants are ethylene glycol, dimethyl sulfoxide, sorbitol, glycerol and, recently, polyethylene glycol. To date, it is still unclear which one of these secondary dopants is best suited for OECTs. We will investigate various formulations, with the aim to elucidate the correlation between the conductivity and the effectiveness of the doping/dedoping process.
VPP of PEDOT will be performed as follows. A layer of oxidant solution (Fe(III) p-toluenesulfonate tosylate, TOS) and pyridine dissolved in isopropanol will be deposited on the substrate. After annealing to 80 °C, the substrate will be transferred to the VPP vacuum chamber and exposed to vapors of the EDOT momoner, which will polymerize when in contact with the oxidant. Finally the substrate will be rinsed in ethanol (to remove excess of oxidant) and dried for a few hours at 50°C. This process yields PEDOT doped with the counter ion TOS-.
