Organic materials like conductive polymers (CPs) have become one of the most popular choices for bioelectronic devices especially when it comes to engineer cell-chip interfaces. In addition to their low Young’s modulus (20 kPa-3 GPa), CPs exhibit a peculiar conduction mechanism based on ionic-to-electronic current transduction, as such materials have the intrinsic ability to convert an ion flow to different electronic conduction states, unlike inorganic materials which are not permeable to ions. Among CPs, we mainly use poly (3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS), which has been considered a promising candidate as a printable electrode material due to its mechanical flexibility, solution process-ability, and biocompatibility. In particular, PEDOT:PSS is used as channel material in organic electrochemical transistors (OECTs), three-terminal devices whose electrical operation depends on the injection of ions from an electrolyte into the bulk of an organic semiconductor channel. These CP-based OECTs found extensive application as biosensors to continuously monitor biological processes at the interface.
Organic electrochemical transistors
Organic electrochemical transistors (OECTs) were firstly reported by White in 1984. These devices share a similar three terminal device configuration with conventional field effect transistors (FETs) comprising source, drain and gate electrode but using a conducting polymer (PEDOT/PSS) as channel material. These polymer transistors facilitate high transconductances at low voltage operation which is critical for the amplification of weak electrophysilogical signals in in vitro and in vivo detection settings. Different from other FETs, OECTs feature mixed electronic and ionic conductivity enabling ion‐to‐electron conversion means translating the ion based communication of cells into electronic signals. Besides, these polymer devices possess a high biocompatibility, mechanical flexibility, and thin architecture but they are also optically transparent, which is benifical for in vitro and in-vivo applications.
OECTs operated as biosensor
Typically, OECTs are operated in an liquid gate configuration where potential changes in the Helmholtz layer are registered via alterations of the source-drain current. Interestingly, the device is similarly sensitive to variations in the Helmholtz layer of both the gate electrode as well as the polymer channel – electrolyte interface. Thus, complicated modification procedures of the PEDOT/PSS polymer by receptor molecules can be avoided. Instead, the receptor units, such as ssDNA aptamers, can be attached to a gold gate electrode similar to the fabrication procedures of conventional amperometric aptasensors. These highly charged polynucleotide receptors bind specifically their corresponding target molecules resulting in alterations of the composition of the electrochemical double layer which are sensitively registered by the OECT as a variations of the source-drain current. Interestingly, the attachment of the aptamer receptors on the gold gate electrode permits a direct comparison of the performance of the amperometric and the OECT transducer of the same sensor electrode. In our studies we found that the OECT transducer outperforms the amperometric detection in terms of detection limit and detection range for the ATP and dopamine sensing due to the amplification capabilities and the high transconductance of OECTs.
Furthermore, we exploit the neuromorphic properties of PEDOT:PSS-based OECTs to engineer in-vitro biohybrid synapses where neurotransmitters released from biological cells can modulate the conductive state of the device on the short and long-time scale, emulating synaptic plasticity of neurons. Additionally, we work on combining light-responsive materials, as azobenzene-based polymers with CPs to engineer OECTs where the synaptic conditioning can be achieved with a double modulation (photo and electrical).
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Tuning Channel Architecture of Interdigitated Organic Electrochemical Transistors for Recording the Action Potentials of Electrogenic Cells. Liang, Y. et al., Advanced Functional Materials, 29 (2019), 1902085
Amplification of aptamer sensor signals by four orders of magnitude via interdigitated organic electrochemical transistors. Liang, Y. et al., Biosensors and Bioelectronics 144 (2019) 111668
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