Interdigitated organic electrochemical transistors for biosensing

The development of transistors with high gain is crucial for applications where extracellular potentials of electrogenic cells or chemical signals are recorded. Organic electrochemical transistors (OECTs) have recently emerged as versatile electrophysiological sensors due to their distinct advantages compared to other sensing systems, including direct communication via ions between the conducting polymer channel and electrolyte, biocompatibility, transparent channel material, and high transconductance. The transconductance of OECTs critically depends on the width-to-length ratio of the drain-source channel and the thickness of organic conducting polymer. Conventional OECTs require large channel dimensions and thick polymer film, which results in slow operation. Therefore, in this thesis, interdigitated OECTs (iOECTs) were developed to obtain high transistor performance and high device density and were utilized for biological and chemical sensing.First, the effect of channel geometry on the electrical performance of interdigitated OECTs (iOECTs) was investigated. A superior device performance is achieved by systematically optimizing the electrode layout regarding channel length, number of electrode fingers, and electrode width. Interestingly, the maximum transconductance (gmax) does not straightforwardly scale with the channel width-to-length ratio, which is different from planar OECTs. This deviation is caused by the dominating influence of the source-drain series resistance Rsd for short channel devices. Noteworthy, there is a critical channel length (15 µm), above which, the channel resistance Rch becomes dominant and the device characteristics converge towards those of planar OECTs. Design rules for engineering the performance of iOECTs are proposed and tested by recording action potentials of cardiomyocyte-like HL-1 cells with high signal-to-noise ratio. The above results demonstrate that interdigitated OECTs meet two requirements of bioelectronic applications, namely high device performance and small channel dimensions. Flexible and transparent electronic devices possess crucial advantages over conventional silicon based systems for bioelectronic applications since they are able to adapt to non-planar surfaces, cause less chronic immunoreactivity, and facilitate easy optical inspection. Therefore, the iOECTs were embedded in a flexible matrix of polyimide to record cardiac action potentials. The wafer-scale devices were firstly fabricated. Considering the transfer characteristics of iOECTs, the flexible device exhibits transconductances (12 mS/V) and drain-source on/off ratio (~105) comparable to state of the art non-flexible and superior to other reported flexible OECTs. The transfer characteristics of the device are preserved even after experiencing extremely high bending strain and harsh crumpling. The excellent device performance is proved by mapping the propagation of cardiac action potentials with high signal-to-noise ratio. These results demonstrate that the electrical performance of flexible OECTs can compete with hard-material-based OECTs and thus potentially be used for in vivo application. IOECTs do not only have great potential for recording of electrophysiological signals but also for chemical sensing. Conventional electrochemical aptamer receptor/transducer systems for the detection of various kinds of targets are often limited by the low density of receptors attached to the sensor surface and high background signals. An iOECTs-based aptasensor is used as a transducer to detect the light molecule (ATP) and compared with a conventional amperometric aptasensor regarding sensitivity and dynamic detection range. When operated as a sensor, the OECT senses modulations of the gate electrode potential induced by binding of the analyte to the aptamer receptor. Even minor changes of the gate potential cause a pronounced response of the channel current due to the amplification characteristics of the OECT. This novel aptasensor can selectively detect ATP with ultrahigh sensitivity down to the concentration of 10 pM, which is several orders of magnitude lower than the detection limit of the same aptasensor using an amperometric transducer principle (limit-of-detection of 93 nM) and even most of other previously reported electrochemical sensors. Furthermore, sensor regeneration is demonstrated, which facilitates reusability of the aptasensor. The small device size in combination with high transconductances paves the way for the development of highly sensitive integrated micro-biosensors for point-of-care applications