The selective detection of dopamine (DA) is currently a subject of significant interest. Its real-time determination is of great importance in the diagnosis of neurological disorders, such as Parkinson’s disease. DA concentration is very low in biological samples and the common chemical sensors do not exhibit an adequate sensitivity for in vivo applications. A very interesting device, that has recently attracted large attention for its high sensitivity and low limit of detection, is the organic electrochemical transistor (OECT). Thanks to the intrinsic signal amplification due to transistor configuration, OECT could be a tool for a reliable detection of dopamine in biological fluids. Nevertheless, its widespread use in real-life applications is hindered by the lack of selectivity due to some interfering compounds like uric and ascorbic acids. Potentiodynamic techniques are commonly employed to separate the redox waves associated to different analytes, for amperometric sensors, but this approach had not yet been explored for OECT sensing. This work explores a new approach to selectively identify and determine the contributions of different analytes to the OECT electrical output signal through a linear scan of the gate potential. We have used OECTs entirely made of PEDOT:PSS (both conductive channel and gate) in order to take advantage of the peculiar electrochemical properties of the conducting polymer. We assessed how OECTs can profit from PEDOT:PSS electrochemical features by selectively detecting the electro-oxidation of three different analytes (ascorbic acid, uric acid and DA) present in the same solution, as it occurs at different gate potentials. The signal related to each one of the three analytes can be individually detected and resolved by recording the trans-conductance, obtaining a linear response for all the analytes. The here reported results also demonstrate that all-PEDOT:PSS OECTs sensitivities and limits of detection are comparable or even higher than sensitivities of the differential pulse voltammetry (DPV), a technique that employs a sophisticate potential wave and read-out system and that can hardly be considered a viable readout method in practical applications. An increase of the selectivity can be alternatively obtained through a proper chemical functionalization of the PEDOT gate electrode. In particular our research is addressed toward the covalent immobilization of Ferrocene-terminated alkyl chains of different lengths in the PEDOT film via “click chemistry” (i.e. copper-catalyzed azide–alkyne cycloaddition). Our results show that the presence of ferrocene redox mediator on the gate electrode allows to eliminate the interference of ascorbic acid in DA detection.
The here described results offer new perspectives on a simple tool that greatly improves the performance of all-PEDOT:PSS OECTs, demonstrating their selectivity and, thus, the possibility to employ them as bioanalytical sensors in real-life applications.