Fabrication and Use of Organic Electrochemical Transistors for Sensing of Metabolites in Aqueous Media

Abstract

Featured Application: Sensing organic molecules in water.Abstract: This review first recalls the basic functioning principles of organic electrochemical transistors (OECTs) then focuses on the transduction mechanisms applicable to OECTs. Materials constituting the active semiconducting part are reviewed, from the historical conducting polymers (polyaniline, polypyrrole) to the actual gold standard, poly-3,4-ethylenedioxythiophene: polystyrene sulfonic acid (PEDOT:PSS), as well as the methods used to fabricate these transistors. The review then focuses on applications of OECTs for the detection of small molecules and more particularly of metabolites, with a distinction between enzymatic and non-enzymatic transduction pathways. Finally, the few patents registered on the topic of OECT-based biosensors are reviewed, and new tracks of improvement are proposed. applied to biosensing quite early, associated with enzymes [4][5][6]. This is because, due to their functioning mechanism (already identified at the time), they brought into play the same charge carriers as most biological functions, i.e., electrons but also ions, and were therefore sensitive both to electron transfer to/from redox enzymes or ions (e.g., protons) produced from other types of enzymes. This is a characteristic which distinguished them from organic field-effect transistors (OFETs), which started to raise the attention of researchers exactly at the same period (precisely, 1983 for the first polyacetylene-based OFET [7]), and a few years later for applications to gas sensing (halogens, ammonia, oxygen [8-10] or even vapors of H 2 O, NO or H 2 O 2 [11][12][13][14]). However, OFETs never really produced any applicable results in the framework of biosensors in aqueous environment because of high operating voltages, of several tens of volts, which impede any measurements because of water electrolysis. Ion-sensitive organic field-effect transistors (ISOFETs), based on the same working principles of their inorganic counterparts (ISFETs) developed earlier [15,16], solved this problem of high operating potential, the electrolyte being not in direct contact with the semiconductor. However, these devices were confined mostly to protons detection [17][18][19] or, in any case, to charged species.As we show in this review, OECTs use both potentiometry and amperometry in their working principles. In that sense, OECTs are devices that represent a bridge between ISOFETs and classical amperometry, which certainly explains their success. This area of research is very active and several reviews have already been published on OECTs in general, for example by Kergoat et al. [1] or Rivnay et al. [3], or on OECTs applied to biology, for example by Liao and Yan [20], Liao et al. [21], Strakosas et al. [22] or . This present review, after going back to the basic functioning principles, along with details on transductions applicable to OECTs, reviews materials, fabrication techniques and then sensing applications. We focus on the detection of small molecules and more part...

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