## 1 Introduction  
Organic electrochemical transistors (OECTs) have been widely reported in the literature [1; 2], even very recently [3]. From the very first example of OECT (described in 1984) to the devices produced today, progress has been considerable. In 1984, experiments were carried out using chips consisting of three Au electrodes deposited on the SiO\({}_{2}\) layer of a Si wafer. Electrodes were 3 \(\upmu\)m wide \(\times\) 140 \(\upmu\)m long, with a distance between electrodes of 1.4 \(\upmu\)m. The organic semiconducting layer was a 0.12 \(\upmu\)m thick polypyrrole layer made by in situ electropolymerization of the monomer. The _on/off_ switch time was ca. 10 s (operating frequency of about 0.1 Hz), the _on/off_ ratio was around 3000 and the maximum transconductance of about 0.1 mS at V\({}_{\text{GS}}\) = 0.1 V. For a contemporary OECT of similar dimensions, typically fabricated by means of printing technologies using poly(3,4-ethylene-dioxythiophene:polystyrene sulfonate (PEDOT:PSS) and having similar thickness, the transconductance is about 150 times higher and the maximum operating frequency reaches 10\({}^{3}\) Hz, i.e., 4 orders of magnitude higher. It is remarkable to note that, in the 1984 article, the authors wrote that \"[their device] has no immediate practical application\". However, they also wrote that \"There is presently considerable interest in interfacing microelectronic devices with chemical and biological systems for sensor applications [...]\". It was a premonition. Indeed, OECTs have been 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; 9; 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 [PERSON] et al. [1] or [PERSON] et al. [3], or on OECTs applied to biology, for example by [PERSON] and [PERSON] [20], [PERSON] et al. [21], [PERSON] et al. [22] or [PERSON] et al. [23]. 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 particularly of metabolites, with a distinction between enzymatic and non-enzymatic devices. Finally, the few patents registered which deal with OECT-based biosensors are reviewed. From the discussed key points, the conclusion identifies new tracks of improvement.