## 2 Organic Electrochemical Transistors (OECTs)
The most commonly used electrolytes include aqueous, organic, gel-based, biomolecule-based, and polymer electrolytes, each with specific characteristics and applications. Aqueous electrolytes are very common in OECTs due to their high ionic conductivity and easy handling. Inorganic salts such as sodium chloride (NaCl) and potassium chloride (KCl) are popular choices as they provide high ionic mobility without negatively reacting with most organic conducting materials. Additionally, strong acids and bases, such as sulfuric acid (H\({}_{2}\)SO\({}_{4}\)) and sodium hydroxide (NaOH), are employed in situations that require very specific chemical environments, such as those with extremely acidic or basic pH [15]. On the other hand, organic electrolytes include quaternary ammonium salts, such as tetraethylammonium perchlorate (TEAP) and tetrabutylammonium perchlorate (TBAP). These salts are valued for their good solubility in organic solvents and their ability to interact effectively with organic conductive materials. Moreover, ionic liquids, such as bis(trifluoromethylsulfonyl)imide of ethyl-methylimidazolium (EMIM-TFSI), are widely used due to their high ionic conductivity and both thermal and electrochemical stability, making them ideal for applications in more advanced devices. Gel-based electrolytes, such as those based on polymers, are another common option in OECTs. Gels like those of polyacrylamide, polyvinyl alcohol (PVA) with phosphoric acid, or those based on polyethylene glycol (PEG) offer good mechanical stability along with adequate ionic conductivity, in addition to being easy to process in device fabrication. Figure 3: Representative transfer curve and transconductance (gm) of an OECT in depletion-mode operation. The black curve represents the transfer characteristics (indicated by sphere markers), showing an “on” state at V\({}_{\text{C}}\) = \(-0.2\) to 0 V and an “off” state at V\({}_{\text{G}}\) = \(+0.5\) to 0.6 V. The blue curve shows the transconductance (indicated by square markers), with peak gm occurring at \(+0.2\) V. Experimental conditions include V\({}_{\text{DS}}\) = \(-0.4\) V and device dimensions of L = 1 \(\upmu\)m and t = 100 nm [15]. The combination of ionic liquids with polymers can form gels that offer the best of both worlds: the high conductivity and stability of ionic liquids and the ease of handling and shaping of polymers. In bioelectronic applications, biomolecule-based electrolytes are crucial. Biocompatible electrolytes, such as sodium chloride solutions in distilled water or phosphate buffer solutions (PBS), are used to replicate biological conditions and ensure compatibility with living tissues, which is essential in devices designed to interact with biological systems or medical implants. Finally, polymer electrolytes, such as polyelectrolytes, are also used in OECTs. Materials like Nafion or polystyrene sulfonate (PSS) offer an attractive combination of ionic conductivity and mechanical stability, making them preferred choices in devices that require long-term stability. When choosing an electrolyte for OECTs, it is important to consider compatibility with the transistor's active material, such as PEDOT, as well as ionic conductivity, electrochemical stability, and, in the case of medical or biological applications, biocompatibility and the absence of toxicity. Each type of electrolyte has its own advantages and challenges, so the appropriate selection depends on the specifications and requirements of the device in question [15]. Over the past few decades, there has been a growing interest in OECT-based sensors, primarily driven by the increasing demand for high-performance chemical and biological sensing across various domains. These applications span from food safety assessments to agricultural and environmental monitoring, as well as medical, healthcare, and security sectors [27]. Understanding the fundamental distinction between chemical and biological sensors is beneficial.