## 4 Applications
### Enzymatic Biosensors  
#### 4.1.1 Glucose  
Glucose sensing and especially continuous glucose monitoring is of crucial importance. Indeed, for example, rather than having to manually control their blood glucose level several times a day, there is a specific need for diabetic patients to have a permanent and continuous automatic glucose monitoring device that would entail automatic insulin injection. Not only would such automatization considerably facilitate patients' life, but it would also prevent thousands of deaths each year. Even if full automation is still a dream for diabetics, continuous glucose monitoring is now available on the market. All OECTs dedicated to glucose monitoring use glucose oxidase as a specific recognition element. This \(D\)-glucose-specific enzyme oxidizes \(D\)-glucose in \(D\)-gluconolactone, producing H\({}_{2}\)O\({}_{2}\) at the same time; its redox center is concomitantly reduced and recycled using molecular oxygen present in the medium. \(D\)-glucose quantification is based either on the reduction of its own concentration in the solution under test or on the quantification of H\({}_{2}\)O\({}_{2}\) that appears as a result of its oxidation. Historically, the first enzymatic glucose biosensor was developed by [PERSON], using GOx and a conventional oxygen electrode [50]. As comprehensively reviewed in the article by [PERSON] and [PERSON] [123], the very first investigations of glucose sensors on devices which were close to OECTs, even if closer to conducttemetric devices because of the absence of gate as third contact, were reported by [PERSON] et al. in 1992 [124] and [PERSON] et al. in 1993 [125]. However, one can consider that [PERSON] et al. described the first OECT for glucose sensing based on PEDOT:PSS in 2004, with a three-terminal device comprising a platinum gate [89]. In the presence of GOx and glucose, gluconic acid and H\({}_{2}\)O\({}_{2}\) were produced; the latter was oxidized at the platinum gate electrode, which consequently reduced PEDOT\({}^{+}\) in the channel and produced a decrease of the drain current \(I_{D}\). Since this seminal work, most of the OECT architectures towards glucose sensing have been focused on the improvement of several parameters such as sensitivity, selectivity (principally towards ascorbic and uric acids), and lowering \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**PEDOT:PSS Source**} & \multirow{2}{*}{**Conductivity** **(S/cm)**} & **Surface Resistance** & **PEDOT:PSS Ratio (\%)** & **Solids Content** & \multirow{2}{*}{**References**} \\  & & **(f/s/sq)** & & & \\ \hline Baytron P A1 4083 & 0.001 & - & 1.6 & 1.3 to 1.7 & [66,70,72,74] \\ Brayton P (Clevios P) & 1 & - & 1.25 & 1.3 & [52,55,80,89,92,95] \\ Clevios PH500 & 5 & - & 1.25 & 1.2 & [34,90,94,97,98,103,104,121] \\ Clevios PH1000 & 10 & - & 1.25 & 1.2 & [57,73,76–79,82,83,91,105,107–109] \\ Clevios PJet NN2V & 200 & - & - & 0.6 to 1.2 & [1001] \\ Organcan EL-P 3040 & - & 680 & - & 3.4 & [93] \\ Organcan II1005 & - & 110 & - & 0.8 & [116] \\ Gwent C2100629D1 & - & - & - & - & [112] \\ Not applicable * & - & - & - & - & [53,54,67,71,75,81,84–86,96,99,100, 102,110,113–116,118–120] \\ \hline \hline \end{tabular}
* Not specified or not sufficiently described.  
\end{table} 
Table 2: Properties of available commercial PEDOT:PSS inks. the detection limit. Furthermore, obtaining a linear response within the standard human glucose concentrations range, whether in blood (4 to 7 mM) or sweat (5.6 mM to 2.2 mM), was also a target. If the previous example used GOx in solution, in almost all recent publications, GOx was immobilized within the device structure, preventing enzyme leakage and decrease of signal over time. For example, [PERSON] and coworkers developed a screen-printed OECT for both glucose and lactate sensing [111]. Rather than being present in solution, both the ferrocene mediator, which was covalently bound to chitosan, and the GOx were integrated into the sol-gel electrolyte which prevented their further deterioration during measurements.