## 4 Applications
### Enzymatic Biosensors
#### 4.1.1 Glucose
Glucose was detected in a linear range from 23 mM to 2 mM, values almost compatible with the glucose concentration found in human sweat. The response time was within the minute range (a maximum of 350 seconds was observed). On the other hand, neither the stability during storage, nor the selectivity, especially in the presence of ascorbic acid, of this device was assessed. With the objective to improve the sensor's sensitivity, [PERSON] et al. described an OECT where they introduced graphene into the gate electrode (chitosan-graphene/GOx/Pt) [54]; a detection limit of 10 nM was achieved. In a previous work, [53] the same authors achieved one of the best detection limit described so far (5 nM) by modifying the gate electrode with GOx and nanomaterials such as PtNPs and multiwalled carbon nanotubes (MWCNTs), but their OECT suffered from a poor linearity, which is a major drawback for practical applications. In a more recent work, they solved this drawback and their OECT showed a linear response range for glucose from 10 nM to 1 mM. The linearity and selectivity of the device was enhanced with the addition of chitosan that helped the immobilization of GOx on the gate electrode and acted as an exclusion membrane limiting the glucose flux and excluding, at the same time, some interferents such as uric and L-ascorbic acids [126]. In 2015, [PERSON] and co-workers focused on the gate electrode nature, replacing the costly Pt gate electrode by cheaper TiO\({}_{2}\) nanotubes arrays (TNTAs) decorated by Pt nanoparticles [90]. Pt nanoparticles deposited onto TNTAs had previously proved themselves useful when employed for H\({}_{2}\)O\({}_{2}\) sensing; indeed, they were 10 times more sensitive compared to TNTAs alone [127]. This could be due to good electron-transfer behavior and large surface area [128]. The PtNPs were deposited on the TNTAs surface, then the Pt NPs/TNTAs electrodes were immersed into a GOx solution and then in a Nafion solution overnight (Figure 12a). Due to the TNTAs architecture, this protocol allowed to immobilize more GOx than on non-modified surfaces. The detection limit was 100 nM, and this OECT showed a linear response from 100 nM to 5 mM. Among other advantages, this OECT showed an excellent selectivity against both ascorbic and uric acids, and a good reproducibility and stability for at least ten days. This work showed that OECTs can be of interest for glucose measurements in real samples. Figure 12: (**a**) The schematic diagram of the device structure of PEDOT:PSS based OECTglucose sensor using Nafion/GOx/Pt-NPs/TNTAs as a gate electrode. Reprinted from [127]. Copyright 2014, with permission from Elsevier. (**b**) PGMA:PHEMA mixed polymer brush and GOx fabrication. Reproduced from [129] with permission from John Wiley and Sons, Copyright © 2015. In 2015, [PERSON] and co-workers described an OECT architecture where the GOx was covalently linked to the transistor via polymer brushes through the establishment of carbon-nitrogen bounds (Figure 12b) [129]. These covalent bonds were the results of the reaction between the primary amine of the GOx with the epoxy groups borne by the poly(-glycidyl methacrylate: poly (2-hydroxyethyl methacrylate) (PGMA:PHEMA) polymers. Those nanopolymer brushes were grafted on the plasma-activated PEDOT:PSS surface via gas phase functionalization which enabled to keep intact the conductivity of the OSC (>90% of its initial conductivity versus ca. 3% with a standard solution functionalization). The article described the optimal conditions for plasma activation of PEDOT:PSS in order to control its conductivity and the length of polymer brushes. These polymers are compatible with biological media and PHEMA prevents non-specific adsorption. The authors demonstrated the importance of covalent bonds between the GOx and the polymers, which brought stability with time: OECTs retained 100% of their signal after 100 days, compared to a few days for non-covalently bound GOx. In 2015, [PERSON] et al.