## 3 Materials and Fabrication Techniques
### Strategies for Enhancement of PEDOT:PSS Properties
#### 3.2.2 Enhancement of Adhesion Properties of PEDOT:PSS  
A major drawback in the use of PEDOT:PSS in biological sensors is its poor mechanical stability in long-term applications. Some studies were performed to increase the adhesion of PEDOT:PSS to the substrate to resist mechanical stress, notably immersion in aqueous media. Two pathways can be cited. The first one and probably the most used is the addition of a cross-linker such as (3-glycidyloxypropyl)trimethoxysilane (GOPS) or divinyl sulfone (DVS). In 2017, [PERSON] et al. [82] showed that the addition of a small amount of GOPS in a PEDOT:PSS solution (0.1 _v/v_ %) prevents the delamination of PEDOT:PSS on a glass substrate. The involved mechanism is a triple cross-linking: GOPS-GOPS, GOPS-PSS and GOPS-glass (Figure 8). Authors showed that PEDOT:PSS resists ultrasonication during more than one hour in comparison with the untreated deposition where PEDOT:PSS delaminates in less than 2 min.  
However, the use of GOPS as cross-linker decreases the conductivity of PEDOT:PSS due to the formation of a siloxane network. To prevent this effect, [PERSON] et al. [83] used DVS in combination with ethylene glycol to create an insoluble film at low temperature curing (50\({}^{\circ}\)C). Converse to GOPS, the amount of added DVS does not influence the conductivity of PEDOT:PSS. In addition, DVS cross-linked PEDOT:PSS films show perfect results in biocompatibility assays.  
To increase the adhesion of PEDOT to substrates, in particular for metallic or ITO substrates, another strategy was employed first by [PERSON] et al. [84] consisting in the chemisorption of EDOT-acid molecules on activated ITO surfaces, followed by electropolymerization of the EDOT moieties. After 2 min of ultrasonication, PEDOT stayed stable on the ITO surface. The same process was followed by [PERSON] et al. [85] in 2017, by electrografting EDOT-NH\({}_{2}\) followed by an electropolymerization of the EDOT moieties. Such PEDOT coatings turned to be stable after 1 h of ultrasonication compared to pure PEDOT which delaminated on ITO after a few seconds.  
To improve adhesion onto gold surfaces without using any cross-linker or adhesion layer, [PERSON] et al. [86] performed the deposition of PEDOT:PSS onto an iodine-etched gold surface, which creates a rough porous surface. Then, the electropolymerization of EDOT in presence of PSS allows the formation of the polymer into the pores leading to the formation of a PEDOT:PSS layer with  
Figure 8: Schematic illustration of the system of GOPS-crosslinked PEDOT:PSS. GOPS is only reacting with the parts of the PSS-chains (gray), not interacting with PEDOT (dark blue). The zoom in shows three main interactions involving GOPS; GOPS-PSS, GOPS-GOPS and GOPS-glass. **(A)** The chemical bonds between ASOH group of PSS and the epoxy ring of GOPS, as well as the chemical bond between three GOPS molecules. **(B)** The chemical bond between hydroxyl groups at the glass surface and the methoxysilane groups of GOPS. Reproduced from [82] with permission from John Wiley and Sons, Copyright © 20017.  
strong mechanical bond. Ultrasonication tests show a conservation of the PEDOT:PSS layer after more than 10 min.  
In the following paragraphs, the most common techniques employed for the fabrication of OECT-based biosensors are reviewed.