## 4 Applications
### Enzyme-Free Biosensors
#### 4.2.2 Dopamine  
Dopamine is one the most important neurotransmitters in biological organisms [140]. The selective sensing of dopamine is of great significance in the diagnosis of neurological diseases such as Parkinson's disease, autism, and schizophrenia. However, the sensitivity and selectivity of the electrochemical detection of dopamine suffer from the low clinical concentration in human body fluids (in the range of micromolar for urine and nanomolar for plasma) and the presence of endogenous interferences such as uric acid and ascorbic acid.  
[PERSON] et al. have reported OECTs showing high sensitivity within a wide linear range from 5 nm to 1 uM and a LOD down to 5 nM [100]. The selectivity was improved by the modification of the Pt gate with biocompatible polymers such as Nafion and chitosan. Since Nafion and chitosan (pK\({}_{\text{a}}\) = 6.4) are negatively charged in PBS (pH 7.4), the diffusion of anionic electroactive compounds such as AA (pKa = 4.2) and UA (pKa = 5.2) to the gate electrode is significantly hindered. The electrostatic repulsion between the interference substance and the film gives rise to an improved selectivity towards dopamine. The introduction of graphene-based materials leads to enhanced electrochemical reactivity and enlarged active surface due to \(\pi\)-\(\pi\) interactions between the 2D planar hexagonal structure of graphene and the phenyl group of dopamine, facilitating the electron transfer and the oxidation of dopamine (Figure 14).  
The influence of different gate electrode materials (graphite, Au, Pt electrodes and modified graphite or Pt electrodes with MWCNT-chitosan hybrid) and the operating voltage on the performances of OECTs as dopamine sensors have been investigated. [PERSON] et al. [96] reported the highest sensitivity with a Pt gate electrode and a gate voltage of 0.6 V. The limit of detection was lower than 5 nM with a broad linear region from 50 nM to 3 uM.  
[PERSON] et al. used the OECT working principle to amplify the fast scan cyclic voltammetry (FSCV) signals to measure dopamine concentrations in the 10 uM range [141]. Fast scan cyclic voltammetry (FSCV) is an electrochemical technique commonly used in neurosciences for spatiotemporal measurement of the concentration of dopamine and other electroactive species [142]. FSCV involves a very high scan rate (50 to 400 V s\({}^{-1}\)) and a wide bandwidth for low currents and is generally executed inside a Faraday cage. The on-site amplification of the OECT allows the reduction of the electrical noise and enables FSCV measurements in an unshielded environment. A similar motivation guided [PERSON] et al., who described an all-PEDOT:PSS OECT dopamine sensor  
Figure 14: Architecture of the OECT-based dopamine sensor, with electrolyte, gate ad PEDOT:PSS channel (**left**). Detail of the gate structure, with Nafion (or chitosan) and graphene (rGO) flakes immobilized on a Pt electrode. The electro-oxidation of dopamine occurs on the surface of the gate (**right**). Reproduced with permission from [100]. © Materials Research Society 2013.  
displaying a limit of detection of 6 \(\upmu\)M in the presence of interference compounds (AA and UA) [105]. The authors took advantage of a potentiodynamic technique to separate the redox waves associated to different analytes and therefore to selectively identify and determine the contributions of different analytes to the OECT electrical output signal through a linear scan of the gate potential. The sensitivities and the LOD were comparable or even higher than those obtained by differential pulse voltammetry (DPV). Similarly, [PERSON] et al. [143] demonstrated that alternative current (AC) measurements of OECT-based dopamine biosensors exhibit several advantages compared to direct current DC methods such as faster response time (up to 10\({}^{-5}\) s), the ability to collect more information (both transconductance amplitude and phase of channel currents), more stable current and lower LOD down to 1 nM by introducing a lock-in amplifier (Figure 15).