## 4 Applications
### Enzymatic Biosensors
#### 4.1.2 Lactate  
The detection of lactate is of great interest for monitoring several biological mechanisms. Lactate is a relevant indicator of the physiological response to exercise and is a marker in diseases such as diabetes and cancer. The concentration range in sweat is comprised between 9 and 23 mM [131].  
[PERSON] et al. [97] fabricated an OECT with a ionogel solid-state electrolyte incorporating poly(\(N\)-isopropylacrylamide-co-\(N\),\(N^{\prime}\)-methylene-bis(acrylamide)), an ionic liquid (1-ethyl-3-methylimidazolium ethylsulfate) (the ionic liquid was assumed to bring a high conductivity), a ferrocene redox mediator, and an immobilized lactate oxidase. The detection of lactate in a relevant physiological range was achieved from 10 to 100 mM. In another study, [PERSON] et al. [91] reported the ex vivo sensing of lactate levels produced from normal cells and malignant cells culture with a limit of detection of 10 \(\upmu\)M. The biosensor consisted of two differently biofunctionalized OECTs integrated into a Wheatstone bridge. Lactate oxidase was immobilized into the gate electrode of the sensing transistor while a nonspecific protein (bovine serum albumin) was used for the reference OECT ensuring an inherent background subtraction and reducing interferences from the complex cell culture media. [PERSON] et al. [132] also developed a multiplexed OECT platform integrated within a portable \"finger- powered\" microfluidic device for the simultaneous detection of three clinically relevant biomarkers in human saliva, with high sensitivity and selectivity. The gate electrode biofunctionalization of each transistor in the platform was performed with a blend of PEDOT:PSS:PVA polymers and by the immobilization of either glucose oxidase, lactate oxidase (LOx), or cholesterol oxidase (ChOx) for the respective detection of glucose, lactate, or cholesterol. The enzyme has been integrated into a chitosan supporting matrix in the presence of ferrocene redox as a mediator, thus lowering the working potential of the electrodes, and reducing background signal. The OECT array exhibits limits of quantification (LOQ) of 10 \(\upmu\)m for glucose, 50 \(\upmu\)m for lactate, and 10 \(\upmu\)m for cholesterol, corresponding to the physiological ranges in saliva.  
[PERSON] et al. [111] reported screen-printed biosensors on a flexible PEN substrate for glucose and lactate sensing in human sweat. The detection range of the printed sensors extended from 0.1 to 2.3 mM, i.e., the low end of the level range of lactate in sweat, the high end of the range only being accessible by dilution. The modification of the gate electrode of an OECT with poly(\(N\)-vinyl-2-pyrrolidone)-capped platinum nanoparticles (PtNPs) was reported to enhance the sensitivity and to improve the detection limit of glucose and lactate down to 10\({}^{-7}\) and 10\({}^{-6}\) M, respectively [133]. The authors also developed a multisensing device by integrated two single OECTs with two separate microfluidic channels in one chip to reduce the volume of analyte and increase the detection speed. [PERSON] et al. also took advantage of the catalytic efficiency of platinum-modified gate electrode [106]. A negatively charged polymeric hydrogel encompassing the enzyme and covalently bound on top of the gate was reported to lower the diffusion of the glucose and lactate analytes thus extending the linearity ranges, and to block negatively charged interfering species from the complex media, i.e., in vitro live cells in the authors' study.