## 4 Electrochemical bAA Sensors and Biosensors
### Conducting Polymers and b AA Electrochemical (Bio)Sensing  
Conducting polymers (CP) are of interest for their use as sensitive electrode surface coatings on electrochemical sensors and biosensors (electrode surface modifiers) [85]. They are outlined by large electrical conductivity and satisfying electrochemical reversibility and therefore upholding their application on sensor transducer signaling. Furthermore, CPs can be chemically acquired functional groups, which act as \"tags\" because of their qualification to identify biological or chemical items [86; 87].  
Nevertheless, the determination of small (bioactive) analytes such as b AAs remains an open continues to be an unclosed issue, as their mere correlation with detectable groups on the CP substrate is not adequate to generate the necessary electrochemical change for their detection. The key in this case is to build high-specific CP recognition points, which will strengthen selectivity and advance the sensitivity of the nidification procedure. In connection with the specificity of the above process, molecularly imprinted polymers (MIPs) can be administered in the synthesis of polymers with predetermined molecular recognition features and can be used in constructing sensors and biosensors [88; 89]. Molecular imprinting is the innovation of these designs, as they enjoy plentiful improved traits: they are sensible, fast, simple and can be portable [90; 91].  
Meanwhile, cellulose nanocrystals polymers, usually are used to upgrade sensitivity and selectivity of b AAs detection and may be an alternative solution [92]. This strategy counts on a rod-like network of eminently crystalline fibers and owns a huge specific surface area, contributing valuable electrical and optical attributes. Hence, [PERSON] et al. [90] proposed an electrochemical sensor depended on 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanocrystals (TOCNCs). In this study, L-Cys modified Au electrode (TOCNC/L-Cys/Au) was formulated for determination and differentiation of the enantiomers of Phe, Leu, as well as Val. CV and DPV experiments showed that the constructed electrode had a peak current difference for the selected enantiomers. The proposed modifier was outlined by its various interactions with the different enantiomers, obtaining the identification of the enantiomers.  
On the other hand, microfluidic chips (MCs) provide many advantages in diagnostics due to theirs fast response, the use of small quantities of sample with good reproducibility, opening new boundaries on point-of-care diagnostics (POC). In addition, electrochemical transducers are convenient for these devices because of their inherent miniaturization and sensitivity. In this sense, conjugated MCs with electrochemical methods which are simple, rapid, and cost-effective, is an attractive approach for a POC device that can be used in metabolic diseases such as MSUD [30; 93].  
Based on these, an electrochemical microfluidic assay was practiced in the partition, as well as the determination of AAs enantiomers of D-Met and D-Leu by [PERSON] et al. [94]. The proposed device admitted the adjusted microfluidic D AA partition, as well as the particular reaction among D-amino acid oxidase (DAAO) and one by one AA on a sole arrangement of a MC. The proposed system was claimed to be consistent with small sample expenditure, averting the need for supplements for the partition of enantiomers, and the covalent accumulation of the enzyme on the wall channels or on the electrode surface. Hybrid polymer/graphene-based electrodes were end-channel capped to the microfluidic apparatus, advancing the traits of the procedure. D-Leu were fortuitously determined, adopting the introduced system. D-Leu was detected indirectly by the detection of H\({}_{2}\)O\({}_{2}\). The proposed method was claimed to have superlative precision in migration times and in peak heights, revealing the satisfying stability of the proposed sensor. It was found that the proposed sensor had also selectivity, due to the inactiveness of L AAs.