## 4 Electrochemical bAA Sensors and Biosensors
### Enzymatic Approaches and b AA Electrochemical Biosensing  
Over the last few decades enzyme biosensors have been widely developed, and it is evident that they are innovative assays in qualitative, as well in quantitative analysis of numerous analytes ([PERSON] et al., 2018). Electrochemical enzyme biosensors have distinguished advantages, because they are highly sensitive and specific, portable, cheap, and can be miniaturized and in this way can be used in the point-of-care diagnostic, which make them alluring for clinical analysis and routine measurements ([PERSON] et al., 2018). Generally, a biosensor is a device that operates to analyze a sample in the presence of a specific target analyze. Customarily, a biosensor is manufactured from a biological unit, which is roared as molecular recognition element, and a detector based on physicochemical process or transducer ([PERSON] et al., 2018). The biological unit (recognition element) is accumulated on the transducer's surface, interacts with target analyte ([PERSON] et al., 2018). The variations are then noticed by the transducer, and transformed to measurable signals, used to detect the concentration of the target molecule ([PERSON] et al., 2018). Biosensors are divided depending on either the recognition element, such as nucleic acids, antibodies, enzymes and cells, or by the class of the transducer (optical, mass-based, electrochemical, piezoelectric) ([PERSON] et al., 2018). On the other hand, electrochemical biosensors rely on the electrochemical properties of transducers and substances to be analyzed. This kind of biosensors were established as a development of the first glucose enzyme biosensor ([PERSON] et al., 2018). Intrinsically, variation in physicochemical features of elements such as current, voltage, resistance, or superficial charge, risen by oxidation-reduction processes are the output signals. The most popular class of transducers are amperometry, potentiometry, conductor, and impedimetry. Therefore, a recent example of an enzyme-based biosensor is that [PERSON] et al. [PERSON] et al. (2018) developed. The proposed biosensor is claimed to be fast, simple, selective, and sensible. The proposedbiosensor was fabricated with the on-line coupling of millimeter size motors (m-motors) and chronoamperometry for real-time analysis of b AAs in MSUD samples. Thus, an integrated cell device, including reservoirs for motors movement and electrochemical determination, was constructed. L-amino acid oxidase was introduced in m-motors by capillarity without coating chemistry, which is necessary for the enantioselective identification, and afterwards the released H\({}_{2}\)O\({}_{2}\) was electrochemically screened with copper microwires. The results showed that the innate m-motor _\"self-micro mixing\"_ characteristics and the extended delivery of new and free enzyme had the advantage on the one hand of avoiding the agitation or the physical adsorption of the enzyme, as well as its chemical bonding on transducer's surface and on any other way it reduced the investigation time. [PERSON] et al. [82] proposed four amperometric diamond paste-based biosensors and then used in the detection of are for the enantiopurity of Leu [82]. Biosensors' layout used physical accumulation of L- and D-oxidases on the proposed electrodes. In this paper, the characteristics of the different proposed sensors were examined in contrast. The results showed that the sensors were linear in pmol/L to nmol/L level. It was also found that the proposed sensor was reliable in detecting the enantiopurity using Leu as a raw material. Moreover, [PERSON] and [PERSON] [83] reported the fabrication of an amperometric bienzyme biosensor based on screen-printed electrodes and used it in the detection of Leu. The proposed biosensor was constructed by immobilizing p-hydroxybenzoate hydroxylase (HBH) and Leu dehydrogenase (LDH) on the proposed electrode, using NADP\({}^{+}\) and p-hydroxybenzoate as cofactors.