## 4 Electrochemical bAA Sensors and Biosensors
### Metal Nanoparticles and b AA Electrochemical (Bio)Sensing  
The limited use of electrochemical sensors in the detection of b AA could be explained by the fact that these molecules are electrochemically inactive on bare electrodes. This limitation is can be solved by the modification of electrode's surface using metal nanoparticles. On this ground, iron oxides [58; 59] CoNPs [60], strontium nanorods [61] as well as multiwall carbon nanotubes (MWCNTs) [62] have been used on the detection of some branched-chain amino acids. Meanwhile, metal nanoparticles (NPs), such as silver and gold, have exceptional attributes, such as biocompatibility, high conductivity and high surface-to-volume ratio [63]. Thus, they are particularly alluring components for applications in electrochemical sensing and biosensing [64; 65; 66; 67], as they are widely used in shaping the surface of electrodes in order to develop methodologies for detecting species of biological interest or to make diagnostic tools for various pathological conditions. Compared to stabilizers such as plant leaves [68], fruit extracts [69], plant roots [70], glucose [71] and carbonates materials [72], used in their composition, organic and natural dyes impart to the formed nanoparticles improved properties [73; 74; 75]. In addition, organic dyes have an advantage over the above factors because they have specific ionic, polar, non-bond functional groups (-azo dyes, -sulfites, -hydroxyl, and -nitro groups) and are usually systems that have \(\pi\)-conjugates [76] capable of being polymerized. For example, [PERSON] et al. [59] used the magnetic (Fe\({}_{2}\)O\({}_{3}\)) mobile crystalline material-41 (MCM-41) to modify the surface of glassy carbon electrode (GCE). They found that the proposed electrode owned electrocatalytic activity against the electro-oxidation of the studied amino acids. In this work the amino acids at larger concentrations were determined by CV, hydrodynamic amperometry, and flow injection analysis. The linear range of the proposed method was in the range of 97-176 nM and the detection limit was found to be equal to 94 nM in the case of Val. The proposed sensor was shown to have rapid response, great catalytic activity, and ease of preparation. Furthermore, [PERSON] et al. [60] immobilized a Fe (III)-Schiff base on a modified GCE with multiwall carbon nanotubes (MWCNTs). They discovered that the proposed electrodes had great catalytic activity against the oxidation of amino acids at positive potential in acidic solution. The outcomes gave confirmation that these electrodes postured innate stability at a wide pH range, agile response, great sensitivity, low detection limit and a very positive oxidation potential of amino acids that declined the influence of interferences of the detection method. The linear concentration range of Val, the detection limits of Val (LOD), the limits quantization of Val (LOQ) and relative standard deviation of the above-mentioned sensor were found to be 25-1000 \(\upmu\)M, 1.67 \(\upmu\)M, 2.79-27.14 and 2.82%, respectively. Meanwhile, cobalt hydroxide nanoparticles were practiced on the modification of a GCE (CHM-GC) and were employed on the investigation of the electrochemical behavior of some amino acids by [PERSON] et al. [61]. CV, chronoamperometry methods, and steady-state polarization measurements were used on the investigation of the oxidation and its kinetics. The results exposed that cobalt hydroxide sponsors the rate of oxidation by reinforcing the peak current, so the particular bimolecular reactions are oxidized at smaller potentials. CVs and chronoamperometry revealed a catalytic EC mechanism to be employable with the electrogeneration of Co(IV). Strontium oxide nanorods (SrO NR) is another example of metal nanoparticle that can be used in the detection assays of b AAs. Thus, [PERSON] et al. [62] synthesized SrO NR in alkaline medium by a wet-chemical method. Their results showed that a thin-layer of the NR was accumulated on a GCE, fabricating an electrochemical sensor for L-Leu.