**4. Biorecognition Elements**

Biorecognition elements (Figure 1) are the most critical part of the electrochemical biosensor, shouldering specific pollutants' specific recognition in a complex matrix. The effective immobilization of biorecognition elements, including enzymes, antibodies, DNA/RNA, aptamers, and whole cells, facilitates their binding to a noticeably broad range of the target species or analyte of interest. As is shown in Table 2, different kinds of recognition elements were summarized, as well as the analytes, electrodes, type of transducers, and the response of various electrochemical biosensors. Apart from conserving the functionality of the bio-recognition elements, e.g., specific enzymatic activities, it is critical to ensure the biomaterials' accessibility to target analytes. The vicinity between the biomaterials and the solid or metallic electrode surface is also preferred for achieving a fast and effective electron transfer. Several techniques have been suggested, including physical (e.g., electrostatic adsorption), chemical (e.g., self-assembled monolayers, covalent bonding, avidin-biotin binding, hybridization), and electrochemical (e.g., electrochemical adsorption) methods, but the optimal configuration for the biorecognition elements depends on the biomaterial and the modified electrode materials and interface. Besides the immobilization strategies, nonspecific adsorptions that mostly lead to high baseline signals and delayed responses should also be considered. Generally, various highly hydrophilic composites, for instance, poly (ethylene glycol) (PEG) and bovine serum albumin (BSA) and can be considered as additional elements for eliminating the nonspecific binding sites at the electrode and solution interface.

**Figure 1.** (**a**,**b**) Amperometric response of B.P.A. at XOD/GCE in the presence of 0.3 mM hypoxanthine (Reproduced with the permission from [53]); (**c**) Schematic of magnetic beads (MBs) for the analyte and its capturing technique on the electrode surface (Reproduced with the permission from [54]); (**d**) The complete schematic diagram of the nanomaterial-based immunosensor based on ELISA indirect competitive format (Reproduced with the permission from [55]); (**e**) Effect of various blocking agents on background reading by eight percent skimmed milk, one percent BSA, casein, protein-free, and superblock (Reproduced with the permission from [56]); (**f**) Schematic representation, SEM and EIS responses of the fabricated aptasensor (Reproduced with the permission from [57]).
