Chitosan is popular for electrode modifications due to its available amino groups, good adhesion and biocompatibility . For example, Soares et al. have reported a low-cost impedimetric biosensor for detection of spike protein of SARS-CoV-2 virus based on carboxymethyl chitosan which was used as a matrix for the antibodies immobilisation via carbodiimide chemistry. The developed biosensor has a limit of detection of 0.179 fg/mL for SARS-CoV-2 spike protein with high selectivity. As with o-ABA, a porous film is formed allowing reagents to reach the electrode surface, though the electrochemical response is reduced compared to the bare electrode. Non-conducting polymers are limited to thin layers to ensure it is sufficient permeable to the mediator molecule. Often conductive nanostructures are incorporated into the chitosan matrix to reduce the insulative behaviour of the coating. Luo et al. demonstrated that an electrode coated with a chitosan film containing multi-wall carbon nanotubes (MWCNT) exhibited a larger current response in 5 mM Fe(CN) 6 3-/4-than when the electrode was modified with a chitosan film without the MWCNT. Similarly, Zang et al. used a chitosan layer containing single-wall carbon nanotubes (SWCNT) to immobilise anti-aflatoxin B 1 onto the electrode surface, as chitosan enhanced the dispersion of the SWCNTs. Other groups have adopted gold nanoparticles (AuNPs), reduced graphene oxide (rGO), magnetic nanoparticles 100 and other nanoparticles 101 in conjunction with chitosan for biosensors. Electrode modification with nanostructures is discussed further in Section 4.3.