**5. Phages immobilization strategies**

It's obvious from the literature that different approaches have been developed for immobilization of phages on surface of electrodes **Figure 3**. The common phage immobilization strategies on solid surfaces include physical adsorption [44], covalent bonding [45], entrapment of phages in solid matrix [46], etc.

The quantity of randomly oriented phages on solid surfaces is the most straightforward way for enhancing signal in bio-sensing scheme [47]. Deposition

#### **Figure 3.**

*Different ways to potentially orient phages on solid surfaces. Green highlighted-bacterial binding proteins, (a) tailed phages-side-ways, head-down, or tail-down, (b) asymmetric icosahedral phages, (c) filamentous phages-through either side-ways or, pole, (d) filamentous phages are likely to be bundled or aggregated (left). Oriented typically parallel on the substrate (right), adapted from [5].*

of high number of phage particles creates a steric interruption between phage particles [39]. Thus number of phage particles immobilized on solid surface should not surpass a specific threshold per surface area [48]. For T4 phage, estimated optical density was 19 phages/ mm<sup>2</sup> area beyond that clogging was happened, resulting in reduced signal [49]. Phage particles can be simply oriented on the surface of electrode as they bear positive and negative potential on their tail fibers and head respectively. Phage immobilization strategies are briefly highlighted in the following context.
