Fig. 3.

Experimental principles of label-based and label-free assays in the detection of viral DNA: (A) Schematic diagram of DNA hybridization sensor on gold films with enzymatic electrochemical detection of SARS-CoV gene sequence [42]. Adapted with permission from Ref. [42]; (B) Schematic diagram of DNA hybridization sensor using gold nanostructured screen-printed carbon electrodes for SARS-CoV virus detection [43]. Adapted with permission from Ref. [43]; (C) Schematic diagram on a screen-printed electrochemical biosensor for the detection of released purine bases of coronavirus aviair (i) immobilization of ssDNA probe onto the gold electrode, (ii) hybridization with the complementary target sequence, (iii) washing of the electrode surface, (iv) hydrolysis of dsDNA, (v) electrochemical signal detection [49]. Adapted with permission from Ref. [49]; (D) Schematic diagram on a label-free electrochemical genosensor for COVID-19 diagnosis [50]. Reproduced with permission from Ref. [50].