Background and purpose: This study aimed to explore the modification of screen-printed carbon electrode (SPCE) to produce an extensive conductive surface with gold nanoparticles (AuNPs) for the detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) ribonucleic acid (RNA).

Experimental approach: The experiment was carried out using drop casting (DC) and spray coating (SC) methods. Au-S covalent interactions were formed between thiolated single-stranded DNA (ssDNA) and Au surface, which further hybridized with the target RNA to be detected using differential pulse voltammetry (DPV). Optimization of experimental conditions was performed using Box-Behnken design (BBD) on probe ssDNA concentration, probe ssDNA immobilization time, and target hybridization time. The morphology of the modified electrode was characterized using a scanning electron microscope, while the electrochemical behaviour was determined with DPV and electron impedance spectroscopy.

Key results: The results showed that SPCE modification with AuNPs by DC produced a higher peak current height of 12.267 μA with an R _ct value of 2.534 kΩ, while SC improved the distribution of AuNPs in the electrode surface. The optimum experimental conditions obtained using BBD were 0.5 μg mL^-1 ssDNA-probe concentration, an immobilization time of 22 minutes, and a hybridization time of 12 minutes. The limit of SARS-CoV-2 RNA detection at a concentration range of 0.5 to 10 μg mL^-1 was 0.1664 and 0.694 μg mL^-1 for DC and SC, respectively. The T-test results for both methods show that the current response of target RNA with SPCE/AuNP by DC does not show the same result, indicating a significant difference in the current response between those two methods.

Conclusion: SPCE/AuNP by DC is better than SPCE/AuNP by SC for immobilizing inosine-substituted ssDNA, which subsequently hybridizes with viral RNA, enabling label-free detection of guanine from SARS-CoV-2 RNA.