## 1 Introduction
It has also been reported to show high sensitivity and long-term permanence. The S protein is known as the most frequently targeted protein after N protein [18]. Studies in plasma samples or serum antibodies from COVID-19 patients have shown that N-protein-specific antibodies identify SARS-CoV-2 infection earlier than S-protein-specific antibodies. [19]. N proteins are effective antigens for the detection of COVID-19 illness. [20]. It can cause cross-reactivity due to the presence of S proteins on the surface and the common virus in many coronavirus families such as influenza, Ebola, and Zika viruses [21]. The N protein is in the virus. It mutates much less, and the risk of cross-reactivity is eliminated. Therefore, it is more advantageous to work with the N protein. It is critical to discover SARS-CoV-2 as soon as possible. It is critical in preventing the spread of viruses. To identify the virus quickly and accurately, diagnostic procedures are necessary. The SARS-CoV-2 detection techniques are as follows: molecular tests, serological tests, protein tests, special nucleic acid tests, and care point tests [22,23]. Molecular testing is intended to discover the virus's genetics. Serological studies may identify antibodies in human blood and tissues but may not always indicate infection status [24]. The polymerase chain reaction (PCR) is an essential tool for detecting the existence of coronavirus [25]. The reverse transcriptase polymerase chain reaction (RT-PCR) is a technique for detecting the existence of nucleic-acid-based genome information from viruses and different organisms [26]. The disadvantages of these methods are the use of high-cost reagents, the use of expensive laboratory tools, the time spent in evaluating samples and producing results, and the risk of contamination. Biosensors have been developed to eliminate the negative points of diagnostic methods. Electrochemical biosensors are detection devices that use an electrochemical converter to turn chemical data into a signal, with advantages such as easy construction, tremendous responsiveness, inexpensive price, and high downsizing potential [27]. In addition, there are also systems that translate biological processes like antigen-antibody complexes into electrical impulses [28; 29]. The electrodes used in the sensors are important for the fixation of biomolecules [30]. Electrochemical biosensors include reduction-oxidation steps that occur depending on the desired target analyte concentration [31]. In addition, electrochemical sensors are of great importance, with high sensitivity and selectivity, low risk of error in virus measurement, reusability, and portability [24]. Our aim is to develop a sensor system by using a pencil graphite electrode due to their superior sensitivity, modifiable electroactive surface area, reproducibility, cost-effectiveness, and disposability [32]. The surfaces of these electrodes are decorated with the following biochemical agents, and the targeted COVID-19 N antigen can be recognized. The potentiometric electrochemical sensor principle was used in our experiment. The pencil graphite electrodes (PGEs), which we determined as sensors, were modified with hydroxyapatite (HAP) and copper oxide (CuO), and electrochemical measurements were taken using the cyclic voltammetry (CV) method with the help of a potentiostat device. It scans the CV oxidation and reduction steps and is used for pathogen detection [33]. Using the antibody-antigen interaction, PGEs modified with HAP + CuO were functionalized with N protein antibody by the following necessary procedures. Binding was provided between functionalized antibodies on the surface of PGEs and SARS-CoV-2 N proteins. Square wave voltammetry (SWV) was used to investigate the existence and electroactivity of the species in the electrochemical cell [33]. Square wave voltammetry (SWV) is a very sensitive electrochemical technique for detecting reversible redox species [34].