## 2 Materials and Methods
### Modification of Electrodes  
Modification of PGEs with HAP was performed by coating calcium phosphate on the surface. One of the most important properties of calcium phosphate (CaP) is its ability to exchange ions. It also increases the surface area. Ion exchange is provided by the high surface area of calcium phosphate crystals and by loose bonds on the crystal surface. In many studies, by using these properties of HAP, metal ions that are divalent such as Cu\({}^{+2}\), Cd\({}^{+2}\), Zn\({}^{+2}\), Al\({}^{+2}\), and Ni\({}^{+2}\) were studied, and successful results were obtained [35]. It has recently been reported to have an affinity for metal ions, such as Cu\({}^{+2}\) and calcium phosphate salts, which are very often needed in heavy metal removal. With CaP compounds, adsorption of heavy metals deteriorates the surface of the CaP, and the metal ions that bind pass into the CaP from a very dense environment to a less dense environment. The Ca\({}^{+2}\) ions in CaP are replaced by other metal ions and reintegrated into CaP [35]. Based on the literature we obtained, it was thought that using HAP would increase the surface area of our PGEs. The 1 cm area of the pencil tip ends was immersed into the 0.12 M CaCl\({}_{2}\) solution for 5 min. Then, surface coating process was advanced by immersion in 0.06 M (NH\({}_{4}\))\({}_{2}\)HPO\({}_{4}\) solution for 5 min. By this dip method, the surface area of PGEs was increased by coating with hydroxyapatite. Modification of PGEs with copper oxide was performed by attaching copper to the surface. Copper is widely used in different areas due to its powerful physical and chemical properties. Copper is more economical and has excellent stability compared to other metals [36]. Because of its better electrochemical activity and ability to promote electron transport at low potential, nanostructured CuO is a promising choice for biosensing surface modification [37; 38]. [PERSON] et al. reported in their study that when they examined the perphonation of CuO-modified biosensors, a rise in concentration resulted in an increase in sensitivity. The increase in sensitivity was due to a good electron transfer and load transfer [39]. CuO was used because of its easy accumulation on the PGE surface. Based on the literature, using copper oxide solution will achieve the desired degree of accumulation on the surface of PGEs. CuSO\({}_{4}\) and NaOH solutions were prepared at different concentrations and ratio (Table 1). Each PGE was immersed in a CuSO\({}_{4}\)*5H\({}_{2}\)O solution prepared at different concentrations for 5 min and in a NaOH solution for 5 min. Our electrodes were removed from the solution and left to dry overnight indoors. CuO held onto the hydroxyapatite, and Cu\({}^{+2}\) ions were deposited on the surface. For signal measurements, a 5 mM Fe(CN)\({}_{6}^{-3/-4}\) solution containing 0.1 M KCl was prepared from K\({}_{3}\)Fe(CN\({}_{6}\)) and K\({}_{4}\)Fe(CN\({}_{6}\)). Measurement parameters were 1 V/s scanning speed from \(-\)0.5 V and +0.8 V, based on the literature. The electrochemical behavior of PGEs was investigated using the cyclic voltammetry (CV) method. Each measurement was repeated 3 times with different tips. After the electrode surfaces were modified with HAP and copper oxide, they were functionalized with the COVID-19 nucleocapsid antibody. Figure 1 shows the processes necessary for the binding of COVID-19 nucleocapsid antibody to the surface of HAP and copper-oxide-modified electrodes. In order to bind antibodies to the surface of the PGEs, amine groups must be formed on the surface. Cu particles held on HAP interact with ammonia (NH\({}_{3}\)) to form amine groups. As a result of the repulsive electrostatic mechanism between amine groups, it offers higher stability than CuNPs and is employed to bind the N antibody [40; 41]. Modified PGEs were immersed in ammonia for 15 min and then dried in an oven at 60-70 \({}^{\circ}\)C for 15 min [41; 42].