## 3 Results and Discussion
### Antigen Detection by Electrochemical Method
As a result, the modified electrode system we designed provides a very precise measurement for the rapid detection of the N protein of the SARS-CoV-2 virus at very low concentrations (even less than nanograms at 1 mL) with a pencil tip, which is a very easily accessible material. The test time takes 11 min in total, including the incubation time of the modified electrodes with the sample (10 min) and the time taken to take two SWV measurements before and after incubation (1 min). Consequently, the designed diagnostic method offers a fast and inexpensive method for detecting SARS-CoV-2 virus. According to the results shown in Table 2, the performance of HAP-Cu PGE/SARS-CoV-2 antibody and additional electrochemical perception approaches for SARS-CoV-2 diagnosis has been described. Furthermore, HAP-Cu PGE/SARS-CoV-2 antibody enables high detectability (LOD = 5.08 \(\times\) 10\({}^{-10}\) g/mL), quick testing (11.0 min), and cheap production costs ($1.50 per test). Effectively, our electrochemical detection method made it possible to speed up response time, advance the detection limits, and widen the linear range. Of all of them, we were the lowest cost. In addition, N proteins were used to provide high sensitivity for virus detection. Our aim was to modify a PGE, which is one of the parts of an electrochemical system based on electrochemical spectroscopy measurement, to be used in the detection of viral structures and to measure its continuous usability. For virus detection, it was ensured that the surface area was increased, and pores were formed by precipitating HAP crystals on the PGE. The surfaces of the HAP-coated PGE were processed in the form of copper oxide coating modification in different concentrations. Thus, the pencil graphite electrodes were modified with different concentrations of copper sulphate, different concentrations and proportions of sodium hydroxide, and the micro/nanocrystalline structures were accumulated on the surface of the electrodes in the form of copper oxide. Modified electrodes were examined morphologically with SEM; it was observed that copper oxidizes by depositing HAP crystals on the electrode surface, increasing the targeted surface area, and depositing copper oxides on the electrode surface by attaching to HAP crystals. In the electrochemical characterization part, unmodified PGEs and modified PGEs carried out at different solution concentrations and at different rates were characterized by cyclic voltammetry signals. In these studies, solutions of NaOH 1/1, 1/3, and 1/5 in different concentrations and ratios were used with hydroxyapatite in different concentrations of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sensor** & **LOD** & **Goal** & \
\begin{tabular}{c} **Electro-** \\ **Chemical** \\ \end{tabular} & **Linear Range** & **Time (min)** & **Reference** \\ \hline HAP-Cu & & & & & \\ PGE/SARS-CoV-2 antibody & 5.08 \(\times\) 10\({}^{-10}\) g/mL & N-Protein & SWV &
\begin{tabular}{c} 6.4 \(\times\) 10\({}^{-14}\) \(\mathrm{\SIUnitSymbolMicro}\) \\ 10\({}^{-9}\) g/mL \\ \end{tabular} & 11.0 & This study \\ \hline LEAD & 1.96 \(\times\) 10\({}^{-13}\) g/mL & SP & SWV & 1 \(\times\) 10\({}^{-14}\)-1 \(\times\) 10\({}^{-9}\) g/mL & 6.5 & [34] \\ \hline Multiplex RCA & 1 copy/uL & N ve S gene & DPV & 1–1 \(\times\) 10\({}^{9}\) copy/uL & 31.0 & [49] \\ \hline Paper/GO/SARS-CoV-2 antibody & 9.6 \(\times\) 10\({}^{-10}\) g/mL & SARS-CoV-2 IgG ve IgM & SWV & 1 \(\times\) 10\({}^{-9}\)-1 \(\times\) 10\({}^{-6}\) g/mL & 46.0 & [50] \\ \hline SPE/CB/SARS-CoV-2 antibody & 19 \(\times\) 10\({}^{-9}\) g/mL & N ve S protein & DPV & 0.5 \(\times\) 10\({}^{-4}\)–20 \(\times\) 10\({}^{-6}\) g/mL & 31.0 & [51] \\ \hline Au@SCX-TB-RGO-LP-Target/Au@Fe3O4 & 200 copies/mL & SARS-CoV-2 RNA & DPV & 1 \(\times\) 10\({}^{-17}\)-10\({}^{-12}\) mol/L & 180.0 & [52] \\ \hline Cotton-tipped electrode/SARS-CoV-antibody & 8.0 \(\times\) 10\({}^{-13}\) g/mL & N protein & SWV & 1 \(\times\) 10\({}^{-12}\)–1 \(\times\) 10\({}^{-6}\) g/mL & 21.0 & [53] \\ \hline \hline \end{tabular}
\end{table}