\section{Materials and Methods}

\subsection{Reagents and Instruments}

Paracetamol standard (99.9\%), graphite powder, potassium chloride (KCl), potassium ferricyanide (K\(_3\)[Fe(CN)\(_6\)]), potassium ferrocyanide (K\(_4\)[Fe(CN)\(_6\)]), and all other reagents were purchased from Sigma-Aldrich (USA). Phosphate buffer saline (PBS) solutions with various pH values (4.0–9.0) were prepared using 0.1 M Na\(_2\)HPO\(_4\) and 0.1 M NaH\(_2\)PO\(_4\). All solutions were prepared using deionized water (18.2 M\(\Omega\) cm) from a Milli-Q water purification system (Millipore, USA).

Electrochemical measurements were performed using an Autolab PGSTAT302N potentiostat (Metrohm, Netherlands) controlled by Nova 2.1 software. A conventional three-electrode system was used throughout the experiments:
\begin{itemize}[leftmargin=0.6cm]
    \item Working electrode: Glassy carbon electrode (GCE) with 3 mm diameter
    \item Counter electrode: Platinum wire
    \item Reference electrode: Ag/AgCl (saturated with 3 M KCl)
\end{itemize}

\subsection{Fabrication of GO/GCE}

Graphene oxide (GO) was synthesized using the modified Hummers' method \cite{hummers1958}. In a typical synthesis, graphite powder (2 g) was added to a mixture of concentrated H\(_2\)SO\(_4\) (50 mL) and NaNO\(_3\) (1 g) under stirring in an ice bath. KMnO\(_4\) (6 g) was gradually added while maintaining the temperature below 20°C. The mixture was then heated to 35°C and stirred for 30 minutes. Deionized water (100 mL) was slowly added, followed by H\(_2\)O\(_2\) (10 mL, 30\%) to terminate the reaction. The resulting GO product was washed repeatedly with deionized water and HCl (5\%) until the pH became neutral. Finally, the GO was dried at 60°C for 24 hours.

The GO dispersion (1 mg mL\(^{-1}\)) was prepared by ultrasonication of GO powder in deionized water for 30 minutes. Prior to modification, the bare GCE was polished sequentially with 0.3 \(\mu\)m and 0.05 \(\mu\)m alumina slurries on a polishing cloth, then rinsed with deionized water and sonicated in ethanol and deionized water for 5 minutes each. Then, 5 \(\mu\)L of the GO dispersion was drop-casted onto the polished GCE surface and dried at room temperature for 2 hours to obtain the GO/GCE.

\subsection{Electrochemical Measurement}

Cyclic voltammetry (CV) was performed in 0.1 M PBS containing 5 mM [Fe(CN)\(_6\)]\(^{3-/4-}\) at a scan rate of 50 mV s\(^{-1}\) to characterize the modified electrode. The potential range was set from -0.2 V to +0.6 V.

Differential pulse voltammetry (DPV) was used for paracetamol detection. The DPV parameters were optimized as follows:
\begin{itemize}[leftmargin=0.6cm]
    \item Pulse amplitude: 50 mV
    \item Pulse width: 50 ms
    \item Scan rate: 20 mV s\(^{-1}\)
    \item Potential range: 0.2 V to 0.8 V
\end{itemize}

All measurements were conducted at room temperature (25 ± 1°C). Before each measurement, the electrode surface was cleaned by cycling in PBS buffer until a stable background current was obtained.

\subsection{Optimization of Experimental Conditions}

Several experimental parameters were optimized to achieve the best analytical performance:

\subsubsection{pH Optimization}
The effect of pH on paracetamol oxidation was investigated over the range of 4.0–9.0 using 0.1 M PBS containing 50 \(\mu\)M paracetamol. DPV measurements were performed at different pH values, and the peak current was recorded.

\subsubsection{Accumulation Time Optimization}
The accumulation time was optimized from 30 to 120 seconds by measuring the DPV response of 50 \(\mu\)M paracetamol at the optimized pH.

\subsubsection{Scan Rate Study}
The effect of scan rate on the electrochemical behavior of paracetamol was studied using CV at scan rates ranging from 20 to 200 mV s\(^{-1}\) in 0.1 M PBS (pH 7.0) containing 50 \(\mu\)M paracetamol.

\subsection{Analytical Procedure}

Paracetamol standard solutions with various concentrations (0.5–100 \(\mu\)M) were prepared by diluting the stock solution (1 mM) with 0.1 M PBS (pH 7.0). DPV measurements were performed for each concentration, and the peak current was recorded. The calibration curve was constructed by plotting the peak current against paracetamol concentration. The limit of detection (LOD) was calculated using the formula:

\begin{equation}
\text{LOD} = \frac{3\sigma}{m}
\end{equation}

where \(\sigma\) is the standard deviation of the blank measurements (n = 10) and \(m\) is the slope of the calibration curve.

\subsection{Real Sample Preparation}

Commercial paracetamol tablets (Paramex\textregistered, labeled 500 mg per tablet) were purchased from a local pharmacy. Ten tablets were weighed and ground into a fine powder. An amount equivalent to one tablet was dissolved in 100 mL of PBS (pH 7.0) and sonicated for 20 minutes. The solution was then filtered through a 0.45 \(\mu\)m membrane filter. The filtrate was diluted appropriately with PBS to obtain a final concentration within the linear range. Recovery studies were performed by spiking the sample solutions with known amounts of paracetamol standard.

\subsection{Data Analysis}

All electrochemical experiments were performed in triplicate. Data were analyzed using Nova 2.1 software and OriginPro 2021. Results are presented as mean ± standard deviation (SD). Statistical analysis was performed using one-way ANOVA with a significance level of p < 0.05.